Quantum signatures of a molecular nanomagnet in direct magnetocaloric measurements.

Joseph W Sharples,Jürgen Schnack,David Collison,Eric J L Mcinnes,Marco Evangelisti,Elias Palacios

doi:10.1038/ncomms6321

Abstract

Geometric spin frustration in low-dimensional materials, such as the two-dimensional kagome or triangular antiferromagnetic nets, can significantly enhance the change of the magnetic entropy and adiabatic temperature following a change in the applied magnetic field, that is, the magnetocaloric effect. In principle, an equivalent outcome should also be observable in certain high-symmetry zero-dimensional, that is, molecular, structures with frustrated topologies. Here we report experimental realization of this in a heptametallic gadolinium molecule. Adiabatic demagnetization experiments reach ~200 mK, the first sub-Kelvin cooling with any molecular nanomagnet, and reveal isentropes (the constant entropy paths followed in the temperature-field plane) with a rich structure. The latter is shown to be a direct manifestation of the trigonal antiferromagnetic net structure, allowing study of frustration-enhanced magnetocaloric effects in a finite system.

Highlights

Geometric spin frustration in low-dimensional materials, such as the two-dimensional kagome or triangular antiferromagnetic nets, can significantly enhance the change of the magnetic entropy and adiabatic temperature following a change in the applied magnetic field, that is, the magnetocaloric effect
The underlying physics is the magnetocaloric effect (MCE) that can be evaluated by considering the adiabatic temperature change, which is when the system is driven on a constant entropy (S) curve:
We have revealed the richness of the isentropes in Gd7 via direct MCE studies, including the first experimental achievement of sub-Kelvin cooling with a molecular nanomagnet, with experimental and theoretical results in excellent agreement

Summary

Introduction

Geometric spin frustration in low-dimensional materials, such as the two-dimensional kagome or triangular antiferromagnetic nets, can significantly enhance the change of the magnetic entropy and adiabatic temperature following a change in the applied magnetic field, that is, the magnetocaloric effect. Geometric spin frustration can give rise to regions of high density of states (and zero-temperature entropy), very high cooling rates should be achievable, for example, when sweeping across the saturation field in such materials The combination of these features in low-dimensional frustrated magnetic materials, for example, the famous 2D kagome or triangular AF lattices or the 1D saw-tooth AF chain[5,6,7,8,9], makes them attractive targets for enhanced MCE and low-temperature refrigeration. Such effects should be observable in certain 0D systems, that is, molecular clusters of spins in frustrated geometries[10,11,12,13]. These are a subset of the broader class of molecules known as molecular nanomagnets

Methods

Results

Conclusion