FePt Thin Films Research Articles

Self-Induced Spin Pumping and Inverse Spin Hall Effect in Single FePt Thin Films

Self-Induced Spin Pumping and Inverse Spin Hall Effect in Single FePt Thin Films

Large magnetocrystalline anisotropic energy and its impact on magnetostriction of L10-FePt

Abstract The origin of magnetocrystalline anisotropic energy (MAE) guided by spin–orbit coupling in the L10-FePt alloy was analyzed. Correlation of MAE with the calculated magnetoelastic constants in the single and polycrystal model was determined by means of first-principles calculations using density functional theory (DFT). More specifically, a systematic analysis of the large MAE and magnetostriction coefficients (λ’s) were done by means of post-processing of eigenvalues (orbital energies) and eigenfunctions (orbital occupancies). Our numerical analysis includes the convolution of the projected wave-function (density of states) of each orbital of the Fe and Pt sub-lattices into their orbital energies, which in principle should be valid for any solid regardless of the strength of the MAE. The determined anisotropic magnetostriction coefficients are of the λ ∼ 10 − 4 –10–3 order for several crystallographic directions; which is in accord to the known experimental data. However, the polycrystalline model based on the uniform stress approximation leads to a decrease of the λ by two orders in the overall magnetostrictive behavior ( λ ∼ 10 − 5 ). Such a large difference is not unexpected as the magnetostriction for polycrystalline FePt thin films were recorded as very low, thus validating the robustness of the current theoretical proposition of analyzing MAE.

Magnetization Reversal and Direct Observation of Magnetic Domains on FePt Thin Films

Magnetization Reversal and Direct Observation of Magnetic Domains on FePt Thin Films

Microstructure and magnetic properties of FePt thin films for the coating of magnetic force microscopy tips

Microstructure and magnetic properties of FePt thin films for the coating of magnetic force microscopy tips

Oxidation-induced surface stress on texture development of thin films: The (001) FePt case

Residual stress is an inevitable characteristic in all kinds of thin film materials. The incorporation of stress and physical properties may also be of importance as a stress-driven functional characteristics, particularly in the context of engineering texture and anisotropies in magnetic thin film materials. A mechanism explaining the (001) preferred orientation in the rapidly annealed FePt thin films is proposed on the basis of the experimental study of the oxidation-induced surface stress measurements. By adopting a grazing incidence anomalous x-ray diffraction based on synchrotron radiation, the depth-resolved stress distribution of the FePt films was investigated. A substantial in-plane tensile stress induced by surface oxidation was observed in the FePt thin films. The oxidation layer, with an appropriate thickness, maintains the accumulated tensile stress, thereby inducing the (001) preferred orientation in FePt. The surface stress relaxation was observed in the FePt films with the processes lacking oxygen or in a poor vacuum condition.

Thermal evolution of low-temperature magnetic texture modulation in fept thin films by direct visualization

Patterns of ferroic domains and domain walls are being intensively studied to implement new logic schemes. Any technological application of such objects depends on a detailed understanding of them. Using low-temperature magnetic force measurements (10–300 K), the evolution of ferromagnetic stripes on equiatomic FePt thin films is thoroughly analyzed. Since FePt is known to develop a transition from in-plane homogeneous magnetization to stripe domains upon varying its thickness, multiple samples are studied demonstrating the well-established reduction upon thickness decrease and a non-trivial dependence on temperature. Moreover, the room-temperature uniform distribution of the pattern evolves into a distorted one upon temperature cycling. Finally, dissimilar stripe patterns are obtained upon reducing and increasing temperature indicating the states are dependent on the history of applied stimuli rather than the parametric conditions.

The effect of Ag additives on the structure of Fe-Pt films

Homogeneous thin films of FePt and Fe-(Pt,Ag) were obtained by modernized three-electrode ion-plasma sputtering. The film thickness was 220–450 nm. In this case, the calculated cooling rate reached ~1012–1014 K/s. The structure of films was studied by X-ray diffraction and electron microscopy. It was found that nanocrystalline and amorphous phases were formed in freshly deposited films. It was shown that the resulting metastable structures were stable when heated to 670–890 K, depending on the composition. It was found that Ag additives reduced the coercive force of the films. It was shown that heat treatment increased the coercive force up to 36 kA/m in FePt films and up to 10 kA/m in Fe-(Pt,Ag) films. The conditions for obtaining films with low values of the temperature coefficient of electrical resistance (~1.1∙10-5 K-1) were determined.

Quantitative magnetization measurements of magnetic particles with FePt standard samples

Micrometer-sized magnetic particles have been widely used in magnetic force microscopy, magnetic resonance force microscopy, and bio-sensing. To quantitatively interpret the data obtained with magnetic particles, it is important to know the magnetic properties of the particles. However, the magnetic moment of individual particle is usually too small to be measured by common instruments for samples with large volume. Here, we present a method to characterize magnetic microspheres using patterned FePt thin films as standard samples. The FePt thin film in the L10 phase has perpendicular magnetic anisotropy, and the patterned features can be magnetized to near single-domain magnets, which make them suitable standards for magnetic sphere calibration with magnetic force microscopy. Multiple linear regression is used to analyze the frequency shift images and obtain the effective dipole moment of the spheres. The position of the dipole moment is obtained by minimizing the residuals in multiple linear regression with a gradient descent algorithm. Three NdFeB spheres of different diameters were measured. It was found that the magnetization increases with the increase in the diameter of the sphere, possibly due to the weakening of ferromagnetism on the surface.

Magnetization reversal modes and coercive field dependence on perpendicular magnetic anisotropy in FePt thin films

The competition between shape and perpendicular magnetic anisotropies in magnetic thin films gives rise to unusual magnetic behaviors. In ferromagnetic thin films, the presence of an out-of-plane component of the magnetic anisotropy may induce a transition from planar to stripe-like magnetic domains above a critical thickness, . In this article, we present a detailed study of the magnetization switching mechanism in FePt thin films, where this phenomenon is observed. Using micromagnetic simulations and experiments, we found that below the reversal mechanism is well described by the two-phase model while above this thickness the magnetization within each stripe reverses by coherent rotation. We also analyzed the out-of-plane component of the magnetic anisotropy and its temperature dependence, probing that substrate-induced strains are responsible for the abnormal coercive field behavior observed for FePt films with .

Electrodeposition of Equiatomic FePt Permanent Magnets from Non-Aqueous Electrolytes Based on Ethylene Glycol

The highly ordered L10 hard-magnetic phase of the equiatomic FePt alloy is of significant interest for a great number of magnetic applications, ranging from the realization of micromagnets for integrated sensors to the deposition of thin layers for vertical recording media [1]. In this context, the development of wet deposition processes able to yield high-quality layers of FePt represents a topic of considerable industrial relevance. Even though several aqueous-based electrodeposition approaches have been developed for the manufacturing of FePt, most formulations evidenced substantial technological limitations, specifically connected to the use of water as solvent (hydrogen evolution, oxygen contamination of the deposit...).From this point of view, non-aqueous electrolytes represent a promising alternative for the electrosynthesis of metallic layers. These alternative solvents are characterized by attractive applicative properties: they present wide electrochemical windows, are simple to prepare, formulated from relatively cheap components, almost unreactive with water and, in most cases, biodegradable. Thanks to these properties, they have been widely employed for the electrodeposition of metals [2], alloys [3] or composites [4].The present work describes the deposition of FePt from a non-aqueous electrolyte based on ethylene glycol, which presents potential advantages over water-based baths in terms of gas evolution reduction and purity improvement of the obtained coatings. Deposition is carried out using Fe(III) and Pt(IV) as precursors and ammonium chloride as additive to enhance the quality of the coatings and their compositional uniformity. In this way, equiatomic FePt thin films characterized by a good morphology are easily obtained. These layers are subsequently characterized to assess their morphology, phase composition and magnetic properties. After annealing at 600 °C, the microstructure changes and the disordered face centered cubic phase present in the as-plated alloy evolves into the highly magneto-crystalline anisotropic L10 phase. As a consequence, the coercivity of the layers reaches values in excess of 10 kOe.[1] Lodder et al.; Encyclopedia of Materials: Science and Technology, pp. 1-10 (2005)[2] Abbott et al.; Phys. Chem. Chem. Phys. 8, 4265-4279 (2006)[3] Bernasconi et al.; J. Phys. Chem. B 124(47), 10739-10751 (2020)[4] Rosoiu et al.; Metals 10(11), 1455 (2020)

Ferromagnetic resonance in FePt thin films at elevated temperatures

Understanding of damping processes in ferromagnetic thin films at elevated temperatures has significant implications for heat-assisted magnetic recording, spin-transfer torque memory, and magnetic sensors operating at high temperatures. Through cavity-based high-temperature ferromagnetic resonance (FMR) measurements, this work examined the FMR linewidth and damping properties of continuous cubic FePt thin films at elevated temperatures. The data show that the FMR linewidth and the Gilbert damping constant both increase monotonically when temperature is increased from room temperature toward the Curie temperature. This temperature dependence is opposite to that observed previously in FePt thin films that are granular, rather than continuous, and have L10 structure, rather than cubic structure; in those films, the FMR linewidth decreases monotonically with an increase in temperature [PR Applied 10, 054046 (2018)]. These opposite results originate from the difference in the crystalline structure and microstructure of the films. In the previous work, the granular L10-order FePt films hold dense material imperfection and thereby may host strong two-magnon scattering (TMS); the TMS-produced damping decreases with an increase in temperature, giving rise to reduced FMR linewidths at high temperatures. In the current work, the continuous cubic FePt films have much less imperfection and thereby host weak TMS, and the dominant damping mechanism is spin-flip magnon-electron scattering (SF-MES). The SF-MES process becomes stronger with an increase in temperature, giving rise to larger linewidth and higher damping at high temperatures. This work and the previous work together demonstrate that for a given thin-film material, the temperature dependence of the FMR linewidth critically relies on the structural properties of the film. They also indicate that one can engineer damping in magnetic thin films through the control of the structural properties of the films.

Electrodeposition of Equiatomic FePt Permanent Magnets from Non-Aqueous Electrolytes Based on Ethylene Glycol

The highly ordered L10 hard-magnetic phase of the equiatomic FePt alloy is of significant interest for a great number of applications, ranging from the realization of magnetic sensors to the deposition of thin layers for vertical recording. In this context, the development of wet deposition processes able to yield high-quality layers of FePt represents a need of considerable industrial relevance. While several aqueous-based electrodeposition approaches have been developed for the manufacturing of FePt, these formulations evidenced substantial technological limitations, specifically connected to the use of water as solvent. The present work describes the deposition of FePt from a non-aqueous electrolyte based on ethylene glycol, which presents potential advantages in terms of gas evolution reduction and purity improvement of the obtained coatings. Deposition was carried out using Fe(III) and Pt(IV) as precursors and ammonium chloride as additive to enhance the quality of the coatings and their compositional uniformity. In this way, equiatomic FePt thin films characterized by a good morphology were easily obtained. After annealing at 600 °C, their microstructure changed and the disordered fcc phase present in the as-plated alloy evolved into the highly magneto-crystalline anisotropic L10 phase. As a consequence, coercivity reached values in excess of 10 kOe.

Toward Miniaturized Undulators with L10 Phase FePt Micromagnet Array

An undulator is a required part for building free‐electron lasers (FELs) upon which a large body of the literature has expressed interest for the applications in various areas such as the defense industry, material science, and medicine. The current state of the art of X‐FEL uses an undulator period down to 3 cm, which creates an output radiation wavelength down to 1.5 A using ≈14 GeV e‐beam. By developing a mm/sub‐mm period undulator (i.e., a period of 300 μm), it is possible to obtain the same output radiation wavelength with a much less e‐beam energy of ≈1.4 GeV. Thus, compact XFELs could become a reality. In this work, FePt thin films and micromagnets are synthesized by the sputtering deposition technique, and their properties are studied. The X‐ray diffraction (XRD) pattern indicated that the crystal structure transformed from face‐centered cubic (FCC) to L10 face‐centered tetragonal (FCT) phase upon annealing. Room temperature coercivity of the structures reached up to 1.4 Tesla. Undulator prototype is developed and the performance tests are done using Genesis simulation. Simulations yield output radiation with 0.52 nm wavelength and 0.7 GW peak power. Synthesized FePt micromagnets could be utilized in next‐generation miniaturized undulators.

Real-time monitoring of order-disorder transformation of FePt thin films by light scattering

ABSTRACT An optical method is proposed to real-time monitor the order transformation of FePt thin films. By measuring the surface reflection amplitude, onset temperature can be determined correctly. In a heating process, order transformation drastically increases the surface roughness due to recrystallization and grain growth, resulting in rapid drop of surface reflectivity. By tracking this drop, the kinetic ordering temperature can be determined. Experimentally measured kinetic ordering temperature for Fe50.4Pt49.6, Fe46.6Pt53.4, and Fe52.7Pt47.3 thin films of 60 nm in thickness are 396°C, 437°C, and 368°C, respectively. They are in good agreement with the published DSC data.

Preparation, Microstructure, and Magnetic Properties of Electrodeposited Nanocrystalline L10 FePt Films

In this work, we prepared the nanocrystalline near-equiatomic FePt thin films by electrodeposition and explored the effects of annealing temperature and annealing time on morphology, structure, and magnetic properties of FePt film. The fcc-FePt phase starts to form when annealing at 400 °C under the 15% H2 gas mixture. With increasing annealing temperature from 400 to 800 °C, the FePt film experiences a gradual phase transformation from the soft magnetic fcc phase to the permanent magnetic fct L10 phase. The transition degree of the L10 phase is achieved to be 0.94 and coercivity reaches a maximum of 1.5 T for the sample annealed at 800 °C for 4 h. A shoulder behavior in the M(H) curves is found in the electrodeposited FePt films. Through fitting the M(H) curves by the model based on the transition degree of the L10 FePt phase, the shoulder behavior is quantitatively confirmed to originate from the coexistence of the soft magnetic fcc phase and the permanent magnetic L10 phase. The magnetic exchange coupling between the soft magnetic and permanent magnetic nanocrystalline phases is negligible in the electrodeposited FePt films due to that the substrate Ag diffusion into the FePt film separates the FePt nanocrystalline phases.

Influence of pressure on (0 0 1)-preferred orientation and in-plane residual stress in rapidly annealed FePt thin films

The dependence of annealing pressure and background pressure during film deposition on the in-plane residual tensile stress, the crystallographic orientation and the surface chemical state in single-layer FePt films formed with rapid thermal annealing were investigated. On manipulating the annealing pressure and background pressure, the transition from perpendicular magnetic anisotropy to isotropy was found in the rapidly annealed FePt films. The slight surface oxidation of the FePt films during rapid thermal annealing induced an in-plane tensile stress to develop the (0 0 1)-texture and a perpendicular magnetic anisotropy. In contrast, for FePt films prepared at low pressure, the accumulated in-plane tensile stress was insufficient to develop a well aligned (0 0 1)-texture because of a lack of stress induced by a surface oxidation layer.

The effect of Bi additives on the properties of Fe-Pt films

Using the modernized three-electrode ion-plasma sputtering method, homogeneous thin films of FePt and Fe (Pt/Bi) were obtained. Films were deposited on NaCl and glass-ceramic substrates. The film thickness was 120-530 nm. In this case, the calculated cooling rate reached ~ 1012–1014 K/s. The structure of the FePt and Fe (Pt/Bi) films was investigated using X-ray diffraction and electron microscopy methods. It was established that metastable phases were formed in freshly sputtered films, including a supersaturated solid solution, a nanocrystalline and amorphous phases. It was determined that the obtained metastable structures are stable when heated to 540-880 K, depending on the composition. It was established that Bi additives significantly reduce the coercive force of films in the as-sputtered state. It was shown that a heat treatment increased the coercive force up to 36 kA/m in FePt films and up to 10 kA/m in Fe (Pt/Bi) films. The composition of Fe (Pt/Bi) films with a small value of the temperature coefficient of resistance (TCR ~3·10-5 K-1) was determined.

Effect of cooling rate on structure and magnetic properties of FePt thin films

The effects of cooling rate on the structure and magnetic properties of FePt films were studied. The results show that the in-plane coercivities of Fe53.6Pt46.4, Fe51.1Pt48.9 and Fe47.2Pt52.8 films in quenching and furnace cooling increased from 6.1 kOe to 7.0 kOe, 9.0 kOe to 12.0 kOe and 3.5 kOe to 8.8 kOe, respectively. The films in furnace cooling have continuous and uniform microstructure, and has good squareness of hysteresis loops. However, quenching cooling leads to island coalescence, forming a void morphology in the film. This study demonstrates that Fe dissolution and precipitation is greatly affected by cooling rate, and the magnetic properties and morphology of FePt films can be improved by controlling the cooling rate after heat treatment.

Modelling of strain induced magnetic anisotropy in Au additive FePt thin films

L10 ordering at low temperature and controlled grain size are prerequisites for the fabrication of magnetic recording media. This can be achieved by several methods such as addition, under layer or over layer of third element in FePt. In the present work, the kinetics of L10 ordering in the presence of immiscible element Au is investigated by varying the Au concentration at different annealing temperatures. The structural analysis shows that the Au additive not only promotes the ordering at low temperature but also supports the textured growth along (001) plane. The magnetic measurements supported by theoretical simulations using Monte Carlo predicts that the enhancement in coercivity attributed to induced strain anisotropy and weakening of inter-granular exchange coupling may be due to presence of Au at grain boundaries.

Magnetic properties and structure of Gd-implanted L1 FePt thin films

In this study, we have investigated the effect of Gd implantation on composition, chemical order, and magnetic properties of 20 nm thick L10 ordered FePt thin films. We show that upon Gd implantation at 30 keV even a small amount of 1 at. % is sufficient to destroy the L10 order, resulting in a soft magnetic A1 FePt alloy, with the exception of a thin L10 ordered layer located at the film/substrate interface. Additionally, a strong resputter effect is observed which results in a large decrease in film thickness as well as to a reduction in Fe content in the FePt alloy. Post-annealing of samples in Ar atmosphere did not result in a restoration of the L10 order, but leads to a transformation to pure Pt and Fe2O3, facilitated by the presence of a high density of vacancies induced by the implantation process.