  Half-metallic ferromagnetism (FM) in single-crystal
Ba{0.39(1)}Rb{0.61(1)}Mn2As2 below its Curie temperature TC = 103(2) K is
reported. The magnetization M versus applied magnetic field H isotherm data at
1.8 K show complete polarization of the itinerant doped-hole magnetic moments
that are introduced by substituting Rb for Ba. The material exhibits extremely
soft FM, with unobservably small remanent magnetization and coercive field.
Surprisingly, and contrary to typical itinerant FMs, the M(H) data follow the
Arrott-plot paradigm that is based on a mean-field theory of local-moment FMs.
The in-plane electrical resistivity data are fitted well by an activated-T^2
expression for T < TC, whereas the data sharply deviate from this model for T >
TC. Hence the activated-T^2 resistivity model is an excellent diagnostic for
determining the onset of half-metallic FM in this compound, which in turn
demonstrates the presence of a strong correlation between the electronic
transport and magnetic properties of the material. Together with previous data
on 40% hole-doped Ba{0.6}K{0.4}Mn2As2, these measurements establish 61%-doped
Ba{0.39}Rb{0.61}Mn2As2 as a prototype for a new class of half-metallic
ferromagnets in which all the itinerant carriers in the material are
ferromagnetic.
