Large Perpendicular Magnetocrystalline Anisotropy Research Articles

Intrinsic magnetism of an individual rare-earth atom on transition metal dichalcogenide semiconductors

Two-dimensional (2D) structures that exhibit intriguing magnetic phenomena such as perpendicular magnetocrystalline anisotropy (PMA) have become a focus of spintronic research due to their potentials in maximizing the information storage density. Herein we perform density-functional theory plus U (DFT+U) calculations to investigate the binding affinity and intrinsic magnetic properties of an individual rare-earth (RE) Sm atom on WSe2 monolayer. Our calculations show that Sm adatom energetically prefers to adsorb at the W-top site in WSe2 rather than the Se-top and hollow sites. We predict extremely large PMA values of ∼7–33 meV per Sm at the most stable W-top site, depending on U parameter in DFT+U calculations, while it is negligibly small for the Se-top and hollow sites. The underlying mechanism for large PMA is elucidated in terms of the strong spin–orbit coupled Sm 4f – W 5d orbital states and large 4f orbital magnetic moment in the high-spin crystal field. These results provide a viable route to achieving an atomic scale f-electron PMA in 2D structures, opening interesting prospects in two-dimensional semiconducting spintronics.

Tailorable magnetocrystalline anisotropy at the MnPt3(001) surface

We report results of the density functional theory and density functional perturbation theory calculations on thermodynamic stability, magnetic structure, and magnetocrystalline anisotropy (MCA) of a ${\mathrm{MnPt}}_{3}(001)$ thin film. It is predicted that the magnetic ground state of the Pt-terminated ${\mathrm{MnPt}}_{3}$(001) thin films is the ferromagnetic as found in its bulk structure, while the MnPt termination favors the A-type antiferromagnetic order at the surface. Even more drastic effect of the surface termination on MCA is revealed; in contrast to an in-plane magnetization preferred for the MnPt termination, the Pt-terminated ${\mathrm{MnPt}}_{3}$(001) thin films exhibit an extremely large perpendicular MCA (PMCA) up to an order of 10 erg/${\mathrm{cm}}^{2}$ regardless of its film thickness. Moreover, from detailed single-particle energy spectra analyses, this magnetization reversal is mainly determined by an interplay between two in-plane orbital states, ${d}_{\text{xy}}$ and ${d}_{{\text{x}}^{2}\ensuremath{-}{\text{y}}^{2}}$ states, of the Pt atom with induced magnetism at the surface, as a result of their energy level changes associated with the Mn $3d\ensuremath{-}\mathrm{Pt}\phantom{\rule{4pt}{0ex}}5d$ hybridization. This reported thin-film system can act as a prototype for the in-depth study of the importance of the surface termination with large perpendicular magnetocrystalline anisotropy in spintronic applications.

An ab initio study of the magnetic properties of strontium hexaferrite

The magnetic properties of {text{SrFe}}_{12}{text{O}}_{19}, a paradigmatic hexaferrite for permanent magnet applications, have been addressed in detail combining density functional theory including spin–orbit coupling and a Hubbard U term with Monte Carlo simulations. This multiscale approach allows to estimate the Néel temperature of the material from ab initio exchange constants, and to determine the influence of different computational conditions on the magnetic properties by direct comparison versus available experimental data. It is found that the dominant influence arises from the choice of the Hubbard U term, with a value in the 2–3 eV range as the most adequate to quantitatively reproduce the two most relevant magnetic properties of this material, namely: its large perpendicular magnetocrystalline anisotropy and its elevated Néel temperature.

Large Perpendicular Magnetocrystalline Anisotropy at the Fe/Pb(001) Interface.

The search for ultrathin magnetic films with large perpendicular magnetocrystalline anisotropy (PMA) has been inspired for years by the continuous miniaturization of magnetic units in spintronics devices. The common magnetic materials used in research and applications are based on Fe because the pure Fe metal is the best yet simple magnetic material from nature. Through systematic first-principles calculations, we explored the possibility to produce large PMA with ultrathin Fe on non-noble and non-magnetic Pb(001) substrate. Interestingly, huge magnetocrystalline anisotropy energy (MAE) of 7.6 meV was found in the Pb/Fe/Pb(001) sandwich structure with only half monolayer Fe. The analysis of electronic structures reveals that the magnetic proximity effect at the interface is responsible for this significant enhancement of MAE. The MAE further increases to 13.6 meV with triply repeated capping Pb and intermediate Fe layers. Furthermore, the MAE can be tuned conveniently by charge injection.

Effect of capping layer on formation and magnetic properties of MnBi thin films

We report on the effect of varied capping layers on the formation of thin film MnBi, and the associated magnetic and crystalline properties for use in magnetic memory. MnBi thin films with a capping layer of either Ta, SiO2, Cr, or Au were grown, and it was observed that the magnetic properties vary significantly depending on the capping layer. Continuous 20 nm MnBi thin films capped with Ta and SiO2 show ferromagnetism with large perpendicular magnetocrystalline anisotropy, however, films capped with Cr and Au show no ferromagnetic behavior. In this work, MnBi thin films have been characterized utilizing magnetization vs. field, x-ray diffraction, cross-section transmission electron microscopy, and optical microscopy. We show that the capping layer plays a significant role in the formation of the low temperature phase MnBi structure and propose that the underlying cause is due to a surface energy difference for the MnBi//Au and MnBi//Cr interface, which allows for Mn oxidation, and prevents the formation of the low temperature phase. This work demonstrates that continuous ultra-thin film MnBi can achieve large magnetocrystalline anisotropy and theoretical magnetization. We also show that film delamination causes a significant variation in the magnetic performance, and leads to a large surface roughness.

Magnetically Ordered Transition-Metal-Intercalated WSe2.

Introducing magnetic behavior in nonmagnetic transition metal dichalcogenides is essential to broaden their applications in spintronic and nanomagnetic devices. In this article, we investigate the electronic and magnetic properties of transition-metal-intercalated tungsten diselenide (WSe2) using density functional theory. We find that intercalation compounds with composition of T1/4WSe2 (T is an iron-series transition-metal atom) exhibit substantial magnetic moments and pronounced ferromagnetic order for late transition metals. The densities of states of the T atoms and the magnetic moments on the W sites indicate that the moments of the intercalated atoms become more localized with increasing atomic number. A large perpendicular magnetocrystalline anisotropy of about 9 meV per supercell has been found for Fe1/4WSe2. Furthermore, using mean field theory, we estimated high Curie temperatures of 660, 475, and 379 K for Cr, Mn, and Fe, respectively. The predicted magnetic properties suggest that WSe2 may have applications in spin electronics and nanomagnetic devices.

Theory of perpendicular magnetocrystalline anisotropy in Fe/MgO (001)

The origin of large perpendicular magnetocrystalline anisotropy (PMCA) in Fe/MgO (001) is revealed by comparing Fe layers with and without the MgO. Although Fe-O p-d hybridization is weakly present, it cannot be the main origin of the large PMCA as claimed in previous study. Instead, perfect epitaxy of Fe on the MgO is more important to achieve such large PMCA. As an evidence, we show that the surface layer in a clean free-standing Fe (001) dominantly contributes to EMCA, while in the Fe/MgO, those by the surface and the interface Fe layers contribute almost equally. The presence of MgO does not change positive contribution from 〈xz|ℓZ|yz〉, wherease it reduces negative contribution from 〈z2|ℓX|yz〉 and 〈xy|ℓX|xz,yz〉.

Jahn-Teller driven perpendicular magnetocrystalline anisotropy in metastable ruthenium

A new metastable phase of the body-centered-tetragonal ruthenium ({\em bct}--Ru) is identified to exhibit a large perpendicular magnetocrystalline anisotropy (PMCA), whose energy, $E_{MCA}$, is as large as 150 $\mu$eV/atom, two orders of magnitude greater than those of 3$d$ magnetic metals. Further investigation over the range of tetragonal distortion suggests that the appearance of the magnetism in the {\em bct}--Ru is governed by the Jahn-Teller spit $e_g$ orbitals. Moreover, from band analysis, MCA is mainly determined by an interplay between two $e_g$ states, $d_{x^2-y^2}$ and $d_{z^2}$ states, as a result of level reversal associated with tetragonal distortion.

Structure of epitaxial L1-FePt/MgO perpendicular magnetic tunnel junctions

Perpendicular magnetic tunnel junctions (p-MTJs) with MgO barriers are interesting for high-density information-storage devices. Chemically ordered L10-FePt is a potential electrode due to its large perpendicular magnetocrystalline anisotropy. To-date, a single theoretical study on L10-FePt/MgO p-MTJ based on an idealized structure reported significant dependence of spin-dependent tunneling on interface structure. [Y. Taniguchi et al., IEEE Trans. Magn. 44, 2585 (2008).] We report a structural study of epitaxial L10-FePt(001)[110]//MgO(001)[110]//L10-FePt(001)[110] p-MTJs, focusing on the interfaces using aberration-corrected scanning transmission electron microscopy. Interfaces are semi-coherent, with oxygen atomic-columns of MgO located opposite to iron atomic-columns in L10-FePt. Up to three lattice planes show atomic-column steps, the origin of which is attributed to antiphase boundaries in L10-FePt.

AFM, XRD and HRTEM Studies οf Annealed FePd Thin Films

Ferromagnetic FePd L10 ordered alloys are highly expected as forthcoming high-density recording materials, because they reveal a large perpendicular magnetocrystalline anisotropy [1]. The value of the magnetic anisotropy of FePd alloy strongly depends on the alloy composition, degree of alloy order as well as on the crystallographic grain orientation. In particular, to obtain the perpendicular anisotropy, it is necessary to get the films with (001) texture. One of the successful methods, which allows one to obtain highly ordered alloy, is a subsequent deposition of Fe and Pd layers, followed by an annealing at high temperature. This paper presents the study of the FePd thin alloy film structure changing in the result of high temperature annealing. During the annealing in high vacuum, the measurements of electrical resistance were performed, indicating the regions of different structure evolution. Changes in the crystal structure and surface morphology induced by thermal treatment were investigated by X-ray diffraction, atomic force microscopy, as well as high resolution transmission electron microscopy and then compared with electrical resistivity measurement. The slow thermal annealing of the deposited layers leads to the formation of L10 ordered FePd alloy with preferred (111) grain orientation. After the annealing at the highest used temperature, the dewetting process was observed, resulting in a creation of well oriented, regular nanoparticles.

Micromagnetic simulations of magnetisation in circular cobalt dots

Using tri-dimensional micromagnetic simulations, the phase diagram including different magnetic ground states is computed for circular Co dots as a function of the dot dimensions. The presence of a large perpendicular magnetocrystalline anisotropy induces a shift of the boundary between the ground states and increases the number of possible stable states.

Theoretical predictions of magnetic interface anisotropy in Co/Pt superlattices (abstract)

Superlattices with both large perpendicular magnetocrystalline anisotropy and Kerr rotations in the range of visible light are potential materials for high-density magneto-optic recording media. Transition metal superlattices formed from Co and Pt have been the focus of recent interest since they have both of these desirable properties. In this study, the intrinsic magnetocrystalline anisotropy of several Co/Pt superlattices is computed from ab initio local spin-density electronic structure calculations. In the case of nCo/5Pt [111], extrapolating the anisotropy to zero Co thickness yields an interface anisotropy of 0.90 ergs/cm2 in good agreement with previous estimates of 0.85 ergs/cm2 based upon the Neèl model.1 Results for the anisotropy versus Co thickness will be presented for several different growth directions and the results compared to experiment and analyzed in terms of the Neèl model.