New ferromagnets of Sr8ARe3Cu4O24 (A=Sr, Ca) with an ordered perovskite structure

Anion order in perovskites: a group-theoretical analysis

Anion ordering in the structure of cubic perovskite has been investigated by the group-theoretical method. The possibility of the existence of 261 ordered low-symmetry structures, each with a unique space-group symmetry, is established. These results include five binary and 14 ternary anion superstructures. The 261 idealized anion-ordered perovskite structures are considered as aristotypes, giving rise to different derivatives. The structures of these derivatives are formed by tilting of BO6 octahedra, distortions caused by the cooperative Jahn-Teller effect and other physical effects. Some derivatives of aristotypes exist as real substances, and some as virtual ones. A classification of aristotypes of anion superstructures in perovskite is proposed: the AX class (the simultaneous ordering of A cations and anions in cubic perovskite structure), the BX class (the simultaneous ordering of B cations and anions) and the X class (the ordering of anions only in cubic perovskite structure). In most perovskites anion ordering is accompanied by cation ordering. Therefore, the main classes of anion order in perovskites are the AX and BX classes. The calculated structures of some anion superstructures are reported. Comparison of predictions and experimentally investigated anion superstructures shows coherency of theoretical and experimental results.

Half-metallic antiferromagnet as a prospective material for spintronics

Spintronics is expected as the next-generation technology based on the novel notch of spin degree of freedom of electrons. Half-metals, a class of materials which behave as a metal in one spin direction and an insulator in the opposite spin direction, are ideal for spintronic applications. Half-metallic antiferromagnets as a subclass of half-metals are characterized further by totally compensated spin moments in a unit cell, and have the advantage of being able to generate fully spin-polarized current while exhibiting zero macroscopic magnetization. Considerable efforts have been devoted to the search for this novel material, from which we may get useful insights for prospective material exploration.

Possible half metallic antiferromagnet in a hole-doped perovskite cuprate predicted by first-principles calculations

We formulate a scheme to realize a half metallic antiferromagnet (HMAFM), a material conductive in only one spin channel while exhibiting zero macroscopic magnetism, by doping carrier into a class of cuprates. The working rationale is exhibited as taking advantage of Hubbard repulsion of d electrons of Cu atoms and the charge-transfer effect from the associated O ligand to fully polarize the spin of a doped carrier. Specifically, doping one hole into the insulating ferrimagnet Sr8CaRe3Cu4O24 by replacing one of the eight Sr atoms by one Rb atom is predicted to achieve a HMAFM, presumably with room-temperature operation. Since the working rationale is the strong correlations of electrons commonly encountered in cuprates, it is expected that the present findings can shed light on a new way to develop a HMAFM.

Robust half-metallic character and large oxygen magnetism in a perovskite cuprate

The new perovskite cuprate material Sr8CaRe3Cu4O24, which behaves ferrimagnetically and shows an unusually high Curie temperature (T(C) approximately 440 K), is found from density-functional theory calculation to display several surprising properties after hole doping or chemical substitution: (1) Half metal (HM) is realized by replacing Re with W or Mo while T(C) remains high; (2) hole-doped Sr8CaRe3Cu4O24 is also HM with high T(C). Moreover, we find that the O atoms will carry a large magnetic moment after hole doping, which is in sharp contrast to the generally accepted concept that magnetism in solids requires partially filled shells of d or f electrons in cations. The material Sr8CaRe3Cu4O24 is therefore expected to provide a very useful platform for material design and development.

Orbital order and ferrimagnetic properties of Sr8CaRe3Cu4O24

By means of the LSDA+U (local spin density approximation plus Hubbard U) method and the Green function method, we investigate the electronic and magnetic properties of the new material of Sr8CaRe3Cu4O24. Our LSDA+U calculation shows that this system is an insulator of ferrimagnetism with a net magnetic moment of 1.01 micro(B)/f.u., which is in good agreement with the experiment. It is the nonmagnetic Re atoms that induce an orbital order of d electrons of Cu atoms, which is responsible for the strong exchange interaction and the high magnetic transition temperature. Based on the LSDA+U results, we introduce an effective model for the spin degrees of freedom and investigate the finite-temperature properties by the Green function method. The obtained results are consistent with the experimental results, indicating that the spin-alternating Heisenberg model is suitable for this compound.