  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, Ni81Fe19 and Ni50Fe50 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, Ni81Fe19 and Ni50Fe50 samples is of
predominantly extrinsic nature. However, the Co thin film exhibits a sizeable
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 magneto-transport phenomena of standard magnetic thin films on
ultrafast time scales.
