CoPt Films Research Articles

Annealing temperature dependence of the bulk spin–orbit torque in CoPt film

The recent discovery of bulk spin–orbit torques (SOTs) in magnetic single layers has attracted much recent attention. However, currently, it remains elusive as to how to understand and how to tune such bulk SOTs. In this study, we study the tunability of the bulk SOTs in the CoPt films, by the annealing temperature. Our results show that the field-free switching can be realized after annealing and optimized at 450 °C. The switching performance is consistent with the out-of-plane SOT efficiency, which also appears after annealing and maximized at 450 °C. The crystal-axis dependence of the switching performance reveals that besides the threefold modulation by the crystal-axis, the switching ratio also contains a contribution that is independent of the crystal-axis, which is different from that in single-crystal films. Our results can help the understanding of the mechanism of SOT and point to the developing of SOT devices.

Development of Ultra-Thin CoPt Films With Electrodeposition for 3-D Domain Wall Motion Memory

Development of Ultra-Thin CoPt Films With Electrodeposition for 3-D Domain Wall Motion Memory

Controllable unidirectional magnetoresistance in ferromagnetic films with broken symmetry

Spin-related unidirectional magnetoresistance (UMR) exhibits nonreciprocal transport to either charge current or magnetization reversal, providing a potential application for direct electrical readout of in-plane magnetization using simple two-terminal geometry. To achieve such a UMR, it is usually believed that an adjacent heavy metal or other materials with strong spin-orbit coupling (SOC) as the spin polarizer is indispensable, leading to most efforts on multilayer structures rather than single-layer ferromagnetic films. Here, we report the observation of UMR in a single CoPt film by introducing a vertical composition gradient to break the structural inversion symmetry. Moreover, unlike conventional heavy-metal/ferromagnetic-metal bilayer films, the UMR in the CoPt film with a positive Co composition gradient is controllable as a function of current amplitude, which shows the decrease, sign reversal, and then enhancement with the increment of current density. These results reveal two competing UMR mechanisms with opposite signs in electrical current dependency, i.e., bulk Rashba effect induced by composition gradients and anomalous Nernst effect driven by a vertical temperature gradient. Our work provides a promising way to manipulate the UMR in a single ferromagnetic film with the combination of charge-spin conversion and magnetothermal effect.

Creating Skyrmions in Thin CoPt Films with an Atomic Force Microscope Probe

Creating Skyrmions in Thin CoPt Films with an Atomic Force Microscope Probe

Formation of skyrmions in thin CoPt films with an atomic force microscope probe

Methods of magnetic force microscopy have been developed that make it possible to visualize the evolution of the domain structure when scanning a sample with a magnetic probe. These methods were used to study the processes of formation of skyrmions in thin CoPt films, a characteristic feature of which is the presence of the Dzyaloshinskii–Moriya interaction. A change in the position, shape, and size of skyrmions under the action of a spatially inhomogeneous magnetic field of the probe has been experimentally demonstrated.

Artificial single skyrmion and biskyrmion states achieved in magnetic nanodot pairs by micromagnetic simulation

Using the shape anisotropy and magnetic field orientation changes, we realized the chirality and polarity controllable artificial skyrmion through the micromagnetic simulation software OOMMF. Combining the CoPt film with perpendicular magnetic anisotropy, and our self-designed Co nanodot pairs structure carrying magnetic vortex with controlled chirality and polarity, skyrmion states could be achieved by adding the external magnetic field. Single skyrmion state can be formed when 200 mT in-plane magnetic field is introduced; while the magnetic field intensity is reduced to 140 mT, double skyrmions state can be formed. The detail forming process of skyrmion is studied by studying the key nodes in the saturation magnetization and demagnetization process. It is also found that the polarity of skyrmion is stable even under the action of the 0.6 T out-of-plane magnetic field. By comparing the influence of Co disk thickness on the formation of skyrmion, it is found that 45 ∼ 60 nm Co disk thickness is suitable for forming stable skyrmion.

Proximity effect in ferromagnetic structures InGaAs/GaAs/CoPt

Ferromagnetic influence of a thin (~8 nm) surface CoPt layer on the circular polarization of the InGaAs/GaAs quantum well photoluminescence is observed in structure GaAs/InGaAs/GaAs/Al2O3 (1 нм)/CoPt with narrow GaAs spacer dS = 5 nm, while electroluminescence is polarized in the whole range of dS = 5–100 nm. It is suggested that the short-range proximity effect is determined by overlap of electrons wave functions with the nearby ferromagnetic CoPt film.

Manipulating the Micromagnetic Structure of Multiphase CoPt Thin Films by Varying Layer Thicknesses

The magnetic properties and micromagnetic structure of [Co/Pt]10 multilayer magnetic films formed by independent variation of the Co and Pt layer thicknesses have been studied. The possibility of the film magnetization parameters manipulation was shown. It is found that the micromagnetic structure of the layers is significantly modified with a change in the thickness of the layers which correlates with the magnetization data. In particular, magnetic force microscopy revealed a system of magnetic skyrmions for a number of structures. The skyrmion density was found to be dependent on the growth conditions which in turn correlates with the shape of the magnetic hysteresis loop. Changing the thickness of the Co and Pt layers makes it possible to control the density of skyrmions in the range from 0.2 to 10.5 mkm-2. Keywords: micromagnetic structure, skyrmions, CoPt films, electron beam evaporation.

Memristive switching by bulk spin–orbit torque in symmetry-broken ferromagnetic films

Bulk spin–orbit torque (SOT) driven memristive switching is demonstrated in perpendicularly magnetized CoPt alloy films by introducing a composition gradient to break the inversion symmetry in the out-of-plane direction. An analog-like magnetization switching consisting of multiple intermediate states can be robustly formed by applying current pulses with different amplitudes or repetition number. The programmable magnetization manipulation is also presented in a continuous manner to simulate the weight update of biology synapses by means of ramped pulses. Furthermore, controllable switching probability dependent on pulse frequency or repetition number is manifested to emulate the integrate-and-fire function of a biological neuron. With the capability to reproduce both functionalities of synapses and neurons in commonly used CoPt films, it will be a promising candidate to advance the SOT-based neuromorphic hardware.

Quantifying the Dzyaloshinskii-Moriya Interaction Induced by the Bulk Magnetic Asymmetry.

A broken interfacial inversion symmetry in ultrathin ferromagnet/heavy metal (FM/HM) bilayers is generally believed to be a prerequisite for accommodating the Dzyaloshinskii-Moriya interaction (DMI) and for stabilizing chiral spin textures. In these bilayers, the strength of the DMI decays as the thickness of the FM layer increases and vanishes around a few nanometers. In the present study, through synthesizing relatively thick films of compositions CoPt or FePt, CoCu or FeCu, FeGd and FeNi, contributions to DMI from the composition gradient-induced bulk magnetic asymmetry (BMA) and spin-orbit coupling (SOC) are systematically examined. Using Brillouin light scattering spectroscopy, both the sign and amplitude of DMI in films with controllable direction and strength of BMA, in the presence and absence of SOC, are experimentally studied. In particular, we show that a sizable amplitude of DMI (±0.15 mJ/m^{2}) can be realized in CoPt or FePt films with BMA and strong SOC, whereas negligible DMI strengths are observed in other thick films with BMA but without significant SOC. The pivotal roles of BMA and SOC are further examined based on the three-site Fert-Lévy model and first-principles calculations. It is expected that our findings may help to further understand the origin of chiral magnetism and to design novel noncollinear spin textures.

Magneto-transport properties of perpendicular magnetization CoPt/VO2 bilayer films grown on glass substrate

VO2 films were prepared on glass substrates by a two-step method, i.e., magnetron sputtering of vanadium in an atmosphere of Ar + O2 gas mixture and followed by in-situ annealing. CoPt films were then sputter-deposited on the VO2 films to obtain the CoPt/VO2 bilayer films. The VO2 films change from monoclinic phase to rutile phase with increasing temperature from room temperature to 90 °C, accompanied by an abrupt change in resistivity. The CoPt/VO2 bilayer film with 3 nm thick CoPt layer shows strong perpendicular magnetic anisotropy and anomalous Hall effect is observed when the magnetic field is applied in the out-of-plane direction. The magneto-transport properties are studied in relation to the phase transition of the VO2 layer.

Lightweight macroporous Co-Pt electrodeposited films with semi-hard-magnetic properties

Macroporous, partially L10-ordered Co-Pt films with nearly equiatomic composition were successfully synthesized by electrodeposition from an aqueous sulfate–chloride electrolyte on colloidal crystal-templated substrates, followed by annealing in vacuum. The colloids deposited on the substrate consisted of amidine-functionalized polystyrene spheres of 215 ± 13 nm in diameter, which were self-assembled by electrophoresis. As-deposited Co-Pt films obtained after the removal of the spheres showed a highly-packed arrangement of macropores. Structurally, the films showed the A1-disordered face-centered cubic (fcc) Co-Pt solid solution, accompanied by small amounts of fcc/hexagonal close-packed (hcp)–Co. Upon annealing at 600 °C, the A1-disordered phase partly transformed into the L10-ordered (face-centered tetragonal, fct) phase. As a result, the coercivity significantly increased from 148 Oe to 1328 Oe. Importantly, the porosity of the films was preserved after annealing. Optimum annealing temperature and time were selected on the basis of a prior parametric study with electroplated dense counterparts. This work demonstrates that the combination of colloidal crystal templating and electrodeposition is a very convenient pathway towards lightweight semi-hard magnets with potential technological applications in automotive and aerospace industries, portable sensors or spectrometers, magnetic levitation systems, or magnetoelectric devices, among others.

Magnetic Properties of Electrodeposited Cobalt-Platinum (CoPt) and Cobalt-Platinum-Phosphide (CoPtP) Thin Films.

CoPt and CoPtP thin films were synthesized using direct current (DC) aqueous electrodeposition from weak alkaline solutions. The basic plating solutions of binary CoPt thin films consisted of cobalt pyrophosphate [Co2P2O7] and chloroplatinic acid [H2PtCl6]. Various amounts of sodium hypophosphite [NaH2PO2] was added to deposit ternary CoPtP thin films. The film composition was adjusted by varying the several electrodeposition parameters including electrolyte composition, solution pH, and current density and correlated to their microstructure and magnetic property (i.e. coercivity and squareness). For the binary CoPt thin films, the maximum coercivities [in-plane coercivity (Hc,//) = ∼1,600 Oe, and perpendicular coercivity (Hc,⊥) = ∼2,500 Oe] were obtained from electrolytes containing 0.01 M H2PtCl6 + 0.04 M Co2P2O7 at current density (CD) of 7.5 mA cm−2. In the case of ternary CoPtP electrodeposits, the maximum coercivities (Hc,// = ∼2,600 Oe, and Hc,⊥ = ∼3,800 Oe) were achieved from baths containing 0.015 M H2PtCl6, 0.07 M Co2P2O7, 0.8 M NaH2PO2 at CD of 7.5 mA cm− 2 and solution pH 9. It was suggested that microstructure and magnetic properties are affected not only by the type of substrate but also by chemical compositions and electrodeposition conditions.

Study of the Transport Properties and Domain Structure of Thin CoPt Films Obtained by Electron Evaporation

Study of the Transport Properties and Domain Structure of Thin CoPt Films Obtained by Electron Evaporation

Polar Kerr effect and perpendicular magnetic anisotropy in Fabry–Pérot cavity containing CoPt/AZO magneto-optical interference layer

We investigate the magneto-optical (MO) polar Kerr enhancement of Fabry–Pérot cavities with perpendicular magnetic anisotropy for chemical and biological sensing applications. The MO cavities consist of stacked films with a half-mirrored surface layer, a dielectric interference layer, a total-reflection mirror, and a magnetic metal layer. Co80Pt20 and Al-doped ZnO (AZO) thin films were utilized as the magnetic and interference layers, respectively. The cavities produce two types of MO enhancement depending on the thickness of the magnetic layer. The sample with a thick magnetic layer as a total-reflection mirror generates a single peak in the polar Kerr spectrum. Contrarily, inserting a several-nanometer-thick CoPt layer into the AZO layer produces a resonant-type spectrum. Although both samples generate large MO enhancement factors (>50), the magnetic properties significantly differ. The AZO/CoPt/AZO interference layer controls the crystalline and interface conditions and produces a square-shaped out-of-plane hysteresis loop with a large Kerr rotation angle. Moreover, the use of an ultrathin CoPt film significantly reduces the saturation magnetization field. Thus, Fabry–Pérot cavities with a CoPt/AZO interference layer may serve as high-sensitivity, low-energy-consumption sensor elements.

Effect of Ion Irradiation on the Magnetic Properties of CoPt Films

Effect of Ion Irradiation on the Magnetic Properties of CoPt Films

Влияние ионного облучения на магнитные свойства пленок CoPt

The possibility of using He+ ion implantation with an energy of 20 keV for modifying the domain structure and magnetic properties of CoPt films formed by electron beam evaporation with different compositions - Co0.45Pt0.55 and Co0.35Pt0.65 - has been investigated. For the irradiated CoPt samples of both compositions, a decrease in the coercivity (narrowing of the hysteresis loop on the magnetic field dependences of the Faraday angle and magnetization) with an increase in the He+ ion fluence from 2×1014 to 4×1014 cm−2 was found. In this case, the remanent magnetization of the Co0.35Pt0.65 films coincides with the value of saturation magnetization, while for Co0.45Pt0.55, a decrease in the remanent magnetization is observed. Magnetic force microscopy has shown that for the Co0.45Pt0.55 alloy, with an increase in the ion fluence up to 3 × 1014 cm−2, the largest number of isolated circular domains (skyrmions) is formed, while for He+ irradiation with a fluence of 4×1014 cm−2 for Co0.35Pt0.65, in addition to isolated circular domains, 360-degree domain walls (1D skyrmions) are observed. At the same time, the study of CoPt films by the Mandelstam-Brillouin spectroscopy method revealed an increase in the shift between the Stokes and anti-Stokes components of the spectrum and thus a significant increase of the Dzyaloshinsky-Moriya interaction for the irradiated samples. Simulation using the SRIM software showed that the applied ion irradiation causes the asymmetric mixing of Co and Pt atoms and thus, this may underlie the mechanism of the of the ion irradiation on magnetic properties and domain structure in CoPt films.

Enhancing Magneto-Ionic Effects in Magnetic Nanostructured Films via Conformal Deposition of Nanolayers with Oxygen Acceptor/Donor Capabilities

Effective electric-field manipulation of the magnetic properties of nanostructured metallic alloys exhibiting inter-grain porosity (i.e., channels) conformally coated with insulating oxide nanolayers is demonstrated. Nanostructured Co-Pt films are prepared by electrodeposition (ED) and subsequently coated with either AlOx or HfOx by atomic layer deposition (ALD) to promote magneto-ionic effects (i.e., voltage-driven ion migration) during electrolyte-gating. Pronounced variations in coercivity (HC) and magnetic moment at saturation (mS) are observed at room temperature after biasing the heterostructures. The application of a negative voltage results in a decrease of HC and an increase of mS, whereas the opposite trend is achieved for positive voltages. Although magneto-ionic phenomena are already observed in uncoated Co-Pt films (due to the inherent presence of oxygen), the ALD oxide nanocoatings serve to drastically enhance the magneto-ionic effects due to partially reversible voltage-driven oxygen migration across the interface between AlOx or HfOx and the nanostructured Co-Pt film. Co-Pt/HfOx heterostructures exhibit the most significant magnetoelectric response at negative voltages, with an increase of mS up to 76% and a decrease of HC by 58%. The combination of a nanostructured magnetic alloy and a skin-like insulating oxide nanocoating is shown to be appealing to enhance magneto-ionic effects, potentially enabling electrolyte-gated magneto-ionic technology.

Sputtered high perpendicular magnetic anisotropy CoPt thin film on flexible substrate at low temperature

In this study, CoPt alloy thin films with alternative sputtering of 0.4 nm-Co and 0.5 nm-Pt layers, [Co0.4/Pt0.5]n, on Pt underlayers were sputtered on rigid glass and soft PI (Polyimide) substrates at 100°C. The effects of repetition (n) of the [Co0.4/Pt0.5]n films on the magnetic properties and microstructures of [Co0.4/Pt0.5]n films were investigated. Results showed that [Co0.4/Pt0.5]4 film had a high perpendicular magnetic anisotropy (PMA (S⊥>0.98)) and a magnetocrystalline anisotropy constant (Ku∼1.12×107 erg/cm3), which were close to the properties of L11-CoPt film prepared at higher temperatures (∼300°C) as seen in previous works. Increasing repetition also increased residual stress and further reduced perpendicular magnetic properties. Thus, better magnetic properties would be obtained in the thinner case. The outstanding PMA properties of CoPt film still well kept as the rigid glass was replaced by flexible PI substrate. In this work, perpendicular magnetic CoPt film could be prepared at lower temperatures (∼100°C) on soft PI substrate. It strongly enhanced low temperature applications of hard magnetic alloy thin films, such as magnetic electronic components with high Ku and PMA on flexible substrate.

Micromagnetic and Magnetooptical Properties of Ferromagnetic/Heavy Metal Thin Film Structures

The magnetic properties of CoPt, CoPd, and FePd thin films with different contents of a ferromagnetic material fabricated by electron beam evaporation have been compared. The micromagnetic structure investigations have shown a decrease in the magnetic domain size with an increase in the Co content in the CoPt and CoPd films, which has been attributed to the presence of two magnetic phases in the films. The magnetooptical studies have shown anomalously large Faraday angles and the absence of Kerr effect in the CoPt films, which make them attractive for application in optics. In the FePd films, the minimum coercivity values have been obtained from the magnetic-field dependences of the magnetization, which has also been related to the magnetic structure features.