Magnetic susceptibility studies of cobalt-based II-VI and manganese-based chalcopyrite diluted magnetic semiconductors
Abstract
We have made magnetic susceptibility measurements of the diluted magnetic semiconductors Zn$\sb{1-x}$Co$\sb{x}$S, Cd$\sb{1-x}$Co$\sb{x}$S, Cd$\sb{1-x}$Co$\sb{x}$Se, and (CuIn)$\sb{1-x}$Mn$\sb{2x}$Te$\sb{2}$. For the Co-based alloys, measurements were done for temperatures 33 mK $\le T\le$ 4.2 K and for concentrations $0.03\le x\le0.103.$ Transitions from paramagnetism were observed in all samples. The low temperature phase was attributed to spin-freezing. The spin-freezing temperature $T\sb{f}$ was found to be a universal function of x. From a log-log plot of $T\sb{f}$ versus x, a power law spatial dependence of the long-range exchange interaction was found, with an exponent $n=6.3\pm1.2.$ This value is very close to the value n = 6.8 obtained for the equivalent Mn-based materials. We give a detailed comparison of our results for the Co-based alloys with the Mn-based data and discuss the properties of the long-range interaction which leads to spin freezing in both alloy systems. For the (CuIn)$\sb{1-x}$Mn$\sb{2x}$Te$\sb2$ materials, samples with $0.0034\le x\le0.101$ were measured in the temperature range 33 mK $\le T\le300$ K. The samples all exhibited Curie-Weiss behavior. An antiferromagnetic effective nearest-neighbor exchange parameter $J\sb{eff}$ was extracted from the Curie-Weiss temperature and plotted as a function of x. At the highest concentrations, $J\sb{eff}$ was virtually constant, having a value $\approx -18.5$ K. As x was lowered, $J\sb{eff}/k\sb{B}$ fell sharply to the value $-$0.36 K. The sample with the lowest x had a large $J\sb{eff}/k\sb{B}$ value of $-$88.5 K. Spin-freezing transitions were also seen in samples with $x>0.01.$ Hall effect measurements were done which established that the samples all had p type conductivity. The samples having the smallest $J\sb{eff}$ values were the best conductors. We speculate that the behavior of $J\sb{eff}$ could be the result of several different effects, including vacancies, ordering of the Mn ions on the lattice, and the RKKY interaction.
Degree
Ph.D.
Advisors
Crooker, Purdue University.
Subject Area
Condensation
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