Laboratoire National des Champs Magnétiques Pulsés CNRS – INSA

X-ray absorption spectroscopy (XAS) and magnetic circular dichroism (XMCD) in pulsed magnetic field
By coupling the mobile pulsed magnetic field device developed at the LNCMI-T to the fast acquisition
possibilities of the ESRF energy-dispersive XAS beamline ID24, the feasibility of measuring X-ray magnetic
circular dichroïsm (XMCD) in pulsed magnetic fields up to 30 T using long pulses has been investigated.
Our first high field XAS and XMCD experiments on Gd foil (December 2006) and URhGe single crystal
(February 2007) showed that the feasibility and the quality of the measurement strongly depend on the
mechanical stability of the experimental setup. This high sensitivity to mechanical stability derives from the
specific features of the beamline ID24, more particularly the use of a polychromatic dispersive light that is
focused into a very small spot of 5 microns horizontally. In these geometrical constraints, the acquisition of
absorption spectra under in-situ pulsed magnetic field is challenging because of potential vibrations and motions
induced by the production of the high field pulse, therefore requires excellent sample homogeneity and position
stability within the beam focal spot.
To reduce the observed vibrations effects in the setup, modifications of the cryostat design and mechanical
improvements are under study. The vibrations effects have been already drastically reduced form the initial
configuration (where it was exceeding 1mm) by consolidating cryostat internal tube with the chamber holding
the coil. With this new configuration, XMCD measurements have been successfully performed on a
Gd5Si1.8Ge2.2 powder sample. Data analysis are still under progress.
High magnetic field neutron diffraction
The magnetic properties of geometrically frustrated systems are among the hot topics in condensed matter
physics. In such systems, a variety of unconventional phases appears due to a frustrated spin interaction that
couples with lattice, orbital and charge degrees of freedom. We can tune the balance between them using strong
magnetic field through the Zeeman interaction of spins, and various novel magnetic phases have been found in
magnetic fields exceeding 20 T. Neutron scattering is a powerful and valuable technique to directly determine
the space- and time-correlation of magnetic moments, and to have a deeper insight into the origin of the novel
phases in high fields. So far, however, sufficiently high magnetic fields have not been accessible for neutron
scattering studies. In view of the successful X-ray scattering experiments at the ESRF using a mobile pulsed
field installation, capable of generating fields in excess of 30 T, we have decided to attempt a neutron scattering
experiment using this installation, at the Institut Laue Langevin (ILL, Grenoble) in collaboration with Prof.
Nojiri’s group from Sendai (IMR, Tohoku University, Japan).
Neutron diffraction study of the frustrated antiferromagnet TbB 4[6]
This new technique was used to investigate the magnetic structure of the geometrically frustrated system TbB4,
in magnetic fields up to 30 T. The experiment was carried out on the high-flux, low background triple-axis
spectrometer IN22, with a neutron wavelength of 1.53 Å, using a transportable pulsed magnet system built up by
Prof. Nojiri’s group and the mobile capacitor bank developed at the LNCMI-T
At room temperature, TbB4 crystallizes in the tetragonal structure P4/mbm. The network of magnetic Tb ions
consists of squares and triangles whose configuration within the c-plane is characterized by orthogonal dimers
topologically equivalent to the two dimensional Shastry-Sutherland lattice (SSL) [1]. In zero field, TbB4 exhibits
two successive antiferromagnetic transitions at TN1 = 44 K and TN2 = 24 K [2]. The magnetic structure at B = 0 T
is a XY-type noncollinear structure [3]. At low temperature and high fields (between 16 and 30 T), multi-step
magnetization plateaus appear when the magnetic field is perpendicular to the magnetic easy plane (fig. 4) [4].
This behaviour is unusual since successive metamagnetic transitions in rare earth intermetallics are generally
expected when the field is parallel to the Ising easy axis [5]. Therefore the determination of magnetic structure of
TbB 4 in high magnetic field is essential to understand this phenomenon.
Within the constraints of the pulsed magnet system developed for neutron scattering, we have succeeded in
measuring the field dependences of four Bragg reflections ((100), (200), (110) and (220)) up to 30 T. The
neutron counts and magnetic fields were measured simultaneously using a time analysing system so that the field
dependence of Bragg peaks at fixed diffraction angle can be monitored and recorded as a function of time. The
accumulation of 100-200 magnetic field pulses for each reflection was required to acquire statistically relevant
data. As an example, the field dependence of the intensity of the (100) peak is reported on figure 5. The relative
intensity against the zero field intensity I/I 0 and the relative magnetization M/M S are reported in the figure.
Stepwise changes which can be related to the magnetization process are clearly observed in the intensity of the
magnetic (100) reflection.
From these experimental results, different magnetic models have been calculated. They indicate a much weaker
AF correlation in TbB 4 than the expected one in a conventional antiferromagnet and also suggest the presence of
significant anisotropic interactions. A model introducing a biquadratic interaction term which stabilizes an
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