Influence of exchange bias coupling on the single-crystalline FeMn ultrathin film

Polarization dependent x-ray photoemission electron microscopy was used to investigate the influence of the exchange bias coupling on the disordered ultrathin single-crystalline fcc Fe50 Mn50. We find that the critical thickness of the FeMn film, where the antiferromagnetic (AF) order is formed, varies with changing the magnetization direction of the ferromagnetic (FM) layer from out-of-plane to in-plane. Surface magneto-optical Kerr effect measurements (SMOKE) further manifest the shift of the critical thickness with alternating the exchange bias coupling. It indicates that the spin structure of the FeMn layer near the FM layer is modified by the presence of exchange bias coupling and the properties of the coupling. Our results provide direct experimental evidence that the AF spin structure at the interface between the FM and AF layers is strongly influenced by the exchange bias coupling. © 2005 American Institute of Physics.published_or_final_versio

History-Dependent Excitability as a Single-Cell Substrate of Transient Memory for Information Discrimination

Neurons react differently to incoming stimuli depending upon their previous history of stimulation. This property can be considered as a single-cell substrate for transient memory, or context-dependent information processing: depending upon the current context that the neuron “sees” through the subset of the network impinging on it in the immediate past, the same synaptic event can evoke a postsynaptic spike or just a subthreshold depolarization. We propose a formal definition of History-Dependent Excitability (HDE) as a measure of the propensity to firing in any moment in time, linking the subthreshold history-dependent dynamics with spike generation. This definition allows the quantitative assessment of the intrinsic memory for different single-neuron dynamics and input statistics. We illustrate the concept of HDE by considering two general dynamical mechanisms: the passive behavior of an Integrate and Fire (IF) neuron, and the inductive behavior of a Generalized Integrate and Fire (GIF) neuron with subthreshold damped oscillations. This framework allows us to characterize the sensitivity of different model neurons to the detailed temporal structure of incoming stimuli. While a neuron with intrinsic oscillations discriminates equally well between input trains with the same or different frequency, a passive neuron discriminates better between inputs with different frequencies. This suggests that passive neurons are better suited to rate-based computation, while neurons with subthreshold oscillations are advantageous in a temporal coding scheme. We also address the influence of intrinsic properties in single-cell processing as a function of input statistics, and show that intrinsic oscillations enhance discrimination sensitivity at high input rates. Finally, we discuss how the recognition of these cell-specific discrimination properties might further our understanding of neuronal network computations and their relationships to the distribution and functional connectivity of different neuronal types

Manufacture Techniques of Chitosan-Based Microcapsules to Enhance Functional Properties of Textiles

In recent years, the textile industry has been moving to novel concepts of products, which could deliver to the user, improved performances. Such smart textiles have been proven to have the potential to integrate within a commodity garment advanced feature and functional properties of different kinds. Among those functionalities, considerable interest has been played in functionalizing commodity garments in order to make them positively interact with the human body and therefore being beneficial to the user health. This kind of functionalization generally exploits biopolymers, a class of materials that possess peculiar properties such as biocompatibility and biodegradability that make them suitable for bio-functional textile production. In the context of biopolymer chitosan has been proved to be an excellent potential candidate for this kind of application given its abundant availability and its chemical properties that it positively interacts with biological tissue. Notwithstanding the high potential of chitosan-based technologies in the textile sectors, several issues limit the large-scale production of such innovative garments. In facts the morphologies of chitosan structures should be optimized in order to make them better exploit the biological activity; moreover a suitable process for the application of chitosan structures to the textile must be designed. The application process should indeed not only allow an effective and durable fixation of chitosan to textile but also comply with environmental rules concerning pollution emission and utilization of harmful substances. This chapter reviews the use of microencapsulation technique as an approach to effectively apply chitosan to the textile material while overcoming the significant limitations of finishing processes. The assembly of chitosan macromolecules into microcapsules was proved to boost the biological properties of the polymer thanks to a considerable increase in the surface area available for interactions with the living tissues. Moreover, the incorporation of different active substances into chitosan shells allows the design of multifunctional materials that effectively combine core and shell properties. Based on the kind of substances to be incorporated, several encapsulation processes have been developed. The literature evidences how the proper choices concerning encapsulation technology, chemical formulations, and process parameter allow tuning the properties and the performances of the obtained microcapsules. Furthermore, the microcapsules based finishing process have been reviewed evidencing how the microcapsules morphology can positively interact with textile substrate allowing an improvement in the durability of the treatment. The application of the chitosan shelled microcapsules was proved to be capable of imparting different functionalities to textile substrates opening possibilities for a new generation of garments with improved performances and with the potential of protecting the user from multiple harms. Lastly, a continuous interest was observed in improving the process and formulation design in order to avoid the usage of toxic substances, therefore, complying with an environmentally friendly approach

Influence of exchange bias coupling on the single-crystalline FeMn ultrathin film

Polarization dependent x-ray photoemission electron microscopy was used to investigate the influence of the exchange bias coupling on the disordered ultrathin single-crystalline fcc Fe(50)Mn(50). We find that the critical thickness of the FeMn film, where the antiferromagnetic (AF) order is formed, varies with changing the magnetization direction of the ferromagnetic (FM) layer from out-of-plane to in-plane. Surface magneto-optical Kerr effect measurements (SMOKE) further manifest the shift of the critical thickness with alternating the exchange bias coupling. It indicates that the spin structure of the FeMn layer near the FM layer is modified by the presence of exchange bias coupling and the properties of the coupling. Our results provide direct experimental evidence that the AF spin structure at the interface between the FM and AF layers is strongly influenced by the exchange bias coupling

Microspectroscopic two-dimensional Fermi surface mapping using a photoelectron emission microscope

We demonstrate the use of a photoelectron emission microscope in connection with a retarding field electron energy analyzer for the fast acquisition of two-dimensional momentum resolved photoelectron angular distribution patterns. This opens the possibility to combine spatial, momentum, and energy resolution of photoelectrons within the same instrument. We have applied this to observe the Cu(001) Fermi surface from a selected region of the sample. A well defined bulk Fermi surface is quickly mapped in this way

Local exchange bias observed by photoemission microscopy

By using a photoemission electron microscope in combination with X-ray magnetic circular dichroism, the domain configuration in a single crystalline Co film exchange coupled to an Fe50Mn50 film is obtained. The effect of the ferromagnetic/antiferromagnetic interaction is observed to be different in Co domains with different magnetization direction, establishing the fact that the exchange bias' is locally set by the magnetization of the ferromagnetic layer, without external field

Magnetic dichroisms in absorption and photoemission for magnetic characterization in x-ray photoelectron emission microscopy

Magnetic contrast for the operation of a photoelectron emission microscope (PEEM) with synchrotron radiation is provided by magnetic dichroisms. Besides the most frequently employed magnetic dichroism, magnetic circular dichroism in x-ray absorption spectroscopy, energy filtering of photoemitted electrons allows one to also use magnetic dichroisms in photoelectron emission as complementary contrast mechanisms. We demonstrate that it is possible to obtain magnetic contrast in photoemission using PEEM equipped with a simple retarding field electron energy analyzer. Magnetic domain images of an ultrathin film of 10 atomic monolayers of Fe on W(001), obtained by three, different contrast mechanisms (circular magnetic dichroism in x-ray absorption, circular magnetic dichroism in Fe valence band photoemission, and linear magnetic dichroism in Fe 3p photoemission) are presented and compared

Three-dimensional noncollinear antiferromagnetic order in single-crystalline FeMn ultrathin films

We present experimental evidence for a three-dimensional noncollinear antiferromagnetic spin structure in ultrathin single-crystalline fcc Fe50Mn50 layers using magnetic circular dichroism photoelectron emission microscopy and x-ray magnetic linear dichroism. Layer-resolved as-grown domain images of epitaxial trilayers grown on Cu(001) in which FeMn is sandwiched between ferromagnetic layers with different easy axes reveal the presence of antiferromagnetic spin components in the film plane and normal to the film plane. An FeMn spin structure with no collinear order in the film plane is consistent with the absence of x-ray magnetic linear dichroism in Fe L-3 absorption in FeMn/Co bilayers

Magnetic domain investigation in Co/Cu/FeMn trilayers

The magnetic domain patterns of epitaxial single-crystalline Co/FeMn bilayers and Co/Cu/FeMn trilayers were investigated by magnetic circular dichroism domain imaging using photoelectron emission microscopy. The as-grown domain size increases continuously with increasing Cu layer thickness, which is attributed to the decrease of the interlayer exchange coupling between ferromagnetic Co and antiferromagnetic FeMn layers. Domain images of the Co layer acquired after applying different external magnetic fields show a decrease in coercivity with increasing Cu layer thickness, confirming the reduction of magnetic coupling energy with increasing Cu thickness