Spin-wave resonance in three-layer NiFe/DyxCo1−x /NiFe films as a method for detecting structural inhomogeneities in amorphous DyxCo1−x Layers

The standing spin-wave spectrum was studied by spin-wave resonance in three-layer Ni₈₀Fe₂₀/Dy_xCo_1?x /Ni₈₀Fe₂₀ films with an amorphous interlayer of DyCo alloy in the region of compensation compositions. It is shown that the spin-wave resonance (SWR) spectrum in the geometry k‖M is observed only for a planar system with a DyCo layer of precompensation composition. In the k⊥M geometry, the SWR spectrum was observed for the DyCo systems with both pre-and postcompensation compositions. The exchange stiffness was analyzed as a function of the DyCo layer thickness to formulate a model of microheterophase structure for amorphous DyCo alloys in the compensation region, where the magnetic microstructure accounts for the dynamic and static magnetic characteristics of these materials.     

“Aharonov-Bohm antiferromagnetism” and compensation points in the lattice of quantum rings

We investigate the magnetic properties of the lattice of non-interacting quantum rings using the 2D rotator model. The exact analytic expressions for the free energy as well as for the magnetization and magnetic susceptibility are found and analyzed. It is shown that such a system can be considered as a system with antiferromagnetic-like properties. We have shown also that all observable quantities in this case (free energy, entropy, magnetization) are periodic functions of the magnetic flux through the ring's area (as well known, such a behavior is typical for the Aharonov-Bohm effect). For the lattice of quantum rings with two different geometric parameters we investigate the ordinary compensation points (“temperature compensation points”, i.e. points at which the magnetization vanishes at fixed values of the magnetic field strength). It is shown that the positions of compensation points in the temperature scale are very sensitive to small changes in the magnetic field strength.     

The phase diagrams and compensation behaviors of mixed spin Blume-Capel model in a trimodal magnetic field

The phase diagrams and compensation behaviors of mixed spin-1/2 and spin-1 Blume-Capel model in a trimodal magnetic field are investigated in the framework of the effective field theory on simple cubic lattice. The change of negative crystal field and trimodal concentration can affect the TCP, the second-order phase and the magnetic field degeneration at ground state in T-H space. In T-D space, the trajectory of the TCP takes on the acre curve and there exist the two TCPs under certain condition. In addition to giving one or two compensation temperature points in M-T space, the mixed spin Blume-Capel model also provides one or two novel compensation magnetic field points in M-H space. Some results are not revealed in previous works.     

Modern microwave magnetic materials: Recent advances and trends

In recent years, a large variety of microwave magnetic materials has been developed, with different compositions, shapes, and fabrication processes. The physics of the dynamic magnetic responses of these materials is very rich, due to the interplay between the intrinsic magnetic properties of the materials, the domain structure, and dynamic shape effects. These materials are associated to a variety of applications, some of them well-established, for direct interaction with rf waves; others corresponding to etablished uses of magnetic materials, but at higher speeds or higher frequencies; and some in association with hot topics in the magnetic or rf communities including metamaterials, nanoscale structures, and nonlinear devices.     

A compositional fluctuation and properties of Pb(Zr,Ti)O3

Structure and properties of piezoelectric materials of composition Pb(Zr_xTi_1?x)O₃ near the morphotropic phase boundary between tetragonal and rhombohedral phases are studied. When these materials were prepared by a new “wet-dry combination technique”, no coexistence of these two phases was found in any range of compositions. This fact is in contrast with the case of Pb(Zr_xTi_1?x)O₃ prepared by an ordinary dry method. The coexistence behavior is interpreted in terms of compositional fluctuation, on the basis of which various properties of the materials are explained.     

Ground-state magnetism of chromium-substituted LiMn2O4 spinel

Comparative crystal structure and magnetic properties studies have been conducted on quaternary powder spinel samples LiMn_1.82Cr_0.18O₄ obtained by two different synthesis methods, glycine-nitrate (GN) and ultrasonic spray-pyrolysis (SP). Although both samples possess the same spinel structure of the cubic space group Fd3¯m, their low-temperature magnetic properties display significant differences. While the SP sample undergoes only spin-glass transition at the freezing temperature T_f=20 K, the GN sample possesses more complicated low-temperature magnetic behavior of the reentrant spin-glass type with the Néel temperature T_N=42 K and freezing temperature T_f=22 K. High-temperature magnetic susceptibility of both samples is of the Curie–Weiss type with the effective magnetic moments in agreement with the nominal compositions. This fact together with the results of the chemical analysis discards the existence of the diversity in chemical compositions as a possible cause for the observed differences in the low-temperature magnetism. On the other hand, the crystal structure analysis done by the Rietveld refinement of the X-ray powder diffraction data points to the strong influence of the cation distribution on the ground-state magnetism of these systems. An explanation of this influence is proposed within the framework of a collective Jahn–Teller effect.     

Review of the magnetocaloric effect in manganite materials

A thorough understanding of the magnetocaloric properties of existing magnetic refrigerant materials has been an important issue in magnetic refrigeration technology. This paper reviews a new class of magnetocaloric material, that is, the ferromagnetic perovskite manganites (R_1−xM_xMnO₃, where R=La, Nd, Pr and M=Ca, Sr, Ba, etc.). The nature of these materials with respect to their magnetocaloric properties has been analyzed and discussed systematically. A comparison of the magnetocaloric effect of the manganites with other materials is given. The potential manganites are nominated for a variety of large- and small-scale magnetic refrigeration applications in the temperature range of 100–375 K. It is believed that the manganite materials with the superior magnetocaloric properties in addition to cheap materials-processing cost will be the option of future magnetic refrigeration technology.     

Low temperature heat capacities of transition metal phosphidest2

Low temperature heat capacities of transition metal phosphides, T₃P, (T = transition metal) are characterized by large γ values which indicate the existence of a narrow d-band in these materials. Anomalous γ values are observed at compositions close to the onset of a long range magnetic state.     

Dynamic compensation temperature in the kinetic spin-1 Ising model in an oscillating external magnetic field on alternate layers of a hexagonal lattice

The dynamic behavior of a two-sublattice spin-1 Ising model with a crystal-field interaction (D) in the presence of a time-varying magnetic field on a hexagonal lattice is studied by using the Glauber-type stochastic dynamics. The lattice is formed by alternate layers of spins σ=1 and S=1. For this spin arrangement, any spin at one lattice site has two nearest-neighbor spins on the same sublattice, and four on the other sublattice. The intersublattice interaction is antiferromagnetic. We employ the Glauber transition rates to construct the mean-field dynamical equations. Firstly, we study time variations of the average magnetizations in order to find the phases in the system, and the temperature dependence of the average magnetizations in a period, which is also called the dynamic magnetizations, to obtain the dynamic phase transition (DPT) points as well as to characterize the nature (continuous and discontinuous) of transitions. Then, the behavior of the total dynamic magnetization as a function of the temperature is investigated to find the types of the compensation behavior. Dynamic phase diagrams are calculated for both DPT points and dynamic compensation effect. Phase diagrams contain the paramagnetic (p) and antiferromagnetic (af) phases, the p+af and nm+p mixed phases, nm is the non-magnetic phase, and the compensation temperature or the L-type behavior that strongly depend on the interaction parameters. For D<2.835 and H₀>3.8275, H₀ is the magnetic field amplitude, the compensation effect does not appear in the system.     

Dilution effect on the compensation temperature in a honeycomb nano-lattice: Monte Carlo study

In this work, we use Monte Carlo simulations with the Metropolis algorithm to study the dilution effect on the magnetic properties in a honeycomb nano-lattice. The geometry of the system is formed by two sub-lattices of 45+45 = 90 atoms consisting of the spins σ = 3/2 and S = 5/2. The ground state phase diagrams at zero absolute temperature are presented. Also, the variation of the total magnetization with reduced temperature for several values of the reduced exchange coupling are discussed. It is found that the compensation temperature depends on the reduced exchange coupling parameters. Furthermore, the dilution of the S spins affects the compensation temperature and hysteresis loops of the studied system.     

Structure and phases of low-neodymium NdFeB permanent magnets

Structures and phase compositions of two low-neodymium magnetically hard materials, differing by the way of preparation — centrifugal atomization and melt spinning — were compared using Mössbauer spectroscopy, X-ray diffraction and measurements of thermomagnetic curves. Better hard magnetic characteristics of the melt-spun material are explained on the basis of the differences in the content of surface and/or interface Fe(Nd,B) phases. Their remarkable presence in the centrifugally atomized material lowers the content of Fe₃B, Fe₂B, α-Fe, and Nd₂Fe₁₄B phases that are responsible for the magnetic quality of the material. There are only subtle differences in the phase compositions of both materials after thermomagnetic measurement, where the α-Fe phase prevails as a product of the thermal decomposition.     

Giant magnetostriction materials

One of the significant technical developments in magnetism of the early 1970's was the discovery of a new class of rare earth intermetallic compounds, the RFe₂ Laves phases, which were found to exhibit room temperature magnetostrictive strains approaching 2 × 10⁻³, an order of magnitude larger than any previously known. Since that time both the fundamental and technical properties of these materials have been of intense interest, and they remain the subject of active research even today. The large strains available are useful in such applications as production of high amplitude, low frequency sound waves in water, certain types of strain gages, vibration compensation and compensation for temperature induced strains in large laser mirrors. Because the performance of these materials depends critically on such fundamental properties as the magnetic anisotropy, magnetization and grain orientation of the material, there has been a very strong interplay between fundamental studies and applications. In this article we briefly review the fundamental magnetic and magnetostrictive properties of the RFe₂ Laves phases, focusing especially on the complex behavior of the anisotropy and the success of crystal field theory in explaining it. We also present neutron measurements of magnetic excitation spectra and explain how they provide an understanding of the remarkable success of mean field theory for these systems.     

An antiferromagnetic transverse Ising nanoisland; unconventional surface effects

The phase diagrams and temperature dependences of magnetizations in a transverse Ising nanosisland with an antiferromagnetic spin configuration are studied by the use of the effective-field theory with correlations (EFT). Some novel features, such as the re-entrant phenomena with two compensation points being free from disorder induced frustration, are obtained for the magnetic properties in the system.     

A universal DC characterisation system for hard and soft magnetic materials

A fully automatic system has been designed for the accurate measurement of the DC magnetic properties of soft and hard ferromagnetic materials utilising discrete calibrated instruments in order to provide a traceable calibration route separate from the transfer of standard magnetic test samples. Custom written software is used to operate the system in one of three modes, constant dH/dt, variable dH/dt and a second quadrant demagnetisation curve mode. The first two of these modes are utilised for soft magnetic materials with the second mode varying dH/dt in order to keep dB/dt relatively constant. Both modes use cycle times of between 60 and 300 s and may utilise a variety of test configurations including a bar permeameter, electromagnet, ring samples or Epstein frame. The minimum cycle time and the most appropriate mode is dependent on the particular sample and the effect of this on materials with a large dB/dH is significant. Measurements on soft materials include major BH loop, minor BH loops, first-order reversal curves, remanence, coercivity, normal magnetisation curve, peak permeability and loop area. The third mode is used with an electromagnet to measure the demagnetisation curve of hard magnetic materials up to a maximum demagnetisation field of 1.6 MA/m. The measurement algorithm modulates dH/dt depending on dB/dt and incorporates holdback in order to accommodate rare earth materials which exhibit high viscosity.     

Crystal structure,thermochemical stability,electrical and magnetic properties of the two-phase composites in the La0.8Sr0.2MnO3 ± δ–CeO2 system

Crystal structure, thermochemical stability, transport and magnetic properties of compositions in the (100-x) La_0.8Sr_0.2MnO_3 ± δ xCeO₂ (LSMC) system were studied. All compositions in the LSMC series containing more than 2 mol% CeO₂ were two phase and consisted of the modified perovskite constituent with rhombohedral structure (R3?c) and ceria as a secondary phase with cubic structure (Fm3?m). The presence of both Ce⁴⁺and Ce³⁺ cations in LSMC compositions was revealed by X-ray Photoelectron Spectroscopy (XPS). CeO₂ and compositions in the LSMC series showed good thermochemical stability in air and argon. However, in H₂–Ar atmosphere all LSMC compositions underwent reduction followed by decomposition. Transport and magnetic properties change in a non-linear way with the increase in the CeO₂ content. The LSMC2 composition showed enhanced electronic conductivity and magnetic characteristics. Metallic type conductivity was observed for LSMC compositions with x ≤ 36 mol% CeO₂ in a narrow temperature range of 770–900 °C. A small degree of substitution of Ce into LSM was found to change structural, magnetic and electrical properties.     

Magnetization of Fe2+-Cr-Ti spinels

Measurements of magnetization σ in fields up to 14 kOe on four sintered samples of Fe²⁺-Cr-Ti spinels reveal σ-temperature curves which exhibit in part compensation points (T_co). The magnetic moments are low, indicating non Néel spin configuration. Close to T_co viscose magnetic effects have been detected.     

Effects of magnetic annealing on structure and multiferroic properties of pure and dysprosium substituted BiFeO3

In this work, the effects of magnetic annealing on crystal structure and multiferroic properties of BiFeO₃ and Bi_0.85Dy_0.15FeO₃ have been investigated. It is found that the X-ray diffraction patterns of pure BiFeO₃ samples are obviously broadened after magnetic annealing, whereas those of Bi_0.85Dy_0.15FeO₃ samples are almost unchanged. Magnetic field annealing did not affect the magnetic properties of these two kinds of samples much. However, ferroelectric properties of the two materials exhibited different behaviors after magnetic field annealing. For pure BiFeO₃ samples, the remnant polarizations (P_r) are suppressed; in contrast, for Bi_0.85Dy_0.15FeO₃ samples, P_r is greatly enhanced. Possible mechanisms for the effects of magnetic field annealing have been discussed.     

Enhancement of the magnetocaloric effect in composites: Experimental validation

Recent calculations have shown that the refrigerant capacity (RC) of magnetic refrigerants can be enhanced using multiphase materials or composites, which expand the temperature range over which a significant magnetic entropy change can be obtained. This work is a systematic experimental validation of the improvement of RC (RCI) using layered composites comprised of two Fe_88?2yCo_yNi_yZr₇B₄Cu₁ amorphous alloy constituents, with y=8.25 and y=11 compositions. RCI has a nonmonotonic dependence on the applied magnetic field H and the fraction x of the two constituent phases. In contrast to common assumptions, the composite has a smaller RCI than its constituent phases for small values of H and x, and there are critical values of each for which RCI is maximized. This work demonstrates the outstanding agreement between the experimental results and the continuous curves predicted by numerical calculations, indicating that this approach can be used to design magnetic refrigerant materials with enhanced magnetocaloric response for moderate magnetic fields.     

The compensation temperature study of the mixed Ising ferrimagnetic spin system with crystal field in a transverse field

Abstract. The magnetic properties of the nearest-neighbor interaction mixed spin-1/2 and spin-1 Ising ferrimagnetic spin system with crystal field in a transverse field are investigated within the framework of the effective-field theory. Particular emphasis is given to the honeycomb lattice with coordination number Z = 3 for which magnetizations are obtained. If transverse field Ω varies in the certain ranges, we find that the compensation temperature is obtained for the value of the crystal field D in a restricted region. We discuss in detail the influence of the transverse field on the behaviors of the compensation point and magnetization curves in this paper.     

Ferromagnetism modulation by phase change in Mn-doped GeTe chalcogenide magnetic materials

In this work, an effective method to modulate the ferromagnetic properties of Mn-doped GeTe chalcogenide-based phase change materials is presented. The microstructure of the phase change magnetic material Ge_1?x Mn _x Te thin films was studied. The X-ray diffraction results demonstrate that the as-deposited films are amorphous, and the crystalline films are formed after annealing at 350 °C for 10 min. Crystallographic structure investigation shows the existence of some secondary magnetic phases. The lattice parameters of Ge_1?x Mn _x Te (x = 0.04, 0.12 and 0.15) thin films are found to be slightly different with changes of Mn compositions. The structural analysis clearly indicates that all the films have a stable rhombohedral face-centered cubic polycrystalline structure. The magnetic properties of the amorphous and crystalline Ge_0.96Mn_0.04Te were investigated. The measurements of magnetization (M) as a function of the magnetic field (H) show that both amorphous and crystalline phases of Ge_0.96Mn_0.04Te thin film are ferromagnetic and there is drastic variation between amorphous and crystalline states. The temperature (T) dependence of magnetizations at zero field cooling (ZFC) and field cooling (FC) conditions of the crystalline Ge_0.96Mn_0.04Te thin film under different applied magnetic fields were performed. The measured data at 100 and 300 Oe applied magnetic fields show large bifurcations in the ZFC and FC curves while on the 5,000 Oe magnetic field there is no deviation.