| dc.description | The anticipated end of Moore's law scaling in microelectronics has stimulated
researchers to explore alternatives, i.e. to render the individual components multifunctional. One promising field on which the researchers have focused in order to
explore the multifunctionality of the material is multiferroic (MF) magnetoelectric
(ME) materials.
The ideal MF-ME material for device applications would be a single phase material, with high ferroelectric (FE) and ferromagnetic (FM) responses, low leakage
current, and a strong coupling between FE and FM properties that allow the control of magnetic properties via electric elds and vice-versa, at room temperature.
Most existing MF-ME materials have drawbacks: (i) they are MF at low temperature; (ii) they have very low FE and/or FM response; and (iii) they have a low
value ME coupling coe cient for practical applications.
This work is an important development in the state-of-the-art that provides a
novel class of materials, by combining the best qualities of both lead iron tantalate (PFT), lead iron niobium (PFN) and lead iron titanate (PZT) to synthesize (PbZr0.53Ti0.47O3)1-x(PbFe0.5Ta0.5O3)x (PZTFT) (0.1 ≤ x ≤ 0:4) and
(PbZr0.53Ti0.47O3)1-y(PbFe0.5Nb0.5O3)y (PZTFN) (0:1 ≤ y ≤ 0:4), two single
phase ceramic systems that have MF and ME properties at room temperature.
The PZTFT and PZTFN materials were synthesized in ceramic form by the conventional solid state reaction route. The PZTFT and PZTFN ceramic samples are
single phase at the nanoscale, exhibit exceptional properties such as high dielectric
constant (ε ≈ 600-1200 for PZTFT, and ε ≈ 700-1600 for PZTFN), low dielectric loss (tan δ ≈ 0.04-0.18 for PZTFT, and tan δ ≈ 0.02-0.18 for PZTFN), well
saturated FE loop (Pr ≈ 12-15μ C/cm2 for PZTFT, and Pr ≈ 16-30μ C/cm2 for
PZTFN) and a well de ned FM loop (Ms ≈ 0.02-0.06 emu/gr for PZTFT, and
Ms ≈ 0.02-0.05 emu/gr for PZTFN). High ME values were obtained from direct
measurements and changes in FE domain patterns induced by the application of
magnetic and/or electric fields were observed from piezo-force microscopy images
that indirectly confirm the ME properties of these materials. It is generally assumed that if we combine two substances that are magnetic at
temperatures T1 and T2, into a single-phase material, the resulting material will
be magnetic up to some temperature T, which lies between T1 and T2. However,
when PFT (or PFN), where T1=150 K (or 143 K), was mixed with PZT, which
has no magnetic properties, we obtained a new single phase material PZTFT (or
PZTFN) where Tmagnetic > 400K. This result is contrary to the generally expected
result, nor is it obvious to a person skilled in the art. The reason is that PFT
(or PFN) has short-range clusters of magnetic order up to Tmagnetic. Nobody
had previously realized that this could be useful for magnetoelectricity. Similar
behaviour was observed in the FE ordering: PZT lost its FE properties at 667 K
and PFT at 250 K, however the PZTFT ceramic system lost its ferroelectricity
around 1250 K.
Another interesting aspect of the present work is related to the phase transitions,
the PZTFT materials exhibit the following sequential phase transitions on cooling:
cubic-tetragonal (ca. 1300 K), tetragonal-orthorhombic (i.e. 520 K for x=0.3 and
475 K for x=0.4) and orthorhombic-rhombohedral (i.e. 230 K for x=0.3 and 270
K for x=0.4). This is a sequence similar to that in barium titanate but different
to the parental PZT and PFT materials.
The aforementioned properties make these families perfect candidates for room
temperature, multiferroic devices.
Highly oriented PZTFT(x=0.4) thin lms were prepared by pulsed laser deposition. The PZTFT(x=0.4) lms show a near-room-temperature, frequency-dependent
dielectric maximum. The real and imaginary parts of Tm follow nonlinear Volger-Fulcher relations, implying a relaxor nature of the lm. AC conductivity
showed frequency-dependent kinks near Tm and frequency-dependent conductiv-
ity far above the phase transition temperature. High dielectric constant (ε ≈ 600-1200), low dielectric loss (tanδ 0.04-0.18), high polarization (Pr ≈ 70 μC/cm2),
and a weak magnetic moment (Ms ≈ 0.02-0.06 emu/gr) reveal that the multiferroic properties has been augmented for the PZTFT material in thin lm form.
This part of research provides thin lms of single phase PZTFT that can be covered by a conducting layer or layers of material to render it suitable for real life
applications including Ferroelectric Random Access Memories (FeRAMS). | |
