铁磁材料格里菲斯相玻璃性的研究

With the development of miniaturization, intelligence, and high sensitivity of devices, the giant magnetoresistive effect, magnetocaloric effect, and magnetostrictive effect of existing ferromagnetic materials have been unable to meet their low field and wide temperature range high-performance applications. Therefore, the development of high-performance and unusual-properties of ferromagnetic materials is the key issue that to be solved in the application of magnetic materials. Ferromagnets, ferroelectrics and ferroelastics have a high degree of physical parallelism from micro-order parameters, mesoscopic domain structure to macroscopic properties，and are called the ferroic smart materials. Recently, it is found that strain glass in ferroelastics and relaxor in ferroelectrics exhibit unusual properties, such as superelasticity，high-damping, ultrahigh piezoelectricity, giant electrostriction, etc. However, the current understanding of the glass in ferromagnets is quite limited, and the low freezing temperature of spin glass limits its practical application. Based on the new mechanism of high-performance in ferroelastics and ferroelectrics. This project intends to develop a new type of glass phase, Griffith phase, which is potentially high-performance in ferromagnets. The content includes: the induction of Griffiths phase and cluster spin glass by introducing defects into the pure ferromagnet; the construction of a comparable phase diagram of temperature-defect concentrations with ferroelectrics and ferroelastics in ferromagnets, and the study of glass nature of Griffiths phase; establishing the common physical basis among three types of ferroic materials and three types of ferroic glasses, and studying the diffuse ferromagnetic phase transition of Griffiths phase. This project is expected to provide theoretical guidance for the application of ferromagnetic glass phase in their high performance and unusual properties.

随器件向小型化、智能化和高灵敏化发展，现有铁磁材料的巨磁电阻、磁热、磁致伸缩效应已无法满足其宽温区、低场高驱动的应用。因此，开发铁磁体特异高性能是磁性材料应用亟待解决的关键问题。铁磁与铁弹、铁电体从微观序参量、介观畴到宏观性能具有高度物理平行性，被称为铁性智能材料。最新研究发现，铁弹、铁电玻璃相具有宽温区超弹性、高阻尼性、高压电性、巨电致伸缩等优异性能。然而，目前对铁磁玻璃相的理解还相当有限，自旋玻璃低的转变温度限制了其实际应用。本项目拟依据铁弹、铁电提供特异高性能的新机制，开发铁磁中潜在高性能的新型玻璃相——格里菲斯相。内容包括通过构建稀释铁磁体，诱导格里菲斯相和团簇自旋玻璃态，构建铁磁体系平行于铁弹、铁电体系的典型温度-缺陷浓度关系相图，研究格里菲斯相的玻璃性，建立铁性材料、铁性玻璃共同的物理基础，研究格里菲斯相扩散型铁磁相变。该项目预计为铁磁玻璃相的特异高性能应用，提供理论指导意义。

格里菲斯相广泛存在于无序体系中，包括磁性、量子、电子甚至网状系统中，并与它们的巨磁电阻效应，量子关键行为，非费米液体行为和自旋液体行为等相关。然而，在磁性体系中由于格里菲斯相与团簇自旋玻璃非常相似，但目前它们之间的关联尚不明确，导致长期以来格里菲斯相的磁本质一直存在争议。.本项目基于铁磁、铁弹（马氏体或形状记忆合金）、铁电三类铁性智能材料，在微观机理、介观畴、宏观性能方面具有高度物理平行性。通过在简单铁磁体中，引入非磁性缺陷，调控缺陷浓度诱导出格里菲斯相变和团簇自旋玻璃相变。运用宏观、微观磁性能测试，研究冷却过程磁相变随缺陷浓度的变化特征：在低缺陷浓度区域（0.01< x ≤0.09），随温度降低首先出现顺磁到格里菲斯相相变，再到铁磁相的相变；但在高缺陷浓度区域，x > 0.09，铁磁相变消失，而是出现了格里菲斯相到团簇自旋玻璃的冻结过程。表明，当缺陷浓度超过临界值xc（0.09）时，原本无相变的格里菲斯相实际上存在团簇自旋玻璃的冻结过程。我们进一步绘制出铁磁体平行于铁弹体、铁电体的典型的温度-缺陷浓度关系相图，从而给出格里菲斯相玻璃性的本质——未冻结的团簇自旋玻璃的直观实验证据。.运用宏观、微观磁性测量方法和特征物理量热膨胀的测量，建立格里菲斯相与未冻结应变玻璃态、各态遍历弛豫铁电体的物理平行性。依据开发铁弹和铁电体中玻璃态的特异高性能，研究格里菲斯相的扩散型铁磁相变。研究发现，格里菲斯相的磁化率曲线表现出扩散型铁磁相变特征，等温磁滞回线在TG温度以上呈现顺磁曲线特征；在TC < T < TG之间，磁滞回线具有“S”型铁磁回线特征，随温度降低，饱和磁化强度出现台阶式增加；当T < TC后，“S”型铁磁回线的饱和磁化强度随温度降低几乎不变。此外，晶格弹性模量在较宽的温区出现了软化，进一步证实了铁磁相变是一个扩散过程。铁磁扩散型相变为开发铁磁玻璃相宽温区、低场高驱动，及其在磁性功能材料中的应用提供材料基础。