Observation of vortex-nucleated magnetization reversal in individual ferromagnetic nanotubes

The reversal of a uniform axial magnetization in a ferromagnetic nanotube (FNT) has been predicted to nucleate and propagate through vortex domains forming at the ends. In dynamic cantilever magnetometry measurements of individual FNTs, we identify the entry of these vortices as a function of applied magnetic field and show that they mark the nucleation of magnetization reversal. We find that the entry field depends sensitively on the angle between the end surface of the FNT and the applied field. Micromagnetic simulations substantiate the experimental results and highlight the importance of the ends in determining the reversal process. The control over end vortex formation enabled by our findings is promising for the production of FNTs with tailored reversal properties.Comment: 20 pages, 13 figure

Imaging magnetic vortex configurations in ferromagnetic nanotubes

We image the remnant magnetization configurations of CoFeB and permalloy nanotubes (NTs) using x-ray magnetic circular dichroism photo-emission electron microscopy. The images provide direct evidence for flux-closure configurations, including a global vortex state, in which magnetization points circumferentially around the NT axis. Furthermore, micromagnetic simulations predict and measurements confirm that vortex states can be programmed as the equilibrium remnant magnetization configurations by reducing the NT aspect ratio.Comment: 14 pages, 4 figures, link to supplementary informatio

Mast Cell Activation and Signaling in the Autoimmune Disease Bullous Pemphigoid

Bullous pemphigoid (BP) is an autoimmune skin blistering disorder primarily observed in the elderly. Autoantibodies directed against the hemidesmosomal protein BP180 trigger a pathological inflammatory response that causes separation of the epidermis from the underlying dermis. Complement, mast cells (MCs), and polymorphonuclear neutrophils (PMNs) are required for disease in experimental BP, an animal model that closely mimics the clinical, immunological and histological features of human BP. In this work, we investigated MC activation and signaling in experimental BP. MC degranulation occurs downstream of complement activation and the generation of complement component 5a (C5a) in experimental BP. We determined that C5a binds to the C5a receptor (C5aR) on MCs in neonatal mice given disease-inducing antibodies. C5a-C5aR interaction significantly increases phosphorylation of the intracellular signaling protein p38 mitogen-activated protein kinase (p38MAPK). Pharmacologically blocking p38MAPK activation protected mice from MC degranulation and clinical disease. Taken together, we demonstrated that the binding of C5a to C5aR on MCs activates the p38MAPK signaling pathway and leads to MC degranulation and skin blistering. Upon degranulation, MCs release bioactive compounds from their secretory granules, including tumor necrosis factor-alpha (TNF-α). Here, we report that MC-derived TNF-α is required for disease in experimental BP. Mice lacking TNF-α globally or in MCs alone fail to recruit sufficient numbers of PMNs to the skin and do not develop clinical blisters following injection with pathogenic anti-BP180 antibodies. C5aR-deficient mice are protected from blistering and do not exhibit elevated TNF-α levels or MC degranulation. TNF-α receptor 1 (TNFR1) expression on MCs is required for development of experimental BP, suggesting that TNF-α acts in an autocrine fashion on MCs. The findings described in this dissertation refine our understanding of the mechanisms of MC degranulation in experimental BP. MCs are activated by the binding of C5a to C5aR. C5a-C5aR interaction leads to activation of p38MAPK and MC degranulation. MC degranulation releases TNF-α, and TNF-α acts in an autocrine manner on MC TNFR1 to promote disease development. In addition to providing insight into the pathogenesis of BP, our data also suggests that C5a, p38MAPK, and TNF-α may be promising therapeutic targets for treatment of human disease

Upper-Ocean Singular Vectors of the North Atlantic Climate with Implications for Linear Predictability and Variability

The limits of predictability of the meridional overturning circulation (MOC) and upper-ocean temperatures due to errors in ocean initial conditions and model parametrizations are investigated in an idealized configuration of an ocean general circulation model (GCM). Singular vectors (optimal perturbations) are calculated using the GCM, its tangent linear and adjoint models to determine an upper bound on the predictability of North Atlantic climate. The maximum growth time-scales of MOC and upper-ocean temperature anomalies, excited by the singular vectors, are 18.5 and 13 years respectively and in part explained by the westward propagation of upper-ocean anomalies against the mean flow. As a result of the linear interference of non-orthogonal eigenmodes of the non-normal dynamics, the ocean dynamics are found to actively participate in the significant growth of the anomalies. An initial density perturbation of merely $0.02 kg m^{−3}$ is found to lead to a 1.7 Sv MOC anomaly after 18.5 years. In addition, Northern Hemisphere upper-ocean temperature perturbations can be amplified by a factor of 2 after 13 years. The growth of upper-ocean temperature and MOC anomalies is slower and weaker when excited by the upper-ocean singular vectors than when the deep ocean is perturbed. This leads to the conclusion that predictability experiments perturbing only the atmospheric initial state may overestimate the predictability time. Interestingly, optimal MOC and upper-ocean temperature excitations are only weakly correlated, thus limiting the utility of SST observations to infer MOC variability. The excitation of anomalies in this model might have a crucial impact on the variability and predictability of Atlantic climate. The limit of predictability of the MOC is found to be different from that of the upper-ocean heat content, emphasizing that errors in ocean initial conditions will affect various measures differently and such uncertainties should be carefully considered in decadal prediction experiments.Earth and Planetary Science

Imaging stray magnetic field of individual ferromagnetic nanotubes

We use a scanning nanometer-scale superconducting quantum interference device to map the stray magnetic field produced by individual ferromagnetic nanotubes (FNTs) as a function of applied magnetic field. The images are taken as each FNT is led through magnetic reversal and are compared with micromagnetic simulations, which correspond to specific magnetization configurations. In magnetic fields applied perpendicular to the FNT long axis, their magnetization appears to reverse through vortex states, i.e.\ configurations with vortex end domains or -- in the case of a sufficiently short FNT -- with a single global vortex. Geometrical imperfections in the samples and the resulting distortion of idealized mangetization configurations influence the measured stray-field patterns.Comment: 14 pages, 4 figure

Magnetization reversal of an individual exchange biased permalloy nanotube

We investigate the magnetization reversal mechanism in an individual permalloy (Py) nanotube (NT) using a hybrid magnetometer consisting of a nanometer-scale SQUID (nanoSQUID) and a cantilever torque sensor. The Py NT is affixed to the tip of a Si cantilever and positioned in order to optimally couple its stray flux into a Nb nanoSQUID. We are thus able to measure both the NT's volume magnetization by dynamic cantilever magnetometry and its stray flux using the nanoSQUID. We observe a training effect and temperature dependence in the magnetic hysteresis, suggesting an exchange bias. We find a low blocking temperature $T_B = 18 \pm 2$ K, indicating the presence of a thin antiferromagnetic native oxide, as confirmed by X-ray absorption spectroscopy on similar samples. Furthermore, we measure changes in the shape of the magnetic hysteresis as a function of temperature and increased training. These observations show that the presence of a thin exchange-coupled native oxide modifies the magnetization reversal process at low temperatures. Complementary information obtained via cantilever and nanoSQUID magnetometry allows us to conclude that, in the absence of exchange coupling, this reversal process is nucleated at the NT's ends and propagates along its length as predicted by theory.Comment: 8 pages, 4 figure