NaZn_(13)型结构LaFe_(13-x)Al_xC_y化合物的磁熵变与磁相变的研究

Study on the magnetic entropy change and magnetic phase transition of NaZn_(13)-type LaFe_(13-x)Al_xC_y compounds

研究了NaZn13型结构LaFe13-xAlxC0.1(x=1·6,1·8)间隙化合物的磁制冷能力和磁相变.利用麦克斯韦关系式计算得到,高Al含量LaFe13-xAlx碳化物的最大磁熵变值|ΔS|m低于低Al含量碳化物的最大磁熵变值.随Al含量的增加,化合物的磁熵变峰展宽,但由于磁熵变大幅降低,衡量磁制冷能力的q值随之降低.基于朗道相变原理,考虑到自旋涨落的影响,磁自由能可以展开到磁化强度的6次方项,材料的相变类型由磁化强度的4次方项系数a3(T)的符号来进行判断.随着Al含量的增加,研究的碳化物相变由弱的一级相变转为二级相变.

The refrigerant capacity and magnetic phase transition of NaZn13-type LaFe13-xAlxCo.1 （x = 1.6, 1. 8） interstitial compounds are investigated. The magnetic entropy change ｜△S｜m, which is calculated from one of the Maxwell＇s relationships △S（ T, H） = -∫（偏倒dM/偏倒dT）HdH , is lower than that of carbides with low Al content. Although the magnetic entropy change peak broadens with increasing Al content, the value of q, which is used to evaluate the refrigerant capacity, decreases due to the sharp derease of the magnetic entropy change. Taking the spin fluctuation into account, the magnetic free energy can be expanded up to the M6 term based on the Landau phase transition theory. The order of phase transition is distinguished by the the signs of α3 （ T）, which is the coefficient of M4 term. The order of phase transition for LaFe13-xAlx carbides changes from the first one that is weak to the second one with the increase of Al content.