The observation of magnetic domains by magneto-optical microscopy, based on the Kerr and the Faraday effect, is one of the most prominent techniques for the visualization of distributions of magnetization within magnetic materials. The method has gained increased attention due to the possibility to visualize field and current induced phenomena in nanostructured magnetic materials on fast time-scales. Fundamental concepts and recent advances in methodology are discussed in order to provide guidance on the usage of wide-field magneto-optical microscopy in applied magnetism. Recent applications of magneto-optical microscopy in bulk and thin film materials are reviewed at the end.

https://iopscience.iop.org/article/10.1088/0022-3727/48/33/333001/pdf

Progress in magnetic domain observation by advanced magneto-optical microscopy

A method for imaging magnetic domains with spatial resolution of less than 0.5 µm is described. The method employs the magnetooptic Kerr effect and is applicable for observing surface domain structures, even if the magnetic surface of interest is covered by a thick transparent overcoat. Polarized laser light, incident at an oblique angle, is focused to a diffraction limited spot onto a surface whose magnetization is modulated at a fixed frequency by a magnetic field. The modulation in the polarization of the light due to the modulation of the magnetization beneath the spot is measured with the use of a lock-in amplifier. An image of the magnetization changes is then constructed by scanning the spot. Domains are revealed because areas of domain wall motion have larger magnetization changes than areas of magnetization rotation within domains. Since this approach minimizes noise and eliminates nonmagnetic signals, the domain image contrast is greatly enhanced. With 0.3 mW of laser power on a spot and a detection bandwidth of 10 Hz, signal to noise ratios of 65 db and a magnetization rotation sensitivity of 0.06° have been obtained in permalloy. Results of domain images in thin-film inductive heads are presented. This technique is ideally suited for the study of the dynamics of wall motion and magnetization rotation in magnetic films.

Magnetic domain imaging with a scanning Kerr effect microscope

We describe a laser magneto‐optic microscope (LAMOM) system, used to observe magnetic domain dynamics in Permalloy thin‐film devices. Optimization of Kerr magneto‐optic contrast is achieved by using laser illumination in a polarized light microscope in combination with video image processing. High magnetic contrast, a diffraction‐limited (0.25 μm) resolution, and motion picture recording capability at up to 30 frames/s are demonstrated by recent results.

Optical imaging of magnetic domains in motion (invited)

Magnetic noises in rapidly quenched amorphous magnetic materials were examined. Magnetic noises are observed as the fluctuations of the induced voltage wave forms of the magnetic cores on every cycle of the alternating exciting field. Fe, Ni, and Co base alloys were tested, and the following results have been obtained. 1) CoFeSiB amorphous materials show the lowest noise, 2) Water quenching treatment decreased the noise remarkably, 3) The lower Curie temperature leads to the lower magnetic noise. 4) Some Co base amorphous alloys have the performance comparable to or better than that of the conventional polycrystalline material such as Supermalloy.

Noise in amorphous magnetic materials

We describe a digital imaging technique with a spatial resolution of less than 0.5 μm that allows the observation of the nucleation and growth of magnetic domains in metallic films whose surfaces are under less than ideal conditions. A diffraction limited laser spot is scanned across the surface of interest while a periodic magnetic field of saturation strength is applied. The local magnetization curve under each point of the laser scan is measured through the longitudinal magneto-optic Kerr effect and recorded in a computer. From the stored data, the spatial distribution of the magnetization component in one direction as a function of time is obtained. Digital averaging and data normalization are used to enhance the quality of the images which allows the optical imaging of very rough surfaces. To demonstrate the technique, the pole tip region of a film recording head was imaged and the magnetization distribution in that region was observed as a current waveform was applied to the coils of the head. In the present implementation, a time resolution of 50 ns was achieved.

A method for the magneto-optic imaging of magnetization time evolution in thin films

Laborious surface polishing has been inevitably necessary for domain observations of bulk magnetic materials by Kerr effect. We have made it possible to observe the magnetic domain without any surface polishing for bulk magnetic materials. A CCD (charge coupled device) image sensor and a microcomputer did the contrast enhancement and noise reduction. Domains in rapidly quenched amorphous magnetic materials have been observed clearly without any surface polishing.

K. Shirae

Papers

A CCD image sensor and a microcomputer make magnetic domain observation clear and convenient