Thermal expansion in the Anderson lattice system, CeAl2

A detailed study is reported of the finite-field longitudinal thermal expansion of Ce${\mathrm{Al}}_2$ between 1.5 and 10 K. From measurements in moderate magnetic fields, up to 55 kOe, it has been possible to (i) construct a phase boundary (magnetic field vs temperature) of the low-temperature moment-modulated antiferromagnetic (AF) phase; (ii) determine the coupling of the order parameter to length; and (iii) study the reorientation of the AF order parameter in the ordered phase and its fluctuations in the paramagnetic phase as a function of magnetic field. In high magnetic fields, up to 115 kOe, the longitudinal thermal expansion and magnetostriction show (i) a quadratic field dependence analogous to a Van Vleck-type magnetization, and (ii) a Schottky-like temperature anomaly resulting presumably from the Zeeman-split levels of the crystalline-electric-field (CEF) doublet ground state. Analysis of the high-field results, within the framework of a mean-field calculation which includes the combined effects of internal fields and the CEF, suggests that the indirect-exchange interaction between Ce atoms switches sign in high fields.