Magnetic Long-Range Order Induced by Quantum Relaxation in Single-Molecule Magnets

Abstract

Can magnetic interactions between single-molecule magnets (SMMs) in a crystal establish long-range magnetic order at low temperatures deep in the quantum regime, where the only electron spin fluctuations are due to incoherent magnetic quantum tunneling (MQT)? Put inversely: can MQT provide the temperature dependent fluctuations needed to destroy the ordered state above some finite T(c), although it should basically itself be a T-independent process? Our experiments on two novel Mn4 SMMs provide a positive answer to the above, showing at the same time that MQT in the SMMs has to involve spin-lattice coupling at a relaxation rate equaling that predicted and observed recently for nuclear-spin-mediated quantum relaxation.

Quantum Nanomagnets and Nuclear Spins: An Overview

“…3(c)]. In each experiment, a T -independent nuclear T 1 was found below a certain temperature, and both for Fe 8 and Mn 12 this was comparable to the temperature at which, for instance, the magnetic hysteresis loops also became T -independent [45,6,5]. This can be qualitatively understood from Eq.…”

“…The electron-spin transitions involved in finding the ordered state are somewhat trivial (no tunneling), but an additional important result was the confirmation that the nuclear T 1 measured by NMR precisely coincides with the nuclear spin-lattice relaxation time as measured by specific heat [28]. The first quantum nanomagnet to show long-range magnetic ordering was a special type of Mn 4 [8] [ Fig. 3(e)], characterized by fast tunneling rate in zero field.…”

“…Specific heat is obviously the main experimental tool to investigate thermal equilibrium (or lack thereof) in an ensemble of nanomagnets. It was found early on that the magnetic specific heat in Fe 8 would be unmeasurable at very low temperatures and zero external field, because the spin-lattice relaxation time of the electron spins becomes much longer that the typical timescale of the experiment (∼ 10 3 s at most). However, by applying a large transverse magnetic field, the tunneling rate could be made large enough to recover the equilibrium magnetic specific heat [23,24].…”

From single-molecule magnetism to long-range ferromagnetism inHpyr[Fe17O16(OH)Vecchini1, Ryan2, Cranswick3 et al. 2008Phys. Rev. BSelf Cite151100View 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.…”

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.…”