A model for magnetic abnormalies of FeNi Invar alloys

A model based on the idea of localized magnetic moments is presented which allows to calculate the local magnetic moment expectation values of FeNi alloys. The only parameters of the model are the exchange integralsJ _FeFe,J _FeNi,J _NiNi. By assuming a “mixed” exchange interaction the concentration dependence of the exchange integralsJ _FeFe andJ _FeNi is calculated. The model allows the iron magnetic moments to orient parallel or antiparallel to the magnetization axis, depending on the local environment. It explains the magnetic abnormalies of FeNi Invar alloys as for example the concentration dependence of the mean magnetic moment and the Curie temperatures as well as the characteristic “flat” courves of the spontaneous magnetization.     

Metastable compositionally and magnetically modulated state of Fe-Ni Invar and the associated super-moment dynamics from Mössbauer spectroscopy

The presence of magnetic and chemical clustering, in Fe₆₅Ni₃₅ Invar, had previously been inferred from various measurements which we review. We independently confirm both types of clustering and measure the associated supermoment dynamics for the first time. The temperature dependence of the fluctuation frequency is understood in terms of inter-cluster surface exchange energy and leads to a spherical cluster radius of 7.2 Å. The observed super-moment excitations are sufficient to explain the anomalous temperature dependence of the magnetization and are slow enough, compared to lattice response times, to cause the magnetovolume anomalies. The magnetic clusters are seen to persist far above T_c - at least up to 600 K. Using a two-cluster-type approximation, the mean compositional morphology is obtained as: 39 at% Fe₈₅Ni₁₅/61 at% Fe₅₃Ni₄₇. Also, the alloy which is most often referred to as “Invar”, is found to contain ⪅ 2 at% of α-Fe precipitate. A change of + 1 at% in the amount of this precipitate is found to cause a change of ≃ + 50 K in the T_c measured on first heating. The three most common models for Invar - itinerate model, two γ-state model, and mixed exchange model - are discussed in terms of both data available in the literature and our own results. The mixed exchange approach is shown to be the most reasonable. Its crudeness is argued to arise from two factors: (i) since detailed structural data were not available, it has assumed the wrong fine scale schemical morphology, and (ii) it uses only three values of J_ij whereas we expect a broad distribution of exchange energies to arise from a Slater-Bethe-Bozorth-type interaction whose strength is a function of local composition.     

Investigation of a V15 magnetic molecular nanocluster by the Monte Carlo method

Exchange interactions in a V₁₅ magnetic molecular nanocluster are considered, and the process of magnetization reversal for various values of the set of exchange constants is analyzed by the Monte Carlo method. It is shown that the best agreement between the field dependence of susceptibility and experimental results is observed for the following set of exchange interaction constants in a V₁₅ magnetic molecular nanocluster: J = 500 K, J′ = 150 K, J″ = 225 K, J ₁ = 50 K, and J ₂ = 50 K. It is observed for the first time that, in a strong magnetic field, for each of the three transitions from low-spin to high-spin states, the heat capacity exhibits two closely spaced maxima.     

Exchange Interactions and Curie Temperatures in Dilute Magnetic Semiconductors

We discuss the origin of ferromagnetism in dilute magnetic semiconductors based on ab initio calculations for Mn-doped GaN, GaP, GaAs and GaSb. We use the Korringa–Kohn–Rostoker method in connection with the coherent potential approximation to describe the substitutional and moment disorder. Curie temperatures (T _C) are calculated from first-principles by using a mapping on a Heisenberg model in a mean field approximation. It is found that if impurity bands are formed in the gap, as it is the case for (Ga, Mn)N, double exchange dominates leading to a characteristic √c dependence of T _C as a function of the Mn concentration c. On the other hand, if the d-states are localized, as in (Ga, Mn)Sb, Zener's p–d exchange prevails resulting in a linear c-dependence of T _C. In order to have more precise estimations of T _C, effective exchange coupling constants J _ij 's are calculated by using the formula of Liechtenstein et al. It is found that the range of the exchange interaction in (Ga, Mn)N is very short. On the other hand, in (Ga, Mn)As the interaction is weaker but long ranged. Monte Carlo simulations show that the T _C values of (Ga, Mn)N are very low since percolation is difficult to achieve for small concentrations and the mean field approximation strongly overestimates T _C. Even in (Ga, Mn)As the percolation effect is still important.     

Modeling a dimer state in CuGeO3 in the two-dimensional anisotropic Heisenberg model with alternated exchange interaction

The two-dimensional Heisenberg spin-1/2 model with alternated exchange interaction along the c axis and an anisotropic distribution of the exchange interaction in the lattice, J _b/J _c=0.1, is examined. A quantum Monte Carlo method is used to calculate the phase diagrams of the antiferromagnet, the dimer state in a plane, the value of the alternation δ of the exchange interaction, and the anisotropy Δ=1−J ^xy/J ^z of the exchange interaction, Δ∼δ ^0.58(6). The following characteristics are calculated for Δ=0.25: the dependence of the temperature of the dimer-state-paramagnet transition on the alternation of the exchange interaction, T _c(δ)=0.55(4)(δ−0.082(6))^0.50(3), the singlet-triplet energy gap, and the dependence of the magnetization on the external field for some values of δ. The value of the exchange interaction, J _c=127 K, the alternation of the exchange interaction, δ=0.11J _c, and the correlation radius along the c axis, ξ _c≈28c, are determined. Finally, it is found that the temperature dependence of the susceptibility and the specific heat are in good agreement with the experimental data. Zh. éksp. Teor. Fiz. 112, 2184–2197 (December 1997)     

Remanence effects in symmetric and asymmetric spin glass models

A new method is introduced which yields exact results for the zero temperature (sequential) dynamics of 1d spin glasses with field and for Cayley trees with arbitrary branching numbers. The remanent magnetization and energy are calculated explicitly. For asymmetric interactionsJ _ij J _ij numerical simulations are performed which show cyclic behavior of the remanent magnetization.     

Antiferromagnetic exchange mechanism of superconductivity in cuprates

The theory of superconducting pairing due to antiferromagnetic exchange is considered. The strong dependence of the superconducting transition temperature T _c on the lattice constant a observed recently in mercury superconductors is explained within the framework of this theory. Calculations have been performed based on the two-band p-d Hubbard model in the strong correlation limit. The large excitation energy Δ_pd for the antiferromagnetic exchange of two particles from different Hubbard subbands results in the suppression of the retardation effects and in the pairing of all the particles in the conduction subband with the Fermi energy E _F ?Δ_pd:T _c ?E _F exp(?1/λ), where λ∝J. The dependence T _c (a) and the isotope effect are explained by the dependence of the exchange interaction J on a and on zero-point vibrations of oxygen ions.     

Thermodynamic properties of the ferromagnetic spinel ZnpCd1−pCr2Se4

The magnetic properties of the B-spinel ferromagnetic Zn_pCd_1−pCr₂Se₄ compound are studied via a cluster series expansion approximation with nearest and next-nearest exchange integrals J₁ and J₂, respectively. Using the minimization of the free-energy expressions, the magnetization, the magnetic susceptibility, the two-spin correlation functions and the specific heat are obtained and computed numerically as a function of temperature and for each composition of the system. The magnetization curves are used to determine the critical temperatures T_c. Using the power laws in the vicinity of the critical regions the critical exponents β, γ and α associated, respectively, with the magnetization, the magnetic susceptibility and the specific heat are numerically calculated. The critical temperatures T_c obtained are in very good agreement with those predicted by the magnetic measurements and the values of the critical exponents may be compared with other theoretical results based on the 3D Heisenberg model.     

Kinematic spin-fluctuation mechanism of high-temperature superconductivity

We study d-wave superconductivity in the extended Hubbard model in the strong correlation limit for a large intersite Coulomb repulsion V. We argue that in the Mott-Hubbard regime with two Hubbard subbands, there emerges a new energy scale for the spin-fluctuation coupling of electrons of the order of the electron kinetic energy W much larger than the exchange energy J. This coupling is induced by the kinematic interaction for the Hubbard operators, which results in the kinematic spin-fluctuation pairing mechanism for V ? W. The theory is based on the Mori projection technique in the equation of motion method for the Green’s functions in terms of the Hubbard operators. The doping dependence of the superconductivity temperature T _c is calculated for various values of U and V.     

Computer simulation of magnetization relaxation in amorphous magnetics with random anisotropy

On the basis of Heisenberg model, the magnetic properties of amorphous terbium were studied by the Monte Carlo method. The temperature dependences of spontaneous magnetization and magnetic susceptibility were plotted as functions of the fraction of the constant of anisotropy to the exchange constant D/J ₀. The behavior of magnetization in an external magnetic field was studied, and the dependence of a coercive field and residual magnetization on the value D/J ₀ was found. The relaxation of magnetization was investigated after the external magnetic field was switched off.     

The proton relaxation timesT 1 in binuclear copper complexes

Proton spin-lattice relaxation times as a function of temperature for two kinds of binuclear copper complexes with antiferromagnetic exchange interactionJS₁S₂ have been measured in order to understand the relaxation effects in exchange clusters. It is shown that the results reflect the coupling of the nuclear and electron subsystems; in the rangekT<J the coupling of the subsystems is modulated by temperature independent processes, in the rangekT≥J the temperature dependence of the dynamics of the electron subsystem is manifested. The response of the nuclear subsystem to the change in the nature of magnetic excitations is discovered.     

Exchange constants for a V<Subscript>15</Subscript> magnetic molecular nanocluster

The spin structure and ground-state energies of a V₁₅ magnetic molecular nanocluster are calculated using a modified Lanczos method. The exchange interaction constants for the V₁₅ magnetic nanocluster (J=290 K, J′=60 K, J₁=30 K, J′′=200 K, and J₂=68 K) are determined from the comparison of the calculated and experimental data. The exchange constants obtained differ significantly from those predicated earlier and permit one to describe quantitatively the magnetization of the V₁₅ nanocluster in weak and strong magnetic fields.     

Irradiation effects on the YBa2Cu3O7-δ superconducting compound

The effects of neutron irradiation on the magnetic and superconducting properties of the YBa₂Cu₃O_7- (Y123) superconducting compound have been investigated. No significant change of the superconducting transition temperature (T _c) was found. The effect of irradiation on the magnetization critical current density (J _c) was studied. Bean's critical state model was used to estimate J _c. Around 40% increase in J _c was found. The temperature dependence of J _c was also studied.     

The effect of clamped and free boundaries on long range strain coupling in structural phase transitions

In ferroelastic structural phase transitions, the atomic ordering in one cell creates a local strain field which is propagated elastically throughout the material, resulting in an effective or indirect coupling J(R _ij ) between the ordering in cells i and j. With free boundaries on the sample, the J(R _ij ) contains a Zener-Eshelby term J _Z of infinite range, which largely determines the transition temperature T _c. The present paper shows what happens when the boundaries are clamped. On cooling from a high temperature an anomaly takes place at more or less the same temperature as the phase transition for free boundaries. Cooling results in an irregular pattern of domains with positive and negative order parameter whose long range strains cancel. Two cases are distinguished. In the “tweed” case coherent domain boundaries form easily and result in fine lamellar domains. When coherent domain boundaries are not possible (the “non-Sapriel”) case, larger less regular domains are formed. In either case the macroscopic net strain adds up to zero.     

The roles of the exchange and dipole couplings on the magnetoresistance for the nanoparticle arrays

Based on Monte Carlo simulations and resistor network model, the tunneling magnetoresistance (TMR) for the square anisotropic magnetic nanoparticle arrays with the exchange coupling J and dipolar coupling D has been studied. The simulated results reveal that, the coercivity H_c increases with decreasing value of D and increasing magnitude of J; the value of TMR_max decreases with enhancing value of J; for D<0.1, the value of TMR_max decreases with increasing value of D, while in contrast for D>0.1, it increases with increasing value of D. Those behaviours have been interpreted with the competition of the different energies, including the random anisotropy, exchange and dipolar energies.     

Hard axis magnetic field dependence on current-induced magnetization switching in MgO-based magnetic tunnel junctions

We conducted a detailed study of hard axis magnetic field (H_hard) dependence on current-induced magnetization switching (CIMS) in MgO-based magnetic tunnel junctions (MTJs) with various junction sizes and various uniaxial anisotropy fields. The decreases in critical current density (J_c) and the intrinsic critical current density (J_c0) estimated from the pulse duration dependence on J_c in CIMS are observed when applying H_hard for all MTJs. The decrease in energy barrier of CIMS is also observed except for the largest sample. These results indicate that the reduction of J_c is attributable to both the increase of spin-transfer efficiency and the decrease in energy barrier in the case of applying H_hard. The J_c0 decreases with increase in the mutual angle between the direction of magnetization and the easy axis (θ_f), which is consistent with the theoretical prediction proposed by Slonczewski. The degree of the reduction of J_c0 for the same value of H_hard decreases with decreasing size of MTJs. This behavior is considered to be related to not only decrease in θ_f due to the increase in anisotropy field in MTJs, but also to the increase in the variance of the initial angle of magnetization due to the thermally activated magnon excitation. The stable switching endurance related to CIMS was observed in a wide range of MTJ sizes when applying H_hard. Moreover, we proposed a new architecture and a new switching method considering write disturbance. These results would be useful for application to spin memory and other spin-electronic devices.     

Magnetic properties of undoped YBa 2 Cu 3 O 6 + x system

In the present paper, we study the magnetic properties of bilayer cuprate antiferromagnets. In order to evaluate the expressions for spin-wave dispersion, sublattice magnetization, Néel temperature and the magnetic contribution to the specific heat, the double time Green's function technique has been employed in the random phase approximation (RPA). The spin wave dispersion curve for a bilayer antiferromagnetic system is found to consist of one acoustic and one optic branch. The “optical magnon gap” has been attributed solely to the intra-bilayer exchange coupling (J _⊥ ) as its magnitude does not change significantly with the inter-bilayer exchange coupling (J_z). However J_z is essential to obtain the acoustic mode contribution to the magnetization. The numerical calculations show that the Néel temperature (T _N ) of the bilayer antiferromagnetic system increases with the J_z and a small change in J_z gives rise to a large change in the Néel temperature of the system. The magnetic specific heat of the system follows a T² behaviour but in the presence of J_z it varies faster than T². Received 13 July 2000 and Received in final form 14 May 2001     

The transverse spin-1 Ising model with random interactions

The phase diagrams of the transverse spin-1 Ising model with random interactions are investigated using a new technique in the effective field theory that employs a probability distribution within the framework of the single-site cluster theory based on the use of exact Ising spin identities. A model is adopted in which the nearest-neighbor exchange couplings are independent random variables distributed according to the law P(J_ij)=pδ(J_ij−J)+(1−p)δ(J_ij−αJ). General formulae, applicable to lattices with coordination number N, are given. Numerical results are presented for a simple cubic lattice. The possible reentrant phenomenon displayed by the system due to the competitive effects between exchange interactions occurs for the appropriate range of the parameter α.