Abstract

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 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.

Highlights

  • In a comprehensive investigation of ZnO:Co thin films based on the combined measurement of macroscopic and microscopic properties, we find no evidence for carrier-mediated itinerant ferromagnetism

  • We review our results concerning the ZnO system which we obtained within the priority program 1285 (“Semiconductor Spintronics”) of the German Research Foundation (DFG)

  • We find the highest magnetization of up to 1.95 μB/Co for thin films deposited at Tsub = 500 ◦C

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Summary

Feature Article

Ferromagnetism above 300 K has been predicted for wide bandgap semiconductors such as ZnO or GaN substituted with magnetic ions [3] This marked the starting point of an exciting race for room temperature DMS, which pushed ZnO into the focus of materials research [34]. The clarification of the origin of the observed magnetism together with the identification of the control parameters is difficult and still a matter of controversy [43] This is partly caused by the problem that different experimental groups have published incongruous results, ranging from room temperature ferromagnetism [16, 35, 36, 39, 40, 52,53,54] to the absence of intrinsic DMS-type ferromagnetic interactions even at low temperatures [23, 24, 42, 55,56,57,58]. We analyzed the Zn0.95Co0.05O film properties in a first comprehensive investigation [23], combining SQUID magnetometry, XMCD, and AC susceptibility measurements with careful X-ray and high resolution transmission electron www.pss-b.com physica status solidi p s sb

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