Magnetic Susceptibility and Specific Heat of an Anisotropic Spin-Glass:EuxSr1−x

Abstract

The magnetic susceptibility of Eu x Srix As3 spin-glasses is highly anisotropic: For x = 0.24 and x = 0.3 the longitudinal and transverse susceptibilities (with respect to the orientation of magnetic moments in antiferromagnetic EuAs 3 ) exhibit spin-glass-like maxima at different temperatures, with the longitudinal freezing occurring at higher temperature. The magnetic specific heat displays a temperature dependence not previously found in spin-glasses, C M^a TQxp ( -bE/k^T). This indicates a gap around zero energy in the density of states of magnetic excitations, which arises from the anisotropy.PACS numbers: 75.30. Gw, 75.50.Ee Anisotropy in spin-glasses has been of much interest recently. In metallic spin-glasses, the Dzyaloshinsky-Moriya interaction leads to an anisotropy which is believed to be responsible for the remanent magnetization remaining in the direction of the initially applied magnetic field. 1 In addition, single-ion anisotropics may exist which lead to a preferred orientation of magnetic moments with respect to the crystal axes. An exemplary system is dilute Mn-based spin-glasses with different hexagonal hosts, where Heisenberg-like, XK-like, and Ising-like spin-glass behavior was observed. 2 The mean-field phase diagram for anisotropic spin-glasses (with infinite-range interactions) predicts subsequent freezing with decreasing temperature of longitudinal and transverse degrees of freedom for weak positive uniaxial anisotropy, D > 0 (and vice versa for D < 0). M However, experimental evidence for this interesting feature has been evasive. 5 Here we report on a new anisotropic spin-glass EUxSrj-^Asa. Anisotropic behavior with subsequent longitudinal and transverse freezing occurs in this system with welldefined Eu 2+ moments (S«-j, L«0), where-for intermediate and high Eu 2+ concentrations-the magnetic interactions are predominantly of short range.Another current problem in spin-glass research is the nature of low-lying excitations in spin-glasses. As is well known, the magnetic excitations lead to a specific heat which below the freezing temperature varies roughly linearly with temperature. The *'linear specific heat" is thought to be a characteristic and general feature of disordered solids, arising from a constant density of states of the elementary excitations near zero energy, and appearing in "real" glasses 6 and in both metallic 7 and nonmetallic 8 spin-glasses. In this paper a distinctly different temperature dependence of the specific heat, not previously observed in spinglasses, is reported for Eu^Sr^^Asa: Over a wide range of concentrations (0.10^x^0.50) and temperatures (one order of magnitude) the magnetic specific heat can be represented by C M = aTThis gives strong evidence for the existence of a gap in the zero-field density of states which is a new feature of spin-glasses, 9 and is probably related to the anisotropy.EuAsj is a monoclinic semimetal which undergoes two successive magnetic transitions. 10 At 11.3 K a transition to an incommensurate magnetic structure occurs which i...

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