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Page 2 of 15 Keeney et al. Microstructures 2023;3:2023041 https://dx.doi.org/10.20517/microstructures.2023.41

technological potential of Aurivillius phase B6TFMO for future miniaturized memory storage devices. Next-
generation devices based on ultrathin multiferroic tunnel junctions are projected.

Keywords: Scanning probe microscopy, piezoresponse force microscopy, ultrathin films, chemical vapor
deposition, ferroelectrics, multiferroics, atomic force microscopy-based nano-machining, domains, domain walls

INTRODUCTION
As miniaturization of electronic devices continues, a key condition for materials in memory storage
technologies is the extension of their functional properties to ultrathin (sub-10 nm) thicknesses. This is
challenging for multiferroics and ferroelectrics, given that ferroelectricity is a collective effect between
competing short-range covalent repulsions and long-range Coulomb interactions. These interactions will
alter as the collective distortion of unit cells (u.c.) within the lattice is adjusted on decreased thickness, and
the spontaneous ferroelectric polarization is projected only to be supported above a particular critical
[1]
thickness . A consequence of fabricating low-dimensional ferroelectric nanostructures is an increase in
surface area, which can result in enlarged depolarization field strengths (of magnitude inversely
proportional to the thickness of the ferroelectric) if the surface charges are not completely screened. Below
the critical thickness, out-of-plane ferroelectricity is destabilized, tending to a decline in performance and a
[2]
lack of reproducibility. In 1997, this critical thickness was thought to be 20 nm ; however, considerable
advancements in thin film synthesis and characterization over the last two decades have enabled
[3]
demonstration of stable out-of-plane ferroelectricity in high-quality ultrathin films at dimensions much
thinner than this. Exploitation of epitaxial strain and confinement strain, along with effective screening of
interface and surface charges, can counteract the opposing effects of thickness reduction and stabilize out-
of-plane ferroelectricity at reduced dimensions. Out-of-plane ferroelectricity has been measured at a
minimum thickness of 4 nm (10 u.c.) for Pb(Zr Ti )O , 1.2 nm (3 u.c.) for PbTiO , 1 nm for BaTiO ,
[5]
[6]
[4]
3
0.8
3
0.2
3
and 1 nm (2 u.c.) for Hf Zr O .
[7]
0.2
0.8
2
Moreover, the problems pertaining to depolarization field effects are substantially diminished for in-plane
polarization directions. Polarization is lateral to the surface for all in-plane directions and is, therefore, not
impeded by contesting depolarization fields upon scaling down to ultrathin thicknesses . A family of
[8]
-2
materials possessing large in-plane spontaneous polarizations [P (≈ 0.4 C m )] are the Aurivillius phase
s
materials described by the general formula Bi O (A B O 3m+1 )) [9-14] . They are a well-established group of
m1 m
2
2
ferroelectrics and a technologically important class of material. SrBi Ta O (m = 2) and Bi La Ti O
0.75
12
2
3
2
9
3.25
(m = 3) thin films have been exploited for commercial use in ferroelectric random access memory
devices [15-18] . By intermixing differing types of A-site (Bi) and B-site (Ti, Mn, Fe) cations within the
Aurivillius phase scaffold, the chemistries required for ferroelectricity and ferromagnetism can be combined
in a single-phase to create the m = 5 B6TFMO(Bi Ti Fe Mn O ) composition (where x = 2.80 to 3.04;
z
18
y
x
6
y = 1.32 to 1.52; z = 0.54 to 0.64). B6TFMO is a valuable example of a single-phase room temperature (RT)
multiferroic joining other innovatively designed multiferroics such as the compositionally controlled tilt-
[19]
engineered perovskites and the 3D strained vertically aligned nanocomposites of polar and magnetic
materials [20,21] . B6TFMO displays saturation magnetization (M ) values (215 emu/cm ) two orders of
3
S
magnitude larger than widely-studied BiFeO and demonstrates the reversible linear RT magnetoelectric
[22]
3
switching necessary for practical applications in future high data-storage, multi-state memory storage
devices [23,24] . Furthermore, B6TFMO thin films are capable of hosting extraordinary polarization profiles,
including charged domain walls and non-trivial polar vortex-type topologies. This further opens up
technological prospects in nano-electronics, domain wall devices, and ultra-compact data storage .
[25]
Aurivillius layer structures [Figure 1A] are innately two-dimensionally nanostructured with large c-axis
[24]
lattice parameters (e.g., c = 50.26 Å for B6TFMO) . Thus, they are ideal candidates for probing ferroelectric
behavior close to the unit cell level [26-30] .