L10-ordered FePt Research Articles

Lattice constants and magnetism of L10-ordered FePt under high pressure

We studied the relationship between the lattice constant and magnetism of L10-ordered FePt under high pressure by means of first-principles calculations and synchrotron x-ray measurements. Based on our calculations, we found that the c/a ratio shows a local maximum at ∼20 GPa and that the Pt magnetic moment first remains almost unchanged and is sharply suppressed at ∼60 GPa. As for the c/a, we experimentally verified the local maximum at ∼20 GPa by powder x-ray diffraction. We also measured the x-ray magnetic circular dichroism at the Pt L edge up to ∼20 GPa. Any significant change in the Pt magnetic moment was not observed in agreement with the calculations. These results, thus, indicate that magnetic states, where the magnetization of Fe decreased and that of Pt did not change, can be created in L10-ordered FePt by lattice deformation under high pressure.

Atomic migration and ordering phenomena in bulk and thin films of FePd and FePt

Abstract “Order – order” kinetics was studied by means of “in situ” and quasi-residual (REST) resistometry in bulk and thin films of L10-ordered FePd and FePt intermetallics. Substantial effect of magnetic ordering on the activation energy for chemical ordering was revealed in FePd. A discontinuous change of ordering dynamics was detected in FePt between 800 and 830 K. The results are consistent with the data of Fe* diffusion in FePt multilayer examined by means of nuclear resonant scattering in grazing-incidence geometry (GINRS). Monte Carlo (MC) simulations of “order –order” processes in L10-ordered bulk FePd and FePt and nano-layered FePt have been carried out using Glauber dynamics with vacancy mechanism of atomic jumps. Multi-timescale “order – order” relaxations observed in the bulk were predominated in nanolayers by a reorientation of the initial z-variant L10 superstructure into a mixture of x- and y-variants.

First-principles study of electronic structure and magnetic properties of L10-ordered FeNi, FePd, and FePt alloys

The structural, electronic, and magnetic properties of three spin configurations of L10-ordered FeM alloys (M = Ni, Pd, or Pt) were studied using the first-principles method. The calculations were carried out using Quantum ESPRESSO package within the framework of Density Functional Theory (DFT). The exchange-correlation functional potentials were studied using local density approximation (LDA) of Perdew-Zunger (PZ), the generalized gradient approximation (GGA) of Perdew–Burke–Ernzerhof (PBE), Perdew and Wang 91 (PW91), and Perdew-Burke-Ernzerhof revised for solids (PBEsol). We found that the PBE approximation has the most accurate results for lattice parameters compared to the experimental values. Furthermore, our results reveal that the most stable spin configuration for the considered alloys is the ferromagnetic configuration, where all spins are aligned perpendicular to the (001) plane. However, in FePd and FePt alloys, a small variation in the tetragonality ratio c/a (from 0.98 to 0.92) can transform them from ferromagnetic to antiferromagnetic state. In an antiferromagnetic state, a pseudogap is observed just below fermi energy for each alloy. Moreover, our calculations reveal large magnetocrystalline anisotropies for FePt alloy in the order of 3 meV/f.u. On the other hand, FePd and FeNi show relatively lower values in the range of 0.18–0.42 meV/f.u. Finally, Heisenberg exchange interactions are calculated from first-principles and Green's functions formalism.

Dependence of the Growth Mode in Epitaxial FePt Films on Surface Free Energy.

Epitaxial thin films of L10-ordered FePt alloys are one of the most important materials in magnetic recording and spintronics applications due to their large perpendicular magnetic anisotropy (PMA). The key to the production of these required superior properties lies in the control of the growth mode of the films. Further, it is necessary to distinguish between the effect of lattice mismatch and surface free energy on the growth mode because of their strong correlation. In this study, the effect of surface free energy on the growth mode of FePt epitaxial films was investigated using MgO, NiO, and MgON surfaces with almost the same lattice constant to exclude the effect of lattice mismatch. It was found that the growth mode can be tuned from a three-dimensional (3D) island mode on MgO to a more two-dimensional (2D)-like mode on MgON and NiO. Contact angle measurements revealed that MgON and NiO show larger surface free energy than MgO, indicating that the difference in the growth mode is due to their larger surface free energy. In addition, MgON was found to induce not only a flat surface as FePt grown on SrTiO3 (STO), which has a small lattice mismatch, but also a larger PMA than that of STO/FePt. As larger lattice mismatch is favored to induce a higher PMA into the FePt films, MgO substrates are exclusively used, but 3D island growth is indispensable. This work demonstrates that tuning the surface free energy enables us to achieve a large PMA and flat film surface in FePt epitaxial films on MgO. The results also indicate that modifying the surface free energy is pertinent for the flexible functional design of thin films.

Self-diffusion of Fe and Pt in L10-Ordered FePt: Molecular Dynamics simulation

Vacancy-mediated lattice diffusion coefficients of Fe and Pt atoms in the chemically ordered L10-FePt phase at temperatures between 1300 and 1600 K were evaluated by means of Molecular Dynamics (MD) simulations. Due to the anisotropic structure of the L10-ordered FePt phase, Fe and Pt diffusion fluxes and the resulting self-diffusion coefficients were considered along and perpendicular to the [001] crystallographic direction. In view of a very low vacancy concentration in real FePt single crystals, steady state conditions of the simulated process were approximated by specifically scaling the self-diffusion coefficients estimated for higher vacancy concentrations to the equilibrium vacancy concentration. This procedure involved the calculation of vacancy formation energies which appeared temperature dependent. The validity of this approach was thoroughly tested and the final results were analyzed and compared to the relevant literature data. The evaluated temperature dependent Fe and Pt self-diffusion coefficients showed Arrhenius behavior, however, their values were much lower than the reported experimental ones. Apart from the inevitable effect of the applied quasi-empirical potentials, the discrepancies might originate from the fact that while the MD simulations addressed a single crystal of FePt defected exclusively with vacancies and antisites, the existence of fast diffusion paths along linear and planar defects cannot be excluded in real materials.

Disorder dependent spin–orbit torques in L1 FePt single layer

We report spin–orbit torques (SOT) in a L10-ordered perpendicularly magnetized FePt single layer, which is significantly influenced by disorder. Recently, self-induced SOT in the L10-FePt single layer has been investigated, which is ascribed to the composition gradient along the normal direction of the film. However, the determined mechanisms for magnetization switching have not been fully studied. With varying growth temperatures, we have prepared FePt single layers with the same thickness (3 nm) but with different disordering. We have found that nearly full magnetization switching only happens in more disordered films, and the magnetization switching ratio becomes smaller with increasing L10 ordering. The method for deriving effective spin torque fields in the previous studies cannot fully explain the spin current generation and self-induced SOT in the L10-FePt single layer. Combined with the magneto-optical Kerr effect microscopy and anomalous Hall effect measurements, we concluded that the disorder should determine the formation of domain walls, as well as the spin current generation.

Exchange bias effect in L10-ordered FePt and FeCo-based bilayer structure: effect of increasing applied field

The applied magnetic field (HAPP) dependence of the exchange bias (EB) is studied in an exchange-coupled thin-film bilayer composed of a hard ferromagnetic FePt layer in the proximity of a soft ferromagnetic FeCo layer. FePt/FeCo structure is deposited in an ultra-high vacuum chamber, where the FePt layer was first annealed at 823 K for 30 min and subsequently cooled to room temperature in the presence of an in-plane magnetic field, HMAX ~ 1.5 kOe to promote L10-ordered hard magnetic phase with magnetic moments aligned in one of the in-plane directions in the FePt layer. In-situ magneto-optical Kerr effect measurements during different stages of bilayer growth and detailed ex-situ superconducting quantum interference device-vibrating sample magnetometer measurements jointly revealed that due to the interplay between exchange coupling at the interface and dipolar energies of the saturated hard FePt layer, a hysteresis loop of FeCo layer shifts along the magnetic field axis. A clear dependence of EB field (HEB) on increasing maximum value of the HAPP during the hysteresis loop measurement is understood in terms of the magnetic state of soft and hard magnetic layers, where EB increases with increasing HAPP until the hard layer moment remains undisturbed in its remanence state. As soon as the field was sufficient to rotate the spins of the FePt layer, the loop became symmetric with respect to the field axis.

Manipulating the magnetic properties and interphase coupling in FePt/Pt/Fe multilayer films by Pt spacer layer

Single-layer FePt (10 nm) film and trilayer FePt (10 nm)/Pt (t)/Fe (3 nm)/Pt (3 nm) (t = 0 nm, 0.5 nm, 1 nm, 2 nm and 4 nm) films were prepared on MgO (1 0 0) substrate. Their cross-sectional morphologies, ordering and interphase coupling behaviors were studied in detail. The results showed that the interphase coupling stiffness (J) increased at first, and then decreased with increasing thickness of the spacer layer. An exponential fitting of J ~ t dependence was obtained for t ⩾ 1 nm. A discrepancy between experiments and exponential fitting, mainly caused by interfacial diffusion, was seen for t ⩽ 1 nm. With respect to the proposed idea of using a spacer layer to modify the interface, FePt (8 nm)/Pt (0.5 nm)/Fe (3 nm)/Pt (3 nm) anisotropic nanocomposite film exhibited the best magnetic properties, with coercivity 0.87 T and (BH)max 370 kJ m−3, accounting for 95% of the theoretical (BH)max of L10-ordered FePt phase.

Magnetic anisotropy of L1-ordered FePt thin films studied by Fe and Pt L2,3-edges x-ray magnetic circular dichroism

The strong perpendicular magnetic anisotropy of L10-ordered FePt has been the subject of extensive studies for a long time. However, it is not known which element, Fe or Pt, mainly contributes to the magnetic anisotropy energy. We have investigated the anisotropy of the orbital magnetic moments of Fe 3d and Pt 5d electrons in L10-ordered FePt thin films by Fe and Pt L2,3-edge x-ray magnetic circular dichroism (XMCD) measurements for samples with various degrees of long-range chemical order S. Fe L2,3-edge XMCD showed that the orbital magnetic moment was larger when the magnetic field was applied perpendicular to the film than parallel to it and that the anisotropy of the orbital magnetic moment increased with S. Pt L2,3-edge XMCD also showed that the orbital magnetic moment was smaller when the magnetic field was applied perpendicular to the film than parallel to it, opposite to the Fe L2,3-edge XMCD results although the anisotropy of the orbital magnetic moment increases with S like the Fe edge. These results are qualitatively consistent with the first-principles calculation by Solovyev et al. [Phys. Rev. B 52, 13419 (1995)], which also predicts the dominant contributions of Pt 5d to the magnetic anisotropy energy rather than Fe 3d due to the strong spin-orbit coupling and the small spin splitting of the Pt 5d bands in L10-ordered FePt.

Magnetic coupling dependence on MgO interlayer thickness of FePt multilayered dots

L10-ordered FePt (001) circular dot arrays with the FePt/MgO/FePt multilayered structure have been prepared and the relationship between the magnetic properties and the size of the dots or the MgO intermediate layer thickness has also been investigated. The dots were microfabricated from continuous films by electron beam lithography and Ar ion milling. The remarkable steps in the hysteresis loops for multilayer films with tMgO = 3, 5, 8, and 10 nm indicated individual magnetization reversal of the top and bottom FePt layers. In the dots with tMgO = 3–8 nm, the step in the definition curves was not observed for D = 0.3 µmφ. When tMgO = 10 nm, remarkable step were observed not only for D = 0.5–3 µmφ but also for D = 0.5 µmφ.

Relaxation effects of magnetization modulus and precession axis on damping time of precession in L10-ordered FePt alloy film

We investigate the relaxation effects of magnetization modulus and precession axis on the damping time of the precession in L10-ordered FePt alloy film using a time-resolved magneto-optical Kerr effect. It is revealed that their fast relaxation processes during the precession bring about the overestimation of the damping time of the precession. We found that this error becomes more significant as the pump intensity increases. We propose that these corrections need to be considered for materials with the fast relaxation dynamics during the precession.

Patterning of L10 FePt nanoparticles with ultra-high coercivity for bit-patterned media.

L10-ordered FePt nanoparticles (NPs) with ultra-high coercivity were directly prepared from a new metallopolyyne using a one-step pyrolysis method. The chemical ordering, morphology and magnetic properties of the as-synthesized FePt NPs have been studied. Magnetic measurements show the coercivity of these FePt NPs is as high as 3.6 T. Comparison of NPs synthesized under the Ar and Ar/H2 atmospheres shows that the presence of H2 in the annealing environment influences the nucleation and promotes the growth of L10-FePt NPs. Application of this metallopolymer for bit-patterned media was also demonstrated using nanoimprint lithography.

Achievement and expected issues in heat assisted magnetic recording

The main goals in magnetic recording are the continuous increase of the areal recording density and the highest possible signal-to-noise ratio (SNR). The conventional magnetic recording has approached its physical limits. Further growth of the areal density is limited by the superparamagnetic effect and by the limited possibilities to further improve write heads design and pole materials in order to enhance the writing field. The perpendicular magnetic recording (PMR) with pole head and perpendicular media is the best alternative, especially to defer the superparamagnetic limit. PMR is also better situated to face the challenging design trilemma of magnetic recording: To increase the areal recording density, smaller grain volumes are needed, but to ensure the thermal stability of recorded information, the anisotropy should be increased accordingly; or, the increased anisotropy asks for higher writing fields, which are unavailable with the saturation magnetization of the magnetic materials of the current heads. Obviously, an alternative technology is needed to overcome the physical limit of conventional perpendicular recording (CPR). The most promising successor of CPR is the heat–assisted magnetic recording (HAMR), which is a multidisciplinary technology, a combination of magnetic and optical recording technology that proved experimentally areal densities of more than 1 Tb/in2 . HAMR allows the use of very smallgrain media, required for recording at ultra-high densities, with a larger magnetic anisotropy at room temperature, thus assuring a good thermal stability, while the local heating leads to a temporary magnetic softening of the medium. The realization of HAMR involved some challenges from both optical design and media properties. In the optical design the use of near-field transducers (NFT) represents an important advance. Regarding the media, a thermal design was used to produce the heating and cooling of the media within a very short time, about 1 ns, in order to achieve the desired data rate and generate a large thermal gradient for sharp bit edge definition. The structure of a HAMR system is briefly discussed, as well as the processes characterizing the writing process. A special attention is paid to the requirements for the materials needed for this type of recording: the granular L10-ordered FePt:X alloy sputtered on glass disk, the FePt-C and FePtC-Ag granular films and the bitpatterned media. Some important challenges of the HAMR technology are also summarized.

Thickness dependence of magnetization reversal mechanism in perpendicularly magnetized L10 FePt films

We have studied the magnetic switching behavior of L10-ordered FePt films with varying thickness. It was found that coercivity is strongly dependent on the film thickness. The obvious variations of the coercivity in the thin films are confirmed by the measurements of structural and magnetic properties. With increasing thickness, the degree of L10 chemical ordering increased, while the magnetization reversal process transforms from a pinned two-steps magnetization reversal to a comparatively smooth domain wall motion behavior. Although considering anisotropy, exchange interaction and applied magnetic field, the switching behavior in films is quite complex, the main features of the magnetization reversal mechanism can be understood by performing detailed investigation on the effect of the deposition temperature and the angle of magnetic field.

MgO-C interlayer for grain size control in FePt-C media for heat assisted magnetic recording

Currently, the most common recording media used for heat assisted magnetic recording is granular L10 based FePt-C directly deposited on a highly textured polycrystalline MgO underlayer. In this study, a thin granular MgO-C interlayer is inserted between the MgO underlayer and the granular L10-FePt-C magnetic layer to control the grain size of the magnetic media. By varying the deposition conditions of the MgO-C interlayer, we can vary the size of the L10-ordered FePt grains from about 10.5 nm to 7.6nm, while keeping the carbon composition in the magnetic layer unchanged. With the optimized interlayer, the L10-FePt-C media of grain size to 7.6 nm shows good squareness and perpendicular coercivity of 47 kOe as well as an order parameter of S = 0.85.

Perpendicular magnetic properties of ultrathin L10-Mn1.67Ga films grown by molecular-beam epitaxy

Materials with large perpendicular magnetic anisotropies (PMAs) have drawn great attention because of their potential applications in advanced spintronic devices such as spin-transfer-torque magnetic random access memory (STT-MRAM) and ultrahigh-density perpendicular magnetic recording. To date, a large variety of PMA materials have been investigated, such as L10-ordered FePt, CoPt granular films, Co/(Pt,Pd,Ni) multilayers, ultra-thin CoFeB alloys and perpendicularly magnetized Co2FeAl films. Among the various kinds of materials with PMA, MnGa film with L10-structure has received the most attention because it has large PMA (Ku～107 erg/cm3), ultralow Gilbert damping constant (0.008) and theoretically predicted high spin polarization (more than 70%). All these properties make L10-ordered MnGa a good candidate for spintronic devices such as STT-MRAM and spin-torque oscillators. Meanwhile, from the viewpoint of STT related spintronic device, it is necessary to fabricate ultrathin perpendicularly magnetized L10-MnxGa films to lower the critical current for magnetization reversal. However, up to now, in the main researches the ultrathin L10-MnxGa films have been grown on MgO substrates, which makes it difficult to integrate the MnGa-based magnetic tunnel junctions into the semiconductor manufacturing process.In this work, ultrathin L10-Mn1.67Ga films with different thickness values (1-5 nm) are grown on traditional GaAa (001) substrates by a molecule-beam epitaxy system. During the deposition, in situ streaky surface reconstruction patterns are observed from reflection high-energy electron diffraction, which implies high crystalline quality of the L10-Mn1.67Ga film. Only MnGa superlattice (001) and MnGa fundamental (002) peaks can be observed in the X-ray diffraction patterns in a range from 20 to 70, which means that the L10-Mn1.67Ga film is a good single-crystalline with c-axis along the normal direction. The magnetic properties of these films are measured by superconductor quantum interference device magnetometer in a field range of 5 T. The perpendicular M-H curves are almost square, while the in-plane curves are nearly hysteresis-free, each with a remnant magnetization (Mr) of around zero, which clearly evidences the PMA of the ultrathin L10-Mn1.67Ga film. Moreover, as the thickness of L10-Mn1.67Ga film decreases from 5 nm to 1 nm, the ratio of Mr/Ms also decreases from 1 to 0.72, which indicates that the PMA loses as thickness decreases. We also estimate the perpendicular anisotropy constant (Ku) from the relation Ku=Keff+2 Ms2, and the maximum Ku of 14.7 Merg/cm3 is obtained for the 5 nm MnGa film. Although the Ku decreases with thickness decreasing, a Ku value of 8.58 Merg/cm3 is observed in a 2 nm thick film. The obtained results are important for developing the L10-MnGa-based spin-transfer torque Gbit class magnetic random access memory.

Origin of (0 0 1) orientation and superlattice structure identification in L10-FePt/B4C multilayer thin films

(001) textured L10 FePt/B4C multilayer thin films have been prepared on amorphous substrates by magnetron sputtering. High resolution transmission electron microscopy investigation indicates that the Fe and Pt atoms stacked alternately along the c-axis of L10 FePt, confirming the formation of the superlattice structure of L10-ordered FePt. The internal stress calculation and geometrical phase analysis confirm the existence of the in-plane tensile strain in the L10 FePt thin films. The diffusion of the B and C atoms into the FePt layers results in expansion of the FePt unit cells in the interfaces, which induces an in-plane tensile strain in the adjacent deep parts of FePt layer. Such an in-plane tensile strain creates a favorable condition for the FePt films to stabilize the (001) texture because it relaxes the ordering strain energy of FePt during phase transformation.

Optimization of Magnetic Inks Made of L1(0)-Ordered FePt Nanoparticles and Polystyrene-block-Poly(ethylene oxide) Copolymers.

The preparation of magnetic inks stable over time made of L10-ordered FePt nanoparticles, thiol-ended poly(ethylene glycol) methyl ether (mPEO-SH) compatibilizing macromolecules and asymmetric polystyrene-block-poly(ethylene oxide) copolymers (BCP) as a subsequent self-organizing medium was optimized. It was demonstrated that the use of sacrificial MgO shells as physical barriers during the annealing stage for getting the L10-ordered state makes easier and more efficient the anchoring of compatibilizing PEO macromolecules onto the nanoparticles surface. L10-FePt grafted nanoparticles have shown a good colloidal stability and affinity with the PEO domains of the BCP leading to L10-FePt/BCP composite thin layers with individual magnetic dots dispersed in the BCP matrix.

Spin-modified catalysis

First-principle calculations are used to explore the use of magnetic degrees of freedom in catalysis. We use the Vienna Ab-Initio Simulation Package to investigate both L10-ordered FePt and CoPt bulk materials and perform supercell calculations for FePt nanoclusters containing 43 atoms. As the catalytic activity of transition-metal elements and alloys involves individual d levels, magnetic alloying strongly affects the catalytic performance, because it leads to shifts in the local densities of states and to additional peaks due to magnetic-moment formation. The peak shift persists in nanoparticles but is surface-site specific and therefore depends on cluster size. Our research indicates that small modifications in stoichiometry and cluster size are a useful tool in the search for new catalysts.

Overcoming the Trilemma Issues of Ultrahigh Density Perpendicular Magnetic Recording Media by L10-Fe(Co)Pt Materials

L10-ordered FePt and CoPt (collectively called L10- Fe ( Co ) Pt in this review) have become potential materials for future ultrahigh density perpendicular magnetic recording (PMR) media due to their high magnetocrystalline anisotropy, rendering small grains with high thermal stability. However, PMR media using such high anisotropy faces the well-known trilemma issues among thermal stability, signal-to-noise ratio (SNR), and writability. This paper will provide an overview of the impact of L10- Fe ( Co ) Pt on overcoming the superparamagnetic limit and balancing the trilemma issues for ultrahigh density PMR media. Here the research and development of L10- Fe ( Co ) Pt materials will be presented, from the perspectives of enhancing thermal stability, SNR and writability. Furthermore, we will provide some combined approaches to tackle the challenges in balancing the trilemma issues, focusing on materials engineering.