Nano-oxide Layer Research Articles

Influence of the insertion of a nano-oxide layer on the interfacial magnetism of FeMn∕NiFe∕Cu∕NiFe spin valves

We have studied the influence of an inserted nano-oxide layer (NOL) on the interfacial magnetism in spin-valve systems showing the giant magnetoresistance effect. Specifically, we performed a magnetic depth profile of these structures with and without a NOL, using the x-ray magnetic circular dichroism technique. We found that insertion of a NOL into the spin-valve structure is correlated with a stronger reduction of the magnetic moments at the ferromagnetic (FM)∕NOL∕FM interface in comparison with a spin valve without NOL.

Influence of the domain structure of nano-oxide layers on the transport properties of specular spin valves

Specular spin valves show enhanced giant magnetoresistive ratio when compared to other simpler, spin valve structures as a result of specular reflection in nano-oxide layers (NOLs) formed by the partial oxidation of the CoFe pinned and free layers. The oxides forming the NOL were recently shown to order antiferromagnetically below T∼175K. Here we study the training effect in MnIr∕CoFe∕NOL∕CoFe∕Cu∕CoFe∕NOL specular spin valves at low temperatures (15K). We observed that the training effect is related to the nano-oxide layer antiferromagnet ordering and to the evolution of the corresponding domain structure with the number of cycles performed. This allowed us to study the influence of the NOL domain structure on the magnetotransport of specular spin valves.

CPP-GMR films with a current-confined-path nano-oxide layer (CCP-NOL)

We investigated the film performance and nanostructure of current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) spin-valve film with a current-confined-path nano-oxide layer (CCP-NOL). By applying ion-assisted oxidation (IAO) for the CCP-NOL formation, we enhanced the MR ratio to 5.4% at a small RA value of 500 mΩ µm2 for conventional Co90Fe10 layers. Furthermore, the use of bcc-Fe50Co50 also increased the MR ratio to 8.2% at a small RA value of 580 mΩ µm2. A modified Valet–Fert model for the CCP-NOL showed that the MR enhancement by the IAO is due to the improvement in resistivity of the CCP, and that by Fe50Co50 is due to a larger spin-dependent interface scattering effect. Analysis by cross-sectional TEM and three-dimensional atom probe confirmed the formation of the CCP-NOL structure. A reliability test for test element devices showed almost no change even under acceleration stress. The CPP-GMR spin-valve film with the CCP-NOL is extendable to future high-density recording heads due to its potential for a higher MR ratio at a small value of RA.

Current-in-Plane Giant Magnetoresistance Sensor Using a Thin Cu Spacer and Dual Nano-Oxide Layers With a ${\rm DR}$ Greater Than $20$ Ohms/sq.

The magnetoresistance (MR) of the current-in-plane spin-valve, which is currently utilized as the readback sensor in the majority of hard disk drives, has reached a maximum MR of DR/R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">min.</sub> ~20% and DR <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sheet</sub> ~4 Omega/sq. A new sensor film stack will be introduced here that utilizes a trilayer (CoFe\Cu\CoFe) where the Cu interlayer is very thin (~10 Aring) to enhance the MR and where the Cu thickness is chosen such that the ferromagnetic Neel coupling and the antiferromagnetic Ruderman-Kittel-Kasuya-Yosida (RKKY) coupling between the CoFe layers partially cancel one another, to maximize the sensitivity. By changing the Cu interlayer thickness, the overall interlayer coupling was adjusted from about -0.05 erg/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> to -0.4 erg/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> while keeping the MR large. Nano-oxide layers (NOLs) are also incorporated below and above the trilayer sensor to enhance the MR. An example of this sensor is NiFeCr 20 Aring/CoFeO 10 Aring/CoFe 15 Aring/Cu 10.5 Aring/CoFe 15 Aring/AlO 30 Aring and will be referred to as CIP-3L. With the combination of the thin Cu spacer, the NOLs and minimal additional layers to shunt the current around the trilayer sensor (no antiferromagnetic material and no pinned layers), as deposited sheet films with an MR of DR/R >25% and DR/sq. >20 Omega/sq. were achieved. This paper shows the optimization of the sensor stack, such as film thicknesses, NOL material, and oxidation process, the adjustability of the interlayer exchange coupling between the CoFe layers and also shows the repeatability of the sensor deposition

Large Magnetoresistance Variation of Pseudo Spin Valves with Different Nano Oxide Layer Position

AbstractWe have investigated the influence of the nano-oxide layer positions on giant magnetoresistance(GMR) of the NiFe(9nm)/Cu(4nm)/Co(5nm) pseudo spin valves. Nano-oxide layer positions had a several effects on the multilayer structure that changes its magnetotransport behavior. GMR ratio varied between 2.8% and 0.15% depending on the nano-oxide layer positions within the stack. The increase of the GMR ratio was accompanied by increase in resistance change, decrease in sheet resistance, and decrease in surface roughness. These significant variations of GMR ratio was explained by the changes on the spin dependent scattering or current shunting effect. Our results showed that appropriate placement of a nano-oxide layer was essential fo optimize magnetoresistance and properties of spin valves.

Formation of CCP-NOL in CPP-GMR spin valve structure for the enhancement of magnetoresistance

For the MR enhancement in current perpendicular to plane-giant magetoresistance spin valve (CPP-GMR SV), a current-confined path—nano-oxide layer (CCP-NOL)—AlO x was formed on the Cu spacer of half SV structure. In order to form effective current-confining paths, an ultra-thin AlO x layer was deposited on a Cu spacer layer by O 2 reactive sputtering of Al with infra-red (IR) heat treatment on the substrate, and that enable to form an island-structured insulating AlO x layer having holes between AlO x islands. By controlling PO 2 and substrate temperature in the NOL deposition, AlO x layer formation without an oxidizing bottom layer could be achieved.

Insertion of a Specular Reflective and Transmissive Nano-Oxide Layer into Giant Magnetoresistance Spin-Valve Structure

We have studied the influence of the insertion of a nano-oxide layer (NOL) into a magnetic GMR spin-valve. It was found that the spin-valve with NOL has a higher GMR ratio than that of the normal spin-valve without NOL. Naturally formed NOL without vacuum break shows a uniform layer, which effectively suppresses the current shunt, resulting in the reduction of the sheet resistance of GMR. The NOL spin-valve also shows a lower interlayer coupling (Hin) than that of the optimal normal spin-valve, which is consistent with AFM measurement showing lower roughness of NOL formed CoFe surface. Based on the advantage of NOL, we succeeded in lowering Hin while maintaining GMR ratio by insertion of NOL inside the CoFe free layer, where the free layer consists of CoFe/NOL/CoFe/NOL/Capping layer.

Insertion of a Specular Reflective and Transmissive Nano-Oxide Layer into Giant Magnetoresistance Spin-Valve Structure

We have studied the influence of the insertion of a nano-oxide layer (NOL) into a magnetic GMR spin-valve. It was found that the spin-valve with NOL has a higher GMR ratio than that of the normal spin-valve without NOL. Naturally formed NOL without vacuum break shows a uniform layer, which effectively suppresses the current shunt, resulting in the reduction of the sheet resistance of GMR. The NOL spin-valve also shows a lower interlayer coupling (Hin) than that of the optimal normal spin-valve, which is consistent with AFM measurement showing lower roughness of NOL formed CoFe surface. Based on the advantage of NOL, we succeeded in lowering Hin while maintaining GMR ratio by insertion of NOL inside the CoFe free layer, where the free layer consists of CoFe/NOL/CoFe/NOL/Capping layer.

Characterization of a Nano-Oxide Layer in a Pseudo Spin Valve by Complex Magneto-Impedance Spectroscopy

The ac properties of a pseudo spin valve (PSV) consisting of NOL top/Co/Cu/Co/Ni80Fe20/NOLbottom with a top and bottom nano-oxide layer (NOL) were studied as functions of NOL and thickness, d, by magneto impedance spectroscopy. The NOLs were formed by natural oxidizing the top and bottom ferromagnetic or Ta layers in situ. The increase in the thickness of NOL resulting in the increment of effective capacitance caused the shift of roll-off frequency from low to high. The frequency dependence behavior of this NOLtop/PSV/NOLbottom system can be modeled by equivalent circuit theory. PSV with a different NOL structure can be checked by such a nondestructive analysis method

Spin-Torque Noise in CPP-GMR Heads With Current Screen Layer

We investigated the head noise in current perpendicular-to-the-plane giant-magnetoresistive (CPP-GMR) heads with a current screen layer (nano oxide layer having a confined current path) to experimentally evaluate spin-torque noise in the head application. Depending on the direction/magnitude of sensing current, a significant increase in head noise was observed. Some features in noise profiles and a correlation between noise and peak asymmetry are qualitatively reasonable from the perspective of the spin-torque theory. This fact indicates that this extraordinary noise was induced by spin-transfer torque. The spin-torque noise was not observed when the heads were well biased with an orthogonal direction for the free and the reference layer, regardless of whether sensing current was positive or negative. This suggests that the spin-torque noise is not a fatal issue in current screen type CPP-GMR heads with small peak asymmetry.

Exchange biased spin polarizer with an embedded nano-oxide layer for a substantially lower switching current density

The authors demonstrate that the spin polarizer in the form of an exchange biased ferromagnetic lead with an embedded nano-oxide layer can greatly enhance the spin transfer torque for the current induced magnetization switching. By applying it in spin valves, the switching current density (4×106A∕cm2) is one order lower and the resistance change (2.78mΩμm2) is three times higher than those gotten by using a simple spin polarizer. This spin torque enhancement is attributed to the exchange bias pinning acting on the polarizer (the fixed layer) with effective support of the nano-oxide layer, which together lead to a much higher current spin polarization.

Controlled Nano-Oxide Layer Coating on Fine Particles with Multiple Optical-Electrical Functions

SiO2 core/TiO2 shell composite particles were synthesized by four different heterogeneous coagulation techniques. The configurations of the composite particles and the microstructure of assembly were characterized by SEM and TEM. The colloidal stability of nano-particles was also computer-simulated by a discrete element method (DEM) in consideration of Brownian motion occurring on the nano-particles. The results indicated that uniform adsorption of the fine particle absolutely needed fine tuning of surface charging conditions and processing steps.

The influence of nano-oxide layer on magnetostriction of sensing layer in bottom spin valves

The magnetostriction coefficient (λs) of ultrathin sputtered polycrystalline as-deposited and annealed Ta∕Ni81Fe19(t)∕Ta films was studied as a function of the thickness. λs and magnetoresistance (MR) of bottom-type spin valves (SVs) with nano-oxide layer (NOL) added in the pinned layer were investigated by using NiFe, Co90Fe10, and CoFe∕NiFe∕CoFe layers as free layer (FL), respectively. λs of SV with NOL increased slightly except that of CoFe FL. NOLs were added at different positions to study the effects of NOL on λs of CoFe FL. All λs of CoFe FL change from negative to positive and its absolute value also increases significantly with CoFeOx related NOL added below. Our λs and surface roughness results indicated that the structure of the film not the roughness dominates λs of ultrathin FL in SVs.

Influence of thermal annealing on the bulk scattering in giant-magnetoresistance spin-valve with nano-oxide layers

Based upon a bulk scattering model, we investigated the variation of giant magnetoresistance (GMR) behavior after thermal annealing at Ta=250°C as a function of the top free layer thickness of a GMR spin valve with nano-oxide layers (NOLs). It was found that the enhancement of GMR ratio after thermal annealing is explained qualitatively in terms of the increase of active GMR region in the free layer and, simultaneously, the increase of intrinsic spin-scattering ratio. These effects are likely due to the improved specular reflection at the well-formed interface of NOL. Furthermore, we developed a modified phenomenological model for sheet conductance change (ΔG) in terms of the top free layer thickness. This modified model was found to be useful in the quantitative analysis of the variation of the active GMR region and the intrinsic spin-scattering properties. The two physical parameters were found to change consistently with the effects of thermal annealing on NOL.

Structural characterization of Co100−xFex nano-oxide layer

For the structural characterization of a Co100−xFex nano-oxide layer (NOL), the exchange bias properties of the Co100−xFex-natural oxidized NOL in the specular spin-valve (SPSV) system were investigated. The exchange bias energy (Jex) increased monotonically with the increasing Fe content for the Co100−xFex-NOL. The enhancement of both the magnetoresistance ratio and the exchange bias field (Hex) was realized by increasing the Fe content in the Co100−xFex-NOL. It should be mentioned that Hex more than 800Oe is obtained by the insertion of Co30Fe70-NOL, even in NOL-SPSV, which is a remarkably higher pinning field than that ever reported on IrMn-SV. This high exchange bias field is considered to be realized by the formation of an Fe-rich fcc phase at the interface of IrMn.

나노 산화층을 사용한 자기터널접합의 특성

The tunneling magnetoresistance (TMR) ratios of magnetic tunnel junctions (MTJs), in general, decrease abruptly above 250 due to Mn interdiffusion from an antiferromagnet IrMn layer to a ferromagnetic CoFe and/or a tunnel barrier. To improve thermal stability, we prepared MTJs with nano-oxide layers. Using a MTJ structure consisting of underlayer CoNbZr 4/bufferlayer CoFe 10/antiferromaget IrMn 7.5/pinned layer CoFe 3/tunnel barrier AlO/freelayer CoFe 3/capping CoNbZr 2 (nm), we placed a nano-oxide layer (NOL) into the underlayer or bufferlayer. Then, the thermal, structural and magneto-electric properties were measured. The TMR ratio, surface flatness, and thermal stability of the MTJs with NOLs were promoted.

Thermopower in specular spin valves

Specular spin valves show enhanced giant magnetoresistance due to electron specular reflection at the ferromagnetic (FM)/nano-oxide layer (NOL) interfaces. The two NOLs are formed by the partial oxidation of the FM free and pinned layers. Here we study the thermopower of specular MnIr/CoFe/NOL/CoFe/Cu/CoFe/NOL spin valves (300–10 K). We show that the oxides making the NOL order antiferromagnetically at T N ≈ 150 K. This magnetic ordering induces a large change in the magneto-thermopower ( Δ S / S under a magnetic field) as T decreases below T N , and also on the temperature derivative of the electrical resistivity. At low temperatures a quadratic dependence is observed in S ( T ) , likely due to excitation of spin waves.

Uniaxial to unidirectional transition of perpendicular interlayer coupling in IrMn∕CoFe∕NiFeO∕CoFe quadrilayers

We studied the interlayer coupling in the quadrilayer consisting of IrMn∕CoFe (bottom layer)/NiFeOx∕CoFe (top layer). An in-plane perpendicular interlayer coupling is observed between CoFe layers at room temperature. An anisotropy transition from uniaxial to unidirectional in a perpendicular direction is observed around Tt=55K. The nano-oxide layer NiFeOx shows no distinguishable ferromagnetic signal in the high-temperature (uniaxial) phase, while a strong signal appeared in the low-temperature (unidirectional) phase. A possible two-component scenario, in which the nano-oxide layer may contain both amorphous short-range antiferromagnetic domains and superparamagneitc clusters, is proposed to explain the phase transition.

Ｃｕ‐ＡｌＯｘ ＮＯＬ（Ｎａｎｏ‐Ｏｘｉｄｅ‐Ｌａｙｅｒ）形成過程の考察

Current-perpendicular-to-plane giant-magnetoresistance spinvalve with Cu-AlOx NOL (nano-oxide layer) spacer has attracted much attention for its relatively high RA and high MR ratio, which are due to the current-confined metal path in the oxide insulator. To clarify the mechanism for the formation of an NOL, we studied the surface structure of an inhomogeneous AlOx layer on Cu by in-situ conductive-AFM observations in various oxidation conditions. The current intensity images obtained show that different Cu metal paths appear at a Cu surface surrounded by AlOx grains, where the AlOx structure of each NOL is different. The AlOx structure, i. e., the NOL structure, can be controlled by pre-annealing of the Al layer before oxidation. It was found that a thin Al layer on Cu (111) was changed remarkably to fine particles by annealing. The fine particles moved from the Cu (111) terrace to the terrace edge. Therefore, the Al movement from a terrace, where a metal path appears, is caused by a decrease in the Cu surface energy.

TEM Investigation of the Role of a Nano-oxide Layer in Aluminum-Induced Crystallization of a-Si:H

A transmission electron microscopy (TEM) investigation was performed on poly-Si films produced by aluminum-induced crystallization of hydrogenated amorphous silicon (a-Si:H) in the presence of a 2 nm SiO 2 layer. The poly-Si films were fabricated on glass substrates. The study showed that introducing a 2 nm SiO 2 layer produces poly-Si films of large (1-20 μm) grains. The annealing temperatures and annealing times were kept below 450°C and 20 min, respectively. The grain sizes were verified by TEM imaging. Electron diffraction patterns of the grains revealed that they are of good quality and that the grains have no Al layer beneath them.