Structure, Magnetic, and Electrical Properties of La2NiO $_{4+\delta }$ Compounds

We have investigated the structure, magnetic, and electrical properties, and the resistance switching effect (RSE) of La
₂
NiO
_4+δ
compounds with different excess oxygen levels of δ = 0.004 - 0.031. Our results showed the structure changes from tetragonal I4/mmm to orthorhombic Fmmm when reducing δ value from 0.011 to 0.006. The M(H) curves measured at several temperatures exhibit a coexistence of the weak ferromagnetic and antiferromagnetic interactions. Two shoulders on M(T) curves related to the spin and charge ordering have also been observed. These ordering temperatures depend strongly on δ value and they shift toward lower temperature with decreasing δ. All the samples exhibit a semiconductor behavior and their temperature dependences of the resistivity obey the variable-range-hoping model. Interestingly, the typical current-voltage characteristics of the samples show a hysteresis, which suggests an existence of the RSE. We have observed two different states of the resistance corresponding to high (R
_H
) and low (R
_L
) values. The resistance change ratio of the samples defined as 100% x [R
_H
- R
_L
]/R
_L
is found to be about 18%-57%, which depends on the value of δ in the samples.