Magnetic excitations of a Heisenberg spin glass : evidence for a gap in high magnetic fields
Résumé
Measurements of the specific heat C(T) of a spin glass Eu 0.44Sr0.56S 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 excitations of Eu2+ ions in the external magnetic field augmented by some internal field of 3.2 T, the origin of which is discussed.
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