## 2 Materials and Methods
### Micromagnetic Multiparameter, Microstructure and Stress Analysis (3 MA) Method
However, it is important to note that the 3 MA method has limitations when it comes to detecting local changes in the microstructure and the mechanical properties derived from it. The 3 MA-X8 applied in this work is the latest implementation of 3 MA, defined around a minimalistic sensor design, using the magnetization coil on a U-shaped core as the only sensing element [28]. The 3 MA-X8 uses low-frequency excitation (\(f<20\) kHz) and offers high-speed multichannel measurement (\(>\)100 measurements/s on 3, 8 or more channels synchronously, depending on the exact device variant). Due to the lower frequency range compared with previous 3 MA implementations, a higher penetration depth of the magnetic field is reached. This is an important advantage, given the correlation of the magnetic properties with the material properties, as these are also integral values of the examined specimens. Harmonics analyses, incremental permeability analyses and eddy current impedance analyses are conducted by supplying a voltage signal comprising two frequencies to the electromagnet. The 3 MA-X8 is operated via modular measuring software based on LabVIEW. The data acquisition and evaluation module produce a set of 21 magnetic features derived from the eddy current analysis, incremental permeability and the harmonic analysis in the time domain signal of the magnetization current. These 21 features are sensitive to different material characteristics. Some of them are sensitive to microstructural features (e.g., precipitates or dislocations), to residual stresses or to surface roughness, depending on the underlying interaction between the magnetic structure and micro- and macrostructure and depending on the different effects that have an influence on the magnetic Bloch-wall movement [28; 29; 30; 31; 32; 33]. The drive current depends on the material analyzed with the probe, while the voltage is used to extract the parameters that describe the magnetization behavior. This is then correlated to the material properties [10; 28; 29; 30; 31; 32; 33]. A comprehensive overview of the various measurement applications of 3 MA-X8 analysis is given in [18; 28]. In this study, three of the twenty-one magnetic features are illustrated: \(DZmax\) is defined as the maximum value of the incremental permeability, \(W3Z\) is the width of the impedance loop at 3% of the maximum and \(W10Z\) is the width of the impedance loop at 10% of the maximum. Figure 2 shows the 3 MA-X8 device, including the probe. As part of the non-destructive material characterization, a 3 MA sensor (Figure 2--right-hand side) with a concave rounding (R3.5) of the contact surface is processed. This design aims to ensure optimum coupling to the sample geometry (Figure 1) and thus, to enable the acquisition of optimum sensor signals across the entire surface of a specimen. Using a probe holder reduces the impact of position variations, which improves the reproducibility (Figure 2--right-hand side). The samples are magnetized in the longitudinal direction. To define the proper measurement parameters, the micromagnetic investigations are conducted with varying measurement parameters, including the frequency and amplitude of the magnetic field. Finally, the 3 MA-X8 measurements are performed with a magnetization frequency of 200 Hz, a magnetization voltage of 1.2 V and a superimposed higher frequency of 5 kHz with a voltage of 0.5 V.