CoPt Alloy Films Research Articles

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.

Variation of effective damping factor for CoPt-based alloy films with various atomic stacking structures

Ferromagnetic resonance (FMR) was observed for Co82Pt12 films with various process. And relation between effective damping factor (αeff) and compositional modulation (CM) in the atomic layers, and other magnetic properties to investigate the relationship between local variation of the magnetic properties and the αeff. The CM was changed by the substrate temperature Tsub and deposition rate of the Co-Pt alloy film. As a result, it was found that 1) For the Tsub dependence, αeff took maximum value of 0.22 at Tsub = 200 oC. 2) For the rate dependence, aeff linearly increases with 0.12 to 0.35 with increasing deposition rate from 0.3 to 2.0 nm/sec. 3) There was no similar tendency between aeff and CM for the Co-Pt alloy film with Tsub and rate change. Thus the CM is not dominant factor of the aeff for the compositionally modulated Co-Pt alloy film.

Influence of Thickness on the Metastable Ordered Phase Formation in CoPt and Co3Pt Alloy Films

Influence of Thickness on the Metastable Ordered Phase Formation in CoPt and Co3Pt Alloy Films

Control of <inline-formula> <tex-math notation="LaTeX">${\boldsymbol {c}}$ </tex-math></inline-formula>-Axis Orientation of <inline-formula> <tex-math notation="LaTeX">${\boldsymbol {L}}\textbf {1}_{\mathbf{0}}$ </tex-math></inline-formula> Ordered FePt, CoPt, and FePd Alloy Thin Films Deposited on MgO(001) Substrates

L1 0 films formed on (001)-oriented underlayers or substrates may involve (100) crystal with the c-axis lying in the film plane in addition to (001) crystal with the c-axis normal to the film surface. In this paper, FePt, CoPt, and FePd alloy films with L1 0 structure are prepared on MgO(001) single-crystal substrates. The c-axis orientation is accurately controlled to be perpendicular by introducing a thin MgO cap layer on magnetic films with disordered structure prepared by low-temperature deposition at 200 °C and by annealing the bilayer films at 600 °C. The phase transformation preferentially occurs along the perpendicular direction in the MgO/(FePt, CoPt, FePd)/MgO sandwich structures. Furthermore, the cap layer promotes L1 0 ordering. The order degrees of FePt, CoPt, and FePd films with and without cap layers, (S w/ , S w/o ), are (0.82, 0.58), (0.36, 0.30), and (0.72, 0.62), respectively. The FePt film with cap layer shows a strong perpendicular magnetic anisotropy and a very low in-plane coercivity because of which the film does not include L1 0 (100) crystal and possesses a high-order degree.

FePd, FePt, and CoPt alloy epitaxial thin films with flat surface grown on MgO(111) substrate

FePd, FePt, and CoPt alloy epitaxial films of 40 nm thickness are prepared on MgO(111) singlecrystal substrates by employing two different methods. One is a one-step method consisting of hightemperature deposition at 600 °C and the other is a two-step method consisting of low-temperature deposition at 200 °C followed by annealing at 600 °C. Although the preparation method is different, similar final crystal structures are realized for all the film materials. FePd and FePt alloy films grow on the substrates with six L 10 (111) variants, whose c -axes are about 35° canted from the substrate surface and rotated around the film normal by 60° each other. The order degrees of FePd and FePt films prepared by one- and two-step methods, (S one-step , S two-step ), are estimated to be (0.25, 0.33) and (0.08, 0.15), respectively. On the contrary, L 10 ordered phase formation is not recognized for CoPt alloy films. The films prepared by one-step method have rough surfaces surrounded by side facets, whereas the films prepared by two-step method have very flat surfaces with the arithmetical mean roughness of 0.3 nm. The two-step method is useful for preparation of L 10 ordered films with flat surface.

Structure analysis of CoPt alloy film with metastable ordered phases ofL11andBhformed on Ru(0001) underlayer

CoPt alloy films of 40 nm thickness are prepared on MgO(111) substrates with and without Ru(0001) underlayer at 300 °C by radio-frequency magnetron sputtering. CoPt films with the close-packed plane parallel to the substrate surface grow epitaxially on the Ru underlayer as well as on the MgO substrate. Flat surfaces with the arithmetical mean roughness value of 0.2 nm are realized for both films. The crystal structure is determined by considering the atomic stacking sequence of close-packed plane and the order degree. The film formed on MgO substrate consists of an fcc-based L 11 ordered crystal, whereas the film grown on Ru underlayer involves an hcp-based B h ordered crystal in addition to the L 11 ordered crystal. The order degrees of films formed on MgO substrate and Ru underlayer are 0.30 and 0.34, respectively. The L 11 crystal consists of two variants whose stacking sequences of close-packed plane are ABCABC… and ACBACB…, while the B h crystal is a single-crystal with the stacking sequence of ABAB… Formation of B h crystal is promoted on the Ru underlayer. The film formed on Ru underlayer shows a strong perpendicular magnetic anisotropy reflecting the magnetocrystalline anisotropies of L 11 and B h crystals.

Batch Patterning of SubMillimeter Features in Hard Magnetic Films Using Pulsed Magnetic Fields and Soft Magnetizing Heads

In this paper, we report a method to impart spatially defined magnetic patterns into hard magnetic films using pulsed magnetic fields. Differing from prior investigations on magnetic patterning, a single magnetizing head is used rather than a sandwich structure with two magnetizing heads. Arrays of 250μm-wide and 125μm-wide magnetic stripes with alternating out-of-plane polarity (perpendicular magnetization) are patterned in a 10μm-thick Co-rich Co-Pt alloy (Co80Pt20) film using three different microstructured, 2-mm-thick, Fe-Co magnetizing heads and pulsed magnetic fields ranging from 0.1 to 1 T. A magneto-optical imaging film is used to view the magnetization patterns at the surface of the Co-Pt film. Scanning Hall probe measurements of the resulting magnetic structures indicate a ~5mT peak magnetic field 100 m above the surface which compares favorably with numerical simulations of the field patterns at the same height.

Magnetic force microscope tips coated with FePd, FePt, and CoPt alloy films

Magnetic force microscope (MFM) tips are prepared by coating Si tips of 4nm radius with FePd, FePt, and CoPt films of 80nm thickness at room temperature followed by annealing at 600°C. The spatial resolution and the switching field are investigated. The resolution of about 9nm is realized for all the tips. On the contrary, the switching fields of FePd-, FePt-, and CoPt-coated tips are 117.4, 37.8, and 268.6 kA/m, respectively. The crystal structure and the magnetic properties of coated magnetic thin films prepared on flat Si substrates are measured and the interrelationships with the MFM tip characteristics are briefly investigated.

Magnetic Force Microscope Tip with High Resolution and High Switching Field Prepared by Coating Si Tip with L11 Ordered CoPt-Alloy Film

Magnetic Force Microscope Tip with High Resolution and High Switching Field Prepared by Coating Si Tip with L11 Ordered CoPt-Alloy Film

Effect of Ag Additive on Magnetic Properties and Microstructure of the Copt-Sinx/Ag Granular Thin Films

Abstract When the Ag layer was introduced under the CoPt alloy film annealed at 700 °C, the CoPt/Ag films had great out-of-plane squareness (S ⊥ ), out-of-plane coercivity (H c⊥ ), and saturation magnetization (M s ). The SiN x ceramic materials were co-sputtered with CoPt on the Ag under layer then annealed to reduce the grain size of CoPt films. The particle size of CoPt was about 10 nm with 46.2 vol. % SiN x content. Furthermore, Ag was added into the CoPt-SiN x layer of CoPt-SiN x /Ag films to reduce the ordering temperature of L1 0 CoPt (fct phase) films.

Microstructures and Magnetic Properties of ${\rm CoPt{-}SiN}_{x} {-}{\rm Ag}$ Granular Thin Films With Ag Underlayer

When the Ag layer was introduced under the CoPt alloy film which annealed at 700°C, the CoPt/Ag films had large out-of-plane squareness ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">⊥</sub> ), out-of-plane coercivity ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">H</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c⊥</sub> ), and saturation magnetization ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</sub> ). They were 0.95, 1432 kA/m, and 390 emu/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> , respectively. Further, the SiN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> ceramic materials were cosputtered with CoPt on the Ag underlayer at room temperature and then annealed to reduce the grain size of CoPt films. From the field-emission gun high-resolution transmission electron microscope analysis, the particle size of CoPt was about 10 nm as the SiN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> content was 46.2 vol.%. Furthermore, Ag was added into the CoPt-SiN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> films to reduce the transformation temperature of CoPt films from face-centered-cubic to face-centered-tetragonal structure.

Correlation of crystallographic structure and magnetic properties of electrodeposited Co-rich Co-Pt films

Co-rich Co-Pt alloy films have been grown by electrodeposition on Au buffer layer. Detailed texture measurements have been performed in order to understand the dependence of magnetic properties on phase composition and texture. X-ray analysis in Bragg-Brentano geometry shows that films seem to consist of hexagonal closed packed (hcp) or fcc phase. Due to similar lattice spacing of the densely packed planes of both possible phases, the (111) fcc and (002) hcp planes of Co-Pt and also the higher order reflections coincide. As a result, the information supplied by X-ray diffraction was of limited value. Hence pole figure measurements are used which determine the directional distribution of planes with a selected spacing. Here we present an approach to use pole figure measurements for phase identification. We show that this method allow a direct correlation with the magnetic properties, in particular with the magnetocrystalline anisotropy and magnetic texture.

Effects of layering and magnetic annealing on the texture of CoPt films

The effect of magnetic field annealing of magnetron sputtered CoPt alloy films and Co/Pt bilayers on the crystallographic texture of the obtained chemically ordered (L1 0) CoPt films is presented. In CoPt alloy films the main effect of the magnetic field is to suppress (1 1 1) growth in the early stages of L1 0 formation whereas the development of (0 0 1) versus (1 0 0) texture is related to chemical ordering strain. A higher degree of (0 0 1) texture is obtained by magnetically annealing Co/Pt bilayers since the initial (1 1 1) texture in the as-sputtered films is avoided and Co–Pt alloying occurs in the presence of the magnetic field.

A computational study of kinetic phase diagrams for CoPt alloy films during epitaxial growth

We have studied the kinetic processes of the epitaxial growth for CoPt alloy films using the master equation method. The kinetic phase diagrams of CoPt alloy films which show the phase formation conditions during the epitaxial growth are determined. From the kinetic phase diagrams, we find that the [001] ordered structure is much easy to be grown at high temperature while the [100] ordered structure is easy to be grown at low temperature although both the [001] and [100] ordering could be the equilibrium ground states. The atomic deposition, ordering and surface segregation lead to a rich variety of phases in epitaxial growth. The surface segregation is found to enhance the [001] ordering and leads to the formation of the [001] ordered phase at high temperature.

Growth rate dependence of the extrinsic magnetic properties of electrodeposited CoPt films

The magnetic properties and the magnetization process of electrodeposited thick films of Co-rich CoPt alloys are studied with particular emphasis on the effects of growth rate, controlled by varying the plating current density, and of lateral confinement, analyzing patterned micro-cylinders. We find that varying the plating current density has virtually no effect on the composition of the samples, and hence on the intrinsic magnetic properties, a substantial increase of both coercivity and squareness is obtained when the current is raised. The films are fine-grained, oriented polycrystals with typical grain sizes in the range 50–150 nm, depending on the growth rate. The complex magnetization process is studied in detail by Magnetic Force Microscopy and shown to be governed by interaction domains. It is shown that further improvement of the squareness can be obtained by exploiting the lateral confinement in patterned samples.

Nanopatterned CoPt alloys with perpendicular magnetic anisotropy

CoPt alloy films with perpendicular magnetic anisotropy were grown on SiO2 nanoparticle arrays with particle sizes as small as 10 nm. In order to induce perpendicular magnetic anisotropy in the CoPt film, a MgO seed layer was sputter deposited. Despite the fact that neighboring CoPt film caps are interconnected, individual caps appear as single domain and for most of them their magnetization orientation can be reversed individually. This behavior might be caused by domain wall nucleation and pinning preferentially at the rim of each cap. Thus, arrays of magnetic caps with defined pinning sites can be considered as a percolated perpendicular medium.

Controlling the magnetic anisotropy of CoPt∕AlN multilayer films

The magnetic anisotropy of sputter-deposited CoPt∕AlN multilayer films has been studied in contrast with the multilayer films consisting of magnetic and noble metals. It has been found that the as-deposited multilayer film shows enhanced in-plane magnetic anisotropy compared with CoPt alloy film of equivalent thickness. However, upon thermal annealing in a vacuum, the film undergoes a smooth transition of magnetic anisotropy from in-plane to perpendicular direction; moreover, the anisotropy field and coercivity can be controlled by controlling the annealing temperature. Structural characterization has revealed that the perpendicular magnetic anisotropy of the annealed CoPt∕AlN multilayer films is mainly of magnetoelastic origin.

Co/Pt multilayer structures on the crystal MgO and Si substrate as a media for perpendicular magnetic recording

In this work, the crystal structure and hysteretic magnetic properties of equiatomic single-crystal CoPt/MgO films prepared by magnetron sputtering and their modifications after heat treatment are studied. A perpendicular magnetic anisotropy is obtained in annealed films in a film thickness range of 2 < d ≤ 16 nm. The correlation between the magnitude of magnetocrystalline anisotropy constant of CoPt films and the order parameter of the L10 superstructure of these alloys is ascertained. The influence of the single-crystal MgO substrate on the structure and magnetic properties of the films of equiatomic CoPt alloys is also investigated.

Kinetic-Anisotropy-Induced Ordering-Orientation Transition in Epitaxial Growth: A Method to Synthesize Ordering-Orientation Superlattices

The epitaxial growth has a distinct kinetic feature that the lateral surface diffusion is faster than the longitudinal bulk diffusion. We show there is an ordering-orientation transition of alloy films with the change of growth rate due to this kinetic anisotropy. As an example, we have calculated the epitaxial growth of CoPt alloy films on the Pt buffer layer. We show the ordered structure of CoPt films changes from the L1(0) [001] (a compositional modulation along the [001] direction) variant to the L1(0) [100] variant with the increase of growth rate. This ordering-orientation transition also occurs with the decrease of temperature at adequate growth rate. Based on this mechanism, we propose a simple method to synthesize the ordering-orientation superlattices.

Alternate monatomic layer sputter deposition of FCT (L1 0-type) ordered FePt and CoPt films

FePt and CoPt alloy films with FCT(0 0 1) structural phase were deposited by sputtering. The films were formed by periodically alternating monatomic layers deposition of Fe(Co) and Pt, on MgO(1 0 0) substrates, with and without a Pt buffer layer, at substrate temperatures ( T S) ranging from 200 to 500 °C. The X-ray diffraction patterns show the formation of the FCT crystalline structure in FePt films deposited at T S ⩾ 2 0 0 ° C . Hysteresis measurements, using Vibrating Sample Magnetrometry, show that FePt and CoPt films with predominant FCT(0 0 1) phase are perpendicularly magnetized. The long range order parameter S = 0.5 8 demonstrates the good degree of chemical order in FePt film prepared at T S = 4 0 0 ° C . The Experimental results demonstrate that the alternate monatomic layer sputter deposition technique is very effective to grow Fe(Co)Pt ordered alloy at moderate T S.