Effect of infrared illumination on the modulated magnetic structure of the weak ferromagnet FeBO<Subscript>3</Subscript>: Mg

The effect of unpolarized white light on the period and conditions of the existence of the modulated magnetic structure of the FeBO₃: Mg single crystal, which is formed in this easy-plane weak ferromagnet in the low-temperature range during technical magnetization, has been investigated experimentally. It has been revealed that the degree of light action on the magnetic state of the crystal depends on both the irradiation duration and the orientation of the ferromagnetic vector during illumination. It has been established that light with wavelengths in the range 0.8 μm < λ < 0.9 μm has a maximum effect on the parameters of the modulated magnetic structure formed in the FeBO₃: Mg single crystal. The results obtained have been interpreted in the framework of the “magnetic ripple” theory using the model of photosensitivity of anisotropic magnetic centers associated with the presence of Mg impurity ions in the composition of the crystal.     

Magnetic properties of the noncollinear phase of a FeBO<Subscript>3</Subscript>:Mg single crystal

The magneto-optical method is used to investigate magnetization and low-frequency magnetic susceptibility curves of the FeBO₃ single crystal doped with diamagnetic Mg ions. It is demonstrated that substitution of a fraction of Fe ions by the Mg ions in the structure of this easy-plane weak ferromagnet causes a significant intraplane magnetocrystalline anisotropy at low temperatures as well as different forms of magnetic hysteresis loops and field dependences of the magnetic susceptibility recorded for different orientations of the external magnetic field relative to the basal crystal plane. The established special features of the FeBO₃:Mg magnetic properties are explained by the transition of the magnetic crystal from the uniform to spatially modulated state during technical magnetization.     

Effect of the transformation of the magnetic structure of a FeBO<Subscript>3</Subscript>:Mg crystal on its magnetooptical anisotropy

The field dependence of the magnetic linear birefringence (MLB) in easy-plane weak FeBO₃:Mg ferromagnet has been investigated. It is found that MLB nonmonotonically (stepwise) tends to a constant value with an increase in magnetic field at low temperatures. This feature of MLB in FeBO₃:Mg is related to the transformation of the crystal magnetic structure during magnetization.     

Modulated magnetic structure of weak rhombohedral ferromagnets α-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>:Ga and FeBO<Subscript>3</Subscript>:Mg

The effect of diamagnetic impurities on the stability of the homogeneous magnetic state of rhombohedral antiferromagnets with weak ferromagnetism (α-Fe₂O₃:Ga and FeBO₃:Mg) is studied experimentally. It is shown that the application of an external magnetic field in the basal plane in the crystals under study in a certain temperature range induces a magnetic superstructure along the hard magnetization axis, which can be presented in the form of a ripplon phase with the azimuth of the local ferromagnetism vector oscillating about the direction of the field. The preferred orientation of the discovered modulated structures relative to crystallographic directions in the basal plane of α-Fe₂O₃:Ga and FeBO₃:Mg is studied, and the dependence of the spatial period of the superstructure on the applied magnetic field and temperature is analyzed. The magnetic-field-induced transition of the studied crystals from a homogeneous to an inhomogeneous magnetic state is described phenomenologically on the basis of the thermodynamic potential with gradient terms. In the discussion of physical reasons for magnetic order parameter modulation in weak ferromagnetic doped with diamagnetic ions, preference is given to the mechanism associated with the emergence of uniaxial magnetic centers with a random distribution of azimuths of easy axes in the basal plane of the crystal in the vicinity of impurities. A model describing the formation of a modulated magnetic state in α-Fe₂O₃:Ga and FeBO₃:Mg is proposed, according to which the competition between magnetoanisotropic and Zeeman interactions in the inhomogeneous magnetic phase of these crystals leads to periodic deviations in the direction of the local ferromagnetism vector from the direction of magnetization.     

Orientational phase transition into a modulated magnetic state in the weak ferromagnet FeBO<Subscript>3</Subscript>: Mg

The transition from a homogeneous into a modulated magnetic state in the easy-plane weak ferromagnet FeBO₃: Mg is studied by a magnetooptic method. At T < 135 K, the application of a magnetic field in the basal plane of the crystal is shown to excite the modulation of its magnetic order parameter, which manifests itself in a periodic deviation of the local ferromagnetism vector from the magnetization direction. The conditions for the existence of a modulated magnetic superstructure in FeBO₃: Mg are studied, and its preferred orientation in the basal plane of the crystal is analyzed. A magnetic H-T phase diagram that shows the boundaries between the homogeneous and inhomogeneous magnetic states of this weak ferromagnet is constructed. The modulation period and the azimuthal angle specifying the local ferromagnetism vector direction in the structure are studied as a function of temperature and magnetic field. The results obtained are discussed in terms of the theory of magnetic ripple using the model of anisotropic magnetic centers appearing in a crystal doped by magnesium ions.     

Technical magnetization of the nonuniformly stressed iron borate single crystal

Additional spatial nonuniform magnetic anisotropy is induced in the basal plane of a FeBO₃ single crystal using low-symmetry mechanical stresses. The effect of the nonuniform magnetic anisotropy on the magnetic state of this weak easy-plane ferromagnet is studied by a magnetooptic method. When the nonuniformly stressed FeBO₃ crystal is magnetized in the basal plane near a certain preferential direction, the crystal is found to transform from a homogeneous into a spatially modulated magnetic state, which can be represented by a static spin wave in which a local ferromagnetism vector lies in the basal plane and oscillates about the average magnetization direction in the crystal.     

Magnetic properties of the Nd<Subscript>0.5</Subscript>Gd<Subscript>0.5</Subscript>Fe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> single crystal

The magnetic properties of the Nd_0.5Gd_0.5Fe₃(BO₃)₄ single crystal have been studied in principal crystallographic directions in magnetic fields to 90 kG in the temperature range 2–300 K; in addition, the heat capacity has been measured in the range 2–300 K. It has been found that, below the Néel temperature T _N = 32 K down to 2 K, the single crystal exhibits an easy-plane antiferromagnetic structure. A hysteresis has been detected during magnetization of the crystal in the easy plane in fields of 1.0–3.5 kG, and a singularity has been found in the temperature dependence of the magnetic susceptibility in the easy plane at a temperature of 11 K in fields B < 1 kG. It has been shown that the singularity is due to appearance of the hysteresis. The origin of the magnetic properties of the crystal near the hysteresis has been discussed.     

Magnetic-field-induced orientational phase transition in a nonuniformly stressed iron borate single crystal

Spatially nonuniform magnetic anisotropy was induced in the basal plane of an iron borate (FeBO₃) single crystal by applying low-symmetry stresses. The effect of nonuniform magnetic anisotropy on the magnetic state of this weak ferromagnet was studied by magnetooptic methods. It is revealed that, when a nonuniformly stressed FeBO₃ crystal is magnetized in the basal plane along a certain direction (depending on the symmetry of the applied stress), a transition from a uniform to a spatially modulated magnetic state occurs, which is not observed in the crystal in the absence of stresses. The modulated magnetic phase of the crystal can be represented as a static spin wave linearly polarized in the basal plane, with the azimuth of the weak-ferromagnetism vector oscillating about the direction of the mean magnetization. The temperature and field dependences of the spatial period of the modulated magnetic structure and the amplitude of oscillations of the ferromagnetism vector are studied, and the temperature dependence of the field range over which the modulated phase exists in the nonuniformly stressed FeBO₃ crystal is found. The results are discussed in terms of the phenomenological theory of magnetic phase transformations. It is shown that the theoretical model used consistently describes all the experimental results of the study of the noncollinear magnetic phase of the nonuniformly stressed FeBO₃ crystal.     

Magnetic properties of the CuCoAlBO<Subscript>5</Subscript> single crystal

High-quality CuCoAlBO₅ single crystals have been grown, and their crystal structure, magnetic susceptibility, and magnetization have been studied. It has been established that the CuCoAlBO₅ compound is an uncompensated antiferromagnet or ferrimagnet with a small magnetic moment and the magnetic ordering temperature T _N = 28 K. A model has been proposed for the magnetic structure. A strong anisotropy of the magnetic properties has been revealed.     

Structure of the inhomogeneous magnetic state of an FeBO<Subscript>3</Subscript>: Mg easy-plane weak ferromagnet

The magnetic linear birefringence of an FeBO₃: Mg ferromagnetic crystal is investigated as a function of the magnetic field strength, the magnetic field orientation, and the coordinates. The structure of the inhomogeneous magnetic phase of this weak ferromagnet is determined by analyzing the experimental results obtained. It is shown that, in an inhomogeneous magnetic state, the ferromagnetic moment does not deviate from the basal plane of the crystal and the angle of its deviation from the direction of the applied magnetic field is described by a one-dimensional harmonic function of the spatial coordinate along the axis of magnetization.     

Effect of diamagnetic impurities on the magnetic susceptibility of iron borate

The low-frequency magnetic susceptibility of a FeBO₃ single crystal doped with diamagnetic Mg ions was measured by a magnetooptical method. It is shown that the introduction of Mg ions into the composition of this easy-plane weak ferromagnet results in the appearance, at low temperatures, of a strong in-plane magnetocrystalline anisotropy and also causes a marked difference between the field dependences of the magnetic susceptibility measured in magnetic fields applied along different directions in the basal plane of the crystal. The revealed specific features of the field dependence of the magnetic susceptibility of the crystal studied are associated with the transformation of its magnetic structure during magnetization.     

Observation of light-induced anisotropy in ferric borate by acoustic resonance

A light-induced anisotropy of two-fold symmetry in the magnetically easy c-plane has been observed at 77 K in c-plane platelets of FeBO₃ using the method of acoustic resonance. The anisotropy field is shown to be negative (positive) in the direction of the net magnetization during illumination in those samples for which the susceptibility increases (decreases) with light.     

Investigation of the magnetic properties and magnetic structure parameters of nanocrystalline Fe<Subscript>79</Subscript>Zr<Subscript>10</Subscript>N<Subscript>11</Subscript> films

The magnetic properties (magnetization curve, ferromagnetic resonance spectrum) of nanocrystalline Fe₇₉Zr₁₀N₁₁ films obtained by RF magnetron sputtering with subsequent annealing were studied experimentally, along with the fundamental magnetic constants of these films (saturation magnetization M _S, local magnetic anisotropy energy K, and the exchange coupling constant A). The magnetic properties are discussed within the random magnetic model, which determines the correlation of the magnetic properties with the fundamental magnetic constants and nanostructure parameters (grain size, magnetic anisotropy, and correlation radius R _C). The exchange correlation length 2R _L for the film magnetic microstructure was determined by correlation magnetometry.     

Colossal magnetodielectric effect in SmFe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> multiferroic

The colossal (more than threefold) decrease in the dielectric constant ɛ in the easy-plane SmFe₃(BO₃)₄ ferroborate in a magnetic field of ∼5 kOe applied in the basal ab plane of the crystal has been found. A close relation of this effect to anomalies in the field dependence of the electric polarization has been established. It has been shown that this magnetodielectric effect is due to the contribution to ɛ from the electric susceptibility, which is related to the rotation of spins in the ab plane, arises in the region of the antiferromagnetic ordering T < T _N = 33 K, and is suppressed by the magnetic field. A theoretical model describing the main features of the behavior of ɛ and electric polarization in the magnetic field has been proposed, taking into account the additional anisotropy in the basal plane induced by the magnetoelastic stresses.     

Magnetic-resonance and thermophysical studies of the magnetic phase diagram for a GdFe<Subscript>2.1</Subscript>Ga<Subscript>0.9</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> single crystal

The antiferromagnetic resonance, heat capacity, magnetic properties, and magnetic phase diagram of a GdFe₃(BO₃)₄ crystal in which some of the iron ions were substituted by diamagnetic gallium ions have been investigated. It has been found that the Neél temperature upon diamagnetic substitution decreased to 17 K compared to 38 K in the unsubstituted crystal. The effective exchange and anisotropy fields for GdFe_2.1Ga_0.9(BO₃)₄ have been estimated from the field dependences of magnetization and resonance measurements. The magnetic phase diagram of the crystal has been constructed from magnetic and resonance measurements. In GdFe_2.1Ga_0.9(BO₃)₄, there is no spontaneous reorientation and, in the absence of a magnetic field, the crystal remains an easy-axis one in the entire domain of magnetic ordering. The critical field of the reorientation transition to an induced easy-plane state in a magnetic field along the trigonal axis has been found to increase compared to that in the unsubstituted crystal.     

Modulated magnetic structure in quasi-one-dimensional clinopyroxene NaFeGe<Subscript>2</Subscript>O<Subscript>6</Subscript>

The magnetic structure of the NaFeGe₂O₆ monoclinic compound has been experimentally investigated using the elastic scattering of neutrons. At a temperature of 1.6 K, an incommensurate magnetic structure has been observed in the form of an antiferromagnetic helix formed by a pairs of the spins of the Fe³⁺ ions with helical modulation in the ac plane of the crystal lattice. The wave vector of the magnetic structure has been determined and its temperature behavior has been studied. The analysis of the temperature dependences of the specific heat and susceptibility, as well as the isotherms of the field dependence of the magnetization, has revealed the existence of not only the order-disorder magnetic phase transition at the point T _N = 13 K, but also an additional magnetic phase transition at the point T _c = 11.5 K, which is assumingly an orientation phase transition.     

Optical study of the electronic structure and magnetic ordering in a weak ferromagnet FeBO<Subscript>3</Subscript>

Spectral dependences of the relative permittivity ɛ = ɛ₁ − iɛ₂ of a uniaxial weak ferromagnet FeBO₃ are measured for two principal polarizations in the energy range 0.6–5.6 eV. The positions have been determined for the charge-transfer transitions that make the main contribution to absorption above the bandgap and determine the refractive-index dispersion below the bandgap. The isotropic magnetic contribution to the refractive index has been detected by studying the temperature dependence of the refractive index in the range 100–700 K; its value (≈2 × 10⁻²) is found to be record high for magnetic dielectrics. The energy shift of the effective oscillator, which characterizes the shift of the positions of the charge-transfer transitions due to magnetic ordering, is determined from these data within the framework of a single-oscillator model. The value of the exchange striction in FeBO₃ is determined from optical measurements.     

Special features of linear magnetic birefringence in an α-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>:Ga single crystal

Experimental investigations of the field dependence of the linear magnetic birefringence (LMB) in an α-Fe₂O₃:Ga single crystal are presented. It is established that during crystal magnetization in the basal plane near directions perpendicular to the С₂ axes in the region of magnetic field saturation, the LMB changes nonmonotonically. The observed special feature of the field LMB dependence is due to the reorganization of the magnetic α-Fe₂O₃:Ga structure during magnetization.     

Magnetic and thermal properties of CuFeS<Subscript>2</Subscript> at low temperatures

The magnetic moment M, the magnetic susceptibility χ, and the thermal conductivity of chalcopyrite CuFeS₂, which is a zero-gap semiconductor with antiferromagnetic ordering, have been measured in the temperature range 10–310 K. It has been revealed that the quantities χ(T) and M(T) increase anomalously strongly at temperatures below ∼100 K. The temperature dependence M(T) is affected by the magnetic prehistory of the sample. An analysis has demonstrated that the magnetic anomalies are associated with the presence of a system of noninteracting magnetic clusters in the CuFeS₂ sample under investigation. The formation of the clusters is most likely caused by the disturbance of the ordered arrangement of Fe and Cu atoms in the metal sublattice of the chalcopyrite, which is also responsible for the phase inhomogeneity of the crystal lattice. The inhomogeneity brings about strong phonon scattering, and, as a result, the temperature dependence of the thermal conductivity coefficient exhibits a behavior characteristic of partially disordered crystals.     

X-ray diffraction studies of the structure of nanocrystals in Fe<Subscript>73.5</Subscript>Si<Subscript>13.5</Subscript>B<Subscript>9</Subscript>Nb<Subscript>3</Subscript>Cu<Subscript>1</Subscript> soft magnetic alloys before and after thermomechanical treatment

The structure of the Fe_73.5Si_13.5B₉Nb₃Cu₁ soft magnetic alloy has been investigated using X-ray diffraction in transmission geometry. The initial alloy prepared by rapid quenching from the melt has a short-range order (∼2 nm) in the atomic arrangement, which is characteristic of the Fe-Si structure with a body-centered cubic lattice. The alloy subjected to annealing contains Fe-Si nanocrystals with sizes as large as 10–12 nm. The annealing under a tensile load leads to an extension of the nanocrystal lattice so that, after cooling, a significant residual deformation is retained. This can be judged from the relative shifts of the (hkl) peaks in the X-ray diffraction patterns measured for two orientations of the scattering vector, namely, parallel and perpendicular to the direction of the load applied. The deformation is anisotropic: within the accuracy of the experiment, no distortions in the [111] direction are observed and the distortions in the [100] direction are maximum. It is known that crystals with a composition close to Fe₃Si exhibit a negative magnetostriction; i.e., their magnetization induced under a load (Villari effect) applied along the [100] direction is perpendicular to this direction along one of the easy magnetization ([010] or [001]) axes. In the alloy, the orientation of the nanocrystal axes is isotropic and the majority of the nanocrystals have a composition close to Fe₃Si. The direction of magnetization of these nanocrystals is determined by the residual deformation of their lattice and lies near the plane perpendicular to the direction of the tensile load applied during heat treatment. This is responsible for the appearance of transverse magnetic anisotropy of the easy-plane type in the Fe_73.5Si_13.5B9Nb₃Cu₁ alloy.