Enhancement Of Perpendicular Magnetic Anisotropy Research Articles

Strain-tunable electric structure and magnetic anisotropy in monolayer CrSI.

Two-dimensional (2D) ferromagnetic semiconductors provide platforms for studying novel physical phenomena in low dimensional materials. Using first-principles calculations, we systematically investigate the effect of strain on the electronic structure and magnetic anisotropy energy (MAE) of monolayer CrSI. The results demonstrate that the easy axis of unstrained monolayer CrSI is parallel to the in-plane [100] axis and the MAE of monolayer CrSI is mainly contributed by the spin-polarized p-orbitals of nonmetallic I atoms. Remarkably, the strain transforms the ground state of monolayer CrSI from a ferromagnetic semiconductor to ferromagnetic metal. More importantly, the external strain can switch the direction of the easy axis of monolayer CrSI and compressive strain significantly enhances the MAE of the I atom to reach 0.52 meV per atom, which is comparable to that of metallic Fe atoms at Fe/MgO interfaces. Furthermore, we elucidate that the increase of the positive contributions of matrix element differences between the spin-up px and py orbitals as well as spin-up py and pz orbitals of the I atom to MAE with respect to compressive strain is the main cause of the significant enhancement in perpendicular magnetic anisotropy of monolayer CrSI under -10% compressive strain. Our research proves that monolayer CrSI has a good application prospect in magnetic storage devices.

Enhancement of Perpendicular Magnetic Anisotropy Through Fe Insertion at the CoFe/W Interface

Magnetic tunnel junctions (MTJs) based on MgO/CoFeB/W structures are of particular interest for magnetic random access memories due to their high tunnel magnetoresistance, low damping, and stable properties at high annealing temperature. One of the major challenges is to obtain a strong interfacial perpendicular magnetic anisotropy (PMA) for sufficient thermal stability during data storage. Here we investigate magnetic anisotropy energies (MAEs) in the ferromagnet (FM)/W structure with different FM materials and interface insertions. We find that the Fe/W interface exhibits a remarkable PMA, whereas the Co/W interface displays a large in-plane anisotropy, which reduces the PMA in the MgO/CoFeB/W structure. By inserting a thin Fe layer at the CoFe/W interface, the interfacial PMA can be enhanced to 3.34 mJ/m2. Microscopic mechanism behind these phenomena is explained with the changes of orbital-resolved MAEs due to the charge redistribution at the interface. These findings pave a new way for the enhancement of PMA and point toward the possibility of achieving high thermal stability in small-node MTJs.

Origin of focused laser irradiation-induced enhancement of perpendicular magnetic anisotropy in Pt/Co/Pt thin films investigated by spatially resolved x-ray absorption spectroscopy

The origin of the focused single-pulse laser irradiation-induced changes in magnetic anisotropy of a Pt/Co/Pt film is investigated by the x-ray absorption near-edge structure and extended x-ray absorption fine structure techniques combined with the photoelectron emission microscope. A significant increase of the Co–Co bond length in both in-plane and out-of-plane directions is observed on the periphery of the laser spot, at which perpendicular magnetization appears. With increasing laser power density towards the center of the laser spot, anisotropic structural changes are observed accompanied by the reappearance of in-plane magnetization. The enhancement of perpendicular magnetization is attributed to the lattice expansion-induced magnetoelastic effect, while the in-plane compressive strain in the Co film is suggested to be the origin of the reappearance of in-plane magnetization at higher laser power densities.

Magnetic properties of L10 FePt thin film influenced by recoverable strains stemmed from the polarization of Pb(Mg1/3Nb2/3)O3–PbTiO3 substrate**Project supported by the National Natural Science Foundation of China (Grant Nos. 11474167, 51701091, and 11504020) and the Start-up Funds of NUIST, China (Grant Nos. 2243141601035 and 2243141601018).

The magnetic properties and magnetization reversible processes of L10 FePt (3 nm)/Pb(Mg1/3Nb2/3)O3–PbTiO3 (PMN–PT) heterostructure were investigated by using the phase field model. The simulation results show that the magnetic coercivities and magnetic domains evolution in the L10 FePt thin film are significantly influenced by the compressive strains stemming from the polarization of single crystal PMN–PT substrate under an applied electric field. It is found that the magnetic coercivities increase with increasing of the compressive strain. A large compressive strain is beneficial to aligning the magnetic moments along the out-of-plane direction and to the enhancement of perpendicular magnetic anisotropy. The variations of magnetic energy densities show that when compressive strains are different at the magnetization reversible processes, the magnetic anisotropy energies and the magnetic exchange energies firstly increase and then decrease, the negative demagnetization energy peaks appear at coercivities fields, and the magnetoelastic energies are invariable at large external magnetic field with the energy maximum appearing at coercivities fields. The variations of the magnetoelastic energies bring about the perpendicular magnetic anisotropy so that the magnetoelastic energy is lower at the large external magnetic fields, whereas the appearance of magnetoelastic energy peaks is due to the magnetization-altered direction from the normal direction of the plane of the L10 FePt thin film at coercivities fields.

Enhancement of perpendicular magnetic anisotropy and its electric field-induced change through interface engineering in Cr/Fe/MgO

Recently, perpendicular magnetic anisotropy (PMA) and its voltage control (VC) was demonstrated for Cr/Fe/MgO. In this study, we shed light on the origin of large voltage-induced anisotropy change in Cr/Fe/MgO. Analysis of the chemical structure of Cr/Fe/MgO revealed the existence of Cr atoms in the proximity of the Fe/MgO interface, which can affect both magnetic anisotropy (MA) and its VC. We showed that PMA and its VC can be enhanced by controlled Cr doping at the Fe/MgO interface. For Cr/Fe (5.9 Å)/Cr (0.7 Å)/MgO with an effective PMA of 0.8 MJ/m3, a maximum value of the voltage-controlled magnetic anisotropy (VCMA) effect of 370 fJ/Vm was demonstrated due to Cr insertion.

Strain induced enhancement of perpendicular magnetic anisotropy in Co/graphene and Co/BN heterostructures

Perpendicular magnetic tunnel junctions in the next-generation magnetic memory using current induced magnetization switching will likely rely on a material design that can enhance the perpendicular magnetic anisotropy of heterojunctions containing only light elements. Using first-principles calculations, we investigated the effect of compressive and tensile strain on the perpendicular magnetic anisotropy of light element heterostructures of Co films, Co/graphene, and Co/BN. We found that the perpendicular magnetic anisotropy of Co/graphene is greatly enhanced compared to the Co films, while that of Co/BN is reduced compared to the Co films. In addition, tensile strain can further enhance perpendicular magnetic anisotropy of Co/graphene and Co/BN heterojunctions by 48.5% and 80.8%, respectively, compared to the unstrained systems. A density of state analysis, combined with layer and orbital magnetic anisotropy contributions obtained from a second-order perturbation theory of the spin-orbit coupling, reveals that the tensile strain effect arises from the increase of the hybridization between same spin ${d}_{xy}$ and ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ states of the surface Co film. Our results suggest that strain engineering is an effective approach to enhance the perpendicular magnetic anisotropy of light element heterostructures.

Ferroelastic Strain-Mediated Nonvolatile Tuning of Perpendicular Magnetic Anisotropy in (Co/Pt)3 /(1 1 1) Pb(Mg1/3Nb2/3)O3-PbTiO3 Multiferroic Heterostructures

Nonvolatile control of perpendicular magnetic anisotropy (PMA) is of great technological importance for low-power spintronic devices. In this study, we successfully achieved ferroelastic strain-mediated nonvolatile tuning of PMA at room temperature in (Co/Pt)3/(1 1 1) Pb(Mg1/3Nb2/3)O3-PbTiO 3 multiferroic heterostructures. Electric field control of magnetic anisotropy was qualitatively studied by ferromagnetic resonance. The anisotropy could be reversibly flipped between low and high perpendicular states by electric field impulses. The application of a negative electric field to positively poled Pb(Mg1/3Nb 2/3)O3-PbTiO3 resulted in a significant enhancement of PMA up to 250 Oe, in which nonvolatile modulation contributed more than 150 Oe. The nonvolatile behavior is due to non-180° ferroelastic domain switching in (1 1 1) Pb(Mg1/3Nb2/3)O3-PbTiO3 as revealed by X-ray diffraction. Ferroelastic domain switching in multiferroic heterostructures is an effective way to nonvolatilely modulate PMA that will enable the realization of high-density and power-efficient spintronic memory.

Enhancement of perpendicular magnetic anisotropy and coercivity in ultrathin Ru/Co/Ru films through the buffer layer engineering

We present results on a study of the interplay between microstructure and the magnetic properties of ultrathin Ru/Co/Ru films with perpendicular magnetic anisotropy (PMA). To induce PMA in the Co layer, we experimentally determined thicknesses of the buffer and capping layers of Ru. The maximum value of PMA was observed for the Co thickness of 0.9 nm with the 3 nm thick capping layer. The effective anisotropy field (Heff) and coercive force (Hc) of the Co layer are very sensitive to the Ru buffer layer thickness (tb). The values of Heff and Hc increase approximately by two and ten times, correspondingly, when tb changes from 6 to 20 nm, owing to an increase in volume fraction of the crystalline phase as a result of the grains’ growth. PMA is found to be mainly enhanced by elastic strains induced by the lattice mismatch on the Ru/Co and Co/Ru interfaces, leading to the deformation of the Co lattice. The surface impact is determined to be less than 10% of the magneto-elastic contribution to the effective anisotropy. Observation of the magnetic domain structure by means of polar Kerr microscopy reveals that out-of-plane magnetization reversal occurs through the nucleation, growth, and annihilation of domains, where the average size drastically rises with the increasing tb.

Off-easy-plane antiferromagnetic spin canting in coupled FePt/NiO bilayer structure with perpendicular exchange bias

We report on the investigation of perpendicular exchange bias in $\mathrm{FePt}(001)/\mathrm{NiO}(\overline{1}\overline{1}1)$ orthogonal exchange couple with FePt partially $\mathrm{L}{1}_{0}$ ordered. From initial magnetization curve measurement and magnetic domain imaging, we find that, for the as-grown bilayer structure, the FePt layer experiences a small-angle magnetization rotation when it is magnetized near to saturation in film normal direction. After field cooling, the bilayer structure shows a significant enhancement of perpendicular magnetic anisotropy, indicating the field mediated coupling between the spins across the FePt/NiO interface. According to Koon's theoretical calculation on the basis of lowest energy ferromagnetic/antiferromagnetic coupling configuration for compensated spins at antiferromagnetic side, we consider slightly slanted Ni spins at the interface off the $(\overline{1}\overline{1}1)$ easy plane can stabilize the spin coupling between FePt and NiO and result in the observed exchange bias in this paper. This consideration was further confirmed by stripe domain width calculation.

Large enhancement of perpendicular magnetic anisotropy and high annealing stability by Pt insertion layer in (Co/Ni)-based multilayers

We have investigated the influence of ultrathin Pt insertion layers on the perpendicular magnetic anisotropy (PMA) and annealing stability of Ta/Pt/(Co/Ni)×3/Co/Pt/Ta multilayered films. When the Pt layers were inserted at the Co/Ni interfaces, the PMA of the multilayered films decreased monotonically as the thickness of the Pt insertion layer (tPt) was increased. However, when the Pt layers were inserted at the Ni/Co interfaces, the PMA increased from 1.39 × 106 to 3.5 × 106 erg/cm3 as tPt increased from 0 to 10 Å. Moreover, the multilayered film containing 6-Å-thick Pt insertion layers that inserted at the Ni/Co interfaces exhibited the highest annealing stability for PMA, which was up to temperature of 480 °C. We hypothesize that the introduced Pt/Co interfaces, due to the Pt insertion layers, are responsible for the enhanced PMA and high annealing stability. This study is particularly important for perpendicularly magnetized spintronic devices that require high PMA and high annealing stability.

Magnetoelastically induced perpendicular magnetic anisotropy and perpendicular exchange bias of CoO/CoPt multilayer films

The effects of magnetoelastically induced perpendicular magnetic anisotropy (PMA) on perpendicular exchange bias (PEB) have been studied in [CoO5nm/CoPt5nm]5 multilayer films. After deposition at room temperature, [CoO5nm/CoPt5nm]5 multilayer films were post-annealed at 100°C, 250°C, 300°C and 375°C for 3h. In-plane tensile stress of CoPt layer was calculated by sin2φ method, and we found it increased gradually upon annealing from 0.99GPa (as-deposited) up to 3.02GPa (300°C-annealed). As to the magnetic property, significant enhancement of PMA was achieved in [CoO5nm/CoPt5nm]5 multilayer films after annealing due to the increase of CoPt layer in-plane tensile stress. With the enhancement of magnetoelastically induced PMA, great improvement of PEB was also achieved in [CoO5nm/CoPt5nm]5 multilayer films, which increased from 130Oe (as-deposited) up to 1060Oe (300°C-annealed), showing the same change tendency as PMA and the strong correlation with CoPt layer in-plane tensile stress. We consider it is the increase of CoPt layer in-plane tensile stress that leads to the enhancement of CoPt layer PMA, which is favorable for the spins in CoPt layer aligning to a more perpendicular direction. And thus the enhanced PMA with more perpendicular spins alignment in CoPt layer results in the improved PEB in [CoO5nm/CoPt5nm]5 multilayer films through enhanced perpendicular spins coupling at CoO/CoPt interfaces.

Impurity-induced enhancement of perpendicular magnetic anisotropy in Fe/MgO tunnel junctions

Using first-principles calculations, we investigated the impact of chromium (Cr) and vanadium (V) impurities on the magnetic anisotropy and spin polarization in Fe/MgO magnetic tunnel junctions. It is demonstrated, using layer resolved anisotropy calculation technique, that while the impurity near the interface has a drastic effect in decreasing the perpendicular magnetic anisotropy (PMA), its position within the bulk allows maintaining high interfacial PMA while reducing the bulk magnetization and correlatively the easy-plane demagnetizing energy. As a result, the effective magnetic anisotropy tends to increase as a function of the Cr or V concentration resulting in an increase in the critical magnetic thickness at which the crossover from out-of-plane to in-plane anisotropy takes place. At the same time, the interfacial spin polarization is not affected by the magnetic layer bulk doping by Cr or V impurities and even enhanced in most situations thus favoring an increase of tunnel magnetoresistance (TMR) amplitude.

Enhancement of perpendicular magnetic anisotropy by compressive strain in alternately layered FeNi thin films

The effect of the lattice strain on magnetic anisotropy of alternately layered FeNi ultrathin films grown on a substrate, Cu(tCu = 0–70 ML)/Ni48Cu52(124 ML)/Cu(0 0 1) single crystal, is systematically studied by means of in situ x-ray magnetic circular dichroism (XMCD) and reflection high-energy electron diffraction (RHEED) analyses. To investigate the magnetic anisotropy of the FeNi layer itself, a non-magnetic substrate is adopted. From the RHEED analysis, the in-plane lattice constant, ain, of the substrate is found to shrink by 0.8% and 0.5% at tCu = 0 and 10 ML as compared to that of bulk Cu, respectively. Fe L-edge XMCD analysis is performed for n ML FeNi films grown on various ain, and perpendicular magnetic anisotropy (PMA) is observed at n = 3 and 5, whereas the film with n = 7 shows in-plane magnetic anisotropy. Moreover, it is found that PMA is enhanced with decreasing ain, in the case where a Cu spacer layer is inserted. We suppose that magnetic anisotropy in the FeNi films is mainly carried by Fe, and the delocalization of the in-plane orbitals near the Fermi level increases the perpendicular orbital magnetic moment, which leads to the enhancement of PMA.

Enhancement of perpendicular magnetic anisotropy and transmission of spin-Hall-effect-induced spin currents by a Hf spacer layer in W/Hf/CoFeB/MgO layer structures

We report that strong perpendicular magnetic anisotropy of the ferromagnetic layer in a W/CoFeB/MgO multilayer structure can be established by inserting a Hf layer as thin as 0.25 nm between the W and CoFeB layers. The Hf spacer also allows transmission of spin currents generated by an in-plane charge current in the W layer to apply strong spin torque on the CoFeB, thereby enabling current-driven magnetic switching. The antidamping-like and field-like components of the spin torque exerted on a 1 nm CoFeB layer are of comparable magnitudes in this geometry. Both components originate from the spin Hall effect in the underlying W layer.

Enhanced perpendicular magnetic anisotropy in Fe/Mn bilayers by incorporating ultrathin ferromagnetic underlayer through magnetic proximity effect

The perpendicular magnetic anisotropy(PMA) was shown to be established in ferromagnetic (FM)/fcc-Mn bilayers through the FM-antiferromagnetic (AFM) exchange coupling. We demonstrate here that such PMA can be further enhanced by incorporating an ultrathin Fe film as an underlayer. In a series of Fe/Mn bilayers, hysteresis loop measurement shows that the thickness of top Fe layer with PMA can be extended to a thicker range while an ultrathin Fe underlayer is inserted. Such enhancement of PMA is attributed to an increase of AFM ordering on the Mn film originated from the magnetic proximity effect with the Fe underlayer.

Perpendicular magnetic anisotropy in a Pt/Co/Pt ultrathin film arising from a lattice distortion induced by ion irradiation

Ga${}^{+}$ irradiation-induced changes in magnetic anisotropy and crystalline structure of a Pt/Co/Pt ultrathin film are investigated by means of x-ray magnetic circular dichroism (XMCD) and extended x-ray absorption fine structure (EXAFS) techniques. The XMCD analysis shows a large orbital moment difference between out-of-plane and in-plane directions, which corresponds to perpendicular magnetic anisotropy (PMA), at moderate Ga${}^{+}$ fluences, while further increased fluences reduce the orbital moment difference resulting in in-plane magnetization. From the EXAFS analysis, enhancement of PMA is directly related to an in-plane lattice expansion caused by ion irradiation and Co-Pt intermixing, which results in a large lattice distortion. Thus, the origin of the ion irradiation-induced changes in magnetic anisotropy is successfully explained.

Enhancement of perpendicular magnetic anisotropy through reduction of Co-Pt interdiffusion in (Co/Pt) multilayers

We demonstrate that the effective magnetic anisotropy of sputtered (Co/Pt) multilayers can be doubled by limiting the interdiffusion occurring at Co/Pt interfaces. We present a way to decrease the interdiffusion by inserting an ultra-thin Cu layer at or near the Co/Pt interfaces. When such a material is sputtered on Co prior to the Pt deposition, the perpendicular magnetic anisotropy, as well as the thermal stability, is enhanced for Co layer thicknesses smaller than 1 nm. This is of great interest for out-of-plane magnetized spintronic devices which require high perpendicular magnetic anisotropy for down-size scalability reasons together with a free layer as thin as possible to reduce the writing energy when switched by spin transfer torque.

Enhancement of magneto-optical Kerr effect for Co ultra-thin film on Pt(1 1 1) with Ag buffer layer

The magnetic properties of Co/1ML Ag/Pt(111) have been monitored by magneto-optical Kerr effect. We find that the magnetization easy axis changes its direction from the out-of-plane to the in-plane when dCo⩾2ML. In the annealing studies of 1ML Co/1ML Ag/Pt(111), the result shows that the enhancement in perpendicular magnetic anisotropy is strongly related to the formation of Co–Pt alloy. The atomic exchange between Co and Ag cause the increase of the coercivity. The possible mechanisms of the enhancement in perpendicular magnetic anisotropy and coercivity are discussed.

Enhancement of magnetic anisotropy of hydrogenated Pd/Co/Pd trilayers

Magnetic and structural properties of Pd/Co/Pd trilayers in hydrogen atmosphere (pressure P H) have been investigated. The Pd layers are well hydrogenated and fully transformed to β-Pd–H phase at P H=1 atm, but this β phase is distorted along the film normal under the restriction of substrate, and the atomic ratio of H/Pd is estimated to be 0.6. Hydrogenation at low P H gives rise to enhancement of perpendicular magnetic anisotropy K eff. For instance, 3 Å-thick Co at P H=1.3 atm exhibits very high K eff (3.5×10 7 erg/cm 3) being two times larger than that of as-prepared one (1.6×10 7 erg/cm 3). Further increase in P H decreases K eff gradually, while K eff recovers to some extent after full dehydrogenation.

Anomalous perpendicular magnetic anisotropy in a Co monolayer on Au(111)

Fully-relativistic spin-polarized local spin density calculations are performed for an Au covered Co monolayer on Au(111). In good agreement with experiments (i) the enhancement of perpendicular magnetic anisotropy for the capped Co layer, and (ii) a characteristic maximum in the anisotropy energy for one covering Au layer are obtained. The close relationship found between the anisotropies of orbital magnetic moments and the anisotropy energies supplies a partial interpretation of our results in terms of familiar perturbation theory. By using this framework the anomalous behavior of the magnetic anisotropy energies can be well explained due to changes in the interfacial hybridization between Co and Au.