Change In Perpendicular Magnetic Anisotropy Research Articles

Electric-field control of skyrmions in multiferroic heterostructure via magnetoelectric coupling

Room-temperature skyrmions in magnetic multilayers are considered to be promising candidates for the next-generation spintronic devices. Several approaches have been developed to control skyrmions, but they either cause significant heat dissipation or require ultrahigh electric fields near the breakdown threshold. Here, we demonstrate electric-field control of skyrmions through strain-mediated magnetoelectric coupling in ferromagnetic/ferroelectric multiferroic heterostructures. We show the process of non-volatile creation of multiple skyrmions, reversible deformation and annihilation of a single skyrmion by performing magnetic force microscopy with in situ electric fields. Strain-induced changes in perpendicular magnetic anisotropy and interfacial Dzyaloshinskii–Moriya interaction strength are characterized experimentally. These experimental results, together with micromagnetic simulations, demonstrate that strain-mediated magnetoelectric coupling (via strain-induced changes in both the perpendicular magnetic anisotropy and interfacial Dzyaloshinskii–Moriya interaction is responsible for the observed electric-field control of skyrmions. Our work provides a platform to investigate electric-field control of skyrmions in multiferroic heterostructures and paves the way towards more energy-efficient skyrmion-based spintronics.

Strain-induced reversible manipulation of orbital magnetic moments in Ni/Cu multilayers on ferroelectric BaTiO3

Controlling magnetic anisotropy by orbital magnetic moments related to interfacial strains has considerable potential for the development of future devices using spins and orbitals. For the fundamental physics, the relationship between strain and orbital magnetic moment is still unknown, because there are few tools to probe changes of orbital magnetic moment. In this study, we developed an electric field (E)-induced X-ray magnetic circular dichroism (EXMCD) technique to apply E to a ferroelectric BaTiO3 substrate. We reversibly tuned the interfacial lattice constants of Ni/Cu multilayers on BaTiO3 using this technique. As the domain structures in BaTiO3 are modulated by E, EXMCD measurements reveal that the changes in the magnetic anisotropy of Ni/Cu films are induced through the modulation of orbital magnetic moments in Ni with magneto-elastic contributions. The strained Ni layer that induces the perpendicular magnetic anisotropy without E is released at E = 8 kV/cm, and in-plane magnetization also occurs. We observed that EXMCD measurements clarified the origin of the reversible changes in perpendicular magnetic anisotropy and established the relationship between macroscopic inverse magnetostriction effects and microscopic orbital moment anisotropy.

Domain wall energy and strain in Pt/Co/Ir thin films on piezoelectric transducers

We study the energy and creep velocity of magnetic domain walls in perpendicularly magnetised Pt/Co/Ir thin films under strain. We find that the enhancement of domain wall creep velocity under strain from piezoelectric transducers is largest in films with the thinnest Co layers (0.56 nm), in which the strain causes the smallest relative change in perpendicular magnetic anisotropy and the largest relative change in domain wall creep velocity. We show how domain wall energy is predictive of the sensitivity of domain wall creep velocity to changes in strain, and thus provide a route to designing magnetic thin film systems for optimum strain control.

Electric field effect on magnetic anisotropy for Fe-Pt-Pd alloys

The electric field effect on magnetic anisotropy was investigated for the FePt1-xPdx alloy films with perpendicular magnetic anisotropy. The polar magneto-optical Kerr (p-MOKE) loops were measured under the electric field application in order to evaluate the electric field-induced perpendicular magnetic anisotropy change per area (Δεperpt). A clear change in the saturation field of p-MOKE loop was observed for FePt by varying the applied electric field (ΔE). In the case of FePt, Δεperpt divided by ΔE was evaluated to be -129 (fJ/Vm). We found that the magnitude of Δεperpt / ΔE was significantly reduced with increasing x.

Modification of Pt/Co/Pt film properties by ion irradiation

We studied the structural modifications of a Pt/Co/Pt trilayer epitaxial film under ${\mathrm{Ga}}^{+}$ 30-keV ion irradiation by means of classical molecular dynamics and Monte Carlo simulations. The semiclassical tight-binding second-moment approximation potential was adjusted to reproduce the enthalpies of formation, the lattice constants, and the order-disorder transition temperatures for Co-Pt alloys. We found that during irradiation, the sandwich-type Pt(fcc)/Co(hcp)/Pt(fcc) film structure underwent a transition to the new solid solution $\ensuremath{\alpha}\ensuremath{-}\mathrm{Co}/\mathrm{Pt}(\mathrm{fcc})$ phase. Our analysis of the short-range order indicates the formation, within a nanosecond time scale, of a homogeneous chemically disordered solution. The longer time-scale simulations employing a Monte Carlo algorithm demonstrated that the transition from the disordered phase to the ordered $L{1}_{0}$ and $L{1}_{2}$ phases was also possible but not significant for the changes in perpendicular magnetic anisotropy (PMA) observed experimentally. The strain analysis showed that the Co layer was under tensile strain in the lateral direction at the fluences of $1.5\ifmmode\times\else\texttimes\fi{}{10}^{14}--3.5\ifmmode\times\else\texttimes\fi{}{10}^{14}\phantom{\rule{0.28em}{0ex}}{\mathrm{ions}\phantom{\rule{0.16em}{0ex}}\mathrm{cm}}^{\ensuremath{-}2}$; this range of fluences corresponds to the appearance of PMA. This strain was induced in the initially relaxed hcp Co layer due to its partial transformation to the fcc phase and to the influence of atomic layers with larger lattice constants at upper/lower interfaces.

Magnetic characteristics of Tb x (FeCoV)100−x films

The magnetic characteristics of TbFeCoV films deposited at different sputtering conditions were investigated using magnetic force microscope, vibrating sample magnetometer and other testing equipments. A minimum out-of-plane saturation magnetization and a maximum out-of-plane coercivity were obtained for the Tb x (FeCoV)100−x films near the point of compensation x = 23. As x = 23–45, perpendicular magnetization component could be observed, while the second largest saturation magnetization and a relatively low coercivity were obtained in the x = 40 sample. For the amorphous Tb40(FeCoV)60 films, with the increase of sputtering pressure or power, the out-of-plane saturation magnetization increases to a maximum and then decreases while the out-of-plane coercivity shows a monotonously decreasing trend as sputtering power is increased from 43 to 64 W. The change of perpendicular magnetic anisotropy can be confirmed by the difference of magnetic domain for the samples deposited at different sputtering pressures or powers.

Opposite signs of voltage-induced perpendicular magnetic anisotropy change in CoFeB|MgO junctions with different underlayers

We report a voltage-induced perpendicular magnetic anisotropy (PMA) change in sputter-deposited Ta|CoFeB|MgO and Ru|CoFeB|MgO junctions. The PMA change is quantitatively evaluated by the field dependence of the tunneling magnetoresistance for various bias voltages. We find that both the sign and amplitude of the voltage effect depend on the underlayer, Ta or Ru, below the CoFeB layer. The rf voltage-induced ferromagnetic resonance spectra also support the underlayer-material-dependent direction of the voltage torque. The present study shows that the underlayer is one of the key parameters for controlling the voltage effect.

Pulse voltage-induced dynamic magnetization switching in magnetic tunneling junctions with high resistance-area product

We investigated pulse voltage-induced dynamic magnetization switchings in magnetic tunneling junctions with a high resistance-area product of 2 kΩ μm2. We found that bistable switching and the oscillatory behavior of switching probability as a function of voltage pulse duration are realized at a lower current density (−1.1 × 105 A/cm2) than in conventional spin-transfer-torque-induced magnetization switching. In addition, the switching probability at different voltage pulse strengths confirmed the existence of a voltage torque induced by a change in perpendicular magnetic anisotropy. This voltage-induced magnetization switching can be a useful technique in future spintronics devices with fast and highly reliable writing processes.

Large change of perpendicular magnetic anisotropy in Cobalt ultrathin film induced by varying capping layers

The magnetic films of Co with Si/SiO2/Pt/Co/molecule structure were fabricated and their structural properties and magnetic anisotropy were investigated by varying both Co (0.5–1.8 nm) thickness and molecular capping layers of 5,6,11,12-tetraphenylnaphthacene (rubrene) and copper phthalocyanine (CuPc), respectively. The crystal structures were characterized using x-ray diffraction (XRD) and the magnetization curves were measured using vibrating sample magnetometer with an applied field both in parallel and perpendicular to a film plane. It was found that the thickness of Co for the maximum perpendicular magnetic anisotropy (PMA) is around 0.7 nm for both group films. However, the estimated effective magnetic anisotropy energy for Co was 2.9 ± 0.3 × 106 erg/cc for rubrene-capped sample, which was smaller than the value of 4.9 ± 0.4 × 106 erg/cc for CuPc-capped sample. The XRD patterns showed the crystal structure of rubrene layer was of amorphous structure and CuPc layer was polycrystalline. The different interface effects of Co/CuPc and Co/rubrene were discussed to analyze the change of PMA.

Large change in perpendicular magnetic anisotropy induced by an electric field in FePd ultrathin films

The magnetic properties of FePd ultrathin films and their variation under the influence of an electric field are investigated by magneto-optical Kerr effect (MOKE) measurements. L10-ordered FePd shows a spin reorientation transition when varying the thickness. The easy axis of magnetization is found to be normal to the plane at thicknesses above 9 monolayers (MLs) and in-plane below 9 ML. The coercive field, the perpendicular magnetic anisotropy and the MOKE signal at saturation vary with the applied electric field. The sensitivity of the interface magnetic anisotropy is estimated to be 602 fJ/V m.

Voltage-induced perpendicular magnetic anisotropy change in magnetic tunnel junctions

A voltage-induced perpendicular magnetic anisotropy change in an ultrathin FeCo layer was observed in an epitaxial magnetic tunnel junction (MTJ) structure. A spin-transfer induced ferromagnetic resonance measurement technique was used under various bias voltage applications to evaluate the anisotropy change. From the peak frequency shifts, we could estimate that a surface magnetic anisotropy change of 15 μJ/m2 was induced by an electric field application of 400 mV/nm in the MTJ with a 0.5 nm thick FeCo layer. The realization of voltage-induced anisotropy changes in an MTJ structure should have a large impact on the development of electric-field driven spintronic devices.

Co/Pd 다층막구조가 수직자기터널접합의 자기저항에 미치는 영향

Co/Pd 다층막을 이용한 수직자기터널접합에서 Co 전극 및 Co, Pd 다층막의 두께변화가 터널링 자기저항비에 미치는 영향에 대해 조사하였다. Co 전극의 경우 0.5nm 두께 부근에서 최대 자기저항비 값을 얻을 수 있었으며, 이는 터널배리어층 부근의 계면영역이 터널링 스핀분극에 주요한 역할을 하기 때문으로 보인다. 다층막내의 Co층의 두께가 증가함에 따라 자기저항비는 다소 복잡한 거동을 나타내었으며, 이는 Co층의 두께 증가에 따른 수직자기이방성의 변화와 계면거칠기 감소에 따른 접합저항의 감소가 복합적으로 작용하기 때문이다. Pd층의 경우 Co층과는 달리 자기저항변화(<TEX>${\Delta}R$</TEX>)감소가 자기저항비의 거동에 영향을 주었으며, 이는 비자성층인 Pd층의 증가에 따라 스핀산란이 증가하기 때문이다. We investigated the magnetoresistance of perpendicularly magnetized magnetic tunnel junction composed of Co/Pd multilayers. The magnetoresistance was maximized with Co electrodes of about 5 nm thickness, which evidenced the important role of the interface in tunneling process. Both the change in perpendicular magnetic anisotropy and improvement of junction resistance were observed with changing Co sublayers, while the spin scattering became dominant with increasing Pd sublayers.

Magnetization reversal process of DyFeCo films under an addition of mechanical stress for application of MRAM

Magnetization reversal process of DyFeCo films with stress was evaluated by the anomalous Hall effect. Anomalous Hall effect measurement is suitable to observe the change of perpendicular magnetic anisotropy of DyFeCo films when the mechanical tensile stress is applied to them. Stress applied to the RE-transition metal (TM) thin films significantly reduced the perpendicular coercivity of the film in comparison with that of the film in stress-free condition.

Effect of interface structure modifications on the properties of some magnetic multilayers

The effect of post-deposition thermal annealing on the interface structures of crystalline Fe–Tb multilayers and the associated changes in perpendicular magnetic anisotropy (PMA) has been studied using X-ray reflectivity, X-ray diffuse scattering and conversion electron Mössbauer spectroscopy. The main effect of annealing is a demixing at the interfaces attributed to a positive heat of mixing between Fe and Tb. Demixing is accompanied by a decrease in PMA due to increased shape anisotropy of the crystalline Fe layer. The results are compared with the effect of heavy ion irradiation in the same system.

Structural Anisotropy in Sputter-and Vacuum-Deposited Dy/Fe Multilayers

Local structural anisotropy was estimated using extended X-ray absorption fine structure (EXAFS) spectroscopy for Dy/Fe multilayers prepared by sputtering and vacuum deposition methods. Fourier transformed Fe-EXAFS indicated a difference in the atomic coordination between in-plane and out-of-plane. Both the structural anisotropy and the perpendicular magnetic anisotropy change depending on the bilayer period and the preparation methods. The variation of the structural anisotropy was found corresponds closely to that of the perpendicular magnetic anisotropy