Magneto-optic measurements on uneven magnetic layers on cardboard

The magneto-optic Kerr effect (MOKE) is a common, simple and inexpensive method to measure magnetic behavior of surfaces, bulk samples and regularly arranged nanoparticles in research and industrial laboratories.^1,2 It is, however, not common for investigations of surfaces with an irregular disorderly shaped surface features higher than the light wavelength.

Surface roughness is well-known to strongly influence the magnetic properties of samples.^3–6 Several studies have pointed out the relation between surface roughness and domain sizes, resulting in modified magnetization reversal behaviour.^7–9 Nevertheless, these studies concentrated on surfaces with an average roughness smaller than the wavelength of the laser light.

Investigation of cracked and uneven surfaces, however, would be of large interest to gain more information about the influence of such roughness on the magnetic properties of layers. Additionally, several complex surfaces, such as biological samples or irregular nano- or micro-structured patterns, e.g. irregularly distributed nanowires, could be investigated by MOKE easier and faster than by SQUID or VSM. Furthermore, in a MOKE setup, the sample can usually be rotated, allowing investigating possible anisotropies.¹⁰ 

To our knowledge, MOKE investigations of an irregular rough surface are only described in one research article,¹¹ pointing out the possibility to measure magnetic properties of industrial steel samples with average roughness up to 12 μm. Thus, the now presented article enlightens the possibilities to measure magnetic properties of a magnetic layer, consisting of iron flakes, spray-coated or hand-coated on a cardboard.

For sample preparation, the “FERRICON^® 160” iron flakes (produced by Eckart GmbH, Hartenstein / Germany) were used. Figure 1 depicts the pure pigment deposited as powder on a sample holder. This image is recorded by scanning electron microscopy (SEM) using a Tabletop TM 3000 from Hitachi. The flake pigments have a diameter around 20 micrometers and a shape which is also referred to as “corn-flake” structure.¹² The anisotropic shape with the very low thickness of pigments is also clearly visible in Figure 1.

Additionally to topography images, the SEM was used for an EDS (electron dispersive spectroscopy) investigation. EDS enables the quantitative determination of chemical elements on sample surfaces. Here a Quantax unit from Bruker was used. Figure 2 shows the EDS spectrum gained from the pure Ferricon pigments. Beside iron, also the elements silicon, carbon and oxygen are detected. The detected amount of those elements of together nearly 20 wt-% is significant (Table I). According to supplier information the pigments are coated with a layer of silicon of ∼ 7 μm thickness to avoid corrosion by oxidation.¹³ Such protective coatings are commonly used for metal pigments and can be of polymeric or inorganic nature.^14–16 

These flakes are coated onto cardboard using two different standard solvents, CSR and LQ S3-3, with 10 % flakes in the solution. Layers on cardboard are created by spray-coating two different layer thicknesses of 26 μm and 50 μm, respectively, resulting in relatively homogeneous surfaces, and by hand-coating with a thickness of ∼ 50 μm using a doctor knife, resulting in less regular surfaces.

The surface structures of spray-coated layers are shown exemplarily in Fig. 3 for layers of thickness 50 μm. Depending on the solvent, the maximum roughness is ∼ 8 μm or 4.5 μm, respectively. Similar values were measured for a layer thickness of 26 μm.

Crosscuts of samples of different thickness, produced with different solvents, were investigated by SEM and EDS-method. Fig. 4 shows images of samples with thicknesses 26 μm and 50 μm, spray-coated with CSR as solvent. In the thinner layer, the flakes are densely stacked, resulting in a relatively homogeneous layer. In the thicker layer, however, areas without magnetic flakes are visible in the coating. Additionally, the flakes are oriented less parallel to each other, several particles are apparently canted. This finding can be assumed to modify the magnetic properties of the complete layer.

MOKE measurements were performed with the external magnetic field oriented parallel to the sample plane. The 15 mW, 532 nm incident laser beam was linearly polarized in s-polarization, the angle between incident beam and sample surface normal was kept constant at ∼ 22.5°. The reflected beam was separated in two orthogonal beams in a Glan-Thompson polarizing beam splitter, both of which were focused onto the photo-diodes of a diode bridge. The diode bridge was balanced before each measurement using a half-wave plate; the difference between the intensities of both diodes was then proportional to the Kerr rotation.¹⁷ During the measurements there was no need to use a lock-in amplifier, while the hysteresis loops were averaged over several consecutive measurements instead.

Fig. 5 depicts hysteresis loops, measured on samples with different solvents and different layer thickness. Hysteresis loops were averaged over 4 measurements each to reduce the influence of single outliers.

The solvent strongly influences the measured Kerr rotation which differs by a factor of 3-4. This can be explained by the Energy Dispersive X-Ray Analysis (EDX) analyses of the samples under investigation. While the amount of iron in the EDX spectra of samples prepared with LQ S3-3 is (73 ± 2) %, the samples prepared with CSR as solvent show only (29 ± 1) % of iron.

Additionally, for both cases, the curves for the thinner layer show the larger Kerr rotation. This can be attributed to the distinct in-plane orientation of the iron flakes in the thinner layer, as revealed by SEM (Fig. 4), while they are partly canted in the thicker layer. Due to the typical strong shape anisotropy of small particles, the magnetization can be expected to be oriented approximately collinear with the flakes. Since in our MOKE setup, we detect the longitudinal – and not the polar – magnetization component, the reduced signal in the thicker layer is comprehensible.

After these tests with spray-coated layers, another experiment was made measuring MOKE on a hand-coated magnetic layer on cardboard. The layer thickness is 50 μm, the maximum roughness is ∼ 11 μm. Despite the increased irregularities, measuring a hysteresis loop was again possible. Fig. 6 depicts the result of averaging over 8 measurements.

Although the saturation values of the Kerr rotation are quite similar to those obtained for spray-coating with the same parameters, the coercive field and the hysteresis loop shape differ clearly from loop measured on the spray-coated sample. This shows clearly the influence of surface roughness on the magnetic properties of a sample – and the necessity to perform more experiments on magnetic structures with microscopic roughness.

In summary, different magnetic coatings were prepared on cardboard and investigated using MOKE. It could be shown that MOKE measurements on theses surfaces with roughness of approximately 4-11 micron were successful even without using a lock-in amplifier.

Moreover, changing the solvent and the layer height resulted in similar coercive fields and hysteresis loop shapes, while changing the coating method led to a reduced coercive field and a modified loop shape.

This shows that measuring magnetic properties of samples with micrometer-sized roughness by MOKE is possible and should be intensified to gain more information about the influence of microscopic structures on magnetic sample properties. It should also be mentioned that the exact theoretical description of the Kerr effect in highly irregular ferromagnetic media requires future development, while it can be of industrial importance.

The authors would like to thank Eckart GmbH for providing the iron flakes for hand-coating and preparing the spray-coated samples. This work was partially supported by the internal project BK-243/RIF/2016 of the Institute of Physics – CSE, Silesian University of Technology. For funding of the electron microscopic equipment, the authors acknowledge very gratefully the program FH Basis of the German federal country North Rhine-Westphalia.