Temperature dependence of Mn-Mn nearest-neighbor distances in (a) CoMnSi and (b) Co0.95Ni0.05MnSi. Crystallographic positions are shown in (c), together with the closest commensurate version of the helical spin Co and Mn sublattices
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‘Smart’ materials display a plethora of potentially useful properties, and these can interact with each other giving rise to new phenomena. In magnetic systems such as ferrromagnetic shape memory materials and magnetic refrigerants, magnetism and crystal structure are intimately linked with each other; that is, a change in one can alter the other.
This feedback mechanism – known as magneto-elastic coupling – is usually quite weak, with atomic separations changing by fractions of a percent as the temperature of the material changes. However, using the high resolution powder diffractometer (HRPD) at ISIS, we see that the separation of magnetic centres in CoMnSi can change by up to 2% as it is heated.
Such large changes in crystal structure have urged us to postulate a novel mechanism for large magneto-elastic coupling. We conclude that the manganese atoms experience a competition between energetically similar magnetic states resulting in quite different atomic separations. Thus, temperature (or magnetic field) can ‘tip the balance’ in these novel materials.
A Barcza (Cambridge University), Z Gercsi (Imperial College London), KS Knight (ISIS), KG Sandeman (Imperial College London)
Research date: January 2010
Contact: Dr K Sandeman, email@example.com
Further information: A Barcza et al., Phys Rev Lett 104 (2010) 247202
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