Magnetocrystalline anisotropy in ferrites of the form Zn2X and Co2X

Measurements of the magnetocrystalline anisotropy have been made in Ba
2
Zn
2
Fe
28
O
46
and Ba
2
Co
2
Fe
28
O
46
, two materials of the
X
phase of hexagonal ferrites. The measurements were made by ferromagnetic resonance techniques at room temperature. The anisotropy of Zn
2
X was also measured by a static technique. These materials have rhombehedral symmetry, and thus are similar to the
Y
phase. The lowest terms in the anisotropy energy consistent with this symmetry are
E_{A} = K_{1} \sin^{2}\theta + K_{2} \sin^{4}\theta + K_{2}' \sin^{3}\theta\cos\theta\cos 3\phi
. Zn
2
X shows uniaxial anisotropy, with
K_{1} = +2.57 \times 10^{6}
erg/cm
³
, K
2
and
K_{2}'
zero. The static measurement yielded
K_{1} = 2.50 \times 10^{6}
erg/cm
³
. In Co
2
X
K_{1} = -2.6 \times 10^{5}
erg/cm
³
,
K_{2} = +3.8 \times 10^{5}
erg/cm
³
,
K_{2}' = 1.2 \times 10^{5}
erg/cm
³
so the unique axis becomes a hard direction. Comparison of the measured change in anisotropy on substitution of cobalt for zinc with the theory of Slonczewski shows that to second order the theory predicts just the three terms shown, and gives numerical values larger by a factor of two than the measured values. Linewidths are also reported.