Tetragonal Distortion Due to the Jahn–Teller Effect and Coercivity of CuₓCo1–xFe₂O₄ Synthesized at Various Annealing Temperatures

The crystal structure and magnetic properties of Cu
_x
Co
_1–x
Fe
₂
O
₄
samples synthesized by a solid-phase reaction method at various annealing temperatures (
${T}_{\mathrm {a}} =1073$
–1223 K) were investigated. Tetragonal distortion due to the Jahn–Teller effect of Cu
²⁺
ions was observed in the
${x} =0.8$
sample synthesized at
${T} _{\mathrm {a}} =1223$
K. As the magnetic anisotropy constant is enhanced by such distortion, the Cu concentration dependence of the coercivity value
${H} _{\mathrm {c}}$
exhibits a maximum. A decrease in
${T} _{\mathrm {a}}$
caused the enhancement of tetragonal distortion and the reduction of crystallite size. As a result, the largest value of
${H} _{\mathrm {c}} = 2100$
Oe was obtained for the
${x} =0.8$
sample synthesized at
${T} _{\mathrm {a}} =1073$
K. It is clear that the control of both the Cu concentration and annealing temperatures during synthesis is important to obtain large coercivity in Cu
_x
Co
_1–x
Fe
₂
O
₄
.