On the theory of static magnetic susceptibility of NiF2

The magnetic susceptibility of the weak ferromagnet NiF₂ is calculated using self-consistent microscopic calculations. It is shown that a comparatively large longitudinal susceptibility can be due to the rotation of a spin subsystem under the action of a magnetic field. The expression for the piezomagnetic moment of NiF₂ is given.     

Neutron diffraction determination of the magnetic structures of NpCo2Si2 and NpCu2Si2

A small polycrystalline ingot sample of NpCo₂Si₂ (weight ≈ 1.5 g) has been studied by neutron diffration between 2 and 160 K on the multi-detector D1B of ILL, Grenoble. At 100 K, the crystal structure is body-centered tetragonal (space group 14/mmm) with a = 3.886 Å and c =9.649 Å. Below T_N = (44 ± 2) K, seven superlattice lines are observed which correspond to a simple tetragonal lattice with lattice constants as above. They are consistent with a type I antiferromagnetic structure of the Np (2a) sublattice, with (001) ferromagnetic sheets coupled antiferromagnetically according to the sequence +-+-. At 6 K, the neptunium moment obtained from the diffracted intensities is: (1.48 ± 0.20)_μuB, and makes an angle 52° ± 15° with the c axis. The cobalt moment is certainly smallet than 0.3_μuB. The Np moment correlates well with the ²³⁷Np hyperfine field deduced from Mos?sbauer spectroscopy; the sublattice magnetization-temoperature curve follows very well the $\text{J=}\text{1}\text{2}$ brillouin curve. The magnetism is therefore probably of lovalized character in this compound. An isomorphous sample of NpCu₂Si₂ (a = 3.990 Å c = 9.920 Å) was shown to be ferromagnetic below (41 ± 2) K, with the Np moment [1.5 ± 0.2)_μuB] aligned along the c axis.     

The theory of a noncollinear ferromagnetism of the U3X4 compounds

In order to describe magnetic properties of U₃X₄ compounds a model is proposed, which contains, besides isotropic exchange, uniaxial exchange anisotropy and three-axial crystal field term. Semiclassical considerations lead to a noncollinear, three-axial, ground state configuration and stability conditions for such an ordering are found. The behaviour of the system with the external magnetic field of different orientations is discussed at zero-temperature. There is no saturation in any direction and asymptotic formulas for magnetization at the high fields and for the initial susceptibility tensor are given. An expression for the Curie temperature is obtained in the simplest molecular field approximation. The model seems to explain qualitatively the experimental data.     

On the theory of optical properties of anisotropic magnetic crystals in external magnetic field

The calculation of dielectric permeability tensor which are based on the theory of excitons in magnetoordered crystal are made. The dipole exciton energies of crystal are obtained and their dependence on external magnetic field H at arbitrary relation between exchange and single-ion anisotropy constants is established. Refractive index of tetragonal antiferromagnetic is determined and it is shown that it (index) is an odd function of H for a light polarized transverse to the fourth-order symmetry axis of crystal.     

Effective field theory approach to light propagation in an external magnetic field

The recent PVLAS experiment observed rotation of polarization and ellipticity when a linearly polarized laser beam passes through a transverse magnetic field. The phenomenon cannot be explained in conventional QED. We attempt to accommodate the result by employing an effective theory for the electromagnetic field alone. No new particles with a mass of order the laser frequency or below are assumed. To quartic terms in the field strength, a parity-violating term appears besides the two ordinary terms. The rotation of polarization and ellipticity are computed for parity-asymmetric and -symmetric experimental set-ups. While rotation occurs in an ideal asymmetric case and has the same magnitude as ellipticity, it disappears in a symmetric set-up like PVLAS. This would mean that we have to appeal to some low-mass new particles with nontrivial interactions with photons to understand the PVLAS result. PACS 12.20.-m; 12.20.Fv; 42.25.Lc; 42.25.Ja     

Crystal and magnetic structures of Y2CrS4

Chromium(II) sulfide, Y₂CrS₄, prepared by a solid-state reaction of Y₂S₃ and CrS, showed an antiferromagnetic transition at 65 K. The neutron diffraction patterns at 10 and 90 K were both well refined with the space group Pca2₁. At 90 K, cell parameters were a=12.5518(13) Å, b=7.5245(8) Å, and c=12.4918(13) Å. At 10 K, magnetic peaks were observed, which could be indexed on the same unit cell. Magnetic moments of chromium ions were parallel to the b-axis and antiferromagnetically ordered in each set of the 4a sites.     

On the magnetic ordering of CeAl2

The magnetic behaviour of CeAl₂ at low temperature is not apparent : contradictory experiments have assumed either an antiferromagnetic ordering or a moment reduction of the Ce³⁺ ion. Using the multidetector D1B at the I.L.L. high flux reactor, we have measured neutron diffraction diagrams, above and below the transition temperature of 3.8 K. At 1.9 K, we have found very weak magnetic reflections corresponding to an antiferromagnetic structure sinusoǐdally modulated, according to the propagation vector (0.612, 0.388, 0.5). The moment reduction in such a modulated structure at 1.9 K can hardly be explained by a thermal partial disordering of the moments. It is rather due to the existence of a singlet ground state resulting from the negative exchange between the 4f electron of Ce³⁺ and the conduction electrons.     

Influence of external pressure on magnetic phase transitions in NdRu2Ge2

Magnetic phase transitions in NdRu₂Ge₂ under external pressure are studied. The (p, T) phase diagram is presented. An additional phase is observed for pressures p ≥ 4.3 kbar.     

Thermal and magnetic properties and crystal structures of CeGe2 and CeSi2

In continuation of our investigation of α-ThSi₂ type rare earth intermetallic compounds, studies on CeSi₂ and CeGe₂ are reported. The results suggest that Ce in CeSi₂ is in the intermediate valence state. The existence or non-existence of superlattice structures are also discussed.     

Study of the excitation properties of the Si3O2 cluster under an external electric field

The structure of the Si₃O_x (x=2, 3) cluster is investigated; we find that the geometry of Si₃O₂ is similar to that of Si₃O₃ except for the oxygen-deficient defect structure (Si-Si band) which exists only in the Si₃O₂ cluster. It is known that oxygen-deficient defects are used to explain visible luminescence (especially blue, purple and ultraviolet light) from silicon-based materials, which are directly bound up with the excited states of the molecules. Therefore the excitation properties of the two clusters are also studied. Our results show that the absorption spectrum of Si₃O₂ is concentrated in the visible light region. In contrast, the absorption spectrum of Si₃O₃ is mainly located in the ultraviolet light region. The calculations are perfectly consistent with experimental data and also support the theory of oxygen-deficient defects.     

Magnetic excitations of HoAl2 in a magnetic field

The ground state spin wave excitation energies of single crystalline HoAl₂ have been studied at T= 4.2 K in external magnetic fields up to 7 T by means of inelastic neutron scattering. The results have been interpreted in terms of a cubic crystalline electric field using the parameters determined by magnetization measurements and an exchange interaction with the exchange parameters taken from the zero field measurements.     

Electronic and magnetic structures of ternary iron telluride KFe2Te2

We examine the electronic and magnetic structures of iron telluride KFe₂Te₂ using first-principle calculations. We demonstrate that the ground state of this compound is in bicollinear antiferromagnetic order with Fe local moments (~ 2.6 μ_B) that are ferromagnetically aligned along a diagonal direction and antiferromagnetically aligned along the other diagonal in the Fe-Fe square lattice, similar to the alignment discovered in the parent compound of superconductor α-FeTe. This bicollinear antiferromagnetic order results from the interplay among the nearest, next-nearest, and next-nextnearest neighbor exchange interactions, which are mediated by Te 5p orbitals. This finding may aid our understanding of the interplay between magnetism and superconductivity in the family of iron-based materials.     

On a theory of surface waves in a smoothly inhomogeneous plasma in an external magnetic field

A theory of surface waves in a magnetoactive plasma with smooth boundaries has been developed. A dispersion equation for surface waves has been derived for a linear law of density change at the plasma boundary. The frequencies of surface waves and their collisionless damping rates have been determined. A generalization to an arbitrary density profile at the plasma boundary is given. The collisions have been taken into account, and the application of the Landau rule in the theory of surface wave damping in a spatially inhomogeneous magnetoactive collisional plasma has been clarified.     

Current-density functional theory study of the H2 molecule evolving under a strong ultrashort magnetic field

Hydrogen molecule in a strong ultrashort magnetic field is investigated through a current-density functional theory (CDFT) and quantum fluid dynamics (QFD) based approach employing current-density dependent vector exchange-correlation potential and energy density functional derived with a vorticity variable. The numerical computations through the CDFT based approach are performed for the H₂ molecule, starting initially from its field-free ground state, in a parallel internuclear axis and magnetic field-axis configuration with the internuclear separation R ranging from 0.1 a.u. to 14.0 a.u., and the strength of the time-dependent (TD) magnetic field varying between 0−10¹¹ G over a few femtoseconds. The numerical results are compared with that obtained using an approach based on the current-density independent approximation under similar computational constraints but employing only scalar exchange-correlation potential dependent on the electronic charge-density alone. The current-density based approach yields exchange- and correlation energy as well as electronic charge-density of the H₂ molecule drastically different from that obtained using current-independent approach, in particular, at TD magnetic field-strengths >10⁹ G during a typical time-period of the field when the magnetic-field had attained maximum applied field-strength and is switched to a decreasing ramp function. This nonadiabatic behavior of the TD electronic charge-density is traced to the TD vorticity-dependent vector exchange-correlation potential of the CDFT based approach. The interesting electron dynamics of the H₂ molecule in strong TD magnetic field is further elucidated by treating electronic charge-density as an ‘electron-fluid’. The present work also reveals interesting real-time dynamics on the attosecond time-scale in the electronic charge-density distribution of the hydrogen molecule.     

Multilayer structures of the iron/chromium type with almost ideal interfaces in an external magnetic field

The magnetic phase diagram of the Fe/Cr/Fe three-layer structure with almost ideal interlayer boundaries was constructed. The effective interlayer interaction in this structure was described by the “half-angle coupling” model. Various system configurations were analyzed taking into account crystalline anisotropy, and the ground state of the system was determined. The behavior of the structure in an external magnetic field applied along easy and hard magnetic axes was studied. The magnetization curves M(H) characteristic of structures with various interface roughness parameter and interlayer exchange values were described and analyzed. The experimental situation is discussed.     

The magnetic structures of LaTiO3 and CeTiO3

The basic features of the magnetic structures of LaTiO₃ and CeTiO₃ were determined by powder neutron diffraction. LaTiO₃ (T_N = 125 K) is a type G antiferromagnet with a moment on Ti (III) of 0.45(5)μ_B at 10 K. As bulk magnetic measurements indicate a weak ferromagnetic moment, a G_zF_x or G_xF_z configuration is implied. CeTiO₃ (T_N) = 116 K) shows more complex behaviour. At 81 K only G and F type reflections are observed. The most consistent interpretation is to assign G-type configuration to Ti(III) and an induced F_z on Ce(III). Moments are 0.36μ_B on Ti(III) and 0.4(1)μ_B on Ce(III). It is also possible to assign both G and F components to Ti(III). This demands a “canting angle” of 34° to explain the F moment. Below 80 K a C-type component develops. A model assuming a G_zF_x configuration for Ti(III) and a C_yF_x configuration for Ce(III) provides a good fit to the data. This assignment is consistent with Bertaut's symmetry considerations. Other models which violate Bertaut's rules also fit the data.     

The magnetic structures of HoTiO3 and ErTiO3

The magnetic structures of rare-earth titanium perovskites, ErTiO₃ and HoTiO₃, have been determined at 4.2 K by neutron diffraction. The Er³⁺ moment of (8.5 ± 0.5) μ_B lies along [001] and is colinear with the titanium moment of (-0.7 ± 0.3) μ_B. The Ho³⁺ moment of (8.1 ± 0.5) μ_B is inclined at an angle of 24° to the bc plane and 32° to the ab pla so as to produce an antiferromagnetic ordering of the x component and a ferromagnetic ordering of the y and the z components. The titanium moment of (-0.55 ± 0.3) μ_B lies in the bc plane but its precise direction has not been determined.     

Variation of H2 bond length with magnetic field

We find a new effect, namely, the variation of the ratio of concentrations of ortho- and para-isomers of hydrogen in thermal equilibrium in a uniform external magnetic field with field strength and temperature, that can be observed experimentally. The observation can determine the variation of bond length with the magnetic field strength.     

Induced moment antiferromagnet in an external magnetic field—I: Molecular field theory

The behavior of a two-level induced moment antiferromagnet in an external magnetic field is investigated in the molecular field approximation. A significant reduction in the critical field and in the sublattice magnetizations is shown. However, the total magnetization rises more rapidly with field and can remain at large value in an external field even at T = 0. The magnetic susceptibility also remains finite at T = 0 in contrast to the case of a permanent moment Ising antiferromagnet. The effects of a ferromagnetic next-nearest neighbor interaction are then examined. It is shown that, in contrast to the usual Ising antiferromagnets, the ferromagnetic coupling has to exceed a certain value depending on the crystal field strength and the antiferromagnetic interaction, to allow for a first order phase transition in a field to occur even at zero temperature.