Magnetic field induced random pulse trains of magnetic and acoustic noises in martensitic single-crystal Ni2MnGa

Abstract

Jerky magnetic and acoustic noises were evoked in a single variant martensitic Ni 2 MnGa single crystal (produced by uniaxial compression) by application of an external magnetic field along the hard magnetization direction. It is shown that after reaching the detwinning threshold, spontaneous reorientation of martensite variants (twins) leads not only to acoustic emission but magnetic two-directional noises as well. At small magnetic fields, below the above threshold, unidirectional magnetic emission is also observed and attributed to a Barkhausen-type noise due to magnetic domain wall motions during magnetization along the hard direction. After the above first run, in cycles of decreasing and increasing magnetic field, at low-field values, weak, unidirectional Barkhausen noise is detected and attributed to the discontinuous motion of domain walls during magnetization along the easy magnetization direction. The magnetic noise is also measured by constraining the sample in the same initial variant state along the hard direction and, after the unidirectional noise (as obtained also in the first run), a two-directional noise package is developed and it is attributed to domain rotations. From the statistical analysis of the above noises, the critical exponents, characterizing the power-law behavior, are calculated and compared with each other and with the literature data. Time correlations within the magnetic as well as acoustic signals lead to a common scaled power function (with β = −1.25 exponent) for both types of signals.

Acoustic Emissions during Structural Changes in Shape Memory Alloys

“…It is worth noting that these values are also in very good agreement with the energy exponents (ε ∼ = 1.5) of ME determined from noises induced by cyclic bending deformations in the same alloy [48,49]. Figure 14 [9]. The exponents are 1.4 ± 0.1 and 1.5 ± 0.1 for ME and AE, respectively.…”

“…In addition, in [53], AE and ME noises were detected during both the first superplastic part of the deformation and the rubber-like region. It was obtained that the energy exponents for the stress-induced rearrangements (superplasticity) were again the same as the error bars (ε = 1.7 ± 0.1 and ε = 1.6 ± 0.1 for AE and ME, respectively) and also agreed well with these values for magnetic-field-induced superplasticity (ε = 1.5 ± 0.1) [9]. Interestingly, the energy exponents obtained from the rubber-like regime were also the same as those belonging to the stress-induced superplasticity.…”

“…Furthermore, in previous studies of magnetic-field-induced detwinning, no noise measurements were implemented, but rather the emphasis was put on the observation of the initial and/or final magnetic domain structure [42][43][44][45][46][47]. A detailed analysis of simultaneously measured AE and ME jerks was carried out during magnetic-field-induced superplastic deformation in a single-crystalline Ni 2 MnGa shape memory alloy with a 10 M modulated martensite structure [9]. For the details of this work, we refer the reader to the original article.…”

“…The time correlations between subsequent events were also analysed in [9] according to the relation (11). Figure 16 shows the P(n;τ m ) functions for τ m = 1 s, together with the distributions of a randomized data set (shuffled data).…”

“…. P(n; t m ) distributions of a sequence of n AE and ME events belonging to the same burst with t < 1 s and P ind (n; t m ) distributions of randomized data sets (shuffled data) [9]. The solid lines show a power law with β = −1.25, and the dashed lines show Equation (11) (independent events) with a = 0.976 for AE and a = 0.988 for ME.…”