102,243 research outputs found
Spin-dependent conductivity of nanosized magnetic inhomogeneities
No full textSpin-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
NEW INSIGHTS OF MIR-145 FUNCTION AND REGULATION IN HUMAN BREAST CANCER.
No full textmiR-145 is down-regulated in the majority of human cancers, including breast cancer (BC).
However, its role remains largely unknown. Here, I provide evidence for miR-145 induced
anti-proliferative and pro-apoptotic effect in several BC cell lines, which was not detected in
BC cells lacking a functional TP53 gene and exhibiting an estrogen receptor alfa (ESR1)
negative status. I found that miR-145 anti-proliferative effects were dependent upon TP53
activation and that activation of TP53 could in turn stimulates miR-145 expression. I also
found that miR-145 could repress the expression of ESR1 protein by direct interaction with
two sites within its gene coding sequence. My findings support the existence of a positive
regulatory loop where miR-145 directly targets ESR1 and indirectly activates TP53, which in
turn sustains miR-145 expression and reinforces miR-145 overall effects on proliferation and
apoptosis
Concentration dependence of interclusters interaction role in sputtered Fe-Ag nanogranular samples
No full textAn 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
Verso la costruzione dell’Euroregione Alpino-Adriatica. La pianificazione congiunta e condivisa del territorio come pratica di coesione transfrontaliera
No full textThermal 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
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
Total radiation losses and emissivity profiles in RFX
No full textIn the framework of adding new information to the reversed field pinch (RFP) confinement database, the radiation emitted by RFX plasmas has been investigated using an eight chord bolometric camera, whose detectors have been calibrated absolutely. From the experimental data the emissivity profiles have been reconstructed by means of a generalized tomography reconstruction algorithm. This analysis confirms that the radiation emitted in RFX is systematically concentrated at the edge. The dependence of the emitted power on the plasma density shows that the radiation increases approaching the high density regime, but it rarely goes beyond 30% of the input power for stationary discharges. This behaviour is strongly dependent on the concentration of impurities but, in any case, in RFX there is no evidence of disruptions. A simple local energy balance allows a preliminary evaluation of the radial heat flux profile to be obtained. These measurements indicate that an active impurity screening mechanism is acting in the edge and that transport is the major energy loss mechanism in RFX
Interplay between GMR intensity and efficiency in the FeAg nanogranular system
No full textThe dependence of electronic conductivity on the electron spin state was observed in nanogranular systems where magnetic (M) nanoparticles are dispersed into a non-magnetic (NM) matrix. The nanogranular systems 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. When the magnetoresistive properties of an ensemble of independent nanogranular particles [2] are considered, GMR is proportional to (m/msat)^2, where m is the sample magnetization. The proportionality constant γ can be interpreted as the GMR efficiency, i.e. as the overall GMR variation in correspondence with a unit change of reduced square magnetization, so it can be seen as an indication of how effective is the granular structure in producing GMR. We investigate how γ and the GMR intensity change with the Fe concentration.
We focused on FexAg1-x nanogranular films, where x is the Fe atomic relative concentration and ranges from 0 up to 0.50, as measured by Rutherford Backscattering Spectrometry. The samples were deposited on Si substrates using dc-magnetron sputtering in cosputtering configuration and Ar atmosphere. The Fe-Ag phase diagram indicates 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].
X-ray diffraction measurements have been performed to investigate the structural properties of the samples. The granular films exhibit three different kind of structures: for x 0.32 there are bcc Fe cluster and Fe-Ag saturated solid solution. On the other hand, for all the concentrations, magnetization data show the presence of Fe precipitates whose size increases with x and the Mössbauer investigation confirms this picture. The GMR intensity is maximum for x = 0.32, while the maximum of γ is observed for x = 0.26. The maximum GMR effect is the best arrangement between a structure that displays the γ maximum, a not-saturated solid solution with very small Fe clusters and a structure with a high concentration of magnetic material. In particular, when the maximum GMR effect is observed, the distance between the magnetic clusters is of the order of the electron spin diffusion length.
[1] A. E. Berkowitz et al., Phys. Rev. Lett. 68 (1992) 3745.
[2] S. Zhang and P. M. Levy, J. Appl. Phys. 73 (1993) 5315
[3] J.Q. Wang, G. Xiao, Phys. Rev. B 49 (1994) 3982
Spin waves in exchange-coupled NiFe/IrMn/NiFe trilayers
No full textMagnetic films consisting of an antiferromagnetic (AFM) layer sandwiched between two non-equivalent ferromagnetic (FM) layers have recently attracted a great interest. In fact, in this configuration, each interface of the AFM layer is exchange-coupled to a different FM layer and information on the AFM magnetic structure may be obtained by the analysis of the exchange bias effect [1].
In this work, we focus on the spin wave properties dynamics of samples (deposited by DC magnetron sputtering in a magnetic field of 400 Oe) with layer-stacking sequence Si/Cu[5 nm]/Py[5 nm]/IrMn[10 nm]/Cu[tCu]/Py[10 nm], where Py is Ni80Fe20, IrMn is Ir25Mn75, and tCu, the nominal thickness of a Cu spacer, is varied in the 0 - 0.2 nm range. Since tCu is lower than the Cu lattice parameter, Cu is expected to grow in form of islands at the top AFM/FM interface, thus providing another tool, a part from the FM layer thickness, to diversify the top from the bottom interface and to tune the strength of the exchange coupling. As a consequence, the magnetization loops of the samples, measured by magneto-optic Kerr effect (MOKE) magnetometry, actually feature two distinct loops, corresponding to the magnetization process of the two Py layers. The relative orientation of the magnetization vectors of the Py layers (parallel P or antiparallel AP) can be thus controlled by an external magnetic field H. The spin-wave properties were studied by Brillouin Light Scattering (BLS): spectra were acquired at room temperature at an angle of incidence of 40° by sweeping H over the upper branch of the hysteresis loop (from negative to positive saturation) and encompassing both the P and AP alignment of the Py layers.
The BLS results were satisfactorily reproduced by a theoretical model we developed for exchange-coupled AFM/FM bilayers [2]. To exemplify the information obtainable through this approach, in Fig. 1 the results for the sample with tCu = 0 Å are shown. In the P states the Stokes (S) and the anti-Stokes (AS) modes are degenerate, so only the difference between optical and acoustic branches is observed. The calculated frequencies are in good agreement with the BLS ones apart from slight discrepancies, especially observed for the high-frequency mode. The S-AS degeneracy is removed in the AP state, between -0.05 kOe and -0.15 kOe, where four modes are found, well reproduced by the analytical model: the agreement is good for the high frequency modes, but only satisfactory for the low frequency ones. Similar results were detected for tCu = 0.1 nm and 0.2 nm. The Cu insertion induced just a change of the extension of the field regions corresponding to the P and AP configurations, whilst the frequency values were nearly unchanged, so the frequency values in the AP configuration are still not well approximated. This discrepancy can be tentatively ascribed to the fact that in the AP state the IrMn spins must comply the exchange coupling with the spins of the two Py layers, which have opposite orientations. This last process is expected to directly involve the interfacial IrMn spins, but reasonably also the ‘bulk’ ones will be implicated. Hence, compared to the case of AFM/FM bilayers, a more accurate analytical description of the IrMn magnetic configuration, that is the aim of this investigation, is necessary.
This work was partially supported by MIUR-PRIN 2010–11 Project No. 2010ECA8P3 “DyNanoMag” and by MIUR-FIRB2010 Project No. RBFR10E61T “NANOREST”
[1] A. N. Dobrynin, D. Givord, Phys. Rev. B 85 (2012) 014413
[2] G. Gubbiotti, S. Tacchi, L. Del Bianco, E. Bonfiglioli, L. Giovannini, M. Tamisari, F. Spizzo, R. Zivieri, J. Appl. Phys. 117 (2015) 17D150
Characterization of metal quantum-dot composites by optical absorption spectroscopy
No full textComposite materials formed by nanometre-sized Ag or Cu clusters in glass are studied by optical absorption spectroscopy. The technique is applied to composites prepared by recently developed new methodologies, showing peculiar spectral features depending on the preparation route. Tests of optical absorption theoretical models are presented
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