Effect of Annealing on Magnetic Properties of FePd ...

Effect of Annealing on Magnetic Properties of FePd and FePdPt Nanoparticles Sachil Sharma, N. S. Gajbhiye, and R. S. Ningthoujam Citation: AIP Conf. Proc. 1313, 125 (2010); doi: 10.1063/1.3530464 View online: http://dx.doi.org/10.1063/1.3530464 View Table of Contents: http://proceedings.aip.org/dbt/dbt.jsp?KEY=APCPCS&Volume=1313&Issue=1 Published by the American Institute of Physics.

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Effect of Annealing on Magnetic Properties of FePd and FePdPt Nanoparticles Sachil Sharma1,a), N. S. Gajbhiye1,b) and R. S. Ningthoujam2 1

2

Department of Chemistry, Indian Institute of Technology, Kanpur, 208 016, India Chemistry Division, Bhahba Atomic Research Centre, Mumbai, 400085, India

Abstract. Nanoparticles of FePd and FePdPt with the average size of ~ 3 nm have been prepared by modified polyol route. As-prepared nanoparticles are crystallized in fcc phase, whereas, 550-600 °C annealed nanoparticles are crystallized in fct phase. As-prepared samples are superparamagnetic at 300 K, whereas, annealed samples are strongly ferromagnetic at 300 K. As compared to fct FePd nanoparticles (Hc =1180 Oe), the fct FePdPt nanoparticles show significantly high coercivity (Hc = 4675 Oe) and squareness ratio (σr/σs =0.71) and thus, the addition of Pt in FePd nanoalloy improves the magnetic anisotropy significantly. The Curie temperature of FePd nanoalloy increases with increasing annealing temperature because of increase of atomic ordering in fct phase. Keywords: Magnetic anisotropy, Phase transition, Coericivity PACS: 75.50.Bb, 75.50. Ss, 75.30.Gw

INTRODUCTION

EXPERIMENTAL DETAILS The details of synthesis of FePd nanoparticles have been described elsewhere.6 The procedure involved simultaneous chemical reduction of palladium (II) acetyl acetonate and thermal decomposition of iron pentacarbonyl (Fe(CO)5) in the presence of oleic acid and oleyl amine as surfactants. We extended the same approach to prepare 3 nm FePdPt particles.6 In a typical procedure, 0.005 M solution of platinum (II) acetylacetonate (39 mg) in diphenylether (20 mL) is heated in the presence of 0.18 M polyethylene glycol-200 (PEG-200) (2 mL) as a reducing agent and 0.02 M oleic acid (0.16 mL) and 0.02 M oleylamine (0.16 mL) as surfactants in 100 mL round bottom flask. Upon heating at 200 oC, color of reaction solution turned black indicating start of chemical reduction of platinum (II) acetylacetonate. At this temperature, 0.005 M solution of palladium (II) acetylacetonate (31 mg) (prepared in a 3 mL of diphenyl ether) is injected into reaction vessel through septum. This is readily followed by injection of 0.02 M of iron pentacarbonyl (0.052 mL) into the reaction solution through septum. The reaction solution is mixed by stirring. Further, temperature of reaction solution is raised to 260 oC and refluxed for 50 min. The black colored solution is cooled down to room temperature under argon gas atmosphere. The product FePdPt nanoparticles are precipitated by adding

The 3d-4d/5d magnetic metal-alloy nanoparticles such as FePt, CoPt, FePd and FePdPt with tetragonal L10 phase are of fundamental interest mainly because of their high uniaxial anisotropy (Ku).1-4 The uniaxial anistotropy (Ku) of L10 phase of FePt and CoPt is of order ~ 5-7 × 107ergs/cm3, while the L10 phase of FePd has Ku of the order ~1.8 × 107 ergs/cm3, which is nearly one third value of L10 FePt phase.4 In L10 FePd, FePt and FePdPt nanosystems, the magnetic properties arise from hybridizaion and strong spin-orbit coupling of 3d and 4d/5d states of Fe and Pd/Pt respectively.5 As a result of this, a large asymmetric electron distribution occurs around the Fermi level responsible for strong ferromagnetism. In our previous work, phase transition, magnetic study and surface effects have been reported for the self-assembled FePd nanoparticles.6 In present study, we explored further the effect of annealing on the magnetic properties of 3 nm FePd particles. The effect on magnetic properties on the addition of Pt to FePd alloy nanoparticles (i.e. Fe51Pd26Pt23 alloy nanoparticles) is also studied. The L10 (fct) phase of Fe51Pd26Pt23 alloy nanoparticles show very high coericivity (Hc) and squareness ratio (σr/σs) as compared to L10 FePd nanoparticles a), b)

Authors to whom correspondence should be addressed. Electronic mails: a)[email protected]; b)[email protected]

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ethanol as a flocculating agent. The 3 nm FePdPt particles are extracted by centrifugation at 6000 rpm for 15 min. The powder is dried under vacuum. As-prepared 3 nm FePd and FePdPt particles (25 mg) are transferred to an alumina boat and placed in the tubular heating furnace. The boat is purged with a mixture of N2 + H2 (86% + 14%) gas for 15 min and is then, heated to 450, 550 and 600 oC under continuous flow of N2 + H2 (86% + 14%) mixture gas at heating rate of 10 oC/min for 1 h. As-prepared FePd nanoparticles are also heated at 550 oC in N2/Ar gas atmosphere only for 1 h. The powder X-ray diffraction (XRD) patterns of the samples were recorded using a Phillips powder Xray diffractometer (model PW 1071) with Ni-filtered CuKα radiation. The morphology of nanoparticles was determined using transmission electron microscope (TEM, model JEOL JEM-2000FX). The field and temperature dependent magnetization measurements are carried out using vibrating sample magnetometer, Model DMS ADE-EV7. The composition of alloy was confirmed by energy dispersive X-ray analysis (EDAX) attached with Scanning electron microscope (SEM, model JEOL JSM-840A).

The XRD pattern of 550°C annealed FePd nanoparticles under N2/Ar gas (Fig. 1 (d)) shows the small Fe3O4 peaks along with fcc peaks of FePd. This indicates that reducing atmosphere of H2 along with N2 is essential for the phase transformation from fcc to fct. The hydrogen atoms have been proposed to occupy interstitial sites of FePd lattice, inducing a local strain to enhance Fe and Pd mobility and structural ordering. The XRD patterns of as-prepared and 600 oC annealed FePdPt nanoparticles under N2 + H2 are shown in Fig. 2 (a, b). As-prepared FePdPt nanoparticles show fcc structure (Fig. 2(a)) with the lattice parameter a = 3.901(2) Å. Average crystallite size calculated by using the Scherrer’s relation is found to be about 3 nm. While 600 oC annealed FePdPt nanoparticles show face centered tetragonal (fct) structure (Fig. 2(b)) with the lattice parameters a = 3.867(2) Å, c = 3.692(2) Å and axial ratio, c/a = 0.955.

RESULTS AND DISCUSSIONS XRD patterns of as-prepared, 450 and 500 °C annealed FePd nanoparticles under N2 + H2 are shown in Fig. 1 (a, b, c). As-prepared FePd nanoparticles show face centered cubic (fcc) structure (Fig. 1(a)) with the lattice parameter a = 3.904(2) Å. Average crystallite size calculated by using the Scherrer’s relation is found to be about 3 nm. While 550 °C annealed sample shows the face centered tetragonal (fct) structure (Fig. 1(c)) with the lattice parameters a = 3.859(1) Å, c = 3.716(1) Å and axial ratio (c/a) = 0.962, which is close to the reported value of 0.966 for L10 phase of FePd alloy.7

FIGURE 2. XRD patterns of (a) as-prepared fcc and (b) 600°C annealed fct FePdPt nanoparticles.

Figure 3 shows field dependent magnetization (σH) curves for as-prepared and 450, 550 and 600 °C annealed FePd nanoparticles at 300 K. As synthesized fcc FePd nanopartilces exhibit the saturation magnetization (σs) of 18.22 emu/g and zero coercivity and thus, are superparamagnetic at room temperature. Interestingly, 550 °C annealed fct Fe43Pd57 nanoparticles show the magnetization (σs) = 65.2 emu/g, remanence (σr) = 24.6 emu/g and coericivity (Hc) =1180 Oe at room temperature (Fig. 3). The values of coericivity (Hc ~ 1300 Oe) and saturation magnetization (σs ~ 93.6 emu/g ) are found to be larger for 600 oC annealed fct FePd nanoparticles compared to that of 550 °C annealed fct sample. This is due to better chemical ordering of fct phase in 600 °C annealed nanoparticles.6 Usually, Curie temperature (Tc) is dependent on particle size and chemical ordering in nanoparticles. In our study, Tc of 550 oC annealed fct FePd nanoparticles

FIGURE 1. XRD patterns of (a) as-prepared fcc, (b) 450 °C, (c) 550 °C annealed FePd nanoparticles under N2 + H2 (86% + 14%) gas and (d) 550 °C annealed FePd nanoparticles under N2 gas only.

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is found to be around 454 oC (Fig. 4), which is less than Tc of bulk value (~ 487 °C) for fct (L10) phase of

Figure 5 shows the field dependent magnetization (σH) curves recorded for as-prepared fcc Fe51Pd26Pt23 and 600 °C annealed fct Fe51Pd26Pt23 nanoparticles at 300 K. The σ-H curve of as-prepared fcc Fe51Pd26Pt23 nanoparticles exhibit the saturation magnetization of 17.6 emu/g and zero coercivity at 300 K. Thus, asprepared Fe51Pd26Pt23 nanoparticles are superparamagnetic at room temperature. The fct (L10) Fe51Pd26Pt23 nanoparticles exhibit high Hc, σs and σr of the values of 4675 Oe, 62 emu/g and 44.2 emu/g, respectively (Fig. 5).

CONCLUSIONS As-prepared 3 nm fcc FePd particles are annealed at 450, 550 and 600 °C under N2 + H2 (86 % + 14%) gas for 1 h. XRD patterns of annealed FePd nanoparticles show that phase transformation from fcc to fct occurs at 550 °C. The 550 °C annealed Fe43Pd57 nanoparticles exhibit Hc, squareness ratio (σr/σs) and Tc of the values 1186 Oe, 0.38 and 454 °C, respectively. The coericivity of 600 °C annealed FePd nanoparticles is found to be 1300 Oe, however, σr/σs = 0.37 remains almost constant. The fct Fe51Pd26Pt23 nanoparticles exhibit Hc and σr/σs of the values 4675 Oe and 0.71, respectively. Hence, we conclude that anisotropy of L10 FePd nanoparticles can be significantly increased after addition of Pt.

FIGURE 3. σ-H curves of as-prepared fcc, 450, 550 and 600 °C annealed FePd nanoparticles.

ACKNOWLEDGMENTS FIGURE 4. σ-T measurements for 450 and 550 oC annealed Fe43Pd57 nanoparticles at 50 Oe applied field.

Authors acknowledge DST, New Delhi for providing financial support. S.S. thanks U.G.C. New Delhi for providing him S.R.F.

FePd.8 Whereas, Tc of 450 oC annealed fcc FePd nanoparticles is found to be around 268 oC (Fig. 4). Thus, after phase transition from fcc to fct, Tc increases due to chemical ordering and increase in the particle size.

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FIGURE 5. σ-H curves of as-prepared fcc and 600 °C annealed fct Fe51Pd26Pt23 nanoparticles

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