Andrew Boothroyd

Abstract:

Soft X-ray resonant diffraction is a new technique and there exist several examples of large resonant enhancements of charge and magnetic scattering that can be obtained at the Ledges of 3d transition metal oxides. Here we have employed resonant soft X-ray scattering at the manganese L edges which provide a direct measurement of the orbital ordering. We have studied the layered manganite La0 5Sr1 5MnO4 that displays charge. spin, and orbital ordering. Energy scans at constant wavevector show that there are two separate contributions to the observed scattering, direct Goodenough orbital ordering and strong cooperative Jahn-Teller distortions of the Mn3+ ions. Finally we will show how that the charge, spin and orbital degrees of freedom are strongly correlated in these materials.

Abstract:

We report the temperature dependence of susceptibility for various pressures, magnetic fields and constant magnetic field of 5 T with various pressures on La2-2xSr1+2xMn2O7 single crystal to understand the effectiveness of pressure and magnetic field in altering the magnetic properties. We find that the Curie temperature, T c, increases under pressure (dTc/dP=10.9 K/GPa) and it indicates the enhancement of ferromagnetic phase under pressure up to 2 GPa. The magnetic field dependence of Tc is about 26 K for 3 T. The combined effect of pressure and constant magnetic field (5 T) shows dT c/dP=11.3 K/GPa and the peak structure is suppressed and broadened. The application of magnetic field of 5 T realizes 3D spin ordered state below Tc at atmospheric pressure. Both peak structure in χc and 3D spin ordered state are suppressed, and changes to 2D-like spin ordered state by increase of pressure. These results reveal that the pressure and the magnetic field are more competitive in altering the magnetic properties of bilayer manganite La1.25Sr1.75Mn2O7 single crystal. © 2005 Elsevier Ltd. All rights reserved.

Abstract:

An ac photopyroelectric calorimeter has been used to measure the thermal diffusivity and specific heat of the perovskite manganites RMnO3 (R ≤ La, Pr, Nd) close to their magnetic transitions. Taking into account that the inverse of the thermal diffusivity has the same critical behaviour as the specific heat, the critical exponent α of the magnetic transition has been obtained by means of both magnitudes. The results in all cases are consistent with the Heisenberg model (α ≤ -0.11), irrespective of the rare-earth ion. © 2005 IOP Publishing Ltd.