Exchange-biased quantum tunnelling in a supramolecular dimer of single-molecule magnets

Abstract

Various present and future specialized applications of magnets require monodisperse, small magnetic particles, and the discovery of molecules that can function as nanoscale magnets was an important development in this regard. These molecules act as single-domain magnetic particles that, below their blocking temperature, exhibit magnetization hysteresis, a classical property of macroscopic magnets. Such 'single-molecule magnets' (SMMs) straddle the interface between classical and quantum mechanical behaviour because they also display quantum tunnelling of magnetization and quantum phase interference. Quantum tunnelling of magnetization can be advantageous for some potential applications of SMMs, for example, in providing the quantum superposition of states required for quantum computing. However, it is a disadvantage in other applications, such as information storage, where it would lead to information loss. Thus it is important to both understand and control the quantum properties of SMMs. Here we report a supramolecular SMM dimer in which antiferromagnetic coupling between the two components results in quantum behaviour different from that of the individual SMMs. Our experimental observations and theoretical analysis suggest a means of tuning the quantum tunnelling of magnetization in SMMs. This system may also prove useful for studying quantum tunnelling of relevance to mesoscopic antiferromagnets.

Calculations of the onset temperature for tunneling in multispin systems

“…The calculated crossover temperature for the dimer is T c = 0.40 K in close agreement with experimental measurements [40]. Due to the weak exchange interaction every spin flips almost independently.…”

“…where bothĤ 1 andĤ 2 have the same form as (17) with the parameters taken from [39] and J = 0.12 K is isotropic superexchange [40]. The transverse term is chosen to include both 4th-and 2nd-order anisotropies: C(Ŝ 2 + +Ŝ 2 − ) and E(Ŝ 4 + +Ŝ 4 − ), with C = 0.032 K and E = −7.5 × 10 −5 K, in order to match the experimental results of the rate above T c .…”

“…(17) for the monomer and (19) for the dimer (dashed vertical lines) with parameters taken from experimental measurements are also shown. Experimentally measured transition rate for the monomer [38] is shown with gray triangles and for the dimer [40] with black triangles. Below T c , the tunneling rate is expected to be similar to the HTST rate at T c .…”

Direct measurement of dysprosium(III)˙˙˙dysprosium(III) interactions in a single-molecule magnet

“…Compound 3 shows very different physics from the exchange bias type of compounds, originally described by Wernsdorfer et al 33 , and also seen in {Dy 2 } complexes 16 , where a remnant magnetization is present in the open hysteresis loops; a feature not observed here because of rapid zero-field relaxation. The hypothetical compound where the tilt angle is 0°may possibly show the characteristic QTM steps at the crossing points in Supplementary Fig.…”

Relaxation dynamics in a Fe7nanomagnet

“…7,8 In addition, MNMs exhibit a wide range of intriguing quantum phenomena, such as quantum tunneling of the magnetization, [9][10][11] decoherence, 12,13 or quantum entanglement between distinct cores. [14][15][16][17] MNMs also present great advantages from the standpoint of theoretical modeling. Indeed, it has been demonstrated that the effective spin Hamiltonian is a powerful model to describe magnetic properties of these clusters.…”