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Magnetic order and multiferroicity in hybrid tetrachlorocuprate-organic layered perovskites

Metal-organic compounds with perovskite crystal structure provide a fertile playground for design of the multifunctional materials. Some properties can be mutually dependent, and some orders mutually coupled, paving the way to design of the magnetoelectric multiferroics. Interesting example of the hybrid layered perovskite multiferroic is ethylammonium tetrachlorocuprate, which consists of the ferromagnetic layers of corner sharing [CuCl4]2− octahedra connected by two layers of polarizable organic ions of C2H5NH3+. Besides the known ferroelectric transition around 245 K, it shows rich magnetic behaviour below 10.5 K, including magnetic anisotropy and transitions between different magnetic states. Although the search for magnetoelectric effect was not successful, a slight change of magnetization with the structural changes above 300 K was observed. Composition of this metal- organic perovskite was changed and the accompanied changes of the crystal structure and magnetic properties were studied. Up to now, magnetization is measured in DC regime using SQUID magnetometer in temperature range 2–300 K at different magnetic fields up to 5 T. Large impact of geometric and electronic changes of the organic cation structure on magnetic properties was observed in following two groups. Novel series of the solid-state architectures consisting of tetrachlorocuprate units and ortho-, meta- and para-anisidinium were prepared and in their crystal structure considerable change of geometry was observed: from the discrete square planar tetrachlorocuprate anions in the ortho- anisidinium compound to the Ruddlesden-Popper perovskite phase with slightly distorted layers built from CuCl6 octahedra in the para- anisidinium compound. Spins 1/2 per Cu2+ ion remain in the paramagnetic state down to the lowest temperatures in the ortho-anisidinium tetrachlorocuprate. However, there are transitions to the ferromagnetic state measured at 4.2 K and 9.5 K for the meta-anisidinium and para-anisidinium tetrachlorocuprates, respectively. Their ferromagnetic-like hysteresis loops are very soft, without the observable coercivity. Surprise happens in difference between the chloroethylammonium tetrachlorocuprate and bromoethylammonium tetrabromocuprate: the first one has antiferromagnetic transition at 7.8 K with metamagnetic transition in fields above 150 Oe, while the second one has ferromagnetic transition at 11 K and broad hysteresis loop with coercivity around 100 Oe at 2 K. Polar order is predicted in theoretical calculations, and magnetoelectric experiments could be challenging. This research showed that a small change in structure has drastic influence on the geometry and magnetism of the cuprate units. At the moment it seems to us that the 3D metal-organic perovskites have brighter perspective for magnetoelectric coupling than the 2D. If having on mind wide possibility of combining the building blocks with different physical properties like elastic, (super)conducting, optical, polar, magnetic, the rich field of applications can be foreseen for 2D perovskites, including also influence of the the light on the magnetic and electric response. Some of those will be of our future interest.

multiferroics ; magnetoelectrics ; perovskites ; metal-organic compounds

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