Feptau And Coptau Coreshell

Journal of Magnetism and Magnetic Materials 226}230 (2001) 1915}1917

Magnetic properties of FePt /Au and CoPt /Au core-shell V V nanoparticles Charles J. O'Connor*, Jessica A. Sims, Amar Kumbhar, Vladimir L. Kolesnichenko, Weilie L. Zhou, Joan A. Wiemann Advanced Materials Research Institute, University of New Orleans, New Orleans, LA 70148, USA

Abstract The gold-coated nanocomposites with magnetic core based on the alloys FePt, Fe Pt, CoPt and CoPt were   synthesized using the reverse micelles technique. They were characterized using X-ray di!ractometry, SQUID magnetometry and electron microscopy (TEM). It was found that the Fe Pt/Au samples exhibited higher coercivity than  FePt/Au, 2885 Oe vs. 500 Oe at 2 K. For the cobalt-based materials, the values of blocking temperature and coercivity increase as the Pt content increases. Annealing of the Co/Au and CoPt /Au composites at 4003C did not cause any  signi"cant change in the magnetic behavior. Annealing of FePt/Au and CoPt/Au samples caused increase of coercivity, H "194 vs. 0 Oe for the former, and H "500 Oe vs. 25 Oe for the later composite at 300 K. We suppose that annealing ! ! of both 1 : 1 alloys caused ordering in their cores, that a!ected their magnetic properties.  2001 Elsevier Science B.V. All rights reserved. Keywords: Nanoparticles; Transition metal; Ferromagnetic materials; Composites

1. Introduction Iron/platinum and cobalt/platinum alloys exhibit enhanced ferromagnetic behavior as compared to pure Fe or Co [1]. Dimensional reduction to nanoscale, makes these alloys very useful in the design of novel storage media. A reverse micelles technique has proven to be a su$cient synthesis method for fabricating nanosize disordered metal alloys. Heat treatment of disordered FePt alloys increase the degree of long-range order in the cores, which decreases the antiferromagnetic interactions between Fe}Fe pairs [1}3]. Here, we present the results of a study of core/shell nanocomposite alloys and demonstrate the e!ect of thermal treatment on their magnetic properties. 2. Experiment Chemicals: FeSO ) 7H O, PtCl , HAuCl ) 3H O,      cetyltrimethylammonium bromide (CTAB), NaBH ,  * Corresponding author. Fax: #1 504-280-3185. E-mail address: [email protected] (C.J. O' Connor).

n-octane and n-butanol from Aldrich, chloroform and methanol from EM Scienti"c and CoCl ) 6H O (99.9%)   from Alfa Aesar, were used as received. All syntheses were performed in argon using a Schlenk technique. Microemulsion systems consisted of an oilphase octane, surfactant CTAB, co-surfactant BuOH and an aqueous phase containing the reactants (for
"16, the weight ratio, respectively: 12 : 6 : 5 : 5). Initially, two such solutions were prepared: one with metal salts (0.425 mmol per 2.5 g of aqueous solution) and another with reducing agent NaBH (2.9 mmol per 2.5 g of aque ous solution). Then both solutions were mixed and stirred for 2 h [4,5]. Excess of borohydride was used to suppress oxidation of Fe and Co by water. Then, the micelles were expanded to accommodate a passivating gold shell by reducing 0.06 M aqueous gold salt. The magnetic particles were isolated using magnetic "eld, washing with chloroform/methanol (1 : 1) and drying in vacuum. Some solids were annealed at 4003C for 4 h under argon. The crystallite size and structure was analyzed by a Phillips-X'PERT X-ray di!ractometer with a scintillation detector. Electron microscopy (TEM), EDS and

0304-8853/01/$ - see front matter  2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 0 6 6 1 - 2

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C.J. O+Connor et al. / Journal of Magnetism and Magnetic Materials 226}230 (2001) 1915}1917

SAD were preformed on a JEOL 2010. Magnetic measurements were done using MPMS SQUID magnetometer.

3. Results and discussion XRD demonstrated that both FePt and Fe Pt have  FCT crystal structures, which agrees with published data [1,6]. Gold peaks were also observed. The crystallite size ranged from 6 to 20 nm for the 2 : 1 alloy and the 1 : 1 alloy, respectively. TEM images showed isolated spherical core-shell structures (3 to 5 nm) with a uniform size distribution of less than 10%. Also, TEM and SAD demonstrated well-de"ned single crystals as compared to particles from unannealed FePt samples. EDS has detected Fe, Pt and Au in the specimens. TEM for CoPt shows that particle sizes are &15 nm  with a size distribution of less than 10%. EDS show the presence of 16.2 wt% of Pt, 52.5 wt% of Co and 31.3 wt% of Au. In the XRD plot, characteristic peaks of Au, Co, CoPt and CoPt appear in their respective samples. The  average crystallite size is &15 nm using Sherrer formula and comparing the full-width at half-maximum (FWHM) of the most intense peak of the sample to an internal reference standard.

Table 2 Summary of magnetic data for CoPt /Au alloys V Parameter

Co/Au

CoPt/Au

CoPt /Au 

¹ (K) at 100 Oe H (Oe) at 10 K !

60 20

80 200

106 415

Table 3 Magnetic properties of the annealed and unannealed CoPt/Au samples Parameter

CoPt/Au unannealed

CoPt/Au annealed

2 K hysteresis (Oe) 10 K hysteresis (Oe) 300 K hysteresis (Oe)

375 200 25

720 645 500

Table 1 Summary of magnetic data for FePt /Au alloys V Parameter

FePt/Au unannealed

FePt/Au annealed

Fe Pt/Au  unannealed

¹ (K) at 100 Oe M (emu/g) at 10 K 0 H (Oe) at 2 K ! H (Oe) at 10 K ! H (Oe) at 300 K !

70 2.3 500 500 0

300 18 1912 1032 194

53 1.6 2885 2013 0

Fig. 1. Hysteresis loop for the annealed FePt/Au, 300 K.

Fig. 2. Hysteresis loop for the annealed CoPt/Au, 300 K.

Magnetic properties of FePt/Au and Fe Pt/Au are  summarized in Table 1. Both are superparamagnetic below the blocking temperature, ¹ "70 K for the former and 53 K for the later. The Fe Pt/Au samples exhib ited higher coercivity than FePt/Au, 2885 Oe vs. 500 Oe at 2 K. Annealing of the FePt/Au composite at 4003C leads to the higher coercivities of 1912 Oe at 2 K and 194 Oe at 300 K (Fig. 1). Magnetic properties of cobalt-based materials are summarized in Table 2. The values of both blocking temperature and coercivity increase as the Pt content increases. Annealing of the Co/Au and CoPt /Au com posites at 4003C did not cause any signi"cant change in the magnetic behavior. In contrast, the CoPt/Au samples showed an increase in coercivity (Table 3), which is especially noticeable at higher temperature: H "500 Oe vs. ! 25 Oe at 300 K (Fig. 2). We suppose that the observed e!ect of annealing on the magnetic properties for both

C.J. O+Connor et al. / Journal of Magnetism and Magnetic Materials 226}230 (2001) 1915}1917

1 : 1 alloys, FePt/Au and CoPt/Au, is caused by ordering in their cores.

Acknowledgements This work was supported by the Advanced Materials Research Institute, through DARPA Grant No. MDA972-97-1-0003.

References [1] J.W. Bowles, Mineral. Petrol. 43 (1990) 37. [2] L. Benner, S. Meguro, Precious Metals Science and Technology, The International Precious Metal Institute, 1991, (Chapter 3).

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[3] T. Sasaki, S. Chikazumi, J. Phys. Soc. Japan 46 (6) (1979) 1732. [4] E.E. Carpenter, J.A. Sims et al., J. Appl. Phys. 87 (2000) 5615. [5] E.E. Carpenter, C.T. Seip, C.J. O'Connor, J. Appl. Phys. 85 (1999) 5184. [6] Reference X-ray Powder Di!raction Patterns, International Center for Di!raction Data, Newtown Square, PA, 1996.