Pulsed Laser Ablated Dilute Magnetic Semiconductors and Metalic Spin Valves

Abstract

Spintronics (spin based electronics) is a relatively new topic of research which is important both from the fundamental and technological point of view. In conventional electronics charge of the electron is manipulated and controlled to realize electronic devices. Spintronics uses charge as well as the spin degree of freedom of electrons, which is completely ignored in the charge based devices. This new device concept brings in a whole new set of device possibilities with potential advantages like higher speed, greater efficiency, non-volatility, reduced power consumption etc. The first realization of the spintronic device happened in 1989, owing to the discovery of the Giant Magneto-resistive (GMR) structure showing a large resistance change by the application of an external magnetic field. Nobel Prize in Physics is awarded for this discovery in 2007. In less than ten years, such devices moved from the lab to commercial devices, as read head sensors in hard disc drives. This new sensor led to an unprecedented yearly growth in the area l density of bits in a magnetic disc drive. Since 2005, another spintronic device known as Magnetic Tunnel Junction (MTJ) which shows a better performance replaced the existing GMR structures in the read heads. Another device which can potentially replace Si based Dynamic Random Access Memory (DRAM) is Magneto-resistive Random Access Memory (MRAM). Being magnetic it is non-volatile, which means not only it retains its memory with the power turned off but also there is no constant power required for frequent refreshing. This can save a lot of power(~ 10-15 Watts in a DRAM), which is quite significant amount for any portable device which runs under battery. Prototype of a commercial MRAM is also made during 2004-2005 by Infineon and Freescale Semiconductors. Recent development has shown switching of magnetic moment by spin-polarised currents (known as spin transfer torque), electric fields, and photonic fields. Instead of Oersted field switching in the conventional MRAM devices, spin torque effect can also be used to switch a magnetic element more efficiently. Recently Spin-Torque MRAM has gained lot of interest due to it’s less power consumption during the writing process. A continuous research effort is going on in realizing other proposed spintronic devices, such as Spin Torque Oscillator, Spin Field Effect Transistor , Race Track Memory etc. which are yet to get realized or yet to make their entry in the commercial devices. Spintronics can be divided in to two broad subfields viz.(1) Semiconductor Spintronics and (2) Metallic Spintronics. Most of the devices belong to the second class whereas the former one is rich in fundamental science and not yet cleared its path towards the world of application. Any spintronic device requires ferromagnetic material which is generally the source of spin polarized electrons. For semiconductor spintronic devices, the main obstacle is the non-existence of the ferromagnetic semiconductor above room temperature (RT). So the development in this direction is very much dependent on the material science research and discovery of novel material systems. Almost a decade back, Dilute Magnetic Semiconductors (DMS) are proposed to behaving RT ferromagnetism. As a result an intense theoretical and experimental research is being carried out since then on these materials. Still a general consensus is lacking both in terms of theory as well as experiment. There are many methodologies and thin film deposition protocols have been followed by different research groups to realize spintronic device concepts. The deposition techniques such as magnetron sputtering, molecular beam epitaxy have been found very efficient for growing metallic spintronic devices. For semiconductor spintronics especially in the area of Dilute Magnetic Semiconductors (DMS) pulsed laser ablation is also considered to be a viable technique. Even though pulsed laser ablation is a very powerful technique to prepare stoichiometric multi-component oxide films, it’s viability for the growth of metallic films and multilayer is considered to be limited. In this regard, we have used pulsed laser ablation to prepare pure and Co doped ZnO films, to examine the magnetic and magneto-transport behavior of these oxides. In addition extensive work has been carried out to optimize and reproducibly prepare metallic multilayer by Pulsed Laser Deposition to realize Spin Valve (SV) effect, which proves the viability of this technique for making metallic multilayer. This thesis deals with the study of Pulsed Laser Deposition(PLD) deposited DMSs and metallic SVs. The thesis is organized into seven chapters as described below: • Chapter:1 This chapter gives an introduction to Spintronics and the different device structures. It is followed by a brief description of the motivation of the present work. Since magnetism is at the heart of the spintronics, next we attempt to introduce some of the basic concepts in magnetism, which are related to the topics discussed in the following chapters. We discuss about various exchange interactions responsible for the long range ferromagnetic ordering below Curie temperature in different compounds. Other magnetic properties are also discussed. Then another important phenomenon called magnetic anisotropy is brought in. We discuss the origin of different types of anisotropy in materials. These anisotropies are also responsible for magnetic domain formation. Then a description of the different types of domain walls are introduced. Unlike conventional electronics, spintronics deals with spin polarized current. A short description of spin polarization from the band picture and concept of half-metal is introduced. The next part (Section-I) of this chapter gives an overview of the challenges in semiconductor spintronics. The spin injection efficiency from a ferromagnetic metal to a semiconductor is found to be poor. This problem is attributed to the conductivity mismatch at the interface. DMS materials can be potential candidates in order to solve this problem. Ferromagnetism in these proposed materials cannot be explained in terms of the standard exchange mechanisms. A model was first proposed for the hole doped system based on Zener model. A more apt model for the n-doped high dielectric materials is then proposed based on Bound Magnetic Polarons (BMP). These models for the unusual ferromagnetism are briefly discussed. Although ferromagnetism is observed by different groups, often questions are raised about the intrinsic origin of this behavior and the topic is still under debate. In this study we have tried to correlate the magnetic property with the transport property as the transport properties are generally not affected much by the presence of external impurities and probes the intrinsic property of the material. Transport and the magneto-transport in disordered materials in general are discussed. A specific model proposed for degenerate semiconductors, which is used for fitting our experimental data is explained. As the ferromagnetism in these materials are generally found to be related to the defects, different types of possible defects are described. Section-II deals with the metallic SV devices. In the history of spintronics, this is one of the most basic and most studied structures, but still having a lot of interest both fundamentally and technologically. A brief history of this discovery and a chronological progress in the device structure is discussed. Our work focuses on the metallic spin valve (SV) structures. Different types of SVs and their properties are explained. In a SV structure one of the ferromagnets (FM) is pinned using an adjuscent antiferromagnetic layer by an effect called exchange bias. A brief description of exchange bias and the effects of different parameters is given. This is followed by a discussion about the theory of GMR which deals with the spin dependent scattering at the bulk and at the interfaces, their relative contributions, effect of the band matching etc. A simple resistor model is used to explain the qualitative behavior of these SVs. The chapter is concluded with a brief summery and applications. • Chapter:2 This chapter provides a brief description of some of the experimental apparatus that are used to perform various experiments. The chapter is organized according to the general functionality of the techniques. This includes different thin film deposition techniques which are used depending on the requirements and also for comparing the properties of the samples, grown by different techniques. Structural, spectroscopic, magnetic and different microscopy techniques which are extensively used throughout, are discussed and their working principles are explained. This work also involves nano/microstructuring of devices. Mainly two structuring techniques are used viz. e-beam lithography and optical lithography by laser writer. In this section we will be discussing about these two techniques and other associated techniques like lift-off, etching etc. Effect of different parameters on the device structures are highlighted. • Chapter:3 Chapter-3 deals with the synthesis and characterization of the pure and 5% Co doped ZnO bulk samples. First a brief introduction about the ZnO crystal structure, band structure and other properties are given followed by the synthesis technique followed in our study. Synthesis is done by low temeperature in organic co-precipitation method. This liquid phase synthesis gives better homogeniety. As-grown sample is also sintered at a higher temperature. Structural study confirms the proper synthesis of the intended compound. Spectroscopic as well as magnetic study of the bulk doped sample indicates the presence of Co nano clusters in the low temperature synthesized sample, whereas after sintering indication of Co2+ is observed which reflects in the magnetic property as well. These samples are used as target material for laser ablation. • Chapter:4 Chapter-4 presents the results of the pure and Co doped ZnO thin film samples. Thin films are grown by PLD method on r-plane Sapphire substrates. Details of the growth technique and the deposition parameters are explained. Our result shows that 5% Co doped ZnO thin film is ferromagnetic in nature as expected in a DMS material, although the film is grown using a paramagnetic target. We also report that pure ZnO grown in an oxygen deficient condition giving ferromagnetic behavior. Not only that, the obtained saturation moment is much higher compared to the Co doped sample. We have demonstrated that the FM can be tuned by tuning the oxygen content and FM disappears when the film is annealed in an oxygen environment .But for the Co doped sample magnetic property could not be tuned much as Co doping stabilizes the surface states. To exclude the possibilities of the extrinsic origin we have done a detailed magneto-transport study for both doped and undoped films. For ZnO, we have shown a one to one correlation of the magnetic and magneto-transport data which further supports the fact that the obtained magnetic behavior is intrinsic. Fitting of the magnetorsistance (MR) data for the pure and Co doped ZnO samples is done using a semi-empirical formula, consisting of both positive and negative MR terms originally proposed for degenerate semiconductors .Excellent agreement of the experimental data is found with the formula. For pure ZnO sample we have extracted the mobility, carrier concentration etc .by Hall measurement. The fabrication steps of Hall bar sample which involves optical lithography and ion beam etching are discussed. 3D e-e interaction induced transport mechanism is found to be dominant in case of oxygen deficient pure ZnO. • Chapter:5 Chapter-5 demonstrates the tuning of band gap of ZnO by alloying with MgO. By changing the ZnO:MgO ratio in PLD grown films, we could tune the band gap over a wide range. Composition alanalysis is done by Rutherford Back-Scattering. Structural and spectroscopic studies are carried out, which shows tuning of band gap upon alloying with MgO. We could tune ZnO band gap from 3.3eV to 3.92eV by30% MgO alloying, while retaining the Wurtzite crystal structure. • Chapter:6 Chapter-6 demonstrates the metallic Pseudo Spin Valve (PSV) structures grown by sputtering and by PLD. Main focus of this chapter is to show that, PLD can be aviable technique for making metallic PSV and Spin Valve (SV) structures. This is almost an unexplored technique for growing metallic thin film SVs, as it is evident in the literature. NiFe and Co are used as the soft and hard FM layers respectively, Au and Cu are used as the spacer layer. FeMn is used for pinning the Co layer in case of the SV structures. The first section describes the properties of these materials and then substrate preparation, deposition parameters etc. are explained in details. Properties of sputter deposited PSV structures are also described. Thickness variation of different layers, double PSV structure and angular variation of the MR properties are presented. Generally two measurement geometries are followed for the SV measurements viz.(1) Current In Plane (CIP) and (2) Current Perpendicular to Plane(CPP). We have carried out MR studies in both the measurement geometries. Measurement in CPP geometry is much more involved than CIP and need structuring with multiple lithography steps. CPP measurement geometry scheme and the process steps are discussed. For this measurement a special ac bridge technique is followed which is also discussed. In the next part we have demonstrated PSV and SV structures, grown, using PLD in an Ultra High Vacuum (UHV) system. Not only that, we have obtained a CIPMR as high as 3.3%. PLD is generally thought to be a technique for oxide deposition and metallic multilayers are not deposited due to particulate formation, high enegy of the adatom species which can lead to inter-mixing at the interface etc. But in this study we have shown that by properly tuning the deposition parameters, it is possible to grow SVs using PLD. We have found the roughness of the PLD grown films are much lower compared to the sputtered films. For top SV structures we have obtained exchange bias even in the absence of applied field during deposition. This effect is observed by performing magnetic and magneto-resistance measurements. Effect of different layer thicknesses, field annealing etc. are discussed. Two different spacer layers are used and their properties are compared. We have found that the interface engineered structures are giving highest MR among the different samples. Then a conclusion of our study is presented followed by a discussion on the difficulties and challenges faced for optimizing the PLD grown SVs. • Chapter:7 Finally, in Chapter-7, various results are summarized and a broad outlook is given. Perspectives for the continuation of the present work is also given.