Intracluster interactions in “butterfly” {Fe3LnO2} molecules

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Abstract

The magnetization contributions of the Fe3 and Ln subcluster in the “butterfly” molecule [Fe3Ln(μ3-O)2(CCl3COO)8(H2O)(THF)3], in brief {Fe3LnO2}, with Ln=Lu, Gd, Tb, Dy and Ho, have been determined by a combination of vibrating sample magnetometry and x-ray circular magnetic dichroism at low temperature and magnetic field up to 14 T. These contributions have been explained in terms of an effective spin model where the Fe3 is described by a SFe3=5/2 spin, Gd by an isotropic J=7/2, Dy by a Kramers doublet, and non-Kramers ions Tb and Ho by a ligand field split doublet. The intracluster interactions JFeLn have been found to amount to a few K.

Introduction

The search for new polynuclear coordination compounds that behave as single molecule magnets (SMMs) has attracted special attention in the past decade, due to the possibility to use them as magnetic storage or quantum computing elements. Besides, they provide a variety of new chemical–physical properties, such as the possibility to chemically induce magnetic anisotropy and to exploit intramolecular exchange interactions. In the last decade the “butterfly” molecules {Fe3LnO2} have become a paradigmatic example of such type of polynuclear molecules, where the Ln(III) may be substituted by ions with very different anisotropies [1].

The present paper deals with the microscopic characterization of the magnetic behavior of the {Fe3LnO2} compounds. The main objective is to study the interaction between the Fe3 subcluster and the Ln within the {Fe3LnO2} cluster, by applying a competing external field at low temperatures to provoke the polarization of the Fe and Ln magnetization within the molecule.

A major problem in this type of compounds is that their magnetic properties are complicated by the large spin–orbit coupling effects of the Ln(III) ions, hampering the quantitative determination of the magnitude of the exchange parameters within these molecules, and the description of the resulting ground state [2].

Therefore, new techniques which explore these molecules at an element selective level are needed to provide information about the microscopic arrangement of magnetic moments and the mechanisms of relative spin orientation within a molecule [3]. An example of these element sensitive probes is the x-ray magnetic circular dichroism (XMCD) technique, which provides element specific magnetization curves that may be compared to cluster magnetization acquired by VSM magnetometry. The combination of XMCD and VSM is able to resolve very small magnetic coupling values in 3d–4f clusters. A detailed magnetic study involving XMCD and VSM magnetometry was carried out to characterize the exchange coupling and magnetic anisotropies of the present “butterfly” molecules. In particular, the evolution of the local magnetization from the 4f states with the magnetic field was explored experimentally by means of XMCD measurements as a function of the applied field at the Ln L2,3 edges, and theoretically, by simulation of the Ln and Fe3 subcluster magnetization dependence on applied field and temperature separately.

In this work we review and compare the results obtained with the methods developed in [3] on Ln substitutions: Lu, non-magnetic ion; Gd, isotropic Kramers ion; Dy, anisotropic Kramers ion, and Tb and Ho as anisotropic non-Kramers ions.

Section snippets

Structure and experimental details

The molecule [Fe3Ln(μ3-O)2(CCl3COO)8(H2O)(THF)3] [1], in brief {Fe3LnO2} has a “butterfly” type structure, with two Fe2Ln(μ3-O) triangular wings sharing a Ln–Fe body and a dihedral angle between the wings of approximately 146.5° (see Fig. 1). The magnetic core of the molecule can be considered as a triangular pyramid where the basis is an obtuse isosceles triangle with three Fe(III) ions located at the vertices, and the Ln ions located at the pyramid apex.

The total magnetization measurements M (

Experimental results

The M (H) isotherms of {Fe3LnO2} Ln=Lu, Gd, Dy, Tb and Ho at low temperature, measured with the VSM technique are shown in Fig. 2a. From direct comparison with the M (H) for the non-magnetic Lu ion {Fe3LuO2} it is evident that the magnetic Ln ions give an extra contribution to M (H). The determination of this contribution MLn (H) at low fields has been the experimental keypoint of this work.

To explain the method an example is given in Fig. 3, namely the Ln=Dy case. The XMCD spectra at the L2,3

Discussion

For {Fe3LuO2} Mtot (H)=MFe3 (H), since Lu is not magnetic. This corresponds to the contribution of the three Fe(III) S=5/2 magnetic moments antiferromagnetically coupled [2]. The Fe(1)–Fe(2) and Fe(2)–Fe(3) average coupling is strong (J/kB=−50 K) in terms of the Heisenberg–Dirac–van Vleck (HDVV) Hamiltonian for Fe sitting at the vertices of an isosceles triangle with negligible interaction between the Fe moments in the base of the triangle Fe(1)–Fe(3) (see Fig. 1). The MFe3(H) is well fitted to

Conclusions

The experimental (VSM combined with XMCD) and simulation methods devised have been successful in determining the intracluster interactions in the magnetic “butterfly” molecules {Fe3LnO2}. The resulting interaction constants are very small. Although these molecules might show SMM behavior, as hinted from ac susceptibility measurements [2], any hysteresis effect would appear only at temperatures below 2 K.

Acknowledgments

The projects MINECO (MAT2011/23791, MAT2011/27233-C02-02 and MAT2014/53921-R), DGA IMANA E34 and Alexander Von Humboldt Foundation (D.P.) are acknowledged for financial support.

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1

Prof. Turta passed away on the 23rd March 2015.

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