Evidence of surface spin-glass behavior in NiFe2O4 nanoparticles determined using magnetic resonance technique

Nanosized nickel ferrite (NiFe₂O₄) is successfully synthesized by the sol-gel method using citric acid (C₆H₈O₇) as fuel. The X-ray diffraction pattern of the as-synthesized sample shows formation of NiFe₂O₄ nanoparticles (NPs) as the main phase, with mean crystalline size ∼50 nm. While transmission electron microscopy (TEM) reveals nearly spherical morphology with mean particle diameter of 41 ± 5 nm, high-resolution TEM shows inter-planar distance (2.55 Å) corresponding to the (1 1 3) plane of the spinel structure. Additionally, the Raman spectrum exhibits five Raman active modes (two A_1g at 575 cm⁻¹ and 710 cm⁻¹; two E_g at 345 cm⁻¹ and 670 cm⁻¹; and one T_2g at 497 cm⁻¹) typical of NiFe₂O₄. X-band magnetic resonance (MR) data reveal a single broad resonance line in the whole temperature range (3.8 K ≤ T ≤ 300 K), with g-value decreasing monotonically from 3.10 ± 0.01 at 3.8 K to 2.77 ± 0.01 at 300 K. The temperature dependence of both resonance field and resonance linewidth show a remarkable change in the range of 60–70 K, herein credited to surface spin-glass behavior. The model picture used to explain the MR data assumes NPs with a core-shell structure. Below about 60–70 K the shell's spin system progressively reveals a paramagnetic to spin-glass-like transition upon cooling, with a freezing temperature estimated at 2.1 ± 0.1 K. However, above about 60–70 K spins in the paramagnetic shell align along the NP's core dipolar field, resulting in an effective enlarged nanoparticle size.