Antiferromagnetic Single Domain L12 FePt3 Nanocrystals - The

Jan 25, 2010 - Tuning the Ferroelectric and Piezoelectric Properties of 0.91Pb(Zn1/3Nb2/3)O3-0.09PbTiO3 Single ... Tuning crystal structure and magnet...
1 downloads 0 Views 5MB Size
Download PDF
2512

J. Phys. Chem. C 2010, 114, 2512–2518

Antiferromagnetic Single Domain L12 FePt3 Nanocrystals Andrew T. Heitsch, Doh C. Lee,† and Brian A. Korgel* Department of Chemical Engineering, Texas Materials Institute, and Center for Nano- and Molecular Science and Technology, The UniVersity of Texas at Austin, Austin, Texas 78712-0231 ReceiVed: October 31, 2009; ReVised Manuscript ReceiVed: January 1, 2010

Compositionally ordered, single domain, antiferromagnetic L12 FePt3 nanocrystals were synthesized by coating colloidally grown Pt-rich Fe-Pt nanocrystals (Fe0.27Pt0.73) with thermally stable SiO2 and annealing at 700 °C in forming gas (7% H2 in N2). Without the silica coating, the nanocrystals transform predominately into the L10 FePt phase due to interparticle diffusion of Fe and Pt atoms. Magnetization measurements of the L12 FePt3 nanocrystals revealed two antiferromagnetic transitions near the bulk Nee´l temperatures of 100 and 160 K. Combining L12 FePt3 nanocrystals with L10 FePt nanocrystals was found to produce the constriction in field-dependent magnetization loops that has been observed near zero applied field in ensemble measurements of single domain silica-coated L10 FePt nanocrystals [Lee, D. C.; et al. J. Phys. Chem. B 2006, 110, 11160]. Introduction A wide variety of magnetic nanocrystals with narrow size and shape distributions can be produced using chemical methods, and they have been explored for various applications, including magnetic tags for biological imaging, separations, chemical and biological sensing, and high-density magnetic memory storage media.1-7 Among these materials, FePt has been one of the most extensively studied.6,8-11 FePt nanocrystals can be synthesized with diameters less than 10 nm and narrow size distributions. The as-synthesized nanocrystals have compositionally disordered cores and are weak ferromagnets. By performing a post-synthesis annealing at a relatively high temperature, between about 550 and 650 °C, the compositionally ordered L10 FePt with high magnetocrystalline anisotropy, as desired for high-density nonvolatile memory applications, can be obtained.12-15 This annealing process, however, sinters the nanocrystals, and one is left with essentially a nanocrystalline film of L10 FePt, as opposed to a collection of discrete nanocrystals. To prevent sintering, the nanocrystals can be coated prior to annealing with a layer of thermally stable material, such as silica. This approach can work very well, and single magnetic domain L10 FePt nanocrystals with blocking temperatures well above room temperature can be obtained.16-19 Right next to L10 FePt on the Fe-Pt phase diagram is antiferromagnetic L12 FePt3. The phase boundary between these two phases is, in fact, relatively Fe-rich, with a composition of 43 at. % Fe (Figure 1).20-22 In practice, the Fe-Pt composition of a nanocrystal sample can often lie close to the L10/L12 phase boundary.23,24 Nonetheless, there have been only a few reports of antiferromagnetic, colloidally grown L12 FePt3 nanocrystals, all of which have been sintered films of particles, and to date, single domain antiferromagnetic FePt3 nanocrystals have not been made.25-27 Here, we report the synthesis of single domain antiferromagnetic L12 FePt3 nanocrystals. The key to obtaining single magnetic domain L12 FePt3 nanocrystals is to coat Pt-rich Fe-Pt nanocrystals with a thermally stable silica shell, followed by * Corresponding author. Phone: 512-471-5633. Fax: 512-471-7060. E-mail: [email protected]. † Present address: Chemistry Division, C-PCS, MS-J567, Los Alamos National Laboratory, Los Alamos, NM 87545.

Figure 1. Fe-Pt phase diagram.21,22 Fe-Pt phases: L12 FePt3 is cubic and antiferromagnetic; L10 FePt is tetragonal and ferromagnetic; L12 Fe3Pt is cubic and ferromagnetic. Chemically disordered Fe-Pt has fcc structure and is weakly ferromagnetic. Dashed lines are predicted phase boundaries from ref 21.

annealing at high temperature (>550 °C) under forming gas. The silica coating prevents Fe and Pt interdiffusion between nanocrystals during annealing, which appears to be the reason that there are few reports of antiferromagnetic L12 FePt3 obtained from nanocrystals,25-27 thus retaining the initial Fe-Pt composition distribution after annealing. Nanoscale antiferromagnetic metal oxides and sulfides, such as CoO,28,29 NiO,30-32 MnO,33-35 ferritin,36-39 MnS,40,41 and NiS,42,43 have been extensively studied, but there are few reports of antiferromagnetic intermetallic nanocrystals because of the need for compositional order in this class of materials.44 The single magnetic domain L12 FePt3 nanocrystals exhibited magnetic transitions near the two expected Nee´l transition temperatures of bulk L12 FePt3 at 100 and 160 K and had slightly higher magnetic susceptibility than the bulk. We also demonstrate that mixing antiferromagnetic L12 FePt3 nanocrystals with ferromagnetic L10 FePt nanocrystals produces a constriction in the magnetization hysteresis loops, like those previously reported for silica-coated single domain L10 FePt nanocrystals.16

10.1021/jp910410x  2010 American Chemical Society Published on Web 01/25/2010

L12 FePt3 Nanocrystals Experimental Methods Chemicals. Platinum(II) acetylacetonate (Pt(acac)2, 97%), 1,2-hexadecanediol (90%), iron pentacarbonyl (Fe(CO)5, 99.999%), oleylamine (70%), oleic acid (99%), Igepal CO-520, tetraethyl orthosilicate (TEOS, 98%), ethanol (ACS grade), methanol (ACS grade), and hexanes (ACS grade) were purchased from Sigma-Aldrich. Dioctyl ether (g97%) and cyclohexane (ACS grade) were purchased from Fluka. Aqueous ammonium hydroxide (NH4OH, 30%) solution was purchased from EMD Chemicals. All chemicals were used as received. Fe-Pt Nanocrystal Synthesis. Fe-Pt nanocrystals were synthesized with an average diameter of 7 nm and composition near 1:1 and 1:3 using a modification of a previously published procedure.13 On a greaseless Schlenk line under inert (N2) atmosphere, 0.197 g (0.50 mmol) of Pt(acac)2 was added to 10 mL of dioctyl ether in a 25 mL three-neck flask and stirred (600 rpm). The solution was degassed under vacuum (200 K), as shown in Figure 5. In eq 1, H is the applied field, N is the number concentration of nanoparticles, T is temperature, Tc is the critical temperature, and kB is the Boltzmann constant. The collective quantity, Nµ2/3kB, is known as the Curie constant. The Fe0.27Pt0.73@SiO2 and Fe0.42Pt0.58@SiO2 nanoparticles (prior to annealing) exhibited magnetic moments of 3781 µB/nanoparticle and 9221 µB/nanoparticle, respectively, which correspond to average moments per Fe atom in the range of 3-4 µB reported in bulk materials.47-51 The magnetic properties of the nanoparticles changed significantly upon annealing. After converting the Fe0.42Pt0.58@SiO2 nanoparticles to L10 FePt@SiO2 nanoparticles, TB exceeded room temperature, and the coercivity measured at 5 K had increased by an order of magnitude from 2.5 to 17.5 kOe (Figure 4B and D). This is consistent with the much higher magnetocrystalline anisotropy of the L10 FePt phase as compared to the random Fe-Pt alloy.12,16 Annealing and converting the Fe0.27Pt0.73@SiO2 nanoparticles to L12 FePt3 resulted in a significant decrease in magnetic response at low field (0.1 T), from 750 to 160 µB/nanoparticle at 5 K (Figure 4A and C). At high fields (5 T), the total magnetization at 5 K (∼3000 µB/nanoparticle) was similar to the nanoparticles prior to annealing (Figure 6). The low value of the low-field susceptibility is consistent with antiferromag-

Figure 3. (A) XRD (λ ) 1.5418 Å) of (i, ii) FePt0.27Pt0.73@SiO2 and (iii, iv) FePt0.42Pt0.58@SiO2 nanoparticles (i, iii) before and (ii, iv) after annealing in forming gas for 4 h at 700 °C. The broad peak at 2θ ) 23° corresponds to silica; the thicker silica shell (12.5 vs 8.5 nm) of the FePt0.27Pt0.73@SiO2 gives rise to a more significant low-angle diffraction peak. (B) Magnification of the {111} diffraction peaks. Reference peak positions are provided in A for (red dashed line) cubic L12 FePt3 (PDF #01-089-2050) and (green dashed line) tetragonal L10 FePt (PDF #01-089-2051). The Miller indices are labeled for L12 FePt3. The unique diffraction peaks that appear only for L10 FePt (compared to L12 FePt3) are labeled with asterisks. Annealing did not sharpen the diffraction peaks, confirming that the silica shells prevented sintering of the Fe-Pt domains.

L12 FePt3 Nanocrystals

J. Phys. Chem. C, Vol. 114, No. 6, 2010 2515

Figure 4. Temperature-dependent (O) zero field cooled (ZFC) and (b) field-cooled (FC) magnetization (0.1 T applied field) of as-prepared (A) Fe0.27Pt0.73@SiO2 and (B) Fe0.42Pt0.58@SiO2 nanoparticles and annealed (4 h at 700 °C in forming gas) (C) Fe0.27Pt0.73@SiO2 (L12 FePt3) and (D) Fe0.42Pt0.58@SiO2 (L10 FePt) nanoparticles. Insets: Field-dependent magnetization measured at 5 K.

Figure 5. Field-cooled inverse magnetic susceptibility χV-1 versus temperature of the (b) Fe0.27Pt0.73@SiO2 and (O) Fe0.42Pt0.58@SiO2 nanoparticles prior to annealing. The solid lines are best fits of eq 1 to the high-temperature magnetization data, giving Curie constants of 0.74 and 3.3 mK and Tc ) 193 and 199 K for the FePt0.27Pt0.73@SiO2 and Fe0.42Pt0.58@SiO2 nanoparticles, respectively.

Figure 6. Field-dependent magnetization measured at 5 K of the Fe0.27Pt0.73@SiO2 nanoparticles (b) before and (O) after annealing in forming gas at 700 °C for 4 h. (Inset) Magnification of the magnetization between (0.5 T.

netism.36 Also consistent with antiferromagnetism was an observed linear field-dependence of the magnetization at low fields (Figure 6), as opposed to the Langevin dependence of

the magnetization on applied field typical for superparamagnetic particles.30,31,36,41,53 The FC and ZFC magnetization scans of the L12 FePt3@SiO2 nanoparticles in Figure 4C also exhibit some features characteristic of antiferromagnetic L12 FePt3. In particular, the FC and ZFC scans diverge when the temperature drops below 200 K, and the ZFC magnetization scan has a peak-like feature at 75 K. These temperatures are close to the Ne´el temperatures of bulk L12 FePt3 of 160 and 100 K.48,54,55 The overlapping FC and ZFC scans at temperatures above 200 K overlap and exhibit Curie law behavior that is consistent with a paramagnetic magnetic response.46 The mass magnetic susceptibility in this temperature range was χm ) 1.1 × 10-4 emu/g of FePt3 · Oe, which is an order of magnitude larger than that of bulk L12 FePt3, χm ) 2 × 10-5emu/g · Oe between 200 and 300 K.56 The large values of χm might reflect the presence of uncompensated surface spins or might indicate that the nanoparticles are superantiferromagnetic.36,38,57 It is also possible that a fraction of residual paramagnetic material may still be present in the sample after annealing. More study is required to identify the precise reason for the high values of χm. Below 200 K, the FC and ZFC scans diverge, indicating the freezing of the spin orientation. In Figure 7, χV-1 is plotted against temperature and compared to Curie law behavior. There is a transition from paramagnetism to antiferromagnetism at 160 K as χV-1 deviates from a linear temperature dependence. This corresponds to the higher Ne´el temperature of L12 FePt3.25 At 75 K, there is another deviation in χV-1 with decreasing temperature, corresponding to the lower-temperature antiferromagnetic transition. Then there is a peak in χV-1 at about 35 K, corresponding to a paramagnetic or ferromagnetic transition.25 Sintered films of L12 FePt3 nanocrystals have also exhibited similar temperature dependence of the magnetization, including the peak near 75 K in the ZFC magnetization and an increase in magnetization at low temperature.25 In fact, many other nanoscale antiferromagnetic53 systems, including Co3O4,58

2516

J. Phys. Chem. C, Vol. 114, No. 6, 2010

Figure 7. The inverse magnetic susceptibility, χV-1, extracted from the temperature-dependent (O) ZFC and (b) FC magnetization measurements of L12 FePt3@SiO2 nanoparticles. The solid line is a best fit of eq 1 to the high temperature (T > 200 K) FC magnetization data, taking Tc ) 0 K. (Inset) Magnification of χV-1 between 5 and 30 K.

Heitsch et al.

Figure 9. (A) XRD patterns of Fe0.27Pt0.73 nanocrystals without encapsulation in silica (i) before and (ii) after annealing in forming gas for 4 h at 700 °C. L10 FePt reference peak positions (PDF #01089-2051) are labeled on the figure. The peak labeled with an asterisk indexes to Fe (PDF #01-085-1410).

Figure 8. Field-dependent magnetization measured at 5 K for mixtures of L12 FePt3@SiO2 (dNC ) 6.2 ( 0.8 nm, tSiO2 ) 12.5 ( 0.6 nm) and L10 FePt@SiO2 nanoparticles (dNC ) 7.2 ( 0.8 nm, tSiO2 ) 9.7 ( 0.7 nm): (b) pure L12 FePt3@SiO2; (1) 1 part L10 FePt@SiO2/2 parts L12 FePt3@SiO2 by weight; (9) 2 parts L10 FePt@SiO2/1 part L12 FePt3@SiO2 by weight; and ([) pure L10 FePt@SiO2.

CoO,29,52,59 MnS,41 and MnO and NiO,35 have shown similar magnetization characteristics. At 5 K, a slight magnetic hysteresis was also observed (Figure 6 inset). This permanent magnetization is most likely the result of uncompensated surface spins at the surface of the L12 FePt3 nanocrystals, similar to what has been observed before in other antiferromagnetic nanocrystals.28,30,39 The small coercivity of only 0.3 kOe at 5 K (Figure 6 inset) is of similar magnitude as what has been previously observed in those cases.30,38,52 Constriction in the L10 FePt Nanocrystal Hysteresis Loops. We have previously shown that L10 FePt@SiO2 nanoparticles often exhibit a constriction in their magnetic hysteresis loops, as in the sample shown in Figure 4D. We tentatively assigned this to magnetic dipole coupling between neighboring nanoparticles.16 However, others have observed such a constriction in Fe-Pt nanocrystal samples, as well,27,60-63 and have attributed it to an additional soft magnetic phase present in the material.16,27,60,62,64-69 Recently, we showed by magnetic force microscopy measurements that magnetic dipole coupling is, in fact, not significant.17 Therefore, on the basis of our findings about the synthesis of antiferromagnetic L12 FePt3@SiO2 nanoparticles, we sought to determine if the presence of a small fraction of these nanoparticles in a sample of L10 FePt@SiO2 nanoparticles could give rise to the constriction. Indeed, we found that mixing L12 FePt3@SiO2 nanoparticles with L10 FePt@SiO2 nanoparticles leads to significant constriction of the hysteresis loops, as shown in Figure 8. The constriction in the hysteresis loops measured at 5 K (normalized to the saturation magnetization of each sample) becomes more significant as more L12 FePt3@SiO2 nanoparticles are added to the L10 FePt@SiO2 nanoparticles.

Figure 10. Schematic illustration of Fe-Pt nanocrystal transformation during synthesis and annealing. Red and gray spheres represent Fe and Pt atoms, respectively.

The formation of a small amount of L12 FePt3 nanocrystals in some L10 FePt samples is perhaps not unexpected, given that the phase boundary between L12 FePt3 and L10 FePt is relatively Fe-rich (at 43% Fe) and the average Fe composition of Fe-Pt nanocrystal samples is often close to this value. However, without a protective coating to prevent sintering, even Fe-

L12 FePt3 Nanocrystals deficient nanocrystal samples convert predominantly to L10 FePt. Figure 9 shows the XRD of Fe0.27Pt0.73 nanocrystals annealed at 700 °C without a silica coating. The nanocrystals sinter and transform predominantly to the L10 FePt phase as indicated by the sharpened diffraction peaks and evolution of the {002} and {202} peaks of tetragonal L10 FePt. The magnetic response is also ferromagnetic and characteristic of L10 FePt. Without nanocrystal encapsulation in thermally stable SiO2, Fe and Pt diffusion occurs between nanocrystals, leading to a significant proportion of L10 FePt, which suppresses evidence of L12 FePt3 formation. Figure 10 illustrates the overall transformation of Fe-Pt nanocrystals, from synthesis to silica encapsulation and high temperature annealing. In the reaction mixture, core-shell Pt@Fe nanocrystals are initially formed,1,13 followed by intraparticle Fe diffusion into the Pt core to lead to alloyed nanocrystals. The silica coating around each nanocrystal then prevents Fe and Pt diffusion between neighboring nanocrystals during annealing, therefore, leading to a retention of the initial Fe/Pt composition distribution in the nanocrystal sample. Without silica encapsulation, a significant amount of Fe and Pt interdiffusion occurs as the particles sinter, leading to L10 FePt phase formation. These results explain why there has been little discussion about antiferromagnetic L12 FePt3 nanocrystals in the literature. Conclusions Single magnetic domain L10 FePt@SiO2 or L12 FePt3@SiO2 nanoparticles can be made by coating colloidal Fe-Pt nanocrystals with silica and annealing under forming gas (7% H2 in N2) at 700 °C. Prior to annealing, the nanocrystals behave as typical superparamagnets with magnetic properties that depend only on the Fe content in the nanocrystals. When the nanoparticles are annealed to induce compositional order, the magnetic properties become dramatically different, depending on the Fe/ Pt composition: L10 FePt@SiO2 nanoparticles are ferromagnetic with large magnetic anisotropy and the L12 FePt3@SiO2 are antiferromagnetic. The constriction in the hysteresis loops of L10 FePt@SiO2 nanoparticles that we previously observed16 is due to the presence of antiferromagnetic L12 FePt3 nanoparticles in the sample. The effect is simply one of overlapping, independent, magnetic signals of an impurity in the sample. Exchange coupling between ferromagnetic L10 FePt and antiferromagnetic L12 FePt3 nanoparticles does not occur in those samples because of the thick silica barrier between neighboring particles. However, it is interesting to consider what the properties of exchange-coupled L10 FePt and antiferromagnetic L12 FePt3 nanoparticles might be, and perhaps it might be possible to induce exchange coupling between single domain ferromagnetic and antiferromagnetic nanoparticles by shrinking the silica coating thickness or somehow eliminating it. It would certainly require a creative strategy to obtain nanocrystals of exchangebiased L10 FePt and L12 FePt3 because of the tendency for Fe-Pt interdiffusion during annealing, but if it were possible to make these nanocrystals, it would be a very interesting system for spintronic applications. At any rate, it is now possible to make single magnetic domain nanocrystals of hard magnetic L10 FePt and antiferromagnetic L12 FePt3. Acknowledgment. This research was supported by funding from The Robert A. Welch Foundation (F-1464), the Air Force Research Laboratory (FA8650-07-2-5061), and the National Science Foundation (DMR-0807065). We also thank Allan

J. Phys. Chem. C, Vol. 114, No. 6, 2010 2517 MacDonald, Alex DeLozanne, Changbae Hyun, and Keeseong Park for helpful discussions. References and Notes (1) Lee, D. C.; Smith, D. K.; Heitsch, A. T.; Korgel, B. A. Annu. Rep. Prog. Chem., Sect. C: Phys. Chem. 2007, 103, 351–402. (2) Latham, A. H.; Williams, M. E. Acc. Chem. Res. 2008, 41, 411– 420. (3) Shinkai, M. J. Biosci. Bioeng. 2002, 94, 606–613. (4) Arruebo, M.; Ferna´ndez-Pacheco, R.; Ibarra, M. R.; Santamarı´a, J. NanoToday 2007, 2, 22–32. (5) Frey, N. A.; Peng, S.; Cheng, K.; Sun, S. H. Chem. Soc. ReV. 2009, 28, 2532–2542. (6) Xu, C. J.; Sun, S. H. Polym. Int. 2007, 56, 821–826. (7) Alivisatos, P. Nat. Biotechnol. 2004, 22, 47–52. (8) Heitsch, A. T.; Smith, D. K.; Patel, R. N.; Ress, D.; Korgel, B. A. J. Solid State Chem. 2008, 181, 1590–1599. (9) Patel, R. N.; Heitsch, A. T.; Hyun, C.; Smilgies, D.-M.; de Lozanne, A.; Loo, Y.-L.; Korgel, B. A. ACS Appl. Mater. Interfaces 2009, 1, 1339–1346. (10) Sun, S. H. AdV. Mater. 2006, 18, 393–403. (11) Hyun, C.; Lee, D. C.; Israel, C.; Korge, B. A.; de Lozanne, A. IEEE Trans. Magn. 2006, 42, 3799–3802. (12) Sun, S. H.; Murray, C. B.; Weller, D.; Folks, L.; Moser, A. Science 2000, 287, 1989–1992. (13) Chen, M.; Liu, J. P.; Sun, S. H. J. Am. Chem. Soc. 2004, 126, 8394–8395. (14) Elkins, K. E.; Vedantam, T. S.; Liu, J. P.; Zeng, H.; Sun, S. H.; Ding, Y.; Wang, Z. L. Nano Lett. 2003, 3, 1647–1649. (15) Wang, J. P. Proc. IEEE 2008, 96, 1847–1863. (16) Lee, D. C.; Mikulec, F. V.; Pelaez, J. M.; Koo, B.; Korgel, B. A. J. Phys. Chem. B 2006, 110, 11160–11166. (17) Hyun, C.; Lee, D. C.; Korgel, B. A.; de Lozanne, A. Nanotechnology 2007, 18, 055704. (18) Li, D. R.; Poudyal, N.; Nandwana, V.; Jin, Z. Q.; Elkins, K.; Liu, J. P. J. Appl. Phys. 2006, 99, 08E911. (19) Yamamoto, S.; Morimoto, Y.; Ono, T.; Takano, M. Appl. Phys. Lett. 2005, 87, 032503. (20) Vacancies and interstitials make up the difference in composition within chemically disordered Fe-Pt alloys. (21) Moffatt’s Handbook of Binary Phase Diagrams; Genium Publishing Corporation: Schenectady, NY, 1997. (22) Okamoto, H. Fe-Pt (Iron-Platinum), Binary Alloy Phase Diagrams, II ed.; ASM International: Materials Park, OH, 1990; Vol. 2. (23) Srivastava, C.; Nikles, D. E.; Thompson, G. B. J. Appl. Phys. 2008, 104, 064315. (24) Yu, A. C. C.; Mizuno, M.; Sasaki, Y.; Kondo, H. Appl. Phys. Lett. 2004, 85, 6242–6244. (25) Song, H. M.; Kim, W. S.; Lee, Y. B.; Hong, J. H.; Lee, H. G.; Hur, N. H. J. Mater. Chem. 2009, 19, 3677–3681. (26) Rong, C. B.; Li, Y.; Liu, J. P. J. Appl. Phys. 2007, 101, 09K505. (27) Zeng, H.; Li, J.; Liu, J. P.; Wang, Z. L.; Sun, S. H. Nature 2002, 420, 395–398. (28) Ghosh, M.; Sampathkumaran, E. V.; Rao, C. N. R. Chem. Mater. 2005, 17, 2348–2352. (29) An, K.; Lee, N.; Park, J.; Kim, S. C.; Hwang, Y.; Park, J. G.; Kim, J. Y.; Park, J. H.; Han, M. J.; Yu, J. J.; Hyeon, T. J. Am. Chem. Soc. 2006, 128, 9753–9760. (30) Jagodic, M.; Jaglicic, Z.; Jelen, A.; Lee, J. B.; Kim, Y. M.; Kim, H. J.; Dolinsek, J. J. Phys.: Condens. Matter 2009, 21, 215302. (31) Makhlouf, S. A.; Parker, F. T.; Spada, F. E.; Berkowitz, A. E. J. Appl. Phys. 1997, 81, 5561–5563. (32) Kodama, R. H.; Makhlouf, S. A.; Berkowitz, A. E. Phys. ReV. Lett. 1997, 79, 1393–1396. (33) Park, J.; Kang, E. A.; Bae, C. J.; Park, J. G.; Noh, H. J.; Kim, J. Y.; Park, J. H.; Hyeon, T. J. Phys. Chem. B 2004, 108, 13594–13598. (34) Seo, W. S.; Jo, H. H.; Lee, K.; Kim, B.; Oh, S. J.; Park, J. T. Angew. Chem., Int. Ed. 2004, 43, 1115–1117. (35) Ghosh, M.; Biswas, K.; Sundaresan, A.; Rao, C. N. R. J. Mater. Chem. 2006, 16, 106–111. (36) Gilles, C.; Bonville, P.; Rakoto, H.; Broto, J. M.; Wong, K. K. W.; Mann, S. J. Magn. Magn. Mater. 2002, 241, 430–440. (37) Silva, N. J. O.; Amaral, V. S.; Carlos, L. D. Phys. ReV. B 2005, 71, 184408. (38) Silva, N. J. O.; Millan, A.; Palacio, F.; Kampert, E.; Zeitler, U.; Rakoto, H.; Amaral, V. S. Phys. ReV. B 2009, 79, 104405. (39) Makhlouf, S. A.; Parker, F. T.; Berkowitz, A. E. Phys. ReV. B 1997, 55, 14717–14720. (40) Kurz, T.; Chen, L.; Brieler, F. J.; Klar, P. J.; von Nidda, H. A. K.; Froba, M.; Heimbrodt, W.; Loidl, A. Phys. ReV. B 2008, 78, 132408. (41) Kan, S. H.; Felner, I.; Banin, U. Isr. J. Chem. 2001, 41, 55–61. (42) Ghezelbash, A.; Sigman, M. B.; Korgel, B. A. Nano Lett. 2004, 4, 537–542.

2518

J. Phys. Chem. C, Vol. 114, No. 6, 2010

(43) Tian, L.; Yep, L. Y.; Ong, T. T.; Yi, J. B.; Ding, J.; Vittal, J. J. Cryst. Growth Des. 2009, 9, 352–357. (44) Morup, S.; Madsen, D. E.; Frandsen, C.; Bahl, C. R. H.; Hansen, M. F. J. Phys.: Condens. Matter 2007, 19, 213202. (45) Magnetic Nanostructures; American Scientific Publishers: Stevenson Ranch, CA, 2002. (46) Klabunde, K. J. Nanoscale Materials in Chemistry; Wiley-Interscience: New York, 2001. (47) Bartholomew, C. H.; Anderson, J. H.; Boudart, M. J. Chem. Soc., Faraday Trans. 1979, 75, 257. (48) Bacon, G. E.; Crangle, J. Proc. R. Soc. London, Series A 1963, 272, 387–405. (49) Kouvel, J. S. Effects of Atomic Order-Disorder on Magnetic Properties. In Magnetism and Metallurgy; Berkowitz, A. E., Kneller, E., Eds.; Academic Press: New York, 1969; Vol. 2; pp 523-575. (50) Paudyal, D.; Saha-Dasgupta, T.; Mookerjee, A. J. Phys.: Condens. Matter 2004, 16, 2317–2334. (51) Margeat, O.; Tran, M.; Spasova, M.; Farle, M. Phys. ReV. B 2007, 75, 134410. (52) Tracy, J. B.; Weiss, D. N.; Dinega, D. P.; Bawendi, M. G. Phys. ReV. B 2005, 72, 064404. (53) Madsen, D. E.; Morup, S.; Hansen, M. F. J. Magn. Magn. Mater. 2006, 305, 95–99. (54) The two Neel temperatures in L12 FePt3 correspond to two different spin alignments: (1) below 160 K, the Fe atoms align along the 〈100〉 easy axis in alternating ferromagnetic (110) sheets; (2) below 100 K, the Fe atoms align in alternating ferromagnetic (100) sheets.53 (55) Maat, S.; Hellwig, O.; Zeltzer, G.; Fullerton, E. E.; Mankey, G. J.; Crow, M. L.; Robertson, J. L. Phys. ReV. B 2001, 63, 134426.

Heitsch et al. (56) Crangle, J. Nature 1958, 181, 644–645. (57) Guyodo, Y.; Banerjee, S. K.; Penn, R. L.; Burleson, D.; Berauo, T. S.; Seda, T.; Solheid, P. Phys. Earth Planet. Inter. 2006, 154, 222–233. (58) Salabas, E. L.; Rumplecker, A.; Kleitz, F.; Radu, F.; Schuth, F. Nano Lett. 2006, 6, 2977–2981. (59) Tracy, J. B.; Bawendi, M. G. Phys. ReV. B 2006, 74, 184434. (60) Tamada, Y.; Morimoto, Y.; Yamamoto, S.; Hayashi, N.; Takano, M.; Nasu, S.; Ono, T. Jpn. J. Appl. Phys., Part 2 2006, 45, L1232–L1234. (61) Qiu, J. M.; Bai, J. M.; Wang, J. P. Appl. Phys. Lett. 2006, 89, 222506. (62) Elkins, K.; Li, D.; Poudyal, N.; Nandwana, V.; Jin, Z. Q.; Chen, K. H.; Liu, J. P. J. Phys. D.: Appl. Phys. 2005, 38, 2306–2309. (63) Yamamoto, S.; Morimoto, Y.; Tamada, Y.; Takahashi, Y. K.; Hono, K.; Ono, T.; Takano, M. Chem. Mater. 2006, 18, 5385–5388. (64) Pan, Z. Y.; Lin, J. J.; Zhang, T.; Karamat, S.; Tan, T. L.; Lee, P.; Springham, S. V.; Ramanujan, R. V.; Rawat, R. S. Thin Solid Films 2009, 517, 2753–2757. (65) Tamada, Y.; Morimoto, Y.; Yamamoto, S.; Takano, M.; Nasu, S.; Ono, T. J. Magn. Magn. Mater. 2007, 310, 2381–2383. (66) Lyubina, J.; Gutfleisch, O.; Muller, K. H.; Schultz, L.; Dempsey, N. M. J. Appl. Phys. 2004, 95, 7474–7476. (67) Liu, J. P.; Luo, C. P.; Liu, Y.; Sellmyer, D. J. Appl. Phys. Lett. 1998, 72, 483–485. (68) Zeng, H.; Sun, S. H.; Vedantam, T. S.; Liu, J. P.; Dai, Z. R.; Wang, Z. L. Appl. Phys. Lett. 2002, 80, 2583–2585. (69) Tamada, Y.; Yamamoto, S.; Takano, M.; Nasu, S.; Ono, T. Appl. Phys. Lett. 2007, 90, 162509.

JP910410X