论文标题
用于检查磁性材料中的热性能的混合方法
Hybrid ab initio method for examining thermal properties in magnetic materials
论文作者
论文摘要
提出了一种用于检查未知熵磁系统中的热性能的杂化缩写理论方法。通常使用的理论方法从Gibbs/Helmholtz自由能询问热特性，这需要精确的磁相互作用模型。目前的方法可以通过计算来自热无序微晶体的系统压力来避免这种要求，这些系统在每个温度以及这些子系统之间的耦合下正确地包含了振动和自旋子系统。该方法代替了磁相互作用的特定模型，整合了研究材料的温度依赖性磁化的测量值。我们采用该方法来计算声子模式，并研究经典infar合金的异常低热膨胀，FE_0.65NI_0.35。计算出的Invar的声子分散与测量数据非常吻合。向显示，向内热膨胀显示在50 K和室温之间保持较小，这与实验观察到的低热膨胀值在同一温度范围内保持一致。这种异常的小热膨胀直接连接到晶格热疾病的较小积极贡献，而晶格热疾病几乎被大型的负磁性疾病贡献所取消。相比之下，BCC FE的计算显示出更大的热膨胀，这与实验一致，这是由晶格热疾病的巨大贡献所占据主导的，而晶格热疾病的贡献仅略微降低，而磁性的负面影响很小。这些发现提供了对Invar和Fe中磁性和自旋晶格耦合的异常性质的见解，它们支持提出的新方法，作为研究磁性材料热性能的一种补充方法。
A hybrid ab initio theoretical approach for examining thermal properties in magnetic systems of unknown entropy is presented. Commonly used theoretical approaches interrogate thermal properties from Gibbs/Helmholtz free energies, which require an accurate model of magnetic interactions. The present approach avoids this requirement by instead calculating system pressure from thermally disordered microstates that properly incorporate vibrational and spin subsystems at each temperature as well as the coupling between these subsystems. In place of a specific model for magnetic interactions, the approach integrates measurements of temperature dependent magnetization of the studied material. We apply the approach to calculate phonon modes and to investigate the anomalously low thermal expansion of the classical Invar alloy, Fe_0.65Ni_0.35. The calculated phonon dispersions for Invar are in excellent agreement with measured data. The Invar thermal expansion is shown to remain small between 50 K and room temperature, consistent with the experimentally observed low thermal expansion value in this same temperature range. This anomalously small thermal expansion is directly connected to a small positive contribution from lattice thermal disorder that is nearly canceled by a large negative magnetic disorder contribution. By contrast, calculations for bcc Fe show a much larger thermal expansion, consistent with experiment, which is dominated by a large contribution from lattice thermal disorder that is reduced only slightly by a small negative contribution from that of magnetism. These findings give insights into the unusual nature of magnetism and spin-lattice coupling in Invar and Fe, and they support the presented new methodology as a complementary way to investigate thermal properties of magnetic materials.
