Strong Perpendicular Magnetic Anisotropy Research Articles

High performance perpendicular magnetic tunnel junction with Co/Ir interfacial anisotropy for embedded and standalone STT-MRAM applications

High volume spin transfer torque magnetoresistance random access memory (STT-MRAM) for standalone and embedded applications requires a thin perpendicular magnetic tunnel junction (pMTJ) stack (∼10 nm) with a tunnel magnetoresistance (TMR) ratio over 200% after high temperature back-end-of-line (BEOL) processing up to 400 °C. A thin reference layer with low magnetic moment and strong perpendicular magnetic anisotropy (PMA) is key to reduce the total thickness of the full pMTJ stack. We demonstrated strong interfacial PMA and a perpendicular Ruderman-Kittel-Kasuya-Yosida exchange interaction in the Co/Ir system. Owing to the additional high PMA at the Ir/Co interface in combination with a conventional CoFeB/MgO interface in the Ir/Co/Mo/CoFeB/MgO reference layer, the full film pMTJ showed a TMR ratio over 210% after annealing at 400 °C for 150 min. The high TMR ratio can be attributed to the thin stack design by combining a thin reference layer with the efficient compensation by a thin pinned layer. The annealing stability may be explained by the absence of solid solution in the Co-Ir system and the low oxygen affinity of Mo in the reference layer and the free layer. High device performance with a TMR ratio over 210% was also confirmed after subjecting the patterned devices to BEOL processing temperatures of up to 400 °C. This proposed pMTJ design is suitable for both standalone and embedded STT-MRAM applications.

A very large perpendicular magnetic anisotropy in Pt/Co/MgO trilayers fabricated by controlling the MgO sputtering power and its thickness

The perpendicular magnetic anisotropy (PMA) properties of Pt/Co/MgO trilayers are investigated as a function of the MgO sputtering power (PMgO) and its thickness (tMgO), both of which are important parameters affecting the degree of oxygen interpenetration into Co during sputtering. A strong PMA is achieved at small values of PMgO and tMgO, where the oxygen interpenetration into Co is expected to be small. The range of oxygen interpenetration is relatively extended in such a way that it affects both the Pt/Co and Co/MgO interfaces. The PMA properties of as-deposited samples are improved by post-annealing for temperatures up to 400 °C examined in this study, probably due to the diffusion of the interpenetrated oxygen atoms toward the Co/MgO interface. In a structure of Pt/Co (0.6 nm)/MgO (2 nm), which is fabricated at PMgO = 50 W and then annealed at 400 °C, a huge saturation field is achieved (over 40 kOe) indicating a very strong PMA. Between the two interfaces of Pt/Co and Co/MgO, the PMA is mainly due to the former in the as-deposited state, but the contribution of the latter increases with the increase in the annealing temperature.

Anisotropic sensor and memory device with a ferromagnetic tunnel barrier as the only magnetic element

Multiple spin functionalities are probed on Pt/La2Co0.8Mn1.2O6/Nb:SrTiO3, a device composed by a ferromagnetic insulating barrier sandwiched between non-magnetic electrodes. Uniquely, La2Co0.8Mn1.2O6 thin films present strong perpendicular magnetic anisotropy of magnetocrystalline origin, property of major interest for spintronics. The junction has an estimated spin-filtering efficiency of 99.7% and tunneling anisotropic magnetoresistance (TAMR) values up to 30% at low temperatures. This remarkable angular dependence of the magnetoresistance is associated with the magnetic anisotropy whose origin lies in the large spin-orbit interaction of Co2+ which is additionally tuned by the strain of the crystal lattice. Furthermore, we found that the junction can operate as an electrically readable magnetic memory device. The findings of this work demonstrate that a single ferromagnetic insulating barrier with strong magnetocrystalline anisotropy is sufficient for realizing sensor and memory functionalities in a tunneling device based on TAMR.

Spin transfer torque magnetization reversal in a hard/soft composite structures

Current induced magnetization manipulation in a spin valve structure where the free layer is a magnetic hard/soft composite structure is studied using micromagnetic simulations. In this structure where the hard layers has strong perpendicular magnetic anisotropy, a domain wall can be nucleated in the soft layer due to the spin transfer torque effect. Depending on the magnetic properties of the layers and the current intensity the domain wall can induce the free layer reversal or be pinned by the hard layer. For these non-uniform magnetic configurations both bulk and interface spin transfer torques need to be considered. The potential reduction of the critical current observed in this geometry is of potential technological interest.

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.

Magnetotransport properties of Cr1−δTe thin films with strong perpendicular magnetic anisotropy

P-type ferromagnetic Cr1-δTe thin films with the Curie temperature of 170K were epitaxially grown on GaAs substrate. Low-temperature magnetotransport study reveals that the film has a strong perpendicular magnetic anisotropy (PMA) and an anisotropic magnetoresistance (AMR) ratio up to 8.1%. Furthermore, reduced anomalous Hall effect is observed at low temperatures in Cr1-δTe, suggesting the possible crossover of the contribution to AHE from the intrinsic mechanism to extrinsic skew scattering. Distinctive from conventional transition metal ferromagnets, the AMR ratio is also greatly suppressed at low temperatures. Our work demonstrates that epitaxial Cr1-δTe films are interesting platforms for studying the physics underlying the strong PMA and large AMR.

Formation of $L1_{0}$ -FePt(001) Ultra-Thin Films With Flat Surfaces Using VC and VN Underlayers

FePt alloy films of 2 to 10 nm thicknesses are prepared on (001) single-crystal underlayers of MgO, VN, and VC by employing a two-step method consisting of low-temperature deposition at 200 °C followed by high-temperature annealing at 600 °C. MgO, VN, and VC crystals are selected as underlayer materials, since the crystals have the same NaCl-type crystal structure and similar lattice constants of about 0.42 nm but different surface energies. The single-crystal underlayers are prepared by RF sputtering on the single-crystal substrates of MgO and SrTiO3 at 600 °C. The influences of underlayer material and FePt film thickness on the $L1_{0}$ ordering degree, the magnetic properties, and the surface morphology are systematically investigated. Epitaxial films involving $L1_{0}$ (001) crystal structure with the $c$ -axis normal to the substrate surface are formed on all the underlayers. As the film thickness decreases, the ratio of out-of-plane to in-plane lattice constant $c/a$ decreases and the ordering degree $S$ increases. Stronger perpendicular magnetic anisotropy is observed for an FePt film with a higher $S$ value. The film thickness trends of $c/a$ , $S$ , and magnetic properties are similar to those for FePt films thicker than 5 nm formed on MgO underlayers. However, when the thickness is decreased to 2 nm, de-wetting of deposited atoms occurs on MgO underlayers during annealing at 600 °C. The surfaces of 2-nm-thick films formed on MgO underlayers are composed of isolated islands. On the contrary, continuous surfaces are realized for the investigated film thickness range of 2–10 nm in the cases of VC and VN underlayers. The surface roughness of FePt film formed on VN underlayer ( $R_{a}$ : 0.1–0.2 nm) is lower than that of film formed on VC underlayer ( $R_{a}$ : 0.1–0.4 nm). This paper shows that the employment of a VN underlayer is effective in the preparation of $L1_{0}$ ordered FePt ultra-thin film with flat surface.

The Spin Wave Gap and Switching Field in Thin Films with In-Plane Anisotropy

In this paper, we have calculated the spin wave gap and the angular dependence of magnetization reversal in a single-layer thin magnetic film that includes the strong perpendicular magnetic anisotropy and in-plane anisotropy. The film is assumed to be under the influence of the out-of-plane direction of the applied magnetic field at zero temperature. Using the quantum model, it is shown that the calculated equations present a nonzero spin wave gap at zero magnetic field which is strongly affected by anisotropies. The effects of the in-plane anisotropy and the role of the applied field were examined. We also discussed a simple theoretical model for the angular variation of switching field by using a quasi-classical argument. We used some constants in connection with experimental data which are reported for chromium telluride thin films grown by molecular beam epitaxy.

Magnetic anisotropy of L1-ordered FePt thin films studied by Fe and Pt L2,3-edges x-ray magnetic circular dichroism

The strong perpendicular magnetic anisotropy of L10-ordered FePt has been the subject of extensive studies for a long time. However, it is not known which element, Fe or Pt, mainly contributes to the magnetic anisotropy energy. We have investigated the anisotropy of the orbital magnetic moments of Fe 3d and Pt 5d electrons in L10-ordered FePt thin films by Fe and Pt L2,3-edge x-ray magnetic circular dichroism (XMCD) measurements for samples with various degrees of long-range chemical order S. Fe L2,3-edge XMCD showed that the orbital magnetic moment was larger when the magnetic field was applied perpendicular to the film than parallel to it and that the anisotropy of the orbital magnetic moment increased with S. Pt L2,3-edge XMCD also showed that the orbital magnetic moment was smaller when the magnetic field was applied perpendicular to the film than parallel to it, opposite to the Fe L2,3-edge XMCD results although the anisotropy of the orbital magnetic moment increases with S like the Fe edge. These results are qualitatively consistent with the first-principles calculation by Solovyev et al. [Phys. Rev. B 52, 13419 (1995)], which also predicts the dominant contributions of Pt 5d to the magnetic anisotropy energy rather than Fe 3d due to the strong spin-orbit coupling and the small spin splitting of the Pt 5d bands in L10-ordered FePt.

Current induced multi-mode propagating spin waves in a spin transfer torque nano-contact with strong perpendicular magnetic anisotropy

Current induced spin wave excitations in spin transfer torque nano-contacts are known asa promising way to generate exchange-dominated spin waves at the nano-scale. It has been shown that when these systems are magnetized in the film plane, broken spatial symmetry of the field around the nano-contact induced by the Oersted field opens the possibility for spin wave mode co-existence including a non-linear self-localized spin-wave bullet and a propagating mode. By means of micromagnetic simulations, here we show that in systems with strong perpendicular magnetic anisotropy (PMA) in the free layer, two propagating spin wave modes with different frequency and spatial distribution can be excited simultaneously. Our results indicate that in-plane magnetized spin transfer nano-contacts in PMA materials do not host a solitonic self-localized spin-wave bullet, which is different from previous studies for systems with in plane magnetic anisotropy. This feature renders them interesting for nano-scale magnonic waveguides and crystals since magnon transport can be configured by tuning the applied current.

Is perpendicular magnetic anisotropy essential to all-optical ultrafast spin reversal in ferromagnets?

All-optical spin reversal presents a new opportunity for spin manipulations, free of a magnetic field. Most of all-optical-spin-reversal ferromagnets are found to have a perpendicular magnetic anisotropy (PMA), but it has been unknown whether PMA is necessary for spin reversal. Here we theoretically investigate magnetic thin films with either PMA or in-plane magnetic anisotropy (IMA). Our results show that spin reversal in IMA systems is possible, but only with a longer laser pulse and within a narrow laser parameter region. Spin reversal does not show a strong helicity dependence where the left- and right-circularly polarized light lead to the identical results. By contrast, the spin reversal in PMA systems is robust, provided both the spin angular momentum and laser field are strong enough while the magnetic anisotropy itself is not too strong. This explains why experimentally the majority of all-optical spin-reversal samples are found to have strong PMA and why spins in Fe nanoparticles only cant out of plane. It is the laser-induced spin–orbit torque that plays a key role in the spin reversal. Surprisingly, the same spin–orbit torque results in laser-induced spin rectification in spin-mixed configuration, a prediction that can be tested experimentally. Our results clearly point out that PMA is essential to spin reversal, though there is an opportunity for in-plane spin reversal.

Spin–orbit torque switching in MgO/CoFeB/Ta/CoFeB/MgO heterostructures with a critical current density of 105 A/cm2

Spin–orbit torque-induced magnetization switching in heavy metal/ferromagnetic metal/oxide heterostructures has been extensively studied. However, thus far, the critical current density for magnetization switching in these systems is at least ∼106 A/cm2. Here, we investigate the spin–orbit torque switching in a substrate/MgO/CoFeB/Ta/CoFeB/MgO structure where the upper CoFeB layer exhibits a strong perpendicular magnetic anisotropy in contrast to the relatively weak perpendicular anisotropy in the lower CoFeB layer. By varying the thickness of Ta, we observe that the critical current required for switching differs markedly. Specifically, for the sample with a 3-nm-thick Ta layer, with an external field of 100 Oe applied along the current direction to break the symmetry, the critical current density is ∼2 × 105 A/cm2. This could be ascribed to the minimum coercivity and perpendicular anisotropic field in the sample with a 3-nm-thick Ta layer.

Perpendicular magnetic anisotropy and microstructure properties of nanoscale Co/Au multilayers

We investigated the role of microstructure and Co layer thickness on the perpendicular magnetic anisotropy of as-deposited and annealed Ta (5 nm)/[Co (tCo)/Au (2 nm)] × N = 20 multilayers with 1 ⩽ tCo ⩽ 2 nm prepared using dc-magnetron sputtering. These multilayers were characterized using a vibrating sample magnetometer, a p-MOKE magnetometer and a microscopy magnetometer, small angle x-ray reflection (XRR), and wide angle x-ray diffraction (XRD) analysis. These multilayers demonstrated strong perpendicular magnetic anisotropy with their saturation magnetization close to the bulk magnetization of Co. Magnetization and magnetic anisotropy increased with annealing and this increase is directly linked to the strain relaxation and sharpening of the interfaces after annealing. Using XRR analysis before and after annealing, and fitting these XRR data, the multilayer periodicities are extracted and the refined layer thickness and surface roughness are determined. Using XRD analysis and fitting these XRD spectra, information regarding both the average lattice spacing of atoms and the strain developed on an individual layer were determined.

Study on the domain structure and tunable spin orientation in L11-CoPt/NiFe exchange springs with Ta-spacer

In this study, we investigate the domain structure and tunable spin orientation in L11-CoPt(10nm)/NiFe(tNiFe) with insertion of 2-nm thick Ta-spacer and by varying NiFe-layer thickness, tNiFe=1.0, 2.5, 5.5, 7.0, 8.5 and 10nm. The L11-CoPt layer exhibits a strong perpendicular magnetic anisotropy and NiFe holds in-plane magnetic anisotropy and both were separated by the non-magnetic Ta-spacer. Upon increasing tNiFe, the magnetic hysteresis loops become more and more tilted along with moderate decrease in both Hc⊥, and SQR⊥ values; while the estimated tilt angle magnetization (θM) values tend to increase from 0 to 60°. Interestingly, with the insertion of Ta-spacer, significant enhancement in θM (=45°) is achieved with tNiFe of ∼3nm in the CoPt/NiFe layers – indicating that increase in tNiFe induces tilting of CoPt spins even in the presence of Ta-spacer. Magnetic force microscopy studies on the the L11-CoPt/Ta/NiFe films revealed maze-like domain patterns and their magnetic phase contrasts tend to decrease with dominant topographic features upon increasing tNiFe. The results obtained with the micro-magnetic simulation on the tilt in anisotropy axis are found to be consistent with the experimental values of θM.

Structure, site-specific magnetism, and magnetotransport properties of epitaxial D022 -structure Mn2FexGa thin films

Ferrimagnetic Mn$_2$Fe$_x$Ga $(0.26 \leq x \leq 1.12)$ thin films have been characterised by X-ray diffraction, SQUID magnetometry, X-ray absorption spectroscopy, X-ray magnetic circular dichroism and M\"{o}ssbauer spectroscopy with the aim of determining the structure and site-specific magnetism of this tetragonal, D0$_{22}$-structure Heusler compound. High-quality epitaxial films with low RMS surface roughness ($\sim 0.6$ nm) are grown by magnetron co-sputtering. The tetragonal distortion induces strong perpendicular magnetic anisotropy along the $c$-axis with a typical coercive field $\mu_0 H\sim 0.8$ T and an anisotropy field ranging from $6$ to $8$ T. Upon increasing the Fe content $x$, substantial uniaxial anisotropy, $K_\mathrm{u} \geq 1.0$ MJ/m$^3$ can be maintained over the full $x$ range, while the magnetisation of the compound is reduced from $400$ to $280$ kA/m. The total magnetisation is almost entirely given by the sum of the spin moments originating from the ferrimagnetic Mn and Fe sublattices, with the latter being coupled ferromagnetically to one of the former. The orbital magnetic moments are practically quenched, and have negligible contributions to the magnetisation. The films with $x=0.73$ exhibit a high anomalous Hall angle of $2.5$ % and a high Fermi-level spin polarisation, above $51$ %, as measured by point contact Andreev reflection. The Fe-substituted Mn$_2$Ga films are highly tunable with a unique combination of high anisotropy, low magnetisation, appreciable spin polarisation and low surface roughness, making them very strong candidates for thermally-stable spin-transfer-torque switching nanomagnets with lateral dimensions down to $10$ nm.

Heusler compounds with perpendicular magnetic anisotropy and large tunneling magnetoresistance

In the present work we suggest a recipe for finding tetragonal Heusler compounds with perpendicular magnetic anisotropy (PMA) that also exhibit large tunneling magnetoresistance (TMR) when used as electrodes in magnetic tunnel junction devices with suitable tunneling barrier materials. We performed density-functional theory calculations for 286 Heusler compounds and identified 116 stable tetragonal compounds. Ten of these compounds are predicted to have strong PMA and, simultaneously, exponentially increasing TMR with increasing tunneling barrier thickness due to the so-called Brillouin zone spin filtering effect. Experimental measurements performed for 25 Heusler compounds theoretically identified as tetragonal show that ten of these compounds indeed have tetragonal structure with PMA.

Angular dependence of magnetization reversal in epitaxial chromium telluride thin films with perpendicular magnetic anisotropy

We investigate magnetic anisotropy and magnetization reversal mechanism in chromium telluride thin films grown by molecular beam epitaxy. We report existence of strong perpendicular magnetic anisotropy in these thin films, along with a relatively strong second order anisotropy contribution. The angular variation of the switching field observed from the magnetoresistance measurement is explained quantitatively using a one-dimensional defect model. The model reveals the relative roles of nucleation and pinning in the magnetization reversal, depending on the applied field orientation. Micromagnetic simulations are performed to visualize the domain structure and switching process.

Structure and magnetic properties of Co/Pd multilayers prepared on porous nanotubular TiO2 substrate

We used porous nanotubular templates of TiO2 for fabrication of Co/Pd antidot arrays with strong perpendicular magnetic anisotropy. The morphology of porous multilayers followed the features of the initial template demonstrating a pronounced relief consisting of the cells with periodic pores with small inclination. We confirmed the formation of Co0.4Pd0.6 alloy at the Co/Pd interface. We observed the conservation of perpendicular magnetic anisotropy in the Co/Pd porous film with coercive field HC=2.7kOe, enhanced with respect to the continuous film due to the pinning of magnetic moments on the nanopore edges. From angular dependence of the coercive field HC we deduced the change of the magnetization reversal mechanism from domain wall motion in the continuous film to the predominantly coherent rotation mechanism in the porous film.

Hydrogenation-induced reversible spin reorientation transition in Co50Pd50 alloy thin films

Abstract Because of the Pd-catalyzed hydrogen dissociation and absorption, magnetic Pd-alloys provide a model system for the investigation of the critical hydrogenation effect on magnetism. In this study, Co 50 Pd 50 (CoPd) alloy thin films were fabricated by e-beam-heated co-evaporation on Al 2 O 3 (0001) substrates. These films exhibited a thickness-dependent spin reorientation transition (SRT) from perpendicular direction to in-plane direction with increase of thickness. For 10-30 nm-thick CoPd alloy films with perpendicular magnetic anisotropy (PMA), hydrogenation triggered a SRT to an in-plane anisotropy. The reversibility of SRT was demonstrated by cyclicly changing the hydrogen gas pressure. Furthermore, hydrogenation-induced SRT randomized the magnetic domain orientation. The surface morphology of the CoPd thin films was composed of nanoclusters, which may play a crucial role in hydrogen dissociation and affect PMA. In comparison with a bare CoPd film, a stronger PMA and a less pronounced hydrogenation-induced SRT were observed in a Pd-capped CoPd film. These observations suggest that the hydrogen content in CoPd alloy films can drastically and reversibly modify PMA, inferring the possible hydrogenation-induced charge transfer and modulation of electronic structure in CoPd.

Perpendicular magnetization of Co2FeAl full-Heusler alloy films induced by Pt insertion

The perpendicular magnetization of Co2FeAl (CFA) full-Heusler alloy films was achieved by intentional insertion of metal layer Pt into Co2FeAl thin film to induce the interfacial perpendicular magnetic anisotropy (PMA). Experimental results indicate that 0.8 nm Pt insertion can greatly improve the ratio of Hc(out-of-plane)/Hc(in-plane), indicating the generation of PMA in the Pt inserted Co2FeAl thin film. At the same time, the effective damping constant of the Co2FeAl film with 0.8 nm Pt insertion layer is 0.0007, which is low enough for spintronic application. Theoretically, based on the first-principles, total energy calculations also prove that the 0.8 nm Pt insertion into Co2FeAl film can generate the PMA and agrees with the experimental results very well. This work reveals that 0.8 nm Pt layer insertion in Co2FeAl film might pave a way towards possessing both strong PMA and 100% theoretical spin polarization in Heusler alloy films.