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1.
The Heisenberg ferromagnet with the biquadratic exchange interaction is studied. The low temperature expansion of magnetization, the Curie temperature and the susceptibility at high temperatures are obtained.  相似文献   

2.
《Physica A》1998,250(1-4)
A system consisting of several layers of magnetic ions interacting by both bilinear and biquadratic Heisenberg exchange is studied within the framework of the mean-field approximation. It is shown that for S = 1 there exist two types of ordering: ferromagnetic and ferroquadrupolar. The stability of phases as the function of temperature, biquadratic exchange and surface exchange is discussed analytically and numerically and it was shown that similar to bulk samples there appear first- and second-order transitions and a tricritical point may appear depending on system parameters.  相似文献   

3.
The pair- and four-spin correlation functions and susceptibility have been calculated rigorously for a classical Heisenberg linear chain with biquadratic exchange interaction by extending Fisher's method.  相似文献   

4.
《Physics letters. A》1986,116(3):139-144
Green function technique with random phase approximation has been utilized to obtain the Curie temperature of a mixed spin system for an anisotropic Heisenberg model with uniaxial anisotropy. The results obtained have been compared with the isolated spin system of a regular lattice.  相似文献   

5.
6.
In this paper the effect of biquadratic exchange on Curie temperature has been discussed on the basis of double-time temperature-dependent Green-function formalism with the utilization of Callen decoupling scheme. Calculation has been done for a spin-1 bcc Heisenberg ferromagnet by the method of Flax and Raich and it has been observed that the variation of Curie temperature with the biquadratic exchange parameter α obeys a law prescribed by the previous authors.  相似文献   

7.
The effect of dipole-octupole interaction on different magnetic parameters for the spinS=3/2 system in the presence of biquadratic exchange has been investigated. The Green function technique with random phase approximation (RPA) has been employed to obtain analytical results. These results are in good agreement with the results of the molecular field approzimation (MFA).  相似文献   

8.
The many-body Green's function theory developed in our previous work for treating the reorientation of the magnetization of thin ferromagnetic films is extended to include the exchange anisotropy. This leads to additional momentum dependencies which require some non-trivial changes in the formalism. The theory is developed for arbitrary spin values S and for multilayers. The effects of the exchange anisotropy and the single-ion anisotropy, which was treated in our earlier work, on the magnetic properties of thin ferromagnetic films are compared. Received 31 October 2002 Published online 7 May 2003  相似文献   

9.
The Green function technique has been employed to study the dynamic susceptibility of a spin-1 system above the Curie temperature. Bilinear and biquadratic terms have been added in writing the Hamiltonian of the system. Higher order Green functions have been decoupled by RPA and Callen type decouplings. The dependence of spin wave energy, intensity and susceptibility on wavevector and biquadratic parameter have been studied.  相似文献   

10.
The Green functions of the anisotropic Heisenberg model are studied by a method which was applied previously to the reduced density matrices. Integral equations are used to prove the existence of the infinite volume limit of the Green functions, and some analyticity properties with respect to the fugacity (or magnetic field), the potentials, and the complex times.Research supported by the National Science Foundation.  相似文献   

11.
Ground-state energies for antiferromagnetic Heisenberg models with exchange anisotropy are estimated by means of a local-spin approximation made in the context of the density functional theory. Correlation energy is obtained using the non-linear spin-wave theory for homogeneous systems from which the spin functional is built. Although applicable to chains of any size, the results are shown for small number of sites, to exhibit finite-size effects and allow comparison with exact-numerical data from direct diagonalization of small chains.  相似文献   

12.
With the pair model approximation, the Curie temperature of a spin-one Heisenberg ferromagnet with biquadratic exchange interaction J' and uniaxial anisotropy term D is calculated for both positive as well as negative values of J' and D.  相似文献   

13.
K.G. Chen  H.H. Chen  C.S. Hsue  F.Y. Wu 《Physica A》1977,87(3):629-632
Seven coefficients in the high temperature series expansions for the zero-field susceptibility and the specific heat are derived for the planar classical Heisenberg model with biquadratic interactions. The critical temperatures and the susceptibility exponents are determined for cubic lattices.  相似文献   

14.
For spin-one Ising and Heisenberg ferromagnets with biquadratic exchange interaction J' and uniaxial anisotropy D, the magnetic susceptibility has been calculated by the use of the pair model approximation and its dependences on J' and D are discussed in comparative way.  相似文献   

15.
The Green's function theory for disordered Heisenberg Ferromagnets developed by Montgomery et al. has been extended for anisotropic systems. The densities of spin-wave states for such systems exhibit split-bands. This behavior differs qualitatively from that of the isotropic case and offers a partial explanation for the neutron scattering experiment of Buyers et al. in which they observe two well defined branches of magnetic excitations in disordered substitutional anti-ferromagnetic alloys.  相似文献   

16.
Combining the two-site cluster approximation with the discretized path-integral representation (DPIR) and Suzuki-Trotter formalism, the critical properties of anisotropic Heisenberg ferromagnet with a crystal field are studied. We find that the tricritical point can appear in the case of negative crystal field. And the exchange anisotropy not only reduces the critical temperature and tricritical point, but also makes re-entrant phenomena instead of the existence of the tricritical point when the exchange anisotropic parameter is larger than a critical value.  相似文献   

17.
Ai-Yuan Hu  Yuan Chen 《Physica A》2008,387(14):3471-3476
We study the phase diagram of the anisotropic ferromagnetic Heisenberg model on a square lattice. We use the double-time Green’s function method within the Callen decoupling approximation. The dependence of the Curie temperature Tc on the spin S and on the anisotropy parameter Δ (Δ=0 and 1 correspond to the isotropic Heisenberg and Ising model, respectively) is obtained explicitly. Our results are in agreement with results obtained from other theoretical approaches.  相似文献   

18.
Vijay Kumar  K C Sharma 《Pramana》1977,8(3):286-291
A formulation, using the double time Green’s function method, is presented for the anisotropic spin-one ferromagnetic system in the presence of Ising part of the biquadratic exchange and an expression is given for the critical temperatureT c in terms of two parameters. One is associated with the uniaxial anisotropy (treated exactly) and the other one corresponds to the biquadratic interaction (treated in RPA). The bilinear Heisenberg exchange term is treated in Callen’s approximation emphasizing the role of intersite correlation for small parameters. The prediction thatT c increases with the biquadratic exchange is found to be consistent with the recent calculation based on high temperature series expansion.  相似文献   

19.
A spin one Ising system with biquadratic exchange, is investigated, using Green's function technique in random phase approximation (RPA). Transition temperature Tc and <(Sz)2> at Tc, are found to increase with biquadratic exchange parameter α for sc, bcc and fcc lattices. The variation of <(Sz)2> at Tc with α is found to be the same for the above lattices.  相似文献   

20.
A formulation, using the double time Green's function method, is presented for the anisotropic spin-one ferromagnetic system in the presence of biquadratic exchange and an expression is given for the critical temperature Tc in terms of parameters associated with the uniaxial anisotropy (treated exactly) and the biquadratic interaction (only the longitudinal part, treated in RPA). The prediction that Tc increases with the biquadratic exchange is found to be consistent with the recent calculation.  相似文献   

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