Experimental and modeling studies on phase stability of nanocrystalline magnetic Sm2Co7

Materials Science and Engineering B 178 (2013) 971–976

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Experimental and modeling studies on phase stability of nanocrystalline magnetic Sm2 Co7 Wenwu Xu, Xiaoyan Song ∗ , Zhexu Zhang, Haining Liang College of Materials Science and Engineering, Key Laboratory of Advanced Functional Materials, Ministry of Education of China, Beijing University of Technology, 100124 Beijing, PR China

a r t i c l e

i n f o

Article history: Received 10 December 2012 Received in revised form 13 April 2013 Accepted 13 May 2013 Available online 29 May 2013 Keywords: Nanocrystalline alloy Thermodynamic modeling Phase stability Phase transformation

a b s t r a c t In contrast to the conventional polycrystalline low-Co Sm–Co alloys that have very weak permanent magnetic properties, the Sm2 Co7 alloy has been found to have fairly promising permanent magnetic performance when its grain size is reduced to the nanoscale. It was discovered that the crystal structure of the nanocrystalline Sm2 Co7 has a strong nanograin-size-dependent stability. The rhombohedral structure of Sm2 Co7 phase which is metastable at temperatures lower than 1435 K in conventional polycrystalline system can exist stably at room temperature in the nanocrystalline system. To understand the phase stability of the nanocrystalline Sm2 Co7 , the experimental and nanothermodynamic analyses were combined to describe quantitatively the phase transformation behavior of Sm2 Co7 on the nanoscale. The results are important for the development of nanostructured Sm–Co permanent magnets. © 2013 Elsevier B.V. All rights reserved.

1. Introduction In the Sm–Co alloys, the Co ions are responsible for the high Curie temperature Tc and the high saturation magnetization Ms , while the Sm ions are responsible for the large uniaxial anisotropy [1,2]. Among the Sm–Co intermetallics, the Co-rich alloys such as the stable-phase Sm2 Co17 and SmCo5 alloys and the metastablephase SmCo7 alloy have been widely investigated because of their excellent permanent magnetic properties [3–5]. However, few studies have been focused on the low-Co Sm–Co intermetallics such as Sm2 Co7 , SmCo3 and SmCo2 alloys, the reason lies in their relatively low Curie temperatures, which makes these alloys less promising in respect of permanent magnetic applications. With the development of nanoscience and nanotechnology, the nanostructured Sm–Co alloys have attracted increasing interests in recent years owing to their significantly enhanced magnetic properties, as compared with those of the conventional polycrystalline counterparts [6–10]. Developments of the nanostructured magnets have revealed new physics [11,12] (e.g. nanomagnetism [11]) and relevant applications [13–15]. Nowadays, experimental investigations on nanostructured magnets are mainly focused on the low-dimensional nanomagnets, e.g. nanoparticles [16], nanowires [17], nanofilms [18], and nanocrystalline powders [19]. The nanocrystalline bulk magnets, which may have even wider applications, however, are hardly

∗ Corresponding author. Tel.: +86 10 67392311; fax: +86 10 67392311. E-mail address: [email protected] (X. Song). 0921-5107/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mseb.2013.05.009

obtained by the conventional preparation approaches because of the difficulties in controlling the coarsening of the grain structure during the heat-treatment processes [20]. As a reason, the comprehensive studies on characteristics of microstructures and phase stabilities, as well as their influences on the magnetic performance, for the nanocrystalline bulk magnets are very scarce in the literature at the present time. The present work is aimed to investigate systematically the low-Co Sm–Co intermetallic alloy, using Sm2 Co7 as an example. The studies contain the preparation of fully dense nanocrystalline bulk material, characterizations of the nanostructure and magnetic performance, and thermodynamic analysis on the phase stability. 2. Experimental procedures The conventional polycrystalline Sm2 Co7 cast ingot was prepared by induction melting of elemental Sm and Co (in an atomic ratio of 2:7) both with 99.99% purity in the water-cooled copper hearth under the purified argon atmosphere. An excess of 3.0 wt.% Sm was used to compensate its loss during melting. To achieve a homogeneous distribution of the composition, the ingot was subjected to re-melting for four times and then annealing in the high vacuum at temperature of 1423 K (which is about 100 K below the melting point) for 24 h. The nanocrystalline Sm2 Co7 bulk material was prepared using the spark plasma sintering (SPS) technique [21]. In SPS process, simultaneous applications of a large pressure and a high heating rate generated by the pulsed electrical current facilitate a rapid densification without obvious microstructure coarsening [22,23]. In the present experiments, the Sm2 Co7

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Fig. 1. XRD patterns of the Sm2 Co7 alloy samples at different preparation states: (a) as-cast ingot, (b) as-milled powder, and (c) as-sintered bulk.

ingot was firstly crushed into coarse powder with a typical particle size of approximately 500 ␮m. In order to avoid contamination, the following procedures were all performed in a home-made “oxygenfree” (the environmental oxygen concentration was controlled to below 0.5 ppm) in situ fabrication system [24], which combined the powder treatment with the SPS technique in an entirely closed system filled with highly purified argon gas. The powder was firstly milled in a planetary mill using the vial and balls made of tungsten carbide. The amorphous powder of the alloy was produced by the high-energy ball milling process with a ball-to-power weight ratio of 14:1 for 18 h at a constant rotational speed of 500 rpm. Then the as-milled amorphous powder was fed into the high-strength cermet die in the glove box and immediately sent to the sintering chamber of the SPS equipment by a sliding rail. To obtain the ultrafine nanocrystalline Sm–Co bulk from the amorphous powder, the sintering parameters were optimized as: a constant external pressure 500 MPa, a heating rate 50 K/min, a sintering temperature 793 K, and no isothermal holding time. The phase constitutions and crystal structures of samples at different preparation stages were detected by the X-ray diffraction (XRD) with Cu K␣ radiation. Microstructures were observed by the transmission electron microscopy (TEM) operated at 300 kV. The statistical results of grain size and its distribution for the nanocrystalline sample were measured by the linear intercept method based on a large number of TEM and high-resolution TEM (HRTEM) images, where a recently developed method for recognition of nanograins in HRTEM images of nanocrystalline materials was applied to separate the overlapping grains [25,26]. Phase stability was detected by the differential scanning calorimetry (DSC) at a heating rate of 10 K/min under a steady argon gas flow (a piece of ∼50 mg sample was used for the measurement). The magnetic properties were measured at room temperature in a magnetic field of 70 kOe (5573 kA/m) using a quantum design physical properties measurement system (PPMS) magnetometer. 3. Characterizations of phase constitution and microstructure The XRD analyses on phase constitution and crystal structure of the Sm2 Co7 alloy samples at different preparation states are shown in Fig. 1. As seen in the curve (a), the Sm2 Co7 as-cast ingot (conventional polycrystalline alloy) has a single phase with a hexagonal Ce2 Ni7 -type structure (␣-Sm2 Co7 ) at the room temperature, which is in agreement with the report in the literature [27]. The curve (b) indicates that the milled powder has an amorphous structure. As shown by curve (c) in Fig. 1, the diffraction peaks of the assintered bulk sample are clearly broadened with respect to those of the as-cast ingot, indicating a fairly fine grain structure. The

Fig. 2. TEM analysis on the microstructure of as-sintered Sm2 Co7 bulk: (a) brightfield image of the microstructure, (b) SADP and its indexing, and (c) grain size distribution in the nanocrystalline bulk.

phase constitution of the as-sintered sample is identified as coexisting phases, i.e. the hexagonal ␣-Sm2 Co7 as a dominant phase and the rhombohedral ␤-Sm2 Co7 with the Gd2 Co7 -type crystal structure as a secondary phase. It should be noted that in the conventional polycrystalline alloys, the ␤-Sm2 Co7 phase can exist stably only at temperatures higher than 1435 K [28]. Thus the nanocrystalline Sm2 Co7 has different phase stability as compared with the conventional coarse-grained polycrystalline alloy. Moreover, the grain size and microstrain in nanocrystalline materials can be estimated from the XRD pattern using the Williamson–Hall [29] (WH) method. By carrying a least-squares fit of the full width at half maximum (FWHM) values in terms of the equation FW(S)Cos() = K/d + 4ε Sin() where K is the shape factor and  is the wave length, the grain size d and microstrain ε in the nanocrystalline sample are obtained as about 20.4 ± 0.9 nm and 0.57%, respectively. The TEM analyses for the microstructure of the as-sintered Sm2 Co7 bulk are shown in Fig. 2. It is seen that the as-sintered bulk has a nanocrystalline grain structure with a homogeneous grain size distribution (GSD), as the results shown in Fig. 2(c) from which the average grain size is obtained as about 21 nm which is in agreement with the result obtained from the XRD analysis. Moreover, as indexed from the selected area electron diffraction pattern (SADP) shown in Fig. 2(b), the nanocrystalline bulk consists of ␣and ␤-Sm2 Co7 phases, which confirms the XRD result of the phase constitution in the as-sintered bulk (see curve (c) in Fig. 1). Fig. 3 shows the high-resolution TEM (HRTEM) image of a local region in the nanocrystalline Sm2 Co7 bulk, which indicates a completely crystallized structure with clean (precipitation-free) grain boundaries (as marked by the dashed lines) and distinct orientations of individual nanograins (as indicated by the white double arrows). The electron diffractions of individual nanograins (marked as ‘1’ and ‘2’ in the HRTEM image in Fig. 3) were simulated by the Fast Fourier Transformation (FFT) model [30], and the results are shown at the bottom of Fig. 3. As revealed by the indexing of FFT patterns, the nanograins ‘1’ and ‘2’ have the hexagonal (␣-Sm2 Co7 ) and rhombohedral (␤-Sm2 Co7 ) crystal structures, respectively. Moreover, a fully coherent relationship between the

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Fig. 4. Magnetization hysteresis loops of the conventional polycrystalline and nanocrystalline Sm2 Co7 alloys in a magnetic field of up to 70 kOe (5573 kA/m) at room temperature.

Fig. 3. HRTEM image of local microstructure in the nanocrystalline Sm2 Co7 bulk, with FFT patterns and its indexing for individual nanograins ‘1’ and ‘2’.

two lattice structures is observed, as marked by the dotted line for the phase boundary. 4. Characterization of magnetic performance The magnetic properties of the as-cast ingot (conventional polycrystalline) and the as-sintered bulk (nanocrystalline) of the Sm2 Co7 alloy were characterized in terms of the magnetization behavior. Fig. 4 shows the magnetization hysteresis loops of the conventional polycrystalline and nanocrystalline Sm2 Co7 alloys in a 70 kOe (5573 kA/m) magnetic field at room temperature. It is seen that the hysteresis loop of the nanocrystalline Sm2 Co7 alloy exhibits a good squareness, and the demagnetization curve is smooth. This implies that the sample has homogeneous grain structure, which is in agreement with the TEM observation (see Fig. 2), and that there exists good intergrain exchange interactions within the same phase and among different phases [2,9]. In contrast to the conventional polycrystalline Sm2 Co7 alloy that has little coercivity (Hci ), the nanocrystalline alloy has a large intrinsic coercivity as Hci = 36 kOe (2866 kA/m) and a high remanence ratio as Mr /Ms-70 kOe = 0.74 (where Ms-70 kOe is the magnetization with the external magnetic field of 70 kOe), showing significantly enhanced permanent magnetic properties. The increase of magnetic properties of nanocrystalline magnets is mainly attributed to the ultrafine nanograin microstructure [31–33]. On one hand, it has been found that the coercivity of nanocrystalline alloy increases first and then decreases with the

decrease of grain size on the nanoscale [34]. The high coercivity can be achieved when the grain size of the hard magnetic phase is close to the size of a single domain. In the present case that the nanocrystalline Sm2 Co7 bulk sample is of mean grain size about 21 nm, it is believed that most of the grains in the sample are tend to exhibit single domain under external magnetic field. On the other hand, as seen from the early stage of magnetization (Fig. 4), the initial magnetization of the nanocrystalline Sm2 Co7 alloy increases slowly with the increase of the applied magnetic field, this implies a pinning mechanism of the coercivity [35]. This is due to the large volume fraction of grain boundaries in the nanocrystalline bulk sample acts as the magnetic domainwall pinning centers [36]. Both aspects result in a high coercivity in the nanocrystalline Sm2 Co7 bulk as compared with the conventional polycrystalline counterpart. Moreover, the high Mr /Ms-70 kOe ratio indicates a strong intergranular exchange coupling among the nanograins [37]. The decrease of saturation magnetization in the nanocrystalline Sm2 Co7 alloy as compared with the polycrystalline counterpart is attributed to the reduction of atomic coordination at nanograin boundaries which induces a strong disorder of spins.

5. Modeling of phase stability As shown in Section 4, the permanent magnetic property of nanocrystalline Sm2 Co7 alloy is clearly superior to that of the conventional polycrystalline alloy. As the phase constitution is different in the coarse-grained and nanocrystalline Sm2 Co7 alloys, it is important to investigate the phase stability in the nanocrystalline alloy. Previous studies have shown that the crystal structure of Sm2 Co7 phase can be developed from the SmCo5 structure by replacing certain Co atoms with Sm atoms [28,38]. In the RECo5 alloy systems, the strong magnetic anisotropy mainly results from the Co atoms at the ‘2c’ site in the crystal structure. For the Sm2 Co7 alloy, the axis ratio is c/a = 4.82 for the ␣-Sm2 Co7 phase and c/a = 7.22 for the ␤-Sm2 Co7 phases, respectively [28]. This implies that the ␤-Sm2 Co7 phase may have different magnetocrystalline anisotropy from that of the ␣ phase. It is considered that the difference in the crystal structure of the Sm2 Co7 alloy can lead to different permanent magnetic properties of the alloy, therefore, in the present work a series of experimental and thermodynamic studies have been performed on the phase stability and phase transformation behavior of the nanocrystalline Sm2 Co7 alloy.

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Fig. 5. Thermal analysis of as-sintered nanocrystalline Sm2 Co7 alloy with DSC measurements.

Fig. 6. Calculated Gibbs free energies (G␣ , G␤ ) for the ␣- and ␤-Sm2 Co7 phases and the difference (G␤−␣ = G␤ − G␣ ) in the nanocrystalline alloy as a function of grain size at the room temperature.

5.1. Experimental finding of phase stability As demonstrated in Figs. 1–3, the ␤-Sm2 Co7 phase, which is stable only at temperatures higher than 1435 K in the conventional polycrystalline system, can exist stably at the room temperature in the nanocrystalline alloy with an average grain size of ∼21 nm. Thus the Sm2 Co7 phase has an abnormal stability at the nanoscale. The high-temperature phase stability of the as-sintered nanocrystalline Sm2 Co7 alloy was examined by the DSC measurement from 298 to 1473 K in the purified Ar gas atmosphere, as shown in Fig. 5. It is observed that a broadened endothermal peak appears at about 1200 K. The start and finish temperatures of the peak are distinguished from the derivative curve of the heat flow with respect to the temperature, which are 1165 and 1310 K, respectively. The endothermal peak is considered to be caused by the transformation between ␣- and ␤-phase in the nanocrystalline Sm2 Co7 alloy. The DSC measurement shows that the ␣ ↔ ␤ phase transformation temperature in the nanocrystalline Sm2 Co7 alloy decreases from that of the conventional polycrystalline counterpart. Moreover, the phase transformation heat (i.e. the phase transformation enthalpy, H␤−␣ ) was estimated as about 926.7 J/mol based on the measured area of the endothermal peak in the DSC curve. 5.2. Thermodynamic modeling of phase stability In order to understand more profoundly the special phase stability in the nanocrystalline Sm2 Co7 alloy, a quantitative thermodynamic analysis on the phase transformation mechanism in the nanocrystalline Sm2 Co7 alloy was performed, using a developed nanothermodynamic model for nanocrystalline alloys (for the model details and applications see Refs. [39–42]). In this model, the nanocrystalline material is considered to consist of two components–the grain boundary and the grain interior, based on the “dilated crystal” assumption [43–46]. At grain boundaries, the excess enthalpy (Hb ), excess entropy (Sb ) and excess Gibbs free energy (Gb ) per atom are expressed as functions of nanograin size (d) and temperature (T). In grain interiors, the molar enthalpy (Hi ), entropy (Si ) and Gibbs free energy (Gi ) can be described approximately by those for the coarse-grained polycrystalline alloy

system, which can usually be obtained from the known database (e.g. the Scientific Group Thermodata Europe, SGTE [47]). Therefore, by introducing the atomic fraction at nanograin boundaries (xb ) which is a function of the grain size [40,41], the enthalpy H(d,T) = Nxb Hb (d,T) + Hi (T), the entropy S(d,T) = Nxb Sb (d,T) + Si (T), and the Gibbs free energy G(d,T) = Nxb Gb (d,T) + Gi (T) of one mole (N denotes the number of atoms in the system) nanocrystalline alloy can be calculated. Through the nanothermodynamic model calculations, the fundamental thermodynamic functions of nanocrystalline compounds can be predicted as a function of grain size and temperature. In the case of Sm2 Co7 alloy, the input parameters used in the nanothermodynamic calculations for ␣- and ␤-phases are listed in Table 1. It is noted that parameters of volume expansion coefficient, bulk elastic modulus and Debye temperature for the ␤-phase were calculated according to the methods proposed in the literature [41], because the high temperature phase of Sm2 Co7 is metastable at room temperature in the coarse-grained polycrystalline system [28]. Fig. 6 shows the calculated Gibbs free energies (G␣ and G␤ ) of the ␣- and ␤-Sm2 Co7 phases and the difference between the two phases (G␤−␣ = G␤ − G␣ ) as a function of grain size at the room temperature. The Gibbs free energies of both phases increase with the decrease of grain size. This implies that the degree of stability of ␣- and ␤-phases decreases with decreasing the grain size. Especially, when the grain size decreases to a few tens of nanometers (e.g. ∼50 nm, see Fig. 6), the degree of stability of ␣- and ␤-phases decreases greatly. It is particularly noted that the relative stability between ␣- and ␤-phases changes when the grain size is reduced to lower than a critical value (dC , as indicated in Fig. 6). As a consequence, phase transformation between ␣- and ␤-Sm2 Co7 may take place. From the calculations, the critical grain size for the ␣ ↔ ␤ transformation in the nanocrystalline Sm2 Co7 alloy is dC = 24 nm at the room temperature. This implies that at the room temperature, theoretically the nanocrystalline Sm2 Co7 phase with grain sizes smaller than 24 nm will have the Gd2 Co7 -type crystal structure (␤-Sm2 Co7 ), while the coarser grained Sm2 Co7 phase will have the Ce2 Ni7 -type crystal structure (␣-Sm2 Co7 ). In the real material having a certain grain size distribution, the nanocrystalline Sm2 Co7

Table 1 Parameters used in thermodynamic calculations for the nanocrystalline Sm2 Co7 .

a

Phase

Volume expansion coefficient ˛0 (10−6 K−1 )

Bulk elastic modulus B0 (GPa)

Debye temperature 0 (K)

Wigner–Seitz radius r0 (Å)

␣-Sm2 Co7 ␤-Sm2 Co7

40.14 [39] 41.91a

111.36 [39] 110.00a

286 [39] 291a

1.56 [39] 1.55 [28]

Parameters were calculated using the methods described in the Ref. [41].

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Fig. 7. Changes of the ␣ ↔ ␤ phase transformation temperature with the critical grain size in the nanocrystalline Sm2 Co7 alloy. Upper and lower regions indicate the thermodynamically stable ranges of grain size and temperature for the ␤- and ␣-Sm2 Co7 phases, respectively.

alloy will have a coexisting phase constitution of ␣- and ␤-Sm2 Co7 phase. The experimental finding that the ␣ and ␤ phases coexist in the nanocrystalline Sm2 Co7 alloy with a grain size distribution of 5–40 nm (Figs. 1 and 2) is consistent with the model prediction. From the grain-size-dependent Gibbs free energies (G␣ , G␤ ) for the ␣- and ␤-Sm2 Co7 phases at each temperature, the Gibbs free energy differences (G␤−␣ = G␤ − G␣ ) as a function of grain size can be obtained for any given temperature, thus the correlation of phase transformation (where G␤−␣ = 0) temperatures and critical grain sizes can be found. Fig. 7 shows the changes of the ␣ ↔ ␤ phase transformation temperature with the critical grain size in the nanocrystalline Sm2 Co7 alloy. It is seen that the critical grain size decreases monotonically when the ␣ ↔ ␤ phase transformation temperature decreases. Thus the thermodynamically stable range of the grain sizes and the temperatures for the ␣- and ␤Sm2 Co7 phases can be determined, as denoted in Fig. 7. The solid curve is the limit of the critical values, the upper and lower regions indicate the thermodynamically stable ranges (in both respects of temperature and grain size) for the ␤- and ␣-Sm2 Co7 , respectively. It is found that at 1165 K the critical grain size for ␣ ↔ ␤ phase transformation in the nanocrystalline Sm2 Co7 alloy is 50.6 nm. The comparison of the endothermal peak in the DSC curve of the as-sintered bulk (Fig. 5) and the nanothermodynamic calculation results (Fig. 6) indicates that the critical grain size increases very slowly below 1165 K during the DSC measurement, however, it increases significantly when the temperature is further increased. Fig. 8 shows the dependence of the enthalpies (H␣ , H␤ ) of the ␣- and ␤-Sm2 Co7 phases at the ␣ ↔ ␤ transformation temperature

Fig. 8. Enthalpies (H␣ , H␤ ) at the ␣ ↔ ␤ phase transformation temperature and its difference (H␤−␣ ) as a function of grain size of the nanocrystalline Sm2 Co7 alloy.

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and its difference (H␤−␣ ) on the nanograin size. It is seen that the enthalpy of the phase transformation decreases with the decrease of nanograin size. This may account for the decreased temperature of ␣ ↔ ␤ phase transformation with the decrease of the grain size in the nanocrystalline alloy. As compared with the experimentally obtained phase transformation enthalpy of the nanocrystalline alloy sample, the value H␤−␣ = 926.7 J/mol corresponds to a grain size of about 26 nm according to the theoretical results (see Fig. 8). This may suggest that an increase of grain size accompanying with the allotropic phase transformation process during the DSC measurement since the initial mean grain size of the nanocrystalline sample is measured to be about 21 nm. Combine the magnetic performance and the special phase stability, the nanocrystalline Sm2 Co7 containing ␤ phase with rhombohedral crystal structure can be considered as an important new candidate for the permanent magnets. The quantified relationship between the phase stability and the grain size and further the correlation between the phase stability and the magnetic performance, will assist essentially the design and development of the nanocrystalline Sm2 Co7 alloy, as well as many other nanostructured Sm–Co alloys, as candidates for permanent magnets. 6. Conclusions The nanocrystalline Sm2 Co7 alloy bulk with an average grain size of 21 nm was prepared. It was discovered that the nanocrystalline Sm2 Co7 alloy has remarkably enhanced permanent magnetic properties as compared with the conventional polycrystalline Sm2 Co7 alloy. Furthermore, distinctly different phase stability was found in the nanocrystalline Sm2 Co7 alloy with respect to the conventional polycrystalline counterpart, which is dependent on the nanograin size. The thermodynamically stable ranges of the grain size and the temperature for the ␣- and ␤Sm2 Co7 phases on the nanoscale were determined by coupling the experimental and nanothermodynamic studies. The ␣ and ␤ phases can coexist over a wide range of temperature for certain grain size distributions in the nanocrystalline Sm2 Co7 alloy. The understanding of the mechanisms for the phase stability will facilitate the design and development of the nanostructured Sm–Co alloys as candidates for permanent magnets. Acknowledgments This work was supported by the National Key Program for Fundamental Research and Development of China (2011CB612207) and the Beijing Natural Science Foundation (2112006). References [1] T. Methasiri, S. Thongmee, S. Varamit, I.M. Tang, Modern Physics Letters B 15 (2001) 97–103. [2] Z.X. Zhang, X.Y. Song, W.W. Xu, M. Seyring, M. Rettenmayr, Scripta Materialia 62 (2010) 594–597. [3] Y. Khan, B. Mueller, Journal of the Less-Common Metals 32 (1973) 39–45. ´ ´ [4] W. Szmaja, K. Polanski, I. Piwonski, A. Ilik, J. Balcerski, Vacuum 81 (2007) 1363–1366. [5] M.Q. Huang, W.E. Wallace, M. McHenry, Q. Chen, B.M. Ma, Journal of Applied Physics 83 (1998) 6718–6720. [6] Y.P. Wang, Y. Li, C.B. Rong, J.P. Liu, Nanotechnology 18 (2007) 465701. [7] X.Y. Song, N.D. Lu, W.W. Xu, Z.X. Zhang, J.X. Zhang, Journal of Applied Crystallography 42 (2009) 691–696. [8] P. Saravanan, G. Venkata Ramana, K. Srinivasa Rao, B. Sreedhar, V.T.P. Vinod, V. Chandrasekaran, Journal of Magnetism and Magnetic Materials 323 (2011) 2083–2089. [9] Z.X. Zhang, X.Y. Song, W.W. Xu, Acta Materialia 59 (2011) 1808–1817. [10] P. Saravanan, S.V. Kamat, B. Sreedhar, Journal of Magnetism and Magnetic Materials 324 (2012) 1201–1204. [11] S.D. Bader, Reviews of Modern Physics 78 (2006) 1–15. [12] P. Dev, H. Zeng, P.H. Zhang, Physical Review B 82 (2010) 165319. [13] N.A. Frey, S. Peng, K. Cheng, S.H. Sun, Chemical Society Reviews 38 (2009) 2532–2542.

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