Exploring magnetism within extreme magnetic fields

04-01-2008

The feasibility of measuring X-ray magnetic circular dichroism (XMCD) within very high magnetic fields has been investigated using an energy-dispersive X-ray absorption spectrometer at the ESRF's energy-dispersive XAS beamline ID24.

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By coupling a pulsed magnetic field device developed at the ESRF (Figure 1) [1] to the fast acquisition capabilities of ID24, the Re L2 and L3 XMCD signals were measured in a Ca2FeReO6 perovskite (Figure 2) at up to 30 T and in the temperature range 10-250 K [2]. Knowledge of the field and temperature dependence of both spin and orbital magnetic moments of Re under extreme magnetic fields could help answer enigmatic questions on the magnetism of these distorted double perovskites, such as why they don’t reveal saturation, whether this phenomenon is only due to anisotropy in the grain boundaries or whether it originates from the bulk.

pulsed magnetic field device at beamline ID24  

Figure 1: Peter van der Linden setting up the pulsed magnetic field device at beamline ID24.

Considerable research effort is being made to understand properties of matter under extreme conditions. Static high pressures up to the multimegabar regime can now be reached with diamond-anvil cells, as well as temperatures from the milli-Kelvin to thousands of Kelvin using dilution refrigerators and laser heating. The exploration of ever widening P–T diagrams has lead to the discovery of a multitude of new chemical and physical phenomena - such as the discovery of the (Mg, Fe) SiO3 postperovskite with significant geophysical implications for the earth mantle’s nature and dynamics - leading to a fundamental understanding as well as technological applications.

The distorted double perovskite Ca2FeReO6.

Figure 2: The distorted double perovskite Ca2FeReO6.

Studies of materials under extreme magnetic fields have the same potential impact. Applying high magnetic fields leads to a variety of phenomena like structural or magnetic phase transitions and the discovery of previously unexplored quantum critical points. To date, the combination of synchrotron experiments with pulsed high magnetic fields has led to interesting achievements mainly using X-ray diffraction [3].

The ID24 experiments represent the first attempt at measuring XAS and XMCD spectra for materials under extreme magnetic fields. These methods give highly complementary information to diffraction methods, namely yielding information on the local and electronic structure and on the ordered magnetic moment on the absorber atom. The energy dispersive geometry allows the whole energy range of the XMCD/XAS spectrum to be recorded in parallel using a position sensitive detector. This is crucial for measurements under high magnetic fields in pulsed mode as the lifetime of the coil limits drastically the number of possible measurement cycles (as a rule of thumb, the lifetime is reduced by a factor of 10 for a 10% increase in maximum field).  The acquisition time per spectrum can be easily tailored to the magnetic pulse length, and is dependent on the ability to detect the spatial distribution of the intensity of the beam transmitted by the sample with sufficient speed. Fast position sensitive detectors featuring readout times in the order of the microsecond, such as the XH [4] used for these measurements, represent a real advantage for such experiments.  

In Figure 3 we show examples of XAS (top panel) and XMCD signals (bottom panel) at the L2 and L3 edges of Re at 30 T and 10 K. The magnetic pulse FWHM being of the order of 350 microseconds, the XMCD data was acquired during the central 75 microseconds and averaged over a number N of magnetic pulse pairs (N=10 for the L2 and N=50 for the L3) to improve signal to noise, each pair consisting of two measurements using opposite helicity of the incoming X-rays.

 

XAS and XMCD Re L2 and L3 edges at 30 T and 10 K on Ca2FeReO6.

Figure 3: XAS (top) and XMCD (bottom) Re L2 and L3 edges at 30 T and 10 K on Ca2FeReO6.

 

References
[1] [1] P. van der Linden, in preparation.
[2] M. Sikora, O. Mathon, P. van der Linden, T. Neisius, J.M. Michalik, Cz. Kapusta, J. Headspith, J.M. De Teresa and S. Pascarelli,  in preparation.         
[3] P. Frings, J. Vanacken, C. Detlefs, F. Duc, J.E. Lorenzo, M. Nardone, J. Billette, A. Zitouni, W. Bras and G.L.J.A. Rikken, Rev. Sci. Instrum. 77, 063903 (2006).
[4] XH is a a hard X-ray compatible, fast, linear detector, developed at the STFC Daresbury and Rutherford Laboratories and based on a germanium based microstrip detector head which was built at the Lawrence Berkeley National Laboratory by Dr Paul Luke (J. Headspith, J. Groves, P.N. Luke, M. Kogimtzis, G. Salvini, S.L. Thomas, R.C. Farrow, J. Evans, T. Rayment, J.S. Lee, W.D. Goward, M. Amman, O. Mathon, S. Diaz-Moreno, proceedings of NSS-MIC2007).

 

Principal publications and authors
XAS and XMCD under high magnetic field and low temperature on the energy-dispersive beamline of the ESRF, O. Mathon (a), P. van der Linden (a), T. Neisius (b), M. Sikora (a), J.M. Michalik (c), C. Ponchut (a), J.M. De Teresa (d) and S. Pascarelli (a), J. Synchrotron Rad. 14, 409 (2007).
(a) ESRF
(b) CP2M, Université Paul Cezanne, Marseille (France)
(c) Department of Solid State Physics, AGH University of Science and Technology, Krakow (Poland)
(d) Instituto de Ciencia de Materiales de Aragon, Universidad de Zaragoza-CSIC, Zaragoza (Spain)