Long-Range Ferromagnetism ofMn12Acetate Single-Molecule Magnets under a Transverse Magnetic Field

Abstract

We use neutron diffraction to probe the magnetization components of a crystal of Mn12 single-molecule magnets. Each of these molecules behaves, at low temperatures, as a nanomagnet with spin S = 10 and strong anisotropy along the crystallographic c axis. The application of a magnetic field H(perpendicular) perpendicular to c induces quantum tunneling between opposite spin orientations, enabling the spins to attain thermal equilibrium. For T approximately < 0.9(1) K, this equilibrium state shows spontaneous magnetization, indicating the onset of ferromagnetism. These long-range magnetic correlations nearly disappear for mu0H(perpendicular) approximately > 5.5 T, possibly suggesting the existence of a quantum critical point.

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

“…The large molecular spins in these crystals take the place of the individual magnetic ions in conventional materials. The intermolecular magnetic coupling results from either purely dipolar [6][7][8][9] or mixed dipolar and/or weak superexchange 5,[10][11][12][13] interactions. 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.…”

“…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.…”

Dipolar Magnetic Order in Crystals of Molecular Nanomagnets

“…Most of the results described in this chapter refer to work done, and published [44][45][46][47][48][49], in the course of the past decade. Yet, it contains a few original aspects too, in particular the determination of the interaction fields in Mn 12 acetate that is included in section 1.3.…”

“…Notice that, at T = θ, χ zz,exp no longer diverges but approaches χ max = 1/N zz . An additional attractive feature of mean-field models is that they can readily include effects of quantum fluctuations, induced by either the magnetic anisotropy or transverse magnetic fields [46], and of molecular disorder [10,50], both of which are cumbersome to deal with using Monte Carlo calculations. These effects give rise to interesting physical phenomena and are also particularly relevant to experimental situations met with some molecular crystals, such as those described below in sections 1.5 and 1.6.…”

Phases of anisotropic dipolar antiferromagnets: Mean field theory and Monte Carlo simulations

“…7 Long-range order has already been observed experimentally. [8][9][10][11][12] Because uniaxial anisotropy is very large, Ising spins with dipolar interactions are reasonable models for these systems. 13 Early rigorous work by Luttinger and Tisza 14 established which type of magnetic order obtains at low temperature T in dipolar Ising models in each of the cubic lattices.…”