Giordano Benacchio1 Ivan Titov1 Denis Mettus1 Inma Peral1 2 Dirk Honecker3 Elliot Gilbert4 Mauro Coduri5 Mehmet Acet6 Andreas Michels1

1, Université du Luxembourg, Luxembourg, , Luxembourg
2, Luxembourg Institute of Science and Technology, Belvaux, , Luxembourg
3, Institut Laue-Langevin, Grenoble, , France
4, Bragg Institute, ANSTO, Kirrawee, New South Wales, Australia
5, ESRF, Grenoble, , France
6, Faculty of Physics, Duisburg, , Germany

Heusler alloys are known for their functionalities related to the magnetic shape memory, giant magnetoresistance, barocaloric, magnetocaloric, and exchange bias effects [1]. The stoichiometry of the materials showing these effects lies in a narrow compositional range where a martensitic transition takes place from a high-temperature cubic austenite state, with long-range ferromagnetic (FM) ordering, to a low temperature modulated tetragonal martensite state, where the ordering is usually short-range frustrated antiferromagnetic (AF).
Recently, a new functional property has been observed in an off-stoichiometric Heusler-based alloy [2]. An AF martensitic Heusler Ni50Mn45In5, when annealed at high temperatures under a magnetic field of about 1 T, segregates and forms Ni50Mn25In25 precipitates embedded in a NiMn matrix. The precipitates are paramagnetic (PM) at the annealing temperature, whereas the matrix is AF. The spins at the interface with the NiMn matrix align with the field during their growth and become strongly pinned in the direction of the applied field during annealing, whereas the core spins become PM. This gives rise to a PM precipitate with a FM shell. The remanent pinning exists below 600 K and an estimated field of 20 T is required to reverse the magnetization of the shell. The occurrence of such permanent memory, despite the PM core, provides possibilities for new applications related both to non-volatile memory withstanding high temperatures at reduced dimensions and to permanent magnets.
Small-angle neutron scattering (SANS) is one of the most important methods for microstructure investigation, which is utilized in a wide range of scientific disciplines, and particularly in magnetism. SANS allows characterizing structures or objects on the nanometer scale, typically between a few and a few hundred of nanometers. By means of magnetic SANS, we have investigated the presumed shell-ferromagnetism in Ni50Mn45In5 samples. By combining unpolarized and polarized SANS (POLARIS), we demonstrate that a number of important conclusions regarding the microscopic spin structure can be made. In particular, the analysis of the magnetic neutron data suggests that precipitates with an effective ferromagnetic component form on annealing. We also provide a real-space analysis of the data by computing the correlation function of the total cross section, giving information on the characteristic length scales (both nuclear and magnetic). We show that the annealing treatment of the samples results in a decrease of the correlation length, the latter is related to the change of microstructural properties of these samples.
[1] A. Planes, L. Mañosa, and M. Acet, J. Phys. Condens. Matter 21, 233201 (2009).
[2] A. Çakir, M. Acet, and M. Farle, Sci. Rep. 6, 28931 (2016)