Then, 63 vol.% of particles and 37 vol.% of wax were mixed together and pressed into a coaxial cylindrical specimen, in which the magnetic particles were randomly SAHA HDAC order dispersed. Electron spin resonance (ESR) measurements were performed with a Bruker ER200D spectrometer (JEOL, Tokyo, Japan). Results and discussion The XRD patterns of NiFe2O4 NPs annealed

at 700°C to 1,000°C for 2 h are depicted in Figure 1. All diffraction peaks of the samples can be well indexed to the standard spinel phase without any additional peak. The average crystallite size of the synthesized powders is estimated by the X-ray peak broadening of the (400) diffraction peak, via the Scherrer equation [23]. The results indicate that the powders are nanocrystalline with an average crystallite size of 31 to 46 nm for S700 to S1000. Figure 2a,b,c,d

shows the SEM images of NiFe2O4 NPs. It is clearly seen that all the NiFe2O4 NPs are partly accumulated together with different sizes, and the size of the sample particles increases obviously with the thermal treatment temperature. The average particle size is about 60 nm for S700 (200 nm for S1000), which is much larger than the crystallite size estimated by XRD. These results indicate that the obtained sample particles are polycrystalline. Figure 1 X-ray diffraction patterns for samples S700, S800, S900, and S1000. Figure 2 SEM images of samples S700 (a), S800 (b), S900 (c), and S1000 (d). The room temperature magnetic properties of NiFe2O4 NPs were studied using VSM. Olopatadine Figure 3a shows the hysteresis PS-341 cell line loops of the samples, and the inset of Figure 3a shows the initial magnetization curves. It is found that M s is a monotonic function of the annealing temperature, and the value of M s is 38.7, 41.1, 42.6, and 45.8 emu/g for S700 to S1000, respectively. Generally, the M s of NiFe2O4 NPs is lower than that of the bulk form (56 emu/g) [24, 25], which can be attributed to the greater fraction of surface spins in NPs that tend to be canted or the spin disorder with a smaller net moment [26]. The spin disorder is due to the presence of considerable defects which can destroy the superexchange interaction. M s increases as the sintering temperature increases,

which is due to the reduction of the specific surface area. The initial magnetization curves suggest that the initial magnetic permeability increases with increasing annealing temperature. Figure 3 M – H curves of the samples and XPS spectra of S700. (a) Magnetic hysteresis loops of the samples (inset: the initial magnetization curves), (b) XPS survey spectrum of sample S700, and (c) fitted XPS spectra of O 1s of sample S700. The vertical axis represents the signal intensity. KCPS, kilo counts per second; B.E., binding energy. The evidence for the composition of products in the surface was obtained by XPS. Figure 3b shows the XPS survey scan spectrum of a representative sample, S700, indicating that no impurities were detected in the sample within the detection limit.