Insights into the evolution from ferromagnetism to antiferromagnetism: A doping-dependent study of NaCrSixGe2−xO6(0≤

Abstract

NaCrGe2O6 and NaCrSi2O6 are isostructural compounds exhibiting different magnetic ground state. NaCrGe2O6 adopts a ferromagnetic ground state with T c=6 K, whereas NaCrSi2O6 orders antiferromagnetically below TN =3.4 K. Although it has been proposed that the intriguing magnetic behavior in Cr-based pyroxenes involves competition between antiferromagnetic direct exchange and ferromagnetic super-exchange interactions-a delicate balance that is sensitive to Cr-Cr distance and local distortion, no spectroscopy study has been done to determine the microscopic interactions in these compounds. To delve deeper on the evolution from ferromagnetism to antiferromagnetism, we performed a doping-dependent study to investigate how the substitution of Ge by Si affects the magnetic properties of NaCrSixGe2−xO6 (x=0, 0.5, 1, 1.5, 2). Neutron diffraction and magnetization measurements show that replacing larger Ge with smaller Si simultaneously suppresses the ferromagnetic order. The lattice constants and the unit-cell volume contract, i.e., chemical pressure effect, and the Cr-Cr distance within the chain gradually decreases with increasing Si-doping. High resolution inelastic neutron scattering studies of the spin waves of NaCrGe2O6 and NaCrSi2O6 indicate that replacing Ge with Si has profound effect to the intrachain coupling, whereas it has neglectable effect to the interchain couplings. We compare our results, which indicate NaCrGe2O6 is magnetic quasi-1D and NaCrSi2O6 is 3D, with LiCr(Si,Ge)2O6 where LiCrSi2O6 is proposed to be magnetic quasi-1D and LiCrGe2O6 is 3D, and discuss the different behavior in magnetic dimensionality crossover in the context of how substituting Ge with Si fine-tunes the relative ratio between the intrachain and interchain couplings that defines the magnetic dimensionality in these materials.