Perpendicular Magnetic Anisotropy Energy Research Articles

Unveiling elevated curie temperature and exceptional perpendicular magnetic anisotropy in chlorinated monolayer CrI3

Due to the finite band gap and a ferromagnetic ground state, the CrI3 monolayer is attracting great research interests. However, the low Curie temperature of 46 K is one of the major obstacles for spintronics applications. Here, we have investigated the magnetic properties of chlorinated CrI3 monolayer with varying concentration of Cl. We investigated the dynamical and thermal stability by phonon dispersion and ab-initio molecular dynamic simulation. The band gap was decreasing with increasing Cl concentration and a half-metallic state is obtained at Cl concentration of 4.22 × 1014/cm2. The pristine CrI3 monolayer had perpendicular magnetic anisotropy energy of 0.84 meV/Cr. We find that this perpendicular magnetic anisotropy energy can be even enhanced twice by chlorination. Due to the hybridization of I and Cr atom, we found the spin asymmetric density of states in the iodine atom and this strong enhancement in the magnetic anisotropy originated from asymmetric spin features in iodine atom. We propose that the chlorination can increase the Curie temperature up to 204 K and this is almost four times enhanced value. Based on these finding, we propose that the chlorinated CrI3 system can be a promising material for spintronics applications.

Above-room-temperature intrinsic ferromagnetism in ultrathin van der Waals crystal Fe3+xGaTe2

Two-dimensional (2D) van der Waals (vdW) magnets are crucial for ultra-compact spintronics. However, so far, no vdW crystal has exhibited tunable above-room-temperature intrinsic ferromagnetism in the 2D ultrathin regime. Here, we report the tunable above-room-temperature intrinsic ferromagnetism in ultrathin vdW crystal Fe3+xGaTe2 (x = 0 and 0.3). By increasing the Fe content, the Curie temperature (TC) and room-temperature saturation magnetization of bulk Fe3+xGaTe2 crystals are enhanced from 354 to 376 K and 43.9 to 50.4 emu·g−1, respectively. Remarkably, the robust anomalous Hall effect in 3-nm Fe3.3GaTe2 indicates a record-high TC of 340 K and a large room-temperature perpendicular magnetic anisotropy energy of 6.6 × 105 J m−3, superior to other ultrathin vdW ferromagnets. First-principles calculations reveal the asymmetric density of states and an additional large spin exchange interaction in ultrathin Fe3+xGaTe2 responsible for robust intrinsic ferromagnetism and higher TC. This work opens a window for above-room-temperature ultrathin 2D magnets in vdW-integrated spintronics.

Ni thin-films on Pd surfaces and effects of oxygen adsorption: Ab-initio study of structures, electronic properties, magnetic anisotropy

We report first-principles electronic structure calculations of the structural, electronic, and magnetic properties of model epitaxial layers consisting of nickel (Ni) atomic layers deposited on palladium (Pd) substrate, i.e., Ni(001)m∣Pd(001)n where m=1,2,6 and n=3,10, are layer thicknesses. We also investigate the effect of oxygen adsorption on the calculated properties. We found variation in magnetization of between ≈0.6μB to 1.00 μB across the nickel layers. Also, finite magnetic moments albeit of small values of between 0.2 μB and 0.3 μB is found on the Pd at the interface. This magnetic moment on an otherwise non-magnetic Pd atoms has been adduced to interfacial strain due to lattice mismatch between the Ni and Pd layers at the Ni|Pd interface. The effect of adsorbed oxygen on the Nim∣Pdn is that it increases the magnetic moment on the nickel layers. Also, regarding the magnitude of magnetic anisotropy energy (MAE), we found a high perpendicular values of 1.63 meV and 1.37 meV per unit cell respectively for Nim∣Pd10 (m=2,6) which are relatively higher than those reported for other transition metal epitaxial layers. However, the presence of oxygen atom on the Ni∣Pd changes the direction and magnitude of MAE. Indeed, O adsorption favours or enhances in-plane magnetization direction depending on the thickness of the Ni layers for a fixed Pd thickness. Plots of local density of states (LDOS) which include the effect of spin–orbit coupling (SOC), show that in the case of Ni∣Pd having perpendicular MAE, there appears a new SOC-induced electronic states below and above the Fermi level. These states appears to stabilize this type of magnetic anisotropy. On the other hand, in-plane MAE is characterized by SOC-induced localized states below the Fermi level (EF) as well as the lowering of the DOS at the EF. Our work explores the physical, magnetic and electronic properties that may be useful in designing Ni∣Pd-based superlattices for magnetic or spintronic applications.

FenB2+2n (n = 1, 2), an intrinsic room temperature ferromagnetic MBene with significant magnetic performance enhancement achievable by carbon substitution

From spintronics to data storage technology, two-dimensional (2D) ferromagnetic materials show great promise for various applications. This work reports a series of stable ferromagnetic transition metal boride FenB2+2n (n = 1, 2), where robust long-range ferromagnetic exchange coupling and large perpendicular magnetic anisotropy energy (MAE) allow the ferromagnetic transition temperature (Tc) of the FenB2+2n monolayer to reach above room temperature. The metallic FenB2+2n exhibits n- and p-type Dirac transport in both spin channels with a high Fermi velocity. Furthermore, the application of biaxial compressive strain and electron doping can greatly increase the ferromagnetic coupling and MAE of FeB4 monolayers. On this basis, the FeB2C2 alloy with a high concentration of carbon substitution has been designed, which allows the nonvolatile integration of in-plane compressive strain and electron doping. As expected, this substitution doping resulted in a significant increase in the Tc and MAE of the system. Our findings provide perspectives for the study of 2D magnetic materials.

External manipulation of the spin-orbit torque and magnetization switching in gradient CoPd single layer via hydrogen

Recent findings regarding spin-orbit torques (SOTs) and current-induced magnetization switching in ferromagnetic (FM) single layers have attracted substantial attention due to the advantage of not necessitating the use of heavy-metal layers. Nevertheless, despite prior studies on the interior structural engineering of the SOT, the external techniques for manipulating the SOT in the FM single layer remains elusive, which is indispensable for the practical application of the single layer SOT devices. Here, we demonstrate external manipulation of SOT generation in CoPd single layer through the fabrication of CoPd film with a composition gradient, utilizing the H2-absorption property of Pd. It is found that the H-induced strain within the CoPd film plays a pivotal role in generating SOT. Meanwhile, we demonstrate that the critical current density required for the current-induced magnetization switching is markedly diminished with the application of H2 due to the enhanced SOT generation and reduced perpendicular magnetic anisotropy energy. Our findings offer a straightforward method for external manipulation of single layer SOT devices, and hold the potential for applications of the spintronic devices.

Low Gilbert damping and high perpendicular magnetic anisotropy in an Ir-coupled L10-FePd-based synthetic antiferromagnet

Thin ferromagnetic films possessing perpendicular magnetic anisotropy derived from the crystal lattice can deliver the requisite magnetocrystalline anisotropy density for thermally stable magnetic memory and logic devices at the single-digit-nm lateral size. Here, we demonstrate that an epitaxial synthetic antiferromagnet can be formed from L10 FePd, a candidate material with large magnetocrystalline anisotropy energy, through insertion of an ultrathin Ir spacer. Tuning of the Ir spacer thickness leads to synthetic antiferromagnetically coupled FePd layers, with an interlayer exchange field upwards of 0.6 T combined with a perpendicular magnetic anisotropy energy of 0.95 MJ/m3 and a low Gilbert damping of 0.01. Temperature-dependent ferromagnetic resonance measurements show that the Gilbert damping is mostly insensitive to temperature over a range of 20 K up to 300 K. In FePd|Ir|FePd trilayers with lower interlayer exchange coupling, optic and acoustic dynamic ferromagnetic resonance modes are explored as a function of temperature. The ability to engineer low damping and large interlayer exchange coupling in FePd|Ir|FePd synthetic antiferromagnets with high perpendicular magnetic anisotropy could prove useful for high performance spintronic devices.

Giant Magneto-Optical Effect in van der Waals Room-Temperature Ferromagnet Fe3GaTe2

Abstract The discovery of ferromagnetic two-dimensional (2D) van der Waals (vdWs) materials provides an opportunity to explore intriguing physics and to develop innovative spin electronic devices. However, the main challenge for practical applications of vdWs ferromagnetic crystals lies in the weak intrinsic ferromagnetism and small perpendicular magnetic anisotropy (PMA) above room temperature. Here, we report the intrinsic vdWs ferromagnetic crystal Fe3GaTe2, synthesized by the self-flux method, exhibiting a Curie temperature (T C) of 370 K, a high saturation magnetization of 33.47 emu/g, and a large PMA energy density of approximately 4.17 × 105 J/m3. Furthermore, the magneto-optical effect is systematically investigated in Fe3GaTe2. The doubly degenerate E 2g (Γ) mode reverses the helicity of incident photons, indicating the existence of pseudoangular-momentum (PAM) and chirality. Meanwhile, the non-degenerate non-chiral A 1g(Γ) phonon exhibits a significant magneto-Raman effect under an external out-of-plane magnetic field. These results lay the groundwork for studying phonon chirality and magneto-optical phenomena in 2D magnetic materials, providing the feasibility for further fundamental research and applications in spintronic devices.

Half-metal Mn2GeI2 monolayer with high Curie temperature and large perpendicular magnetic anisotropy

Two-dimensional (2D) intrinsic ferromagnetic (FM) materials with high Curie temperatures (Tc), large perpendicular magnetic anisotropy (PMA), and large spin polarization are desirable for atomically thin spintronic devices. Herein, we propose a 2D intrinsic FM material Mn2GeI2 monolayer with thermodynamical stability and outstanding FM properties using first-principles calculations. Our calculations show that Mn2GeI2 monolayer is a half-metal with a spin gap of 1.76 eV, which ensures 100% spin polarization ratio at the Fermi level. Importantly, Mn2GeI2 monolayer has a large PMA energy of 2.90 meV and a high Tc of 648 K, ideal for practical applications at room temperature. Further in-depth investigation of microscopic coupling in the Mn2GeI2 monolayer reveals that the robust ferromagnetism mainly resulted from the synthetic effect of Ruderman–Kittel–Kasuya–Yosida exchange interaction between the Mn ion layers and superexchange interaction within the Mn ion layers. Our results give insights into the structure and electronic and magnetic properties of Mn2GeI2 monolayer and provide a promising candidate for 2D spintronic devices.

Write error rate analysis of field-free spin-orbit torque switching in conically magnetized free layer nanomagnet

Enhancing the performance of magnetic random access memories (MRAMs) is crucial, considering speed, energy efficiency, and endurance. Spin-orbit torque-based MRAMs offer ultrafast operation and enhanced reliability. Still, the energy efficiency and external magnetic field requirement for deterministic switching of nanomagnets with perpendicular magnetic anisotropy (PMA) are a significant hurdle. To address these issues, one of the proposed solutions employs a conically magnetized free layer magnetic tunnel junction device featuring second-order PMA. This approach can minimize the required switching current density by leveraging the interplay between first- and second-order PMA energies. Additionally, coupling with exchange bias from the antiferromagnet can eliminate the need for an external field entirely. Our analysis investigates the impact of current density (magnitude and rise/fall time), first- and second-order PMA fields, exchange bias, and field-like torque on the device's switching performance. By optimizing the perpendicular magnetic anisotropies, we report improvement in the write error rate from 10−4 to 10−7. Our findings hold promise for magnetic memory applications.

Oxygen adsorption on pristine MnBi and Ta-capped MnBi thin-films: Ab-initio study of structural and electronic properties, and magnetic anisotropy

We have studied the structural and electronic properties, as well as magnetic anisotropy in oxidized MnBi thin film with and without tantalum (Ta) atom as a capping layer. Spin-polarized density-functional theory calculations with the generalized gradient approximation (GGA) including U+J Hubbard correction, i.e., DFT+U+J have been employed. We found that the oxygen (O) atom binds more strongly to the MnBi surface when co-present with the Ta atom. We postulate that the strong binding prevents the O atoms from diffusing into the MnBi film layer. Thus, the Ta layer prevents surface oxidation by inducing strong binding of the O atoms to the Mn surface. Furthermore, in cases where molecular O2 is adsorbed on the Ta-capped MnBi, the O2 molecule fragments into two atomic components such that each of the O atoms relaxes to a different lattice sites on the MnBi film. Electronic structure analysis performed with this structure shows that the Ta atom on which one of the O atom is directly attached is weakly binded to the Mn. This makes it less feasible for the O atom to attach to MnBi surface. We conclude therefore, that two different mechanisms contrive to prevent MnBi surface oxidation. These are the strong binding of the O atom to the Mn which prevents the O diffusion into the MnBi film and the weak electronic bonding between the Ta and O which prevent the O atom from binding to the MnBi surface. Furthermore, we found that the presence of either Ta or O, or both, largely preserves the lattice structure of the MnBi film. With respect to the magnetic anisotropy energy (MAE), the presence of Ta capping layer restores the in-plane parallel magnetization of the pristine MnBi thin film after the latter’s oxidation. Where the direction of MAE is reversed from the in-plane to perpendicular, the perpendicular MAE is order of magnitude smaller than the in-plane MAE. This affirms the tendency of Ta capping layer to preserve either strongly or weakly, the direction of magnetization in oxidized MnBi. Our work provides the missing but highly crucial atomic level insights into an experimental study (M.Y. Sun et.al., Journal of Alloys and Compounds 672 (2016) 59-63) and has implications for the modeling of oxidation prevention via capping of magnetic thin film with external metallic atoms.

Large anomalous transverse transport properties in atomically thin 2D Fe3GaTe2

Anomalous transverse conductivities, such as anomalous Hall conductivity (AHC), anomalous Nernst conductivity (ANC), and anomalous thermal Hall conductivity (ATHC), play a crucial role in the emerging field of spintronics. Motivated by the recent fabrication of two-dimensional (2D) ferromagnetic thin film Fe3GaTe2, we investigate the thickness-dependent anomalous transverse conductivities of the 2D Fe3GaTe2 system (from one to four layers). The atomically ultrathin 2D Fe3GaTe2 system shows above-room-temperature ferromagnetism with a large perpendicular magnetic anisotropy energy. Furthermore, we obtain a large AHC of −485 S/cm in the four-layer thickness, and this is further enhanced to −550 S/cm with small electron doping. This AHC is seven times larger than the measured AHC in thicker 2D Fe3GaTe2 (178 nm). The ANC also reaches 0.55 A/K.m in the four-layer structure. Along with these, the four-layer system exhibits a large ATHC (−0.105 ~ −0.135 W/K.m). This ATHC is comparable to the large ATHC found in Weyl semimetal Co3Sn2S2. Based on our results, the atomically ultrathin 2D Fe3GaTe2 system shows outstanding anomalous transverse conductivities and can be utilized as a potential platform for future spintronics and spin caloritronic device applications.

Perpendicular magnetic anisotropy properties of Co2FeSi/Pt multilayers deposited on amorphous dielectric Ta2O5

Perpendicular magnetic anisotropy (PMA) is the mostsignificant characteristic for the development ofnonvolatile magnetic storage systems with superior performance. Hence, we investigate PMA of Co2FeSi (CFS)/Pt structures deposited on amorphous Ta2O5 of high dielectric constant. This design has the potential to be beneficial for low-power voltage-controlled spintronic devices. Strong PMA around ∼ 106 erg/cm3 is noticed in Ta2O5/[CFS/Pt]3 stacks with CFS thickness ranging from 0.6 ∼ 1.0 nm. The interfacial PMA energy density is found to be ∼ 0.63 erg/cm2. The PMA rises in the beginning with increase in number of repetitions (n), reaches a maximum of1.4x106 erg/cm3 at n = 3, and then falls. Pronounced thickness effects of Ta2O5 buffer on magnetic properties are observed. We clearly demonstrate the process of engineering PMA in the CFS/Pt bilayer structures deposited on Ta2O5 dielectric. Our findings might be useful for designing CFS/Pt multilayers on high-K dielectric for low power consumption spintronic devices.

Interfacial Fe segregation and its influence on magnetic properties of CoFeB/MgFeO multilayers

Abstract We investigated effects of Fe segregation from partially Fe-substituted MgO (MgFeO) on the magnetic properties of CoFeB/MgFeO multilayers. X-ray photoelectron spectroscopy and magnetic measurements revealed that segregated Fe reduced to metallic Fe and ferromagnetism was exhibited at the CoFeB/MgFeO interface. The CoFeB/MgFeO multilayer showed a more than 2-fold enhancement in perpendicular magnetic anisotropy (PMA) energy density compared with that of a standard CoFeB/MgO multilayer. The PMA energy density was further enhanced by insertion of an ultrathin MgO layer between the CoFeB and MgFeO layers. Ferromagnetic resonance measurements also revealed a remarkable reduction of magnetic damping in the CoFeB/MgFeO multilayers.

Manipulation of spin–orbit torque and Dzyaloshinskii-Moriya interaction by varying Mn concentration in Pt1-xMnx/Co bilayer

Reducing critical current density (JC) of current-driven magnetization switching via power-efficient spin–orbit torque (SOT) is a key challenge in spin-based memory and logic devices. The Pt1-xMnx alloy shows prospect for enhancing the damping-like SOT efficiency (ξDL) while maintaining relatively low resistivity and power consumption. In this work, we investigated the Mn concentration x dependence of ξDL in Pt1-xMnx/Co bilayer and observed a nonmonotonic variation with the maximum ξDL ≈ 0.25 appearing around x = 0.2. The increase of the Pt1-xMnx layer resistivity is the main contribution to the enhancement of ξDL. Mn atoms doped in Pt not only modulate ξDL originating from the spin Hall effect of the Pt1-xMnx layer, but also affect the interfacial phenomena at the Pt1-xMnx/Co interface. We find that the related interfacial Dzyaloshinskii-Moriya interaction (DMI) is also tunable by x with the maximum appearing around x = 0.15. The significantly reduced JC of Pt1-xMnx/Co bilayer is mainly due to the enhancement of ξDL and the attenuation of effective perpendicular magnetic anisotropy energy density. Meanwhile, Pt1-xMnx/Co bilayer maintains low power consumption and necessary thermal stability. Our work highlights the potential of alloying Pt with Mn as a valuable means to achieve high-performance SOT spintronic devices.

Current-driven magnetic skyrmion diodes controlled by voltage gates in synthetic antiferromagnets

Magnetic skyrmions, as promising candidates in various spintronic devices, have been widely studied owing to their particle-like properties, nanoscale size, and low driving current density. Here, we numerically and theoretically investigate the dynamics of current-driven skyrmion passing through a voltage gate in a synthetic antiferromagnetic racetrack. It is found that the critical current required for skyrmion to pass through the voltage gate positively is much less than that for skyrmion to pass through the gate negatively. Furthermore, we systematically study the linear dependence of the minimum velocity of skyrmion on the driving current density and perpendicular magnetic anisotropy (PMA) gradient, and the calculation results are quite consistent with the simulation results. Finally, we find that the variation of the PMA energy with the position of skyrmion can help us to compare the magnitude of resistance force when the skyrmion passes through different voltage gates. Our results can be beneficial for the design and development of skyrmion diodes.

Spontaneous spin and valley polarizations in a two-dimensional Cr2S3 monolayer

Valleytronics has attracted much attention due to its potential applications in information progress and data storage. In this paper, monolayer Cr2S3 is proven to be a ferromagnetic (FM) semiconductor by using first-principles calculations. Moreover, monolayer Cr2S3 exhibits a perpendicular magnetic anisotropy energy of 30 μeV/f.u. Surprisingly, monolayer Cr2S3 presents spontaneous valley polarization, which means that it will be nonvolatile for data storage. Notably, monolayer Cr2S3 changes to an antiferromagnetic (AFM) state from the original FM state under biaxial tensile strain, and its easy axis will be reorientated from out-of-plane to in-plane when the compressive strain is larger than −2%. Importantly, for AFM monolayer Cr2S3, the valley polarization reversion can be realized by an external electric field along the z direction. In brief, valley polarization has been achieved in both FM and AFM monolayer Cr2S3, which is very rare in other valleytronics research. The present research provides a tantalizing candidate for realizing and manipulating valley and spin physics.

Magnetic anisotropy of Fe/MgO interfaces inserted with alkali halide layers

Perpendicular magnetic anisotropy (PMA) at the Fe/MgO interface has been widely studied for memory applications, and further improving PMA has been one of the central challenges. In this study we fabricate epitaxial Fe/alkali halide (LiF, NaCl, or CsI)/MgO multilayers using molecular beam epitaxy and investigate how the alkali halide layer insertion can enhance PMA at the Fe/MgO interface. We find that insertion of an ultrathin LiF layer (0.1--0.4 nm) enhances interfacial PMA, but thicker LiF layer insertion (0.6--1 nm) weakens it. For the CsI and NaCl cases, interfacial PMA energy decreases monotonically with CsI or NaCl thickness. The present results indicate that well-defined Fe-anion orbital hybridization achieved by good lattice matching between the Fe and insulator layers is essential to have strong PMA. It is also suggested that anion atoms with high electronegativities, such as F, are beneficial for interfacial PMA, probably because of the weaker Fe-F hybridization and stronger electron localization at the interface. Our study shall serve as a guiding principle for designing a new dielectric layer to achieve stronger PMA in ultrathin Fe films.

Perpendicular magnetic anisotropy, tunneling magnetoresistance and spin-transfer torque effect in magnetic tunnel junctions with Nb layers

Nb and its compounds are widely used in quantum computing due to their high superconducting transition temperatures and high critical fields. Devices that combine superconducting performance and spintronic non-volatility could deliver unique functionality. Here we report the study of magnetic tunnel junctions with Nb as the heavy metal layers. An interfacial perpendicular magnetic anisotropy energy density of 1.85 mJ/m2 was obtained in Nb/CoFeB/MgO heterostructures. The tunneling magnetoresistance was evaluated in junctions with different thickness combinations and different annealing conditions. An optimized magnetoresistance of 120% was obtained at room temperature, with a damping parameter of 0.011 determined by ferromagnetic resonance. In addition, spin-transfer torque switching has also been successfully observed in these junctions with a quasistatic switching current density of 7.3 times ;10^{5} A/cm2.

Two dimensional Janus RuXY (X, Y = Br, Cl, F, I, X ≠ Y) monolayers: ferromagnetic semiconductors with spontaneous valley polarization and tunable magnetic anisotropy.

Two-dimensional (2D) ferromagnetic (FM) materials with valley polarization are highly desirable for use in valleytronic devices. The 2D Janus materials have fascinating physical properties due to their asymmetrical structures. In this work, the electronic structure and magnetic properties of Janus RuXY (X, Y = Br, Cl, F, I, X ≠ Y) monolayers are systematically studied using first-principles calculations. RuBrCl, RuBrF, and RuClF monolayers are all FM semiconductors. The valley polarization is present in the band structure and this is determined by the spin orbit coupling (SOC). The valley splitting energy of the RuClF monolayer is as large as 204 meV, with a perpendicular magnetic anisotropy (PMA) energy of 1.918 mJ m-2 and a Curie temperature of 316 K. Therefore, spontaneous valley polarization at room temperature will be seen in the RuClF monolayer. The Curie temperature of the RuBrF monolayer is higher than that of the RuClF, but the magnetic anisotropy energy (MAE) is in-plane magnetic anisotropy (IMA). The valley splitting energy of the RuBrCl monolayer is higher and the PMA energy is lower than that of the RuClF monolayer. The Curie temperature was only 197 K. The valley polarization was modulated in the RuXY monolayers at different biaxial strains, during which the semiconductor properties are still maintained. The PMA of the RuClF and RuBrCl monolayers is enhanced by the biaxial compressive strains, which are mainly attributed to the variation of the (dyz, d2z) orbital matrix elements of the Ru atoms. The MAE of the RuBrF monolayer is tuned from IMA into PMA at a biaxial strain of -6%. These results show an example of a 2D Janus ferrovalley material.

High temperature ferromagnetic metal: a Janus CrSSe monolayer.

Two-dimensional (2D) ferromagnets are popular in fields such as spintronic devices, but their low Curie temperature (TC) limits their practical application. In this work, by using a global optimization evolutionary algorithm and density functional theory method, a Janus CrSSe ferromagnetic monolayer was predicted systematically. Monte Carlo simulations show that the Curie temperature of the Janus CrSSe monolayer is about 272 K and can be adjusted to 496 K under a small tensile biaxial strain. Besides, this monolayer possesses large magnetic anisotropy energy (1.4 meV Cr-1). The magnetic order can be changed from ferromagnetic to antiferromagnetic order under compressive strain. What's more, this monolayer possesses the lowest energy in the 2D search space and excellent thermal, dynamic, and mechanical stabilities. Considering its excellent properties and current experimental techniques, it is possible to synthesize CrSSe monolayer experimentally.