Abstract
Polarized neutron diffraction is used to study in depth the magnetic properties of the heterometallic compound [NH2(CH3)2][FeIIIFeII(HCOO)6] and give insight into its magnetic behaviour, addressing open questions that will contribute to a better understanding of this attention-grabbing material and other related ones. Previous results revealed that upon cooling, the magnetic moments of the FeII and FeIII sites do not order simultaneously: the magnetization of the FeII site increases faster than that of the FeIII sites. Unpolarized neutron diffraction measurements at 2 K with no external field revealed some discrepancies in the saturation value of the magnetic signal on the FeIII sites and in the ferromagnetic moment along the c axis. These discrepancies could be related to the actual distribution of magnetic moment, since unpolarized neutron diffraction gives information on the magnetic moment localized only on the magnetic ions. Polarized neutron diffraction allows an analysis of the magnitude of the spin density over magnetic and non-magnetic ions (the organic ligand and the counterion), which can give a clue to explain the low saturation on the FeIII sites and the correlation with the physical measurements. The present study also contributes to the understanding of the magneto-electric behaviour of this compound, giving insight into the role of metal disorder in the origin of the structural phase transition, which is responsible for its antiferrolelectric order, and into the influence of spin-density delocalization on its magneto-electric properties, allowing a discussion of the alternative explanations given so far for its electric properties at low temperature.
Highlights
Metal-organic frameworks combining two or more properties have been the focus of interest of several research groups in the last few decades [see, for example, Thomas et al (1996), Ruiz et al (1997), Wang et al (2004), Zhou et al (2012), Berg et al (2012), Cucinotta et al (2012), Adhikary et al (2014)]
The magnetic moments obtained from the neutron measurements at 2 K along the c axis, is 0.3 B (Canadillas-Delgado et al, 2012). This value is slightly lower than that obtained from the magnetometry study by Zhao et al (2010)
Polarized neutron diffraction measurements were performed on the D3 diffractometer at ILL, in normal beam geometry with a 9 T external magnetic field applied vertically along the c axis, to be sensitive to the magnetic moment contribution
Summary
Metal-organic frameworks combining two or more properties have been the focus of interest of several research groups in the last few decades [see, for example, Thomas et al (1996), Ruiz et al (1997), Wang et al (2004), Zhou et al (2012), Berg et al (2012), Cucinotta et al (2012), Adhikary et al (2014)]. A good approach to achieve this combination has been the design of perovskite-like metal-organic frameworks where a three-dimensional network made of metallic centres linked through organic ligands, which presents magnetic order, accommodates counterions responsible for electric order [see, for example, Chitnis et al (2018), Maczka et al (2017), Malik et al (2018), Hughey et al (2018), Mazzuca et al (2018), Stroppa et al (2013), Fu et al (2011), Jain et al (2009)] One of these examples is the mixed-valence iron(II)–iron(III) formate compound [NH2(CH3)2][FeIIIFeII(HCOO)6], (1). The electric behaviour of this compound has been the subject of alternative explanations, such as the presence of spinarrangement defects promoting a magnetoelectric coupling effect (Guo et al, 2017) or a symmetry breaking compatible with a small polarization mainly from the off-centred FeIII—O octahedron (Tang et al, 2019)
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