pyroxenes*

NaCrGe₂O₆ and NaCrSi₂O₆ are isostructural compounds exhibiting different magnetic ground state. NaCrGe₂O₆ adopts a ferromagnetic ground state with T_c=6K, whereas NaCrSi₂O₆ orders antiferromagnetically at T_N=3K. The complex magnetic behavior in Cr-based pyroxenes involves competition between direct and super-exchange magnetic interactions - a delicate balance that is sensitive to the Cr-Cr distance and chemical pressure. Here we present a systematic neutron scattering study of NaCrSi_xGe_2-xO₆ that reveals how the substitution of Ge by Si affects the evolution from ferromagnetism to antiferromagnetism in theses compounds.