Table of contents

We present a mean-field theory of the conduction electron spin resonance g-factors in β-YbAlB₄. The temperature-dependent shift in the g-factors is attributed to an isotropic exchange interaction between the spin of the conduction electrons and the spin of the Yb³⁺ ion. In the mean field approximation, the difference between the parallel and perpendicular g-factors is proportional to the difference between the parallel and perpendicular susceptibilities. Using experimental values for the susceptibilities, and fitting the data at 4.2 K, we predict the temperature dependence of the g-shifts at higher temperatures.

We summarize recent theoretical and experimental work in the field of magnetic small-angle neutron scattering (SANS) of bulk ferromagnets. The response of the magnetization to spatially inhomogeneous magnetic anisotropy and magnetostatic stray fields is computed using linearized micromagnetic theory, and the ensuing spin-misalignment SANS is deduced. Analysis of experimental magnetic-field-dependent SANS data of various nanocrystalline ferromagnets corroborates the usefulness of the approach, which provides important quantitative information on the magnetic-interaction parameters such as the exchange-stiffness constant, the mean magnetic anisotropy field, and the mean magnetostatic field due to jumps ΔM of the magnetization at internal interfaces. Besides the value of the applied magnetic field, it turns out to be the ratio of the magnetic anisotropy field H_p to ΔM, which determines the properties of the magnetic SANS cross-section of bulk ferromagnets; specifically, the angular anisotropy on a two-dimensional detector, the asymptotic power-law exponent, and the characteristic decay length of spin-misalignment fluctuations. For the two most often employed scattering geometries where the externally applied magnetic field H₀ is either perpendicular or parallel to the wave vector k₀ of the incoming neutron beam, we provide a compilation of the various unpolarized, half-polarized (SANSPOL), and uniaxial fully-polarized (POLARIS) SANS cross-sections of magnetic materials.

Using density-functional calculations (DFT) and a tight-binding model, we investigate the origin of distinct favorable geometries which depend on the type of graphyne used. The change in the H geometry is described in terms of the tuning of the hopping between sp²-bonded C atoms and sp-bonded C atoms hybridized with the H atoms. We find that the different preferred geometry for each type of graphyne is associated with the electronic effects due to different symmetries rather than a steric effect minimizing the repulsive interaction between the H atoms. The band gaps are significantly tuned as the hopping varies, except in α-graphyne, in agreement with the result of our previous DFT study (Koo J et al 2013 J. Phys. Chem. C 117 11960). Our model can be used to describe the geometry and electronic properties of hydrogenated graphynes.

The effect of temperature on the luminescence properties of LiMgPO₄ doped with Eu³⁺ and Eu²⁺ are presented. Depending on the excitation wavelength, luminescence spectra consist of two distinct broad emission bands peaking at 380 nm and 490 nm related to 4f⁶5d¹ → 4f⁷ (⁸S_7/2) luminescence of Eu²⁺ and to europium-trapped exciton, respectively, and/or several sharp lines between the 580 nm and 710 nm region, ascribed to the ⁵D₀ → ⁷F_J (J = 0, 1, 2, 3 and 4) transitions in Eu³⁺. To explain all the features of the Eu²⁺ and Eu³⁺ luminescence we discussed the existence of two different Eu sites substituting for Li⁺, with short and long distance compensation. The evident effect of increasing the intensity of the Eu²⁺ luminescence with increasing temperature was observed. It was considered that the charge compensation mechanism for Eu³⁺ and Li⁺ as well as Eu²⁺ replacing Li⁺ in the LiMgPO₄ is a long distance compensation that allows for the existence of some of the europium ions either as Eu³⁺ at low temperature or as Eu²⁺ at high temperature. We concluded that Eu²⁺ in the Li⁺ site with long distance compensation yields only 4f⁶5d¹ → 4f⁷ luminescence, whereas Eu²⁺ in the Li⁺ site with short distance compensation yields 4f⁶5d¹ → 4f⁷ luminescence and europium-trapped exciton emission.

We present comparative analysis of microscopic mechanisms relevant to plastic deformation of the face-centered cubic (FCC) metals Al, Cu, and Ni, through determination of the temperature-dependent free energies of intrinsic and unstable stacking faults along [1  $\bar{1}$ 0] and [1  $\bar{2}$ 1] on the (1 1 1) plane using first-principles density-functional-theory-based calculations. We show that vibrational contribution results in significant decrease in the free energy of barriers and intrinsic stacking faults (ISFs) of Al, Cu, and Ni with temperature, confirming an important role of thermal fluctuations in the stability of stacking faults (SFs) and deformation at elevated temperatures. In contrast to Al and Ni, the vibrational spectrum of the unstable stacking fault $({\rm USF}_{[1\,\bar{2}\,1]})$ in Cu reveals structural instabilities, indicating that the energy barrier (γ_usf) along the (1 1 1)[1  $\bar{2}$ 1] slip system in Cu, determined by typical first-principles calculations, is an overestimate, and its commonly used interpretation as the energy release rate needed for dislocation nucleation, as proposed by Rice (1992 J. Mech. Phys. Solids40 239), should be taken with caution.

The temperature dependences of the unit cell parameter and the atomic displacement parameters (adp) for galena (PbS) have been measured using high resolution neutron powder diffraction in the temperature interval 10–350 K. No evidence has been found for the anomalous behaviour recently reported in a total scattering study of galena, in which the temperature variation of both the unit cell and the adp for lead are reported to undergo a dramatic reduction at a temperature of ~250 K. The linear thermal expansion coefficient calculated from the powder diffraction study is found to be in excellent agreement with literature values over the entire temperature interval studied, and approximately 25% greater at room temperature than that determined by analysis of the pair distribution function (pdf) derived from the total scattering data. This discrepancy is shown to be attributable to a linear, temperature-dependent offset from the published temperatures in the total scattering study, and has arisen from the sample temperature being significantly lower than the experimental set point temperature. Applying this correction to the adps of the lead cation removes the anomalous temperature dependence and shows the pdf results are in agreement with the neutron powder diffraction results. Application of the identical temperature offsets to the results of the pdf analysis of data collected on altaite (PbTe) eliminates the anomalous behaviour in the unit cell and the adp for lead, bringing them in line with literature values. Contrary to the conclusions of the pdf analysis, adps for the lead cation in both galena and altaite can be described in terms of Debye-like behaviour and are consistent with the partial phonon density of states.

We introduce a new framework for designing a transition metal (TM) d-electrons dominant Dirac cone spectrum based on the hybridization between graphene and a modulated TM d impurity band. The obtained Dirac cone behaves like a 'copy' from graphene, insensitive to the TM coverage and order. First-principles calculations reveal such a system of Mn intercalated epitaxial graphene on SiC(0 0 0 1), dubbed manganosine. The robustness of the Dirac cone is discussed in terms of the possible imperfection of Mn atoms. The mechanism at work is expected to be rather general and may open the door to designing new d- or f-character Dirac systems.

CaLaSiN₃ samples doped with Eu, Yb, Sm, Ce and Pr have been prepared via solid-state reaction synthesis and the optical properties have been studied. Both Yb and Sm were only observed in the trivalent state due to the fact that their Ln²⁺ ground states are located inside or very close to the conduction band of the CaLaSiN₃ host lattice. Doping with Ce³⁺ or Eu²⁺ resulted in a very low energy Ce³⁺ or Eu²⁺ 4f-5d absorption band around 1.9 eV (650 nm) and 1.4 eV (885 nm), respectively. The Ce³⁺ 5d-4f emission appeared to be quenched, just as the Eu²⁺ 5d-4f emission, which can be explained as the result of auto-ionization.

A systematic investigation of the luminescence spectroscopy of Y(P,V)O₄:Bi³⁺ is presented. The emission spectra and the decay curves are measured as a function of the host morphology, composition, temperature, excitation wavelength, and doping concentration. On this basis, the nature of the excited states and the radiative and non-radiative relaxation processes are discussed. Colour coordinates and quantum yield measurements are also carried out to provide information about the potential applications of the studied materials.

We have studied the structural and physical properties of the La_2-xTb_xCoMnO₆ series. The crystal and magnetic structures of these compounds were determined by x-ray and neutron diffraction techniques. All samples belong to the family of double perovskites with space group P2₁/n, but the Co/Mn ordering is not perfect, and antisite defects are formed. The concentration of these defects increases for intermediate compositions, indicating that La/Tb disorder influences the Co/Mn arrangement. A ferromagnetic ground state is established due to the strong Mn⁴⁺-O-Co²⁺ superexchange interaction. For the intermediate compositions and at low temperature, the Co/Mn ordering is accompanied by the ordering of Tb³⁺ moments in the ab-plane, indicating a mutual polarization between both sublattices. Macroscopic magnetic properties reveal that Curie temperature decreases as Tb content increases in correlation with the increase of the structural distortion. All samples show semiconducting behaviour, and overall the electrical resistivity increases with decreasing La-content. The dielectric constant (ε') has a value of around 12 at low temperatures for all samples, revealing the lack of permanent dipoles. The temperature dependence of ε' on warming exhibits a strong increase that depends heavily on the frequency of the electric field. This effect is ascribed to non-intrinsic effects such as contacts or internal barrier-layers.

The effect of stoichiometry on magnetocrystalline anisotropy energy (MAE) of Fe_1+xPt_1−x and Co_1+xPt_1−x (−0.5 < x < 0.5) is studied by use of first-principles method. The calculated MAEs show maxima at x = 0 for both fully L1₀-ordered systems. Compared with that, the MAEs of partially L1₀-ordered systems reduce but their composition dependences do not change, without shift of the maximum MAE to Fe/Co-rich alloy as found in experiment at room temperature. In the off-stoichiometric alloys, the misoccupied Fe/Co and Pt show large MAEs, which is explained by the enhanced in-plane hybridization between Fe/Co and Pt. The composition dependence of the atom-resolved MAE is governed by the varying number of heterogeneous ligands around the atom. The MAE(T)/MAE(0) is discussed based on spontaneous magnetization and Curie temperature, which suggests that the temperature effect may contribute to the discrepancy between calculation and experiment in the composition dependence of MAE.

The thermodynamics of a spin-1/2 magnetic multilayer system with antiferromagnetic interplanar couplings is studied using the pair approximation method. Special attention is paid to magnetocaloric properties, quantified by isothermal entropy change. The multilayer consists of two kinds of magnetic planes, one of which is diluted. The intraplanar couplings in both planes have arbitrary anisotropy ranging between Ising and isotropic Heisenberg interactions. The phase diagram related to the occurrence of magnetic compensation phenomenon is constructed and discussed. Then the isothermal entropy change is discussed as a function of interaction parameters, magnetic component concentration and external magnetic field amplitude. The ranges of normal and inverse magnetocaloric effect are found and related to the presence or absence of compensation.