Materials Science and Engineering B-Solid State Materials For Advanced Technology, 2007
Ferromagnetism of La 0.67 Sr 0.33 MnO 3 is extremely sensitive to external perturbations like sub... more Ferromagnetism of La 0.67 Sr 0.33 MnO 3 is extremely sensitive to external perturbations like substrate-induced strain, charge injection and chemical interactions with neighbour layers. In this paper we discuss the perturbation induced by the presence of a metallic overlayer, typically deposited for electric contacts, in the prototypical case of the Au/La 0.67 Sr 0.33 MnO 3 interface. In particular we found a sizable decrease of the Curie temperature in thin films of La 0.67 Sr 0.33 MnO 3 after gold capping: 65 K for 5 nm thickness of the manganite. Apart from chemical reactions at the interface, charge injection-depletion induced by the difference in the work function between Au and La 0.67 Sr 0.33 MnO 3 could partially explain this phenomenon.
The authors report about La0.67Sr0.33MnO3 single crystal manganite thin films in interaction with... more The authors report about La0.67Sr0.33MnO3 single crystal manganite thin films in interaction with a gold capping layer. With respect to uncoated manganite layers of the same thickness, Au-capped 4nm thick manganite films reveal a dramatic reduction (≃185K) of the Curie temperature TC and a lower saturation low temperature magnetization M0. A sizable TC reduction (≃60K) is observed even when an inert SrTiO3 layer is inserted between the gold film and the 4nm thick manganite layer, suggesting that this effect might have an electrostatic origin.
The effect of the AlOx barrier thickness on magnetic and morphological properties of Ta/Co/(AlOx)... more The effect of the AlOx barrier thickness on magnetic and morphological properties of Ta/Co/(AlOx)/Alq3/Si hybrid structures was systematically studied by means of atomic force microscopy, superconducting quantum interference device magnetometry, and nuclear magnetic resonance (NMR). All used techniques pointed out that the barrier thickness of 2 nm is required to obtain a magnetically good cobalt layer on top of Alq3. C59o NMR measurements revealed that the AlOx barrier gives rise to the formation of an interface layer with “defective” cobalt favoring growth of “bulk” cobalt with good magnetic properties.
The effect of the AlOx barrier thickness on magnetic and morphological properties of Ta/Co/(AlOx)... more The effect of the AlOx barrier thickness on magnetic and morphological properties of Ta/Co/(AlOx)/Alq3/Si hybrid structures was systematically studied by means of atomic force microscopy, SQUID magnetometry and nuclear magnetic resonance (NMR). All used techniques pointed out that the barrier thickness of 2 nm is required to obtain a magnetically good cobalt layer on top of Alq3. 59 Co NMR measurements revealed that the AlOx barrier gives rise to the formation of an interface layer with "defective" cobalt favouring growth of "bulk" cobalt with good magnetic properties.
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.
We have measured by X-ray resonant magnetic scattering the stripe domain nucleation field and the... more We have measured by X-ray resonant magnetic scattering the stripe domain nucleation field and the stripe domain critical width of FePd epitaxial films in a wide temperature range. A detailed quantitative analysis of our data in the framework of the rigorous theory of domain nucleation in thin films, allowed to determine the temperature dependence of both the exchange stiffness A and the anisotropy constant K u . The proposed approach can be applied to thin magnetic films with perpendicular magnetic anisotropy, thus providing a way to measure A in a relevant class of magnetic materials. r
Technological procedures able to produce high quality electrodes from magnetic oxides in vertical... more Technological procedures able to produce high quality electrodes from magnetic oxides in vertical organic-inorganic hybrid devices is a challenging task in the field of organic spintronics. Thin films of magnetite ͑Fe 3 O 4 ͒ have been successfully grown directly on top of organic semiconductor layers, tris͑8-hydroxyquinoline͒aluminium͑III͒ ͑Alq 3 ͒, by pulsed-electron ablation technique. The films show ferromagnetic behavior and good structural quality, properties detected by magneto-optical Kerr effect, superconductor quantum interference device, micro-Raman spectroscopy, and Atomic Force Microscopy. The ferromagnetic behavior persists even for 10 nm thick films. Charge injection at magnetite-organic interface has been finally demonstrated by detecting electroluminescence from Alq 3 .
We present a magnetic characterisation of MnAs films grown by MOVPE on semiconductor substrates, ... more We present a magnetic characterisation of MnAs films grown by MOVPE on semiconductor substrates, for comparing the effect of thickness and substrate on their magnetic and magneto-thermal properties, with particular focus on the nature of the magneto-structural transition between the ferromagnetic hexagonal a-phase and the paramagnetic orthorhombic b-phase. The nature of this transformation depends on the presence of strain and turns out to be of second-order type for epitaxial-crystalline MnAs/(0 0 1)GaAs films and closer to a first-order type for polycrystalline MnAs/SiO 2 /Si films. The magnetocaloric effect at the transition has been estimated for thin film samples, resulting appreciably lower than that of bulk MnAs.
We present the study of Co/organic semiconductor (OS) stacks both from the morphological and magn... more We present the study of Co/organic semiconductor (OS) stacks both from the morphological and magnetic point of view. Co has been successfully used up to now as top contact of hybrid vertical devices. While the properties of Co grown on amorphous layers are well established, its deposition on soft materials presents critical aspects such as interfacial damage that affects its electrical and magnetic properties. In this work we focus on the influence of the morphology of the organic underlayer in the magnetic behavior of a Co thin film: tris(8-hydroxyquinoline) aluminum (Alq 3 ) grown in different conditions by molecular beam evaporation have been considered. A further considered aspect is the effect of the presence of a thin oxide barrier (Al 2 O 3 ) on the Co magnetic properties.
A thorough micromagnetic analysis of the exchange-spring problem is reported with special emphasi... more A thorough micromagnetic analysis of the exchange-spring problem is reported with special emphasis on multilayers constituted by hard-soft exchange-coupled phases. The developed one-dimensional micromagnetic model leads to a complete magnetic phase diagram in terms of layer thicknesses. Both perpendicular and parallel configurations are considered. 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. The model has been applied to a variety of hard magnetic phases (e.g., FePt, CoPt, SmCo and NdFeB), coupled to different soft materials (e.g., Fe, FeCo, FeRh or permalloy) both in the form of bilayers and multilayers. The most significant results will be highlighted. r
The magnetic properties and the magnetization process of electrodeposited thick films of Co-rich ... more The magnetic properties and the magnetization process of electrodeposited thick films of Co-rich CoPt alloys are studied with particular emphasis on the effects of growth rate, controlled by varying the plating current density, and of lateral confinement, analyzing patterned micro-cylinders. We find that varying the plating current density has virtually no effect on the composition of the samples, and hence on the intrinsic magnetic properties, a substantial increase of both coercivity and squareness is obtained when the current is raised. The films are fine-grained, oriented polycrystals with typical grain sizes in the range 50-150 nm, depending on the growth rate. The complex magnetization process is studied in detail by Magnetic Force Microscopy and shown to be governed by interaction domains. It is shown that further improvement of the squareness can be obtained by exploiting the lateral confinement in patterned samples.
The magnetic anisotropy of LaCo-hexaferrites has been investigated by the use of the singular poi... more The magnetic anisotropy of LaCo-hexaferrites has been investigated by the use of the singular point detection and the second harmonic techniques in the low-temperature range. Below 200K a steep rise of the anisotropy field has been observed together with a strong distortion of the magnetization curve which is interpreted as due to a negative K2 and a large positive K3 anisotropy constants. The results are analyzed in terms of an extended Stoner–Wohlfarth model and the obtained values of the anisotropy coefficients χ2,0,χ4,0,χ6,0 are reported versus temperature.
The demagnetization processes in epitaxially grown SmCo 5 / Fe/ SmCo 5 trilayers have been analyz... more The demagnetization processes in epitaxially grown SmCo 5 / Fe/ SmCo 5 trilayers have been analyzed in the framework of a one-dimensional micromagnetic model, exploiting the well-defined orientation of the easy magnetization axis present in both SmCo 5 layers. The applied magnetic field is considered along easy, hard, and tilted directions. For the case of hard direction, the nucleation field equation and the analytical expression of the critical susceptibility are given. Due to the observed separate switching behavior of the two hard layers in the realized trilayers, the analysis also considers demagnetization processes in which the two hard layers have opposite magnetic polarization. Calculations based on the nominal values of the magnetic and geometric parameters for the ideal symmetric trilayer are only in qualitative agreement with the corresponding experimental curves. In order to explain the discrepancies, the influence of variations in the parameters has been deeply analyzed, as well as the effect of a distribution of tilting angles of the hard layers anisotropy axes. In addition, we have extended the model to treat the case of a real asymmetric trilayer, characterized by slightly different anisotropy properties of the two hard layers, and we have derived the corresponding nucleation field equation for the case of easy direction. This analysis shows that a better agreement between calculations and measurements concerning the reversible behavior of the trilayers can be achieved by slightly modifying the nominal soft layer thickness and by assigning different tilting angles to the hard layers. Moreover, it shows that the remaining discrepancies can be ascribed to imperfections, which cannot be considered in detail in the one-dimensional model.
We have measured by X-ray resonant magnetic scattering the stripe domain nucleation field and the... more We have measured by X-ray resonant magnetic scattering the stripe domain nucleation field and the stripe domain critical width of FePd epitaxial films in a wide temperature range. A detailed quantitative analysis of our data in the framework of the rigorous theory of domain nucleation in thin films, allowed to determine the temperature dependence of both the exchange stiffness A and the anisotropy constant K u . The proposed approach can be applied to thin magnetic films with perpendicular magnetic anisotropy, thus providing a way to measure A in a relevant class of magnetic materials. r
The micromagnetic continuum theory has been applied to perfect soft/hard multilayers characterize... more The micromagnetic continuum theory has been applied to perfect soft/hard multilayers characterized by antiferromagnetic interface coupling. The soft and hard phases have uniaxial anisotropy with a common direction, along which the external field is applied. The model assumes a nonuniform rotation of the magnetization, and it also considers an interface coupling that is reduced with respect to the strong-limit case. It is found that the deviation of the magnetization from the saturated antiparallel state can occur at two distinct nucleation fields, which mainly involve only one of the two phases. Moreover, in the case of a reduced interface coupling, the saturated parallel state becomes accessible and thus the nucleation from this state is taken into account. The critical equations have been deduced, allowing us to identify the conditions for which the nucleation regime changes from reversible to irreversible as a function of the intrinsic and extrinsic parameters. The results of the model, applied to a typical soft/hard system with planar anisotropy, have been summarized in suitable phase diagrams, as a function of the layer thicknesses and of the strength of the interface coupling. The analysis, supported by additional static and dynamic micromagnetic simulations, shows the occurrence of a rich variety of magnetization curves. As a secondary result we have found that, in the parallel nucleation process, the influence of the interface coupling extends inside the two phases to distances appreciably larger than the corresponding Bloch wall widths.
The phenomenon of stripe domain nucleation is deeply investigated both theoretically and experime... more The phenomenon of stripe domain nucleation is deeply investigated both theoretically and experimentally in FePd films by the rigorous micromagnetic theory of domain nucleation and x-ray resonant magnetic scattering. The critical domain width and the nucleation field are determined by measuring the magnetic satellite peak position and integrated intensities in a wide temperature interval up to 400°C ͑0.9T c ͒ at varying in-plane magnetic fields for each temperature value. We develop and demonstrate a procedure that allows us to determine directly from the micromagnetic treatment the exchange stiffness constant A and the first order anisotropy constant K u as a function of temperature. The proposed procedure, based on linearized micromagnetic equations at the critical field, is valid for magnetic films with perpendicular magnetic anisotropy, and is therefore effective to measure A and K u in a technologically relevant class of materials.
Repeatable magnetization reversal under purely electrical control remains the outstanding goal in... more Repeatable magnetization reversal under purely electrical control remains the outstanding goal in magnetoelectrics. Here we use magnetic force microscopy to study a commercially manufactured multilayer capacitor that displays strain-mediated coupling between magnetostrictive Ni electrodes and piezoelectric BaTiO(3)-based dielectric layers. In an electrode exposed by polishing approximately normal to the layers, we find a perpendicularly magnetized feature that exhibits non-volatile electrically driven repeatable magnetization reversal with no applied magnetic field. Using micromagnetic modelling, we interpret this nominally full magnetization reversal in terms of a dynamic precession that is triggered by strain from voltage-driven ferroelectric switching that is fast and reversible. The anisotropy field responsible for the perpendicular magnetization is reversed by the electrically driven magnetic switching, which is, therefore, repeatable. Our demonstration of non-volatile magnetic switching via volatile ferroelectric switching may inspire the design of fatigue-free devices for electric-write magnetic-read data storage.
We have studied the thermal stability of L1 0 FePt thin films and Fe-FePt exchange-spring (ES) bi... more We have studied the thermal stability of L1 0 FePt thin films and Fe-FePt exchange-spring (ES) bilayers grown on (1 0 0) MgO by RF sputtering. The viscosity curves showed both for FePt films and bilayers a clear logarithmic decay of magnetization. Moreover, it was possible to evaluate the viscosity coefficient S for different applied reverse fields and the activation volumes at the coercivity. The latter values were then related to structural, magnetic and morphological measurements performed on the samples. r
Materials Science and Engineering B-Solid State Materials For Advanced Technology, 2007
Ferromagnetism of La 0.67 Sr 0.33 MnO 3 is extremely sensitive to external perturbations like sub... more Ferromagnetism of La 0.67 Sr 0.33 MnO 3 is extremely sensitive to external perturbations like substrate-induced strain, charge injection and chemical interactions with neighbour layers. In this paper we discuss the perturbation induced by the presence of a metallic overlayer, typically deposited for electric contacts, in the prototypical case of the Au/La 0.67 Sr 0.33 MnO 3 interface. In particular we found a sizable decrease of the Curie temperature in thin films of La 0.67 Sr 0.33 MnO 3 after gold capping: 65 K for 5 nm thickness of the manganite. Apart from chemical reactions at the interface, charge injection-depletion induced by the difference in the work function between Au and La 0.67 Sr 0.33 MnO 3 could partially explain this phenomenon.
The authors report about La0.67Sr0.33MnO3 single crystal manganite thin films in interaction with... more The authors report about La0.67Sr0.33MnO3 single crystal manganite thin films in interaction with a gold capping layer. With respect to uncoated manganite layers of the same thickness, Au-capped 4nm thick manganite films reveal a dramatic reduction (≃185K) of the Curie temperature TC and a lower saturation low temperature magnetization M0. A sizable TC reduction (≃60K) is observed even when an inert SrTiO3 layer is inserted between the gold film and the 4nm thick manganite layer, suggesting that this effect might have an electrostatic origin.
The effect of the AlOx barrier thickness on magnetic and morphological properties of Ta/Co/(AlOx)... more The effect of the AlOx barrier thickness on magnetic and morphological properties of Ta/Co/(AlOx)/Alq3/Si hybrid structures was systematically studied by means of atomic force microscopy, superconducting quantum interference device magnetometry, and nuclear magnetic resonance (NMR). All used techniques pointed out that the barrier thickness of 2 nm is required to obtain a magnetically good cobalt layer on top of Alq3. C59o NMR measurements revealed that the AlOx barrier gives rise to the formation of an interface layer with “defective” cobalt favoring growth of “bulk” cobalt with good magnetic properties.
The effect of the AlOx barrier thickness on magnetic and morphological properties of Ta/Co/(AlOx)... more The effect of the AlOx barrier thickness on magnetic and morphological properties of Ta/Co/(AlOx)/Alq3/Si hybrid structures was systematically studied by means of atomic force microscopy, SQUID magnetometry and nuclear magnetic resonance (NMR). All used techniques pointed out that the barrier thickness of 2 nm is required to obtain a magnetically good cobalt layer on top of Alq3. 59 Co NMR measurements revealed that the AlOx barrier gives rise to the formation of an interface layer with "defective" cobalt favouring growth of "bulk" cobalt with good magnetic properties.
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.
We have measured by X-ray resonant magnetic scattering the stripe domain nucleation field and the... more We have measured by X-ray resonant magnetic scattering the stripe domain nucleation field and the stripe domain critical width of FePd epitaxial films in a wide temperature range. A detailed quantitative analysis of our data in the framework of the rigorous theory of domain nucleation in thin films, allowed to determine the temperature dependence of both the exchange stiffness A and the anisotropy constant K u . The proposed approach can be applied to thin magnetic films with perpendicular magnetic anisotropy, thus providing a way to measure A in a relevant class of magnetic materials. r
Technological procedures able to produce high quality electrodes from magnetic oxides in vertical... more Technological procedures able to produce high quality electrodes from magnetic oxides in vertical organic-inorganic hybrid devices is a challenging task in the field of organic spintronics. Thin films of magnetite ͑Fe 3 O 4 ͒ have been successfully grown directly on top of organic semiconductor layers, tris͑8-hydroxyquinoline͒aluminium͑III͒ ͑Alq 3 ͒, by pulsed-electron ablation technique. The films show ferromagnetic behavior and good structural quality, properties detected by magneto-optical Kerr effect, superconductor quantum interference device, micro-Raman spectroscopy, and Atomic Force Microscopy. The ferromagnetic behavior persists even for 10 nm thick films. Charge injection at magnetite-organic interface has been finally demonstrated by detecting electroluminescence from Alq 3 .
We present a magnetic characterisation of MnAs films grown by MOVPE on semiconductor substrates, ... more We present a magnetic characterisation of MnAs films grown by MOVPE on semiconductor substrates, for comparing the effect of thickness and substrate on their magnetic and magneto-thermal properties, with particular focus on the nature of the magneto-structural transition between the ferromagnetic hexagonal a-phase and the paramagnetic orthorhombic b-phase. The nature of this transformation depends on the presence of strain and turns out to be of second-order type for epitaxial-crystalline MnAs/(0 0 1)GaAs films and closer to a first-order type for polycrystalline MnAs/SiO 2 /Si films. The magnetocaloric effect at the transition has been estimated for thin film samples, resulting appreciably lower than that of bulk MnAs.
We present the study of Co/organic semiconductor (OS) stacks both from the morphological and magn... more We present the study of Co/organic semiconductor (OS) stacks both from the morphological and magnetic point of view. Co has been successfully used up to now as top contact of hybrid vertical devices. While the properties of Co grown on amorphous layers are well established, its deposition on soft materials presents critical aspects such as interfacial damage that affects its electrical and magnetic properties. In this work we focus on the influence of the morphology of the organic underlayer in the magnetic behavior of a Co thin film: tris(8-hydroxyquinoline) aluminum (Alq 3 ) grown in different conditions by molecular beam evaporation have been considered. A further considered aspect is the effect of the presence of a thin oxide barrier (Al 2 O 3 ) on the Co magnetic properties.
A thorough micromagnetic analysis of the exchange-spring problem is reported with special emphasi... more A thorough micromagnetic analysis of the exchange-spring problem is reported with special emphasis on multilayers constituted by hard-soft exchange-coupled phases. The developed one-dimensional micromagnetic model leads to a complete magnetic phase diagram in terms of layer thicknesses. Both perpendicular and parallel configurations are considered. 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. The model has been applied to a variety of hard magnetic phases (e.g., FePt, CoPt, SmCo and NdFeB), coupled to different soft materials (e.g., Fe, FeCo, FeRh or permalloy) both in the form of bilayers and multilayers. The most significant results will be highlighted. r
The magnetic properties and the magnetization process of electrodeposited thick films of Co-rich ... more The magnetic properties and the magnetization process of electrodeposited thick films of Co-rich CoPt alloys are studied with particular emphasis on the effects of growth rate, controlled by varying the plating current density, and of lateral confinement, analyzing patterned micro-cylinders. We find that varying the plating current density has virtually no effect on the composition of the samples, and hence on the intrinsic magnetic properties, a substantial increase of both coercivity and squareness is obtained when the current is raised. The films are fine-grained, oriented polycrystals with typical grain sizes in the range 50-150 nm, depending on the growth rate. The complex magnetization process is studied in detail by Magnetic Force Microscopy and shown to be governed by interaction domains. It is shown that further improvement of the squareness can be obtained by exploiting the lateral confinement in patterned samples.
The magnetic anisotropy of LaCo-hexaferrites has been investigated by the use of the singular poi... more The magnetic anisotropy of LaCo-hexaferrites has been investigated by the use of the singular point detection and the second harmonic techniques in the low-temperature range. Below 200K a steep rise of the anisotropy field has been observed together with a strong distortion of the magnetization curve which is interpreted as due to a negative K2 and a large positive K3 anisotropy constants. The results are analyzed in terms of an extended Stoner–Wohlfarth model and the obtained values of the anisotropy coefficients χ2,0,χ4,0,χ6,0 are reported versus temperature.
The demagnetization processes in epitaxially grown SmCo 5 / Fe/ SmCo 5 trilayers have been analyz... more The demagnetization processes in epitaxially grown SmCo 5 / Fe/ SmCo 5 trilayers have been analyzed in the framework of a one-dimensional micromagnetic model, exploiting the well-defined orientation of the easy magnetization axis present in both SmCo 5 layers. The applied magnetic field is considered along easy, hard, and tilted directions. For the case of hard direction, the nucleation field equation and the analytical expression of the critical susceptibility are given. Due to the observed separate switching behavior of the two hard layers in the realized trilayers, the analysis also considers demagnetization processes in which the two hard layers have opposite magnetic polarization. Calculations based on the nominal values of the magnetic and geometric parameters for the ideal symmetric trilayer are only in qualitative agreement with the corresponding experimental curves. In order to explain the discrepancies, the influence of variations in the parameters has been deeply analyzed, as well as the effect of a distribution of tilting angles of the hard layers anisotropy axes. In addition, we have extended the model to treat the case of a real asymmetric trilayer, characterized by slightly different anisotropy properties of the two hard layers, and we have derived the corresponding nucleation field equation for the case of easy direction. This analysis shows that a better agreement between calculations and measurements concerning the reversible behavior of the trilayers can be achieved by slightly modifying the nominal soft layer thickness and by assigning different tilting angles to the hard layers. Moreover, it shows that the remaining discrepancies can be ascribed to imperfections, which cannot be considered in detail in the one-dimensional model.
We have measured by X-ray resonant magnetic scattering the stripe domain nucleation field and the... more We have measured by X-ray resonant magnetic scattering the stripe domain nucleation field and the stripe domain critical width of FePd epitaxial films in a wide temperature range. A detailed quantitative analysis of our data in the framework of the rigorous theory of domain nucleation in thin films, allowed to determine the temperature dependence of both the exchange stiffness A and the anisotropy constant K u . The proposed approach can be applied to thin magnetic films with perpendicular magnetic anisotropy, thus providing a way to measure A in a relevant class of magnetic materials. r
The micromagnetic continuum theory has been applied to perfect soft/hard multilayers characterize... more The micromagnetic continuum theory has been applied to perfect soft/hard multilayers characterized by antiferromagnetic interface coupling. The soft and hard phases have uniaxial anisotropy with a common direction, along which the external field is applied. The model assumes a nonuniform rotation of the magnetization, and it also considers an interface coupling that is reduced with respect to the strong-limit case. It is found that the deviation of the magnetization from the saturated antiparallel state can occur at two distinct nucleation fields, which mainly involve only one of the two phases. Moreover, in the case of a reduced interface coupling, the saturated parallel state becomes accessible and thus the nucleation from this state is taken into account. The critical equations have been deduced, allowing us to identify the conditions for which the nucleation regime changes from reversible to irreversible as a function of the intrinsic and extrinsic parameters. The results of the model, applied to a typical soft/hard system with planar anisotropy, have been summarized in suitable phase diagrams, as a function of the layer thicknesses and of the strength of the interface coupling. The analysis, supported by additional static and dynamic micromagnetic simulations, shows the occurrence of a rich variety of magnetization curves. As a secondary result we have found that, in the parallel nucleation process, the influence of the interface coupling extends inside the two phases to distances appreciably larger than the corresponding Bloch wall widths.
The phenomenon of stripe domain nucleation is deeply investigated both theoretically and experime... more The phenomenon of stripe domain nucleation is deeply investigated both theoretically and experimentally in FePd films by the rigorous micromagnetic theory of domain nucleation and x-ray resonant magnetic scattering. The critical domain width and the nucleation field are determined by measuring the magnetic satellite peak position and integrated intensities in a wide temperature interval up to 400°C ͑0.9T c ͒ at varying in-plane magnetic fields for each temperature value. We develop and demonstrate a procedure that allows us to determine directly from the micromagnetic treatment the exchange stiffness constant A and the first order anisotropy constant K u as a function of temperature. The proposed procedure, based on linearized micromagnetic equations at the critical field, is valid for magnetic films with perpendicular magnetic anisotropy, and is therefore effective to measure A and K u in a technologically relevant class of materials.
Repeatable magnetization reversal under purely electrical control remains the outstanding goal in... more Repeatable magnetization reversal under purely electrical control remains the outstanding goal in magnetoelectrics. Here we use magnetic force microscopy to study a commercially manufactured multilayer capacitor that displays strain-mediated coupling between magnetostrictive Ni electrodes and piezoelectric BaTiO(3)-based dielectric layers. In an electrode exposed by polishing approximately normal to the layers, we find a perpendicularly magnetized feature that exhibits non-volatile electrically driven repeatable magnetization reversal with no applied magnetic field. Using micromagnetic modelling, we interpret this nominally full magnetization reversal in terms of a dynamic precession that is triggered by strain from voltage-driven ferroelectric switching that is fast and reversible. The anisotropy field responsible for the perpendicular magnetization is reversed by the electrically driven magnetic switching, which is, therefore, repeatable. Our demonstration of non-volatile magnetic switching via volatile ferroelectric switching may inspire the design of fatigue-free devices for electric-write magnetic-read data storage.
We have studied the thermal stability of L1 0 FePt thin films and Fe-FePt exchange-spring (ES) bi... more We have studied the thermal stability of L1 0 FePt thin films and Fe-FePt exchange-spring (ES) bilayers grown on (1 0 0) MgO by RF sputtering. The viscosity curves showed both for FePt films and bilayers a clear logarithmic decay of magnetization. Moreover, it was possible to evaluate the viscosity coefficient S for different applied reverse fields and the activation volumes at the coercivity. The latter values were then related to structural, magnetic and morphological measurements performed on the samples. r
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Papers by M. Ghidini