Magnetism

• Ferrimagnet  to Papers

The copper oxide material Sr8CaRe3Cu4O24 is an insulator with a perovskite structure (Fig. 1), which has spontaneous magnetization even at room temperature. The magnetic transition temperature of this material (Tc=440K) is much higher than those of other ferromagnetic cuprates (Tc≈10K). We have shown by a first-principle calculation that an orbital order appears where copper orbitals (Cu1: eg orbital, Cu2: dx2-y2 orbital) and oxygen orbitals (O2: p orbital) form strong pdσ bonds as shown in Fig. 2. This is the reason why the transition temperature is so high. Based on the numerical results, we proposed a spin-alternating Heisenberg model in three dimensions as an effective model for magnetism. The ground-state magnetization of this effective model is proven to be exactly 1μB by the Marshall-Lieb-Mattis theorem. This is consistent with experimental result (M ≈ 0.95μB). We have investigated the properties of this model by a quantum Monte Carlo simulation, and successfully explained the behavior of magnetization observed experimentally (Fig. 3). Furthermore, we predicted behaviors of other physical quantities that have not been measured experimentally yet. Numerical results also suggest that a half-metallic state would be realized by chemical substitution or hole-doping for this material.

Fig. 1: Lattice structure of Sr8CaRe3Cu4O24 Fig. 2: Schematic picture of orbital and spin orderings Fig. 3: Temperature dependence of magnetization (Experimental result (…), Numerical result(―))