Applied-Field Dependent Hyperfine Structure and Magnetic Properties of ${\rm Ni}_{0.8-x}{\rm Cu}_{x}{\rm Zn}_{0.2}{\rm Fe}_{2}{\rm O}_{4}$

The crystal structure of Ni_0.8-xCu_xZn_0.2Fe₂O₄ (x = 0.2, 0.4, 0.6) sample is determined to be cubic spinel with space group Fd-3 m by Rietveld refinement. The lattice constant a₀ increased linearly with the Cu concentration from x = 0.2 to 0.6. With increasing Cu concentration, the saturation magnetization and coercivity decreased from 101.3 emu/g, 112 Oe to 88.7 emu/g, 83 Oe, respectively, at 4.2 K. Ni and Cu ions in NiCuZn ferrite prefer octahedral sites (B) and Zn ions prefer tetrahedral sites (A). Based on the distribution probability, we have analyzed Mössbauer spectra measured at 4.2 K as 5 sets with six-lines. Hyperfine fields at A and B sites at 4.2 K with zero magnetic field were H_hf(A)=500 kOe, and H_hf(B₀) =545 kOe, H_hf(B₁) = 525 kOe , H_hf(B₂) = 516 kOe, H_hf(B₃) = 483 kOe, for x = 0.6. Applied-field Mössbauer spectra of Ni_0.8-xCu_xZn_0.2Fe₂O₄ were measured parallel to the γ-ray direction under 5 T at 4.2 K. Hyperfine field H_hf(A) at A site under 5 T was 516 kOe, larger than that under zero applied-field, and at B site the average value of hyperfine field 〈H_hf(B)〉 was 440 kOe, smaller than 〈H_hf(B)〉 = 517 kOe under zero applied-field, for x = 0.6 . Then the hyperfine field at A and B sites decreased with increasing Cu content x = 0.2 to 0.6. Also, we noticed that the second and fifth absorption lines of Mössbauer spectra completely disappeared above 1 T, indicating that the spins of Fe ions at A and B sites were collinea- - r to the applied-field. The Fe valence state was determined to be ferric from the isomer shift values.