LOW TEMPERATURE CALORIMETRIC INVESTIGATION OF THE SPIN GLASSES: MERCURY(1-X)MANGANESE(X)TELLURIDE AND COBALT(X)GALLIUM(1-X); AND OF THE COMPOUNDS: MERCURY-TELLURIDE, ALPHA - MERCURY SULFIDE, BETA - MERCURY SULFIDE, THALLIUM(3)ARSENIC SELENIDE(3), THALLIUM(3)ANTIMONY SULFIDE(3), SILVER-THALLIUM-SULFIDE, AND SILVER-THALLIUM-SELENIDE
Abstract
A systematic study of the low-dc-field magnetic susceptibility and the specific heat has been carried out on mixes Hg(,1-x)Mn(,x)Te crystals, in the composition range 0 (LESSTHEQ) x (LESSTHEQ) 0.35. The alloy with x = 0.35 showed spin-glass behavior below T = 10.9 K. The observed spin-glass phase is ascribed to the frustration of the antiferromagnetic interactions. For x (LESSTHEQ) 0.25, the Hg(,1-x)Mn(,x)Te samples remain paramagnetic down to 1 K. Experimental results for the specific heat and the susceptibility for x < 0.1 are discussed in terms of a cluster model which leads to an estimated value of the antiferromagnetic exchange constant J/k (DBLTURN) -0.7 (+OR-) 0.3 K. When a random distribution of Mn ions over the fcc sublattice is assumed, calculated values for the specific heat and the susceptibility differ substantially from the experimental results for the low Mn concentration, leading to the conclusion that the magnetic ions prefer to cluster rather than to remain isolated in Hg(,1-x)Mn(,x)Te. Low temperature specific heat of the spin glass compounds Co(,x)Ga(,1-x). with x between 0.49 and 0.58 are presented. For all samples the excess specific heat in zero magnetic field initially contains a term linear in temperature, and for x > 0.52 it also has contributions by the cobalt nuclei, proportional to T('-2), and a spin wave contribution proportional to T('3/2). This last term indicates the coexistence of spin glass and ferromagnetic properties. A simple two level system model fits the spin glass specific heat very well. The agreement between experimental and calculated specific heat shows that individual AS defects are responsible for the thermal properties. To explain hysteresis and remanence objects containing thousands of AS defects have been proposed. On increasing the temperature some objects become unfrozen. We speculate that the individual AS defects in the unfrozen objects can adjust themselves over their own two levels and so contribute to the thermal properties. Specific heats of mercury chalcogenides (HgTe, HgSe, (alpha)-HgS, (beta)-HgS) and red HgI(,2) have been measured in the temperature range of 0.4 - 50 K. All materials display well defined maxima in CT('-3) which indicate the presence of low-lying modes described by Einstein oscillators. The specific heats of Tl(,3)AsSe, Tl(,3)SbS(,3), AgTlS, and AgTlSe have been measured between 1 and 50 K. The Debye temperatures are, respectively: 140, 145, 160, 140 K. Above 2.5 K an additional contribution is noticed which indicates low-lying optical modes.
Degree
Ph.D.
Subject Area
Condensation
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