Features of Cr$^{3+}$ ions ESR spectra temperature behavior in Dirac 3D semimetal Cd$_3$As$_2$

The temperature behavior of chromium impurity in the Dirac semimetal $\alpha$-Cd$_3$As$_2$ was studied using the electron spin resonance (ESR) method on the Cr$^{3+}$ ion. Small intensity of the ESR spectra on the Cr$^{3+}$ ions definitely points that the main valence state of chromium, as well as other impurities, in this material is the Cr$^{2+}$ state. Some of these ions turn into the valence state of Cr$^{3+}$. Cr$^{3+}$ ions located in tetrahedral vacancies turn into the anti-ferromagnetic phase at the temperatures above 127 K. The transition is accompanied by a sharp narrowing of the ESR spectrum and a collapse of its hyperfine structure. At the temperatures below 50 K, the remaining magnetic ions located in the positions of cadmium ions turn into the ferromagnetic phase, the appearance of which is indicated by a sharp broadening of the exchange-narrowed resonance line and its strong shift to low resonance fields. The observed inversion of the shape of the exchange-narrowed line of the hyperfine structure of the spectrum requires more detailed study.