单晶NdMnO3的比热研究

研究了低温下NdMnO₃单晶的比热随温度和磁场的变化(2K≤T≤200K，0T≤H≤8T ).对应于 Mn磁矩亚晶格的A型反铁磁(A-AF)相变，零场下的比热曲线在85K附近出现尖锐的λ形峰，随 着磁场的增加，此λ峰降低展宽而且平滑变化，这与此温度附近磁化强度的变化规律一致. 与磁有序相变相关的熵变约为理论值的26％，这可能是由于磁有序涨落延续在较大温区造成 的.在20K以下，比热曲线出现了明显的肩膀形状的Schottky反常，其峰值随着磁场的增加而 逐渐向高温移动.考虑了低温下比热的各种贡献，根据Nd³⁺位有效分子场(H _mf) 引起的Nd^3＋基态双重态(GSD)劈裂对上述现象进行了解释.通过对2K≤T≤2 0K，0Ｔ≤ H≤8T范围内比热数据的拟合，得到了样品的GSD劈裂，德拜温度和A-AF自旋波劲度系数以及 它们对磁场的依赖关系.发现GdFeO₃型八面体旋转引起的A-AF结构中Mn磁矩亚晶 格的铁磁成分可能是H_mf的来源. 关键词： 比热 Schottky反常 反铁磁相变

Specific heat of single-crystal NdMnO3

The temperature and field dependences of the specific heat of single-crystal NdMnO₃ were investigated from 2K to 200K under different fields up to 8 T.Corresponding to the Type-A antiferromagnetic (A-AF) phase transition in the Mn sub-lattice,the specific heat under zero field exhibited a sharp λ-shaped peak at about 85K,which was depressed and broadened and changed smoothly with the increa se of magnetic field.But the entropy change due to the magnetic order is only 26% of the expected value,which might arise from the extension of the magnetic flu ctuation in a larger temperature range.The specific heat curves exhibited a shou lder-shaped Schottky-like anomaly below 20K,whose maximum shifts gradually to higher temperatures with the magnetic field.By considering the contributions to the specific heat at low temperatures,the above phenomena can be interpreted in terms of the model of effective molecular field (H_mf) at Nd ³⁺ site that causes the splitting of the ground state doublet (GSD) of Nd^{3+.The specific heat below 20K under different magnetic fields was successfully fitted,through which the meaningful parameters,including the splitting of the GSD of Nd3+,Debye temperature and the A-AF spin wave stiffness coefficient s,as well as their field dependence were obtained.It was found that the ferromagnetic component of the Mn sub-lattice caused by the GdFeO₃-type octahedron ro tation could be the origin of the H_mf.}