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.     

Magnetic and Mössbauer Studies of Fe<Subscript>76</Subscript>Mo<Subscript>8</Subscript>Cu<Subscript>1</Subscript>B<Subscript>15</Subscript> Amorphous Alloy

“Zero field”-Mössbauer and magnetization measurements have been performed on an amorphous Fe₇₆Mo₈Cu₁B₁₅ alloy in the temperature range of (10-340) K. The room-temperature Mössbauer spectrum exhibits magnetic dipole and electric quadrupole interactions. At approximately 306 K, the magnetic interactions vanish and the alloy shows fully paramagnetic behavior. On the other hand, the relative representation of paramagnetic component becomes weak with decreasing temperature and below 220 K the magnetic dipole interactions prevail. Below this temperature an anomaly in the low-temperature dependencies of ac susceptibility and of magnetization, measured during cooling the specimen from 340 K down to 20 K is observed. The anomaly on the magnetization curve vanishes in the field of 200 Oe.     

Muon-spin-relaxation investigation of EuMn<Subscript>2</Subscript>O<Subscript>5</Subscript>

The magnetic properties of the EuMn₂O₅ multiferroic (samples consisting of single crystals and ceramic samples) have been investigated by the muon-spin-relaxation (μSR) method in the temperature range of 10–300 K. Below the magnetic ordering temperature T _N = 40 K, the loss of the polarization of muons and the effect of the external magnetic field have been observed. Both phenomena can be explained by an additional channel of the depolarization of muons owing to the appearance of muons in a medium with a low electron density due to the charge separation process (the redistribution of the electron density in the phase transition process). The “memory” phenomenon has been revealed in a sample in the external magnetic field; the memory relaxation time depends on the size of the structure units of the samples (single crystals or ceramic grains).     

Magnetic and thermal properties of single-crystal NdFe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript>

The neodymium ferroborate NdFe₃(BO₃)₄ undergoes an antiferromagnetic transition at T _N = 30 K, which manifests itself as a λ-type anomaly in the temperature dependence of the specific heat C and as inflection points in the temperature dependences of the magnetic susceptibility χ measured at various directions of an applied magnetic field with respect to the crystallographic axes of the sample. Magnetic ordering occurs only in the subsystem of Fe³⁺ ions, whereas the subsystem of Nd³⁺ ions remains polarized by the magnetic field of the iron subsystem. A change in the population of the levels of the ground Kramers doublet of neodymium ions manifests itself as Schottky-type anomalies in the C(T) and χ(T) dependences at low temperatures. At low temperatures, the magnetic properties of single-crystal NdFe₃(BO₃)₄ are substantially anisotropic, which is determined by the anisotropic contribution of the rare-earth subsystem to the magnetization. The experimental data obtained are used to propose a model for the magnetic structure of NdFe₃(BO₃)₄.     

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.     

Magnetic and Mössbauer Studies of Fe<Subscript>76</Subscript>Mo<Subscript>8</Subscript>Cu<Subscript>1</Subscript>B<Subscript>15</Subscript> Nanocrystalline Alloy

The influence of different degrees of crystallinity on the magnetic behaviour of heat-treated nanocrystalline Fe₇₆Mo₈Cu₁B₁₅ alloy has been investigated using a combination of Mössbauer spectrometry and magnetic measurements. The evolution of magnetically active regions and their growth with rising contents of nanocrystals are followed by distributions of hyperfine interactions. Combined electric quadrupole and magnetic dipole interactions corresponding to non-magnetic and magnetic regions inside the amorphous phase, respectively, were revealed. A deterioration of the soft-magnetic properties takes place for the samples exhibiting low fraction of crystallinity. The very good soft-magnetic behaviour is regained for the samples where the primary crystallization process is almost finished.     

Electron acoustic imaging of Mn<Subscript>50</Subscript>Ni<Subscript>28</Subscript>Ga<Subscript>22</Subscript> ferromagnetic shape memory alloy

The novel ferromagnetic shape memory alloy Mn₅₀Ni₂₈Ga₂₂ exhibits a single martensite phase with tetragonal structure at room temperature. Its martensite structure was investigated by scanning electron acoustic microscopy. Stripe twin variants exist in every grain and exhibit the configurations of the typical self-accommodation arrangement. The interfaces between twin variants are straight and clear. However, the magnetic domain walls obtained by the Bitter method coincide well with the twin variant boundaries. The first direct evidence of the coincidence between micron-scale magnetic domains and martensite variants is presented. These results will be beneficial for the development of ferromagnetic shape memory alloys and for the understanding of the correlation between magnetic domains and crystallographic twin variant domains as well. PACS  68.37.Hk; 75.70.Kw; 75.60.Ch; 73.50.Rb; 43.58.+z     

Kinetics of magnetization reversal in a thin La<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> manganite film

The kinetics of magnetization reversal of a thin LSMO film has been studied for the first time. It is shown that the magnetic domain structure critically depends on the conditions of structure formation. In the demagnetized state (after zero-field cooling from T _c ), a maze-like domain microstructure with perpendicular magnetization is formed in the film. However, after field cooling and/or saturating magnetization by a field of arbitrary orientation, the [110] direction of spontaneous magnetization in the film plane is stabilized; this pattern corresponds to macrodomains with in-plane magnetization. Further film magnetization reversal (both quasi-static and pulsed) from this state is implemented via nucleation and motion of 180° “head-to-head” domain walls. Upon pulse magnetization reversal, the walls “jump” at a distance proportional to the applied field strength and then undergo thermally activated drift. All dynamic characterisitcs critically depend on the temperature when the latter varies around the room temperature.     

Observation of the magnetic domain structure in Cu<Subscript>0.47</Subscript>Ni<Subscript>0.53</Subscript> thin films at low temperatures

We report on the first experimental visualization of domain structure in films of weakly ferromagnetic Cu_0.47Ni_0.53 alloy with different thickness at liquid helium temperatures. Improved high-resolution Bitter decoration technique was used to map the magnetic contrast on the surface of the films well below the Curie temperature T_Curie (∼60 K). In contrast to magnetic force microscopy, this technique allowed visualization of the domain structure without its disturbance while the larger areas of the sample were probed. Maze-like domain patterns, typical for perpendicular magnetic anisotropy, were observed. The average domain width was found to be about 100 nm. The article is published in the original.     

Effect of a magnetic field on the thermal expansion of Ni<Subscript>2.08</Subscript>Mn<Subscript>0.96</Subscript>Ga<Subscript>0.96</Subscript> alloys

The effect of a magnetic field on the temperature dependence of thermal expansion of Ni_2.08Mn_0.96Ga_0.96 alloy in the range of structural-phase transformation is investigated. It is found that, when the sample is cooled in a magnetic field with an intensity of 110 kA/m, it shortens by 0.21% more strongly than upon cooling in the absence of the magnetic field. The additional shortening of the sample under the action of the magnetic field depends on the intensity of the magnetic field applied to the sample.     

Magnetic properties of the quasi-two-dimensional crystal (CH<Subscript>3</Subscript>NH<Subscript>3</Subscript>)<Subscript>2</Subscript>CuBr<Subscript>4</Subscript>

The magnetic properties of (CH₃NH₃)₂CuBr₄ quasi-two-dimensional crystals were studied experimentally. The magnetic-field and temperature dependences of magnetization were measured for various magnetic field orientations relative to the crystallographic axes. Possible reasons for features in the behavior of the magnetization are discussed.     

Magnetic resonance of the RbMnBr<Subscript>3</Subscript> antiferromagnet under pressure

The antiferromagnetic resonance spectrum was experimentally studied for a noncollinear RbMnBr₃ antiferromagnet under a mechanical pressure applied to the sample in various directions. It is shown that the incommensurate magnetic phase existing in the initial system in the presence of regular crystallographic distortions is sensitive to the pressure applied along one of the sample axes. The critical transition field to the commensurate phase decreased under pressure. It was also found that the pressure influences the uniaxial anisotropy appearing in the crystal basal plane in the presence of orthorhombic distortions. These effects were analyzed with allowance for the domain structure of the sample.     

Influence of the special features of the effective magnetic anisotropy on the temperature dependences of the magnetoimpedance of nanocrystalline Fe<Subscript>73.5</Subscript>Si<Subscript>16.5</Subscript>B<Subscript>6</Subscript>Nb<Subscript>3</Subscript>Cu<Subscript>1</Subscript> strips

The influence of the thermal treatment type on the temperature dependences of the magnetoimpedance of nanocrystalline Fe_73.5Si_16.5B₆Nb₃Cu₁ alloy strips is investigated. The main mechanisms determining the temperature behavior of the magnetoimpedance of strips with induced magnetic anisotropy having various special features are established. The prospects for application of the alloy strips nanocrystallized in the presence of a magnetic field as sensitive elements of temperature sensors and special magnetic field detectors are demonstrated.     

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.     

Structural,magnetic, and magnetothermal properties of the Tb<Subscript>0.3</Subscript>Dy<Subscript>0.7</Subscript>Co<Subscript>2</Subscript> compound

A complex investigation of the structural, magnetic, and magnetothermal properties of the Tb_0.3Dy_0.7Co₂ compound synthesized with the use of high-purity rare-earth metals has been performed. The phase composition has been controlled using the X-ray structural analysis, and the topology of the alloy surface has been investigated using atomic-force microscopy. It has been established that the Tb_0.3Dy_0.7Co₂ compound is single-phase, while the samples selected for measurements possess a clearly pronounced texture. The magnetization has been measured using a vibrating-sample magnetometer in the fields up to 100 kOe in a temperature range from 4.2 to 200 K. The Curie temperature of the compound is 170 K. The data on the temperature dependence of heat capacity of Tb_0.3Dy_0.7Co₂ have been obtained. The magnetocaloric effect ΔT has been measured by a direct method in the fields up to 18 kOe applied both along and perpendicularly to the texture axis. The anisotropic behavior of the magnitude ΔT for this compound, which possesses the cubic structure, has been found. The maximum value of the magnetocaloric effect ΔT = 2.3 K (ΔH = 18 kOe) has been observed upon applying the magnetic field along the texture axis.     

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.     

Effect of atomic disordering and iron admixture on the structure and properties of the Cu<Subscript>3</Subscript>Pd alloy

It is shown that the periodic antiphase domain structure of the Cu₃Pd alloy disappears as a result of disordering and doping with iron. The observed changes in the electrical, optical, and magnetic properties indicate an insignificant reconstruction of the electronic spectrum near the Fermi level as a result of the alloy disordering and a more significant reconstruction due to doping with iron.     

Structural and magnetic properties of layered Ca<Subscript>3</Subscript>Co<Subscript>4</Subscript>O<Subscript>9</Subscript> thin films

Layered cobalt oxides Ca₃Co₄O₉ thin films have been grown directly on c-cut sapphire substrates using pulsed laser deposition. X-ray diffraction and transmission electron microscopy characterizations show that the deposited films present the expected monoclinic structure and a texture along the direction perpendicular to the Al₂O₃(001) plane. The Ca₃Co₄O₉ structure presents six variants in the film plane. Rutherford backscattering spectroscopy shows that the films are stoichiometric and that the film thickness agrees with the nominal value. The susceptibility χ of the films, recorded along the c-axis of the substrate, after field cooling and zero field cooling in an applied field of 1 kOe shows two magnetic transitions at 19 and 370 K which agree well with previous findings on single crystal samples. In turn, at low temperature (5 K), the magnetization curve along the c-axis exhibits coercive field and remanent magnetization much smaller than those reported for bulk samples, which can be related to the influence of structural variants and structural defects.     

Large successive magnetocaloric effects around room temperature in Ni<Subscript>50</Subscript>Mn<Subscript>34</Subscript>In<Subscript>15</Subscript>Al alloy

Two successive magnetocaloric effects consisting of inverse magnetocaloric effect around martensitic transition and negative magnetocaloric effect around magnetic transition of austenitic phase have been observed in Ni₅₀Mn₃₄In₁₅Al alloy. Large inverse magnetic entropy change ΔS_m ( ~ 21.3 J kg^?1 K^?1), small thermal and magnetic hysteresis of martensitic transition give rise of large net refrigerant capacity ( ~ 152.3 J kg^?1) under a magnetic field of 50 kOe, which is comparable with that ( ~ 157.9 J kg^?1) of second-order transition. The large combined magnetocaloric effects make the Ni₅₀Mn₃₄In₁₅Al alloy as a promising candidate material for room temperature magnetic refrigeration.