86,556 research outputs found
Characterization of magnetic IrMn/NiFe nanostructures
No full textIt is now largely demonstrated that magnetic systems structured on a nanometric length scale may exhibit peculiar magnetic properties, generally indicated as disordered magnetism effects, originating from the concomitant presence of topological disorder and competing magnetic interactions, as a consequence of the lack of structural periodicity. It is to be expected that this complex mix of magnetic effects play a relevant role also in the behavior of exchange coupled antiferromagnetic (AFM)/ferromagnetic (FM) systems, where the torque action exerted by the interfacial AFM spins on the FM ones brings about the insurgence of an unidirectional exchange anisotropy for the FM magnetization, and then the exchange bias (EB) effect.
In this work the EB properties of the IrMn/NiFe system in form of continuous film and of arrays of square dots with different size (1000, 500 and 300 nm), produced by e-beam lithography and dc-sputtering deposition, have been investigated. The stacking of the final samples was Si (substrate)/Cu[5 nm]/IrMn[10 nm]/NiFe[5 nm] (in square brackets the nominal thickness of each layer is reported). Structural investigations by TEM of the continuous IrMn/NiFe film indicate that the bulk of the AFM layer consists of nanograins with mean size ~ 10 nm, whereas the NiFe phase appears amorphous. In particular, the TEM analysis reveals the existence of a structurally disordered IrMn region (2-3 nm thick) at the interface between the FM phase and the bulk of the AFM layer. This finding, namely the evidence of structural disorder, strongly supports the spin-glass like magnetic character of the interfacial IrMn region, deduced by SQUID measurements on the continuous film, Figure 3. The existence of a low temperature frozen collective regime of the interfacial AFM spins and its break up above a critical temperature, that we have schematically settled at T = 100 K, determine the thermal evolution of the EB properties. In fact, below 100 K, the exchange coupling between IrMn and NiFe is ruled by magnetically correlated, frozen AFM spins, subjected to a strong effective local anisotropy, which results in high Hex (exchange field) and HC (coercivity) values. Above 100 K, the AFM/FM coupling is governed by a fraction of interfacial AFM spins, magnetically polarized by the AFM nanograins. Hence, Hex and HC decrease more and more with rising T, reflecting the progressive thermal instability of the AFM nanograins, assumed as essentially non-interacting. The spatial confinement, namely the passage from the continuous film to the dot structure, affects the exchange coupling mechanism when the dot size enters the sub-micron regime [1].
This research work has been carried out in the framework of the project FIRB 2010 “Tailoring the magnetic anisotropy of nanostructures for enhancing the magnetic stability of magnetoresistive devices”-NANOREST
References
1. F. Spizzo, E. Bonfiglioli, M. Tamisari, A. Gerardino, G. Barucca, A. Notargiacomo , F. Chinni, L. Del Bianco, “Magnetic exchange coupling in IrMn/NiFe nanostructures: from the continuous film to dot arrays”, Physical Review B, 91, pp.064410 1-9, 2015
On the Sub-Supersolution Approach for Dirichlet Problems driven by a (p(x), q(x))-Laplacian Operator with Convection Term
No full textThe method of sub and super-solution is applied to obtain existence and location of solutions to a quasilinear elliptic problem with variable exponent and Dirichlet boundary conditions involving a nonlinear term f depending on solution and on its gradient. Under a suitable growth condition on the convection term f, the existence of at least one solution satisfying a priori estimate is obtained
Tailoring the exchange coupling in IrMn/NiFe films and nanodots by interface confinement
No full textThe scientific interest towards antiferromagnetic (AFM) materials has been increasing continuously mainly because of their crucial role in the operating principle of modern, miniaturized spintronic devices. In spin valves and tunnel junctions, a fine control of the magnetization reversal process in the ferromagnetic (FM) electrodes is usually achieved through the interface exchange coupling with an AFM layer [1]: the torque action exerted by the interfacial AFM spins on the FM ones brings about the insurgence of an unidirectional exchange anisotropy for the FM magnetization, and then of the exchange bias (EB) effect. Moreover, since the strategic importance of the EB effect in the technology of magnetoresistive spin valves and tunnel junctions and the increasing demand of miniaturization of modern devices (magnetic sensors, high-density data storage media), it is admittedly crucial to expand the description of the EB mechanism so as to include the effects of spatial confinement.
The exchange interaction between AFM and FM interfacial spins depends, in polycrystalline systems, on the magnetic anisotropy of the bulk AFM phase and on the size distribution of the crystalline AFM grains [2]. Recent investigations have proposed the presence of disordered AFM spins at the AFM/FM interface, with spin-glass-like magnetic properties [3]. With this respect, we have recently observed that, at low temperature, these disordered AFM spins are frozen in a magnetic disordered state and are collectively involved in the exchange coupling with the FM moments, showing a magnetic correlation length, lambda [4]. With increasing temperature, lambda progressively shortens (we have established that at T ~ 100 K the frozen collective regime breaks up) even if the AFM spins do not enter the full paramagnetic regime due to the polarizing action of adjacent FM and AFM spins. Due to that, when interface confinement is observed, namely passing from a continuous film to a nanodot or when the morphology of the AFM/FM interface is modulated at the nanoscale, lambda is expected to play a role in the EB effect.
In this contribution, we present our study on the mechanism of the magnetic exchange coupling in the Ir25Mn75/Ni20Fe80 system. The interface confinement has been accomplished in different ways: by producing that system in form of arrays of dots with different size D = 1000, 500, 300, 140 nm and by inserting, in the continuous films, a Cu spacer with a nominal thickness, tCu, of the order of 1Å. Due to the small tCu value, Cu islands, whose presence was confirmed by X-Ray Absorption Fine Structure investigations, are obtained at the AFM/FM interface. The EB properties of the samples, i.e. exchange field HEX and coercivity HC, and their thermal dependance, were investigated by SQUID and MOKE magnetometers in the 5-300 K temperature range. The role of in the dots arrays was reflected by the strong dependence of HEX on D. In more detail, at 5 K HEX ~ 750 Oe when D = 1000 nm, whilst HEX ~ 1100 Oe when D = 1000 nm; when D = 140 nm, HEX decreases down to ~ 100 Oe [5]. In the continuous films with the Cu insertion at the interface, we observed that, at high temperature, the change in the HEX value may be explained just in terms of a dilution effect, namely in terms of the reduction of the extension of the AFM/FM interface. Differently, at low temperature, i.e. when lambda approaches the interdistance between Cu islands, the HEX values strongly depend on tCu, namely on Cu islands size/interdistance. These findings will be presented and discussed, taking also into account the results of micromagnetic calculations. [Research sponsored by MIUR Italy, project RBFR10E61T-NANOREST.]1. C. Chappert, A. Fert, F.N.V. Dau, Nature Mater. 6, 813 (2007)
2. G. Lhoutellier et al. J. Appl. Phys. 120, 193902 (2016)
3. V. Baltz et al. Phys. Rev. B 81, 052404 (2010).
4. F. Spizzo et al., Phys. Rev. B. 91, 064410 (2015)
5. F. Spizzo et al., J. Magn. Magn. Mater. 400, 242 (2016
Glass vessels from the monastery of Saint Severo in Classe (Ravenna – Italy): archaeological aspects and archaeometrical analyses of 13th- 16th century Italian glass
No full textMagnetic study of nanocomposite films consisting of ultrafine Co particles embedded in a ferromagnetic AuCo alloy
No full textNowadays, the search for innovative nanocomposite systems consisting of at least two different magnetic phases is attracting remarkable attention. Indeed, the intimate mixing of the different phases at the nanoscale level may give rise to new materials showing unique properties. In particular, a fine tuning of their overall magnetic anisotropy may be obtained and, accordingly, of their magnetic hysteretic properties.
In this framework, we present an in-depth study of the magnetic properties of a set of three AuCo films, ~ 30 nm thick, with different Au:Co concentration ratio (2:1, 1:1, 1:2). The samples were grown by magnetron sputtering co-deposition technique on naturally oxidized (100)-Si substrates, and it turns out that this method allows the alloying of Au and Co and the production of a bimetallic compound is achieved to a good extent. The samples mainly consist of a structurally disordered ferromagnetic alloy in which segregated Co particles (~ 2 nm in size) are dispersed and the two phases are finely intermixed.
Magnetization measurements, performed with a SQUID magnetometer in the 6 K - 300 K temperature range, have pointed out a peculiar hysteretic behavior, especially well visible in samples Au1Co1 and Au1Co2, characterized by in-plane anisotropy and crossed branches in the loops measured along the hard magnetization direction (Fig. 1(a)).
To elucidate the origin of this behavior, micromagnetic calculations have been carried out using the object-oriented micromagnetic framework (OOMMF) code. The calculations have been performed for a simplified system made of two exchange-coupled ferromagnetic phases: an AuCo matrix surrounding a Co cluster, i.e. an aggregate of smaller Co particles. Indeed, our experimental results suggest that the exchange coupling of the Co particles with the surrounding matrix may result in the formation of magnetic clusters. So our model emphasizes both the nanocomposite nature of the investigated samples and the role of interparticle magnetic interactions. The main features of the hysteretic behavior are qualitatively well reproduced provided that the two phases have almost orthogonal magnetic anisotropy axes. We hypothesize a dominant magnetoelastic character of the anisotropy in both phases and we discuss how this requirement can be plausibly fulfilled
Polymorphic miRNA-mediated gene contribution to inhibitor development in haemophilia A
No full textDevelopment of inhibitory antibodies is perhaps the most serious complication of FVIII replacement therapy, precluding efficient clinical management of patients with haemophilia A (HA). The development and function of immune system are also regulated by microRNAs (miRNAs). Mutations and changes in the level of expression of some miRNA genes have been associated with the onset and progression of immunological disorders. The aim of this study was to investigate new genetic polymorphisms in loci for miRNA and their targets to evaluate whether these SNPs may confer susceptibility to inhibitor development in patients with HA. Italian HA patients with and without inhibitors and healthy controls were recruited in this study. For SNP analysis, standard DNA sequencing method was used. We have studied four SNPs, i.e. rs36101366, rs34683807, rs1803603 and rs3024496 located in the 3′UTR of F8 and IL-10 genes. These SNPs have been checked for their frequencies in patients with and without inhibitors, but no statistically significant differences were found. Then, we have searched for other genetic variants in loci for haematopoietic-specific miRNAs, i.e. hsa-mir-150, hsa-mir-155, hsa-mir-146a, hsa-mir-142, hsa-mir-181a and in a specific miRNA, hsa-mir-1184, i.e. predicted to be located in the intron 22 of F8 gene. For all miRNAs selected, we did not identify any sequence variation in our study population. This is the first study to demonstrate that there was no association between selected SNPs in miRNAs and their targets and the susceptibility to inhibitor development in people affected by H
Idrossiapatite di sintesi come sostituto osseo: il gradiente di porosità e la carbonatazione a confronto.
No full textVariations on the Author
No full text“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Interplay between magnetic anisotropies in CoAu and Co films and antidot arrays: effects on the spin configuration and hysteretic behavior
No full textWe studied (i) a set of three Co: Au continuous films, grown by sputtering co-deposition (B80 nm thick)
with concentration ratios of 2 : 1, 1 : 1 and 1 : 0 (i.e., a pure Co film was also included), and (ii) a
corresponding set of antidot arrays, produced by nanosphere lithography with the same hexagonal
pattern (nominal lattice periodicity B520 nm). The samples were investigated by atomic and magnetic
force microscopy and SQUID magnetometry. A twofold aim was fulfilled: to gain information on the
magnetism of the CoAu compound (saturation magnetization, effective in-plane and out-of-plane
anisotropy, exchange stiffness constant and magnetostrictive behavior) and to compare the magnetic
behavior of the continuous and patterned samples. The continuous films exhibited a variety of hysteretic
behaviours and magnetic configurations, ruled by the interplay between different magnetic anisotropy
terms (magnetocrystalline, magnetoelastic and shape). The Co1Au1 film was anisotropic in the plane,
whereas Co2Au1 and Co were isotropic and had an out-of-plane magnetization component; stripe
domains were observed in Co2Au1, resulting in a transcritical hysteresis loop. A key role in determining
these properties was ascribed to the magnetoelastic anisotropy term. Unlike the continuous films, the
antidot arrays showed a similar hysteretic behavior and important similarities in the spin configuration
were pointed out, despite the different compositions. We argue, also based on micromagnetic
simulations, that this occurred because the nanopatterning enabled a local modification of the shape
anisotropy, thus smoothing out the differences observed in the continuous films
Thermal and spatial confinement effects in exchange coupled IrMn/NiFe dot arrays
No full textWe present a comprehensive study of the exchange bias phenomenon (EB) in an antiferromagnetic (AF)/ferromagnetic (FM) continuous film and in arrays of square dots with different size (D), aimed at elucidating thermal and spatial confinement effects on the AF/FM exchange coupling and their correlation with the AF structural and magnetic properties.
For this purpose, an AF/FM Ir25Mn75[10 nm]/Ni80Fe20[5 nm] continuous film and arrays of square dots (D = 1000 nm, 500 nm and 300 nm) were prepared by electron beam lithography and lift-off using dc-sputtering.
Structural investigations by electron microscopy techniques indicated that the AF layer consists of nanograins (mean size ~ 10 nm), but also clearly revealed the existence of a structurally disordered IrMn region (2-3 nm thick) at the interface with the NiFe phase.
The magnetic properties, in particular the temperature dependence of the exchange field Hex and coercivity HC, were studied by SQUID and MOKE measurements. At room temperature, Hex decreases with reducing the size of the dots and it is absent in the smallest ones, whereas the opposite trend is visible at T = 10 K (Hex ~ 1140 Oe for D = 300 nm).
The EB mechanism and its thermal evolution have been explained through a phenomenological model [1] that combines spatial confinement effects with other crucial items concerning the AF phase: the magnetothermal stability of the IrMn nanograins, the glassy magnetic nature of the structurally disordered IrMn region, the stabilization of a low-temperature (T < 100 K) frozen collective regime of the IrMn interfacial spins, implying the appearance of a length of magnetic correlation among them. The model predictions have been supported by micromagnetic calculations, satisfactorily reproducing the experimental findings.
This research work has been sponsored by MIUR under project FIRB2010-NANOREST.
[1] F. Spizzo et al., Phys. Rev. B 91 (2015) 06441
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