Mechanism of Polarization Fatigue in BiFeO3: the Role of Schottky Barrier

By using piezoelectric force microscopy and scanning Kelvin probe microscopy, we have investigated the domain evolution and space charge distribution in planar BiFeO3 capacitors with different electrodes. It is observed that charge injection at the film/electrode interface leads to domain pinning and polarization fatigue in BiFeO3. Furthermore, the Schottky barrier at the interface is crucial for the charge injection process. Lowering the Schottky barrier by using low work function metals as the electrodes can also improve the fatigue property of the device, similar to what oxide electrodes can achieve

Variable Sample Size Control Charts for Monitoring the Multivariate Coefficient of Variation Based on Median Run Length and Expected Median Run Length

The monitoring of a well-functioning process system has always held significant importance. In recent times, there has been notable attention towards employing control charts to oversee both univariate and multivariate coefficients of variation (MCV). This shift is in response to the concern of erroneous outcomes that can arise when traditional control charts are applied under the condition of dependent mean and standard deviation, as highlighted by prior research. To address this, the remedy lies in adopting the coefficient of variation. Furthermore, this study underscores the application of MCV in scenarios where multiple quality attributes are simultaneously under surveillance within an industrial process. This aspect has demonstrated considerable enhancement in chart performance, especially when incorporating the variable sample size (VSS) feature into the MCV chart. Adaptive VSS, evaluated through metrics like median run length (MRL) and expected median run length (EMRL), is also integrated for MCV monitoring. In contrast to earlier studies that predominantly focused on average run length (ARL), this research acknowledges the potential inaccuracies in ARL measurement. In this study, two optimal designs for VSS MCV charts are formulated by minimizing two criteria: firstly, MRL; and secondly, EMRL, both accounting for deterministic and unknown shift sizes. Additionally, to assess the distribution's variability in run lengths, the study provides the 5th and 95th percentiles. The research delves into two VSS schemes: one with a defined small sample size (nS), and another with a predetermined large sample size (nL) for the initial subgroup (n(1)). The approach taken involves the development of a Markov chain method for designing and deriving performance measures of the proposed chart. These measures include MRL and EMRL. Moreover, a comparative analysis between the proposed chart's performance and the standard MCV chart (STD) is presented in terms of MRL and EMRL criteria. The outcomes illustrate the superiority of the proposed chart over the STD MCV chart for all shift sizes, whether they are upward or downward, and when n(1) equals nS or nL

Scanning SQUID-on-tip microscope in a top-loading cryogen-free dilution refrigerator

The scanning superconducting quantum interference device (SQUID) fabricated on the tip of a sharp quartz pipette (SQUID-on-tip) has emerged as a versatile tool for nanoscale imaging of magnetic, thermal, and transport properties of microscopic devices of quantum materials. We present the design and performance of a scanning SQUID-on-tip microscope in a top-loading probe of a cryogen-free dilution refrigerator. The microscope is enclosed in a custom-made vacuum-tight cell mounted at the bottom of the probe and is suspended by springs to suppress vibrations caused by the pulse tube cryocooler. Two capillaries allow in-situ control of helium exchange gas pressure in the cell that is required for thermal imaging. A nanoscale heater is used to create local temperature gradients in the sample, which enables quantitative characterization of the relative vibrations between the tip and the sample. The spectrum of the vibrations shows distinct resonant peaks with maximal power density of about 27 nm/Hz$^{1/2}$ in the in-plane direction. The performance of the SQUID-on-tip microscope is demonstrated by magnetic imaging of the MnBi$_2$Te$_4$ magnetic topological insulator, magnetization and current distribution imaging in a SrRuO$_3$ ferromagnetic oxide thin film, and by thermal imaging of dissipation in graphene.Comment: Submitted to Review of Scientific Instrument

MAGNETISM WITH STRONG SPIN-ORBIT COUPLING IN PEROVSKITE IRIDATES

Ph.DDOCTOR OF PHILOSOPH

A Robust High-temperature Multiferroic Device with Tunable Topological Hall Effect

We fabricated Pt/La0.5Ba0.5MnO3 ultrathin films with integration to the PbZr0.2Ti0.8O3 ferroelectric. Strong Topological Hall Effect can be measured across a wide temperature range, which can be turned on and off corresponding to the ferroelectric polarization switching driven by short voltage pulses, indicating creation and annihilation of magnetic Skyrmions. However, its magnetoelectric coupling polarity was found to be opposite to the earlier consensus, which involves a phase transition between ferromagnetic and A-type antiferromagnetic in manganites due to electrostatic doping into the eg-orbitals by the ferroelectric bound charges. In our case, we propose a novel yet complementary picture where the transition occurs between C-type antiferromagnetic and ferromagnetic. The distinction between the earlier consensus and our picture lies at the tetragonal distortion of the manganite unit cell, resulting in an opposite magnetoelectric coupling polarity. We support the perceived relationship between the phase transition and tuning of Topological Hall Effect by micromagnetic simulations.Comment: 12 pages, 3 figure

Skyrmionics in correlated oxides

10.1557/s43577-021-00227-9MRS Bulletin46111053-106

Switchable photovoltaic response from polarization modulated interfaces in BiFeO3 thin films

The switchable photovoltaic effect in BiFeO3 thin films capacitors has been studied extensively. However, the origin of the photovoltaic response is still under debate. Both bulk depolarization field and interface effects have been used to explain the observations. In this work, we fabricate BiFeO3 epitaxial films on SrTiO3 substrate with La0.7Sr0.3MnO3 and Pt as electrodes. Much larger switchable photovoltaic response can be observed in the Pt/BiFeO3/La0.7Sr0.3MnO3 samples, as compared with La0.7Sr0.3MnO3/BiFeO3/La0.7Sr0.3MnO3. Moreover, the photovoltaic voltage of the Pt/BiFeO3/La0.7Sr0.3MnO3 samples is nearly independent of the thickness of the La0.7Sr0.3MnO3 bottom electrode. We suggest that the Schottky barrier modulation by ferroelectric polarization at the Pt/BiFeO3 interface is mainly responsible for the photovoltaic effect, with very small contribution from the bulk depolarization field.Published versio

Origin of the uniaxial magnetic anisotropy in La0.7Sr0.3MnO3 on stripe-domain BiFeO3

The La0.7Sr0.3MnO3/BiFeO3 (LSMO/BFO) heterostructure has been a model system to study the interaction between ferroic order parameters at a complex oxide interface. In this study, uniaxial magnetic anisotropy is artificially induced in LSMO thin film grown on BFO with electrically patterned stripe domains. Variable-field magnetic force microscopy is exploited to investigate the in situ magnetic switching dynamics and subsequently determine the magnetic easy axis of the LSMO thin film. Intriguingly, one-to-one correspondence between the magnetization of LSMO and the polarization of BFO is found. The observed uniaxial magnetic anisotropy is attributed to the magnetocrystalline anisotropy of the LSMO, which is induced by the shear strain of the BFO lattice, rather than the interfacial magnetic coupling which would be more naturally assumed. This finding highlights the crucial role of lattice coupling at a complex oxide interface. When multiple-order parameters come into play at the heterointerface, special care is needed to deconvolute their effects on the related physical properties.NRF (Natl Research Foundation, S’pore)MOE (Min. of Education, S’pore)Published versio

Oxygen Octahedral Tilt Controlled Topological Hall Effect in Epitaxial and Freestanding SrRuO3/SrIrO3 Heterostructures

The fabrication technique of freestanding oxide flakes by epitaxial lift-off has made significant contributions to the multifunctional oxide thin film research. Several highly impactful work have recently demonstrated the robustness of freestanding oxide flakes retaining their desirable properties after detachment from the substrate by dissolving away the water-soluble Sr3Al2O6 buffer layer. Firstly, in epitaxial SrRuO3/SrIrO3 systems showing Hall-humps reminiescence of Topological Hall Effect, first-principle calculations revealed that octahedral tilts can modify the sign of Dzyaloshinskii-Moriya interaction and avoid cancellation in a symmetric trilayer structure. Secondly, freestanding flakes of the trilayer is also able to retain the Hall-humps across a wide temperature range. The behaviour of humps' peak field does not vary with field direction rotation away from surface normal, consistent to a micromagnetic simulation result of N\'eel-type magnetic Skyrmions. The layer-resolved octahedral tilts in the freestanding heterostructures also crucially controls the occurrence of Hall-humps, consistent to the insight from the epitaxial ones. This work offers a new perspective to understanding Hall-humps in perovskite oxides, as well as demonstrates the fabrication of oxide heterostructure membranes with high interfacial quality