Inhomogeneous field calibration of a magneto-optical indicator film device

Abstract

A concept for the traceable calibration of magneto-optical indicator film (MOIF) based magnetic field imaging devices is presented and discussed for the example of a commercial MOIF device with a 60 × 45 mm2 sensor. The calibration facilitates a quantitative and fast characterization of magnetic microstructures combining relatively high spatial resolution with large imaging areas. The macroscopic calibration is performed using the homogeneous magnetic stray field of a pre-characterized electromagnet with a large pole shoe diameter of 250 mm. However, this calibration alone cannot yet account for the vectorial and spatially fast decaying stray fields of magnetic microstructures. For that, a forward simulation approach is pursued, based on the temperature-dependent magnetic parameters of the MOIF material as resulting from superconducting quantum interference device magnetometry and ferromagnetic resonance measurements. This is complemented by a transfer function-based approach to correct the impact of the sensor thickness and in-plane stray field components. The validity of the combined calibration and simulation approach is proven by means of a quantitative characterization of a magnetic scale. For the commercial MOIF device a 28.4 µm spatial resolution and 1.18 mT field resolution is achieved. The calibration is validated by a comparison to scanning Hall probe microscopy results. Furthermore, the uncertainty budget is discussed.

Magneto-Optical Indicator Films: Fabrication, Principles of Operation, Calibration, and Applications

“…Here, F m = −µ 0 (M•H) is the Zeeman energy of the magnetization in the external field H, F d = −µ 0 (M•H d )/2 is the energy in the self-demagnetizing field H d , and F A is the anisotropy energy (magnetocrystalline or induced) which consists of the uniaxial anisotropy K u sin 2 θ (θ is the angle between M and the anisotropy axis) and cubic anisotropy F c . In ferrite garnets, cubic anisotropy is very low and can be neglected [14].…”

“…Of course, because of the resolution limit of optical microscopes and the finite thickness of MOIFs, the spatial resolution in real experimental conditions is lower. The actual resolution also depends on the magnetic pattern being observed; its precise determination requires complicated calculations using a transfer function [14], but a rough estimate of the resolution in the range of the order of the MOIF thickness generally works well [33]. The normal component of M can, thus, be detected optically in light transmitted through the MOIF via changes in the light polarization induced by the Faraday effect.…”

“…Finally, another advantage of in-plane MOIFs, which also owes its origin to the absence of domains, is their completely reproducible and reversible response to magnetic fields. This makes it possible to precisely calibrate them and, thus, they can be used in various quantitative measurements and sensor applications [14,15,33]. All this makes in-plane MOIFs a very good choice to be used as a standardized technique for local measurements of stray magnetic fields at the micrometer to millimeter scales.…”

“…[13]. However, substantial theoretical advancements in understanding the physics of MOIFs have recently been made [14,15], which, in turn, have led to new developments in MOIF calibration and quantitative measurement techniques. As a result, new advanced industrial applications have been demonstrated.…”

A protocol for mapping transport current density of REBCO-coated conductor by magneto-optical imaging

“…The other triplet-rotation-angle approach [40] was effective to obtain a high resolution of the magnetic field with magnitude and sign simultaneously, with the aid of an external device to precisely control the rotation plane of polarizing light, which to some extent complicated the MOI system. By practically calibrating a polynomial fit to the relationship between the intensity and magnetic field, we can get rid of the main hindrance of non-uniform illumination [41,42]. The parameters for calibration strongly depend on the conditions of the light source and camera settings, making it hard to be comparable between different utilizers.…”