Relaxation of spin echo signals of 53Cr nuclei in Ag-doped CdCr 2Se 4

Abstract

The frequency dependences of the relaxation times of NMR spin echo signals of the quadrupole nuclei 53Cr were measured in the ferromagnetic semiconductor Cd 0.985Ag 0.015Cr 2Se 4 at the temperature T=4.2 K . The experimental results were well explained by the developed theory of the two-pulse echoes relaxation. The main assumption of this theory is the assumption that the temporal fluctuations in the electron magnetization due to the fluctuations in the hyperfine and quadrupole Hamiltonians lead to the relaxation of the echo signals. It was shown that in Cd 0.985Ag 0.015Cr 2Se 4 there are two kinds of the quadrupole nuclei 53Cr, which have quite different relaxation times. The existence of two kinds of the nuclei 53Cr ( 53Cr(I) and 53Cr(II)) was connected with doping of the cadmium selenochromite with Ag + ions. The nuclei 53Cr(II) are sited in the crystal ranges where the rapid electron exchange between the Cr 4+ and Cr 3+ ions leads to the rapid fluctuations in the local electron magnetization vector. The nuclei 53Cr(I) are located far from these dynamical defects. The observed frequency dependence of the relaxation rate of the usual Hahn's echo signal from the nuclei 53Cr(I) was explained by the secular theory of the echo relaxation. The nonsecular relaxation theory well explains the frequency dependence of the relaxation rate of multiquantum echo signal from the nuclei 53Cr(II).