Academic Staff - Dr Mark Green - Inorganic

Colossal magnetoresistance (CMR) in transition metal oxides, sulphides and selenides.

Magnetoresistance in materials is of enormous technological importance, as they can be used in a wide range of commercial products such as read heads for hard disks, magnetic storage and sensing devices. The effectiveness of these materials is directly related to the percentage change of the resistance in an applied magnetic field. The current compounds, permalloys, have around 2-3% change in low fields. Magnetoresistance in multilayers such as Co/Cu have improved the effect to up to 40%. More recently, there has been an enormous interest in the magnetoresistance of mixed valence manganates. These perovskite oxides offer nearly 100% effects and therefore vast improvements in the effectiveness of any device. However, these effects are limited to low temperature and/or high magnetic fields, which prevent operating in an industrial environment. Therefore, we propose to study new materials with the goal of optimising the properties for improved performance.
Two distinct mechanisms for colossal magnetoresistance are known to produce colossal magnetoresistance. Firstly, those based on the double exchange process, where mixed valance leads a concomitant ferromagnetic ordering and metal-insulator transition. Secondly, that which is realised in the Mn 4+ pyrochlore, Tl 2 Mn 2 O 7 , which contains distinct metallic and magnetically ordered electrons, and it is the interference between these two sorts of electrons in the form of spin scattering, which leads to a magnetoresistance effect. One of the key areas of study is the detailed structure and physical properties of CuCr 2 X 4 (X=S,Se) and related systems for the following key reasons:

CuCr 2 Se 4 is thought to undergo the double exchange interaction in a fundamentally different way to that of the manganates: Cr ions in these systems have a mixed electronic configuration of d 3 and d 2 . Therefore, exchange occurs between the t 2g electrons, which, despite the 90-degree interaction present in a spinel structure, have sufficient overlap to create conduction. However, it is unclear how this will affect the magnetoresistance properties of the system and will constitute an extremely important comparison with the manganate perovskites and layered perovskite systems.
The ferromagnetic ordering temperature of the CuCr 2 Se 4 is around 460 K, thereby allowing this transition to be tuned to room temperature with chemical substitutions.
CuCr 2 Se 4 crystallises in the spinel structure. The spinel structure is one of the key lattice types in materials chemistry as it shows a wide ranging properties, many of them of great technological importance, from superconductivity (LiTi 2 O 4 ), semiconducting oxides and magnetoresistance, FeCr 2 S 4 .
There are an enormous number of possibilities to subtly tune the interaction to the desired electronic characteristics. Chemical substitutions can be made to the both the A (tetrahedral) and B (octahedral) sites. e.g. (a) The A site can be modified with a higher valent ion to reduce the Cr oxidation state. (b) The A site can be replaced with a magnetic ion to create ferrimagnetic A-B exchange. (c) The B site can be substituted to directly manipulate the Cr-Cr magnetic exchange and orbital overlap, which will affect the electron conduction pathways.

Structure of the spinel lattice

This page last modified 9 August, 2010