Perpendicular Magnetic Anisotropy Energy Research Articles

Bimetal single-molecule magnets supported on benzene with large magnetic anisotropy and unquenched orbital moment

Single-atom magnets, the ultimate limit of high-density magnetic storage, have attracted widespread attention over recent decades. However, they are far from being applicable as a storage medium because of their low magnetic stability. In this paper, we describe a strategy to induce huge magnetic anisotropy in ``bimetal magnets'' on a benzene (Bz) substrate based on the electron filling of $d$ orbitals. Our first-principles calculations reveal that OsX-Bz ($X=\mathrm{Fe}$, Ru, and Os) exhibits high structural stability, large unquenched orbital moments ($1.55--1.59\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{B}$), and huge perpendicular magnetic anisotropy energy (MAE) above 54.8 meV. The synergistic effect of two transition metal components toward this huge MAE and preserved orbital moment is discussed in depth, which is mainly attributed to the d-d coupling induced energy level rearrangement. Our work may provide insights into the underlying physical mechanism and bimetal molecule magnet design.

Voltage-Gate-Assisted Spin-Orbit-Torque Magnetic Random-Access Memory for High-Density and Low-Power Embedded Applications

Voltage-gate assisted spin-orbit torque (VGSOT) writing scheme combines the advantages from voltage control of magnetic anisotropy (VCMA) and spin-orbit torque (SOT) effects, enabling multiple benefits for magnetic random access memory (MRAM) applications. In this work, we give a complete description of VGSOT writing properties on perpendicular magnetic tunnel junction (pMTJ) devices, and we propose a detailed methodology for its electrical characterization. The impact of gate assistance on the SOT switching characteristics are investigated using electrical pulses down to 400ps. The VCMA coefficient ({\xi}) extracted from current switching scheme is found to be the same as that from the magnetic field switch method, which is in the order of 15fJ/Vm for the 80nm to 150nm devices. Moreover, as expected from the pure electronic VCMA effect, {\xi} is revealed to be independent of the writing speed and gate length. We observe that SOT switching current characteristics are modified linearly with gate voltage (V_g), similar as for the magnetic properties. We interpret this linear behavior as the direct modification of perpendicular magnetic anisotropy (PMA) and nucleation energy induced by VCMA. At V_g = 1V, the SOT write current is decreased by 25%, corresponding to a 45% reduction in total energy down to 30fJ/bit at 400ps speed for the 80nm devices used in this study. Further, the device-scaling criteria are proposed, and we reveal that VGSOT scheme is of great interest as it can mitigate the complex material requirements of achieving high SOT and VCMA parameters for scaled MTJs. Finally, how that VGSOT-MRAM can enable high-density arrays close to two terminal geometries, with high-speed performance and low-power operation, showing great potential for embedded memories as well as in-memory computing applications at advanced technology nodes.

Structure distortion related magnetic anisotropy in 5d transition-metal dimer adsorbed g-C3N4 monolayers

Abstract Among the various two-dimensional (2D) materials, g-C3N4 monolayer has a novel two-dimensional sheet structure with unique characteristics and wide applications. The pristine g-C3N4 is semiconductor with an indirect-band-gap of 1.21 eV. The g-C3N4 adsorbed other materials has proven to be an effective method to change the performance of g-C3N4. Here, we investigated the adsorption energy, stable geometry, electronic structure and magnetic properties of Os, Ir atoms and its dimers adsorbed 2D g-C3N4 monolayers by first-principles calculations. We have considered several possible adsorption sites and select the center of vacancy site after the structure optimization to carry out the further calculations. The Os atom and Os–Ir dimer adsorbed g-C3N4 monolayers are n-type semiconductors, and the Ir–Os, Os–Os and Ir–Ir dimer adsorbed g-C3N4 monolayers are half-metallic. The larger magnetic anisotropy energy (MAE) in the perpendicular direction is more suitable for application than the MAE in the horizontal plane. The MAE of Os adsorbed g-C 3N4 monolayer shows a large perpendicular MAE of 5.192 mJ/m 2 at the tensile strain of 4%. The Ir–Ir adsorbed g-C 3N4 monolayer has the in-plane MAE at compressive strain of −4% and turn into perpendicular MAE of 0.275, 1.516 and 0.361 mJ/m2 at a strain of −6%, 0% and 4%, the Os and Ir–Ir dimer adsorbed g-C3N4 monolayer is promising candidates for room temperature applications. Our results reveal that 5d TM atoms and its dimers adsorbed g-C3N4 monolayers have potential applications in spintronic devices.

The 3d transition-metals doping tunes the electronic and magnetic properties of 2D monolayer InP3

Introducing a magnetic moment into nonmagnetic two-dimensional semiconducting materials is significant for them in spintronic application. Here, we systematically study the electronic structures and magnetism of 3d transition-metals (TMs) doped two-dimensional (2D) InP3 monolayer (m-InP3) using density functional theory. Our results indicate that all the 3d TMs doping can induce different magnetism (except for Sc), and it mainly originates from the unpaired d electrons filled in the non-bonding orbitals. In particular, Fe doping system obtains the largest total magnetic moment of 5 μB. Furthermore, the V, Mn, Co, Ni and Zn doped 2D m-InP3 appear half-metallicity, while Ti, Cr, Fe, Cu doping possess dilute magnetic semiconductor (DMS) property. For the two identical 3d TM atoms doping systems, the ferromagnetic couplings are found in the Ti, V and Fe doping. Remarkably, the largest perpendicular magnetic anisotropy energy (MAE) with 340.38 μeV is found in the Fe doped system and its physical origin is analyzed by the projected density of states and the d-orbital resolved MAE based on the second-order perturbation theory. These findings provide a promising pathway to realize 2D m-InP3-based materials application in future spintronic devices.

Néel-Type Elliptical Skyrmions in a Laterally Asymmetric Magnetic Multilayer.

Magnetic skyrmions, topological-chiral spin textures, have potential applications in next-generation high-density and energy-efficient spintronic devices for information storage and logic technologies. Tailoring the detailed spin textures of skyrmions is of pivotal importance for tuning skyrmion dynamics, which is one of the key factors for the design of skyrmionic devices. Here, the direct observation of parallel aligned elliptical magnetic skyrmions in Pt/Co/Ta multilayers with an oblique-angle deposited Co layer is reported. Domain wall velocity and spin-orbit-torque-induced out-of-plane effective field analysis demonstrate that the formation of unusual elliptical skyrmions is correlated to the anisotropic effective perpendicular magnetic anisotropy energy density (Keff u ) and Dzyaloshinskii-Moriya interaction (DMI) in the film plane. Structural analysis and first-principles calculations further show that the anisotropic Keff u and DMI originate from the interfacial anisotropic strain introduced by the oblique-angle deposition. The work provides a method to tune the spin textures of skyrmions in magnetic multilayers and, thereby, a new degree of freedom for the design of skyrmionic devices.

Enhancement of magnetic coupling and magnetic anisotropy in MTJs with multiple CoFeB/MgO interfaces for high thermal stability

Magnetic coupling between two CoFeB layers through the W insertion layer is important in the conventional double CoFeB/MgO interface, magnetic tunneling junctions (MTJs) (double-MTJs) with MgO/CoFeB/W/CoFeB/MgO free layer stack because it increases the effective magnetic volume of the free layer. The magnetic coupling energy constant per unit area, Jcpl, between two CoFeB layers through the W layer and the effective perpendicular magnetic anisotropy (PMA) energy constant per unit area, Kefft*, were investigated for conventional double-MTJs with various W insertion layer thicknesses. As the W layer thickness increased, Kefft* increased and Jcpl decreased. There exists a trade-off relationship between Jcpl and Kefft*. In conventional double-MTJs with a single W insertion layer, large values for Jcpl and Kefft* were difficult to obtain simultaneously. To improve this tradeoff, we employed a free layer stack with a thin ferromagnetic layer (ferromagnetic bridge layer: FBL) located in the W insertion layer. In the double-MTJs with FBL annealed at 400 °C, a large Jcpl value of 0.37 mJ/m2 was achieved while maintaining the maximum values of Kefft*. Accordingly, the MTJ with FBL provides an MTJ stack structure for obtaining high thermal stability.

Deterministic magnetization switching by spin–orbit torque in a ferromagnet with tilted magnetic anisotropy: A macrospin modeling

Field-free magnetization switching by in-plane current injection via spin–orbit interactions in heavy-metal/ferromagnet bilayer systems is of great interest for applications of spin–orbit torque (SOT). Here, we demonstrate theoretically how to deterministically switch the magnetization of CoFeB with a tilted perpendicular magnetic anisotropy (PMA) by using the SOT effect from the viewpoint of energy landscape. It is found that magnetization switching is dependent both on the polar angle θ and the azimuthal angle φ of the tilted PMA. A tiny remnant magnetization component that deviates away from the film plane during the spin current pulse impulsion period is a key point for the subsequent switching. The underlying mechanism is attributed to the shift of PMA energy barrier with the magnetization angle, which dominates the state of the tiny magnetization component and thereafter the bipolar deterministic switching. Based on the simulation results, a phase diagram of deterministic switching regarding θ and φ is established. Moreover, the positive and negative critical switching current densities are demonstrated to exhibit an astroid-like characteristic as a function of θ. This study provides a fundamental insight into the nature of SOT-induced deterministic switching with the tilted PMA method.

Thermal stability for domain wall mediated magnetization reversal in perpendicular STT MRAM cells with W insertion layers

We present an analytical model for calculating the energy barrier for the magnetic field-driven domain wall-mediated magnetization reversal of a magneto-resistive random access memory cell and apply it to study thermal stability factor Δ for various thicknesses of W layers inserted into the free layer (FL) as a function of the cell size and temperature. We find that, by increasing the W thickness, the effective perpendicular magnetic anisotropy (PMA) energy density of the FL film monotonically increases, but at the same time, Δ of the cell mainly decreases. Our analysis shows that, in addition to saturation magnetization Ms and exchange stiffness constant Aex of the FL film, the parameter that quantifies the Δ value of the cell is its coercive field Hc, rather than the net PMA field Hk of the FL film comprising the cell.

Spin-Resolved Contribution to Perpendicular Magnetic Anisotropy and Gilbert Damping in Interface-Engineered Fe/MgAl2O4 Heterostructures

The coexistence of a low magnetic Gilbert damping constant and large perpendicular magnetic anisotropy (PMA) in ferromagnetic thin films is critical for high-speed and energy-efficient spintronics devices. To clarify the spin-resolved contributions of damping and PMA, a flat, lattice-matched interface is developed in an $\mathrm{Fe}$(0.7 nm)/${\mathrm{Mg}\mathrm{Al}}_{2}{\mathrm{O}}_{4}$(oxide)(3 nm) bi-layer film by varying the ex situ annealing temperature, which acts as a catalyst to vary the PMA energy densities according to the oxidation degree. Here, the optimized procedure for interface engineering is implemented to achieve an extremely low magnetic Gilbert damping constant (0.013) and a strong interfacial PMA energy $(0.8\phantom{\rule{0.1em}{0ex}}\mathrm{MJ}\phantom{\rule{0.1em}{0ex}}{\mathrm{m}}^{\ensuremath{-}3})$ for epitaxial thin films. By employing different interfacial atomic configurations in a first-principles calculation, this study explains the origin of the PMA energy and damping constant. The d(yz) and d(zx) orbitals of the interfacial $\mathrm{Fe}$ atoms in the minority-spin states play a role in the orbital moment and its anisotropy. Furthermore, the matrix elements between these two orbitals in the non-spin-flip term predominately contribute to damping. These detailed findings provide a clear insight into the development of materials with significantly improved PMA energies and low damping characteristics, thereby facilitating promising future spintronic applications.

Giant perpendicular magnetic anisotropy induced by double degeneracy of d-orbitals in W/MgO/Ag heterojunction

Large perpendicular magnetic anisotropy energy (MAE) is crucial for designing high-density information storage devices. Here, employing first-principles calculations, it has shown the possibility to boost the magnetic anisotropy using a single W atom functionalized by radical of hydroxy (OH) on the MgO(001)/Ag(001) heterojunction. The OH-functionalized system exhibited the giant perpendicular MAE of 214.76 meV, which outperformed all the reported heterojunctions. The underlying mechanism for the enormous MAE is attributed to the doubly degenerated d-orbitals around the Fermi level caused by a special OH-ligand field which alters the spin-orbit coupling Hamiltonian and enhances the magnetic anisotropy. Meanwhile, the small spin magnetic moment of 1.829 μB is obtained due to the strong OH-ligand field. This work provides a new approach to generate high perpendicular MAE that is very promising for nanoscale magnetic devices.

Perpendicular magnetic anisotropy of a Ta/NiFeB/MgO system and its magnetic coupling with [Pt/Co] multilayers

The perpendicular magnetic anisotropy (PMA) and crystalline structure of Ta/NiFeB/MgO stacks are investigated. When the stack is annealed, PMA is induced and the magnetic moment increases due to the crystallization of bcc NiFe from amorphous NiFeB. This structure is combined with [Pt/Co]6 multilayers to study the change in the magnetic coupling and PMA behavior with the change in the Ta thickness inserted in-between and the annealing temperature. Among the [Pt/Co]6/Ta/NiFeB/MgO stacks, the stack with a 0.5-nm-thick Ta layer shows the strongest PMA with an effective PMA energy density of 4.4 × 106 erg/cm3 upon annealing at 400 °C. Further, the X-ray diffraction study of the crystalline structures confirms the formation of the NiFe (001) phase, which is essential for achieving a high tunneling magnetoresistance ratio.

Large magnetic anisotropy of single transition metal adatoms on WS2

We performed systematic density functional theory calculations to study the structural stability and magnetic properties of single transition metal adatoms on a WS2 monolayer. It was found that Re adatom takes the hollow site on WS2 and has a large perpendicular magnetic anisotropy energy (MAE) up to ~30 meV/atom. Electronic structure and rigid band model analyses revealed that this large MAE originates from the spin-orbital coupling interaction between the dxz and dyz orbitals in the spin down and through cross spin channels. This work provides a practical single-atom magnetic system for potential applications and gives useful insights for further developments.

Magnetic anisotropy and Curie temperature of two-dimensional VI3 monolayer

Recently, it was reported that the VI3 had a Mott insulator nature and also displayed the structural and magnetic phase transition at low temperature. Here, we explored the magnetic properties of the two-dimensional (2D) monolayer structure using the density functional theory. We found that the 2D VI3 had an enhanced lattice constant compared with that in the bulk structure. Besides, the 2D monolayer had an indirect band gap of 0.98 eV, and this band gap was increased (decreased) with tensile (compressive) strain up to ±3%. The monolayer structure had a ferromagnetic ground state and this nature was preserved under both tensile and compressive strains. We obtained that the monolayer structure had a perpendicular magnetic anisotropy energy of 0.29 meV/cell. The perpendicular magnetic anisotropy still remained even after applying the tensile and compressive strains although the magnitude of magnetic anisotropy was slightly changed. Using the Metropolis Monte Carlo simulations, we found that the monolayer had a Curie temperature of 46 K. This Curie temperature was increased to 57 K with 3% tensile strain whereas it was decreased to 35 K with 3% compressive strain. Overall, we found that the magnetic property of 2D VI3 monolayer was robust under the strain.

Perpendicular magnetic anisotropy of Pd/Co55Mn25Si20/NiO/Pd sputtering films

This paper carries out research on perpendicular magnetic anisotropy (PMA) of the Pd/Co55Mn25Si20(CMS)/NiO trilayer stack, which is a structure innovatively combining heavy metal (HM)/ferromagnetic (FM) interface (Pd/CMS), FM/oxide interface (CMS/NiO), and FM/antiferromagnetic (AFM) interface (CMS/NiO). The maximum effective PMA energy, obtained from the Pd (6 nm)/Co55Mn25Si20(5.5 nm)/NiO(1.0 nm) trilayer film which is sputter-deposited on glass at a substrate temperature of 350 °C, is 0.695 Merg/cm3. By appropriately increasing the Co content, a wider FM layer thickness range (3–8 nm) that achieved PMA was obtained. The oxide in the traditional FM/oxide structure was replaced by the antiferromagnetic NiO to obtain a wider oxide layer thickness of 0.4–7.6 nm, or even wider.

High Curie temperature and strain-induced semiconductor-metal transition with spin reorientation transition in 2D CrPbTe3 monolayer

One of the major obstacles for Cr-based 2D materials such as CrI3, CrSiTe3 and CrGeTe3 for spintronics applications is their low Curie temperature. Herein, we investigated the strain-induced magnetic properties of 2D CrPbTe3 (CPT) monolayer belonging to members of the Cr-based 2D family. We explored the possibility of the fabrication of 2D layer through the mechanical stability, dynamical stability, formation energy, cohesive energy and thermal stability calculations. We found ferromagnetic ground state and the pristine CrPbTe3 monolayer had an indirect band gap of 0.25 eV with an in-plane magnetic anisotropy of −1.37 meV cell−1. The Curie temperature was 110 K and this is much larger than that of CrI3, CrSiTe3 and CrGeTe3. Under 4% tensile strain, the band gap was increased to 0.45 eV and the Curie temperature was increased to 150 K. We found strain-induced semiconductor-metal transition at 3% compressive strain and also spin reorientation transition from in-plane to perpendicular magnetic anisotropy at 4% compressive strain, and the perpendicular magnetic anisotropy energy was almost three times larger than that of the CrGeTe3 layer. Our finding may suggest that the CrPbTe3 system can be utilized for spintronics and straintronics applications.

Effect of tungsten doping on perpendicular magnetic anisotropy and its voltage effect in single crystal Fe/MgO(0 0 1) interfaces

Heavy metal elements with strong spin–orbit coupling generally play an important role for perpendicular magnetic anisotropy (PMA) in magnetic heterostructures. We examined interface PMA and related properties in single crystal Fe/MgO(0 0 1) heterostructures in which a very thin W interface layer was nominally inserted as doping. The PMA energy density was reduced with increasing the W layer thickness, in contrast to the results in a similar study using poly-crystal FeB/MgO heterostructures (Nozaki et al 2018 APL Mater. 6 026101), suggesting that the detailed mechanisms of PMA are different between single crystal Fe/MgO and poly-crystal FeB/MgO. X-ray magnetic circular dichroism measurements revealed that anisotropy in the Fe orbital magnetic moments decreased with the PMA energy density, i.e. the PMA observed was interpreted in the framework of Bruno’s model (Bruno 1989 Phys. Rev. B 39 R865). The coefficient of the voltage effect of PMA arising in the Fe/MgO heterostructure with 0.05 nm thick W interface insertion was determined to be ~190 fJ Vm−1, showing a small quantity of reduction as well as the PMA. This study gives an insight into the role of heavy metal elements in the PMA for single crystal Fe/MgO heterostructures.

Magnetic properties of Co film in Pt/Co/Cr2O3/Pt structure

Magnetic properties of Co film in Pt/Co/α-Cr2O3/Pt/α-Al2O3 structure were investigated. Co layer thickness tCo dependence of perpendicular magnetic anisotropy energy density K reveals that the bulk magnetic anisotropy plays an important role in the system in addition to the interfacial anisotropy. Damping constant α monotonically increases with the decrease of tCo but not proportionally to 1/tCo. Both K and α increase with the increase of Pt layer thickness tPt from 3 nm to 5 nm and keeps almost constant in the tPt range between 5 nm to 20 nm. These results are of importance to understand the magnetization switching behavior driven by the magneto-electric (ME) effect as well as to design the spintronics device using the ME effect.

Anatomy of large perpendicular magnetic anisotropy in free-standing Co/Ni (1 1 1) multilayer

We investigated the magnetic anisotropy energy (MAE) in the free-standing Co/Ni (111) multilayer as both Ni thickness dependence (tNi = 1-4MLs) and number of multilayer repetition times (N = 1–3) by means of a first-principles electronic structure calculation based on spin density functional theory. We included both contributions to the MAE from magnetocrystalline anisotropy energy (MCAE) originating from spin-orbit coupling and shape magnetic anisotropy energy (SMAE) originating from spin dipole-dipole interaction. The MCAE part was evaluated from both methods of total energy (TE) and grand-canonical force theorem (GCFT). The SMAE part was calculated by using a spin density approach (SDA). All MCAE values from the TE are well reproduced by those from the GCFT method. In N = 1, the total MAE (MCAE + SMAE) for tNi showed a perpendicular MAE (PMAE) with a maximum value of 1.67 mJ/m2 at tNi = 2MLs. The PMAE increases with increasing N. The series of tNi = 3MLs showed a linear behavior as N dependence with an increasing ratio of 0.68 mJ/m2, which is in good agreement with experimental measurement. By using the GCFT, we evaluated the atom-resolved and k-resolved MCAEs. The atom-resolved MCAE indicates that the Co/Ni interface is the main origin of PMAE. The PMAE is mainly located at Γ¯-K¯ line in the two dimensional Brillouin zone. This is attributed to large components of the d-orbitals extending along the multilayer plane on Co and Ni near the Fermi energy. We also calculated the SMAE using a discrete approach (DA) and found that there is a reduction of SMAE part in the SDA, compared to the DA. This reduction originates from a prolate quadrupole component of spin density distribution. The present comprehensive study may provide a better understanding of magnetic properties in Co/Ni multilayers as widely used in spintronic devices.

Electronic and magnetic properties of 5d transition metal substitution doping monolayer antimonene: within GGA and GGA + U framework

The structure, electronic, and magnetic properties of 5d series transition metal atoms (TMs) (Hf to Hg) substitution doping antimonene are systematically investigated within the framework of the generalized gradient approximation density functional theory (DFT-GGA) calculations. Our results indicate that identical magnetic ground states are obtained both GGA and GGA with on-site Coulomb repulsion (GGA + U) methods, and the magnetic moment mainly arises from the localized 5d electrons of the 5d TMs. By selective doping different 5d TM atom, antimonene can be regulated from a spin non-polarized to spin-polarized semiconductor and metal. Interestingly, based on GGA + U level, we have demonstrated the spin-polarized semiconducting properties of Hf, Ta, W, Re, Os-substituted antimonene systems, and they can further transform into magnetic half-metallic by electronic or hole doping. Besides these, the weak magnetic coupling is found in the two identical TM impurity antimonene systems. The largest perpendicular magnetic anisotropy energy (MAE) manifests in Re@Sb, up to 18.33 meV in our research systems, and its physics origin of the large MAE is explored in detail in this article. These findings provide an efficient method to tune the physics properties of antimonene, supporting the potential application in the field of spintronics.

Perpendicular orbital and quadrupole anisotropies at Fe/MgO interfaces detected by x-ray magnetic circular and linear dichroisms

We investigated interfacial perpendicular magnetic anisotropy (PMA) in ultrathin Fe/MgO(001) using both x-ray magnetic circular dichroism and magnetic linear dichroism (XMLD). We developed the XMLD technique for detecting the signals from the PMA samples. The PMA energy and quadrupole moments at an Fe/MgO interface were deduced from the XMLD sum rules, whose values explain the microscopic origin of PMA. We found that orbital moment anisotropy is dominant at the Fe/MgO interfacial PMA and the contribution of quadrupole moments is small but finite at the lattice distorted interfaces.