Top CoFe Research Articles

Epitaxial growth of CoFe/Ge stacked structures on a perpendicularly magnetized MnGa alloy

Abstract For high density and low-power-consumption MRAM applications, the combination of a low resistive tunnel barrier and perpendicular magnetized magnetic layers is required. Here, we experimentally explore the growth of an all-epitaxial spin-valve structure with a perpendicularly magnetized MnGa alloy and semiconductor Ge. Using magnetron sputtering, solid phase epitaxy (SPE), and molecular beam epitaxy (MBE) methods, we stack the MnGa, Ge, and ferromagnetic CoFe layers, respectively. Although an unintentional Mn-based oxide layer is formed between MnGa and Ge, the Ge thin layer is able to be epitaxially grown even on the MnGa alloy. From the magnetometry, we find that the top CoFe and the bottom MnGa layers are magnetically decoupled and spin-valve like magnetization reversals are seen. This study enables us to fabricate a CoFe/Ge stacked structure on MnGa.

Systematic investigation of the effect of layer thickness on the linear sensing characteristics of asymmetric structured CoFe/Rh/CoFe/Cu/CoFe fully epitaxial CIP-GMR based magnetic sensors

Giant magnetoresistance (GMR) devices known for their widespread application as magnetic field sensors face challenges owing to their nonlinear resistance response with the magnetic field. Here, we have developed CoFe(3 nm)/Rh(tRh nm)/CoFe(tmidCoFe nm)/bcc Cu(1.6 nm)/CoFe(ttopCoFe nm)/MgO(2 nm) based current in plane (CIP)-GMR devices and demonstrated linear MR curves with tunable characteristics. The MR ratio and saturating magnetic field were tunable between 3.9% and 23.5%, and 0.013 T and 1.5 T. The numerical simulation modeling confirmed biquadratic coupling between the bottom and middle CoFe layers through Rh and antiferromagnetic coupling between the middle and top CoFe layers through a Cu spacer, providing the linear MR. The fine-tuning of thicknesses of the middle and top CoFe layers provides supplementary routes for optimizing the sensitivity and operation field range of the CIP-GMR devices to specific magnetic sensor applications in the low and medium magnetic field range. The above results demonstrate that CIP-GMR devices with the asymmetric structure demonstrated in this study are promising for the next generation linear magnetic sensors.

Reduction in magnetic exchange bias in CoFe/FeMn/CoFe trilayers due to reduced pinned uncompensated moments in AFM layer

Abstract The M-H hysteresis curves of field cooled CoFe/FeMn bilayers and CoFe/FeMn/CoFe trilayers were studied to understand the exchange bias phenomena in these systems. The measured data revealed that the values of the exchange bias corresponding to a bottom CoFe layer reduced by about 23.5% with an addition of another CoFe layer at the top of bilayer stack. It was also observed that, while this reduction in exchange bias of a bottom CoFe layer (calculated in %) depends on thicknesses of a top CoFe layer and an antiferromagnetic FeMn layer, it is independent of the thickness of bottom CoFe layer. As the strength of exchange bias depends on the presence of pinned uncompensated moments in an antiferromagnetic layer, our observations indicate that the FeMn layer consists comparatively lower amount of pinned uncompensated moments in trilayers. This reduction in pinned uncompensated moments of FeMn layer in trilayers is then co-related with the domain wall suppression in the FeMn layer in CoFe/FeMn/CoFe trilayers.

Effects of target bias voltage in magnetic tunnel junctions grown by ion beam deposition

Magnetic tunnel junctions (MTJs) with an AlOx barrier were fabricated by a biased target ion beam deposition (BTIBD) sputtering technique using a low energy ion source (0–50 eV) and voltage biased targets. The BTIBD system applies a bias voltage directly onto the desired targets, providing enough sputtering energy and avoiding overspill contamination during film deposition. The successful deposition of AlOx-MTJs demonstrated the capability of the BTIBD to make multilayer structures with good film quality. MTJ thin film surface roughness and intermixing between layers are among the key problems leading to low tunneling magnetoresistance (TMR) performance. Here we study the effects of bias voltage on MTJ properties via the measurements of the Néel coupling field and TMR. We suggest that a lower bias voltage reduces the intermixing that occurs when a top CoFe free layer is deposited on an AlOx barrier, but produces relatively high surface roughness. On the other hand, higher energy deposition enhances both interlayer mixing and surface flattening. Such understanding of the bias voltage effects on film properties could be used to optimize MTJ performance.

Magnetization Switching of Magnetic Submicron Structure Fabricated by Atomic Force Microscope

AbstractMagnetization switching of a CoFe submicron structure was studied. A patterned stripe‐shaped film consisting of CoFe/Cu/Co multilayer was prepared. By fabricating two oxide nanowires on the top CoFe layer by a nano‐oxidation technique using an atomic force microscope (AFM), a rectangular isolated region of CoFe was obtained. The magnetization of the CoFe rectangle was successfully switched by applying a pulsed current of 100 mA with 1‐ms duration through this stripe film. This switching was reproducible and observed with a threshold current of around 90 mA. Copyright © 2008 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

Ruthenium nano-oxide layer in CoFe-Ru-CoFe trilayer system: An x-ray reflectivity study

A grazing incidence x-ray reflectivity technique is used to determine the electron density profile as a function of depth in CoFe-Ru-CoFe and CoFe-Ru nano-oxide layer (NOL)–CoFe trilayers. Four trilayers with ruthenium thicknesses of 8, 8.5, and 9Å and one with Ru 8.5Å NOL, prepared by a dc planetary sputtering system, were investigated. For all samples, the electron density profile (EDP) shows a central peak that is related to the Ru layer. Natural oxidation in all of the samples introduces a graded EDP of the top CoFe layers, which decreases gradually to zero. The large surface resistivity of Ru 8.5Å NOL as compared to Ru 8.5Å is related to the remarkable difference between their EDPs. EDP changes have also been investigated in Ru NOL trilayers after annealing at 280°C. The Ru phase in the EDP was observed to confirm the thermal stability of the spacer layer after annealing.

Effect of Anti-Diffusion Oxide Layer on Enhanced ThermalStability of Magnetic Tunnel Junctions

Magnetic tunnel junctions (MTJs) with one proper oxidized FeOx layerplaced between the Al oxide barrier and the top CoFe pinned layer showlarge tunnelling-magnetoresistance (TMR) signals as high as 39% afteranneal at 380°C. The increased TMR signal may originate from theas-deposited Fe/FeOx (non-magnetic) layers changing toFe+magnetic FeOy layer (some Fe3O4 and mostly other kindof magnetic Fe oxide) after high temperature anneal. The maximum TMRvalue (TMRmax) and the corresponding temperature Tswhere the TMRmax occurs upon annealing are closely associatedwith the oxidation time of the AlOx and FeOx layers, toolong oxidation for the Fe layers is detrimental for the TMR value. Inaddition to the enhanced AlOx barrier quality upon anneal, theimproved thermal stability is also attributed to the Mn diffusionretardation by the presence of the FeOx layer which acts as anantidiffusion layer. For MTJs without the interposed FeOx layer,the TMR signal reduction at 300°C originates from the MnIr/CoFepartially decoupling and CoFe/AlOx interface polarization lossdue to the significant Mn diffusion.

Role of interfacial roughness on bias-dependent magnetoresistance and transport properties in magnetic tunnel junctions

The effects of metal-insulator interfacial roughness, modulated by Ar+ irradiation, on bias dependence of tunnel magnetoresistance (TMR) and electrical transport of CoFe–AlOx–CoFe magnetic tunnel junctions (MTJs) have been studied. Reduction of TMR ratio and asymmetric TMR falloff curves as a function of dc bias have been observed for Ar+-irradiated MTJs. The results are analyzed by x-ray reflectivity together with complex impedance techniques, indicating interfacial roughness which likely results in a proportional rising trap state density (TSD). Increasing TSD for Ar+-irradiated MTJs increases an unpolarized current which decreases TMR ratio. The asymmetric TMR falloff curves are attributed to the different TSDs of bottom and top CoFe–AlOx interfaces in tunneling process.

40% tunneling magnetoresistance after anneal at 380 °C for tunnel junctions with iron–oxide interface layers

Spin tunnel junctions fabricated with one interposed Fe–FeOx layer between the Al2O3 barrier and the top CoFe pinned electrode show large tunneling magnetoresistance (TMR) (40%) for anneals up to 380 °C. The annealing temperature TTMR*, where maximum TMR occurs, increases with the inserted Fe–FeOx layer thickness. For samples with thicker inserted layer, the pinned layer moment (which usually starts to decay below 300 °C in the normal junctions) increases with annealing temperature up to 380 °C and remains at a maximum until 450 °C. The large TMR at high temperature is related with the diffusion of extra Fe (from the Fe–FeOx layer) into the electrode interfacial region and the as-deposited paramagnetic FeOx decomposition into metallic Fe, and possibly the formation of some Fe3O4, which compensate the interface polarization loss associated with Mn interdiffusion. Rutherford backscattering spectrometry analysis confirms partial Fe diffusion into the top CoFe electrode after anneal. Meanwhile, x-ray photoelectron spectra for the Fe 2p core level show that the FeOx contribution in the upper part of the inserted layer decreases upon annealing, while it increases in the inner part near the barrier, suggesting the FeOx decomposition and the oxygen diffusion toward the inner metallic Fe and Al barrier. The study of R×A values and barrier parameters versus annealing temperature for samples with 7 and 25 Å Fe–FeOx also reflects the above structural changes in the inserted layer.

Annealing effect of magnetic tunnel junctions with one FeOx layer inserted at the Al2O3/CoFe interface

Spin tunnel junctions with one interposed Fe oxide layer between the Al2O3 barrier (tAl=8–9 Å) and the top CoFe pinned layer show large tunneling magnetoresistance (TMR) values (39%) after 40 min anneal at 380 °C. The as-deposited TMR is low and does not increase until 350 °C (<10%), but then increases sharply, peaking at 380°C. Further anneals at this temperature (380 °C), lead to TMR decrease to 20% with a diffusion constant of 34 min. At 360 °C, the diffusion constant is about 60 min. Samples without this inserted FeOx layer show TMR<5% after prolonged anneals at 380 °C. The different techniques utilized to probe the barrier and electrode changes during the annealing processes indicate that from the initial Fe–FeOx layer, part of the Fe diffuses into the CoFe electrode, and the remaining FeOx probably decomposes into a pure interfacial Fe layer at high temperature, responsible for the large TMR. The observed TMR values and barrier parameters, seem consistent with a standard CoFe/Al2O3/CoFe barrier formed at high temperature.