CoFe Layer Research Articles

Influence of Cr interlayer with different thickness on transition of magnetoresistance effect of Gd/FeCo thin films

As one of the most representative features characterizing the spin valve structure, magnetoresistance is an important method to study the interlayer coupling in multilayers. Considering the induced magnetism of rare earth at room temperature due to the coupling and magnetic proximity effect in the structure of rare earth/magnetic transition metal, an intermediate nonmagnetic metal can be inserted to form the spin valve structure to regulate the interlayer coupling, which expands the scope of applications of rare earth in spintronics. In this work, the interlayer exchange coupling and interfacial effects of Gd (4 nm)/Cr (<i>t</i><sub>Cr</sub>)/FeCo (5 nm) trilayers with different Cr layer thickness (<i>t</i><sub>Cr</sub>) are studied by means of in plane magnetoresistance. Compared with FeCo film, Gd/FeCo film obtains more obvious anisotropic magnetoresistance. While the magnetoresistance value obtained for the configuration of <i>I</i>⊥<i>H</i> shows a minimum value at the peak due to the insertion of Cr layer, and this minimum value becomes more pronounced with the increase of <i>t</i><sub>Cr</sub>. When<i> t</i><sub>Cr</sub> = 3 nm, the negative spin valve effect almost totally overcomes the anisotropic-magnetoresistance effect. Different spin asymmetries of scattering that are formed in FeCo layer and Cr/Gd layers are mainly responsible for creating the negative spin valve magnetoresistance, in which the resistance becomes smaller near the coercive, while the resistance becomes larger at high field parallel to magnetic moment. The oscillation of magnetoresistance with <i>t</i><sub>Cr</sub> at <i>I</i> // <i>H</i> and the hysteresis loops at 5 K further confirm the existence of interlayer coupling both at room temperature and 5 K.

Increasing the Efficiency of a Spintronic THz Emitter Based on WSe2/FeCo.

We report an increase in terahertz (THz) radiation efficiency due to FeCo/WSe2 structures in the reflection geometry. This can be attributed to an absorption increase in the alloy FeCo layer at the input FeCo/WSe2 interface due to constructive interference, as well as to the backward transport of hot carriers from FeCo to WSe2. In contrast to the transmission geometry, the THz generation efficiency in the reflection is much less dependent on the magnetic layer thickness. Our results suggest a cheap and efficient way to improve the characteristics of THz spintronic emitters with the conservation of a full set of their important properties.

Thermally robust synthetic antiferromagnetic fixed layers containing FeCoB for use in STT-MRAM devices

In this work we measure the bilinear and biquadratic interlayer exchange coupling, J1 and J2, for several different synthetic antiferromagnetic structures containing FeCoB both before and after annealing at 350°C, with Ru spacer layers ranging in thicknesses from 0.4 to 1 nm. We show that when FeCoB is on top of the Ru spacer layer, in a Co/Ru/FeCoB trilayer structure, it is able to maintain antiferromagnetic coupling after annealing at 350°C, for Ru spacer layer thicknesses of 1 nm or less. This is in contrast to coupling in FeCoB/Ru/FeCoB trilayer structures that becomes strongly ferromagnetically coupled after annealing above 300°C. This difference is thought to be caused by boron diffusion into the spacer layer, which in turn enhances the diffusion of magnetic atoms into the spacer layer. We also show that coupling in a FeCoB/Co/Ru/FeCo/FeCoB structure remains AFC after annealing at 350°C, for Ru spacer layer thicknesses of 0.8 nm or less. This is thought to be caused by the Co and FeCo layers acting as diffusion barriers and reducing the diffusion of boron into the Ru spacer layer during the annealing process. We also show that, in a Co/Ru/Co trilayer structure, J1 can be increased in magnitude by replacing the top Co layer with FeCo, for spacer layer thicknesses of 0.5 nm or less. After these samples have been annealed at 350°C, the replacement of the top Co layer with FeCo was found to result in a two fold increase in the magnitude of J1, for Ru spacer layer thicknesses of 0.5 nm or less.

Hf thickness dependence of perpendicular magnetic anisotropy, damping and interfacial Dzyaloshinskii-Moriya interaction in Pt/CoFe/Hf/HfO2

Vibrating sample magnetometery (VSM) combined with Brillouin light scattering (BLS) were used to investigate the Dzyaloshinskii-Moriya interaction (iDMI), the perpendicular magnetic anisotropy (PMA) and the damping in ${\mathrm{Co}}_{0.5}{\mathrm{Fe}}_{0.5}$-based systems grown by sputtering on $\mathrm{Si}/{\mathrm{SiO}}_{2}$ substrates, using Pt or Cu buffer layers and Hf capping layer of variable thickness $(0\ensuremath{\le}{t}_{\mathrm{Hf}}\ensuremath{\le}2\phantom{\rule{0.28em}{0ex}}\mathrm{nm})$. VSM measurements revealed a significant decrease of the areal magnetic moment at saturation as ${t}_{\mathrm{Hf}}$ increases, most likely due to the increase of the magnetic dead layer thickness at CoFe/Hf interface up to $0.73\ifmmode\pm\else\textpm\fi{}0.06\phantom{\rule{0.28em}{0ex}}\mathrm{nm}$ for ${t}_{\mathrm{Hf}}=2\phantom{\rule{0.28em}{0ex}}\mathrm{nm}$. Damping measurements allowed concluding to the weaker spin pumping contribution by the CoFe/Hf interface compared to Pt/CoFe. The analysis of the ${t}_{\mathrm{Hf}}$ dependence of damping within a model considering the contribution of each interface allowed us to determine the spin diffusion length of Hf, found to be 1.7 nm. The Hf thickness dependence of iDMI and PMA constants revealed significant (weak) contribution of the CoFe/Hf interface to PMA (iDMI). The surface iDMI constant of CoFe/Hf interface was estimated to be $(\ensuremath{-}0.37\ifmmode\pm\else\textpm\fi{}0.12)\ifmmode\times\else\texttimes\fi{}{10}^{--7}\phantom{\rule{0.28em}{0ex}}\mathrm{erg}/\mathrm{cm}$. This suggests that the iDMI sign of Hf and Pt are opposite, and an additive behavior can be obtained when they are placed in opposite sides of the CoFe layer, allowing thus to strengthen the iDMI, needed for some applications. Quadratic correlations between PMA and iDMI constants and linear dependence of damping versus PMA constant were obtained confirming their relationship with the spin-orbit coupling.

Enhanced annealing stability of ferrimagnetic Tb/FeCo multilayers

In this study, we have investigated amorphous 20-nm-thick Tb/Fe80Co20 multilayer samples with different individual layer thicknesses and their corresponding alloy counterpart. In particular, the structural and magnetic properties were analyzed upon post-annealing. Up to a certain critical thickness of the individual layers in the multilayer, no significant difference between the multilayers and the alloy is observed in their as-deposited states, which indicates the importance of interfacial intermixing. With a further increase in thicknesses of the individual layers, regions with significant larger Tb content emerge, resulting in a reduced effective Tb moment. The loss in perpendicular magnetic anisotropy upon annealing seems to be delayed for multilayers with thin individual layers compared to the alloy sample. We contribute this behavior to the underlying anisotropic short-range order enforced by the multilayer structure, which hinders the structural relaxation process. At higher temperatures, the multilayers strongly intermix and Fe and Co diffuse through the capping layer. This process leads to a strong enhancement of the saturation magnetization at a certain annealing temperature, due to the formation of separated FeCo and Tb-rich TbFeCo alloy layers, until the sample gets oxidized.

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.

High thermal stability of anti-ferromagnetic coupled molecules with FeCo layers

We propose the optimization of the magnetic remanence and the thermal stability of Mn phthalocyanine coupled with a ferromagnetic substrate, by exploiting interlayer exchange coupling within an advanced organic spin interface architecture, constituted by a FeCo film covered by a graphene membrane, hosting the MnPc molecular layer. The challenge to obtain magnetic remanence for molecular systems stable up to room temperature has been accomplished thanks to a super-exchange path, mediated by the π orbital of the organic ligands of the molecule and of the graphene sheet, favoring an antiferromagnetic (AFM) alignment for the MnPc molecules with the FeCo film. This spin interface with a strong AFM coupling mediated by a graphene spacer is optimized against thermal fluctuations, presenting a well defined remanence even at room temperature, as demonstrated by the persistent dichroic signal in temperature-dependent circularly polarized x-ray absorption spectra.

Spin-orbit torques and magnetotransport properties of α−Sn and β−Sn heterostructures

Topological insulators have emerged as an important material class for efficient spin-charge interconversion. Most topological insulators considered to date are binary or ternary compounds, with the exception of $\alpha$-Sn. Here we report a comprehensive characterization of the growth, magnetotransport properties, and current-induced spin-orbit torques of $\alpha$-Sn and $\beta$-Sn-based ferromagnetic heterostructures. We show that $\alpha$-Sn grown with a Bi surfactant on CdTe(001) promotes large spin-orbit torques in a ferromagnetic FeCo layer at room temperature, comparable to Pt, whereas $\alpha$-Sn grown without Bi surfactant and the non-topological phase, $\beta$-Sn, induce lower torques. The dampinglike and fieldlike spin-orbit torque efficiency in $\alpha$-Sn with Bi are 0.12 and 0.18, respectively. Further, we show that $\alpha$-Sn grown with and without Bi presents a spin Hall-like magnetoresistance comparable to that found in heavy metal/ferromagnet bilayers. Our work demonstrates direct and efficient charge-to-spin conversion in $\alpha$-Sn ferromagnetic heterostructures, showing that $\alpha$-Sn is a promising material for current-induced magnetization control in spintronic devices.

Interplay between superconductivity and the Kondo effect on magnetic nanodots

We study the interplay of superconductivity, ferromagnetism, and the Kondo effect in a single system, using vertical geometry and planar magnetic tunnel junction devices, in which a thin CoFe layer is inserted in the middle of the MgO layer, forming a quantum dot like system. It is shown that the Kondo resonance peak at the zero bias coexists with a sharp Bardeen-Cooper-Schrieffer gap on double tunnel junctions, Al/MgO/CoFe nanodot/MgO/Al. It is also found that the competition between superconductivity and the Kondo effect is tunable with magnetic fields and the temperature. The coexistence of Kondo screening and superconductivity survives long range magnetic order in CoFe nanodots with a spin polarization of 0.2; however, it disappears when the CoFe layer becomes a continuous film with a spin polarization of 0.5. The competition between SC and the Kondo effect in the presence of magnetic ordering opens exciting possibilities to control information in nanomagnets.

Narrowing of d bands of FeCo layers intercalated under graphene

We report on the electronic properties of an artificial system obtained by the intercalation of equiatomic FeCo layers under graphene grown on Ir(111). Upon intercalation, the FeCo film grows epitaxially on Ir(111), resulting in a lattice-mismatched system. By performing density functional theory calculations, we show that the intercalated FeCo layer leads to a pronounced corrugation of the graphene film. At the same time, the FeCo intercalated layers induce a clear transition from a nearly undisturbed to a strongly hybridized graphene π-band, as measured by angle-resolved photoemission spectroscopy. A comparison of experimental results with the computed band structure and the projected density of states unveils a spin-selective hybridization between the π band of graphene and FeCo-3d states. Our results demonstrate that the reduced dimensionality, as well as the hybridization within the FeCo layers, induces a narrowing and a clear splitting of Fe 3d-up and Fe 3d-down-spin bands of the confined FeCo layers with respect to bulk Fe and Co.

Large enhancement of magnetic damping in the presence of domain walls in spin valves

Magnetic damping of the free layer of CoFeB in the spin valve IrMn/CoFe/Cu/CoFeB with large exchange bias has been characterized by frequency-swept ferromagnetic resonance under a series of fixed magnetic fields. The damping constant shows little difference between the parallel and antiparallel magnetization configurations, consistent with the theoretical prediction. Remarkably, in the intermediate states of the pinned CoFe layer under reversal, the effective damping constant of the CoFeB layer is significantly enhanced from 0.0119 up to 0.0292. This enhancement, exceeding the effect of the pumped spin current appreciably, is mainly due to the inhomogeneous broadening and/or two-magnon scattering caused by the stray field emerging from the domain walls (DW) of the pinned CoFe layer when its magnetization is partially reversed. Meanwhile, a resonance frequency shift is also observed in the presence of DW. Our result confirms the strong influence of the pinned layer DW on the magnetic damping in spin valves, which should be properly excluded while dealing with the nonlocal spin-transport-induced damping in heterostructures.

Enhanced giant magneto-impedance effects in sandwich FINEMET/rGO/FeCo composite ribbons

This paper designed and regulated a unique sandwich FINEMET/rGO/FeCo composite ribbons (/F/GFC) with giant magneto-impedance (GMI) effect. The reduced graphene oxide (rGO) was synthesized on the FINEMET ribbons by one-step electroless plating, and FeCo layer was obtained by magnetron sputtering method. The study on morphology, structure, GMI effect and magnetic properties of /F/GFC illustrated that the currents passed through the highly conductive layer of rGO at high frequency, which greatly reducing the skin effect. In addition, the FeCo layer increased the permeability and provided a closed magnetic circuit, which can finally gain a large GMI effect at low frequency. The /F/GFC owned enhanced GMI effect, with the maximum GMI ratio reaching 70.32%, which was 2.1 times of the bare FINEMET ribbon, 1.4 times of the FINEMET/rGO composite ribbon and 1.2 times of the FINEMET/FeCo composite ribbon, respectively. The result can be explained by the regulating effect of magnetic dipole interaction on the magnetic properties of soft magnetic materials.

Effect of Mo monoatomic interlayer on magnetic properties of in-plane anisotropic Nd-Fe-B/Mo/FeCo nanocomposite multilayered films

The Nd-Fe-B/Mo/Fe-Co multilayered films on MgO(001) were fabricated by UHV sputtering method. The as-deposited films showed soft magnetic properties with weak perpendicular magnetic anisotropy, and after annealing above 650°C, these films showed in-plane magnetic anisotropy, good squareness, and hard magnetic properties with high coercivity of about 8 kOe. The (220) peak of Nd2Fe14B was identified in the XRD pattern after annealing of the film, which means that the c-axis, which is the easy axis of magnetization, points in the plane of the film, consistent with the measurement of the magnetization curve. The positive peak of ΔM plot of in-plane direction of the film annealed at 800°C was shown. This result suggests existence of exchange coupling between the Nd-Fe-B and Fe-Co layers in the in-plane anisotropic multilayered film with monoatomic Mo interlayer.

Spin wave excitations in exchange biased IrMn/CoFe bilayers

Using an atomistic spin model, we have simulated spin wave injection and propagation into antiferromagnetic IrMn from an exchange coupled CoFe layer. The spectral characteristics of the exited spin waves have a complex beating behavior arising from the non-collinear nature of the antiferromagnetic order. We find that the frequency response of the system depends strongly on the strength and frequency of oscillating field excitations. We also find that the strength of excited spin waves strongly decays away from the interfacial layer with a frequency dependent attenuation. Our findings suggest that spin waves generated by coupled ferromagnets are too weak to reverse IrMn in their entirety even with resonant excitation of a coupled ferromagnet. However, efficient spin wave injection into the antiferromagnet is possible due to the non-collinear nature of the IrMn spin ordering.

Modulating Multiferroic Control of Magnetocrystalline Anisotropy Using 5d Transition Metal Capping Layers.

Electric-field control of magnetocrystalline anisotropy energy (MAE) is important for the optimal performance of the tunnel junction components of the STT-MRAM. In such a device, a high MAE of the free magnetic layer improves storage robustness, whereas a low MAE is also useful to keep energy expenditure in the switching process at a minimum. Using the frozen potential method to calculate the MAE of the CoFe layer, the electric-field control of MAE in the BaTiO3/CoFe/(Hf, Ta, W, Re, Os, Ir, Pt, or Au) heterostructure is studied. Electric field tuning of MAE is determined to be possible through switching the direction of BaTiO3 ferroelectric polarization, although both the tuning effect and the MAE depend strongly on the choice of the 5d transition metal element in the capping layer. The results predict a complicated behavior of both MAE and the underlayer polarization effect as we progress down the 5d series of elements as the choice of the capping layer element. Using the second-order perturbation theoretical framework, this behavior can nevertheless be explained by mechanisms including CoFe/capping layer interface hybridization and 5d band-filling trends in the capping layer.

Germanium pn junctions between ferromagnetic CoFe and Fe3Si layers for spintronic applications

We demonstrate germanium (Ge) pn junctions between ferromagnetic alloys, CoFe and Fe3Si, on Si(111) for vertically stacked spin-valve devices. Uniform and all-epitaxial CoFe/undoped Ge/Sb-doped Ge/Fe3Si layer structures with a spin-valve like magnetization reversal process can be grown by using a low-temperature molecular beam epitaxy technique. Current-voltage characteristics of the small-sized vertical CoFe/undoped Ge/Sb-doped Ge/Fe3Si devices show evident rectifying behavior, indicating pn diode, even at room temperature. We discuss future subjects for achieving Ge-based vertical spintronic devices.

The role of boron on exchange coupling in NiFe/Ru1−xBx/FeCo trilayer structures

In this work, we study the interlayer exchange coupling, J, between two NiFe and FeCo layers in a series of NiFe/Ru1−xBx(d)/FeCo synthetic antiferromagnet (SAF) samples, where the thickness of the spacer layer, d, is varied from 0.4 nm to 0.9 nm, and the boron concentration, x, is varied from 0 to 15 at. %. The samples are studied as deposited and after being annealed at 250°C. B is deposited into the Ru spacer layer to investigate what occurs after annealing a FeCoB/Ru/FeCoB SAF structure, which is commonly used in modern nanoscale magnetic devices, in which the FeCoB layer crystallizes to FeCo and B diffuses to adjacent layers. We find that J in as-deposited samples is relatively unaffected by adding up to 15% B into the Ru spacer layer. However, after annealing at 250°C, J changes the sign from antiferromagnetic coupling to ferromagnetic coupling for spacer layers thinner than 0.45nm for 5% and 10% B and thinner than 0.525nm for 15% B. We used transmission electron microscopy energy-dispersive x-ray spectroscopy in order to investigate the diffusion of atoms within a similar Ta(2.5nm)/NiFe(0.8nm)/Ru1−xBx(23 nm) layer structure. We find that after annealing at 250°C, the sample containing 15% B within the Ru85B15 layer had significantly more diffusion of Fe into the Ru85B15 layer, from the NiFe layer, as compared to the sample with 0% B. Thus, the presence of B within the spacer layer enhances diffusion of Fe into the spacer layer.

Determination of fine magnetic structure of magnetic multilayer with quasi antiferromagnetic layer by using polarized neutron reflectivity analysis

We carried out polarized neutron reflectivity (PNR) analysis to determine the fine magnetic structure of magnetic multilayers with quasi-antiferromagnetic (quasi-AFM) layers realized by 90-deg coupling using two Co90Fe10 layers, and quantitatively evaluated the magnetization of quasi-AFM layers. Two types of samples with different buffer layers, Ru buffer and a NiFeCr buffer, were investigated and the average angles between the respective magnetization of the two Co90Fe10 layers were estimated to be +/− 39 degrees and +/− 53 degrees. In addition, less roughness was found in the NiFeCr buffer sample resulting stronger 90-deg coupling. A perfect quasi-AFM is expected to be realized by a flat interface of the magnetic multilayer.

Micromagnetic model of exchange bias: effects of structure and AF easy axis dispersion for IrMn/CoFe bilayers

A micromagnetic model of an exchange bias bilayer is used to examine the impact of the physical structure and the easy axis dispersion of the antiferromagnetic (AF) layer on the exchange bias field (HEB) in an IrMn/CoFe system. Because of the different timescales, the magnetization dynamics of the IrMn and CoFe layers are modelled using respectively a kinetic Monte Carlo (kMC) approach and Landau–Lifshitz–Gilbert (LLG) equation. The easy axis dispersion is modelled using a Gaussian distribution. The calculations show that HEB increases with increasing IrMn thickness and grain size, in agreement with experimental work. Moreover, the model allows the visualization of the switching process at the micromagnetic level to reveal the reversal mechanism. We find that the effect of AF easy axis distribution not only strongly affects the reduction of HEB but also drives non-coherent behaviour in the reversal mechanism. This confirms that the easy axis distribution is an important factor with strong impact on the magnetic properties and exchange bias field of an exchange bias system.

Comparative study of spin-dependent transport in Co2FeAl/MgAl2O4/CoFe magnetic tunnel junctions with and without thin CoFe interface insertion: an elastic and inelastic scattering model analysis

We compare structural and transport properties of Co2FeAl (CFA)/MgAl2O4 (MAO)/CoFe magnetic tunnel junctions (MTJs) with and without a 1 nm thick CoFe insertion layer. While the two MTJs show a large difference in resistance area (RA) product, there is no significant difference in their stacked structures such as the thickness of an MAO barrier. Temperature (T) dependence of tunneling conductance (GP and GAP for parallel and antiparallel configurations, respectively) was investigated for the two MTJs, as well as their tunnel magnetoresistance that exceeds 200%. The most striking feature of the temperature dependence is that non-monotonic change appeared by inserting the CoFe layer, i.e. negative dGP/dT emerged only in the CFA/1 nm CoFe/MAO/CoFe MTJ. The observed results of GP and GAP were analyzed in a model proposed by Hu et al (2016 Phys. Rev. B 94, 094428), in which elastic (spin-conserved) and inelastic (spin-flip scattering) terms were considered. We found that the non-monotonic change observed in T dependence of GP is associated with dominance of the elastic tunneling term. This study demonstrated that the model analysis of T dependence of GP can give insight into the microscopic transport mechanism.