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Register a new userEnhanced Granular Magnetoresistance due to Ferromagnetic Layers
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Giant magnetoresistance (GMR) of sequentially evaporated Fe-Ag structures have been investigated. Direct experimental evidence is given that inserting ferromagnetic layers into a granular structure significantly enhances the magnetoresistance. The increase of the GMR effect is attributed to spin polarization effects. The large enhancement (up to more than a fourfold value) and the linear variation of the GMR in low magnetic fields are explained by scattering of the spin polarized conduction electrons on paramagnetic grains.
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Skyrmions are localized, topologically non-trivial spin structures which have raised high hopes for future spintronic applications. A key issue is skyrmion stability with respect to annihilation into the ferromagnetic state. Energy barriers for this collapse have been calculated taking only nearest neighbor exchange interactions into account. Here, we demonstrate that exchange interactions beyond nearest neighbors can be essential to describe stability of skyrmionic spin structures. We focus on the prototypical film system Pd/Fe/Ir(111) and demonstrate that an effective nearest-neighbor exchange or micromagnetic model can only account for equilibrium properties such as the skyrmion profile or the zero temperature phase diagram. However, energy barriers and critical fields of skyrmion collapse as well as skyrmion lifetimes are drastically underestimated since the energy of the transition state cannot be accurately described. Antiskyrmions are not even metastable. Our work shows that frustration of exchange interactions is a route towards enhanced skyrmion stability even in systems with a ferromagnetic ground state.
Free electron theory tells us that resistivity is independent of magnetic field. In fact, most observations match the semiclassical prediction of a magnetoresistance that is quadratic at low fields before saturating. However, a non-saturating linear magnetoresistance has been observed in exotic semiconductors such as silver chalcogenides, lightly-doped InSb, N-doped InAs, MnAs-GaAs composites, PrFeAsO, and epitaxial graphene. Here we report the observation of a large linear magnetoresistance in the ohmic regime in commonplace commercial n-type silicon wafer. It is well-described by a classical model of spatially fluctuating donor densities, and may be amplified by altering the aspect ratio of the sample to enhance current-jetting: increasing the width tenfold increased the magnetoresistance at 8 T from 445 % to 4707 % at 35 K. This physical picture may well offer insights into the large magnetoresistances recently observed in n-type and p-type Si in the non-ohmic regime.
The B20 chiral magnets with broken inversion symmetry and C4 rotation symmetry have attracted much attention. The broken inversion symmetry leads to the Dzyaloshinskii-Moriya that gives rise to the helical and Skyrmion states. We report the unusual magnetoresistance (MR) of B20 chiral magnet Fe0.85Co0.15Si that directly reveals the broken C4 rotation symmetry. We present a microscopic theory, a minimal theory with two spin-orbit terms, that satisfies all the symmetry requirements and accounts for the transport experiments.
We report the observation of the antisymmetric magnetoresistance (MR) in perpendicular magnetized CoTb films with inhomogeneous magnetization distribution driven by gradient magnetic field. By synchronously charactering the domain pattern evolution during transport measurements, we demonstrate that the nonequilibrium currents in the vicinity of tilting domain walls give rise to such anomalous MR. Moreover, theoretical calculation and analysis reveal that the geometry factor of the multidomain texture plays a dominant role in generating the nonequilibrium current. The explicitly established interplay between the anomalous transport behaviors and the particular domain wall geometry is essential to deepening understanding of the antisymmetric MR, and pave a new way for designing novel domain wall electronic devices.
Current-induced magnetization excitation is a core phenomenon for next-generation magnetic nanodevices, and has been attributed to the spin-transfer torque (STT) that originates from the transfer of the spin angular momentum between a conduction electron and a local magnetic moment through the exchange coupling. However, the same coupling can transfer not only spin but also energy, though the latter transfer mechanism has been largely ignored. Here we report on experimental evidence concerning the energy transfer in ferromagnet/heavy metal bilayers. The magnetoresistance (MR) is found to depend significantly on the current direction down to low in-plane currents, for which STT cannot play any significant role. Instead we find that the observed MR is consistent with the energy transfer mechanism through the quantum spin-flip process, which predicts short wavelength, current-direction-dependent magnon excitations in the THz frequency range. Our results unveil another aspect of current-induced magnetic excitation, and open a channel for the dc-current-induced generation of THz magnons.
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