CoFe Layer Research Articles

Effects of submonolayer Mg on CoFe–MgO–CoFe magnetic tunnel junctions

CoFe–MgO–CoFe magnetic tunnel junctions (MTJs) were fabricated by means of ion beam sputtering of MgO as well as subsequent thin films on CoFe. We demonstrated that when a submonolayer of Mg was deposited on the CoFe layer prior to the deposition of MgO, profound improvement in the performance of the CoFe–Mg–MgO–CoFe MTJ was achieved compared to that without the Mg layer. ac impedance measurement indicated that the interfacial Mg layer resulted in fivefold increase in the magnetoresistance ratio from 0.76% for MTJs without it to 4% for that with it. Complex-capacitance (CC) spectra of MTJs showed that the contribution in the relaxation frequency by textured MgO increased. This indicated that the submonolayer Mg improved the texture of the subsequently deposited MgO. Bias-dependent complex-impedance (CI) spectra reveal that interfacial defects were also suppressed due to the submonolayer of Mg. Results obtained from x-ray reflectivity and transmission electronic microscopy were also consistent with those of CI and CC.

Annealing Effect on Exchange Bias in NiFe/FeMn/CoFe Trilayer Thin Films

We investigated the exchange bias fields at the NiFe/FeMn and FeMn/CoFe interfaces in 18.9-nm NiFe/15.0-nm FeMn/17.6-nm CoFe trilayer thin films as the annealing temperature was varied from room temperature to <TEX>$250^{\circ}C$</TEX> in a vacuum for 1 hour in a magnetic field of 150 Oe. Interestingly, magnetic hysteresis (M-H) measurements showed that NiFe/FeMn/CoFe trilayer thin films exhibited a completely contrasting variation of the exchange bias fields at both the NiFe/FeMn and FeMn/CoFe interfaces with annealing temperatures. High-angle X-ray diffraction (XRD) measurements indicated the absence of any discernible effect of thermal treatment on the NiFe(111) and FeMn(111) peaks. The compositional depth profile obtained from X-ray photoelectron spectroscopy (XPS) results presented the asymmetric compositional depth profiles of the Mn and Fe atoms throughout the FeMn layer. We contend that this asymmetric compositional depth profile and the preferential Mn diffusion into the NiFe layer, compared to that into the CoFe layer, are conclusive experimental evidence of the contrasting variation of the exchange bias fields at two interfaces having a common polycrystalline FeMn(111) layer.

A TEM structural study of thermal stability of magnetic tunnel junctions integrated with CMOS devices

We present a TEM/STEM study of the structure and thermal stability of Co/AlOx/Si, CoFe(B)/MgO/Si and FeNi/SiOx/Si magnetic junctions deposited directly on patterned Si. In all three types of junctions the films are uniform and continuous. Annealing at 300 at 550 °C does not change their structure. CoFe and CoFeB layers remained amorphous after the annealing process.

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 &amp; Sons, Inc.

Training effect in specular spin valves

Specular spin valves show an enhanced giant magnetoresistive (GMR) ratio due to specular reflection in nano-oxide layers (NOLs) formed by the partial oxidation of CoFe pinned and free layers. The oxides that form the (pinned layer) NOL were recently shown to antiferromagnetically order at $T\ensuremath{\sim}175\text{ }\text{K}$. Here, we study the training effect (TE) in MnIr/CoFe/NOL/CoFe/Cu/CoFe/NOL/Ta specular spin valves in the 300--15 K temperature range. The exchange bias direction between the MnIr and CoFe layers impressed during annealing is taken as the positive direction. The training effect is observed in antiferromagnetic (AFM)/ferromagnetic (FM) exchange systems and related to the rearrangement of interfacial AFM spin structure with the number of hysteretic cycles performed $(n)$, resulting in the decrease of the exchange field $({H}_{\text{exch}})$. Here, in the studied specular spin valve, TE was only observed for $T&lt;175\text{ }\text{K}$ and is thus related to the pinned layer NOL-AFM ordering and to the evolution of the corresponding spin structure with $n$. We show that FM spins that are strongly coupled to AFM domains do not align with the applied positive magnetic field $(H)$, giving rise to a residual MR at $H\ensuremath{\gg}0$. Such nonsaturating MR will be related with a spin-glass-like behavior of the interfacial magnetism induced by the nano-oxide layer. The observed dependence of the training effect on the field cooling procedure is also likely associated with the existence of different spin configurations available in the magnetically disordered oxide. Furthermore, anomalous magnetoresistance cycles measured after cooling runs under $\ensuremath{-}500\text{ }\text{Oe}$ are here related to induced NOL exchange bias/applied magnetic field misalignment. The temperature dependence of the training effect was obtained and fitted by using a recent theoretical model.

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 Ir–Mn composition on exchange bias and thermal stability of spin valves with nano-oxide layers

The Ir–Mn bottom-pinned spin valves with nano-oxide layers (NOLs), Ta∕Ni81Fe19∕Ir–Mn∕Co90Fe10∕NOL∕Co90Fe10∕Cu∕Co90Fe10∕NOL∕Ta, were fabricated by dc magnetron sputtering. The magnetoresistance (MR), magnetization, and exchange bias have been studied as a function of Ir–Mn composition and annealing temperature. It was observed that the spin valves with the Ir–Mn layer containing relatively low Mn content (58.9–72.4at.% Mn) show the best thermal endurance. For these samples, the Mn diffusion is effectively hampered by the NOL with a large MR value of about 12.5% even after annealing at 300°C. On the other hand, the exchange bias field of the pinned CoFe layer shows a maximum at Mn content of about 72.4at.%, which is different from the widely adopted composition, Ir-80at.% Mn, optimized from the top-pinned NiFe∕Ir–Mn system. Moreover, the blocking temperature of the Ir–Mn∕CoFe system with 72.4at.% Mn is higher than that with 80.6at.% Mn. The present results suggest that the Ir–Mn∕CoFe pinning system with Mn content at about 72% renders the most favorable exchange bias and the best thermal stability for the bottom-pinned specular spin valves.

Ultrafast optical modulation of exchange coupling in FePt∕CoFe composite structure

Photoinduced magnetization dynamics in FePt∕CoFe bilayer is investigated by optical pump-probe measurements. The transient Kerr loops exhibit an instantaneous separated switching behavior due to exchange decoupling between FePt and CoFe layers induced by photoexcitation, which subsequently recover to the original uniform switching at a certain delay time of 10–20ps. By analyzing the temporal dependence of magnetization ratio of FePt to CoFe and coercivity of FePt phase, we infer that the decoupling and slow magnetization recovery phenomena are ascribed to the laser-created spin disordering mainly in FePt layer, while the following onset of interfacial coupling results from spin/lattice cooling process.

Effect of heat sink layer on ultrafast magnetization recovery of FeCo films

For FeCo alloy thin films with Ag, Cu, Pt, Ta and Cr as heat sink layers, ultrafast demagnetization and recovery processes of transient magnetization have been studied by the time-resolved magneto-optical Kerr effect. For all samples, the ultrafast demagnetization process is accomplished within almost the same time interval of 500 fs, which is independent of the heat sink layer material and the pump fluence. The recovery rate of the FeCo film grown on the Si(1 0 0) substrate is enhanced with a heat sink layer. In addition, the recovery rate is found to be independent of the heat sink layer thickness; it decreases with increasing pump fluence. Among all heat sink layers, the sample with the Cr layer achieves the highest recovery rate because it has the same bcc structure as that of the FeCo layer and the small lattice mismatch. The sample with the Ta layer, has the largest damage threshold of pump fluence because of the highest melting point.

Ferromagnetic resonance studies of surface and bulk spin-wave modes in a CoFe∕PtMn∕CoFe multilayer film

We studied exchange-dominated surface and bulk spin-wave modes in a single period of CoFe∕PtMn∕CoFe trilayer film grown on a seed layer of Ru with CoFe film compositions being Co-16at.%Fe. The thickness of the ferromagnetic CoFe layers is ∼400Å and that of the antiferromagnetic layer is 120Å. Multimode spin-wave spectra were observed using the ferromagnetic resonance technique, as the sample plane was rotated with respect to the direction of the magnetic field. The effective magnetic anisotropy parameters and the g factor of the magnetic film were calculated from the field corresponding to the main (strongest) resonance peak at different angles. In addition, we identified a high-order standing spin wave in our spectra and found a “critical angle” in the multilayer sample. As H is significantly rotated away from the normal, there is a critical orientation where only a single acoustic spin-wave mode can be observed. We included an effective surface anisotropy field to describe our results. From the surface anisotropy, we are able to analyze the spin-wave resonance spectra in terms of the dynamic surface spin pinning. This allows us to determine the exchange interaction stiffness in the CoFe layers, J∼2.7meV.

Integrated planar spiral inductors with CoFe and NiFe ferromagnetic layer

AbstractInductance increase up to 28% was obtained for on‐chip spiral inductors by employing DC‐sputtered CoFe and NiFe ferromagnetic (FM) layer on top of the spirals. It was also observed that slit‐patterned FM layers exhibited better quality factors than blanket FM layers, leading to values comparable to that of the inductors without FM layer. © 2008 Wiley Periodicals, Inc. Microwave Opt Technol Lett 50: 676–678, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.23180

垂直ＭＲＡＭ用スピン偏極増強用ＦｅＣｏ層およびＭｇＯバリア層の結晶性向上

Rare-earth-transition metal (RE-TM) alloy films, such as TbFeCo and GdFeCo layers were used as perpendicularly magnetized layers for pinned and free layers in a perpendicular magnetic tunneling junction (p-MTJ). (100) oriented MgO tunneling barrier layer can be deposited on (100) oriented Fe (and FeCo) buffer layers. The (100) texture of the Fe (and FeCo) layers were enhanced by depositing them on the RE-TM layers. X-ray diffraction study clarified that MgO layers deposited on the Fe (and FeCo) layers revealed apparent (100) orientation at the early stage of the film growth. 3nm-thick MgO layer deposited on GdFeCo(100nm)/Fe(3nm) exhibited (100) texture.

Spin Polarization of Sub-monolayer Cu Embedded in Fe70Co30 Films

Spin polarization of Cu embedded in Fe70Co30 films has been investigated by means of transmission x-ray magnetic circular dichroism (XMCD). [Fe70Co30(dFeCo) / Cu(dCu)]n multilayered films were fabricated on Si-N membrane substrate, fixing total thickness of Fe-Co and Cu layers. Magneto-optical sum-rules were applied to the measured absorption and XMCD spectra to determine quantitatively the spin and orbital moments of Cu. With decreasing the Cu layer thickness, the spin moment of Cu increases but exhibits to be saturated about 0.1 μB / atom. The small value of spin moment (0.1 μB), comparing to that of the 3d transition metals such as Fe and Co, implies that only the top part of 3d down-spin band of Cu is unoccupied in these specimens.

Study of synthetic antiferromagnetic Ni81Fe19/Co90Fe10/Ru/Co90Fe10/Ni81Fe19

A series of synthetic antiferromagnets and spin valves with synthetic antiferrimagnets as free layers were fabricated by DC magnetron sputtering. It is found that inserting a thin Co90Fe10 layer of proper thickness between Ni81Fe19 and Cu is an effective means to strengthen the antiferromagnetic coupling in the synthetic antiferrimagnet, and obtain synthetic antiferromagnets with excellent magnetic properties, i.e., higher saturation magnetic field, lower saturation magnetization value and better thermal stability. In addition, spin valves using the synthetic antiferrimagnet as a free layer have better stability.

Pseudo spin-valves with Al or Nb as spacer layer: GMR and search for spin switch behaviour

The magnetoresistance of several Ferromagnet/Normal metal/Ferromagnet spin-valve type structures has been investigated using Al as normal spacer layer. A magnetoresistance ratio up to 4.1% at room temperature and 5.7% at 0.3 K is found for the sandwich with both Co layers, while slightly lower signals are found for the structures involving CoFe and NiFe layers. The magnetoresistance dependence for Co/Al/Co, Co/Al/CoFe and Co/Al/NiFe on the spacer layer thickness exhibits the familiar non monotonic behaviour with second peak slightly larger than the one reported for Cu based pseudo spin valves. At cryogenic temperatures, preliminary results on the onset of spin switch effects in Co/Al/Co and the full spin switch effect in Co/Nb/Co are also reported here.

Soft X-ray Resonant Magnetic Reflectivity Study on Induced Magnetism in [Fe70Co30/Pd]nSuper-Lattice Films

We performed soft x-ray resonant magnetic reflectivity experiments on [FeCo/Pd]n super-lattice films to study the depth profile of magnetization near the interface. From the analysis of the reflectivity measured at the Fe-LIIIabsorption edge, we obtained the magnetization profiles for the samples with different FeCo layer thicknesses. The profiles can be divided into three different zones: (A) Mixed region at the Pd interface where the magnetization decreased due to the structural randomness, (B) Enhanced region where the magnetization increased, and (C) Non-enhanced region. We estimated the thickness where the FeCo magnetization is enhanced by the adjacent Pd layer is about 4 atomic monolayers.

Exchange Bias Characteristics in Specular Spin Valve System With Embedded (Co–Fe)/Cr Nano-Oxide-Layer in Pinned Ferromagnetic Layer

The insertion of a nano-oxide-layer (NOL) into a spin valve system is well known to be very effective in enhancing the magnetoresistance ratio at both of current-in-plane (CIP) and current-perpendicular-to-plane (CPP) geometries. Recently, a clear NOL pinning assist effect is reported to be observed in the low temperature range below ~150 K by using the magnetic field cooling method from room temperature (R.T.) in the Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.9</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.1</sub> pin-NOL, wherein the NaCl-type CoO-FeO antiferromagnetic component exists. In this paper, the NOL pinning assist effect in a specular spin valve system with (Co-Fe)/Cr pin-NOL is presented. An ultra-thin Cr layer deposit on the Co-Fe layer before oxidation leads to increase the exchange bias field still more with the increment of the blocking temperature (T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</sub> )of the (Co-Fe)-NOL biasing in the low temperature range below R.T. and to fully exploit the superior pinning properties of the metallic antiferromagnet (Ir-Mn) at R.T. in the Co-rich region. The effect of ultra-thin Cr layer on the NOL exchange bias is considered from the aspect of the complicated oxide formation and its magnetism.

FeCo/Cu and FeCo/Ru Laminated Structures by Electrodeposition

Studies were carried out on the laminated structures FeCo/Cu and FeCo/Ru prepared by two-bath plating, one-bath plating and plating-sputtering hybrid method. Zero remnant magnetization as in antiferromagnetically coupled structures made by sputtering was not achieved. A slow magnetization was observed for structures plated from two baths. A hypothesized mechanism was proposed, involving a superparamagnetic component due to the displacement and corrosion reactions. This effect is less prominent in the structures prepared with one bath plating. Lower magnetic moments were observed, probably due to the inclusion of Cu in FeCo layers as well as a displacement reaction. All the structures prepared by plating-sputtering method behave like a bulk ferromagnetic material. An increase and decrease of the magnetic moment was observed for FeCo/Cu and FeCo/Ru, respectively, probably due to the different effect on the corrosion reaction by Cu and Ru.

Nanoscale Calorimetry Using a Suspended Bridge Configuration

Abstract A new setup for small-scale differential scanning calorimetry (DSC) studies based on a suspended bridge configuration is presented. The new setup has three major advantages over previously reported DSC setups: 1) superior temperature uniformity in the bridge cross section; 2) less heat loss to the surroundings by at least two orders of magnitude; and 3) a faster transient response by three orders of magnitude. This paper includes a thermal analysis to support these improvements. A major contribution of the new thermal analysis over previous reports is the inclusion of the thermal mass of the substrate in calculations, which makes thermal design more detailed, dramatically affecting accuracy and sensitivity in measurements. Furthermore, the new thermal analysis more accurately accounts for heat loss to the substrate and the surroundings in efforts to resolve suspected inconsistencies in previously reported data. Experimental validation of the new setup is presented by measuring the specific heat of thin layers of Si02 and CoFe. The specific heat of Si02 was found to be 2.2 times 106 Jm -3 K-1 which is nearly 10% different from the literature values of bulk specimens. For CoFe, the specific heat value of 3.16 x 106 Jm -3 K-1 is obtained using differential Cu/Si02 and Cu/Si02/CoFe structures compared to the value of 3.5 times 106 Jm -3 K-1 obtained using single CoFe suspended structure.

Double Exchange Biased Magnetic Force Microscopy Tip and Comparison of Its Imaging Performance with Commercial Tips

A double exchange biased magnetic force microscopy tip, which is coated with an antiferromagnet (IrMn)/ferromagnet (CoFe)/antiferromagnet (IrMn) multilayer on one side of a bare tip, has been fabricated and studied. In this design, the CoFe layer can be very thin due to its high moment, which leads to high spatial resolution without sacrificing the signal intensity. The exchange coupling between IrMn and CoFe stabilizes the magnetization of the latter which enables this kind of tip to have a better sensitivity and thermal stability as compared to conventional single ferromagnet layer tips. The performance of the double exchange biased tips has been evaluated through imaging the magnetic patterns recorded on longitudinal magnetic recording media. Micromagnetic modeling has been conducted to understand the underlying mechanism of the tip performance improvement.