L10 Transformation Research Articles

Atomic level mechanism of disorder-order transformation kinetics at nanoscale in FePt based systems

L10 ordered FePt is one of the most promising materials for spintronic and recording media applications. In the present work, the mechanism of L10 phase transformation in FePt based films with varying initial structures is examined at the nanoscale to understand the ordering process using synchrotron based GIXRD, MOKE, VSM, and techniques with sub nanometer depth selectivity like XRR and SIMS. Precisely controlled compositions of the films are deposited using magnetron sputtering. Rapid thermal annealing is used for post-deposition processing. It is evaluated experimentally that for a shorter annealing time of 70 s at 400 °C, besides volume diffusion, short circuit diffusion paths along the intercrystallite region owing to the presence of nanostructured grains play a dominant role in alloying behavior. A study of the L10 ordering process reveals the crucial role of film structure in controlling the transformation kinetics, texturing of nanograins, and magnetic coercivity. Diffusion studies disclose that type B diffusion kinetics is activated for the annealing time during which L10 transformation occurs in the films.

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.

The thermally activated distortion with amplification effect and related variant selection in red gold alloys

Variant selection during the A1→L10 transformation in a polycrystalline red gold alloy close to equiatomic Au-Cu composition has been extensively studied by Electron Backscatter Diffraction (EBSD) in our previous work. The use of a mathematical description of the lattice distortion and the maximal work criterion allowed us to quantify the degree of selection. With the same approach, we investigate here an interesting shape distortion effect, discovered twenty years ago in equiatomic AuCu-Ga. The shape distortion of thin samples placed in bending condition and then heat-treated under stress is studied in details. The singular shape memory effect and the remarkable distortion amplification, which we call TADA effect, are explored by monitoring the sample radius of curvature and the advancement of the transformation. The underlying mechanisms of variant selection are revealed by EBSD analysis across the samples. The experimental crystallographic variant selection distribution is compared with the expected profile calculated with the Euler-Bernoulli beam theory. The good agreement demonstrates that variant selection during the transformation is at the origin of the macroscopic distortion of red gold alloys. The TADA effect was found to occur when external stresses are released, and strongly depends on the stress at the initial stage of the transformation. This unusual effect is assumed to result from the persistence of variant selection throughout the transformation.

Fabrication and characterization of IrMn/PtMn/PtCr/PtMn antiferromagnets in terms of the role of each layer

Herein, we investigated the exchange couplings in ferromagnet/antiferromagnet film systems of structures CoFe/Pt50Cr50, CoFe/interface-Pt50Mn50/Pt50Cr50, CoFe/interface-Pt50Mn50/Pt50Cr50/backed-Pt50Mn50, and CoFe/Ir20Mn80/interface-Pt50Mn50/Pt50Cr50/backed-Pt50Mn50. These investigations revealed that the backed(b)-PtMn(2.5–3.5 nm), the interface(i)-PtMn(2 nm), and the IrMn(0.8 nm)/i-PtMn(1.2 nm) significantly increased the unidirectional anisotropy constant (Jk) up to 0.67 erg/cm2. The insertion of an ultra-thin IrMn(0.8–1.2 nm) layer at the CoFe interface improved the CoFe hysteresis, i.e. the ratio of exchange bias field (Hex) to coercivity (Hc) was increased. The effects associated with thin b-PtMn, i-PtMn, and IrMn/i-PtMn layers were presumably due to a mismatch between lattice constant and a/c ratio of the PtCr. In other words, the larger lattice constants and lattice deformation during L10 transformation of PtMn may promote a dynamic-stress-induced PtCr ordering-assisting effect, though the PtCr thickness was reduced to 13 nm. The real cause of the effect of ultra-thin IrMn layers has not been properly explained to date. The blocking temperature (Tb) for the CoFe/i-PtMn(2 nm)/PtCr(25 nm)/b-PtMn(3 nm) film was 500 °C. Although the additional insertion of IrMn(0.8 nm) reduced Tb to 410 °C, the Tb and the normalized Hex did not depend on the PtCr thickness, which varied from 13 to 25 nm. Remarkably, an insertion of IrMn(0.8 nm) had no influence on the normalized Hex at temperatures below 300 °C.

Effect of the A1 to L10 transformation on the structure and magnetic properties of polycrystalline Fe56Pd44 alloy thin films produced by thermal evaporation technique

X-ray diffraction, conversion electron Mössbauer spectroscopy and superconducting quantum interference device magnetometer techniques have been used to characterize the evolution of the microstructure and of the magnetic properties of phase transformation from disordered FePd face-centred cubic (A1) phase, to an ordered phase L10 FePd of tetragonal structure. This study was carried out by isothermally annealing in vacuum at 550 °C, as a function of annealing time, a Fe56Pd44 thin film alloy deposited on silicon substrate.The structural transformations are accompanied by a decrease of the (c/a) ratio and by an increase of the coercive field as a function of annealing time. After 10 min of annealing, the (c/a) ratio is equal to 0.945, which indicates that the disordered FePd phase is completely transformed into the ordered L10 FePd phase. By increasing annealing time, the microstructure of the alloy evolves, antiphase boundaries and twin boundaries develop. In contrast to the relationship most commonly described in the literature, that the highest coercivity corresponds to a two-phase ordered/disordered mixture, in ours studies the maximum value for coercivity corresponds to the fully ordered state. Furthermore, we propose that the high coercivity in these films is due to pinning centres of magnetic domain walls by antiphase boundaries and twin boundaries.

Enhancement of L1 transformation in Fe/Pt multilayer by Cu addition

Enhancement in L10 transformation kinetics in FePt is achieved by incorporating an optimum concentration of ternary element Cu, which has limited solubility in the fcc FePt phase, into the FePt multilayer stack. Two different multilayer structures were deposited. In first multilayer Cu is deposited at one interface of Fe/Pt and in other Cu is alloyed with Fe and Pt layers by co-sputtering. One Fe42.5Pt42.5Cu15 alloy film is also prepared and detailed study of evolution of structural and magnetic properties as a function of isochronal annealing is done using XRD and Magneto Optic Kerr Effect (MOKE) measurements respectively. Annealing up to 200oC results only in intermixing in the multilayer structure, with no sign of L10 transformation. Annealing at 300oC for 1h results in partial transformation to L10 phase as evidenced by appearance of (001) superlattice peak as well as large increase in the coercivity. It is found that in the Fe(Cu)/Pt(Cu) multilayer exhibits significantly faster L10 transformation as compared to Fe/Pt/Cu multilayer or FePtCu alloy film. Inter-diffusion study using x-ray reflectivity measurements reveals that constant for interdiffusion in Fe(Cu)/Pt(Cu) is only marginally higher than that in Fe/Pt/Cu multilayer. The observed enhancement in L10 transformation rate in Fe(Cu)/Pt(Cu) multilayer is discussed in terms of possible enhancement of diffusivities of constituent species in fcc FePt phase.

Special Features of L21–L10 Thermoelastic Martensitic Transformations in Ni54Fe19Ga27 Single Crystals in Tension

Special features of L21–(14М)–L10 martensitic transformations (MT) are experimentally established for [001]-oriented Ni54Fe19Ga27 (аt.%) single crystals under tensile stress, including staging of superelasticity curves, stress hysteresis, and strain level of martensite formation depending on the test temperature and MT sequence. An additional stage of inelastic strain with high strain hardening coefficient and a stress drop arise on the σ(e) curves in the interval of changes of the stress-induced MT sequence from L21–14M–L10 to L21–L10 (320–400 K) at superelasticity (SE). Special features and physical reasons of this stage and of the stress drop related with the processes of origin and detwinning of L10 martensite during stress-induced L21–L10 transformation are established. The stage of inelastic strain and the stress drop are absent at the L21–14М–L10 MT in the [001]-oriented single crystals or at the L21–L10 MT at other orientations, where detwinning of L10 martensite is suppressed by geometrical reasons.

Mössbauer study of structural inhomogeneities formed upon the FCC–L10 transformation in equiatomic FePd alloy from different initial states

ABSTRACTA comparative Mössbauer study of structural inhomogeneities that arise in the course of А1→ L10 phase transformation in nondeformed (as cast and quenched from 950°C and melt-spun from the ingot) and severely deformed samples of equiatomic FePd alloy upon ordering annealing at Т = 450°С has been performed. According to the known experimental works, the chosen temperature of annealing is optimal for achieving the highest coercive force Hc in both quenched and deformed samples. It is shown that in the high-coercivity state both quenched and deformed samples FePd possess an inhomogeneous tetragonal structure, which is preserved even after quite a prolonged (40–100 h) annealing. All the samples contain, along with the configurations of the nearest neighbourhood that are assigned to the ordered L10 phase, significant volume fractions of configurations typical of nonequiatomic compositions. This conclusion is inconsistent with the commonly accepted concept on single-phase L10 type alloys with maximal values of Hc. An inference is made that the structural inhomogeneities detected in the samples under study result from the mismatch of the position of the point of congruent А1→ L10 transformation (≈58 at.% Pd) in the phase diagram of the FePd system to equiatomic composition.

Quantitative Kinetic Models of the A1 to ${\rm L}1_{0}$ Transformation in FePt and Related Ternary Alloy Films

Quantitative kinetic models for the A1 to L10 transformation in FePt and related ternary alloy films are derived for three nucleation conditions for both isothermal and constant heating rate annealing. The nucleation conditions are: 1) continuous nucleation, 2) burst nucleation, and 3) pre-existing nuclei. For the first nucleation condition, new nuclei are formed continuously throughout the transformation. For the second nucleation condition under isothermal annealing, nucleation is constrained to occur at a constant rate over only a short period of time. In the limit of this time period going to zero, the nucleation sites are saturated very rapidly and nucleation can be considered to have occurred as a burst. For the third nucleation condition, the nuclei are pre-existing, having formed prior to the start of the transformation. No new nuclei form during the transformation. To identify the nucleation condition that best describes the transformation in FePt and related ternary alloy films, model predictions must be compared with experimental kinetic results.

Time-temperature-transformation measurements of FePt thin films in the millisecond regime using pulse laser processing

A section of the time-temperature-transformation (TTT) curve has been measured in the millisecond regime to describe the A1 to L10 transformation of 10 nm FePt thin films. Short time annealing was accomplished using a pulsed neodymium-doped yttrium aluminum garnet laser operating at a wavelength of 1064 nm. The temperature-time profile of the films was measured using an optical pyrometer and a platinum thin film resistor and it was numerically modeled. Effective thermal pulse widths were determined from the time dependence of the atomic diffusion coefficient calculated from the measured temperature profile. The measured TTT diagram involving average order parameter is consistent with theoretical predictions of TTT diagrams involving ordered volume fraction and shows that partial ordering can be obtained with a single effective thermal pulse as short as 1.1 ms.

Effect of addition of N on L10 transformation and atomic diffusion in FePt films

Abstract The effect of nitrogen incorporation on the kinetics of L1 0 transformation in FePt alloy films was studied. Films with a maximum possible amount of nitrogen were prepared by sputtering a FePt compound target with pure nitrogen. As-prepared films consisting of nano-grains of Pt and Fe 0.7 N 0.3 transformed to face-centered cubic FePt phase with a certain amount of nitrogen incorporated in the grains after annealing at 573 K. It was found that the presence of N results in faster kinetics of L1 0 transformation compared with pure FePt alloy films, as well as a substantial reduction in grain size. Detailed structural and diffusion measurements were carried out to elucidate the mechanism of enhanced transformation kinetics as well as grain size reduction. The activation energy for volume diffusion of Fe in FePtN films was found to be 0.27 ± 0.14 eV, while that in FePt was 0.50 ± 0.17 eV. Faster atomic diffusion in nitrogen-containing films was the cause of an enhanced rate of L1 0 transformation. Further studies reveal that, in partially transformed FePtN alloy films, N is incorporated mainly in the grain boundary region, which hinders grain growth and results in a smaller grain size.

Effect of the structural state of the FePd equiatomic alloy on the temperature dependence of the initial magnetic susceptibility and the curie temperature

The temperature dependence of the initial magnetic susceptibility of a quenched equiatomic FePd alloy with an A1 structure and the variations of its Curie temperature and anisotropy of hysteresis loops have been studied during the ordering annealing of single-crystal samples at 550°C. The micro- and mesostructure, relief patterns, and magnetic-powder patterns on {100} surfaces of the single crystal were studied using electron microscopy and the polarization-optical method. The correlation between the temperature dependence of the magnetic susceptibility and the Curie temperature of the FePd alloy and the peculiarities of its elastically stressed nanocrystalline state during the A1 → L10 transformation is discussed.

Phase transformations and magnetic structure of nanocrystalline Fe–Pd and Co–Pt alloys studied by in situ neutron powder diffraction

The formation of the ordered L10 structure from the A1 phase in nanocrystalline Fe100−xPdx (x=50–60) and Co100−xPtx (x=45–55) alloys is followed by in situ neutron powder diffraction. The A1→L10 transformation starts at about 620 and 730K in Fe50Pd50 and Co50Pt50, respectively, and proceeds faster in the Fe–Pd alloys. In both Fe–Pd and Co–Pt, a deviation from the equiatomic composition leads to the deceleration of the ordering process. The magnetic moment of iron in the L10 FePd phase decreases with increasing Fe concentration from 3.3μB in Fe40Pd60 to 3.0μB in Fe50Pd50. In the Co–Pt alloys, the Co magnetic moment is found to be independent of the composition and degree of chemical order and is equal to 1.9μB.

Formation mechanism and composition distribution of FePt nanoparticles

Self-assembled FePt nanoparticle arrays are candidate structures for ultrahigh density magnetic storage media. One of the factors limiting their application to this technology is particle-to-particle compositional variation. This variation will affect the A1 to L10 transformation as well as the magnetic properties of the nanoparticles. In the present study, an analysis is provided for the formation mechanism of these nanoparticles when synthesized by the superhydride reduction method. Additionally, a comparison is provided of the composition distributions of nanoparticles synthesized by the thermal decomposition of Fe(CO)5 and the reduction of FeCl2 by superhydride. The latter process produced a much narrower composition distribution. A thermodynamic analysis of the mechanism is described in terms of free energy perturbation Monte Carlo simulations.

Time-temperature-transformation diagrams for the A1 to L1 phase transformation in FePt and FeCuPt thin films

The kinetics of the A1 to L10 transformation are given in the form of time-temperature-transformation diagrams. These diagrams are calculated using experimentally measured data for 1μm thick FePt and FeCuPt thin films and three variations of the Johnson-Mehl-Avrami-Kolmogorov kinetic theory. For the FePt alloys, an increase in Pt content from 45.6 to 52.5 at.% results in a >300 times increase in the annealing time needed to reach a fraction transformed of 0.95 at 300°C. Extension of the calculations for 1μm thick films to 10nm thick films using geometric corrections shows not only that thinner films result in slower kinetics for all temperatures, but also that film composition has a significant impact on the transformation behavior. Transformation in nanoparticles is highly dependent on the nucleation of the L10 phase.

Ordering of nanocrystalline Fe–Pt alloys studied by in situ neutron powder diffraction

The ordering in nanocrystalline Fe100−xPtx (x=40, 45, 50, and 55) alloys prepared by mechanical ball milling was studied by neutron powder diffraction during in situ heat treatment and differential scanning calorimetry. Independent of milling time and stoichiometry the onset of ordering is at around 300°C. For alloys with a multilayer-type microstructure an enhanced kinetics of ordering is observed, characterized by an activation energy of 1.03±0.08eV. The transformation to the L10 phase proceeds via a metastable state, which is a mixture of highly ordered L10 and L12 phases. For samples with homogeneous A1 phase, the A1→L10 transformation is characterized by a lower driving force and higher activation energy. For nonequiatomic compositions, the A1→L10 transformation is considerably decelerated.

The A1 to L1 transformation in FePt and FeCuPt thin films: Determination of isothermal transformation kinetics from nonisothermal measurements

Time–temperature-transformation diagrams describing the A1 to L10 transformation in 1μm thick FePt and FeCuPt alloy films are constructed from data obtained using nonisothermal differential scanning calorimetry in conjunction with a limited number of isothermal anneals. For the FePt alloys, an increase in Pt content from 45.6 to 52.5 at.% results in a >300× increase in the annealing time needed to reach a fraction transformed of 0.95 at 300 °C. Using geometric corrections, the results are extended to 10 nm thick films, which are found to transform approximately five times more slowly than thicker films.

Phase transformation in sputter-deposited PdMn and PdPtMn thin films

The phase transformations of PdMn and PdPtMn films were investigated using differential scanning calorimetry (DSC), x-ray diffraction (XRD), and transmission electron microscopy (TEM). The enthalpy for the fcc to L10 transformation in the PdMn and PdPtMn thin films has been measured by DSC as −5.4 and −7.6kJ∕molat., respectively. The fcc to L10 phase transformation was identified by XRD and TEM on as-deposited and annealed samples. The transition temperature for the PdPtMn is approximately 40 °C lower than that for PdMn. PdPtMn thin films have better corrosion resistance than PdMn.

L10 Ordered Intermetallics for Ultrahigh Density Magnetic Recording Media: Phase Formation and the Role of Alloy Chemistry and Composition

AbstractIn this paper we provide a summary review of our research regarding the effect of composition on the A1 to L10 transformation in binary FePt with compositions in the range of 39.3 to 55.3 at.% Pt and ternary FeCuPt alloys with Cu additions in the range of 1.0 to 19.4 at.%. These binary and ternary alloy films have been studied using non-isothermal differential scanning calorimetry (DSC) combined with the calculation of TTT diagrams generated via experimentally validated Johnson-Mehl-Avrami-Kolmogorov (JMAK) analysis. Composition is found to play a significant role in the transformation kinetics in both FePt and FeCuPt. However, when comparing equivalent FePt and FeCuPt alloys, the addition of Cu has no significant effect. TTT diagrams show that as little as a 90 °C increase in peak temperature from the DSC measurements can result in a >300× increase in the time required for full transformation using isothermal anneals. Our results on the effect of Cu additions are compared with those reported in the literature.

Differential scanning calorimetry studies of the effect of Cu on the A1 to L10 transformation in FePt thin films

Abstract Ternary FeCuPt alloys with 1–5 at.% Cu produced no advantage over equivalent binary FePt alloys in terms of lowering the kinetic ordering temperature. The Curie temperature decreased with increasing Cu content. The transformation enthalpy and activation energy were not affected.