In addition to the ground-breaking discovery of high temperature superconductivity in the iron-based compound LaFeAsO1-xFx (Tc - 26 K in x = 0.05), a plethora of other interesting properties have been observed in the analogous manganese pnictide series. Magnetoresistant materials provide the foundation for many magnetic memory based applications and are defined by the change of electrical resistivity ρ in an applied magnetic field, H, so that MR = (ρ(H)-ρ(0))/ρ(0). We have comprehensively studied the Ln1-xAxMnAsO1-xFx system (Ln = La, Nd, Pr, Ce. A = Sr), analysing the magnetic and nuclear structures with a combination of variable temperature x-ray synchrotron data and high resolution neutron diffraction. Colossal magnetoresistance (CMR) was achieved in electron doped NdMnAsO1-xFx, with a maximum CMR of -95% observed at 3 K for x = 0.05 in a 9 T field (Figure 1). In contrast to this, hole-doping on the rare earth site to obtain the phases Nd1-xSrxMnAsO (x = 0.05, 0.1) was shown to induce positive MR behaviour. These materials crystallise in the ZrCuSiAs-type tetragonal structure. Most of the materials studied show no change in crystal structure down to 4 K, except for PrMnAsO1-xFx. The Pr variant exhibits a structural transition from tetragonal (P4/nmm) to orthorhombic symmetry (Pmmn) below 35 K, coincident with a sizeable negative MR. All of the compounds are antiferromagnetic (TN(Mn) 300-370 K), yet, the role of the rare earth lanthanide ion appears to be important in the development of the magnetic properties. All of the aforementioned compouinds, with the exception of LaMnAsO, display a spin reorientation (SR) transition at low temperature, as Mn2+ moves from allignment along the c axis into the ab plane. This suggests that the transition arises from the coupling of magnetic lanthanide ions with Mn. Interestingly, the CeMnAsO1-xFx series does not show MR but instead displays a semi-conductor-insulator transition at 100 K, which is several orders of magntitude in size (Figure 2). The structure and properties of these compounds will be explored in order to demonstrate that various MR mechanisms and other emergent phenomena are apparent in novel Mn oxyarsenides.

Multiple mechanisms for magnetoresistance in LnMnAsO pnictides

254th National Meeting (2017)

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