Lorentz microscopy of small magnetic structures (invited)

Domains and domain walls in micron and submicron sized magnetic elements ca n be studied at high resolution using Lorentz microscopy in the transmissio n electron microscope. In situ magnetizing experiments are possible in whic h magnetization reversal processes can be viewed directly in the presence o f varying magnetic fields. These techniques have been used to investigate s mall magnetic structures fabricated by electron beam lithography on electro n transparent membrane substrates. Patterned elements as small as 200 x 40 nm have been imaged magnetically. Detailed studies have been carried out in to the properties of high aspect ratio (acicular) elements of Co and a soft NiFe alloy. It has been found that the coercivity increases as the element s become narrower, down to ultrasmall elements with a width of 40 nm. Eleme nt length has no effect so long as the aspect ratio is sufficiently high. M agnetization reversal in acicular elements is known to begin from the ends of the elements, therefore the shape of the ends - flat, elliptical, or poi nted - has a significant effect on the coercivity. The magnetic environment of an element is also highly important in determining its properties. A on e-dimensional array of closely spaced elements has the same average switchi ng field as an isolated element but the spread in values is greatly increas ed when the gap between elements is made smaller than the width of an eleme nt. Adding rows of elements to make a two-dimensional array also has an eff ect, even if the rows are spaced further apart than the length of the eleme nts. (C) 1999 American Institute of Physics. [S0021-8979(99)78508-7].