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Complex magnetic interactions in off-stoichiometric NiMnGa alloys

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2010 J. Phys.: Condens. Matter 22 346001

(http://iopscience.iop.org/0953-8984/22/34/346001)

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IOP PUBLISHING	JOURNAL OF PHYSICS: CONDENSED MATTER
J. Phys.: Condens. Matter 22 (2010) 346001 (6pp)	doi:10.1088/0953-8984/22/34/346001

Complex magnetic interactions in off-stoichiometric NiMnGa alloys

Subhradip Ghosh1 and Biplab Sanyal2,3

1Department of Physics, Indian Institute of Technology Guwahati, Guwahati-781039, India

2Division of Materials Theory, Department of Physics and Astronomy, Uppsala University, Box-516, 75120 Uppsala, Sweden

E-mail: [e-mail]

Received 25 May 2010, in final form 3 July 2010

Published 2 August 2010

Online at stacks.iop.org/JPhysCM/22/346001

Abstract

Using first-principles density functional theory, the magnetic pair interactions between various pairs of chemical specie have been calculated and the trends in magnetism with varying compositions and chemical ordering are analyzed for three off-stoichiometric NiMnGa alloys in their austenite phases. The experimentally observed trend of decreasing magnetization with increasing Mn concentration is attributed to the antiferromagnetic interactions among Mn atoms occupying sublattices other than the original Mn one. The role of chemical ordering on magnetization is also analyzed by total energy results and exchange interactions. We are able to explain the recently published neutron scattering experiments with our theoretical analyses.

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1. Introduction

Ferromagnetic Heusler alloy Ni2MnGa exhibits the shape memory effect and recoverable magnetic field induced strain. These materials are quite promising as high sensitivity magnetic sensors and actuators. The structural transformations, magnetic properties and the interplay of these two effects have been widely investigated for the Ni2MnGa system at stoichiometric composition [1–4]. However, from the point of view of practical applications the alloy with stoichiometric composition of 50:25:25 has severe limitations. The thermoelastic martensitic transformation temperature of the stoichiometric alloy being 200 K makes it unfavorable for application at room temperature. A high saturation magnetization is also desirable for the alloy to establish a large magnetic field induced strain. In order to circumvent

these limitations, the	introduction of	chemical	disorder	in
a controlled manner	is a necessity.	This is	due to	the

fact that a small degree of chemical disorder can introduce defects of a structural and magnetic nature that may alter the characteristic temperatures associated with the martensitic and magnetic transformation, the net magnetic moment and/or affect the output strain. These intuitions are substantiated by recent experiments in which it was observed that the highest

3 Author to whom any correspondence should be addressed.

known magnetic field induced strain of 10% is achieved for Ni1.95Mn1.19Ga0.86 at ambient temperature in a magnetic field of 1 T [5, 6] and that the martensitic transformation temperature can be increased beyond room temperature for Ni2.15Mn0.85Ga [7].

In order to facilitate the technological breakthrough, a detailed understanding of the magnetic interactions and the nature of chemical ordering in different sublattices of the disordered samples is necessary. This is more so in the light of the unexpected trends in total magnetization of Mnrich alloys. In the stoichiometric sample, the saturation magnetization of 4.1 μB has contributions of about 3.5 μB from the Mn sublattice; therefore it was expected that the saturation magnetization will increase on increasing the Mn concentration. In recent magnetization measurements of various Mn-excess samples [8], an opposite trend has been observed. From these counter-intuitive results, it was inferred that the excess Mn atoms occupying sublattices other than the Mn ones are coupled antiferromagnetically with respect to the ones occupying the original Mn sublattice, thereby reducing the net magnetization upon increasing Mn content. This was further confirmed by total energy calculations [8] on the Ni2Mn1.25Ga0.75 system where the existence of both ferromagnetic and antiferromagnetic alignments of Mn atoms was observed and the calculated energies of the antiferromagnetic configuration

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