First-order structural transition in the multiferroic perovskite-like formate [(CH3)2NH2][Mn(HCOO)3]

In this work we explore the overall structural behaviour of the [(CH3)2NH2][Mn(HCOO)3] multiferroic compound across the temperature range where its ferroelectric transition takes place by means of calorimetry, thermal expansion measurements and variable temperature powder and single crystal X-ray diffraction. The results clearly proof the presence of structural phase transition at Tt ~187 K (temperature at which the dielectric transition occurs) that involves a symmetry change from R-3c to Cc, twinning of the crystals, a discontinuous variation of the unit cell parameters and unit cell volume, and a sharp first-order-like anomaly in the thermal expansion. In addition, the calorimetric results show a 3-fold order-disorder transition. The calculated pressure dependence of the transition temperature is rather large (dTt/dP = 4.6 $\pm$ 0.1 K/kbar), so that it should be feasible to shift it to room temperature using adequate thermodynamic conditions, for instance by application of external pressure

Strongly Coupled Magnetic and Electronic Transitions in Multivalent Strontium Cobaltites

The topotactic phase transition in SrCoOx (x = 2.5-3.0) makes it possible to reversibly transit between the two distinct phases, i.e. the brownmillerite SrCoO2.5 that is a room-temperature antiferromagnetic insulator (AFM-I) and the perovskite SrCoO3 that is a ferromagnetic metal (FM-M), owing to their multiple valence states. For the intermediate x values, the two distinct phases are expected to strongly compete with each other. With oxidation of SrCoO2.5, however, it has been conjectured that the magnetic transition is decoupled to the electronic phase transition, i.e., the AFM-to-FM transition occurs before the insulator-to-metal transition (IMT), which is still controversial. Here, we bridge the gap between the two-phase transitions by density-functional theory calculations combined with optical spectroscopy. We confirm that the IMT actually occurs concomitantly with the FM transition near the oxygen content x = 2.75. Strong charge-spin coupling drives the concurrent IMT and AFM-to-FM transition, which fosters the near room-T magnetic transition characteristic. Ultimately, our study demonstrates that SrCoOx is an intriguingly rare candidate for inducing coupled magnetic and electronic transition via fast and reversible redox reactions

Magnetic neutron diffraction study of the charge ordered chain compounds Rb11Mn8O16 and Cs3Mn2O4

The magnetic ordering patterns of Rb11Mn8O16 and Cs3Mn2O4 were determined by neutron powder diffraction with and without applied magnetic fields. The crystal structures of these compounds exhibit infinite chains of edge sharing MnO4 tetrahedra with periodically alternating Mn2 and Mn3 valence. Both Rb11Mn8O16 and Cs3Mn2O4 show collinear magnetic order with antiferromagnetic alignment of Mn moments along the chains below the N el temperatures TN 38 1 K and 13.5 5 K, respectively. In Cs3Mn2O4 the Mn2 and Mn3 moments could be separately refined. The full magnetic structure in zero magnetic field can be viewed as a set of ferrimagnetic chains whose net moments are coupled antiferromagnetically perpendicular to the chain direction. For this compound, we further observe a magnetic field induced transition into a high field phase with uniformly aligned ferrimagnetic moment

First-order structural transition in the multiferroic perovskite-like formate [(CH3)(2)NH2][Mn(HCOO)(3)]

In this work we explore the overall structural behaviour of the [(CH 3)2NH2][Mn(HCOO)3] multiferroic compound across the temperature range where its ferroelectric transition takes place by means of calorimetry, thermal expansion measurements and variable temperature powder and single crystal X-ray diffraction. The results clearly prove the presence of a structural phase transition at Tt ~ 187 K (the temperature at which the dielectric transition occurs) that involves a symmetry change from R3c to Cc, twinning of the crystals, a discontinuous variation of the unit cell parameters and unit cell volume, and a sharp first-order-like anomaly in the thermal expansion. In addition, the calorimetric results show a 3-fold order-disorder transition. The calculated pressure dependence of the transition temperature is rather large (dTt/dP = 4.6 ± 0.1 K kbar-1) in that it should be feasible to shift it to room temperature under adequate thermodynamic conditions, for instance by application of an external pressure. © 2014 the Partner Organisations

Structural details and magnetic order of La_1-xSr_xCoO₃ (x=0.3)

The crystallographic structure and the magnetic order of the distorted perovskite La1-xSrxCoO3 (0.10≤x ≤0.30) has been studied by neutron diffraction, high-resolution electron microscopy, and magnetic-susceptibility measurements. The results give direct evidence for an inhomogeneous distribution of the Sr2+ ions and the segregation of the material into hole-rich ferromagnetic regions and a hole-poor semiconducting matrix at lower values of x. The holes introduced by Sr doping are attracted to the Sr2+ ions where they stabilize to lowest temperatures an intermediate-spin state at neighboring trivalent cobalt. The antibonding e electrons so stabilized increase the mean unit-cell volume and are delocalized over the cobalt atoms of the cluster where they couple the localized t5 configurations ferromagnetically. Long-range ferromagnetic order between clusters is realized even for Sr doping as low as x=0.10. The transition to a spin glass state is observed only for Sr concentrations smaller than 0.10. The volume of a hole-rich cluster grows in a magnetic field, and the origin of the large negative magnetoresistance observed near Tc for 0.15 ≤ x ≤ 0.25 appears to be due to a growth of the clusters to a percolation threshold. For x=0.30, the σ* band of the intermediate-spin state below Tc is at the threshold of a transition from itinerant to polaronic conduction and, above Tc, the system transforms smoothly to a cluster state. © 1999 The American Physical Society