Combining insights from neutrons and muons to explain magnetic transitions

​​EMU instrument at ISIS​

​​EMU instrument at ISIS​

In the panel, magnetization is plotted as a function of temperature in zero-field-cooled heating (ZFCH), field cooling (FC), and field-cooled heating (FCH) conditions in the presence of 50 Oe of external magnetic field. The martensitic transformation TMS is clearly visible around 300 K, where large thermal hysteresis is present. TCA, TCM, and TB indicate the austenite Curie point, martensite Curie point, and exchange bias blocking temperature, respectively.

(Magnetic states of Ni-Mn-Sn based shape memory alloy: A combined muon spin relaxation and neutron diffraction study
J. Sannigrahi, S. Pramanick, S. Chatterjee, J. S. Lord, D. Khalyavin, A. D. Hillier, D. T. Adroja, and S. Majumdar
Phys. Rev. B 99, 224401, 3 June 2019)

Using muon spin relaxation experiments on EMU and HiFi alongside neutron powder diffraction on WISH has enabled the characterisation of the complex phases of a metamagnetic shape memory alloy: a material that can produce force under a magnetic field.

Thanks to support from the India-RAL collaboration, a group of scientists have used multiple beamlines to study the alloy Ni2.04Mn1.4Sn0.56. This compound is a metamagnetic shape memory alloy (MSMA), which means that it can produce motion or force under a moderate magnetic field. This kind of material has applications in areas including energy harvesting, computation and communication, as well as combating environmental pollution.

To understand their functionality and potential applications, it is important to investigate their physical properties. This material takes a structure at higher temperatures called austenite, which has cubic lattice symmetry, and forms a low-temperature phase called martensite, which has a tetragonal/orthorhombic/modulated structure. The temperature at which this change occurs is called TMS.

In each of these structural phases, a magnetic transformation takes place at a certain temperature: TCS and TCM. Another important temperature for this material is TB, the temperature below which exchange bias is observed.

Previous investigations have studied the structure and the magnetism of the material, but the nature of the magnetic state just below TMS remains uncertain, and solving this ambiguity was the focus of part of this project. The other focus of the experiment is into the magnetic ground state of the alloy, which previous results have suggested is a glassy magnetic state.

The muon spin relaxation measurements on EMU and HiFi confirmed the complexity of the magnetic phases present in the material but, by combining their results with the neutron powder diffraction data from WISH, the scientists were able confirm that the alloy forms a paramagnetic state just below TMS.

The muon spin relaxation measurements also enabled the group to assign the temperature TB as the spin-freezing temperature. This is caused by both antiferromagnetic, and ferromagnetic correlations combined with the presence of disorder in the system. By studying the neutron data alongside these results, they were able to show that the antiferromagnetic correlation is short-range, whereas there is evidence of long-range ferromagnetic order in the ground state. The remarkable phenomenon of exchange bias observed in Ni-Mn-Z alloys, such as the one studied here, is due to the coupling between the interfacial spins of the spin-glass and ferromagnetic phases.

Original publication

Combining insights from neutrons and muons to explain magnetic transitions. 07 Oct 2019 – Rosalind Davies

The full paper can be viewed at” DOI: 10.1103/PhysRevB.99.224401