Giant Isotope Effect in the Incoherent Tunneling Specific Heat of the Molecular NanomagnetFe8

Abstract

Time-dependent specific heat experiments on the molecular nanomagnet Fe8 and the isotopic enriched analogue 57Fe8 are presented. The inclusion of the 57Fe nuclear spins leads to a huge enhancement of the specific heat below 1 K, ascribed to a strong increase in the spin-lattice relaxation rate gamma arising from incoherent, nuclear-spin-mediated magnetic quantum tunneling (MQT) in the ground doublet. Since gamma is found comparable to the expected tunneling rate, the MQT process has to be inelastic. A model for the coupling of the tunneling spins to the lattice is presented. Under transverse field, a crossover from nuclear-spin-mediated to phonon-induced tunneling is observed.

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 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.…”

Extended Debye model for molecular magnets

“…In recent Ref. 18 Fig. 1 go as C ∝ T at elevated temperatures, and the heat capacity per molecule by far exceeds the value 3k B of a crystal with one atom per unit cell at T Θ D .…”

Quantum Nanomagnets and Nuclear Spins: An Overview

“…That the nuclear spins should play a role in this process is clear from the discussions above, and it has been experimentally verified in Fe 8 , again by playing with isotopic substitutions. While a "natural" Fe 8 sample falls out of thermal equilibrium at low-T in zero field, a sample enriched with 57 Fe shows a much larger magnetic specific heat that almost reaches the full equilibrium value [9] [ Fig. 3(f)].…”

“…(e) Long-range magnetic ordering in Mn 4 and equilibrium hyperfine specific heat at low-T [8]. (f) Isotope effect in the magnetic specific heat of Fe 8 [9].…”