engleski

Low temperature heat capacity in some low-dimensional systems with unusual magnetic order

We present the temperature and magnetic field dependence of the heat capacity Cp of quasi one-dimensional system CuSeO3 and quasi two-dimensional system Fe8Te12O32Cl6. CuSeO3 [1] has an orthorhombic structure with linear Cu2+ chains very similar to CuGeO3 [2], a well known spinPeierls system [3]. We have observed a BCS-like transition at Tc=8.2 K with very similar features as in CuGeO3 [4], including the broad maximum in the magnetic contribution to Cp well above Tc [5]. However, the transition did not change even in the magnetic field of 8 T whereas in CuGeO3 both Tc and DCp at Tc decrease by 10% for in the same field [6]. Moreover, the magnetic susceptibility of CuSeO3 indicates AFM ordering and magnetic anisotropy even more complex diagram with probable tetramerization of Cu2+ spins along the chain. Fe8Te12O32Cl6 is a quasi two-dimensional system with the magnetically active Fe atoms arranged on the distorted honeycomb lattice within the layer [7]. Although Fe8Te12O32Cl6 is a good insulator, the low temperature Cp shows a strong linear contribution. However, this linear contribution ceases at the transition temperature Tc=30 K. The transition is rather small and wide, and the overall entropy change is only 3 J/mole K. Tc corresponds nicely with the narrow maximum in the magnetic susceptibility which does not resemble any standard transition. Both Tc and DCp at Tc decrease strongly with applied magnetic field and at 0.5 T it is hardly observable in the background of the lattice contribution. Magnetic susceptibility is similarly strongly dependent on applied magnetic field below 30 K, however at lower temperatures shows clearly the spin-flop transition characteristic for AFM order. We assign tentatively the results of Cp to the strongly fluctuating one-dimensional AFM order which sets in at 30 K and supports spin wave excitations below Tc. [1] K. Kohn et al., J. Solid State Chem. 18 (1976) 27 [2] H. Völlenkle et al., Monatsch. Chem. 98 (1967) 1352 [3] K. Uchinokura, J. Phys.: Condens. Matter 14 (2002) R195 [4] S. Sahling et al., Solid State Commun, 92 (1994) 423 [5] M. Weiden Z. Phys. B 98 (1995) 167 [6] G. Remenyi et al., J. Low Temp. Phys. 107 (1997) 243 [7] R. Becker, M. Johnsson, J. Solid State Chem. 180 (2007) 1750

heat capacity; low-dimensional systems; magnetic order

nije evidentirano