Lattice anomaly associaterd with the magnetic order in a re-entrant superconductor Ho_<1-x>Y_xNi_2B_2C

在RNi2B2C(R=稀土元素)中，稀土元素的超导电性和磁序共存(或竞争)，其中超导电性、磁性和晶格动力学三者密切相关。当RE=Ho时，超导转变温度(T_C=7K)接近磁转变温度(T_N=5K)，超导和磁性的温度-磁场相图是复杂的，包括重入超导相和反铁、无公度磁相。本项目的目的是通过超声波测量和单轴应力下的晶格形变来阐明…_2B_2C的超导电性和磁性之间的相互关系。我们仔细地检查了由熔融炉制备的单晶的超导电性与样品的关系，发现多个超导相存在于晶格中超声测量表明，沿[110]方向的弹性常数C&lt；66&gt；随温度从80K降低到T_n，有较大幅度的软化，最高可达60%。与[001]应变相关的弹性常数C&lt；33&gt；随温度从80K降低到T_N，表现出类似的软化，但其变化比C&lt；66&gt；小两个数量级。此外，C_&lt；33&gt；在2K下显示出与亚磁相变相关的阶跃软化。我们认为，沿[001]方向的应变与c面间Ho磁矩的磁关联相耦合。单轴应力下的磁性测量表明，沿[110]方向施加应力后，T_N显著增加。这一速率比在水压下的速率大一个数量级。另一方面，在本精度范围内，T_N不随[001]方向的应力变化。除了上述研究外，我们还在Spring-8上利用同步辐射进行了初步的x射线磁散射实验。用磁布拉格散射证实了2K时反铁磁性相和铁磁性相的周期性。较少

In RNi_2B_2C(R=rare earth elements) the superconductivity and the magnetic order of the rare-earth elements coexist, (or compete), where the superconductivity, the magnetism, and the lattice dynamics are closely related to the others. When RE =Ho, the superconducting transition temperature (Tc=7K) is close to the magnetic transition temperature (T_N=5K), and the temperature vs magnetic-field phase diagram for the superconductivity and the magnetism is complicated involving the reentrant superconducting phase and antiferro-, ferri-, ferro-, and incommensurate magnetic phases. The purpose of the present project is to elucidate the interrelation between the superconductivity and the magnetism of HoNi_2B_2C through "lattice" by the ultrasonic measurement and by lattice deformation under uniaxial stresses.We carefully checked the sample dependence of the superconductivity of the single crystals prepared by a froating-zone infraned furnace, and found that multiple superconducting phases ofte … More n coexist even in a single crystal.The Ultrasonic measurements revealed that the elastic constant C_<66>, relevant to the strain along [110], shows a large softening up to 60% with decreasing temperature from 80 K to T_N. We conclude that the strain couples with the ferromagnetic fluctuation of Ho magnetic moments within the c-plane. The elastic constant C_<33>, relevant to the strain along [001], shows a similar softening with decreasing temperature from 80 K to T_N although its change is smaller by two orders of magnitude than C_<66>. In addition, C_<33> shows stepwise softenings associated with the meta-magnetic transitions at 2 K. We conclude that the strain along [001] couples with the magnetic correlation of Ho magnetic moments between the c-planes. We deduced the dependence on the c-spacing of the exchange interactions between the c-planes.The magnetic measurements under uniaxial stress revealed that T_N increases remarkably by applying a stress along [110]. The rate is larger by 1 order of magnitude than that in the hydro-pressure. On the other hand, T_N dose not change by the stress along [001] within the present precision. We did not find any appreciable change of Tc by the stresses in all directions.In addition to the above investigation, we performed preliminary x-ray magnetic scattering experiment using synchrotron radiation in Spring-8. We confirmed the periodicity of the antiferro- and ferri- magnetic phases at 2 K by magnetic Bragg scattering. Less