Magnetic study of spin freezing in the spin glass BaCo6Ti6O19: Static and dynamic analysis

Abstract

ac and dc magnetic susceptibility as a function of frequency, magnetic field, and temperature have been studied in polycrystalline BaCo 6 Ti 6 O 19 . This oxide displays all the common features of spin-glass-like behavior. dc magnetic susceptibility at low fields shows a sharp peak at ca. 13.6 K, which signals the onset of irreversibility between the zero-field-cooled and field-cooled (FC) processes. Isothermal magnetization curves obtained from FC data have been fitted in terms of odd powers of ( χ 0 H). The first two coefficients of the nonlinear susceptibility increase by more than three orders of magnitude when approaching T c from above, thus indicating the occurrence of an equilibrium phase transition. Static scaling analysis of the nonlinear susceptibility leads to the following set of critical parameters: δ=7.9±0.3, φ=5.7±0.5, T c =13.21±0.05. The reliability of this set has been checked by reference to the asymptotic behavior of the scaling function. The unusually high values of the δ and φ exponents could be related to some degree of reduction of the spin dimensionality, because of the planar contribution to the anisotropy of Co 2 + ions when located at octahedral sites. Dynamic scaling has been developed from ac susceptibility data assuming power-law divergence of the relaxation time, yielding the following set of parameters (zν=8.0±0.5, β=0.75±0.10, T c =13.20±0.05). The value of zν has also been confirmed by studying the dependence of the freezing temperature on frequency. The β, δ, φ values roughly accomplish the scaling relation β=φ/δ. Thermoremanent-magnetization data show the common aging phenomena in disordered magnetic systems, as expected in spin glasses.