【作者】 刘艳；

【导师】 田明亮； 杨继勇；

【作者基本信息】 中国科学技术大学 ， 凝聚态物理， 2017， 博士

【摘要】 强关联电子体系中,含有高轨道4d电子态的钙钛矿钌氧化合物（Ca,Sr）n+1RunO3n+1中存在多种自由度,包括电荷、自旋、晶格和轨道间的复杂相互作用,蕴含着丰富的物理现象,如自旋三重态配对（p波）的超导电性（Sr2Ru04）、Mott绝缘体态（Ca2Ru04）、磁场诱导的量子临界和自旋密度波（Sr3Ru207）、奇异的巨磁阻效应（Ca3Ru207）等等。而原胞中包含3层Ru06八面体单元的Sr₄Ru₃O₁₀表现出很复杂的磁行为,是该体系中被研究过的最复杂的材料中的一种。随着温度的降低,该材料在居里温度TC= 105 K处发生铁磁转变,随后在温度TM～50 K发生另一个磁转变,并在输运上伴随着电阻异常。在TM以下,该材料的磁化强度沿着c方向表现出典型的铁磁行为,而沿着ab方向却类似于顺磁或者反铁磁行为。Sr₄Ru₃O₁₀材料合成至今已经被研究了近20年,但是对该材料中的复杂磁特性仍然缺乏清晰的认识。本论文采用机械剥离单晶的办法首次将Sr₄Ru₃O₁₀单晶的厚度推进到纳米尺度,并系统的研究了该材料在纳米尺度下的输运特性,澄清了引起Sr₄Ru₃O₁₀中复杂磁行为的机理,得到了如下创新性成果:1)发现Sr₄Ru₃O₁₀的磁相变具有厚度依赖特性通过对不同厚度样品的输运测量发现,Sr₄Ru₃O₁₀发生铁磁转变的居里温度与厚度没有依赖关系,但是其发生第二次磁转变的温度TM却严重的受到样品厚度的调制,TM由50 K（块材单晶）降至25 K（30nm）左右。进一步的测量表明,其正常霍尔效应以及反常霍尔效应的标度关系与厚度的变化无关,但是其沿c方向磁化的饱和场却随着厚度的减小而增大,表明,样品越薄其磁矩越容易沿ab方向排列。该结果表明,铁磁转变是该材料的本征特性,由磁交换作用决定,而第二个磁转变极有可能与材料中Ru磁矩的重排布有关系。2)发现纳米尺度Sr₄Ru₃O₁₀面内具有铁磁序通过平面霍尔效应对纳米尺度Sr₄Ru₃O₁₀的面内磁阻行为进行了系统的研究。我们发现,在铁磁转变温度TC以下,样品的横向磁阻上总会出现类似于"自旋阀"效应的电阻跳变行为,表明,纳米尺度下,该材料的ab面内含有具有各向异性特征的铁磁序。进一步的面内转角测量结果表明,[110]轴为该面内铁磁序的易磁化轴,而[110]轴为亚稳轴。这个结果与块材的磁结构相悖,因为以往在块材的ab面内既没有发现铁磁序也没有发现反铁磁序,如此证明厚度的降低有利于Ru磁矩在面内呈铁磁排列。对变温平面霍尔效应的分析发现,该面内铁磁序在另一磁转变温度TM以上呈单畴态,在TM以下反而呈多畴态。3)找到Sr₄Ru₃O₁₀中自旋随温度发生自发重取向的证据通过对纳米尺度Sr₄Ru₃O₁₀各向异性磁阻的变温测量发现,随着温度的降低,当磁场沿着ab面时,低场磁阻在另一磁转变温度TM以上表现出负磁阻行为,在TM以下表现出正磁阻行为。但是,当磁场沿着c方向时,其低场磁阻却在TM以上表现出正磁阻行为而在TM以下表现出负磁阻行为。该结果表明,Sr₄Ru₃O₁₀中的自旋随着温度的降低发生了自发重取向,在TM处从主要沿ab面排列转向为主要沿c轴方向排列。这种自旋重取向行为是引起Sr₄Ru₃O₁₀发生第二个磁转变的原因。对磁阻的进一步分析还发现,该材料中的自旋轨道耦合在决定其磁特性上扮演了非常重要的角色。更多还原

【Abstract】 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.更多还原