SCIENTIFIC HIGHLIGHTS
2008 • 2007 • 2006
Large Coupled Magnetoresponses in Oxynitride Perovskites A.B. Jorge, J. Oró-Solé, A.M. Bea, A. Fuertes
JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 130, (2008), 12572-12573
Materials in which magnetism is coupled to large changes in electronic conductivity (magnetoresistance effect) or electrical polarization (magnetocapacitance) are of fundamental interest and practical importance for new memory and sensor technologies. Perovskites containing divalent europium and tetravalent transition metals are particularly interesting as the order of Eu2+ S=7/2 spins can couple to the magnetic and electronic properties derived from the dn configuration. Substitution of oxide by nitride in perovskites AMO3 (A= divalent cation, M=early transition metal) allows the stabilization of new compounds where the transition metal shows high oxidation states that may be intermediate between +4 and +5 or +5 and +6. To discover new compounds with magnetoelectric coupled effects we have synthesized the perovskite oxynitrides EuM(O,N)3 (M =Nb, Ta, W). They constitute a versatile group of new materials where the magnetic and electrical properties can be tuned by controlling the nitrogen stoichiometry and combining different valence states of europium and the transition metal showing d1, d0 or mixed configurations. EuM(O,N)3 compounds are conveniently synthesised by the ammonolysis of EuMO4 (Ta, Nb) or Eu2W2O9 precursors at temperatures between 600ºC and 950ºC. These appear to be cubic perovskites, but
splittings of synchrotron X-ray diffraction peaks reveal slight tetragonal lattice distortions c > a is consistent with some anion ordering and octahedral tilting. Eu2+(Nb5+1-yNb4+y)O2+yN1-y with y=0.14 shows colossal (>99%) magnetoresistances at low temperatures and an apparent giant (20%) magnetocapacitance. For EuWO1+xN2-x, with x between 0.5 and -0.2, and possible valence states Eu2+, Eu3+, W5+ and W6+, large negative magnetoresistances are observed at low temperatures, up to 70% in the x = 0.09 material. Non-ohmic conduction emerges below the 12 K Curie transition. The magnetotransport properties of EuNbO2N and EuWON2 arise from the coupling of the carriers to those of the localised Eu2+ S = 7/2 spins. This is analogous to the double exchange mechanism observed in manganites, although the carriers and core spins in EuM(O,N)3 are associated with different cations. The coupling of an apparent giant magnetocapacitance to the CMR in EuNbO2N is a notable result. However, this response is shown to arise from microstructural effects rather than an intrinsic multiferroism.
MAGNETIC FIELD DEPENDENCE OF (A) THE MAGNETISATION (PLOTTED AS M2), (B) THE MAGNETORESISTANCE AND (C) THE MAGNETOCAPACITANCE (MEASURED AT 100 KHZ FREQUENCY) FOR EUNBO2N.
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