Dual Behavior of Antiferromagnetic Uncompensated Spins in NiFe/IrMn Exchange Biased Bilayers

We present a comprehensive study of the exchange bias effect in a model system. Through numerical analysis of the exchange bias and coercive fields as a function of the antiferromagnetic layer thickness we deduce the absolute value of the averaged anisotropy constant of the antiferromagnet. We show that the anisotropy of IrMn exhibits a finite size effect as a function of thickness. The interfacial spin disorder involved in the data analysis is further supported by the observation of the dual behavior of the interfacial uncompensated spins. Utilizing soft x-ray resonant magnetic reflectometry we have observed that the antiferromagnetic uncompensated spins are dominantly frozen with nearly no rotating spins due to the chemical intermixing, which correlates to the inferred mechanism for the exchange bias.Comment: 4 pages, 3 figure

Probing amyloid protein aggregation with optical superresolution methods: from the test tube to models of disease.

The misfolding and self-assembly of intrinsically disordered proteins into insoluble amyloid structures are central to many neurodegenerative diseases such as Alzheimer's and Parkinson's diseases. Optical imaging of this self-assembly process in vitro and in cells is revolutionizing our understanding of the molecular mechanisms behind these devastating conditions. In contrast to conventional biophysical methods, optical imaging and, in particular, optical superresolution imaging, permits the dynamic investigation of the molecular self-assembly process in vitro and in cells, at molecular-level resolution. In this article, current state-of-the-art imaging methods are reviewed and discussed in the context of research into neurodegeneration.This work was funded by grants from the Wellcome Trust, the Medical Research Council UK, the Alzheimer Research UK Trust, the Engineering and Physical Sciences Research Council UK, the Biotechnology and Biological Sciences Research Council, and the Swiss National Science Foundation.This is the final version of the article. It first appeared from the Society of Photo-optical Instrumentation Engineers via http://dx.doi.org/10.1117/1.NPh.3.4.04180

Synaptic tau: A pathological or physiological phenomenon?

In this review, we discuss the synaptic aspects of Tau pathology occurring during Alzheimer's disease (AD) and how this may relate to memory impairment, a major hallmark of AD. Whilst the clinical diagnosis of AD patients is a loss of working memory and long-term declarative memory, the histological diagnosis is the presence of neurofibrillary tangles of hyperphosphorylated Tau and Amyloid-beta plaques. Tau pathology spreads through synaptically connected neurons to impair synaptic function preceding the formation of neurofibrillary tangles, synaptic loss, axonal retraction and cell death. Alongside synaptic pathology, recent data suggest that Tau has physiological roles in the pre- or post- synaptic compartments. Thus, we have seen a shift in the research focus from Tau as a microtubule-stabilising protein in axons, to Tau as a synaptic protein with roles in accelerating spine formation, dendritic elongation, and in synaptic plasticity coordinating memory pathways. We collate here the myriad of emerging interactions and physiological roles of synaptic Tau, and discuss the current evidence that synaptic Tau contributes to pathology in AD.G.S.K.S. acknowledges funding from the Wellcome Trust (065807/Z/01/Z) (203249/Z/16/Z), the UK Medical Research Council (MRC) (MR/K02292X/1), Alzheimer Research UK (ARUK) (ARUK-PG013-14), Michael J Fox Foundation (16238) and Infnitus China Ltd. M.A.R acknowledges funding from the Engineering and Physical Sciences Research Council (EP/L015889/1)

Observation of Devil's Staircase in the Novel Spin Valve System SrCo6_6O11_{11}

Using resonant soft x-ray scattering as a function of both temperature and magnetic field, we reveal a large number of almost degenerate magnetic orders in SrCo6O11. The Ising-like spins in this frustrated material in fact exhibit a so-called magnetic devil's staircase. It is demonstrated how a magnetic field induces transitions between different microscopic spin configurations, which is responsible for the magnetoresistance of SrCo6O11. This material therefore constitutes a unique combination of a magnetic devil's staircase and spin valve effects, yielding a novel type of magnetoresistance system.Comment: 5 pages, 5 figure

Opening a nodal gap by fluctuating spin-density-wave in lightly doped La2−x_{2-x}Srx_xCuO4_4

We investigate whether the spin or charge degrees of freedom are responsible for the nodal gap in underdoped cuprates by performing inelastic neutron scattering and x-ray diffraction measurements on La$_{2-x}$Sr$_x$CuO$_4$, which is on the edge of the antiferromagnetic phase. We found that fluctuating incommensurate spin-density-wave (SDW) with a the bottom part of an hourglass dispersion exists even in this magnetic sample. The strongest component of these fluctuations diminishes at the same temperature where the nodal gap opens. X-ray scattering measurements on the same crystal show no signature of charge-density-wave (CDW). Therefore, we suggest that the nodal gap in the electronic band of this cuprate opens due to fluctuating SDW with no contribution from CDW

Magnetic domain fluctuations in an antiferromagnetic film observed with coherent resonant soft x-ray scattering

We report the direct observation of slow fluctuations of helical antiferromagnetic domains in an ultra-thin holmium film using coherent resonant magnetic x-ray scattering. We observe a gradual increase of the fluctuations in the speckle pattern with increasing temperature, while at the same time a static contribution to the speckle pattern remains. This finding indicates that domain-wall fluctuations occur over a large range of time scales. We ascribe this non-ergodic behavior to the strong dependence of the fluctuation rate on the local thickness of the film.Comment: to appear in Phys. Rev. Let

Advanced fluorescence imaging of in situ protein aggregation.

The aggregation of intrinsically disordered proteins is a hallmark of neurodegenerative diseases, such as Alzheimer's, Parkinson's and Huntington's disease. Although we currently have a good molecular level understanding on how protein aggregation occurs in vitro, the details of its self-assembly in live cells are still mainly unknown. During the last ten years, we have witnessed the rapid development of advanced imaging techniques, especially super-resolution and fluorescence lifetime-based microscopy, in different areas of cell biology. These methods have been revolutionising our understanding of how proteins aggregate, providing unprecedented high spatial-temporal resolution which permits us to capture the kinetics of aggregate seeding and expansion, the motion and distribution of individual aggregates within the cells, and its structural change. In this article, we will review the study of in situ protein aggregation using advanced imaging techniques, with the focus on protein aggregate structure and its assembly dynamics

An Easy-to-Implement Protocol for Preparing Postnatal Ventral Mesencephalic Cultures.

Postnatally derived cultures of ventral mesencephalic neurons offer several crucial advantages over embryonic ventral mesencephalic cultures, including a higher content of TH-positive cells and the ability to derive cells from the substantia nigra, which contains the neurons most vulnerable to Parkinson's disease. On the other hand, these cultures are more challenging to produce consistently. Here, we provide an easy-to-implement protocol for culturing postnatal ventral mesencephalic cells from the substantia nigra (SN) and the ventral tegmental area using commercially available media, dishes, and general lab equipment, avoiding extensive material and equipment purchases. The protocol can be completed in about 5 h and provides ventral midbrain neuron cultures on cortex glia feeder layers in three weeks' time. The protocol uses an optimized protease digestion, tissue storage in Hibernate A during dissection and purification of neurons on an OptiPrep density gradient

Electronic structure, magnetic and dielectric properties of the edge-sharing copper-oxide chain compound NaCu2_{2}O2_{2}

We report an experimental study of \nco, a Mott insulator containing chains of edge-sharing CuO$_4$ plaquettes, by polarized x-ray absorption spectroscopy (XAS), resonant magnetic x-ray scattering (RMXS), magnetic susceptibility, and pyroelectric current measurements. The XAS data show that the valence holes reside exclusively on the Cu$^{2+}$ sites within the copper-oxide spin chains and populate a $d$-orbital polarized within the CuO$_4$ plaquettes. The RMXS measurements confirm the presence of incommensurate magnetic order below a N\'eel temperature of $T_N = 11.5$ K, which was previously inferred from neutron powder diffraction and nuclear magnetic resonance data. In conjunction with the magnetic susceptibility and XAS data, they also demonstrate a new "orbital" selection rule for RMXS that is of general relevance for magnetic structure determinations by this technique. Dielectric property measurements reveal the absence of significant ferroelectric polarization below $T_N$, which is in striking contrast to corresponding observations on the isostructural compound \lco. The results are discussed in the context of current theories of multiferroicity.Comment: 7 pages, 7 figure

A Comparison of Stripe Modulations in La1.875_{1.875}Ba0.125_{0.125}CuO4_4 and La1.48_{1.48}Nd0.4_{0.4}Sr0.12_{0.12}CuO4_4

We report combined soft and hard x-ray scattering studies of the electronic and lattice modulations associated with stripe order in La$_{1.875}$Ba$_{0.125}$CuO$_4$ and La$_{1.48}$Nd$_{0.4}$Sr$_{0.12}$CuO$_4$. We find that the amplitude of both the electronic modulation of the hole density and the strain modulation of the lattice is significantly larger in La$_{1.875}$Ba$_{0.125}$CuO$_4$ than in La$_{1.48}$Nd$_{0.4}$Sr$_{0.12}$CuO$_4$ and is also better correlated. The in-plane correlation lengths are isotropic in each case; for La$_{1.875}$Ba$_{0.125}$CuO$_4$, $\xi^{hole}=255\pm 5$ \AA\ whereas for La$_{1.48}$Nd$_{0.4}$Sr$_{0.12}$CuO$_4$F, $\xi^{hole}=111\pm 7$ \AA. We find that the modulations are temperature independent in La$_{1.875}$Ba$_{0.125}$CuO$_4$ in the low temperature tetragonal phase. In contrast, in La$_{1.48}$Nd$_{0.4}$Sr$_{0.12}$CuO$_4$, the amplitude grows smoothly from zero, beginning 13 K below the LTT phase transition. We speculate that the reduced average tilt angle in La$_{1.875}$Ba$_{0.125}$CuO$_4$ results in reduced charge localization and incoherent pinning, leading to the longer correlation length and enhanced periodic modulation amplitude.Comment: 6 pages, 4 figure