Oriented and well-isolated L 10 -FeCuPd ternary alloy nanoparticles have been fabricated by electron-beam evaporation followed by postdeposition annealing. A single L 10 phase was formed in the FeCuPd nanoparticles with (Fe+Cu) content lower than 48 at. %. A strong preferential c -axis orientation along the film normal direction was achieved by Cu addition, which leads to a strong perpendicular magnetic anisotropy. Also, a lowering of the ordering temperature by 50 K compared to the binary L 10 -FePd nanoparticles was achieved by Cu addition. By contrast, composite particles composed of the bcc Fe and the L 10 -FeCuPd were formed when the (Fe+Cu) content was higher than 52 at. %. Coexistence of the bcc Fe and the L 10 -FeCuPd was confirmed by high-resolution transmission electron microscopy and nanobeam electron diffraction. It was found that perpendicular magnetic anisotropy of the L 10 -FeCuPd nanoparticles on the NaCl substrate is sensitive to the alloy composition. © 2006 American Institute of Physics.Hiroshi Naganuma, Kazuhisa Sato, and Yoshihiko Hirotsu, "Fabrication of oriented L1₀-FeCuPd and composite bcc-Fe/L1₀-FeCuPd nanoparticles: Alloy composition dependence of magnetic properties", Journal of Applied Physics 99, 08N706 (2006) https://doi.org/10.1063/1.2165604

Hirotsu, Yoshihiko

Naganuma, Hiroshi

Sato, Kazuhisa

English

Osaka University Knowledge Archive

TitleFabrication of oriented L1₀-FeCuPd andcomposite bcc-FeL1₀-FeCuPd nanoparticles: Alloycomposition dependence of magnetic propertiesAuthor(s) Naganuma, Hiroshi; Sato, Kazuhisa; Hirotsu,YoshihikoCitation Journal of Applied Physics. 99(8) p.08N706Issue Date 2006-04-15oaire:version VoRURL https://hdl.handle.net/11094/89396rightsThis article may be downloaded for personal useonly. Any other use requires prior permissionof the author and AIP Publishing. This articleappeared in Hiroshi Naganuma, Kazuhisa Sato,and Yoshihiko Hirotsu, "Fabrication of orientedL1₀-FeCuPd and composite bcc-Fe/L1₀-FeCuPdnanoparticles: Alloy composition dependence ofmagnetic properties", Journal of AppliedPhysics 99, 08N706 (2006) and may be found athttps://doi.org/10.1063/1.2165604.NoteOsaka University Knowledge Archive : OUKAhttps://ir.library.osaka-u.ac.jp/Osaka UniversityJOURNAL OF APPLIED PHYSICS 99, 08N706 2006Fabrication of oriented L10-FeCuPd and composite bcc-Fe/L10-FeCuPdnanoparticles: Alloy composition dependence of magnetic propertiesHiroshi Naganuma,a Kazuhisa Sato, and Yoshihiko HirotsuThe Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki,Osaka 567-0047, JapanPresented on 2 November 2005; published online 20 April 2006Oriented and well-isolated L10-FeCuPd ternary alloy nanoparticles have been fabricated byelectron-beam evaporation followed by postdeposition annealing. A single L10 phase was formed inthe FeCuPd nanoparticles with Fe+Cu content lower than 48 at. %. A strong preferential c-axisorientation along the film normal direction was achieved by Cu addition, which leads to a strongperpendicular magnetic anisotropy. Also, a lowering of the ordering temperature by 50 K comparedto the binary L10-FePd nanoparticles was achieved by Cu addition. By contrast, composite particlescomposed of the bcc Fe and the L10-FeCuPd were formed when the Fe+Cu content was higherthan 52 at. %. Coexistence of the bcc Fe and the L10-FeCuPd was confirmed by high-resolutiontransmission electron microscopy and nanobeam electron diffraction. It was found thatperpendicular magnetic anisotropy of the L10-FeCuPd nanoparticles on the NaCl substrate issensitive to the alloy composition. © 2006 American Institute of Physics.DOI: 10.1063/1.2165604I. INTRODUCTIONThe assembly of the isolated L10-ordered FePt or FePdnanoparticles has attracted much interest as one of the can-didate for future high-density magnetic recording media be-cause of their high uniaxial magnetic anisotropy energy.1However, a high-temperature heat treatment at temperaturesaround 873 K is necessary to obtain the highly ordered L10phase of nanoparticles with high coercivity. One of the cur-rent issues of these L10 materials is the addition of thirdelements in order to reduce the annealing temperature. In thecase of FePt thin films, Cu is well known as one of the mosteffective elements for reducing the ordering temperature.2On the contrary, there have been few reports on theL10-FePd thin films or nanoparticles with Cu additives sofar. Quite recently, we found that the perpendicular magneticanisotropy was enhanced by addition of Cu to FePdnanoparticles.3 The origin of the appearance of such a per-pendicular anisotropy has not been revealed so far. It is con-sidered that the magnetic properties of the L10-FeCuPdnanoparticles are sensitive to the alloy composition as wellas the Cu site in the ternary L10 phase. However, the alloycomposition dependence of the magnetic properties of theFeCuPd nanoparticles is still an open question.The present study aims at the synthesis and the charac-terization of L10-FeCuPd nanoparticles with different Fecontent by transmission electron microscopy and electrondiffraction. The nanostructure and magnetic properties werestudied as a function of the alloy composition.II. EXPERIMENTAL PROCEDUREFeCuPd nanoparticles were fabricated by successivedeposition4 of Pd, Cu, and Fe using an electron-beam evapo-aElectronic mail: hirosi22@sanken.osaka-u.ac.jp0021-8979/2006/998/08N706/3/$23.00 99, 08N70Downloaded 20 Apr 2006 to 133.1.93.172. Redistribution subject to Aration technique onto NaCl 001 substrates kept at 673 K.After the deposition of Fe, an amorphous Al2O3 thin filmwas further deposited to protect the particles from oxidation.Annealing of these specimens for the formation of theL10-ordered nanoparticles was made in a high-vacuum fur-nace at 823 K for 3.6 ks. Compositional analyses of thesespecimens were performed by energy-dispersive x-ray spec-troscopy EDS attached to a transmission electron micro-scope TEM operated at 300 kV JEM-3000F. The alloycompositions were changed by adjusting the nominal thick-ness of each element. The fabricated specimens had Fe con-tent between 36 and 55 at. % with a fixed Cu content ofabout 7 at. %, namely, FexCu7Pd93−x36x55. The nano-structure and morphology of the FeCuPd nanoparticles werecharacterized by TEM, selected area electron diffractionSAED, and nanobeam electron diffraction NBD. Themagnetic properties were measured by a superconductingquantum interference device SQUID magnetometer.III. RESULTS AND DISCUSSIONA. Alloy composition dependence of the magneticpropertiesFigure 1 shows the Fe+Cu concentration dependenceof the coercivity for the FexCu7Pd93−x nanoparticles after an-nealing at 823 K for 3.6 ks. High perpendicular coercivityvalues of more than 3 kOe were obtained for the specimenwith the Fe+Cu content between 43 and 48 at. %. In thiscomposition range, the coercivity values for the in-plane di-rection were much lower than those of the perpendicularones, indicating the enhancement of the strong perpendicularmagnetic anisotropy. The annealing temperature necessaryfor obtaining a coercivity higher than 3 kOe was 50 K lowerthan that of the binary L10-FePd nanoparticles. Dissolution5of Cu into Pd leads to a decrease of the melting temperature,© 2006 American Institute of Physics6-1IP license or copyright, see http://jap.aip.org/jap/copyright.jsp08N706-2 Naganuma, Sato, and Hirotsu J. Appl. Phys. 99, 08N706 2006which will results in an enhancement of the chemical diffu-sion constant D since the D value is inversely proportionalto the melting temperature of the material. By contrast, thecoercivity in the perpendicular direction abruptly decreasedto the same level as the in-plane coercivity at the Fe+Cucontent of 49 at. %.Figure 2 shows the magnetization curves of theFeCu48Pd52 and the FeCu62Pd38 nanoparticles measuredat 300 K a and 10 K b, respectively. The magnetic fieldwas applied perpendicular to the film plane. The coercivityand the remanence of the FeCu48Pd52 nanoparticles werehigher than those of the FeCu62Pd38 nanoparticles. At 10 K,coercivities were almost two times higher than those at 300K for both specimens, indicating the suppression of thermalagitation as well as the increase of the magnetic anisotropyenergy at low temperature.B. Alloy composition dependence of nanostructureFigures 3a and 3b show the SAED patterns, the high-resolution TEM HRTEM images, and the bright-field TEMimages for the FeCu48Pd52 and the FeCu62Pd38 nanopar-ticles after annealing at 823 K for 3.6 ks, respectively. In theSAED pattern shown in Fig. 3a, superlattice reflectionsfrom the L10-type ordered structure are visible. ManyFIG. 1. Fe+Cu concentration dependence of the coercivity for theFexCu7Pd93−x nanoparticles measured at 300 K after annealing at 823 K for3.6 ks with the external field both perpendicular solid circles and parallelopen circles to the film plane.FIG. 2. Magnetization curves of the FeCu48Pd52 and the FeCu62Pd38nanoparticles measured at 300 K a and 10 K b, respectively.Downloaded 20 Apr 2006 to 133.1.93.172. Redistribution subject to AL10-FeCuPd nanoparticles have a squarelike shape with110 facets with a preferential c-axis perpendicular orienta-tion showing strong 110 superlattice reflections in the SAEDpattern. Actually, this specimen showed a strong perpendicu-lar anisotropy as shown in Fig. 1. By contrast, in the case ofthe FeCu62Pd38 nanoparticles, not only the superlattice re-flections but also the reflections from the bcc Fe can be seenin the SAED pattern Fig. 3b. The HRTEM image in Fig.3b shows two types of lattice fringes in the same nanopar-ticle corresponding to the L10-FeCuPd and the bcc Fe. Theformation of the nanocomposite particles was also confirmedby NBD as shown in Fig. 4. These NBD patterns correspondto local regions in a nanoparticle with the bcc Fe a and theL10-FeCuPd b with the c axis oriented perpendicular to thefilm planes. These results indicate a formation of nanocom-posite particles composed of the bcc-Fe and the L10-FeCuPdphases. The average lattice parameters and the axialratios are shown in Fig. 5. The axial ratios for the presentFeCuPd nanoparticles are in the range between 0.923 andFIG. 3. SAED patterns, HRTEM images, and bright-field TEM images forthe FeCu48Pd52 nanoparticles a and the FeCu62Pd38 nanoparticles bafter annealing at 823 K for 3.6 ks.0.928 and are much smaller than that of the binary L10-FePdIP license or copyright, see http://jap.aip.org/jap/copyright.jsp08N706-3 Naganuma, Sato, and Hirotsu J. Appl. Phys. 99, 08N706 2006nanoparticles c /a=0.959±0.004 Ref. 4 and close to thevalue reported in the L10-FeCuPt2 compound c /a=0.923±0.001.6 The composition of each nanoparticle wasindependent of the particle size, and also the alloy composi-tion distribution was small 2 at. % according to thenanobeam EDS analysis. Although, the additive Cu isthought to be responsible for the observed smaller axial ratio,the role of Cu for reducing the axial ratio is still an openquestion. It should be noted that the lattice parameter and theaxial ratio are independent of the alloy composition, indicat-ing the fact that the degree of order is independent of theexistence of bcc Fe in the case of the bcc Fe/L10-FeCuPd,although the coercivity of the nanocomposite particles waslower than that of the L10-FeCuPd nanoparticles. The ob-served disappearance of the strong perpendicular magneticanisotropy in the nanocomposite particles can be attributedto the existence of bcc Fe with cubic anisotropy.Figure 6 shows the diffracted beam intensity profiles forthe FeCuPd nanoparticles measured along the 001* direc-tion in the SAED patterns as a function of the Fe+Cucontent. The annealing condition was 823 K for 3.6 ks forevery specimen. Both 001FeCuPd and weak 200Fe reflectionscoexisted in the specimens with the Fe+Cu compositionsof 62, 57, and 52 at. %, while the intensity of the 200Fereflection decreased at 52 at. % and disappeared at 48 at. %,indicating that the amount of the bcc-Fe phase decreased asthe composition reached the equiatomic composition ofFeCu50Pd50 Ref. 7. It can be considered that the additiveCu replaces the Fe site in the L10-FeCuPd ternary alloynanoparticles. In other words, although the substitution onFIG. 4. NBD patterns for the FeCu62Pd38 nanoparticles after annealing at823 K for 3.6 ks. The NBD patterns correspond to the local regions in thenanoparticles with bcc Fe a and the L10-FeCuPd of the c axis orientedperpendicular to the film plane b.FIG. 5. Fe+Cu content dependence of the lattice parameters and the axialratios for the L10-FeCuPd nanoparticles. The averaged measurement errorwas ±0.004 nm for a and c axes and ±0.004 for c /a.Downloaded 20 Apr 2006 to 133.1.93.172. Redistribution subject to Athe Fe site by Cu effectively reduces the ordering tempera-ture, the decrease of Fe atoms will hinder the magnetizationwhen the Cu content becomes larger. The quantitative evalu-ation of the magnetocrystalline anisotropy constant andthe saturation magnetization are necessary for further discus-sion.IV. CONCLUSIONL10-FeCuPd nanoparticles and bcc-Fe/L10-FeCuPdnanocomposite particles were fabricated by successive depo-sition of Pd, Cu, and Fe followed by postdeposition anneal-ing at 823 K for 3.6 ks. A high coercivity more than 3 kOewith perpendicular magnetic anisotropy was obtained for thespecimens with Fe+Cu content between 43 and 48 at. %.The perpendicular coercivity abruptly decreased when theFe+Cu content is higher than 49 at. %. The drastic changeof the coercivity is related to the formation of nanocompositeparticles composed of bcc Fe and the L10-FeCuPd. The lat-tice parameter and the axial ratio were independent of thealloy composition, although the coercivity values of thenanocomposite particles were lower than those of theL10-FeCuPd nanoparticles. It has been shown that the per-pendicular magnetic anisotropy as well as nanostructure canbe changed by controlling the alloy composition in the ter-nary L10-FeCuPd nanoparticles.ACKNOWLEDGMENTSThis study was partly supported by the Center of Excel-lence COE program at ISIR, Osaka University and by theGrant-in-Aid for Scientific Research S Grant No.16106008 from the Ministry of Education, Culture, Sports,Science and Technology, Japan.1D. Weller and M. F. Doerner, Annu. Rev. Mater. Sci. 30, 611 2000.2T. Maeda, T. Kai, A. Kikitsu, T. Nagase, and J. Akiyama, Appl. Phys. Lett.80, 2147 2002.3H. Naganuma, K. Sato, and Y. Hirotsu unpublished.4K. Sato and Y. Hirotsu, J. Appl. Phys. 93, 6291 2003.5T. B. Massalski, H. Okamoto, P. R. Subramanian, and L. Kacprzak, BinaryAlloy Phase Diagrams, 2nd ed. ASM International, Materials Park, Ohio,1990, p. 1454.6M. Shahmiri, S. Murphy, and D. J. Vaughan, Miner. Mag. 49, 547 1985.7FIG. 6. Diffracted beam intensity profiles for the FeCuPd nanoparticlesmeasured along the 001* direction in the SAED patterns as a function ofthe Fe+Cu content.J. Kawamura, K. Sato, and Y. Hirotsu, J. Appl. Phys. 96, 3906 2004.IP license or copyright, see http://jap.aip.org/jap/copyright.jsp

Fabrication of oriented L1₀-FeCuPd and composite bcc-FeL1₀-FeCuPd nanoparticles: Alloy composition dependence of magnetic properties

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Fabrication of oriented L1₀-FeCuPd and composite bcc-FeL1₀-FeCuPd nanoparticles: Alloy composition dependence of magnetic properties

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