The Average Valence Electron Number: Is it a relevant parameter for Ni 2 MnGa Alloys?
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The Average Valence Electron Number: Is it a relevant parameter for Ni2MnGa Alloys?
Kristin Neumann1, Khaled Elmashai1, Tilmann Hickel2, Klaus-Ulrich Neumann1, Bachir Ouladdiaf3, Kurt R.A. Ziebeck1 1
Dept. of Physics, Loughborough University, Loughborough LE11 3TU, United Kingdom
2
Max Planck Institut für Eisenforschung, Max Planck Str. 1, 40074 Düsseldorf, Germany
3
Institute Laue-Langevin, Rue des Martyres, 38074 Grenoble, Cedex, France
ABSTRACT The influence is investigated of the average valence electron number on the systematic changes of a Ni2MnGa based alloy series. The experimental investigation focuses on an isoelectronic alloy series Ni2Mnx(CrFe)(1-x)/2Ga for which the average valence electron number is unchanged for any value of x. Based on the changes of physical properties of alloys in this series compared to the properties of Ni2MnGa it is argued that local lattice distortions are more relevant for driving the change in alloy characteristics, such as the martensitic phase transition temperature or the ferromagnetic ordering temperature, than the band filling by valence electrons.
INTRODUCTION In the search for new ferromagnetic shape memory materials it is generally a promising avenue to explore the change of properties with alloying. This allows, on the one hand, to establish the systematic variation of some physical characteristics, such as the structure of the martensitic phase and the martensitic transition temperature, thereby enabling the possibility to arrive at a material with a more desirable set of features as determined by the intended application. On the other hand, from a theoretical point of view, it is important to establish a more systematic description of the basic features. As such for the Heusler alloys in general, and the ferromagnetic shape memory compound Ni2MnGa in particular, alloying investigations have been carried out, both experimentally [1, 2] as well as theoretically [3,4]. From such investigations it has emerged that one parameter to characterise the variation of characteristic temperatures (such as the ferromagnetic transition temperature TC or the martensitic transition temperature Tm) for an alloy series is the number of available valence electrons per formula unit. This seems to correlate with the observed changes as a function of x in series such as Ni2+xMn1-xGa [4, 5], Ni2Mn1+xIn1-x [4, 5] and others [4-6]. While the observation of an experimental correlation between an x-value in an alloy series and the systematic change of a particular characteristic physical entity is indicative of an underlying connection it is less obvious to actually establish the causality between, for example, electron concentration and magnetic transition temperature. The difficulty with identifying a unique cause for such changes lies in the complexity of the alloying process itself. If, for example, one element is replaced by another one with a different valency, the replacing atom almost certainly will also affect the lattice as it has generally a slightly different
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