Antiferromagnetic Oxide Layer Research Articles

Effect of interfacial oxidation layer in spin pumping experiments on Ni80Fe20/SrIrO3 heterostructures

SrIrO3 with its large spin–orbit coupling and low charge conductivity has emerged as a potential candidate for efficient spin–orbit torque magnetization control in spintronic devices. Here we report on the influence of an interfacial oxide layer on spin pumping experiments in Ni80Fe20 (NiFe)/SrIrO3 bilayer heterostructures. To investigate this scenario, we have carried out broadband ferromagnetic resonance (BBFMR) measurements, which indicate the presence of an interfacial antiferromagnetic oxide layer. We performed in-plane BBFMR experiments at cryogenic temperatures, which allowed us to simultaneously study dynamic spin pumping properties (Gilbert damping) and static magnetic properties (such as the effective magnetization and magnetic anisotropy). The results for NiFe/SrIrO3 bilayer thin films were analyzed and compared to those from a NiFe/NbN/SrIrO3 trilayer reference sample, where a spin-transparent, ultra-thin NbN layer was inserted to prevent the oxidation of NiFe. At low temperatures, we observe substantial differences in the magnetization dynamics parameters of these samples. In particular, the Gilbert damping in the NiFe/SrIrO3 bilayer sample drastically increases below 50K, which can be well explained by enhanced spin fluctuations at the antiferromagnetic ordering temperature of the interfacial oxide layer. Our results emphasize that this interfacial oxide layer plays an important role for the spin current transport across the NiFe/SrIrO3 interface.

Reentrant superconductivity and superconducting critical temperature oscillations in F/S/F trilayers of Cu41Ni59/Nb/Cu41Ni59 grown on cobalt oxide

Ferromagnet/Superconductor/Ferromagnet (F/S/F) trilayers constitute the core of a superconducting spin valve. The switching effect of the spin valve is based on interference phenomena occurring due to the proximity effect at the S/F interfaces. A remarkable effect is only expected if the core structure exhibits strong critical temperature oscillations, or most favorable, reentrant superconductivity, when the thickness of the ferromagnetic layer is increased. The core structure has to be grown on an antiferromagnetic oxide layer (or such layer to be placed on top) to pin by exchange bias the magnetization-orientation of one of the ferromagnetic layers. In the present paper, we demonstrate that this is possible, keeping the superconducting behavior of the core structure undisturbed.

Effect of aging on the magnetic characteristics of nickel nanowires embedded in polycarbonate

Single crystalline Ni nanowires (with a controlled diameter of ∼14 nm) have been fabricated by electrochemical deposition in etched ion-track polycarbonate templates. The exchange bias effect in these nanowires has been studied at 10 K under a cooling field of 10 kOe applied parallel to the wire axis for different time intervals starting from the as-fabricated to ∼2 yr old samples. A continuous drop in the saturation magnetization has been observed with time due the formation of an antiferromagnetic oxide layer at the periphery of nanowires. A model for the drop of magnetization has been used to calculate the thickness of the oxide layer in nickel nanowires. The observed variation in exchange bias and saturation magnetization has been explained by taking into account the exchange interactions at the Ni-NiO interface and the development of antiferromagnetic NiO at the expense of metallic Ni.

Magnetic phase diagram of the spin-valve structure with an antiferromagnetic oxide layer

The magnetic phase diagram of a three-layer ferromagnet-antiferromagnet-ferromagnet structure with an antiferromagnetic oxide layer of variable thickness has been investigated. It has been predicted that there are three ranges of thicknesses in which domain walls created by the frustration of the exchange interaction between the layers have different structures. The structure of these domain walls induced by edges of atomic steps at the interfaces between the layers has been described. The predictions of the theory have been compared with the available experimental results, in particular, with the data indicating the existence of the nanodomain state.

Time-Resolved Spin-Torque Switching and Enhanced Damping in Permalloy/Cu/Permalloy Spin-Valve Nanopillars

We report time-resolved measurements of current-induced reversal of a free magnetic layer in Permalloy/Cu/Permalloy elliptical nanopillars at temperatures T=4.2 K to 160 K. Comparison of the data to Landau-Lifshitz-Gilbert macrospin simulations of the free layer switching yields numerical values for the spin torque and the Gilbert damping parameters as functions of T. The damping is strongly T dependent, which we attribute to the presence of an antiferromagnetic oxide layer around the perimeter of the Permalloy free layer. This adventitious antiferromagnetic oxide can have a major impact on spin-torque phenomena.

An aerosol-mediated magnetic colloid: Study of nickel nanoparticles

A method is presented for the synthesis of high-quality nickel nanoparticles. Laser-driven decomposition of nickel carbonyl vapors is used to produce particles in the form of an aerosol, followed by exposure to a solvent containing an appropriate surfactant to yield a stable dispersion of particles. This method is scalable and yields a substantially monodisperse distribution of particles at a relatively high rate of production. The particles produced by this method are subjected to a detailed characterization using transmission electron microscopy, atomic force microscopy, energy dispersive spectroscopy, and dc magnetization. They have an average diameter of 5 nm, and the observed magnetization curves show no hysteresis above 200 K. The normalized magnetization curves follow a scaling law proportional to the quotient of the applied field over temperature. This data indicates the presence of randomly oriented superparamagnetic particles. The measured magnetization is significantly smaller than that of the bulk, probably due to an effective surface anisotropy and spin canting. The coercivity is the same in either direction of the applied field which indicates that there is negligible exchange coupling between the nickel particles and any possible antiferromagnetic oxide layer on their surfaces.

Exchange biasing of the ferromagnetic semiconductor (Ga,Mn)As by MnO (invited)

We provide an overview of progress on the exchange biasing of a ferromagnetic semiconductor (Ga1−xMnxAs) by proximity to an antiferromagnetic oxide layer (MnO). We present a detailed characterization study of the antiferromagnetic layer using Rutherford backscattering spectrometry, x-ray photoelectron spectroscopy, transmission electron microscopy, and x-ray reflection. In addition, we describe the variation of the exchange and coercive fields with temperature and cooling field for multiple samples.

New Trends in Magnetic Exchange Bias

New Trends in Magnetic Exchange Bias

Shift in the magnetic percolation threshold of phase separated Co-rich CoAg very thin films due to reduced dimensionality

An investigation of the structural and magnetic properties of phase separated Co-rich CoAg very thin films (5 nm–50 nm) as a function of film thickness and Co concentration is presented. In the as-deposited state the films are very fine grained and the coercive field is relatively low (Hc<20 Oe). However, after annealing at 420 °C for half an hour in high vacuum, grain growth is promoted and a drastic change in the magnetic properties is observed. The coercivity now has a strong thickness dependence with a maximum at approximately 15 nm. The largest room temperature value of coercivity, Hc=850 Oe, was found for a CoAg 70:30 volume percent alloy. The concentration of maximum coercivity is usually associated with the percolation threshold, xp, of the ferromagnetic element and is much higher than the ∼50 vol % value normally observed in thick film and bulk granular ferromagnets. Values of xp≲0.65 were found for 30 nm thick films increasing up to xp∼0.75 for 10 nm films. This behavior is explained as a shift in the percolation limit to higher values associated with the reduced dimensionality of very thin films. This shift implies that the volume fraction of the ferromagnetic component should be larger than its bulk counterpart if granular ferromagnets are to be used as thin film media for magnetic recording. The temperature dependence of Hc was also studied and coercivities exceeding 1800 Oe have been found at 50 K. The presence of a thin antiferromagnetic oxide layer is manifested as a shift in the low temperature hysteresis loops due to the anisotropic exchange coupling interaction with ferromagnetic Co.

Magnetic hysteresis and Mössbauer studies in ultrafine iron particles

The magnetic hysteresis behavior of ultrafine Fe particles has been examined by SQUID magnetometry and Mössbauer spectroscopy. Ultrafine Fe particles have been prepared by vapor deposition under argon atmosphere in pressures from 1 to 6 Torr and passivated by exposure to oxygen at low pressure. Particles with size below 10 nm showed a small coercivity (∼100 Oe) at room temperature, which increased drastically upon cooling to cryogenic temperatures (1000–1500 Oe). The larger particles had the highest coercivity at room temperature (∼1000 Oe) which increased by 100% at 10 K. Mössbauer studies revealed that the Fe particles are surrounded by an Fe oxide layer composed of ultrafine Fe3O4 or γ-Fe2O3 grains. At room temperature the oxide component gives a very broad absorption superimposed on an α-Fe spectrum. The enhanced coercivity at room temperature may be attributed to anisotropy induced by exchange interaction between the ferromagnetic core and the ferri- or antiferromagnetic oxide layer.