摘要
基于格林函数方法,通过采用Tyablikov和Callen近似分别对系统最近邻交换耦合项和单粒子各向异性项进行退耦,研究了三维各向异性的混合自旋亚铁磁系统的磁性质,详细探讨了单粒子各向异性对系统补偿温度的影响,并对系统出现补偿温度的机理进行了分析.结果表明:当小自旋的单粒子各向异性D_a增大时,其子晶格磁化强度随温度下降的速度要比大自旋子晶格的磁化强度慢.当D_a增大到一定值时,如D_a^(min),系统出现补偿点,且补偿点随着D_a的增大而减小.同时,当其他参数取值变化时,D_a^(min)的大小也随之变化.这表明D_a^(min)的取值是依赖系统其他参数取值的.但如果D_a为零时,无论其他参数为何值,系统都不会出现补偿温度.因此,对于本文所研究的亚铁磁模型,出现补偿温度的前提条件是必须考虑小自旋的单粒子各向异性,且其取值存在一个最小值,即:D_a^(min).当且仅当D_a≥D_a^(min)时,系统出现补偿温度,否则补偿温度消失.
In this paper, we use the double-time Green＇s function method to study the properties of the mixed spin-1 and spin-3/2 Heisenberg ferrimagnets with exchange anisotropy and single-ion anisotropy on a three-dimensional lattice. We derive the equation of motion of the Green＇s function by a standard procedure. In the course of this, the higher order Green functions have to be decoupled. For the terms concerning exchange interaction in the Hamiltonian, a Tyablikov or random phase approximation decoupling are used to decoupling the higher order Green functions. For the terms concerning the single-ion anisotropy, we adopt the Anderson-Callen decoupling to decoupling. Based on the above procedure, the effects of the exchange anisotropy and the single-ion anisotropy on the critical and compensation temperature are investigated. The cause of compensation temperature appearance of a spin-1 and spin-3/2 ferrimagnetic system is discussed in detailed. Our results show that, when the large spin Sb single-ion anisotropy is equal to zero, i.e., Db=0, the drop of the sublattice magnetization ｜ma｜ with the increase of temperature is slower than the sublattice ｜mb｜magnetization of large spin as the single-ion anisotropy Da of small spin increases. Under general condition, Sb is larger than Sa so that ｜ma｜ is always smaller than ｜mb｜. It means that the difference between ｜ma｜ and ｜mb｜ decreases with the increasing of Da below the critical temperature. Therefore, as Da increases a certain value Da^（min）, we obtain ma=-mb≠0 below the critical temperature. It shows that the compensation point appears. And the compensation temperature decreases with increasing Da. The value of Da^min will alter with changing of other parameters in the Hamiltonian. It means that the value of Da^（min） depends on the other parameters in the Hamiltonian. Nevertheless, for Da=0, the drop of the sublattice magnetization ｜mb｜ with the increase of temperature is slower than the sublattice ｜ma｜ magnetization of large
作者
胡爱元
HU AiYuan(Chongqing Key Laboratory of Photoelectric Functional Materials, College of Physics and Electronic Engineering, Chongqing Normal University, Chongqing 401331, China)
出处
《科学通报》
EI
CAS
CSCD
北大核心
2016年第28期3155-3162,共8页
Chinese Science Bulletin
基金
国家自然科学基金(11404046)
重庆市自然科学基金(cstc2014jcyjA50013)
重庆市教委科研项目(KJ130615)资助
关键词
亚铁磁体
补偿温度
相变温度
各向异性
格林函数
ferrimagnetisms
compensation temperature
critical temperature
anisotropy
Green＇s function method
