EXPERIMENTAL ATTEMPTS TO MEASURE NONCOLLINEAR LOCAL MAGNETISATION

P.J. Browna

aInstitut Laue Langevin, BP 156, 38042 Grenoble, France (brown@ill.fr)

The representation of atomic magnetic moments as simple vectors, shown as arrows in the pictorial representation of a magnetic structure, has proved a very useful one; but it should not be pushed too far. In a solid, the magnetisation is a continuous function, a property of the electron wave-functions, and there is no requirement that it should be collinear. Nevertheless the assumption of a collinear magnetisation distribution is widely made and one should therefore give some thought to the circumstances in which it might not be valid. The first and most obvious is that of a non-collinear magnetic structure. When the atomic moments themselves are not parallel it would be naive to suppose that there is a well defined boundary between atoms at which the magnetisation changes abruptly from one general direction to another. A second example occurs in anisotropic systems; if the local easy axes of magnetisation of atoms are not parallel to one another then the magnetisation distribution induced by an applied field will be non-collinear. Furthermore even in very simple systems in which the orbital moment is not fully quenched there may be non-collinearity due to spin-orbit interaction. In fact the spin-orbit interaction is by far the most important cause of non-colinearity in magnetic systems, being the principal means by which an atom's spin direction is coupled to the orientation of its ligand environment. In all the examples which will be given, in which a non-collinear magnetisation distribution has been sought or found, this non-collinearity is primarily due to the presence of orbital magnetisation.

Several experimental techniques are available to probe non-collinearity of the magnetisation distribution. The most direct is to measure both the magnitude and direction of the magnetic interation vectors which give its fourier components. Such measurements are now becoming possible for antiferromagnets with the advent of a new generation of neutron polarimeters which will allow both greater geometric flexibility and higher precision. However up to now non-collinear magnetisation distributions have been revealed by more indirect means. Polarised neutron flipping ratio measurements can give only a single component of the magnetic interaction vector directly. However the special geometric properties of the interaction vector and the symmetry breaking properties of an applied field can be exploited to obtain evidence of non-collinearity in the magnetisation distribution even from such limited data.