ESR and magnetization of the spin-glass $\mathrm{Cu}\mathrm{Mn}$ at low concentrations

We have determined the resonance frequency $\omega$ versus resonant field $H$ diagram for $\mathrm{Cu}\mathrm{Mn}$ with Mn concentrations 2, 3.5, and 5 at.%, both when $\overrightarrow{\mathrm{H}}$ is parallel and antiparallel to the cooling field ${\overrightarrow{\mathrm{H}}}_c$. From the parallel measurements we find anisotropy energy density constants $K$ that are in agreement with $K$ values determined from transverse susceptibility and torque measurements. However, the $K$ values determined from the remanence reversal of dc magnetization are smaller by a factor of approximately 2. We suggest that this is caused by a breakdown of rigid-body spin rotations. By combining the parallel data with the antiparallel data, we obtain sets of $K$ values for the vector model and the triad model which are unphysical. We attribute this also to nonrigid spin rotations. In addition, we present the full angle dependence of ESR measurements for the 5-at.% sample, which shows that the quantitative agreement with the two models breaks down when the angle ${\theta }_R$ between $\overrightarrow{\sigma }$ and ${\overrightarrow{\mathrm{H}}}_c$ is greater than $\approx {25}^{\circ }$ for this concentration. Finally, we have carefully searched for the predicted ${\omega }^{-}$ mode in small external fields, but we have not observed it. By using the concept of oscillator strength we proceed to calculate this effect for the vector anisotropy model. We find the ESR intensity of the expected ${\omega }^{-}$ mode to be $\approx 1\%$ of that of the ${\omega }^{+}$ mode. Nevertheless, under our experimental conditions this should not preclude its observation.