Nano-oxide Layer Research Articles

Tailored microwave absorption performance through interface evolution by in-situ reduced Fe nanoparticles on the surface of FeSiAl microflake

Poor impedance matching and low-frequency magnetic resonance are great challenge for Fe-based micron-scale materials as microwave absorber in GHz range. Here, we propose in situ interface engineer strategy for tailoring the interface through in situ reduced Fe nanoparticles and introduce planar anisotropy for flattening particles with high aspect ratio during high-energy vertical ball milling process, which can synchronously enhance impedance matching and exceed Snoke’s limit. With increasing milling time, the interface experiences the following in situ evolution: the nano oxide layer-micro particles interface, the partial interface between nano oxide layer and Fe nanoparticles, the integrity Fe nanoparticles-oxide layer interface, the nano oxide layer-microflake interface. Meanwhile, the aspect ratio is up to 74.93. The evolution of interface and high aspect ratio can effectively tailor permittivity and permeability. Due to the novel microstructure that FeSiAl flake are covered by ∼ 30 nm oxide layer containing ∼ 10 nm Fe nanoparticles, natural resonance induced by planar anisotropy, eddy current loss of metallic microflake, relaxation loss induced by interface polarization, conductive loss from electric resistivity together led to good microwave absorption performance (effective absorbing bandwidth of 3.12 GHz at 3.2 mm and minimal reflection loss of −44.47 dB at 7.44 GHz and 3.3 mm). The present finding might highlight for developing the composite absorbers with multi loss type coupling across nano-micro scales.

Nanoscale Thermal Transport Model of Magnetic Tunnel Junction (MTJ) Device for STT-MRAM

Spin-transfer-torque magnetic random access memory (STT-MRAM) based on magnetic tunnel junction (MTJ) device has attracted significant attention from academics and industries. Nevertheless, the thermal effect under bias operation affects the overall performance and stability of a device. To evaluate the thermal phenomenon in MTJ devices accurately, the non-equilibrium effect between the electron and the phonon near the electrode-barrier interface cannot be ignored. The existence of the interface leads to additional thermal resistance hindering the heat conduction of tunnel junction. Therefore, an effective equivalent model concerning the interface effect is necessary to represent the heat transportation of the device. In this article, a nanoscale thermal transport model of the MTJ device is proposed. The influence of the thermal conductivity of the nano-oxide layer on the temperature distribution is discussed. The interface energy balance transport model is used to clarify the non-equilibrium relationship between the phonon and the electron at the CoFeB/MgO/CoFeB interfaces. A parameterization study is conducted to illustrate the temperature distribution in the nanoscale thermal transport model. Furthermore, the phonon-electron coupling distance ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\delta$ </tex-math></inline-formula> ) based on the thermal conductivity of the phonon ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\kappa _{p}$ </tex-math></inline-formula> ) and the electron ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\kappa _{e}$ </tex-math></inline-formula> ) of the device is implemented to construct the effective equivalent model using finite element modeling (FEM), which could realistically reflect the thermal transport under working conditions. The established equivalent model has a guiding role in exploring the thermal transportation in the nanoscale MTJ device.

Evolution of magnetoresistance behaviour at low temperatures in naturally oxidised specular spin valve systems

Abstract The temperature dependent magnetoresistive behaviour of field cooled naturally oxidised specular spin valve systems has been studied in the temperature range of 300–10 K. Inconsistent to the non-specular spin valve system, an anomalous behaviour was evolved with large exchange bias and higher coercivity, below 200 K. The structural investigations inferred the formation of magnetic oxides with higher density gradient in the pinned layer, and the observed anomalous behaviour at low temperatures was correlated with the antiferromagnetic ordering of these oxides in spin glass state. The uncompensated interfacial magnetism of the nano-oxide layer was further confirmed by comparing with low temperature magnetoresistive behaviour of non-magnetic oxide based specular spin valve systems.

Lowering the coefficient of martensite steel by forming a self-lubricating layer in dry sliding wear

In this paper, the dry sliding wear behavior of martensite steel under different friction conditions was studied. The sliding contact surface was found to form a self-lubricating layer composed of nano-oxide particles under specific strain and strain rate conditions. The heat and force field environments of surface oxidation were analyzed from the perspectives of heat transfer and contact mechanics. The protective mechanism of the self-lubricating layer was elaborated based on the 3D topography of the laser confocal results of the worn surface at different times. The essential causes of the changes in the dry coefficient of friction (COF) under different friction conditions (normal load, speed and time) were analyzed from the perspective of the formation and destruction of the nano-oxide self-lubricating layer. This study shows that in the process of dry sliding wear, martensite experienced large plastic deformation in the surface layer, and nanolamellar martensite was formed in the severe plastic deformation area. The self-lubricating layer composed of a large number of nano-oxide particles with a depth of 1 μm was formed at the contact surface due to plastic deformation and frictional heat. The self-lubricating layer reduced the dry COF from 0.5 to 0.28 as well as the surface roughness and wear trace depth.

Electric field effect on the Néel temperature of cobalt oxide formed at an alumina nano-oxide layer

The electric control of surface magnetism is important for spintronic applications. Due to the screening of electric field by conduction electrons in metals, an electric field can be applied only at a surface layer. However, the electric field can be applied on metallic contacts smaller than the electron’s mean free path. We report on the electric modulation of the phase-transition temperature of CoO that is present at Co/Pt nano-contacts through a thin AlOx barrier. We found a 50-K/V change that can be attributed to the injection of electrons through nano-contacts.

TEM and AES investigations of the natural surface nano-oxide layer of an AISI 316L stainless steel microfibre.

The chemical composition, nanostructure and electronic structure of nanosized oxide scales naturally formed on the surface of AISI 316L stainless steel microfibres used for strengthening of composite materials have been characterised using a combination of scanning and transmission electron microscopy with energy-dispersive X-ray, electron energy loss and Auger spectroscopy. The analysis reveals the presence of three sublayers within the total surface oxide scale of 5.0-6.7 nm thick: an outer oxide layer rich in a mixture of FeO.Fe2 O3 , an intermediate layer rich in Cr2 O3 with a mixture of FeO.Fe2 O3 and an inner oxide layer rich in nickel.

An Early Cancer Diagnosis Platform based on Micro-magnetic Sensor Array Demonstrates Ultra-high Sensitivity

In this paper a highly sensitive detection methodology for obtaining the concentration of protein tumor markers via the selective incorporation of super paramagnetic iron oxide nanoparticles (MNP) onto an antigen is reported. The tumor marker concentration is measured with a nano-oxide layer inserted giant magneto-resistive (GMR) sensor that determines the attenuation of an external magnetic field that is induced by the MNP. The 15 nm MNPs, used herein, exhibit a super paramagnetic behavior with saturation magnetization of 36 emu/g. The output voltage signal of the Specular GMR sensor is 500 μV/Oe. The patterned GMR multiple-bio-sensor array, has demonstrated real-time measurements of carcinoembryonic antigen (CEA) protein concentrations down to femtomole level (10-15 mole) in a variety of clinically relevant media with a linear dynamic range of over five orders of magnitude. The sensitivity of the GMR bio-array platform is 1000 times higher than conventional enzyme-linked immunosorbent assay (ELISA) technique. The arrays of magnetoresistive sensors offer great promise in applications for early cancer diagnosis.

The dependence of nano-contact magnetoresistance on the bulk scattering spin asymmetry in CoFe alloys with oxidation impurities.

Nano-contact magnetoresistance (NCMR) spin-valves (SVs) using an AlO x nano-oxide-layer (NOL) have numerous nanocontacts in the thin AlOx oxide layer. The NCMR theoretically depends on the bulk scattering spin asymmetry ([Formula: see text]) of the ferromagnetic material in the nanocontacts. To determine the relationship between NCMR and [Formula: see text], we investigated the dependence of NCMR on the composition of the ferromagnetic material Co1-xFex. The samples were annealed at 270 °C and 380 °C to enhance the MR ratio. For both annealing temperatures, the magnetorsistance ratio in the low-resistance area product region at less than 1 Ω μm2 was maximized for Co0.5Fe0.5. To evaluate [Formula: see text] exactly, we fabricated current-perpendicular-to-plane giant magnetoresistance SVs with Co1-xFex/Cu/Co1-xFex layers and used Valet and Fert's theory to solve the diffusion equation of the spin accumulation for a ferromagnetic layer/non-ferromagnetic layer of five layers with a finite diffusion length. The evaluated [Formula: see text] for Co1-xFex was also maximized for Co0.5Fe0.5. Additionally, to determine the difference between the experimental MR ratio of NCMR SVs and the theoretical MR ratio, we fabricated Co0.5Fe0.5 with oxygen impurities and estimated the decrease in [Formula: see text] with increasing oxygen impurity concentration. Our Co0.5Fe0.5 nano-contacts fabricated using ion-assisted oxidation may contain oxygen impurities, and the oxygen impurities might cause a decrease in [Formula: see text] and the MR ratio.

Formation and anisotropic magnetoresistance of Co/Pt nano-contacts through aluminum oxide barrier

We report on the observation of anisotropic magnetoresistance (AMR) in vertical asymmetric nano-contacts (NCs) made through AlOx nano-oxide layer (NOL) formed by ion-assisted oxidation method in the film stack of Co/AlOx-NOL/Pt. Analysis of NC formation was based on in situ conductive atomic force microscopy and transmission electron microscopy. Depending on the purity of NCs from Al contamination, we observed up to 29% AMR ratio at room temperature.

Positive exchange bias in thin film multilayers produced with nano-oxide layer

We report a positive exchange bias in thin film multilayers produced with nano-oxide layer. The positive exchange bias resulted from an antiferromagnetic interfacial exchange coupling between the ferromagnetic CoFe and the antiferromagnetic CoO layers, which spontaneously forms on top of the nano-oxide layer during the subsequent deposition of a CoFe layer. The shift in the hysteresis loop along the direction of the cooling field and the change in the sign of exchange bias are evidence of antiferromagnetic interfacial exchange coupling. The high temperature positive exchange bias observed for our system results from magnetic proximity effects between CoFe and CoO.

Demonstration of magnetoelectric effect in ultrathin Cr2O3/Fe2O3 nano-oxide layer by training effect

We identified magnetoelectric effect in an ultrathin (Cr2O3/Fe2O3) nano-oxide layer structure (thickness ∼1 nm) formed on the surface of a Co0.9Fe0.1 metal layer by investigating the training effect as a function of sense current/bias voltage and its direction. A system-dependent constant κMR, which reflects changes in the surface spin, increases (decreases) with increasing current in the pinned (inverse pinned) direction. Furthermore, κMR varies for temperatures up to ∼200 K with current despite the extreme thinness, thus suggesting the current contributes to the surface spin changes. The results indicate the demonstration of the magnetoelectric effect in ultrathin Cr2O3.

Modification of Magnetic Nanocontact Structure by a Bias-Voltage-Induced Stress and Its Influence on Magnetoresistance Effect in TaOx Nano-Oxide Layer Spin Valve

A magnetic nanocontact spin valve (NCSV) was fabricated by inserting a TaOx nano-oxide layer (NOL) as the spacer layer. Current-perpendicular-to-film-plane (CPP) measurements revealed that the SV had a positive magnetoresistance (MR) ratio. When a high bias voltage was applied to the SV, the fine structure of the NOL changed i.e., the resistance and MR ratio of the device changed irreversibly. The change in device characteristics is attributed to a proportional change in the number of nonmagnetoresistive and magnetoresistive conductive channels in the SV upon high bias voltage application. The decrease in MR ratio accompanied the disappearance of the magnetic nanocontact, suggesting that the positive MR effect was partially due to the presence of magnetic nanocontacts.

Oxidation and Annealing Process: Morphological Change and Nanocontact MR in Spin Valves With FeCo–AlO$_{x}$ NOL Spacer

We studied the formation mechanism of ferromagnetic nanocontacts (NCs) around 1-2 nm in diameter in thin AlOx nano-oxide layer (NOL) through in situ conductive atomic force microscopy (c-AFM), which is the only method suitable for measuring morphological changes in features this small. Spin valves (SVs) with Fe0.5Co0.5NCs in AlOx NOL fabricated by ion-assisted-oxidation (IAO) irradiated onto 1.3-nm-thick Al had nanocontact magnetoresistance (NCMR) characteristics with the MR ratio of 5% for 1 Ω μm2. According to measurements with c-AFM, these NCs were mostly located at the valleys between the AlOx grains or the edge of the grain. In other words, NCs were formed in the oxidation and cohesion process of Al by the IAO. Moreover, we found that the high-temperature annealing process improved the MR ratio to around 10% for 0.5 Ω μm2. The high-temperature annealing process may involve the AlOx morphological change and have led the Al atoms in NCs to migrate to AlOx by combining with oxygen as impurities. Then, the morphological changes and migration of Al caused NCs to be the low resistivity and the short length.

Suppression of the spin pumping in Pd/Ni81Fe19 bilayers with nano-oxide layer

We demonstrate that the spin pumping effect can be effectively suppressed with a nano-oxide layer. Spin pumping effect manifests itself by an enhancement of the Gilbert damping parameter in normal metal/ferromagnetic hetero-structures, while many spintronics devices prefer smaller damping parameter. Since the spin pumping effect is directly related with the spin dependent interface conductance, we can modify the spin pumping by altering the interface conductance with the nano-oxide layer. We prepared series of Pd/Ni81Fe19 bilayers with different pausing time between Pd and Ni81Fe19 depositions in order to control the interface conductance. The Gilbert damping parameters are determined from the line-width measurements in the ferromagnetic resonance spectra for each pausing time sample. They are 0.0490, 0.0296, 0.0278, and 0.0251 for 0, 6, 30, and 60 s pausing time, respectively. We find that the damping parameter of Pd/Ni81Fe19 is almost recovered to one of the Cu/Ni81Fe19 bilayer with 60 s pausing time, while the static magnetic properties are not noticeably changed.

Effect of the tip–contact interaction on the MFM image of magnetic nanocontact

MFM images of magnetic nanocontacts fabricated in nano-oxide layers are evaluated with special attention to the tip-contact interaction. We show that a sharp ring or arc signal with a diameter on the order of 100 nm appears while the diameter of the contact is on the order of 1 nm. We also show that the position of the sharp signal is determined by the competition between the forces due to the magnetic anisotropy in the contact and due to the tip-contact interaction.

Spin-transfer torque driven ferromagnetic resonance in nano-contact magnetoresistive devices and its characteristics

We investigated the spin-transfer torque driven ferromagnetic resonance in nano-contact magnetoresistive (NCMR) devices with a current-perpendicular-to-plane spin-valve structure consisting of a FeCo layer, a FeCo-AlOx nano-oxide layer, a FeCo multilayer for the reference, a spacer, and free layers. In contrast with tunneling magnetoresistance devices with an AlOx spacer layer that shows a single resonant frequency, NCMR devices showed multi-resonant frequencies caused by free-layer resonance and other types of resonance appeared in a frequency region higher than that of the free-layer. We discuss these characteristic results in terms of the magnetic structure of the NCMR device, especially the free and reference layers around the nano-contacts.

Spin accumulation and mistracking effects on the magnetoresistance of a ferromagnetic nano-contact

We theoretically studied spin accumulation and mistracking effects on the current-perpendicular-to-plane magnetoresistance of a domain wall confined in a nano-contact which is experimentally fabricated as current-confined-path structure made in a nano-oxide layer between ferromagnetic metal layers. We calculated the magnetoresistance (MR) ratio by evaluating the voltage drop due to the spin accumulation and additional resistivity due to mistracking. We showed that both the spin accumulation and mistracking give the same order of magnitude to the MR ratio for the nano-contact with 2 nm diameter.

Effect of nano-oxide layers on giant magnetoresistance in pseudo-spin-valves using Co 2FeAl electrodes

We studied the pseudo-spin-valves (PSVs) with a structure of Ta/Co 2FeAl/NOL 1/Co 2FeAl/Cu/Co 2FeAl/NOL 2/Ta, where NOL represents the nano-oxide layer. Compared with the normal Co 2FeAl (CFA) PSV with a structure of Ta/Co 2FeAl/Cu/Co 2FeAl/Ta, which shows only a current-in-plane (CIP) giant magnetoresistance (GMR) of 0.03%, the CFA PSV with NOLs shows a large CIP-GMR of 5.84%. The enhanced GMR by the NOLs inserted in the CFA PSV is due to the large specular reflection caused by [(CoO)(Fe 2O 3)(Al 2O 3)] in NOL 1 and [(Fe 2O 3)(Al 2O 3)(Ta 2O 5)] in NOL 2. Another reason is that the roughness of the interface between Ta and CFA is improved by the oxidation procedure.

Enhancement of magnetoresistance by hydrogen ion treatment for current-perpendicular-to-plane giant magnetoresistive films with a current-confined-path nano-oxide layer.

We have enhanced magnetoresistance (MR) for current-perpendicular-to-plane giant-magnetoresistive (CPP-GMR) films with a current-confined-path nano-oxide layer (CCP-NOL). In order to realize higher purity in Cu for CCPs, hydrogen ion treatment (HIT) was applied as the CuO(x) reduction process. By applying the HIT process, an MR ratio was increased to 27.4% even in the case of using conventional FeCo magnetic layer, from 13.0% for a reference without the HIT process. Atom probe tomography data confirmed oxygen reduction by the HIT process in the CCP-NOL. The relationship between oxygen counts and MR ratio indicates that further oxygen reduction would realize an MR ratio greater than 50%.

Direct Measurement for Electric Resistance of Ferromagnetic Metal- Nano-Contact in Oxide Layer

Nanocontacts Magneto-Resistive (NCMR) Spin-Valve devices using NOL (Nano-Oxide Layer) have been reported to show some kind of a new MR effect. If the NC resistivity nears to the bulk's value without limit, the NC resistance will be almost dominated by the sharvin resistance at the NC with 1 to 2nm size, which conductance regime dominates the NC resistance of our NCMR attract much interest for understanding MR mechanism of our NCMR, where the NC size is certainly just 1 to 2nm but the NC resistivity is still unknown. So, in this study we constructed the home-made circuit for directly measuring each conductive channel-current through each nano-contact with in-situ conducting AFM and tried to evaluate quantitatively the resistivity of each single NC in the NOL.