INVESTIGATION OF EXCHANGE BIAS PHENOMENON IN MULTIFUNCTIONAL MAGNETIC THIN FILMS AND BILAYER

Abstract

Heusler alloys are one of the important multifunctional alloys that are also magnetically ordered and have properties such as magnetic shape memory effect (MSME), magnetic super elasticity, magnetocaloric effect (MCE) and magnetostriction. These magnetically ordered alloys were first discovered in 1903 by Heusler, Stark and Haupt in Cu2MnAl showing strong ferromagnetism. The crystal structure of these Heusler alloy has been described as four interpenetrating f.c.c. (L21 structure) sublattice with stiochiometric composition X2YZ. Further research indicated that the magnetic properties originating in these alloys system are dependent on the chemical composition, crystal structure, and to the ordering of the atoms on an f.c.c. sublattice. These alloys exhibit most of the properties of metals but have the structure of an ordered compound in which magnetism may originate from several different magnetic exchange mechanisms leading to magnetic properties that are ferromagnetic / antiferromagnetic or intermediate. Consequently Heusler alloys provide an opportunity for investigation of the effects of chemical and structural order on the transport and magnetic exchange interactions in these multifunctional magnetic alloy systems. In this thesis particular attention has been given to exchange biased thin film systems where the exchange coupling occurs at the interface between ferromagnetic (FM) and antiferromagnetic (AF) layers. This an active field of research due its technological applications in magnetic read heads. The study of magnetic behaviour of a material in thin film form helps in exploring fundamental magnetization phenomenon occurring in the sample. Thin film form also offers the most suitable system for the development of practical systems and devices. With reducing dimension, the properties of a material start deviating from the bulk form. The properties of thin film are different from those of the bulk form of same material and when the thickness of the film reaches nanoscale dimension, the properties are usually completely different from bulk form of same material. Thin films and multilayers / heterostructures offer the advantage of controlling thickness / interface and study the critical phenomenon originating from interfacial effects which is not possible in other low dimension systems like very fine particles. From the early 1980s thin films have played a crucial role in understanding magnetic phenomenon and developing devices like higher-density nonvolatile random access memory and giant magnetoresistance based II systems. Owing to the far reaching consequences of giant magnetoresistance effect in developing devices, thin film has been established as one of the ideal model for understanding some fundamental physical mechanism. The results can then be used for the design of material with improved performance. Magnetic properties of a FM material are drastically altered in the vicinity of an antiferromagnet giving rise to the phenomenon of exchange bias. Exchange bias has been observed in many systems with antiferromagnet – ferromagnet interfaces such as spin glasses, nanoparticles and bilayers. A class of multifunctional materials with high potential for applications are Heusler alloys. Heusler alloys are one of the important multifunctional alloys that are also magnetically ordered and have properties such as magnetic shape memory effect (MSME), magnetic superelesticity, magnetocaloric effect (MCE) and magnetostriction. Historically Heusler alloys are of two classes: (i) the full Heusler alloys with the general formula X2YZ and (ii) the half Heusler alloys that have general formula XYZ. The half Heusler alloys are found to be of importance for spintronic materials and are not of interest for this thesis. Among full Heusler alloys (Ni-Mn-X, X= Sn, In, Ga, Sb, Al), Ni-Mn-Al alloys are interesting as their mechanical properties are superior and they are potential high temperature shape memory alloy. Further Ni– Mn–Al system has so far received much less attention. In this thesis exchange bias properties of Ni-Mn-Al off-stoichiometric full Heusler alloy thin films have been studied in detail. Along with this Ni-Mn-Al / Ni bilayer system and Ni-Mn-Al-In thin films have also been explored for their exchange bias effect. Investigation on exchange bias properties of Heusler alloys and bilayers by several research group including ours reveal that the exchange bias properties exhibited by Heusler alloys are very sensitive to the thinkness, composition and microstructure of the alloy. In order to further elucidate this point, in this thesis, we attempt to study exchange bias in Ni-Mn-Al system, Ni-Mn- Al-In system and Ni-Mn-Al / Ni bilayer system. Chapter 1 The literature survey, important findings and discussions relavent to the thesis have been discussed. Definition and various models of exchange bias suggested have been mentioned. Details of work carried out for NiMnAl and NiMnIn have also been discussed. The basis of the present thesis work has been discussed and the important results obtained for various systems have been given in tabulated form. III Chapter 2 In this chapter various characterization tools have been discussed which are carried out during this thesis work. This includes XRD, SEM, TEM, Nanoindentation and SQUID. The principal behind every technique and the information obtained have also been discussed. Chapter 3 Exchange Bias property of Ni rich and Mn rich thin Films have been explored. The films have been cosputter deposited using elemental targets of Ni, Mn & Al. It has been observed that Ni rich compositions exhibit a mixed L21 + B2 structure at thickness ≤ 1.5 μm. For larger thickness, a pure B2 structure has been observed. The Mn rich compositions exhibit a pure B2 structure independent of thickness. The structure and magnetic properties of Ni rich and Mn rich Heusler alloys have been studied in detail. Martensitic transition has been observed for the films exhibiting B2 as well as mixed (B2+ L21) structure. For very thin films no martensitic transition has been observed in both the systems. Owing to the presence of ferromagnetic (L21) and antiferromagnetic (B2) phase in film with mixed structure (L21+ B2), the exchange bias has been observed in these films. The exchange bias observed in Ni rich samples has been explained in terms of the existence of a spin glass state at low temperature. Exchange bias has also been observed in B2 phase films associated with the field induced ferromagnetic type ordering occurring in the antiferromagnetic matrix. An increase in exchange bias with increasing film thickness has been observed in Ni rich and Mn rich compositions. Large exchange bias has been observed Mn rich compositions. Chapter 4 In this chapter the effect of ferromagnetic (Ni) layer on Ni rich Ni57Mn24Al19 compostion and Mn rich Ni50Mn31Al19 compositionhas been studied in detail. Thickness of both NiMnAl layer and Ni layer has been varied. The effect of cooling field and varying ferro – antiferro layer has been studied in detail. For the Ni layer deposited on Ni rich compositions, exchange bias increases with increasing cooling field. For a particular thickness of Ni rich NiMnAl layer, effect of Ni variation is non monotonic. The thickness of Ni layer at which maximum exchange bias occurs depends on the thickness of NiMnAl layer. Maximum EB has been obtained for Ni layer thickness of 60nm. For the Ni layer deposited on Mn rich compositions, exchange bias shows non-monotonic variation with increasing cooling field. For a particular thickness of Mn rich NiMnAl layer, effect of Ni variation is non monotonic. The thickness of Ni layer at which maximum exchange bias occurs depends on the thickness of NiMnAl layer. For 1 μm thick film maximum HEB has been IV obtained for 30 nm thick film. For 2.5 μm thick film maximum HEB has been obtained for 60 nm thick film. The exchange bias increases with Ni layer for NiMnAl thickness of 1 μm, for other films HEB decreases with Ni layer. Variation of EB with temperature has also been discussed in terms of existing models. Chapter 5 In this chapter exchange bias properties of NiMnAlIn thin films have been studied. Thin films have been deposited by co-sputtering from target of NiMnAl and In. Thin films of thickness have been deposited. Owing to the ferromagnetic nature of NiMnIn films and antiferromagnetic NiMnAl films, coexisting ferro – antiferro interactions occur in the deposited films which give rise to exchange bias interactions. The effect of cooling field and temperature on exchange bias properties has been discussed. Training effect has also been done to study the exact nature of exchange bias phenomenon in this alloy system. Large exchange bias of 475 Oe at 10 K has been obtained for the film with thickness 1.2 μm. Chapter 7 Summarizes the results and findings of the present work discussed in this thesis. The future directions in which these studies can be extended have been suggested at the end.