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Half-metallic NiCo2O4 with high spin polarizability has great potential applications in spintronics. The manipulation of magnetic anisotropy is crucial for spintronics based on spin-transfer or spin–orbit torques, as it is directly related to the critical switching current density. Here, we report epitaxial growth of metallic NiCo2O4 film with perpendicular magnetic anisotropy on MgAl2O4 single crystal substrates. The modulation of the magnetic anisotropy was achieved by changing the growth conditions (deposition temperature and thickness) of NiCo2O4 films and by means of protonation. Strong dependence of magnetic coercivity on deposition temperature (350–500 °C) has been observed due to variable phase configuration. Furthermore, the magnetic coercive field can also be effectively controlled by the film thickness (3–78 nm) through strain relaxation. More importantly, spin reorientation transition has been achieved by proton and electron doping in the NiCo2O4 films, resulting in reconfigured valence states of Ni and Co cations and a magnetic easy axis rotation from out-of-plane to in-plane. The effective modulation of the magnetic anisotropy provides important insights into the functional design of NiCo2O4-based spintronics with ultralow energy dissipation. [ABSTRACT FROM AUTHOR]

Unidirectional magnetic anisotropy induced by exchange bias (EB) between ferromagnetic and antiferromagnetic layers is of paramount importance in various spintronic devices. Traditional setting and optimization of EB involve cooling through the Néel temperature of the antiferromagnets under biasing field in vacuum furnace which takes hours of time and cannot realize localized control of EB. Herein, an alternative process to configure EB by injection of microsecond current impulse for locally heating of the heterostructure is reported. 180‐degree exchange bias switching has been realized in both anisotropic magnetoresistance and giant magnetoresistance units, resulting in tunable magnetic field sensing functionalities. This mechanism for pinpoint control of EB provides more freedom for the design and optimization of various sensing and logic spintronics. [ABSTRACT FROM AUTHOR]

Highlights: The (001)-oriented ferromagnetic La0.67Sr0.33MnO3 films are stuck onto the (011)-oriented ferroelectric single-crystal 0.7Pb(Mg1/3Nb2/3)O3–0.3PbTiO3 substrate with 0° and 45° twist angle. By applying a 7.2 kV cm−1 electric field, the coexistence of uniaxial and fourfold in-plane magnetic anisotropy is observed in 45° Sample, while a typical uniaxial anisotropy is found in 0° Sample. Manipulating strain mode and degree that can be applied to epitaxial complex oxide thin films have been a cornerstone of strain engineering. In recent years, lift-off and transfer technology of the epitaxial oxide thin films have been developed that enabled the integration of heterostructures without the limitation of material types and crystal orientations. Moreover, twisted integration would provide a more interesting strategy in artificial magnetoelectric heterostructures. A specific twist angle between the ferroelectric and ferromagnetic oxide layers corresponds to the distinct strain regulation modes in the magnetoelectric coupling process, which could provide some insight in to the physical phenomena. In this work, the La0.67Sr0.33MnO3 (001)/0.7Pb(Mg1/3Nb2/3)O3–0.3PbTiO3 (011) (LSMO/PMN-PT) heterostructures with 45º and 0º twist angles were assembled via water-etching and transfer process. The transferred LSMO films exhibit a fourfold magnetic anisotropy with easy axis along LSMO < 110 >. A coexistence of uniaxial and fourfold magnetic anisotropy with LSMO [110] easy axis is observed for the 45° Sample by applying a 7.2 kV cm−1 electrical field, significantly different from a uniaxial anisotropy with LSMO [100] easy axis for the 0° Sample. The fitting of the ferromagnetic resonance field reveals that the strain coupling generated by the 45° twist angle causes different lattice distortion of LSMO, thereby enhancing both the fourfold and uniaxial anisotropy. This work confirms the twisting degrees of freedom for magnetoelectric coupling and opens opportunities for fabricating artificial magnetoelectric heterostructures. [ABSTRACT FROM AUTHOR]

Measurement of 3D vector magnetic field is of vital importance for the development of magnetic navigation, biomedical diagnosis, and microimaging. Traditional 3D magnetic sensors require cooperation of multiple sensors on three orthogonal planes, resulting in disadvantages of bulky size and low spatial resolution. Recently proposed spin orbit torque sensor based on ferromagnetic/heavy‐metal heterostructures can detect three magnetic field components individually due to the different symmetries of current‐polarity‐dependent magnetization dynamic. However, the large driving current density and complex driving procedure hinder their practical application, especially in AC magnetic field detection. Herein, 3D magnetic sensors with dramatically reduced driving current density are reported, one fifth of the original value, by exquisite engineering of the magnetic anisotropy in Pt/Co/Ta heterostructures. With further reduced perpendicular magnetic anisotropy, the sensor in the easy‐cone state demonstrates a record‐high sensitivity of 31196 V A−1 T−1. More importantly, the easy‐cone state sensor can work with an ultralow driving current density of 3.8 kA cm−2, which is three orders lower than previous results. Although easy‐cone state sensor can only measure the z‐axis field, highly compact 3D magnetic sensor can be realized by adoption of two anisotropic magnetoresistance sensors, promising great potential application in space‐ and energy‐restricted scenarios. [ABSTRACT FROM AUTHOR]

Zn/Al co-doped nickel ferrites (NZAFO) with ultralow magnetic damping and large magnetostriction are critically needed for use in tunable spintronic devices and dissipationless quantum computation. However, the growth of epitaxial NZAFO thin films with optimized properties is difficult due to their tendency to form anti-site defects and antiphase boundaries, even on isostructural spinel substrate. Here, growth temperature-dependent ferromagnetic resonance (FMR) and magnetization have been systematically investigated. The saturate magnetization and damping linewidth have decreased from 114 to 70 emu/cc and 321 to 12 Oe as deposition temperature increased from 500 °C to 650 °C, respectively. With further optimization of the film thickness, a minimum FMR linewidth of 7.5 Oe has been observed, only 1.74% of that of bulk form NZAFO materials. Further increasing of deposition temperature to 700 °C results in a dramatic increase in the magnetic coercive field and damping linewidth. The magnetostriction and magnetocrystalline anisotropy are relatively stable in a large temperature range. The strong dependence between magnetic properties and the deposition temperature has been explained in terms of cation redistribution and evaporation. The insulating ferrimagnetic NZAFO with ultralow microwave loss and strain tunable static and dynamic magnetic properties serve as a promising candidate to be used in various tunable electronic devices. [ABSTRACT FROM AUTHOR]

Voltage-tunable RF/microwave devices attract great attention due to their low power consumption; however, they still suffer from the fundamental dilemma between large tunability and low insertion loss. In this paper, yttrium iron garnet (YIG) polycrystalline thin films with low microwave loss (<3 × 10−4) were directly deposited on mica substrates via a van der Waals force and then transferred onto PMN-PT substrates. The in-plane magnetic anisotropy of YIG was tuned by the in-plane biaxial stress of PMN-PT, and an improved voltage tunability (175 MHz) of in-plane FMR fr was obtained due to the removal of the substrate clamping effect. Moreover, our samples still represent good frequency stability (4–12 GHz) during the tuning process. It is of significance for exploiting novel, portable, low-loss RF/microwave devices in the future. [ABSTRACT FROM AUTHOR]

Wireless strain sensors have received extensive attention owing to their wide application prospects in structural health monitoring, industrial automation, human activity monitoring, and intelligent robotic systems. Here, a wireless strain sensor prototype based on the magnetoelectric heterostructure of ferromagnetic thin films on a piezoelectric substrate has been developed. The ferromagnetic resonance (FMR) frequency of the sensor is strongly dependent on external strain due to the large magnetostriction of the film. The piezoelectric substrate with a programmable voltage has been used as a strain source for the characterization of the wireless strain sensor. The limit of detection of the wireless strain sensor is 0.54 με, which is comparable with that of commercial metal-foil sensors that need connection wires. More importantly, the FMR strain sensor shows a sensitivity of 65.46 ppm/με, indicating more than a 60 fold improvement than that of traditional wireless strain sensors based on patch antenna and RLC resonators whose frequency shift is mainly due to the strain induced dimension change. [ABSTRACT FROM AUTHOR]

The magnetic field sensors based on anisotropic magnetoresistance (AMR) effect have been widely used in data storage, navigation, and medical diagnosis. However, the AMR effect of metal materials is relatively weak with an AMR ratio below 2%, which results in low voltage output. In order to improve the sensitivity of weak magnetic fields, we optimize the structure of the AMR sensor with a specific photolithographic process. We use two different designs of Hall bar and Wheatstone bridge with similar barber pole structures, and investigate the angular dependence as well as magnetic field dependence of AMR ratio and voltage output. With Wheatstone bridge, the magnetoresistance leads to a voltage output without dc components. The NiFe magnetic layer and the Au electrode are patterned into a highly conductive barber poles structure, and a high voltage output ratio of about 80% is obtained. At the same time, we achieve a high sensitivity of about 4.3 Oe−1, which implies potential applications in AMR effect-based magnetic field sensors. [ABSTRACT FROM AUTHOR]

An orthogonal biaxial magnetic anisotropy configuration was built in the FeCoAlO/PZN–PT multiferroic composite using two perpendicular uniaxial magnetic anisotropies (UMAs). One is stable and induced by composition gradient, and the other is electric field (E-field) dependent resulting from the magnetoelectric coupling between FeCoAlO ferromagnetic film and the PZN–PT ferroelectric substrate. As a result, the easy axis of a FeCoAlO soft magnetic film can be rotated nearly 90° by the E-field due to the competition between the two UMAs. The magnitude and direction of the ferromagnetic resonance frequency can be manipulated by E-field, and very high resonance frequency up to 9 GHz is achieved in this orthogonal biaxial multiferroic composite. [ABSTRACT FROM AUTHOR]

Magnonics or spin waves have the potential to serve as the carrier for future information communication. A controllable spin wave resonance (SWR) device is demonstrated in a Au/[DEME]+[TFSI]−/LSMO/STO capacitor heterostructure, which could be regulated by ionic liquid gating (ILG) method. The SWR critical angle φC, excitation position to perform uniform precession, is shifted in a reversible manner (thus recording "off" and "on") with +1.5 V gating voltage (Vg), measured by quantitative angular dependent electron spin resonance (ESR) spectroscopy. Based on the modified Puszkarski's surface inhomogeneity model, the ILG control SWR at low Vg (Vg < 1.5 V) can be explained by a charge‐doping‐induced effective surface magnetic anisotropy change. Applying a higher Vg (Vg > 1.5 V) enhances the surface mode SWR and gradually diminishes the body mode SWR. Oxygen vacancies generate at higher Vg (Vg > 1.5 V) resulting in the modulation of superexchange between the Mn ions, evidenced by X‐ray photoelectron spectroscopy and secondary ion mass spectroscopy characterization. This ILG control SWR presents a solution for energy efficient and low‐voltage control of magnonics and spin wave devices. [ABSTRACT FROM AUTHOR]