Dipolar ordering in Fe ₈ ?

Abstract

We show that the low-temperature physics of molecular nanomagnets, contrary to the prevailing one-molecule picture, must be determined by the long-range magnetic ordering due to many-body dipolar interactions. The calculations here performed, using Ewald's summation, suggest a ferromagnetic ground state with a Curie temperature of about 130 mK. The energy of this state is quite close to those of an antiferromagnetic state and to a glass of frozen spin chains. The latter may be realized at finite temperature due to its high entropy.

From single-molecule magnetism to long-range ferromagnetism inHpyr[Fe17O16(OH)Vecchini1, Ryan2, Cranswick3 et al. 2008Phys. Rev. B151100View full textAdd to dashboardCite

“…The second set of neutron diffraction measurements consisted of a more complete and detailed temperature dependent survey of the magnetic signal and was performed on the constant wavelength C2 DualSpec spectrometer ͑located at the Canadian Neutron Beam Centre, Chalk River, Canada͒ on the same sample of deuterated Fe 17 . An incident neutron wavelength of 2.3723 Å was used and a modified Oxford Instruments Heliox 3 He cold stage was adapted to fit into an orange cryostat that was mounted on the spectrometer.…”

“…The former are, in principle, much simpler to understand because magnetic phase transitions driven solely by long-range dipolar interactions can be predicted without involving any adjustable parameters. 8,[14][15][16][17][18] Unfortunately, very few examples of purely dipolar LRMO are known to date, and hence the investigation of magnetic phase transitions induced by dipolar interactions presents a most pertinent field of research in molecular nanomagnets.…”

“…The presence of long-range ferromagnetic order in the prototypical molecular magnet Mn 12 was recently confirmed by singlecrystal neutron diffraction. 13 The molecular magnet Hpyr͓Fe 17 25,26 The Fe 17 molecules are bound together in the crystal solely by van der Waals forces, hence prohibiting any intermolecular superexchange pathway. An interesting and possibly unique characteristic of Fe 17 is that once the constituent ligands have been chosen, the molecules can be arranged in different crystal packings without affecting the individual molecules themselves, 9 keeping the surrounding ligands, the molecular high-spin ground state, and the magnetic anisotropy unaltered.…”

“…This system is characterized by a uniaxial anisotropy of only D Ӎ −0.02 K, in addition to the S =35/ 2 giant-spin ground state. This combination of high spin and low anisotropy makes Fe 17 an ideal candidate for investigating dipolar-induced long-range magnetic order. In what follows, we shall present the magnetic structure of Fe 17 as induced by dipolar interactions solely and determined by means of neutron powder diffraction measurements.…”

Tunable Dipolar Magnetism in High-Spin Molecular Clusters

“…In parallel, dipolar interactions provide the only source of coupling between the molecular spins arranged in crystallographic lattices. Assuming each molecule as a high-spin pointlike dipole, the macroscopic properties of dipolar magnets can be precisely predicted because dipole-dipole interactions are calculated without involving any adjustable parameter [3][4][5][6][7][8]. These ideal materials are, however, very difficult to obtain.…”