For some AFM applications, a "magnetic" substrate, such as steel is the most desirable kind of substrate. The SPI Supplies brand of substrates for AFM is noted for the smoothness of the edges of the small discs and the surfaces are quite flat. Of course they are not atomically smooth flat such as would be the case for the V-1 grade of HOPG or possibly even mica but the steel product is a reasonably decent quality and is certainly inexpensive.