Antiferromagnetic NiO Layer Research Articles

High-Efficiency Magnon-Mediated Magnetization Switching in All-Oxide Heterostructures with Perpendicular Magnetic Anisotropy.

The search for efficient approaches to realize local switching of magnetic moments in spintronic devices has attracted extensive attention. One of the most promising approaches is the electrical manipulation of magnetization through electron-mediated spin torque. However, the Joule heat generated via electron motion unavoidably causes substantial energy dissipation and potential damage to spintronic devices. Here, all-oxide heterostructures of SrRuO3 /NiO/SrIrO3 are epitaxially grown on SrTiO3 single-crystal substrates following the order of the ferromagnetic transition metal oxide SrRuO3 with perpendicular magnetic anisotropy, insulating and antiferromagnetic NiO, and metallic transition metal oxide SrIrO3 with strong spin-orbit coupling. It is demonstrated that instead of the electron spin torques, the magnon torques present in the antiferromagnetic NiO layer can directly manipulate the perpendicular magnetization of the ferromagnetic layer. This magnon mechanism may significantly reduce the electron motion-related energy dissipation from electron-mediated spin currents. Interestingly, the threshold current density to generate a sufficient magnon current to manipulate the magnetization is one order of magnitude smaller than that in conventional metallic systems. These findings suggest a route for developing highly efficient all-oxide spintronic devices operated by magnon current.

Origin of Perpendicular Magnetic Anisotropy Enhancement in Co/Ni Bilayer Due to Plasma Oxidation

In spintronics and magnonics, materials that offer tunable perpendicular magnetic anisotropy (PMA) along with low Gilbert damping and high spin polarization are particularly important. Therefore, a lot of studies are performed on Co/Ni films, that aim at satisfying all these requirements. Moreover, the Dzyaloshinskii−Moriya interaction is induced in them by surrounding the magnetic layers with heavy metal or oxide layers. For this reason, the study of the oxidation of Co/Ni is of particular interest. Therefore, the magnetic properties of Co/Ni bilayers after plasma oxidation (PO) are investigated. It is shown that the magnetic anisotropy of these bilayers can be tuned, not only by the thicknesses of Co and Ni layers, but also by oxidation time varied in the range between 15 and 220 s. After PO, the effective thickness of ferromagnetic Ni is reduced, but it does not oxidize more than ≈2 nm, even for longer oxidation. However, the contribution to PMA increases for the entire range of oxidation times. This indicates that the thickness reduction of the Ni layer is not the only source of PMA enhancement. This additional contribution is attributed to the exchange bias coupling between the ferromagnetic (Co/Ni) and the antiferromagnetic NiO layers.

Magnetoionic control of perpendicular exchange bias

Voltage control of magnetism via ionic motion (magnetoionic control) is currently a thriving approach for low-power magnetic nanodevices. Recently, fast switching and high reversibility have been achieved for proton (H+)-based magnetoionic control in all-solid devices. In this work, the authors extend such (H+)-based magnetoionic control to a perpendicular exchange bias (EB) systems by functionalizing the ferromagnetic Co layer with an underlying antiferromagnetic NiO layer. They reveal the importance of an interfacial Pd layer for the existence of perpendicular EB in this system. Importantly, they demonstrate reversible (H+)-based magnetoionic control of remanence and coercivity in such a perpendicular EB system and thoroughly discuss the underlying mechanism.

Electrical Detection of DC Spin Current Propagation Through an Epitaxial Antiferromagnetic NiO Layer

A method for detecting dc spin current propagation through an epitaxial antiferromagnetic (AFM) NiO layer is presented. Spin current is generated by spin pumping from an adjoining ferromagnetic (FM) layer and detected in a non-magnetic metallic layer by the inverse spin Hall effect. Comparison is made with a YIG/Pt bilayer, where only the Pt layer is electrically conducting, but for which spin Hall magnetoresistance makes an additional contribution to the measured signal. The signal obtained from the multilayered stack containing the AFM NiO layer is found to contain additional contributions due to anisotropic magnetoresistance. By exciting the sample with out-of-plane rf magnetic field and making measurements with a static field applied at different orientations within the plane of the sample, a signal associated with the dc spin current may be identified.

Current-Induced Magnetization Switching of Exchange-Biased NiO Heterostructures Characterized by Spin-Orbit Torque

In this work, we study magnetization switching induced by spin-orbit torque in W(Pt)/Co/NiO heterostructures with variable thickness of heavy-metal layers W and Pt, perpendicularly magnetized Co layer and an antiferromagnetic NiO layer. Using current-driven switching, magnetoresistance and anomalous Hall effect measurements, perpendicular and in-plane exchange bias field were determined. Several Hall-bar devices possessing in-plane exchange bias from both systems were selected and analyzed in relation to our analytical switching model of critical current density as a function of Pt and W thickness, resulting in estimation of effective spin Hall angle and perpendicular effective magnetic anisotropy. We demonstrate in both the Pt/Co/NiO and the W/Co/NiO systems the deterministic Co magnetization switching without external magnetic field which was replaced by in-plane exchange bias field. Moreover, we show that due to a higher effective spin Hall angle in W than in Pt-systems the relative difference between the resistance states in the magnetization current switching to difference between the resistance states in magnetic field switching determined by anomalous Hall effect ($\Delta R/\Delta R_{\text{AHE}}$) is about twice higher in W than Pt, while critical switching current density in W is one order lower than in Pt-devices. The current switching stability and training process is discussed in detail.

Interface exchange coupling induced fourfold symmetry planar Hall effect in Fe3O4/NiO bilayers

An unexpected fourfold symmetry planar Hall effect was observed in Fe3O4/NiO bilayers. As the thickness of the antiferromagnetic layer exceeds 37 nm, the planar Hall effect of the bilayer further shifts to twofold symmetry, which is ascribed to the dying interfacial coupled effect with increasing antiferromagnetic NiO layer thickness. According to the fitting based on the Stoner–Wohlfarth model, it was notable that an extra cubic anisotropic field in the bilayer structure was obviously amplified by attenuating the thickness of the antiferromagnetic layer. First principle calculations reveal that the amplified cubic anisotropic field was ascribed to the synergistic effect from interfacial bonding structure and charge transfer.

Imprinting antivortex states from ferromagnetic Fe into antiferromagnetic NiO in epitaxial NiO/Fe/Ag(001) microstructures

Magnetic vortex and antivortex are the two basic topological states in magnetic systems. While the ferromagnetic (FM) vortex state can be formed spontaneously and be imprinted into an antiferromagnetic (AFM) layer in AFM/FM disks, the antivortex state has never been realized in AFM films. By fabricating single crystalline NiO/Fe/Ag(001) microstructures, we show that the magnetic antivortex state can be created in the Fe microstructures and imprinted into the AFM NiO layer.

Interlayer exchange coupling of antiferromagnetic NiO layers through Cu spacer

The interlayer exchange coupling (IEC) of two antiferromagnetic (AF) NiO layers separated by the nonmagnetic (NM) spacer layer is observed by investigating exchange bias effect in the structure of ferromagnetic (FM)/AF/NM/AF. The IEC shows an oscillatory behavior with a period of about 1.1nm as the spacer layer thickness increases. The oscillation amplitude and phase also depend on the NiO layer thickness. A qualitative explanation of the experimental results is provided by the RKKY-like IEC between the AF uncompensated spins at the AF/NM interfaces rather than AF itself.

Ferromagnetic resonance study of Permalloy/Cu/Co/NiO spin valve system

Ferromagnetic resonance, in 7 to 18 GHz frequency range, is used to investigate a series of rf sputtered Permalloy (Py)/Cu/Co trilayers and Py/Cu/Co/NiO spin valve system. The experimental data, frequency versus applied magnetic field, agree quit well with the theoretical model. The magnetic coupling between Py and Co is found to be ferromagnetic with effective magnetic coupling parameter values increasing from 0.05 to 0.1 erg/cm2 as the interlayer Cu thickness decreases from 10 to 2 nm. While the magnetic coupling leads to a decrease in the resonance fields, HR, of the modes, the exchange anisotropy at the Co/NiO interface shifts the mode upwards. However the shift is more important for the optical mode than for the acoustic one. Hysteresis curves, qualitatively, confirm the ferromagnetic coupling. The antiferromagnetic NiO layer leads to a slight increase in the coercive field.

Self-Formed Exchange Bias of Switchable Conducting Filaments in NiO Resistive Random Access Memory Capacitors

We report on the ferromagnetism of conducting filaments formed in a NiO thin film, which exhibited a typical bistable resistive switching characteristic. The NiO thin film showed an antiferromagnetic hysteresis loop for a high resistive state (R(OFF)). However, for a low resistive state (R(ON)), the conducting filaments exhibited a ferromagnetic hysteresis loop for the field cooling. The ferromagnetic hysteresis behavior of the R(ON) state reveals switchable exchange coupling between the ferromagnetic Ni conducting filaments and the antiferromagnetic NiO layer.

Dependence of exchange coupling on NiO grain size in NiO/NiFe bilayers

The texture and grain-size effects on the exchange bias in sputtered polycrystalline NiO/NiFe bilayers were studied. Two oriented antiferromagnetic NiO layers along (111) and (200) planes were fabricated on SiO2/Si(100) substrates by varying the Ar/O2 ratio. An exchange anisotropy field Hex was detected in both NiO/NiFe bilayers with a NiO(111) plane where Ni moments are in ferromagnetic (FM) order, and with a NiO(200) plane where Ni moments are in fully compensated antiferromagnetic (AF) order. In order to clarify the presence of the Hex in a NiO(200)/NiFe bilayer, we prepared NiO(200) layers with different grain sizes by controlling a total pressure at a constant Ar/O2 ratio in a sputter chamber. We observed that the Hex of the bilayer films with small grains of NiO(200) is larger than the Hex with large grains. This observation is consistent with a model that the exchange interaction is caused by the reorientation of the moments with AF layer spins rotating, rather than FM layer spins rotating at the interface of the bilayer.

Optimization of planar Hall resistance using biaxial currents in a NiO/NiFe bilayer: Enhancement of magnetic field sensitivity

We present the optimized planar Hall resistance (PHR) obtained by using biaxial currents in a NiO (30 nm)/NiFe (30 nm) bilayers. The measured PHR, Rxy, had a drift resistance due to the intrinsic and extrinsic characteristics caused by magnetization and sample geometry, respectively. The drift voltage due to drift resistance restricted the PHR ratio and could be compensated for by using the auxiliary current Ix for the sensing current Iy to enhance PHR ratio. A huge PHR ratio over 3000% (±1500%) with the linearity and small hysteresis for the magnetic field experimentally obtained using biaxial currents and could be explained by the anisotropic characteristic of the magnetoresistance, which is influenced by the exchange coupling field (Hex) induced by the antiferromagnetic NiO layer.

New memory effect in ferro/antiferromagnetic multilayers

Previous magnetization and ferromagnetic resonance measurements showed that when coupling was observed between ferromagnetic layers separated by an antiferromagnetic NiO layer, it was always ferromagnetic. Here we report magnetization measurements that explain this result. We find that the coupling is only strongly ferromagnetic when the sample is cooled with the two ferromagnetic layer magnetizations parallel. Cooling with the magnetizations antiparallel causes the coupling to become nearly zero. Thus, the coupling retains a memory of the relative orientation of the ferromagnetic layer magnetizations during cooling.

Current aging effect on anisotropic magnetoresistance in NiO spin valves

The current aging on the profiles of anisotropic magnetoresistance (MR) in parallel and perpendicular directions to magnetic easy axis was observed for NiO spin valves by the application of DC current over the critical valve. The critical current to cause the aging effect on the MR profiles showed a linear dependence on the exchange coupling field, suggesting that the effects is related to the disappearance of anisotropic exchange coupling between the pinned NiFe and antiferromagnetic NiO layer due to current induced magnetic field.

Enhanced giant magnetoresistance in spin-valves sandwiched between insulating NiO.

We have investigated the giant magnetoresistance of artificial structures containing metallic Co/Cu/Co and ${\mathrm{Ni}}_{80}$${\mathrm{Fe}}_{20}$/Cu/${\mathrm{Ni}}_{80}$${\mathrm{Fe}}_{20}$ spin-valves confined within insulating antiferromagnetic NiO layers. The observed enhanced magnetoresistance as compared to conventional all-metal spin-valves is interpreted with a semiclassical approach in which specular reflections at the metal/insulator barrier are included.

Spin-valve heads utilizing antiferromagnetic NiO layers

Spin-valve heads utilizing antiferromagnetic NiO layers as the pinning layers were investigated. Advantages of using NiO are (1) superior corrosion resistance, (2) relatively high blocking temperature and (3) reduction in the heat generation due to current shunting. Spin-valve films of a structure of NiO/NiFe/Cu/NiFe show large /spl Delta//spl rho///spl rho/ (approximately 4%) and good sensitivity. Thin Co layers inserted between NiFe and Cu improve both /spl Delta//spl rho///spl rho/ and the thermal stability. To explore the feasibility of spin-valves with NiO, unshielded sensors with hard bias structure having longitudinally magnetized permanent magnets were fabricated. Irregular response in the transfer curves can be suppressed when the permanent magnet strength M/sub r/t is 0.5 memu/cm/sup 2/ or more. The linear response region of the spin-valve sensor was optimized by adjusting the thickness of the pinned layers and the sensor height. Finally, by incorporating these results a shielded spin-valve head with a track width of 2.4 /spl mu/m, a gap length of 0.3 /spl mu/m and a sensor height of 0.7 /spl mu/m was fabricated The response was noise-free and the output obtained with a small sense current of 0.41 mA was approximately 1 mVp-p, demonstrating a high sensitivity of a spin-valve head utilizing NiO.

Implications of Interfacial Coupling and Strain on the Magnetic and Structural Ordering of Fe3O4/NiO Superlattices

AbstractHere we review recent work on the preparation and characterization of magnetically ordered oxide Fe3O4/NiO superlattices. The materials were prepared by oxygen plasma-assisted molecular beam epitaxy. Their structural ordering was studied by x-ray, neutron, and RHEED electron diffraction techniques, and the superlattices are found to form as highly coherent strained-layer modulated single crystals. The magnetic ordering studies, using SQUID magnetometry, ferromagnetic resonance, and neutron diffraction, indicated strong interfacial coupling between the ferrimagnetic Fe3O4 layers and the antiferromagnetic NiO layers, with the magnetic ordering in each layer altered by the proximity to the magnetic moments in the adjacent layer. Strain and other layer-thickness effects are also evident in these magnetic layered structures. The special influence of interlayer coupling and strain on the Fe3O4 Verwey transition are discussed.