Magnetic and optical studies of some nanostructured materials for various applications

Muddisetti, Venkatanarayana (2017) Magnetic and optical studies of some nanostructured materials for various applications. PhD thesis, Indian Institute of Technology Hyderabad.

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Abstract

Physics of nanostructured materials under dimensional constraints has been of great interest since they play a major role in building various technological devices. Among various nanostructured materials, magnetic and optical materials h ave been found ample interest due to their applications in magnetic refrigeration, spintronic and optoelectronic devices. Fundamentally, novel properties emerge in magnetic elements as the materials dimensions become comparable to or smaller than certain c haracteristic length scales, such as spin diffusion length, carrier mean free path, magnetic exchange length, domain wall width, etc. Such a strong dependence on the geometry allows one then to tailor the magnetization configuration by appropriately engine ering the geometry. In addition, structuring the materials down to characteristic length scales lead s to pronounced modifications of the general response of the system. In addition, exchange bias effect in magnetic system has been found much attention due to its implications in the devices such as spin valve and magnetic random access memory. Hence, we are especially concerned in the systems where the interplay of different physical mechanisms can lead to entirely new phenomena. Hence, objectives of the pre sent thesis are (a) Study the magnetocaloric properties in flakes of DyCu 2 DyCu 2 is a model system which exhibits anti - ferromagnetic nature below 30 K and exhibits large MCE properties. On top of that DyCu 2 is very ductile in nature as a result, it would be e xtremely difficult to reduce its dimension and study its physical properties. Hence, our first aim of this thesis is to propose a method to prepare small flakes of the DyCu 2 . As a result of the size reduction, physical properties may alter in a very impres sive manner from their bulk counterpart, which might essentially be useful for various applications. Nevertheless, calculation of the MCE at reduced dimensions for the DyCu 2 has not been attempted until now, which is the main focus of the present work. In addition to the reduced size, correlation between the ferromagnetic (FM) and antiferromagnetic (AFM) phases which would arise due to the field induced transitions may alter the nature of magnetic transition. Such viii microscopic studies would be of great inter est to understand the physics behind a strongly anisotropic DyCu 2 system in its reduced form. Hence, in this work, we also demonstrate our calculations pertinent to the MCE and the nature of magnetic transitions related to small flakes of DyCu 2 . (b) Band gap tuning of graphene oxide under compressive strain An attempt has been made to reduce the GO by removing oxygen functional groups using a dry ball mill in an inert atmosphere. Essentially, ball milling process is advantageous to remove the functional grou ps and creates the defects in addition to in - plane defects. In the present case, we strongly believe that a de - oxygenation phenomenon is helping in inducing defects via ball milling technique. At this juncture, in recent past, it has been believed that wet ball milling method is more efficient than dry milling in attaining low particle size distribution and in maintaining low temperatures. It is also well know that using wet ball milling method one can maintain low particle size distribution and less temper ature. In addition, wet ball milling is more economical compared with dry ball milling method. Hence, in the present study we would like to demonstrate our efforts in tuning the optical properties of the GO marginally using wet ball milling. (c) Exchange bias effects in IrMn/YCo thin films Thin films based on rare earth (R) - transition metal (TM) systems have been of great interest due to their usage in various applications such as transducers, sensors, magneto optical recording media and actuators apart fro m their fundamental properties. R – TM systems also found to show magnetic field induced phase transition from the paramagnet to ferromagnetic state by the application of magnetic field and Pauli paramagnetism. Such intriguing phenomenon has been evidenced in a system, YCo 2 . It has been believed that in a sputtering system, deposition processing parameters such as Ar gas pressure and sputtering power may play a vital role in influencing magnetic and morphological properties. Hence, in the present work we wo uld like to study the effect of Ar gas pressure and sputtering power on magnetic and morphological properties of YC O films. In addition, it is also very much essential to develop EB effects in as deposited films as the field is highly ix demanding. Hence, we also tried to explore if we can induce exchange bias in IrMn/YCo b ilayers. In addition, we also would like to see the effect of perpendicular field annealing on the EB properties of IrMn/YCo b ilayers. (d) Exchange bias effects in graphene nanoribbons (GNRs) Due to the high carrier mobility, long spin life time resulting from the low spin - orbit coupling (~ 10 - 4 eV) and the low hyperfine interaction of graphene at room temperature, it has been believed that it is promising material for the spintronic applicatio ns. Evidenced negative exchange bias effect in the GNRs upon a positive cooling field has been attributed to the presence of the AFM regions mixed with a FM phase. A plethora of experimental investigations have proved that negative magnetic field cooling ( NFC) would have pronounced effects on exchange bias properties. Such marked results pertinent to exchange bias have been attributed to existence of pinned and rotatable spins at the interfaces. Hence, in the present work, we would like to explore and demon strate the effect of negative magnetic field cooling on exchange bias properties of the GNRs.

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