L10 FePt Films Research Articles

Fluence and Temperature Dependences of Laser-Induced Ultrafast Demagnetization and Recovery Dynamics in L10-FePt Thin Film.

The fundamental mechanisms of ultrafast demagnetization and magnetization recovery processes in ferromagnetic materials remain incompletely understood. The investigation of different dynamic features which depend on various physical quantities requires a more systematic approach. Here, the femtosecond laser-induced demagnetization and recovery dynamics in L10-Fe0.5Pt0.5 alloy film are studied by utilizing time-resolved magneto-optical Kerr measurements, focusing on their dependences of excitation fluence and ambient temperature over broad ranges. Ultrafast demagnetization dominated by Elliott-Yafet spin-flip scattering, and two-step magnetization recovery processes are found to be involved in all observations. The fast recovery time corresponding to spin-lattice relaxation is much shorter than that of many ferromagnets and increase with excitation fluence. These can be ascribed to the strong spin-orbit coupling (SOC) demonstrated in FePt and the reduction of transient magnetic anisotropy, respectively. Surprisingly, the demagnetization time exhibits no discernible correlation with ambient temperature. Two competitive factors are proposed to account for this phenomenon. On the other hand, the spin-lattice relaxation accelerates as temperature decreases due to enhanced SOC at lower ambient temperature. A semiquantitative analysis is given to get a visualized understanding. These results offer a comprehensive understanding of the dynamic characteristics of ultrafast demagnetization and recovery processes in iron-based materials with strong SOC, highlighting the potential for regulating the magnetization recovery process through temperature and laser fluence adjustments.

Influence of nano-twins on {001} texture of L10-FePt thin films

Influence of nano-twins on {001} texture of L10-FePt thin films

Field-free switching through bulk spin — orbit torque in L10-FePt films deposited on vicinal substrates

L10-FePt distinguishes itself for its ultrahigh perpendicular magnetic anisotropy (PMA), enabling thermally stabile memory cells to scale down to 3 nm. The recently discovered “bulk” spin-orbit torques in L10-FePt provide an efficient and scalable way to manipulate the L10-FePt magnetization. However, the existence of an external field during the switching limits its practical application, and therefore field-free switching of L10-FePt is highly demanded. In this manuscript, by growing the L10-FePt film on vicinal MgO (001) substrates, we realize the field-free switching of L10-FePt. This method is different from previously established strategies as it does not need to add other functional layers or create asymmetry in the film structure. The dependence on the vicinal angle, film thickness, and growth temperature demonstrates a wide operation window for the field-free switching of L10-FePt. We confirm the physical origin of the field-free switching is due to the tilted anisotropy of L10-FePt induced by the vicinal surface. We also quantitatively characterize the spin-orbit torques in the L10-FePt films. Our results extend beyond the established strategies to realize field-free switching, and potentially could be applied to mass production.

Magnetic properties and microstructure of FePt (Ag, C) films with MgTiOBN/CrRu underlayer

The (001) textured L10 FePt film is the promising candidate material for coming heat assisted magnetic recording technology. The material of epitaxial intermediate layer for FePt based granular film was considered by the surface energy and lattice mismatch between FePt and MgO-based materials. The (00L) textured FePt (Ag, C)/MgTiOBN/CrRu film was prepared and the MgTiOBN intermediate layer was prior explored in this study. This layer was sputtered by MgTiOBN target at 435 °C and formed the (002) texture on CrRu (002)/glass. The 6–12 nm thick FePt films were finally deposited on MgTiOBN at 470 °C. The FePt (Ag, C) films illustrate high perpendicular magneto- crystalline anisotropy (Ku) (3.85–3.97 × 107 erg/cm3) and out-of-plane coercivity (17.3–20.1 kOe). The lower in-plane magnetization is evidenced by the lower remanence ratio, [Mr(in-plane)/Mr(out-of-plane) ~0.08–0.14]. The saturation magnetization (Ms) of FePt films are around 752–775 emu/cm3 after co-sputtering with 40 vol% (Ag, C) segregants. From cross-sectional transmission electron microscopy image, the FePt shows island-like grains with contact angle between 35 and 67° and the average FePt grains size is around 20 nm estimated from plane view microstructural images. The reason for nonuniform trapezoidal islands is that diffusive C can penetrate and separated the FePt grains but agglomerated again during thin film formation and the lower grains contact angles was due to close surface energy between MgTiOBN and FePt layer. The release of lattice strain energy was formed the misfit dislocation in between FePt/MgTiOBN interface and epitaxial domain match was evidenced in the (111) plane system.

Theoretical estimation of surface magnetic anisotropy on [formula omitted]-FePt thin films: Case of perfect and defect Surfaces.

We carried out first-principle calculations of magnetocrystalline anisotropy of L10-FePt thin films terminated by either perfect Fe (Pt) layers or defect surfaces, including vacancies or substituted surface atoms. Inspired by the experimental strategy for determining magnetic anisotropy, we present here a theoretical model for analyzing volume and surface magnetic anisotropies for L10-FePt thin films with different surface terminations. We demonstrate that surface anisotropy does not depend solely on the film thickness, but also on the atomic environment of the surface layer. Perpendicular surface anisotropy of L10-FePt thin films reaches its maximum with full Pt termination, which decreases with increasing Pt vacancies and becomes parallel to the surface for a full Fe surface. In the case of mixed surfaces, surface anisotropy of (3Pt,1Fe) is enhanced compared to platinum-rich surface with one vacancy, and tends to be more in-plane for an iron-rich surface (3Fe,1Pt). Our results could be used to study surface anisotropy effects on L10-FePt surfaces by mean of classical magnetic models, and they are very promising for the development of high density magnetic data storage such as magnetic memory.

Influence of thickness on current-induced magnetization switching in L10-FePt single layer* *Project supported by the National Key Research and Development Program of China (Grant No. 2019YFB2005801), the National Natural Science Foundation of China (Grant Nos. 51971027, 51731003, 51971023, 51927802, and 51971024), and the Fundamental Research Funds for the Central Universities, China (Grant No. FRF-TP-19-001A3).

The thickness dependent spin–orbit torque (SOT) in an L10-FePt single layer is investigated in this work. As the thickness increases from 8 nm to 16 nm, the magnetization switching ratio in the L10-FePt film with higher chemical ordering becomes smaller. It is noted that compared with 3-nm-thick L10-FePt film, 8-nm-thick L10-FePt film can switch much magnetization with the increase of chemical ordering. When the FePt film is thick enough, the SOT in FePt is closely related to the L10-ordered structure, which indicates a bulk nature. Therefore, the disordering plays an important role in the magnetization switching only for the ultra-thin FePt films, while the structural gradient may play an important role for thicker films. However, both of the two mechanisms cannot fully explain the process of magnetization switching and the spin current generation. Although many factors influence SOT, here in this work we emphasize only the bulk nature of strong SOC in L10-FePt through density functional theory calculations, which should generate large spin current due to spin Hall effect.

Magnetic properties and structure of L10 FePtC films prepared by using the electric treatment

We successfully fabricated L10 ordered FePt and FePt-C(30 vol%) films without heat treatment by using electric treatment. The magnetic properties and structure of L10 FePt and FePtC films with different applied voltage and treatment time(t) have been investigated. With the increase of t, the out-of-plane coercivity (Hc) and chemical order parameter(S) of FePt films increase first and then decrease. The maximum values of Hc and S of FePtC films are larger than those of FePt films treated at 30 V. FePtC films treated at higher applied voltage show excellent (001) texture, higher Hc, S, smaller grain size and better perpendicular magnetic anisotropy. Electric treatment will not like thermal treatment which causes excessive grain growth. This study provides a new method for the ordering of other alloy thin films except for heat treatment.

Temperature assistance of electric field-controlled spin-orbit torque-based magnetization switching in PMN-PT/FePt heterostructures.

We report the temperature assistance of electric field (E-field)-controlled spin–orbit torque (SOT)-based magnetization switching of L10-FePt films grown on a PbMg1/3Nb2/3O3–PbTiO3 (PMN–PT) (011) substrate, which generates considerable strain via piezoelectric effects of the PMN–PT substrate under E-field. Owing to large strain-induced effective field and weak perpendicular magnetic anisotropy (PMA) at a high temperature, E-field controls the PMA- and SOT-based magnetization switching more effectively. Driven by E-field, magnetization switching is detected by a magnetic optical Kerr (MOKE) microscope under a fixed perpendicular magnetic field. Furthermore, E-field modulates change of anomalous Hall resistance regularly, which enables us to achieve the bidirectional transmission of data by designing an E-field controlled SOT-based logical circuit. This study indicates an efficient way to fabricate potential E-field-controlled spintronic applications at high temperatures.

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.

L10 FePt Films with Optimal (001) Texture on Amorphous SiO2/Si Substrates for High-Density Perpendicular Magnetic Recording Media

Highly ordered L10 FePt films with the highest degree of [001] preferred orientation were prepared by multilayer deposition on amorphous oxidized Si substrates and subsequent rapid thermal annealin...

Thickness dependence of anomalous Hall conductivity in L10-FePt thin film

L10 ordered alloys are ideal models for studying the anomalous Hall effect (AHE), which can be used to distinguish the origin from intrinsic (from band structure) or from extrinsic effects (from impurity scatterings). In the bulk limit of L10 ordered FePt films, the AHE is considered to be dominated by the intrinsic contribution, which mainly comes from the strong spin–orbit interaction (SOI) of Pt atoms and exchange-splitting of Fe atoms. The study of anomalous Hall conductivity (AHC) of L10-FePt thin films is of particular interest for its application in spintronic devices. In order to reduce the effects of defects such as grain boundaries, we chose SrTiO3 as the substrate, which has a very small mismatch (0.1%) with L10-FePt films to obtain high quality continuous thin films. We show that the contribution from phonon scattering should be included to fully interpret the AHE signals. For our 6 nm FePt film, the AHE mainly originates from the contribution of phonon scattering. While for the thicker films, the origin for AHE is mainly intrinsic Berry curvature contribution. Both experimental results and calculation results show that AHC increases with thickness and reaches the saturation when the film thickness approaches a critical value.

Comparison of microstructures and magnetic properties in FePt alloy films deposited by direct current magnetron sputtering and high power impulse magnetron sputtering

Single-layered FePt films of 30 nm thickness with random orientation are produced by direct current magnetron sputtering (DCMS) deposited directly onto Corning 1737 glass substrates and then post-annealed at 700 °C for 30 min. In films produced by high power impulse magnetron sputtering (HiPIMS) however, a strong (001) texture of L10 FePt film with isolated island structure is achieved and presents high perpendicular magnetic anisotropy after post-annealing under the same conditions. This is due to the compressive stress in as-deposited FePt films being enhanced greatly by HiPIMS. The films achieved by HiPIMS possess perpendicular coercivity, saturation magnetization and perpendicular squareness of 14.2 kOe, 620 emu/cm3 and 0.98, respectively, which reveal HiPIMS as a good technique to produce single-layered FePt films with high perpendicular magnetic properties.

L10 FePt films with high T capping layer for Heat Assisted Magnetic Recording (HAMR)

Abstract The effect of a 1.5 nm thick CoX layer (CAP) of high Curie temperature TC, deposited on top of an L10 Fe50Pt50 layer (MAG) on the properties of the MAG/CAP bilayer is investigated. Two series of samples are studied: “w/o CAP” are single MAG layers of thickness in the 3.8–10.5 nm range and “w/CAP” are MAG/CAP bilayers of MAG thickness in the 3.8–10.5 nm range and 1.5 nm CAP. For both series, magnetization reversal is investigated at TRT = 300 K and is shown to be coherent rotations up to a MAG thickness threshold of tcr ∼ 7.5 nm and incoherent, domain-wall mediated, above tcr. The dependence of d H RC / d T T = T C , where HRC is the remanence coercivity, on the MAG thickness indicates tcr is poorly dependent on temperature from TRT up to the MAG TC. The MAG TC distribution, of average 〈TC〉 and standard deviation σTC, is evaluated for both series. The results indicate 〈TC〉 is higher and σTC/〈TC〉 is lower w/CAP than w/o, particularly at low MAG thicknesses. This behavior is interpreted as a consequence of spin exchange hardening in the MAG layer in the proximity of the MAG/CAP interface. For the w/CAP case, the saturation field of the Thermoremanent magnetization exhibits a non-motononic dependence on the MAG thickness, displaying a maximum. This behavior indicates the CAP assists the applied field to set the MAG layer magnetization during a HAMR process, the assist effect becoming more efficient the lower is the MAG thickness.

Ultrafast laser induced magnetization reversal in L1 FePt films with different chemical orders

Using an ultrafast laser pulse as a substitution of continuous-wave laser to excite a magnetic film leads to high spin temperature and fast demagnetization, beneficial to the rapid and efficient photo-assisted magnetization (M) reversal. Here, ultrafast laser induced M reversal in perpendicular magnetic anisotropy L10 FePt films with different chemical ordering parameter (from S<0.6 to S>0.9) was investigated using magneto-optical Kerr effect (MOKE). It was found that the coercive field (Hc) without laser excitation increases from ∼0.8 to ∼4 kOe with increasing S, but Hc becomes smaller for enhanced laser fluence (F) and reaches an analogous small value of ∼0.2 kOe for F>12 mJ cm-2. Despite such a significant softening in films with high S, the laser induced M reversal rate is slightly less than 1 even for the applied field (H) much larger than 0.2 kOe. This reveals a small portion of spins are photo inactively pinned, as confirmed by magnetic force microscopy measurements. Such pinning sites may be reduced with improved chemical order and morphology. We also found an approximately inverse linear relation between the H and the corresponding threshold F to induce the M reversal in the film of high S, which can be described by an ultrafast thermal activated spin flip model.

Experimental study and micro-magnetic modeling of magnetization dynamics in L10-FePt thin film

Abstract Among different magnetic thin films, L10 FePt due to high magnetocrystalline anisotropy is attracting much attention for applications in new generation of magnetic recording media. In this work, switching time and switching mechanism of magnetization as essential properties of L10 FePt film was studied by magneto-optical Kerr effect (MOKE) and time-resolved magneto-optical Kerr effect (TR-MOKE). For this purpose, static in plane and out of plane magnetic hysteresis loop of the 20-nm thick L10 FePt film on (1 0 0) MgO was measured and modeled using polar and longitudinal MOKE and mumax code, respectively. Furthermore, the switching time of magnetization was studied using laser-induced ultrafast demagnetization and relaxation of the sample by TR-MOKE, in which for the first time, the magnetic field was applied in the plane of the sample for this measurement.

Impact of ultrafast demagnetization process on magnetization reversal in L1 FePt revealed using double laser pulse excitation

Ultrafast laser induced magnetization reversal in L10 FePt films with high perpendicular magnetic anisotropy was investigated using single- and double-pulse excitations. Single-pulse excitation beyond 10 mJ cm−2 caused magnetization (M) reversal at the applied fields much smaller than the static coercivity of the films. For double-pulse excitation, both coercivity reduction and reversal percentage showed a rapid and large decrease with the increasing time interval (Δt) of the two pulses in the range of 0–2 ps. In this Δt range, the maximum demagnetization (ΔMp) was also strongly attenuated, whereas the integrated demagnetization signals over more than 10 ps, corresponding to the average lattice heat effect, showed little change. These results indicate that laser induced M reversal in FePt films critically relies on ΔMp. Because ΔMp is determined by spin temperature, which is higher than lattice temperature, utilizing an ultrafast laser instead of a continuous-wave laser in laser-assisted M reversal may reduce the overall deposited energy and increase the speed of recording. The effective control of M reversal by slightly tuning the time delay of two laser pulses may also be useful for ultrafast spin manipulation.

Structure and Magnetic Properties of Graded (001)-Oriented FePt Films Prepared by Magnetron Sputtering and Rapid Thermal Annealing

L10-FePt/[Fe/Pt]27 multilayer films were prepared on (100)-oriented single-crystal MgO substrates. Structural analysis indicates epitaxial growth of the L10-FePt films during high-temperature deposition. The room-temperature-deposited [Fe/Pt]27 multilayer exhibits the fcc-FePt structure grown epitaxially on the L10-FePt base layer. The degree of chemical order and thereby the anisotropy of the top layer has been controlled by varying the temperature of a post-deposition annealing process. Compared with the as-deposited films, coercivities of the films annealed below 400 ∘C are substantially decreased by introducing the formation of a graded anisotropy in the partially ordered top layer. Coercivities decrease for annealing temperatures that are greater than 400 ∘C due to improved ordering of the top layer.

Preparation of c-axis perpendicularly oriented ultra-thin L1-FePt films on MgO and VN underlayers

Ultra-thin L10-FePt films of 2 nm average thickness are prepared on (001) oriented MgO and VN underlayers epitaxially grown on base substrate of SrTiO3(001) single crystal. Detailed cross-sectional structures are observed by high-resolution transmission electron microscopy. Continuous L10-FePt(001) thin films with very flat surface are prepared on VN(001) underlayer whereas the films prepared on MgO(001) underlayer consist of isolated L10-FePt(001) crystal islands. Presence of misfit dislocation and lattice bending in L10-FePt material is reducing the effective lattice mismatch with respect to the underlayer to be less than 0.5 %. Formation of very flat and continuous FePt layer on VN underlayer is due to the large surface energy of VN material where de-wetting of FePt material at high temperature annealing process is suppressed under a force balance between the surface and interface energies of FePt and VN materials. An employment of underlayer or substrate material with the lattice constant and the surface energy larger than those of L10-FePt is important for the preparation of very thin FePt epitaxial thin continuous film with the c-axis controlled to be perpendicular to the substrate surface.

Effects of oxide isolation layer on magnetic properties of L10 FePt film grown on Si substrate

Magnetic force microscope (MFM) is a powerful tool to subtly detect the stray field distribution of magnetic film or particles on a sub-micrometer scale. Due to its huge uniaxial magnetocrystalline anisotropy (Ku~7107 erg cm-3) and high Currie temperature (TC~500℃), FePt alloy in an L10 phase is expected to be coated on the MFM tip to display high coercive force (Hc) and to improve the magnetic stability and MFM resolution. A grain size of~3 nm will be enough to overcome the super paramagnetism. However, the growing fresh FePt films must experience a high temperature annealing (exceeding 700℃) in order to transform their structures thoroughly from a soft A1 phase into the desired hard L10 phase. This brings the risk of diffusion between FePt coating layer and the underneath Si cantilever. Several admixtures have been attempted by other researchers to obtain granular films with FePt grains separated by oxides, with the purpose to prevent the diffusion from happening between FePt and Si. But apparently, it will be very difficult to fabricate a separated FePt grain exactly on the top of MFM tip. This is a critical factor to affect the MFM resolution. And discussion about the influence of the interface diffusion is avoided in most of published papers. Alternatively, some oxide isolation layers with higher melting temperature can be useful for separating the top FePt film from the bottom Si crystal. In this paper, MgO and SiO2 are selected as isolation layers, deposited by magnetron sputtering. Subsequently, the FePt films are deposited at 400℃ and annealed at different temperatures (500℃ to 800℃) for 2 h. The experimental results indicate that the diffusion between FePt and Si substrate always occurs in the absence of any isolation layer, leading to a reluctant maximum Hc of~5 kOe for 50 nm FePt film. However, the coercive force could remarkably exceed 10 kOe if an isolation layer is used. In the case of MgO, a maximum Hc of~12.4 kOe for 50 nm FePt could be stably measured. However, the annealing temperature must be lower than 600℃ to hold back the occurrence of brittle cracks in isolation layer. Because of the smaller lattice mismatch and expansion coefficient difference between SiO2 isolation layer and Si substrate, the highest annealing temperature could exceed 800℃ when replacing MgO with SiO2. The Hc of FePt film could be adjusted in a range from~5 kOe to~15 kOe by changing the annealing temperature. These findings greatly benefit the fabrication of FePt-based MFM tips with high Hc. And it is expected to be able to effectively enhance the resolution of MFM image.

The effect of film thickness on Curie temperature distribution and magnetization reversal mechanism for granular L10 FePt films

Structural and magnetic properties of granular L10 FePt films are investigated as a function of film thickness in the range 3–11 nm. Scanning transmission electron microscopy observations indicate an increasing contribution of in-plane variants to the L10 FePt crystalline structure with increasing film thickness, which may explain the noticed degradation of the perpendicular anisotropy. The temperature dependence of coercivity suggests a progressive deviation of the magnetization reversal mechanism from coherent rotations with increasing film thickness above ~6 nm, corresponding to a domain-wall width. The anisotropy field distribution (〈HK〉, ) is estimated using Monte Carlo (MC) simulations of AC transverse susceptibility (ACTS). The average anisotropy field 〈HK〉 displays a similar temperature dependence to (MS)1.1, where MS is the film saturation magnetization, consistent with the theoretical predictions for L10 FePt. The Curie temperature distribution (〈TC〉, ) is evaluated using two methods: fast (ns) thermal erasure (ThEr) of remanent magnetization and MC modeling of the temperature dependence of AC susceptibility (ACS). The values evaluated by the two methods are in agreement, indicating an increase of in the range 2–6% with decreasing film thickness. The observed increase of 〈TC〉 in the range 620–680 K with increasing film thickness is explained using the finite-size scaling law with the critical exponent set to the theoretical value for L10 FePt. A general procedure to extract 〈TC〉 and using MC modeling of ACS, essentially independent of the magnetization reversal mechanism, is proposed. MC simulations of ACS confirm the deviation of the magnetization reversal mechanism from coherent rotations with increasing film thickness above ~6 nm.