International and National Journals
G. Bertotti, R. Bonin, A. Magni, I.D. Mayergoyz, C. Serpico
Energy equation for the analysis of magnetization relaxation to equilibrium
Journal of Magnetism and Magnetic Materials, Vol. 286, pp. 282-285, 2005.
Magnetization relaxation starting from a generic non-equilibrium state is analytically described. An equation for the energy decay is obtained. On this basis, an approximate expression for the magnetization motion during the ringing process is obtained in terms of Jacobi elliptic functions with time-dependent parameters.
M. Lo Bue, V. Basso, C.P. Sasso, G. Bertotti
Entropy and entropy production in magnetic systems with hysteresis
Journal of Applied Physics, vol. 97, 10E513, 2005.
A thermodynamic model of hysteresis is presented based on an internal variable reinterpretation of the Preisach model. Explicit expressions are obtained for the entropy and the entropy production for arbitrary open field histories. The model is applied to the prediction of nonmonotonic heat exchange along the hysteresis loop measured for ferromagnets.
G. Bertotti, A. Magni, R. Bonin, I.D. Mayergoyz, C. Serpico
Analytical description of magnetization relaxation to equilibrium
Journal of Applied Physics, vol. 97, 10E315, 2005.
Approximate analytical expressions are obtained for the magnetization relaxation to equilibrium in a thin film element with in-plane anisotropy subject to a constant external field applied along the easy axis. First, exact analytical solutions are obtained for the constant-energy motions taking place in the limit case where the damping constant is zero. Then a separate equation is derived and solved for the slow relaxation to equilibrium of the system energy. Final expressions for the magnetization relaxation are obtained by using the ensuing time-dependent energy in the solutions for the undamped dynamics.
G. Bertotti, C. Serpico, I.D. Mayergoyz, A. Magni, M. d'Aquino, R. Bonin
Magnetization switching and microwave oscillations in nanomagnet driven by spin-polarized currents
Physical Review Letters, Vol. 94, no. 12, 127206, 2005.
A novel theoretical approach to magnetization dynamics driven by spin-polarized currents is presented. Complete stability diagrams are obtained for the case where spin torques and external magnetic fields are simultaneously present. Quantitative predictions are made for the critical currents and fields inducing magnetization switching, for the amplitude and frequency of magnetization self-oscillations, and for the conditions leading to hysteretic transitions between self-oscillations and stationary states.
M. d'Aquino, C. Serpico, G. Miano, I.D. Mayergoyz, G. Bertotti
Numerical integration of Landau-Lifshitz-Gilbert equation based on the midpoint rule
Journal of Applied Physics, Vol. 97, 10E319, 2005.
The midpoint rule time discretization technique is applied to Landau-Lifshitz-Gilbert LLG equation. The technique is unconditionally stable and second-order accurate. It has the important property of preserving the conservation of magnetization amplitude of LLG dynamics. In addition, for typical forms of the micromagnetic free energy, the midpoint rule preserves the main energy balance properties of LLG dynamics. In fact, it preserves LLG Lyapunov structure and, in the case of zero damping, the system free energy. All the above preservation properties are fulfilled unconditionally, namely, regardless of the choice of the time step. The proposed technique is then tested on the standard micromagnetic problem No. 4. In the numerical computations, the magnetostatic field is computed by the fast Fourier transform method, and the nonlinear system of equations connected to the implicit time-stepping algorithm is solved by special and reasonably fast quasi-Newton technique.
I.D. Mayergoyz, M. Dimian, G. Bertotti, C. Serpico
Critical fields and pulse durations for precessional switching of perpendicular media
Journal of Applied Physics, vol. 97, 10E509, 2005.
Precessional switching of magnetization in perpendicular media induced by short rectangular pulses applied at arbitrary angles with respect to anisotropy axis, are studied. The expressions for critical magnetic fields and pulse durations that guarantee precessional magnetization reversals are derived.
V. Basso, M. LoBue, G. Bertotti
Temperature hysteresis and thermal remagnetization in magnetic materials
Journal of Applied Physics, Vol. 97, 10E501, 2005.
A temperature-dependent Preisach model is introduced and discussed for the case where the Preisach distribution depends on temperature through scaling factors. By expressing the model in terms of temperature-independent variables, we derive expressions for the hysteretic magnetization response to cyclic temperature variations as a function of past field history. We apply these results to the interpretation of thermal remagnetization phenomena experimentally observed in hard magnetic materials.
I.D. Mayergoyz, M. Dimian, G. Bertotti, C. Serpico
Inverse problem approach to precessional switching in perpendicular media
Journal of Applied Physics, Vol. 97, 10A703, 2005.
The "inverse problem" approach to the analysis of precessional switching of magnetization in perpendicular media is presented. This approach is purely algebraic in nature, and it leads to the direct calculations of expressions for magnetic-field pulses that guarantee the precessional switching. The effectiveness of the developed technique is illustrated by examples.
V. Basso, G. Bertotti, M. Lo Bue, C.P. Sasso
Theoretical approach to the magnetocaloric effect with hysteresis
Journal of Magnetism and Magnetic Materials, Vol. 290-291, Part 1, pp. 654-657, 2005.
In this paper a thermodynamic model with internal variables is presented and applied to ferromagnetic hysteresis. The out-of-equilibrium Gibbs free energy of a magnetic system is expressed as a function of the internal state of the Preisach model. Expressions for the system entropy and the entropy production are derived. By this approach it is possible to reproduce the characteristic features of the experimentally observed temperature changes (of the order of 104 K around room temperature) induced by the magnetic field along the hysteresis loop performed in iron under adiabatic condition.
C. Serpico, M. d'Aquino, G. Bertotti, I.D. Mayergoyz
Analytical approach to current-driven self-oscillations in Landau-Lifshitz-Gilbert dynamics
Journal of Magnetism and Magnetic Materials, Vol. 290-291, Part 1, pp. 502-505, 2005.
Nonlinear magnetization dynamics in uniformly magnetized bodies subject to spin-polarized current, is described by Landau-Lifshitz-Gilbert (LLG) equation with an additional spin-transfer torque term. The resulting magnetization dynamics may exhibit self-oscillatory regimes, i.e. limit cycles. By using the fact that spin-transfer torque and Gilbert damping are small perturbations of the conservative LLG dynamics, the analysis of limit cycles is carried out by an appropriate perturbation method known as Melnikov-function technique. The technique is then applied to the analysis of a typical current-driven switching process in magnetic thin film. Analytical formulas for frequency, amplitude of limit cycles in function of the injected current are derived along with critical values of current which characterized the switching process. Finally, the accuracy of the perturbative technique is tested by comparing analytical results with numerical solutions.
G. Bertotti, A. Magni, R. Bonin, I.D. Mayergoyz, C. Serpico
Bifurcation analysis of magnetization dynamics driver by spin transfer
Journal of Magnetism and Magnetic Materials, Vol. 290-291, Part 1, pp. 522-525, 2005.
Nonlinear magnetization dynamics under spin-polarized currents is discussed by the methods of the theory of nonlinear dynamical systems. The fixed points of the dynamics are calculated. It is shown that there may exist 2, 4, or 6 fixed points depending on the values of the external field and of the spin-polarized current. The stability of the fixed points is analyzed and the conditions for the occurrence of saddle-node and Hopf bifurcations are determined.
M. Lo Bue, V. Basso, C. Beatrice, G. Bertotti, G. Durin, C.P. Sasso
Barkhausen jumps and magnetic viscosity in NdFeB magnets
Journal of Magnetism and Magnetic Material, Vol. 290-291, Part 2, pp. 1184-1187, 2005.
We present the analysis of Barkhausen noise measured on a Nd-Fe-B sintered sample during viscosity experiments at constant applied field H. The measured jump sizes give direct evidence that the dominant magnetization process is the reversal of a single or of a small group of grains. The Barkhausen noise average amplitude allows to evaluate the viscosity coefficient S and the related activation volume va. We found va1/3 ~ 6 nm which is comparable to the domain wall width, in agreement with previous interpretations. The Barkhausen jump sizes show a power-law distribution independent of the time elapsed from the beginning of the relaxation. This fact validates the interpretation of the observed relaxations as due to thermal activation over a distribution of energy barriers.
S. Zapperi, C. Castellano, F. Colaiori, G. Durin
Signature of effective mass in crackling-noise asymmetry
Nature Physics, Vol. 1, no. 1, pp. 46-49, 2005.
Crackling noise is a common feature in many dynamic systems, the most familiar instance of which is the sound made by a sheet of paper when crumpled into a ball. Although seemingly random, this noise contains fundamental information about the properties of the system in which it occurs. One potential source of such information lies in the asymmetric shape of noise pulses emitted by a diverse range of noisy systems, but the cause of this asymmetry has lacked explanatio. Here we show that the leftward asymmetry observed in the Barkhausen effect - the noise generated by the jerky motion of domain walls as they interact with impurities in a soft magnet-is a direct consequence of a magnetic domain wall's negative effective mass. As well as providing a means of determining domain-wall effective mass from a magnet's Barkhausen noise, our work suggests an inertial explanation for the origin of avalanche asymmetries in crackling-noise phenomena more generally.
G. Bertotti, C. Serpico, I.D. Mayergoyz, R. Bonin, A. Magni, M. d'Aquino
Magnetization self-oscillations induced by spin-polarized currents
IEEE Transactions on Magnetics, Vol. 41 (10), pp. 2574-2576, 2005.
The effect of a spin-polarized current on magnetization dynamics is described by the Landau-Lifshitz-Gilbert (LLG) equation with the addition of Slonczewski spin-transfer term. Detailed predictions about the existence, position, and stability of magnetization self-oscillations induced by the spin-polarized current are obtained by an analytical perturbative method for different levels of current injection and externally applied field. The two cases of opposite direction of current flow are discussed.
C. Serpico, G. Bertotti, M. d'Aquino, R. Bonin, I.D. Mayergoyz
Transient dynamics leading to self-oscillations in nanomagnets driven by spin-polarized currents
IEEE Transactions on Magnetics, Vol. 41 (10), pp. 3100-3102, 2005.
Magnetization dynamics in uniformly magnetized nanomagnets subject to spin-polarized currents is studied by Landau-Lifshitz-Gilbert (LLG) equation with a spin-transfer torque term. This kind of magnetic system may exhibit stationary states and self-oscillatory regimes. By using the fact that spin-transfer torque and Gilbert damping are small perturbations of the conservative LLG dynamics, the analysis of self-oscillations is carried out by an appropriate perturbation technique. Averaging technique is then used to derive an approximated model for the energy dynamics which enables one to study the transient leading to self-oscillatory regimes. The accuracy of the proposed analytical technique is tested by comparison with numerical solutions of the LLG equation.
M. Pasquale, M. Coisson, S. Perero, S.H. Lim
Microwave properties and anisotropy field distribution in nanogranular Fe-Co-Al-O films
IEEE Transactions on Magnetics, Vol. 41 (10), pp. 3508-3510, 2005.
We have investigated the microwave properties of a set of Co-Fe-Al-O films with different thicknesses. The films show a well-defined in-plane anisotropy due to the bias field applied during deposition. The resonance frequency is determined through the scattering parameters measured with a vector network analyzer and a probing station connected to a coplanar waveguide. The results show that the resonance peak features depend on the direction of the applied/RF field and on the applied field history.
M. Pasquale, C.P. Sasso, L.H. Lewis, L. Giudici, T. Lograsso, D. Schlagel
Magnetostructural transition and magnetocaloric effect in Ni55Mn20Ga25 single crystals
Physical Review B, Vol. 72 (9), 094435, 2005.
A first order transition from a paramagnetic-austenite phase to a ferromagnetic martensite phase occurring in off-stoichiometric single crystals of Ni2MnGa at 313 K presents unique features due to the multifunctional character of the magnetic shape memory alloy. A remarkable magnetocaloric effect, associated with an entropy change up to ΔS~-86 J kg-1K-1 and an adiabatic temperature change ΔT~2.2 K, accompained by mechanical strain Δε≥ 3% have been observed in samples subjected to changes of the applied magnetic field ΔH = 4·106 A/m (≈ 5 T). The effects of magnetic field, temperature, and stress on the entropy variation ΔS are quantified and compared.
M.M. Tehranchi, M. Ghanaatshoar, S.M. Mohseni, M. Coisson, M. Vazquez
Temperature dependence of magnetoimpedance in annealed Co-based ribbons
Journal of Non-Crystalline Solids, Vol. 351 (37-39), pp. 2983-2986, 2005.
The study on the temperature dependence of magnetoimpedance (MI) response of soft magnetic materials is particularly important for controlling the thermal stability of MI-based sensor devices. In this paper, the MI effect in non-magnetostrictive Co67Fe4Si14.5B14.5 amorphous ribbons at the frequency of 250 kHz in the range of temperature from -170 ºC to 66 ºC after particular DC and AC current annealing procedures is investigated. Temperature dependence of MI response of DC current annealed sample is different from that of as-quenched one. A nearly temperature independent behavior is observed after subsequent AC current annealing, seemingly as a consequence of the structural changes at the surface of the ribbons by partial devitrification.
M. Pasquale, C.P. Sasso, A. Magni, F. Celegato, J.C. Sohn, S.H. Lim
Magnetomechanical properties of nanogranular Co-Fe-Al-O films
Journal of Applied Physics, Vol. 97 (10), art. No. 10n306 part 3, 2005.
Using different experimental techniques we analyze the interplay between the structure and the magnetomechanical behavior of soft nanogranular CoFeAlO films with thickness ranging from 125 to 550 nm. The sputtered film is characterized by 40-nm FeCo grains and a saturation magnetization of 16 kG. The results show that, due to the role of the mechanically stiff and insulating Al-O matrix, an increase of the resistivity leads to an increase of the anisotropy field and also of the ferromagnetic resonance frequency. The 125-nm-thick films possess the best combination of anisotropy field and resistivity leading to an estimated ferromagnetic resonance frequency around 1.9 GHz
C. Appino
Hysteresis and nucleation field in two-dimensional magnetization process
Journal of Magnetism and Magnetic Materials, Vol. 290, part 1, pp. 471-474, sp. Iss., 2005.
We have investigated the vector nature of the magnetization process in disk shaped Co-based amorphous samples, providing a joint description of spin rotation and domain wall motion. Thermo-magnetic treatments have been exploited to induce a macroscopic uniaxial anisotropy in the specimen plane and a regular domain pattern. An alternating field Ha has been applied to the specimens along a direction forming with the easy axis an angle θa between 0º and 90º. The two orthogonal components of the magnetization M and of the effective field H have been measured by means of compensated windings and many-turn H-coils placed on the sample surface. We have studied the evolution of the hysteresis loop as a function of θa and verified that with a two-dimensional magnetization process the coercive field must be replaced by a vector Hc(Hc//,Hc⊥), being Hc// and Hc⊥ the projections of Hc along the easy axis and perpendicular to it. We have experimentally worked out the relation Hc//(Hc⊥), and found that Hc//, starting from a finite value for Hc⊥=0, passes through a maximum and drops to zero when Hc⊥ equals the anisotropy field. We have demonstrated that this behavior can be partly ascribed to the fact that the nature of the walls, in low anisotropy materials, is deeply modified by the spin rotations in the adjacent domains, induced by the applied field. These results have been interpreted on the basis of the Stoner-Wohlfarth model, with modifications describing the domain wall displacement.
P. Allia, F. Celegato, M. Coisson, A. Da Re, F. Ronconi, F. Spizzo, P. Tiberto, F. Vinai
Temperature dependence of spontaneous magnetisation in granular Au80Fe20 films
Journal of Magnetism and Magnetic Materials, Vol. 290, part 1, pp. 580-583, Sp. Iss., 2005.
A granular Au80Fe20 film was deposited on Si substrates by DC magnetron cosputtering in Ar atmosphere. Magnetic quantities (isothermal magnetisation, initial and high-field susceptibilities, FC/ZFC curves) were obtained between 10 and 400 K using an extraction method and vibrating-sample magnetometry. Magnetization curves exhibit an apparent superparamagnetic behaviour with a small hysteresis which disappears above 270 K. FC/ ZFC curves become separate below 50 K. The anhysteretic curves are fitted by a Langevin function with a single, temperature-dependent magnetic moment. Above blocking temperature, the system is an interacting superparamaexperience a ferromagnetic-to-paramagnetic transition.
E. Olivetti, M. Baricco, E. Ferrara, P. Tiberto, L. Martino
Effect of annealing on the magnetic properties of Nd70Fe20Al10 bulk metallic glasses
Journal of Magnetism and Magnetic Materials, Vol. 290, part. 2, pp. 1214-1216, Sp. Iss., 2005.
In this work, the influence of thermal treatments on the hysteresis behaviour of Nd70Fe20Al10 bulk metallic glasses is studied. Two samples obtained applying different quenching rates have been characterized: (a) master alloy ingots, prepared through arc melting, and (b) cone-shaped ingots obtained by copper mould casting. DSC measurements have been performed on both alloys. Selected samples have been submitted to subsequent annealing. Hard magnetic properties have been observed at room temperature either in the as-cast master alloy or in the cone-shaped ingots. High values of coercivity are still observed after treatment at temperatures close to the crystallisation temperature (up to 500 ºC). The different magnetic behaviour of the samples is discussed in terms of differences in the residual amorphous phase composition.
M.J. Alava, V. Basso, F. Colatori, L. Dante, G. Durin, A. Magni, S. Zapperi
Ground-state optimization and hysteretic demagnetization: The random-field Ising model
Physical Review B, Vol. 71 (6), 064423, 2005.
We compare the ground state of the random-field Ising model with Gaussian distributed random fields, with its nonequilibrium hysteretic counterpart, the demagnetized state. This is a low-energy state obtained by a sequence of slow magnetic-field oscillations with decreasing amplitude. The main concern is how optimized the demagnetized state is with respect to the best-possible ground state. Exact results for the energy in d = 1 show that in a paramagnet, with finite spin-spin correlations, there is a significant difference in the energies if the disorder is not so strong that the states are trivially almost alike. We use numerical simulations to better characterize the difference between the ground state and the demagnetized state. For d>=3, the random-field Ising model displays a disorder induced phase transition between a paramagnetic and a ferromagnetic state. The locations of the critical points Rc(DS) and Rc(GS) differ for the demagnetized state and ground state. We argue based on the numerics that in d = 3 the scaling at the transition is the same in both states. This claim is corroborated by the exact solution of the model on the Bethe lattice, where the critical points are also different.
A. Magni, V. Basso
Study of metastable states in the random-field Ising model
Journal of Magnetism and Magnetic Materials, Vol. 290, pp. 460-463, 2005.
The random-field Ising model (RFIM) provides a convenient framework to investigate complex energy landscapes. We use an out-of-equilibrium T=0 single spin flip dynamics induced by varying the applied field, starting from locally stable configurations {si}. If the configuration considered cannot be reached by a field history from saturation, it is possible to define a set of connected states which is defined as a basin. A whole hierarchy of basins is found when the field is increased outside the limits of the initial basin. The resulting structure has the topology of an oriented graph. The properties of the graph cast new light on properties of the ground state (GS) and the possibility to reach the GS by a field history from saturation. We have numerically determined the graphs in RFIM realizations of finite size in one dimension for arbitrary selected states and for the GS. Remarkable differences between them are found in the critical path of the corresponding graphs, the GS being nearer to the field reachable states than a generic state.
P. Allia, M. Coisson, P. Tiberto, F. Vinai, D. Bisero, F. Spizzo
Magnetoresistance analysis of nanoscale magnetic correlation in cosputtered Fe100-x Agx film
IEEE Transactions on Magnetics, Vol. 41, no. 10, pp. 3412-3414, 2005.
Nanogranular films of composition Ag Fe (x: 10, 14, 20, 26, 30 at %) were prepared by dc plasma sputtering deposition. All films exhibit a negative magnetoresistance whose intensity decreases monotonically with temperature, with no apparent saturation up to 50 kOe. A magnetic correlation range is obtained from analysis of the GMR versus curves. The role of interparticle correlation is investigated as a function of both Fe concentration and temperature.
O. Bretcanu, S. Spriano, E. Verné, M. Coisson, P. Tiberto, P. Allia
The influence of crystallised Fe3O4 on the magnetic properties of coprecipitation-derived ferrimagnetic glass-ceramics
Acta Biomaterialia, Vol. 1, pp. 421-429, 2005.
Ferrimagnetic glass-ceramics are potential candidates for magnetic induction hyperthermia, which is one form of inducing deepregional hyperthermia, by using a magnetic field. The aim of this work was to analyse the influence of the amount of crystallised magnetite on the magnetic properties of glass-ceramic samples. Thus, two different ferrimagnetic glass-ceramics with the composition of the system Na2O-CaO-SiO2-P2O5-FeO-Fe2O3 were prepared by melting at 1500 ºC for 30 min of the coprecipitationderived starting products. The X-ray diffraction patterns show the presence of nanometric magnetite crystals in a glassy matrix after cooling from melting temperature. The estimated amount of crystallised magnetite varies between 20 and 45 wt.%, as a function of the chemical composition. The morphology of the crystals was studied by scanning electron micrography and transmission electron micrography. Glass transition temperature and thermal stability were investigated by differential thermal analysis. Magnetic hysteresis cycles were analysed using a vibrating sample magnetometer with a maximum applied field of 17 kOe, at room temperature, in quasi-static conditions. Calorimetric measurements were carried out using a magnetic induction furnace. The power losses estimated from calorimetric measurements under a magnetic field of 40 kA/m and 440 kHz are 65 W/g for the glass-ceramic with lower iron oxides content and 25 W/g for the glass-ceramic with higher iron oxide content.
Conference Proceedings
P. Allia, F. Celegato, M. Coisson, P. Tiberto, F. Vinai, F. Spizzo
Magnetic nanoparticle aggregation states in Ag100-xFex cosputtered granular films investigated by magnetic and magnetotransport measurements
Material Research Society Symposium Proceedings, Vol. 877E S5.4.1, 2005.
Magnetoresistance and magnetization measurements have been performed on nanogranular, cosputered Ag100-xFexfilms (x = 10 to 30) in the 4 K - 300 K temperature interval. The analysis reveals that the films with x ≤ 14 are interacting superparamagnets, characterized by a magnetic correlation length of the order of the electronic mean free path λ. Films with x ≥= 26 behave as concentrated magnets with strong competing interactions among magnetic moments (frustrated ferromagnets) and magnetic correlation length much larger than λ.
G. Bertotti, V. Basso, M. Lo Bue, A. Magni
Thermodynamics, hysteresis and micromagnetics
Chapter 1 in Science of Hysteresis, vol. II (G. Bertotti and I. Mayergoyz eds.), pp.1- 106, Elsevier 2005.
The connection between thermodynamics, hysteresis, and micromagnetics is discussed. Formulations of equilibrium and non-equilibrium thermodynamics are reviewed, with particular emphasis on formulations in terms of internal variables, leading to rate-independent hysteresisis in the limit of low temperatures. Subsequently, idealized model systems are considered, consisting of ensembles of independent two-level sub-systems (TLS), the two states of each TLS unit being separated by an energy barrier hindering spontaneous transitions from one level to the other. The non-equilibrium behavior of the ensemble is described by a thermodynamic formulation with internal variables exhibiting many similarities with the thermodynamics of chemical reactions. It is shown that in the limit of low temperatures the ensemble exhibits Preisach-type rate-independent hysteresis. Viscosity and rate-dependent effects induced by thermal fluctuations are also analyzed. Finally, the onset of hysteresis in micromagnetic systems is considered in the case where the problem can be reduced to the study of independent degrees of freedom moving in random one-dimensional pinning-field profiles. The problem is cast into a functional integration form amenable to full analytical treatment in the case where the pinning field profiles are sample functions of some Markovian stochastic process. It is proven that when the pinning field profiles are Markovian and spatially homogeneous, the ensuing average hysteretic behavior of the system is of Preisach type.
C.P. Sasso, V. Basso, M. LoBue, G. Bertotti
Carnot cycle for magnetic materials: the role of hysteresis
Physica B, Vol. 372, pp. 9-12, 2005.
The role of hysteresis in a refrigeration thermodynamic cycle involving ferromagnetic materials is discussed. A model allowing to calculate magnetization, entropy and entropy production in systems with hysteresis is used to compute a non-ideal Carnot cycle performed on a ferromagnetic material.
V. Basso, A. Magni, G. Bertotti
First steps towards a state classification in the random-field Ising model
Physica B, Vol. 372, pp. 243-246, 2005.
The properties of locally stable states of the random-field Ising model are studied. A map is defined for the dynamics driven by the field starting from a locally stable state. The fixed points of the map are connected with the limit hysteresis loops that appear in the classification of the states.
V. Basso, C.P. Sasso, M. Lo Bue, G. Bertotti
Thermodynamic models for magnetic materials
Refrigeration Science and Technology Proceedings (P. Egolf ed.) IIR/IIF, pp.155-162, 2005.
In this paper we review the current developments in the field of hysteresis models applied to magnetic materials with magnetocaloric effect. It is discussed how these models may account for the temperature dependence of hysteresis phenomena and how this reflects in a thermodynamic formulation valid for out-of-equilibrium systems. The developed approach is applied to discuss the heat fluxes along the hysteresis loop and to include the role of hysteresis in a refrigeration thermodynamic cycle. As an example the model is employed to compute a non-ideal Carnot cycle performed on a ferromagnetic material.
M. Pasquale, C.P. Sasso, A. Magni, S. Perero, F. Celegato, V. Teppati, J.C. Sohn, S.H. Lim
Nanostructure and microwave properties of CoFeAlO films
Proceedings of Third International Workshop on High Frequency Micromagnetic Devices and Materials (MMDM3), pp. 13-14, Tohoku University (Japan), April 2005.
G. Durin, S. Zapperi
Two dimensional models for Barkhausen noise
Proceedings of SPIE International Symposium "Fluctuations and Noise", Vol. 5843, pp. 31-39, Austin (Texas, USA), May 2005 (on cd rom).
We analyze the current experimental and theoretical research about the magnetization (Barkhausen) noise in magnetic thin films. We observe that, in respect to three-dimensional systems, the situation is much more complicated, and many details still have to be analyzed and understood. In particular, the critical exponents must be correctly analyzed when compared with the theoretical predictions. We observe, for instance, that the measured magnetization steps in optical measurements do not actually correspond to the avalanche size, as claimed, but to a different quantity having a different critical exponent. We also observe as the lack of a significative demagnetizing field does not assure to have a stationary Barkhausen signal, which again implies a different set of exponents. For comparison, we report in detail all the possible theoretical values of the critical exponents calculated for a single DW model.
Technical Reports
D. Patroi, M. Coisson, E. Ferrara
Atomic force microscopy: an overview
IEN Technical Report no. 693, August 2005.
This work is an introductory presentation of the Atomic Force Microscope (AFM) system, effectuated by eng. phys. Delia Patroi during her formative stage in Turin, in the frame of international collaboration for scientific research and development between the IEN Galileo Ferraris and the National Institute for Research and Development in Electrical Engineering - Advanced Research (INCDIE ICPE-CA) from Bucharest, Romania.
P. Fisicaro, E. Ferrara
Studio pilota CCQM-P82 Determinazione del valore di pH, mediante cella di Harned, su soluzione tampone di borato
Rapporto Tecnico IEN n. 696, dicembre 2005.
Questo lavoro raccoglie le principali osservazioni teoriche e gli sviluppi sperimentali che hanno portato l'IEN, nel periodo agosto-settembre 2005, alla partecipazione al confronto internazionale di misura CCQM-P82 (Pilot study): "pH determination on borate buffer by Harned cell measurements". Nella prima parte, in italiano, sono riassunti sia i principi base che fanno della cella di Harned un sistema primario di riferimento per la misura di pH sia le considerazioni necessarie per associare ad essa un bilancio di incertezza completo. Tutti i diversi contributi d'incertezza sono stati definiti in modo univoco ed il loro valore calcolato. Nella seconda parte del documento, in inglese, sono illustrati i risultati ottenuti dall'IEN nel corso dello studio pilota CCQM-P82.