Zinc oxide : from dilute magnetic doping to spin transport (Phys. Status Solidi B 9/2014)

Matthias Opel, Sebastian T. B. Goennenwein, Matthias Althammer, Karl-Wilhelm Nielsen, Eva-Maria Karrer-Müller, Sebastian Bauer, Konrad Senn, Christoph Schwark, Christian Weier, Gernot Güntherodt, Bernd Beschoten, Rudolf Gross
2014 Physica status solidi / B (2019). doi:10.1002/pssb.201900269  
This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. During the past years, there has been renewed interest in the wide-bandgap II-VI semiconductor ZnO, triggered by promising prospects for spintronic applications. First, ferromagnetism was predicted for dilute magnetic doping. In a comprehensive investigation of ZnO:Co thin films based on the
more » ... s based on the combined measurement of macroscopic and microscopic properties, we find no evidence for carrier-mediated itinerant ferromagnetism. Phase-pure, crystallographically excellent ZnO:Co is uniformly paramagnetic. Superparamagnetism arises when phase separation or defect formation occurs, due to nanometer-sized metallic precipitates. Other compounds like ZnO:(Li,Ni) and ZnO:Cu do not exhibit indication of ferromagnetism. Second, its small spin-orbit coupling and correspondingly large spin coherence length makes ZnO suitable for transporting or manipulating spins in spintronic devices. From optical pump/optical probe experiments, we find a spin dephasing time of the order of 15 ns at low temperatures, which we attribute to electrons bound to Al donors. In all-electrical magnetotransport measurements, we successfully create and detect a spin-polarized ensemble of electrons and transport this spin information across several nanometers. We derive a spin lifetime of 2.6 ns for these itinerant spins at low temperatures, corresponding well to results from an electrical pump/optical probe experiment. 1 ZnO for spintronic applications Classical semiconductor-based devices rely on the controlled transport and storage of electrical charge. In semiconductor spintronics, however, both the electrons' charge and spin degree of freedom are exploited, providing fascinating perspectives for novel devices with improved performance. Moreover, since the spin degree of freedom usually is coupled much more weakly to the environment than the charge degree of freedom, spin-based devices are promising for future quantum devices making use of quantum superposition and entanglement. Therefore, both classical and quantum spintronics are rapidly growing areas of basic and applied research [1] . More than two decades ago, the exploitation of the spin degree of freedom in electronic devices started with the field of magnetoelectronics, in which ferromagnetic metals have been used in passive spin-dependent devices like spin valves, magnetic tunnel junctions, or magnetic sensors [2] . Magnetoelectronics then has been extended to the much broader field of spintronics, which is considered a replacement technology providing improved spin-active devices with the goal to encode information in single spins. The control of these spins (read, write, transport, and manipulate) is realized by magnetic fields, photonic fields, electric fields, or electric currents [1] . With regard to materials and starting from metal-based devices in the late 1980s, the field of spintronics soon expanded to the use of transition-metal oxides [3] [4] [5] [6] [7] . Within the past two decades, the growth of high-quality oxide thin films and heterostructures progressed enormously concerning sample and interface quality [8] [9] [10] . Among the various material systems for spintronic applications, the II-VI semiconductor ZnO [11] is of interest due to several reasons. First,
doi:10.18154/rwth-2015-00182 fatcat:lpy3r37yavaw7o5c4vcxtwnucm