Neutron and magnetization study of La1.8Sr0.2NiO4

Magnetization and neutron-diffraction experiments with both polarized and unpolarized neutrons have been performed on single crystals of La1.8Sr0.2NiO4. The susceptibility is highly anisotropic at low temperature, χab (field applied perpendicular to long axis) showing a peak at ∼18 K. However, no evidence has been found for long-range magnetic order. It is speculated that the low-temperature state is a spin glass, with 18 K being the freezing temperature. Polarized neutron diffraction has been used to measure the induced magnetization density in a field of 4.6 T and at 20 K. The results can be understood by using an expanded Ni2+ form factor and invoking a small positive susceptibility at the La site. There is also evidence for covalency between the Ni and O(2) atoms along the c axis. The material distorts from tetragonal (I4/mmm) to orthorhombic (Bmab) at 120±5 K. The structural parameters are similar to those of La2NiO4