R. Welter et al., HIGH RARE-EARTH SUBLATTICE ORDERING TEMPERATURES IN RMNSI COMPOUNDS (R=LA-SM, GD) STUDIED BY SUSCEPTIBILITY MEASUREMENTS AND NEUTRON-DIFFRACTION, Journal of alloys and compounds, 206(1), 1994, pp. 55-71
Investigations by bulk magnetization and neutron diffraction measureme
nts are reported on the ternary silicides RMnSi (R = La-Nd). All these
compounds crystallize in the well-known tetragonal structure of the C
eFeSi type (space group P4/nmm). This structure, which is closely rela
ted to the ThCr2Si2- and TbFeSi2-type structures, can be described as
isolated ''ThCr2Si2 blocks'' connected via R-R contacts. The R, Mn and
Si atoms are arranged in alternate layers stacked along the c axis in
the sequence RSi(Mn2)SiRRSi(Mn2)SiR. All these compounds are antiferr
omagnetic below T(N) almost-equal-to 310, 240, 265 and 280 K for La-,
Ce-, Pr- and NdMnSi respectively. Their magnetic structures are charac
terized by a stacking of antiferromagnetic (001) Mn layers. The Mn mag
netic moments (about 3 mu(B)) are at 45-degrees to c axis (LaMnSi) or
in the (001) plane. At lower temperature PrMnSi and NdMnSi show additi
onal transitions which correspond to an antiferromagnetic ordering of
the rare earth sublattice (T1 = 130 and 185 K for Pr and Nd respective
ly) followed by a spin reorientation process simultaneously with a cry
stallographic phase transition (tetragonal to orthorhombic symmetry) a
t about 80 K for both compounds. At 2 K the antiferromagnetic (AF) str
ucture of PrMnSi and NdMnSi consists of a stacking of ferromagnetic an
d antiferromagnetic (001) layers of R and Mn respectively in the seque
nce Mn(AF) R(+)R(+) Mn(AF) R(-)R(-); the moments are in the layers, co
llinear in PrMnSi and in quadrature in NdMnSi. In CeMnSi the ferromagn
etic (001) cerium layers order antiferromagnetically below about 175 K
and are coupled with the manganese sublattice according to the follow
ing scheme: Ce(+) Mn(AF) Ce(+)Ce(-) Mn(AF) Ce(+); at 2 K the Ce moment
value is mu(Ce) = 0.78(4) mu(B). The results are discussed and compar
ed with those obtained previously for the corresponding ternary silici
des and germanides of the CeFeSi-, TbFeSi2- and ThCr2Si2-type structur
es.