OBSERVATION OF ORBITAL MOMENT IN NiO

V. Fernandez¹, C. Vettier¹, F. de Bergevin², C. Giles³, W. Neubeck¹

¹ESRF, European Synchrotron Radiation Facility, BP 220, 38043 Grenoble Cedex , France;
²CNRS, Laboratoire Cristallographie, BP 166 38042 Grenoble Cedex 9, France
³Universidade Estadual de Campinas, C.P. 6165, 13083-970 Campinas, Brazil
e-mail:neubeck@esrf.fr

Keywords: magnetic X-ray Scattering, transition metal oxides (NiO), separation of spin and orbit, magnetisation density

The spin and orbital moment contributions to the total magnetic density have been measured using magnetic X-ray scattering methods [1].

Unlike neutrons which measure only the total magnetisation density, X-rays can separate orbital and spin contributions to the total magnetic moment density. This is done by performing polarisation analysis of the scattered beam. We have used this technique to measure the orbital moment in NiO. NiO is a type II antiferromagnet. The propagation vector corresponds to a doubling of the unit cell in the (111) direction and the spins lie perpendicular in the (11^-2) directions. The Neel temperature is at 523 K, which allows to perform experiments at room temperature. It has been evidenced that 17% of the total magnetic density are coming from the orbital moment. This large contribution implies that the orbital moment should not be considered as quenched, but taken into account in discussion on electronic and magnetic properties.

It has also been shown that the NiO crystal that was investigated was single T-domain in the near surface region, but all the three S domains were populated. By performing Renninger scans their modulation has been observed. The polarisation analysis of these scans evidenced that spin and orbital moment are collinear.

Resonant enhancements in the scattering cross-section occur when the incident photon energy is tuned through an atomic absorption edge. In the case of transition metals the K-edge can be studied. This enhancement has been observed in NiO. Similar experiments are in work on other transition metal oxides (MnO and CoO), where it is also expected to separate L and S.

[1] V. Fernandez et al., Phys. Rev. B. 57 (1998)