Thermal evolution of ferromagnetic metallic glasses

A traditional TG apparatus was modified by placing two permanent magnets producing a controlled magnetic field (TG(M): Magneto Thermogravimetry). This technique proved to be useful to study both structural relaxation and crystallisation of ferromagnetic metallic glasses. Results obtained for the amorphous alloys Fe₄₀Ni₄₀P₁₄B₆ and Fe_62.5Co₆Ni_7.5Zr₆Nb₂Cu₁B₁₅, are reported in this paper. Structural relaxation can be evaluated by measuring changes in Curie temperature induced by thermal treatments. Crystallisation in TG(M) is detected through a change in the measured apparent mass (difference between the sample mass and magnetic force driving it upward). These results were confirmed by DSC analysis. Whether the obtained crystalline phase is ferromagnetic, it can be identified through its Curie temperature, measured by TG(M). In fact the value of 770°C measured as Curie temperature of crystallised Fe_62.5Co₆Ni_7.5Zr₆Nb₂Cu₁B₁₅led to conclude that the only ferromagnetic crystalline phase is a-Fe. These hypothesis was confirmed by XRD analysis, showing that the first crystallisation yields to a-Fe nanocrystals. This revised version was published online in July 2006 with corrections to the Cover Date.     

Magnetic and structural properties of nanocomposite Co-Ni-Cr-Al-Y-N thin films

The relation between structural and magnetic properties of Co-Ni-Cr-Al-Y-N thin films deposited by reactive r.f. magnetron sputtering was investigated. A marked change in the magnetic behaviour of the films with the different nitrogen partial pressure in the Ar/N₂ deposition atmosphere was observed and qualitatively explained in correlation with the phase composition. The nanocrystalline metal solid-solution obtained at low N₂ content and the nanocrystalline nitride/amorphous composite obtained at high N₂ content are not magnetic, whereas the amorphous phase produced for intermediate N₂ pressures behaves like a ferromagnetic semi-permanent material. The results demonstrate the possibility of modulating the magnetic properties of r.f. magnetron sputtered Co-Ni-Cr-Al-Y-N thin films, thus opening a new route for magnetic multilayer deposition. PACS 68.55.-a; 75.70.Ak; 75.75.+a; 85.70.-w     

Multifractal statistics of Lagrangian velocity and acceleration in turbulence

The statistical properties of velocity and acceleration fields along the trajectories of fluid particles transported by a fully developed turbulent flow are investigated by means of high resolution direct numerical simulations. We present results for Lagrangian velocity structure functions, the acceleration probability density function, and the acceleration variance conditioned on the instantaneous velocity. These are compared with predictions of the multifractal formalism, and its merits and limitations are discussed.     

Effect of structural relaxation on Curie temperature and magnetostriction investigated by magnetoelastic waves in metglas

Summary Magnetoelastic wave amplitude, a, was measuredvs. the temperature during thermal cycles in Metglas 2826. When the Curie temperature,T _C, has been reached, theA value vanishes due to the fall of the magnetoelastic coupling in the paramagnetic state. This allows evaluation of theT _c temperature. The latter increases after the iterated thermal treatments while the magnetic anisotropyK _u decreases. Also theA amplitude, measured at room temperature after the subsequent thermal treatments, shows an increasing behaviour. The values ofK _u ,T _c andA approach saturation after the same number of thermal cycles; this suggests that the structural relaxation produced by annealing is the microscopic mechanism governing all the three physical quantities. In particular we explain the connection betweenK _u andA by means of the longitudinal magnetostriction.

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Riassunto L'ampiezza delle onde magnetoelastiche,A, è stata misurata in funzione della temperatura eseguendo cicli termici nel Metglas 2826. Raggiunta la temperatura di Curie,T _C l'ampiezzaA si annulla a causa della transizione dell'accoppiamento magnetoelastico nello stato paramagnetico. Ciò ci mette in grado di misurare la temperaturaT _C; quest'ultima aumenta in seguito ai ripetuti trattamenti termici mentre l'anisotropia magneticaK _u diminuisce. Anche l'ampiezzaA, misurata a temperatura ambiente dopo i trattamenti termici, mostra un andamento crescente. I valori diK _u,T _c eA tendono alla saturazione dopo lo stesso numero di cicli termici; ciò suggerisce che il rilassamento strutturale prodotto dalla ricottura è il meccanismo microscopico che regola le tre grandezze fisiche menzionate. In particolare noi mostriamo cheK _u eA sono legati dalla magnetostrizione longitudinale.

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Резюме Измеряется зависимость амплитуды магнитноупругой волны,A, от температуры в течение температурных циклов в металлическом стекле 2826. При достижении температуры Кюри,T _c, величинаA обращается в нуль из-за уменьшения магнитноупругой связи в парамагнитном состоянии. Это позволяет оценить темпаратуру Кюри,T _c. Температура Кюри увеличивается после повторных температурных обработок, тогда как магнитная анизотропияK _u уменьшается. Также отмечается увеличение амплитудыA, измеренной при комнатной температуре после последовательных температурных обработок. ВеличиныK _u,T _c иA приближаются к насьщению после одинакового числа температурных циклов. Это результат предполагает, что структурная релаксация, обусловленная отжигом, представляет микроскопический механизм, определяющий все три физические величины. В частности, мы объсняем связь междуK _u иA с помощью продольной магнитострикции.

     

Magnetoelastic coupling coefficient and anisotropy induced by magnetic annealing in strips of noncrystalline material

Summary The anisotropy induced by magnetic annealing in a transversal field is correlated with the Young modulus, the longitudinal magnetostriction and the magnetoelastic coupling coefficient. Subsequently it is shown that the value of the anisotropy deriving from directional short-range ordering,k _DSRO, can be calculated in an original manner by means of the mentioned parameters. Thisk _DSRO evaluation is in increasing agreement with its directly measured value, when the magnetic-annealing effects approach saturation. The magnetic heat treatment influences can be described by a small rise of the internal stress, in comparison with conventional annealing, and a large increase of longitudinal magnetostriction.

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Riassunto L'anisotropia indotta da trattamento termico in campo magnetico trasversale si correla con il modulo di elasticità lineare, la magnetostrizione longitudinale ed il coefficiente di accoppiamento magnetoelastico, in provini nastriformi di metallo amorfo. Successivamente si dimostra che l'anisotropia magnetica prodotta per ordine direzionale a corto raggio,k _DSRO, può essere calcolata in modo originale attraverso i menzionati parametri fisici. La valutazione, cosí ottenuta in maniera semiteorica, è in crescente accordo con i risultati della misura diretta dik _DSRO quanto piú gli effetti del trattamento termomagnetico si avvicinano alla loro saturazione. Detti effetti possono essere sintetizzati in una presenza di stress residui, lievemente maggiore rispetto a quanto avviene dopo il solo trattamento termico, ed in un grande aumento del coefficiente di magnetostrizione longitudinale.

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Резюме Анизотропия, индуцированная отжигом в поперечном магнитном поле, коррелирует с модулем Юнга, продольной магнитострикции и коэффициентом магнитоупрутой связи. Показывается, что величина анизотропии, полученная из ориентационного короткодействующего упорядочения,k _DSRO, может быть вычислена с помощью вышеуказанных параметров. Эта оценкаk _DSRO согласуется с непосредственно измеряемой, величиной, когда происходит насыщение эффектов магнитного отжига. Влияние магнитного теплового эффекта может быть описано с помощвю небольшого увеличения внутреннего напряжения, по сравнению с обычным отжигом, и большого увеличения продольной магнитострикции.

     

Stress sensitivity coefficient in polycrystalline and amorphous ferromagnetic materials below room temperature

Summary The piezomagnetic coupling coefficient (the ?stress sensitivity?) is quantitatively investigated below room temperature and a comparison is made among polycrystalline samples and amorphous ones. The technique used is an original one. It experimentally consists in the measurement of the amplitude of magnetoelastic waves, their resonance frequency and differential susceptibility; theoretically, it consists in coupling Newton’s law of force with the piezomagnetic state equations. The results clearly show different responses of polycrystalline samples with respect to the amorphous ones; in particular the nonmonotonic behaviour of the ?stress sensitivity? for metallic glasses is interesting for establishing their actual magnetic states at low temperature.     

Relaxation of young modulus by thermal treatment in metglas ribbons

Summary The Young modulus dependence on thermal treatment and magnetization state was analysed on various metallic glasses. Propagation of standing magnetoelastic waves was used as an investigation technique. A simple model was elaborated to predict the magnetoelastic coupling. The explanation of the results was suggested on the basis of macroscopic and microscopic relaxation effects and is also revealed by the measurement of internal friction and magnetoelastic-coupling constant.

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Riassunto Si è studiato il comportamento del modulo di Young nei vetri metallici in dipendenza del trattamento termico e dello stato di magnetizzazione dei provini. La tecnica sperimentale è basata sulla rilevazione delle onde magnetoelastiche stazionarie. Un semplice modello è usato per spiegare l'accoppiamento magnetoelastico. L'interpretazione dei risultati è data sulla base degli effetti di rilassamento macroscopici e microscopici che sono stati evidenziati da misure di frizione interna e di costante di accoppiamento magnetoelastico.

     

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.     

Partial anisotropy induced by low-field thermomagnetic treatment of noncrystalline ribbons

Summary The initial development of the transverse anisotropy induced by magnetic annealing is investigated for magnetizing field values which are much smaller than the ones generally employed. Measurements of Young modulus and longitudinal magnetostriction not only show their relation with induced anistotropy, as predicted in a previous paper, but also enable us to calculate the easy magnetization direction. The results are in agreement with the distributed relaxation time theory.

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Riassunto La cinetica dell’anisotropia trasversale, indotta da trattamenti termomagnetici, è stata studiata per valori del campo magnetico notevolmente piú piccoli di quelli generalmente impiegati. Si dimostra la relazione del modulo di elasticità lineare e la magnetostrizione longitudinale con l’energia di anisotropia indotta, come già predetto in un precedente lavoro. In base alla coerenza dei risultati abbiamo potuto stimare la direzione di facile magnetizzazione indotta da trattamenti termomagnetici parziali. I risultati sono in accordo con l’ipotesi teorica di tempi di rilassamento distribuiti.

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Резюме Исследуется начальное развитие поперечной анизотропии, индуцированной магнитным отжигом, в магнитных полях, напряженности которых очень малые по сравнению с обычно используемыми. Измерения модуля Юнга и продольной магнитострикции не только показывают связь с индуцированной анизотропией, но также позволяют вычислить направление легкого намагничивания. Полученные результаты согласуются с теорией.

     

Structure dependence of the magnetic-susceptibility behaviour at low magnetizing field

Summary The magnetic susceptibility during hysteresis cycles was measured in both amorphous and crystalline metallic ribbons. The analysis of the dependence on magnetizing field evidences a different behaviour due to the structural differences. The fundamental importance of the amplitude of the magnetizing field used for dynamic evaluation of initial susceptibility is demonstrated and related to the measured effects as well. Moreover, both temperature and frequency effects on the initial susceptibility confirm some peculiar properties related to the amorphous character.