Shape memory effect and magnetostriction of sputtered NiMnGa thin films

摘要

NiMnGa thin films have been deposited by magnetron sputtering on Mo substrates using a Ni_(50)Mn_(30)Ga_(20) powder metallurgical target. Independent from variation of substrate temperature during the sputtering process the deposited films are found to be polycrystalline. X-ray diffraction patterns show a decreasing peak width and a shift to slightly higher Bragg angles with increasing substrate temperature during sputtering, which is even amplified when subsequent rapid thermal annealing is applied. Annealing temperatures above 500℃ lead to a remarkable enhancement of the shape memory effect as well as of the magnetostriction. Temperature induced martensitic transformations have been measured by a cantilever deflection technique and a cantilever resonance method. Martensitic start temperatures (M_s) range between 50 and 90℃ depending on composition and annealing temperature. Stress relief Δσ upon the martensitic transformation ranges between 200 and 300 MPa whereas the magnetostrictive coupling constant b is about 2 MPa. Magnetization measurements and Curie temperature determination reveal ferromagnetic behavior within the temperature range of the martensitic transformation.