Magnetism under extreme condition using the ODE beamline at SOLEIL

F.Baudelet¹, A. Congeduti¹, Q. Kong¹, A. Monza¹, J.P.Itié¹, S. Chagnot¹, A. Polian², M. D’Astuto², B. Couzinet², J.C. Chervin²

¹ Synchrotron SOLEIL - L'Orme des Merisiers Saint-Aubin - BP 48 91192 GIF-sur-YVETTE
² IMPMC 140 rue de Lourmel 75015 Paris 

 
The X-ray Magnetic Circular Dichroism (XMCD) under pressure technique is a probe of microscopic magnetic properties. XMCD is a selective probe, which can access to a large variety of elements. The dispersive EXAFS station at SOLEIL (ODE-beamline) give the possibility to perform numerous pressure XMCD experiments with an excellent statistic.
The classical method of recording absorption spectra is the step by step measurement of the absorption coefficient for each energy point. An other way is the use of a bent monochromator to eliminate the stepwise scanning of the X-ray energy. This is the so-called “dispersive mode”^1,2, Figure1.
The main advantages of Dispersive XAFS are the focusing optics, the short acquisition time (few ms) and the great stability during the measurements due to the absence of any mechanical movement.

These advantages allow the study of small samples, 100μm at SOLEIL, which is mandatory in the case of high pressure studies. X-ray Magnetic Circular Dichroism is a technique which uses the polarization properties of X-rays to probe the microscopic magnetism. Intense linearly or circularly polarized X rays produced in synchrotron sources have allowed to develop new probes of the magnetic structure of materials. Magnetic resonant scattering and magnetic X ray dichroism use linearly or circularly polarized photons with energy near an absorption edge, and then give access selectively to local magnetic moments.3 The first expected pressure effect is of course a reduction of inter-atomic distances. The relation between inter-atomic distances and magnetic properties is illustrated by the Slater-Néel curve4 and the RKKY function for the transition metals and the rare-earths respectively. An other important effect is to change the energy position and energy width of the valence or conduction electron in compounds: For magnetic 3d insulators, these variations may induce insulator-metal transition accompanied by a collapse of the magnetic moment.

Figure1: Dispersive XAFS set-up installed at SOLEIL

In the special case of rare-earths compounds, the effect of pressure is to modify the 4f density of state at the Fermi level, which can lead to heavy fermion feature, mixed-valence compounds or Kondo systems.

The modification of the inter-atomic distances may lead to a large variety of magnetic transitions. Nevertheless, pressure can also induce more subtle changes in crystallographic structures like bond angle variations or in electronic structures like in the electronic density at the Fermi level, leading to dramatic changes in magnetic properties.

Figure 2: First XMCD measurement at the iron K-edge on the ODE beamline Figure 3: High pressure, Low temperature and high field XMCD set-up

We measured XMCD signals under hydrostatic high pressure, with a non-magnetic high-pressure anvil-cell small enough to be inserted between the polar pieces of a magnetic dipole. This allows studying the fundamental aspect of magnetism of conduction bands, but also opens the possibility of studying directly the magnetic properties of geological materials. A first study has been made on the invar problem5, at the previous LURE synchrotron. More recently we succeed in measuring very small XMCD signal under pressure in the pure iron system6 at ESRF. We present here the new ODE beamline at SOLEIL and the sample environments combination: high magnetic fields, high pressure and low temperature. The ODE beamline is almost complete and started to welcome users during 2008 for classical XAS applications. The commissioning of the optics is almost finished and the first XMCD spectra at the iron K-edge of the iron foil has been achieved, figure2. The future combination of external parameters includes a 7T magnetic field, a 2K cryostat and a membrane high pressure cell. Figure 3. These experimental conditions will be available on the beam line during the first semester of 2009. Each component is actually tested separately and will be bound together. The first applications will be done on quantum critical point systems. Quantum criticality occurs when system are close to a phase transition and the transition temperature falls near zero, so that quantum fluctuation replace classical thermal fluctuation. In these condition, new exotic state of the matter arise, driven by this strongly fluctuating state as for heavy fermion, and new coherent phases may arise as superconductivity. This picture, formulated about 30 years ago, has been recently reviewed thanks to new discovery. In particular, the original picture applied essentially for antiferromagnetic states, while recently ferromagnetic system has been found showing Quantum criticality.