Bristol and Munich physicists use spin-polarised positron annihilation to probe a Heusler alloy, in an echo from Professor H H Potter 80 years ago

Fermi surface of Cu2MnAl J A Weber (Technical University of Munich)

12 November 2015

In a neat connection with pioneering work in Bristol over 80 years ago, a team of physicists from Bristol and Munich have used spin-polarised positron annihilation to probe the electronic structure of a Heusler alloy, Cu2MnAl. [See note 1 below]

First discovered more than 100 years ago, these Heusler alloys were initially of interest because they exhibited strong ferromagnetism even though the constituent elements were themselves not ferromagnetic [2].

Cu2MnAl was the first Heusler alloy, but today there are a wide range of these alloys which exhibit a broad range of useful physical properties ranging from ferromagnetic shape-memory behaviour to half-metallicity (a metal which only has a Fermi surface in one spin channel). While Cu2MnAl is commonly used to polarise beams of neutrons, Heuslers such as Ni2MnGa can be used as a magnetic actuator, and Co2FeSi is finding spintronic applications as a source of highly spin-polarised electrons.

Physicists from the H H Wills Physics Laboratory in Bristol and the Technical University of Munich worked together on a paper, Spin-resolved Fermi surface of the localised ferromagnetic Heusler compounds Cu2MnA measured with spin-polarised positron annihilation published in Physical Review Letters and selected as an Editors' Suggestion.

Positrons which are emitted from a radioisotope source have a net spin polarisation due to the parity violation in the beta decay mediated by the weak interaction. This polarisation can be used to preferentially annihilate majority and minority spin electrons and thus measure the Fermi surfaces of the two spins separately.

The theoretical calculations which support the Munich experiments were performed by Bristol Ph.D. student David Ernsting, together with his supervisor Professor Stephen Dugdale.

Professor Dugdale said: 'There is a rather nice connection with some pioneering work performed in the laboratory more than 80 years ago. The starting point for our electronic structure calculations was the crystal structure of these alloys. That the atoms in the Heusler structure occupy four interpenetrating face-centred cubic lattices was suggested by Dr Harold Herbert Potter in 1928, on the basis of careful X-ray measurements made in the H H Wills Physics Laboratory.' [3]

[1] J A Weber et al., Phys. Rev. Lett. 115 206404 (2015)

[2] F Heusler, W Starck, and E F A Haupt, Ver. Deut. Phys. Ges. 5, 219 (1903)

[3] H H Potter Proc. Phys. Soc. 41 135 (1928)