d-zero Magnetism in Nanoporous Amorphous Alumina Membranes

TL;DR

This study reports a novel form of zero-field magnetism in nanoporous amorphous alumina membranes, linked to open pore surfaces and likely caused by giant orbital paramagnetism rather than traditional magnetic ordering.

Contribution

It demonstrates that nanoporous amorphous alumina exhibits temperature-independent magnetism originating from pore surfaces, a phenomenon not explained by impurities or conventional ferromagnetism.

Findings

Magnetism is independent of impurity levels.

Magnetism correlates with open pore surface area.

Chemical treatments reduce the magnetic effect.

Abstract

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.