Abstract:
This thesis presents studies on magnetic and magnetoelectric behavior of composite
multiferroics, materials that are strong candidates for next generation multifunctional
devices. These investigations have been carried out on matrix based composite
magnetoelectric multiferroics consisting of magnetostrictive cobalt ferrite (CoFe2O4)
and piezoelectric barium titanate (BaTiO3) or bismuth ferrite (BiFeO3), with the
magnetostrictive phase embedded in a three dimensional matrix of the piezoelectric
phase.
The direct magnetoelectric (ME) effect has been investigated in 0-3 composites of
BaTiO3 (BTO) and CoFe2O4 (CFO). The ME response has been found to depend non-
linearly on the ratio of the magnetostrictive (CFO) content and it is proportional to the
strength of the applied magnetic field. The non-linear behavior is explained in terms
of the compound effect of the increasing CFO content and the number density of
CFO-BTO interfaces.
Epitaxial self-assembled nanocomposite thin films were grown by laser ablating
targets of magnetostrictive (CoFe2O4) and piezoelectric (BiFeO3 or BaTiO3) phases,
on (001) oriented substrates using a combinatorial approach. The surface morphology
of the nanocomposite films reveal CFO nanopillars protruding out of a flat
piezoelectric matrix. Structural properties were investigated by a four-circle
diffractometer, which revealed the presence of the epitaxially grown phases and was
used to find the in-plane and out-of-plane lattice parameters of the constituent phases.
The magnetization hysteresis loops were measured by a SQUID magnetometer at
room temperature. The highly magnetostrictive CFO phase under epitaxial strain from
the piezoelectric matrix, exhibits a strong perpendicular anisotropy in the magnetic
properties. Changes in the magnetic anisotropy were investigated under different
strain conditions induced by (i) post growth annealing, (ii) exploiting phase transitions
in the BaTiO3 substrate, and (iii) applying an electric field to the electromechanical
substrates.
It has been shown that by exploiting the hysteretic properties of a suitable
electromechanical substrate, one can control volatile or non-volatile magnetic states
by means of an electric field. These results make an important contribution towards
the understanding and potential applications of magnetoelectric multiferroics.