The large anomalous Hall effect (AHE) in antiferromagnetic(AFM) Weyl semimetal Mn3Sn attracts intensive attentions in spintronics. Here, we report the structural property of high quality Mn3Sn thin film on insulator substrate MgO(1 1 0) by molecular beam epitaxy (MBE), and AHE in control of residual mismatch strain between Mn3Sn film and substrate. We are able to grow strain-free Mn3Sn(10 1 0) films or alternatively strained Mn3Sn(11 2 0) films via a three-step process. The strain-free Mn3Sn film has large anomalous Hall conductivity up to 30 & omega;-1cm- 1 at room temperature, which is comparable to bulk Mn3Sn. In contrast, AHE is switched off in strained Mn3Sn film due to piezomagnetic effect under a uniaxial compress strain of-2.0%. These findings provide a deeper understanding on AFM spintronic applications.

The synchronization of the spin Hall nano-oscillator (SHNO) device driven by the pure spin current has been investigated with micromagnetic simulations. It was found that the power spectra of nanowire-based SHNO devices can be synchronized by varying the current flowing in the heavy metal (HM) layer. The synchronized signals have relatively high power and narrow linewidth, favoring the potential applications. We also found that the synchronized spectra are strongly dependent on both the number and length of nanowires. Moreover, a periodic modulation of power spectra can be obtained by introducing interfacial Dzyaloshinskii–Moriya interaction (iDMI). Our findings could enrich the current understanding of spin dynamics driven by the pure spin current. Further, it could help to design novel spintronic devices.

We report an unconventional carrier-dependent anomalous Hall effect (AHE) with a quadratic scaling relation in epitaxial films of a ferromagnetic semiconductor (ZnCo)O with a high Co concentration. We show the co-existence of AHE together with the nonlinear ordinary Hall effect (NLHE) and the separation of NLHE by using a two-conducting channels model in the expression of Hall resistivity. We found that the NLHE depends strongly on both temperature and carrier density n and dominates at low temperature when n <1.0×1020cm−3, indicating the very necessary of separation them from each other for a proper scaling of the AHE. The anomalous Hall resistivity is nearly independent on carrier density and longitudinal resistivity regardless the underlying transport mechanisms and thermal properties. Very interestingly, a quadratic scaling relation between anomalous Hall conductivity and longitudinal conductivity σxyAHE∝σxx2 is obtained. The further analysis shows that σxyAHE∝n2 with a frozen electron mobility. In addition, sign reversal of the AHE has been observed by reducing magnetization via growth controlling engineering. Our results reveal a carrier-dependent AHE in low conductivity regime and provide an experimental evidence for the itinerant ferromagnetism in ferromagnetic semiconductor (ZnCo)O.

Novel spintronic devices with low power consumption, nonvolatility, and high storage density are highly desired to meet the rapid development of modern information storage and communication technology, which poses a great challenge to both material and device researches. To overcome this challenge, our group has focused our research on the following two aspects. On one hand, we have explored novel magnetoresistance effects in a variety of materials and devices, aiming to get a deep understanding about the spin dependent transport and obtain an effective control of spin-dependent transport. On the other hand, we have tried to control the magnetoresistance based on multi-physical field effects, aiming to obtain spintronic prototype devices that can be used for multi-state data storage. Regarding the novel magnetoresistance explorations, this article will introduce: (1) The negative magnetoresistance in amorphous condensed magnetic semiconductors. The spin dependent variable range hopping model is proposed, which can quantitatively explain the temperature and magnetic field dependent transport behavior in the condensed magnetic semiconductors. In addition, this model provides an alternative way to detect the spin polarization ratio of the magnetic semiconductors. (2) The positive magnetoresistance in single-crystal CoZnO magnetic semiconductors. By quantitative analysis of the transport properties of CoZnO films, it is observed that the positive magnetoresistance in the "hard gap" regime is the result of the carrier wavefunction shrinkage under applied magnetic field. (3) The rectification magnetoresistance in non-magnetic Schottky heterojunctions and the tunneling rectification magnetoresistance in magnetic tunnel junctions. A brand new rectification magnetoresistance is observed in nonmagnetic Al/Ge Schottky heterojunctions: The application of a pure small sinusoidal alternating current to the nonmagnetic Schottky heterojunctions can generate a significant direct-current voltage, and this rectification voltage strongly varies with the external magnetic field. Moreover, by using CoO-ZnO composite tunneling barrier, the charge-related rectification and spin-dependent tunneling magnetoresistance are integrated into the Co/CoO-ZnO/Co magnetic tunneling junctions to realize the tunneling rectification magnetoresistance. The observation of rectification magnetoresistance and tunneling rectification magnetoresistance not only adds new members to the magnetoresistance family, but also provides a promising way to control the devices' properties by using alternating current. In terms of multistate data storage application, this review will introduce: (1) The electrical and magnetic field controllable 4 resistance states in oxide heterojunctions. In Co/CoO-ZnO/Co magnetic tunneling junctions, by integrating the electrical field induced resistance switching and the magnetic field induced tunneling magnetoresistance effects, four nonvolatile resistance states are demonstrated. (2) The remanent magnetization controllable 10 resistance states in magnetic heterojunctions. Here, a general remanent magnetism engineering method is proposed for realizing multiple reliable magnetic and resistance states, not depending on a specific material or device structure. Especially, as a proof-of-concept demonstration, ten states of nonvolatile memory based on the manipulation of ferromagnetic remanent magnetization have been revealed in both Co/Pt magnetic multilayers with strong perpendicular magnetic anisotropy and MgO-based magnetic tunneling junctions at room temperature. (3) The spin-orbit torque controllable 10 resistance states in single-layer magnetic alloy. A repeatable bulk spin-orbit torque switching of the perpendicularly magnetized CoPt alloy single-layer films is realized by introducing a composition gradient in the thickness direction to break the inversion symmetry. Moreover, a ten states nonvolatile memory is illustrated solely by changing the electrical current to control the multi-domain states of the CoPt alloy.

Via exploring the spin dynamics and transport properties at interfaces of magnetic material/non-magnetic metal bilayers, spintronics advanced many techniques on generation, detection. and manipulation of spin currents. which laid down a foundation for designing spintronic devices. New ideas and methods for manipulating spin are required in the spintronics community. Merging with cavity quantum electrodynamics, magnetic resonance in ferromagnets inside of a microwave cavity, realizes a strong magnon-microwave coupling at room temperature, opening an avenue to manipulate spin and spin current for the spintronics community. Spin-microwave coupling strength was significantly improved by replacing a few spins in a paramagnetic spin system with magnetization in ferromagnetic materials. Thus. a strongly coupled magnon-microwave system offers a hybrid quantum platform with many measures to tune the coupled system, such as an exchange magnetic field, an anisotropy field, controlled coupling strength, controlled damping parameters, and so on. Many devices were proposed and demonstrated with potential applications. In this review, we briefly introduce the concept and mechanism of magnon-microwave coupling, a classical electrodynamical coupling model for the coupling, spin currents produced by the coupled magnon, and manipulation of the spin current via the strong coupling. In ferromagnets, magnon can couple to a microwave mode as a magnon-polariton propagating in the materials. The strongly coupled magnon-microwave system is a magnon-polariton in a cavity where the microwave was confined as a resonance mode with a high quality factor. Using the Landau-Lifshitz-Gilbert equation and Maxwell equations, we introduce the classical electrodynamical coupling model and reveal that the magnon is driven by a torque produced by the microwave magnetic field on the magnetization and the microwave is feedback via Faraday's law. Therefore, the coupling strength of the magnon-microwave system can be controlled by changing the torque. Magnon-microwave coupling is experimentally studied using many techniques, such as microwave transmission, Brillouin light scattering. Faraday rotation measurement, and spin pumping electrical detection. Spin pumping is well explored in the spintronics community as a spin current produced by magnon and detected as a voltage via inverse spin Hall effect in a novel metal. Combining with microwave transmission of the coupled microwave subsystem, spin pumping electrical detection by directly detecting the spin current produced by the magnon subsystem in the coupled system, illustrates a more complete profile of the coupled magnon-microwave system. In addition, the strongly coupled magnon-microwave system offers a new technique to manipulate spin currents. We experimentally demonstrate that spin currents produced by the strongly coupled system are correlated with each other mediated by the microwave cavity, and can be manipulated remotely. Two almost identical magnet samples am used to couple to a cavity and the coupling strengths between each magnet and the cavity are tunable individually. By using electrical detection to locally detect each magnet sample respectively, we find that the spin currents produced by two magnet samples are coherently correlated. By tuning the coupling strength of one magnet sample to the cavity. the spin current produced by the second magnet sample is manipulated distantly over a few centimeters, which is far longer than the distance of spin-orbit interaction or exchange interaction. This distant control is only limited by the coherence length of the microwave and the dimension of the cavity. This flexibility opens the door to improve spin current generation and manipulation for cavity spintronic devices.

In this paper, we demonstrate the ferroelectric gate control of Rashba–Dresselhaus spin–orbit coupling (R–D SOC) in a hybrid heterostructure consisting of a ferromagnetic semiconductor channel (Zn, Co)O(0001) and a ferroelectric substrate PMN-PT(111). The R–D SOC causes a transverse spin current via the charge-spin conversion, which results in unbalanced transverse spin and charge accumulations due to the spin-polarized band in the ferromagnetic (Zn, Co)O channel. By the reversal of gated ferroelectric polarization, we observed 55% modulation of the R–D SOC correlated Hall resistivity to the magnetization correlated anomalous Hall resistivity and 70% modulation of the low-field magnetoresistance at 50 K. Our experimental results pave a way toward semiconductor-based spintronic-integrated circuits with an ultralow power consumption in ferromagnetic semiconductors.

Manipulation of oxygen vacancies (V-o) in transition metal oxides (TMOs) can largely alter their physical and chemical properties, such as electrical conductivity, magnetic state, optical band-gap, and electrocatalytic reactivity. Many experimental and theoretical works have been conducted to study the formation/annihilation of V-o and its corresponding effect on the properties in TMOs. In this paper, a solid-state approach to modulate the oxygen stoichiometry in high quality SrCoOx epitaxial thin films was demonstrated. Dependence of the magnetic and electrical conducing properties on V-o was investigated. Room temperature reversible phase switching between brownmillerite antiferromagnetic insulating SrCoO2.5 and perovskite ferromagnetic metallic SrCoO3-delta was achieved by electric-field induced oxygen non-stoichiometry. This room temperature reversible phase switching indicates that SrCoOx thin films are a promising candidate for practical applications in resistive random access memory and spintronic devices.

Heterostructures consisting of ferromagnetic metal and antiferromagnetic insulator have exhibited novel properties in interface exchange coupling, magnetoresistance and spin injection due to their fundamental physics of magnetism. However, as compared with bilayer heterostructures, it is very challenging in experiments to prepare single crystal epitaxial ultrathin superlattices consisting of ferromagnetic metal and antiferromagnetic insulator, where the interface exchange coupling, magnetic order, and electrical transport properties are expected to be greatly modulated due to the multiple interfaces and periodical structures. Here we prepared single crystal epitaxial [Co/CoO]n superlattices consisting of ferromagnetic metal Co and antiferromagnetic insulator CoO by molecular beam epitaxy. It is found that the saturation magnetization of [Co 0.6 nm/CoO 1.2 nm]5 superlattice is robust against temperature increase, which only shows a slight decrease of 1.5% from 5 K to 300 K. Moreover, we found that the longitudinal resistivity and anomalous Hall resistivity of [Co/CoO]n superlattices are strongly modulated by the short-period Co/CoO heterostructure, which are quite distinguished from the electrical transport properties of Co bulk ferromagnetic metal. The [Co/CoO]n superlattices provide a new insight into heterostructures consisting of ferromagnetic metal and antiferromagnetic insulator, which are beneficial for designing future spintronics devices.

Recently antiferromagnetic materials become very attractive due to their new applications in spintronic devices. High-quality single crystal epitaxial ZnxCo1-xO (1 1 1) thin films were grown by co-evaporating Zn and Co and simultaneously oxidizing in oxygen plasma. It is found that the exchange bias fields of Co/as-prepared ZnxCo1-xO bilayers decrease with increasing Zn composition. When as-prepared ZnxCo1-xO layer was annealed in oxygen plasma to remove oxygen vacancies, the exchange bias field of Co/oxygen-plasma-annealed ZnxCo1-xO bilayers can be further enhanced. Moreover, a weak ferromagnetism was observed at 300 K in the as-prepared antiferromagnetic ZnxCo1-xO film with oxygen vacancies, but it did not exist in oxygen-plasma-annealed ZnxCo1-xO film. This indicates that oxygen vacancies can simultaneously weaken the antiferromagnetism but enhance the ferromagnetism of ZnxCo1-xO layer. Therefore, we offer a method of manipulating the antiferromagnetism of ZnxCo1-xO films by changing Zn composition and oxygen vacancies, which is useful for designing antiferromagnetic spintronic devices.

We present an experimental observation of interfacial spin accumulation induced by anomalous Hall effect (AHE) in ferromagnets (FMs) in a multilayer structure of FM/YIG/Pt, where the direction of charge current injection in FM layer is perpendicular to the direction of voltage detection in Pt layer. In this structure, the magnon-mediated drag voltage (Vdrag) due to the interfacial spin accumulation induced by AHE, can be unambiguously separated from spin Seebeck voltage (VSSE) by sweeping or rotating the applied magnetic field. Field-dependent spin accumulation induced by AHE has been observed by comparison of nonlocal voltages between Ni/Cu/YIG/Pt and Pt/YIG/Pt samples. Furthermore, we demonstrate that the AHE voltage strongly depends on the spin polarization of conductivity and spin Hall angles of electrons with opposite spins in FMs. Our results show a prospect for FMs to be field-control spin generators via AHE, and provide a new viewpoint to realize the AHE.

We report that the unusual behavior of anomalous Hall effect (AHE) in permalloy (Fe20Ni80, Py) thin film with silicon oxide (SiO2) as an adjacent layer. It is found that sign of AHE changes from positive to negative with decrement in thickness of Py. Through the AHE measurement, we demonstrate that this unusual behavior originates from the interface of Py/SiO2 rather from the structural inversion asymmetry. In the Pt/YIG/Cu/Py/SiO2 multilayer structure with a charge current injected in the Py layer, the information on spin accumulation at the YIG/Cu interface induced by the AHE in Py, can be obtained from the nonlocal voltage measured in the N layer. Through the nonlocal measurement, it is found, however, the electrons with same spin for interfacial Py near the SiO2 layer and bulk Py are scattered to the same direction. Hence we can deduce that the sign of spin polarization of the conductivity closely related to the asymmetry in density of states at the Fermi level for interfacial Py and bulk Py should be opposite to each other, which is responsible for the abnormal AHE observed. Our results demonstrate that oxide layer can drive the change of sign of AHE of Py, which provide an alternative way of manipulating the spin-related transport.

Ferromagnetic noncollinear spin valve effect has been observed in a magnetic multilayer structure of IrMn/NiFe/Cu/Y3Fe5O12 (YIG). In this structure, the detected amplitude of spin current emitted by spin pumping depends on the NiFe magnetic states (saturation or unsaturation). This yields a spin valve-like behavior with reliable amplitude changes up to 110% and the behavior may originate from the difference of spin absorption when the NiFe layer is magnetically saturated or not. Our results provide a ferromagnetic noncollinear spin valve, paving the road towards ultralow-dissipation spintronic devices based on pure spin current.

The magnetization-direction-dependent inverse spin Hall effect (ISHE) has been observed in NiFe film during spin Seebeck measurement in IrMn/NiFe/Cu/yttrium iron garnet (YIG) multilayer structure, where the YIG and NiFe layers act as the spin injector and spin current detector, respectively. By using the NiFe/IrMn exchange bias structure, the magnetization direction of YIG (M YIG) can be rotated with respect to that of NiFe (M NiFe) with a small magnetic field, thus allowing us to observe the magnetization-direction-dependent inverse spin Hall effect voltage in NiFe layer. Compared with the situation that polarization direction of spin current (σ s) is perpendicular to M NiFe, the spin Seebeck voltage is about 30% larger than that when σ s and M NiFe are parallel to each other. This phenomenon may originate from either or both of stronger interface or bulk scattering to spin current when σ s and M NiFe are perpendicular to each other. Our work provides a way to control the voltage induced by ISHE in ferromagnets.

We demonstrate the large spectral range nonlinear optical response of a Co:ZnO epitaxial thin film (with single crystal structure) with the absorption range from 1.0 μm to 2.0 μm, for the first time to our knowledge. The third-order nonlinear optical properties and saturable absorption of Co0.15Zn0.85O (Co:ZnO) were studied with the modulation depth of 0.68% and saturation fluence of 1.532 μJ/cm2. Passive Q-switched lasers at the wavelengths of a 1.4 μm Nd:YGG laser and a 1.9 μm Tm:LuVO4 laser were realized with the Co:ZnO epitaxial thin film as the saturable absorber. In the case where the wavelength is 1.4 μm, we obtained the shortest pulse width, the largest pulse energy and the highest peak power, which were recorded at 89 ns, 2.3 μJ and 25.8 W, respectively. Moreover, when the laser output wavelength is 1.9 μm, we got the smallest pulse width of 274 ns. This work provides a promising saturable absorber with a large spectral range response.

The spin transparency at the normal/ferromagnetic metal (NM/FM) interface was studied in Pt/YIG/Cu/FM multilayers. The spin current generated by the spin Hall effect (SHE) in Pt flows into Cu/FM due to magnetic insulator YIG blocking charge current and transmitting spin current via the magnon current. Therefore, the nonlocal voltage induced by an inverse spin Hall effect (ISHE) in FM can be detected. With the magnetization of FM parallel or antiparallel to the spin polarization of pure spin currents ( σ sc ), the spin-independent nonlocal voltage is induced. This indicates that the spin transparency at the Cu/FM interface is spin-independent, which demonstrates that the influence of spin-dependent electrochemical potential due to spin accumulation on the interfacial spin transparency is negligible. Furthermore, a larger spin Hall angle of Fe20Ni80 (Py) than that of Ni is obtained from the nonlocal voltage measurements.

Direct evidence of purely interfacial effects on resistance switching is demonstrated in Au/BiFeO3/Nd:SrTiO3(001) (Au/BFO/NSTO) Schottky junctions by reducing the thickness of ferroelectric interlayer BFO.

We have carried out a detailed investigation of the magnetism, valence state, and magnetotransport in VSe2 bulk single crystals, as well as in laminates obtained by mechanical exfoliation. In sharp contrast to the ferromagnetic behavior reported previously, here, no ferromagnetism could be detected for VSe2 single crystal and laminate from room temperature down to 2 K. Neither did we find the Curie paramagnetism expected due to the 3d1 odd-electronic configuration of covalent V4+ ions. Rather, intrinsic VSe2 is a non-magnetic alloy without local moment. Only a weak paramagnetic contribution introduced by defects is noticeable below 50 K. A weak localization effect due to defects was also observed in VSe2 single crystals for the first time.

Magnetic doping of semiconductors has been actively pursued because of their potential applications in the spintronic devices. Central to these efforts is a drive to control the mutual interactions between their magnetic properties (supported by d electrons of the magnetic ions) and their semiconductor properties (supported by s and/or p electrons) at room temperature (RT). Despite the long, intensive efforts, the experimental evidence of thermally robust s, p–d coupling in a semiconductor remains scarce and controversial. Here, we report the enhancement of RT ferromagnetic s, p–d exchange interaction by means of carrier doping in single crystalline Co0.4Zn0.6O epitaxial films with a high Co concentration. Magneto-transport measurements reveal that spin-polarized conducting carriers are produced at RT and are increased with the carrier density through Ga3+ doping, owing to the s, p–d coupling between Ga (4s), O (2p), and Co (3d) orbitals. With the ability to individually control carrier density and magnetic doping, single crystalline Ga(Co, Zn)O films can lay a solid foundation for the development of practical semiconductor spintronic devices operable at RT.

Spin pumping in yttrium-iron-garnet (YIG)/nonmagnetic-metal (NM) layer systems under ferromagnetic resonance (FMR) conditions is a popular method of generating spin current in the NM layer. A good understanding of the spin current source is essential in extracting spin Hall angle of the NM and in potential spintronics applications. It is widely believed that spin current is pumped from precessing YIG magnetization into NM layer. Here, by combining microwave absorption and DC-voltage measurements on thin YIG/Pt and YIG/NM1/NM2 (NM1 = Cu or Al, NM2 = Pt or Ta), we unambiguously showed that spin current in NM, instead of from the precessing YIG magnetization, came from the magnetized NM surface (in contact with thin YIG), either due to the magnetic proximity effect (MPE) or from the inevitable diffused Fe ions from YIG to NM. This conclusion is reached through analyzing the FMR microwave absorption peaks with the DC-voltage peak from the inverse spin Hall effect (ISHE). The voltage signal is attributed to the magnetized NM surface, hardly observed in the conventional FMR experiments, and was greatly amplified when the electrical detection circuit was switched on.