Microstructure and magnetic properties of FePt and Fe/FePt ...

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476 J. Appl. Phys., Vol. 96, No. 1, 1 July 2004 Takahashi et al.FIG. 1. XRD patterns for various thicknesses of FePt films: 10 nm a, 30nm b, 38nmc, and 100 nm d.was 6.010 7 Pa, and a high-purity argon of 0.1 Pa wasflown during sputtering. The growth rate of FePt was controlledto 0.10 nm/s and the nominal thickness was evaluatedbased on the sputtering time. To produce Fe/FePt nanocomposites,Fe layers of 20%, 40%, and 60% of FePt thicknesswere deposited on the FePt film at room temperature. Thefilm structures were characterized by x-ray diffractionXRD and transmission electron microscopy TEM. Thecompositions of the FePt estimated by energy dispersivex-ray spectroscopy were about Fe 50 Pt 50 for all the films. Themagnetic properties of the films were examined by a superconductingquantum interference device magnetometer.III. RESULTSA. FePt filmFigure 1 shows x-ray diffraction patterns for the FePtfilms with various thicknesses, t10 nm a, 30nmb, 38nm c, and 100 nm d. Superlattice diffraction lines of001 and 112 are clearly observed around 224° and60°, respectively. All the films are ordered to the L1 0 structurein the as-deposited state. The other unlabeled sharppeaks are due to the Si substrate. The relative integratedintensities of 001 and 111 indicates that all the films havea weak preferred orientation to 001 in the perpendiculardirection to the film plane.Figure 2 shows TEM bright field images of the FePtfilms with various thicknesses. The inset selected area electrondiffraction SAED patterns show that all the films are inthe L1 0 ordered state, because the superlattice diffractionrings of 001 and 110 are clearly observed as indicated bythe arrows. The average particle size of the 10 nm thick filmis about 50 nm. The FePt particles are completely isolatedwith each other. With increasing the film thickness, the averageparticle size increases. The films that are thicker than60 nm have an interconnected network structure. This morphologicalchange is the same as that observed in the FePtfilm deposited on a MgO 001 single crystal substrate. 14Twins are commonly observed in the particles as indicatedby the arrows. Figure 3 shows a typical 011 nanobeamdiffraction pattern obtained from one of the grains that containa twin. The twin was identified as the (111¯) twin, so thisis different from the transformation twins that form when fccdisordered phase transform to the L1 0 tetragonal lattice. 18Hence, these twins are believed to be growth twins whichcommonly occur in the fcc crystals during growth by variousthin film processing. The 111 twins in the L1 0 ordered FePtthin films were also reported by Hong et al. 19Figure 4 shows magnetization curves of the FePt filmswith t10 nm a, 20nmb, 30nmc, 38nmd, 60nme, and 100 nm f. The filled and open circles show themagnetization curves in the perpendicular and in-plane directionsto the film, respectively. The magnetization curves arenot corrected with the demagnetization factors. The filmsthat are thicker than 30 nm show weak perpendicular anisotropydue to the development of the 001 preferred orientationduring the film growth. Because of the high magnetocrystallineanisotropy of the isolated particles, theFIG. 2. TEM images and SAED patterns for various thicknesses of FePt films: 10 nm a, 20nmb, 30nmc, 38nmd, 60nme, and 100 nm f.Downloaded 26 Jun 2004 to 144.213.253.14. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp
J. Appl. Phys., Vol. 96, No. 1, 1 July 2004 Takahashi et al.477FIG. 5. Change of H c as a function of the film thickness.FIG. 3. 011 nanobeam diffraction pattern obtained from one of the grainsthat contains a twin.magnetization curves of the films with t10 and 20 nm donot saturate even at a magnetic field of 55 kOe. A largecoercivity (H c ) of about 23 kOe was obtained from the 10nm thick film. H c gradually decreases with increasing thefilm thickness, but still shows a high H c of about 13 kOe att100 nm. The initial magnetization curves for the 10 nmand 20 nm thick films are characteristic of the rotation magnetization,because large magnetic field is required to magnetizethe particles. On the other hand, those for 60 and 100nm thick films are characteristic of the domain wall displacementbecause the initial magnetization curve indicate that themagnetization progress easily at the low magnetic field. Figure5 shows the change of H c as a function of the filmthickness. The H c of the FePt films deposited on MgO001 14 and MgO 110 20 single crystal substrates are alsoshown in the same figure. With increasing film thickness, theH c for the films deposited on the SiO 2 substrates decreasesgradually, while the H c for the single crystal films depositedon MgO 001 substrates shows a drastic decrease when thefilm thickness is more than 40 nm. The gradual decrease ofH c in the polycrystalline film as a function of film thicknessis similar to that observed in the FePt film deposited on aMgO 110 substrate. 20B. FeÕFePt filmTo obtain a higher energy product by making a nanocompositewith a soft phase, Fe was deposited on 10 and 38nm thick FePt particulate films. Figures 6 and 7 show thein-plane and cross section TEM bright field images of: aFePt 10 nm film, b Fe 2 nm/FePt 10 nm, c Fe 4 nm/FePt10 nm, and d Fe 6 nm/FePt 10 nm bilayer films. The Fe110 diffraction ring is clearly observed in Fig. 6d. Theplan-view image shows that the film morphology is particu-FIG. 4. Magnetization curves with initial magnetization curve of variousthicknesses of FePt films. Filled and opened circles show the magnetizationcurves in the perpendicular and in-plane direction to the film, respectively.FIG. 6. TEM images and SAED patterns of Fe/FePt 10 nm bilayer films.Downloaded 26 Jun 2004 to 144.213.253.14. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp
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