By fitting, we obtained three peaks at 529.8, 531.2, and 532.4 eV. The dominant peak located at 529.8 ± 0.2 eV (Oa), which corresponds to O2− ions of the pure composites [27, 28], and the highest binding energy peak at 532.4 ± 0.2 eV (Oc) can be attributed to the chemisorbed oxygen of surface hydroxylation, oxygen atoms in carbonate ions, and adsorbed H2O

or O2[29]. Furthermore, the medium find more binding energy component (Ob) located at 531.2 ± 0.2 eV (Oc) is associated with the O2− ions in the oxygen-deficient regions (O vacancies) [30]. The result obviously demonstrates the presence of oxygen defects in the surface, and the oxygen defects can destroy the superexchange interaction. This indicates that surface and internal magnetic states are different, and the surface magnetic state can show a strong surface anisotropy [14]. Figure 4 shows the complex permeability μ of the NiFe2O4/wax with 63 vol.%. At a frequency of 0.1 GHz, the real part of the complex permeability (μ’; Figure 4a) increases from 2.0 to 2.8 with the increase

of sintering temperature. The spectra of the imaginary part (μ”) are shown in Figure 4b; it is worth noting that a resonance phenomenon in the effective permeability is observed at around 1 ~ 3 GHz for NiFe2O4 NPs. Thiazovivin datasheet Meanwhile, with the increase of sintering temperature, selleck kinase inhibitor continuous modification in the resonance frequency of the samples in the range of BCKDHB 1.45 to 2.54 GHz has been achieved, which is much higher than previously reported [31]. Pascard and Globus reported that the magnetic resonance frequency is approximately 102 MHz for NiFe2O4 microparticles [32]. Based on the Landau-Lifshitz-Gilbert equation, the resonance frequency is f r = (1 + α 2) × γ × H a /2π (α is the magnetic damping parameter, γ is the

gyromagnetic ratio, H a is the magnetic effective anisotropy field), and Vittoria et al. reported that α is less than 0.01 [33]. As a result, an approximately effective anisotropy field is 900, 760, 610, and 510 Oe for S700, S800, S900, and S1000, respectively. The data unambiguously show that the magnitude of the effective anisotropy field is on the decline with the increase of sintering temperature. For NiFe2O4 NPs, a strong effective anisotropy has been obtained, which is consistent with previous theoretical results [14–16]. This effective anisotropy field is much bigger than the magnetocrystalline anisotropy field for NiFe2O4; therefore, it is related to the strong surface anisotropy for NPs. The magnitude of this surface anisotropy is related to the concentration of the defects in the surface and the fraction of broken exchange bonds relative to the total number of neighboring pairs of surface cations [14], for an individual particle.