Perpendicular Anisotropy Films Research Articles

Spin–Orbit Torque Switching in a Nearly Compensated Heusler Ferrimagnet

Ferrimagnetic materials combine the advantages of the low magnetic moment of an antiferromagnet and the ease of realizing magnetic reading of a ferromagnet. Recently, it was demonstrated that compensated ferrimagnetic half metals can be realized in Heusler alloys, where high spin polarization, zero magnetic moment, and low magnetic damping can be achieved at the same time. In this work, by studying the spin-orbit torque induced switching in the Heusler alloy Mn2 Ru1- x Ga, it is found that efficient current-induced magnetic switching can be realized in a nearly compensated sample with strong perpendicular anisotropy and large film thickness. This work demonstrates the possibility of employing compensated Heusler alloys for fast, energy-efficient spintronic devices.

Magnetization reversal of antiferromagnetically coupled perpendicular anisotropy films driven by current

By inserting an ultrathin Pt layer at Co/Ru interface, we established antiferromagnetic coupling with out-of-plane magnetization in Co/Ru/Co film stacks fabricated by sputtering. To achieve configuration suitable for free layer, the magnetic properties of the stacks have been investigated by changing the thickness of Co, Ru and Pt layers using an orthogonal wedges technique. It is found that magnetic properties for upper Co layer thinner than 0.5nm are sensitive to little change in Ru thickness. Improving continuity of upper Co layer by slightly increasing the thickness can effectively increase the squareness of minor loop. The switching magnetization of synthetic antiferromagnetic (SAF) structure is achieved by DC current under an in-plane static magnetic field of±500Oe. This structure is very promising for free layer in spintronic application.

Chirality-Dependent Transmission of Spin Waves through Domain Walls.

Spin-wave technology (magnonics) has the potential to further reduce the size and energy consumption of information-processing devices. In the submicrometer regime (exchange spin waves), topological defects such as domain walls may constitute active elements to manipulate spin waves and perform logic operations. We predict that spin waves that pass through a domain wall in an ultrathin perpendicular-anisotropy film experience a phase shift that depends on the orientation of the domain wall (chirality). The effect, which is absent in bulk materials, originates from the interfacial Dzyaloshinskii-Moriya interaction and can be interpreted as a geometric phase. We demonstrate analytically and by means of micromagnetic simulations that the phase shift is strong enough to switch between constructive and destructive interference. The two chirality states of the domain wall may serve as a memory bit or spin-wave switch in magnonic devices.

On the theory of a supercritical state of ferromagnetic films with uniaxial perpendicular anisotropy and (001) single-crystal films with positive constants of magnetocrystalline anisotropy

A model of the magnetization distribution in thin ferromagnetic films that are in the so-called supercritical state has been considered. The model makes it possible to take into account the real distribution of the magnetization more precisely in ferromagnetic films than open and closed models. The theoretical results have been compared to the experimental data.

Microwave assisted magnetization switching in Co/Pt multilayer

In this study, we have experimentally investigated the microwave assisted magnetization by switching (MAS) on the microstructured Co/Pt multilayer. The sample exhibits the typical magnetization curve peculiar to perpendicular anisotropy films, that is, a steep reversal initiated by nucleation of a reversed domain followed by its subsequent gradual expansion by the domain wall displacement. By applying microwaves with the frequency of GHz order, the nucleation field Hn is significantly reduced at three frequencies. Taking into account the effective anisotropy field of our sample, the first dip of Hn at the lowest frequency probably corresponds to the Kittel mode excitation, and the other two dips at higher frequencies correspond to unidentified excitation modes other than the Kittel mode. Among them, the last dip of Hn at the highest frequency reaches about 1/3 of that without microwave application. These results suggest the existence of more effective excitation modes for MAS than the Kittel mode.

FePt-C nanogranular films for perpendicular magnetic recording

We report the study of L10 ordered FePt-C nanogranular perpendicular anisotropy films and a media consisting of an 8 nm thick FePt-C hard magnetic layer covered with a 2 nm thick soft magnetic layer (CoFe and FePt-C). All the films were fabricated on oxidized Si substrates with a (100) textured polycrystalline MgO intermediate layer of 10 nm in thickness. Cosputtering of Fe, Pt, and C on the MgO layer results the formation of L10 ordered FePt nanogranular films with a strong c-axis texture and the average FePt particle size of about 5 nm with a narrow particle size distribution of 1.4 nm in standard deviation. Perpendicular coercivity is controllable between 7 and 12 kOe. The coercivity and the saturation field of the FePt-C granular films can be reduced by 55% and 38%, respectively, with the capping of thin soft magnetic layer. This study demonstrates that the FePt-C system accomplishes suitable nanogranular structures and magnetic properties for ultrahigh-density perpendicular recording media.

Magnetic properties and microstructures of perpendicular anisotropy (SmCo 7/Cu) n films

Pseudobinary SmCo 7 − x Cu x intermetallic compound films with a TbCu 7-type structure were prepared by sputtered (SmCo 7/Cu) n multilayer. After annealing, the spacer layer Cu (0.5–1.5 nm) diffused into the SmCo 7 matrix and stabilized the SmCo 7 meta-stable phase. The resulting microstructure was investigated by transmission electron microscopy. Perpendicular anisotropy was obtained by introducing a Cu/Ti dual underlayer during sputtering, resulting in prefer-orientated SmCo 7 (00L) X-ray diffraction peaks. This study observes a maze-like domain pattern in perpendicular anisotropy films and finds single- and multi-domain particles in correlated AFM and MFM images.

Thermal Properties of Spin Torque Oscillators: Singularity in Effective Energy

It is important to be able to calculate steady-state probability distributions in spin torque systems, for such applications as spin-torque oscillators and thermal switching of MRAM. It has been shown using a Fokker-Planck formulation of this problem that the steady state probability can be expressed in terms of an effective energy, which can be calculated analytically for uniaxial systems (including perpendicular-anisotropy thin film elements). For elements with in-plane anisotropy or in-plane fields, the effective energy has been calculated numerically and fit to a polynomial in the energy. The fit was not very good, and in the present paper we show that this is because the effective energy has a logarithmic singularity at the saddle point. Fitting to a form with the singularity built in gives a much better fit, using the same number of parameters. This makes computation of oscillator linewidths and switching rates in spin-torque systems much easier.

Magnetic anisotropy and evolution of ground-state domain structures in bccFe81Ni19/Co(001)superlattices

The magnetic anisotropy and evolution of striped magnetic domain structures in bcc ${\text{Fe}}_{81}{\text{Ni}}_{19}/\text{Co}(001)$ superlattices with the total thickness ranging from 85 to 1370 nm has been studied by magneto-optical Kerr effect and magnetic force microscopy. At a thickness of about 85 nm [25 bilayers (BL)] the domains appear as stripe domains, typical for perpendicular anisotropy films, with the weak cubic anisotropy of the in-plane magnetization component stabilizing the stripe direction. The magnetic domain period strongly depends on the thickness of the superlattice. As the thickness increases, the equilibrium magnetization orients at oblique angles with respect to the film plane and continuously varies with the thickness from in-plane to out-of-plane. We first apply a simple phenomenological model which correctly predicts the transition from in-plane to out-of-plane magnetization as well as increasing domain period and saturation field with increasing BL number. The results indicate the presence of partial flux-closure domains at the film surface with the tilt angle continuously varying with the superlattice thickness. By solving a linearized Landau--Lifshitz equation together with Maxwell's equations in magnetostatic approximation for samples consisting of up to 1000 individual layers, we calculate the spin-wave dispersion and determine the stability conditions for the saturated ferromagnetic state. From these results the dependence of the saturation field on the number of layers is inferred and agrees well with the experiment. The uniaxial bulk anisotropy is attributed to distortions along the $c$ axis and the results further show evidence for the presence of an easy-plane interface anisotropy in these samples.

Direct observation of field and temperature induced domain replication in dipolar coupled perpendicular anisotropy films

Dipolar interactions in a soft/Pd/hard CoNi/ Pd 30 / Pd/ Co/ Pd 20 multilayer system, where a thick Pd layer between two ferromagnetic units prevents direct exchange coupling, are directly revealed by combining mag-netometry and state-of-the-art layer resolving soft x-ray imaging techniques with sub-100-nm spatial resolution. The domains forming in the soft layer during external magnetic field reversal are found to match the domains previously trapped in the hard layer. The low Curie temperature of the soft layer allows varying its intrinsic parameters via temperature and thus studying the competition with dipolar fields due to the domains in the hard layer. Micromagnetic simulations elucidate the role of CoNi/Pd magnetization, exchange, and anisotropy in the duplication process. Finally, thermally driven domain replication in remanence during temperature cycling is demonstrated.

New Trends in Magnetic Exchange Bias

New Trends in Magnetic Exchange Bias

Intrinsic perpendicular anisotropy and exchange biasing in CoO/permalloy multilayers

For permalloy/CoO multilayers, the exchange field and the coercivity of the perpendicular and longitudinal exchange biasing have been found to decrease with increasing temperature. After zero field cooling, perpendicular magnetic anisotropy is observed. For small permalloy layer thickness, perpendicular anisotropy film can be established at low temperatures.

Perpendicular exchange bias and magnetic anisotropy in CoO/permalloy multilayers

For permalloy/CoO multilayers, after perpendicular field cooling, perpendicular exchange bias has been observed below room temperature. For out-of-plane hysteresis loops, the exchange field and the coercivity at fixed temperatures are proportional to ${1/t}_{\mathrm{FM}}$ and ${1/t}_{\mathrm{FM}}^{3/2}$ ${(t}_{\mathrm{FM}}=\mathrm{permalloy}$ layer thickness), respectively. At the same time, the remanent ratio increases with decreasing ${t}_{\mathrm{FM}}.$ The induced effective uniaxial anisotropy changes as a linear function of ${1/t}_{\mathrm{FM}},$ as a symbol of an interface anisotropy. The interface anisotropic energy increases with decreasing temperatures. For samples with small ${t}_{\mathrm{FM}},$ perpendicular anisotropy films can be established at low temperature. The established perpendicular magnetic anisotropy and its remarkable temperature dependence might facilitate perpendicular magnetic recording and magneto-optical storage techniques.

Magnetic coercivity patterns for magnetic recording on patterned media

Magnetic patterns were created by locally altering the magnetic coercivity of a perpendicular anisotropy film by ion irradiation. The resulting pattern, after dc magnetizing, consists of regions having alternating up/down magnetization patterns in remanence. In this case, the magnetization of the soft magnetic material in the exposed segments is reversed and stabilized by the demagnetization fields of the adjacent unexposed media. A magnetic pattern generated in this manner can be used as a type of servo pattern for recording head positioning. An accuracy in positioning of better than 10 nm was demonstrated and deliberate magnetic patterns were written and read back by addressing individual 80 nm single-domain nanostructures.

Effective demagnetizing factors in ferromagnetic resonance equations

A simple method of deriving effective demagnetizing factors ( N x e, N y e) for the use of Kittel's ferromagnetic resonance formula is reported. The effective demagnetizing factors are expressed by an anisotropy energy ( G) and a static field direction ( θ, φ). By this derivation method, the resonance equations of thin films having a uniaxial or a four-fold anisotropy are obtained when a static field is rotated in the film plane. Six arrangements are calculated: (1) perpendicular anisotropy, (2) in-plane anisotropy, (3) cubic-crystal (0 0 1) face, (4) cubic-crystal (0 1 1) face, (5) cubic-crystal (1 1 1) face, and (6) oblique anisotropy films.

Effects of Improved Background Pressure on the Deposition of Co-Cr Perpendicular Anisotropy Films

The effects of improved background pressure on the deposition of Co-Cr perpendicular anisotropy films were investigated. To obtain a high vacuum, an aluminum alloy chamber was used, whose inside wall was subjected to the EL process ro reduce outgassing from the chamber. The background pressure reached 4×10-8 Torr after chamber baking. The pressure was changed by the addition of nitrogen to the atmosphere. In spite of the nitrogen addition, the percentage of H2O in the atmosphere was greater than that of N2. As a result, we were able to obtain a high perpendicular coercive force and a high crystallinity for the deposition of Co-Cr at a low background pressure. As regards the recording performance, a perpendicular double-layered medium with the Co-Cr recording layer deposited in a high vacuum showed larger output than one deposited in a low vacuum.

ＥＣＲスパッタ法により作製したＣｏ‐Ｃｒ垂直磁気異方性膜の磁気特性

Co-Cr single-layer perpendicular anisotropy films were prepared on polyimide films by sputtering using electron cyclotron resonance (ECR) microwave plasma. The dependence of the crystallographic and magnetic properties of the Co-Cr films on the Ar gas pressure, PAr, and the target-to-substrate distance, DT-S, was investigated. An 0.09,μm thick Co-Cr film with excellent c-axis orientation—namely, Δθ50 of 3.35°—, a high perpendicular magnetic anisotropy field of 4.5 kOe, and a high perpendicular coercivity Hc⊥ of 1530 Oe was achieved at a relatively high PAr of 8×10-2 Pa and a large DT-S of 230 mm. It may be surmised that ion bombardment with moderate energy in the PAr and DT-S conditions described above is suitable for the deposition of Co-Cr perpendicular anisotropy films.

Domains and domain nucleation in magnetron-sputtered CoCr thin films

The magnetic domain configurations in magnetron-sputtered CoCr thin films have been examined by Lorentz transmission electron microscopy. The thinnest (10 nm) films display in-plane 180 degrees domain walls, while thicker (50 nm) films exhibit out-of-plane dot-type domain structures. The dot domains were observed even in films that had not yet developed a columnar morphology. Intermediate thickness films show a featherlike contrast, indicating that both in-plane and out-of-plane magnetization components are present. Magnetization reversal is seen to occur by domain wall motion in films displaying in-plane anisotropy and by rotation for perpendicular anisotropy films. Intrinsic film stress was found to play a major role in determining the preferred magnetization direction, and thus the resulting magnetic domain configurations.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Method for providing a perpendicular recording medium for use with a ring-shaped recording and reproducing head

A perpendicular magnetic recording method uses a perpendicular magnetic recording medium. This medium comprises a perpendicular anisotropy film having perpendicular remanent magnetization Mr and longitudinal remanent magnetization Mr.sub.∥, wherein the value of the ratio Mr.sub.⊥ /Mr.sub.∥ is not less than 0.2, but is less than 1.

Vacuum deposition of Co-Cr perpendicular anisotropy films on polymer substrates treated with ion beam

The author investigated the effect of ion-beam treatment of polymer substrate surfaces on magnetic properties, microstructure, adhesive force, and recording performances of vacuum-deposited Co-Cr films. Films which are deposited on the ion-beam-treated substrates have higher H/sub c perpendicular to / than that of the films on untreated substrates without the treatment and have H/sub keff/ values high enough for perpendicular magnetic recording applications. With ion-beam treatment the adhesive force of the Co-Cr films is considerably improved, becoming of practical strength. The films deposited on the treated substrates reproduced voltage as high as that of Co-Cr films with Ti underlayer. Ion-beam treatment of the polymer substrates therefore makes both glow discharge treatment and the use of a Ti underlayer unnecessary and promises high manufacturing productivity.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>