Structures, Magnetic Properties And Magnetoresistance Effect Of Rare-Earth Mn-Based Compounds RMn₆Sn₆ And Their Derivatives

Structures, magnetic structures, intrinsic magnetic and magnetotransport properties of the RMn₆Sn₆ (R: rare-earth elements) compounds and their derivatives have been investigated systemically by means of x-ray diffraction, neutron diffraction, magnetic and transport measurements in this dissertation.Firstly, the structural and magnetic properties of the RMn₆Sn_6-xGa_x (R=Er, Gd; x =0-2.0) compounds have been studied. According to the study, the hexagonal HfFe₆Ge₆-type structure still remains in the ErMn₆Sn_6-xGa_x compounds by the substitution Ga for Sn. However, the GdMn₆Sn_6-xGa_x series shows three distinct crystal structures with increasing the Ga content: 1) the hexagonal HfFe₆Ge₆ type for x=0-0.6, 2) the orthorhombic HoFe₆Sn₆ type for x=0.9-1.2, and 3) the orthorhombic TbFe₆Sn₆ type for x=1.5-2.0. In both ErMn₆Sn_6-xGa_x and GdMn₆Sn_6-xGa_x series, the Ga substitution leads to an isotropy contraction of their unit cells.The strong hard magnetic properties at low temperatures have been observed in the HfFe₆Ge₆-type ErMn₆Sn_6-xGa_x compounds, e.g., the compound with x=2.0 shows a large coercivity field H_c≥22 kOe. The sign of the second-order crystalline electric-field (CEF) coefficient A₂₀ of the Er sublattice reverses due to the change of coordination configuration of Er³⁺ resulted from a small amount of Ga substitution for Sn. Thus the negative second-order CEF parameter B20 is obtained, leading to the easy-axial behavior in the compounds. Moreover, the higher order CEF coefficients, i.e., the fourth-order CEF coefficient A40 and sixth-order Stevens factor γ_J, play an importance role in the determination of the easy magnetic direction in the series.The Mn magnetic moment prefers easy planar alignment in both hexagonal and orthorhombic structures. The magnetocrystalline anisotropy of Mn sublattices can be weakened when the temperature increases, or the symmetry and the coordination configuration of Mn atoms change. At low temperatures, the magnetocrystalline anisotropy constant of Mn sublattice is in the order of-10 K, which is considerablysmall compared with that of the R sublattice. Therefore, the magnetocrystalline anisotropy of Mn sublattice can be neglected at low temperatures.Doping of H, N, C and B has been carried out in the RMn6Sn6 (R=Y, Gd, Tb) and ErMn6Sn6-xGax compounds. However, the formation of RMneSn^-based interstitial compounds has not been observed.Secondly, the magnetic structures, matamagnetic transition and magnetoresistance (MR) effect have been studied in the HfFe6Ge6-type (Y Ho)Mn6Sn6 and ErMn6Sn6 compounds. It has been found that the compounds show a variety of magnetic behaviors with the variation of constituent elements and temperature. The magnetic structures of (Y,Ho)MneSn6 display a helical antiferromagnetic (AF)-spiral ferrimagnetic (FI)-(conical) FI transition with the increase of Ho content, and show the spiral Fl-helical AF-FI transition with increasing temperature, e.g. Yo.5Hoo.5Mn6Sri6, which is of an incommensurate-incommensurate magnetic transition, indicated by the neutron diffraction measurements. ErMn6Sn6 also displays the complicated magnetic behaviors, i.e., the FI, helical AF, and paramagnetic ordering with increasing temperature. The spiral magnetic structures are related to interlayer Mn-Mn coupling, i.e., not only the nearest interplanar Mh-Mn interaction (?/) but also the next nearest one (n2) should be taken into consideration. The exchange interactions obey the relationship m=-4ri2cos^, where $ is the degree between interplanar Mn magnetic moments. In those magnetic spiral structures, there is an effective magnetic anisotropy K^d^-ft-costf)2, which drives the moments align within planes, /^shows an unusual thermal behavior, i.e., it is strengthened with increasing temperature.In the AF state, the AF-FI transition can be induced by an application of external magnetic field in these two series. In the Yi.xHoxMn6Sn6 compounds, the metamagnetic transition can be achieved in a fairly small field with increasing Ho content, e.g. x=0.5, Hth<6 kOe. No thermal or field hysteresis has been observed at the metamagnetic transition, indicating the transition is of the second order. The metamagnetic transition is accompanied by a large MR effect, e.g. forY0.5Hoo.5Mn6Sn6 MR=-16% at 100 K and for ErMneSne MR=-9.8% at 135 K, respectively. Moreover, the weakly positive ordinary MR effect appears in the FI stateThirdly, the new HfFe6Ge6-type Yi.xLaxMn6Sn6 (x<0.2), Yi.xCexMn6Sn6 (x<0.3) and Tbi.xCexMn6Sn6 (x<0.3) compounds have been prepared through partly substitution of light rare earth (e.g. La and Ce) at the R site. The helical AF magnetic structure is not changed by the weakly La/Ce substitution. In the AF state, the metamagnetic transition can also be induced by the application of magnetic fields, accompanied by a large MR effect, e.g., MR=-37% for Yi-xCexMn6Sn6(x=0.175) at 5 K and in 50 kOe field.