  Nanoporous alumina membranes produced by mild or hard anodisation in oxalic
acid at potentials ranging from 5 - 140 V have a controllable pore surface area
of up to 200 times the membrane area. They exhibit a saturating magnetic
response that is temperature-independent and almost anhysteretic below room
temperature. The magnetism cannot be explained by the ~1 ppm of
transition-metal impurities present in the membranes. The magnetic moment
increases with the area of the open nanopores, reaching values of 0.6 Bohr
magnetons per square nanometer for mild anodisation and 8 muB/nm2 for hard
anodisation where the growth rate is faster. Crystallization of the membrane or
chemical treatment with salicylic acid reduces the effect by up to 90 %. The
magnetism is therefore associated with the open pore surfaces of the amorphous
Al2O3, and it is independent of the orientation of the applied field with
respect to the membrane surface. Possible explanations in terms of electrons
associated with oxygen vacancies (F or F+ centres) are discussed and it is
concluded that the effect is due to giant orbital paramagnetism rather than
collective ferromagnetic spin order.
