Magnetic excitations of a Heisenberg spin glass : evidence for a gap in high magnetic fields

Measurements of the specific heat C(T) of a spin glass Eu 0.44 Sr 0.56 S single crystal have been performed between 0.7 and 10 K, and with applied magnetic fields (H=0, 4, 6 and 7 T). The application of a large magnetic field strongly modifies the C(T) behaviour, with a large decrease at low temperatures: the quasilinear variation observed in zero field is replaced by an exponential variation characteristic of the opening of a gap in the excitation spectrum. These results, together with those of magneto-thermal resistivity obtained previously, can be described with the assumption of two kinds of magnetic excitations: (i) very low energy collective excitations and (ii) single spin excitation of Eu 2+ ions in the external magnetic field augmented by some internal field of 3.2 T, the origin of which is discussed Mesures de chaleur massique sur un monocristal de verre de spin isolant Eu 0,44 Sr 0,56 S entre 0,7 et 10 K, sous champ magnetique H(0 a 7 T): effet important du champ dur C(T) avec une tres forte diminution a basse temperature, la variation quasi lineaire en champ nul etant remplacee par une variation exponentielle, caracteristique de l'apparition d'une bande interdite dans le spectre des excitations. A l'aide de resultats anterieurs sur la conductivite thermique en champ magnetique, interpretation par deux types d'excitations magnetiques: collectives et d'ions Eu 2+