High magnetic fields for neutron diffraction

A magnetic field often induces an unexpected magnetic structure when various interactions are competing. However only moderate dc magnetic fields are currently available for neutron diffraction, with a current limit of 17 Tesla. Recently, we made a breakthrough by combining a 33 Tesla portable miniature pulsed magnet and the world brightest steady neutron source at the ILL.

The device presented here demonstrates that neutron diffraction in very strong magnetic fields is feasible at the expanse of a relatively cheap and portable miniature pulsed magnet.

Pulsed Magnetic Field up to 33 Tesla

Magnet: group of professor H. Nojiri (IMR, Tohoku University, Sendai, Japan).

Capacitor bank: group of professor P. Frings (LNCMI, CNRS-Toulouse, France).

By using a portable miniature pulsed magnet, we have succeeded in performing neutron diffraction in very strong magnetic fields up to 33 Tesla.

We have applied the technique to determine the magnetic structure of the frustrated antiferromagnet TbB₄, which shows unusual multiple magnetisation plateau behaviour. This investigation demonstrates the great potential of this technique to perform neutron diffraction measurements in pulsed magnetic fields. The magnet itself, together with the compact insert system designed to fit into a standard ILL cryostat (see figure 1) was actually carried in the pocket of a member of the experimental team when they flew from Japan.

: Fig. 1: The mobile miniature pulsed magnet system
The tube on the left is the bottom of the cryostat insert. The magnet is cooled by liquid nitrogen and is separated from the sample space by the insulation vacuum between the two SUS corn pipes. The sample is attached to the sample rod made of sapphire and BN ceramics.
Courtesy Professor H. Nojiri (IMR, Tohoku University, Sendai, Japan).

In a pulsed magnet, a current as large as several kA is supplied by a large capacitor bank, generating a magnetic field of several tens of Tesla for several milliseconds. It is impossible to install a large conventional capacitor bank into the limited space of neutron spectrometer.
However, the compact design of our miniature magnet, coupled with very compact mobile capacitor bank provided through the High Magnetic Field Laboratory in Toulouse enables us to use it for neutron diffraction without any modification of the spectrometer (see figure 2).

: Fig. 2: IN22 equiped with the mobile miniature pulsed magnet system
IN22: 1) monochromator housing; 2) orange cryostat containing the pulsed micro magnet and the sample; 3) analyser shielding
Magnet: 4) capacitor banks (P. Frings, LNCMI, CNRS-Toulouse, France); 5) bank controller; 6) control computer.
Photo: CEA, L.-P. Regnault.