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-x
Cu
_x
Zn
_0.2
Fe
₂
O
₄
(
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
a
₀
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
(
B
₀
) =545 kOe,
H
_hf
(
B
₁
) = 525 kOe ,
H
_hf
(
B
₂
) = 516 kOe,
H
_hf
(
B
₃
) = 483 kOe, for
x
= 0.6. Applied-field Mössbauer spectra of Ni
_0.8-x
Cu
_x
Zn
_0.2
Fe
₂
O
₄
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 collinear to the applied-field. The Fe valence state was determined to be ferric from the isomer shift values.