LAFO超薄 films中低温自旋动力学：从立方各向异性到TLS限制的相干性
Low-temperature spin dynamics in LAFO thin films: from cubic anisotropy to TLS-limited coherence
AI总结：
研究揭示LAFO超薄 films中低温自旋动力学，发现立方各向异性与TLS限制相干性的关系，为磁子材料设计提供新见解。
摘要：
我们利用从0.44 K到68 K的宽频铁磁共振（FMR）光谱学，研究了 epitaxial 硫酸锂铝铁（LAFO）超薄 films 的低温自旋动力学。结果揭示了在较高温度下从传统立方各向异性主导行为向低温下显著线宽展宽和更高阶各向异性贡献的转变。当磁场沿[100]晶格方向时，共振被四重平面和垂直各向异性所捕捉。相比之下，沿[110]方向测量揭示了异常大的六阶立方各向异性项，该项在[100]方向下被对称性抑制，但在这种磁场取向下在超低温下变得明显，表明各向异性景观有显著变化。独立线宽分析显示在8 K附近有明显峰值，并在低于2 K的温度下有微妙的单调增强，这与由反位缺陷和局域Fe³+自旋引起的TLS浴引起的损耗一致。与TLS基模型的比较表明，交换耦合杂质和近自由的顺磁中心都对观察到的阻尼有贡献。我们的结果确立LAFO为一个模型铁氧体系统，其中由于无序引起的TLS限制自旋相干性在超低温下起作用，为各向异性工程、磁弛豫和铁磁绝缘体的相干磁子设计提供了新见解。这些发现为未来优化生长条件提供了框架。
英文摘要：
We investigate the low-temperature spin dynamics of epitaxial lithium aluminum ferrite (LAFO) thin films using broadband ferromagnetic resonance (FMR) spectroscopy from 0.44 K to 68 K. The results reveal a crossover from conventional cubic anisotropy-dominated behavior at higher temperatures to pronounced linewidth broadening and higher-order anisotropy contributions at cryogenic temperatures. With the magnetic field oriented along the [100] crystallographic direction, the resonance is well-captured by four-fold in-plane and out-of-plane uniaxial anisotropies. In contrast, measurements with the field along the [110] direction reveal the presence of an unusually large sixth-order cubic anisotropy term that is symmetry-suppressed for [100] but becomes apparent under this field orientation at ultralow temperatures, indicating a substantial modification of the anisotropy landscape. Independent linewidth analysis shows a pronounced peak near 8 K and a subtle monotonic enhancement with decreasing temperatures below 2 K, features consistent with dissipation mediated by a bath of two-level systems (TLS) arising from antisite defects and localized Fe$^{3+}$ moments. Comparison with TLS-based models demonstrates that both exchange-coupled impurities and nearly free paramagnetic centers contribute to the observed damping. Our results establish LAFO as a model ferrite system where disorder-induced TLS limit spin coherence at ultralow temperatures, providing new insights into anisotropy engineering, magnetic relaxation, and the design of ferrimagnetic insulators for coherent magnonics. These findings offer a framework for future optimization of growth conditions.
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