Perpendicular Magnetic Films Research Articles

Measuring and tailoring the Dzyaloshinskii-Moriya interaction in perpendicularly magnetized thin films

We investigate the Dzyaloshinskii-Moriya interactions (DMIs) in perpendicularly magnetized thin films of Pt/Co/Pt and Pt/Co/Ir/Pt. To study the effective DMI, arising at either side of the ferromagnet, we use a field-driven domain wall creep-based method. The use of only magnetic field removes the possibility of mixing with current-related effects such as spin Hall effect or Rashba field, as well as the complexity arising from lithographic patterning. Inserting an ultrathin layer of Ir at the top Co/Pt interface allows us to access the DMI contribution from the top Co/Pt interface. We show that the insertion of a thin Ir layer leads to reversal of the sign of the effective DMI acting on the sandwiched Co layer, and therefore continuously changes the domain wall structure from the right- to the left-handed N\'eel wall. The use of two DMI-active layers offers an efficient way of DMI tuning and enhancement in thin magnetic films. The comparison with an epitaxial Pt/Co/Pt multilayer sheds more light on the origin of DMI in polycrystalline Pt/Co/Pt films and demonstrates an exquisite sensitivity to the exact details of the atomic structure at the film interfaces.

Spin-wave resonances in the presence of a Bloch wall

In-plane magnetized films of CoTaZr demonstrate standing spin-wave resonances in the complex permeability spectra well below the ferromagnetic resonance frequency. From detailed analysis of the static field dependence of the modes combined with observation of the domains using Kerr microscopy, we show that these modes originate from spin waves within the Bloch walls where the magnetization rotates perpendicular to the plane of the film. Unlike spin waves typically described in perpendicularly magnetized films, the frequencies involved are much lower due to unique field dynamics inside the wall. This theory can be fully understood by incorporating these observations into an analytical model using the Landau-Lifshitz-Gilbert equation, with good agreement between model and experimental measurement.

Controlled nucleation of topological defects in the stripe domain patterns of lateral multilayers with perpendicular magnetic anisotropy

Magnetic lateral multilayers have been fabricated on weak perpendicular magnetic anisotropy amorphous Nd-Co films in order to perform a systematic study on the conditions for controlled nucleation of topological defects within their magnetic stripe domain pattern. A lateral thickness modulation of period $w$ is defined on the nanostructured samples that, in turn, induces a lateral modulation of both magnetic stripe domain periods $\lambda$ and average in-plane magnetization component $M_{inplane}$. Depending on lateral multilayer period and in-plane applied field, thin and thick regions switch independently during in-plane magnetization reversal and domain walls are created within the in-plane magnetization configuration coupled to variable angle grain boundaries and disclinations within the magnetic stripe domain patterns. This process is mainly driven by the competition between rotatable anisotropy (that couples the magnetic stripe pattern to in-plane magnetization) and in-plane shape anisotropy induced by the periodic thickness modulation. However, as the structural period $w$ becomes comparable to magnetic stripe period $\lambda$, the nucleation of topological defects at the interfaces between thin and thick regions is hindered by a size effect and stripe domains in the different thickness regions become strongly coupled.

Microscopic origin of the reduced magnetocrystalline anisotropy with increasing oxide content in Co80Pt20 : oxide thin films

Angle-dependent x-ray magnetic circular dichroism at the Co L2.3 edges has been utilized to systematically study Co80Pt20 : WO3 perpendicular magnetic recording thin films, in which the magnetocrystalline anisotropy significantly drops as the oxide volume fraction increases. The microscopic origin of this phenomenon in the studied films can be mainly attributed to an increase in orbital moment normal to the grain–oxide interface, with increasing oxide volume fraction, which arises from a more pronounced effect of symmetry breaking at the grain–oxide interface in smaller grains.

CoTaZr/Pd multilayer with perpendicular magnetic anisotropy

We report a novel perpendicularly magnetized thin film [Co91.5Ta4.5Zr4/Pd]5 multilayer, which exhibits strong perpendicular magnetic anisotropy when grown on 5 nm of Pd and Ru seed layers. The Pd-seeded multilayer annealed at 300 °C shows an effective uniaxial anisotropy constant, Keff = 1.1 MJ m−3, with an anisotropy field as high as 1.6 T. The perpendicular anisotropy is sustained on annealing at 400 °C for 1 h. X-ray diffraction on multilayers with 30 repeats suggests that the use of amorphous CoTaZr reduces the stress of the stack, compared to [Co/Pd] multilayer.

Temperature dependence of remanent magnetization of thin films at the interface to a nonmagnetic material: Cu/Ni/Cu(100)

Temperature dependence of remanent magnetization of Ni thin films at the interface to Cu is investigated by means of the depth-resolved x-ray magnetic circular dichroism technique. Although magnetization at the interface is smaller than that in the inner layers, no detectable difference in Curie temperature, at which remanent magnetization vanishes, is found between the interface and the inner layers. In contrast, the magnetization-temperature curves at the interface and in the inner layers show a small difference, which cannot be explained by using a simple mean-field theory if the interface layer is assumed to have the same exchange-coupling constant as that in the inner layers. It is also shown that the critical exponent for the in-plane magnetized film is significantly larger than that for the perpendicularly magnetized film due to the difference in the degree of freedom for the magnetic moment.

Energy barriers to magnetization reversal in perpendicularly magnetized thin film nanomagnets

Understanding the stability of thin film nanomagnets with perpendicular magnetic anisotropy (PMA) against thermally induced magnetization reversal is important when designing perpendicularly magnetized patterned media and magnetic random access memories. The magnetization reversal rate depends primarily on the energy barrier the system needs to surmount in order for reversal to proceed. In this paper, we study the reversal dynamics of these systems and compute the relevant barriers using the string method of E, Vanden-Eijnden, and Ren. We find the reversal to be often spatially incoherent; that is, rather than all parts of the element switching simultaneously, reversal proceeds instead through a soliton-like domain wall sweeping through the system. We show that for square nanomagnetic elements, the energy barrier increases with element size up to a critical length scale, beyond which the energy barrier is constant. For circular elements, the energy barrier continues to increase indefinitely, albeit more slowly beyond a critical size. In both cases, the energy barriers are smaller than those expected for coherent magnetization reversal.

Damping of CoxFe80−xB20 ultrathin films with perpendicular magnetic anisotropy

We use vector network analyzer ferromagnetic resonance to study the perpendicularly magnetized CoFeB films. We report the dependence of the anisotropy, the g-factor, and the damping upon the Fe-Co compositional ratio in the amorphous and crystalline states. The damping and the anisotropy increase upon crystallization but vary little with composition on the Fe-rich side. At high cobalt content, the anisotropy lowers while the damping and the sample inhomogeneity increase. The compositional dependences seem to extrapolate from the properties of bulk CoFe alloys, with differences that can be understood from the correlated impacts of spin-orbit interaction on anisotropy, g-factor, and damping.

Magnetic domain structure in thin CoPt perpendicular magnetic anisotropy films

The relation between thickness and domain structure of Co80Pt20 perpendicular magnetic anisotropy films was investigated through experiments and micromagnetic simulation. The films with thickness over 10 nm exhibited clear maze domain structure, while for the films thinner than 10 nm the domain structure abruptly changed from maze domain to irregular and large domain as the thickness became thinner. The irregular domain had narrower domain wall width than maze domain.

Structural characterization and magnetic properties of perpendicularly magnetized MnAl films grown by molecular-beam epitaxy

Perpendicularly magnetized MnAlx thin films with different Al contents have been epitaxied on GaAs (001) substrates by a molecular-beam epitaxy system. Crystalline quality of MnAlx films is closely related to Al content, and magnetic properties of MnAlx films are improved as crystalline quality of MnAlx fims increases. MnAl0.9 film shows the best crystalline quality and magnetic property among all samples. So we grew MnAl0.9 films at different growth temperatures to further optimize growth conditions. With increasing temperature, the chemical order parameter increases and the full width at half maximum of the τ(002) peak decreases, which reveal the improvement of crystalline quality. Higher perpendicular magnetization, coercivity and magnetic anisotropy are found as growth temperature increases. The best crystalline quality and perpendicularly magnetized properties are found at 350℃; the coercivity of 8.3 kOe, saturation magnetization of 265 emu/cm3, Mr/Ms of 0.933 and perpendicular magnetic anisotropy constant of 7.74 Merg/cm3 are achieved. These tunable perpendicularly magnetized properties and good compatibility associated with semiconductor materials make the noble-metal-free and rare-earth-free MnAl films attractive in the application of spintronic devices.

Optimization of perpendicular magnetic anisotropy tips for high resolution magnetic force microscopy by micromagnetic simulations

Magnetic Force Microscopy (MFM) tip coated with perpendicular magnetic anisotropy film (PMA tip) is one of the choices for high resolution imaging at low scan height (SH), since it has negligible tip–sample interaction related to its stable magnetic state, sharp, and small tip stray field. In this work, detailed micromagnetic studies are carried out to understand the effect of geometrical and magnetic parameters including the cone angle θ of the PMA tip, intergrain exchange constant \(A_{2}^{*}\), saturation magnetization M s and uniaxial crystalline anisotropy constant K 1 of the tip coating on the MFM tip resolution. To evaluate the resolution performance of the optimized PMA tip, MFM images of high-density granular recording media and patterned media are simulated. We find that, for the PMA tip and its coating, a cone angle in a range of 36.9∘ to 53.1∘, a saturation M s of 700 emu/cm3, a large uniaxial crystalline anisotropy constant K 1 (>4.9×106 erg/cm3) and a high intergrain exchange constant \(A_{2}^{*}\) of (0.3–1.0)×10−6 erg/cm are optimized conditions for high resolution imaging. The optimized PMA tip has an excellent performance on imaging of high-density thin film media (bit size of 8×16 nm2) at low SH of 2–8 nm and bit pattern media with a pitch of 50 nm, edge-edge spacing of 5–15 nm at SH of 8–15 nm.

Influence of various underlayers on [001] texture and magnetic properties in FePt and FexNi1 − xPt thin films

Perpendicular magnetic recording thin films require proper crystallographic growth to ensure that the magnetic easy-axis is aligned normal to the growth plane. The texture and in situ growth stresses of (001) FePt and FexNi(1−x)Pt thin films on various underlayer combinations are studied. These were correlated with the magnetic properties allowing self consistent direct comparisons of the various parameters to be made. It was found that the (001) texture, L10 chemical ordering and perpendicular magnetic alignment was improved with the smaller lattice misfit that accompanied a combination of Pt/Cr multilayered and compositionally graded underlayers on a (001) MgO substrate. A Cr underlayer allowed the Pt film to deposit in a relatively strain-free lattice condition from which the FePt based films could grow in a similar strain-free epitaxial condition. In situ stress measurements confirmed that these underlayers reduced the intrinsic compressive growth stresses to approximately one-fifth the FePt stress value for being directly deposited onto the MgO surface. The incorporation of Ni to the (001) growth of FePt facilitated in-plane and out-of-plane c-variant growth for the L10 structure. Finally, a linear compositional gradient FexNi0.48−xPt0.52 (0<x<0.48) thin film was deposited, which exhibited good [001] orientation and perpendicular magnetic alignment. This has been proposed as a magnetic switching architecture for high density, thermally stable but switchable magnetic media grains using current write head fields.

Topological Defects and Misfit Strain in Magnetic Stripe Domains of Lateral Multilayers With Perpendicular Magnetic Anisotropy

Stripe domains are studied in perpendicular magnetic anisotropy films nanostructured with a periodic thickness modulation that induces the lateral modulation of both stripe periods and in-plane magnetization. The resulting system is the 2D equivalent of a strained superlattice with properties controlled by interfacial misfit strain within the magnetic stripe structure and shape anisotropy. This allows us to observe, experimentally for the first time, the continuous structural transformation of a grain boundary in this 2D magnetic crystal in the whole angular range. The magnetization reversal process can be tailored through the effect of misfit strain due to the coupling between disclinations in the magnetic stripe pattern and domain walls in the in-plane magnetization configuration.

A study of the perpendicular magnetic microstructure of the L10 FePt epitaxial film using electron holography

The magnetic microstructure of the L10 FePt perpendicular magnetic film, grown epitaxially on MgO substrate, was studied using high-resolution electron holography. It was shown that the hologram-fringe information of stray-fields can be used to determine the sense of stray-fields and thus, the sense of perpendicular magnetizations inside the film. Furthermore, we were able to directly reveal the detailed perpendicular magnetic structure of L10 FePt epitaxial film by constructing unwrapped phase-image and induction mapping. The perpendicular induction-strength was varying with the domain size. The variation of induction-strength was due to a variation of the degree of ordering in the weakly ordered film.

Demonstration of microwave assisted magnetic reversal in perpendicular media

Microwave assisted magnetic reversal (MAMR) has not yet been experimentally demonstrated in hard perpendicular media. We study MAMR in perpendicular bilayer magnetic thin films and characterize the behavior as a function of frequency, power, and temperature. We find a broad frequency response with little dependence on temperature from 4 to 300 K.

Study on Micro-magnetic Simulation of Magnetic State of TbFeCo/Anodized Aluminum Oxide Film

Based on the experimental study of the AAO underlayer’s effect on the magnetic properties and structure, combining micro-magnetic theory as well, TbFeCo/AAO film magnetic properties and magnetic reversal characteristics were analyzed by finite-element micro-magnetic calculation software Magpar. With limited hardware conditions for the computer, a TbFeCo perpendicular magnetization films model with 9×9 matrix particles was simulated. It was found that for the pinning effect of non-magnetic porous AAO barrier film on TbFeCo particle film, the coercivity of isolated TbFeCo/AAO film (2100 kA/m) was larger than the coercivity of continuous TbFeCo film (2000 kA/m). The exchange coupling among the film particles, which was affected by AAO barrier film, led to a longer magnetization reversal time of isolated TbFeCo films than that of continuous ones.

CoPt垂直磁気異方性膜の磁気特性の膜厚依存性

The relation between thickness and magnetic properties of CoPt perpendicular magnetic anisotropy film was investigated through experiments and micromagnetic simulation. The perpendicular nucleation field and saturation field decreased as the film thickness decreased, whereas the in-plane squareness ratio increased as the thickness decreased. The increase in in-plane squareness ratio with decreasing thickness was due to an expansion of the domain wall region that occurred with the decrease in thickness.

Research on enhanced perpendicular magnetic anisotropy in CoFe/Pd bilayer structure

A series of CoFe/Pd bilayer thin films is fabricated by introducing a native oxide layer to the interface or to the inside of CoFe layer in this paper. The results indicate that the magnetic anisotropy of the film is transformed from in-plane to out-of-plane after annealing by introducing the native oxide layer. For the samples with the introduction of native oxide layer into CoFe layer, the strong perpendicular magnetic anisotropy is maintained in a wide range of the effective thickness (1.22 nm) of magnetic layer. For the perpendicular magnetic films, the thickness of CoFe layer in this special bilayer structure is at least 1.4 nm, thicker than in common CoFe/Pd multilayer structure. The results in this paper are beneficial for the fabrication of the electrodes in perpendicular magnetic devices with high thermal stability.

Influence of Pt Doping on Gilbert Damping in Permalloy Films and Comparison with the Perpendicularly Magnetized Alloy Films

Effects of Pt doping on magnetic properties and Gilbert damping are investigated for Ni80Fe20 permalloy films to compare with damping in alloy films containing Pt with a large perpendicular anisotropy. Gilbert damping constant α and g-factor g for (Ni80Fe20)100-xPtx (x = 0–34 at. %) are evaluated from out-of-plane angular variations of ferromagnetic resonance (FMR) linewidth and resonance field with an analysis based on the Landau–Lifshitz–Gilbert equation. Data of angular dependence of the FMR linewidth are fitted reasonably well by a theoretical model without having to take into account any extrinsic influences on linewidth, thereby allowing us to determine precise values of α. The α values show variation with increasing Pt concentration rising by ∼0.06 at a Pt concentration of 34 at. %, which is very close to those in perpendicularly magnetized CoCrPt and FePt film reported recently. Nevertheless, Gilbert damping rate G for the Pt doped permalloy films is smaller than those in CoCrPt and FePt films. These experimental results are discussed with a spin–orbit torque theory.

Threshold photoemission magnetic circular dichroism of perpendicularly magnetized Ni films on Cu(001): Theory and experiment

Threshold photoemission magnetic circular dichroism of perpendicularly magnetized Ni films on Cu(001) was measured in total electron yield and used to observe the magnetic domain structure in a photoemission electron microscope. Spin-polarized relativistic Korringa-Kohn-Rostoker Green's function calculations including a dynamical mean-field theory approach within the one-step-photoemission model reproduce the measured asymmetry in the photocurrents for left and right circularly polarized light. In addition, a three-step photoemission model calculation based on the same ab initio calculation is used to quantitatively explain the MCD effect near the photoemission threshold.