We review recent experimental work falling under the broad classification of colossal magnetoresistance (CMR), which is magnetoresistance associated with a ferromagnetic-toparamagnetic phase transition. The prototypical CMR compound is derived from the parent compound, perovskite LaMnO 3. When hole doped at a concentration of 20–40% holes/Mn ion, for instance by Ca or Sr substitution for La, the material displays a transition from a high-temperature paramagnetic insulator to a low-temperature ferromagnetic metal. Near the phase transition temperature, which can exceed room temperature in some compositions, large magnetoresistance is observed and its possible application in magnetic recording has revived interest in these materials. In addition, unusual magneto-elastic effects and charge ordering have focused attention on strong electron–phonon coupling. This coupling, which is a type of dynamic extended-system version of the Jahn–Teller effect, in conjunction with the double-exchange interaction, is also viewed as essential for a microscopic description of CMR in the manganite perovskites. Large magnetoresistance is also seen in other systems, namely Tl 2Mn2O7 and some Cr chalcogenide spinels, compounds which differ greatly from the manganite perovskites. We describe the relevant points of contrast between the various CMR materials.

REVIEW ARTICLE: Colossal magnetoresistance

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.

Composite Titanium Dioxide Nanomaterials

Properties of materials determined by their size are indeed fascinating and form the basis of the emerging area of nanoscience. In this article, we examine the size dependent electronic structure and properties of nanocrystals of semiconductors and metals to illustrate this aspect. We then discuss the chemical reactivity of metal nanocrystals which is strongly dependent on the size not only because of the large surface area but also a result of the significantly different electronic structure of the small nanocrystals. Nanoscale catalysis of gold exemplifies this feature. Size also plays a role in the assembly of nanocrystals into crystalline arrays. While we owe the beginnings of size-dependent chemistry to the early studies of colloids, recent findings have added a new dimension to the subject.

Size‐Dependent Chemistry: Properties of Nanocrystals

Exploitation of the hydrogen bond: recent developments in the context of crystal engineering

We have examined the magnetotransport properties and the structure, by Rietveld refinement of powder X-ray data, of the phases RE1.2Sr1.8Mn2O7(RE = La, Pr, Nd). We find that on cooling, La2Sr1.8Mn2O7 undergoes a transition to a nearly perfect ferromagnet with 90% magnetization at 1.45 T, as reported by earlier workers, but the Pr and Nd phases show only a small magnetization that grows gradually as the temperature is decreased. There seems to be significant correlation between electrical transport and the Jahn-Teller elongation of the apical Mn–O bonds in these systems. The elongation of the apical Mn-O bonds forces the nine-coordinate rock-salt site to be occupied preferentially by the smaller rare-earth-metal cations. This preferential occupation is reliably obtained from the X-ray refinement. All three title phases show a magnetoresistance ratio of about 4 (corresponding to a magnetoresistance, [R(0)–R(H)]/R(0), of about 75%) at a field of 7 T and temperatures around 100 K.

Structure and magnetotransport properties of the layered manganites Re1.2Sr1.8Mn2O7(RE = La, Pr, Nd)

Based on in situ X-ray diffraction and EXAFS studies, it is shown that NiTiO3 formed by the reaction of TiO2(anatase) with Ni2+ during the calcination of Ni/TiO2, gives rise to Ni metal and TiO2(rutile) on reduction, a process that may be responsible for the strong interaction between the metal and the oxide support.

Promotion of the metal–oxide support interaction in the Ni/TiO2 catalyst. Crucial role of the method of preparation, the structure of TiO2 and the NiTiO3 intermediate

The structure and properties of metal clusters are currently subjects of intense research. Many fundamental aspects of metal clusters need to be understood, such as the dependence of the metallicity of the clusters on the size.

Dependence of the Reactivity of Ag and Ni Clusters Deposited on Solid Substrates on the Cluster Size

Evidence is presented for the strong interaction of nitrogen and oxygen with buckminsterfullerene.

C. N. Ramachandra Rao

Papers

Structure and magnetotransport properties of the layered manganites Re1.2Sr1.8Mn2O7(RE = La, Pr, Nd)

Promotion of the metal–oxide support interaction in the Ni/TiO2 catalyst. Crucial role of the method of preparation, the structure of TiO2 and the NiTiO3 intermediate

Dependence of the Reactivity of Ag and Ni Clusters Deposited on Solid Substrates on the Cluster Size

Interaction of nitrogen and oxygen with C60

Abhängigkeit der Reaktivität von Ag- und Ni-Clustern auf festen Trägern von der Clustergröße