Perpendicular Easy Axis Research Articles

Strain-induced giant enhancement and transformation of magnetocrystalline anisotropy in 1T-FeSe2 monolayer

The development of magnetic materials with large magnetic anisotropy energy (MAE) is crucial for magnetic storage and spintronics applications. The electronic structure and magnetic properties of 1T-FeSe2 monolayer have been systematically investigated by first-principles computational methods. The monolayer 1T-FeSe2 is confirmed to be a ferromagnetic semimetal with 100% spin polarization and exhibits pronounced magnetic anisotropy. It shows an easy magnetization plane in the two-dimensional plane and its MAE is as high as 8.134 meV. The MAE mainly originates from the contributions of transitions between Fe dyz and dz2, dxy and dx2−y2 orbitals, while the contributions of transitions between p x and p y, p x and p z orbitals of Se are also obvious. The biaxial strain significantly regulates the magnetic properties and electronic structures of the 1T-FeSe2 monolayer. Interestingly, the MAE increases significantly to 77.680 meV when the tensile strain increases to 3.5%, we can expect to achieve a reasonable value of MAE up to 98.951 meV by applying 5% tensile strain. In addition, the easy magnetization plane changes to perpendicular easy magnetization axis direction at −6% compressive strain or 7% tensile strain, which leads to a significant change in the band structure near the Fermi level. The 100% spin-polarized ferromagnetic semi-metallic behavior and the huge strain-induced magnetic anisotropy make the 1T-FeSe2 monolayer material promising for spintronics and magnetic data storage.

Stable skyrmions in Co/Ni-based nanopillars with perpendicular magnetization anisotropy

Micromagnetic simulations have been conducted to explore the formation rules of stable skyrmions in Co/Ni-based perpendicular magnetization anisotropy nanopillars with Dzyaloshinskii–Moriya Interaction (DMI). The results show that an appropriate perpendicular magnetic field can generate a robust Néel-type skyrmion magnetic configuration in the free layer with a 5° tilted easy axis. The creation and stability of skyrmion states also depends on the strength of the DMI and the size of nano-disk. Furthermore, we observed intriguing behavior in the case of the free layer with a perfectly perpendicular easy axis. Skyrmion states appeared in two distinct regions of perpendicular magnetic field when both perpendicular and in-plane magnetic fields were applied. This discontinuity arises from the nucleation of initial Bloch-type skyrmions. These results contribute to comprehending the formation mechanism of stable skyrmions in perpendicular magnetic anisotropy multilayers and may promote the development of skyrmion-based nanodevices.

Nonlinear conductance in nanoscale CoFeB/MgO magnetic tunnel junctions with perpendicular easy axis

Magnetic tunnel junctions (MTJ) exhibit spin-dependent conductance that governs their performance in various applications. While the transport characteristics are known to show nonlinearity, their behavior and underlying mechanism have not yet understood well. Here we investigate nonlinear conductance at a low bias regime in nanoscale MTJs with a perpendicular magnetic easy axis and various junction sizes, by measuring current-voltage ($IV$) characteristics and ferromagnetic resonance. We evaluate $IV$ properties as $I={G}_{1}V+{G}_{2}{V}^{2}+{G}_{3}{V}^{3}$ under various external magnetic fields and examine the correlations among ${G}_{1}, {G}_{2}$, and ${G}_{3}$. We find that ${G}_{2}$ increases with decrease in the junction size, ${G}_{3}$ has a negative correlation with ${G}_{1}$, and $\ensuremath{\delta}{G}_{3}/\ensuremath{\delta}{G}_{1}\phantom{\rule{4pt}{0ex}}(=k)$ has a positive correlation with ${G}_{2}$. These results can be explained by considering the spin flip during the tunneling and a modulation of material properties at the device edge caused by the nanofabrication process. Ferromagnetic resonance measurements support the physical picture suggested by the transport measurements. Our findings shed light on the mechanism of electron transport in nanoscale MTJs and facilitate the establishment of a rigorous model describing their nonlinear conductance.

Effect of annealing on the magnetic anisotropy of GaMnAsP layers with graded P concentration

We have investigated the effect of annealing on the magnetic anisotropy of MBE-grown GaMnAs1−yPy film in which phosphorus content varies from 0% to 24% along the growth direction. Such variation is achieved by growing a series of GaMnAs1−yPy layers in which y is successively increased. Hall effects measurements on an as-grown graded film reveal that the bottom 80% of the film has in-plane easy axes, 10% has both in-plane and perpendicular easy axes, and the remaining 10% has a vertical easy axis. Such gradual change of magnetic anisotropy in the film from in-plane to perpendicular with increasing P concentration is in accordance with the continuous variation of strain from compressive to tensile as the P concentration increases the bottom of the film to tensile toward its tip surface. However, thermal annealing significantly changes the magnetic anisotropy of the graded GaMnAs1−yPy film. In particular, the intermediate region having both in-plane and perpendicular easy axes nearly disappears in the film after annealing, so the film is divided into two types of layers having either only in-plane or only perpendicular anisotropy. These dramatic changes in magnetic anisotropy of the graded GaMnAs1−yPy film introduced by annealing suggest that one can strategically use this process to realize orthogonal magnetic bilayers consisting of in-plane and perpendicular easy axes.

Reduced effective magnetization and damping by slowly relaxing impurities in strained γ-Fe2O3 thin films

Magnetically ordered insulators are of key interest for spintronics applications, but most of them have not yet been explored in depth regarding their magnetic properties, in particular with respect to their dynamic response. We study the static and dynamic magnetic properties of epitaxially strained γ-Fe2O3 (maghemite) thin films grown via pulsed-laser deposition on MgO substrates by SQUID magnetometry and cryogenic broadband ferromagnetic resonance experiments. SQUID magnetometry measurements reveal hysteretic magnetization curves for magnetic fields applied both in- and out of the sample plane. From the magnetization dynamics of our thin films, we find a small negative effective magnetization in agreement with a strain induced perpendicular magnetic anisotropy. Moreover, we observe a non-linear evolution of the ferromagnetic resonance-linewidth as a function of the microwave frequency and explain this finding with the so-called slow relaxor model. We investigate the magnetization dynamics and non-linear damping mechanisms present in our samples as a function of frequency and temperature and in particular, observe a sign change in the effective magnetization from the transition of the magnetic anisotropy from a perpendicular easy axis to an easy in-plane anisotropy for reduced temperatures. Its nonlinear damping properties and strain-induced perpendicular anisotropy render γ-Fe2O3 an interesting material platform for spintronics devices.

Nanometer-thin L1-MnAl film with B2-CoAl underlayer for high-speed and high-density STT-MRAM: Structure and magnetic properties

The material development of magnetic tunnel junction with a perpendicular easy axis is in great demand to advance spin-transfer torque magnetoresistive random access memory (STT-MRAM) technologies. To realize high-speed and high-density STT-MRAM, a thin-film magnetic material with large perpendicular anisotropy and small spontaneous magnetization has great potential. Here, we develop a thin-film deposition technique for a-few-nanometer-thin L10-MnAl by sputtering and investigate its structure and magnetic properties. Utilization of the B2-CoAl buffer layer allows us to grow L10-MnAl with a large crystalline anisotropy of 8.5 × 105 J/m3, the small spontaneous magnetization of 0.62 T, and the tolerance for 400 °C annealing even at the MnAl thickness of 2 nm. We calculate the device properties based on the obtained material parameters and find that high retention properties, high-speed switching, and low write-error rate can be obtained at the single-digit-nm region, which are not readily achieved by conventional material systems. The results show the potential of L10-MnAl for high-density and high-speed STT-MRAM.

Temperature dependence of intrinsic critical current in perpendicular easy axis CoFeB/MgO magnetic tunnel junctions

Current induced magnetization switching in CoFeB/MgO-based magnetic tunnel junctions (MTJs) with a perpendicular easy axis is studied above room temperature. The intrinsic critical current IC0 of the MTJs decreases with increasing temperature. From a vector-network-analyzer ferromagnetic resonance measurement with a heating system, temperature dependence of magnetic anisotropy and damping constant is evaluated. We find that the reduction of IC0 at elevated temperature is mainly due to a decrease in magnetic anisotropy. A slight increase in the damping constant with temperature rise is also observed, consistent with the mechanism considering electron scattering through the inter-band transition.

Sigmoidal curves of stochastic magnetic tunnel junctions with perpendicular easy axis

We investigate the physical mechanism governing the sigmoid-like time-averaged response of stochastic magnetic tunnel junctions (s-MTJ), which is a promising building block for probabilistic computers. We measure the time-averaged resistance of perpendicular easy-axis s-MTJs with various free-layer thicknesses and diameters as functions of an external magnetic field and current. The time-averaged response shows no significant dependence on the free-layer thickness, whereas significantly varies with the diameter. Based on the Néel-Arrhenius law, we derive an analytical expression of the time-averaged response against both the magnetic field and current and discuss the underlying mechanism accounting for the obtained results. We show that the experimental results are well explained by considering magnetically active and electrically active volumes of the superparamagnetic free layer in s-MTJs. The obtained finding provides an important design guideline of s-MTJs for probabilistic computers.

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.

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.

Chiral magnetic domain walls under transverse fields: A semi-analytical model

An analytical model for the domain wall structure in ultrathin films with perpendicular easy axis and interfacial Dzyaloshinskii-Moriya interaction, submitted to an arbitrary in-plane magnetic field, is presented. Its solution is simplified to the numerical minimization of an analytic function of just one variable. The model predictions are compared to numerical micromagnetic simulations, using parameters of existing samples, revealing a very good agreement. Remaining differences are analyzed, and partly corrected. Differences with the predictions of the simplest model, usually found in the literature, in which only the domain wall moment’s in-plane orientation can vary, are exemplified. The model allows accurate computations, as a function of in-plane field module and orientation, of the domain wall tension and width, quantities controlling the creep motion of domain walls in such films.

Thermodynamically stable skyrmion lattice in a tetragonal frustrated antiferromagnet with dipolar interaction

Motivated by recent experimental results on GdRu$_2$Si$_2$ [Khanh, N.D., Nakajima, T., Yu, X. et al., \emph{Nat. Nanotechnol.} \textbf{15}, 444-449 (2020)], where nanometric square skyrmion lattice was observed, we propose simple analytical mean-field description of the high-temperature part of the phase diagram of centrosymmetric tetragonal frustrated antiferromagnets with dipolar interaction in the external magnetic field. In the reciprocal space dipolar forces provide momentum dependent biaxial anisotropy. It is shown that in tetragonal lattice in the large part of the Brillouin zone for mutually perpendicular modulation vectors in the $ab$ plane this anisotropy has mutually perpendicular easy axes and collinear middle axes, what leads to double-Q modulated spin structure stabilization. The latter turns out to be a square skyrmion lattice in the large part of its stability region with the topological charge $\pm 1$ per magnetic unit cell, which is determined by the frustrated exchange coupling, and, thus, nanometer-sized. In the presence of additional single-ion easy-axis anisotropy, easy and middle axes can be swapped, which leads to different phase diagram. It is argued that the latter case is relevant to GdRu$_2$Si$_2$.

Effect of surface modification treatment on top-pinned MTJ with perpendicular easy axis

We investigated the effects of surface modification treatment (SMT) on the perpendicular magnetic anisotropy (PMA) and the thermal tolerance of top-pinned magnetic tunnel junctions (MTJs) with a Co/Pt synthetic ferrimagnetic coupling reference layer. Applying an SMT to the bottom Pt layer increased the PMA of the overlying Co/Pt multilayer. X-ray diffraction spectrum analysis revealed that the SMT resulted in a higher crystallinity and smaller lattice spacing in the Co/Pt multilayer in the thinner bottom Pt layer, which may have increased the PMA in the Co/Pt multilayer. The tunnel magnetoresistance (TMR) ratio of the MTJ with SMT increased as the annealing temperature was increased up to 400 °C; conversely, the TMR ratio of the MTJ without SMT decreased at an annealing temperature of 400 °C. Evaluation of the m-H loops revealed that, after annealing at 400 °C, the reference layers in the MTJs after SMT possessed better magnetic properties than those in the MTJs without an SMT; this is attributable to the higher PMA of the reference layers with SMT. EDX line analysis revealed that SMT suppressed Pt diffusion to the MgO barrier, resulting in a higher thermal tolerance and larger PMA of the reference layer.

Magnetic response and electronic states of well defined Graphene/Fe/Ir(111) heterostructure

We investigate a well defined heterostructure constituted by magnetic Fe layers sandwiched between graphene (Gr) and Ir(111). The challenging task to avoid Fe-C solubility and Fe-Ir intermixing has been achieved with atomic controlled Fe intercalation at moderate temperature below 500 K. Upon intercalation of a single ordered Fe layer in registry with the Ir substrate, an intermixing of the Gr bands and Fe $d$ states breaks the symmetry of the Dirac cone, with a downshift in energy of the apex by about 3 eV, and well-localized Fe intermixed states induced in the energy region just below the Fermi level. First principles electronic structure calculations show a large spin splitting of the Fe states, resulting in a majority spin channel almost fully occupied and strongly hybridized with Gr $\ensuremath{\pi}$ states. X-ray magnetic circular dichroism on the Gr/Fe/Ir heterostructure reveals an ordered spin configuration with a ferromagnetic response of Fe layer(s), with enhanced spin and orbital configurations with respect to the bcc-Fe bulk values. The magnetization switches from a perpendicular easy magnetization axis when the Fe single layer is lattice matched with the Ir(111) surface to a parallel one when the Fe thin film is almost commensurate with graphene.

Probing edge condition of nanoscale CoFeB/MgO magnetic tunnel junctions by spin-wave resonance

We investigate spin-wave resonance in nanoscale CoFeB/MgO magnetic tunnel junctions (MTJs) with a perpendicular easy axis and various free-layer sizes. Two types of MTJs are fabricated by different process conditions, and the spin-wave resonance is measured with homodyne-detected ferromagnetic resonance. We focus on the distance between resonance frequencies of the uniform and spin-wave modes as a function of the free-layer size in order to examine the effect of the edge state of MTJs. A marked difference is observed between the two types of MTJs, and the result is consistently reproduced by a model assuming free- or fixed-edge boundary conditions with or without reduced magnetic properties near the pattern edge for each MTJ. The obtained results indicate that the edge state of nanoscale MTJs is crucially affected by the process condition, and spin-wave resonance can serve as a sensitive probe for the edge condition.

Origin of Perpendicular Magnetic Anisotropy in Yttrium Iron Garnet Thin Films Grown on Si (100)

We report the magnetic properties of yttrium iron garnet (YIG) thin films grown by pulsed laser deposition technique. The films were deposited on Si (100) substrates in the range of 15-50 nm thickness. Magnetic characterizations were investigated by ferromagnetic resonance spectra. Perpendicular magnetic easy axis was achieved up to 50 nm thickness. We observed that the perpendicular anisotropy values decreased by increasing the film thickness. The origin of the perpendicular magnetic anisotropy (PMA) was attributed to the texture and the lattice distortion in the YIG thin films. We anticipate that perpendicularly magnetized YIG thin films on Si substrates pave the way for a cheaper and compatible fabrication process.

Structural Analysis of CoFeB/MgO-Based Perpendicular MTJs With Junction Size of 20 nm by STEM Tomography

We have successfully observed the process-induced microstructure of the ultrafine CoFeB-MgO-based magnetic tunnel junctions with a perpendicular easy axis (p-MTJs) down to junction size of less than 20 nm using scanning transmission electron microscope (STEM) tomography combined with energy-dispersive X-ray spectroscopy (EDX). This made it possible to examine 3-dimensionally the precise structural information inside the entire MTJ. By this, the root cause of the change in the resistance of the MTJ was found to be the altered region consisting of Fe-dipping attached to the edge of the MgO and ultrathin TaO layer uniformly formed on the MTJ surface. This altered region is caused by etching during patterning of the MTJ, and the resistance value of the MTJ changes depending on the degree of adhesion of insulating TaO and Fe atoms at the edge of the MgO barrier layer. Furthermore, it was found that the unevenness of the MgO/CoFeB interface inside the MTJ was not a factor that caused the change in the thickness of the MgO barrier layer in the current process. Our results show that STEM tomography is an effective tool for failure analysis to clarify the relationship between the detailed structure of fine MTJs and magnetic properties. Unlike the conventional cross-sectional STEM observation, since the STEM tomographic observation includes the whole view of the MTJ, this enables higher resolution observation as the MTJ size decreases without overlooking the cause of failure inside the MTJ.

Influence of Hard Mask Materials on the Magnetic Properties of Perpendicular MTJs With Double CoFeB/MgO Interface

We investigated the influence of hard mask (HM) materials on tunnel magnetoresistance (TMR) properties and magnetic properties for the magnetic tunnel junctions with perpendicular easy axis (p-MTJs) with double CoFeB/MgO interface annealed at 400 °C for 0.5-10 h. After annealing at 400 °C for 5 h, the TMR ratio of the MTJ with TiN HM was significantly decreased, whereas the TMR ratio of the MTJ with Ta-HM was maintained at a value of more than 120%. The analysis results by secondary-ion mass spectrometry (SIMS) and energy-dispersive X-ray spectroscopy (EDX) revealed that Ru in Ru-cap for the MTJ with TiN-HM diffused into the free layer through the MgO-cap layer, resulting in the degradation of the magnetic and TMR properties of the MTJ with TiN-HM. In contrast, Ru did not diffuse into the free layer for the MTJ with Ta-HM, which results from suppression of Ru diffusion into the free layer due to the formation of Ta-Ru alloy. Furthermore, in both the MTJs with TiN-HM and Ta-HM annealed at 400 °C for 10 h, EDX analysis revealed Pt diffusion into the CoFeB reference layer, which could degrade the magnetic properties of the reference layer, resulting in the degradation of TMR properties. To obtain a double CoFeB/MgO interface p-MTJ with higher thermal tolerance, it is necessary to design an HM material that suppresses the diffusion of the cap material into the free layer in addition to suppressing the Pt diffusion of the reference layer.

Exotic Transverse-Vortex Magnetic Configurations in CoNi Nanowires.

The magnetic configurations of cylindrical Co-rich CoNi nanowires have been quantitatively analyzed at the nanoscale by electron holography and correlated to local structural and chemical properties. The nanowires display grains of both face-centered cubic (fcc) and hexagonal close packed (hcp) crystal structures, with grain boundaries parallel to the nanowire axis direction. Electron holography evidences the existence of a complex exotic magnetic configuration characterized by two distinctly different types of magnetic configurations within a single nanowire: an array of periodical vortices separating small transverse domains in hcp-rich regions with perpendicular easy axis orientation and a mostly axial configuration parallel to the nanowire axis in regions with fcc grains. These vastly different domains are found to be caused by local variations in the chemical composition modifying the crystalline orientation and/or structure, which give rise to change in magnetic anisotropies. Micromagnetic simulations, including the structural properties that have been experimentally determined, allow for a deeper understanding of the complex magnetic states observed by electron holography.

Influence of antiphase and ferroelastic domain boundaries on ferromagnetic domain wall width in multiferroic Ni-Mn-Ga compound

This paper reports on the experimental measurements of magnetic domain wall (DW) widths by Lorentz transmission electron microscopy in Ni-Mn-Ga ferromagnetic shape memory alloys. The wall widths were measured and compared in both austenitic and martensitic states, at positions pinned on antiphase boundaries (APB) or coinciding with the twin boundaries (TB) together with walls at APB-free and TB-free positions. The thickest DW widths of about 28 nm were measured in APB-free positions in austenite. DW in APB-free and TB-free positions in martensite had widths of about 10 nm for in-plane and 13 nm for perpendicular easy axis orientation. The narrowest DWs with widths about 7–8 nm were measured for walls pinned on APBs in austenite or for walls coinciding with the TBs in martensite. The measurements are broadly in agreement with theoretical wall width predictions in thin foil.