Magnetic structures of RAgSb 2 compounds

Abstract

Results of neutron diffraction measurements carried out for RAgSb 2 (R=Ce, Pr, Nd, Tb, Dy, Ho, Er, Tm) compounds are reported. All compounds crystallize in the tetragonal ZrCuSi 2-type crystal structure and, except CeAgSb 2, they are collinear antiferromagnets with the Néel temperatures between 11.1 K for R=Tb and 2.0 K for R=Tm. Below T c=9.5 K CeAgSb 2 is a ferromagnet with the Ce magnetic moment equal to 0.33 μ B and parallel to the c-axis. PrAgSb 2 has an antiferromagnetic structure of AF II type with the magnetic unit cell ( a, a, 2c ). In the case of NdAgSb 2 a sine-wave-modulated magnetic ordering, described by the wave vector k=(0.5435, 0, 0) , is observed. The compounds with R=Tb, Dy and Er order antiferromagnetically with the magnetic unit cell ( 2a, a, c ). At T=1.4 K the magnetic structure of HoAgSb 2 is similar to the one observed for TbAgSb 2 and DyAgSb 2 while at T=2.7 K the peaks of magnetic origin correspond to a new incommensurate magnetic phase with the propagation vector k=(k x, 0, k z) . Up to the Néel temperature equal to 5.5 K a commensurate antiferromagnetic structure and an incommensurate sine-wave-modulated one coexist. Two magnetic phases are also observed in TmAgSb 2 at T=1.4 K. An increase in the temperature causes that the commensurate phase with the magnetic unit cell ( 2a, a, c ) disappears. The magnetic moment is parallel to the b-axis in the case of R=Tb, to the [1 1 0] direction for R=Dy and parallel to the c-axis in the case of R=Er and Tm ones which correlates well with the respective signs of CEF B 0 2-parameter. In HoAgSb 2 the magnetic moment lies in the b– c plane and forms the angle ϕ b =13.5° with the b-axis at 1.4 K. With increasing temperature the angle ϕ b increases up to 32° about the Néel temperature. The obtained data are analysed in terms of the exchange interaction theory and crystal electric field.