Ferromagnetic Resonance Absorption in Magnetite Single Crystals

The microwave resonance absorption technique, at both 1.25 and 3.3 cm wave-lengths, was used to study the ferromagnetic crystalline anisotropy characteristics and $g$-factor of magnetite ${\mathrm{Fe}}_3$ ${\mathrm{O}}_4$. The experiments were performed on single crystals, both synthetic and natural, from room temperature to -195°C. Depending upon the temperature, magnetite single crystals were found to have magnetic anisotropy characteristics similar to those of single crystals of the three ferromagnetic elements. From room temperature to -143°C, they behave like nickel; between -143° and the transition which magnetite is known to undergo at $\mathrm{ca}$. -160°C, like iron; and below the transition, somewhat like cobalt. At room temperature, values of $g=2.12\mathrm{}$ and $K_1$ (first-order anisotropy constant) = -1.10×${10}^4$ joule/${\mathrm{m}}^3$ (-1.10×${10}^5$ erg/${\mathrm{cm}}^3$) were obtained. It was found that below about -90°C the absolute value of $K_1$ decreases with decreasing temperature, reaching zero at $\mathrm{ca}$. -143°C. Between -143° and the transition $K_1$ is positive and increases with decreasing temperature. The vanishing of crystalline anisotropy at -143°C accounts for the peak in initial permeability found near this temperature. The $g$-value was found to decrease gradually and monotonically with decreasing temperature. The behavior of magnetite in the resonance experiments below the transition seems to indicate that the magnetic symmetry is uniaxial in this temperature region. This conclusion is consistent with the findings of other investigators. Below the transition the magnetic axis is that [100] direction most nearly parallel to a strong magnetic field applied to the crystal as it is cooled through the transition. At temperatures not far below the transition it is possible to change the magnetic axis from one [100] direction to another by means of a strong magnetic field.