Critical Magnetic Properties and Exchange Interactions in EuO

The critical magnetic exponents are determined for EuO and found to be $\beta =0.\mathrm{}368\pm 0.005$, $\delta =4.\mathrm{}46\pm 0.1$, and $\gamma =1.\mathrm{}29\pm 0.01$. Within experimental error these values are consistent with the predictions of scaling theory. It is shown that the reduced magnetic field is expressible as a function only of the reduced magnetization in the critical region, with a functional dependence which differs for $T$ above and below $T_c$. However, the linear model of Schofield et al. leads to a single equation of state which gives a good approximation to our data throughout the entire critical region. Our results were analyzed in terms of high-temperature expansion for fcc lattices with nearest $J_1$ and next-nearest-neighbor $J_2$ interactions. It is shown that straightforward extrapolation procedures can lead to serious discrepancies. However, the analysis can be used in conjunction with our susceptibility measurements both immediately above $T_c$ and up to 370°K and with the observed ${\gamma }^{*}$ behavior to determine the value of the magnetic interactions. Our results indicate that $J_2$ is ferromagnetic and equal to $(0.\mathrm{}5\pm 0.2)J_1$. The disagreement between this and the previously reported value is discussed.