SYNTHESIS AND CHARACTERIZATION OF Ni-Mn-X (X: In, Sb) FSMA THIN FILMS

Abstract

In recent years, Ni-Mn-X (X: In, Sb) based ferromagnetic shape memory alloys (FSMAs) showing a reversible first-order martensitic phase transformation between high symmetry austenitic phase to low symmetry martensitic phase accompanied with a macroscopic shape change, have attracted increasing scientific attention as multifunctional materials due to their large magnetic-field-induced strains (MFIS) by the rearrangement of twin variants in the martensite and as potential candidates for high sensitivity magnetic actuators and sensors. These alloys exhibit various magnetic field-driven properties such as magnetic shape memory effect, martensitic phase transformation, exchange bias effect and magnetocaloric effect etc. So far, numerous groups have studied these magnetic field-driven properties in bulk FSMAs. For emerging micro devices such as magnetically driven microelectromechanical systems (MEMS) and even microscopic machines, high quality FSMA thin films grown on semiconductor substrates are required. It is desirable to establish the process for producing high quality thin films of these materials for their integration into emerging technologies. The main objective of the present work was to synthesize nanostructured ferromagnetic shape memory alloy thin films of Ni-Mn-X (X: In, Sb) on Si (100) substrate using DC magnetron sputtering technique to investigate the structural, phase transformation, magneticand mechanical properties of these films. In addition Ni-Mn-Sb/CrN heterostructures were fabricated and the effect of varying thickness of CrN layer on various properties of heterstructure was studied. A chapter - wise summary of the thesis is given below: Chapter 1 gives an overview of ferromagnetic shape memory alloys and material background. This chapter includes literature survey on physical properties of Ni-Mn-X (X: In, Sb) ferromagnetic shape memory alloys and their thin films. The fourth element addition, exchange bias and magnetocaloric effect properties of FSMAs have been discussed. Chapter 2 presents the details of experimental techniques, which we have used for the synthesis and characterization of these films. The synthesis of thin films in present thesis has been carried out by DC magnetron sputtering technique. Various sputtering parameters were optimized in order to obtained good quality FSMA films. X-ray diffraction technique has been used for phase identification and crystallite size analysis. The surface morphology and microstructure of thin films were studied using Atomic Force Microscopy (AFM) and Field Emission Scanning Electron Microscopy (FESEM). The film thickness was measured using cross-sectional FESEM. The temperature and field dependent magnetization (M-T and M-H) measurements of thin films were studied using Cryofree Vibrating Sample Magnetometer iv (VSM) and SQUID. The electrical properties of these films were measured using four probe resistivity set up in the temperature range from 30 K to 400 K. The nanoindentation was used to study the mechanical properties such as hardness (H), elastic modulus (E), plasticity index (H/E) and resistance to plastic deformation (H3/E2) of thin films. Chapter 3 describes the growth and characterization of nanostructured Ni-Mn-In ferromagnetic shape memory alloy thin films. In this chapter the influence of film thickness on structural, magnetic, electrical and mechanical properties of nanostructured Ni-Mn-In thin films was studied. The film thickness was varied from ~ 90 nm to 655 nm.XRD analyses revealed that the films exhibit austenitic phase with L21 structure at room temperature. The grain size and crystallization extent increased with corresponding increase in film thickness. The temperature dependent magnetization and electrical measurements demonstrated the absence of phase transformation in the film with lower thickness ~ 90nm which could be due to small grain size of film. For thickness greater than 153 nm, the films show first order martensitic phase transition with thermal hysteresis width, which increases with further increase in film thickness. The field dependent magnetization curves also show the increase in saturation magnetization (SM). The value of refrigeration capacity (RC) which is an important figure of merit has been found to be 155.04 mJ/cm3. Maximum exchange bias of 0.0096 T and large magnetic entropy change ΔSM =15.2 mJ/cm3 K (field 2 T) at martensitic transition was obtained for film thickness of 655 nm which makes them useful for microelectromechanical systems (MEMS) applications. Further, nanoindentation studies revealed the higher values of hardness of 7.2 GPa and elastic modulus of 190 GPa for the film thickness of 153 nm. These findings indicate that the Ni-Mn-In thin film is potential candidate for various multifunctional properties. Chapter 4 describes the growth and characterization of Ni-Mn-Sb-Al and Ni-Mn-In-Cr FSMA thin films. This chapter has been divided into two sections. The first section (section 4.1) describes the effect of aluminium (Al) content on the martensitic transformations and magnetocaloric effect (MCE) in Ni-Mn-Sb ferromagnetic shape memory alloy (FSMA) thin films. An increase in martensitic transformation temperature (TM) with increasing Al content was observed from magnetic (M-T) and electrical (R-T) measurements. From the study of isothermal magnetization (M-H) curves, a large magnetic entropy change (ΔSM) of 23 mJ/cm3 K was found in N49.8Mn32.97Al4.43Sb12.8. A remarkable enhancement of MCE has been attributed to the significant change in the magnetization of Ni-Mn-Sb films with increasing Al content. Furthermore, a high refrigerant capacity (RC) was observed in Ni-Mn-Sb-Al thin films as compared to pure Ni-Mn-Sb. The substitution of Al for Mn in Ni-Mn-Sb thin films with field v induced MCE are potential candidates for micro length scale magnetic refrigeration applications where low magnetic fields are desirable. Section 4.2 describes the influence of Cr addition on the structural, magnetic, and mechanical properties as well as magnetocaloric effect for magnetron sputtered Ni-Mn-In ferromagnetic shape memory alloy thin films. X-ray diffraction studies revealed that Ni-Mn-In-Cr thin films possessed purely austenitic cubic L21 structure at lower content of Cr, whereas higher Cr content, the films exhibited martensitic structure at room temperature. The temperature-dependent magnetization (M-T) and resistance (R-T) results confirmed the monotonous increase in martensitic transformation temperatures (TM) with the addition of Cr content (0 - 4.5 at %). Further the addition of Cr content significantly enhanced the hardness (28.2±2.4 GPa) and resistance to plastic deformation H3/E2 (0.261) in Ni50.4Mn34.96In13.56 Cr1.08 film as compared with pure Ni-Mn-In film, which could be due to reduction in grain size, and has been explained in terms of the grain boundary strengthening mechanism. Further, from the study of isothermal magnetization (M-H) curves, the magnetocaloric effect (MCE) around the martensitic transformation has been investigated. The magnetic entropy change ΔSM of 7.0 mJ/cm3 K was observed in Ni51.1Mn34.9In9.5Cr4.5 film at 302 K in an applied field of 2 T. Finally, the refrigerant capacity (RC) was also calculated for all the films in an applied field of 2 T. Chapter 5 describes the deposition of Ni50Mn36.8Sb13.2/CrN heterostructure thin films on Si (100) substrate to improve the exchange bias and mechanical properties of Ni-Mn-Sb ferromagnetic shape memory alloy thin films. The antiferromagnetic CrN thickness was varied from 15 nm-80 nm. The shift in hysteresis loop up to 51 Oe from the origin was observed at 10 K when pure Ni-Mn-Sb film was cooled under a magnetic field of 0.1 T. The observed exchange bias has been attributed to the coexistence of antiferromagnetic (AFM) and ferromagnetic (FM) exchange interactions in the martensitic phase of the film. On the other hand, a significant shifting of hysteresis loop was observed with AFM CrN layer in Ni50Mn36.8Sb13.2/CrN heterostructure. The exchange coupled 140 nm Ni50Mn36.8Sb13.2/35nm CrN heterostructure exhibited relatively large exchange coupling field of 138 Oe at 10 K compared to other films, which could be due to uncompensated and pinned AFM spins at FM-AFM interface and different AFM domain structure for different thicknesses of CrN layer. Further nanoindentation measurements revealed the higher values of hardness and elastic modulus of about 12.7±1.25 GPa and 179±0.12 GPa in Ni50Mn36.8Sb13.2/CrN heterostructures making them promising candidate for various multifunctional MEMS devices. vi Chapter 6 presents the summary and conclusion of the entire work presented in the thesis and also proposes the future directions in which these studies can be extended.