In-plane Exchange Interactions Research Articles

Novel intermediate phase in Heisenberg helimagnets

The ground state of a classical Heisenberg spin system with competing in-plane exchange interactions is studied, with particular attention to the phase boundary between two helical phases one with vector along a nearest neighbor direction, the other along a next-nearest neighbor direction. This phase transition either can be discontinous, or can occur via a novel intermediate phase in which the wave vector changes continuously from one orientation to the other.

Magnetic order and spin fluctuations in oxide superconductors

A brief review is given of both the rare earth ( R ) and Cu magnetism in the R Ba 2Cu 3O 6+ x , R Ba 2Cu 4O 8, (La-Sr) 2CuO 4 and (Nd-Ce) 2CuO 4 systems. The Cu magnetism is dominated by the strong in-plane exchange interactions, which give rise to two-dimensional magnetic behavior in both the antiferromagnetically ordered insulating phase as well as in the superconducting state. The rare earth ions, on the other hand, order at low temperatures irrespective of the presence or absence of superconductivity. In the 1-2-3 and 2-4-8 materials, 2-d behavior is observed, while in the (Nd-Ce) 2CuO 4 system there is a substantial magnetic interaction between the Nd and Cu sublattices.

EPR study of the randomly diluted two-dimensional heisenberg ferromagnet K 2Cu xZn 1−xF 4

We report a room temperature EPR study of K 2Cu x Zn 1− x F 4, a randomly site-siluted system of the two-dimensional Heisenberg ferromagnet K 2CuF 4. Since the ferromagnetism of K 2CuF 4 comes from the orbital ordering of d z 2− x 2 and d z 2− y 2 due to the cooperative Jahn-Teller distortion of F −-octahedra, particular attention has been paid to the x-dependences of g-values in order to see how the orbital ordering is disturbed by Zn 2+ ions which have no Jahn-Teller effect. From the almost unchanged g-values obtained for x≳0.4, we infer that the orbital ordering remains for x≳0.4, which assures the in-plane exchange interactions to be ferromagnetic over this range of x. Both the linewidth of the main-line and the intensity of the satellite (2ω 0)-line show different aspects above and below x≊0.4. This value of x coincides with the percolation limit 0.41 for the next-nearest-neighbor interactions on a square lattice. Taking account of the appearance of T c for x<0.59, the percolation limit for the nearest-neighbor interactions, observed in the susceptibility-measurement by Okuda et al., we conclude that x≊0.4 should be the critical concentration and thus the next-nearest-neighbor exchange plays an important role in the present diluted system.

Antiferromagnetism in a Layer Structure by Green Function Techniques

A Green function method has been used to investigate antiferromagnetic ordering in a layer structure. Taking a lattice with an in-plane ferromagnetic exchange interaction and a between-plane antiferromagnetic interaction, the method is used to consider a Hamiltonian which there is reason to believe is applicable to the `metamagnetic' salts Co${\mathrm{Cl}}_{2}$ and Ni${\mathrm{Cl}}_{2}$ and which includes a uniaxial anisotropy. It is shown that the Green function techniques are well able to deal with Hamiltonians of this more complex type, and an expression is derived for the sublattice magnetization as a function of temperature. In particular, the antiferromagnetic transition temperature is calculated as a function of the exchange parameters. In the following paper these results are used to assist in the calculation of the actual exchange interactions which are present in Co${\mathrm{Cl}}_{2}$ and Ni${\mathrm{Cl}}_{2}$.