An electron in a solid, that is, bound to or nearly localized on the specific atomic site, has three attributes: charge, spin, and orbital. The orbital represents the shape of the electron cloud in solid. In transition-metal oxides with anisotropic-shaped d-orbital electrons, the Coulomb interaction between the electrons (strong electron correlation effect) is of importance for understanding their metal-insulator transitions and properties such as high-temperature superconductivity and colossal magnetoresistance. The orbital degree of freedom occasionally plays an important role in these phenomena, and its correlation and/or order-disorder transition causes a variety of phenomena through strong coupling with charge, spin, and lattice dynamics. An overview is given here on this "orbital physics," which will be a key concept for the science and technology of correlated electrons.

http://science.sciencemag.org/content/sci/288/5465/462.full.pdf

Orbital Physics in Transition-Metal Oxides

Giant and isotropic magnetoresistance as huge as −53% was observed in magnetic manganese oxide La0.72Ca0.25MnOz films with an intrinsic antiferromagnetic spin structure. We ascribe this magnetoresistance to spin‐dependent electron scattering due to spin canting of the manganese oxide.

31p-S-4 Giant Magnetoresistance in Magnetic Manganese Oxide with Intrinsic Antiferromagnetic Spin Structure

Spintronics is a multidisciplinary field involving physics, chemistry, and engineering, and is a new research area for solid-state scientists. A variety of new materials must be found to satisfy different demands. The search for ferromagnetic semiconductors and stable half-metallic ferromagnets with Curie temperatures higher than room temperature remains a priority for solid-state chemistry. A general understanding of structure-property relationships is a necessary prerequisite for the design of new materials. In this Review, the most important developments in the field of spintronics are described from the point of view of materials science.

Spintronics: a challenge for materials science and solid-state chemistry.

The manganese oxides of general formula RE1−xMxMnO3 (RE = rare earth, M = Ca, Sr, Ba, Pb) have remarkable interrelated structural, magnetic and transport properties induced by the mixed valence (3+–4+) of the Mn ions. In particular, they exhibit very large negative magnetoresistance, called colossal magnetoresistance (CMR), in the vicinity of metal–insulator transition for certain compositions. In this review paper, we summarize the most important features of the physics of the CMR manganites. The growth techniques for manganese oxide thin films, which are the basic material for potential applications, are reviewed and their structure and morphology examined in relation to growth parameters. The effects of epitaxial strains on the physical properties are discussed. Early works on superlattices and devices are presented.

http://iopscience.iop.org/article/10.1088/0022-3727/36/8/201/pdf

CMR manganites: physics, thin films and devices

The study of magnetoelectric materials has recently received renewed interest, in large part stimulated by breakthroughs in the controlled growth of complex materials and by the search for novel materials with functionalities suitable for next generation electronic devices. In this Progress Report, we present an overview of recent developments in the field, with emphasis on magnetoelectric coupling effects in complex oxide multiferroic composite materials.

Magnetoelectric coupling effects in multiferroic complex oxide composite structures.

A systematic study of the magnetic properties of the manganites Ln0.5(A,A′)0.5MnO3 (A,A′=Ba, Ca, Sr) has been carried out. The variations of TC and TN vs two parameters, the average size of the interpolated cations 〈rA〉 and the mismatch effect represented by the variance σ2, have been studied for the series of manganites La0.5(A,A′)0.5MnO3, Pr0.5(A,A′)0.5MnO3, Nd0.5(A,A′)0.5MnO3 and Sm0.5(A,A′)0.5MnO3 (A,A′=Ba, Ca, Sr), involving at most three different cations on the perovskite A site. The results obtained for other (Ln, Ln′)0.5Sr0.5MnO3(Ln, Ln′=Gd, Sm Y, Pr) have been taken into consideration. A σ2−〈rA〉 diagram has been established, which displays the different types of magnetic behaviors that can be exhibited by Ln0.5A0.5MnO3 manganites, i.e., when the temperature is decreasing, either a paramagnetic (PM) to ferromagnetic (FM) transition, or PM to FM followed by FM to antiferromagnetic transitions, or a PM to weak ferromagnetic transition.

Cation disorder and size effects upon magnetic transitions in Ln0.5A0.5MnO3 manganites

Field-induced magnetization jumps with similar characteristics are observed at low temperature for the intermetallic germanide ${\mathrm{Gd}}_{5}{\mathrm{Ge}}_{4}$ and the mixed-valent manganite ${\mathrm{Pr}}_{0.6}{\mathrm{Ca}}_{0.4}{\mathrm{Mn}}_{0.96}{\mathrm{Ga}}_{0.04}{\mathrm{O}}_{3}.$ We report that the field location\char22{}and even the existence\char22{}of these jumps depends critically on the magnetic field sweep rate used to record the data. It is proposed that, for both compounds, the martensitic character of their antiferromagnetic-to-ferromagnetic transitions is at the origin of the magnetization steps.

Field-induced magnetization steps in intermetallic compounds and manganese oxides: The martensitic scenario

We have studied the variation of the paramagnetic to ferromagnetic transition temperature TC and of the ferromagnetic to antiferromagnetic transition temperature TN as a function of the average size of the A site cations (Ln, Sr) in the manganite perovskites Ln0.5Sr0.5MnO3 (Ln=Gd, Sm, Y, Pr, La). For our investigation we used dc resistivity, magnetization, susceptibility and magnetoresistance measurements. Results show that the Curie temperature TC increases from 85 to 310 K when 〈rA〉 varies from 1.221 to 1.263 A, whereas TN remains stable around 150 K. A magnetic phase diagram temperature-〈rA〉 has been established, allowing five regions with different magnetic behaviours to be evidenced: paramagnetic insulating, ferromagnetic metallic, antiferromagnetic insulating, canted antiferromagnetic metallic, and weak ferromagnetic insulating. The extension of these regions is determined by the value of the applied magnetic field.

Cation size-temperature phase diagram of the manganites Ln0.5Sr0.5MnO3

Magnetic phase diagram of Ru-doped Sm 1 − x Ca x MnO 3 manganites: Expansion of ferromagnetism and metallicity

For certain combinations of temperature and magnetic field, the evolution with time of the magnetization of some phase-separated manganese oxides exhibits a unique steplike feature. This jump in the magnetization is proposed to correspond to a burstlike growth of the ferromagnetic fraction at the expense of the antiferromagnetic component, driven by the evolution of the strains at the interfaces between the two kinds of domains. These results bear a striking similarity with the phenomenon of an ``incubation time'' encountered in standard martensitic transformations.

C. Martin

Papers

Cation disorder and size effects upon magnetic transitions in Ln0.5A0.5MnO3 manganites

Field-induced magnetization steps in intermetallic compounds and manganese oxides: The martensitic scenario

Cation size-temperature phase diagram of the manganites Ln0.5Sr0.5MnO3

Magnetic phase diagram of Ru-doped Sm 1 − x Ca x MnO 3 manganites: Expansion of ferromagnetism and metallicity

Observation of spontaneous magnetization jumps in manganites