Magnetic susceptibility of La$_2$NiO$_{4+\delta}$ single crystals with $\delta=0.095$ and $0.105$. Ordering of holes and ferromagnetic heisenberg chains

The temperature dependence of the magnetic susceptibility of La₂NiO_4+δ single crystals with δ = 0.095 and 0.105 has been analyzed. The anomalous dependence of χ(T) characteristic of stripe structures previously found in La_1.8Sr_0.2NiO₄ has been found. Investigation of the dependence χ(T) on the impurity and charge disorder degrees makes it possible to separate the contribution associated with the ordering of holes and impurity oxygen. It has been shown that the contribution from the ordering of holes is well described by the model of ferromagnetic Heisenberg chains of spins S = 1/2. The length of the chains and intrachain interaction J _N have been determined from the temperature dependence χ(T).