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Register a new userBroadband terahertz probes of anisotropic magnetoresistance disentangle extrinsic and intrinsic contributions
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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.
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Identifying the intrinsic and extrinsic origins of magneto-transport in spin-orbit coupled systems has long been a central theme in condensed matter physics. However, it has been elusive owing to the lack of an appropriate experimental tool. In this work, using terahertz time-domain spectroscopy, we unambiguously disentangle the intrinsic and extrinsic contributions to the anisotropic magnetoresistance (AMR) of a permalloy film. We find that the scattering-independent intrinsic contribution to AMR is sizable and is as large as the scattering-dependent extrinsic contribution to AMR. Moreover, the portion of intrinsic contribution to total AMR increases with increasing temperature due to the reduction of extrinsic contribution. Further investigation reveals that the reduction of extrinsic contribution is caused by the phonon/magnon-induced negative AMR. Our result will stimulate further researches on other spin-orbit-interaction-induced phenomena for which identifying the intrinsic and extrinsic contributions is important.
We conducted a systematic angular dependence study of nonlinear magnetoresistance in NiFe/Pt bilayers at variable temperature and field using the Wheatstone bridge method. We successfully disentangled magnon magnetoresistance from other types of magnetoresistances based on their different temperature and field dependences. Both the spin Hall/anisotropic and magnon magnetoresistances contain sine phi and sine 3 phi components with phi the angle between current and magnetization, but they exhibit different field and temperature dependence. The competition between different types of magnetoresistances leads to a sign reversal of sine 3 phi component at a specific magnetic field, which was not reported previously. The phenomenological model developed is able to account for the experimental results for both NiFe/Pt and NiFe/Ta samples with different layer thicknesses. Our results demonstrate the importance of disentangling different types of magnetoresistances when characterizing the charge-spin interconversion process in magnetic heterostructures.
We investigate intrinsic and extrinsic decay of edge magnetoplasmons (EMPs) in graphene quantum Hall (QH) systems by high-frequency electronic measurements. From EMP resonances in disk shaped graphene, we show that the dispersion relation of EMPs is nonlinear due to interactions, giving rise to intrinsic decay of EMP wavepacket. We also identify extrinsic dissipation mechanisms due to interaction with localized states in bulk graphene from the decay time of EMP wavepackets. We indicate that, owing to the unique linear and gapless band structure, EMP dissipation in graphene can be lower than that in GaAs systems.
We investigate an excitonic peak appearing in low-temperature photoluminescence of monolayer transition metal dichalcogenides (TMDCs), which is commonly associated with defects and disorder. First, to uncover the intrinsic origin of defect-related excitons, we study their dependence on gate voltage, excitation power, and temperature in a prototypical TMDC monolayer, $MoS_2$. We show that the entire range of behaviors of defect-related excitons can be understood in terms of a simple model, where neutral excitons are bound to ionized donor levels, likely related to sulphur vacancies, with a density of $7cdot10^{11} cm^{-2}$. Second, to study the extrinsic origin of defect-related excitons, we controllably deposit oxygen molecules in-situ onto the surface of $MoS_2$ kept at cryogenic temperature. We find that in addition to trivial p-doping of $3cdot10^{12} cm^{-2}$, oxygen affects the formation of defect-related excitons by functionalizing the vacancy. Combined, our results uncover the origin of defect-related excitons, suggest a simple and conclusive approach to track the functionalization of TMDCs, benchmark device quality, and pave the way towards exciton engineering in hybrid organic-inorganic TMDC devices.
The terahertz spectral regime, ranging from about 0.1 to 15 THz, is one of the least explored yet most technologically transformative spectral regions. One current challenge is to develop efficient and compact terahertz emitters/detectors with a broadband and gapless spectrum that can be tailored for various pump photon energies. Here we demonstrate efficient single-cycle broadband THz generation, ranging from about 0.1 to 4 THz, from a thin layer of split-ring resonators with few tens of nanometers thickness by pumping at the telecommunications wavelength of 1.5 micrometer (200 THz). The terahertz emission arises from exciting the magnetic-dipole resonance of the split-ring resonators and quickly decreases under off-resonance pumping. This, together with pump polarization dependence and power scaling of the terahertz emission, identifies the role of optically induced nonlinear currents in split-ring resonators. We also reveal a giant sheet nonlinear susceptibility $sim$10$^{-16}$ m$^2$V$^{-1}$ that far exceeds thin films and bulk non-centrosymmetric materials.
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