Physical Properties Of The Geometry-confined Ruthenate Sr₄Ru₃O₁₀

The 4d Ruthenium oxides?Ca,Sr?n+1RunO3n+1 possess perovskite-type structure are typical strongly correlated materials,involving complex interactions between the charge,spin,orbit and lattice degrees of freedom.Their ground states present a rich of exotic physical properties,such as the spin-triplet superconductivity?Sr2RuO4?,Mott insulating?Ca2RuO4?,the quantum criticality and spin density wave?Sr3Ru2O7?,exotic giant magnetoresistance?Ca3Ru2O7?,etc.Sr₄Ru₃O₁₀ is the n=3 member of the Srn+1RunO3n+1 family,which performs a complex magnetic behavior,and it is one of the most complex materials in this system.With decreasing temperature,this material shows a ferromagnetic?FM?transition at a Curie temperature Tc= 105 K,and then,followed by another transition at a characteristic temperature TM?50 K.Below TM,it shows typical FM behavior along the c-axis while paramagnetic or anti ferromagnetic behavior along the ab-plane.Sr₄Ru₃O₁₀ has been studied for nearly 20 years since it is synthesized,but its complex magnetic property remains elusive.In this thesis,we have,for the first time,exfolited the Sr₄Ru₃O₁₀ bulk single crystal into nanosheets,and studied their transport properties systematically.Now,several longstanding issues concerning the complex magnetic property is well understood,based on our results presented as follows:1)The second magnetic transition of Sr₄Ru₃O₁₀ is thickness dependentBy measuring the transport properties of different-thick samples,we found that the FM transition temperature TC of Sr₄Ru₃O₁₀ is independent on the thickness,but the second magnetic transition temperature TM is significantly modulated by the thickness.As the thickness is reduced to 30 nm,the TM reaches to 25 K.Further measurements show that both the normal Hall effect and the scale relation of the anomalous Hall effect are independent on the thickness,but the saturation field along the c-axis increases with the decrease of thickness,indicating that thinner sample is easier to be magnetized along the ab-plane.These results show that the ferromagnetic transition is the intrinsic characteristic of Sr₄Ru₃O₁₀,which is determined by the magnetic exchange interaction,while the second magnetic transition is likely to be related to the rearrangement of Ru moments.2)These exists in-plane FM order in the Sr₄Ru₃O₁₀ nanosheetThe in-plane magnetic behavior of Sr₄Ru₃O₁₀ nanosheets has been systematically studied by planar Hall effect.Large "spin-valve"-like switching behavior has always been observed on transverse magnetoresistance below the FM transition temperature TC,indicating a strongly anisotropic FM order in the ab-plane of the nanosheets.The in-plane field angle dependent measurement reveals that the[110]axis is the magnetic easy axis and the[110]axis is a metastable axis.This result is in contrast to the magnetic structure of the bulk,where neither FM order nor antiferromagnetic order were found in the ab plane,indicating that the reduction of thickness favors the Ru moments FM-aligned in the ab-plane.Further temperature dependent planar Hall effect measurement demonstrates that the domain structure of the in-plane FM order in the nanosheet transforms from a single domain state into a multi-domain state below TM.3)There is a spontaneous spin reorientation process in Sr₄Ru₃O₁₀ as the temperature changesThe anisotropic magnetoresistance?MR?measurement reveals that,when the magnetic field is applied along the ab plane,above TM,the low field MR shows a negative magnetoresistance behavior,and below TM,the MR shows positive MR behavior.While the field is applied along the c-axis,this MR is just reverse.In other words,there is a reversed MR effect as a function of temperature when the magnetic field is applied along both the c-axis and the ab-plane.Analysis of the data indicates that above TM the magnetization is predominantly oriented in the ab-plane at the ground state,while below TM it changes into the c-direction.This temperature-induced magnetic anisotropy reversal is a result of the completion between the size effect and spin-orbit coupling,where the former forces the spins aligned in the ab-plane while the latter drives the spins rearranged along the c-axis.This result naturally explains the second transition and clarifies the physical meaning of TM in Sr₄Ru₃O₁₀.