Figure 6B shows an EPR spectrum of Zn1-xCoxO (x )
0.021) nanowires at various temperatures. In Figure 6C, the
reciprocal intensity of the EPR signal is plotted versus
temperature. A clear deviation from Curie law behavior is
seen in the temperature range 25-30 K, with a similar
deviation from paramagnetic behavior seen in SQUID
magnetometry experiments. Although characterization of the
cobalt-doped ZnO wires suggests uniform doping, we note
the existence of complex magnetic behavior that raises the
possibility of multiple magnetic phases. Transition-metaldoped
ZnO is predicted to be ferromagnetic at room
temperature,61,62 but experimental observations on the magnetism
vary widely, ranging from no magnetic ordering63 to
superparamagnetic behavior64 to ferromagnetism above 500
K.65 The conflicting reports suggest either that magnetism
cannot be modeled by a simple ferromagnetic or paramagnetic
mechanism or that sample quality is variable in these
cases. This inconsistency is the subject of ongoing research