Characterization of Softmagnetic Thin Layers using Barkhausen Noise Microscopy
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Characterization of Softmagnetic Thin Layers using Barkhausen Noise Microscopy Jochen Hoffmann1, Norbert Meyendorf1, Iris Altpeter2 Center for Materials Diagnostics, University of Dayton, Dayton, OH 45469-0121, U.S.A. Fraunhofer Institute for nondestructive Testing IZFP, Saarbruecken, Germany ABSTRACT Ferromagnetic materials are essential for data recording devices. For inductive or magnetoresistive (MR) sensors softmagnetic thin layer systems are used. Optimal performance of these layers requires homogeneous magnetic properties, especially a pronounced uniaxial magnetic anisotropy. Furthermore, microstructural imperfections and residual stresses influence the magnetic structure in the layer system. Barkhausen Noise Microscopy enables the characterization of such thin layers. By cycling the magnetic hysteresis of ferromagnetic material electrical voltages (the Barkhausen noise) are induced in an inductive sensor. Miniaturization of the sensor and the scanning probe technique provides resolution down to few micrometers. Two materials were examined in terms of their structure, thickness, residual stresses and heat treatment condition: Sendust, used in inductive sensors and nanocrystalline NiFe, used in MR-sensors. In quality correlations to Barkhausen noise parameters were found. For representative sample a quantification of residual stress distribution could be established employing X-ray stress analysis.
INTRODUCTION Currently there are several sophisticated methods available to image and characterize the magnetic structure of ferromagnetic thin layers. Especially Magnetic Force Microscopy MFM and Kerr-optical measurements are very popular. For dynamic magnetization processes ferromagnetic resonance spectroscopy or Brillouin scattering are frequently used. An important issue is the influence of mechanical properties, e.g. residual stress on the magnetic performance of the layer system. Residual stresses in such layers are due to non-optimized process conditions, undesired phase transitions or insufficient ductile adaptation to the substrate. They deteriorate the signal to noise ratio and thus the sensor sensitivity. Barkhausen Noise Microscopy provides the possibility to characterize mechanical and magnetic properties of softmagnetic layers both with high accuracy and lateral resolution.
EXPERIMENTAL DETAILS By tracing the hysteresis curve of a ferromagnetic material, electrical impulses are induced in an electromagnetic inductive probe: the magnetic Barkhausen noise. Barkhausen events occur when domain wall movement has to overcome microstructural obstacles. Usually most noise activity can be measured in the vicinity of the coercivity HC. The main parameters derived from the Barkhausen noise signal are the Barkhausen noise maximum MMAX and the coercive field U1.5.1
a)
0.1-5 µm
b) Sample
MMAX
5 µm HCM Electromagnet
Figure 1. a) Schematic of miniaturized inductive sensor
b) Barkhausen noise signal
strength HCM, which is the position of MMAX within the magnetic field H (Fig. 1b) [1]. The Barkhausen Noise an
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