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1. Introduction
Smart and powerful electronic devices enrich our daily life. Computers, mobile phones and
entertainment devices are performing gigantic amounts of logical operations and saving Terabytes
of data. This stunning development of modern information technology largely relies on
intelligent device concepts and functional materials. First and foremost, semiconductors are
used for logic operations, i. e. the manipulation of charge currents, which enable electrical
switching in transistors, diodes, etc. Ferromagnets, in contrast, provide the non-volatile spin
degree of freedom, which is the basis for long-term data storage in computer hard disks.
A milestone for information technology was the discovery of the giant magnetoresistance
(GMR), 1 which gave birth to the prospering field of spintronics. 2 By a combination of ultrathin
ferromagnetic and non-magnetic (NM) layers, the GMR allows one to electrically read
out magnetic information. 219 Substituting the NM layer by an insulating tunnel barrier has
significantly increased the magnetoresistance in these ferromagnetic multilayer structures. 3
In this way, the tunnel magnetoresistance (TMR) is the basis for nowadays devices, such as
hard disk read heads or magnetic random access memory (MRAM). 4 The significant technological
impact of these spintronic devices impressively illustrates the recent progress of
spintronics research.
A further important step in spintronics is the integration of ferromagnets with semiconductors,
which exploits the non-volatile memory functionality of ferromagnets and the ability
to process information of semiconductor logics in a single device. A model system for this
combination is the spin-field effect transistor (spin-FET), which has been proposed by Datta
and Das (1990). 5 The spin-FET concept relies on the injection of spin-polarized electrons
through a ferromagnet into silicon and their subsequent electrical manipulation and detection.
Economically, this is advantageous, since switching the spin information is expected
to cost much less energy than switching a conventional charge-based transistor. This makes
silicon spintronics 6,7 a potentially energy saving alternative information technology.
Materials with high electron spin polarization are the key for efficiently operating spintronic
devices. In particular, ferromagnetic insulators can provide a spin-polarization of up to 100%
when utilized as spin-functional tunnel contacts. Thus, magnetic tunnel contacts integrated
with silicon provide one possible route towards high-efficiency spin-FETs in silicon spintronics.
For spin-functional tunneling, a magnetic tunnel contact must have particular properties:
(i) The magnetic oxide has to provide an unoccupied conduction band above the Fermi
level, which is spin-split by exchange interaction. This permits spin-dependent tunneling by
the so-called spin filter effect. 10 (ii) When combining a ferromagnetic oxide with Si, one has
to account for its thermodynamic stability. This requires the control of interface oxidation
and diffusion between these highly reactive compounds. (iii) Spin injection into silicon using
metallic ferromagnets is of negligible efficiency due to a large conductance mismatch. 11
Recent endeavors are pushing forward ferromagnetic insulators for all-oxide spintronics in which ferromagnetic
and ferroelectric properties may be functionally interrelated. 8,9
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