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COMPLEX OXIDES

Interfaces on stageThe broken symmetry at an interface between two different oxides is a source of unexpected behaviour. For instance, the modifi ed orbital physics at the interface can lead to induced magnetism in a superconductor.

JACOBO SANTAMARÍAis in the Grupo de Física de Materiales Complejos, Departamento de Física Aplicada III, Universidad Complutense, 28040 Madrid, Spain. e-mail: jacsan@fi s.ucm.es

Complex oxide systems with the perovskite structure can be found in almost every possible solid physical state — as superconductors,

metals, insulators, ferromagnets, ferroelectrics, multiferroics and more. Many are transition metal oxides with strong electron correlations that lead to fi erce competition between lattice, orbital, charge and spin interactions. Owing to the balance between kinetic energy and Coulomb repulsion, these systems tend to stabilize diff erent phases under small perturbations in temperature, electric or magnetic fi elds, strain and so forth. On page 244 of this issue1, Jacques Chakhalian et al. investigate what happens at the interface between a superconductor YBa2Cu3O7 (YBCO) and a ferromagnet La0.7Ca0.3MnO3 (LCMO) in a thin-fi lm superlattice, where, in a sense, the perturbation stabilizing unusual phases is the interface itself. What they fi nd is induced magnetism in the superconductor and altered domain structure in the ferromagnet at the superconducting transition temperature, showing that both of these long-range cooperative phenomena, magnetism and superconductivity, are modifi ed at the interface. Th is work demonstrates how interfaces between complex oxides can show properties not seen in the individual constituents, and illustrates a possibility of engineering the electronic structure to design new functionalities.

Candidates for heteroepitaxial thin-film growth of complex oxides can be chosen from a long list of materials with very different properties and with nearly perfect lattice matching and good chemical compatibility, so that highly structurally ordered heterostructures can be grown with negligible interdiffusion. In the same way that the layer-by-layer growth of semiconductor hetero-interfaces is controlled by the highly directional covalent bond, the growth properties of complex oxides are governed by the constraint of preserving charge neutrality and stoichiometry imposed by the ionic character. The minimum growth unit is thus a complete unit cell, which gives rise to the cell-by-cell growth mechanism that allows the formation of atomically sharp interfaces if the process is carefully controlled.

Most of these compounds share nearly full ionic bonding, a type of bonding characterized by charge-transfer processes within the length scale of the unit cell. Modifi ed charge transfer due to valence or polarity mismatch is a fi rst source of altered physical properties at the interface. In this context the self-doping of interfaces — resulting from the modifi ed charge transfer — could turn the interface between two insulators into a metal2. Furthermore, modifi ed bonding at the interface may aff ect the spin properties because of the strong interaction between orbital and spin degrees of freedom3. Unexpected phenomena, which cannot be understood in terms of conventional band pictures, may thus appear. Within this framework, the fi nding of Chakhalian and co-authors1 constitutes a solid example of electronic

Mn

Cu Cu Cu Cu

CuCu Cu

CuCu CuCu

Cu

Mn Mn Mn

Mn Mn Mn Mn

Mn Mn Mn Mn

LCMO

YBCO

Figure 1 Magnetic proximity effect. A magnetic moment is induced in the interfacial Cu plane of the YBCO (represented by the dotted line) by spin canting. Arrows denote the direction of the magnetic moments. Note the 180º (antiparallel) alignment of Mn and in-plane component of the Cu moments, in agreement with Anderson–Goodenough–Kanamori rules for determining whether coupling through a non-magnetic intermediary ion is ferromagnetic (parallel) or antiferromagnetic (antiparallel) in character.

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230 nature physics | VOL 2 | APRIL 2006 | www.nature.com/naturephysics

reconstruction at the interface, triggering deep modifi cations of the electronic properties4.

Broken lattice symmetry modifi es orbital physics at the interface, and consequently a magnetic moment is induced in the interfacial Cu plane of the YBCO, oriented antiparallel to the Mn in LCMO. According to the Anderson–Goodenough–Kanamori rules (which predict whether the superexchange interaction between two spins mediated by a non-magnetic ion has a ferromagnetic or antiferromagnetic character), the observed antiferromagnetic coupling mediated by a 180° Mn–O–Cu exchange path at the interface indicates preferential occupancy of the Mn dx2– y2 orbitals, rather than the random occupation of the dx2–y2 and d3z2–r2 orbitals found in the bulk. Th e modifi ed orbital occupancy at the interface establishes the antiferromagnetic interaction that induces the Cu moment by spin canting (see Fig. 1).

Another interesting aspect of the work by Chakhalian et al.1 is the stripe-like domain pattern, detected by off -specular neutron refl ectivity, appearing in the ferromagnet at the superconducting transition. Th is indicates the interplay between ferromagnetism and superconductivity. Ferromagnetism (F) and (singlet) superconductivity (S) are antagonistic phenomena in the sense that one tends to align spins in parallel, and the other antiparallel. Th e strong competition between the two long-range

orders makes them incompatible and diffi cult to fi nd on the same sample.

Artifi cial F/S hybrids have previously been used to study the eff ect of an exchange fi eld of the ferromagnet on the superconducting state5. Th e result of Chakhalian et al. indicates that not only the superconducting but also the ferromagnetic state can be modifi ed at the F/S interface. Th e eff ect recalls the ‘old’ non-uniform ferromagnetic (cryptoferromagnetic) state6 proposed for a superconductor undergoing a ferromagnetic transition, characterized by a magnetic domain size that is much smaller than the superconducting coherence length of the pairs. Th e eff ect of the exchange fi eld on the coherent volume is averaged out and the superconductor ‘feels’ an eff ective antiferromagnet, allowing the peaceful coexistence of both long-range phenomena. Here, in contrast, the micrometre-sized domains are much larger than the nanometre-ranged in-plane coherence length, so the situation must be diff erent. Th is is an intriguing result that may be pointing to new physics at the interface between half-metals and superconductors.

REFERENCES1. Chakhalian, J. et al. Nature Phys. 2, 244–248 (2006).2. Ohtomo, A., Muller, D. A., Grazul, J. L, & Hwang H. Y. Nature 419,

378–380 (2002).3. Tokura, Y. & Nagaosa, N. Science 288, 462–468 (2000).4. Okamoto, S. & Millis, A. J. Nature 428, 630–633 (2004).5. Buzdin, A. I. Rev. Mod. Phys. 77, 935–976 (1959).6. Anderson, P. W. & Suhl, H. Phys. Rev. 116, 898–900 (1959).

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