Synthesis of single-crystal Sm-Co nanoparticles by cluster beam deposition
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- Akdogan, O., Li, W., Hadjipanayis, G.C. et al. J Nanopart Res (2011) 13: 7005. doi:10.1007/s11051-011-0612-8
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Single-crystal Sm-Co nanoparticles have been successfully produced by a cluster beam deposition technique. Particles have been deposited by DC magnetron sputtering using high Ar pressures on both single-crystal Si substrates and Au grids for the magnetic and structural/microstructural properties, respectively. Oxidation of the particles is prevented by using carbon buffer and cover layers. Nanoparticles have a uniform size distribution with an average size of 4.2, 6 and 7 nm at 1, 1.5 and 2 Torr of Ar pressure, respectively. At 1 Torr, the particles have the disordered 1:7 structure and a high coercivity of 19 kOe at 10 K. These particles show a superparamagnetic behavior with a blocking temperature of TB = 145 K. From this value of TB and the particle volume, the value of anisotropy constant K is estimated to be around 2.2 × 107 ergs/cc. Heat is introduced to the particles during their flight to the substrate to increase the particle size. Nanoparticles of SmCo5 with an average size of 15 nm and high room temperature coercivity have been produced. No change in magnetic and structural properties of the samples has been observed even after 10 months. Cluster beam deposition could play a key role for the production of rare earth nanoparticles for many applications.
KeywordsSputteringCoercivityNanoparticlesSm-CoRare earth metals
Synthesis of high-anisotropy magnetic nanoparticles with desired structure and size is of great fundamental and technological interest (McHenry and Laughlin 2000). Rare earth intermetallic compounds (RE-TM) are known to have very high anisotropy; however, their high reactivity makes it harder to produce nanoparticles of these materials with the desired properties. Direct production of these nanoparticles with good magnetic properties without post-annealing could be the key to use these particles for potential applications (Hadjipanayis 1999). Previously, good magnetic properties were achieved in thin films made by physical vapor deposition, but these required a subsequent post-annealing (Sayama et al. 2004; Zhang et al. 2011). Very few studies exist on the preparation of magnetically hard nanoparticles of RE-TM compounds including those via ball milling (Kirkpatrick et al. 1996; Akdogan et al. 2009a, Akdogan et al. 2009b, Akdogan et al. 2010), chemical synthesis (Gu et al. 2003; Hou et al. 2007; Matsushita et al. 2010) and cluster gun (CG) (Stoyanov et al. 2003a; Tuaillon-Combes et al. 2003). The latter technique was successfully used to fabricate FePt (CoPt) (Stoyanov et al. 2003b; Qiu and Wang 2006; Liu et al. 2011) and Co/CoO(Skumryev et al. 2003) core–shell nanoparticles with excellent magnetic properties. These results are promising since the same technique could be used to produce RE-TM nanoparticles with good magnetic properties. Recently, single-crystal YCo5 nanoparticles with excellent magnetic properties have been produced with this technique (Balasubramanian et al. 2011a), which opened the way for the fabrication of Sm-Co nanoparticles; however, the latter are expected to be harder to synthesize due to the amorphization and oxidation problems of Sm-based nanoparticles. One of the main advantages of this technique is that it can be easily scaled up to produce larger amounts of nanoparticles with excellent material purity and without the presence of chemical by-products (Binns et al. 2005; Wegner et al. 2006).
Sm-Co compounds are very attractive for nanofabrication (Gutfleisch et al. 2011), because they have a large anisotropy in bulk and it is easier to control their composition through cluster beam deposition. Especially, fabrication of well-separated, single-crystal nanoparticles with moderate coercivity plays a key role in the development of high-density recording media (Frey and Sun 2010). The SmCo7 alloys have properties between those of SmCo5 (1:5) and Sm2Co17 (2:17). SmCo7 (1:7), with TbCu7 structure, is a metastable phase, which could only be formed during mechanical alloying and sputtering (Liu et al. 2005).
For many applications, control of the particle size is one of the main problems. Most often, post-annealing is necessary to increase the particle size. However, control on the agglomeration of the particles is very limited, which makes the end product non-uniform with much bigger agglomerates than desired. In the case of FePt nanoparticles, Colak and Hadjipanayis (2009) used silica coating to prevent agglomeration of chemically synthesized nanoparticles. In cluster beam deposition samples, the agglomeration problem is solved by using a more confined plasma (through a special iron pole piece) and applying high sputtering powers (Qiu et al. 2006, Qiu and Wang 2006; Liu et al. 2011). However, these techniques have not yet been applied to RE-Co nanoparticles. Another problem with post-annealing is the oxidation, which is especially important for RE-TM nanoparticles. In order to control the particle size and oxidation, we introduce in-flight annealing of the particles (which is explained in next section in detail).
Previous attempts to produce high anisotropy Sm-Co nanoparticles with CBD failed; Stoyanov et al. (2003a) produced Sm-Co nanoparticles that lacked crystallinity and gave 5 mT coercivities even at 5 K, compared to 19 kOe we got at 10 K. Tuaillon-Combes et al. (2003) also produced Sm-Co nanoparticles with CBD technique. However, X-ray photoelectron spectroscopy of the deposited clusters showed that the Sm atoms were segregated on the cluster surface. They achieved RT coercivity of 700 Oe only after ex situ annealing at 570 °C in Nb matrix. In this study, we have produced Sm-Co nanoparticles with good magnetic properties from 1:5 and 2:17 targets using inert gas condensation in a CG.
The base pressure in the sputtering chamber was 2 × 10−7 Torr, and high-purity Ar (99.9999%) was used for the deposition with a pressure of 5 mTorr inside the main chamber and 1–2 Torr inside the CG. A DC power of 25 Watts was applied to both the 1:5 and 2:17 alloy targets. Samples were sputtered on 500-μm-thick Si (100) wafers for magnetic measurements and on Au grids for transmission electron microscopy (TEM). The samples were coated with carbon using a DC power of 24 Watts (sputtering rate = 2.6 Å/sec) in order to prevent oxidation. Microstructure characterization and composition analyses of the samples were performed with JEOL JEM-3010 TEM and JSM 6330F SEM. Magnetic measurements at room temperature and below were taken with a Quantum Design Versalab vibrating sample magnetometer (VSM) with a maximum field of 3 T and with a quantum design squid (superconducting quantum interference device) with a maximum field of 5.5 T. X-ray diffraction (XRD) measurements were taken with Rigaku Ultima IV.
Results and discussion
Particles from the SmCo5 Target
Particles from the Sm2Co17 Target
In-flight annealing of the particles
Sm-Co nanoparticles with different sizes have been successfully produced with the cluster beam deposition technique. Particle size has been controlled by introducing in-flight heat to the particles. During RT synthesis, 3.5-nm superparamagnetic 1:7 nanoparticles have been successfully produced with different lattice constants a and c from 1:5 and 2:17 targets. With the heater addition to the system (at 750 °C), the particle size distribution becomes bimodal with the small particles having an average size of 7 nm and the big nanoparticles having an average size of 15 nm covered with 5 nm Sm-O shell. The sample has 0.7 and 3 kOe at 300 and 50 K, respectively. Our current efforts are focused on increasing the amount of the big nanoparticles in the sample. Cluster beam deposition could be the key technique to produce high-coercivity RE-TM nanoparticles without post-annealing.
The authors would like to thank A. M. Gabay for helpful discussions. The authors also thank Dr. Melania Marinescu and EEC for providing the targets. Work supported by DOE DE-FG02-04ER4612.