Detection of the Spin Reorientation and Glassy Dynamics in La$_{1.55}$Sr$_{0.45}$NiO$_4$

The magnetism of charge stripe ordered La$_{1.55}$Sr$_{0.45}$NiO$_4$ was studied by the combined techniques of neutron diffraction, muon spin relaxation and mass susceptibility. Magnetic ordering was observed at a lower temperature by muon relaxation than neutron diffraction, indicating a glassy transition to the ordered phase, in agreement with the susceptibility. Neutron diffraction determined a second magnetic transition that is observed by all techniques, to be a spin reorientation. On cooling below $T{\rm_{SR}} = 42$\, K the spins re-orientate from lying 33.7$\pm 0.6 ^{\circ}$ away from the stripe direction at 70\,K to 57.4 $\pm 0.4^ {\circ}$ at 10\, K. The magnetic order was observed by neutron diffraction to be of both anisotropic 3D and 2D (without any correlation on the $c$ axis) character. Muon relaxation confirmed this to be consistent with a single magnetically ordered spin stripe phase. The muon site associated with the spin stripe indicated critical behavior expected from a two dimensional magnetically ordered state, whereas the site associated with the charge stripe showed three dimensional critical behavior indicating spin frustration at the charge stripe.