Perpendicular Magnetocrystalline Anisotropy Research Articles

Magnetocrystalline Anisotropy of d0-Magnetic Material NaN(001) Thin Films: A Density Functional Study.

A bulk d0 NaN of rocksalt or zinc-blende structure was predicted to be a ferromagnetic half metal and furthermore the half-metallicity would be retained in thin films. Such half metallicity of d0 ferromagnetic NaN is attractive for possible application in a spintronics device, such as a spin transfer torque magnetic random access memory. In this study, we carried out first-principles calculations on magnetocrystalline anisotropy rocksalt structured NaN thin films with different thicknesses, using Vienna Ab-initio Simulation Package code. It was found that the NaN(001) thin films have perpendicular magnetization with quite low magnetocrystalline anisotropy energies of order of 10 µeV, but capping of a 5d-transition metal Ta monolayer over the NaN(001) thin films enhances the perpendicular magnetocrystalline anisotropy energies significantly, more than 10 times. Furthermore, the 1 (Ta)/NaN(001) systems retain their half-metallicity except the NaN layer just below Ta.

Thickness effect on magnetocrystalline anisotropy of Co/Pd(111) films: A density functional study

In this study, we carried out first-principles calculations on magnetocrystalline anisotropy (MCA) of Co/Pd thin films by adopting two different systems of (i) n-Co/3-Pd and (ii) n-Pd/3-Co. In one system, we vary the thickness of Co layer, fixing the thickness of the Pd layer to 3-monolayers, and in the other system vice versa. MCA is mainly governed by the surface and interface Co atoms, while contributions from other Co atoms are smaller. MCA energy (EMCA) of the Co/Pd thin film shows oscillatory behavior with the thickness of the Co layer, but is insensitive to the thickness of the Pd layer. In particular, the n-Co/3-Pd films of n = 2, 4, and 6 exhibit strong perpendicular MCA of about 1 meV. Our results suggest that controlling the thickness of the Co layer in Co/Pd (111) is crucial in achieving strong perpendicular MCA.

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.

Electrical detection of millimeter-waves by magnetic tunnel junctions using perpendicular magnetized L10-FePd free layer.

Spin dynamics excited by spin-polarized current in magnetic tunnel junctions (MTJs) is potentially useful in nanoscale electrical oscillation sources and detection devices. A spin oscillator/detector should work at a high frequency, such as that of a millimeter-wave, where the quality of a semiconductor device is restricted by carrier mobility, the CR time constant, and so on. Developers of spin systems for practical use need to find out how to excite spin dynamics (i) in the millimeter-wave region, (ii) with low power consumption (ex: no external magnetic field, low damping material), and (iii) for broad frequency modulation. Here L10-ordered FePd alloy with perpendicular magnetocrystalline anisotropy (PMA) and a low damping constant, 0.007, was used for the free layer in the MTJs, and a homodyne-detected ferromagnetic resonance (FMR) signal was obtained at around 30 GHz together with the possibility of one-octave frequency modulation. The FMR signal in out-of-plane magnetized L10-ordered FePd free layer could be excited without an external magnetic field by injecting in-plane spin polarized alternating current. This study shows the potential utility of L10-ordered alloy materials such as FePt, CoPt, MnAl, and MnGa in a variety of millimeter-wave spin devices.

Magnetism and magnetocrystalline anisotropy of 3dtransition metal monolayers on Pt(001): a density-functional study.

We systematically investigate the effects of having Pt as a substrate and/or capping layer on the magnetism and magnetocrystalline anisotropy (MCA) of 3d transition metal (TMs; Cr, Mn, Fe, and Co) monolayers (MLs) by using a first-principles calculationl method. We found that Fe and Co MLs are ferromagnetic (FM) on a Pt(001) surface, but Mn and Cr MLs are antiferromagnetic (AFM). The magnetic moments are quite robust with additional Pt-capping. Furthermore, Pt-capping enhances the small perpendicular MCA (meV) of Fe/Pt(001) significantly to 4.44 meV. Our electronic structure analyses indicate that strong hybridization between Pt-5d and TM-3d orbitals plays a crucial role in determining magnetic ordering and MCA. For comparison we also calculated magnetism and MCA of 3d TM MLs on Ta(001) with and without Ta-capping.

Magnetocrystalline anisotropy and its electric-field-assisted switching of Heusler-compound-based perpendicular magnetic tunnel junctions

Employing density functional theory combined with the non-equilibrium Greenʼs function formalism, we systematically investigate the structural, magnetic and magnetoelectric properties of the Co2FeAl(CFA)/MgO interface, as well as the spin-dependent transport characteristics of the CFA/MgO/CFA perpendicular magnetic tunnel junctions (p-MTJs). We find that the structure of the CFA/MgO interface with the oxygen-top FeAl termination has high thermal stability, which is protected by the thermodynamic equilibrium limit. Furthermore, this structure is found to have perpendicular magnetocrystalline anisotropy (MCA). Giant electric-field-assisted modifications of this interfacial MCA through magnetoelectric coupling are demonstrated with an MCA coefficient of up to 10−7 erg V−1 cm. In addition, our non-collinear spin transport calculations of the CFA/MgO/CFA p-MTJ predict a good magnetoresistance performance of the device.

Current-driven dynamics of Dzyaloshinskii domain walls in the presence of in-plane fields: Full micromagnetic and one-dimensional analysis

Current-induced domain wall motion along high perpendicular magnetocrystalline anisotropy multilayers is studied by means of full micromagnetic simulations and a one-dimensional model in the presence of in-plane fields. We consider domain wall motion driven by the spin Hall effect in the presence of the Dzyaloshinskii-Moriya interaction (DMI). In the case of relatively weak DMI, the wall propagates without significant tilting of the wall plane, and the full micromagnetic results are quantitatively reproduced by a simple rigid one-dimensional model. By contrast, significant wall-plane tilting is observed in the case of strong DMI, and a one-dimensional description including the wall tilting is required to qualitatively describe the micromagnetic results. However, in this strong-DMI case, the one-dimensional model exhibits significant quantitative discrepancies from the full micromagnetic results, in particular, when high longitudinal fields are applied in the direction of the internal domain wall magnetization. It is also shown that, even under thermal fluctuations and edge roughness, the domain wall develops a net tilting angle during its current-induced motion along samples with strong DMI.

CoFe 박막의 자성과 자기결정이방성에 대한 제일원리계산

We investigate magnetism and magnetocrystalline anisotropy of CoFe thin films, using VASP code in GGA. In this study Coterminated and Fe-terminated 5-layer CoFe thin films are employed. The Co-terminated CoFe thin film shows two total energy minima at 2-dimensional lattice constants of 2.45 A and 2.76 A. The film of 2.45 A has fcc-like structure and the film of 2.76 A has bcc-like structure similarly to a bulk CoFe alloy. And the fcc-like film is more stable by the energy difference of about 160 meV compared to the bcc-like film. The Fe-terminated CoFe film shows very complicated behaviour of total energy which is suspected to be closely related to its complex magnetic structure. The Co-terminated CoFe film of 2.76 A shows perpendicular magnetocrystalline anisotropy (MCA), while the film of 2.45 does parallel MCA. The Fe-terminated CoFe film also exhibits similar MCA behaviour.

Electric field control and effect of Pd capping on magnetocrystalline anisotropy in FePd thin films: A first-principles study

Using ab initio electronic structure calculations, we have investigated the effect of an electric field and of a heavy-metal cap of Pd on the magnetocrystalline anisotropy (MCA) of FePd ultrathin film. Analysis of the energy- and $k$-resolved distribution of the orbital character of the minority-spin band reveals that the perpendicular MCA of the uncapped film mainly arises from the spin-orbit coupling (SOC) between unoccupied Fe ${d}_{xy}$ and occupied Fe ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ states. On the other hand, the SOC between the Pd- and Fe-derived $d$ states yields negative contributions to the MCA. We find that the sensitivity of the surface anisotropy energy to the applied electric field is 18 fJ/(Vm) and is due to changes in the occupation of the surface Fe atoms ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ and (${d}_{xz},{d}_{yz}$) orbitals. We demonstrate that the thickness of the Pd cap has a dramatic effect on the MCA and can even switch the magnetization from out-of- to in-plane orientation. The underlying origin is the change of the position and orbital character of the spin-polarized quantum well states induced in the Pd cap by varying its thickness. These results have important implications for exploiting heavy metals with large spin-orbit coupling (Ru, Pd, Ta, Pt, or Au) as contacts with ferromagnetic thin films to tailor the magnetic switching of spintronic devices by tuning the cap thickness.

Magnetocrystalline anisotropy of the Fe-sublattice in Y2Fe14B systems

To reveal the role of Fe sublattice in the coercivity of R2Fe14B, we investigated the origin of perpendicular magnetocrystalline anisotropy (MCA) of Y2Fe14B using first-principles density-functional calculations. We found that the perpendicular MCA of Y2Fe14B arises predominantly from Fe sites with higher symmetry (16k1,2 and 8j1,2). On the other hand, the Fe(4c) sites show a significant contribution to the in-plane MCA. This can be attributed to the localized character of Fe(4c)-d orbitals in R2Fe14B. Furthermore, the MCA energy of Y2Fe14B increases as the number of valence electrons increases within the rigid band model, indicating that the partial substitution of Fe by Co can enhance the MCA energy of Y2Fe14B.

Magnetic Properties of Fe/WRe (001) Multilayers

Following recent theoretical predictions on perpendicular uniaxial magnetocrystalline anisotropy in multilayers of 5 monolayers of Fe and 2 monolayers of W x Re100–x with tetragonally strained components, we have measured the magnetization and detailed atomic structure in a series of such samples with x = 20–80 at%. Although the achieved strain in the WRe layers is smaller than in the modelled case, we do observe a positive uniaxial magnetocrystalline anisotropy of around 0.6 MJ/m3 at 40 K, after accounting for the in-plane shape anisotropy. This causes the out-of-plane saturation field to be reduced considerably compared to a pure Fe thin film, while the saturation magnetization is maintained at a value which is enhanced by at least 30% from what would be expected from the multilayer composition alone. The coercivity is low, on the order of 6 kA/m in all samples.

Extremely large perpendicular magnetic anisotropy of an Fe(001) surface capped by5dtransition metal monolayers: A density functional study

Significant enhancement of the magnetocrystalline anisotropy (MCA) of an Fe(001) surface capped by 4$d$ and 5$d$ transition metal monolayers is presented in this study using first principles density functional calculations. In particular, an extremely large perpendicular MCA of $+$10 meV/Ir was found in Ir-capped Fe(001), which originates not from the Fe but from the large spin-orbit coupling of the Ir atoms. From the spin-channel decomposition of the MCA matrix and electronic structure analyses, we find that strong 3$d$--5$d$ band hybridization in the minority spin state is responsible for the sign changes of the MCA from parallel to perpendicular.

Role of interfacial B impurity in the magnetocrystalline anisotropy at MgO/Fe interface

The structural stability and the magnetocrystalline anisotropy (MCA) at MgO/Fe(001) interfaces with the incorporation of interfacial B atoms were investigated by using first-principles calculations. Total energy calculations predicted that the B atoms favored segregating to the interface. The calculated MCA energy for an ideal abrupt MgO/Fe interface (with no B atom) indicated that the magnetization strongly favored pointing along the perpendicular direction. However, when B atoms were incorporated at the MgO/Fe interfaces, the perpendicular MCA almost disappeared.

Domain Wall Dynamics in Asymmetric Stacks: The Roles of Rashba Field and the Spin Hall Effect

The influence of spin-orbit torques, either Rashba and/or spin Hall effects, on the current-driven domain wall motion along high perpendicular magnetocrystalline anisotropy stacks, is analyzed by means of micromagnetic simulations and the one-dimensional model. The study indicates that the domain wall motion can be either in the direction of the current or opposing to it depending on the strength of the spin Hall effect in the presence of the Rashba spin-orbit torque, and therefore, it points out that both spin-orbit torques could be essential ingredients to describe recent experiments.

A first-principles study on magnetocrystalline anisotropy at interfaces of Fe with non-magnetic metals

Magnetocrystalline anisotropy (MCA) of Fe(001) interfaces with various non-magnetic metals (Hf, Zr, Ti, Ta, Nb, V, Ir, Rh, Pt, Pd, Au, Ag, Cu, and Zn) was investigated by first-principles calculations. We found that Fe interfaces with non-magnetic metals with fully occupied d states tend to show perpendicular MCA. The spin-orbit coupling in interfacial Fe atoms plays an important role in perpendicular MCA. Conversely, Fe interfaces with non-magnetic metals with partially occupied d states exhibit in-plane MCA. The Hf/Fe(001) interface shows an exceptionally large perpendicular MCA energy of 1.5 mJ/m2, which corresponds to that of the MgO/Fe(001) interface. In these cases, contributions from interfacial Fe atoms to MCA are relatively small, and the large spin-orbit coupling of non-magnetic atoms is the primary contribution to MCA. We conclude that formation of Hf/Fe(001) interfaces will enhance the perpendicular magnetization of MgO/CoFeB-based magnetic tunnel junctions.

First-principles study of the structural stability of cubic, tetragonal and hexagonal phases in Mn3Z (Z=Ga, Sn and Ge) Heusler compounds

We investigate the structural stability and magnetic properties of the cubic, tetragonal and hexagonal phases of Mn3Z (Z=Ga, Sn and Ge) Heusler compounds using first-principles density-functional theory. We propose that the cubic phase plays an important role as an intermediate state in the phase transition from the hexagonal to the tetragonal phases. Consequently, Mn3Ga and Mn3Ge behave differently from Mn3Sn, because the relative energies of the cubic and hexagonal phases are different. This result agrees with experimental observations for these three compounds. The weak ferromagnetism of the hexagonal phase and the perpendicular magnetocrystalline anisotropy of the tetragonal phase obtained in our calculations are also consistent with experiment.

Structural and magnetic properties of epitaxial CrO2 thin films grown on TiO2 (001) substrates

The structural and magnetic properties of epitaxial CrO2 thin films grown on (001)-oriented TiO2 substrates by atmospheric pressure chemical vapor deposition are investigated. Due to the competition between demagnetization and a relatively weak perpendicular magnetocrystalline anisotropy, the deposited CrO2 (001) films exhibit magnetic properties that are significantly different from CrO2 (100) and CrO2 (110) films grown on TiO2 substrates. Based on the thickness dependence of M-H curves, a surface anisotropy is confirmed to exist, likely originating from strain in the film. The out-of-plane hysteresis curves can be well described by a distribution of effective anisotropy that may be due to a varying local demagnetizing field and a distribution of strain across the film. For the in-plane magnetization, the hysteresis curves are consistent with stripe or vortex domain structures of an almost closed flux configuration at remanence.

Energy product and coercivity of a rare-earth-free multilayer FeCo/FePt exchange spring magnet

Using the full potential linearized augmented plane wave (FLAPW) method, we have explored the energy product and the coercivity field of rare-earth-free FeCo/FePt(001) multilayered exchange spring magnet systems. We have considered 5 and 7 monolayers (ML) of a FePt hard layer and 3, 5, 7, and 9 ML of a FeCo soft layer. The FeCo soft layers are found to show close to half metallic features, while the FePt hard layers manifest conventional metallic behavior. A giant perpendicular magnetocrystalline anisotropy energy (EMCA) is observed. For instance, an EMCA of 27.24 meV/cell is found in FeCo(9 ML)/FePt(7 ML) multilayer structure. The energy product almost linearly increases with increasing FeCo thickness, while the coercivity filed shows the opposite behavior. Interestingly, we have obtained that the multilayer structures display very large energy product and coercivity field. For example, FeCo(9 ML)/FePt(5 ML) multilayer has an energy product of about 82 MGOe and a coercivity field of about 130 KOe. Moreover, we find that the multilayer system may show enhanced coercivity field compared with that found in FeCo/FePt bilayer film structures, while the energy product is comparable to that observed in bilayer films. Therefore, our results may imply that the FeCo/FePt multilayer can be employed as a potential rare- earth-free permanent magnet material.

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.

The origin of perpendicular magneto-crystalline anisotropy in L10–FeNi under tetragonal distortion

We investigated the origin of perpendicular magneto-crystalline anisotropy (MCA) in L10-ordered FeNi alloy using first-principles density-functional calculations. We found that the perpendicular MCA of L10-FeNi arises predominantly from the constituent Fe atoms, which is consistent with recent measurements of the anisotropy of the Fe orbital magnetic moment of L10-FeNi by means of x-ray magnetic circular dichroism. Analysis of the second-order perturbation of the spin–orbit interaction indicates that spin-flip excitations between the occupied majority-spin and unoccupied minority-spin bands make a considerable contribution to the perpendicular MCA, as does the spin-conservation term for the minority-spin bands. Furthermore, the MCA energy increases as the in-plane lattice parameter decreases (increasing the axial ratio c/a). The increase in the MCA energy can be attributed to further enhancement of the spin-flip term due to modulation of the Fe d(xy) and d(x2 − y2) orbital components around the Fermi level under compressive in-plane distortion.