Novel room temperature ferromagnetic semiconductors

Sammanfattning: In today's information world, bits of data are processed bysemiconductor chips, and stored in the magnetic disk drives.But tomorrow's information technology may see magnetism (spin)and semiconductivity (charge) combined in one 'spintronic'device that exploits both charge and 'spin' to carry data (thebest of two worlds). Spintronic devices such as spin valvetransistors, spin light emitting diodes, non-volatile memory,logic devices, optical isolators and ultra-fast opticalswitches are some of the areas of interest for introducing theferromagnetic properties at room temperature in a semiconductorto make it multifunctional. The potential advantages of suchspintronic devices will be higher speed, greater efficiency,and better stability at a reduced power consumption. This Thesis contains two main topics: In-depth understandingof magnetism in Mn doped ZnO,and our search and identificationof at least six new above room temperature ferromagneticsemiconductors. Both complex doped ZnO based new materials, aswell as a number of nonoxides like phosphides, and sulfidessuitably doped with Mn or Cu are shown to give rise toferromagnetism above room temperature. Some of the highlightsof this work arediscovery of room temperature ferromagnetismin     ZnO:Mn     ZnO doped with Cu (containing no magnetic elements init!)     GaP doped with Cu (again containing no magnetic elementsin it!)     Enhancement of Magnetization by Cu co-doping inZnO:Mn     CdS doped with Mn, and a few others not reported in thisthesis. We discuss in detail the first observation of ferromagnetismabove room temperature in the form of powder, bulk pellets, in2-3 μm thick transparent pulsed laser deposited films ofthe Mn (<4 at.% ) doped ZnO. High-resolution transmissionelectron microscopy (HRTEM) and electron energy lossspectroscopy (EELS) spectra recorded from 2 to 200nm areasshowed homogeneous distribution of Mn substituting for Zn a 2+state in the ZnO lattice. Ferromagnetic Resonance(FMR) technique is used to confirm the existence offerromagnetic ordering at temperatures as high as 425K. The abinitio calculations were found to be consistent with theobservation of ferromagnetism arising from fully polarized Mn 2+state. The key to observed room temperatureferromagnetism in this system is the low temperatureprocessing, which prevents formation of clusters, secondaryphases and the host ZnO from becoming n-type. The electronicstructure of the same Mn doped ZnO thin films studied usingXAS, XES and RIXS. revealed a strong hybridization between Mn3d and O 2p states, which is an important characterstic of aDilute magnetic Semiconductor (DMS). It is shown that the various processing conditions likesintering temperature, dopant concentration and the propertiesof precursors used for making of DMS have a great influence onthe final properties. Use of various experimental techniques toverify the physical properties, and to understand the mechanisminvolved to give rise to ferromagnetism is presented. Methodsto improve the magnetic moment in Mn doped ZnO are alsodescribed. New promising DMS materials (such as Cu doped ZnOare explored). The demonstrated new capability to fabricate powder,pellets, and thin films of room temperature ferromagneticsemiconductors thus makes possible the realization of a widerange of complex elements for a variety of new multifunctionalphenomena related to Spintronic devices as well asmagneto-optic components.