Structure, Magnetifc Properties And Magnetoresistance Effect Of RMn₆Ge₆ Rare-earth Mn-based Compounds

In this thesis, the structure , magnetic properties and magnetoresistancc (MR) of R1-xGdxMn6Ge6(R=Dy,Er,Ho) and R1-xSmxMri6Ge6(R=Dy,Er) are measured by x-ray diffraction (XRD), superconducting quantum interface device (SQUID), physical property measure system (PPMS) and vibrating sample magnetometer (VSM) ea al.. The effect of the substitution of rare-earth elements On the magnetic properties and transport properties is investigated.1. According to the XRD patterns, R1-xGdxMn6Ge6(R=Dy, Er, Ho) and R1-xSmxMn6Ge6 (R=Dy, Er x 0.4) crystallize in the HfFe6Ge6-type structure, but R1-xSmxMn6Ge6 (R=Dy , Er x 0.6) crystallizes in the YCo6Ge6-type structure (space group P6/mmm). The lattice constants a, c and the unit pell volume v of them increase with increasing Gd and Sm content. The coupling between k- and Mn-sublattice is strengthed and leads to a transition from an antiferromagnetic state to a ferrimagnetic/ferromagnetic state. The magnetic ordering temperature and the second-transition temperature also increase with the increase of the Gd and Sm content.2. We confirm 1 that the MR effects always accompany with the magnetic transition by investigating the MR effects of Dy0.4Gdo.6Mn6Ge6 , Er0.1 Gd0.9Mn6Ge6 and Ho0.1 Gd0.9Mn6Ge6, for example, with increasing temperature, Dy0.4Gd0.6Mn6Ge6 change from a ferrimagnetic state to an antiferromagnetic state and then to a ferrimagnetic state. It displays the MR effects around the transition temperature. Thus, the MR effects can be used to judge whether any magnetic transition takes place in the materials,3. The MR effects of Dy0.4Sm0.6Mn6Ge6 and Er0.2Sm0.8Mn6Ge6 are analyzed. For Dy0.4Sm0.6Mn6Ge6, there are inflexions in the MR-H curves at low temperatures and the phenomenon disappears with increasing temperature; for Er0.2Sm0.8Mn6Ge6, the magnitude of MR is found to be pesitive below 55 K and gradually increase to a relatively large value of about 5.02% at 5 K in a field of 5 T as the temperature is lowered. The experimental results can be interpreted as due to the influence of magnetic field on the Fermi surface or due to thecompetition between scatting of magnetic moments and the effect of Lorentz force on the conduction electrons.