magnetic and structural investigation of growth induced magnetic anisotropies in fe50co50 thin films

In this paper, we investigate the magnetic properties of Fe50 Co50 polycrystalline thin films, grown by dc-magnetron sputtering, with thickness (t) ranging from 2.5 nm up to 100 nm. We focused on the magnetic properties of the samples to highlight the effects of possible intrinsic stress that may develop during growth, and their dependence on film thickness. Indeed, during film deposition, due to the growth technique and growth conditions, a metallic film may display an intrinsic compressive or tensile stress. In our case, due to the Fe50Co50 magnetolastic properties, this stress may in its turn promote the development of magnetic anisotropies. Samples magnetic properties were monitored with a SQUID magnetometer and a magneto–optic Kerr effect apparatus, using both an in–plane and an out–of–plane magnetic field. Magnetoresistance measurements were collected, as well, to further investigate the magnetic behavior of the samples. Indications about the presence of intrinsic stress were obtained accessing samples curvature with an optical profilometer. For t ≤ 20 nm, the shape of the in-plane magnetization loops is squared and coercivity increases with t, possibly due to fact that, for small t values, the grain size grows with t. The magnetoresistive response is anisotropic in character. For t > 20 nm, coercivity smoothly decreases, the approach to saturation gets slower and the shape of the whole loop gets less and less squared. The magnetoresistive effect becomes almost isotropic and its intensity increases of about one order of magnitude. These results suggest that the magnetization reorientation process changes for t > 20 nm, and are in agreement with the progressive development of an out-of-plane easy axis. This hypothesis is substantiated by profilometric analysis that reveals the presence of an in-plane compressive stress

BULLKID: Monolithic array of particle absorbers sensed by Kinetic Inductance Detectors

We introduce BULLKID, an innovative phonon detector consisting of an array of dices acting as particle absorbers sensed by multiplexed Kinetic Inductance Detectors (KIDs). The dices are carved in a thick crystalline wafer and form a monolithic structure. The carvings leave a thin common disk intact in the wafer, acting both as holder for the dices and as substrate for the KID lithography. The prototype presented consists of an array of 64 dices of 5.4x5.4x5 mm$^3$ carved in a 3" diameter, 5 mm thick silicon wafer, with a common disk 0.5 mm thick hosting a 60 nm patterned aluminum layer. The resulting array is highly segmented but avoids the use of dedicated holding structures for each unit. Despite the fact that the uniformity of the KID electrical response across the array needs optimization, the operation of 8 units with similar features shows, on average, a baseline energy resolution of $26\pm7$ eV. This makes it a suitable detector for low-energy processes such as direct interactions of dark matter and coherent elastic neutrino-nucleus scattering

Observation of a nuclear recoil peak at the 100 eV scale induced by neutron capture

Coherent elastic neutrino-nucleus scattering and low-mass Dark Matter detectors rely crucially on the understanding of their response to nuclear recoils. We report the first observation of a nuclear recoil peak at around 112 eV induced by neutron capture. The measurement was performed with a CaWO$_4$ cryogenic detector from the NUCLEUS experiment exposed to a $^{252}$Cf source placed in a compact moderator. The measured spectrum is found in agreement with simulations and the expected peak structure from the single-$\gamma$ de-excitation of $^{183}$W is identified with 3 $\sigma$ significance. This result demonstrates a new method for precise, in-situ, and non-intrusive calibration of low-threshold experiments

The regulatory and market developments of covered bonds in Europe

This work analyses the reasons behind the growing success of covered bonds in the relation to the recent evolution in bank funding tools in Europe. The efficiency implications of the adoption of covered bonds for banks are explored, together with the impact this has on the financial system as a whole

Spin-dependent conductivity of nanosized magnetic inhomogeneities

Spin-dependent scattering (SDS) originates at the interface between magnetic (M) and non-magnetic (NM) regions, and gives rise to the giant magnetoresistive effect (GMR), that is observed when M and NM regions are interleaved at the nanoscale level. The GMR intensity, i.e. the change in resistivity observed due to the application of an external magnetic field, is affected by the SDS strength that, in its turn, is inversely proportional to the lateral size of the magnetic regions [1]. Such a lateral scale is the effective size of the magnetic regions [2], Deff, that is the result of the effect of magnetic interactions on the real size of the magnetic regions, D. Similarly, if the magnetic morphology of the system is not uniform, the SDS strength changes with the lateral scale of the magnetization inhomogeneities [3]. The dependence of GMR on the external field is the counterpart of Deff, so direct indications about D are not easily accessed. However, if we resort to the GMR efficiency, gamma, [4], i.e. the change in GMR for a unit change of squared magnetization, the comparison between the values of gamma measured at low and at high applied field as a function of Deff enables one to evidence D with respect to Deff. Indeed, if Deff is larger than D, as soon as the external applied field overcomes the effect of magnetic interactions, the efficiency of the magnetic structure is expected to change, as the large effective magnetic volumes break into smaller parts. As a consequence of that, the lateral scale of the system decreases, and gamma is expected to increase accordingly. In this work, we study different FexAg1-x nanogranular systems, where x is the relative Fe atomic concentration, 0.1 < x < 0.5. Under equilibrium conditions, Fe and Ag are not miscible, so using an out-of-equilibrium technique, in our case dc-magnetron sputtering, we obtain a deep intermixing of the two species. In this way, as a function of x, different samples with a different average magnetic length scale, namely with a different Deff, can be produced [3]. The evolution of the magnetic morphology of the systems was followed with zero-field-cooled and field-cooled magnetization measurements. Magnetization and GMR loops were recorded at two different temperatures, at 300 K and at 4 K. Indeed they represent two conditions were the contribution of interparticle interactions to systems dynamics is expected to be different. In this way, the effect of Deff can be better appreciated. We present the gamma dependence on x, measured both at low and at high applied field, gamma_L and gamma_H, respectively. For low values of x, gamma_L and gamma_H display the same dependence as a function of x, whilst for higher values gamma_L shows a broad maximum whilst gamma_L has a monotonic dependence that eventually approaches saturation. These data are presented and discussed and compared to magnetization loops and diffraction data in order to give an estimation of the Deff of the different samples. [1] S. Zhang and P. M. Levy, J. Appl. Phys. 73, 5315, 1993. [2] P. Allia, M. Coisson, F. Spizzo, P. Tiberto, F. Vinai, Phys. Rev. B 73 (2006) 054409 [3] P. Vavassori, E. Angeli, D. Bisero, F. Spizzo, F. Ronconi, J. Magn. Magn. Mater. 262 (2003) 52 [4] M. Tamisari, F. Spizzo, F. Ronconi, M. Sacerdoti, G. Battaglin, submitted to Journal of Applied Physics

Tailoring the exchange bias properties of Ni/NiO nanogranular samples by the structure control

The exchange bias (EB) effect has been studied in Ni/NiO nanogranular samples obtained by an original method that combines mechanical milling and hydrogen partial reduction of NiO. In this procedure, precursor NiO powder is ball-milled to reduce the grain size to the nanometric scale; then, the milled powder is subjected to high-temperature treatments in H2, inducing the reduction to metallic Ni. Typically, the samples consist of Ni nanocrystallites (size of the order of 10 nm) dispersed in a nanocrystalline NiO matrix, as observed by electron microscopy (HRTEM) [1]. In particular, Ni/NiO samples have been prepared by annealing in H2, at selected temperatures (200 < Tann < 300 °C), NiO powder previously milled for 5, 10, 20 and 30 hours. The structural features of the samples have been investigated by X-ray diffraction and the low-temperature magneto-thermal behavior and EB properties have been analyzed by SQUID magnetometry. The structure and composition of the Ni/NiO samples can be satisfactorily controlled during the synthesis procedure by varying both Tann and the milling time of the precursor NiO powders [2]. By increasing this last parameter, the mean grain size of the NiO phase reduces down to the final value of 16 nm and the microstrain increases, which is consistent with an enhancement of the structural disorder. The structure of the milled NiO matrix strongly affects the process of nucleation and growth of the Ni nanocrystallites, so that, Tann being equal, the amount and the mean grain size DNi of the Ni phase vary substantially in samples having different milling times. Such features of the Ni phase determine the extent of the Ni/NiO interface and consequently the magnitude of the exchange field Hex: the highest value (~ 940 Oe) has been measured at T = 5 K in a sample containing ~7 wt % Ni and with DNi = 19 nm. However, in Ni/NiO samples with very different structural characteristics and different values of Hex at T = 5 K, the EB effect vanishes at the same temperature (~ 200 K) and the same thermal dependence of Hex is observed. We consider that the evolution of the EB effect with temperature is ultimately determined by the microstructure of the Ni/NiO interface, which cannot be substantially modified by changing the synthesis parameters, milling time and Tann. [1] L. Del Bianco, F. Boscherini, A.L. Fiorini, M. Tamisari, F. Spizzo, M. Vittori Antisari, E. Piscopiello, Phys. Rev. B 77 (2008) 094408 [2] L. Del Bianco, F. Spizzo, M. Tamisari, J. Magn. Magn. Mater. (2009, in press

X-ray diffraction analysis on Fe-Ag nanocrystalline superparamagnetic films

The dependence of electronic conductivity on the electron spin state was first ob-served in multilayer systems made of magnetic (M) and non-magnetic (NM) layers [1]. It was later observed in nanogranular systems, as well, where M nanoparticles are dispersed into a NM matrix [2]. Thanks to this phenomenon, large variations of sample resistivity, rho, can be obtained by applying an external magnetic field, H, and this effect is called giant magnetoresistance (GMR). GMR is defined as (rho(H) - rho(0))/rho(0), where the numerator is affected by the magnetic properties of the sample through the spin dependent scattering whilst the denominator is the effect of the spin independent scattering sources, i.e. rough-ness or crystalline defects. As both terms are related to the structural properties of the nanogranular films, x-ray diffraction is a useful technique to probe the connection between growth conditions and GMR properties. We focused on FexAg100-x nanogranular films, where x is the Fe atomic relative con-centration and ranges from 0 up to 40, deposited on (100) Si substrates using dc-magnetron sputtering in cosputtering configuration and Ar atmosphere. The Fe/Ag phase diagram in-dicates that the two elements are not mutually soluble for any relative concentration but thanks to that out-of-equilibrium deposition technique it is possible to produce a system that at room temperature behaves like a magnetic nanogranular one [3]. Diffraction data show reflections ascribed to a FCC crystalline lattice. When x is 0, the values of the lattice parameter, d, are the same as the Ag bulk, but the ratio of the inten-sities I111/I200 is very large indicating a preferred (111) orientation. With increasing x, d values decrease, the full width at half maximum (FWHM) of the peaks increases while the intensity of the 200 peak progressively decreases. Finally, as a function of x, up to now no clear evidence of a BCC or FCC Fe crystalline phase is found. The shift of FCC peaks angular position with Fe concentration points out that iron and silver could possibly give rise to a solid solution, where Ag lattice undergoes a com-pressive strain due to the intermixing of Fe atoms [4]. The stress of the mixed Fe/Ag FCC lattice therefore possibly increase with x, and this fact could explain the FWHM increase, i.e. the reduction of the average crystalline grains size. On the other hand, for all the con-centrations, magnetization data show the presence of Fe precipitates whose size increases with x and goes from a few nanometers up to tens of nm. However, low temperature data support the presence of a Fe-Ag solid solution. In this work, all these data will be compared and discussed to find out a model of the samples that is in agreement with the presented magnetic and structural pictures of the me-tallic films. Eventually, new measurements performed on miscut Si substrates will be pre-sented to point out the possible presence of FCC or BCC Fe reflections, that could be actu-ally hidden by the 400 Si peak. [1] M. N. Baibich et al, Phys. Rev. Lett. 61 (1988) 2472. [2] A. E. Berkowitz et al., Phys. Rev. Lett. 68 (1992) 3745. [3] J.-Q. Wang, G. Xiao, Phys. Rev. B 49 (1994) 3982. [4] N. Kataoka et al., J. Phys. F, Met. Phys. 15 (1985) 140

Concentration dependence of interclusters interaction role in sputtered Fe-Ag nanogranular samples

An ideal nanogranular sample is an ensemble of independent nanosized magnetic (M) particles dispersed in a non magnetic (NM) matrix. The magnetic particles can be seen as large magnetic moments; in case their dynamics is affected just by thermal energy contributions, the whole system is referred to as a superparamagnet (SP) [1]. Real systems, due to growth conditions, often differ from this picture and interparticle magnetic interactions, e.g. dipolar or exchange ones, are one of the main sources of deviation from the ideal SP behaviour. If M and NM metallic species are used, the samples display a spin-dependent electronic resistivity that remarkably decreases if an external magnetic field, H, is applied, i.e. they show the so called giant magnetoresistance (GMR) [1]. GMR is ascribable to the magnetic ordering effect induced by H, so the higher the degree of disorder at zero field the larger the GMR effect. Interactions induce correlations among the magnetic moments, in particular when H is small, viz. an higher degree of order. As a consequence, the overall resistivity change is reduced. It’s therefore important to study interactions effects when H ≈ 0. In this work, we have studied dc-cosputtered nanogranular FexAg100-x thin films with a volume Fe concentration, x, varying from 10 up to 30 as measured by Rutherford Backscattering Spectrometry. At room temperature, for x < 20 a SP behaviour is observed. A recently devel-oped model [2], based on the simultaneous investigation of magnetic and GMR data, has pointed out that magnetic interactions affect samples dynamics for all concentrations. The correlation length, λ, is always larger than particles average distance and increases with tem-perature and x [2]. These systems are therefore suitable to study interparticle interactions and their effect on low-field magnetic configuration. The investigation was performed with sus-ceptibility measurements in field-cooled (FC) and zero-field-cooled (ZFC) configuration, re-laxation and Mössbauer measurements; X-Ray diffraction data were collected, as well. When x < 18, FC and ZFC data display the typical lambda shape but, for temperatures lower than the blocking temperature, FC signal displays an unexpected maximum at about 40 K. This effect is less and less pronounced as x increases and it vanishes starting from x = 18. The comparison between ZFC/FC curves and magnetic relaxation data confirms that interparticle interactions have a remarkable influence on low-field dynamics and this finding is supported by low-temperature Mössbauer measurements. However, the kind of interactions seems to change with x. Indeed, for low Fe concentration the samples possibly behave like a cluster-glass system, where frustrated interactions produce the FC maximum. Whilst approaching x ≈ 18, the interactions turn to dipolar and for higher concentrations the samples approach a re-entrant ferromagnetic behaviour. Eventually, X-Ray diffraction data suggest that the whole transition is related to the effects induced on samples structure/morphology by the change in iron concentration. [1] A. E. Berkowitz et al, Phys. Rev. Lett. 68 (1992) 3745 [2] P. Allia, M. Coisson, F. Spizzo, P. Tiberto, and F. Vinai, Phys. Rev. B 73 (2006) 05440