The critical magnetic behavior of the new Heusler CoXO2 alloys (X=Cu or Mn): Monte Carlo Study

In this paper, we study the effect of different physical parameters on the critical magnetic behavior of the new full Heusler CoXO2 (X=Cu or Mn) alloys, using Monte Carlo simulations (MCS) under the Metropolis algorithm.To reach this goal, we have proposed a Hamiltonian describing and modeling these compounds. The CoCuO2 alloy is formed with the spin variables of cobalt Co atoms (S_i=±2, ±1 or 0) and the copper Cu atoms modeled by the spins σ_i=±1/2. While, the compound CoMnO2 is modeled the spins of Co atoms (Si=±2, ±1 or 0) and the Mn atoms represented by the spins (Q_i=±5/2, ±3/2 or ±1/2).In fact, we have studied and discussed the ground state phase diagrams of the new Heusler alloys CoXO2(X=Cu or Mn) in different physical parameters. Moreover, we have presented and discussed the thermal behavior of the total magnetizations and the magnetic susceptibilities, for non-null temperature values.To complete this study, we have elaborated and discussed the hysteresis loops of the studied alloys when fixing the temperature values.     

Magnetic and magnetocaloric properties of perovskite manganite Pr0.55Sr0.45MnO3

In this paper, we have studied the magnetic and magnetocaloric properties of the perovskite manganite Pr_0.55Sr_0.45MnO₃. It shows a sharp paramagnetic-ferromagnetic phase transition at 291 K and possesses a moderate magnetic entropy change near room temperature. In addition, a large relative cooling power (143.64 J/kg) and a wide temperature range (84 K) have been found in this material. Compare with the Landau model, we find that the itinerant electrons mainly contribute the larger magnetic entropy change at paramagnetic region.     

First and second order quantum phase transitions in multi-band superconductors

In multi-band and inter-metallic materials superconductivity can be destroyed by applying external pressure in these systems. In many cases the critical temperature is driven continuously to zero, the superconducting to normal transition being associated with a superconducting quantum critical point (SQCP). In this paper we propose a model for this type of SQCP based on the increase of hybridization as pressure is applied in the material. We study a two-band superconductor with hybridization V between these bands. We use a BCS approximation and include both inter- and intra-band attractive interactions. We show that for negligible inter-band interactions, as hybridization increases there is a second order phase transition from a superconductor to a normal state at zero temperature at a critical value of the hybridization V_c. This SQCP can be reached by pressure, since this external parameter controls hybridization in the system. We also find discontinuous transitions at zero temperature and the appearance of a gapless superconducting (GS) phase in a certain range of hybridization in the case of inter-band interactions being dominant.     

Magnetodielectric coupling by exchange striction in Y<Subscript>2</Subscript>Cu<Subscript>2</Subscript>O<Subscript>5</Subscript>

We have studied the magnetodielectric response of Y₂Cu₂O₅, the so-called blue phase in the Y₂O₃-CuO-BaO phase diagram. Based on symmetry principles, we predict and demonstrate magneto-dielectric coupling on a single crystal sample. We report an anomaly in the dielectric constant at the ordering temperature of the Cu spins. We probe the magnetic field-induced phase transitions between four different magnetic phases using magneto-capacitance measurements, demonstrating relatively strong magnetodielectric coupling. We observe an increase in dielectric constant in the spin-flip phase where there exists spontaneous magnetization. We construct a detailed magnetic phase diagram. The magnetodielectric coupling is analyzed in terms of striction induced by symmetric superexchange and optical phonon frequency shifts.     

Magnetocaloric effect in polycrystalline (La0.5Gd0.2)Sr0.3MnO3

In this paper, we present a study of magnetocaloric effect in the colossal magnetoresistance material (La_0.5Gd_0.2)Sr_0.3MnO₃. From the measurements of isothermal magnetization at different temperatures, we have discovered a large magnetic entropy change with a broad peak around Curie temperature (270.5 K) in (La_0.5Gd_0.2)Sr_0.3MnO₃ polycrystalline sample. Moreover, the maximum of magnetic entropy change exhibits a nearly linear dependence with applied high magnetic field. These results suggest that this material is a suitable candidate as working substance in magnetic refrigeration near room temperature.     

Determination of the magnetoelectric coupling coefficient from temperature dependences of the dielectric permittivity for multiferroic ceramics Bi5Ti3FeO15

In the multiferroic materials, the dielectric and magnetic properties are closely correlated through the coupling interaction between the ferroelectric and magnetic order. We attempted to determine the magnetoelectric coupling coefficient from the temperature dependences of the dielectric permittivity for multiferroic Bi₅Ti₃FeO₁₅. Multiferroic ceramics Bi₅Ti₃FeO₁₅ belong to materials of the Aurivillius-type structure. Multiferroic ceramics Bi₅Ti₃FeO₁₅ was synthesized via sintering the Bi₂O₃ and Fe₂O₃ mixture and TiO₂ oxides. The precursor material was ground in a high-energy attritorial mill for 5 hours. This material was obtained by a solid-state reaction process at T = 1313 K. We investigated the temperature dependences of the dielectric permittivity for the different frequencies. From the dielectric measurements, we determined the temperature of phase transition of the ferroelectric-to-paraelectric type at about 1013 K. Based on dielectric measurements and theoretical considerations, the values of the magnetoelectric coupling coefficient were specified.     

Mössbauer study of Zn x Ni5/3−x Fe1Sb1/3O4 spinel ferrite

The antimony substituted nickel ferrite Zn_xNi_5/3–xFe₁Sb_1/3O₄ with different values ofx are prepared, checked by x-ray and studied with⁵⁷Fe Mössbauer spectroscopy over a wide temperature range. Characteristic spectra of paramagnetic, magnetic and electronic relaxation types for the different compositions have been observed. The interpretation of the spectra allows the cation distribution of the compounds to be deduced. The Mössbauer effect parameters at different temperatures are calculated and their dependence on the substitution of non-magnetic Zn⁺² for the magnetic nickel ions are discussed. The temperature dependence of the hyperfine magnetic fields and the respective Néel temperature points are obtained.     

Field dependent specific heat and longitudinal magnetization of the quantum magnet layer Cs2CuCl4: Anisotropy effects

We have addressed the specific heat and magnetization of an anisotropic spin-1/2 triangular Heisenberg antiferromagnet Cs₂CuCl₄ in the presence of magnetic field at finite temperature. We have investigated the behavior of thermodynamic properties by means of excitation spectrum in terms of a hard core bosonic representation. The effect of in-plane anisotropy on thermodynamic properties has also been studied via the bosonic model by Green’s function approach. This anisotropy is considered for exchange constants that couple spin components perpendicular to magnetic field direction. We have found the temperature dependence of the specific heat and longitudinal magnetization in the gapped field induced spin-polarized phase for various magnetic fields and anisotropy parameters. Furthermore we have studied the magnetic field dependence of specific heat and magnetization for various anisotropy parameters. Our results show temperature dependence of specific heat includes a peak so that its temperature position goes to higher temperature with increase of magnetic field. We have found the magnetic field dependence of specific heat shows a monotonic decreasing behavior for various magnetic fields due to increase of energy gap in the excitation spectrum.     

Structural,electronic, magnetic properties and critical behavior of the equiatomic quaternary Heusler alloy CoFeTiSn

In this article, we study the exchange coupling interactions of the equiatomic quaternary Heusler alloy CoFeTiSn, using the two methods: Monte Carlo simulations and the ab-initio method. In a first step, we use the ab-initio calculations to investigate the structural, the electronic and the magnetic properties of this alloy under the GGA method. The analysis of the energy dependence on the lattice parameter a (Å) of the equiatomic quaternary Heusler alloy CoFeTiSn, is discussed for different atomic configurations. The ferromagnetic configuration is found to be the more stable one, with an optimal lattice parameter value 6.00 Å. On the other hand, the electronic structure results show that the compound CoFeTiSn exhibits a half-metallic character and a spin polarization of 100% at the Fermi-level. The total magnetic moment of this alloy is found to be equal to 2.00 μ_B which follows the Slater Pauling rule. Our results support the half-metallic behavior of the studied material. In order to complete this study, we reported the dependence of the critical transition temperature as a function of the parameter α of the equiatomic quaternary Heusler alloy CoFeTiSn. We showed that the critical temperature increases almost linearly with an increase of the values of the parameter α.     

Temperature and magnetic field induced structural transformation in Si-doped CeFe2: An in-field X-ray diffraction study

Using the X-ray powder diffraction technique at various temperatures and applied magnetic fields, we have studied the magnetostructural properties of Ce(Fe_0.95Si_0.05)₂. The X-ray diffraction data establish quantitative relationships between bulk magnetization and the evolution of structurally distinct phases with magnetic field and temperature, and confirm the distinct features of a first-order phase transition such as supercooling and superheating, metastability, and phase co-existence of different structural polymorphs. We observe the lattice volume mismatch across the structural phase transition, which appears to be the cause for the step behavior of the magnetization isotherms at low temperatures. The present study shows that the lattice distortion has to be treated explicitly, like spin, along with the effects of lattice–spin coupling to account for the magnetization behavior of this system. This structure template can resolve the issue of kinetics in this material as observed in different time scale measurements and with different experimental protocols.     

Mössbauer study of ferromagnetic metallic glasses irradiated by swift heavy ions at temperatures 100 and 300 K

The effect of swift heavy ion irradiation on ferromagnetic metallic glasses Fe₄₀Ni₃₈Mo₄B₁₈ and Fe₇₈Si₉B₁₃ has been studied. The ion beams used are 100 MeV ¹²⁷I and 180 MeV ¹⁹⁷Au. The specimens were irradiated at fluences ranging from 3 × 10¹² to 1.5 × 10¹⁴ ions/cm². The irradiations have been carried out at temperatures 100 and 300 K. The magnetic moments are sensitive towards the irradiation conditions such as irradiation temperature and stopping power of incident ion beam. The irradiation-induced effects have been monitored, by using Mössbauer spectroscopy. The modifications in magnetic anisotropy and hyperfine magnetic field distributions, as an effect of different irradiation temperature as well as different stopping power have been discussed. After irradiation, all the samples remain amorphous and magnetic anisotropy considerably changes from its original in-plane direction. The results show enhancement in magnetic anisotropy in the specimen irradiated at 100 K, as compared to that of irradiated at 300 K. It is expected that at low temperature, the stresses produced in the material would remain un-annealed, compared to the samples irradiated at room temperature and therefore, the modification in magnetic anisotropy would be enhanced. A distribution of hyperfine magnetic field, of the samples irradiated at low temperature, show a small but distinct peak at ～?11 Tesla, indicating Fe-B pairing.     

Low-field DC-magnetization study of Ho-doped Mn–Zn ferrite ferrofluid

The physical and magnetic properties of magnetic nanoparticles are crucial for their effectiveness and reliability in biomedical applications. In this article, we report the synthesis of a stable Ho-substituted Mn–Zn ferrite ferrofluid and its physical and magnetic properties. Substitution by rare earth metal plays an important role in determining the magneto-crystalline anisotropy in 4f-3d inter-metallic compounds. Ho³⁺ substitution not only enhanced the magnetic anisotropy but also produced strong spin frustration at low temperature. The field dependence of blocking temperature shows H^2/3 dependency in the entire range of field, i.e., 10–700 Oe, indicating the emergence of Ising spins characteristics in the present system.     

Magnetic properties of naphthalene-like nano-structure with RKKY interactions: Monte Carlo simulations

In this paper, we have studied the magnetic properties of a bi-layer naphthalene-like nano-structure with RKKY (Ruderman-Kittel-Kasuya-Yoshida) interactions, using Monte Carlo simulations. The system consists of two naphthalene-like magnetic planes, with mixed spins σ = 7/2 and S = 1, separated by L non-magnetic layers. In the first part of this study, the ground state phase diagrams for zero temperature are reported and discussed. In the second part, we investigate the effect of the RKKY interactions on the magnetizations and susceptibilities of the system, for non-zero temperature values. It is found that the value of the blocking temperature decreases when increasing the number of non-magnetic layers. Furthermore, the effect of the ferrimagnetic parameter (J_Sσ) on the blocking temperature behavior has been investigated. Finally, the effect of different physical parameters, namely the number of non-magnetic layers, the ferrimagnetic parameter, the crystal field and the temperature, on the magnetic hysteresis cycles are illustrated.     

熔融织构(Nd-Eu-Gd)-Ba-Cu-O超导体的各向异性磁通跳跃及其不稳定性研究

对c轴择优取向的熔融织构样品(Nd_0.33Eu_0.33Gd_0.33) Ba₂Cu₃O_7-δ（含Gd(211)相）的磁通跳跃现象进行 了系统研究.结果表明，在外加磁 场平行于样品c轴条件下，在2到3K的温度范围内明显观测到了部分磁通跳跃现象，而 在5K及以上温区并未出现.在磁场垂直于样品的c轴情况，在2K到T_c的整个温 区都没有观察到磁通跳跃现象.这种各向异性磁通跳跃现象可归因于各向异性钉扎力和几何 退磁因子的结果.随着温度的增加，磁通跳跃数目减少，且M(H)曲线的第三象限是磁通 跳跃的最不稳定过程.最后，研究了磁通跳跃对磁场扫描速率的依赖关系，并讨论了磁通蠕 动对磁通跳跃的影响. 关键词： 0.33Eu_0.33Gd_0.33)Ba₂Cu₃O_7-δ超导体')" href="#">(Nd_0.33Eu_0.33Gd_0.33)Ba₂Cu₃O_7-δ超导体 OCMG方法 磁通跳跃     

Toroidal moments as indicator for magneto-electric coupling: the case of BiFeO<Subscript>3</Subscript> versus FeTiO<Subscript>3</Subscript>

In this paper we present an analysis of the magnetic toroidal moment and its relation to the various structural modes in R3c-distorted perovskites with magnetic cations on either the perovskite A or B site. We evaluate the toroidal moment in the limit of localized magnetic moments and show that the full magnetic symmetry can be taken into account by considering small induced magnetic moments on the oxygen sites. Our results give a transparent picture of the possible coupling between magnetization, electric polarization, and toroidal moment, thereby highlighting the different roles played by the various structural distortions in multiferroic BiFeO₃ and in the recently discussed isostructural material FeTiO₃, which has been predicted to exhibit electric field-induced magnetization switching.     

Inhomogeneous ferrimagnetic-like behavior in Gd2/3Ca1/3MnO3 single crystals

We present a study of the magnetic properties of Gd_2/3Ca_1/3MnO₃ single crystals at low temperatures, showing that this material behaves as an inhomogeneous ferrimagnet. In addition to small saturation magnetization at 5 K, we have found history dependent effects in the magnetization and the presence of exchange bias. These features are compatible with microscopic phase separation in the clean Gd_2/3Ca_1/3MnO₃ system studied.     

Magnetic-relaxation study in melt-textured ErBa2Cu3O7−δ ceramic material

We have studied the zero-field-cooled magnetic relaxation in the melt-textured ErBa₂Cu₃O_7– ceramic material. The magnetic relaxation rate dM/d (ln t) exhibits strong temperature and field dependence in the temperature range up to 80 K and the field range up to 5.5 T. The magnetic relaxation behavior observed in the melt-textured ErBa₂Cu₃O_7– ceramic sample is similar to that in single crystals. The magnetization to noise ratio is much improved, in particular, for temperatures approaching the superconducting transition temperature and for low magnetic fields. Several theoretical models were explored to fit the experimental data.     

Structurally sensitive magnetic and electrical properties of Fe15Co20Ni65 thin films

We have studied the influence of thickness, magnetic annealing temperature, and substrate material on a number of structurally sensitive magnetic and electrical properties of Fe₁₅Co₂₀Ni₆₅ films. It is shown that the main microstructure parameter which determines the resistivity, its temperature coefficient, the anisotropy of the magnetoresistivity, the coercive force, and the induced magnetic anisotropy field is the crystallite dimension d. We establish the conditions for producing a structural state (d=10 nm) which, from a practical standpoint, has an optimal relationship between the magnetic and magnetoresistance properties.Physics and Applied Mathematics Scientific-Research Institute, Urals State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 61–66, June, 1993.