Giant exchange coupling and field-induced slow relaxation of magnetization in Gd2@C79N with a single-electron Gd-Gd bond

Abstract

Magnetic properties of the azafullerene Gd₂@C₇₉N are studied by SQUID magnetometry. The effective exchange coupling constant j_Gd,e between the Gd spins and the spin of unpaired electron residing on the single-electron Gd-Gd bond is determined to be 170 ± 10 cm^-1. Low temperature AC measurements revealed field-induced millisecond-long relaxation of magnetization.

Fig. 1. (a) Magnetization curves of Gd₂@C₇₉N measured at different temperatures. (b) Magnetization versus the quotient μ₀HT^–1. Dots are experimental points, solid lines are simulations using eqn (1) and the jeffGd,e value of 170 cm^–1, the dash line in (b) is a simulation for the [Gd³⁺–e–Gd³⁺] system with non-interacting spins. The inset in (b) shows molecular structure and DFT-computed spin density distribution in Gd₂@C₇₉N (see footnote §) visualized with isovalues ±0.015 a.u. (solid) and ±0.0012 a.u. (semi-transparent). Three well-seen maxima of the spin density correspond to Gd atoms and unpaired electron spin (see also Fig. S3, ESI†).

Fig. 2. (a) The product χ·T (χ is magnetic susceptibility) measured for Gd₂@C₇₉N in the field of 1 T (dots) and compared to the simulations with different values of the exchange coupling constant jeffGd,e (coloured lines; the values of jeffGd,e are given in cm^–1). (b) Comparison of experimental χ·T curves measured in different magnetic fields from 0.5 to 7 T (dots) to the results of simulations with the jeffGd,e constant of 170 cm^–1. The inset in (b) shows thermal populations of the giant-spin states, in particular S = 15/2 (black), 13/2 (red), and 11/2 (blue).

Fig. 3. AC magnetometry studies of Gd₂@C₇₉N. (a) Out-of-phase susceptibility χ′′ measured at 1.8 K in different constant DC fields. (b) Same as (a), but showing the out-of-phase signal susceptibility χ′′ versus in-phase susceptibility χ′ (Cole–Cole plots). (c) Relaxation times of magnetization as a function of the magnetic field. (d) Out-of-phase susceptibility χ′′ measured at different temperatures in the constant field of 0.3 T. (e) Same as (d), but showing the Cole–Cole plots. (f) Relaxation times of magnetization in the field of 0.3 T as a function of temperature; the lines are fits to the Orbach relaxation mechanism (red, U^eff = 6.5 K) or to the power law (blue, n = 3.2). The dots in (a,b,d and e) are experimental values, lines are the fits with generalized Debye model, the latter is also used to determine relaxation time from AC data.