Diluted magnetic semiconductors – which are semiconductors in which some of the non-magnetic cations have been replaced with paramagnetic ions – are often touted as being the ideal building blocks for spin-based electronics, or "spintronics". The materials are interesting thanks to the interactions between the magnetic dopants and mobile charge carriers (conduction-band electrons or valence-band holes). ZnO in particular has attracted a flurry of attention recently as a possible spintonics material for high-temperature applications. Until now, however, no-one had ever directly measured the dopant-electron interactions in these materials.

Daniel Gamelin and colleagues of the University of Washington, Seattle, together with Rudolf Bratschitsch and colleagues at the University of Konstanz, Germany, have used time-resolved Faraday rotation to measure the ultrafast spin dynamics in ZnO thin films with and without cobalt doping. The films were prepared by rapid solution processing, which allows for precise control of the magnetic dopant concentration in these materials. Indeed the concentrations of Co2+ can be finely controlled even at very low levels, say the researchers.

Measuring dopant-electron interaction strengths
Time-resolved Faraday rotation is a pump-probe spectroscopic technique that allows you to measure dopant-electron interaction strengths by probing the transient magnetization of a sample. The pump beam, which is circularly polarized, photoexcites spin-polarized electrons whose magnetic response is measured by looking at how they then rotate a linearly polarized probe beam. This phenomenon is called the Faraday effect.

"The electron spins precess around an externally applied magnetic field, leading to an oscillatory response of the Faraday rotation that slowly decays," explained Gamelin. The frequency of the oscillations is directly proportional to the effective "Landé g-factor" and the decay is characterized by the "spin dephasing", or decoherence, time constant, T2* – the time it takes for quantum-mechanical interference between spins to be destroyed. "The g-factor basically depends on the strength of the interactions between the magnetic dopants and electrons, and on the dopant concentration, while knowledge of T2* allows us to determine relevant dephasing processes in the material," he added.

Using this technique, the researchers obtained accurate values for the g-factor and the T2* in a series of ZnO-based thin films. By performing a systematic study on 10 samples with different doping levels, they were able to determine the mean-field electron-Co2+ exchange energy in Zn1–xCoxO for the first time as being +0.25 +/–0.02eV. They also found that minuscule amounts of Co2+ dopants greatly accelerate electron spin dephasing.

Room temperature observations
"Surprisingly, in undoped films, the T2* increases with increasing temperature, allowing spin precession to be observed even at room temperature!" Bratschitsch told nanotechweb.org. "Indeed this anomalous T2*, which is attributed to thermally activated hole trapping at grain surfaces – reaches 1.2 ns at room temperature. This is the longest room-temperature optically generated spin coherence time yet observed in any ZnO-based material."

The results represent the first direct measurement of carrier spin dynamics in any member of the transition-metal doped ZnO family of diluted magnetic semiconductors, he adds.

"Beyond providing fundamental insights into the ultrafast spin dynamics of magnetically doped ZnO, our work shows the potential of solution-processed thin films for new optoelectronic and spintronic device structures," said Bratschitsch. "And, because diluted magnetic semiconductor research has attracted attention in physics, chemistry and material science fields, we think that our work is of general interest to all of these areas."

The team now hopes to extend these studies to probe the interactions between other dopants and carriers in such solution-processed films, as well as to obtain more detailed information about spin-dephasing mechanisms in other complex materials.

The results were detailed in Nano Letters.