Large Perpendicular Magnetic Anisotropy Research Articles

Engineering Interfacial Perpendicular Magnetic Anisotropy in Fe2CoSi/Pt Multilayers with Interfacial Strain and Orbital Hybridization

We report a large interfacial perpendicular magnetic anisotropy (PMA) in [Fe2CoSi/Pt]n (FCS/Pt) multilayers deposited on thermally oxidized Si substrates. A maximum anisotropy energy density, Ku, of 2.64 Merg/cm3 is achieved with an optimized stack. We are able to tune the PMA by adjusting the Pt and FCS film thicknesses and number of periods of the multilayer structure, as these significantly affect the lattice structure of FCS/Pt stack. Both high-resolution transmission electron microscopy and structural relaxation of the atomistic model using first-principles calculations reveal that while the multilayer structure assumes a monoclinic structure (α = 84°), the FCS films undergo a local orthorhombic distortion from the bulk structure due to the strain imposed by the FCS/Pt interface. By analyzing the orbital-resolved density of states of the system, we propose that both the orthorhombic distortion of the FCS and orbital interactions between interface Pt and Fe atoms are responsible for the high PMA. For ...

Microscopic origin of large perpendicular magnetic anisotropy in an FeIr/MgO system

The FeIr/MgO multilayer system was found to exhibit large interfacial perpendicular magnetic anisotropy (PMA) and voltage-controlled magnetic anisotropy effects. We herein report the x-ray magnetic circular dichroism spectroscopy of both the Fe and Ir absorption edges in the FeIr/MgO system. Although Ir insertion at the Fe/MgO interface increased the PMA energy in the system, the orbital magnetic-moment anisotropy of Fe remained approximately constant. We found significant anisotropy in the orbital magnetic moment but not in the magnetic-dipole ${T}_{\mathrm{z}}$ term in the Ir atoms. Our experimental and theoretical results demonstrated that the large PMA of FeIr/MgO originates from second-order perturbation of the spin-orbit interaction energy of Ir. Moreover, the energy mainly originates from the spin-conserved virtual excitation process terms from the majority-spin bands of the Ir atoms located in the second atomic layer from MgO.

Electric field induced modulation to the magnetic anisotropy of Fe/silicene heterostructures: First-principles study

Abstract In view of the importance of inducing magnetism and large perpendicular magnetic anisotropy in silicene, the influence of an external electric field on the magnetism and the magnetic anisotropy of the Fe(15, 18, 21 monolayers)/silicene heterostructures are investigated by using first-principles calculations. The results show that the magnetic moments and the magnetic anisotropy of the Fe/silicene heterostructures are mainly from Fe films. With increasing the thickness of Fe film, the easy magnetization axis of the Fe/silicene heterostructure changes from out-of-plane to in-plane direction. In addition, the magnetic moments in all the Fe/silicene heterostructures are insensitive to the electric field. Particularly, under the positive electric field, the perpendicular magnetic anisotropy of the Fe(15 monolayers)/silicene can be enhanced, the magnetic anisotropy of the Fe(18 monolayers)/silicene increases from in-plane magnetic anisotropy to perpendicular magnetic anisotropy, and the magnetic anisotropy of the Fe(21 monolayers)/silicene also increases although its easy magnetization axis is still along in-plane direction. The mechanism of the electric-field-driven modulation of magnetic anisotropy is mainly from the increase of the positive contribution from spin-orbit coupling interaction between opposite spin d z 2 and d yz orbitals of the second Fe layer from the Fe/silicene interface. Our study reveals that the perpendicular magnetic anisotropy can be achieved or enhanced by applying positive electric field if the thickness of Fe film in Fe/silicene heterostructure is suitable.

Tunable perpendicular magnetic anisotropy in epitaxial Y3Fe5O12 films

High quality epitaxial (111) Y3Fe5O12 (YIG) films are fabricated by annealing amorphous precursor films that are sputtering deposited on three kinds of single crystal garnet substrates with lattice constants exceeding that of YIG by a ratio from 0.76% to 1.58%. The effective perpendicular magnetic anisotropy (PMA) in the YIG films is significantly altered by the epitaxial strain induced magnetoelastic anisotropy. Large PMA is demonstrated in the fully strained thin YIG films on substrates with lattice mismatch from 1.05% to 1.58% due to the overwhelming of the magnetoelastic anisotropy. Less-strained YIG films, corresponding to partial strain relaxation at larger YIG thickness or smaller substrate lattice mismatch at 0.76%, show substantial but insufficient magnetoelastic anisotropy to overcome shape anisotropy. Magnetotransport characterization on YIG/Pt bilayers shows that the surface of YIG with either in-plane or perpendicular magnetization allows efficient equilibrium and/or nonequilibrium spin interexchange across the heterostructure interface.

All-optical probe of sub-THz spin precession in a L10 MnGa nanolayer

We measured an all-optical time-resolved magneto-optical Kerr effect for L10 MnGa ultrathin films fabricated with a low-temperature hetero-epitaxy technique. Spin precession was observed with a frequency over 0.1 THz due to a large perpendicular magnetic anisotropy (PMA) field of ∼3 T. The effective Gilbert damping constant αeff was sensitive to the deposition temperature. A minimum value of αeff of 0.017 was observed, which was about twice as large as that reported for much thicker films of MnGa with high chemical ordering but was comparable to those reported in other nanometer-thick PMA material films.

Structural and Magnetic Properties of Mn3Ge Grown on a Thin Polycrystalline MgO Seed Layer

D022‐Mn3Ge has been studied recently for its large perpendicular magnetic anisotropy (PMA) and low magnetization, which enables it as a potential alternative material for the scaling of magnetic tunnel junctions (MTJs) used in spin‐transfer‐torque magnetic random access memory (STT‐MRAM). Most of the research was carried out on D022‐Mn3Ge deposited by molecular beam epitaxy (MBE) on single crystalline MgO substrates. As the use of single crystalline substrates is not compatible with an industrial integration flow, we developed a polycrystalline MgO seed layer by physical vapor deposition (PVD) on Si. PMA is observed in MBE‐grown D022‐Mn3Ge with coercive field up to 4.5 Tesla, even when the PVD‐MgO seed is only 1 nm thick. It indicates that PVD‐MgO is able to serve as the seed layer for D022‐Mn3Ge integration to MTJs. However, no PMA is observed in Mn3Ge grown by PVD, which is attributed to different kinetics taking place during PVD deposition.

Tetragonal Mn3Sn Heusler films with large perpendicular magnetic anisotropy deposited on metallic MnN underlayers using amorphous substrates

Tetragonal Heusler compounds that exhibit large perpendicular magnetic anisotropy are promising materials for advanced spintronic devices. A prerequisite are thin films whose tetragonal axis is oriented perpendicular to the plane of the films. Here we show that highly textured, (001) oriented, tetragonal Mn3Sn layers can be prepared using metallic zinc-blende (ZB) MnN as underlayers. Moreover, we show that these layers can be deposited on amorphous substrates using reactive magnetron sputtering. The ferrimagnetic Mn3Sn layers exhibit perpendicularly magnetized hysteresis loops with coercive fields of ∼2 T. Stoichiometric ZB-MnN underlayers share an “equivalent” Mn-Mn layer at the interface with Mn3Sn, thus promoting their oriented growth. Other nitride underlayers are not effective due to their rock-salt (RS) crystal structure and the absence of Mn. Density functional theory calculations confirm that tetragonal Mn3Sn Heusler films are energetically stable when interfaced with ZB-MnN underlayers and not with any of the other RS nitride underlayers considered here. Such Heusler compounds have much promise as electrodes for magnetic tunnel junction memory elements for deeply scaled magnetic random access memories.

Colossal electric field control of magnetic anisotropy at ferromagnetic interfaces induced by iridium overlayer

Voltage-induced magnetization switching can lead to a new paradigm enabling ultralow-power and high density instant-on nonvolatile magnetoelectric random access memory (MeRAM) devices. Two major challenges for future MeRAM devices are to achieve large perpendicular magnetic anisotropy (PMA) and high voltage-controlled magnetic anisotropy (VCMA) coefficient of heavy-metal/ferromagnet/insulator heterostructures (HM/FM/I). Employing ab initio electronic structure calculations we have investigated the effect of epitaxial strain and thickness of both FeCo and Ir layers as design parameters to optimize the PMA and VCMA of Ir/FeCo/MgO. We predict that the Ir cap layer can induce both large PMA and colossal VCMA efficiency which depend on the strain. More importantly, we predict that a single Ir cap monolayer gives rise to a VCMA efficiency one order of magnitude higher than that of thicker Ir layers. The underlying mechanism is the synergistic effects of the emergence of Ir local moments, the large Ir spin-orbit coupling (SOC), and the large modulation of the magnetic anisotropy at the Ir/vacuum interface. These results provide useful guiding rules in the design of the next generation of high performance MeRAM memory devices.

Magnetic Anisotropy and Ferromagnetic Resonance in Nitrogen-Incorporated NiFe2O4Thin Films

The nitrogen-incorporated nickel ferrite (N-NFO) thin films deposited by radio-frequency magnetron sputtering were found to exhibit interesting magnetic properties upon heat treatment. The in-plane (IP) and out-of-plane (OP) magnetizations of the heat-treated N-NFO thin films measured at 20 K were seen not to saturate even at higher fields, indicating a large perpendicular magnetic anisotropy (PMA). The coercive field value was found to decrease upon heat treatment, due to the weakened pinning of domain walls upon decrease in grain boundary volume. The IP magnetization exhibits a tendency to saturate at high fields, whereas the OP magnetization does not, indicative of the existence of easy magnetization direction in the plane of the film. The room temperature ferromagnetic resonance (FMR) spectra in both IP and OP configurations indicated the increase of FMR linewidth upon nitrogenation, revealing the increased distribution of magnetic anisotropy induced by nitrogen in the lattice. An increase in the resonance field value was observed for the N-NFO film grown at 100 W compared with that of NFO in the IP configuration, while a decrease in the value was observed for OP, which is attributed to an enhanced IP magnetic anisotropy. The films grown at 120 and 140 W have shown decrease in a resonance field value in the IP configuration and increase in OP, which is attributed to the increase in thickness-dependent PMA.

Role of the Interfacial Rh-layer on Robust Ferromagnetism and Large Perpendicular Magnetic Anisotropy of FeRh Films on MgO(0 0 1)

Abstract Employing first-principles calculations we reveal the strong interface termination dependences of the magnetic order and perpendicular magnetic anisotropy (PMA) in FeRh films on MgO(0 0 1). While the magnetic ground state of the Fe-terminated FeRh(0 0 1) is an antiferromagnetic, the Rh-termination favors ferromagnetism within the underlying FeRh layers. This ferromagnetism can persist even at the interface with MgO, which in turn leads to a few times enhancement of PMA compared with the Fe-interface in FeRh/MgO(0 0 1). The underlying mechanism for this large PMA is an interplay between the spin-polarized Rh d xy and d x 2 - y 2 orbital states with large spin-orbit coupling at the interface. These results unveil a crucial role of the interface termination on magnetism of FeRh in antiferromagnet spintronics.

Evidence of a strong perpendicular magnetic anisotropy in Au/Co/MgO/GaN heterostructures.

We report a strong perpendicular magnetic anisotropy (PMA) in Au/Co/MgO/GaN heterostructures from both experiments and first-principles calculations. The Au/Co/MgO heterostructures have been grown by molecular beam epitaxy (MBE) on GaN/sapphire substrates. By carefully optimizing the growth conditions, we obtained a fully epitaxial structure with a crystalline orientation relationship Au(111)[1̄10]//Co(0001)[112̄0]//MgO(111)[101̄]//GaN(0002)[112̄0]. More interestingly, we demonstrate that a 4.6 nm thick Co film grown on MgO/GaN still exhibits a large perpendicular magnetic anisotropy. First-principles calculations performed on the Co (4ML)/MgO(111) structure showed that the MgO(111) surface can strongly enhance the magnetic anisotropy energy by 40% compared to a reference 4ML thick Co hcp film. Our layer-resolved and orbital-hybridization resolved anisotropy analyses helped to clarify that the origin of the PMA enhancement is due to the interfacial hybridization of O 2p and Co 3d orbitals at the Co/MgO interface. The perpendicularly magnetized Au/Co/MgO/GaN heterostructures are promising for efficient spin injection and detection in GaN based opto-electronics without any external magnetic field.

Controlled coexcitation of direct and indirect ultrafast demagnetization in Co/Pd multilayers with large perpendicular magnetic anisotropy

Ever since its discovery in 1996, ultrafast demagnetization has ignited immense research interest due to its scientific rigor and technological potential. A flurry of recent theoretical and experimental investigations has proposed direct and indirect excitation processes in separate systems. However, it still lacks a unified mechanism and remains highly debatable. Here, we demonstrate that instead of either direct or indirect interaction, simultaneous and controlled excitation of both direct and indirect mechanisms of demagnetization is possible in multilayers composed of repeated Co/Pd bilayers. Moreover, we are able to modulate demagnetization time (from \ensuremath{\sim}350 to \ensuremath{\sim}750 fs) by fluence and thickness-dependent indirect excitation due to heat current flowing vertically downward from top layers, which is combined with an altogether different scenario of direct irradiation. Finally, by regulating the pump wavelength, we can effectively control the contribution of indirect process, which gives a confirmation to our understanding of the ultrafast demagnetization process.

Anomalous Hall Effect of Fe2CoSi/Pt Multilayers With Large Perpendicular Magnetic Anisotropy

In this paper, the anomalous Hall effect (AHE) of [Fe2CoSi/Pt] n multilayers with large perpendicular magnetic anisotropy has been investigated. We found that both skew scattering (SS) and side jump contribute to the AHE, while SS is the dominant contribution to the AHE. Moreover, the contribution of SS to the AHE becomes less significant when either decreasing the thickness of Pt layer or increasing the number of periods $n$ .

Control of Magnetic Anisotropy by Lattice Distortion in Cobalt Ferrite Thin Film

We have investigated the properties of cobalt ferrite (CFO)(001) epitaxial thin films grown by RF magnetron sputtering method on various buffer layers to induce large perpendicular magnetic anisotropy (PMA) through the magneto-elastic effect. Among five different compounds examined as the buffer layers, we found that only CFO thin films grown on (Mg, Sn)3O4 buffer layers showed a strong PMA, and the value of the saturation magnetization was 460 emu/cm3, which is larger than the 425 emu/cm3 for bulk CFO. Torque measurements revealed the PMA energy $K_{u} = 24.6$ Merg/cm3 at room temperature, which is extraordinarily large for a ferromagnetic oxide.

Emission of Coherent Propagating Magnons by Insulator-Based Spin-Orbit-Torque Oscillators

We experimentally demonstrate generation of coherent propagating magnons in ultra-thin magnetic-insulator films by spin-orbit torque induced by dc electric current. We show that this challenging task can be accomplished by utilizing magnetic-insulator films with large perpendicular magnetic anisotropy. We demonstrate simple and flexible spin-orbit torque devices, which can be used as highly efficient nanoscale sources of coherent propagating magnons for insulator-based spintronic applications.

Quantitative analysis and optimization of magnetization precession initiated by ultrafast optical pulses

Magnetic storage and magnetic memory have recently shifted towards the use of magnetic thin films with large perpendicular magnetic anisotropy (PMA) to simultaneously satisfy the requirements in storage density and thermal stability. Understanding the magnetic switching process and its dependence on the Gilbert damping (α) of materials with large PMA is crucial for developing low-power consumption, fast-switching, and high-thermal stability devices. The need to quantify α of materials with large PMA has resulted in the development of the all-optical ultrafast Time-Resolved Magneto-optical Kerr Effect (TR-MOKE) technique. While TR-MOKE has demonstrated its capability of capturing magnetization dynamics of materials with large PMA, a quantitative analysis regarding the operational optimization of this emerging technique is still lacking. In this paper, we discuss the dependence of the TR-MOKE signal on the magnitude and angle of the applied field, by utilizing a numerical algorithm based on the Landau-Lifshitz-Gilbert equation. The optimized operational conditions that produce the largest TR-MOKE signals are predicted. As an experimental verification, we conduct TR-MOKE measurements on a representative sample of a tungsten-seeded CoFeB PMA thin film to show the excellent agreement of the model prediction with measurements. Our analysis results in a better understanding of the external field influence on the magnetization precession processes. The results of this work can also provide guidance on selecting operational conditions of the TR-MOKE technique to achieve optimal signal-to-noise ratios and thus more accurate measurements of magnetization dynamics.

Thickness and composition dependencies of magnetization and perpendicular magnetic anisotropy of Heusler-like alloys based [formula omitted] superlattices

Finding materials that possess high perpendicular magnetic anisotropy (PMA) and low magnetization is a key challenge for the potential application to the magnetic tunnel junction (MTJ). Through systematic density functional theory (DFT) calculations, we have studied a series of MnxGa|Co2FeAl (x = 1, 1.66, 3) superlattice (SL) models by individually changing the thickness of MnxGa hard-magnetic layers and Co2FeAl soft-magnetic layers as well as controlling the interface magnetic couplings. Our results demonstrated that total magnetic moments of the SLs can be well balanced to a small (zero) value as the antiferromagnetically coupled MnxGa and Co2FeAl layers have the matching thickness and appropriate interface. Strong PMAs of MnxGa bulks can be well preserved in SLs, being up to 0.38, 0.37, 0.22 meV/Mn for x = 1, 1.66, 3, respectively, which are weakly dependent on the layered thickness (flexible lattice constants of SLs) but strongly on the magnetic coupling with Co2FeAl layers. The ferromagnetic/antiferromagnetic coupling at Mn-Co interface often generates small/large negative interface PMA and thus it would diminish the PMA of SLs; however, Ga-Co interface was found to give large/small positive contribution to the PMA of SLs. Typically, Ga-Co (MnGa-Co) interface is proposed to be the most favorable one for experimentally heteroepitaxial growth of MnxGa|Co2FeAl SLs, because it can guarantee these SLs to present high stability, large PMAs, and tunable magnetic moments.

First-principles study of magnetization reorientation and large perpendicular magnetic anisotropy in CuFe2O4/MgO heterostructures

Herein, using first-principles calculations we predict magnetization reorientation and large perpendicular magnetic anisotropy (PMA) in spinel ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$/MgO heterostructure by replacing the octahedral Fe ions with Cu. The substitutional ${\mathrm{Cu}}^{2+}$ ions prefer the octahedral site within the xy-plane layer in an inverse spinel structure, which is associated with the Jahn-Teller tetragonal and $xy$-plane twisted distortions. While magnetization of ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$/MgO is significantly reduced in ${\mathrm{CuFe}}_{2}{\mathrm{O}}_{4}$/MgO, the presence of the substitutional ${\mathrm{Cu}}^{2+}$ ions reorients magnetization from an in-plane to perpendicular magnetic anisotropy. More remarkably, PMA further increases gradually with the film thickness of ${\mathrm{CuFe}}_{2}{\mathrm{O}}_{4}$ layers in the ${\mathrm{CuFe}}_{2}{\mathrm{O}}_{4}$/MgO heterostructure. The underlying mechanism for this large PMA is the interplay of the spin-orbit-coupled Cu ${d}_{xy}--{d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ states in the center layers and the Fe ${d}_{{z}^{2}}$--O ${p}_{z}$ hybridization at the interface. These findings point toward the feasibility of reducing magnetization and enhancing PMA in spinel structures for spintronics applications.

Ultra-low damping insulating magnetic thin films get perpendicular

A magnetic material combining low losses and large perpendicular magnetic anisotropy (PMA) is still a missing brick in the magnonic and spintronic fields. We report here on the growth of ultrathin Bismuth doped Y3Fe5O12 (BiYIG) films on Gd3Ga5O12 (GGG) and substituted GGG (sGGG) (111) oriented substrates. A fine tuning of the PMA is obtained using both epitaxial strain and growth-induced anisotropies. Both spontaneously in-plane and out-of-plane magnetized thin films can be elaborated. Ferromagnetic Resonance (FMR) measurements demonstrate the high-dynamic quality of these BiYIG ultrathin films; PMA films with Gilbert damping values as low as 3 × 10−4 and FMR linewidth of 0.3 mT at 8 GHz are achieved even for films that do not exceed 30 nm in thickness. Moreover, we measure inverse spin hall effect (ISHE) on Pt/BiYIG stacks showing that the magnetic insulator’s surface is transparent to spin current, making it appealing for spintronic applications.

Large exchange coupling field in perpendicular synthetic antiferromagnetic structures with CoPt alloy

We have investigated the magnetic properties and thermal stability of perpendicular synthetic antiferromagnetic (p-SAF) layers with a [Co/Pt]x/Ru/[Co/Pt]x structure. A very large exchange coupling field (Hex) of 7.0 kOe was obtained in as-deposited p-SAF structures owing to the large perpendicular magnetic anisotropy of [Co/Pt] and exchange coupling with Ru. We found that a smooth surface of the Ru buffer layer is effective for improving the thermal robustness of [Co/Pt] and demonstrating a large Hex of 2.6 kOe even after annealing at 400 °C, which is as high as the integration temperature for Si-based circuits. Such a thermally robust p-SAF layer is promising for developing magnetic sensor devices.