Neutron Scattering Study of Weak Antiferromagnets

磁研究的最新进展之一是研究金属磁体自旋涨落的SCR理论的发展。目前该理论已能定量地解释MnSi、Ni_3Al等弱铁磁体的性质。对弱反铁磁体进行了定性计算，并进行了相应的实验研究。另一方面，实验表明弱反铁磁体的性质与铁磁体有很大的不同，如金属反铁磁体的自旋波的自旋斜率很陡，反映了其电子能带结构，我们试图研究简单到足以与SCR理论相比较的弱反铁磁体。在本研究中，我们选取了FeSb和Mn_3Pt。在FeSb的情况下，我们可以制作一个Fe_ Sb单晶<1027>，在其上进行中子散射测量，发现自旋波的斜率非常陡。Mn_3Pt是金属反铁磁体，在TN=480 K时呈共线自旋结构，在Tt=415 K时转变为三角形自旋结构。我们还制备了Mn_3Pt单晶，研究了其磁结构和自旋波激发。磁致伸缩的立方对称性和自旋波色散的各向同性性质表明Mn_3Pt的磁结构是立方对称的多Q结构，而不是以前报道的单轴结构。Mn_3Pt自旋波色散的斜率比分子场理论预测的陡约两倍。

One of recent progresses in research of magnetism is due to development of SCR theory which deals spin fluctuation in metallic magnets. Now the theory can explain quantitatively the properties of weak ferromagnets of MnSi and Ni_3Al etc. In the present stage of the theory, the main theme goes to the weak antiferromagnets. Qualitative calculations have been done on the weak antiferromagnets, and the corresponding experiments are earnestly desired to do. On the other hand experiments shows the properties of the weak antiferromagnets being greatly different from those of ferromagnets, for example the slope of spin of spin wave in metallic antiferromagnets is very steep reflecting the electron band structure of them.We try to study on the weak antiferromagnets which are simple enough to compare with the SCR theory. In the present investigation we picked up FeSb and Mn_3Pt. In the case of FeSb, we could make a single crystal of Fe_<1027>Sb on which we made neutron scattering measurements and found slope of the spin wave being very steep. Mn_3Pt is a metallic antiferromagnet with TN=480 K taking collinear spin structure, and transforms to a triangular spin structure at Tt=415 K. We also made single crystals of Mn_3Pt, and studied the magnetic structure and the spin wave excitations. Cubic symmetry of the magnetostriction and isotropic nature of the spin wave dispersion suggest the magnetic structures of Mn_3Pt to be multi-Q structures of cubic symmetry for high and also low temperature phases instead of the uni-axial structure proposed in the previous reports. The slope of the spin wave dispersion of Mn_3Pt is about two times steeper compared with the prediction by molecular field theory.