L10 FePt Research Articles

Exploring Structural, Electronic, Vibrational, and Thermophysical Properties of Fe–Pt, Fe3–Pt, and Fe–Pt3 Alloys: A Density Functional Theory Study

Utilizing density functional theory, the structural, electronic, vibrational, and thermophysical properties of L10 FePt, L12 Fe3Pt, and L12 FePt3 alloys are meticulously analyzed. Employing projected augmented wave pseudopotentials alongside the Perdew–Burke–Ernzerhof exchange‐correlation function, equilibrium lattice constants are computed, aligning closely with existing data, thus validating our approach. To ascertain the dynamical stability of these alloys, phonon frequencies and density of states across high symmetry directions of the Brillouin zone are computed, affirming their stability with positive phonon frequencies throughout. Furthermore, the electronic band structure, the total and projected density of states, electronic charge density, and Fermi surfaces of the alloys are delved. The thorough analysis of phonon dispersion curves, electronic band structures, and the density of states, charge densities, and Fermi surfaces provides conclusive insights into the properties and behavior of the alloys. In essence, comprehensive investigation offers valuable insights into the thermophysical properties of L10 FePt, L12 Fe3Pt, and L12 FePt3 alloys, spanning equilibrium lattice constants, phonon characteristics, and electronic properties. These findings significantly augment the understanding of the structural stability, phonon dynamics, and electronic behavior exhibited by these alloys.

Efficient calculation of magnetocrystalline anisotropy energy using symmetry-adapted Wannier functions

Magnetocrystalline anisotropy, a crucial factor in magnetic properties and applications like magnetoresistive random-access memory, often requires extensive k-point mesh in first-principles calculations. In this study, we develop a Wannier orbital tight-binding model incorporating crystal and spin symmetries and utilize time-reversal symmetry to divide magnetization components. This model enables efficient computation of magnetocrystalline anisotropy. Applying this method to L10 FePt and FeNi, we calculate the dependence of the anisotropic energy on k-point mesh size, chemical potential, spin-orbit interaction, and magnetization direction. The results validate the practicality of the models to the energy order of 100[μeV/f.u.] for these systems.

Segregation effects of doped VN and ZrN on magnetic and microstructural characteristics of FePt-based films

Segregants are essential materials for forming the complexation or granular microstructure in thin films for magnetic recording media. One such film, FePt-BN, has a metallic L10 FePt main phase as its matrix, in addition to having BN and C segregants at the grain boundaries. Amorphous BN has sufficient mechanical strength to support the columnar growth of FePt grains, however, it has a lower deposition rate resulting in columnar grains with a nonuniform aspect ratio. Moreover, some B diffuses in the interstitial site of FePt lattices. To overcome these limitations, this study added the transition nitride VN and ZrN with equal volume percentages to replace some BN segregants in FePt(BN, Ag, C) films: substantially different segregation behaviors were observed. Microstructural imaging mapping revealed that although VN segregated the FePt grains boundary, ZrN was diffused in the lattice. The FePt-BN-ZrN film thus had more disordered grains contributing to a higher in-plane magnetization and wider hard axis magnetic hysteresis loops than did FePt-BN-VN. The FePt-BN-VN film has the highest perpendicular magnetic anisotropy constant (Ku= 2.17 × 106 J/m3) and coercivity (Hc= 4 T); moreover, parallel remanence magnetization was minimal with a linear in-plane magnetization loop. The FePt-BN-VN film was sputtered directly on MgO(100) but depositing the FePt-BN-ZrN film required a 2-nm-thick FePt nucleation layer. The high atomic number of Zr and high sputter yield ratio (YN/Ymetal) of ZrN may increase the sites occupied by N, potentially affecting ZrN segregation.

Lithographically Ordered FePt L10 Dots with High Coercivity for Logic-Conditioned Magnetic Nanostructures

In order to develop the building blocks for future biosensing and spintronic applications, an engraving technique using electron beam lithography is employed in order to develop nanomagnetic pre-patterned structures with logic potential. The paper describes the realization and morphological and magnetic characterization of potentially logic-conditioned substrates, a building block to be further used as an integration platform upon which nanodevices, such as magnetic wires, or various geometrical shapes, circles, triangles, can be considered as pre-requisite for full integration into logic devices. As a proof of concept, regular arrays of FePt circles or magnetic dots were devised and structural characterization by X-ray diffraction and transmission electron microscopy proved the occurrence of the tetragonal L10 phase. Moreover, the magnetic characterization provided more insight into the potential of such arrays of magnetic devices as the hysteresis provided good values of magnetic coercivity, remanent and saturation magnetization. These findings show good potential for developing regular arrays of uniformly shaped magnetic entities with encouraging magnetic performances in view of potential applications in various applications.

Temperature and size dependence of energy barrier for magnetic flips in L10 FePt nanoparticles: A theoretical study

L10 FePt nanoparticle is among the most promising materials for magnetic recording and nanomagnetic applications. In nanoparticles, the superparamagnetic effect, wherein the thermal fluctuation is comparable to the energy barrier for magnetic flips, strongly affects the stability of magnetic recording. The temperature dependence of the magnetic anisotropy constant in L10 FePt nanoparticles is a crucial factor for estimating the relaxation time of magnetic flips in a magnetic nanoparticle. However, comprehensive simulations of the atomic level for energy barrier for magnetic flips in the L10 FePt nanoparticles are lacking. This study entailed a simulation that quantitatively reproduced the size dependence of the Curie temperature of nanoparticles in the experimental studies. Moreover, the surface effect, wherein the magnetization decreases owing to the loss of magnetic exchange interaction pairs on the surface, was also clarified. The temperature and size dependences of energy barrier for the magnetic flips of L10 FePt nanoparticles were elucidated.

FePt–BN granular HAMR media with high grain aspect ratio and high L1 ordering on corning LotusTM NXT glass

Fabricating highly ordered tall L10–FePt with a small grain pitch distance on a commercially available glass substrate is crucial to realize heat-assisted magnetic recording (HAMR) media for industrial manufacture. We have realized tall FePt grains surrounded by crystalized h–BN on Corning NXT™ glass deposited at elevated temperatures in the presence of radio frequency (RF) bias. In this paper, we discuss the effect of deposition temperature on the order parameter of L10–FePt–BN granular media. Well-isolated L10–FePt–BN granular media with a grain diameter of 6.5 nm and height of 11 nm is achieved. These films exhibit a high order parameter of 0.85 with a perpendicular coercivity of 35 kOe.

Spin–Orbit Torque-Driven Memristor in L10 FePt Systems with Nanoscale-Thick Layers for Neuromorphic Computing

In this study, a memristor driven by spin–orbit torque (SOT) is realized in the nanoscale thickness L10 FePt systems with high perpendicular magnetization anisotropy (PMA). Due to the domain nucleation and expansion driven by current pulses, multilevel Hall resistance states can be continuously tuned by current density, where the memristive states are retained by the domain wall pinning effects. The properties of multilevel resistance states for samples with different structures are associated with the magnitude of field-like torque, and the larger efficiency of field-like torque enhances multiple resistance characteristics. Furthermore, the stable memristive behavior is obtained in the FePt/MgO/NiFe heterostructure. Finally, a three-layer multilayer perceptron (MLP) neural network is built to perform the MNIST handwritten digit recognition task based on the device’s memristive behaviors, and the accuracy of weight update can reach up to 88.55%. These results pave the way for the application of L10 FePt nanomaterials in neuromorphic computing.

Cu interfaced Fe/Pt multilayer with improved (001) texture, enhanced L10 transformation kinetics and high magnetic anisotropy

In the present work, enhanced L10 ordering kinetics with improved crystallographic texturing in [001] direction and high magnetic coercivity of about 6 kOe is observed in L10 ordered Fe(1-x)CuxPt films, after successive isochronal vacuum annealing of multilayer structure [(Fe(1-x)/Cux)/Pt]x10. Synchrotron grazing incidence x-ray diffraction (GIXRD) measurements, performed using area detector confirms the onset of L10 ordering at 200 °C and presence of high degree of ordering at 300 °C. In-plane and out-of-plane GIXRD measurements validates texturing of magnetic easy axis [001] in out of film plane direction. Enormous increase in magnetic coercivity is observed from MOKE measurements in L10 ordered state. Lower ordering temperature with enhance (001) texturing and high magnetic coercivity is desirable for potential device applications of FePt. Effect of Cu and annealing temperature on structural and magnetic properties is investigated systematically in Fe(1-x)CuxPt films. Structural investigation discloses that lattice distortion in L10 ordered state caused by Cu addition and high interfacial energy associated with initial multilayer structure drives faster interdiffusion. Both of these aspects collectively work for FCC to FCT transformation, hence results in enhanced kinetics and texturing effects of L10 FePt.

DFT calculation of intrinsic properties of magnetically hard phase L10 FePt

Due to its strong magnetocrystalline anisotropy, FePt L1$\mathrm{_0}$ phase is considered as a promising magnetic recording media material. Although the magnetic properties of this phase have already been analyzed many times using density functional theory (DFT), we decided to study it again, emphasizing on full potential methods, including spin-polarized relativistic Korringa-Kohn-Rostoker (SPR-KKR) and full-potential local-orbital (FPLO) scheme. In addition to the determination of exact values of the magnetocrystalline anisotropy constants K$\mathrm{_1}$ and K$\mathrm{_2}$, the magnetic moments (m), the Curie temperature, and the magnetostriction coefficient, we focused on the investigation of the magnetocrystalline anisotropy energy (MAE) dependence on the magnetic moment values using the fully relativistic fixed spin moment (FSM) method with various exchange-correlation potentials. We present nearly identical MAE(m) curves near the equilibrium point, along with different equilibrium values of MAE and magnetic moments. For a magnetic moment reduced by about 10%, we determined a theoretical MAE maximum in the ground state (0 K) equal to about 20.3 MJ m$\mathrm{^{-3}}$ and independent of the choice of the exchange-correlation potential form. These calculations allow us to understand the discrepancies between the previous MAE results for different exchange-correlation potentials.

A study on the relationship of magnetic moments orientation in L10 FePt network nanostructured film by electron energy-loss magnetic chiral dichroism using semi-core excitation spectra

In this study, we applied electron energy-loss magnetic chiral dichroism (EMCD), an electron counterpart of X-ray magnetic circular dichroism (XMCD), to a network nanostructured FePt L10 ordered alloy film to examine the relative orientation of magnetic moments between neighboring Fe and Pt atoms using the Fe-M2,3, Pt-O2,3, and Pt-N6,7 semi-core excitation spectra with transmission electron microscopy and electron energy-loss spectroscopy. EMCD signals were successfully extracted from a large number of spectra using a dedicated data analysis procedure to obtain sufficient noise statistics. Results showed that the relative sign relation of the EMCD signals between the Fe and Pt absorption edges was consistent with that of the theoretical dielectric tensor while assuming that parallel magnetic moments exist between neighboring Fe and Pt. We believe the results of this study can be applied to alloys with different nanostructures to determine whether the spin configuration depends on the size and geometry of the nanostructures.

Current-Induced Magnetic Switching in an L10 FePt Single Layer with Large Perpendicular Anisotropy Through Spin–Orbit Torque

In this study, current-induced partial magnetization-based switching was realized through the spin–orbit torque (SOT) in single-layer L10 FePt with a perpendicular anisotropy (Ku⊥) of 1.19 × 107 erg·cm−3 (1 erg·cm−3 = 0.1 J·m−3), and its corresponding SOT efficiency (βDL) was 8 × 10−6 Oe·(A·cm−2)−1 (1 Oe = 79.57747 A·m−1), which is several times higher than that of the traditional Ta/CoFeB/MgO structure reported in past work. The SOT in the FePt films originated from the structural inversion asymmetry in the FePt films since the dislocations and defects were inhomogeneously distributed within the samples. Furthermore, the FePt grown on MgO with a granular structure had a larger effective SOT field and efficiency than that grown on SrTiO3 (STO) with a continuous structure. The SOT efficiency was found to be considerably dependent on not only the sputtering temperature-induced chemical ordering but also the lattice mismatch-induced evolution of the microstructure. Our findings can provide a useful means of efficiently electrically controlling a magnetic bit that is highly thermally stable via SOT.

A high throughput study of both compositionally graded and homogeneous Fe–Pt thin films

Compositionally graded Fe–Pt thin films were prepared on stationary 100 mm Si substrates by magnetron sputtering a base target of Fe on which a piece of Pt is asymmetrically positioned. Energy dispersive X-ray analysis was used to map the variation in film composition across the substrate, as a function of the size of the Pt piece. A scanning polar Magneto-Optical-Kerr-Effect system was used to probe the influence of composition and post-deposition annealing conditions (temperature and time) on coercivity. In this way, the maximum coercivity achievable for the sputtering system used could be established in a high throughput fashion. The evolution in coercivity with composition was correlated with the formation of L10 FePt and changes in its lattice parameters, as determined by scanning X-ray diffraction. High throughput coercivity mapping was then carried out on homogeneous Fe–Pt thin films of different compositions treated to different annealing conditions. This study serves as a step towards the integration of coercive FePt films into collectively fabricated devices.

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.

A Study on the Relationship of Magnetic Moments Orientation in L10 Fept Network Nanostructured Film by Electron Energy-Loss Magnetic Chiral Dichroism Using Semi-Core Excitation Spectra

A Study on the Relationship of Magnetic Moments Orientation in L10 Fept Network Nanostructured Film by Electron Energy-Loss Magnetic Chiral Dichroism Using Semi-Core Excitation Spectra

Role of Disordered Precursor in L10 Phase Formation in FePt-Based Nanocomposite Magnet

In order to prove the usefulness of having a structurally disordered precursor to the formation of FePt L10 phase and to facilitate the co-existence of exchange coupled hard and soft magnetic phases with optimized magnetic properties in various conditions of annealing, a Fe-Pt-Zr-B melt spun alloy has been synthesized and detailed structural and magnetic investigations have been undertaken to probe its phase evolution during annealing. The dynamics of formation of the hard magnetic L10 phase during the gradual disorder–order phase transformation has been monitored by using a complex combination of X-ray diffraction methods and 57Fe Mössbauer spectroscopy methods, over a wide range of annealing temperatures. Multiple phases co-existing in the annealed sample microstructures, observed in XRD, have been reconfirmed by the Mössbauer spectra analysis and, moreover, accurate quantitative data have been acquired in what concerns the relative abundance of each of the observed crystalline phases in every stage of annealing. It is shown that the formation of the hard magnetic phase, emerging from the chemically disordered precursor, is gradual and occurs via complex mechanisms, involving the presence of a disordered Fe-Zr-B-rich intergranular region which contributes to an increase in the abundance of the L10 phase for higher annealing temperatures. Magnetic measurements have confirmed the good performances of these alloys in terms of coercivity and remanence. These results contribute to the development of these alloys as the next generation of rare earth, free permanent magnets.

Influence of Cu on the Improvement of Magnetic Properties and Structure of L10 FePt Nanoparticles.

L10 ordered FePt and FePtCu nanoparticles (NPs) with a good dispersion were successfully fabricated by a simple, green, one-step solid-phase reduction method. Fe (acac)3, Pt (acac)2, and CuO as the precursors were dispersed in NaCl and annealed at different temperatures with an H2-containing atmosphere. As the annealing temperature increased, the chemical order parameter (S), average particle size (D), coercivity (Hc), and saturation magnetization (Ms) of FePt and FePtCu NPs increased and the size distribution range of the particles became wider. The ordered degree, D, Hc, and Ms of FePt NPs were greatly improved by adding 5% Cu. The highest S, D, Hc, and Ms were obtained when FePtCu NPs annealed at 750 °C, which were 0.91, 4.87 nm, 12,200 Oe, and 23.38 emu/g, respectively. The structure and magnetic properties of FePt and FePtCu NPs at different annealing temperatures were investigated and the formation mechanism of FePt and FePtCu NPs were discussed in detail.

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.

The effect of Cu additions in FePt–BN–SiO2 heat-assisted magnetic recording media

Structural and chemical order impact magnetic properties of solids, which are governed by spin–orbit coupling and exchange interaction. The ordered L10 phase of FePt is a key material to heat-assisted magnetic recording; to enable high storage density, a solid understanding is needed of structural and chemical disorder at small length scales, as well as associated modifications of the electronic band structure. Here, we investigate the effect of boron and copper additions (≲6 mol% Cu) on structural and magnetic properties of L10 FePt granular media. Two copper-driven mechanisms, although competing, can lead to improvements in both structural and magnetic properties. In particular, the Cu substitution on the Fe-site leads to a degradation of magnetic properties due to the delocalized electron orbitals originating from a larger Cu d-orbital occupancy. At the same time, Cu substitution leads to an enhanced crystallographic order and consequently magneto-crystalline anisotropy, which offsets the former effect to a large extent. Our study is based on magnetometry, x-ray absorption spectroscopy, ab-initio calculations and a phenomenological theory of disordered FePt granular media. We do not observe a sizable modification to Fe moments and electronic configuration; Cu reveals two different resonances associated with the presence and absence of Cu–B bonds that vary with total Cu concentration.

Incoherent Magnetic Switching of L1₀ FePt Grains

In this article, we present a novel micromagnetic model for L10 ordered FePt grains based on their atomic structure. In this model, each monolayer of Fe atoms is represented by a magnetic moment vector and the entire grain consists of a stack of magnetic moments coupled by magnetostatic and nearest-neighbor exchange fields. The grain model enables simulation of incoherent magnetization switching, especially near the Curie temperature. The model is applied to simulate the heat-assisted recording process in granular L10 FePt thin-film media. One of the unique findings is that grains at transition centers could have a domain wall trapped within, provided sufficient grain height. The existence of these grains, whose magnetic moments could be fractional at transition centers, resulting in significant reduction of medium noise. The noise reduction is effectively equivalent to an increase of the number of grains across the read track width. Enhancing and controlling this mechanism by material engineering could lead to significant increase of area density capability, specifically, increase of track density, without decreasing grain size.

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.