Authors: Wei Tian | Jiaqiang Yan | А. И. Колесников
Source: Physical review. B./Physical review. B (article, 2019-02-20)
Field: Materials Science — Electronic, Optical and Magnetic Materials

Insights into the evolution from ferromagnetism to antiferromagnetism: A doping-dependent study of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>NaCrSi</mml:mi><mml:mi>x</mml:mi></mml:msub><mml:msub><mml:mi>Ge</mml:mi><mml:mrow><mml:mn>2</mml:mn><mml:mo>−</mml:mo><mml:mi>x</mml:mi></mml:mrow></mml:msub><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mn>6</mml:mn></mml:msub><mml:mspace width="0.16em"/><mml:mrow><mml:mo>(</mml:mo><mml:mn>0</mml:mn><mml:mo>≤</mml:mo><mml:mi>x</mml:mi><mml:mo>≤</mml:mo><mml:mn>2</mml:mn><mml:mo>)</mml:mo></mml:mrow></mml:mrow></mml:math> ${\mathrm{NaCrGe}}_{2}{\mathrm{O}}_{6}$ and ${\mathrm{NaCrSi}}_{2}{\mathrm{O}}_{6}$ are isostructural compounds exhibiting different magnetic ground states. ${\mathrm{NaCrGe}}_{2}{\mathrm{O}}_{6}$ adopts a ferromagnetic ground state with $Tc=6\phantom{\rule{0.28em}{0ex}}\mathrm{K}$, whereas ${\mathrm{NaCrSi}}_{2}{\mathrm{O}}_{6}$ orders antiferromagnetically below ${T}_{N}=3.4\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. Although it has been proposed that the intriguing magnetic behavior in Cr-based pyroxenes involves competition between antiferromagnetic direct exchange and ferromagnetic superexchange 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 into 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 ${\mathrm{NaCrSi}}_{x}{\mathrm{Ge}}_{2\ensuremath{-}x}{\mathrm{O}}_{6}$ ($x=0$, 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 ${\mathrm{NaCrGe}}_{2}{\mathrm{O}}_{6}$ and ${\mathrm{NaCrSi}}_{2}{\mathrm{O}}_{6}$ indicate that replacing Ge with Si has profound effect on the intrachain coupling, whereas it has negligible effect on the interchain couplings. We compare our results, which indicate ${\mathrm{NaCrGe}}_{2}{\mathrm{O}}_{6}$ is magnetic quasi-one-dimensional (1D) and ${\mathrm{NaCrSi}}_{2}{\mathrm{O}}_{6}$ is three-dimensional (3D), with $\mathrm{LiCr}{(\mathrm{Si},\mathrm{Ge})}_{2}{\mathrm{O}}_{6}$, where ${\mathrm{LiCrSi}}_{2}{\mathrm{O}}_{6}$ is proposed to be magnetic quasi-1D and ${\mathrm{LiCrGe}}_{2}{\mathrm{O}}_{6}$ is 3D, and discuss the different behaviors 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. Antiferromagnetism ; Superexchange ; Ferromagnetism ; Neutron diffraction ; Isostructural ; Condensed matter physics ; Magnetization ; Crystallography ; Ground state ; Magnetism ; Physics ; Materials science ; Chemistry ; Crystal structure ; Atomic physics Multiferroics and related materials ; Magnetic and transport properties of perovskites and related materials ; Advanced Condensed Matter Physics