arXiv subjects

David L. Cortie

Publications and source records attributed to David L. Cortie.

16 recordsLinked to original sources

arXiv2026

Customized spin spirals in ferromagnetic thin films

The advancement of spintronic nanoscale devices hinges on the ability to flexibly engineer magnetic spin structures in thin-film stacks with precision and control. Meeting this demand remains a challenge for stable non-collinear spin configurations and, more specifically, vertical spin spirals in thin films. Innovative methods are required for their fabrication, stabilization and control. Here, we use oblique-incidence deposition to design and stabilize vertical spin spirals at room temperature and without an external field in magnetic thin films. We induce two crossed uniaxial magnetic anisotropies at the thin film boundaries. These anisotropies are tunable in direction and strength, thus providing control over the angular range and depth profile of the resulting spin spiral. The combination of polarized neutron reflectometry and nuclear resonant scattering enables precise and direct determination of the depth-dependent spin configurations. Our results establish a single-film design approach, in which the surface anisotropies independently serve as controllable design parameters for tailoring the vertical spin-spiral profile. Potential applications include nanoscale energy-storage devices, magnetic sensors, and ferromagnetic-resonance filters, advancing all-spin-based device engineering in general.

cond-mat.mtrl-sci↗

arXiv2026

Ferroelectric--Superconducting Interaction in Epitaxial YBa2Cu3O7-δ/BaTiO3 Films

Microscopic theories predict that the critical temperature of a superconducting layer can be strongly modified in proximity to a ferroelectric; however, experimental evidence is lacking. We report on BaTiO3 (BTO)/YBa2Cu3O7-δ (YBCO) heterostructures grown on SrTiO3 (001) substrates using pulsed laser deposition (PLD), with precise control of growth conditions and oxygen stoichiometry. Piezoresponse force microscopy confirms ferroelectric switching within both single-layer and heterostructured films. Neutron reflectometry measurements show low roughness parameters for the buried interfaces, which is critical for mediating the interfacial coupling between superconductivity and ferroelectric polarization. The superconducting transition temperature ($T_{\mathrm{C}}^{R}$) measured from the $R(T)$ curve of the single-layer YBCO was approximately 86 K, and a transition temperature ($T_{\mathrm{C}}^{M}$) of approximately 80-85 K was observed from the $M(T)$ curve. For the heterostructure film (YBCO/BTO/YBCO) grown under the same conditions, both resistivity and magnetization measurements indicate lower transition temperatures of approximately 75 K and approximately 55 K, respectively, as well as a lower overall susceptibility. These findings highlight the need to combine multiple techniques to characterize BTO/YBCO heterostructures and identify subtle features of the ferroelectric--superconducting coupling. This establishes BTO/YBCO heterostructures as promising systems for tunable oxide electronics and reconfigurable superconducting devices.

cond-mat.supr-con↗

arXiv2025

Phonon density of states of magnetite (\ce{Fe3O4}) nanoparticles via molecular dynamics simulations

This study presents a comprehensive computational investigation of magnetite nanoparticles, systematically evaluating a range of force fields against experimental results. We analyze the influence of particle size, temperature, and surface-adsorbed water molecules on the structural and dynamic properties of the nanoparticles. We performed classical molecular dynamics of nanoparticles and bulk magnetite and utilized density functional theory calculations for bulk magnetite for comparison. Our results reveal that nanoparticle size and the presence of adsorbed water molecules have a pronounced impact on the density of states. Specifically, as the nanoparticle size is decreased, phonon modes exhibit significant broadening and softening, which is attributable to reduced phonon lifetimes resulting from enhanced boundary scattering. The incorporation of water further broadens the density of states and extends the spectra to higher energy regions. Temperature variations result in a slight broadening and softening of the phonon density of states, particularly in the oxygen-dominated region, which is attributed to phonon anharmonicity.

cond-mat.mtrl-sci↗

arXiv2025

Tuning the Surface States of $Fe_3O_4$ Nanoparticles for Enhanced Magnetic Anisotropy and Induction Efficacy

Magnetite ($Fe_3O_4$) nanoparticles are crucial for biomedical applications, including magnetic hyperthermia, targeted drug delivery, and MRI contrast enhancement, due to their biocompatibility and unique physicochemical properties. Here, we investigate how surface states influence their induction performance. Heat treatment removes surface water and FeOOH, forming a $γ$-$Fe_2O_3$ shell, as confirmed by synchrotron powder diffraction, neutron powder diffraction, thermogravimetric analysis, X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, and time-of-flight inelastic neutron spectroscopy. AC magnetic susceptibility measurements reveal that this surface modification enhances magnetic anisotropy and reduces the spin relaxation time, leading to a 140% increase in the specific absorption rate. Additionally, the increased anisotropy suppresses the low-temperature clustered spin-glass transition and raises the blocking temperature. These findings highlight surface-state engineering as a powerful approach to optimizing $Fe_3O_4$ nanoparticles for biomedical applications.

cond-mat.mes-hall↗

arXiv2025

Lithium-ion dynamics in synthetic quartz studied via the NMR of implanted $^{8}$Li$^{+}$

We report $β$-detected nuclear magnetic resonance ($β$-NMR) measurements of implanted $^{8}$Li$^{+}$ in a synthetic single crystal of $α$-SiO$_2$ (quartz). At 6.55 Tesla, the spectrum is comprised of a large amplitude broad resonance and a quadrupolar multiplet that is only revealed by an RF comb excitation. The quadrupole splitting is surprisingly small, increases with temperature, and provides information on the implantation site. Supercell density functional theory calculations show that the small EFG is consistent with an in-channel interstitial site (Wyckoff 3$a$). The spin-lattice relaxation is unexpectedly fast and strongly temperature dependent with a diffusive peak above 200 K and a second more prominent relaxation peak at lower temperature. Analysis of the diffusive relaxation yields an activation barrier 178(43) meV for the isolated Li$^{+}$, in the range of other measurements and calculations. To account for many of the other features of the data, it is suggested that some of the implanted ions trap an electron forming the neutral Li$^{0}$, which is stable over a narrow range of temperatures.

cond-mat.mtrl-sci↗

arXiv2025

Liquid Metal-Exfoliated SnO$_2$-Based Mixed-dimensional Heterostructures for Visible-to-Near-Infrared Photodetection

Ultra-thin two-dimensional (2D) materials have gained significant attention for making next-generation optoelectronic devices. Here, we report a large-area heterojunction photodetector fabricated using a liquid metal-printed 2D $\text{SnO}_2$ layer transferred onto CdTe thin films. The resulting device demonstrates efficient broadband light sensing from visible to near-infrared wavelengths, with enhanced detectivity and faster photo response than bare CdTe photodetectors. Significantly, the device shows a nearly $10^5$-fold increase in current than the dark current level when illuminated with a 780 nm laser and achieves a specific detectivity of around $10^{12} \, \text{Jones}$, nearly two orders of magnitude higher than a device with pure CdTe thin film. Additionally, temperature-dependent optoelectronic testing shows that the device maintains a stable response up to $140^\circ \text{C}$ and generates distinctive photocurrent at temperatures up to $80^\circ \text{C}$, demonstrating its thermal stability. Using band structure analysis, density functional theory (DFT) calculations, and photocurrent mapping, the formation of a $p$-$n$ junction is indicated, contributing to the enhanced photo response attributed to the efficient carrier separation by the built-in potential in the hetero-junction and the superior electron mobility of 2D $\text{SnO}_2$. Our results highlight the effectiveness of integrating liquid metal-exfoliated 2D materials for enhanced photodetector performance.

cond-mat.mtrl-sci↗

arXiv2024

In-situ Investigation of the Phase Formation and Superconductivity in V$_3$Si Thin Films at High Temperatures

Vanadium silicide (V$_3$Si) is a promising superconductor for integration with silicon-based electronics, however the interfacial growth kinetics have a strong influence on the resulting superconducting properties and are not yet fully understood. In this study, we have used neutron reflectometry to reveal the phase transformation during thin film growth driven by different annealing strategies. We examined the silicide formation when a thin layer of vanadium undergoes reactive diffusion with a silicon dioxide film on silicon at temperatures from 650-800 °C. To further investigate the time evolution of different phases under various annealing temperatures, a chemical model was developed and subsequent simulations were performed. The results of this model were validated using X-ray diffraction and cross-sectional TEM analysis. Correlations were observed between the structure and superconducting properties. Over-annealing films leads to complete depletion of the SiO$_2$ barrier layer, forming diffuse interfaces and driving the formation of undesirable silicon-rich silicides. Avoiding this by controlling time and temperature, allows higher quality superconducting films to be achieved. The $T_c$ of the films was found to be 13 K, and the annealing conditions influenced the critical fields and the paramagnetic Meissner effect near $T_c$. For optimally-annealed films, superconducting order parameters were calculated. Ginzberg-Landau theory was applied to explain flux penetration.

cond-mat.supr-con↗

arXiv2024

Thermal Nanoquakes: Terahertz Frequency Surface Rayleigh Waves in Diamond Nanocrystals

Mechanical THz vibrations in nanocrystals have recently been harnessed for quantum sensing and thermal management. The free boundaries of nanocrystals introduce new surface wave solutions, analogous to the seismic waves on Earth, yet the implications of these surface waves on nanocrystals have remained largely unexplored. Here, we use atomistic molecular dynamics simulations and experimental neutron spectroscopy to elucidate these THz-scale features in nanodiamond. Our key insight is that thermally induced Rayleigh surface phonons, which have a low group velocity and an amplitude that decays exponentially away from the surface, are responsible for the previously observed but unexplained linear scaling of the low-energy vibrational density of states in nanocrystals. Large thermal atomic displacements, relative to the nanoparticle radius, induce perpetual surface quakes, even at ambient conditions. Normalised to the radius, the surface displacement ratio in diamond nanocrystals exceeds that of the largest recorded earthquakes by a factor of $10^{5\pm1}$. We explicate how these dramatic Rayleigh waves coexist with other distinctive features including confined lattice phonons, soft surface modes, the acoustic gap, Love waves, and Lamb modes, thereby offering a complete framework for the vibrational dynamics of nanocrystals.

cond-mat.mes-hall↗

arXiv2024

Two dimensional magnets: Forgotten history and recent progress towards spintronic applications

The recent discovery of two-dimensional magnetic order in van-der Waals materials has stimulated a renaissance in the field of atomically-thin magnets. This has led to promising demonstrations of spintronic functionality such as tunneling magnetoresistance. The frantic pace of this emerging research, however, has also led to some confusion surrounding the underlying phenomena of phase transitions in two-dimensional (2D) magnets. In fact, there is a rich history of experimental precedents beginning in the 1960s with quasi-2D bulk magnets and progressing to the 1980s using atomically-thin sheets of elemental metals. This review provides a holistic discussion of the current state of knowledge on the three distinct families of low-dimensional magnets: quasi-2D, ultra-thin films and van-der Waals crystals. It highlights the unique opportunities presented by the latest implementation in van-der Waals materials. By revisiting the fundamental insights from the field of low-dimensional magnetism, this review will highlight factors that can be used to enhance material performance. For example, the limits imposed on the critical temperature by the Mermin-Wagner theorem can be escaped in three separate ways: magnetocrystalline anisotropy, long range interactions and shape anisotropy. Several recent experimental reports of atomically-thin magnets with Curie temperatures above room temperature are highlighted.

cond-mat.mtrl-sci↗

arXiv2023

Depth-resolved measurement of the Meissner screening profile in a niobium thin film from spin-lattice relaxation of the implanted $β$-emitter $^{8}$Li

We report measurements of the Meissner screening profile in a Nb(300 nm)/Al$_{2}$O$_{3}$ thin film using $^{8}$Li $β$-detected nuclear magnetic resonance ($β$-NMR). The NMR probe $^{8}$Li was ion-implanted into the Nb film at energies $\leq$ 20 keV, corresponding to mean stopping depths comparable to Nb's magnetic penetration depth $λ$. $^{8}$Li's strong dipole-dipole coupling with the host $^{93}$Nb nuclei provided a "cross-relaxation" channel that dominated in low magnetic fields, which conferred indirect sensitivity to the local magnetic field via the spin-lattice relaxation (SLR) rate $1/T_{1}$. From a fit of the $1/T_{1}$ data to a model accounting for its dependence on temperature, magnetic field, and $^{8}$Li$^{+}$ implantation energy, we obtained a magnetic penetration depth $λ_{0}$ = 51.5(22) nm, consistent with a relatively short carrier mean-free-path $\ell$ = 18.7(29) nm typical of similarly prepared Nb films. The results presented here constitute an important step towards using $^{8}$Li $β$-NMR to characterize bulk Nb samples with engineered surfaces, which are often used in the fabrication of particle accelerators.

cond-mat.mtrl-sci↗

arXiv2023

Advice on describing Bayesian analysis of neutron and X-ray reflectometry

Driven by the availability of modern software and hardware, Bayesian analysis is becoming more popular in neutron and X-ray reflectometry analysis. The understandability and replicability of these analyses may be harmed by inconsistencies in how the probability distributions central to Bayesian methods are represented in the literature. Herein, we provide advice on how to report the results of Bayesian analysis as applied to neutron and X-ray reflectometry. This includes the clear reporting of initial starting conditions, the prior probabilities, and results of any analysis, and the posterior probabilities that are the Bayesian equivalent of the error bar, to enable replicability and improve understanding. We believe that this advice, grounded in our experience working in the field, will enable greater analytical reproducibility among the reflectometry community, as well as improve the quality and usability of results.

stat.AP↗

arXiv2022

Increased Phase Coherence Length in a Porous Topological Insulator

The surface area of Bi2Te3 thin films was increased by introducing nanoscale porosity. Temperature dependent resistivity and magnetotransport measurements were conducted both on as-grown and porous samples (23 and 70 nm). The longitudinal resistivity of the porous samples became more metallic, indicating the increased surface area resulted in transport that was more surface-like. Weak antilocalization (WAL) was present in all samples, and remarkably the phase coherence length doubled in the porous samples. This increase is likely due to the large Fermi velocity of the Dirac surface states. Our results show that the introduction of nanoporosity does not destroy the topological surface states but rather enhances them, making these nanostructured materials promising for low energy electronics, spintronics and thermoelectrics.

cond-mat.mtrl-sci↗

arXiv2020

Local electronic and magnetic properties of the doped topological insulators Bi$_{2}$Se$_{3}$:Ca and Bi$_{2}$Te$_{3}$:Mn investigated using ion-implanted $^{8}$Li $β$-NMR

We report $β$-detected nuclear magnetic resonance ($β$-NMR) measurements in Bi$_{2}$Se$_{3}$:Ca (BSC) and Bi$_{2}$Te$_{3}$:Mn (BTM) single crystals using $^{8}$Li$^{+}$ implanted to depths on the order of 100 nm. Above $\sim 200$ K, spin-lattice relaxation (SLR) reveals diffusion of $^{8}$Li$^{+}$, with activation energies of $\sim 0.4$ eV ($\sim 0.2$ eV) in BSC (BTM). At lower temperatures, the nuclear magnetic resonance (NMR) properties are those of a heavily doped semiconductor in the metallic limit, with Korringa relaxation and a small, negative, temperature-dependent Knight shift in BSC. From this, we make a detailed comparison with the isostructural tetradymite Bi$_{2}$Te$_{2}$Se (BTS) [McFadden et al., Phys Rev. B 99, 125201 (2019)]. In the magnetic BTM, the effects of the dilute Mn moments predominate, but remarkably the $^{8}$Li signal is not wiped out through the magnetic transition at 13 K, with a prominent critical peak in the SLR that is suppressed in a high applied field. This detailed characterization of the $^{8}$Li NMR response is an important step towards using depth-resolved $β$-NMR to study the low-energy properties of the chiral topological surface state (TSS). With the bulk NMR response now established in several Bi$_{2}$Ch$_{3}$ tetradymite topological insulators (TIs), the prospect of directly probing their chiral TSS using the depth resolution afforded by $β$-NMR remains strong.

cond-mat.mtrl-sci↗

arXiv2019

Local Metallic and Structural Properties of the Strongly Correlated Metal LaNiO$_{3}$ using $^{8}$Li $β$-NMR

We report $β$-detected NMR of ion-implanted $^{8}$Li in a single crystal and thin film of the strongly correlated metal LaNiO$_{3}$. In both samples, spin-lattice relaxation measurements reveal two distinct local metallic environments, as is evident from $T$-linear Korringa $1/T_{1}$ below 200 K with slopes comparable to other metals. A small, approximately temperature independent Knight shift of $\sim 74$ ppm is observed, yielding a normalized Korringa product characteristic of substantial antiferromagnetic correlations, but, we find no evidence for a magnetic transition from 4 to 310 K. Two distinct, equally abundant $^{8}$Li sites is inconsistent with the widely accepted rhombohedral structure of LaNiO$_{3}$, but cannot be simply explained by either of the common alternative orthorhombic or monoclinic distortions.

cond-mat.str-el↗

arXiv2019

Ionic and electronic properties of the topological insulator Bi$_2$Te$_2$Se investigated using $β$-detected nuclear magnetic relaxation and resonance of $^8$Li

We report measurements on the high temperature ionic and low temperature electronic properties of the 3D topological insulator Bi$_2$Te$_2$Se using ion-implanted $^8$Li $β$-detected nuclear magnetic relaxation and resonance. With implantation energies in the range 5-28 keV, the probes penetrate beyond the expected range of the topological surface state, but are still within 250 nm of the surface. At temperatures above ~150 K, spin-lattice relaxation measurements reveal isolated $^8$Li$^{+}$ diffusion with an activation energy $E_{A} = 0.185(8)$ eV and attempt frequency $τ_{0}^{-1} = 8(3) \times 10^{11}$ s$^{-1}$ for atomic site-to-site hopping. At lower temperature, we find a linear Korringa-like relaxation mechanism with a field dependent slope and intercept, which is accompanied by an anomalous field dependence to the resonance shift. We suggest that these may be related to a strong contribution from orbital currents or the magnetic freezeout of charge carriers in this heavily compensated semiconductor, but that conventional theories are unable to account for the extent of the field dependence. Conventional NMR of the stable host nuclei may help elucidate their origin.

cond-mat.mtrl-sci↗

arXiv2017

Microscopic Dynamics of Li$^{+}$ in Rutile TiO$_{2}$ Revealed by $^{8}$Li $β$-detected NMR

We report measurements of the dynamics of isolated $^{8}$Li$^{+}$ in single crystal rutile TiO$_{2}$ using $β$-detected NMR. From spin-lattice relaxation and motional narrowing, we find two sets of thermally activated dynamics: one below 100 K; and one at higher temperatures. At low temperature, the activation barrier is $26.8(6)$ meV with prefactor $1.23(5) \times 10^{10}$ s$^{-1}$. We suggest this is unrelated to Li$^{+}$ motion, and rather is a consequence of electron polarons in the vicinity of the implanted $^{8}$Li$^{+}$ that are known to become mobile in this temperature range. Above 100 K, Li$^{+}$ undergoes long-range diffusion as an isolated uncomplexed cation, characterized by an activation energy and prefactor of $0.32(2)$ eV and $1.0(5) \times 10^{16}$ s$^{-1}$, in agreement with macroscopic diffusion measurements. These results in the dilute limit from a microscopic probe indicate that Li$^{+}$ concentration does not limit the diffusivity even up to high concentrations, but that some key ingredient is missing in the calculations of the migration barrier. The anomalous prefactors provide further insight into both Li$^{+}$ and polaron motion.

cond-mat.mtrl-sci↗