FePtCu Alloy Research Articles

Evolution of structural and magnetic properties of FePtCu alloy films on annealing of FePt/Cu multilayers.

Thin films of ternary (FePt)100-xCux alloys were obtained by annealing of FePt (100 Å)/Cu (d Å) multilayers with d = 50 and 100 Å deposited by sputtering at room temperature on Si substrates. The evolution of structural and magnetic properties of these multilayers induced by isochronal and isothermal annealing in a vacuum has been studied using depth dependent characterization techniques. Isochronal annealing for 0.5 h at different temperatures (300 to 600 °C) showed very low interdiffusion at the interfaces with no signature of alloy phase formation. However, isothermal annealing of multilayers at 600 °C for longer times (1.5-6.5 h) showed significantly large interdiffusion accompanied by the formation of polycrystalline ternary alloy and iron silicide phases. The iron silicide formed at the substrate-film interface assists the growth of the L10 ordered ternary alloy phase, which showed different stoichiometry for different multilayers. The L10 phase formed with higher Cu content showed drastically different magnetic properties with a reduction in saturation magnetization and an increase in coercivity (∼6 kOe) at room temperature. The iron silicide formed on high temperature annealing showed ferromagnetic nature with a magnetization of ∼140 emu cm-3 at room temperature.

Core/shell Cu/FePtCu nanoparticles with face-centered tetragonal texture: An active and stable low-Pt catalyst for enhanced oxygen reduction

Low-Pt based nanocrystals demonstrate potential as highly active catalysts for the oxygen reduction reaction (ORR), but they suffer from undesirable structural degradation. Therefore, it is highly challenging to optimize their surface and interfacial structures to tune their catalytic properties for both activity and stability. Here core–shell Cu/FePtCu nanoparticles with a face-centered-tetragonal phase are prepared by a facile one–pot polyol method at 320 °C. The optimized core–shell Fe45Pt35Cu20 catalyst with Pt-enriched surface exhibits 0.5 A/mgPt mass activity, which is a factor of 4 better than that of commercial Pt/C (0.13 A/mgPt). In addition, the current density of the catalyst drops only 3.0% after 1000 cycles, which is much better than Pt/C (34.2% decay). Using aberration-corrected scanning transmission electron microscopy and atomically resolved elemental mapping, the morphology and structure evolving between the FePtCu alloy and core–shell Cu/FePtCu could clearly be explained. This work demonstrates that an ordered and core–shell FePtCu catalyst is highly promising for ORR and other electrochemical processes.

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.

Room temperature superparamagnetism in ternary (Fe50Pt50)0.42Cu0.58 phase at interfaces on annealing of Fe50Pt50/Cu multilayer

The face- centered- tetragonal (FCT) or L10 phase of Fe50Pt50 (FePt) film is obtained on annealing FePt films at high temperature (∼600 °C), that usually grows as face-centered- cubic (FCC) phase. Cu as an additive up to ∼20 atomic%, which form ternary alloy FePtCu on mixing, is used to reduce the ordering temperature. We studied the formation of FePtCu ternary alloy: its structure, magnetic properties and grain growth at the interfaces of a FePt/Cu multilayer, as a function of annealing temperature (100 °C–600 °C), which is very less as compared to their melting temperatures. The study indicates the growth of a ternary alloy with composition of (FePt)0.42Cu0.58 in FCT phase on annealing the FePt/Cu multilayer at a moderate annealing temperature (∼400 °C). With a large addition of Cu at 56 atomic% in FePt, the ordering temperature reduces by ∼200 °C and form a ternary alloy with distinct magnetic properties. We observed rapid growth of ordered FCT phase with increase in crystallite size from 50 Å to 150 Å on annealing the multilayer in the range of 400 °C–600 °C. Interestingly, from macroscopic magnetization measurements we could identify the existence of a super-paramagnetic (SP) phase at room temperature with a typical particle size of ∼12 Å of ternary alloy on annealing the multilayer at 400 °C, which is much smaller than the crystallite size (∼50 Å). Annealing at 600 °C reduces the multilayer to a disordered magnetic phase. The room temperature SP nanoparticles of FePtCu alloy phase formed on annealing the multilayer at 400 °C are potential candidates in biomedical applications.

(Invited) Structural Control of Electrodeposited L10-FePt Nanodot Arrays Towards Reduction of Ordering Temperature

Electrodeposition has been widely used as a fabrication method of micro/nano structures due to its precise control of growth and high deposition uniformity at nanoscale. Utilizing these features, we have attempted to fabricate ferromagnetic nanodot arrays [1] by combining electron beam lithography (EBL) and electrodeposition for use in bit patterned media (BPM), which have been suggested as one of the few designs capable to achieve ultrahigh density in magnetic recording medium [2]. L10-FePt ordered alloy has been considered as one of the most promising candidates for the use in BPM due to its high magnetocrystalline anisotropy, Ku [2]. In previous work, we have succeeded to fabricate FePt nanodot arrays with 35 nm in pitch [1], however, high temperature annealing at ca. 600 oC was needed for the phase transition to L10 ordered structure, which caused a deterioration of nanodot arrays. In order to solve this problem, in the present work, we have attempted to fabricate uniform L10-FePt nanodot arrays by applying multilayered FePt structure [3] and FePt-Cu alloy structure [4] to lower the ordering temperature. Nanopore patterned substrates were fabricated by EBL onto sputter deposited Ru (60 nm thick) / Ti (5 nm thick) / n-Si (100) substrate. Utilizing these substrates, FePt was potentiostatically electrodeposited under the conditions summarized in Table 1 to fabricate FePt nanodot arrays. Post annealing treatment of deposited FePt was carried out in argon atmosphere (Ar 90 % and H2 10 %) using rapid thermal annealing system. Multilayered FePt were fabricated by pulse deposition of Fe-rich layer and Pt-rich layer with -1.4 and -0.8 V (vs. Ag/AgCl), and the thickness of each layer was set to around 2 nm by adjusting the pulse duration. For the fabrication of FePt-Cu alloy, CuSO4 was added into the bath condition of Table1. In order to analyze the effect of multilayered structure on phase transition and coercivity of FePt, magnetic properties and crystal structures of multilayered FePt film and continuous FePt film, which was deposited with constant potential of -1.0 V (vs. Ag/AgCl), with thickness of 20 nm after annealing at 450 oC for 1 h were analyzed. As a result, perpendicular coercivity increased from 1.7 kOe to 6.6 kOe in the multilayered structure compared to continuous film, and x-ray diffraction (XRD) patterns of multilayered film showed clear diffraction peaks of L10 ordered structure, whose intensity were very small in the continuous film. In addition, ordering parameter, S, of FePt films were calculated form the axial ratio (c/a), which was determined from the XRD patterns, and it was found that S was increased from 0.24 to 0.80 in multilayered structure, indicating that phase transition of FePt was facilitated in the multilayered structure resulting in the increase of coercivity at low ordering temperature. Based on these results, fabrication of multilayered FePt nanodot arrays with 35 nm in pitch was attempted, and uniform nanodot arrays without deterioration of dots were successfully formed after post annealing process at 450 oC for 1 h. Subsequently, lowering the ordering temperature of FePt was attempted by addition of Cu into FePt films to form ternary FePt-Cu alloy. XRD patterns of FePt-Cu films after annealing at 450 oC for 1 h showed peaks of L10 ordered structure without elemental Cu peaks, and (001) peak of L10 structure was shifted to higher 2θ angle compared to FePt binary alloy, providing evidence that Cu was alloyed with FePt to shrink the lattice in the c-axis. In addition, perpendicular coercivity of FePt-Cu film was higher than that of FePt film at annealing temperature of 450 oC, indicating that reducing the ordering temperature was successfully attained by the addition of Cu to FePt. These results demonstrated a successful fabrication of uniform L10-FePt nanodot arrays with Tbit/in2 level density, by controlling the fine structure of FePt nanodot arrays by applying multilayered structure or addition of Cu to lower the ordering temperature. This work was financially supported in part by JSPS KAKENHI Grant Number 25249104. [1] T. Homma, S. Wodarz, D. Nishiie, T. Otani, S. Ge, and G. Zangari, Eletrochem. Soc. Trans., 64, 1-9 (2015) [2] B. D. Terris, and T. Thomson, J. Phys. D: Appl. Phys., 38, R199-R222 (2005) [3] Y. Endo, N. Kikuchi, O. Kitakami, and Y. Shimada, J. Appl. Phys., 89, 7065-7067 (2001) [4] T. Maeda, T. Kai, A. Kikitsu, T. Nagase, and J. Akiyama, Appl. Phys. Lett., 80, 2147-2149 (2002) Figure 1

Scaling dependence and tailoring of the pinning field in FePt-based exchange coupled composite media

We studied exchange coupled composite (ECC) media with an out-of-plane easy axis consisting of hard magnetic L10 chemically ordered FePtCu alloy films and magnetically softer [Co/Pt]N multilayer stacks. By tailoring the structural properties of the ternary FePtCu alloy and [Co/Pt]N multilayers, we can tune the magnetic parameters of the composite in a wide range. This allowed us to address experimentally one of the most crucial properties determining the performance of ECC media, namely the pinning field of the magnetic domain wall present at the interface between the hard and soft layers. We demonstrate that the pinning field is proportional to the difference of the magnetic anisotropy constants of the hard and soft layers, which confirms the theoretical predictions. Furthermore, we show that the pinning field can be efficiently decreased after an additional annealing step. Transmission electron microscopy investigations indicated that the origin of the observed effect is due to a heat-induced phase transformation of iron oxide present at the interface between the hard and soft layers. This study reveals that tailoring the properties of the hard/soft interface is another efficient tuning knob for optimization of the performance of ECC media.

Perpendicular FePt-based exchange-coupled composite media

Exchange-coupled composite media were realized by combining perpendicular hard magnetic FePtCu alloy films with perpendicular Co/Pt multilayers which are magnetically softer. We demonstrate that the switching field of the hard layer can be efficiently altered by modifying the material properties of the soft layer by varying the number of Co/Pt bilayers. Moreover, the possibility of effectively tuning the interlayer exchange coupling using rapid thermal annealing was shown. These studies were supported by theoretical modeling revealing the relevant factors to reduce the switching field of the hard layer which are important for future media design.

Structural and magnetic properties of (FePt) 1− xCu x nanowires

Highly ordered (FePt) 1− x Cu x nanowire arrays have been fabricated successfully by double-pulse electro-deposition into the pores of a porous anodic aluminum oxide template. We have found that the addition of Cu to FePt nanowires is an effective approach for reducing the ordering temperature of FePt. The coercivity of the (FePt) 88Cu 12 nanowire arrays is around 2 kOe after annealing at 300 °C for 1 h, whereas that of FePt nanowire arrays shows several hundred Oe. X-ray diffraction characterization showed that only one set of L1 0 diffraction peaks appeared and no elemental Cu diffraction peaks were visible. This result, along with a varying c/ a-lattice parameter ratio, suggests that Cu substitutes Fe or Pt in the ternary FePtCu alloy nanowires.

Nanostructure and magnetic properties of highly (001) oriented L1 (Fe49Pt51)1−xCux films

We report on nonepitaxially grown L10 Cu-alloyed FePt thin films with strong (001) texture. The FePt films with different Cu contents were deposited directly on Si wafers with a Fe49Pt51∕Cu multilayer structure. The Cu content was varied from 0to13at.%. All films were annealed at 600°C for 5min. X-ray-diffraction characterization showed that only one set of L10 diffraction peaks appeared and no elemental Cu diffraction peaks were visible. This result, along with a varying c∕a lattice-parameter ratio, suggests that Cu substitutes Fe or Pt in the L10 lattice and ternary FePtCu alloy films are formed. (001) texture was enhanced with the increase of Cu content. Transmission electron microscope images showed that the grain size of FePtCu was about 10nm. For FePt film with 11at.% Cu substitution, coercivity was about 5kOe, which is suitable for writing in a practical perpendicular-recording film.

Influence of oxygen content on the reduction of the ordering temperature of L1/sub 0/ FePtCu alloy

The influence of the residual oxygen on the ordering (transformation from fcc to fct) temperature of sputtered Fe-Pt-Cu films was investigated. Secondary ion mass spectroscopy analysis revealed that high oxygen content increases the onset temperature of the ordering process. The oxygen is brought into an Fe-Pt-Cu film during the deposition process and forms stable Fe-O compound. This might increase the ordering temperature as well as prevent the coalescence of grains, that is, the nucleation of phase transformation. A hot-pressed Fe-Pt composite target which contains oxygen of 50 ppm was developed. This target is also effective for reducing the ordering temperature of Fe-Pt films.

Reduction of ordering temperature of an FePt-ordered alloy by addition of Cu

We have found that the addition of Cu to an FePt alloy film is an effective approach for reducing the ordering temperature of FePt. The coercivity of the FePtCu film is around 5 kOe after annealing at 300 °C, whereas that of FePt shows several hundred Oe. In the FePtCu film annealed at 700 °C, the coercivity is almost the same as for the FePt films. Therefore, the FePtCu film displays a hard-magnetic property similar to that of the FePt film. The results of x-ray diffraction indicate that a ternary FePtCu alloy is formed. Thus, the formation of the ternary FePtCu alloy is considered to play an important role in reducing the ordering temperature.