Field-dependent antiferromagnetism and ferromagnetism of the two copper sublattices in Sr2Cu3O4Cl2

The Cu3O4 layer in Sr2CU3O4Cl2 is a variant of the square CuO2 lattice of t he high-temperature superconductors, in which the center of every second pl aquette contains an extra Cu2+ ion, The ions that make up the conventional CuO2 network, called CuI, have CuI-CuI exchange energy approximate to 130 m eV, and order antiferromagnetically at about 380 K; the Cull-Cull exchange is only approximate to 10 meV, and the Cull's order at approximate to 40 K. A study is reported here of the dependence of the magnetization on held, t emperature, and crystallographic orientation for this interesting system. W e show that the small permanent ferromagnetic moment, that appears when the CuI spins order, and the unusual spin rotation transitions seen most clear ly for one particular direction of the magnetic field, are the result of se veral small bond-dependent anisotropic terms in the spin Hamiltonian that a re revealed because of the frustration of the isotropic Heisenberg interact ion between CuI and Cull spins. These include a term which favors collinear ity of the CuI and CuII spins, which originates from quantum fluctuations, and also the pseudodipolar interaction. Some of these small interactions al so come into play in other lamellar cuprates, connected with the high-T-c s uperconductivity materials, and in many spin-chain and spin-ladder compound s. [S0163-1829(99)03218-X].