Spin correlations in a spin-glass system studied by light scattering: EuxSr1−xS (abstract)

Abstract

Raman measurements in all magnetic phases of Eux Sr1−xS show a new type of q≠0 magnetic scattering at low temperatures. The intensity can be quantitatively described by using in the scattering cross section a modified Ornstein–Zernike ansatz for the spin correlation function. A consistent interpretation of the Raman data in correspondence with the magnetic phase diagram has been achieved. Eux Sr1−xS shows a rather unusual magnetic phase diagram. Ferromagnetism breaks down below x=0.51. Spin-glass behavior is observed at low temperatures for 0.13<x<0.65. Most interest has been attracted recently by the reentrant phase boundary between the (‘‘frustrated’’) ferromagnet and the spin glass for 0.51<x<0.65. We performed Raman scattering on Eux Sr1−xS for x=0.65, 0.54, and 0.40 at temperatures between 1.5 and 300 K and in applied magnetic fields up to 7 T using 5145-Å laser excitation. Besides the well-known spin-disorder induced scattering from the LO(L) phonons near the zone boundary in EuS, one observes in the (frustrated) ferromagnetic phase and in the spin-glass regime additional scattering intensity which is of magnetic origin. A consistent theoretical description of our results is given by the one-phonon one-spin scattering cross section used for EuS. For the spin correlation function, we used a modified Ornstein–Zernike ansatz, with an additional exponent y which describes the spin system from uncorrelated ( y=0) via statistically coupled ( y=2) to ferromagnetically aligned spins ( y>2). Systematic variations of the exponent y as a function of composition and temperature in high magnetic fields characterize the spin correlations and yield a consistent interpretation of the Raman data in correspondence with the magnetic phase diagram. For example, for Eu0.54 Sr0.46S, we observe in the (frustrated) ferromagnetic phase with decreasing temperature an increase of y from y=2.0 to y=2.6 near the reentrant phase boundary and a decrease to y=2.0 in the spin-glass phase. The latter value is also found in the spin glass phase of Eu0.40Sr0.60S. In conclusion, we would like to point out that Raman measurements in Eux Sr1−xS show a new type of q≠0 magnetic scattering at low temperatures and in applied magnetic fields, not observed in the ferromagnetic state of EuS. The deduced exponent y of the spin correlation function agrees well with that deduced from diffuse magnetic neutron scattering. In addition, our measurements in external magnetic fields up to 7 T provide information complementary to the zero-field neutron measurements and yield a microscopic understanding of the magnetic phase diagram.