Magnetic resonance in the spin-glass $\left(La\mathrm{Gd}\right){\mathrm{Al}}_2$

ESR has been measured in the spin-glass $\left({\mathrm{La}}_{1-x}{\mathrm{Gd}}_x\right){\mathrm{Al}}_2$, $1\le x\le 15$ at.% for 1, 3, 9, and 35 GHz, corresponding to different applied magnetic fields and in the temperature range of 100 mK to 30 K. The comparison of data for different frequencies shows that the linewidth depends very much on the frequency and/or field and therefore the temperature $T_{min}$ of the linewidth minimum is not an intrinsic parameter and the shift of the resonance position is almost independent of frequency. The former effect is analyzed and theoretically explained in terms of the temperature dependence of the measured susceptibility at the field for resonance instead of a Curie-Weiss susceptibility. When the resonance position does shift, close to the glass temperature, this is identified to be an internal-field, rather than a $g$-factor, effect. The temperature dependence of this internal field differs from that of the bulk magnetization.