Magnetoelectric Coupling in Single-Phase Multiferroics at Room Temperature via Scanning Probe Microscopy

In ceramic samples of the material, (BiFe1-xCoxO3)0.4-(Bi1/2K1/2TiO3)0.6 (BFCBKT),
a new kind of nano-sized region, so called multiferroic clusters (MFC)
were found at room-temperature. These exhibit the largest reported
magnetoelectric (ME) coupling coefficient, α ≈ 1.0 x 10-5 s/m (corresponding
to a Voltage coefficient dE/dH ≈ 1300 V/(cm Oe)) for a single phase
multiferroic so far, to the best of the author’s knowledge. Furthermore, they
are ferroelectric and ferrimagnetic at the same time, which has not yet been
observed directly in one material. Using a broad range of experimental
techniques, formation of the MFC has been understood in terms of the nonergodic
relaxor properties and ferrimagnetism in inherent Bi(Fe,Co)O3 rich
regions. Furthermore, the presence of magnetic nanoregions (MNR) was
confirmed and their formation and role in ME coupling are discussed.
Solid solutions of the system (PbZr0.52Ti0.48O3)1-x-(PbFe2/3W1/3O3)x (PZTPFW)
were prepared which, however, did not show ME effects.
Nevertheless, analogies to BFC-BKT concerning occurrence of
characteristic magnetic phases were established.
Since piezoresponse force microscopy (PFM) played an important role in
this study, the method was subject to detailed investigations concerning e.g.
the signal stability. Among others, it was found that it is highly beneficial to
use diamond coated tips in PFM.
In addition, a useful and easily applicable technique for distinguishable
marking of micro-sized areas has been developed.
It is expected that especially the findings on BFC-BKT will have wider
implications, as they enable an entirely new perspective for such relaxortype
ferrimagnetic multiferroics, which were barely considered in research so
far. The relaxor properties are expected to play an important role in the
strong ME coupling. Since the MFC are well-separated and offer large ME
coupling, they are suitable to serve as bits in an electrically controlled
magnetic nanodot storage device (MERAM). Various possibilities, the for
realization of such a device are discussed.