In-plane Easy Axis Research Articles

Carrier doping-induced strong magnetoelastic coupling in 2D lattice.

The realization of intertwined ferroelasticity and ferromagnetism in two-dimensional (2D) lattices is of great interest for broad nanoscale applications but still remains a remarkable challenge. Here, we propose an alternative approach to realize the strongly coupled ferromagnetism and ferroelasticity by carrier doping. We demonstrate that prototypical 2D β-PbO is dynamically, thermally and mechanically stable. Under hole doping, 2D β-PbO possesses ferromagnetism and ferroelasticity simultaneously. Moreover, the robustness of ferromagnetic and ferroelastic orders is doping tunable. In particular, 2D β-PbO features an in-plane easy magnetization axis that is coupled with the lattice direction, enabling the ferroelastic manipulation of the spin direction. Furthermore, the efficient ferroelastic control of the anisotropic optical property and spin splitting in 2D β-PbO are also clarified. Our study highlights a new direction for 2D magnetoelastic research and enables the possibility for multifunctional devices.

Electronic and magnetic properties of [formula omitted] monolayer: DFT+U investigation

A previous theoretical study predicted the stable distorted octahedral (T′) phase of RuO2 sheet using first-principles calculations based on density functional theory (DFT). It is a non-magnetic semiconductor with a bandgap of 0.74 eV using hybrid functionals. However, the Ru atom is located in 4d transition metal in the periodic table, and it has half-filled d orbitals. To correct the self-interaction errors which can be seen in this kind of material (which has half-filled d orbitals) Coulomb interactions should be included by using Hubbard + U parameters in the calculations. So, in this study, it is found that the T′−RuO2 sheet prefers an antiferromagnetic ground state with a direct bandgap of 1.77 eV by using DFT + U. In addition, the antiferromagnetic state is switched when charging and discharging the T′−RuO2 sheet. The T′−RuO2 sheet exhibits a large magnetic anisotropy (MAE) of 2.42 meV per Ru atom with an in-plane easy axis (EA). The EA can be tuned into out-of-plane direction by charging the RuO2 sheet. Mean-field approximation based on the 2D classical Heisenberg model predicts a high Néel temperature (TN) of the T′−RuO2 sheet up to 335 K. The findings expand the potential of the T′−RuO2 sheet for antiferromagnetic spintronic devices.

Elliptical skyrmion moving along a track without transverse speed

It was recently demonstrated that introducing the shape anisotropy of a skyrmion is an alternative strategy for controlling the skyrmion Hall effect. For example, an elliptical skyrmion shows anisotropic motion under the applied driving force. Here, we study the fundamental properties of an elliptical skyrmion induced by uniaxial in-plane magnetic anisotropy. The dynamics of a skyrmion under various parameters, such as uniaxial in-plane magnetic anisotropy, interfacial Dzyaloshinskii-Moriya interaction, and the damping constant, are investigated, and we employ Thiele's theory and derive expressions for the velocity of the skyrmion that match very well with micromagnetic simulation results. We find that the skyrmion Hall angle can reach ${90}^{\ensuremath{\circ}}$ with the optimized angle between the in-plane easy axis and the current. Our results offer a potential application of racetrack memory based on skyrmions, and a type of racetrack configuration was designed.

First-principles and Monte Carlo investigation of magnetic properties of two-dimensional transition metal alloyed boron-carbide [formula omitted] sheet

We identify a new two-dimensional (2D) tetragonal phase of transition metal alloyed boron-carbide (t-CrFeBC) sheet through combined first-principles calculations and Monte Carlo (MC) simulations. The t-CrFeBC sheet prefers a ferromagnetic ground state with the metallic electronic property. Also, the t-CrFeBC sheet is dynamically and thermally stable. t-CrFeBC exhibits sizable magnetic anisotropy energy (MAE) of 120 μeV per CrFe alloy with an in-plane easy axis (EA) magnetization direction. Moreover, hysteresis loops and other hysteresis related properties (coercivity and remanent magnetization) which are evidence of existence of ferromagnetism in the t-CrFeBC sheet are presented for a wide range of temperature. MC simulation results indicate that t-CrFeBC sheet is soft magnetic material with a small coercieve field and narrow rectangular shaped hysteresis curve near the room temperature. All results show that 2D t-CrFeBC sheet holds a unique promise for advanced magnetic device applications.

A new ternary pentagonal monolayer based on Bi with large intrinsic Dzyaloshinskii–Moriya interaction

We systematically investigate the structural, electronic, and magnetic properties of a new pentagonal CoBiS monolayer using first-principles and Monte Carlo simulations. We find that Penta-CoBiS is stable mechanically, dynamically, and thermally and is an antiferromagnetic semiconductor with an indirect band gap of 0.5 eV with HSE functional. In addition, the band-gap increased by applying in-plane biaxial strain. We further show that this monolayer has an in-plane easy axis and possesses large intrinsic Dzyaloshinskii–Moriya interaction because of the broken inversion symmetry, and strong spin–orbit coupling originated from the Bi atoms. Moreover, the Néel temperature is also predicted using Monte Carlo simulations. An out-of-plane magnetic field B is then applied to compensate the in-plane anisotropy. It is found that for B = 1.72 T the spins are fully polarized to the out-of-plane direction. Our results demonstrate that Penta-CoBiS monolayer may find numerous applications in flexible spintronics and electronics.

Room Temperature Ferromagnetism of Monolayer Chromium Telluride with Perpendicular Magnetic Anisotropy.

The realization of long-range magnetic ordering in 2D systems can potentially revolutionize next-generation information technology. Here, the successful fabrication of crystalline Cr3 Te4 monolayers with room temperature (RT) ferromagnetism is reported. Using molecular beam epitaxy, the growth of 2D Cr3 Te4 films with monolayer thickness is demonstrated at low substrate temperatures (≈100 °C), compatible with Si complementary metal oxide semiconductor technology. X-ray magnetic circular dichroism measurements reveal a Curie temperature (Tc ) of v344 K for the Cr3 Te4 monolayer with an out-of-plane magnetic easy axis, which decreases to v240 K for the thicker film (≈7nm) with an in-plane easy axis. The enhancement of ferromagnetic coupling and the magnetic anisotropy transition is ascribed to interfacial effects, in particular the orbital overlap at the monolayer Cr3 Te4 /graphite interface, supported by density-functional theory calculations. This work sheds light on the low-temperature scalable growth of 2D nonlayered materials with RT ferromagnetism for new magnetic and spintronic devices.

Structure and magnetic properties of ferrimagnetic [Gd/Fe]n multilayer and GdxFe100−x thin films

The structural and magnetic properties of two series of [Gd(2, 4 nm)/Fe(t)]n multilayer films with varying Fe thickness were investigated and compared to those of amorphous ferrimagnetic GdFe alloys of the same corresponding composition. Transmission electron microscopy studies confirmed the high interface quality of both multilayer series. Furthermore, the microstructure was analyzed, revealing polycrystallinity in both Gd and Fe layers with strong (101̄0)-oriented textured growth of Gd particularly for the multilayer series with 2 nm Gd. Magnetic measurements confirm an out-of-plane magnetic easy axis in the alloy samples and an in-plane magnetic easy axis in all multilayer samples. Twisted spin states in samples with a low remanent magnetization were identified. Magnetic compensation points of both multilayer series are compared to those of the alloy samples. It was found that the dependence of the magnetic compensation point on effective Gd concentration in the series with 2 nm Gd closely resembles the strong dependence observed in the alloy counterparts. In contrast, a weaker dependence is revealed for the multilayer series with 4 nm Gd, which makes this system more robust against variations in composition required for applications.

Wide-Range Epitaxial Strain Control of Electrical and Magnetic Properties in High-Quality SrRuO3 Films

Epitaxial strain in 4d ferromagnet SrRuO3 films is directly linked to the physical properties through the strong coupling between lattices, electrons, and spins. It provides an excellent opportunity to tune the functionalities of SrRuO3 in electronic and spintronic devices. However, a thorough understanding of the epitaxial strain effect in SrRuO3 has remained elusive due to the lack of systematic studies. This study demonstrates wide-range epitaxial strain control of electrical and magnetic properties in high-quality SrRuO3 films. The epitaxial strain was imposed by cubic or pseudocubic perovskite substrates having a lattice mismatch of -1.6 to 2.3% with reference to bulk SrRuO3. The Poisson ratio, which describes the two orthogonal distortions due to the substrate clamping effect, is estimated to be 0.33. The Curie temperature (TC) and residual resistivity ratios of the series of films are higher than or comparable to the highest reported values for SrRuO3 on each substrate, confirming the high crystalline quality of the films. A TC of 169 K is achieved in a tensile-strained SrRuO3 film on the DyScO3 (110) substrate, which is the highest value ever reported for SrRuO3. The TC (146-169 K), magnetic anisotropy (perpendicular or in-plane magnetic easy axis), and metallic conduction (residual resistivity at 2 K of 2.10 - 373 {\mu}{\Omega}cm) of SrRuO3 are widely controlled by epitaxial strain. These results provide guidelines to design SrRuO3-based heterostructures for device applications.

Enhanced magnetic moment with cobalt dopant in SnS2 semiconductor

We report the strong ferromagnetic order in van der Waals (vdW) layered SnS2 induced by cobalt substitution. The single-crystal Co-doped SnS2 grown by a self-flux method reveals a relatively high Curie temperature (TC) of ∼131 K with an in-plane magnetic easy axis and a large saturation magnetization of ∼0.65 emu g−1 for the 2 at. % Co concentration, which is two orders of magnitude larger than the previously reported value for transition-metal-doped SnS2. The average magnetic moment per Co atom, as high as 1.08 µB, is consistent with the calculated value based on density functional theory, i.e., 1 µB, indicating a negligible antiferromagnetic coupling between Co atoms. Magnetoresistance shows a change in sign from positive to negative, which further confirms the ferromagnetic order in Co-doped SnS2. Our s-p hybridized vdW layered SnS2 serves as a host semiconductor material to search for a suitable magnetic dopant with a high magnetic moment and room temperature TC for next-generation spintronics.

Role of charge doping and strain in the stabilization of in-plane ferromagnetism in monolayer at room temperature

We study the origin of in-plane ferromagnetism in monolayer VSe2 focusing on the effect of charge doping and mechanical strain. We start from an anisotropic spin Hamiltonian, estimate its parameters from density functional calculations, and determine the spectrum of spin-wave excitations. We show that 1T-VSe2 is characterized by relatively strong on-site Coulomb repulsion (U ≃ 5 eV), favoring an antiferromagnetic ground state, which contradicts experimental observations. We calculate the magnetic phase diagram as a function of charge doping and strain, and find a transition to the ferromagnetic state with in-plane easy axis under moderate hole doping (∼1014 cm−2). Analysis of spin-wave excitations in doped monolayer VSe2 reveals a gap due to the in-plane anisotropy, giving rise to long-range magnetic order well above 300 K, in agreement with recent experiments. Our findings suggest that experimentally available 1T-VSe2 monolayer samples might be intrinsically or extrinsically doped, which opens up the possibility for a controllable manipulation of their magnetic properties.

Large amplitude spin-Hall oscillations due to field-like torque

Large amplitude spin-Hall oscillations are identified in a ferromagnetic layer with two perpendicular in-plane easy axis in the presence of field-like torque without any polarizer and external field. The analytical study confirms the possibility of oscillations in the presence of field-like torque. The investigation shows that the oscillation frequency can be tuned from ∼2 GHz to ∼80 GHz by current and enhanced by field-like torque. Further, the enhancement of frequency along with the Q-factor by current and field-like torque is also observed.

Out-of-plane magnetization oscillation in spin Hall device assisted by field-like torque

An excitation of a large-amplitude out-of-plane magnetization oscillation in a ferromagnet by the spin Hall effect is of great interest for practical applications such as microwave generators and neuromorphic computing. However, both experimental and theoretical works have revealed that only small-amplitude oscillation around an in-plane easy axis can be excited via the spin Hall effect. Here, we propose that an out-of-plane oscillation can be excited due to an assistance of field-like torque. We focus on an in-plane magnetized ferromagnet with an easy axis parallel to the current direction. We notice that the field-like torque with an appropriate sign provides an additional field, modifying the dynamic trajectory of the magnetization, and drives the auto-oscillation. The condition on the sign of the field-like torque is satisfied for a typical nonmagnet used in spin Hall devices such as tungsten.

Correlation between Ferromagnetic Layer Easy Axis and the Tilt Angle of Self Assembled Chiral Molecules

The spin–spin interactions between chiral molecules and ferromagnetic metals were found to be strongly affected by the chiral induced spin selectivity effect. Previous works unraveled two complementary phenomena: magnetization reorientation of ferromagnetic thin film upon adsorption of chiral molecules and different interaction rate of opposite enantiomers with a magnetic substrate. These phenomena were all observed when the easy axis of the ferromagnet was out of plane. In this work, the effects of the ferromagnetic easy axis direction, on both the chiral molecular monolayer tilt angle and the magnetization reorientation of the magnetic substrate, are studied using magnetic force microscopy. We have also studied the effect of an applied external magnetic field during the adsorption process. Our results show a clear correlation between the ferromagnetic layer easy axis direction and the tilt angle of the bonded molecules. This tilt angle was found to be larger for an in plane easy axis as compared to an out of plane easy axis. Adsorption under external magnetic field shows that magnetization reorientation occurs also after the adsorption event. These findings show that the interaction between chiral molecules and ferromagnetic layers stabilizes the magnetic reorientation, even after the adsorption, and strongly depends on the anisotropy of the magnetic substrate. This unique behavior is important for developing enantiomer separation techniques using magnetic substrates.

Two-dimensional uranium halide monolayers UX3 (X[dbnd]Cl, Br) with high Curie temperatures

The successful fabrication of two-dimensional ferromagnetic materials, such as monolayer CrI3 and bilayer Cr2Ge2Te6, has drawn much attention in recent years. Here, we found that UX3 (with XCl, Br) monolayers are ferromagnetic materials with high Curie temperatures. In this study we explored the electronic structures and magnetic properties of UX3 by using density functional theory. UX3 has a ferromagnetic order with in-plane magnetic easy axis (i.e., along x direction), and its magnetic anisotropy is 31.2∼34.2 meV per U atom. We performed Monte Carlo calculations to predict the Curie transition temperatures. They are as high as 160 and 125 K for UCl3 and UBr3 monolayers, respectively, which are higher than that of the UI3 found before. Therefore, UX3 monolayers offer new platforms for the study of spintronic devices.

Tailoring magnetism in self-intercalated Cr1+δTe2 epitaxial films

Magnetic transition metal dichalcogenide (TMD) films have recently emerged as promising candidates to host novel magnetic phases relevant to next-generation spintronic devices. However, systematic control of the magnetization orientation, or anisotropy, and its thermal stability, characterized by Curie temperature (Tc) remains to be achieved in such films. Here we present self-intercalated epitaxial Cr1+{\delta}Te2 films as a platform for achieving systematic/smooth magnetic tailoring in TMD films. Using a molecular beam epitaxy (MBE) based technique, we have realized epitaxial Cr1+{\delta}Te2 films with smoothly tunable over a wide range (0.33-0.82), while maintaining NiAs-type crystal structure. With increasing {\delta}, we found monotonic enhancement of Tc from 160 to 350 K, and the rotation of magnetic anisotropy from out-of-plane to in-plane easy axis configuration for fixed film thickness. Contributions from conventional dipolar and orbital moment terms are insufficient to explain the observed evolution of magnetic behavior with {\delta}. Instead, ab initio calculations suggest that the emergence of antiferromagnetic interactions with {\delta}, and its interplay with conventional ferromagnetism, may play a key role in the observed trends. To our knowledge, this constitutes the first demonstration of tunable Tc and magnetic anisotropy across room temperature in TMD films, and paves the way for engineering novel magnetic phases for spintronic applications.

Switching induced by spin Hall effect in an in-plane magnetized ferromagnet with the easy axis parallel to the current

Magnetization switching in a fine-structured ferromagnet of nanoscale by the spin-transfer torque excited via the spin Hall effect has attracted much attention because it enables us to manipulate the magnetization without directly applying current to the ferromagnet. However, the switching mechanism is still unclear in regard to the ferromagnet having an in-plane easy axis parallel to the current. Here, we develop an analytical theory of the magnetization switching in this type of ferromagnet, and reveal the threshold current formulas for a deterministic switching. It is clarified that the current should be in between a certain range determined by two threshold currents because the spin-transfer torque due to a large current outside the range brings the magnetization in an energetically unstable state, and causes magnetization precession around the hard axis.

The strain induced magnetic and anisotropic variations of LaCoO3 thin films

The emergence of ferromagnetism in tensile strained LaCoO3 (LCO) films is still being researched due to the lack of any uniform explanation. Herein, the magnetic anisotropy and the precise ratio of Co2+ ions in LCO films growth on different substrates are reported. The emergence of ferromagnetic state can be observed irrespective of the tensile or compressive strain on the LCO films. A small magnetization along the out-of-plane easy axis is exhibited in the LCO film under compressive strain. The maximum magnetization (1.11 μB/u.c.) is observed in the LCO film over tensile strained LSAT substrate along the in-plane easy axis, which is around four times compared to the magnetization of LCO film on compressive strain. In addition, the exact percentage of Co2+ ions in different LCO films deviated from 12.8% to 10.6% as the strain changed from tensile to compressive. This little variation has a relatively weak effect on the hugely different magnetization of LCO films. Subsequently, the magnetization of LCO film grown on STO buffer layer has a great increase compared to the film direct on LAO substrate. All these results support the proposition that the tensile strain should induce more IS (t2g5eg1) or HS (t2g4eg2) spin states of Co3+ ions, which plays the key role for enhancement of ferromagnetism.

Size dependent chaotic spin–orbit torque induced magnetization switching of a ferromagnetic layer with in-plane anisotropy

Understanding the magnetization switching dynamics induced by the spin–orbit torque (SOT) in a ferromagnetic layer is crucial to the design of the ultrafast and energy-saving spin–orbit torque magnetic random access memory. Here, we investigate the SOT switching dynamics of a ferromagnetic layer with in-plane anisotropy with various elliptic sizes in different easy-axis orientations using micro-magnetic simulations. The reliable and ultrafast magnetization switching can be realized by tilting the easy axis to an optimum angle with respect to the current injecting direction. The switching time, in general, decreases smoothly with an increasing current density, and the optimum tilting angle is determined for small device sizes with width smaller than 100 nm. This optimum angle is a small angle deviating from a case when the in-plane easy axis is orthogonal to the current direction. It depends on the size, the current density, and also the damping constant. However, with the device increasing to a certain size (e.g., 250 nm), especially at small tilting angles, we observe chaotic switching behavior where the switching times fluctuate locally with the current density. We attribute this size dependent chaotic switching phenomenon to the nucleation and formulation of complex multi-domains during switching. This chaotic phenomenon can be alleviated by enhancing the field-like torque in the device and thus decreasing the switching times. Consequently, the shape and size of the devices should be carefully taken into account while designing a practical fast switching and low power SOT device with in-plane anisotropy.

Voltage-controlled magnetic anisotropy in antiferromagnetic L10-MnPt and MnPd thin films

The efficient electrical control of the magnetic states of antiferromagnets is one of the main focus in antiferromagnetic spintronics. In this work, the voltage-controlled magnetic anisotropy (VCMA) effect in two representative antiferromagnets L10-type MnPt and MnPd thin films has been investigated by employing first-principles calculations. Our results indicate that both of the MnPt and MnPd films show in-plane magnetic anisotropy and the magnetic easy axis points either along [1 0 0] or [1 1 0] direction depending on the film thickness. The applied electric field in the range of −0.3 V/Å and + 0.3 V/Å (vacuum as dielectric) leads to the change of magnetocrystalline anisotropy of both films at the order of tens of μJ/m2. Especially, for MnPt film with thickness of two unit cells on Pt(0 0 1), it shows that the moderate electric field is able to switch the in-plane magnetic easy axis between [1 0 0] and [1 1 0] directions. For MnPd thin films, the VCMA effect shows generally linear dependence on the electric field and the VCMA coefficients are estimated in the range of 2.7 to 22.6 fJ/V/m. Our calculation results may stimulate future investigations on the VCMA effect of metallic antiferromagnetic films and pave the possible applications in memory device.

Intrinsic skyrmions in monolayer Janus magnets

Skyrmions are localized solitonic spin textures with protected topology, which are promising as information carriers in ultra-dense and energy-efficient logic and memory devices. Recently, magnetic skyrmions have been observed in magnetic thin films, and are stabilized by the extrinsic interfacial Dzyaloshinskii-Moriya interaction (DMI) and/or external magnetic fields. The specific effects in magnetic monolayer materials have not been thoroughly studied. Here, we investigate the intrinsic magnetic skyrmions in a family of monolayer Janus van der Waals magnets, MnSTe, MnSeTe, VSeTe, and MnSSe, by the first-principles calculations combined with the micromagnetic simulations. The monolayer Janus MnSTe, MnSeTe, and VSeTe with out-of-plane geometric asymmetry and strong spin-orbit coupling (SOC) have a large intrinsic DMI, which could stabilize a sub-50 nm intrinsic skyrmions in monolayer MnSTe and MnSeTe at zero magnetic field. While monolayer VSeTe with in-plane easy axis forms magnetic domain rather than skyrmions. Moreover, the size and shape of skyrmions can be tuned by an external magnetic field. Therefore, our work motivates a new vista for seeking intrinsic skyrmions in atomic-scale magnets.