Magnetic and structural study ofLa1.8Sr0.2NiO4

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Abstract
Neutron-diffraction experiments with both polarized and unpolarized neutrons have been performed on single crystals of (La1.8Sr0.2)NiO4. The material is tetragonal at room temperature (I4/mmm) but distorts to the orthorhombic (Bmab) modification at 120 +/- 5 K with a second-order phase transition. The structural parameters are similar to those in the pure La2Ni)4. The susceptibility is highly anisotropic at low temperature, with X(ab) (field applied perpendicular to the long axis) showing a peak at approximately 17 K. However, we have found no evidence for long-range magnetic order. We speculate that the low-temperature state is an anisotropic spin glass, with 17 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 a slightly 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. Surprisingly no unusual effects are found in the ab plane where we might expect strong covalency between the Ni d(x2-y2) and O P(x),P(y) orbitals. The total moment as determined by these neutron experiments is in good agreement with that determined by magnetization experiments.