Abstract

Reverse first-order magnetostructural phase transitions have been theoretically analyzed within the model of interacting parameters of magnetic and structural orders. A characteristic feature of these transitions is stepwise occurrence of the magnetic order upon cooling (as in the case of the first-order phase transition) and its smooth disappearance upon heating (as in the conventional second-order phase transition). These transitions are observed in some alloys of the Mn_1 – xCr_xNiGe magnetocaloric systems under pressure (x = 0.11) and without it (x = 0.18) and are accompanied by specific magnetic and magnetocaloric features. These specific features are phenomenologically described within the concept of soft mode for a structural subsystem undergoing the first-order structural phase transition (P6₃/mmc–Pnma) and the Heisenberg model for a spin subsystem. It is shown for systems with magnetostructural instability within the molecular-field approximation for a spin subsystem and the approximation of biased harmonic oscillator for a lattice subsystem that reverse phase transitions occur when the magnetic disordering temperature is in the range of temperature hysteresis of the P6₃/mmc–Pnma first-order structural phase transition. It is also shown that the two-peak form of isothermal entropy (characteristic of reverse transitions) is due to separation of contributions from the structural and magnetic entropies.

中文翻译：

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具有反向一阶跃迁的系统中的磁和磁热效应

摘要

在磁性和结构级相互作用参数的模型中，理论上分析了反向一阶磁结构相变。这些转变的一个特征是在冷却时逐步出现磁序（如在一级相变的情况下）和在加热时平滑消失（如在传统的二级相变中）。在压力 ( _{x  =} 0.11 )和没有压力( _x= 0.18）并伴有特定的磁性和磁热特征。这些特定特征在经历一阶结构相变 ( P 6 ₃ / mmc – Pnma ) 的结构子系统的软模式概念和自旋子系统的海森堡模型的概念中进行了现象学描述。对于在自旋子系统的分子场近似和晶格子系统的偏置谐振子近似内具有磁结构不稳定性的系统，当磁无序温度在P 6的温度滞后范围内时会发生反向相变₃ / mmc – Pnma一阶结构相变。还表明，等温熵的双峰形式（反向跃迁的特征）是由于结构熵和磁熵的贡献分离。