A nanocomposite material is obtained by coupling superparamagnetic magnetite nanoparticles (Fe 3 ... more A nanocomposite material is obtained by coupling superparamagnetic magnetite nanoparticles (Fe 3 O 4 NP) and vapor phase grown zinc oxide nanostructures with 'tetrapod' morphology (ZnO TP). The aim is the creation of a multifunctional material which retains the attractive features of ZnO (e.g. surface reactivity, strong UV emission, piezoelectricity) together with added magnetism. Structural, morphological, optical, magnetic and functional characterization are performed. In particular, the high saturation magnetization of Fe 3 O 4 NP (above 50 A m 2 kg −1 ), the strong UV luminescence and the enhanced photocatalytic activity of coupled nanostructures are discussed. Thus the nanocomposite turns out to be suitable for applications in energy harvesting and conversion, gas-and bio-sensing, bio-medicine and filter-free photocatalysis.
A combined experimental and theoretical Brillouin light scattering study of thermally excited spi... more A combined experimental and theoretical Brillouin light scattering study of thermally excited spin waves in Fe/Co multilayers with three and five magnetic layers in direct contact is presented. A large number of discrete, well-resolved peaks, classified as either surface or bulk standing modes of the stack, appear in the measured spectra. The investigation of their frequency dependence on the magnetic field intensity and incidence angle of light allowed us to determine the complete set of magnetic parameters of the multilayer. The interlayer exchange coupling and perpendicular interface anisotropy are discussed in detail. The profiles of the dynamic magnetization associated with different spin wave modes are calculated and utilized for the calculation of the Brillouin scattering cross section. The result, compared with experimental spectra, allowed the determination of the ratio between the magneto-optic constants of Fe and Co.
The magnetic behavior of the perpendicular exchange-spring bilayer and multilayer, constituted of... more The magnetic behavior of the perpendicular exchange-spring bilayer and multilayer, constituted of a hard and a soft phase that are exchange-coupled on a nanometric scale, is analyzed by a one-dimensional micromagnetic model leading to a complete magnetic phase diagram in terms of layer thicknesses. The validity of the one-dimensional assumption for the perpendicular situation is demonstrated. The phase diagram provides information on the type of demagnetization processes and the critical fields at which nucleation and reversal take place, depending on the intrinsic properties of the chosen soft and hard materials. An analytical expression of the reversal field is deduced for relatively large thicknesses. Moreover, the effect of a reduced interlayer coupling is also taken into account, leading to slight modifications of both the magnetic phase diagram and the hysteresis loops. A series of Fe/ FePt bilayers, prepared by sputtering, has been used to evaluate the predictions of the model, which has also been tested with the available literature data on FeRh/ FePt bilayers. Both systems have a particular relevance for potential applications in magnetic recording as well as magnetic microelectromechanical systems.
The magnetization reversal mechanism in perpendicular soft/hard Fe/FePt exchange-coupled bilayers... more The magnetization reversal mechanism in perpendicular soft/hard Fe/FePt exchange-coupled bilayers has been investigated as a function of the soft layer thickness (t Fe = 2, 3.5, 5 nm) combining magnetization loops at variable angle, magnetic domain analysis by magnetic force microscopy and numerical micromagnetic simulations. The analytical model proposed in the literature can properly account for some features of the reversal mechanism, such as positive nucleation fields and the reduction of the perpendicular coercive field and remanence by increasing the soft layer thickness, but cannot satisfactorily describe the magnetization process of real systems. We showed that for a thickness of the soft layer exceeding the FePt exchange length (∼2 nm), numerical micromagnetic calculations are needed to reproduce experimental observations. Indeed, just above the coercive field, the magnetization reversal does not proceed in single step switching, as predicted by the analytical model, but according to a more complex process: evolution of nucleated magnetic domains whose magnetization is approximately along the surface normal in the hard layer and slightly out of the film plane in the soft layer, followed by rotation of Fe moments along the field direction.
A combined experimental and theoretical Brillouin light scattering study of thermally excited spi... more A combined experimental and theoretical Brillouin light scattering study of thermally excited spin waves in Fe/Co multilayers with three and five magnetic layers in direct contact is presented. A large number of discrete, well-resolved peaks, classified as either surface or bulk standing modes of the stack, appear in the measured spectra. The investigation of their frequency dependence on the magnetic field intensity and incidence angle of light allowed us to determine the complete set of magnetic parameters of the multilayer. The interlayer exchange coupling and perpendicular interface anisotropy are discussed in detail. The profiles of the dynamic magnetization associated with different spin wave modes are calculated and utilized for the calculation of the Brillouin scattering cross section. The result, compared with experimental spectra, allowed the determination of the ratio between the magneto-optic constants of Fe and Co.
The effect of pressures up to 1.2 GPa on saturated magnetization M S of Ni 2+x Mn 1Àx Ga (0pxp0.2... more The effect of pressures up to 1.2 GPa on saturated magnetization M S of Ni 2+x Mn 1Àx Ga (0pxp0.2) alloys has been studied at temperatures from 5 to 400 K. The pressure parameter, dln M S /dp at 5 K, varies from À0.3 Â 10 À2 GPa À1 up to À1.3 Â 10 À2 GPa À1 . At temperature T M of martensitic transition (MT), dln M S /dp changes its sign. M S and the Curie temperature T C of the cubic phase of compounds increase with increasing pressure. These results are consistent with a theoretical picture of band structure of Ni 2 MnGa assuming a broadening and a depletion of the dominant minority Ni e g band under pressure. r
Superparamagnetic Fe(3)O(4) diphenyl nanoparticles were prepared according to a solvothernal proc... more Superparamagnetic Fe(3)O(4) diphenyl nanoparticles were prepared according to a solvothernal procedure and characterized by X-ray diffraction, infrared spectroscopy, surface area measurements, scanning electron microscopy, X-ray photoelectron spectroscopy and transmission electron microscopy. The magnetic phases present in the nanoparticle samples were analyzed by thermomagnetic analysis and the samples' magnetic properties were studied by vibrating sample magnetometry. The resulting nanoparticles having an average diameter of 200 nm were then used as solid-phase extraction sorbent for the determination of polycyclic aromatic hydrocarbons in urine samples. Method validation proved the feasibility of the developed beads for the quantitation of the investigated analytes at trace levels obtaining lower limit of quantitation values in the ng/l range. A good precision with coefficients of variations always lower than 15% was obtained. Finally, the superior extraction performance of the synthesized nanoparticles with respect to commercially available beads was proved.
Magnetometric and magnetic force microscopy (MFM) measurements were performed on [Co(6 Å)/Au(27 Å... more Magnetometric and magnetic force microscopy (MFM) measurements were performed on [Co(6 Å)/Au(27 Å)]27 multilayers grown at different sputtering pressures (0.8-9 Pa). All the as-prepared samples display perpendicular magnetic anisotropy that has its maximum value at intermediate pressures, while a monotonic decrease of the saturation magnetization was found with increasing pressure. As found by MFM, a relevant characteristic of these systems is the magnetic stripe domain structure. The application of the stripe domain model of Draaisma and de Jonge [H. J. G. Draaisma and W. J. M. de Jonge, J. Appl. Phys. 62, 3318 (1987)] allows us to satisfactorily reproduce the magnetization processes and to determine the anisotropy constants (Keff) starting from the domain width. The obtained Keff values display the same behavior and order of magnitude as those deduced from the area enclosed between parallel and perpendicular magnetization curves. The measurement of the stripe domain size with MFM thus results in an easy and reliable method for the evaluation of the anisotropy constants in multilayers with perpendicular anisotropy.
We report the magnetic and magnetocaloric properties in Co-doped Ni-Mn-Ga Heusler alloys around t... more We report the magnetic and magnetocaloric properties in Co-doped Ni-Mn-Ga Heusler alloys around the Mn-rich composition Ni 50 Mn 30 Ga 20 . The presence of Co affects profoundly the critical temperatures and alters the exchange interactions of martensite and austenite to different extents; by varying the composition it is possible to tune the critical temperatures and to induce a paramagnetic gap between the magnetically ordered martensite and magnetic austenite, thus giving rise to a reverse magnetostructural transformation. Contrary to the Co-free alloys, the saturation magnetization moment of austenite is strongly enhanced by Co with respect to the martensitic one: thus the magnetocaloric effect turns from direct into inverse. Remarkable values of the magnetic properties related to the magnetocaloric effect, e.g. the saturation magnetization jump at the transformation (DM) and the field dependence of the transformation temperature (dT/dH) are reported, together with high positive values of the isothermal magnetic entropy change (DS). The conditions for enhancing the magnetocaloric properties and triggering the sign reversal of the magnetocaloric effect are discussed.
We investigate equilibrium properties of an exchange-spring magnetic system constituted of a soft... more We investigate equilibrium properties of an exchange-spring magnetic system constituted of a soft layer (e.g. Fe) of a given thickness on top of a hard magnetic layer (e.g. FePt). The magnetization profile M (z) as a function of the atomic position ranging from the bottom of the hard layer to the top of the soft layer is obtained in two cases with regard to the hard layer: i) in the case of a rigid interface (the FePt layer is a single layer), the profile is obtained analytically as the exact solution of a sine-Gordon equationwith Cauchy's boundary conditions. Additional numerical simulations also confirm this result. Asymptotic expressions of M (z) show a linear behavior near the bottom and the top of the soft layer. In addition, a critical value of the number of atomic planes in the soft layer, that is necessary for the onset of spin deviations, is obtained in terms of the anisotropy and exchange coupling between the adjacent plane in the soft layer. ii) in the case of a relaxed interface (the FePt layer is a multilayer), the magnetization profile is obtained numerically for various Fe and FePt films thicknesses and applied field.
A nanocomposite material is obtained by coupling superparamagnetic magnetite nanoparticles (Fe 3 ... more A nanocomposite material is obtained by coupling superparamagnetic magnetite nanoparticles (Fe 3 O 4 NP) and vapor phase grown zinc oxide nanostructures with 'tetrapod' morphology (ZnO TP). The aim is the creation of a multifunctional material which retains the attractive features of ZnO (e.g. surface reactivity, strong UV emission, piezoelectricity) together with added magnetism. Structural, morphological, optical, magnetic and functional characterization are performed. In particular, the high saturation magnetization of Fe 3 O 4 NP (above 50 A m 2 kg −1 ), the strong UV luminescence and the enhanced photocatalytic activity of coupled nanostructures are discussed. Thus the nanocomposite turns out to be suitable for applications in energy harvesting and conversion, gas-and bio-sensing, bio-medicine and filter-free photocatalysis.
A combined experimental and theoretical Brillouin light scattering study of thermally excited spi... more A combined experimental and theoretical Brillouin light scattering study of thermally excited spin waves in Fe/Co multilayers with three and five magnetic layers in direct contact is presented. A large number of discrete, well-resolved peaks, classified as either surface or bulk standing modes of the stack, appear in the measured spectra. The investigation of their frequency dependence on the magnetic field intensity and incidence angle of light allowed us to determine the complete set of magnetic parameters of the multilayer. The interlayer exchange coupling and perpendicular interface anisotropy are discussed in detail. The profiles of the dynamic magnetization associated with different spin wave modes are calculated and utilized for the calculation of the Brillouin scattering cross section. The result, compared with experimental spectra, allowed the determination of the ratio between the magneto-optic constants of Fe and Co.
The magnetic behavior of the perpendicular exchange-spring bilayer and multilayer, constituted of... more The magnetic behavior of the perpendicular exchange-spring bilayer and multilayer, constituted of a hard and a soft phase that are exchange-coupled on a nanometric scale, is analyzed by a one-dimensional micromagnetic model leading to a complete magnetic phase diagram in terms of layer thicknesses. The validity of the one-dimensional assumption for the perpendicular situation is demonstrated. The phase diagram provides information on the type of demagnetization processes and the critical fields at which nucleation and reversal take place, depending on the intrinsic properties of the chosen soft and hard materials. An analytical expression of the reversal field is deduced for relatively large thicknesses. Moreover, the effect of a reduced interlayer coupling is also taken into account, leading to slight modifications of both the magnetic phase diagram and the hysteresis loops. A series of Fe/ FePt bilayers, prepared by sputtering, has been used to evaluate the predictions of the model, which has also been tested with the available literature data on FeRh/ FePt bilayers. Both systems have a particular relevance for potential applications in magnetic recording as well as magnetic microelectromechanical systems.
The magnetization reversal mechanism in perpendicular soft/hard Fe/FePt exchange-coupled bilayers... more The magnetization reversal mechanism in perpendicular soft/hard Fe/FePt exchange-coupled bilayers has been investigated as a function of the soft layer thickness (t Fe = 2, 3.5, 5 nm) combining magnetization loops at variable angle, magnetic domain analysis by magnetic force microscopy and numerical micromagnetic simulations. The analytical model proposed in the literature can properly account for some features of the reversal mechanism, such as positive nucleation fields and the reduction of the perpendicular coercive field and remanence by increasing the soft layer thickness, but cannot satisfactorily describe the magnetization process of real systems. We showed that for a thickness of the soft layer exceeding the FePt exchange length (∼2 nm), numerical micromagnetic calculations are needed to reproduce experimental observations. Indeed, just above the coercive field, the magnetization reversal does not proceed in single step switching, as predicted by the analytical model, but according to a more complex process: evolution of nucleated magnetic domains whose magnetization is approximately along the surface normal in the hard layer and slightly out of the film plane in the soft layer, followed by rotation of Fe moments along the field direction.
A combined experimental and theoretical Brillouin light scattering study of thermally excited spi... more A combined experimental and theoretical Brillouin light scattering study of thermally excited spin waves in Fe/Co multilayers with three and five magnetic layers in direct contact is presented. A large number of discrete, well-resolved peaks, classified as either surface or bulk standing modes of the stack, appear in the measured spectra. The investigation of their frequency dependence on the magnetic field intensity and incidence angle of light allowed us to determine the complete set of magnetic parameters of the multilayer. The interlayer exchange coupling and perpendicular interface anisotropy are discussed in detail. The profiles of the dynamic magnetization associated with different spin wave modes are calculated and utilized for the calculation of the Brillouin scattering cross section. The result, compared with experimental spectra, allowed the determination of the ratio between the magneto-optic constants of Fe and Co.
The effect of pressures up to 1.2 GPa on saturated magnetization M S of Ni 2+x Mn 1Àx Ga (0pxp0.2... more The effect of pressures up to 1.2 GPa on saturated magnetization M S of Ni 2+x Mn 1Àx Ga (0pxp0.2) alloys has been studied at temperatures from 5 to 400 K. The pressure parameter, dln M S /dp at 5 K, varies from À0.3 Â 10 À2 GPa À1 up to À1.3 Â 10 À2 GPa À1 . At temperature T M of martensitic transition (MT), dln M S /dp changes its sign. M S and the Curie temperature T C of the cubic phase of compounds increase with increasing pressure. These results are consistent with a theoretical picture of band structure of Ni 2 MnGa assuming a broadening and a depletion of the dominant minority Ni e g band under pressure. r
Superparamagnetic Fe(3)O(4) diphenyl nanoparticles were prepared according to a solvothernal proc... more Superparamagnetic Fe(3)O(4) diphenyl nanoparticles were prepared according to a solvothernal procedure and characterized by X-ray diffraction, infrared spectroscopy, surface area measurements, scanning electron microscopy, X-ray photoelectron spectroscopy and transmission electron microscopy. The magnetic phases present in the nanoparticle samples were analyzed by thermomagnetic analysis and the samples' magnetic properties were studied by vibrating sample magnetometry. The resulting nanoparticles having an average diameter of 200 nm were then used as solid-phase extraction sorbent for the determination of polycyclic aromatic hydrocarbons in urine samples. Method validation proved the feasibility of the developed beads for the quantitation of the investigated analytes at trace levels obtaining lower limit of quantitation values in the ng/l range. A good precision with coefficients of variations always lower than 15% was obtained. Finally, the superior extraction performance of the synthesized nanoparticles with respect to commercially available beads was proved.
Magnetometric and magnetic force microscopy (MFM) measurements were performed on [Co(6 Å)/Au(27 Å... more Magnetometric and magnetic force microscopy (MFM) measurements were performed on [Co(6 Å)/Au(27 Å)]27 multilayers grown at different sputtering pressures (0.8-9 Pa). All the as-prepared samples display perpendicular magnetic anisotropy that has its maximum value at intermediate pressures, while a monotonic decrease of the saturation magnetization was found with increasing pressure. As found by MFM, a relevant characteristic of these systems is the magnetic stripe domain structure. The application of the stripe domain model of Draaisma and de Jonge [H. J. G. Draaisma and W. J. M. de Jonge, J. Appl. Phys. 62, 3318 (1987)] allows us to satisfactorily reproduce the magnetization processes and to determine the anisotropy constants (Keff) starting from the domain width. The obtained Keff values display the same behavior and order of magnitude as those deduced from the area enclosed between parallel and perpendicular magnetization curves. The measurement of the stripe domain size with MFM thus results in an easy and reliable method for the evaluation of the anisotropy constants in multilayers with perpendicular anisotropy.
We report the magnetic and magnetocaloric properties in Co-doped Ni-Mn-Ga Heusler alloys around t... more We report the magnetic and magnetocaloric properties in Co-doped Ni-Mn-Ga Heusler alloys around the Mn-rich composition Ni 50 Mn 30 Ga 20 . The presence of Co affects profoundly the critical temperatures and alters the exchange interactions of martensite and austenite to different extents; by varying the composition it is possible to tune the critical temperatures and to induce a paramagnetic gap between the magnetically ordered martensite and magnetic austenite, thus giving rise to a reverse magnetostructural transformation. Contrary to the Co-free alloys, the saturation magnetization moment of austenite is strongly enhanced by Co with respect to the martensitic one: thus the magnetocaloric effect turns from direct into inverse. Remarkable values of the magnetic properties related to the magnetocaloric effect, e.g. the saturation magnetization jump at the transformation (DM) and the field dependence of the transformation temperature (dT/dH) are reported, together with high positive values of the isothermal magnetic entropy change (DS). The conditions for enhancing the magnetocaloric properties and triggering the sign reversal of the magnetocaloric effect are discussed.
We investigate equilibrium properties of an exchange-spring magnetic system constituted of a soft... more We investigate equilibrium properties of an exchange-spring magnetic system constituted of a soft layer (e.g. Fe) of a given thickness on top of a hard magnetic layer (e.g. FePt). The magnetization profile M (z) as a function of the atomic position ranging from the bottom of the hard layer to the top of the soft layer is obtained in two cases with regard to the hard layer: i) in the case of a rigid interface (the FePt layer is a single layer), the profile is obtained analytically as the exact solution of a sine-Gordon equationwith Cauchy's boundary conditions. Additional numerical simulations also confirm this result. Asymptotic expressions of M (z) show a linear behavior near the bottom and the top of the soft layer. In addition, a critical value of the number of atomic planes in the soft layer, that is necessary for the onset of spin deviations, is obtained in terms of the anisotropy and exchange coupling between the adjacent plane in the soft layer. ii) in the case of a relaxed interface (the FePt layer is a multilayer), the magnetization profile is obtained numerically for various Fe and FePt films thicknesses and applied field.
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Papers by F. Casoli