MEASUREMENT 2011, Proceedings of the 8th International Conference, Smolenice, Slovakia 107 Analysis of Magnetic Fields on the Surface of Grain Oriented Electrical Steel 1 A. J. Moses, 1 P. I. Anderson, 1 N. Chukwuchekwa, 1 J. P. Hall, 1 P. I. Williams and 1 X. T. Xu 1 Wolfson Centre for Magnetics, School of Engineering, Cardiff University, Cardiff, UK Email: Mosesaj@cf.ac.uk Abstract. Magnetic properties of grain oriented electrical steel are often measured using well established surface field sensors. Use of magnetic domain observation, magnetic force microscopy (MFM) and Barkhausen noise measurement for studying magnetic properties and microstructure is demonstrated. It is shown that uncertainty over the region in which Barkhausen noise is detected needs clarification. It is further shown that different domain observation techniques appear to identify different types of micro structural features and that uniaxial magnetic field sensors may not reveal the total loss occurring in the steel. The origin of unusual MFM images is discussed. Keywords: Electrical Steels, Magnetic Domains, Barkhausen Noise, Magnetic Losses, Magnetic Force Microscopy, Magnetic Field Detectors 1. Introduction Grain oriented electrical steel (GOSI) is a key component of electrical machine cores. It is produced in thin strip form, typically 0.30 mm thick following a complex sequence of rolling and heat treatment schedules together with application of electrical insulating coatings [1]. The steel comprises large grains (up to around 100 mm 2 surface area) with a strong {110}<001> texture. When laminations are magnetised at power frequencies along their rolling directions, as in transformer cores operating in electrical power systems, magnetic losses occur during domain wall motion [2]. Such losses are responsible for up to 10% of all electrical energy generated [3]. More knowledge based measurement and interpretation of losses is necessary to quantify metallurgical and processing parameters which control the losses. The underpinning dependence on composition, internal stress, thickness, impurities, grain size and texture cannot be fully quantified without a deeper knowledge of magnetisation processes. Two measurable physical features closely associated with the loss process are magnetic domain motion and Magnetic Barkhausen emission (MBE). Although an IEC standard for loss measurement for commercial grading of GOSI is well established [4], its use as a research tool for investigating loss mechanisms is limited. Other methods based on surface magnetic field sensors are commonly used for localised loss evaluation. Magnetic domain observation (MDO) and magnetic force microscopy (MFM) are useful tools for surface magnetic field characterisation. Domain images obtained from the commonly used Bitter technique and the Kerr magneto optic (KMO) effect are presented to show how these techniques can imply the presence of different domain structures. It is shown that a better understanding of MBE measurements is needed to develop quantitative relations between microstructure and magnetic properties.