TbFe Layer Research Articles

Inversion symmetry breaking in spin–orbit torque-induced magnetization switching to improve the recording density of multi-level magnetoresistive random-access memory

In this study, we prepared a multi-layer Tb–Fe/Pt/Tb–Fe wire to develop a multi-level magnetic memory. By applying current, magnetizations of the Tb–Fe layers were inversion symmetrically switched by spin– orbit torque (SOT) generated from the middle Pt layer. Measurements of SOT showed that its efficiency had opposite polarities in the top and bottom Tb–Fe layers. The switching current density of the top and bottom Tb–Fe layers shifted in opposite directions under a uniform perpendicular magnetic field. Because the perpendicular magnetic field broke the inversion symmetry of SOT generated from the middle Pt layer, it could be used to control the switching current. Our results prove that the additional uniform and perpendicular magnetic field can enhance the controllability of the magnetization state in case of multi-level SOT-induced magnetization switching.

Chiral Coupling between Magnetic Layers with Orthogonal Magnetization.

We report on the occurrence of strong interlayer Dzyaloshinskii-Moriya interaction (DMI) between an in-plane magnetized Co layer and a perpendicularly magnetized TbFe layer through a Pt spacer. The DMI causes a chiral coupling that favors one-handed orthogonal magnetic configurations of Co and TbFe, which we reveal through Hall effect and magnetoresistance measurements. The DMI coupling mediated by Pt causes effective magnetic fields on either layer of up to 10-15mT, which decrease monotonically with increasing Pt thickness. Ru, Ta, and Ti spacers mediate a significantly smaller coupling compared to Pt, highlighting the essential role of Pt in inducing the interlayer DMI. These results are relevant to understand and maximize the interlayer coupling induced by the DMI as well as to design spintronic devices with chiral spin textures.

Effect of Ti underlayer thickness on the magnetic anisotropy of TbFe thin films

In this study, we address the impact of Ti underlayer thickness (UL: 0-40 nm) on the structural, magnetic, and microscopic properties of TbFe thin films. The structural analysis confirmed the intermixing at interfaces of the Ti and TbFe layer with the increment of UL thicknesses. Out-of-plane (OOP) coercivity (\({H_{\rm{c}}}\)), and saturation field (\({H_{\rm{s}}}\)) gradually increased with an increase in UL thickness. For UL = 10 nm, the domain contrast and OOP stray field strength were enhanced, which may be due to the extent of d-d hybridization dominated over the influence of interfacial roughness. While for UL = 20, and 40 nm, the extent of interfacial roughness dominated the hybridization effects. In the case of UL: 20 nm, \({H_{\rm{c}}}\) increased by nearly 6 times more than the bare TbFe system. We observed a state with high OOP \({H_{\rm{c}}}\) combined with nearly zero OOP stray fields that are found to coexist in the sample. The magnetization reversal studies on a large area reveal domain nucleation followed by domain wall motion in all the films. The idea of tuning magnetic properties by varying thicknesses of Ti UL may be useful in spintronics applications.

Synthetic domain walls in [TbFeGa/TbFe]2 multilayers

Here we report the possibility of creating synthetic domain walls in nominal [Fe72Ga28/Tb33Fe67]2 multilayers. The magnetization as a function of the temperature reveals the absence of Compensation temperature in the samples which can be understood considering an interdiffusion process that results in the formation of TbFeGa alloys at the nominal FeGa layers. Therefore, samples actually comprise TbFeGa and TbFe layers. The hysteresis loops exhibit magnetization steps related to the nucleation of domain walls formed because of the competition between different interactions: (i) the antiferromagnetic exchange coupling between the heavy rare earth (Tb) and the transition magnetic metal (Fe) inside each layer, (ii) the antiferromagnetic coupling between Tb and Fe at the interfaces, and (iii) the Zeeman energy. In good agreement with the experimental values, the nucleation field of the domain walls has been theoretically calculated taking into account the tilt of the magnetization with respect to the sample plane. Our experimental results indicate that by a proper thickness adjustment, it can be tuned, both the value of the nucleation field and the layer in which this firstly occurs. Experimental values for the exchange bias field have also been calculated.

Effective Damping Constant and Current Induced Magnetization Switching of GdFeCo/TbFe Exchange-Coupled Bilayers

The Gilbert damping constants <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\alpha$</tex-math></inline-formula> and effective perpendicular magnetic anisotropies <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$K_{{\rm eff}}$</tex-math> </inline-formula> of amorphous GdFeCo ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$10 - {x}$</tex-math></inline-formula> nm)/TbFe ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$x$</tex-math></inline-formula> nm) exchange-coupled bilayers with various TbFe layer thicknesses <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$x$</tex-math></inline-formula> were measured. The products <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\alpha \times K_{{\rm eff}}$</tex-math> </inline-formula> of the bilayers were compared with the critical current densities <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$J_{{\rm c}}$</tex-math></inline-formula> for current-induced magnetization switching (CIMS) in giant magnetoresistance nanopillars with a GdFeCo/TbFe memory layer. The damping constant <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\alpha$</tex-math></inline-formula> of the bilayers, estimated from time-resolved magneto-optical Kerr effect measurements, was 0.051 for <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${x} = 0$</tex-math> </inline-formula> , and was significantly enhanced to 0.23 for <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${x} = 1$ </tex-math></inline-formula> . The anisotropy constant <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$K_{{\rm eff}}$ </tex-math></inline-formula> and the product of saturation magnetization times anisotropy field <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> s</sub> <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">H</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</sub> also increased with increasing TbFe thickness <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$x$</tex-math></inline-formula> . The CIMS of the GdFeCo (9 nm)/TbFe (1 nm) bilayer exhibited 1.6 times larger <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> than that of GdFeCo (10 nm), while the product <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\alpha \times K_{{\rm eff}}$</tex-math></inline-formula> increased by a factor of 10.

Role of TbFe on Perpendicular Magnetic Anisotropy and Giant Magnetoresistance Effect in [Co/Ni]N-Based Spin Valves.

The exchange-coupled [Co/Ni]N/TbFe nano-magnetic films can display strong perpendicular magnetic anisotropy (PMA) which depends on the Tb:Fe component ratio, TbFe layer thickness and the repetition number N of [Co/Ni]N multilayer. Perpendicular spin valves in the nano thickness scale, consisting of a [Co/Ni]3 free and a [Co/Ni]5/TbFe reference multilayer, show high giant magnetoresistance (GMR) signal of 6.5 % and a large switching field difference over 3 kOe. However, unexpected slanting of the free layer magnetization, accompanied by a reduced GMR ratio, was found to be caused by the presence of a thick Fe-rich or even a thin but Tb-rich TbFe layer. We attribute this phenomenon to the large magnetostriction effect of TbFe which probably induces strong stress acting on the free layer and hence reduces its interfacial PMA.

Influence of Ni thin layer on a static and dynamic magnetostrictive behavior in TbFe multi-layered film

Static and dynamic magnetostrictive behaviors of the TbFe/Ni/TbFe film are compared with a tri-layered TbFe film to discover the effects of a Ni thin layer on the improvement of magnetic moment and magnetostriction, as well as the reduction of dynamic response time under a low magnetic field. The Heisenberg model and Landau–Lifshitz–Gilbert equation were used to simulate magnetic domain motion. Time consumptions, in particular, were calculated to determine a uniformly magnetized state in Ni and TbFe layers. In addition, the magnetic moment together with the magnetostriction was examined with a fabricated magnetostrictive actuator. Contrary to a tri-layered conventional TbFe film, the TbFe/Ni/TbFe showed a higher magnetostrictive behavior even though a lower magnetic field was applied, corresponding to a low coercive force within the range of 0–10kA/m. The dynamic magnetostriction response time, in particular, was also reduced to about 27ms compared to the tri-layered TbFe film.

Thermally Assisted Magnetization Switching on Magnetic Tunnel Junctions With Perpendicularly Magnetized TbFe Layer

Thermally Assisted Magnetization Switching on Magnetic Tunnel Junctions With Perpendicularly Magnetized TbFe Layer

Magnetostrictive Performance in Py/TbFe Coupled Bilayers: Dependence on Hard Layer Thickness

Magnetostrictive TbFe was chosen as hard layer, and the spin dynamics in the coupled bilayers Py(10 nm)/TbFe(t nm) (Py-permalloy; t=4, 7, 10, 30) were investigated by taking advantage of the engineering magnetostrictive strain under the constraints of the measurement. The dependence of the engineering magnetostrictive strain on TbFe layer thickness reflects a competitive process between the interfacial exchange interaction and the TbFe anisotropy. For the dominant interfacial exchange interaction in the coupled bilayers with 4 and 7 nm TbFe layer, coherent magnetization rotation of Py and TbFe layers is very similar to the behavior described by the Stoner-Wolfarth model. Magnetic hysteresis in the coupled bilayers with 10 nm TbFe layer and the decrease in the maximum engineering magnetostrictive strain in the coupled bilayers with 30 nm TbFe layer were observed.

Angular Response of Magnetostrictive Thin Films

The magnetostrictions of the single TbFe layer and coupled Py/TbFe2 bilayers were measured by using laser deflectometry. The dependences of the magntostriction performance on the driving magnetic field direction have been investigated. The relationship studies between the saturation bending angle and torsion angle of the single layer with perpendicular anisotropy and coupled bilayers with in‐plane uniaxial anisotropy have been conducted. Interesting “jump” reflecting the spin dynamics is observed in the magnetostriction loops of the coupled bilayers.

Magnetostriction of FM/TbFe (FM=Fe, Py-permalloy, FeCo) coupled bilayer

Choosing TbFe as the magnetostrictive layer and using soft ferromagnetic materials (FM=Fe, Py-permalloy, FeCo) as the inductive layer, we report the magnetization and magnetostriction of the S/FM(10nm)/TbFe(10nm) coupled bilayer (S-glass substrate). The magnetization of the coupled bilayer is attributed to the soft ferromagnetic layer and there is no contribution from the TbFe layer. Interfacial exchange interaction plays an important role in determining the magnetostriction of the coupled bilayer. The effect coming from the deposition Argon pressure on the magnetostriction of the coupled bilayer was also investigated.

Investigation on the structural and magnetic properties of sputtered TbFe2/Fe3Ga heterostructures

We have analyzed the structural and magnetic properties of as-grown and annealed [TbFe2/Fe3Ga]n heterostructures grown by sputtering. Evidence of the bcc structure in the Fe3Ga layers has been found. The diffraction peak related to this structure shifts to high angles with the annealing temperature. Also, we have observed a change in the microstructure of the Tb-Fe layers when the thickness layer is reduced in the as-grown heterostructures. Moreover, the Tb content is lower than 33% of the TbFe2 Laves phase and it depends on the layer thickness. The thermal treatments promote the increase of the Tb content, but only in the heterostructures with thick layers. The strong lattice mismatch between the Tb-Fe and Fe-Ga layers seems to prevent a complete Tb diffusion upon the annealing process. Thus, the crystallization of the TbFe2 Laves phase is inhibited in the heterostructures with thin layers, although our experimental results indicate the presence of potential magnetostrictive TbFeGa alloys.

Exchange-bias training effect inTbFe∕GdFe: Micromagnetic mechanism

We present magnetization and Kerr microscopy measurements of the exchange-bias training effect in hard/soft, ${\mathrm{Tb}}_{12}{\mathrm{Fe}}_{88}$/${\mathrm{Gd}}_{40}{\mathrm{Fe}}_{60}$, bilayers. These experimental results, compared with micromagnetic simulations, unambiguously show the role of the soft GdFe layer reversal on irreversible magnetic changes in the hard TbFe layer. After a partial reversal of the GdFe magnetization layer, the next field cycle exhibits a double hysteresis loop which is due to the existence of two types of domains in the sample, and only one of these domains has been subject to the training effect. The antiferromagnetically interfacial coupling and positive exchange-bias shift permit us to exclude any direct contribution from the magnetic field to this mechanism.

Photon pressure and magnetic moment transfer in multilayered nanosize films

The effect of nanosecond laser pulses with λ = 355 nm on the Kerr and Faraday angles in nanosize layered magnetic films Al2O3/TbFe/Al2O3/TbFe/Al2O3 and Al2O3/TbFe/Au/TbFe/Al2O3 is studied experimentally. The first ferrimagnetic TbFe layer in these films with transverse anisotropy is enriched by terbium, while the second Tb22Fe78 layer is enriched by iron relative to the composition at the compensation point. When the ferrimagnetic TbFe layers in the film are magnetized in the same direction, the magnetooptical characteristics of each TbFe layers and of the film as a whole depend on the laser radiation power, which is in good agreement with the temperature characteristics. When the TbFe layers are magnetized in opposite directions for a high power of nanosecond pulses, the values of the Faraday and Kerr angles for the output TbFe layer sharply change and even reverse their signs. These results are explained by the effect of electrons with a high spin polarization, which are injected by radiation due to the photon pressure. In the case of antiparallel magnetizations of the TbFe layers, a high concentration of such electrons not only changes the local magnetization of the output layer, but also causes its magnetization reversal due to magnetic moment transfer by such electrons.

Influence of lateral domains and interface domain walls on exchange-bias phenomena inGbFe∕TdFebilayers

The effect of lateral magnetic domains on exchange-bias phenomena in a soft/hard Gd 40 Fe 60 100 nm / Tb 12 Fe 88 50 nm bilayer with antiferromagnetic interface exchange coupling is investigated. At low temperature , domains in the hard TbFe layer are found to behave independently of each other and to give rise to their own exchange-bias fields. Double hysteresis loops are observed when the sample is cooled with domains present in both the hard TbFe layer and the soft GdFe layer, whereas single hysteresis loops are observed when domains are only present in the hard layer. These features are understood by taking into account the presence of both lateral domains and planar interface domain walls in this strongly coupled system.

Training Effect in an Exchange Bias System: The Role of Interfacial Domain Walls

Polarized neutron reflectivity (PNR) is used to obtain the magnetic depth profile of an antiferromagnetically coupled ferrimagnetic/ferrimagnetic bilayer, Gd40Fe60/Tb12Fe88. This system shows a transition from positive to negative exchange bias field H(E) as the cooling field H(cf) is increased from small to large positive value. It also exhibits training behavior upon field cycling which affects H(E) and the coercive field H(C). From the PNR measurements at room temperature and at 15 K, we confirm that the magnetic configuration inside the TbFe layer is frozen when the sample is cooled in various H(cf). The thickness and pitch of the magnetic twist inside the TbFe layer depend on H(cf) and give rise to the observed differences in the bias field. Irreversible reorganization of the TbFe magnetization at the interface occurs upon GdFe magnetization reversal and is found to explain the training effect as well as the overall reduction in coercivity.

Effect of annealing on the structural and magnetic properties of giant magnetostrictive multilayers

Exchange-coupled multilayered thin films which combine giant magnetostriction and soft magnetic properties are of growing interest for applications. TbFe 2/Fe and TbFe 2/FeCo nanometric multilayers were grown by RF-sputtering onto glass substrates. A bias field of 250 Oe was applied along the longitudinal axis, inducing an in-plane uniaxial magnetic anisotropy. For es-deposited multilayers, TbFe layers are amorphous while soft layers are nanocrystallized. The effect of annealing was investigated by 57Fe Mössbauer spectrometry, magnetic and magnetostriction measurements in the temperature ( T ann) range from 150 to 350 °C. After annealing, the Mössbauer analysis evidences for an increase of the crystallized iron component, probably due to a thinning of the interfaces. A maximum of the magnetoelastic coefficient is found for T ann around 250 °C, with the highest value obtained in the case of TbFe/FeCo. Furthermore, stress relaxation within the film reduces the anisotropy field, leading to a large magnetoelastic susceptibility of about 250 MPa/kOe.

Magnetization reversal in exchange-coupled GdFe/TbFe studied by x-ray magnetic circular dichroism

This work is dedicated to the study of magnetization reversal processes in a ferromagnetically exchange coupled ferrimagnetic/ferrimagnetic bilayer system made of a soft magnetic GdFe layer and a hard TbFe layer. Regular magnetization measurements and x-ray magnetic circular dichroism (XMCD) measurements performed at the ESRF on a GdFe (100 nm)/TbFe (3 nm) bilayer at different temperatures are presented and compared. The XMCD measurements allowed us to study separately the magnetic contribution of Gd, Tb, and Fe at the system interface. Evidence of the creation of an interface domain wall and its compression and pinning are given. In addition to quasistatic measurements, the interface magnetization reversal was also studied using XMCD aftereffect measurements. Both magnetization and XMCD measurements are explained by considering the formation of lateral domains during the interface magnetization reversal.

Tunnel Magnetoresistance Effect and the Magnetization Process of Gd Fe/Al oxide/Co/TbFe Junctions

Magnetic tunnel junctions (MTJs) consisting of GdFe/Al oxide/Co/TbFe layers were fabricated. The GdFe and TbFe layers were perpendicularly magnetized, and the TbFe and Co layers were exchange-coupled with each other. Magnetoresistance (MR) curves were measured for MTJs with Co layers of various thicknesses while applying a field perpendicular to the film plane. The MR ratio showed a maximum of 12.1% when the Co layer thickness was 0.5 nm. Calculation using a noncontinuum model showed that the direction of the Co magnetization in the exchange-coupled film was perpendicular to the film plane when the thickness of the Co layer was less than 0.4 nm, which was consistent with measured magnetization curves as well as the results of simulation.

Exchange magnetostriction in TbFe/FeCuNbSiB multilayers

The magnetostrictive changes arising from the magnetic transition have been investigated in TbFe/FeCuNbSiB multilayer systems. The spin-flop behavior of the magnetization has clearly been observed in the samples which possess in-plane anisotropy. The magnetostriction measured in the longitudinal (transverse) direction shows an up and down (down and up) change associated with the spin-flop. The spin-flop field has been found to decrease with increasing the Tb–Fe layer thickness. Furthermore, the spin-flop field decreases and the magnetic softness represented by susceptibility and coercivity are improved on annealing.