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The strongly doped semiconductor Fe_1−xCo_xSi displays a dome of helimagnetic order for 0.05 ≤ x ≤ 0.7. We report small angle neutron scattering of the magnetic structure in the skyrmion lattice phase of Fe_1−xCo_xSi for x = 0.2 and magnetic field parallel to a crystallographic (100) direction. We observe twelve equally spaced maxima of scattering intensity on a ring, with an underlying six-fold symmetry of two sets of six spots. The intensity distribution suggests the formation of two degenerate skyrmion lattice domain populations with respect to the four-fold symmetry of the (100) directions in the scattering plane.

The structure of the complex intermetallic compound T-Al₃Mn, an approximant of the decagonal quasicrystal, persists by substitution of Pd or Fe for Mn. Magnetically spin glass behaviour is obtained. Relative to the mother compound substitution of Pd (Fe) decreases (increases) the freezing temperature T_f. The time dependence of the magnetic moment points to broad activation energy distributions with similar shape but different centre of gravity. For comparable temperatures the Fe containing compounds exhibit larger height of the barriers than those without and with Pd substitution. For the decagonal quasicrystal the mean effective activation energy is reduced compared to the one for the T-phase compounds.

The anisotropy of magnetoresistance (MR) is studied in magnetic phases of Pr¹¹B₆ and NdB₆ by sample rotation technique in fixed magnetic field (μ₀H≤8T) to be perpendicular to the axis of rotation I||<110>. The obtained ρ(ϕ, T₀, H₀) data allowed us to estimate the maximal value of MR anisotropy which approaches the values ~18% and ~8% for PrB₆ and NdB₆ in commensurate (C) phase (T< T_M~4.6K for PrB₆ and T<T_N~7.7K for NdB₆) at T=2.8K in the field interval 3T≤μ₀H≤4T. In the vicinity of phase boundary to incommensurate (IC) magnetic phase of PrB₆ (T_M<T<T_N~6.7K) the strong enhancement MR anisotropy along the direction H||<110> is obtained. The behavior of the field dependencies of MR for PrB₆indicates an existence of new magnetic phase at T<T_M for the direction H||<110>. The analysis of Δρ/ρ(ϕ, T₀, H₀) data allowed us to conclude about a significant role of 5d-spin polarization in the formation of magnetically ordered state in these compounds.

Magnetite (Fe₃O₄) is a magnetic material with potentially interesting properties for applications in spintronics. The promising properties of this oxide are unfortunately affected by several kinds of structural defects which locally modify the electronic structure. We have calculated the density of states in the vicinity of the important defects which are the antiphase boundaries. The calculations have been performed with the LSDA+U approximation. This approximation is suitable to study transition metal oxides with important correlation effects between 3d electrons, as we have first shown for perfect bulk magnetite, by comparing the density of states calculated with the LSDA, LSDA+U, and PBE0 approximations. Our calculations show that antiphase boundaries are responsible for the existence of new localised electron states (defect states) which modify the electric properties, the energy gaps and the Fe magnetic moments.

In the presented paper we are focused on the magnetic properties of the hexagonal Yb_xGd_1-xNi₅ intermetallic compounds. Based on wide-ranging SQUID magnetometer (Quantum Design MPMS, temperature from 1.9K to 400K and magnetic field up to 7T) series of different magnetic measurements were carried out. It was shown that saturation magnetization and the Curie temperature strongly depends of Yb concentration. Moreover, the so-called field cooling – zero field cooling (FC-ZFC) curves reveal an unexpected existence of negative magnetization M in a certain temperature range. The possible origin of the diamagnetic behaviour, observed in doped samples, has been rather ascribed to the interaction between more than one opposite aligned magnetic sublattices.

Neutron scattering experiments with polarization analysis have established that Fe_100−xZr_x glasses have non-collinear ferromagnetic structures. The magnitudes of the non-collinear components are enhanced by small fractions of α-Fe precipitated in these glasses. Mössbauer spectroscopy has therefore been used to measure the fractions of α-Fe in two samples each of Fe₉₀Zr₁₀ and Fe₉₂Zr₈ metallic glasses, which are confirmed to contain up to 6 atomic % α-Fe. The measured fractions and their influence on the magnetic structures are discussed in the context of the existing literature on nanocrystalline FeZr.

A study of the low field magnetization and specific heat in magnetic fields up to 9 T of Nd_2/3Ca_1/3MnO₃ perovskite in the 2-30 K temperature range has been done. All the specific heat data show broadened Schottky-like anomaly below 20 K. We suppose that such a behavior originates from the Nd magnetic ordering caused by the splitting of the Nd³⁺ ions ground-state doublet (GSD) in the effective molecular field H_ex of Mn spin system supplemented by an applied external magnetic field. The zero field GSD splitting is an evidence of a strong exchange coupling between Nd and Mn magnetic subsystems. The Nd-ions magnetic ordering introduces an additional contribution to the ferromagnetic moment producing anomalies of the field-cooled and zero-field-cooled magnetizations of the system below 28 K. The broadened Schottky-like anomalies found are fitted for every field by a set of three Schottky functions. Applied magnetic field extends the anomaly region and shifts it to the higher temperatures. Splitting of the higher crystal field Kramers doublets gives an additional contribution to the heat capacity under magnetic fields. The GSD g-factors g_|| and g_⊥ was estimated as 3.4 and 2.2, respectively, and H_ex as 9 T.

A three-dimensional Monte Carlo model has been used for theoretical description of magnetic and magnetocaloric properties of Ni_2.18Mn_0.82Ga Heusler alloys undergoing first order magnetostructural phase transition. For this alloy, calculated temperature dependence of magnetization, magnetic contribution to the total specific heat and magnetic entropy change ΔS_mag are in good qualitatively agreement with available experimental data.

The magnetic structure of Er₃Cu₄Ge₄ was previously studied down to 1.5 K by Wawrzyńska et al. who found that the Er(2d) sublattice orders at 8 K in a commensurate, doubled structure (k_d = [0 1/2 0]) with an Er moment close to the free-ion value of 9 μ_B. By contrast, the Er(4e) sublattice ordered at 3.5 K in an incommensurate structure (k_e = [0 0.883(2) 0]) with a greatly reduced moment (about one-third of the free-ion value). We have extended this study by high-resolution neutron powder diffraction down to 0.34 K in an effort to establish if the Er(4e) sublattice eventually locks-in and attains its free-ion moment (in keeping with our ¹⁶⁶Er Mössbauer work). We observed a change in the Er(4e) order, beginning at 1.4 K, leading to a magnetic structure at 0.34 K which has almost locked-in but with (k_e = [0.1 2/3 0]) and a still-reduced Er moment.

In Er(Co,Mn)O₃ an avalanche mechanism occurs under the application of moderate magnetic fields, leading to the rotation of the ferromagnetic domains in one or more steps, depending on the rate of variation of the applied field. In order to deeply investigate the influence of the microstructure, we have elaborated nanosized ErMn_0.5Co_0.5O₃ compound by a citrate method, prepared at 700 °C and further calcined at increasing temperatures which assure the formation of very reactive grains of small size (~25 nm) and facilitate their full oxidation. Grains progressively grow from 20-30 nm up to several hundred nm, reaching good percolation at 950 °C (100 nm average size) for which a sudden jump is observed in the magnetization cycles M(H). We have simulated domains rotation by freezing a gel-type solution prepared from nanosized grains, orienting the individual spins on the paramagnetic state and subsequently freezing the gel to form single ferromagnetic domains.

The magnetic behaviour of Ni₂Mn_1+xSn_1−x ferromagnetic (FM) shape memory alloys are investigated with a focus on the exchange coupling and the ground state magnetic properties. Large exchange bias in the field cooled state of x = 0.36, 0.44 alloys is observed, which strongly varies with the cooling field. The observed exchange coupling was found to originate right from a characteristics step like anomaly in the zero-field-cooled magnetization versus temperature data. The step like anomaly shows clear frequency shift in the ac susceptibility data, indicating the development of spin frustration arising from the incipient antiferromagnetic (AFM) correlations in the otherwise FM alloy. Interestingly, the zero field cooled magnetic hysteresis loops in the studied samples show constricted double loop structure, which on field cooling shifts asymmetrically depending upon the sign of the cooling field. This indicates the complex AFM domain structure originating from the FM/AFM coupling.

NdCoSi and NdCoGe order antiferromagnetically at T_N = 7.5 and 8.3 K respectively. These ternary compounds absorb easily hydrogen at 523 K under a hydrogen pressure of 4 MPa; the resulting hydrides are stable in ambient conditions and crystallize in the tetragonal ZrCuSiAs-type structure. Investigation of the hydrides by magnetization and electrical resistivity measurements and also by neutron powder diffraction reveals their ferromagnetic behaviours appearing below T_C = 20.5 and 15.9 K respectively for NdCoSiH and NdCoGeH. Their magnetic structures are collinear with Nd-moments (2.28 and 2.35 μ_B at 1.5 K for NdCoSiH and NdCoGeH respectively) aligned along the c-axis as observed previously for NdFeSi. The antiferromagnetic-ferromagnetic transition evidenced during the hydrogenation can be explained by a change of the number of conduction electrons linked to the H-insertion as observed by electronic structure determination.

The real energy-free levitation exists with the help of diamagnetic material. Its ultra-high sensitivity to force is particularly attractive to micro/nano force sensing. A key parameter: Levitation Stabilizing Local Range, LR (allowable moving range of the floater) is critical to the load and self-rotating performance. Besides, larger LR reduces the energy loss due to the eddy current and has greater application potential. Recently, an idea of extending the LR by a modulating coil array has been validated using numerical simulation. This paper takes the next move to carry out an experimental study on the shape effect of stacked coil arrays with different currents on LR.

We present a theoretical study on the structural, electronic and magnetic properties of Mn-doping in amorphous group IV-semiconductors. In order to understand the effect of local disorder on the properties of the system, we consider the ST12 structure, a 12-atom unit cell which is known to locally reproduce the coordination of amorphous semiconductors. Based on ab-initio calculations we predict enhanced magnetic properties and higher solubility of Mn ions in model amorphous systems suggesting that amorphous semiconductors can represent an alternative route for spintronic applications.

The most general form of the exchange interaction for Mn atoms in TbMnO₃is derived, including nearest neighbour and next-nearest neighbour interactions. It is found that for each exchange path there are six independent exchange terms, leading to a rather complicated Hamiltonian. Experimentally determined constraints on the Hamiltonian are discussed. Degeneracy lifting in the spin-wave excitation spectrum is also discussed.

Ternary mixed magnetic Co_1−xMn_yNi_x−yCl₂·2H₂O has as its components three well studied antiferromagnets. Each is characterized by MCl₂MCl₂M...chemical and structural chains, with intrachain exchange interactions antiferromagnetic for the Mn component but ferromagnetic for the other two components. Competing ferromagnetic and antiferromagnetic intrachain exchange interactions occur in two different pairwise combinations. Reported here is the magnetic behavior of an equimolar mixture of the three components. One maximum appears in the magnetic susceptibility vs temperature, at 4.85 ± 0.05 K, a quite interesting result since decidedly lower than the locations of susceptibility maxima in the pure components. A pronounced upturn in the susceptibility below 2.3 K also appears. Magnetization vs field isotherms display increasingly strong convex upward curvature and associated hysteresis with decreasing temperature. All of these characteristics differ markedly from those of the pure components.

A new ternary compound Np₃Co₂Si₇ was found to crystallize in the orthorhombic La₃Co₂Sn₇-type structure, space group Cmmm (N°65) with room temperature lattice parameters determined from powder diffraction as follows: a = 4.000 (1) Å, b = 24.491 (8) Å, c = 4.020 (2) Å, (V = 393.8 ų). Np₃Co₂Si₇ orders ferromagnetically with a Curie temperature of about 32 K. U₃Co₂Si₇ has also been prepared for comparison. It is isostructural to Np₃Co₂Si₇with lattice parameters a = 4.010 (6) Å, b = 24.389 (2) Å, c = 4.022 (5) Å. An annealing of U₃Co₂Si₇ at 1223 K for 10 days is required to obtain it as a single phase. U₃Co₂Si₇ was reported in the literature as a ferromagnet. However, our measurements show a paramagnetic behavior and the presence of a weak ferromagnetic impurity.

Magnetic structures and properties of the quasi-ternary layered intermetallic La_1-xDy_xMn₂Si₂ (0 ≤ x ≤ 1) compounds were investigated using magnetic measurements on single crystals and neutron powder diffraction. It was shown that various magnetic structures and magnetic phase transitions, which are observed in the compounds with different Dy content, arise as result of changes in the intralayer Mn-Mn distances and competitions of the Mn-Mn, Mn-Dy and Dy-Dy exchange interactions.

The ESR measurements of the EuB₆ single crystal samples were executed on frequency 9.25 GHz in TE₁₀₂ rectangular cavity in the temperature range from 15 to 300 K. We used samples of identical form and size, but different crystallografic orientation to estimate their magnetization. At T = 30 – 40K was observed the magnetic phase separation, which, most likely, is accompanied also by charging separation. The anisotropy magnetization of more intensive magnetic phase (with anti-Kondo interaction) along the different crystallographic directions was found above a temperature of the ferromagnetic transition. We conclude that this result connect with existence the ferronic states and charging separation in the EuB₆ single crystal. Estimations of angular distribution of the magnetic moment of ferrons in EuB₆ are made.

Electronic structure calculations in the local spin-density approximation have been carried out on the recently synthesized trimer (CrCl₃)₃ exhibiting antiferromagnetic ordering below T_N =55 K. The calculated magnetic coupling constant, the paramagnetic Curie temperature, and the decrease of the magnetic moment below T_N are well reproduced by the calculations. An interpretation of the spin structure is given that is consistent with all experimental data currently available.

The dynamic antiferromagnetic phase present in CeFe₂ gets stabilized with Ga and Si substitutions. Phenomena such as strain-induced first order jumps in the magnetization curves, asymmetry between the M-H curves during the increasing and decreasing field cycles, the envelope curve being inside the virgin curve, occur in these compounds. Temperature and time dependences of magnetization show that the compounds possess glassy behavior at low temperatures. Multi-step magnetization behavior, unusual relaxation effect, thermal and magnetic history dependence, which are signatures of the martensitic scenario due to the strong magneto-structural coupling, are found to be present in this system. We also show that one can induce the magnetization steps with the help of appropriate measurement protocol. Detailed magnetization relaxation studies have been carried out to understand the dynamics of magnetic phase transition.

The stacked triangular lattice antiferromagnet GdPd₂Al₃ has a helical magnetic structure of extremely long incommensurate modulation below T_N2 (= 13.3 K). In order to observe the dependence of the magnetic structure on magnetic field applied in the spin plane, we performed resonant magnetic x-ray diffraction experiments in magnetic fields up to 0.15 T at 4 K and observed a first-order incommensurate-to-commensurate transition around 0.04 T. This result indicates that the free energy has a double-well structure around the K-point.

The magnetic structure of CePd₅Al₂, crystallizing in the tetragonal ZrNi₂Al₅-type structure, was studied by neutron diffraction technique using single crystals. This compound exhibits antiferromagnetic (AF) orderings at T_N1 = 4.1 K and T_N2 = 2.9 K. Below T_N1 magnetic peaks were observed at Q = (0.235, 0.235, 0) and its equivalent positions. Our analysis of the Q-dependence of the peak intensities suggests a sinusoidally modulated magnetic structure where the magnetic moments of 2.0(1) μ_B/Ce align along the c-axis. Third-order harmonics were also observed at temperatures from 3 K to 0.8 K, suggesting that the magnetic moments square up in the ground state. Inelastic neutron scattering measurements revealed well-defined crystalline electric field (CEF) excitations at 21.3 and 22.4 meV. The derived wave functions of the CEF ground state consist primarily of |±5/2⟩, which reproduce the anisotropic behavior of the magnetic susceptibility and isothermal magnetization in the paramagnetic state.

We have prepared a NdCr₂Si₂C single crystal by Czochralski technique and investigated it by heat capacity and magnetization measurements in the temperature range 2-300 K and magnetic fields up to 6 T applied parallel to the principal crystallographic axes. A strongly anisotropic ferromagnetism has been observed with the T_C = 24 K and the ordered magnetic moment oriented entirely along the c-axis of the tetragonal structure whereas the basal plane response is weak paramagnetic. We also studied the electronic structure of the compound by first-principles DFT calculations, which predict existence of a stable Cr magnetic moment. Results of our present experiments do not allow resolving the question on the existence of the ordered Cr moments unambiguously.

An orbital ordering effect is observed in YbVO₃ around 170 K while the crystal structure is orthorhombic (space group pnma). A monoclinic transition has been reported below T_N = 104 K, while according to recent specific heat measurements, it occurs at 170 K. The crystal structure of YVO₃ at 300 K is also orthorhombic. It becomes monoclinic at T_c = 200 K and back orthorhombic at T = 77 K. Spins order into the C-type antiferromagnetic structure below T_N1 = 116 K and the order changes into the G-type antiferromagnetic structure below T_N2 = 77 K. Controversial interpretations of YVO₃ Raman active excitations have been reported. For instance the 489 and 679 cm^-1 excitations have been assigned either to phonons or orbitons in two recent studies. In this communication we present a micro-Raman study of YbVO₃ and YVO₃ Raman active excitations as a function of temperature in order to trace the multiple phase transitions. Also by comparing the two single crystals spectra and previous studies in rare-earth manganites, high energy Raman active excitations are tentatively assigned.

In MnSi the application of a small magnetic field destabilizes the helimagnetic order in a narrow temperature interval just below the helimagnetic ordering temperature and stabilizes the formation of a hexagonal lattice of skyrmions, i.e., a lattice composed of a type of magnetic vortex lines. We have studied the skyrmion lattice in MnSi using a cold triple-axis spectrometer. Our data suggests that the skyrmion lattice represents a three-dimensional spin structure. The collective spin excitations of the skyrmion lattice are strongly reminiscent of the rich spectrum of helimagnon bands, recently shown to be a universal property of the helimagnetic state of MnSi in zero magnetic field.

We report on the magnetic structure in PrCuAl as determined by powder neutron diffraction. This compound crystallizes in the hexagonal ZrNiAl-type structure and the bulk measurements indicated an antiferromagnetic order in PrCuAl below T_N = 7.9 K. Our neutron-diffraction data confirmed the antiferromagnetic order below T_N and revealed the propagation vector (1/3, 1/3, 1/3). The Pr magnetic moments lie within the basal plane and form a triangular structure. This magnetic structure differs from antiferromagnetic structures observed in other RCuAl or RNiAl compounds that are mostly characterized by (1/2, 0, q) propagation.

Thermal expansion and magnetostriction measurements of the superconducting ferromagnet UGe₂ under pressure were carried out. The temperature dependence of the thermal expansion coefficient shows a peak at the Curie temperature. When pressure is varied, the peak exhibits a maximum in the vicinity of a tricritical point (TCP), which separates the second-order phase transition from the first-order transition. From results of these measurements, we first construct the magnetic phase diagram including the TCP (P_TCP ~ 12.5 kbar). We also show that two lines characterizing the metamagnetism and the magnetic susceptibility emerge from the TCP. We argue that these magnetic properties in the vicinity of the TCP can be understood within a phenomenological frame of spin fluctuations.

The substitution effects have been measured on the magnetization and lattice parameters of Gd_100-xZr_x and Gd_100-yY_y alloys. The magnetic transition temperatures are affected by Zr more effectively than by Y. The Curie temperature decreases with increasing Zr content; the coefficient is -3.6 K/at.%. The effect on spin-reorientation transition temperature is of particular note. With increasing Zr content, it increases with the coefficient +7.9 K/at.%, which is about three times as large as that of Gd_100-yY_y alloy. From the measurement of lattice parameters, it is confirmed that Zr-substitution affects the temperature dependence of c-axis value more than that of a-axis one.

We present here the first-principles dynamical CPA (coherent potential approximation) combined with the tight-binding LMTO LDA+U method towards quantitative calculations of the electronic structure and magnetism at finite temperatures in transition metals and compounds. The theory takes into account the single-site dynamical charge and spin fluctuations using the functional integral technique as well as an effective medium. Numerical results for Fe, Co, and Ni show that the theory explains quantitatively the high-temperature properties such as the effective Bohr magneton numbers and the excitation spectra in the paramagnetic state, and describes the Curie temperatures semiquantitatively.

The results of powder neutron diffraction studies of an antiferromagnet YbCo₂Si₂with T_N,L=0.9 K and T_N,H=1.7 K were reported. An appearance of several superlattice peaks was observed at 0.48K, which characterizes the antiferromagnetic ground state in YbCo₂Si₂. By heating up to 1.1 K, to the intermediate phase, a substantial change in the magnetic reflection from the ground state was revealed. The magnetic peak shifts to lower angle, indicating longer periodicity in the intermediate phase. In addition, the intensity ratio of the magnetic peaks shows significant change between two phases. In contrast to other magnetic RCo₂Si₂, the simple two-sublattice magnetic structure characterized by an existence of the magnetic 1 00 reflection cannot account for the observed magnetic peaks at both temperatures, which suggests more complex magnetic structures in YbCo₂Si₂.

We analyzed the magnetic susceptibilities of several Cr spinels using two recent models for the geometrically frustrated pyrochlore lattice, the Quantum Tetrahedral Mean Field model and a Generalized Constant Coupling model. Both models can describe the experimental data for ACr₂O₄ (with A = Zn, Mg, and Cd) satisfactorily, with the former yielding a somewhat better agreement with experiment for A = Zn, Mg. The obtained exchange constants for nearest and next-nearest neighbors are discussed.

Effect of Mn substitution in paramagnetic metal CaRuO₃ was studied by magnetization measurements. Development of ferromagnetic order was observed for x ≥ 0.2 in CaRu_1-xMn_xO₃. The ratio of the effective moment and the spontaneous magnetization P_eff/M(0) is much larger than one in this ferromagnetic state. At low temperatures the antiferromagnetic order coexists with the ferromagnetic one and the magnetization is significantly suppressed with decreasing temperature. The magnetic phase diagram in CaRu_1-xMn_xO₃ with 0.3 ≤ x ≤ 0.8 estimated from the magnetization measurements is reported.

Ternary TmPt₂B crystallizes in the hexagonal CePt₂B-type structure (space group P6₂22). Magnetic and specific heat measurements reveal long range magnetic order at 12.5 K. Field dependent results of both quantities clearly evidence a predominant ferromagnetic phase. The total angular momentum associated with the Tm³⁺ electronic configuration becomes split by crystalline electric field effects. The overall splitting turns out to be larger than 170 K.

The time-dependent phase transition of a random magnetic system is investigated using a dynamic Monte Carlo simulation. The present random system is a simple dilution of magnetic atoms with Ising spins in a square lattice. The time-dependent order parameters are calculated as a function of temperature for several concentrations of magnetic atoms. The results indicate that as the averaging time becomes shorter, the phase transition becomes more gradual. This tendency is most pronounced around the percolation concentration. These results are compared to results of a recent NMR spin echo experiment on Mn_xCd_1-x(HCOO)₂·2(NH₂)₂CO reported by Kubo et al. (2007). Although the present simulation includes essentially no adjustable parameter, the results are in satisfactory agreement.

Magnetic properties, Curie temperatures, and transport properties of semi-Heusler alloys Cu_xNi_1−xMnSb are calculated as a function of the alloy composition from first-principles. The transition from the ferromagnetic state (NiMnSb) to the antiferromagnetic state (CuMnSb) gives rise to an abrupt change in the concentration dependence of magnetic moments and resistivity at about x = 0.7 while the Curie temperatures decrease monotonically with the Cu-content. Such behaviour can be understood as due to the onset of disorder in orientations of Mn-spins at x = 0.7. A simple account of magnetic disorder as based on the uncompensated disordered local moment picture provides also a good quantitative explanation of available experimental data.

We have successfully used magneto-transport measurement to study magnetization reversal of domain structure in single sub-micron magnetic disks of various sizes, 300nm ≤ diameter ≤ 5μm and 36nm ≤ thickness ≤ 48nm. The magnetoresistance (MR) resulted from the anisotropic magnetoresistance effect can reveals the magnetization reversal of a single disk as confirmed by the magnetic force microscopic results. For a fixed thickness, the remanent domain state can transits from the multi-domain to vortex states by decreasing the disk diameter. Here, the nucleation and annihilation fields of vortex core are determined from disk's hysteretic MR and analytically described by the micromagnetic LLG calculation incorporated with the "rigid" vortex model developed by Guslienko et al. under the consideration of magnetostatic, exchange, and Zeeman energies.

Influence of successive thermal annealing and magnetic field on First order antiferro (AFM) to ferromagnetic (FM) transition in the Pd substituted FeRh has been studied. With successive thermal annealing CsCl type bcc phase increases at the expense of fct (pseudo fcc) phase. Resistivity measurements do not show any transition in as-cast sample in contrast to annealed samples. AFM to FM transition temperature (T_N)is found to decrease with higher annealing temperature. With the application of magnetic field, T_N shift to lower temperature. These measurements show anomalous thermomagnetic irreversibility besides showing giant magnetoresistance across magnetic field induced first order AFM to FM transition.

A direct approach for calculating magnetic coupling constants is presented. For the double-bridged copper dimer [Cu₂F₆]^2- the results compare well with fully numerical calculations in local spin-density approximation.

In the case of 5f magnetism, the net increase of T_c due to the interstitial doping by hydrogen is generally understood as due to the increase of the density of states at the Fermi level, resulting from 5/band narrowing. The record value of T_c among UTX compound can be achieved in hydrides of U(Co,Fe)Sn series (T_c = 108 K for 15% of Fe). This concept, based on the 5f band formed by periodic array of U sites, does not work for Th doped compounds. In the case of U sublattice diluted by a non-magnetic element the expansion cannot have dominant effect, partly because already doping by non-magnetic Th leads to an expansion.

The rare earth tetraborides REB₄ with RE = Ho, Er, Tm, crystallize in a tetragonal lattice where the positions of the RE ions can be mapped to a Shastry Sutherland lattice. We have investigated the magnetic properties by means of magnetisation and specific heat experiments in a magnetic field. All compounds are anisotropic, with RE= Er, Tm they are strong Ising magnets, for RE = Ho we find xy anisotropy. In magnetic field we find complex behaviour with a number of different phases as a function of applied field, field direction and temperature. Remarkable is the observation of fractional magnetisation plateaux for magnetic field || (001) in HoB₄ and TmB₄.

The dynamical magnetic properties of a Cu(Mn) Heisenberg spin glass have been investigated by SQUID magnetometry. The zero-field-cooled magnetization has been recorded as a function of the temperature and time after specific cooling protocols, in order to study in detail the so-called memory effect displayed by spin-glasses.

A uniaxial pressure technique is developed to study the dependence of the magnetization up to 12 kbar using a SQUID magnetometer. The technique has been tested for the magnetic properties of a Metal-Organic Framework, Co(H₂O)₆(C₁₀H₄O₈), where C₁₀H₄O₈ is benzene-1,2,4,5-tetracarboxylate, along three orthogonal axes of single crystals. It displays anisotropic paramagnetic properties between 2 and 300 K without any phase transition. The isothermal magnetizations at 5 K show decrease with pressure when the field is applied along the b-axis but an increase along the a-and c*-axes. The results are interpreted as a change in the g-value tensor as a function of pressure due to compression along the Jahn-Teller distorted axis.

The phase diagram including the cubic, two kinds of tetragonal and the orthorhombic phases of the antiferromagnets Mn_1−xA_x (A = Ni, Ga, Rh and Au) which are known as the first-kind antiferromagnets, is explained theoretically. Our Hamiltonian is composed of the polynomial of the variables describing multiple spin density wave (MSDW) states, the coupling between MSDW states and lattice distortion and the elastic energy. The polynomial is derived from symmetry consideration. By calculating the partition function and the free energy, we show that the phase diagram is reproduced and elucidate the structure of MSDW at each phase and the condition of the appearance of the orthorhombic phase.

We consider (by the example of the structure Fe/Ga(Mn)As, investigated experimentally in [1]) magnetic properties of the planar nanostructure consisting of a ferromagnetic metal and a diluted magnetic semiconductor. In the framework of the mean field theory, it has been shown there occurs the considerable amplification of the ferromagnetism, induced by the ferromagnetic metal, due to the indirect interaction of magnetic impurities. That results in significant broadening the temperature range of the existence of the perceptible magnetization in the near-interface semiconductor area (run far beyond the interval limited above by the intrinsic Curie temperature of the magnetic semiconductor).

Precise magnetization processes of ZnCr₂O₄ were obtained by means of the Faraday rotation method subjected to ultra high magnetic fields up to 190 T generated by a single turn coil technique. In magnetization processes of ZnCr₂O₄, there observed a coplanar 2:1:1 canted state (the cant 2:1:1 phase) which is predicted by the theoretical model including a spin-lattice coupling. A magnetic-field width of the 1/2 plateau widens as the temperature increases.

We have performed the specific heat study in a new geometrically frustrated system, clinoatacamite Cu2Cl(OH)3 with the corner-sharing tetrahedron structure of the Cu²⁺ ions with 5=1/2 Heisenberg spin. At H=0 T, two anomalies are observed at T₂=18.1 K and at T₂=6.2 K. The specific heat decreases rapidly below T₂ and shows no anomaly down to T=150 mK despite the existence of the spin fluctuation shown in the μSR experiments. As the magnetic field is increased, the sharp peak at T₂ is broadened and shows a small reentrant behavior in the T – H phase diagram. On the other hand, the peak at T₁ shows no obvious change up to H=5 T. The entropy at T₁ is estimated as ~0.35Rln2 at H=0 T. These features may be caused by the two dimensional nature of the kagome antiferromagnets which are weakly coupled via Cu²⁺ ions at the triangular sites located in between the kagome layers.

CeIr₃Si₂ shows successive magnetic transitions at T_N1=4.1 K and T_N2=3.3 K. At T < T_N2 it shows three-step metamagnetic transitions below H=1.43 T. In this non-diluted compound a long-time variation of magnetic structure has been detected by means of magnetc susceptibility and time-resolved neutron scattering measurements. When a sample is rapidly cooled below T_N2, the magnetic Bragg peaks corresponding to the intermediate temperature phase (T_N2 < T < T_N1) are observed. The amplitude of these Bragg peaks gradually decreases with time. On the other hand, another group of Bragg peaks corresponding to the low temperature phase (T < T_N2) gradually grow with time. The characteristic time for these variations follows the Arrhenius law with an activation energy E_a/k_B =4 K. This is the first observation of long-time variation of magnetic structure in non-diluted uniform magnets.

The structure and magnetic properties of TbNi₂Mn_x compounds have been studied by the X-ray and neutron powder diffraction, magnetization and magnetostriction measurements. It was found that the compounds for x < 1 crystallize in a cubic MgCu₂ Laves phase structure, in spite of the R to 3d-metal ratio changes from 1:2 to 1:3. TbNi₂Mn shows a superstructural ordering (F3m space group). Magnetic measurements revealed a substantial growth of the Curie temperature from 29 K for TbNi₂ up to a maximum value of 160 K for TbNi₂Mn_0.5. Monotonous decrease of the magnetization, magnetostriction and increase of the coercivity are observed with the Mn alloying. The obtained data are interpreted assuming the formation of a magnetic moment at Ni site and a non-collinear arrangement of the Tb-ion magnetic moments.

The measurements of electrical resistivity ρ and thermopower S of the single-crystalline EuCo₂P₂ have been performed at temperatures from 2 K to 300 K under hydrostatic pressures up to 3 GPa. The temperature dependence of ρ and S show drastic changes at the critical pressure P_c, indicating a large modification of electronic structure around the Fermi level due to a pressure-induced structural and magnetic phase transition. The magnetic phase transition temperature increases linearly with increasing pressure, and shows a sudden increase at the critical pressure P_c, which correspond to the change of magnetic state from the localized Eu(4f) sub-lattice magnetism into the itinerant Co(3d) sub-lattice magnetism.

In a search for promising ferromagnets exhibiting a giant magnetocrystalline anisotropy (MCA) change by an external electric field, the MCA in the Fe-Co(001) alloy monolayers is determined by means of full-potential linearized augmented plane-wave method. A large MCA change by the electric field is found to appear in the Fe_0.75Co_0.25 monolayer, and a reasonable origin from a band structural change in the minority-spin d states is obtained, where a position of Fermi level relative to the d band level is a key factor for designing the giant MCA change.

The hyperfine coupling constant for ⁵⁹Co in the ferromagnetic state of a Heusler alloy Co₂TiGa in the high-field range has been estimated to be 170 kOe per 1 μ_B of the moment of cobalt atom from the high-field magnetic susceptibility of 2.3 × 10⁻⁶ emu/gOe measured at 5 K in a field range from 20 to 70 kOe and a reported positive high-field NMR shift of +0.83 percent. The value of the hyperfine coupling constant has been found to be even larger in the case of Co₂VGa. These features suggest that the positive hyperfine field at Co nucleus in the Co-based Heusler alloys is due to conduction electron polarization rather than a transferred hyperfine field.

The field dependence of the magnetization of a ferromagnetic Heusler alloy Co₂CrGa has been measured close to the Curie temperature of 488.0 (0.3) K up to an applied field of 50 kOe. The critical exponent δ has been determined to be 4.93 (0.30). The result is described by the theory by Takahashi for a weakly ferromagnetic itinerant electron system rather than by a conventional molecular field theory.

The aging behaviour of spin glass semiconductor Cd_1-xMn_xTe subjected to bond perturbation was studied by means of the direct change in the spin–spin interaction ΔJ using photo illumination, without change in temperature. Under the ΔJ perturbation, the maximum of relaxation rate of magnetization S at the time t_max is suppressed and t_max becomes longer with increasing the light intensity. The former is consistent with the droplet picture, but the latter indicates that the ΔJ perturbation accelerates the magnetic relaxation so as to contradict with the chaotic nature of spin configuration under the perturbation.

We report the study of magnetic and transport properties of binary intermetallic compounds RPd₃ (R: Tb and Er) and boron-filled perovskite compounds RPd₃B. Our results suggest that the magnetic and transport behavior of boron-filled compositions is substantially different compared to that of undoped compounds. For example, TbPd₃ and ErPd₃ exhibit negative magnetoresistance, while boron-filled TbPd₃B and ErPd₃B shows positive magnetoresistance. In addition, our results also suggest that there exists a strong correlation between magnetic and electrical-transport behavior of these systems.

Basic physical properties of the compound TmCu₂Ge₂, crystallizing with the tetragonal ThCr₂Si₂-type crystal structure, were studied down to 350 mK. The temperature dependencies of the magnetic susceptibility, the specific heat and the electrical resistivity exhibit distinct anomalies at T_N = 3.8 K that can be associated with the onset of an antiferromagnetic ordered state. Above this temperature the reciprocal susceptibility obeys the Curie–Weiss law with the paramagnetic Curie temperature θ_p equal to 1.2 K and the effective magnetic moment μ_eff equal to 7.12 μ_B. In the same region, the electrical resistivity shows a metallic character. In the ordered state, another clear peak in the specific heat is observed at T_t = 1.7 K that likely manifests a change in the magnetic structure.

The measured heat capacity C of solids is thought in general to involve the change of the internal energy which is a state property. Therefore the electron density of states at the Fermi level is derived from the low-temperature data C=γ T + β T³. The La(Fe_0.88Al_0.12-xSi_x)₁₃ compounds are the antiferromagnets (AFM) T_N=200 K with 0<x<0.03 and the ferromagnets (FM) with 0.03<x<0.12 T_C=194 K. For x=0.024 and x=0.027 a metamagnetic phase transition from AFM to the field-induced FM (FIFM) occurs in low external magnetic field. The FIFM state of the sample remains if the magnetic field turns off. The measurements of the low-temperature heat capacity in this FIFM state have shown an increase of coefficient γ in comparison with the value both the AFM and the FM. The measured heat capacity depends on a thermodynamic path and thus is not a state property. The change of the work of the magnetostrictive strain of the sample as a part of the low-temperature heat capacity is discussed.

We present results of specific heat, magnetization and AC susceptibility measurements of SmPd₂Al₃ single crystal. The compound exhibits strong uniaxial magnetocrystalline anisotropy with the easy-magnetization direction along the hexagonal c-axis. A transition to a ferromagnetic ordering has been observed at T_C = 12.4 K followed by several order-order magnetic phase transitions below 5 K. The magnetization curves measured below 4 K exhibit a series of metamagnetic transitions in fields < 1 T, which indicates that the AF ground state is gradually destroyed with applying a magnetic field along the c-axis.

We propose the new approach to the problem of electrically controlled magnetic state: the electric field driven domain wall motion. The effect is demonstrated in iron garnet films in ambient conditions. The theoretical model based on inhomogenous magnetoelectric interaction provides with the necessary criteria of the effect and the way to maximize it.

The electrical resistivity ρ and thermopower S of metallic ferromagnets CuCr₂X₄ (X=S,Se or Te) are measured at low temperatures and in an external magnetic field of 1.5 Tesla. These properties are dominated by strong scattering effects in their ferromagnetic phase, resulting in (a) ρ(T) ~A T²dependence with very large value of coefficient A ≈ 1 − 3×10⁻⁸ Ω-cm/K² in resistivity and (b) a peak in the thermopower around T_C/3 . Thermopower is further increased by crystalline order and by the alignment of magnetic domains in an external magnetic field as has been found for Cu_1+x Cr₂Te₄. We have interpreted these effects as due to strongly coherent momentum conserving electron-magnon scattering giving a large T² – contribution in resistivity and a large drag effect in thermopower.

Heat capacity measurements of the cubic intermetallic RECu₅ (RE – heavy rare earths) compounds have been performed in the applied magnetic field up to 9 T and in the temperature range from 0.4 to300 K. The pollycrystalline samples have been prepared in ribbon shape by melt-spinning and subsequent annealing for stabilisation of the cubic phase. We have first observed the phase transitions into the magnetic ordered states in TmCu₅ (1.3 K), HoCu₅ (3 K) and ErCu₅ (1.6 K).

We propose a model which suggests that structural martensitic transitions are related to significant changes in the electronic structure, and are effected by high-magnetic fields. The magnetic field dependence is considered unusual as many influential investigations of martensitic transitions have emphasized that the structural transitions are primarily lattice dynamical and are driven by the entropy due to the phonons. We provide a theoretical framework which can be used to describe the effect of high magnetic field on the transition and lattice dynamics in which the field dependence originates from the dielectric constant. The model is compared with some recent experimental results.

We performed polarized neutron reflectometry (PNR) experiments on a 29 nm thick Au₉₃Fe₇ film. The measurements were done in a temperature range from 300 K to 2 K in a magnetic field up to 6 T applied in the sample's plane, at 2 K we took PNR data up to 11 T. The magnetization as determined by PNR can be described with a Brillouin function from 295 K down to 50 K assuming the magnetic moment of isolated Fe atoms, i.e. 4 |μ_B per Fe atom. Below 50 K the onset of the spin-glass freezing was observed as a strong reduction of the magnetic moment compared to the isolated Fe atom.

The current report is devoted to the study of magnetic properties of MnGeO3 single crystals with orthorhombic symmetry Pbca. MnGeO₃ single crystals have been grown by a flux method. A new data of magnetic measurements in magnetic fields up to 80 kOe are presented. A spin-flop transition is observed at H_sf=35 kOe and T=4.2 K along a-axis. The exchange and anisotropy fields are determined.

The B-T magnetic phase diagrams for TbCu₂Si₂ single crystal, having the tetragonal ThCr₂Si₂-type crystal structure, have been constructed from magnetic measurements. The compound orders antiferromagneticaly below T_N=11.9 K and shows another magnetic transition at T_t=9.1 K. In the easy magnetization direction of [110], a six-step metamagnetic process appears at low temperatures. Above T_t, it becomes a two-step metamagnetic one. There are seven ordered magnetic phases below the paramagnetic region in the B₁₁₀-T magnetic phase diagram. In the [100] magnetization process, three metamagnetic transitions appear at low temperatures. The magnetization process becomes a one-step one above T_t. The B₁₀₀-T phase diagram, composed of four phases, is very peculiar; a quad-critical point may appear. The magnetic structure for each phase has been discussed.

We have carried out ⁵⁵Mn and ¹⁴⁹Sm NMR measurements on SmMn_2−xFe_xGe₂ (0.05 ≤ x ≤ 0.6) at 4.2 K in order to elucidate the Fe substitution effect on the magnetic properties of SmMn_2−xFe_xGe₂. The NMR signals of Mn have been observed in the frequency range 180 – 210 MHz. The resonance frequency plotted against the Fe concentration shows an abrupt change between x = 0.3 and 0.4. This discontinuity suggests that the magnetic structure changes around x = 0.35 in the low temperature range where Sm and Mn sublattices order magnetically.

We study the exchange interaction between two magnetic impurities on the (110) surface of Cu. We find that the interaction is oscillatory and decays as 1/d² for large distances d of the impurities. We also show that the oscillations are governed by the Shockley-type surface state around the point of the Surface Brillouin zone. The asymmetry of the dispersion of this surface electronic band implies an anisotropic exchange interaction with respect to the relative position of the impurities.

Magnetization, specific heat, magnetic resonance and neutron diffraction measurements are used to study the magnetic structure of KFe(MoO₄)₂. This stacked triangular antiferromagnet (T_N=2.5 K) demonstrates an unusual breaking of the spin system into two intercalated and almost independent 2D subsystems. One is a collinear antiferromagnet with a simple spin-flop behavior. The other is a spiral magnet. The spin structure may be explained assuming two types of inequivalent magnetic planes with distorted triangular lattices of Fe³⁺(S=5/2) ions.

The specific heat (c_p) of the dilute antiferromagnet Fe_xZn_1-xF₂ has been measured in absence of external magnetic fields for x = 0, 0.26, 0.31, 0.34, 0.36, 0.38, 0.41, 0.45, 0.56, 0.88, 0.97, and 1.0. For x > 0.45, a sharp peak associated to the antiferromagnetic (AF) phase transition at T_N(x) is the only observed feature. For 0.31 ≤ x ≤ 0.41, this peak becomes smaller with decreasing x and a rounded bump appears at higher temperatures T. Closer to the percolation concentration (x_p = 0.24), the peak characteristic of the AF phase transition disappears and the rounded bump becomes the only observed feature. The low-Tc_p behavior confirms a crossover from AF long range order at large x to a spin glass behavior close to x_p. At intermediate x, the low-T joint signature of both phenomena indicates a coexistence of AF order and a cluster-glass phase. The x-dependence of the Néel temperature was accounted for using a simple phenomenological model.

In this paper we report the study of boron substituted SmFe₂ compounds belonging to the family of giant magnetostrictive materials RFe₂(R=Tb, Dy, Sm). The dc magnetization up to fields of 60 kOe and low field ac sucsceptibility has been measured in the temperature range 5-300 K. In SmFe₂, the temperature dependence of magnetization shows two magnetic transitions at 190 K and 80K in the zero field cooled state. The lower temperature transition is suppressed by the addition of even 1at%B as observed from the dc magnetization. The ac susceptibility shows a pronounced maximum close to 80 K in both the parent compound and in 1 at% B substituted one. This transition presumably occurs due to freezing of domain wall motion.

We report on the first multi-parameter investigation of a well-characterized sample of PrCuSi. The crystal structure is hexagonal, with a unique lattice site for each of the 3 atoms. Specific heat measurements indicate long-range magnetic ordering, and data of magnetic susceptibility confirm a paramagnetic-to-antiferromagnetic type transition at T_N = 5.1 K, in contrast to what was claimed previously in exploratory work on this compound. There is a well-defined anomaly in the electrical resistivity ρ(T) at T_N, below which a spin-density-wave expression accurately accounts for ρ(T) according to AFM ordering. A field-driven instability of the ordered state is furthermore indicated by a sharp feature in ρ(B) at B ≊ 0.75 T. Footnote for author AMS: Part of this work was performed during a research visit of AMS to the Max Planck Institute for Chemical Physics of Solids in Dresden, Germany.

X-ray diffraction, dc magnetization and magnetic susceptibility measurements were performed on polycrystalline Tb_3-xHo_xCu₄Sn₄ (x = 0-3) intermetallics. The X-ray data indicate that all the compounds crystallize in the orthorhombic crystal structure of the Gd₃Cu₄Sn₄-type and are antiferromagnets at low temperatures. Their reciprocal magnetic susceptibility obeys the Curie-Weiss law above the appropriate Néel temperatures with negative values of the paramagnetic Curie temperature, which is linear function of the de Gennes factor. Anomalous concentration dependence of the Néel temperature is observed.

We performed mechanical milling of the A15-type superconductor Nb₃Sn with transition temperature T_c = 18K. T_c decreased with increasing milling time, a result similar to that of Di, Loeff and Bakker, except that T_c seemed to decrease discontinuously with milling time. X-ray diffraction (XRD) patterns were taken using a conventional X-ray diffractometer with CuKa radiation at room temperature. The height of the Bragg peaks lowered and their width broadened after milling, but the patterns of the A15 structure persisted for up to 48 h of milling. We compared the XRD patterns of Nb₃Sn with simulations using REATAN-2000, and obtained evidence that the site random disorder of Nb and Sn atoms in the A15 structure caused the decrease in T_c.

The ternary silicides RE₆Ti_1.67Si₃ (RE = Ce, Pr, Nd, Gd, Tb and T = Co, Ni) crystallizing with the hexagonal Ce₆Ni_1.67Si₃-type structure has been recently extensively studied. It has been shown that some of these compounds exhibit interesting magnetocaloric properties. In this work, the magnetic structures of the Nd₆Ni_1.67Si₃ compound were determined by means of neutron powder diffraction. According to previous magnetization and specific heat measurements, this compound exhibits two successive magnetic transitions at 84K and 38K. The first transition corresponds to a ferromagnetic arrangement of the Nd-moments along the c-axis and the second one to a non-collinear ferromagnetic arrangement yielding a conical structure. The magnetic structures of Nd₆Ni_1.67Si₃ are compared to those of the Nd₆Co_1.67Si₃homologous ternary silicide.

We have investigated the magnetic orderings and the magnetic interactions in garnet antiferromagnets AgCa₂M₂V₃O₁₂(M=Mn, Co, Ni) and NaPb₂Mn₂V₃O₁₂, using the powder neutron diffraction and magnetization measurements. In the neutron diffraction measurements, we revealed that the magnetic structures in all samples measured are simple two sublattice antiferromagnetic structures with collinear magnetic moments. We also determined the exchange interactions from the critical magnetic fields H^sat observed in the high field magnetization processes up to 30 T.

We report magnetic, transport and neutron diffraction studies of the solid solution Mn_1−xCo_xSi. For the Mn rich compounds, a sharp decrease of the Curie temperature is observed upon cobalt doping and neutron elastic scattering shows that the helimagnetic order of MnSi persists up to x = 0.06 with a shortening of the helix period. For higher Co concentrations (0.06 < x < 0.90), the Weiss temperature changes sign and the system enters an antiferromagnetic state upon cooling (T_N=9K for x = 0.50). In this doping range, the antiferromagnetic coupling leads to a Kondo effect marked by a minimum in the resistivity. This scenario is supported by the scaling of the magnetoresistance with a T_K ≈ 6.5 K, close to the change in curvature of the resistivity and in agreement with the Weiss temperature from magnetic susceptibility. The sign change of the Weiss temperature and the transition from a helimagnetic to an antiferromagnetic ground state, with increasing the Co doping, point toward the existence of a quantum critical point at the composition Mn_0.94Co_0.06Si.

Electrical resistivity ρ and thermopower S of Nd_1−xTb_xCo₂ Laves phase quasibinary alloys (0 ≤ x ≤ 1) are investigated at temperatures from 2 K to 300 K. The magnetic transition temperature T_C, determined from resistivity magnetic anomaly, increases linearly with increasing x. The low temperature thermopower changes its sign from negative to positive at the critical composition where the relative orientation of total magnetization and cobalt 3d moment is changed. We propose that this change is related to the dependence of s-d scattering rate on relative polarization of conduction electrons and cobalt 3d band.

The C14 Laves phase TaFe₂ is a paramagnet. The magnetic properties and phase diagrams of V, Cr, Mn, Co and Ni substituted systems for Fe in TaFe₂ have been investigated using magnetization and NMR measurements. Antiferro- or/and ferromagnetic states appeared by the V, Cr and Mn substitutions. In the Cr and Mn cases ferromagnetic states appeared first (very weak in Cr case) and then the both change to be antiferromagnetic. In the V case only antiferromagnetic state appeared from the beginning of substitution. The susceptibilities of the slightly V-substituted compounds were, however, exchange-enhanced at low temperatures. The above mentioned magnetic changes are qualitatively the same to the previously reported Nb(Fe_1−xT_x)₂ systems. The electronic condition of the appearance of ferro- and antiferromagnetism can systematically be interpreted in both systems.

Manganese phosphide MnP has been investigated for decades because of its rich magnetic phase diagram. It is well known that the MnP exhibits the ferromagnetic phase transition at T_C = 292 K and the helical magnetic phase below T_N = 47 K at zero field. Recently, a novel magnetic phase transition was observed at T* = 282 K when the magnetic field is lower than 5 Oe. However, the nature of the new phase has not been illuminated yet. In order to reveal it, we performed the AC and the DC magnetization measurements for a single crystal MnP at very low field. A divergent behavior of the real and the imaginary part of the AC susceptibility and a sharp increase of the DC magnetization was observed at T*, indicating the magnetic phase transition at T*. Furthermore a peculiar temperature hysteresis was observed: namely, the magnetization depends on whether cooling sample to the temperature lower than T_N or not before the measurements. This hysteresis phenomenon suggests the complicated nature of the new phase and a strong relation between the magnetic state of the new phase and the helical structure.

The wave-vector (q) and doping dependences of the magnetic energy, iron moment, and effective exchange interactions in LaFeAsO, BaFe₂As₂, and SrFe₂As₂ are studied by self-consistent LSDA calculations for co-planar spin spirals. For the undoped compounds, the calculated total energy, E(q), reaches its minimum at q corresponding to stripe anti-ferromagnetic (AF) order. In LaFeAsO, this minimum becomes flat already at low levels of electron-doping and shifts to an incommensurate q at δ=0.2, where δ is the number of additional electrons (δ > 0) or holes (S < 0) per Fe. In BaFe₂As₂ and SrFe₂As₂, stripe order remains stable for hole doping down to δ = −0.3. Under electron doping, on the other hand, the E(q) minimum shifts to incommensurate q already at δ=0.1.