Pr1-xNaxMnO3 solid solutions up to x = 0.25 were prepared. Irrespective of
magnetic order all samples are electrically insulating. The Curie temperatu
re is maximum at T-C = 125 K for x = 0.15; the samples with x greater than
or equal to 0.2 are charge-ordered antiferromagnets. The x = 0.2 antiferrom
agnetic charge-ordered insulator (AFCO) can be switched to the ferromagneti
c metallic state (FMM) by application of a relatively weak external magneti
c field. This is accompanied by a huge CMR effect, irreversible at low temp
eratures. The neutron diffraction data, which show a decoupling of charge o
rder (T-CO = 215 K) and long range AF order of the "pseudo" CE (T-N = 175 K
), are supported by specific heat measurement. The specific heat at 14 T co
nfirmed the transformation of the x = 0.2 sample to FMM state and the natur
e of the AFCO-FMM transition was probed by magnetic moment and electrical r
esistivity relaxation measurements. This comprehensive characterization of
the sample led to a complete determination of the magnetic phase diagram wh
ich demonstrated the near degeneracy of the ferro- and antiferromagnetic gr
ound states in Pr0.8Na0.2MnO3. (C) 2001 American Institute of Physics.