Anisotropic magnetoresistance (AMR) is a ubiquitous and versatile probe of magnetic order in contemporary spintronics research. Its origins are usually ascribed to extrinsic effects (i.e., spin-dependent electron scattering), whereas intrinsic (i.e., scattering-independent) contributions are neglected. Here, we measure AMR of polycrystalline thin films of the standard ferromagnets Co, Ni, ${\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$, and ${\mathrm{Ni}}_{50}{\mathrm{Fe}}_{50}$ over the frequency range from dc to 28 THz. The large bandwidth covers the regimes of both diffusive and ballistic intraband electron transport and, thus, allows us to separate extrinsic and intrinsic AMR components. Analysis of the THz response based on Boltzmann transport theory reveals that the AMR of the Ni, ${\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$, and ${\mathrm{Ni}}_{50}{\mathrm{Fe}}_{50}$ samples is of predominantly extrinsic nature. However, the Co thin film exhibits a sizable intrinsic AMR contribution, which is constant up to 28 THz and amounts to more than $2/3$ of the dc AMR contrast of 1%. These features are attributed to the hexagonal structure of the Co crystallites. They are interesting for applications in terahertz spintronics and terahertz photonics. Our results show that broadband terahertz electromagnetic pulses provide new and contact-free insights into magnetotransport phenomena of standard magnetic thin films on ultrafast timescales.

中文翻译：

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各向异性磁电阻宽带太赫兹探针的外源和本征贡献。

各向异性磁阻（AMR）是当代自旋电子学研究中普遍存在的磁序探针。它的起源通常归因于外在效应（即自旋相关的电子散射），而固有的（即独立于散射的）贡献却被忽略了。在这里，我们测量标准铁磁体Co，Ni，${你}_{81}{铁}_{19}$， 和 ${你}_{50}{铁}_{50}$在直流至28 THz的频率范围内。大带宽涵盖了扩散性和弹道带内电子传输的机制，因此使我们能够分离外部和固有AMR分量。根据玻尔兹曼输运理论对太赫兹响应的分析表明，镍的AMR${你}_{81}{铁}_{19}$， 和 ${你}_{50}{铁}_{50}$样本主要是外部性质。但是，Co薄膜表现出可观的固有AMR贡献，在最高28 THz时恒定不变，并且超过$\mathrm{2个}/3$直流AMR对比度的1％。这些特征归因于Co微晶的六边形结构。它们对于太赫兹自旋电子学和太赫兹光子学中的应用很有趣。我们的结果表明，宽带太赫兹电磁脉冲在超快时间尺度上为标准磁性薄膜的磁传输现象提供了新的和无接触的见解。