交变力磁力显微镜动态成像技术的研究

近年来磁力显微镜(magnetic force microscopy,MFM)对动态磁场信号的测量与分析由于其特殊的工业要求和重要用途而受到广泛关注,本文旨在利用交变磁力对磁性探针的周期性调制发展一种交变力磁力显微镜技术,为磁信息存储工业等重要领域关键技术的发展提供新型的有力的工具.与目前标准MFM采用的设计思路不同,本文的关键在于合理利用MFM频率调制机理,优化设计MFM磁性探针,并且引入动态信号处理模块,实现对交变磁场信号的MFM成像.为达到这些目的,需要从理论上研究MFM探针的频率调制机理,并由实验上设计出动态信号提取模块,二者相辅结合优化设计出具有动态信号测试和分析能力的交变力磁力显微镜技术,由此来测量和解释纳米尺度磁畴结构.     

复杂氧化物中电子相分离的量子调控

复杂氧化物可以呈现出高温超导、庞磁阻以及多铁效应等诸多新奇的物理现象.这类材料中的电荷/自旋/轨道和晶格自由度之间的强耦合相互作用,可以导致多种相互竞争且能量非常接近的电子态的空间共存,这就是电子相分离现象.如果可以将材料的空间尺寸缩小到电子相分离的特征长度,其物理性质甚至电子关联作用本身都会发生根本的变化,从而有可能实现复杂氧化物中的量子调控.本文综述了我们课题组在过去几年中针对复杂氧化物中电子相分离的量子调控取得的进展,内容包括：发现了锰氧化物边缘电子态,通过氧化物微纳加工技术,实现了量子态空间分布的调控,提高了庞磁阻锰氧化物的临界温度;研究了当材料空间尺度小于其电子相分离特征尺度时电子相分离的表现,确定了在电子相分离消失以后体系的磁结构;通过超晶格生长技术调控了材料中的掺杂有序度,对锰氧化物中大尺度的电子相分离的物理机理从实验上给出了解释.     

范德瓦尔斯磁性材料CrOCl变磁性相变的磁力显微镜研究

近年来,二维范德瓦尔斯磁性材料因为在自旋电子学的应用前景而吸引了广泛的关注.CrOCl是一种范德瓦尔斯磁性材料,理论预言其单层具有高达160 K的居里温度,因此吸引了广泛的关注。为了更好的理解这一材料的磁性,我们利用磁力显微镜研究了CrOCl变磁性相变中磁畴结构随磁场的变化。实验发现,在2K下CrOCl样品表面出现随磁场变化的方格条纹,给出了变磁性相比中反铁磁相和铁磁相竞争的图样,并通过二维快速傅里叶变换证实了CrOCl磁性的各向异性。我们的结果为后续研究CrOCl薄层的磁性提供了参考依据。     

双层锰氧化物薄膜的制备及其物理性质

用脉冲激光沉积(PLD)方法成功地制备了双层钙钛矿结构的La2-2xSr1+2xMn2O7(x=032)单相薄膜.这种薄膜生长在具有不同晶格参数的两种衬底上.测量发现，两种衬底上生长的La2-2xSr1+2xMn2O7(x=032)薄膜具有迥然不同的金属-绝缘体转变温度TM-I及其他物性.界面应力的研究表明这是衬底晶格常数不同引起膜内应变的结果.在衬底的压应力下，薄膜的电阻-温度曲线的峰值(TM-I)向高温移动且电阻率(ρ)下降；相反，对于衬底张应力下的薄膜，TM-I下降ρ上升.这些结果可以用双交换模型做很好的解释. 关键词： CMR 双层锰氧化物薄膜 PLD 应力     

纳晶锰氧化物对CO氧化反应的催化活性研究

用γ－射线辐射方法制备了ＭｎＯ２,Ｍｎ２Ｏ３和Ｍｎ３Ｏ４３种纳晶锰的氧化物,它们的平均粒径分别为１０,２０和２２ｎｍ,通过与常规晶粒的相应粉末对比研究发现。纳晶粒末对ＣＯ的氧化反应的催化活性远远高于常规晶粒。经过一次催化实验这后,纳晶锰的氧化物的平均颗粒尺寸虽然有一定程度的增加,但仍保持在纳米范围内,这说明纳晶锰的氧化物作为ＣＯ氧化反应的催化剂是相对稳定的,因此,可以作为廉价和易得的清除ＣＯ的催化     

锰氧化物相竞争的理论研究

势函数是在平衡态统计物理中应用得十分成功的理论,将势函数理论推广到非平衡系统是非平衡态统计物理的一项十分重要的任务。本文综述了非平衡态势函数理论的一个重要方面,即利用对随机过程的研究来建立系统的势函数,本文阐述了非平衡系统势函数的定义和概念;论证势函数在系统演化过程中的单调性质;详细介绍了各种计算非平衡势函数的方法;同时介绍了利用势函数解析地研究在噪声作用下的非线性系统的非定态演化行为。整个研究包含了一维和多维系统的广泛系统。     

锰氧化物相竞争的理论研究

锰氧化物属于典型的强关联电子材料,具有包括庞磁电阻、电荷/轨道有序、电子相分离、多铁性等奇特的物理特性。这些现象涉及一系列凝聚态物理学基本问题,是近年来研究者一直关注的热点和难点。并且这些奇异的电磁性质也为开发量子调控器件提供了基本素材。虽然近20年来对锰氧化物的研究取得了丰硕成果,全世界的研究者仍在为理解并应用其特性作着孜孜不倦的努力。本综述将主要从理论角度,重点关注钙钛矿结构锰氧化物中多种相竞争和调制。由于有着多种竞争相互作用和多重量子自由度,锰氧化物有着丰富的相,这些相物理特性迥异,而自由能却可能相当接近。因此,自发或人为调制导致的相竞争是锰氧化物研究的一个核心问题,也是整个强关联物理领域中一个很有意义的课题。本综述将以电致电阻、多铁性和异质结界面处电子重组这三个具体实例,介绍如何采用蒙特卡罗模拟等方法研究其中的相竞争和调制。     

基于LabVIEW实时研究辐照效应对锰氧化物薄膜电阻的影响

基于LabVIEW编程开发出了一套自动电输运测量系统,并利用该自动测量系统实时测量了γ射线辐照对钙钛矿锰氧化物薄膜La0.67Ca0.33MnO3(LCMO)的电阻率的影响,结果发现LCMO薄膜电阻率在短时间辐照时降低,随着辐射时间增加,电阻率转而单调增加.在同等条件下还测量了1 wt%Nb掺杂SrTiO3(NSTO)的电阻的射线辐射效应,对比发现NSTO的电阻未出现明显变化.通过辐射损伤机理及锰氧化物的强关联电子特征对结果做出了解释.     

巨磁电阻锰酸盐中纳米尺度的相分离

准电子概念是固体能带论的核心 .处于周期晶格中的电子 ,其电荷仍为ｅ ,自旋仍是 1 2 ,但由于晶格周期场的影响 ,质量却不同于自由电子的ｍｅ,而是表现为有效质量ｍ .在超导体中 ,当库珀对被拆散时 ,准电子跨越能隙被激发到费米能级ＥＦ 以上 .准电子的寿命τ可以用光发射谱 (ＰＥＳ)的能量宽度ΔＥ表征 ,因为按照不确定性关系 ,τΔＥ≈ .高温超导铜氧化物的ＰＥＳ谱通常很宽 ,这表明准电子的寿命非常短 .加之 ,这种材料的赝能隙表现出强的各向异性 (在ＣｕＯＣｕ键方向能隙宽 ,在转角 4 5°方向能隙降为零 ) ,对传统准电子概念提出了…     

La0.33Pr0.34Ca0.33MnO3薄膜的应变效应

采用脉冲激光沉积法分别在(100)LaAlO3和(100)SrTiO3基片上生长了La0.33Pr0.34Ca0.33MnO3 薄膜,并通过磁测量和电输运测量对生长在不同基片上的La0.33Pr0.34Ca0.33MnO3薄膜的物性进行了研究.结果表明,基片和薄膜之间的压应力导致La0.33Pr0.34Ca0.33MnO3薄膜容易出现铁磁相,而电荷有序相则被抑制.基片和薄膜之间的张应力则导致La0.33Pr0.34Ca0.33MnO3薄膜容易出现电荷有序相,铁磁相被抑制.导致该现象的可能原因之一是应力使得晶格中Mn-O键角发生改变而引起双交换作用的改变.     

Magnetocaloric effect in La0.67Sr0.33MnO3 manganite above room temperature

The La_0.67Sr_0.33MnO₃ composition prepared by sol-gel synthesis was studied by dc magnetization measurements. A large magnetocaloric effect was inferred over a wide range of temperature around the second-order paramagnetic-ferromagnetic transition. The change of magnetic entropy increases monotonically with increasing magnetic field and reaches the value of 5.15 J/kg K at 370 K for Δμ0H=5 T. The corresponding adiabatic temperature change is 3.3 K. The changes in magnetic entropy and the adiabatic temperature are also significant at moderate magnetic fields. The magnetic field induced change of the specific heat varies with temperature and has maximum variation near the paramagnetic-ferromagnetic transition. The obtained results show that La_0.67Sr_0.33MnO₃ could be considered as a potential candidate for magnetic refrigeration applications above room temperature.     

Phase separation, ferromagnetism and magnetic irreversibility in La1−xSrxMn1−yFeyO3

Magnetic susceptibility, χ(T), is investigated in ceramic La_1−xSr_xMn_1−yFe_yO₃ (LSMFO) samples with x=0.3 and y=0.15−0.25. A ferromagnetic (FM) transition observed in LSMFO is accompanied with an appreciable decrease of the transition temperature with increasing y, which is connected to breaking of the FM double-exchange interaction by doping with Fe. Strong magnetic irreversibility, observed in low (B=10 G) field, gives evidence for frustration of the magnetic state of LSMFO. The FM transition, which is expanded with increasing B, is more pronounced in the samples with y=0.15-0.20 and broadens considerably at y=0.25, where the irreversibility is increased. Well above the transition, χ(T) exhibits a Curie-Weiss asymptotic behavior, yielding very large values of the effective Bohr magneton number per magnetic ion, incompatible with those of Mn or Fe single ions. At y=0.15 and 0.20 a critical behavior of χ⁻¹(T)∼(T/T_C−1)^γ in the region of the FM transition is characterized by influence of two different magnetic systems, a 3D percolative one with γ₌γ_p≈1.8 and T_C=T_C^(p), and a non-percolative 3D Heisenberg spin system, with γ=γ_H≈1.4 and T_C=T_C^(H), where T_C^(p)<T_C^(H). At y=0.25 the percolative contribution to the critical behavior of χ(T) is not observed. The dependence of χ on T and y gives evidence for phase separation, with onset already near the room temperature, leading to generation of nanosize FM particles in the paramagnetic host matrix of LSMFO. The ferromagnetism of LSMFO is attributable to percolation over the system of such particles and generation of large FM clusters, whereas the frustration is governed presumably by a system of smaller weakly-correlated magnetic units, which do not enter the percolative FM clusters.     

Imaging the Magnetic Reversal of Isolated and Organized Molecular‐Based Nanoparticles using Magnetic Force Microscopy

In the race towards miniaturization in nanoelectronics, magnetic nanoparticles (MNPs) have emerged as potential candidates for their integration in ultrahigh‐density recording media. Molecular‐based materials open the possibility to design new tailor‐made MNPs with variable composition and sizes, which benefit from the intrinsic properties of these materials. Before their implementation in real devices is reached, a precise organization on surfaces and a reliable characterization and manipulation of their individual magnetic behavior are required. In this paper, it is demonstrated how molecular‐based MNPs are accurately organized on surfaces and how the magnetic properties of the individual MNPs are detected and tuned by means of low‐temperature magnetic force microscopy (LT‐MFM) with variable magnetic field. The magnetization reversal on isolated and organized MNPs is investigated; in addition, the temperature dependence of their magnetic response is evaluated.     

Magnetic domain observations in Fe-Ga alloys

The domain structure of Fe-Ga bulk alloys is investigated with magnetic force (MFM) and magneto-optic Kerr microscopy. Published domain observations on this class of materials predominantly reveal maze-like domain patterns that indicate out-of-plane magnetization, i.e. out-of-plane anisotropy. Contrary to the belief that this anisotropy is due to the presence of nanoscale heterogeneities [1] and [2] (Bai et al., 2005, 2009), we show that it is due to a damaged surface layer caused by standard mechanical polishing. The surface conditions in Fe-Ga alloys are more sensitive to stress-induced damage than in pure α-Fe. This is explained as being due to increased magnetostriction. We demonstrate that the damaged surface layer can be removed with an additional polishing step using colloidal amorphous silica. On (0 0 1) bulk crystal surfaces, the domain structures, obtained after the removal of the damaged surface layer, reveal in-plane magnetization with sharp and straight 90° and 180° domain walls that are expected in these alloys.     

Thermochromism and Magnetic Phase Transition in an Organic Radical 1,3,5-Trithia-2,4,6-Triazapentalenyl

Thermochromic and magnetic properties of an organic radical, 1,3,5-trithia-2,4,6-triazapentalenyl (TTTA) crystal were investigated through polarized reflection and EPR measurements. TTTA shows a first-order magnetic phase transition with a wide thermal hysteresis loop near room temperature. The high temperature (HT) phase is paramagnetic due to a uniform one-dimensional stacking of the radical molecules, while the low temperature (LT) phase is diamagnetic due to strong dimerization along the stacking direction. The colors of the HT and LT phases are purple and green, respectively, and both phases show a strong dichroism. We also found that part of the LT phase changes to the HT phase by laser light irradiation. The experimental results suggest the possibility of optical control of magnetic bistabitity in TTTA crystal.     

Phase transitions and the magnetocaloric effect in Mn rich Ni–Mn–Ga Heusler alloys

In Mn rich polycrystalline Heusler alloys, Ni₅₀Mn_25+−xGa_25−x, prepared by Arc melting, it is found that the structural/first-order magnetic transition temperature T_m increases as the Mn content increases. The Curie temperature T_c is higher than that of Ni rich alloys (Ni_50+xMn_25−xGa₂₅ ) of the same series, and is less affected by composition x. Magnetic entropy change of |ΔS_M| also increases as Mn content increases, while behaviour of the field dependence of ΔS_M is similar to that of single crystal Ni_52.6Mn_23.1Ga_24.3.     

应用曲面镜成像原理仿真高温超导体磁悬浮力

文章首次探讨了应用曲面镜成像原理 ,对高温超导体悬浮力进行仿真计算 ,并且比较了实验数据与计算结果 ,结果表明了此方法的有效性。文章还讨论了此方法今后的工作方向。