Giovanni Asti | Università degli Studi di Parma (Italy) (original) (raw)

Papers by Giovanni Asti

Research paper thumbnail of Phase and frequency control in the vibrating wire magnetic susceptometer

Sensors and Actuators A: Physical, 2000

ABSTRACT

Research paper thumbnail of Vibrating wire magnetic susceptometer

Review of Scientific Instruments, 1996

ABSTRACT

Research paper thumbnail of Discontinuous free rotations in uniaxial ferrimagnets

Journal of Magnetism and Magnetic Materials, 1999

ABSTRACT

Research paper thumbnail of Magnetization process in thin Ni sheets: Effect of cold-rolling and recrystallization annealing

Journal of Applied Physics, 2001

Reversible and irreversible magnetization processes have been investigated in thin Ni sheets by m... more Reversible and irreversible magnetization processes have been investigated in thin Ni sheets by means of flux-metric and alternating-gradient-force magnetometer techniques. Cold-rolled samples exhibit a rotation-dominated process at low fields, and the value of the associated susceptibility, related to the stress-induced and the magnetocrystalline anisotropies, can provide an estimate of the residual stress in the material. Reversible displacements of the domain walls come into play and combine with rotations in the annealed samples. A method is devised, by which domain-wall and rotational contributions to the reversible susceptibility can be singled out. The Rayleigh law always accounts for the behavior of hysteresis loops and losses at low fields, but rotations engender some peculiar evolution of the related parameters and their relationship with the coercive field. Magnetic softening by stress relief is assessed, as a whole, through Preisach analysis. It is found that the local stochastic coercive fields can be described by means of a Lorentzian distribution function, whose relative width is minimum in the cold-rolled material. By removing the internal stresses through recrystallization annealing, the contribution of grain boundaries and surface effects to coercivity is put in evidence, with the latter apparently posing the major restraints to domain-wall motion for grain sizes larger than the sheet thickness.

Research paper thumbnail of Singularities in the AC energy losses in hard magnetic materials

IEEE Transactions on Magnetics, 2000

The imaginary part of the reversible susceptibility and its harmonics are measured on hard magnet... more The imaginary part of the reversible susceptibility and its harmonics are measured on hard magnetic materials by the modulation technique in a DC bias field. A singular point is observed in the second harmonic at the anisotropy field. An interpretation is given based on singular point detection approach

Research paper thumbnail of Force on slowly moving variable electric or magnetic dipoles in vacuo

The force on electric and magnetic dipoles moving in vacuo is discussed in the general case of ti... more The force on electric and magnetic dipoles moving in vacuo is discussed in the general case of time-variable non-uniform fields and time-variable dipole moments, to first order in v/c and neglecting radiation reaction. Emphasis is given to the symmetry between electric and magnetic dipoles, justifying in general Ampère's equivalence principle, and showing that the difference between gilbertian and amperian dipoles (in vacuo) is only a question of interpretation. The expression for the force can be expressed in a variety of different forms, and each term of each form is susceptible of specific physical interpretations. Terms not described in the literature are pointed out. A possible experiment for verifying the (dual-Lorentz) force of an electric field on a magnetic current (and then " hidden momentum ") is proposed.

Research paper thumbnail of Nucleation of weak stripe domains: Determination of exchange and anisotropy thermal variation

Physical Review B, 2007

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.

Research paper thumbnail of Effect of annealing on the magnetisation processes in cold-rolled thin Ni sheets

Journal of Magnetism and Magnetic Materials, 2003

The magnetisation process has been investigated in thin Ni sheets by means of fluxmetric and alte... more The magnetisation process has been investigated in thin Ni sheets by means of fluxmetric and alternating force techniques. As-rolled samples exhibit a rotation-dominated reversible magnetisation process, related to stress-induced and magnetocrystalline anisotropies. After annealing the rotations combine with substantial reversible displacements of the domain walls. It is always found that the transverse reversible susceptibility passes through a maximum at a field of the order of the anisotropy field. r

Research paper thumbnail of Interaction between an electric charge and a magnetic dipole of any kind (permanent, para-or dia-magnetic or superconducting)

arXiv, 2015

The interaction between point charge and magnetic dipole is usually considered only for the ca... more The interaction between point charge and magnetic dipole is usually considered
only for the case of a rigid ferromagnetic dipole (constant-current): here the analysis of force,
momentum and energy (including the energy provided by the internal current generator) is
generalised to any magnetic dipole behaviour: rigid, paramagnetic, diamagnetic or
superconducting (perfectly diamagnetic).

Research paper thumbnail of Micromagnetic analysis of exchange-coupled hard-soft planar nanocomposites

Physical Review B, 2004

A complete magnetic phase diagram for exchange-coupled planar hard-soft nanocomposites has been o... more A complete magnetic phase diagram for exchange-coupled planar hard-soft nanocomposites has been obtained in the frame of a one-dimensional micromagnetic model describing the dependence of the properties along the growth direction. The phase diagram in terms of layer thicknesses provides information on the type of demagnetization processes and the critical fields at which nucleation and reversal take place. The basic criterion to this purpose is the analytical expression we have obtained of the critical susceptibility at the nucleation field. The phase diagram is divided into three regions: the exchange-spring magnet ͑ES͒, the rigid composite magnet ͑RM͒, and the decoupled magnet ͑DM͒. The main boundary line is an U-shaped line corresponding to divergence of the critical susceptibility. The diagram also reports the isocritical field lines both for the nucleation and the reversal field. These lines bifurcate along the RM boundary line. The essential characteristics of the phase diagram are directly connected with the intrinsic properties of the chosen soft and hard materials. With increasing ratio between the anisotropy constants of soft to hard phases the ES region is reduced until it disappears at a critical value. The model includes as limiting cases the classical problems of the planar soft inclusion in a bulk magnet and of the domain-wall depinning at the hard-soft interface.

Research paper thumbnail of Flux reversal in hard-soft composite magnets

Journal of Magnetism and Magnetic Materials, 2004

We analyze the reversal process and calculate the critical fields in the relevant case of a hard ... more We analyze the reversal process and calculate the critical fields in the relevant case of a hard bulk enclosing a soft layer of varying thickness. The system behaves like a rigid magnet, an exchange spring magnet or a decoupled magnet in well-defined intervals of the layer thickness. The analysis is performed by considering a one-dimensional micromagnetic model of the hard-soft multilayer magnet. In particular, an analytical expression of the domain wall depinning field is obtained. In the case of the NdFeB/Fe system, the depinning field is about 10% of the hard-phase anisotropy field. r

Research paper thumbnail of A wide temperature range susceptometer

IEEE Transactions on Magnetics, 1996

A new type of high-sensitivity magnetic susceptometer is proposed. It utilises a long thin vibrat... more A new type of high-sensitivity magnetic susceptometer is proposed. It utilises a long thin vibrating wire as a sample support, which is driven at resonance by an alternating field gradient, with no applied dc field. The gradient is generated by a tiny soft ferromagnetic rod close to the sample, The chosen configuration is particularly suitable to perform measurements on thin

Research paper thumbnail of Electrodynamics in one dimension: radiation and reflection

European Journal of Physics, 2011

Problems involving polarized plane waves and currents on sheets perpendicular to the wavevector i... more Problems involving polarized plane waves and currents on sheets perpendicular to the wavevector involve only one component of the fields, so it is possible to discuss electrodynamics in one dimension. Taking for simplicity linearly polarized sinusoidal waves, we can derive the field emitted by currents (analogous to dipole radiation in three dimensions) and reflection and transmission (analogous to Thomson scattering). Some aspects of the results are not intuitive, and for a physical understanding we need to see the problem in three dimensions. Eventually we apply these results to a linear model of the sheet, and we discuss the limit from a thick sheet.

Research paper thumbnail of Phase and frequency control in the vibrating wire magnetic susceptometer

Sensors and Actuators A: Physical, 2000

ABSTRACT

Research paper thumbnail of Vibrating wire magnetic susceptometer

Review of Scientific Instruments, 1996

ABSTRACT

Research paper thumbnail of Discontinuous free rotations in uniaxial ferrimagnets

Journal of Magnetism and Magnetic Materials, 1999

ABSTRACT

Research paper thumbnail of Magnetization process in thin Ni sheets: Effect of cold-rolling and recrystallization annealing

Journal of Applied Physics, 2001

Reversible and irreversible magnetization processes have been investigated in thin Ni sheets by m... more Reversible and irreversible magnetization processes have been investigated in thin Ni sheets by means of flux-metric and alternating-gradient-force magnetometer techniques. Cold-rolled samples exhibit a rotation-dominated process at low fields, and the value of the associated susceptibility, related to the stress-induced and the magnetocrystalline anisotropies, can provide an estimate of the residual stress in the material. Reversible displacements of the domain walls come into play and combine with rotations in the annealed samples. A method is devised, by which domain-wall and rotational contributions to the reversible susceptibility can be singled out. The Rayleigh law always accounts for the behavior of hysteresis loops and losses at low fields, but rotations engender some peculiar evolution of the related parameters and their relationship with the coercive field. Magnetic softening by stress relief is assessed, as a whole, through Preisach analysis. It is found that the local stochastic coercive fields can be described by means of a Lorentzian distribution function, whose relative width is minimum in the cold-rolled material. By removing the internal stresses through recrystallization annealing, the contribution of grain boundaries and surface effects to coercivity is put in evidence, with the latter apparently posing the major restraints to domain-wall motion for grain sizes larger than the sheet thickness.

Research paper thumbnail of Singularities in the AC energy losses in hard magnetic materials

IEEE Transactions on Magnetics, 2000

The imaginary part of the reversible susceptibility and its harmonics are measured on hard magnet... more The imaginary part of the reversible susceptibility and its harmonics are measured on hard magnetic materials by the modulation technique in a DC bias field. A singular point is observed in the second harmonic at the anisotropy field. An interpretation is given based on singular point detection approach

Research paper thumbnail of Force on slowly moving variable electric or magnetic dipoles in vacuo

The force on electric and magnetic dipoles moving in vacuo is discussed in the general case of ti... more The force on electric and magnetic dipoles moving in vacuo is discussed in the general case of time-variable non-uniform fields and time-variable dipole moments, to first order in v/c and neglecting radiation reaction. Emphasis is given to the symmetry between electric and magnetic dipoles, justifying in general Ampère's equivalence principle, and showing that the difference between gilbertian and amperian dipoles (in vacuo) is only a question of interpretation. The expression for the force can be expressed in a variety of different forms, and each term of each form is susceptible of specific physical interpretations. Terms not described in the literature are pointed out. A possible experiment for verifying the (dual-Lorentz) force of an electric field on a magnetic current (and then " hidden momentum ") is proposed.

Research paper thumbnail of Nucleation of weak stripe domains: Determination of exchange and anisotropy thermal variation

Physical Review B, 2007

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.

Research paper thumbnail of Effect of annealing on the magnetisation processes in cold-rolled thin Ni sheets

Journal of Magnetism and Magnetic Materials, 2003

The magnetisation process has been investigated in thin Ni sheets by means of fluxmetric and alte... more The magnetisation process has been investigated in thin Ni sheets by means of fluxmetric and alternating force techniques. As-rolled samples exhibit a rotation-dominated reversible magnetisation process, related to stress-induced and magnetocrystalline anisotropies. After annealing the rotations combine with substantial reversible displacements of the domain walls. It is always found that the transverse reversible susceptibility passes through a maximum at a field of the order of the anisotropy field. r

Research paper thumbnail of Interaction between an electric charge and a magnetic dipole of any kind (permanent, para-or dia-magnetic or superconducting)

arXiv, 2015

The interaction between point charge and magnetic dipole is usually considered only for the ca... more The interaction between point charge and magnetic dipole is usually considered
only for the case of a rigid ferromagnetic dipole (constant-current): here the analysis of force,
momentum and energy (including the energy provided by the internal current generator) is
generalised to any magnetic dipole behaviour: rigid, paramagnetic, diamagnetic or
superconducting (perfectly diamagnetic).

Research paper thumbnail of Micromagnetic analysis of exchange-coupled hard-soft planar nanocomposites

Physical Review B, 2004

A complete magnetic phase diagram for exchange-coupled planar hard-soft nanocomposites has been o... more A complete magnetic phase diagram for exchange-coupled planar hard-soft nanocomposites has been obtained in the frame of a one-dimensional micromagnetic model describing the dependence of the properties along the growth direction. The phase diagram in terms of layer thicknesses provides information on the type of demagnetization processes and the critical fields at which nucleation and reversal take place. The basic criterion to this purpose is the analytical expression we have obtained of the critical susceptibility at the nucleation field. The phase diagram is divided into three regions: the exchange-spring magnet ͑ES͒, the rigid composite magnet ͑RM͒, and the decoupled magnet ͑DM͒. The main boundary line is an U-shaped line corresponding to divergence of the critical susceptibility. The diagram also reports the isocritical field lines both for the nucleation and the reversal field. These lines bifurcate along the RM boundary line. The essential characteristics of the phase diagram are directly connected with the intrinsic properties of the chosen soft and hard materials. With increasing ratio between the anisotropy constants of soft to hard phases the ES region is reduced until it disappears at a critical value. The model includes as limiting cases the classical problems of the planar soft inclusion in a bulk magnet and of the domain-wall depinning at the hard-soft interface.

Research paper thumbnail of Flux reversal in hard-soft composite magnets

Journal of Magnetism and Magnetic Materials, 2004

We analyze the reversal process and calculate the critical fields in the relevant case of a hard ... more We analyze the reversal process and calculate the critical fields in the relevant case of a hard bulk enclosing a soft layer of varying thickness. The system behaves like a rigid magnet, an exchange spring magnet or a decoupled magnet in well-defined intervals of the layer thickness. The analysis is performed by considering a one-dimensional micromagnetic model of the hard-soft multilayer magnet. In particular, an analytical expression of the domain wall depinning field is obtained. In the case of the NdFeB/Fe system, the depinning field is about 10% of the hard-phase anisotropy field. r

Research paper thumbnail of A wide temperature range susceptometer

IEEE Transactions on Magnetics, 1996

A new type of high-sensitivity magnetic susceptometer is proposed. It utilises a long thin vibrat... more A new type of high-sensitivity magnetic susceptometer is proposed. It utilises a long thin vibrating wire as a sample support, which is driven at resonance by an alternating field gradient, with no applied dc field. The gradient is generated by a tiny soft ferromagnetic rod close to the sample, The chosen configuration is particularly suitable to perform measurements on thin

Research paper thumbnail of Electrodynamics in one dimension: radiation and reflection

European Journal of Physics, 2011

Problems involving polarized plane waves and currents on sheets perpendicular to the wavevector i... more Problems involving polarized plane waves and currents on sheets perpendicular to the wavevector involve only one component of the fields, so it is possible to discuss electrodynamics in one dimension. Taking for simplicity linearly polarized sinusoidal waves, we can derive the field emitted by currents (analogous to dipole radiation in three dimensions) and reflection and transmission (analogous to Thomson scattering). Some aspects of the results are not intuitive, and for a physical understanding we need to see the problem in three dimensions. Eventually we apply these results to a linear model of the sheet, and we discuss the limit from a thick sheet.