Magnetic properties and concurrence for fluid 3He on kagome lattice

We present the results of magnetic properties and entanglement for kagome lattice using Heisenberg model with two- and three-site exchange interactions in strong magnetic field. Kagome lattice correspond to the third layer of fluid ³He absorbed on the surface of graphite. The magnetic properties and concurrence as a measure of pairwise thermal entanglement are studied by means of variational mean-field like treatment based on Gibbs-Bogoliubov inequality. The system exhibits different magnetic behaviors depending on the values of the exchange parameters (J ₂, J ₃). We have obtained the magnetization plateaus at low temperatures. The central theme of the paper is comparing the entanglement and magnetic behavior for kagome lattice. We have found that in the antiferromagnetic region behavior of the concurrence coincides with the magnetic susceptibility one.     

Disordered correlated Kondo-lattice model

We propose a self-consistent approximate solution of the disordered Kondo-lattice model (KLM) to get the interconnected electronic and magnetic properties of ‘local-moment’ systems like diluted ferromagnetic semiconductors. Aiming at (_A1-xMx)$(A_{1-x}{M}_x)$ compounds, where magnetic (M)$(M)$ and non-magnetic (A)$(A)$ atoms distributed randomly over a crystal lattice, we present a theory which treats the subsystems of itinerant charge carriers and localized magnetic moments in a homologous manner. The coupling between the localized moments due to the itinerant electrons (holes) is treated by a modified RKKY-theory which maps the KLM onto an effective Heisenberg model. The exchange integrals turn out to be functionals of the electronic self-energy guaranteeing self-consistency of our theory. The disordered electronic and magnetic moment systems are both treated by CPA-type methods. We discuss in detail the dependencies of the key-terms such as the long-range and oscillating effective exchange integrals, ‘the local-moment’ magnetization, the electron spin polarization, the Curie temperature as well as the electronic and magnonic quasiparticle densities of states on the concentration x of magnetic ions, the carrier concentration n, the exchange coupling J, and the temperature. The shape and the effective range of the exchange integrals turn out to be strongly x-dependent. The disorder causes anomalies in the spin spectrum especially in the low-dilution regime, which are not observed in the mean field approximation.     

The high temperature expansion of the classical XYZ chain

The β$\beta$-expansion of the Helmholtz free energy (HFE) up to order β¹²${\beta }^{12}$ of the classical XYZ model with a single-ion anisotropy term and external magnetic field is calculated and compared to the numerical solution of Joyce's [Phys. Rev. Lett. 19 (1967) 581] for the XXZ   classical model, with neither single-ion anisotropy term nor external magnetic field. This comparison shows that the derived analytical expansion is valid for intermediate temperatures such as kT/_Jx≈0.5$\mathit{kT}/J_x\approx 0.5$. The specific heat and magnetic susceptibility of the S=2$S=2$ antiferromagnetic chain can be approximated by their respective classical results, within an error of 2.5%$2.5\%$, up to kT/J≈0.8$\mathit{kT}/J\approx 0.8$. For a vanishing external magnetic field the ferromagnetic and antiferromagnetic chains are shown to have the same classical HFE; their behaviour in a non-vanishing external magnetic field is also described.     

Exchange-induced phase separation in Ni–Cu films

Magneto-structural properties of films of diluted ferromagnetic alloys Ni_xCu_1−x in the concentration range 0.7<x<1.0$0.7<x<1.0$ are studied experimentally. Films deposited by magnetron sputtering show partial phase separation, as evidenced by structural analysis and ferromagnetic resonance measurements. The phase diagram of the Ni_xCu_1−x bulk system is obtained using numerical theoretical analysis of the electronic structure, taking into account the interatomic exchange interactions. The results confirm the experimentally found partial phase separation, explain it as magnetic in origin, and indicate an additional metastable region connected with the ferromagnetic transition in the system.     

Persistent spin current in mesoscopic antiferromagnet spin chain with Dzyaloshinskii–Moriya interaction

By using the modified spin-wave and gauge invariant methods, we show that at zero temperature in the presence of an inhomogeneous magnetic field with magnitude B gives rise to a persistent magnetization current around a mesoscopic antiferromagnetic Heisenberg spin ring with the DM (Dzyaloshinskii–Moriya) interaction. The results show that the persistent magnetization current is vanishing at large $D_s/J$ ($D_s$ is reduced DM interaction and J is nearest exchange coupling) with $\alpha >1$ (α is a constant describing the energy gap of the spin system). The result also shows that under the homogeneous magnetic field there exists a non-zero spin current in the spin ring.     

Magnetic properties of electron-doped La0.23Ca0.77MnO3 nanoparticles

Magnetic properties of electron-doped La_0.23Ca_0.77MnO₃ manganite nanoparticles, with average size of 12 and 60?nm, prepared by the glycine?Cnitrate method, have been investigated in the temperature range 5?C300?K and magnetic fields up to 90?kOe. It is suggested that weak ferromagnetic moment results from ferromagnetic shells of the basically antiferromagnetic nanoparticles and from domains of frustrated disordered phase in the core. Assumption of two distinct sources of ferromagnetism is supported by the appearance of two independent ferromagnetic contributions in the fit of the T ^3/2 Bloch law to spontaneous magnetization. The ferromagnetic components, which are more pronounced in smaller particles, occupy only a small fraction of the nanoparticle volume and the antiferromagnetic ground state remains stable. It is found that the magnetic hysteresis loops following field cooled processes, display size-dependent horizontal and vertical shifts, namely, exhibiting exchange bias effect. Time-dependent magnetization dynamics demonstrating two relaxation rates were observed at constant magnetic fields upon cooling to T?<?100?K.     

Crystal field effects on the saturation magnetic moment of Sm3+ ion in ferronagnetic samarium compounds

The effects of cubic crystal fields on the saturation magnetic moment of Sm³⁺ ion in ferromagnetic compounds have been investigated. In samarium compounds with magnetic elements, the exchange fieldH _ex acting on Sm³⁺ ion is taken to be proportional to the sublattice magnetization of the magnetic element, while in compounds with nonmagnetic elementsH _ex is taken to be proportional to the spin average of the Sm³⁺ ion and is determined self-consistently. In both types of compoundsH _ex is assumed to be along [001] direction. The saturation magnetic moment is calculated by taking into account the admixture of excited (J=7/2 andJ=9/2) levels into the ground (J=5/2) level of Sm³⁺ ion by crystal fields and exchange fields. It is shown that depending upon the strength, the crystal fields quench or enhance the magnetic moment from the free ion value, and in some cases force Sm³⁺ ion to behave effectively like an (L+S) ion rather than an (L−S)ion. The crystal fields may have important bearing on the performance of samarium compounds as permanent magnet materials.     

The magnetic spin-reorientation transitions in the RGa (R=rare earth) intermetallic compounds studied by measurements of the hyperfine interactions of the Sn probe atoms

The magnetic and electric hyperfine interactions of the probe nucleus ¹¹⁹Sn on the Ga site of the ferromagnetic rare-earth (R) gallium compounds RGa (R=Pr–Er) have been investigated by Mössbauer spectroscopy technique. For all of the compounds, the directions of the magnetic moments of the R³⁺ ions have been determined as a function of temperature in the range from 5 K to T_C. For NdGa, SmGa, HoGa, and ErGa compounds, the magnetic reorientation transitions due to the competition between the exchange interaction and the interaction with crystal field have been investigated. At high temperatures, when the electric interaction dominates, the orientation of the magnetic moments is unambiguously determined by the sign of the quadrupole moment of 4f shell of the R³⁺ ion. With decreasing temperature, the magnetic moments rotate gradually from the bc-plane toward the crystallographic a-axis. In the temperature range 5 K?T<100 K, the ferromagnetic structure of the GdGa compound is noncollinear. At 5 K the magnetic moments of the Gd³⁺ ions point in two distinct directions with respect to the crystallographic a  -axis (_θ1≈30°${\theta }_1\approx 30°$ and _θ2≈60°${\theta }_2\approx 60°$).     

Multicritical behaviors of the antiferromagnetic Blume–Emery–Griffiths model with the external magnetic field on the Bethe lattice

The antiferromagnetic Blume–Emery–Griffiths model in an external magnetic field is studied by using the exact recursion relations on the Bethe lattice for arbitrary values of biquadratic and for negative values of bilinear interactions. We have studied the thermal variations of two-sublattice magnetizations belonging to spin-1 BEG model to obtain the phase diagrams on the (H/|J|,kT/|J|)$(H/|J|,\mathit{kT}/|J|)$ plane. As a result, we have found that the system presents second- and first-order phase transitions, therefore, tricritical points for appropriate values of K/|J|$K/\left|J\right|$, D/|J|$D/\left|J\right|$ and q  . We have also found that the second-order phase transition lines exhibit reentrant phenomena in temperature, besides it also shows reentrant phenomena for the first-order phase lines in external magnetic field for q=4$q=4$ and 6.     

Monte Carlo study of a superlattice with alternate layers

A Monte Carlo (MC) simulation was used to observe the magnetic behavior of a superlattice Ising Model, in the presence of both an external and crystal magnetic fields. The system is made up to layers σ=±1/2$\sigma =\pm 1/2$ and S=±1,0$S=\pm 1,0$. The effect of the exchange interaction coupling _Jp$J_p$ between the spin configurations σ$\sigma$ and S$S$ is investigated for different values of temperature at fixed values of the crystal field. We found that this parameter increases the magnetization of the system at high temperature. Also, the critical temperature is calculated, for each spin configuration as function of temperature using the MC technique. The thermal behavior magnetizations and susceptibilities are studied. Finally, the response of the magnetization to the field shows a hysteresis behavior.     

3He/graphite commensurate bilayer films in the antiferromagnetic regime

We have measured the nuclear magnetic susceptibility of second layer³He films adsorbed on graphite in the antiferromagnetic regime. The nuclear exchange parameter J is found to be constant over a coverage range centered around that expected for a 4/7 commensurate phase. The ratio of the magnetization of the first and second layers provides a direct test of the bilayer commensuration.     

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.     

The spin-1 Blume–Capel model on the Bethe lattice in ± J distribution with an adjustable parameter between FM and AFM phases

The spin-1 Blume–Capel (BC) model is studied on the Bethe lattice (BL) for the ?±? J distribution with a competing adjustable parameter α which alters the strength of bilinear exchange interaction parameter for the ferromagnetic phase (J?>?0) with respect to antiferromagnetic phase (J?<?0). The J?>?0 and αJ?<?0 values are also distributed throughout the BL with probabilities p and $1?p,$ respectively. The order-parameters are obtained on the BL in terms of exact recursion relations (ERR’s) and their temperature (T) variations are studied to calculate the phase diagrams on the (α, T) planes for given values of p, crystal field (D) and coordination number $q=3$ corresponding to honeycomb lattice. It is found that the model gives both first- and second-order phase transitions and also tricritical points. In addition to the well known ordinary phases and TCP’s, the spin glass phase and two more special points are also observed.     

Strongly correlated electron system in the magnetic field

In the range of hole concentrations 0.08<x<0.18$0.08<x<0.18$ the density of states of the two-dimensional t–J model reveals oscillations with changing the magnetic field. Oscillation frequencies correspond to large Fermi surfaces. However, the oscillations are modulated with a frequency which is smaller by an order of magnitude. The modulation is related to van Hove singularities in the Landau subbands, which traverse the Fermi level with changing the field. The singularities are connected with bending the subbands due to strong electron correlations. The frequency of the modulation is of the same order of magnitude as quantum oscillation frequencies in underdoped cuprates.     

Thermodynamics of 3He solid monolayers in the Heisenberg model

The two-dimensional Heisenberg model is applied to the interpretation of the experimental data on the thermodynamic and magnetic properties of ³He monoatomic films in the millikelvin temperature range, i.e., under conditions when these properties are completely governed by the dynamics of the nuclear spin subsystem. The theoretical results obtained make it possible to describe the internal energy E, the heat capacity C _s, and the magnetic susceptibility χ of the two-dimensional spin-1/2 Heisenberg ferromagnets and antiferromagnets on a triangular lattice within the unified approach over the entire range of temperatures. The data available in the literature on the heat capacity and magnetic susceptibility of ³He films are interpreted in the framework of the advanced theory. Most attention is concentrated on the layers characterized by the ferromagnetic exchange. Comparative analysis of theoretical and experimental data is carried out with the use of two fitting parameters: the exchange interaction constant J and the number of “active” spins n ₂ in the layer that is determined from the entropy of the system in the limit T → ∞. It is demonstrated that, for the ferromagnetic layers, the theoretical results obtained within the Heisenberg model are in very good agreement with the experimental data reported by different authors.     

Magnetization and Lyapunov exponents on a kagome chain with multi-site exchange interaction

The Ising approximation of the Heisenberg model in a strong magnetic field, with two, three and six spin exchange interactions is studied on a kagome chain. The kagome chain can be considered as an approximation of the third layer of ³He absorbed on the surface of graphite (kagome lattice). By using dynamical approach we have found one- and multi-dimensional mappings (recursion relations) for the partition function. The magnetization diagrams are plotted and they show that the kagome chain is separating into four sublattices with different magnetizations. Magnetization curves of two sublattices exhibit plateaus at zero and 2/3 of the saturation field. The maximal Lyapunov exponent for multi-dimensional mapping is considered and it is shown that near the magnetization plateaus the maximal Lyapunov exponent also exhibits plateaus.     

Magnetic properties of the bond and crystal field dilution spin-3/2 Blume–Capel model in an external magnetic field

Magnetic properties of the bond and crystal field dilution spin-3/2 Blume–Capel model in an external magnetic field (h)$(h)$ on simple cubic lattice are studied by using the effective field theory. In the m−T$m-T$ plane, the degeneracy of the magnetization (m)$(m)$ is affected by the concentration of bond or crystal field dilution at low temperature (T)$(T)$. The magnetization curves can appear to fluctuate in certain regions of negative crystal field. In the m−h$m-h$ plane, the initial magnetization curve has an irregular behavior due to the introduction of bond dilution. The crystal field dilution has the influence on the process of magnetic domain displacement. In the χ−h$\chi -h$ plane, there exists one susceptibility (χ)$(\chi )$ shoulder and one step for different negative crystal field. The susceptibility curve takes on the feature of multi-peaks distribution under bond and crystal field dilution conditions.