Magnetic and Magneto-Optic Properties of FeRh and CrO2

For polycrystalline FeRh and single-crystal CrO2 the complex index of refraction N and the complex quantity Q (Q=Q0e−iq is the off-diagonal component of the complex permittivity tensor) were determined as a function of wavelength from 425 to 1000 mμ. The largest value for the magneto-optic amplitude Q0 was 0.008 at 700 mμ for FeRh, which corresponds to a Faraday rotation, F = π | NQ |/λ0, of 90 500 deg cm−1 and an absorption coefficient α, I/I0 = e−αx, of 329 000 cm−1 for a normalized Faraday rotation, f = F/α, of 0.28 deg. The largest value of Q0 for CrO2 was 0.025 at 1000 mμ which corresponds to F of 135 000 deg cm−1, α = 102 000 cm−1, and f = 1.3 deg. The wavelength dependence of the complex dielectric constant ε indicates a resonance close to 1000 mμ, which would explain the rather high magneto-optic effect. These values were measured at room temperature for CrO2 and for FeRh just above the first-order magnetic phase transition at 60°C. For both materials the temperature dependence of the coercive force was measured. The coercive force for FeRh increases with decreasing temperature and goes to infinity at the magnetic transition, while Hc for the CrO2 single crystal was temperature-independent.