Istituto dei materiali per l'elettronica ed il magnetismo     
Albertini F., Casoli F., Fabbrici S., Righi L., Chernenko V., Besseghini S., Gambardella A. Coincidence of magnetic and martensitic transition in NiMnGa thin films obtained by changing growth parameters. In: Intermag 2008 (Madrid, Spain, 4-8 maggio 2008).
NiMnGa alloys have attracted a great deal of interest for huge magnetic-field-induced strains and giant magnetocaloric effects [1]. Both effects are linked to the occurrence of a martentesitic transformation between a cubic and a lower-symmetry phase in the ferromagnetic state. The interplay between structural and magnetic degrees of freedom is furtherly enhanced when structural and Curie transition coincide: i.e. first order transition from lower-symmetry ferromagnet to cubic paramagnet. Such a condition has been obtained in bulk systems by means of suitable composition changes and exploited to improve the magnetocaloric and magnetomechanical performances [2,3]. Up to now a lot of work aimed at understanding the fundamental properties and at tailoring the giant effects has been done mainly on bulk material. The research on NiMnGa thin film represents a new field of interest thanks to important possible applications in the field of sensors, actuators, and refrigeration materials [4]. In this case the properties can be tailored by means of composition and growth parameters changes, substrate choice and orientation, growth and annealing tempera- tures, thickness, and nano- and micro-scale structure. Aim of the present paper is to study the effects of growth parameters on the main properties of NiMnGa thin films of different thickness in order to tailor magnetic and structural transitions and to make them coincide. NiMnGa thin films of thickness ranging from 20 to 100 nm were deposited by r.f. sputtering on oxidized Si and MgO(100) substrates. The Ni2Mn1.2Ga0.8 target was obtained by induction melting; its composition and homogeneity were confirmed by means of Energy Dispersive X-ray (EDX) analysis. Film growth was performed at a substrate temperature of about 400C and Ar pressure p=1.4X10-2 mbar, with different target voltage V from 700 to 1200 V, giving rise to growth rates ranging from 2.45 nm/min for V=700 V to 7.5 nm/min for V= 1200 V. Structural and morphological characterization were performed by means of X-Ray Diffraction, Scanning Electron and Atomic Force Microscopy. The magnetic properties were studied by Alternating Gradient Force Magnetometery. Thermomagnetic analysis (magnetic susceptibility vs. temperature) and resistivity measurements as a function of temperature were also performed in order to study magnetic and structural transitions. The films were found to be polycrystalline with a granular morphology with mean grain size dependent on thickness and growth parameters. Magnetization mesurements confirm the ferromagnetic state of samples with saturation magnetization values at RT ranging from 350 to 400 emu/cm3, as expected for Mn-rich NiMnGa alloys. Selected results on transition temperatures are reported in Table I. It was found that Curie temperature (Tc) decreases by decreasing thickness and by increasing target voltage, independently of substrate. An increase of Ni to Mn ratio by increasing target voltage has been evidenced by EDX analyses. The decrease of Tc by increasing Ni/Mn ratio is in qualitative agreement with the results reported on bulk Mn-rich systems [3]. A simultaneous increase of martensitic temperature has also been found. As an example in samples of thickness t=75 nm the martensite to austenite transformation (TAM) increases of 44C by increasing target voltage from 900 to 1200V. Taking advantage of these results, the coincidence of magnetic and structural transition has been induced by modifying target voltage. In Figure 1 resistivity measurements versus temperature are reported for films of thickness t=75 nm grown at V=900 V and V=1200 V respectively. In the first curve (a) the martensitic transformation (step with thermal hysteresis) and Curie transition (kink) are well separated while in the second one (b) they merge (TMA =83.7 C) and show a thermal hysteress of 15 C. A suitable way to decrease both transition temperature and thermal hysteresis is to increase the sample thickness (see curve c for 100 nm thick sample). [1] A.Sozinov et al. Appl. Phys. Lett. 80 (2002) 1746 ; F.Hu et al. Appl. Phys. Lett. 76 (2000) 3460 [2] L.Pareti et al. Eur. Phys. J B 32 (2003) 303, A.A.Cherechukin et al. Phys. Lett. A 291 (2001) 175 [3] F. Albertini et al. J. Appl. Phys. 100 (2006) 023908 [4] V.A.Chernenko et al. Smart Mater. Struct. 14 (2005) S245, M.Khol et al. Mat. Sci. Eng. A 438 (2006) 940
Subject Ni2MnGa
Magnetoelastic properties
Magnetic thin films

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