SYNTHESIS, CHARACTERIZATION AND IRRADIATION OF FERROMAGNETIC SHAPE MEMORY ALLOY THIN FILMS

Abstract

Ferromagnetic shape memory alloys (FSMAs) have attracted much attention in recent years as intelligent and functional material due to their exceptional magnetoelastic properties and large magnetic field induced strain that arises due to the rearrangement of twin variants in the martensitic phase. The shape memory effect in these materials can not only be controlled by changing the temperature, as it occurs in traditional shape memory alloys, but also by varying the magnetic field upto moderate field values. The later makes them of noteworthy interest for developing new thermal or magnetically driven actuators. Many attempts have been made to investigate the shape memory effect of bulk FSMA but the knowledge of such phenomenon in thin films is sparse as limited studies have been reported on the growth and magnetic properties of FSMA thin films. However, the applications of these materials in emerging microdevices such as magnetically driven microelectromechanical systems (MEMS) require a high quality of FSMA thin films grown on semiconductor substrate. Hence, a systematic study is required to understand the associated phase transformation mechanisms of FSMA thin films for scientific and industrial purposes. The main aim of the thesis was to synthesize Ni-Mn-Sn ferromagnetic shape memory alloy thin films on silicon substrate by DC magnetron sputtering technique in order to (i) understand the role of composition, substrate temperature and film thickness on texture, surface morphology and martensitic transformation of these films; (ii) investigate the exchange bias properties of these films (iii) study the low energy and high energy ion irradiation effects on structural, electrical and magnetic properties of Ni-Mn-Sn films. A chapter-wise summary of the thesis is given below: Chapter 1 gives an overview of ferromagnetic shape memory alloys and material background. The chapter includes the literature survey on the synthesis and properties of FSMA. The structure, phase diagram and magnetic properties of Ni-Mn-Sn FSMA have been discussed. The effect of ion beam irradiation on shape memory behavior has also been included. Chapter 2 presents the details of experimental techniques which we have used for the synthesis and study of the properties of FSMA thin films. This chapter is divided into two sections which deal with synthesis and irradiation techniques, structural characterization and measurement of magnetic and electrical properties. Section 2.1-The synthesis of thin films in present thesis has been carried out by DC magnetron sputtering technique. The various sputtering parameters were optimized to obtain good quality Ni-Mn-Sn thin films. The irradiation of these films done by using low and high energy ion irradiation facilities are also described in detail. Section 2.2-It gives an overview of characterization techniques used for the present work. X-ray diffractometer (XRD) has been used for the phase identification and texture analysis. Surface morphology and microstructure were studied using field emission electron microscopy (FE-SEM) and atomic force microscopy (AFM). Transmission electron microscopy (TEM) was used for phase identification and high resolution imaging. The film thickness was measured using surface profilometer and cross-sectional FESEM. Magnetic properties of these films were studied using superconducting quantum interference device (SQUID) magnetometer. Phase transformation behavior of these films was studied using four probe electrical resistivity set-up. Further mechanical properties were studied using nanoindenter. Chapter 3 describes the composition and grain size effect on the phase transformation behavior of Ni-Mn-Sn thin films. This chapter is divided into two sections. The first section (Section 3.1) mainly describes the effect of composition on structural and magnetic properties of Ni-Mn-Sn thin films. Films of compositions Ni52 6Mn23 7Sn23.6 (Si) and Ni51 5Mn26 1Sn22 2 (S2) are observed to exhibit austenite phase with mixed L21/A2+132 structure while the films of compositions Ni58.9Mn28 OSn13.0 (S3) and Ni58.3Mn29.0Sn12 6 (S4), with higher Mn content, show martensitic phase with monoclinic structure. Substitution of Mn for Sn in these films causes a structural instability that has a considerable impact on the magnetic properties of these films. Temperature dependent magnetization measurements demonstrate the influence of magnetic field on the structural phase transition temperature. The martensitic transition temperature was observed to increase with increase in Mn content. Section 3.2-describes the grain size dependence of martensitic transformation temperature in Ni-Mn-Sn thin films. The grain sizes were found to have a considerable effect on phase transformation behavior of the films. The grain size and the crystallization II extent increases with increase in substrate temperature. XRD pattern reveals the formation of austenite phase with preferential (220) reflection for the films with grain size in the range between 4.6 and 21 nm while the films with grain size 23.2 nm exhibit martensite phase with monoclinic structure at room temperature. Both the microstructure and grain size of the films are found to depend on the substrate temperature. It is observed that the fine nanostructure of crystalline films resulting from deposition at low temperature < 450 °C did not allow the occurrence of phase transformation until a critical grain size is obtained during the coalescence process that took place for deposition at temperature > 500 °C. The magnetic behavior of the films differs in martensite and austenite phases considerably which is due to the structural change as well as the formation of martensitic variants in the low temperature phase. The pronounced improvement of the magnetic properties in terms of saturation magnetization and coercivity with increasing grain size is related to the reduction in excess free volume associated with grain boundaries. Chapter 4 describes the exchange bias behavior of Ni-Mn-Sn FSMA thin films. This chapter is divided into two sections. The first section (Section 4.1) describes the exchange bias effect in the martensitic state of Ni498Mn36 ISni3, film at low temperatures below blocking temperature (TB) which is attributed to the unidirectional anisotropy that arises due to the coupling between antiferromagnetic (AFM) and ferromagnetic (FM) interactions in the martensite phase of the film. Shift in hysteresis loops of up to 41 Oe is observed in the 2 Tesla field cooled film. Both exchange bias field HE and coercivity ifc are found to strongly depend on temperature. The double shifted hysteresis loop observed at 5 K during zero field cooling, confirms the existence of exchange bias effect in the film at low temperature. This study give a possibility of the application of Ni-Mn-Sn FSMA films for device applications since most of the applications based on exchange bias effect are in thin film form. Section 4.2- describes the thickness dependent martensitic transformation and exchange bias effect in Ni-Mn-Sn FSMA thin films. XRD pattern reveals that the films exhibit austenite phase with L21 cubic crystal structure at room temperature and the grain size and crystallization extent increases with increase in film thickness upto — 1400 nm above which structural disorders appear in the films due to the formation of MnSn2 and Ni3Sn4 precipitates. The improvement in the crystallinity of the film with thickness is due to the reduction in film-substrate interfacial strain resulting in preferred oriented growth of the III films. Temperature dependent magnetization measurements as well as electrical measurements demonstrates the complete absence of phase transformation for the film of thickness 120 nm which was due to its lower grain size (9.1 nm) below critical value required for the occurrence of phase transition. Film with thickness 1412 nm possesses the highest magnetization with the smallest thermal hysteresis among all the films and therefore best suited for the actuators based on first-order structural phase transformation. However, the thick film of 2022 nm thickness displays the degraded phase transformation behavior with shift in transformation temperatures towards lower temperatures due to the MnSn2 precipitate formation and the film of 2518 nm thickness shows complete suppression of shape memory behavior due to the increased formation of MnSn2 precipitate and evolution of Ni3Sn4 precipitate. Chapter 5 describes the ion irradiation induced modifications of Ni-Mn-Sn FSMA thin films. This chapter is divided into two sections. In the first section (Section 5.1), we have systematically investigated the effect of low energy (450 keV) Ar ions irradiation on the first order phase transformation behavior of Ni50Mn356Sn144 films. XRD pattern reveals the increase in the crystallinity of pristine film upto a critical fluence of 1 x 1015 ions/cm2 above which crystallinity degrades resulting in complete amorphization at a fluence of 3 x 1016 ions/cm2 which was further confirmed by FESEM micrographs, TEM bright field images and electron diffraction patterns. The electrical resistance versus temperature plots and thermomagnetic measurements reveal the variation of phase transformation behavior. Section 5.2- describes swift heavy ion irradiation induced modifications of Ni-Mn-Sn FSMA thin films. In the present study, we report the effect of 200 MeV Au ions irradiation on the structural, magnetic and electrical properties of Ni-Mn-Sn FSMA thin films. In order to understand the role of initial microstructure of the films with respect to high energy ion irradiation, irradiation was done on two types of Ni-Mn-Sn FSMA film, one in martensite phase at room temperature and other in austenite phase at room temperature. This type of study is also important to investigate the applications of these materials in radiation zones such as in space or near reactors. Chapter 6 presents the summary and conclusions of the entire work presented in the thesis and also propose the future directions in which these studies can be further extended. IV