科研之友
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摘要
Neutron diffraction and muon spin relaxation (mu SR) studies are presented for the newly characterized polymorph of NiNb2O6 (beta-NiNb2O6) with space group P4(2)/n and mu SR data only for the previously known columbite structure polymorph with space group Pbcn. The magnetic structure of the P4(2)/n form was determined from neutron diffraction using both powder and single-crystal data. Powder neutron diffraction determined an ordering wave vector (k) over right arrow = (1/2,1/2,1/2). Single-crystal data confirmed the same (k) over right arrow vector and showed that the correct magnetic structure consists of antiferromagnetically coupled chains running along the a or b axis in adjacent Ni2+ layers perpendicular to the c axis, which is consistent with the expected exchange interaction hierarchy in this system. The refined magnetic structure is compared with the known magnetic structures of the closely related trirutile phases, NiSb2O6 and NiTa2O6. mu SR data finds a transition temperature of T-N similar to 15 K for this system, while the columbite polymorph exhibits a lower T-N = 5.7(3) K. Our mu SR measurements also allowed us to estimate the critical exponent of the order parameter beta for each polymorph. We found beta = 0.25(3) and 0.16(2) for the beta and columbite polymorphs, respectively. The single-crystal neutron scattering data give a value for the critical exponent beta = 0.28(3) for beta-NiNb2O6, in agreement with the mu SR value. While both systems have beta values less than 0.3, which is indicative of reduced dimensionality, this effect appears to be much stronger for the columbite system. In other words, although both systems appear to be well described by S = 1 spin chains, the interchain interactions in the beta polymorph are likely much larger.
- 出版日期2017-10-13
- 单位浙江大学
