Manganite, Magnetite, and Double- Perovskite Thin Films and Heterostructures

Abstract

The physical properties of oxide systems cover a broad range of interesting properties, which have attracted the attention of technologists for quite some time. However, oxides have not yet made the expected impact on the high technology sector, primarily due to a relatively poorer control on their growth and properties in thin film form. This situation appears to be changing quite rapidly with increased use of in situ growth characterization and control, and the growing recognition of the uniqueness and value of the novel electronic, magnetic, and optical responses of oxides in the context of modem micro and nanoelectronics. In this chapter we address the developments in this domain, focusing primarily on the exploration of thin films and device configurations involving ferromagnetic oxides with high spin polarization. In a given ferromagnetic system, the number of carriers with spin up and spin down (w.r.t. the defining field direction) are unequal. The higher this differential the higher the spin polarization as per the definition: P -(Nt -N;)/(Nt + N|), where, Nt and Nj, are the numbers of spin up and spin down electrons. Materials with high spin polarization are considered key to the realization of a number of novel device concepts in the emerging field 154 of spintronics ^"^. This field envisages manipulation of the spin variable of electrons to control their transport in circuits and systems, in contrast to conventional electronics, w^hich uses only the charge property. There are certain specific advantages of spintronics over electronics, some emanating from longer spin relaxation lifetimes and diffusion lengths as compared to charge momentum relaxation times and lengths, which can only be harnessed if the corresponding devices can be realized in practice. This realization not only requires materials with very high (preferably 100%) spin polarization, but also the actual realization of their high spin polarization in a real device configuration involving surfaces and interfaces with other materials. Such other materials may be insulators used as barrier layers in tunneling devices, metals used as contacts or property separating layers, or semiconductors used to configure the signal processing electronics. Highly spin-polarized materials are also of interest in magnetic sensing applications requiring high sensitivity. Another application area of interest is magneto-optics devices. Amongst various ferromagnetic materials, which reportedly have or are predicted to have a high spin polarization, ferromagnetic oxides are of great interest because they can also support a host of interesting physical properties. If suitable oxides with very high spin polarization can be discovered/developed and their spin polarization is maintained in device architectures, a large number of possibilities could unfold for novel device concepts. One key advantage can be epitaxial integration of a magnetic oxide with semiconducting, metallic or superconducting oxides, which can be found within the same class of lattice/chemical systems. Moreover, successes have already been achieved in realizing epitaxial oxide layers on silicon^'^ and GaAs^'^, which can be used as templates for the integration of spintronic/oxide electronic devices with semiconductor electronics. Over the years, a number of magnetic oxides have been predicted to have high spin-polarization (even 100%) based on the mechanism of ferromagnetism and transport therein. For example, the double exchange process in mixed-valent manganites suggests that these materials should have 100% spin polarization^^'^^. Similar predictions have also been made regarding magnetite (Fe304)^'^'^^ and Cr02^^'^'^, with some evidence of experimental verifications. More recently, an exciting new development has occurred wherein high temperature ferromagnetism has been reported in some non-magnetic oxides dilutely doped with magnetic impurities. These so-called diluted magnetic semiconductor (DMS) oxides are currently the focus of intense scientific activity. Among the materials of interest are epitaxial films of Co and Fe doped Ti02^^ • ^^ Sn02^^ LSTO^^ and CuiO^^ Mn doped ZnO^^"^\ etc.