【作者】 苏彦涛；

【导师】 隋郁；

【作者基本信息】 哈尔滨工业大学 ， 光学， 2013， 博士

【摘要】 钙钛矿过渡族氧化物体系中存在许多奇特和有趣的物理现象，以及诸多丰富而复杂的物理内涵，成为当今凝聚态物理学和材料物理学等学科的前沿研究领域。由于存在自旋自由度、轨道自由度与晶格自由度之间的耦合与相互作用，钙钛矿型稀土钛酸盐RTiO₃体系（R=稀土元素或Y）表现出许多奇特的物理性质。通过对RTiO₃体系的充分研究与理解，有助于澄清在强关联电子体系中轨道自由度所起的作用，以及揭示磁性与轨道自由度之间相互耦合的奥秘，进而提升对强关联电子体系中存在的复杂相互作用的认识。在RTiO₃体系中，随着稀土离子半径的减小，Ti3+离子自旋序从反铁磁（R=La, Pr, Nd, Sm）转变为铁磁（R=Gd,…, Lu, Y），该体系中磁有序渡越（crossover）行为的来源还有待探讨；RTiO₃体系在磁有序温度处表现出磁化强度的陡峭变化以及轨道有序-无序转变，因此该体系是否存在大的磁热效应以及磁热效应是否与轨道有序存在关联等问题都非常值得研究；临界行为的研究将有助于加深对RTiO₃中存在的铁磁相变以及磁相互作用类型等问题的理解。但是，由于磁性稀土离子对磁性测量的影响，在利用传统的Arrott图方法研究铁磁相RTiO₃的临界行为时遇到了困难，这就要求我们探索一种新的方法来确定铁磁相RTiO₃的临界行为。因此，综合上述研究背景，本论文主要围绕稀土钛酸盐RTiO₃体系反铁磁到铁磁相转变的渡越行为、体系的磁热效应以及临界行为进行了研究和分析，主要研究内容包括：通过对SmTiO₃低温比热和磁性数据的分析，揭示了处于RTiO₃体系中反铁磁到铁磁相转变边界的SmTiO₃中复杂的磁结构，以及自旋有序、轨道有序和晶格之间的耦合相互作用。研究表明SmTiO₃处于反铁磁到铁磁转变的渡越区，特殊的晶体场环境以及自旋轨道结构导致SmTiO₃的二维各向异性特征。在c轴方向上施加强磁场时出现的比热反常来源于反铁磁背景中出现的铁磁性成分。研究发现RTiO₃（R=Dy-Yb）体系中存在大的低场磁热效应：例如，DyTiO₃材料在2T的磁场下，磁熵变ΔSM达到9.64J kg^-1K^-1，绝热温度改变ΔTad达到4.14K。RTiO₃材料的低场磁熵变在氧化物磁热材料中还是比较可观的，表明该体系材料可以作为潜在的磁制冷工材料。对RTiO₃材料中磁熵变的来源以及物理机制进行了分析，结果表明该体系中轨道有序引起的晶格参数的异常以及磁性与晶格之间的耦合都对磁熵变具有重要的影响。利用标度关系（Scaling Relations）理论对磁熵变随温度和磁场变化关系曲线（ΔSM-H-T）存在的归一化行为进行了解释。利用磁热效应与临界行为之间的关系，将磁热效应标度律应用于磁相变临界行为的研究中。在铁磁相RTiO₃（R=Dy-Yb）中，由于受到磁性稀土离子R3+的影响，利用传统的Arrott图方法研究该体系的临界行为时遇到困难，无法得到正确的临界指数。通过分析RTiO₃的磁热效应以及利用磁热效应标度律方法，得到了准确的临界指数，并确定RTiO₃中的铁磁耦合相互作用可以用三维的海森堡模型来描述，加深了对该体系中存在的铁磁相互作用类型的理解。通过分析GdTiO₃的低温比热对温度和磁场的依赖关系，发现低温下的肖特基（Schotty）比热反常对GdTiO₃的磁制冷能力（Refrigeration Capacity）具有重要影响。零场比热在TC=35K处出现的λ型反常来源于Ti3+离子磁矩亚晶格的铁磁合作有序；在TS≈8K附近存在的肖特基比热反常来源于Gd3+离子基态双重态在Gd-Ti交换场作用下的劈裂。研究表明，Gd3+基态双重态的劈裂对GdTiO₃的磁熵变具有重要贡献，并且展宽了磁熵变随温度变化的曲线，进而增强了GdTiO₃的磁制冷能力。对TbTiO₃中磁场引起的变磁转变现象以及正常磁热效应和反常磁热效应共存的现象进行了分析。研究发现在居里温度TC以下TbTiO₃存在的变磁转变与其特殊的基态磁结构有关；TbTiO₃磁熵变随温度以及外加磁场发生改变，表现为正常磁热效应与反常磁热效应的共存，反常磁热效应正是来源于磁场引起的变磁转变。此外，在20K以下TbTiO₃出现的异常磁滞现象来源于材料中存在的磁晶各向异性能。更多还原

【Abstract】 Perovskite transition metal oxides system has been the forefront research fieldsof condensed matter physics and materials physics and other disciplines, becausethere are many strange and interesting physical phenomena, as well as many otherrich and complex physical connotations. The interplay and coupling between thespin, orbit, and lattice degree of freedom makes the perovskite rare earth titanatesRTiO₃（R=rare earth element or Y） system exhibiting many peculiar physicalphenomena. Sufficient research and understanding of the titanate system will helpresearchers to clarify the role of orbital degrees of freedom in strongly correlatedelectron system, reveal the mysteries of the coupling between magnetic and orbitaldegrees of freedom, and enhance the understanding of the complex interactions ofstrongly correlated electron system. With decreasing the size of R3+ions in theRTiO₃system, the magnetic ordering of Ti3+ions changes from antiferromagneticfor R=La, Pr, Nd, Sm to ferromagnetic for R=Gd,…, Lu, Y. The crossoverbehavior of the magnetic ordering is needed to be further studied. There are thesharp change of the magnetization and the orbital order-disorder transition at themagnetic ordering temperature. Therefore, the existence of large magnetocaloriceffect and magnetocaloric effect associated orbital ordering are very worthresearching in the system. The critical behavior study will contribute to theunderstanding of the ferromagnetic phase transition and the magnetic interactions inthe RTiO₃. Since the magnetization measurements are plagued by the contributionfrom magnetic rare earth, the critical behavior study of the ferromagnetic RTiO₃willmeets some problems by the conventional Arrott method. This requires us to explorea new method to determine the critical behavior of the ferromagnetic RTiO₃, anddeepen the understanding of the types of the ferromagnetic interactions in thesystem. Therefore, in this dissertation we studied the crossover behavior near theantiferromagnetic （AFM） to ferromagnetic （FM） phase transition boundary, themagnetocaloric effect and the critical behavior of the RTiO₃system. The mainresults were listed as following:Through the specific heat and magnetic data, the complex magnetic structureand the coupling between spin, orbital and lattice were revealed in SmTiO₃near theAFM-to-FM phase transition boundary in RTiO₃system. The special crystal fieldenvironment as well as the special spin-orbit structure leads to the two-dimensionalanisotropic characteristics. The abnormal specific heat peak under high magneticfield at c axis was originated from the weak ferromagnetism in SmTiO₃. The RTiO₃（R=Dy-Yb） system exhibits a large low magnetic fieldmagnetocaloric effect. For instance, the ΔSMand ΔTadreaches9.64J kg^-1K^-1and4.14K for DyTiO₃under a magnetic change of2T, respectively. The low-fieldmagnetic entropy change in RTiO₃system is quite impressive in the oxidemagnetocaloric materials. The origin and physical mechnism of the largemagnetocaloric effect was analyzed. The simultaneous anomalies of the latticeparameters due to the orbital order and the coupling of the magnetism and lattice canstrongly influence the magnetic entropy change. The universal behavior was alsofound in the temperature and magnetic field dependence of the magnetic entropychange curves, which was explained by the scaling relation theory.The magnetocaloric effect scaling law method is applied to the study of thecitical behavior of magnetic phase transition based on the relationship of themagnetocaloric effect and the critical behavior. Because of the influence frommagnetic rare earth R3+ions, the Arrott plot method leads to the incorrect criticalexponents. Here we report critical exponents for most ferromagnetic members in theRTiO₃family by measuring magnetocaloric effect and applying the correspondingscaling laws. Our results indicate that the ferromagnetic coupling in the RTiO₃canbe well-described by the3D Heisenberg model.The specific heat and the enhanced magnetic refrigeration capacity in GdTiO₃were studied. The λ-type anomaly occurs at TC=35K in zero-field specific heatcomes from the cooperation behavior of the magnetic sublattice Ti3+ions. TheSchottky specific heat anomaly appears at TS≈8K was derived from the groundstate doublet splitting of the Gd3+ions in the role of Gd-Ti exchange field. Thepresence of low-temperature Schottky-like anomaly arising from the splitting of theground-state doublet of Gd3+ion, which enlarges the temperature span of large MCE,and consequently resulting an enhanced RC.The metamagnetic transition phenomenon and the anomalous （inverse）magnetocaloric effect in TbTiO₃were studied. Below the Curie temperature TC,magnetic field induced metamagnetic transition is associated with the ground statemagnetic structures in TbTiO₃. The change of the magnetic entropy withtemperature as well as the external magnetic field and the performance for thecoexistence of normal and inverse magnetocaloric effect were originated from themetamagnetic transition. In addition, the anomalous hysteresis loop below the20Kwas associated with the magnetic anisotropy energies.更多还原