Quantitative model of large magnetostrain effect in ferromagnetic shapell memory alloys

A quantitative model describing the large magnetostrain effect observed in several ferromagnetic shape memory alloys such as Ni₂MnGa is briefly reported.The paper contains an exact thermodynamic consideration of the mechanical and magnetic properties of similar types of materials. As a result, the basic mechanical state equation including magnetic field effect is directly derived from a general Maxwell relation. It is shown that the magnetic field induced deformation effect is directly connected with the strain dependence of magnetization. A simple model of magnetization and its dependence on the strain is considered and applied to explain the results of experimental study of large magnetostrain effects in Ni₂MnGa. Received 29 September 1999     

Micromagnetic simulation of magnetization reversal in controlled mesoscopic ferromagnetic junction

Magnetisation switching process and corresponding domain structures of mesoscopic ferromagnetic junction with different thickness are studied with micromagnetic simulation using Landau–Lifshiz–Gilbert equations. It is demonstrated by simulation results that the reversal process and switching field are dominated by the wide pad parts and only weakly depend on the wire parts.     

On the gas adsorption effect upon electronic structure of ferromagnetic Co(0 0 0 1)

Effect of C, N or O adsorption on ferromagnetic Co(0 0 0 1) surface on evolution of important electronic features is studied within the local-density functional scheme. At high oxygen or carbon coverage, the spin asymmetry of Co(3d) states at the Fermi level can be reversed; for oxygen, however, the effect is not stable with respect to geometry variation and might correspond to non-equilibrium adatom positions. The concept of exchange splitting for non-homogeneous systems is poorly defined and its presence for well distinguished peaks in the local density of adatom 2p states is expected especially for oxygen. The calculations find, nevertheless, a simple and accurate relation between the spin-splitting of the centre of gravity of 2p electronic sates and value of the magnetic moment induced on adatoms.     

Dynamic stress sensitivity of Fe73.5Cu1(Ta or W)3Si15.5B7 ribbons after heat treatments in vacuum

Summary The effects of heat treatments in vacuum on magnetic permeability, Young's modulus and magnetoelastic wave amplitude were investigated in the amorphous ferromagnetic alloys Fe_73.5Cu₁Ta₃Si_15.5B₇ and Fe_73.5Cu₁W₃Si_15.5B₇ where tantalum or tungsten replaces niobium, which was generally used with the same atomic fractions of the other elements. The stress sensitivity coefficient was also theoretically deduced in its developmentvs. the applied magnetic bias field. Structural relaxation and crystallization consequences are evidenced. Moreover an already predicted relation between the maximum of magnetoelastic wave amplitude and the minimum of stress sensitivity coefficient was confirmed.     

The anomalies of the longitudinal magnetothermoelectric power of metal in the vicinity of electronic topological transition

Summary The longitudinal magnetothermoelectric power (MTEP) is calculated for metal in magnetic field and in the vicinity of electronic topological transition (ETT). The giant oscillations of MTEP as a function of applied magnetic field are found. These oscillations originate from the energy dependence of electron relaxation time in magnetic field and they are peculiar to any normal metal independently of the shape of its Fermi surface. Nevertheless the ETT essentially changes the form and magnitude of such oscillations. The results of recent experimental investigation of MTEP in cadmium monocrystals under the pressure and in magnetic field are discussed from the point of view of the presented theory.

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Riassunto Si calcola la potenza magnetotermoelettrica longitudinale per metalli nel campo magnetico e in vicinanza della transizione topologica elettronica (ETT). Si trovano oscillazioni giganti di MTEP in funzione del campo magnetico applicato. Queste oscillazioni originano dalla dipendenza dall'energia del tempo di rilassamento dell'elettrone nel campo magnetico e sono peculiari di ogni metallo normale indipendentemente dalla forma della sua superficie di Fermi. Cionondimeno ETT cambia essenzialmente la forma e la grandezza di tali oscillazioni. Si discutono i risultati di una recente ricerca sperimentale di MTEP in monocristalli di cadmio sotto pressione e nel campo magnetico dal punto di vista della teoria presentata.

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Резюме Вычисляется продольный коэффициент магнитнотермоэлектродвижущей силы для металла в магнитном поле и вблизи электронного топологигеского перехода. Обнаружены заметные осцилляции коэффициента магнитнотермоэлектродвижущей силы в зависимости от велигины приложенного магнитного поля. Эти осцилляции обусловлены энергетической эависимостью времени релаксации электронов в магнитном поле. Эти осцилляции являются характерными для любого нормального металла, независимо от формы поверхности Ферми. Тем не менее, электронный топологический переход существенно изменяет форму и величину таких осцилляций. В рамках предложенной теории обсуждаются результаты недавних экспериментальных исследований коэффициента магнитнотермоэлектродвижущей силы в монокристаллах кадмия под действием давления и в магнитном поле.

     

Magnetoelastic effects at the interface of CoP nanostructured multilayers

Summary The role of magnetoelastic energy to determine the causes which affect the magnetic anisotropy are reviewed. The simple model used to describe magnetostriction in the new nanostructured materials is reported. Starting from this model the magnetostriction in multilayer samples, of the kind magnetic material/non-magnetic material, is discussed. In particular the fundamental influence of the interlayers is shown. Finally it is demonstrated that by measurement of saturation magnetostriction it is possible to have information on the interlayers thickness and average composition.     

Optimized factor of merit of the magneto-optical Kerr effect of ferromagnetic thin films

This paper deals with the optimization of the factor of merit of the magneto-optical Kerr effect of a resonant multilayer cavity including a ferromagnetic film. This optimization is of interest in the context of optical storage technology. Using numerical simulations based on the Green's dyadic technique, we discuss a route to obtain magneto-optical multilayers with a vanishing ellipticity and factors of merit (with respect to the bulk magnetic material) larger than 3 on a broad range of wavelengths, significantly higher than the actual state of the art. Received 21 May 1999     

Size distribution effect on the magnetization and magneto-resistance in ferromagnetic nanostructured manganites

We present experimental data of magnetization and magneto-resistance of nanostructured La_2/3B_1/3MnO₃ with B=Ca, Sr, which present difference between the coercive field in the magnetization loop with their corresponding maximum value in the magneto-resistance. This difference is described by a model that include, size distribution of magnetic particles, randomly oriented anisotropy axis and electronic transfer between the particles, which is mediated by spin-polarized tunneling process. Also, the model predicts that the maximum magneto-resistance can be, in the magnetic disorder state, two times larger than the experimental value. The model results can be used to estimate the size dispersion of nanoparticles in similar systems.     

Ferromagnetism in laser ablated ZnO and Mn-doped ZnO thin films: A comparative study from magnetization and Hall effect measurements

Room temperature FM was observed in pristine ZnO thin films grown by pulsed laser deposition on Al₂O₃ substrates. It seems to originate from other defects but not oxygen vacancies. Magnetization of thinner films is much larger than that of the thicker films, indicating that defects are mostly located at the surface and/or the interface between the film and the substrate. Data on the Fe:ZnO and Mn:ZnO films show that a transition-metal doping does not play any essential role in introducing the magnetism into ZnO. In the case of Mn doping, the magnetic moment could be very slightly enhanced. Hall effect measurements reveal that an incorporation of Mn does not change the carrier type, but decreases the carrier concentration, and increases the Hall mobility, resulting in more resistive Mn:ZnO films. Since no anomalous Hall effect was observed, it is understood that the observed FM is not due to the interaction between the free-carrier and the Mn impurity.     

Skyrmions on an elastic cylinder

For a spin-polarized electron gas on an elastic cylinder in an external axial magnetic field and an axial electric field we find that the corresponding Euler-Lagrange equation is the double sine-Gordon (DSG) equation with an exact 2π-skyrmion solution. The DSG skyrmion is stabilized, without Coulomb repulsion, by the curvature of the cylinder. It adopts a characteristic length ξ which is smaller than the radius of the cylinder. For an elastic cylinder this mismatch of length scales causes a deformation of the cylinder in the region of the skyrmion. Received 23 October 2001 / Received in final form 8 March 2002 Published online 2 October 2002 RID="a" ID="a"e-mail: rossen.dandoloff@ptm.u-cergy.fr     

The effect of internal stresses on the magnetization curve of amorphous ferromagnetic FeNi alloys

The Rayleigh region, the coercive field and the magnetization curves of the amorphous, ferromagnetic alloys Fe_80−xNi_xB₂₀ and Fe₄₀Ni₄₀P₁₄B₆ have been investigated as a function of the temperature and composition. It is shown that the characteristic parameters of the magnetization curves can be described by the statistical potential theory as developed previously for the movement of domain walls. Our experimental results are compatible with the assumption that the obstacles opposing the domain wall displacements are elastic stress centres produced during the rapid quenching process of the amorphous alloys.     

Determination of the elastic limit in ferromagnetic materials using the mechanical barkhausen effect

Conclusions The proposed method for finding the elastic limit in annealed ferromagnetic samples may be useful for investigating this and short samples, where other methods are inconvenient. In principle, it can also be used to find _l in unannealed samples, but in this case the mechanical Barkhausen effect is very weak, and the accuracy of measurement decreases.     

Micromagnetic study of magnetic domain structure and magnetization reversal in amorphous wires with circular anisotropy

In this work we present a detailed numerical investigation on the magnetic domain formation and magnetization reversal mechanism in sub-millimeter amorphous wires with negative magnetostriction by means of micromagnetic calculations. The formation of circular magnetic domains surrounding a multidomain axially oriented central nucleus was observed for the micromagnetic model representing the amorphous wire. The magnetization reversal explained by micromagnetic computations for the M-H curve is described in terms of a combined nucleation-propagation−rotational mechanism after the saturated state. Results are interpreted in terms of the effective magnetic anisotropy.     

The effect of magnetic stress and stiffness modulus on resonant characteristics of Ni-Mn-Ga ferromagnetic shape memory alloy actuators

The prospect of using ferromagnetic shape memory alloys (FSMAs) is promising for a resonant actuator that requires large strain output and a drive frequency below 1 kHz. In this investigation, three FSMA actuators, equipped with tetragonal off-stoichiometric Ni₂MnGa single crystals, were developed to study their frequency response and resonant characteristics. The first actuator, labeled as A1, was constructed with low-k bias springs and one Ni-Mn-Ga single crystal. The second actuator, labeled as A2, was constructed with high-k bias springs and one Ni-Mn-Ga crystal. The third actuator, labeled as A3, was constructed with high-k bias springs and two Ni-Mn-Ga crystals connected in parallel. The three actuators were magnetically driven over the frequency range of 10 Hz-1 kHz under 2 and 3.5 kOe magnetic-field amplitudes. The field amplitude of 2 kOe is insufficient to generate significant strain output from all three actuators; the maximum magnetic-field-induced strain (MFIS) at resonance is 2%. The resonant MFIS output improves to 5% under 3.5-kOe amplitude. The frequency responses of all three actuators show a strong effect of the spring k constant and the Ni-Mn-Ga modulus stiffness on the resonant frequencies. The resonant frequency of the Ni-Mn-Ga actuator was raised from 450 to 650 Hz by increasing bias spring k constant and/or the number of Ni-Mn-Ga crystals. The higher number of the Ni-Mn-Ga crystals not only increases the magnetic force output but also raises the total stiffness of the actuator resulting in a higher resonant frequency. The effective modulus of the Ni-Mn-Ga is calculated from the measured resonant frequencies using the mass-spring equation; the calculated modulus values for the three actuators fall in the range of 50-60 MPa. The calculated effective modulus appears to be close to the average modulus value between the low twinning modulus and high elastic modulus of the untwined Ni-Mn-Ga crystal.     

On the effect of impurities on the critical resolved shear stress of zinc single crystals

The dependence of the critical resolved shear stress of zinc single crystals on the impurity content and the dislocation density is observed. It is shown that the effect of impurities on the critical resolved shear stress is indirect.

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On the vibration of an elastic plate on an elastic foundation

The forced vibration of an elastic plate under a time harmonic point force is studied. The plate is infinite in extent and supported by an elastic foundation. This study is made on the basis of the improved (Timoshenko) plate theory. The mathematical problem is to seek a fundamental solution (the Green's function) of the time-reduced plate equation of the improved plate theory. Such a fundamental solution is constructed by the distributional Fourier transform method. From the explicit expressions of the fundamental solution, the behavior of the fundamental singularity as a function of the vibration frequency and the foundation stiffness is examined. Conditions under which plate resonance occurs are also determined.     

Size effect of the elastic modulus of rectangular nanobeams:Surface elasticity effect

The size-dependent elastic property of rectangular nanobeams （nanowires or nanoplates） induced by the surface elas- ticity effect is investigated by using a developed modified core-shell model. The effect of surface elasticity on the elastic modulus of nanobeams can be characterized by two surface related parameters, i.e., inhomogeneous degree constant and surface layer thickness. The analytical results show that the elastic modulus of the rectangular nanobeam exhibits a distinct size effect when its characteristic size reduces below 1 O0 nm. It is also found that the theoretical results calculated by a mod- ified core-shell model have more obvious advantages than those by other models （core-shell model and core-surface model） by comparing them with relevant experimental measurements and computational results, especially when the dimensions of nanostructures reduce to a few tens of nanometers.     

Uncompensated magnetization and exchange-bias field in La0.7Sr0.3MnO3/YMnO3 bilayers: The influence of the ferromagnetic layer

We studied the magnetic behavior of bilayers of multiferroic and nominally antiferromagnetic o-YMnO₃ (375 nm thick) and ferromagnetic La_0.7Sr_0.3MnO₃ and La_0.67Ca_0.33MnO₃ (8…225 nm), in particular the vertical magnetization shift M_E and exchange-bias field H_E for different thickness and magnetic dilutions of the ferromagnetic layer at different temperatures and cooling fields. We have found very large M_E shifts equivalent to up to 100% of the saturation value of the o-YMO layer alone. The overall behavior, including XMCD magnetization shift measured at the Mn-L edge of the LSMO layer only, indicates that the properties of the ferromagnetic layer contribute substantially to the M_E shift and that this does not correlate straightforwardly with the measured exchange-bias field H_E.     

On the ability to determine intrinsic switching field distributions from hysteresis loops in the partially correlated magnetization reversal regime

In the present work we use computational analysis based on the interacting hysteron model to address the question in how far intrinsic microscopic materials information can be retrieved from magnetic hysteresis loop data. Specifically, the goal is to understand whether it is possible to determine the intrinsic switching field distribution if exchange and magneto-static interactions of variable strength are simultaneously present in the material. We find that due to an existing degeneracy of hysteresis data sets, it is generally not possible to separately determine contributions from exchange and magneto-static interactions, even if the magnetization reversal is only partially correlated. However, the intrinsic switching field distribution could always be accurately determined, as long as the system remains in the uncorrelated or partially correlated magnetization reversal regime.