CoFe Layer Research Articles

Domain structure and perpendicular magnetic anisotropy in CoFe/Pd multilayers using off-axis electron holography

Multilayers of Co90Fe10/Pd with different bilayer thicknesses, have been deposited by dc-magnetron sputtering on thermally oxidized Si wafers. Transmission electron microscopy showed that the highly textured crystalline films had columnar structure, while scanning transmission electron microscopy and atomic force microscopy respectively indicated some layer waviness and surface roughness. The magnetic domain structure and perpendicular magnetic anisotropy (PMA) of the Co90Fe10/Pd multilayers were investigated by off-axis electron holography and magnetic force microscopy. The Co90Fe10 layer thickness was the primary factor determining the magnetic domain size and the perpendicular magnetization: both decreased as the thickness increased. The strongest PMA was observed in the sample with the thinnest magnetic layer of 0.45nm.

CoFe Layers Thickness and Annealing Effect on the Magnetic Behavior of the CoFe/Cu Multilayer Nanowires

CoFe/Cu multilayer nanowires were electrodeposited into anodic aluminum oxide templates prepared by a two-step mild anodization method, using the single-bath technique. Nanowires with 30 nm diameter and the definite lengths were obtained. The effect of CoFe layers thickness and annealing on the magnetic behavior of the multilayer nanowires was investigated. The layers thickness was controlled through the pulses numbers: 200, 260, 310,360 and 410 pulses were used to deposit the CoFe layers, while 300 pulse for the Cu layers. A certain increase in coercivity and squareness of CoFe/Cu multilayer nanowires observed with increasing the CoFe layer thickness and annealing improved the coercivity and decrease squareness of CoFe/Cu multilayer nanowires. First order reversal curves after annealed showed amount domains with soft magnetic phase, it also shows decreasing spreading of distribution function along the Hu axis after annealed

Strain control magnetocrystalline anisotropy of Ta/FeCo/MgO heterostructures

Using ab initio electronic structure calculations, we have investigated the effect of epitaxial strain on magnetocrystalline anisotropy (MCA) of Ta/FeCo/MgO heterostructure. At small expansive strains on the FeCo layer, the system exhibits perpendicular MCA (PMA). Strain not only has a profound effect on the value of MCA but also induces a switching of magnetic easy axis. Analysis of the energy- and k-resolved distribution of orbital characters of the minority-spin band reveals that a significant contribution to PMA at zero strain arises from the spin-orbit coupling between occupied dx2−y2 and unoccupied dxy states, derived from Fe at the FeCo/MgO interface. The strain effect is attributed to strain-induced shifts of spin-orbit coupled d-states. Our work demonstrates that strain engineering can open a viable pathway towards tailoring magnetic properties for spintronic applications.

Effects of repetitive bending on the magnetoresistance of a flexible spin-valve

A positive magnetostrictive single layer (CoFe) and top-pinned spin-valve structure with positive magnetostrictive free (NiFe) and pinned (CoFe) layers were deposited on flexible polyethylene terephthalate film to investigate the changes in the magnetic properties in flexible environments, especially with a repetitive bending process. It was found that the stress, applied by repetitive bending, changes significantly the magnetic anisotropy of both layers in a single and spin-valve structure depending on the direction of applied stress. The changes in magnetic anisotropy were understood in terms of the inverse magnetostriction effect (the Villari effect) and the elastic recovery force from the flexibility of the polymer substrate. Repetitive bending with tensile stress transverse (or parallel) to the magnetic easy axis was found to enhance (or reduce) the magnetic anisotropy and, consequently, the magnetoresistance ratio of a spin-valve. The observed effects of bending stress in this study should be considered for the practical applications of electro-magnetic devices, especially magneto-striction sensor.

High-frequency magnetic properties of [FeCo/FeCo-SiO2]n multilayered films deposited on flexible substrate

A series of 120 nm [FeCo/FeCo-SiO2]n multilayer thin films with different FeCo layer thicknesses (T = 10–40 nm) and fixed FeCo-SiO2 layer thickness (20 nm) were deposited on flexible substrate (Mylar) by controlling the sputtering time under applied magnetic field. The magnetic properties of the multilayer thin films were investigated. The experimental results showed that the multilayer thin films had obvious in-plane uniaxial magnetic anisotropy, which can be adjusted in a broad range by changing the thickness of FeCo layer. In addition, good soft magnetic performance was obtained. The coercivity in the easy axis Hce of the multilayer films with the T = 10 nm, 20 nm, 30 nm, and 40 nm was 5 Oe, 25 Oe, 10 Oe, and 8 Oe, respectively. In frequency-dependent permeability spectra measurement, two resonance peaks of complex permeability spectrum were observed, especially for T = 20 nm, which can be attributed to the complicated magnetic structure of FeCo and FeCo-SiO2 layer. By introducing multilayers with different effect anisotropy field, one can broaden the absorption peak while maintaining the high permeability, which would open the way for their potential applications as wide-band microwave absorber.

The effect of ferromagnetic and non-ferromagnetic layer thicknesses on the electrodeposited CoFe/Cu multilayers

A series of CoFe/Cu multilayer was electrodeposited as a function of ferromagnetic and non- ferromagnetic layer thicknesses on Ti substrates from a single electrolyte containing Co (from CoSO4), Fe (from FeSO4), Cu (from CuSO4) metal ions under potentiostatic control. The deposition was carried out in a three electrode cell at room temperature. The Cu and CoFe layers were deposited at a cathode potential of −0.3 and −1.5 V with respect to saturated calomel electrode respectively. The structural studies by X-ray diffraction revealed that the multilayers have face-centered-cubic structure. The magnetic characteristics of the films were investigated using a vibrating sample magnetometer and their easy-axis was found to be in the film plane. Magnetoresistance measurements were carried out at room temperature with magnetic fields up to ±955 kA/m using the Van der Pauw method. All multilayers exhibited giant magnetoresistance (GMR). The GMR values up to 22 % and a GMR sensitivity of 52 % per 1 kA/m were obtained.

Probing electric field control of magnetism using ferromagnetic resonance.

Exchange coupled CoFe/BiFeO3 thin-film heterostructures show great promise for power-efficient electric field-induced 180° magnetization switching. However, the coupling mechanism and precise qualification of the exchange coupling in CoFe/BiFeO3 heterostructures have been elusive. Here we show direct evidence for electric field control of the magnetic state in exchange coupled CoFe/BiFeO3 through electric field-dependent ferromagnetic resonance spectroscopy and nanoscale spatially resolved magnetic imaging. Scanning electron microscopy with polarization analysis images reveal the coupling of the magnetization in the CoFe layer to the canted moment in the BiFeO3 layer. Electric field-dependent ferromagnetic resonance measurements quantify the exchange coupling strength and reveal that the CoFe magnetization is directly and reversibly modulated by the applied electric field through a ~180° switching of the canted moment in BiFeO3. This constitutes an important step towards robust repeatable and non-volatile voltage-induced 180° magnetization switching in thin-film multiferroic heterostructures and tunable RF/microwave devices.

Exponentially decaying magnetic coupling in sputtered thin film FeNi/Cu/FeCo trilayers

Magnetic coupling in trilayer films of FeNi/Cu/FeCo deposited on Si/SiO2 substrates have been studied. While the thicknesses of the FeNi and FeCo layers were kept constant at 100 Å, the thickness of the Cu spacer was varied from 5 to 50 Å. Both hysteresis loop and ferromagnetic resonance results indicate that all films are ferromagnetically coupled. Micromagnetic simulations well reproduce the ferromagnetic resonance mode positions measured by experiments, enabling the extraction of the coupling constants. Films with a thin Cu spacer are found to be strongly coupled, with an effective coupling constant of 3 erg/cm2 for the sample with a 5 Å Cu spacer. The strong coupling strength is qualitatively understood within the framework of a combined effect of Ruderman-Kittel-Kasuya-Yosida and pinhole coupling, which is evidenced by transmission electron microscopy analysis. The magnetic coupling constant surprisingly decreases exponentially with increasing Cu spacer thickness, without showing an oscillatory thickness dependence. This is partially connected to the substantial interfacial roughness that washes away the oscillation. The results have implications on the design of multilayers for spintronic applications.

The effect of deposition parameters on the magnetic behavior of CoFe/Cu multilayer nanowires

In this work, CoFe/Cu multilayer nanowires were deposited into an anodic aluminum oxide template using the single-bath method. The effect of deposition parameters as off-time between pulses and reduction/oxidation voltage of both the magnetic CoFe and non-magnetic Cu layers on the magnetic behavior of the multilayer nanowires was investigated. The layers thickness was controlled through the pulse numbers: 30, 50, 100, 200, 400 and 600 pulses were used to deposit the CoFe layers, while 200 and 400 pulses for the Cu layers. Due to the shape variation of the CoFe layer from rod- to disc-like cylindrical (rotation of the magnetic easy axis toward the plane of the disc), decreasing the CoFe pulse numbers led to approaching the in-plane coercivity toward the out of plane. The structural studies revealed that the pulsed alternating electrodeposition method in the single-bath system enabled to deposit CoFe/Cu multilayer nanowires with highly pure segments.

Study of Magnetization Reversal Process in FeCo/Ru/FeCo Exchange Coupled Synthetic Antiferromagnetic Multilayers

FeCo/Ru/FeCo exchange coupled synthetic antiferromagnetic multilayers were prepared with two sputtering modes. One is continuous sputtering mode, and the other mode is layer‐by‐layer sputtering mode. The former mode implies that substrate faced the target and film growth process was continuous when FeCo layers were sputtering, whereas the latter implies that substrate was rotating with the mask at a speed of 5 rpm when FeCo layers were sputtering. It was found that the exchange coupling field Hex of sample sputtered by layer‐by‐layer mode was higher than the one sputtered by continuous mode. Domain structures were measured with applying varied in‐plane magnetic fields along the easy axis in order to study the magnetization reversal process. We found it is a domain wall move process. When the applied field is smaller than Hex, both the two magnetic layers have domain structure and the domain structure of the two layers is reversed correspondingly. When the applied field is varying in the range of −Hex to Hex, the domain wall of the two layers moves correspondingly at the same time.

Magnetization dynamics in an exchange-coupled NiFe/CoFe bilayer studied by x-ray detected ferromagnetic resonance

Exchange-coupled hard and soft magnetic layers find extensive use in data storage applications, for which their dynamical response has great importance. With bulk techniques, such as ferromagnetic resonance (FMR), it is difficult to access the behaviour and precise influence of each individual layer. By contrast, the synchrotron radiation-based technique of x-ray detected ferromagnetic resonance (XFMR) allows element-specific and phase-resolved FMR measurements in the frequency range 0.5–11 GHz. Here, we report the study of the magnetization dynamics of an exchange-coupled Ni0.81Fe0.19 (43.5 nm)/Co0.5Fe0.5 (30 nm) bilayer system using magnetometry and vector network analyser FMR, combined with XFMR at the Ni and Co L2 x-ray absorption edges. The epitaxially grown bilayer exhibits two principal resonances denoted as the acoustic and optical modes. FMR experiments show that the Kittel curves of the two layers cannot be taken in isolation, but that their modelling needs to account for an interlayer exchange coupling. The angular dependence of FMR indicates a collective effect for the modes of the magnetically hard CoFe and soft NiFe layer. The XFMR precessional scans show that the acoustic mode is dominated by the Ni signal with the Co and Ni magnetization precessing in phase, whereas the optical mode is dominated by the Co signal with the Co and Ni magnetization precessing in anti-phase. The response of the Co signal at the Ni resonance, and vice versa, show induced changes in both amplitude and phase, which can be ascribed to the interface exchange coupling. An interesting aspect of phase-resolved XFMR is the ability to distinguish between static and dynamic exchange coupling. The element-specific precessional scans of the NiFe/CoFe bilayer clearly have the signature of static exchange coupling, in which the effective field in one layer is aligned along the magnetization direction of the other layer.

Lattice-matched magnetic tunnel junctions using a Heusler alloy Co2FeAl and a cation-disorder spinel Mg-Al-O barrier

Perfectly lattice-matched magnetic tunnel junctions (MTJs) consisting of a Heusler alloy B2-Co2FeAl (CFA) electrode and a cation-disorder spinel (Mg-Al-O) barrier were fabricated by sputtering and plasma oxidation. We achieved a large tunnel magnetoresistance (TMR) ratio of 228% at room temperature (RT) (398% at 5 K) for the epitaxial CFA/MgAl-O/CoFe(001) MTJ, in which the effect of lattice defects on TMR ratios is excluded. With inserting a ultrathin (≤1.5 nm) CoFe layer between the CFA and Mg-Al-O, the TMR ratio further increased up to 280% at RT (453% at 5 K), which reflected the importance of controlling barrier-electrode interface states other than the lattice matching.

The investigation of interface properties of FeCo/Bepp2 heterostructures

The interface has become an important issue in organic magnetic spin-valves. In this work, Si/FeCo/Ta, Si/FeCo/Bepp2/Ta, and Si/Bepp2/FeCo/Ta samples with the variation of the FeCo layer thickness were fabricated by direct-current magnetron sputtering for FeCo layers and the thermal evaporation method for Bepp2 layers. The interaction at the interfaces between the FeCo magnetic layer and Bepp2 organic layer was investigated by the traditional magnetic properties measurement and X-ray photoelectron spectroscopy with the Argon ion etching technique. The experimental results show that the interfacial reaction is strong when sputtering FeCo on Bepp2 layer due to the high energy of sputtered FeCo atoms in contrast with the weak interfacial reaction when evaporating Bepp2 on FeCo layer. Moreover, the capping layer of Ta is quite important to protect FeCo layer against oxidation from air either by direct contact or through Bepp2 layer. Our experiment is very helpful for fabricating organic spin-valves with high-spin injection efficiency.

Formation of a bcc (001)-textured CoFe layer by the insertion of an FeZr layer in multilayer-based stacks with perpendicular magnetic anisotropy

The effects of inserting an amorphous FeZr layer between the [Pt/Co] multilayer and the CoFeB/MgO layer in stacks possessing perpendicular magnetic anisotropy are examined. A 1-nm-thick FeZr layer is effective in forming a bcc (001)-textured CoFe layer during annealing by suppression of crystallization at the interface with the multilayer, which is terminated in a close-packed plane. Because FeZr is magnetic, it has an advantage over Ta, an alternative material used for the same purpose. Efficient magnetic coupling between the multilayer and CoFeB/MgO occurs even for large FeZr layer thicknesses, so the magnetic properties of the stack are only weakly affected.

Magnetoelectric effect in FeCo/PMN-PT/FeCo trilayers prepared by electroless deposition of FeCo on PMN-PT crystals with various orientations

The magnetoelectric (ME) effect was studied in the FeCo/PMN-PT/FeCo trilayer composites prepared by electroless deposition of the FeCo layers on the single crystal PMN-PT substrates with various crystal cuts. X-ray diffraction reveals that the orientation of PMN-PT substrate has no effect on the growth of FeCo layer. The structures with PMN-PT crystals of various orientations have different acoustic resonance frequencies. FeCo/PMN-PT/FeCo composites with PMN-PT of 〈001〉L×〈01‾1〉W×〈011〉T crystal cut shows superior ME performance, which is due to the highest piezoelectric module of PMN-PT.

Perpendicular magnetic anisotropy and magnetization of L1 FePt/FeCo bilayer films

Fe40Co60 epitaxial thin films are prepared on L10 ordered Fe60Pt40 underlayer by ultrahigh vacuum multiple dc-sputtering systems. Magnetic properties of the Fe60Pt40 (10 nm)/Fe40Co60 (t nm) bilayer films are investigated. When the FeCo thickness is less than 3 nm, the easy magnetization axis of FePt/FeCo bilayer film is perpendicular to the film plane. Compared with FePt/Fe and FePt/Co films, FePt/FeCo bilayer films possess not only higher anisotropy field Hk but also larger magnetic anisotropy energy Ku, which may be due to the perpendicular magnetic anisotropy yielded by the tetragonal distorted FeCo layer on FePt, while the magnetic easy axes of Fe and Co layers lie in the film plane. Meanwhile, saturation magnetization of FePt/FeCo film increases reasonably because of the high Ms value of FeCo component. These results indicate that the FePt/FeCo bilayer films which possess both large magnetic anisotropy energy Ku and high saturation magnetization Ms have great potential for using as the magnetic recording media, and also give a clue to develop a new type of permanent magnet without rare-earth metals.

Magnetic degradation of thin film multilayers during ion milling

We present a study of Ar ion milling-induced damage in exchange biased IrMn/CoFe/Ag-based magnetic multilayer thin films. While process variations determine the change in CoFe magnetic properties, the distance from the ion milling front to the IrMn/CoFe interface dominates the extent of exchange bias damage. Remarkably, the interfacial coupling energy Jk can be reduced by 50% before any removal of the CoFe pinned layer. We attribute the losses to microstructural changes and damage effects where cap material is driven into the CoFe layer below. Disturbance depth estimates from ion impact simulations agree reasonably with the observed length scales of damage.

TUNABLE RESONANT FREQUENCY OF FeCo/PZT/FeCo CYLINDERS

FeCo / PZT / FeCo cylindrical heterostructures were prepared by electroless deposition. Scanning electron microscopy shows that the magnetostrictive FeCo layers are in contact with piezoelectric PZT directly. To control the resonant frequency at which the magnetoelectric coupling shows an obvious improvement, the height of cylinder needs to be taken into consideration. In the present work, three hollow cylinders with different height were polarized along the radial direction. It was shown that with the increase of cylinder height the magnetoelectric coefficient increased. Both experimental and calculated results show that the resonant frequency of FeCo / PZT / FeCo cylindrical layered composites decreases with increasing the cylinder height. Proper resonant frequency and strong ME effect could be obtained by optimizing the configuration.

Magnetoelectric effect in square FeCo/PMN-PT/FeCo trilayer prepared by electroless deposition

A multiferroic square FeCo/PMN-PT/FeCo trilayer has been constructed by electroless depositing. Scanning electron microscopy reveals that the magnetostrictive FeCo layers are in direct contact with the piezoelectric single crystal PMN-PT substrates. It is shown that electroless deposition is an effective way to deposit films on single crystal PMN-PT. Thanks to the excellent piezoelectric performance, the composite adapting PMN-PT as piezoelectric phase possess a higher magnetoelectric coefficient than that of traditional PZT piezoelectric ceramics with the same configuration. For the square FeCo/PMN-PT/FeCo sample, six obvious resonance peaks can be observed in the dependence of the magnetoelectric coefficient on the magnetic field frequency due to the complex vibration mode, which makes this structure suitable for applications in multifunctional devices with multifrequency operation.

Effects of Different Relevant Layers on Magnetic Properties of Bottom Synthetic IrMn Spin Valves

The effects of different relevant layers, especially the insertion layers (which are between Ta buffer layer and IrMn pinning layer) and free layers, on the magnetic properties of IrMn bottom-pinning spin valves are investigated. Spin valve with a NiFe insertion layer exhibits a higher GMR ratio of ~ 6.0% than that of 2.0% for the spin valve with a Cu insertion layer due to a better pinning strength. The spin valves with a CoFe/NiFe composite free layer have relatively better magnetic properties: a higher GMR ratio compared with the spin valve with a single NiFe free layer and a lower free layer coercivity compared with the spin valve with a single CoFe layer.