  We propose a solid state refrigeration technique based on repeated adiabatic
magnetization/demagnetization cycles of a superconductor which acts as the
working substance. The gradual cooling down of a substrate (normal metal) in
contact with the working substance is demonstrated for different initial
temperatures of the substrate. Excess heat is given to a hot large-gap
superconductor. The on-chip refrigerator works in a cyclic manner because of an
effective thermal switching mechanism: Heat transport between N/N versus N/S
junctions is asymmetric because of the appearance of the energy gap. This
switch permits selective cooling of the metal. We find that this refrigeration
technique can cool down a 0.3cm$^{3}$ block of Cu by almost two orders of
magnitude starting from 200mK, and down to about 1mK starting from the base
temperature of a dilution fridge (10mK). The corresponding cooling power for a
1cm$\times$1cm interface are 25 nW and 0.06 nW respectively, which scales with
the area of the interface.
