A. A. Nikonov

Manifestation of ferromagnetism in lightly oxygen-doped La2CuO4 single crystals in magnetic fields H<50 Oe

The temperature and field dependences χ(T,H) in La2CuO4+δ single crystals with δ<0.015 have been investigated in magnetic fields 0.1<H<450 Oe by the differential magnetic susceptibility method. It was found that under oxygen doping conditions ferromagnetic regions are formed. These regions produce a characteristic curve of the magnetic susceptibility χ(T,H), which is observed only in magnetic fields of less than 50 Oe. This can be explained by the formation of ferrons [A. Aharony et al. Phys. Rev. Lett. 60, 1330 (1988); L. I. Glazman and A. S. Ioselevich, Z. Phys. B 80, 268 (1990)] in an antiferromagnetic matrix.

Charge transfer near the Néel temperature in La2CuO4+x

The electrical resistance and the thermoelectric power in the ab plane of a weakly oxygen-doped La2CuO4+x crystal (0.001<x<0.007) and its static magnetic susceptibility were studied in the vicinity of antiferromagnetic transition. The electrical resistance and the thermoelectric power behave anomalously near the Néel temperature, indicating that the transport is strongly affected by the establishment of long-range antiferromagnetic order. Analysis of the obtained data allows the conclusion to be drawn that the doping gives rise to a conduction band as a result of the overlap between the wave functions of deep impurity states that are strongly renormalized due to the correlation and polaron effects.

Weak ferromagnetism and oxygen ordering in La2CuO4+x single crystals

The changes produced in the magnetic properties and structure of La2CuO4+x (0< x<0.015) single crystals by doping with oxygen are investigated by differential magnetic susceptibility and x-ray diffraction methods. It is found that the appearance of a weak ferromagnetism in weak fields H<50 Oe is accompanied by a lowering of lattice symmetry as a result of the oxygen ordering.

Magnetic susceptibility of La2NiO4+δ single crystals with δ = 0.095 and 0.105. Ordering of holes and ferromagnetic heisenberg chains

The temperature dependence of the magnetic susceptibility of La2NiO4+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 La1.8Sr0.2NiO4 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 JN have been determined from the temperature dependence χ(T).

Anomalous charge transfer in La2CuO4 near the Néel temperature: Evidence of the transition from spin liquid to two-sublattice antiferromagnetic state

The temperature dependence of the electrical resistance and thermopower of La2− xSrxCuO4+δ single crystals with x ≅ 0.003 and δ<0.05 has been studied in the temperature range from 100 to 400 K. All crystals exhibiting two-dimensional hopping conductivity via neighboring acceptor sites in the CuO2 plane show a significant difference in the charge transfer below and above the Néel temperature TN. This difference indicates that the loss of a two-sublattice antiferromagnetic order strongly affects the charge transport in the CuO2 plane. The obtained data lead to a conclusion that the crystal above TN occurs in a resonance valence bond state of the Bose type.

Two-dimensional quantum ferromagnet, magnetic chains, and oxygen ordering in La2NiO4.125 single crystals

The temperature dependence of the magnetic susceptibility χ(T) is studied in the region of the magnetic and charge ordering of La2NiO4.125 single crystals that differ in the degree of ordering of the excess oxygen. In the absence of the ordering of the excess oxygen, χ(T) obeys the dependence χ (T)≈C/tT2, which is explained by the formation of 1D FM Heisenberg chains of impurity states with S=1/2. The ordering of oxygen creates favorable conditions for the formation of 2D FM Heisenberg lattices of impurity states with S=1/2 that obey the exponential dependence χ(T)≈C(T)2exp(4π JS2/kT), which is an indication of the formation of a 2D quantum Heisenberg ferromagnet.

Anomalous magnetism of La2CuO4+x single crystals

Measurements of the differential magnetic susceptibility are performed to study the changes in the magnetic properties of La2CuO4+x due to oxygen doping in the concentration interval 0<x<0.011. For crystals with 0.005<x<0.011, a ferromagnetic-type phase transition is observed at Tc>TN, and it occurs even in crystals with TN∼0. The concentration dependence of the transition temperature Tc(x) is obtained.