The suppression of CMR in Nd(Mn_1-xCo_x)AsO_0.95F_0.05

The colossal magnetoresistance (CMR) observed in the oxypnictide NdMnAsO1-xFx has been further investigated. The magnetotransport is dominated by magnetopolarons. Magnetoresistance measurements of the series Nd(Mn1-xCox)AsO0.95F0.05 show that doping with cobalt on the manganese site pins the magnetopolarons and suppresses the CMR, which is completely destroyed by x = 0.047. The chemical doping results in non-stoichiometric samples, with both As and O vacancies. The relationship between the non-stoichiometry, magnetic order, electron doping and CMR is explored. The Nd antiferromagnetic transition and simultaneous reorientation of the Mn spins into the basal plane at 23 K (TSR) is not effected by Co doping. However, there is a significant decrease in TN(Mn) as the antiferromagnetic transition is suppressed from 360 K to 300 K as x increases from 0-0.047. The manganese moment at 10 K is also reduced from 3.86(2)μB to 3.21(2)μB over the same doping range. This reduction in the in-plane Mn moment decreases the electron-electron correlations below TSR and acts to further diminish the magnetoresistance.