## 2 Materials and Methods
### Magnetic Adaptive Testing  
Magnetic adaptive testing is a special way of magnetic hysteresis measurements. Instead of the major hysteresis loop, a series of minor loops are measured with step-by-step increasing amplitude of a triangular waveform magnetizing current. The variation of the magnetizing field is linear with time; consequently, the signal of the pick-up coil is proportional to the sample's differential permeability. The measured minor loops (permeability loops) serve as the input for the further evaluation of data. Each point of permeability loops represents some information about the magnetic behaviour of the sample. During the evaluation process, permeability matrices are calculated from permeability data, and each matrix element is compared with the corresponding element of the reference sample. The elements of permeability matrices are characterized by \(\mu\equiv\mu\) (\(h_{a}\), \(h_{b}\)) values. Here, \(h_{a}\) is the magnetizing field and \(h_{b}\) is the amplitude of minor loop. \(\mu\)-matrix elements are called as \"MAT-descriptors.\" In this way, a big data pool is generated and these descriptors characterize magnetically the investigated material. They are considered as functions of any independent parameter, describing the degradation of the material due to any external influence. The purpose of the MAT evaluation is to choose from these data pools those \(\mu\) (\(x\))-degradation functions that best characterize the actual material degradation. \(x\) is the DBTT transition temperature in the present work. The MAT procedure and evaluation is described in detail in [15].  
The sensitivity and reliability of the chosen--so-called optimal MAT descriptors--is essential for the effective application of MAT. For checking these parameters (sensitivity and reliability), a \"sensitivity map\" of degradation functions is generated, which indicates relative sensitivity of degradation functions with respect to independent variable of the measured material. The degradation function's sensitivity is calculated as the slope of its linear regression. In the graph of the sensitivity map, it is expressed by a colour, giving useful information about the relative modification of the studied magnetic parameter with respect to the independent characteristic. The most sensitive MAT-descriptors are taken from the top area of \"hills\" of the elevated sensitivity values. The sensitivity map also indicates the reliability of the chosen descriptor. Big plateaus are favourable, where the parameters only slightly depend on the actual choice of \(h_{a}\) and \(h_{b}\) field coordinates values. See the sensitivity map in Appendix A.  
The measured samples were magnetized by a yoke, which was put on the top of the sample, opposite to the V-notch. A half transformer core was used as magnetizing yoke. It is made of laminated Fe-Si sheets. Two coils, a pick-up coil and an exciting coil, are wound on yoke legs. The cross-section of the yoke leg was 10 \(\times\) 5 mm\({}^{2}\), corresponding to the sample size. The sample holder was designed for usage in a hot cell, made possible by the replacement of the samples by a manipulator.  
Analyzing carefully the possible error of those MAT descriptors that characterize the material degradation, it was found that the uncertainty of this parameter is less than 1%. Considering the importance of MAT descriptors' uncertainty, a detailed analysis can be found in the Appendix A; they are the possible sources of this error, and the analysis shows how this uncertainty can be determined.