For ferromagnet/antiferromagnet bilayers, rotation of the easy axis has been \textit{for the first time} observed during measurements of training effect and the recovery of exchange bias using FeNi/FeMn system. These salient phenomena strongly suggest irreversible motion of antiferromagnet spins during subsequent measurements of hysteresis loops. It is found that the rotation of the easy axis can partly account for the training effect and the recovery of the exchange bias.
It is well known that anisotropy determines the preferred transport direction of carriers. To manipulate the anisotropy is an exciting topic in two-dimensional materials, where the carriers are confined within individual layers. In this work, it is found that uniaxial strain can tune the electronic anisotropy of the 90°-twisted bilayer phosphorene. In this unique bilayer structure, the zigzag direction of one layer corresponds to the armchair one of the other layer and vice versa. Owing to this complementary structure, the directional (zigzag or armchair) deformation response to strain of one layer is opposite to that of the other layer, where the in-plane positive Poisson's ratio plays a key role. As a result, the doubly degenerate highest valence bands split, followed by a recovery of anisotropy. More interestingly, such an anisotropy, namely, the ratio of the effective mass along the direction to that along the direction, reaches as high as 6 under a small strain of 1%, and keeps nearly unchanged up to a strain of 3%. In addition, high anisotropy only holds for hole carriers as the conduction band is insensitive to strain. These findings should shed new light on the design of semiconducting devices, where the hole acts as the transport carrier.
We report a large but asymmetric magnetoresistance in silicon p-n junctions, which contrasts with the fact of magnetoresistance being symmetric in magnetic metals and semiconductors. With temperature decreasing from 293 K to 100 K, the magnetoresistance sharply increases from 50% to 150% under a magnetic field of 2 T. At the same time, an asymmetric magnetoresistance, which manifests itself as a magnetoresistance voltage offset with respect to the sign of magnetic field, occurs and linearly increases with magnetoresistance. More interestingly, in contrast with other materials, the lineshape of anisotropic magnetoresistance in silicon p-n junctions significantly depends on temperature. As temperature decreases from 293 K to 100 K, the width of peak shrinks from 90° to 70°. We ascribe these novel magnetoresistance to the asymmetric geometry of the space charge region in p-n junction induced by the magnetic field. In the vicinity of the space charge region the current paths are deflected, contributing the Hall field to the asymmetric magnetoresistance. Therefore, the observed temperature-dependent asymmetry of magnetoresistance is proved to be a direct consequence of the spatial configuration evolution of space charge region with temperature.
For Gd45Fe55∕Fe50Mn50 bilayers, both negative and positive exchange biasing have been observed for low and high magnetic cooling field HCF, respectively. These results can be attributed to a competition between antiferromagnetic coupling at GdFe∕FeMn interface and the Zeeman energy of FeMn spins under HCF. In order to reveal the magnetization reversal mechanism, the angular dependence of HE and HC has been investigated. It is found that the negative exchange biasing and the positive one have similar angular dependence that can be described by a magnetization coherent rotation model.
Abstract Multilevel remanence states have potential applications in ultra‐high‐density storage and neuromorphic computing. Continuous tailoring of the multilevel remanence states by spin‐orbit torque (SOT) is reported in perpendicularly magnetized Pt/Co/IrMn heterostructures. Double‐biased hysteresis loops with only one remanence state can be tuned from the positively or negatively single‐biased loops by SOT controlled sign of the exchange‐bias field. The remanence states associated with the heights of the sub‐loops are continually changed by tuning the ratio of the positively and negatively oriented ferromagnetic domains. The multilevel storage cells are demonstrated by reading the remanent Hall resistance through changing the sign and/or the magnitude of current pulse. The synaptic plasticity behaviors for neuromorphic computing are also simulated by varying the remanent Hall resistance under the consecutive current pulses. This work demonstrates that SOT is an effective method to tailor the remanence states in the double‐biased heavy metal/ferromagnetic/antiferromagnetic system. The multilevel‐stable remanence states driven by SOT show potential applications in future multilevel memories and neuromorphic computing devices.
We report on the magnetoresistance in different crystallographic directions of epitaxial ferromagnetic Fe30Co70 thin films with magnetization rotated in the film plane. A negative single crystal anisotropic magnetoresistance (SCAMR) is found when the current is along the easy magnetization axis [110], and the SCAMR can be tuned to the conventional positive one when the current flows along the hard magnetization axis [100]. This finding is explained comprehensively by a magnetocrystalline anisotropy (MCA) symmetry-adapted model expanded along the easy magnetization direction, with which the SCAMR can be represented as a MCA-independent conventional term cos 2φM and a series of MCA-dependent terms cos 2nφA (n≥1). The results show that the MCA-dependent twofold term contributes to the negative SCAMR, which cannot be used as a fingerprint of the half-metallicity. Our finding provides an approach to understand and design the magnetoresistance with ferromagnets by MCA.
The second harmonic voltage has been widely used to measure electrically driven spin torques in ferromagnet--heavy metal heterostructures. Based on the macrospin model, the third harmonic voltage is proposed for studying thermally driven spin torques. Our experiments clearly show that two distinct physical mechanisms dominate such thermal spin torques. The first refers to thermal expansion or contraction, which generates an effective field through the magnetoelastic effect. The second comes from the thermally reduced magnetization of the ferromagnetic layer, which modulates the magnetoresistance of the heterostructure. Our study illustrates the versatile applicability of the third harmonic voltage in investigating the nonlinear effects in ferromagnet--heavy metal heterostructures.
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