Correlation effects in the ground state of Ni-(Co)-Mn-Sn Heusler compounds
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MRS Advances © 2019 Materials Research Society DOI: 10.1557/adv.2019.134
Correlation effects in the ground state of Ni-(Co)Mn-Sn Heusler compounds Bernardo Barbiellini1,2, Aki Pulkkinen1 , Johannes Nokelainen1 , Vasiliy Buchelnikov3,4, Vladimir Sokolovskiy3,4, Olga N. Miroshkina3, Mikhail Zagrebin3,4,5, Katariina Pussi1, Erkki Lähderanta1 and Alexander Granovsky6 1
Department of Physics, LUT University, FI-53851 Lappeenranta, Finland
2
Department of Physics, Northeastern University, Boston, MA 02445, U.S.A.
3
Faculty of Physics, Chelyabinsk State University, 454001 Chelyabinsk, Russia
4
National University of Science and Technology "MISiS", 119049 Moscow, Russia
5
National Research South Ural State University, 454080 Chelyabinsk, Russia
6
Department of Physics, Lomonosov Moscow State University, 119991 Moscow, Russia
Abstract
We present density functional theory calculations to study the interplay between magnetic and structural properties in Ni-Co-Mn-Sn. The relative stability of austenite (cubic) and martensite (tetragonal) phases depends critically on the magnetic interactions between Mn atoms. While the standard generalized gradient approximation (GGA) stabilizes the latter phase, correlation effects beyond GGA tend to suppress this effect. Mn atoms treated as magnetic impurities can explain our results, where a delicate balance between magnetic interactions mediated by Ni d and Sn p orbitals determines the equilibrium structure of the crystal. Finally, we discuss the role of Co doping in stabilizing ferromagnetic austenite phases.
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INTRODUCTION Ni-Co-Mn-Sn based Heusler alloys display remarkable magneto-caloric effects [1-3], which are consequence of the strong coupling between magnetism and structure arising from the martensitic transition as the sample is cooled. Stoichiometric Heusler alloys have a generic formula X2YZ and display cubic structure at high temperature. The magnetism is due to 3d electrons from X and Y transition metals. In Ni-Co-Mn-Sn alloys, the magnetic moments are mostly localized at Mn atoms that are magnetically coupled through an oscillatory interaction mediated by the conduction electrons. Consequently, the magnetic behavior of these alloys is extremely sensitive to the distance between Mn atoms. In this work, we study structural, magnetic and electronic properties of Ni4Mn3Sn and Ni3CoMn3Sn in eight atom simulation cells by using the density functional theory (DFT) with different approximations for describing correlation effects beyond the local density approximation. Standard DFT simulations use the generalized gradient approximation (GGA) while the strongly constrained and appropriately normed (SCAN) meta-GGA contains correlation corrections beyond the GGA [4]. In practice, SCAN behaves like a DFT+U approach but it av
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