Andrew Boothroyd

Abstract:

Magnetic order has been investigated in stripe-ordered La 2-xSrxNiO4 (x=0.275,0.37,0.4) by dc magnetization and by polarized- and unpolarized-neutron diffraction. In the magnetically ordered phase, all three compositions exhibit a magnetic transition consistent with a spin reorientation in the ab plane. For x=0.37, the spin axes rotate from an angle of 37.7°±0.3° to the stripe direction at 71 K, to 52.3°±0.20 at 2 K. The x=0.275 and 0.4 compounds were found to undergo a similar spin reorientation. A spin reorientation has now been observed to occur for five different doping levels in the range of 0.275≤x≤0.5, suggesting that this spin transition is an intrinsic property of the stripe phase.

Abstract:

The thermal diffusivity in the vicinity of the magnetic transition of a set of La1-xSrxMnO3 (0less than or equal toxless than or equal to0.3) single crystals has been measured using the photopyroelectric technique. Taking into account that the inverse of the thermal diffusivity behaves as the specific heat, the critical exponent alpha has been obtained. In the pure and lightly doped samples (x=0,0.05) where the transition is antiferromagnetic-paramagnetic, the critical exponent is consistent with the Heisenberg model (alpha=-0.115). For Sr concentration higher than 0.20, where the low temperature phase is ferromagnetic, a drastic change in the critical exponent has been observed, approaching Ising behavior (alpha=+0.11) at the highest concentration. (C) 2004 American Institute of Physics.

Abstract:

We report magnetization measurements on La2-xSrxNiO4+delta single crystals, with 0less than or equal toxless than or equal to0.5. Glassy behaviour associated with the formation of spin-charge stripes, and a separate spin-glass phase at low temperatures were observed. We have also found a 'memory effect' in the magnetic field-temperature history, which is found to be suppressed in the low-temperature spin state of the x = 0.333 crystal. (C) 2003 Elsevier B.V. All rights reserved.

Abstract:

We report neutron scattering measurements of the spectrum of magnetic excitations in the stripe-ordered phase of La2-xSrxNiO4 (x = 0.275 and 1/3). The propagating spin excitations follow a similar dispersion relation for the two compositions, but the line widths are broader for x = 0.275 than for 1/3. (C) 2003 Elsevier B.V. All rights reserved.