Dipolar Magnets and Frustration

Dnr:

SNIC 2017/1-304

Type:

SNAC Medium

Principal Investigator:

Patrik Henelius

Affiliation:

Kungliga Tekniska högskolan

Start Date:

2017-07-01

End Date:

2018-07-01

Primary Classification:

10304: Den kondenserade materiens fysik

Webpage:

Allocation

Abstract

This proposal aims to investigate frustrated dipolar magnets. The main goals are to calculate the demagnetizing factors necessary for any quantitative study of magnetic susceptibilities in insulating magnets, to understand the maximum that appears in the magnetic susceptibility for certain frustrated magnets, and to investigate the freezing process in the spin ice family of materials. Our first results, currently submitted and under review, show that the demagnetizing factors that have been tabulated, and much cited, so far are not valid for insulating, lattice-based magnets. Furthermore, we find that the maximum in the spin ice susceptibility is a magnetic analog of the Joule temperature in the theory of classic gases. We are currently preparing a manuscript on this topic for submission. The scientific methods we use in this project are tailored to the specific parts. In order to determine the magnetic Joule and Boyle temperatures we are developing a magnetic analog of the classical theory of thermodynamics, and apply large-scale parallel tempering simulations to verify our theory . Finally we have written a massively parallel Monte Carlo code to study the freezing process in the spin-ices. Our algorithm is capable of calculating the neutron structure factor for systems ten times larger than previously studied, with an equal increase in the spatial resolution. Of paramount importance in all parts of the project is a very close collaboration with our experimental colleagues. In the collaborations we develop state-of-the-art computational tools to meet the many challenges encountered when investigating frustrated magnetic systems. Having completed the development of the program, a medium allocation on Beskow would mean that we can could calculate neutron spectra, demagnetizing factors and critical temperatures and publish the results of our current projects. Last year we we received a large allocation of 300 kcoreh/month. We were not able to use the full allocation since the crystal growers were not able to deliver crystals to our experimental colleagues, who were therefore unable to perfrom the measurements we had planned to compare our calcualations with. Now the crystals have been delivered, measurements are well under way, and we are in great need of a 200 kcoreh/month medium allocation.