Transformation behavior of Ni-Mn-Ga/Si(100) thin film composites with different film thicknesses

Stefano Besseghini 1Andrea Gambardella 1Volodymyr A. Chernenko 1,2Michael Hagler 3Chris Pohl 3Peter Müllner 3Makoto Ohtsuka 4

1. Istituto per l'Energetica e le Interfasi Consiglio Nazionale delle Ricerche Unità di Lecco (IENI Lecco), Corso Promessi Sposi, 29, Lecco 23900, Italy
2. Institute of Magnetism, NAS of Ukraine, 36-B Vernadsky str., Kiev 03142, Ukraine
3. Department of Materials Science and Engineering, Boise State University, Boise 83725, United States
4. Institute of Multidisciplinary Research for Advanced Materials (IMRAM), Sendai 980-8577, Japan

Abstract

The Ni-Mn-Ga magnetic shape memory alloys (MSMAs) show large magnetoplasticity and strain-induced change of magnetization. Thin film composites of MSMAs representing Ni-Mn-Ga thin films sputter-deposited on different substrates are prospective materials for (micro)nanosystem applications.

In this work, we have prepared two series of Ni-Mn-Ga/Si(100) thin film composites and studied the influence of the heat treatment and film thickness on the structure, martensitic transformation, ferromagnetic ordering and intrinsic stress state in the films. Each series containing 7 samples with thickness of 0.1; 0.3; 0.5; 0.7; 1; 2 and 3 μm was prepared by r.f. magnetron sputtering using Ni49.5Mn28Ga22.5 and Ni52Mn24Ga24 targets. These targets facilitate the formation of 10M and 14M ferromagnetic martensites at room temperature, respectively. A Si(100) wafer having amorphous 500 nm thick SiNx buffer layer was used as a substrate.

According to X-ray diffraction, a disordered state can be inferred in the as-deposited films while annealed films (in vacuum at 1073K for 1h) show (110)-type fiber texture. Temperature dependencies of the electrical resistivity, ρ(T), of films were measured precisely in the interval 250-450 K. On warming, as-deposited submicron films show a monotonous reversible decrease of resistivity which changes to the monotonous increase for the thicker films. The annealed films demonstrate the anomalies on the ρ(T) curves associated with the martensitic transformation (MT) and Curie temperature (TC) typical for bulk Heusler alloys. The values of MT and TC temperatures are considerably dependent on film thickness in the submicron range. The stress in the films and stress anomalies produced by MT were determined with substrate curvature measurement method. The results of the resistivity and substrate curvature experiments are linked to explain the thickness dependence of the transformation temperatures.

 

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Presentation: Oral at E-MRS Fall Meeting 2007, Symposium E, by Andrea Gambardella
See On-line Journal of E-MRS Fall Meeting 2007

Submitted: 2007-05-02 17:57
Revised:   2009-06-07 00:44