Magnetic domain structure of multidomain magnetite as a function of temperature: observation by Kerr microscopy

This study presents magnetic domain observations of the crystallographic (1 10)- and (Ill)-planes of various monocrystalline multidomain (MD) magnetite s (Fe3O4) using the magneto-optical Kerr effect (MOKE). At room temperature (RT), lamellar domains separated by 180 degrees-, and closure domains with 71 degrees-, and 109 degrees-walls have been observed on the (110)-plane d ue to the two easy directions of magnetization within this plane. Domain st ructure observations on the (Ill)-plane reveal more complex domain patterns , which are formed by arrays of closure domains. Furthermore, in this study , the behaviour of the domain structure as a function of temperature has be en observed. For high temperature domain observations, a special furnace ha s been developed which allows domain observations using the MOKE between ro om temperature and Curie-point (575 degrees C). During observation, the fur nace is evacuated to 10(-3) Pa in order to prevent chemical alteration effe cts of the sample. The temperature dependence of the domain structure on th e (110)-plane has been observed for various (MD) magnetites from Minais Ger ais (Brazil) and the Zillertal (Austria). Magneto-optical domain contrasts could be observed up to 555 degrees C. During heating and cooling, the doma in structure proved to be reversible. The mean domain width at each tempera ture step was determined by two methods in order to obtain an improved appr oximation. At RT, the mean domain width for different crystals varies betwe en 40 and 90 mu m. The heating experiments on Brazilian magnetites show tha t the mean domain width is relatively stable from RT up to 350-400 degrees C; it then increases significantly at higher temperatures up to the Curie-p oint of magnetite. The experimental results are in good agreement with theo retical calculations of the absolute domain width in MD magnetite for a lar ge range of grain sizes and varying temperatures. (C) 1999 Elsevier Science B.V. All rights reserved.