Analyzer-free, intensity-based, wide-field magneto-optical microscopy

Abstract

In conventional Kerr-and Faraday microscopy the sample is illuminated with plane-polarised light and a magnetic domain contrast is generated by an analyser making use of the Kerr-or Faraday rotation. In this paper we demonstrate possibilities of analyser-free magneto-optical microscopy based on magnetisationdependent intensity modulations of the light: (i) The transverse Kerr effect can be applied for in-plane magnetised material, demonstrated for an FeSi sheet. (ii) Illuminating the same sample with circularly polarised light leads to a domain contrast with a different symmetry as the conventional Kerr contrast. (iii) Circular polarisation can also be used for perpendicularly magnetised material, demonstrated for a garnet film and an ultrathin CoFeB film. (iv) Plane-polarised light at a specific angle can be employed for both, in-plane and perpendicular media. (v) Perpendicular light incidence leads to a domain contrast on in-plane materials that is quadratic in the magnetisation and to a domain boundary contrast. (vi) Domain contrast can even be obtained without polariser. In cases (ii) and (iii), the contrasts are generated by MCD (Magnetic Circular Dichroism), i.e. by the differential absorption of left and right circularly polarised light, induced by magnetization components along the direction of light propagation. In case (v) the contrast is caused by MLD (Magnetic Linear Dichroism), i.e. by the differential absorption of linearly polarised light, induced by magnetisation components transverse to the propagation direction. The domain boundary contrast is due to the magneto-optical gradient effect. An explanation of these contrast phenomena is provided in terms of Maxwell-Fresnel theory.

Odd Magneto-Optical Linear Dichroism in a Magnetophotonic Crystal

“…In this case, the polarization state can be switched, thereby implementing the maximum possible modulation without the use of an analyzer in the observation system. In magneto-optical microscopy, the effects of linear dichroism can be efficient in situations where the magnetic circular dichroism vanishes, for example, for the visualization of antiferromagnetic domains [18].…”

“…Magneto-optical effects associated with a change in the intensity of reflected or transmitted light, called intensity effects, are widely used in integrated optics devices to create optical non-reciprocal elements [1][2][3], spin wave detectors [4], chemical sensors and biosensors [5][6][7][8][9][10][11][12], magnetometers [13][14][15][16][17] and visualizations of magnetic structures [18,19].…”

“…Both effects in polar and longitudinal polarizations are odd not only with respect to the magnetization but also with respect to the angle of deviation of the plane of polarization from the p-and s-polarizations. Similar geometries with the light of 45 • polarization allow one to obtain dichroic effects and distinguish the domains with opposite magnetization orientations [18].…”

The 2022 magneto-optics roadmap

“…This progress is supported by the revival of anti-reflection schemes [66] for achieving huge MO contrasts for even nanometre thick films [46]. This helps enabling analyser free domain imaging modes [46,72].…”

“…The challenge has been partly overcome by advanced illumination schemes with simultaneous [64,67] or sequentially alternating illumination schemes [68,69], enabling the separation of the different Kerr effects for the MO image formation and the realization of quantitative Kerr effect micrographs [64,[67][68][69]71]. With the increase of dynamic range and signal-to-noise ratio in current complementary metal-oxide semiconductor (CMOS) camera technology, imaging of the purely transverse Kerr effect with sensitivity only to the in-plane magnetization has become achievable [72]. This progress is supported by the revival of anti-reflection schemes [66] for achieving huge MO contrasts for even nanometre thick films [46].…”