The role of interfaces in CoFe/IrMn exchange biased systems

A trilayer system consisting of an IrMn layer exchanged coupled to two CoFe layers of equal thickness has been studied. A single stage reversal was observed over a wide range of temperatures. Two bilayers with the same thicknesses of the pinning layer but different ferromagnetic thicknesses were also studied. By comparing the magnetic properties of these three stacks the effect of the interfacial area on the exchange field and the coercivity has been determined. We find that the interfacial area has a very minor effect on the exchange field H-ex and the blocking temperature (T-B) but causes a doubling of the coercivity (H-c). This indicates that H-c is dominated by the interface whereas the exchange bias is controlled by volumetric effects

Size effects in submicron exchange bias square elements

The behavior of submicron exchange bias square elements has been investigated for systems containing metallic polycrystalline layers. Numerical simulations using a simple theoretical model show that the exchange bias for such elements can increase and/or decrease depending on the microstructure of the antiferromagnetic layer and, in particular, its grain size distribution. The predictions are based on a granular model of exchange bias that accounts for grain cutting at the edges of the nanoelements that takes place during ion milling/etching. This leads to distributions of exchange bias fields that can be quite broad, especially in sub-250 nm elements

Magnetic and structural properties of antiferromagnetic Mn2VSi alloy films grown at elevated temperatures

80 nm thick polycrystalline Mn2VSi films have been deposited on silicon substrates with an 18 nm silver seed layer and a 3 nm aluminium capping layer using a sputtering system. The best quality film is obtained for 723 K growth. The Mn2VSi thin film is verified to be antiferromagnetic, where an exchange bias is found when a 3 nm ferromagnetic CoFe layer has been deposited on the top of the Mn2VSi layer. The exchange bias is measured to be 34 Oe at 100 K. The blocking and thermal activation temperature (TACT) of Mn2VSi is estimated to be below 100 K and within a range between 100 K and 448 K, respectively. These properties can be improved by substituting the constituent atoms with the other elements (e.g., Co and Al), suggesting a potential of Mn2VSi to be used as an antiferromagnet in a spintronic device

A Web-based Environment Providing Remote Access to FPGA Platforms for Teaching Digital Hardware Design

In this work we present the design and implementation of a Web-based application for remote access to the FPGA boards in a Digital Design Laboratory. It enables students from specialization courses to afford the design exercises at any place and time, even at home, just with an Internet access and a Web browser. At the same time, it opens the possibility of prototyping small designs to the rest of students which have no access rights to the physical Laboratory

Docencia Semipresencial en Laboratorios Docentes para Diseño de Hardware Digital en la ETSIT-UPM

En esta comunicación se describe el planteamiento seguido en la ETSIT-UPM (Escuela Técnica Superior de Ingenieros de Telecomunicación – Universidad Politécnica de Madrid) para la formación de sus alumnos en diseño de hardware digital, y la utilidad que tiene el trabajo práctico con herramientas reales y sistemas de prototipado en cada una de sus etapas. A continuación, se detalla la aproximación seguida en nuestra Escuela para posibilitar el acceso de un mayor número de alumnos a los recursos de laboratorio mediante el empleo de las TIC (Tecnologías de la Información y las Comunicaciones). El objetivo final del trabajo desarrollado es dotar a los alumnos de la capacidad para acceder de forma remota a herramientas software de desarrollo y, especialmente, a plataformas de prototipado reales, todas ellas destinadas al diseño de hardware digital. Finalmente, se introducen algunos detalles técnicos relativos a una parte fundamental del desarrollo, que es la aplicación Web para el acceso remoto a las placas de prototipado- In this paper we describe the approach taken at ETSIT-UPM (School for Telecommunication Engineers – Technical University of Madrid) to teach digital hardware design, together with the possibilities of practising with real software tools and prototyping platforms. Next, we detail our approach to facilitate access to the hardware laboratory to a greater number of students by means of TIC (Information and Communication Technologies). The final goal of our work is providing students with remote access to software development tools and prototyping platforms, all of them devoted to digital hardware design. Finally, we describe some technical details of the Web application providing remote access to the prototyping boards

Grain size effects in polycrystalline exchange biased systems

The capability of a novel sputtering technology known as HiTUS to produce thin films with controlled grain size and distribution has been evaluated. This has been achieved for different materials chosen due to their importance in industrial applications. The grain size of the deposited films was measured from TEM images in bright field mode. All the distributions were lognormal and over five hundred particles were measured to recompose each lognormal function. For a range of materials close control of grain size between 4nm and 30nm was achieved. The grain size was controlled through the deposition rate. This has allowed the investigation of the effect of the antiferromagnetic grain size on exchange bias without the use of underlayers, additives or substrate heating. It was found that the magnitude of the loop shift strongly correlated to the distribution of grain volumes in the antiferromagnetic layer. Using detailed measurements of the grain size distribution the features of exchange bias in these systems can be explained on the basis of a grain volume model with coherent rotation. For example using measurement protocols that ensure reproducibility of data has allowed the development of a new measurement technique for the anisotropy constant of metallic antiferromagnets. This is a key result since the anisotropy is used in all theoretical models of exchange bias as a fitting parameter. The anisotropy was determined only from experimental features and only its temperature dependence had to be assumed to be of the form K(OK)(1-T/TN)3 in order to calculate the values. For IrMn K(293K) =4.14xlO6erg/cc and for FeMn K(293K) = 1.36x106er

Hardware Reuse Improvement through the Domain Specific Language dHDL.

The dHDL language has been defined to improve hardware design productivity. This is achieved through the definition of a better reuse interface (including parameters, attributes and macroports) and the creation of control structures that help the designer in the hardware generation process

Remote (250 km) Fiber Bragg Grating Multiplexing System

We propose and demonstrate two ultra-long range fiber Bragg grating (FBG) sensor interrogation systems. In the first approach four FBGs are located 200 km from the monitoring station and a signal to noise ratio of 20 dB is obtained. The second improved version is able to detect the four multiplexed FBGs placed 250 km away, offering a signal to noise ratio of 6–8 dB. Consequently, this last system represents the longest range FBG sensor system reported so far that includes fiber sensor multiplexing capability. Both simple systems are based on a wavelength swept laser to scan the reflection spectra of the FBGs, and they are composed by two identical-lengths optical paths: the first one intended to launch the amplified laser signal by means of Raman amplification and the other one is employed to guide the reflection signal to the reception system

Thermal stability of exchange bias systems based on MnN

At the present time there is a requirement to identify new antiferromagnetic alloys or compounds which might be suitable for the production of exchange bias systems. The phenomenon of exchange bias remains crucial for the operation of all read heads in hard disk drives and also has potential for use in magnetic random access memory (MRAM) systems. There is also an increasing interest in the use of antiferromagnets themselves in spintronic devices. Generally for applications the alloy IrMn is used, however given that Iridium is one of the rarest, and therefore most expensive elements on Earth, there is a search for alternative materials. In this paper we report on a study of the compound MnN in terms of its thermal stability. We have produced polycrystalline films of this compound with sub 10 nm grains and examined the thermal stability in layers of thicknesses of up to 30 nm. Using thermal activation studies we have determined a room temperature value of the anisotropy constant of this compound in a tetragonal structure of up to (6.3 ± 0.3) × 10 6 erg/cm 3 . The antiferromagnetic grains can be aligned by thermal annealing at an optimum temperature of 380 K. Above this temperature the magnetic properties deteriorate possibly due to nitrogen desorption

HAMR Media Based on Exchange Bias

In this work we describe an alternative strategy for the development of heat assisted magnetic recording (HAMR) media. In our approach the need for a storage material with a temperature dependent anisotropy and to provide a read out signal is separated so that each function can be optimised independently. This is achieved by the use of an exchange bias structure where a conventional CoCrPt-SiO2 recording layer is exchange biased to an underlayer of IrMn such that heating and cooling in the exchange field from the recording layer results in a shifted loop. This strategy requires the reorientation of the IrMn layer to allow coupling to the recording layer. This has been achieved by the use of an ultrathin (0.8nm) layer of Co deposited beneath the IrMn layer. In this system the information is in effect stored in the antiferromagnetic (AF) layer and hence there is no demagnetising field generated by the stored bits. A loop shift of 688 Oe has been achieved where both values of coercivity lie to one side of the origin and the information cannot be erased by a magnetic field