Origin Of Perpendicular Magnetic Anisotropy Research Articles

Perpendicular Magnetic Anisotropy of an Ultrathin Fe Layer Grown on NiO(001)

The magnetic anisotropy and magnetic interactions at the interface between $\mathrm{Fe}$ and $\mathrm{Ni}\mathrm{O}$(001) were investigated. Depending on the growth conditions of the $\mathrm{Ni}\mathrm{O}$(001) layers and the postannealing temperature, the preferential magnetization direction of the ultrathin $\mathrm{Fe}$ layer grown on a $\mathrm{Ni}\mathrm{O}$(001) layer changed from the in-plane direction to a direction perpendicular to the film plane. The lattice constant of the $\mathrm{Ni}\mathrm{O}$(001) layers parallel to the growth direction increased with ${\mathrm{O}}_{2}$ flow rate, while that parallel to the in-plane direction were locked onto the MgO(001) substrate regardless of the growth conditions of the NiO layers. Moreover, perpendicular magnetization was observed only when the NiO layer was grown with ${\mathrm{O}}_{2}$ flow rates higher than 2.0 sccm corresponding to oxygen-rich NiO. X-ray magnetic circular dichroism measurements revealed an enhancement in anisotropic orbital magnetic moments similar to the origin of perpendicular magnetic anisotropy at the $\mathrm{Fe}$/MgO(001) interface. The interfacial magnetic anisotropy energies were 0.93 and $1.02\phantom{\rule{0.2em}{0ex}}{\mathrm{mJ}/\mathrm{m}}^{2}$ at room temperature and at 100 K, respectively, indicating less temperature dependence. In contrast, the coercivity ${H}_{c}$ exhibited a significant temperature dependence. Although no signature of exchange bias or unidirectional loop shift was observed, ${H}_{c}$ was strongly dependent on the NiO layer thickness, indicating that the exchange interaction at the interface between the ferromagnetic and antiferromagnetic layers was not negligible, despite the NiO(001) being a spin-compensated surface.

First-principles study of origin of perpendicular magnetic anisotropy in MgO|CoFeB|Ta structures

Using fist-principles calculations, we study the B atom of diffusion process and the origin of the perpendicular magnetic anisotropy in MgO|CoFe|Ta structures with boron. It is found that B atoms are more likely to enter the Ta layer than FeCo layer and MgO layer. This result is in good agreement with that of the experimental and theoretical results. Furthermore, we elucidate the physical origin of the interfacial PMA Ta|CoFe|MgO structure with B element. We find that interfacial PAM come from both the MgO|CoFe and CoFe|Ta layer interfaces. These findings provide a comprehensive understanding of the PMA and point towards the possibility to achieve the advanced-node STT-MRAM with high thermal stability.

Perpendicular magnetic anisotropy in a sputter deposited nanocrystalline high entropy alloy thin film

High entropy alloys (HEAs) have received great research attention in recent times because of their novel microstructures and properties. Among HEAs, eutectic HEA (EHEA) AlCoCrFeNi2.1 is considered particularly attractive because of its unprecedented mechanical properties. However, the functional properties of EHEA have been hardly explored. In the present work, we have investigated the magnetic properties of nanocrystalline EHEA films grown by RF magnetron sputtering in the as-deposited and after annealing at 200 °C, 400 °C, and 600 °C in vacuum. Further, the magnetic behavior was correlated with structural and chemical characteristics. Grazing incidence X-ray diffraction (GIXRD) of the films shows the presence of FCC and BCC phases similar to the target (bulk) material. Static and dynamic magnetic properties of the HEA films are investigated by using vibrating sample magnetometer (VSM) and ferromagnetic resonance (FMR) techniques, respectively. VSM results reveal the presence of the perpendicular magnetic anisotropy (PMA) in our films annealed at 600 °C, whereas the as-deposited and samples annealed at 200 °C and 400 °C exhibit only in-plane anisotropy behavior. The presence of PMA is further supported by the FMR dynamics results. The origin of PMA in sample annealed at 600 °C is attributed to the formation of Al2O3, which has been confirmed through GIXRD and amply corroborated by X-ray photoelectron spectroscopy (XPS). The interfacial hybridization effect between Co/Fe-3d and the adjacent O-2p orbitals of Al2O3 possibly resulted in PMA in HEA films.

Investigation of perpendicular magnetic anisotropy in Pd/Co2FeSi/MgO structures with evaluations of magnetic property, crystal structure, and elemental mapping

The authors investigated origins of perpendicular magnetic anisotropy (PMA) in a multilayer of full-Heusler Co2FeSi (CFS) alloys, MgO, and Pd. The CFS thickness dependence of the magnetic properties indicated that the bulk magnetic anisotropy of the CFS layers also contributed to the PMA even in the samples where the CFS thickness tCFS was 10 nm. The anisotropy constant was drastically increased at tCFS = 1.8 nm, and 0.7 nm. The crystal structure analysis indicated that CFS layers had the tensile strain for out-of-plane when the thickness was thicker than 1.8 nm, which can induce the bulk magnetic anisotropy via the magnetostrictive effect. The elemental mapping by a scanning transmission electron microscope revealed that approximately 2 nm thick Pd existed at the CFS/MgO interface. The authors considered the whole CFS layers could contain Pd when the thickness of CFS was less than 2 nm. This Pd might cause the bulk anisotropy of the samples with the CFS thickness below 2 nm since some Pd alloys and Pd/Co and Pd/Fe multilayers have large bulk magnetic anisotropy.

Preferential alignment of Co moments at oxide/Co interface in Si/Co/Co-oxide/Cowedge/Pt structure

Magnetization reversal process in Si/Co/Co-oxide/CoWedge/Pt structure with increasing thickness of CoWedge layer has been studied with an aim to explore the effect of interface oxidation on the preferential alignment of Co spins at Co-oxide/CoWedge interface. Oxide layer is prepared at the surface of Co layer by heating it in the presence of oxygen partial pressure. Top Co layer (CoWedge) was deposited in the form of wedge with thickness gradient ~0.11 nm/mm across the full length (55 mm) of the Si substrate. Azimuthal angle dependent magneto-optic Kerr effect and magnetoresistance measurements in both transverse and longitudinal geometries revealed that despite of degraded growth, oxidized interface is responsible for the preferential alignment of Co spins at the interface. During the initial stages of Co deposition on Co-Oxide layer, the possible Co–O hybridization at the interface leads to the Co–O bonds alignments perpendicular to the interface which helped Co layer to grow in hcp phase with c-axis aligned preferentially perpendicular to the film surface. Combined effect of volume and interface magnetic anisotropy is attributed to overcome the shape anisotropy and responsible for the origin of perpendicular magnetic anisotropy (PMA) in the CoWedge layer. Variation in PMA with increasing thickness of CoWedge (0.6 nm to 5.6 nm) could be understood primarily in terms of film morphology, which changed significantly with film thickness.

Microstructural evolution of perpendicular magnetization films with an ultra-thin Co2FeAl/MgAl2O4(001) structure

We investigated the structure of 1-nm-thick Co2FeAl (CFA)/MgAl2O4 (MAO) epitaxial heterojunctions in order to understand the origin of perpendicular magnetic anisotropy (PMA). The CFA/MAO stacks were fabricated by plasma oxidation of Mg-Al grown on the CFA layer and subsequent post-annealing process. Before post-annealing (as-oxidized), the stack showed in-plane magnetization with poor crystallinity of the MAO layer. After post-annealing, the stack showed PMA accompanied by the improved crystallinity in the MAO layer, leading to the formation of a lattice-matched CFA/MAO(001) epitaxial interface. These behaviors indicate that the crystallization of the MAO layer is strongly promoted by atomic templating effect from a lattice-matched underlayer. In addition, we also found that this MAO crystallization effectively enhanced PMA at the interface. At the same time, significant Al atomic diffusion from CFA to MAO was confirmed. This Al diffusion was observed in the as-oxidized stack, and this was further promoted by the post-annealing. Therefore, the strong PMA at the CFA/MAO interfaces is attributed to (i) the reduced in-plane magnetocrystalline anisotropy due to the lattice-matching and (ii) the promoted hybridization between Fe and O orbitals due to the Al re-distribution near the interface. These results include important findings for constructing high performance perpendicularly-magnetized magnetic tunnel junctions for non-volatile magnetic memory applications.

Spatially Resolved Magnetic Anisotropy of Cobalt Nanostructures on the Au(111) Surface.

Understanding the origin of perpendicular magnetic anisotropy in surface-supported nanoclusters is crucial for fundamental research as well as data storage applications. Here, we investigate the perpendicular magnetic anisotropy energy (MAE) of bilayer cobalt islands on Au(111) substrate using spin-polarized scanning tunneling microscopy at 4.6 K and first-principles theoretical calculations. Au(111) substrate serves as an excellent model system to study the effect of nucleation site and stacking sequence on MAE. Our measurements reveal that the MAE of bilayer islands depends strongly on the crystallographic stacking of the two Co layers and nucleation of the third layer. Moreover, the MAE of Co atoms on Au(111) is enhanced by a factor of 1.75 as compared to that reported on Cu(111). Our first-principles calculations attribute this enhancement to the large spin-orbit coupling of the Au atoms. Our results highlight the strong impact of nanometer-scale structural changes in Co islands on MAE and emphasize the importance of spatially resolved measurements for the magnetic characterization of surface-supported nanostructures.

Perpendicular magnetic anisotropy in amorphous NdxCo1−x thin films studied by x-ray magnetic circular dichroism

The origin of perpendicular magnetic anisotropy (PMA) in amorphous ${\mathrm{Nd}}_{x}{\mathrm{Co}}_{1\ensuremath{-}x}$ thin films is investigated using x-ray magnetic circular dichroism (XMCD) spectroscopy at the Co ${L}_{2,3}$ and Nd ${M}_{4,5}$ edges. The magnetic orbital and spin moments of the $3d$ cobalt and $4f$ neodymium electrons were measured as a function of the magnetic field orientation, neodymium concentration, and temperature. In all the studied samples, the magnetic anisotropy of the neodymium subnetwork is always oriented perpendicular to the plane, whereas the anisotropy of the orbital moment of cobalt is in the basal plane. The ratio ${L}_{z}/{S}_{z}$ of the neodymium $4f$ orbitals changes with the sample orientation angle, being higher and closer to the atomic expected value at normal orientation and smaller at grazing angles. This result is well explained by assuming that the $4f$ orbital is distorted by the effect of an anisotropic crystal field when it is magnetized along its hard axis, clearly indicating that the $4f$ states are not rotationally invariant. The magnetic anisotropy energy associated to the neodymium subnetwork should be proportional to this distortion, which we demonstrate is accessible by applying the XMCD sum rules for the spin and intensity at the Nd ${M}_{4,5}$ edges. The analysis unveils a significant portion of neodymium atoms magnetically uncoupled to cobalt, i.e., paramagnetic, confirming the inhomogeneity of the films and the presence of a highly disordered neodymium rich phase already detected by extended x-ray-absorption fine structure (EXAFS) spectroscopy. The presence of these inhomogeneities is inherent to the evaporation preparation method when the chosen concentration in the alloy is far from its eutectic concentrations. An interesting consequence of the particular way in which cobalt and neodymium segregates in this system is the enhancement of the cobalt spin moment which reaches 1.95 ${\ensuremath{\mu}}_{B}$ in the sample with the largest segregation.

On the origin of perpendicular magnetic anisotropy in strained Fe–Co(–X) films

Very high magnetic anisotropies have been theoretically predicted for strained Fe–Co(–X) and indeed several experiments on epitaxial thin films seemed to confirm strain induced anisotropy enhancement. This study presents a critical analysis of the different contributions to perpendicular anisotropy: volume, interface and surface anisotropies. Tracing these contributions, thickness series of single layer films as well as multilayers with Au–Cu buffers/interlayers of different lattice parameters have been prepared. The analysis of their magnetic anisotropy reveals a negligible influence of the lattice parameter of the buffer. Electronic effects, originating from both, the Au–Cu interface and the film surface, outrange the elastic effects. Surface anisotropy, however, exceeds the interface anisotropy by more than a factor of three. A comparison with results from density functional theory suggests, that the experimentally observed strong perpendicular surface anisotropy originates from a deviation from an ideal oxide-free surface. Accordingly, tailored Fe–Co–X/oxide interfaces may open a route towards high anisotropy in rare-earth free materials.

First-principles investigation of magnetocrystalline anisotropy at theL21fullHeusler|MgOinterfaces and tunnel junctions

Magnetocrystalline anisotropy at Heusler alloy$|$MgO interfaces have been studied using first principles calculations. It is found that Co terminated Co$_{2}$FeAl$|$MgO interfaces show perpendicular magnetic anisotropy up to 1.31 mJ/m$^2$, while those with FeAl termination exhibit in-plane magnetic anisotropy. Atomic layer resolved analysis indicates that the origin of perpendicular magnetic anisotropy in Co$_{2}$FeAl$|$MgO interfaces can be attributed to the out-of-plane orbital contributions of interfacial Co atoms. At the same time, Co$_{2}$MnGe and Co$_{2}$MnSi interfaced with MgO tend to favor in-plane magnetic anisotropy for all terminations.

Copper dusting effects on perpendicular magnetic anisotropy in Pt/Co/Pt tri-layers

The effect of Cu dusting on perpendicular magnetic anisotropy of sputter grown Pt/Co/Pt stack in which the Cu layer is in proximity with that of Co is investigated in this work. We used magneto optic Kerr effect microscopy measurements to study the variation in the reversal mechanisms in films with Co thicknesses below 0.8nm by systematically varying their perpendicular magnetic anisotropy using controlled Cu dusting. Cu dusting was done separately above and below the cobalt layer in order to understand the role of bottom and top Pt layers in magnetization reversal mechanisms of sputtered Pt/Co/Pt stack. The introduction of even 0.3nm thick Cu layer below the cobalt layer drastically affected the perpendicular magnetic anisotropy as evident from the nucleation behavior. On the contrary, even a 4nm thick top Cu layer had little effect on the reversal mechanism. These observations along with magnetization data was used to estimate the role of top and bottom Pt in the origin of perpendicular magnetic anisotropy as well as magnetization switching mechanism in Pt/Co/Pt thin films. Also, with an increase in the bottom Cu dusting from 0.2 to 0.4nm there was an increase in the number of nucleation sites resulting in the transformation of domain wall patterns from a smooth interface type to a finger like one and finally to maze type.

Effect of Mg interlayer on perpendicular magnetic anisotropy of CoFeB films in MgO/Mg/CoFeB/Ta structure

The effects of Mg metallic interlayer on the magnetic properties of thin CoFeB films in MgO/Mg (tMg)/CoFeB (1.2 nm)/Ta structures were studied in this letter. Our experimental result shows that the CoFeB film exhibits perpendicular magnetic anisotropy (PMA) when the CoFeB and MgO layers are separated by a metallic Mg layer with a maximum thickness of 0.8 nm. The origin of PMA was discussed by considering the preferential transmission of the Δ1 symmetry preserved by the Mg interlayer in crystallized MgO/Mg/CoFeB/Ta. In addition, the thin Mg interlayer also contributes to enhancing the thermal stability and reducing the effective damping constant and coercivity of the CoFeB film.

Co/Ni(111) superlattices studied by microscopy, x-ray absorption, andab initiocalculations

We explore the origins of perpendicular magnetic anisotropy in epitaxial and textured Co/Ni(111) superlattices using a combination of thin-film growth, structural characterization, x-ray magnetic circular dichroism (XMCD), and ab initio calculations. Transmission electron microscopy and x-ray diffraction experiments allow us to show that the bulk magnetoelastic contribution to the total magnetic energy is small compared to the interface anisotropy. The magnetic properties are studied by using XMCD at the Co and Ni L 2,3 edges. Hysteresis loops performed at the Co L 3 edge confirm the perpendicular magnetization for Co thicknesses up to four monolayers. The spin and orbital moments were deduced using the XMCD sum rules. The results are explained by considering two kinds of magnetic moments for Co, distinguishing the interfaces from the rest of the layers. Both effective spin and orbital moments of Co atoms are found to be enhanced at the Co-Ni interfaces, whereas the magnetic moment of Co surrounded by Co is similar to the bulk value. Ab initio calculations allow us to show a strong enhancement of the dipole operator contribution on Co atoms at the interface that is partly responsible for this high effective spin moment at the interface. Such a moment enhancement is not observed for Ni, with the dipole operator contribution being close to zero. Finally, we observed a very surprising proportionality between the effective spin and orbital moments, independent of the absorption edge or deposition technique used. We assign this peculiar behavior to the fact that the magnetic dipole operator involved in the sum rules is closely linked to the increase of the Co orbital moment at the interface. Based on XMCD results obtained on both molecular beam epitaxy and sputter-deposited samples, this link allows us to show the extreme sensitivity of the perpendicular anisotropy with the chemical ordering at the interface.

First-principles investigation of the very large perpendicular magnetic anisotropy at Fe|MgO and Co|MgO interfaces

The perpendicular magnetic anisotropy (PMA) arising at the interface between ferromagnetic transition metals and metallic oxides are investigated via first-principles calculations. In this work very large values of PMA up to 3 erg/cm$^2$ at Fe$|$MgO interfaces are reported in agreement with recent experiments. The origin of PMA is attributed to overlap between O-$p_z$ and transition metal $d_{z^2}$ orbitals hybridized with $d_{xz(yz)}$ orbitals with stronger spin-orbit coupling induced splitting around the Fermi level for perpendicular magnetization orientation. Furthemore, it is shown that the PMA value weakens in case of over- or underoxidation when oxygen $p_z$ and transition metal $d_{z^2}$ orbitals overlap is strongly affected by disorder, in agreement with experimental observations in magnetic tunnel junctions.

Microscopic origin of perpendicular magnetic anisotropy in amorphous Nd-Co homogeneous and compositionally modulated, thin films studied by XMCD

Amorphous Nd-Co films deposited by DC-magnetron sputtering presented perpendicular magnetic anisotropy (PMA) with energies, KN, of the order of 106 erg/cc at RT. To understand the origin of their PMA, we measured the orbital and spin magnetic moments in Co and Nd by XMCD at the Co L3,2 and Nd M5,4 edges in two kinds of samples of similar thickness (30 nm) and composition: one compositionally modulated Nd/Co film (CM) with strong PMA (KN ~107 erg/cc at 10 K) and a homogenous alloy (A) with not strong enough PMA to see stripe domains for such thickness. The XMCD analysis evidenced the significant role of Nd in the PMA of these films.

Origin of perpendicular magnetic anisotropy and evolution of magnetic domain structure of amorphous Pr–TM–B (TM=Fe, Co) films

In this study, the deposition pressure dependence of the compositional ratio, magnetic domain structure, and perpendicular magnetic anisotropy (PMA) of B-containing PrFe- and PrCo-based films, which are rare-earth–transition-metal (RE–TM) films, was investigated. PrFe- and PrCo-based films were fabricated by magnetron sputtering. The film compositions were controlled in a wide range by varying the deposition pressure. On the basis of experimental results, the residual stress of the films was considered to be the possible origin of their PMA. The films showed strong magneto-optical effects over the entire wavelength range of 300–750 nm. Because of the excellent magnetic and magneto-optical (MO) properties of the films, they have high potential for MO applications at wavelengths of red and blue lasers.

Crystallographic origin of perpendicular magnetic anisotropy in CoPt film: polarized x-ray absorption study

Crystallographic structure, growth induced miscibility gap and strain in Ta/Co100−xPtx (0 ⩽ x ⩽ 43 at%)/Ru/Ta/glass films deposited at ambient temperature were investigated using polarized x-ray absorption spectroscopy to clarify the origin of observed perpendicular magnetic anisotropy (PMA) in Co72Pt28 film. Extended x-ray absorption fine structure spectroscopy data at Co K-edge showed that Co has a similar local atomic environment and averaged interatomic distance in the in-plane and out-of-plane polarization geometries for Co72Pt28, ruling out the contribution of magneto-elastic anisotropy and growth induced structural anisotropy as the origin of PMA. A large PMA in Co72Pt28 film was attributed to the preferred hexagonal close-packed stacking as observed using the x-ray absorption near-edge structure spectroscopy.

X-ray analysis of the magnetic influence of oxygen in Pt∕Co∕AlOx trilayers

X-ray spectroscopy measurements have been performed on a series of Pt∕Co∕AlOx trilayers to investigate the role of Co oxidation in the perpendicular magnetic anisotropy at the Co∕AlOx interface. It is observed that varying the degree of oxidation modifies the magnetic properties of the Co layer, inducing a magnetic anisotropy crossover from in plane to out of plane. The microscopic structural properties are analyzed via x-ray photoelectron spectroscopy measurements. It is shown that increasing the oxidation time enhances the amount of interfacial oxide, which may be at the origin of perpendicular magnetic anisotropy.

Origin of perpendicular magnetic anisotropy of SmCo 5 thin films with Cu underlayer

Effects of the Cu underlayer thickness and the addition of Cu to a Sm–Co layer on magnetic properties and microstructure of SmCo 5 thin films exhibiting perpendicular magnetic anisotropy were studied. The origin of the perpendicular magnetic anisotropy was discussed from these experimental results. A thick Cu underlayer of more than 100 nm brought about high perpendicular magnetic anisotropy leading to the squareness ratio equal to unity. The Cu addition enhanced the perpendicular magnetic anisotropy and reduced the Cu underlayer thickness required to obtain the squareness ratio of unity. X-ray diffractometry showed that the crystalline orientation of the Sm–Co layer did not correlate with that of the Cu underlayer. Auger electron spectroscopy revealed that Cu atoms were diffused up to the Sm–Co layer from the Cu underlayer. From the results, Cu atoms existing in the Sm–Co layer were suggested to be strongly related with an appearance of the perpendicular magnetic anisotropy by introducing the Cu underlayer.

Origin of Perpendicular Magnetic Anisotropy in Tb-Fe Amorphous Alloy

The origin of perpendicular magnetic anisotropy has been investigated in a thick Tb-Fe amorphous alloy plate prepared by dc high-rate sputtering. In the as-prepared samples with and without a Cu substrate, no distinct difference in perpendicular magnetic anisotropy was confirmed. After annealing, the perpendicular magnetic anisotropy in the sample with a Cu substrate was maintained, whereas that disappeared in the sample without a Cu substrate. Inducing thermal strains with an epoxy resin, the perpendicular magnetic anisotropy was restored. Therefore, it is concluded that the strong strains induced by the difference between the thermal expansion of the Tb-Fe amorphous alloy plate and the Cu substrate dominate to the origin of perpendicular magnetic anisotropy.