Magnetic order in YbMn₂Si₂ — Neutron scattering investigation

Abstract

Several mechanisms have been proposed to account for the structural and magnetic behaviour of the rare earth intermetallic compound YbMn₂Si₂ below the transition that occurs around ∼30 K. We have carried out detailed neutron diffraction measurements over the temperature range ∼0.3–52 K and confirm both an antiferromagnetic structure with ferromagnetic Mn (001) planes coupled antiferromagnetically along the c-axis above ∼30 K and the absence of ordering of the Yb sublattice down to 0.3 K. The decrease in intensity of the (111) reflection around ∼30 K together with the appearance of satellite reflections of propagation vector $k = 00\tfrac{1} {2}$ confirm that the magnetic unit cell doubles along the c axis below T _N2. The resultant molecular field acting on half of the ions below ∼30 K removes the degeneracy of the Kramers doublets of the Yb³⁺ ions and accounts for the additional excitations observed below ∼30 K compared with the inelastic neutron scattering above ∼30 K.