Relaxation of spin echo signals of Cr-53 nuclei in Ag-doped CdCr2Se4

The frequency dependences of the relaxation times of NMR spin echo signals of the quadrupole nuclei Cr-53 were measured in the ferromagnetic semicondu ctor Cd0.985Ag0.015Cr2Se4 at the temperature T = 4.2 K. The experimental re sults were well explained by the developed theory of the two-pulse echoes r elaxation. The main assumption of this theory is the assumption that the te mporal fluctuations in the electron magnetization due to the fluctuations i n the hyperfine and quadrupole Hamiltonians lead to the relaxation of the e cho signals. It was shown that in Cd0.985Ag0.015Cr2Se4 there are two kinds of the quadrupole nuclei 53Cr, which have quite different relaxation times. The existence of two kinds of the nuclei Cr-53 (Cr-53(I) and Cr-53(II)) wa s connected with doping of the cadmium selenochromite with Ag+ ions. The nu clei Cr-53(II) are sited in the crystal ranges where the rapid electron exc hange between the Cr4+ and Cr3+ ions leads to the rapid fluctuations in the local electron magnetization vector. The nuclei Cr-53(I) are located far f rom these dynamical defects. The observed frequency dependence of the relax ation rate of the usual Hahn's echo signal from the nuclei Cr-53(I) was exp lained by the secular theory of the echo relaxation. The nonsecular relaxat ion theory well explains the frequency dependence of the relaxation rate of multiquantum echo signal from the nuclei Cr-53(II). (C) 2000 Elsevier Scie nce B.V. All rights reserved.