arXiv subjects

J. P. Whitehead

Publications and source records attributed to J. P. Whitehead.

14 recordsLinked to original sources

arXiv2023

A tale of two faults: Statistical reconstruction of the 1820 Flores Sea earthquake using tsunami observations alone

Using a Bayesian approach we compare anecdotal tsunami runup observations from the 29 December 1820 Flores Sea earthquake with close to 200,000 tsunami simulations to determine the most probable earthquake parameters causing the tsunami. Using a dual hypothesis of the source earthquake either originating from the Flores Thrust or the Walanae/Selayar Fault, we found that neither source perfectly matches the observational data, particularly while satisfying seismic constraints of the region. However, there is clear quantitative evidence that a major earthquake on the Walanae/Selayar Fault more closely aligns with historical records of the tsunami, and earthquake shaking. The simulated data available from this study alludes to the potential for a different source in the region or the occurrence of an earthquake near where both faults potentially merge and simultaneously rupture similar to the 2016 Kaikoura, New Zealand event.

physics.geo-ph↗

arXiv2019

Rigorous bounds on the heat transport of rotating convection with Ekman pumping

We establish rigorous upper bounds on the time-averaged heat transport for a model of rotating Rayleigh-Benard convection between no-slip boundaries at infinite Prandtl number and with Ekman pumping. The analysis is based on the asymptotically reduced equations derived for rotationally constrained dynamics with no-slip boundaries, and hence includes a lower order correction that accounts for the Ekman layer and corresponding Ekman pumping into the bulk. Using the auxiliary functional method we find that, to leading order, the temporally averaged heat transport is bounded above as a function of the Rayleigh and Ekman numbers Ra and Ek according to $Nu \leq 0.3704 Ra^2 Ek^2$. Dependent on the relative values of the thermal forcing represented by $Ra$ and the effects of rotation represented by $Ek$, this bound is both an improvement on earlier rigorous upper bounds, and provides a partial explanation of recent numerical and experimental results that were consistent yet surprising relative to the previously derived upper bound of $Nu \lesssim Ra^3 k^4$.

math-ph↗

arXiv2019

Triangular array of iron-oxide nanoparticles: A simulation study of intra- and inter-particle magnetism

A study of spherical maghemite nanoparticles on a two dimensional triangular array was carried out using a stochastic Landau-Lifshitz-Gilbert (sLLG) approach. The simulation method was first validated with a triangular array of simple dipoles, where results show the expected phase transition to a ferromagnetic state at a finite temperature. The ground state exhibited a continuous degeneracy that was lifted by an order-from-disorder mechanism at infinitesimal temperatures with the appearance of a six-fold planar anisotropy. The nanoparticle array consisted of 7.5 nm diameter maghemite spheres with bulk-like superexchange interactions between Fe-ions in the core, and weaker exchange between surface Fe-ions and a radial anisotropy. The triangular nanoparticle array ordered at the same reduced temperature as the simple dipole array, but exhibited different behaviour at low temperatures due to the surface anisotropy. We find that the vacancies on the octahedral sites in the nanoparticles combine with the surface anisotropy to produce an effective random temperature-dependent anisotropy for each particle. This leads to a reduction in the net magnetization of the nanoparticle array at zero temperature compared to the simple dipole array.

cond-mat.mes-hall↗

arXiv2019

Data Assimilation in Large-Prandtl Rayleigh-Bénard Convection from Thermal Measurements

This work applies a continuous data assimilation scheme---a particular framework for reconciling sparse and potentially noisy observations to a mathematical model---to Rayleigh-Bénard convection at infinite or large Prandtl numbers using only the temperature field as observables. These Prandtl numbers are applicable to the earth's mantle and to gases under high pressure. We rigorously identify conditions that guarantee synchronization between the observed system and the model, then confirm the applicability of these results via numerical simulations. Our numerical experiments show that the analytically derived conditions for synchronization are far from sharp; that is, synchronization often occurs even when the conditions of our theorems are not met. We also develop estimates on the convergence of an infinite Prandtl model to a large (but finite) Prandtl number generated set of observations. Numerical simulations in this hybrid setting indicate that the mathematically rigorous results are accurate, but of practical interest only for extremely large Prandtl numbers.

physics.flu-dyn↗

arXiv2018

Impact of further-range exchange and cubic anisotropy on magnetic excitations in the fcc kagome antiferromagnet IrMn3

Exchange interactions up to fourth nearest neighbor are shown within a classical local-moment Heisenberg approach to be important to model inelastic neutron scattering data on the fcc kagome antiferromagnet IrMn$_3$. Spin wave frequencies are calculated using the torque equation and the magnetic scattering function, $S({\bf Q},ω)$, is determined by a Green's function method, as an extension of our previous work, LeBlanc et al, Phys. Rev. B 90, 144403 (2014). Results are compared with intensity contour data on powder samples of ordered IrMn$_3$, where magnetic Mn ions occupy lattice sites of ABC stacked kagome planes. Values of exchange parameters taken from DFT calculations used in our model provide good agreement with the experimental results only if further-neighbor exchange is included. Estimates of the observed energy gap support the existence of strong cubic anisotropy predicted by DFT calculations.

cond-mat.str-el↗

arXiv2017

Dipolar ferromagnetism in three-dimensional superlattices of nanoparticles

A series of atomistic finite temperature simulations on a model of an FCC lattice of maghemite nanoparticles using the stochastic Landau-Lifshitz-Gilbert (sLLG) equation are presented. The model exhibits a ferromagnetic transition that is in good agreement with theoretical expectations. The simulations also reveal an orientational disorder in the orientational order parameter for $T < 0.5 T_c$ due to pinning of the surface domain walls of the nanoparticles by surface vacancies. The extent of the competition between surface pinning and dipolar interactions provides support for the conjecture that recent measurements on systems of FCC superlattices of iron-oxide nanoparticles provide evidence for dipolar ferromagnetism is discussed.

cond-mat.mes-hall↗

arXiv2017

Kinetic Monte Carlo Simulations of the Effect of the Exchange Control Layer Thickness in CoPtCrB/CoPtCrSiO Granular Media

A hybrid kMC/LLG algorithm is used to simulate experimental MH hysteresis loops for dual layer ECC media. The calculation of the rate coefficients and difficulties arising from low energy barriers, a fundamental problem of the kMC method, are discussed and the methodology used to treat them in the present work is described. The results from simulations are compared with experimental data on dual layer ECC CoPtCrB/CoPtCrSiO media. A quantitative relationship between the thickness of the exchange control layer and the effective exchange constant between the layers is demonstrated.

cond-mat.mtrl-sci↗

arXiv2015

Ergodicity in Randomly Forced Rayleigh-Bénard Convection

We consider the Boussinesq approximation for Rayleigh-Bénard convection perturbed by an additive noise and with boundary conditions corresponding to heating from below. In two space dimensions, with sufficient stochastic forcing in the temperature component and large Prandtl number, we establish the existence of a unique ergodic invariant measure. In three space dimensions, we prove the existence of a statistically invariant state, and establish unique ergodicity for the infinite Prandtl Boussinesq system. Throughout this work we provide streamlined proofs of unique ergodicity which invoke an asymptotic coupling argument, a delicate usage of the maximum principle, and exponential martingale inequalities. Lastly, we show that the background method of Constantin-Doering [CD96] can be applied in our stochastic setting, and prove bounds on the Nusselt number relative to the unique invariant measure.

math.AP↗

arXiv2015

Monte Carlo Simulations of ABC Stacked Kagome Lattice Thin Films

Properties of thin films of geometrically frustrated ABC stacked antiferromagnetic kagome layers are examined using Metropolis Monte Carlo simulations. The impact of having an easy-axis anisotropy on the surface layers and cubic anisotropy in the interior layers is explored. The spin structure at the surface is shown to be different from that of the bulk 3D fcc system, where surface axial anisotropy tends to align spins along the surface [111] normal axis. This alignment tendency then propagates only weakly to the interior layers through exchange coupling. Results are shown for the specific heat, magnetization and sub-lattice order parameters for both surface and interior spins in three and six layer films as a function of increasing axial surface anisotropy. Relevance to the exchange bias phenomenon in IrMn$_3$ thin films is discussed.

cond-mat.str-el↗

arXiv2015

Thermally activated switching at long time scales in exchange-coupled magnetic grains

Rate coefficients of the Arrhenius-Neel form are calculated for thermally activated magnetic moment reversal for dual layer exchange-coupled composite (ECC) media based on the Langer formalism and are applied to study the sweep rate dependence of MH hysteresis loops as a function of the exchange coupling I between the layers. The individual grains are modelled as two exchange coupled Stoner-Wohlfarth particles from which the minimum energy paths connecting the minimum energy states are calculated using a variant of the string method and the energy barriers and attempt frequencies calculated as a function of the applied field. The resultant rate equations describing the evolution of an ensemble of non-interacting ECC grains are then integrated numerically in an applied field with constant sweep rate and the magnetization calculated as a function of the applied field H. MH hysteresis loops are presented for a range of values I and a figure of merit (FOM) that quantifies the advantages of ECC media is proposed. The results are also used to examine the accuracy of certain approximate models that reduce the complexity associated with the Langer based formalism and which provide some useful insight into the reversal. Of particular interest is the clustering of minimum energy states that are separated by relatively low energy barriers into "metastates." It is shown that while approximating the reversal process in terms of "metastates" results in little loss of accuracy, it can reduce the run time of a Kinetic Monte Carlo (KMC) simulation of the magnetic decay of an ensemble of dual layer ECC media by 2~3 orders of magnitude. The essentially exact results presented in this work for two coupled grains are analogous to the Stoner-Wohlfarth model of a single grain and serve as an important precursor to KMC based simulation studies on systems of interacting dual layer ECC media.

cond-mat.mtrl-sci↗

arXiv2014

Spin waves in the anisotropic fcc kagome antiferromagnet

Spin wave calculations demonstrate that the macroscopic continuous degeneracy associated with the two-dimensional kagome Heisenberg spin lattice persists in the case of the stacked fcc structure giving rise to zero energy modes in three dimensions. The addition of an effective local cubic anisotropy is shown to remove this continuous degeneracy and introduce a gap in the spectrum as well as modify the inelastic scattering function $S({\bf q},ω)$. This scenario supports earlier Monte Carlo simulations which indicate that the phase transition to long range $q=0$ magnetic order is driven to be discontinuous by critical fluctuations associated with the large degeneracy in the absence of anisotropy, but becomes continuous with the addition of anisotropy. The results are relevant to Ir-Mn alloys which are widely used in the magnetic storage industry in thin film form as the antiferromagnetic pinning layer in spin-valve structures.

cond-mat.str-el↗

arXiv2012

Micromagnetic simulations of sweep-rate dependent coercivity in perpendicular recording media

The results of micromagnetic simulations are presented which examine the impact of thermal fluctuations on sweep rate dependent coercivities of both single-layer and exchange-coupled-composite (ECC) perpendicular magnetic recording media. M-H loops are calculated at four temperatures and sweep rates spanning five decades with fields applied normal to the plane and at 45 degrees. The impact of interactions between grains is evaluated. The results indicate a significantly weaker sweep-rate dependence for ECC media suggesting more robustness to long-term thermal effects. Fitting the modeled results to Sharrock-like scaling proposed by Feng and Visscher [J. Appl. Phys. 95, 7043 (2004)] is successful only in the case of single-layer media with the field normal to the plane.

cond-mat.mes-hall↗

arXiv2012

Atomic level micromagnetic model of recording media switching at elevated temperatures

An atomic level micromagnetic model of granular recording media is developed and applied to examine external field-induced grain switching at elevated temperatures which captures non-uniform reversal modes. The results are compared with traditional methods which employ the Landau-Lifshitz-Gilbert equations based on uniformly magnetized grains with assigned intrinsic temperature profiles for $M(T)$ and $K(T)$. Using nominal parameters corresponding to high-anisotropy FePt-type media envisioned for Energy Assisted Magnetic Recording, our results demonstrate that atomic-level reversal slightly reduces the field required to switch grains at elevated temperatures, but results in larger fluctuations, when compared to a uniformly magnetized grain model.

cond-mat.mes-hall↗

arXiv2012

Monte Carlo simulations of magnetic ordering in the fcc Kagome lattice

Monte Carlo simulation results are reported on magnetic ordering in ABC stacked Kagomé layers with fcc symmetry for both XY and Heisenberg models which include exchange interactions with the eight near-neighbors. Well known degeneracies of the 2D system persist in the 3D case and analysis of the numerical data provides strong evidence for a fluctuation-driven first-order transition to well-defined long-range order characterized as the layered $q=0$ (120-degree) spin structure. Effects of varying the inter-layer coupling are also examined. The results are relevant to understanding the role of frustration in IrMn$_3$ alloys widely used by the magnetic storage industry as thin-films in the antiferromagnetic pinning layer in GMR and TMR spin valves. Despite the technological importance of this structure, it has not previously been noted that the magnetic Mn-ions of fcc IrMn$_3$ form Kagomé layers.

cond-mat.str-el↗