Multi-step ferroelectric switching and antiferromagnetic states in strained multiferroic phases

Abstract

The research on heteroepitaxial oxide thin films has not only enabled the physical properties of bulk phases to be effectively tuned by the magnitude of misfit strain but also led us to unexpected phases which possess emergent phenomena. Since the first successful epitaxial growth of high-quality bismuth ferrite thin films in 2003 the research on bismuth ferrite which simultaneously shows ferroelectricity and antiferromagnetism at room temperature has deepened our understanding of room-temperature multiferroic oxides through extensive follow-up studies on single-phase magnetoelectric effect, multiferroic domains and domain walls, piezoelectric effect, optical properties, novel multiferroic phases, and phase competition between two multiferroic phases. In addition, its potential for next-generation multifunctional and magnetoelectric devices beyond the current silicon-based nanodevices and memories has attracted many researchers. In this dissertation, the multi-step ferroelectric switching and the antiferromagnetic state of strained multiferroic phases are investigated in epitaxial bismuth ferrite (BiFeO$_{3}$, pseudocubic lattice parameter $a_{pc}$~3.965 $\AA$) thin films grown on gadolinium scandate (GdScO$_{3}$, $a_{pc}$~4.014 $\AA$) and lanthanum aluminate (LaAlO$_{3}$, $a_{pc}$~3.789 $\AA$) substrates using the pulsed laser deposition technique. Laser molecular beam epitaxy, hard x-ray scattering, neutron scattering, soft x-ray absorption spectroscopy and one-electron-Hamiltonian-based single-ion anisotropy calculation that are required to solve any scientific or/and technical issues in investigating strained multiferroic phases are explained. In order to solve a scientific question on the effects of misfit strain and external electric field on multi-step ferroelectric switching and to reveal the relation between the large strain gradient and the antiferromagnetic spin axis of phase-competition-driven mixed-phase nanostructures, four research achievements are described. Accordingly, this dissertation proposes new physical origins which enable us the electric-field control of antiferromagnetic spin states in strained multiferroic phases. In the case of $(110)_{pc}$-oriented bismuth ferrite thin films grown on gadolinium scandate substrates with a strontium ruthenate bottom electrode thinner than 1 nm, phase separation of two different multiferroic phases has been observed as a result of their similar ground state energies at a tensile-strain-driven morphotropic phase boundary of rhombohedral and orthorhombic bismuth ferrite. The crystal structure of two competing phases has been determined by analyzing x-ray reciprocal space maps. Through piezoresponse-force-microscope-based poling experiment, the electric-field-induced switching between the two multiferroic phases has been demonstrated. By employing light-polarization-dependent photoemission electron microscopy, the perpendicular relation between in-plane antiferromagnetic spin axes of the two phases has been revealed, which infers that the electrical 90$^{\circ}$ rotation of the in-plane antiferromagnetic spin axis is possible. This observation provides a pathway to magnetoelectric devices based on phase separation. When the strontium ruthenate bottom electrode becomes thicker than 10 nm, the phase separation between the two multiferroic phases disappears and a rhombohedral phase is stabilized in $(110)_{pc}$-oriented bismuth ferrite thin films deposited on gadolinium scandate substrates. Due to the anisotropic misfit strain from the substrates, a nonvolatile third intermediate ferroelectric state in the films can be created by applying an appropriate external field. Electrical switching among three stable out-of-plane polarizations which occurs via successive 71$^{\circ}$ or 109$^{\circ}$ ferroelastic switching has been realized by the use of an asymmetric external electric field at the step edge of a bottom electrode. Phenomenological Landau theory in conjunction with slow-scan-direction-dependent poling experiment using piezoresponse force microscopy has been employed in order to understand the role of anisotropic misfit strain and an in-plane electric field in stabilization of multiple ferroelectric states and their competition. This finding not only provides a useful insight into multistep ferroelectric switching in rhombohedral ferroelectrics but also introduces a new concept for multilevel polarization devices which possess higher storage density than the conventional ferroelectric memory devices. In addition, by stabilizing 71$^{\circ}$ or 109$^{\circ}$ ferroelastic switching which may accompany the rotation of antiferromagnetic easy plane in rhombohedral bismuth ferrite, the result raises the applicability of rhombohedral bismuth ferrite thin films to future magnetoelectric devices. It has been observed that a new room-temperature multiferroic phase, \textit{i.e.}, a highly-elongated tetragonal-like phase (c/a $\sim$ 1.27), is stabilized in epitaxial bismuth ferrite thin films deposited on lanthanum aluminate (001)\textsubscript{pc} substrates. In addition, when the film thickness of the films becomes thicker than $\sim$35 nm, the strain relaxation of the large misfit strain ($-$4.4\%) results in the creation of compressive-strain-driven morphotropic phase boundary (or mixed-phase boundary). The correlation between an electrically-written straight-stripe mixed-phase boundary and an antiferromagnetic spin axis in La-5\%-doped bismuth ferrite thin films deposited on lanthanum aluminate substrates has been discovered by performing polarization-dependent photoemission electron microscopy in conjunction with a cluster model calculation. A single-ion anisotropy calculation based on a one-electron Hamiltonian has been employed to investigate the microscopic origin of the observed correlation. This observation provides an alternative route toward an electric-field-induced rotation of antiferromagnetic spin axis in bismuth ferrite by 90$^{\circ}$ at room temperature. By stacking a ferromagnetic Co layer on straight-stripe mixed-phase boundaries of La-5\%-doped bismuth ferrite thin films, the exchange coupling between the ferromagnetic spins of Co and the spin state of mixed-phase nanostructures showing a large strain gradient is realized. The exchange coupling in the heterostructures causes the exchange anisotropy in the Co layer, and longitudinal magneto-optic Kerr effect microscopy has been employed to analyze the local exchange anisotropy. In order to compare the exchange anisotropy contribution with the shape anisotropy contribution to the total magnetic anisotropy, the analysis on a control sample where a nonmagnetic Ta spacer is inserted between the La-5\%-doped bismuth ferrite layer and the Co layer has been performed. The observed exchange anisotropy in the heterostructure has been well explained by the perpendicular alignment between the antiferromagnetic spins of the mixed-phase nanostructure and the ferromagnetic spins of the Co layer, which supports the observed correlation between an antiferromagnetic spin axis and a mixed-phase-boundary elongation axis of La-5\%-doped bismuth ferrite thin films.