## 2 Historical Development  
Strictly speaking, the development of micromagnetic methods started more than 80 years ago [4]. In this early work, the electromagnetic properties of the material were correlated to other material properties, like elastic parameters. However, the name [PERSON] dominated the early days of the development of micromagnetic methods [5; 6; 7]. The idea to use micromagnetic NDT for production-integrated testing--in that case, material sorting--was initially reported in 1955 [8]. At least since 1964, micromagnetic methods have been used for the NDT of case depth in carburized steels [9]. Only shortly later, it was shown that magnetic Barkhausen Noise measurements could be used for investigating the stress condition in the magnetic materials [10]. Later, there have been some basic physical investigations, e.g., from [PERSON], providing a better understanding of the relations between the electromagnetic properties of a material and its microstructure [11]. In the early 1970s, [PERSON] and his co-researchers discussed the relationship between Rockwell hardness type C (HRC) and magnetic permeability in technical steels [12]. In addition, they first promoted electromagnetic tools for the quality assurance of metals by characterizing internal stresses via magnetic methods [13]. In the late 1970s, the refinement of the Barkhausen Noise technique marked major advances in the enhancement of microstructure and stress sensitivity. Since then, micromagnetic methods for nondestructive evaluation have attracted the attention of several research groups [14; 15]. At about the same time, the researcher of Fraunhofer IZFP started to investigate and develop micromagnetic NDT methods. A first project was dedicated to the \"determination of microstructure in pressure vessel steels with magnetically induced measuring parameters\" on behalf of the German Federal Ministry for Research and Technology (BMFT) [15]. The main results led to the development of a specific surface sensor with an open U-shaped yoke design and the use of a Hall sensor in the control of a tangential magnetic field in the specimen. The correlation between the micromagnetic measuring parameter and the macro-stress level as the target parameter was reported [16]. It was recognized that different measuring parameters are sensitive to different target quantities, i.e., microstructure and stress parameters in the material. It was concluded that the micromagnetic measuring parameters complement each other in regard to their measuring information [17]. Based on this approach, the separation of microstructure effects from the influence of residual stresses could be discussed for the first time [18]. This research project resulted in the construction of the so-called EM8101--the first prototype of a micromagnetic testing equipment that was developed at Fraunhofer IZFP (see Figure 1). This equipment combined two micromagnetic methods, namely Barkhausen Noise (BN) and incremental permeability (IP). Two micromagnetic measuring parameters, H\({}_{\text{CM}}\) and M\({}_{\text{MAX}}\), have been derived from BN and two parameters, H\({}_{\text{C\mu}}\) and \(\mu_{\text{MAX}}\) from IP. By combining both techniques, it was possible to independently quantitatively characterize residual stress from changes in microstructure [19]. In this early micromagnetic research at IZFP, it became apparent that the use of one single micromagnetic parameter and also the use of parameters from only one micromagnetic method could lead to ambiguous or unreliable results. In fact, each micromagnetic measuring parameter is influenced by a multitude of microstructure- and/or stress-related material features. In some \"simple\" cases, demonstrated by [PERSON], a quantitative evaluation using only one measuring quantity could be successful [20]. This simple approach is restricted to applications where other influences remain constant or their changes can be neglected.