Javier F. Calleja

Transverse susceptibility of nanoparticle systems: The effect of interaction, dispersion, and texture

The effect of texture, dispersion, and interaction on transverse susceptibility has been studied. We have developed a model based on the well-known Stoner-Wohlfarth model, by taking into account the texture, the anisotropy field value distribution, and the intergranular magnetic interactions. This model shows a good agreement with experimental measurements on granular FeCoV thin films and allows us to determine the mean value of the local magnetic anisotropy, intergrain interaction, and texture of these alloys.

TBIS measurements performed by MOKE for the study of soft ferromagnetic thin films and sandwiches: In-plane and biaxial anisotropies, local anisotropy and dispersion

Abstract Magneto-optic effects have been proved to be a premier tool for the study of magnetic thin films. In the case of films thinner than 1000 A and if there is no appreciable variation of the magnetic structure through the thickness, the magneto-optic effects provide complete information. It is noteworthy that transverse biased initial susceptibility (TBIS) measurement is very sensitive, for it allows a precise determination of the anisotropy field and, moreover, to quantify the anisotropy dispersion distinguishing between short- and long-range magnetization fluctuations in the plane of the film the origin of which is different. If the samples are deposited on glass substrates, by performing TBIS measurements with a magneto-optic transverse Kerr effect (MOKE) we can obtain information of the processes occurring at both film/air and glass/film interfaces, providing information of the variation of the magnetic properties through the thickness. We have successfully applied this technique for the study of thin films and sandwiches, focusing our attention on the following items: 1. (1) Uniaxial in-plane anisotropy, local anisotropy and anisotropy dispersion in transition metal-rare-earth thin films. We have investigated the dependence of the uniaxial anisotropy and its dispersion on the rare-earth substituted and on the composition. Using the ripple theory we have evaluated the local anisotropy as a function of the rare-earth content, and we have found a very nice correlation between macroscopic and local anisotropies, allowing us to establish the origin of the macroscopic anisotropy in these alloys. 2. (2) TBIS measurements can also be used as an alternative technique to measure perpendicular anisotropy. We have developed a theoretical model to explain the particular behaviour of TBIS curves in the most general case when both in-plane and perpendicular anisotropies are present (biaxial anisotropy). From the fit of the experimental curves to the theoretical model we can obtain the in-plane and the perpendicular anisotropy fields. 3. (3) When the samples are not magnetically homogeneous, they may present different magnetic phases which may be made evident by our technique. 4. (4) We recently extended this technique to the study of ferromagnetic double-layered and sandwiched films. In these systems the magnetic and magneto-optical properties are strongly influenced by the coupling at the interfaces. The high sensitivity of our procedure allowed us to distinguish between two contributions to the anisotropy dispersion in sandwiched films: a well-known term due to the in-plane anisotropy skew (long-range fluctuations of the in-plane easy axis) and a contribution due to the existence of a weak perpendicular anisotropy.

Transverse Kerr magnetometry for the study of thin films presenting perpendicular and in-plane anisotropy

Abstract A model for the behaviour of transverse initial susceptibility as a function of dc applied field is developed for the case of thin films exhibiting simultaneous in-plane and perpendicular anisotropy. It is used to deduce in-plane and perpendicular anisotropy fields in a very simple way from transverse susceptibility measurements made by the transverse Kerr effect. The results obtained are cross-checked by those obtained from magnetization curves.

An Overview of the Regional Experiments for Land-atmosphere Exchanges 2012 (REFLEX 2012) Campaign

The REFLEX 2012 campaign was initiated as part of a training course on the organization of an airborne campaign to support advancement of the understanding of land-atmosphere interaction processes. This article describes the campaign, its objectives and observations, remote as well as in situ. The observations took place at the experimental Las Tiesas farm in an agricultural area in the south of Spain. During the period of ten days, measurements were made to capture the main processes controlling the local and regional land-atmosphere exchanges. Apart from multi-temporal, multi-directional and multi-spatial space-borne and airborne observations, measurements of the local meteorology, energy fluxes, soil temperature profiles, soil moisture profiles, surface temperature, canopy structure as well as leaf-level measurements were carried out. Additional thermo-dynamical monitoring took place at selected sites. After presenting the different types of measurements, some examples are given to illustrate the potential of the observations made.

Improvements in the estimation of daily minimum air temperature in peninsular Spain using MODIS land surface temperature

Air temperature (Ta) is a key variable in many environmental risk models and plays a very important role in climate change research. In previous studies we developed models for estimating the daily maximum (Tmax), mean (Tmean), and minimum air temperature (Tmin) in peninsular Spain over cloud-free land areas using Moderate Resolution Imaging Spectroradiometer (MODIS) data. Those models were obtained empirically through linear regressions between daily Ta and daytime Terra-MODIS land surface temperature (LST), and then optimized by including spatio-temporal variables. The best Tmean and Tmax models were satisfactory (coefficient of determination (R2) of 0.91–0.93; and residual standard error (RSE) of 1.88–2.25 K), but not the Tmin models (R2 = 0.80–0.81 and RSE = 2.83–3.00 K). In this article Tmin models are improved using night-time Aqua LST instead of daytime Terra LST, and then refined including total precipitable water (W) retrieved from daytime Terra-MODIS data and the spatio-temporal variables curvature (c), longitude (λ), Julian day of the year (JD) and elevation (h). The best Tmin models are based on the National Aeronautics and Space Administration (NASA) standard product MYD11 LST; and on the direct broadcast version of this product, the International MODIS/AIRS Processing Package (IMAPP) LST product. Models based on Sobrino’s LST1 algorithm were also tested, with worse results. The improved Tmin models yield R2 = 0.91–0.92 and RSE = 1.75 K and model validations obtain similar R2 and RSE values, root mean square error of the differences (RMSD) of 1.87–1.88 K and bias = 0.11 K. The main advantage of the Tmin models based on the IMAPP LST product is that they can be generated in nearly real-time using the MODIS direct broadcast system at the University of Oviedo.

Structural and magnetic properties of V∕Cofcc and Cohcp∕V bilayers grown on MgO(100): A comparative study

A detailed study of the structure and magnetic properties of V / Co versus Co/ V bilayers grown on MgO001 substrates with cubic and hexagonal Co crystal structures is presented. Co is found to adopt fcc structure when grown on MgO001, while it adopts hcp structure when grown on V001. The different magnetocrystalline anisotropy associated with each structure gives rise to different magnetic properties for the otherwise equivalent symmetric Co–V bilayers.

Effect of grain size on the soft magnetic properties of FeCoV∕CoNbZr multilayers

Experimental characterization of macroscopic effective anisotropy field and coercive field for nanocrystalline (FeCoV∕CoNbZr)16 multilayers was carried out with transverse susceptibility. The multilayer structure consisting of a FeCoV ferromagnetic nanogranular layer covered with a CoNbZr amorphous layer was prepared by rf sputtering. All multilayers had the same thickness of CoNbZr (3 nm) with FeCoV layer thickness varying between 6 and 30 nm. The properties of the optimized multilayer structured films are related to an appropriate grain size of FeCoV where the CoNbZr layer acts as a very effective grain refiner and reduces the average grain size. The experimental values of the coercive force and the macroscopic uniaxial anisotropy were related to the grain size. The values were analyzed according to a two dimensional random anisotropy model which takes into account the uniaxial anisotropy.

Morphological and magnetic properties of Co nanoparticle thin films grown on Si3N4

The morphological and magnetic properties of Co nanoparticles deposited by triode sputtering on Si3N4 at 550°C are reported. The nominal thickness of Co ranges from 2 up to 15nm, and two different capping layers, Au and Pt, are used. The nanoparticles were characterized by x-ray diffraction and atomic force microscopy. Morphological and structural studies show that the nanoparticles grow in a well-defined nanostructured pattern and adopt a hexagonal closed packed crystalline structure. Moreover, the average particle size and the particle size dispersion increase as the thickness increases, due to percolation. Experimental characterization of effective anisotropy field was carried out with transverse susceptibility. Transverse susceptibility measurements reveal an in-plane isotropic magnetic behavior. Both the effective anisotropy field and the coercive field increase as the particle size increases, following a D6 dependence, which is typical for three-dimensional structures in the framework of the random ...

Induced and local anisotropies in amorphous CoZr–rare earth thin films containing Pr, Nd, and Tb

The induced in‐plane and local magnetic anisotropies of rf sputtered (Co93Zr7)100−xREx thin films were investigated as a function of RE content for RE=Pr, Nd, and Tb by transverse biased initial susceptibility measurements at both film/air and glass/film interfaces. The deposition was performed in a magnetic field. The films exhibit a very well defined in‐plane anisotropy with negligible long‐range fluctuations and a small coercive field Hc, in accordance with the experimental data. The overall variations of the transverse susceptibility are related to the ripple. The variations of the local anisotropy with composition were measured. Both the induced anisotropy Ku and the local anisotropy Kloc increase with the increasing amount of RE in the layers. A clear relationship between Ku and Kloc could be established. The results are discussed in terms of the single‐ion anisotropy of the rare earth. While the value of Ku is the same at both interfaces, the ripple constants were found to be slightly different. Th...

Quantitative determination of mixed anisotropies in Fe(100) thin films and annealing-induced magnetic reorientation

A specific formalism for quantification of magnetic anisotropies using transverse biased initial susceptibility measurements in systems with mixed in-plane biaxial and uniaxial anisotropies has been developed. This formalism has been applied to the study of magnetic anisotropies in epitaxial, partially nitrided Fe thin films that exhibit these coexisting anisotropies. The influence of annealing on the structure and magnetic anisotropies in this system is also studied. A restructurating of N into the Fe lattice with the formation of a nonstoichiometric FeN phase is found after sufficient annealing time. An additional result is a 5° in-plane rotation of the uniaxial anisotropy direction with respect to the as-grown sample, with no reorientation of the film crystal lattice with respect to the substrate. Possible mechanisms to explain the origin of magnetic anisotropy and the rotation upon annealing of the uniaxial anisotropy are proposed.