Table of contents

The surface magnetization reversal has been studied in Co-rich amorphous glass-covered microwires. The studies have been performed by the magneto-optical Kerr effect surface loop tracer for the series of the microwires with diameters of 16.8–5.8 μm. The experiments have been carried out in the crossed axial and circular magnetic fields. The variety of the magnetic behaviour has been found in dependence on the diameter of the microwire, that confirms the existence of the helical domain structure in the Co-rich microwires. The special behaviour has been discovered for the wire with the diameter of 10 μm. From this results we can conclude that the axially magnetized inner core disappears for this value of the microwire diameter and that the domain structure of the microwire consists only of the helical domain structure. The experiments are in good agreement with calculations based on the model taking into account the existence of the helical anisotropy in the surface area of the microwires.

The Finemet alloy prepared by the crystallizaton of an amorphous precursor is a basic model material for study of magnetic nanoparticle structures. It was shown that intensive plastic deformation localized into adiabatic shear bands causes intensive heating and structural changes in the amorphous alloy. These changes influence the magnetic properties of the amorphous precursor and the Finemet crystallization. Direct creep measurements during the crystallization of Finemet alloys were performed and the creep properties of the residual amorphous phase formed during the nanocrystallization were described. It was shown that due to relatively high temperatures the residual amorphous phase undergoes intensive structural relaxation resulting in the obvious embrittlement of these materials.

Formation mechanism of nano-crystalline β-Fe₂O₃ particles was investigated by controlling the preparation conditions such as the mixing ratio of NaFe(SO₄)₂ to NaCl and heating treatment temperature. A single phase of β-Fe₂O₃ was formed regardless of the mixing ratio. The heat treatment temperature strongly affected the particle shapes and sizes. Below 490 °C, they had granular shape with a diameter of about 50 to 100 nm. While, above 490 °C, cubic particles with increased size of about 1 μm were produced. All samples were antiferromagnetic at low temperature. The Néel temperature (T_N) of 119 K for the large cubic particles was decreased to 113 K for the nanoparticles.

The isostructural Co_1.5Cu_0.5(OH)(XO₄) (X = P, As) phases have been prepared from hydrothermal synthesis and characterized from powder X-ray diffraction. The structure consists of a three-dimensional framework in which M(1)O₅-trigonal bipyramid dimers and M(2)O₆-octahedral chains M = (Cu and Co) are simultaneously present. Magnetization measurements of Co_1.5Cu_0.5(OH)(PO₄) show the presence of one maxima at ca. 60 K attributed to a three-dimensional antiferromagnetic ordering. This behaviour was confirmed from the ac measurements. Below T_N, a significant irreversibility appears in ZFC-FC measurements suggesting the presence of some ferromagnetic contributions. When PO₄³⁻ is substituted by AsO₄³⁻ anions, the irreversibility exhibited for the Co-Cu hydroxi-phosphate disappears showing a broad maximum around 44 K accompanied by a large decrease below this temperature.

Strontium hexaferrite (SrFe₁₂O₁₉) nanoparticles have been prepared with co-precipitation in aqueous solutions and precipitation in microemulsion system water/SDS/n-butanol/cyclohexane, using iron and strontium nitrates in different molar rations as a starting materials. The mixed Sr²⁺, Fe³⁺ hydroxide precursors obtained during the reaction between corresponding metal nitrates and tetramethylammonium hydroxide (TMAH), which served as a precipitating reagent, were calcined in a wide temperature range, from 350 °C to 1000 °C in a static air atmosphere. The influence of the Sr²⁺/Fe³⁺ molar ratio and the calcination temperature to the chemistry of the product formation, its crystallite size, morphology and magnetic properties were investigated. It was found that the formation of single phase SrFe₁₂O₁₉ with relatively high specific magnetization (54 Am²/kg) was achieved at the Sr²⁺/Fe³⁺ molar ration of 6.4 and calcination at 800 °C for 3h with heating/cooling rate 5 °C/min. The prepared powders were characterized using X-ray diffractometry (XRD) and specific surface area measurements (BET). The specific magnetization (DSM-10, magneto-susceptometer) of the prepared samples was measured.

We developed a technique to predict flux loss of a magnet with a complicated magnetization pattern using the finite element method. The developed method consists of four steps. At first, the distribution of magnetization under magnetizing field is analyzed (Step 1), and a demagnetization curve of each element is deduced from the result of the first step (Step 2). After removing the magnetizing field, the distributions of magnetization at room and elevated temperatures are analyzed by using demagnetization curves determined in Step 2 (Step 3). Based on a physical model, the distribution of flux loss due to exposure at the elevated temperature is predicted by using the result obtained in Step 3 (Step 4). We applied this technique to a ring magnet with 10 poles, and large flux loss values were predicted at the transition regions between magnetic poles.

The Fe₅₆Pt₂₄B₂₀ powder was prepared by high-energy ball milling of the melt-spun ribbon. The structure of the samples was characterized by X-ray diffraction and transmission Mössbauer spectroscopy. The as-quenched alloy was partially crystalline with a contribution of the disordered cubic FePt solid solution. The ball milling of the ribbon led to the separation of the FeB regions from the disordered precursor. Annealing of the as-milled powder caused the formation of the nanocrystalline ordered tetragonal FePt and Fe₂B phases. Hysteresis loop and magnetization vs. temperature measurements revealed differences in magnetic properties between the as-milled and annealed powders. The latter exhibited hard magnetic properties with coercivity of about 355 kA m⁻¹.

Cobalt(II) compounds [Co(C10C8C2-terpy)₂](BF₄)₂ (1) and [Co(C12C10C2-terpy)₂](BF₄)₂ (2) (CxCyCz-terpy = 4'–5'''-Alkyl–1'''-alkoxy-2,2':6',2''-terpyridine) with branched alkyl chains, based on a terpyridine frame, were synthesized. The cobalt(II) compounds 1 and 2 exhibit the spin transition between low-spin and high-spin with thermal hysteresis loop at the liquid crystal transition temperature (T_1/2↑ = 275 K and T_1/2↓ = 250 K for 1 and T_1/2↑ = 307 K and T_1/2↓ = 296 K for 2), and the spin transition is induced by crystal – liquid crystal transition. The bifunctional materials show synchronizing both spin transition and liquid crystal transition.

We report the novel results on studies of the magneto impedance (MI) effect at GHz-range in 20 μm amorphous glass-coated microwire and 40 μm melt-extracted wire with nearly-zero magnetostriction constant. Both types of wires demonstrated similar behavior although the MI effect is higher in glass-coated microwire. In high field region, when the sample is magnetically saturated, the MI dependence demonstrates typical ferromagnetic resonance behavior. In the low field region the magnetization rotation process is dominant resulting in the highest sensitivity of the impedance to magnetic field. The drawback found in the studied samples is the low field magnetic hysteresis which is undesieble for potential application. The cause of this hysteresis is found to be the helical surface anisotropy in the wires which is characterized by a nonzero angle between the anisotropy easy axis and the wire's transversal plane.

An external high parallel magnetic fields (MF) was imposed on the barium phosphate process of AZ91D magnesium alloy. The influences of the permanent MF on the morphology, the phase composition and the corrosion resistance of the barium phosphate coatings were studied using scanning electron microscope, X-ray diffraction, salt spay test. The results show that the coatings with MF are more compact, uniform, and smooth, and have better the corrosion resistance than without MF.

Open cell aluminum foams are metal cellular structures with a large volume fraction of pores. Due to their high stiffness to weight ratio, they are commonly used in applications for energy absorption and mechanical damping. The stiffness of the aluminum foam was increased by a nanocrystalline nickel coating via an electrodeposition process. The deposition process and thus the coating thickness strongly depend on mass transport limitations. To visualize the coating thickness distribution of the foam, we measured the magnetic flux density distribution by scanning the surface of cuts of coated foams with a commercial Hall probe. By measuring the magnetic flux density distribution, deposition parameters as the current density and flow conditions could be optimized with regard to a more homogeneous coating thickness distribution. Furthermore, a model of the mass transport limitation at a complex three dimensional foam electrode could be evaluated from the magnetic flux density distribution of the nickel coated foam cuts.

Complex permeability μ* and permittivity * spectra of permendur (Co₅₀Fe₅₀) composite materials have been studied in the microwave frequency range considering the application to the left-handed meta-materials and EMC devices. High surface electrical resistance of the permendur particles was achieved by the heat-treatment in order to suppress the eddy current effect in the high particle content composites. For the 82.6 vol.% composite, the μ' is 11 and less than 1 at 100 MHz and 6 GHz, respectively; the μ'' shows the two peaks around 700 MHz and 3GHz due to the domain wall and gyromagnetic spin resonance. On the other hand, the ' is almost constant value of 28 and the '' is almost zero in the frequency range from 100 MHz to 6 GHz. The calculated reflection loss of a single-layer electromagnetic wave absorber (EM absorber) designed by using permendur composites indicates less than −20 dB around the matching frequency of 1 GHz.

A texture analysis is performed by means of the electron-backscatter diffraction technique (EBSD) on melt-spun ribbon-like samples of the composition Ni_52.5Mn_24.5Ga₂₃ (at.-%) were prepared. A dedicated surface treatment is required in order to achieve high quality Kikuchi patterns. For this purpose, mechanical polishing plus ion polishing was employed. EBSD analysis and transmission electron microscopy revealed that the samples have a polycrystalline, granular morphology, with grain sizes around 1 – 2 μm. Several larger grains being present in the region selected for EBSD analysis, and many small grains are found, even embedded in the larger ones. The larger grains exhibit a common direction of elongation, yielding to a specific texture.

The microtexture of differently prepared Ba-hexaferrite samples is investigated by means of electron backscatter diffraction (EBSD). Kikuchi patterns are obtained with a high image quality, enabling a spatial resolution of the EBSD maps of about 20 nm. The spatially highly resolved EBSD mappings provide additional information (individual grain orientation, misorientation angles, grain size distribution) as compared to the standard analysis techniques, which can contribute to an optimization of the growth process. Furthermore, as the crystallographic orientation of each grain is known, an exact analysis of the grain aspect ratio becomes possible which provides further insight to the microstructural dependence of the magnetic properties of ferrites.

Nd₂Fe₁₄B films with the c-axis perpendicular to the film plane were deposited by introducing Mo (110) underlayer on the SiO₂ substrates. When the temperature of the substrate during the deposition of the Nd-Fe-B layer was 600 °C, the demagnetization curve exhibited the coercivity of H_c=11 kOe. We observed an H_c increase of up to 14 kOe by capping the film with the Nd overlayer and post-annealing at 500°C. Cross-sectional TEM observation and NBD analysis suggest that the Nd in the overlayer is in the oxide state with either cubic-NdO₂ or hcp-Nd₂O₃ structures.

Structural and magnetic properties of Fe/C catalysts synthesized by ball milling and deposited onto Al₂O₃, SiO₂ and TiO₂ supports are reported. Ball milling α-Fe and C in the presence of these supports produced peculiar solid solutions in which antiferromagnetic and ferrimagnetic iron phases doped with Al, Si and Ti coexist. Mössbauer spectroscopy and powder X-ray diffraction data show no evidence of any Fe_xC phase. Instead, oxidation took place even though carbon (graphite) was present. All the catalysts were found to exhibit strong metal-support interactions, with the strongest interactions found in the TiO₂ supported catalyst.

The magnetization of sigma and alpha FeCr samples ball-milled in vacuum is reported as a function of milling time. The change of the saturation magnetization with milling time is non-monotonous in the sigma series: after an initial increase due to the transformation to the magnetic alpha phase, then it decreases as in the alpha phase. The results are fully consistent with our previous results from Mössbauer spectroscopy that established the formation of an amorphous phase.

The magnetic properties of dysprosium iron garnet (DyIG) have been studied by performing high resolution powder neutron diffraction experiments and high dc fields magnetizations on single crystals. Among all the reflections (hkl) indexed in the nuclear cubic space group (CSG) Ia d with h+k+l=2n and k=[000], the superstructure lines (hkl)* forbidden by the symmetry (222)* and (622)* are not observed in the patterns at all temperatures. The pattern at 130 K is well interpreted within the magnetic modes F belonging to the irreducible representation (IR) T_1g of the CSG and identified to the room temperature ferrimagnetic Néel model. The high magnetic field behavior of the spontaneous collinear magnetic structure (MS) along the easy axis (EA) <111> is isotropic. Below 130 K, the patterns exhibit additional magnetic superstructure lines. They are associated to the appearance of the spontaneous non collinear MS which is described in the subgroup of the CSG, R c within the IR A_2g. A strong magnetization anisotropy (MA) is observed at 1.5 K in the low symmetry phases <uuw> were the spin reorientation transition (SR) occur at T_RS=14.5 K. The onset of MA is detected below two characteristic temperatures, Ta₁=125 K and Ta₂=75 K respectively to the hard axis (HA) <100> and <110>. Symmetry arguments are used in the framework of the theory of representation analysis (RA) applied to the subgroup of R c, C2/c within the IR A_g. It seems that this MA results essentially from the difference between the spontaneous non collinear MS and the field induced (FI) configurations. All results are discussed with previous neutrons studies.

This study provides the effect of Cu addition on coercivity (H_cJ) and interfacial microstructure in Nd-Fe-B/Nd-rich thin films. All films were deposited by using ultra high vacuum (UHV) magnetron sputtering, and the Nd-Fe-B layer was oxidized under several atmospheres with different oxygen content. Then, the films were annealed at 250–550 °C under UHV. The films oxidized in low vacuum (10⁻²−10⁻⁵ Pa) (under low oxygen state) exhibited the recovery of H_cJ by the annealing at 450 °C. On the contrary, the H_cJ of the films oxidized in Ar (under high oxygen state) decreased with increasing annealing temperature. However, the H_cJ increased drastically at the temperatures above 550 °C. In addition, the Cu added films, which were annealed at temperatures above 350 °C, showed higher coercivities than the films without Cu addition. The XRD analysis suggested the existence of C-Nd₂O₃ phase in the Cu added films annealed at 550 °C. It can be considered that the Cu addition decreases the eutectic temperature of Nd-rich phase and influences the interfacial state between Nd₂Fe₁₄B and Nd-rich phase.

Thin films of FeCuNbSiB have been sputtered on SiO₂/Si substrates with thickness varying from 50 nm to 1.8 microns, then annealed at temperature ranging from 200 to 500 °C. The coercivity globally decreases with the film thickness down to 100 A m⁻¹ for as-deposited samples and 10 A m⁻¹ for annealed samples. However, it appears a local maximum for thickness close to 1 micron (i.e. the exchange length), regardless the annealing temperature. This behavior is supposed to be related to a modification of the magnetic domain structure, as the RAM applied to thin films and the dependence of the transitions points (glass state temperature Tg and crystallization temperature Tx) on the film dimension are monotonous. In addition, the films have been annealed under magnetic field and strong anisotropy energy has been reached, in the range of 150 J m⁻³.

We investigated the effect of substrates for preparing perpendicularly anisotropic Nd-Fe-B thick film magnets prepared by the high-speed PLD (Pulsed Laser Deposition) method with a substrate heating system. Although it was difficult to prepare perpendicularly anisotropic films on W, Zr and Nb substrates, usage of Ta, Mo and Ti substrates enabled us to obtain anisotropic ones. It was found that the preparation of anisotropic PLD-made Nd-Fe-B thick film magnets depends on substrates.

Spontaneous volume magnetostriction of the intermetallic compound GdAl₂ is studied from the X-ray diffraction measurement at low temperature. It was found that GdAl₂exhibits the ferromagnetic transition at T_C ~170 K without lowering of crystal symmetry of a cubic C15 Laves structure. The temperature dependence of the lattice constant a is in good agreement with that of the volume thermal expansion, in which negative spontaneous volume magnetostriction has been observed. It indicates that the magnetic exchange interaction can be described by the distance between the Gd moments. From these experimental results, the origin of the negative magnetostriction is discussed on the basis of the localized spin model of ferromagnetism.

Electron Paramagnetic Resonance (EPR) was used to study, the temperature dependence, of the magnetic behavior of Ni_1−xCo_xFe₂O₄ with 0.0 < x < 0.5, in the temperature range 80 < T < 700 K. Nanoparticles of sizes between 30 and 40 nm were obtained using the sol-gel method. The results show that the resonance field (H_R) decrease while the linewidth (AH_PP) increase, in the temperature range studied, when x is increased. The H_R values for x = 0 are in agreement with a superparamagnetic phase in the temperature range studied, while for x = 0.2, H_R and ΔH_PP are in accordance with a ferri to superparamagnetic transition at T~350 K, where T is related to the EPR blocking temperature of these samples. For sample with x = 0.5 this temperature is T~470 K. These results are in good agreement with the magnetization and MOKE results. MOKE measurements as a function of temperature were made to corroborate EPR results.

Correlation between crystal structure and physical properties for Co₂FeO₄ spinel oxide are studied. There are two coexisting phases, Fe rich and Co rich spinels, for S80, S86, S100 samples. The S90 and S95 samples showed single phase nature. A small signature of second phase is noted for S95 sample from the temperature dependence of magnetization measurement. The single phase with single Curie temperature at about 453 K is confirmed for S90 sample. Interestingly, S95 sample showed minimum magnetic energy loss. The complex magnetic permittivity and dielectric permeability at 4.6 GHz and 7.2 GHz has been measured for all samples. There is a correlation between annealing temperature for the samples and measured electromagnetic properties.

The structural and magnetic properties of Ho₂Fe_17−xMn_x (x=0−5) have been investigated by x-ray diffraction, Mössbauer spectroscopy and DC magnetization measurements (0-5 T) over the temperature range 4.5–350 K. Similar to other ferrimagnetic R₂Fe_17−xMn_x systems, the unit cell volume generally increases with Mn content other than for low Mn values where a slight maximum is detected around x ~ 0.5. The nature of the magnetic phase transitions around T_C is shown by Arrot plot analysis to be second order for all samples. The Curie temperature remains essentially unchanged for Mn contents up to x=1.0 (T_C = 336 K for x=0.0, T_C = 338 K for x=1.0) before decreasing steadily with further increase in Mn content (T_C = 209 K for x=5). The ⁵⁷Fe hyperfine interaction parameters have been determined from variable temperature Mössbauer spectra.

SmFe₅(0001) single-crystal thin films are prepared by molecular beam epitaxy employing Cu(111) single-crystal underlayers on MgO(111) substrates. The Cu atoms diffuse into the Sm-Fe layer and substitute the Fe sites in SmFe₅ structure forming an alloy compound of Sm(Fe,Cu)₅. The Sm(Fe,Cu)₅ film is more Cu enriched with increasing the substrate temperature. The Cu underlayer plays an important role in assisting the formation of the ordered phase.

We have already reported an advanced method for producing a radially-anisotropic rare earth composite bonded magnet with continuously controlled direction of anisotropy. The magnet has been applied to an inner rotor as a practical usage. In this study, the outstanding preparation method was adopted into the preparation of a magnet applied for an outer rotor. An optimized condition of extrusion and compaction at an elevated temperature could be obtained. In addition, a low pressure configuration to the ring-shaped magnet from plural preformed magnets was carried out, which had specific distribution of magnetic anisotropy for internal space for a small motor, by using self recoverability based on the viscous deformation without an alignment field. No deterioration of magnetic properties was detected through the process even if those magnets were miniaturized. Resultantly, the (BH)_max of a ring-shaped magnet with the continuously controlled direction of magnetic anisotropy attained the value of 186 kJ/m³, and we obtained sine-wave magnetic anisotropy distribution, even if those magnets were miniaturized.