Magnetic And Electric Transport Properties Investigation Of Several Mn-based Magnetic Materials

Magnetic skyrmions are the topologically protected novel vortex magnetic domain structures with the sizes of nanometer,and can be modulated by temperature,magnetic and electric fields.Thus,the magnetic skyrmions have become ideal candidates for future high-density,high-velocity,low-energy magnetic information storage and spintronic applications,and accordingly received a lot of attentions from researchers.The interaction between the magnetic skyrmions and conduction electrons would produce the topological Hall effect,which is one of the important methods to explore the existence of magnetic skyrmions in materials.In this thesis,the magnetic and transport properties have been investigated in centrosymmetric Mn-Cu-Ga alloys,HoMn₆Sn₆ single crystals,and non-centrosymmetric Cr-Mn-Ge and Co-Zn-Mn polycrystalline alloys,which would provide theoretical guidance and possible research directions for the development of next-generation novel spintronic devices.The results are summarized as follows:1)The non-centrosymmetric B20 type Cr Ge-based chiral magnets exhibit an abundant magnetic structure at low temperature region by Mn substitution for Cr,while few researches have been reported yet.With the increase of Mn substitution,the ground states of the alloys change from the paramagnetic to ferromagnetic state.The thermomagnetic curves of Cr_0.82Mn_0.18Ge alloy show a bifurcation at low temperature.The results of DC and AC magnetization measurements indicate that this bifurcation originates from the magnetic domain pinning effect.Large topological Hall effect induced by low magnetic field is obtained in Cr_0.82Mn_0.18Ge alloy,which might be derived from the topological spin structure,i.e.skyrmions.The phase diagram is constructed by combining the magnetic,transport and AC magnetization measurements.2)The non-centrosymmetric Co-Zn-Mn alloys are non-B20 type of chiral magnets,in which the skyrmions phase exists stably at the highest temperature region.So,these alloys are great potential for practical applications in skyrmions materials.The series of Co-Zn-Mn alloys were prepared by solid-phase reaction method,and confirmed to be the chiral cubic?-Mn type single phase by the room-temperature XRD.The maximum magnetic entropy change with the value of0.8 Jkg^-1K^-1 is obtained in Co₇Zn₈Mn₅ alloy under an applied field change of 0-2 T.Under the field change of 0-1 T,the peak values of magnetic entropy change are 0.27 and 0.14 Jkg^-1K^-1 for Co₇Zn₉Mn₄ and Co₇Zn₁₀Mn₃ alloys,respectively.The Co₇Zn₈Mn₅ alloy exhibits a re-entrant spin glass behavior at low temperature.Large topological Hall effect is observed in a very wide temperature region.The topological Hall resistance increases with decreasing temperature,and a maximum value of-2.1??cm is obtained at 10 K.In the temperature range of 210 K-220 K,the topological Hall effect is attributed to the skyrmions and scalar spin chirality,while in the temperature region below 210 K it originates from the scalar spin chirality associated with the magnetic spin structure.3)The centrosymmetric HoMn₆Sn₆ single crystal with a two-dimensional kagome structure is an ideal platform to understand the connection between Berry curvature and intrinsic anomalous Hall conductivity in non-colinear magnets.High-quality HoMn₆Sn₆ single crystals were prepared by the flux method,and the XRD pattern showed a single phase with the hexagonal structure.The magnetic measurement results show a spin reorientation transition from easy axis to easy plane for HoMn₆Sn₆ single crystals with increasing the temperature.The characterization of transport was performed by the four-wire method.The anomalous Hall effect dominated by the intrinsic K-L mechanism was observed in HoMn₆Sn₆ single crystals.Large anomalous Hall conductivity(?_xy^int?118.6?^-1cm^-1)and Hall angle(?^A_xy/?_xx?2.4%)were obtained.4)The centrosymmetric Mn-Cu-Ga alloy has a non-collinear magnetic structure and exhibits a wealth of magnetic functional properties that are of great scientific research significance.In this part,the magnetic properties of(Mn_0.7Cu_0.3)₆₅Ga₃₅ alloy at low temperatures have been investigated.An anomalous bump is observed near 170 K in thermomagnetic curve.By calculating the magnetic entropy change,it is confirmed that this bump originates from a second-order magnetic phase transition between a ferrimagnetic state and an incommensurate magnetic phase;The thermomagnetic curves exhibit a significant irreversibility at low temperature range.Combining with the measurements of magnetic relaxation effects,magnetic memory effects,and AC magnetization,a super-spin glassy state is validated at low temperature region,which is consistent with the hierarchical model.A giant exchange bias effect of 308.4 m T was achieved in the(Mn_0.7Cu_0.3)₆₅Ga₃₅ alloy,which would be ascribed to the strong exchange coupling between the ferromagnetic-like and super-spin glass phases.Large topological Hall effect induced by non-coplanar magnetic structure was obtained near room temperature in(Mn_0.7Cu_0.3)₆₅Ga₃₅ alloy.