Magnetovolumic effects are essential in many applications of bulk magnetic materials produced in high quantities and with widespread use. Magnetostrictive properties yield actuators which can be micrometric in size, as in medical applications: valves or motors can be produced with the best force/weight ratio and the unique advantage of use without connecting leads. More recently, the study of element-selective magnetovolumic properties has been triggered from the worldwide interest in nanoparticles grown by epitaxy on surfaces. Due to the epitaxial constraint, a significant reduction in atomic volume occurs when compared with the bulk material.

The volume occupied by a magnetic atom drives its microscopic properties, magnetisation and anisotropy, through its hybridisation with its neighbours. Element selective X-ray Magnetic Circular Dichroism carried out under pressure is a direct way to evaluate the magnetism induced on a specific non-magnetic element (5d) due to the hybridisation with a magnetic (3d) element in an alloy. XMCD is an orbital moment-sensitive probe, and due to the spin-orbit interaction it is also sensitive to spin. Therefore it is able to probe the strength of the spin hybridisation as it varies with pressure, i.e. with interatomic distance.

Thanks to the excellent focusing properties of its energy dispersive spectrometer, and to the availability of a Quarter Wave Plate to tune the helicity of the incoming photons, ID24 can undertake such lines of research. However, such experiments impose severe constraints on beam stability, since a very small signal (the XMCD signal in the hard X-ray range is of the order of 10­3) has to be measured on samples in Diamond Anvil Cells. The increased stability of the beam resulting from the recent installation of the local feedback has finally made possible such experiments on ID24.

The Pt3Cr alloy is a good candidate for the investigation of volume-dependent magnetism. Recent works [1,2] report on the very unusual magnetism of Pt 5d bands in this alloy: the L3 and L2 XMCD signals have the same sign, meaning that the dominant contribution to the magnetic signal is due to the orbital moment.

Figure 73 illustrates the variation with pressure of the Pt L2 and L3 edge XMCD signal in a Pt3Cr alloy. The initial decrease in the signal reflects the attenuation of the Pt 5d moment, due either to the decrease in the Cr 3d moment or to a change of the magnetic order.

The rapid enhancement of both L2 and L3 signals at around 35 kb reflects the increase of Pt 5d band magnetic moment. This is induced by the increasing hybridisation between the 3d Cr band and the 5d Pt band with increasing pressure.

The remarkable result of the present study is the constant branching ratio (L3/L2) along the pressure domain. This means that there is no variation of the orbital/spin magnetic moment ratio of Pt between 0 and 50 kb which is rather surprising for two reasons. First, because the magnetic moment (here largely given by the orbital moment) is known to be strongly influenced by crystallographic order. Second, because this ratio remains surprisingly insensitive to the reduction of the interatomic distance, which is about 0.7% in this pressure range.

[1] W. Grange et al., J. Synchr. Rad., 6, 679-681 (1999).
[2] H. Maruyama et al., J. Magnetism and Magnetic Materials, 43, 140-144 (1995).

F. Baudelet (a), J.P. Itié (a), A. Polian (a), J.P. Kappler (b), A. Fontaine (c), S. Pizzini (c), S. Pascarelli (d), T. Neisius (d), F. Natali (d), S. Diaz Moreno (d).

(a) PMC Jussieu, Paris (France)
(b) IPCMS, Strasbourg (France)
(c) Lab. Louis Néel, Grenoble (France)
(d) ESRF