Helmholtz Zentrum Berlin


hzbHFM • Building the Strongest Magnet­ in the World of Neutron Scattering

Inside matter(s)

authored 3 years ago:

The Test-Experiment is finished – but for Karel physics starts now

The first “friendly user” Karel Prokes has finished his Test-experiment. These experiences have contributed much to improve the complex systems for future users: “Now the connection between software and hardware is much better and also a lot of other issues could be addressed by my colleagues from the HFM Team and the EXED,” he says. For instance, users can now control safely the field with software run directly at the EXED-Neutron instrument. This is much more comfortable.

Karel has got nice results on his sample, which is a crystal based on “5f-electron atoms”. These systems display some of the most unusual behaviors that include superconductivity and heavy-fermion behaviors. Electrons in their 5f-shells are moving extremely fast around nuclei and relativistic effects play an important role in their magnetic properties. "Their study can give us a deeper understanding of the role of relativity throughout the periodic table", he explains. Also, due to their “heaviness” and significant spin-orbit coupling, (spin and orbital parts are the two sources of magnetism in all materials) these electrons are behaving strangely – they cause an extremely large magnetic anisotropy. Without an external magnetic field, their total magnetic moments form in this material a planar pattern in just two dimensions, called non-collinear, as shown in sketch 1:

Bild1 Sketch1: The spins show in opposite directions, so the net magnetic Moment M is zero.

However, there is a clear prediction how this should change under a strong magnetic field B, as shown in the next sketch 2:

Bild2 Sketch2: The external field B is symbolized by a blue arrow. The physicists expect that in this case, one magnetic moment should change its direction and align parallel to the B-field, giving rise of a net magnetic moment M. This should influence the neutron diffraction reflexes in a specific way.

However, the experimental findings are not that clear: “The observed number of reflections is due to restricted geometry not high enough to check the prediction unambiguously. One has to use models to find out which of the moment configurations is the correct one. At this point I can only say that at least one of the moments changes its direction and points towards the field as shown on the second figure”, Karel says.

There is - as expected - a net magnetic moment when the external field goes up. However, what the other two moments under applied field are doing is less clear – Is it possible that they come out of the 2D-Pattern, pointing perpendicular to the plane? ?


“I do not know yet”; Karel says. He is doing combinatorics right now, turning mentally the spins of every atom in his crystal in every possible spatial direction. Isn’t it a bit like playing with Rubik’s cube?

Physicists wanting to know a bit more about what Karel is doing, may want to read the basic papers about the problem in A. Purwanto et al., Phys. Rev. B, Vol. 50, 10, September 1994 and Phys. Rev Letters in 1995