Perpendicular Easy Axis Research Articles

Spin reorientation transition in Co/Au multilayers

We report a study about the spin reorientation transition (SRT) from perpendicular easy axis to in-plane easy axis of magnetization in Co/Au multilayers. A series of multilayers of Si/Au(100Å)/{[Co(tCo)/Au(20Å)]20}/Au(50Å) family were studied, with Co layer thickness varying between 6Å to 30Å. The thickness of the Au layer was chosen large enough in order to minimize the interlayer exchange coupling between Co layers. In such thick Au-layer samples the magnetic properties are mainly the result of competition between interlayer magnetostatic coupling due to stray field, perpendicular magnetic anisotropy and shape anisotropy. The effective anisotropy constant Keff and the second order anisotropy K2 were deduced from the fit of the resonant magnetic field obtained from out of plane dependence Ferromagnetic Resonance (FMR) experiments. To study the SRT, we have plotted the phase diagram between Keff and K2. The results show that SRT occurs through the metastable region with K2≤−½Keff, (Keff>0). It is interesting to note that FMR shows the coexistence of two modes with different anisotropy for small Co thickness, while for thick Co layers the modes have the same anisotropy. Moreover, in thick Co layer samples, volume and surface spin wave resonance (SWR) modes were also excited by the microwave field, around the perpendicular FMR geometry, giving a clear evidence of a magnetic coupling between the Co layers.

Magnetization switching in a CoFeB/MgO magnetic tunnel junction by combining spin-transfer torque and electric field-effect

We propose and demonstrate a scheme for magnetization switching in magnetic tunnel junctions, in which two successive voltage pulses are applied to utilize both spin-transfer torque and electric field effect. Under this switching scheme, a CoFeB/MgO magnetic tunnel junction with perpendicular magnetic easy axis is shown to switch faster than by spin-transfer torque alone and more reliably than that by electric fields alone.

Co/Pt multilayer based reference layers in magnetic tunnel junctions for nonvolatile spintronics VLSIs

We investigated properties of Co/Pt multilayer for reference layer in CoFeB–MgO magnetic tunnel junctions with perpendicular easy axis. The sufficient thermal stability factor of 284 was obtained under zero applied field in 40-nm-diameter Co/Pt multilayer based reference layer annealed at 350 °C. By applying a synthetic ferrimagnetic (SyF) structure to the Co/Pt multilayer based reference layer, the shift of the center of minor resistance-magnetic field curves was suppressed, leading to higher thermal stability of antiparallel magnetization configuration than that without a SyF structure.

Magnetization reversal induced by in-plane current in Ta/CoFeB/MgO structures with perpendicular magnetic easy axis

We investigate in-plane current-induced magnetization reversal under an in-plane magnetic field in Hall bar shaped devices composed of Ta/CoFeB/MgO structures with perpendicular magnetic easy axis. The observed relationship between the directions of current and magnetization switching and Ta thickness dependence of magnetization switching current are accordance with those for magnetization reversal by spin transfer torque originated from the spin Hall effect in the Ta layer.

Controlling the Magnetic Properties of Cr-Implanted Co/Pt Multilayer Films Using Ion Irradiation for Planar Patterned Media

We have investigated the modification of the microstructure and magnetic properties of Co/Pt multilayer films with various layer structures using ion beam irradiation. Cr/[Co/Pt] multilayer films prepared by a magnetron sputtering deposition on a surface oxidized Si substrate were irradiated by 30 keV Kr+ ion beam with various ion doses from 1×1014 to 5×1015 ions/cm2. The modifications of magnetic properties and microstructures due to Kr+ ion irradiation depended on Co/Pt layer structures. The as-prepared [Co(0.4 nm)/Pt(0.45 nm)]15 multilayer films were confirmed to exhibit the square shaped M-H loop and the perpendicular easy axis with a saturation magnetization of 620 emu/cc and coercivity of 1.1 kOe. After the irradiation with ion dose of 5×1015 ions/cm2, the easy axis changes from perpendicular to in-plane direction. The coercivity and saturation magnetization decrease down to 2% and 13% of its original values, respectively. The decrease of coercivity and saturation magnetization is due to the implantation of the Cr atom in the top most layers into Co/Pt multilayer films caused by Kr+ ion irradiation, as confirmed from Auger electron spectroscopy (AES) depth profiles.

Electric Field-Induced Magnetization Switching in CoFeB-MgO—Static Magnetic Field Angle Dependence

We investigate switching probabilities of electric field-induced magnetization switching in CoFeB/MgO based magnetic tunnel junction with perpendicular magnetic easy axis under static magnetic field with various directions. The switching takes place through magnetization precession with the period determined by the magnitude of the in-plane component of magnetic field. The experimental results are well reproduced by macro-spin simulation. The simulation reveals that decoherence induced by thermal fluctuations and/or other effects limit the switching probabilities in the present system.

Influence of thermal agitation on the electric field induced precessional magnetization reversal with perpendicular easy axis

We investigated the influence of thermal agitation on the electric field induced precessional magnetization switching probability with perpendicular easy axis by solving the Fokker-Planck equation numerically with finite difference method. The calculated results show that the thermal agitation during the reversal process crucially influences the switching probability. The switching probability can be achieved is only determined by the thermal stability factor Δ of the free layer, it is independent on the device dimension, which is important for the high density device application. Ultra-low error rate down to the order of 10−9 can be achieved for the device of thermal stability factor Δ of 40. Low damping factor α material should be used for the free layer for high reliability device applications. These results exhibit potential of electric field induced precessional magnetization switching with perpendicular easy axis for ultra-low power, high speed and high density magnetic random access memory (MRAM) applications.

In-plane magnetic field dependence of electric field-induced magnetization switching

Electric field-induced magnetization switching through magnetization precession is investigated as a function of in-plane component of external magnetic field for a CoFeB/MgO-based magnetic tunnel junction with perpendicular easy axis. The switching probability is an oscillatory function of the duration of voltage pulses and its magnitude and period depend on the magnitude of in-plane magnetic field. Experimental results are compared with simulated ones by using Landau-Lifshitz-Gilbert-Langevin equation, and possible factors determining the probability are discussed.

Magnons in ultrathin ferromagnetic films with a large perpendicular magnetic anisotropy

We report on an experimental observation of high-energy magnon excitations in ultrathin ferromagnetic films with a perpendicular easy axis. We demonstrate that a transversally spin-polarized beam can be used to excite and probe the high-energy magnons within spin-polarized electron energy-loss spectroscopy experiments. The magnon dispersion relation and lifetime are probed over the entire surface Brillouin zone for a set of body-centered tetragonal FeCo films with a large perpendicular magnetic anisotropy. First-principles calculations reveal that in addition to the tetragonal distortion, which is the origin of the large perpendicular magnetic anisotropy, the interfacial electronic hybridization also has a considerable impact on the properties of magnons.

Increased Perpendicular TMR in FeCoB/MgO/FeCoB Magnetic Tunnel Junctions by Seedlayer Modifications

By modifying the seedlayer in perpendicular FeCoB/MgO/FeCoB magnetic tunnel junctions (MTJs), we observe an increase in maximum tunneling magnetoresistance (TMR) from 65% up to 138%. It's found that decreasing the Ta deposition rate in Ta/Ru/Ta underlayers allows for greater annealing temperatures (up to 350 ) while still maintaining a perpendicular easy axis. An improvement is also seen at a lower temperature where both seedlayers maintain a perpendicular FeCoB easy axis indicating that the increase in TMR is not solely related to annealing at a higher temperature.

MgO/CoFeB/Ta/CoFeB/MgO Recording Structure in Magnetic Tunnel Junctions With Perpendicular Easy Axis

Junction size (D) dependence of thermal stability (Δ) factor and intrinsic critical current (I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C0</sub> ) were investigated for MgO/CoFeB/Ta/CoFeB/MgO recording structure in magnetic tunnel junctions (MTJs) having a CoFeB reference layer and a synthetic ferrimagnetic (SyF) reference layer. Δ of the recording structure shows almost constant value down to 40 nm, whereas I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C0</sub> shows a linear dependence on the recording layer area, as similarly observed in recording structure with single-interface. Average absolute intrinsic critical current density is 3.5 MA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , which is comparable to previously reported value for recording structure with single-interface. A MgO/CoFeB(1.4)/Ta(0.4)/CoFeB(1.0)/MgO double-interface recording structure in MTJ with SyF reference layer shows Δ of 59 at D = 29 nm.

Atomic-Layer Alignment Tuning for Giant Perpendicular Magnetocrystalline Anisotropy of3dTransition-Metal Thin Films

The magnetocrystalline anisotropy (MA) of Fe-based transition-metal thin films, consisting of only magnetic 3d elements, was systematically investigated from full-potential linearized augmented plane-wave calculations. The results predict that giant MA with a perpendicular magnetic easy axis (PMA) can be achieved by tuning the atomic-layer alignments in an Fe-Ni thin film. This giant PMA arises from the spin-orbit coupling interaction between occupied and unoccupied Ni dx2-y2,xy bands crossing the Fermi level. A promising 3d transition-metal thin film for the MgO-based magnetic tunnel junctions with the giant PMA was, thus, demonstrated.

Magnetic properties of MgO-[Co/Pt] multilayers with a CoFeB insertion layer

Magnetic properties of MgO-[Co/Pt] multilayers with a CoFeB insertion layer were investigated and the structure was adopted as a recording layer in magnetic tunnel junctions (MTJs) with perpendicular magnetic easy-axis to attain a high thermal stability. Perpendicular easy-axis was obtained with the Pt thickness range of 0.6–1.2 nm and the CoFeB thickness range of 0.6–1.5 nm. As-made MTJs employing the CoFeB-[Co/Pt] multilayer structure as a recording layer showed tunnel magnetoresistance of 40% on average. A high thermal stability factor over 200 was obtained in the MTJs with the size of 25 nm in diameter.

Systematic control of stress-induced anisotropy in pseudomorphic iron garnet thin films

Iron garnets are one of the most well-studied magnetic materials that enabled magnetic bubble memories and magneto-optical devices employing films with a perpendicular easy axis. However, most studies have been conducted on rather thick films (>1μm), and it has not been elucidated whether it is possible to align the magnetic easy axis perpendicular to the film plane for much thinner (<100nm) films by overcoming shape anisotropy. We studied the effects of epitaxial strain and film composition on the magnetic properties of 50-nm-thick garnet thin films grown by pulsed-laser deposition. Y3Fe5O12 was selected as the most prototypical garnet and Sm3−xTmxFe5O12 (x=1, 2, 3) was selected in view of its negatively large magnetostriction constants. We employed (111) planes of single crystalline Gd3Ga5O12 and (CaGd)3(MgGaZr)5O12 substrates to tune the epitaxial strain. Thin films with a pseudomorphic structure were fabricated with the in-plane strain (ε//) ranging from −1.5% to +0.5%, corresponding to the stress-induced anisotropy field (HA) ranging from −40kOe to +25kOe, respectively. The magnetization ratio of the out-of-plane to in-plane component (M⊥/M//) systematically varied in accord with HA, yielding M⊥/M// >1 for thin films with HA values larger than 20kOe. Among the films grown, Tm3Fe5O12 on Gd3Ga5O12 showed the largest ε// and HA values of +0.5% and +25kOe, respectively, to realize an apparently perpendicular easy axis, confirmed by a large M⊥/M// value of 7.8. Further, magnetic force microscope images showed a maze pattern typical of a perpendicularly magnetized film. These results reveal a method for tailoring the magnetic anisotropy of garnet ultrathin films by utilizing epitaxial strain. These thin films may be utilized to obtain nanoscale magnetic bubbles for use in novel devices.

The research of the perpendicular magnetic anisotropy in CoFeB/AlOx/Ta and AlOx/CoFeB/Ta structures

The perpendicular magnetic anisotropy (PMA) of the CoFeB/AlOx/Ta structure and the AlOx/CoFeB/Ta structure with different thicknesses of both CoFeB and AlOx is studied. Magnetization curves show that the CoFeB/AlOx/Ta structure has a clear perpendicular magnetic easy axis while the AlOx/CoFeB/Ta structure does not. The cause of the asymmetrical phenomenon in the symmetric structures is analyzed. Dependence of the perpendicular coercivities on the thicknesses of CoFeB and AlOx shows that both of them can affect the strength of the PMA originating from the interfacial interaction. This work will be meaningful for the fabrication of the AlOx-based perpendicular magnetic tunnel junctions.

The Effects of Electrode and Annealing on CoFeB–MgO–TbFeCo Perpendicular Magnetic Tunnel Junctions

In this study, we report the effects of electrode and annealing on the magnetic properties of CoFeB-MgO-CoFeB-TbFeCo structures. The samples of structure bottom electrode/Co20Fe60B20(x nm)/MgO(1)/Co20Fe60B20(0 or 1 nm)/Tb23(Fe80Co20)77(5 nm)/top electrode were deposited on SiO2 substrates at room temperature. The structures were then annealed at the temperature ranging from 250 to 400°Cunder 5 kOe perpendicular applied field for two hours. We found that the magnetic properties of the CoFeB free layer depend on the electrode stack structures. The perpendicular magnetic easy axis of the CoFeB free layer can be maintained after annealing up to 400°C.

Electric field-induced magnetization reversal in a perpendicular-anisotropy CoFeB-MgO magnetic tunnel junction

The electric field-induced ∼180° magnetization reversal is realized for a sputtered CoFeB/MgO-based magnetic tunnel junction with perpendicular magnetic easy axis in a static external magnetic field. Application of bias voltage with nanoseconds duration results in a temporal change of magnetic easy axis in the free layer CoFeB to in-plane, which induces precessional motion of magnetization in the free layer. The magnetization reversal takes place when the bias voltage pulse duration is adjusted to a half period of the precession. We show that the back and forth magnetization reversal can be observed by using successive application of half-period voltage pulses.

RECENT PROGRESS OF PERPENDICULAR ANISOTROPY MAGNETIC TUNNEL JUNCTIONS FOR NONVOLATILE VLSI

We review recent developments in magnetic tunnel junctions with perpendicular easy axis (p-MTJs) for nonvolatile very large scale integrated circuits (VLSIs). So far, a number of material systems such as rare-earth/transition metal alloys, L10-ordered ( Co, Fe )– Pt alloys, Co /( Pd, Pt ) multilayers, and ferromagnetic-alloy/oxide stacks have been proposed as electrodes in p-MTJs. Among them, p-MTJs with single or double ferromagnetic-alloy/oxide stacks, particularly CoFeB–MgO , were shown to have high potential to satisfy major requirements for integration.

Electric field induced magnetization rotation in patterned Ni ring/Pb(Mg1/3Nb2/3)O3](1−0.32)-[PbTiO3]0.32 heterostructures

Electric field induced magnetoelastic anisotropy is shown to rotate the magnetization of a ring-shaped magnet by 90° in a Ni ring/(011) Pb(Mg1/3Nb2/3)O3](1−0.32)-[PbTiO3]0.32 heterostructure. The 2000 nm diameter ring is initially field annealed forming the “onion” magnetization state. A 0.8 MV/m electric field is applied to the substrate creating anisotropic piezostrain and a perpendicular in-plane easy axis. Magnetic force microscopy confirms the 90° rotation of the vortex-type domain walls from the field annealing direction. Rotations are stable without electric field due to remnant strains induced during the poling process, supporting the viability of strain-based magnetic recording methods.

Domain Structure in CoFeB Thin Films With Perpendicular Magnetic Anisotropy

Domain structures in CoFeB-MgO thin films with a perpendicular easy magnetization axis were observed by magneto-optic Kerr-effect microscopy at various temperatures. The domain wall surface energy was obtained by analyzing the spatial period of the stripe domains and fitting established domain models to the period. In combination with SQUID measurements of magnetization and anisotropy energy, this leads to an estimate of the exchange stiffness and domain wall width in these films. These parameters are essential for determining whether domain walls will form in patterned structures and devices made of such materials.