Effect of magnetic field on the TC and magnetic properties for the aligned MnBi compound

In the compound MnBi, a first-order transition from the paramagnetic to the ferromagnetic state can be triggered by an applied magnetic field and the Curie temperature increases nearly linearly with an increase in magnetic field by ∼2 K/T. Under a field of 10 T, T_C increases by 20 and 22 K during heating and cooling, respectively. Under certain conditions a reversible magnetic field or temperature induced transition between the paramagnetic and ferromagnetic states can occur. A magnetic and crystallographic H-T phase diagram for MnBi is given. Magnetic properties of MnBi compound aligned in a Bi matrix have been investigated. In the low temperature phase MnBi, a spin-reorientation takes place during which the magnetic moments rotate from being parallel to the c-axis towards the basal plane at ∼90 K. A measuring Dc magnetic field applied parallel to the c-axis of MnBi suppresses partly the spin-reorientation transition. Interestingly, the fabricated magnetic field increases the temperature of spin-reorientation transition T_s and the change in magnetization for MnBi. For the sample solidified under 0.5 T, the change in magnetization is ∼70% and T_s is ∼91 K.     

Low temperature specific heat of bi-layered manganites La2−2xSr1+2xMn2O7 (x=0.3 and 0.5)

The specific heat (C) of bi-layered manganites La_2−2xSr_1+2xMn₂O₇ (x=0.3 and 0.5) is investigated for the ground state of low temperature excitations. A T^3/2 dependent term in the low temperature specific heat (LTSH) is identified at zero magnetic field and suppressed by magnetic fields for x=0.3 sample, which is consistent with a ferromagnetic metallic ground state. For x=0.5 sample, a T² term is observed and is consistent with a two-dimensional (2D) antiferromagnetic insulator. However, it is almost independent of magnetic field within the range of measured temperature (0.6-10 K) and magnetic field (6 T).     

Magnetic structure of geometrically frustrated compound Co2Cl(OH)3 determined by proton NMR

The magnetic structure of a geometrically frustrated system Co₂Cl(OH)₃ is determined by comparing the observed proton NMR spectrum with numerical calculations based on various magnetic models. The best fit is obtained with a model that the magnetic moments of Co²⁺ ions in the triangular plane are parallel to the principal axis of local crystal field and those of Co²⁺ ions in the kagome lattice plane are randomly disordered in the a-b plane, which nearly bisects the angle between the principal axis of the local field and a line pointing towards the body center of the tetrahedron. The coexistence of the ferromagnetic order in the triangular plane and the random disorder in the kagome plane is consistent with the results of measurements by Zheng et al. However, the magnetic moments of Co²⁺ ions are not directed towards the body center of the tetrahedron as characteristic in the “spin ice” magnetic structure. Furthermore, the Co²⁺ ions in the triangular plane have a smaller magnitude of magnetic moment than those in the kagome plane. Thus, our result suggests that the transition metal compound Co₂Cl(OH)₃ is different from the “spin ice” in magnetic structure, although it is similar to rare-earth pyrochlores in crystal structure.     

Diluted magnetic semiconductors and spintronics

Classification of concentrated and diluted magnetic semiconductors is given and their physicochemical properties that are interesting for spintronics are characterized. The electronic structure of magnetic impurities in semiconductors and the nature of indirect exchange interactions between impurity spins in diluted magnetic semiconductors are considered. On the basis of the proposed theory of kinematic exchange, the Curie temperature T _C for bulk diluted magnetic semiconductors (In,Mn)Sb are estimated.     

Electronic and magnetic phase transitions in (Sm0.65Sr0.35)MnO3 induced by temperature and magnetic field

Temperature (4.2–260 K) and magnetic field (0–50 kOe) dependencies of the DC electrical resistance, DC magnetization, and AC magnetic susceptibility of (Sm_0.65Sr_0.35)MnO₃ prepared from high purity components have been studied. (Sm_0.65Sr_0.35)MnO₃ undergoes a temperature-induced transition between low-temperature ferromagnetic metallic and high-temperature paramagnetic insulating-like states. A magnetic field strongly affects this transition resulting in a metallic state and “colossal” magnetoresistance in the vicinity of the metal↔insulator transition. Magnetic and electric properties of (Sm_0.65Sr_0.35)MnO₃ are different compared to those reported earlier for similar composition, which is attributable to the purity of the starting materials and/or different process of synthesis. The character of phase transformations observed in (Sm_0.65Sr_0.35)MnO₃ is compared to that reported for Gd₅(Si_xGe_4−x) intermetallic alloys with a true first order phase transition.     

Magnetostrictions and Curie temperature measurements of (Fe–Co)91−xMo8Cu1Bx alloys with varying Co/Fe ratio

Amorphous rapidly quenched ribbons of (Fe–Co)₇₉Mo₈Cu₁B₁₂ and (Fe–Co)₇₆Mo₈Cu₁B₁₅ with the ratio of Co/Fe from 0 to 1 and 0 to 2, respectively, were prepared by planar flow casting. The dependence of Curie temperature T_C on Co/Fe ratio was determined from temperature dependencies of sample dilatation measured using a special dilatometer designed for these materials. Due to the presence of the invar effect, it was possible to measure the spontaneous volume magnetostriction in the temperature interval between 300 K and T_C, which is of the order of 10⁻³. Using special disc-shaped samples field dependencies of magnetostriction in parallel and perpendicular directions of the applied magnetic field were obtained by direct measurement. Subsequently, saturation magnetostriction and volume magnetostriction as well as forced magnetostriction were computed. Saturation magnetostriction λ_S increases with increasing Co/Fe ratio from 0 up to 15 and from 0 up to 17 ppm for both alloy systems, respectively, depending both on the Co/Fe ratio and on the shift of T_C with composition. After attaining the maximal value and further increase of the Co/Fe ratio the saturation magnetostriction decreases. Both alloy systems with ratio Co/Fe=0 exhibit T_C near room temperature and the system passes into paramagnetic state. T_C for higher Co/Fe ratios approaches the glass transition region. In paramagnetic state the field dependencies of magnetostriction are practically linear functions of applied field and approach saturation only for high-field values.     

High-temperature ferromagnetism in laser-deposited layers of silicon and germanium doped with manganese or iron impurities

The paper reports on the results of a study of the synthesis conditions effects on magnetic and transport properties of nanosized layers of high-T_c diluted magnetic semiconductors (DMS), such as Ge:Mn, Si:Mn and Si:Fe, fabricated by laser-plasma deposition over a wide range of the growth temperature, T_g=(20-550) °C on single-crystal GaAs or Al₂O₃ substrates. Ferromagnetism of the layers was detected by measurement data of the magneto-optical Kerr effect, anomalous Hall effect, negative magnetoresistance and ferromagnetic resonance (FMR) at 5-500 K. The optimum growth temperature, T_g, for Si:Mn/GaAs layers with T_c≈400 K is shown to be about 400 °C. The Si:Mn/Al₂O₃ layers with 35% of Mn have the metal-type of conductivity with manifestation of magnetization up to room temperature. Different types of uniformly doped structures and digital alloys have been investigated. In contrast to GaSb:Mn films, Si-based ferromagnetic layers have strongly different magnetic and electric properties in case of uniformly doped structures and digital alloys. Positive results of the Fermi level variation effect on the improvement of Si- and Ge-based DMS layers have been gained on the use of additional doping with shallow acceptor Al impurity which contributes to the increase of the hole concentration and the RKKY exchange interaction of 3d-ions. The Ge:(Mn, Al)/GaAs or Ge (Mn, Al)/Si layers grown at 20 °C feature surprising extraordinary angular dependence of FMR.     

Hysteresis loss subdivision

Assuming that different energy dissipation mechanisms are at work along hysteresis, a hysteresis loss subdivision procedure has been proposed, using the induction at maximum permeability (around 0.8 T, in electrical steels) as the boundary between the “low-induction” and the “high-induction” regions. This paper reviews the most important results obtained in 10 years of investigation of the effect of microstructure on these components of the hysteresis loss. As maximum induction increases, the “low-induction loss” increases linearly up to 1.2 T, while the “high-induction loss” is zero up to 0.7 T and then increases as a power law with n=5. Low-induction loss behavior is linearly related to H_c between 0.4 and 1.2 T. Grain size has a larger influence on low-induction losses than on high-induction losses. Texture has a much stronger influence on high loss than on low-induction loss, and it is related to the average magnetocrystalline energy. 6.5%Si steel shows smaller hysteresis loss at 1.5 T than 3.5%Si steel only because of its smaler high-induction component. The abrupt increase in hysteresis loss due to very small plastic deformation is strongly related to the high-induction loss component. These results are discussed in terms of energy dissipation mechanisms such as domain wall movement, irreversible rotation and domain wall energy dissipation at domain nucleation and annihilation.     

A novel technique for measurement of core loss in low permeability materials

A survey of existing techniques for measuring core loss in magnetic materials is presented. It is shown that the most challenging combination is a low permeability material at low measurement frequency. Such materials have application in components of power converters, and thus are of considerable technological significance. Measurement difficulties stem from losses in the windings employed, which are difficult to disentangle from loss in the core, as well as the stringent requirements on phase accuracy of measurement instruments. Another practical problem is the large power capacity required of the drive source to achieve significant induction levels. A new resonant measurement setup, which we call the “virtual impedance” technique, is proposed as a solution to these problems. Its utility is demonstrated with measurements of a commercial composite core with permeability 14μ₀ whose low-core loss defeats conventional methods of measurement.     

Ferromagnetic semiconductor Ge1−x−ySnxMnyTe with phase transformation of ferroelectric type

It is expected that joint existence of ferromagnetic properties and ferroelectric structural phase transition in diluted magnetic semiconductors IV-VI leads to new possibilities of these materials. Temperature of ferroelectric transition for such crystals can be tuned by the change of Sn/Ge ratio. Magnetic susceptibility, Hall effect, resistivity and thermoelectric power of Ge_1−x−ySn_xMn_yTe single crystals grown by Bridgeman method (x=0.083-0.115; y=0.025-0.124) were investigated within 4.2-300 K. An existence of FM ordering at T_C∼50 K probably due to indirect exchange interaction between Mn ions via degenerated hole gas was revealed. A divergence of magnetic moment temperature dependences at T?T_C in field-cooled and zero-field-cooled regimes is obliged to magnetic clusters which are responsible for superparamagnetism at T>T_C≈T_f (freezing temperature) and become ferromagnetic at T_C arranging spin glass state at T<T_f≈T_C. Phase transition of ferroelectric type at T≈46 K was revealed. Anomalous Hall effect which allows to determine magnetic moment was observed.     

Thermomagnetic transitions and coercivity mechanism in bulk composite Nd60Fe30Al10 alloys

The thermomagnetic behaviour (within the temperature range 553-300 K) for the bulk composite Nd₆₀Fe₃₀Al₁₀ alloy is described in terms of a transition from paramagnetic to superferromagnetic state at T=553 K, followed by a ferromagnetic ordering for T<473 K. For the superferromagnetic regime, the alloy thermomagnetic response was associated to a homogeneous distribution of magnetic clusters with mean magnetic moment and size of 1072 μ_B and 2.5 nm, respectively. For T<473 K, a pinning model of domain walls described properly the alloy coercivity dependence with temperature, from which the domain wall width and the magnetic anisotropy constant were estimated as being of ≈8 nm and ≈10⁵ J/m³, typical values of hard magnetic phases. Results are supported by microstructural and magnetic domain observations.     

Transitions involving conical magnetic phases in a model with bilinear and biquadratic interactions

In a previous work a model was proposed for the phase transitions of crystals with localized magnetic moments which at low temperature have a “conical” arrangement that at higher T transforms into a more symmetrical structure (depending on the compound) before becoming totally disordered. The model assumes bilinear and biquadratic interactions between magnetic moments up to the fifth neighbours, and for any given T the structure with the least free energy is obtained by a mean-field approximation (MFA). The interaction constants are derived from ab initio   energy calculations. In this work we improve upon that model modifying the MFA in such a way that a continuous (instead of discontinuous) spectrum of excited states is available to the system. In the previous work, which dealt with _LaMn2Ge2${\mathrm{LaMn}}_2{\mathrm{Ge}}_2$ and _LaMn2Si2${\mathrm{LaMn}}_2{\mathrm{Si}}_2$, we found that transitions to different structures can be obtained for increasing T, in good qualitative agreement with experiment. The critical temperatures, however, were exaggeratedly high. With the new MFA we obtain essentially the same behaviour concerning the phase transitions, and critical temperatures much closer to the experimental ones.     

Ferromagnetism in epitaxial layers of gallium and indium antimonides and indium arsenide supersaturated by manganese impurity

We report on laser synthesis of thin 30–200 nm epitaxial layers with mosaic structure of diluted magnetic semiconductors GaSb:Mn and InSb:Mn with the Curie temperature T_C above 500 K and of InAs:Mn with T_C no less than 77 K. The concentration of Mn was ranged from 0.02 to 0.15. In the case of InSb:Mn and InAs:Mn films, the additional pulse laser annealing was needed to achieve ferromagnetic behavior. We used Kerr and Hall effects methods as well as ferromagnetic resonance (FMR) spectroscopy to study magnetic properties of the samples. The anisotropy FMR was observed for both layers of GaSb:Mn and InSb:Mn up to 500 K but it takes place with different temperature dependencies of absorption spectra peaks. The resonance field value and amplitude of FMR signal on the temperature is monotonically decreased with the temperature increase for InSb:Mn. In the case of GaSb:Mn, this dependence is not monotonic.     

Hole transport anomaly in high-TC ferromagnet (Zn,Mn)GeP2

Anomalous Hall effect and a large negative magnetoresistance (up to −8.5%) have been found in the high-T_C ferromagnetic chalcopyrite (Zn,Mn)GeP₂. The elevated manganese concentration in the top diffusion layer grown on ZnGeP₂ isolated substrate gives rise to the increased hole conductivity and the temperature dependence of electric resistance ρ(T) typical of a metallic trace which indicate the charge carrier degeneration in combination with ferromagnetism. Additionally, we found a hysteresis of magnetoresistance Δρ/ρ₀ vs. H is associated with change in a magnetic order at low temperatures, T<50 K. The effect accompanies the reversal sign of Δρ/ρ₀ and is consistent with a singularity of magnetization vs. temperature. These anomalies observed for the first time in the high-T_C ferromagnetic chalcopyrites II-IV-V₂:Mn are explained by the phenomenological cluster model for ferromagnets.     

EPR studies of linewidth anomalies at phase transitions in [N(C2H5)4]2MnCl4

We have studied [N(C₂H₅)₄]₂MnCl₄ crystal by X-band CW EPR spectra in the temperature range 170-300 K. The angular dependences of linewidth ΔH were measured and described in the light of a double-layer system (2D) with exchange interactions. Two temperature anomalies of linewidth ΔH were found at T₁=225 K and T₂=192 K on cooling. Different behaviors of ΔH anomalies recorded for an external magnetic field parallel and perpendicular to the ab crystallographic plane indicate ordering/disordering of MnCl₄ groups in this plane and their displacement along the c-axis which occurs in the temperature of about 225 K.     

Magnetic and electrical properties of new magnetic phase in Si1−xMnx crystals

We have investigated the magnetic and electrical transport properties of Si_1−xMn_x single crystals grown by the vertical Bridgman method. The alloys with Mn concentrations up to x=0.64 have weak ferromagnetic ordering around T_C∼30 K. However, Si_0.25Mn_0.75 alloys show weak ferromagnetic ordering at 70 K and antiferromagnetic ordering at 104 K, which is confirmed by magnetization and electrical transport studies.     

Very high field magnetization and AC susceptibility of native horse spleen ferritin

The magnetization of native horse spleen ferritin protein is measured in pulsed magnetic fields to 55 T at T=1.52 K. The magnetization rises smoothly with negative curvature due to uncompensated Fe³⁺ spins and with a large high field slope due to the underlying antiferromagnetic ferritin core. Even at highest fields the magnetic moment is only ∼4% of the saturation moment of the full complement of Fe³⁺ in the ferritin molecule. The AC magnetic susceptibility, χ_AC(T,f), responding to the uncompensated spins, reaches a maximum near the superparamagnetic blocking temperature with the temperature of the maximum, T_M, varying with excitation frequency, T_M⁻¹ α log f for 10?f?10⁴ Hz.     

Magnetocaloric effects in Ni-Mn-X based Heusler alloys with X=Ga, Sb, In

The Mn-based Heusler alloys encompass a rich collection of useful materials from highly spin-polarized systems to shape memory alloys to magnetocaloric materials. In this work we have summarized our studies of magnetostructural transitions from paramagnetic austenite to ferromagnetic martesite phases at TMC in Ni₂MnGa-based alloys (Ni₂Mn_0.75Cu_0.25-xCo_xGa, Ni₂Mn_0.70Cu_0.30Ga_0.95Ge_0.05, Ni₂Mn_1-xCu_xGa, Ni_2+xMn_1-xGa, and Ni₂Mn_0.75-xCu_xGa), and martensitic transitions from the ferromagnetic austenite to the martesite state in off-stoichiometric Ni-Mn-(In/Sb) Heusler alloys. The phase transition temperatures and respective magnetic entropy changes (ΔS) depend on composition in these systems and have been determined from magnetization measurements in the temperature interval 5-400 K, and in magnetic fields up to 5 T. It is shown that, depending on the composition and doping scheme the “giant” ΔS=40-60 J/(kgK) (for a field change of 5 T) can be observed in the temperature range (300-360 K) for the Ga-based alloys. The interplay between or coupling of the various transitions in Ni₂Mn(Mn,X) systems with X=Sb and In leads to exchange bias effects, giant magnetoresistance, and both inverse and “normal” magnetocaloric effects.     

Influence of crystallization on the spectral features of nano-sized ferroelectric barium strontium titanate (Ba0.7Sr0.3Tio3) thin films

Ferroelectric barium strontium titanate (Ba_0.7Sr_0.3TiO₃)(BST) thin films have been prepared from barium 2-ethylhexanoate [Ba[CH₃(CH₂)₃CH(C₂H₅)CO₂]₂]_, strontium 2-ethylhexanoate [Sr[CH₃(CH₂)₃CH(C₂H₅)CO₂]₂] and titanium(IV) isopropoxide [TiOCH(CH₃)₂]₄ precursors using a modified sol-gel technique. The precursor except [TiOCH(CH₃)₂]₄ were synthesized in the laboratory. Transparent and crack-free films were fabricated on pre-cleaned quartz substrates by spin coating. The structural and optical properties of films annealed at different temperatures have been investigated. The as-fired films were found to be amorphous that crystallized to the tetragonal phase after annealing at 550 °C for 1 h in air. The lattice constants “a” and “c” were found to be 3.974 A and 3.990 A, respectively. The grain sizes of the films annealed at 450, 500 and 550 °C were found to be 30.8, 36.0 and 39.8 nm respectively. The amorphous film showed very high transparency (∼95%), which decreases slightly after crystallization (∼90%). The band gap and refractive index of the amorphous and crystalline films were estimated. The optical dispersion data are also analyzed in the light of the single oscillator model and are discussed.