摘要
强关联钙钛矿锰氧化物体系的研究表明电荷、自旋、轨道和晶格自由度之间存在相互耦合和竞争,并由此诱发产生了绝缘-金属转变、有序化、相分离和变磁相变等一系列新奇物理现象。由此产生很多对上述现象进行解释的理论和模型进一步推动该领域的发展,使得强关联钙钛矿锰氧化物体系的研究成为二十一世纪科学技术特别是凝聚态物理强关联电子系统的主要研究热点之一。但目前仍有很多磁现象产生的物理根源及它们之间的关系没有完全研究清楚。
本文以具有典型低温相分离特征的Pr_(0.75)Na_(0.25)Mn_(1-x)Fe_xO_3系列样品为具体研究对象,在合成系列单相样品的基础上,通过X-ray衍射(XRD)、X-ray荧光光谱分析(XRF)、扫描电镜(SEM)、红外分析(IR)等多种技术确定了系列样品的微观结构及阳离子比例,用物理性质测量系统(PPMS)对系列样品的M-T、M-H、ρ-T和ρ-H曲线,交流磁化率及弛豫效应等磁电性质进行了系统研究,分析了电荷有序、轨道有序与Fe掺杂量之间的关系,系列样品的低温相分离特征及其与台阶状变磁相变的关系,磁场诱导产生的变磁相变与可能的自旋/轨道量子转变的物理本质。
采用溶胶凝胶法首次合成了单相的多晶Pr_(0.75)Na_(0.25)Mn_(1-x)Fe_xO_3系列样品,结合XRF分析,证明了sol-gel法确实避免了Na元素挥发和锰氧化物析出,这是由于sol-gel法合成样品较固相反应法所需的温度低、时间短,这是本论文的创新之一,为制备含易挥发元素的氧化物陶瓷样品提供了新的思路。SEM照片表明sol-gel法制得的样品致密度较固相反应法制得的样品致密度稍差一些;IR分析表明MnO6八面体的Jahn-Teller畸变随Fe含量的增加而减弱。
sol-gel法制得的样品Pr_(0.75)Na_(0.25)MnO_3的电荷有序(CO)峰的相对强度要明显强于用固相反应法制得的相同名义配比的样品(该样品实际Pr/Na=0.80:0.20)。同时,前者的电阻率也明显比后者高。这可以归于两个原因:(1) sol-gel法制得的样品Pr_(0.75)Na_(0.25)MnO_3中Mn4+/Mn3+～1:1对应较强的电荷有序反铁磁(COOAF)相,从而使与之竞争的铁磁金属态相对较弱。(2) sol-gel法制得的样品晶粒间不致密,使晶界电阻较大。
研究了Fe掺杂对Pr_(0.75)Na_(0.25)MnO_3中CO和磁有序的影响。Fe掺杂抑制了长程CO,并使CO温度向低温移动。在x≤0.05时铁磁成分随Fe含量增加而增加,电阻率随Fe含量增加而下降,然而二者随Fe含量进一步增加均朝相反的趋势变化。这种复杂的现象可以通过几何效应(Fe3+的几何尺寸大小)和电子结构的影响两个方面的竞争来解释。在Pr_(0.75)Na_(0.25)Mn_(1-x)Fe_xO_3中几何效应在低掺杂范围内起主导作用,Fe3+对双交换的削弱作用在高掺杂范围内起主导作用。这一观点为理解强关联体系中的相互作用关系提供了新的机制。x≤0.05样品在2 K出现了台阶状变磁相变现象,这是在相分离框架下由COOAF相向铁磁(FM)相转变引起的。由于Fe掺杂抑制了COOAF相,x≥0.1的样品在2 K下并没有出现台阶状变磁相变。为了考察2 K下无变磁相变现象样品的基态,详细研究了Pr0.75Na0.25Mn0.9Fe0.1O3的基态。分析表明Pr0.75Na0.25Mn0.9Fe0.1O3基态是铁磁团簇和反铁磁基质共存的相分离基态。铁磁团簇体积随外磁场的增加而增大,但是直到5 T场下铁磁团簇仍然没有完全占据该样品。这一结果有力的支持了当今锰氧化物强关联物理中占主导地位的相分离机制。另外,即便是在5 T外场下65 K以下Pr0.75Na0.25Mn0.9Fe0.1O3仍然是绝缘态,65 K以上电阻率表现了磁性半导体的特征。研究了多晶样品Pr0.75Na0.25Mn1-xFexO3(0≤x≤0.30)中2 K下台阶状变磁相变现象。数据分析表明该系列样品中存在三种相。即COOAF、FM、AFII (与COOAF不同类型的一种反铁磁相)。AFII的反铁磁交换作用比COOAF强,首次提出了AFII所占的百分比是临界场Hc大小的决定因素的观点,并用自旋势垒模型解释了多晶锰氧化物中陡峭的台阶状的变磁相变(临界场宽度< 0.01 T)及相关物理现象。自旋阻塞温度以下的台阶状变磁相变和自旋阻塞温度以上的渐变型变磁相变都起源于COOAF相。这两种变磁相变的锐度及临界场大小之间的显著差异主要是由自旋在不同温度下的属性差异引起的。
There exist strong couplings and competitions among the spin, charge, orbital, and lattice degrees of freedom, which can induce a wide variety of newly novel phenomena such as insulator-metal transition, orderings and phase separation and metamagnetic phase transition etc by studying of the strongly correlated perovskite manganites. Therefore, many theories and models have been brought forward in order to explain those phenomena, which will arouse the further development of this research field in the future. This probably makes strongly correlated manganites one of the hotspots in the field of condensed matter physics in the 21st century. While the origins of many magnetic phenomena and the relatation among them are still open questions.
In this dissertation, based on the preparation of single-phase polycrystalline sample series by a sol-gel technique, the phase-separated Pr0.75Na0.25Mn1-xFexO3 (0≤x≤0.30) at low temperatures are investigated in magnetic fields. XRD (X-ray diffraction), XRF (X-ray fluorescence analysis), SEM (scanning electron microscope) and IR (infrared) spectra were used for microstructure and cationic composition analysises. A PPMS (physics properties measurement system) was used for M-T curves, M-H curves,ρ-T curves,ρ-H curves, ac susceptibility and relaxation effect measurements. The interrelation between charge/orbit ordering and Fe doping was analyzed. At the same time, the phase-separated ground state, the interrelation between the ground state and step-like metamagnetic transition, as well as the mechanisms of magnetic induced metamagnetic transition and the possible quantum spin transition were studied.
The single-phase polycrystalline perovskites Pr0.75Na0.25Mn1-xFexO3 (0≤x≤0.30) were prepared for the first time by a sol-gel technique. The analysises of XRF and XRD proved that the volatilization of Na and the precipitation of manganese oxide are avoided by sol-gel technique (which offers shorter sintering time and lower temperature than solid-state reaction method). This is a novelty of this dissertation, which proposed a novel idea to prepare oxide ceramics containing volatile element. The scanning electron microscope (SEM) results reveal that the sol-gel-derived compounds are not as compact as ceramic samples. The analysis of IR shows that the Jahn-Teller distortion of MnO6 octahedron decreases with increasing Fe doping.
The relative intensity of the CO peak of Pr_(0.75)Na_(0.25)MnO_3 synthesized by sol-gel technique is much stronger than that of the sample with the same nominal structural formula synthesized by solid-state method (the real Pr/Na=0.80:0.20). Simultaneously, the resistivity of the former is much larger than that of the latter, which can be ascribed to two factors: First, Mn4+/Mn3+ ~1:1 in the former, which induces stronger charge-orbit ordering antiferromagnetic (COOAF) phase, and weakens the ferromagnetic (FM) phase competed with COOAF. Second, grain boundary effect caused by the granular structure of the sample synthesized by sol-gel technique offers much insulating grain boundary resistance.
The effects of Fe doping at the Mn site upon the charge and the magnetic order in Pr_(0.75)Na_(0.25)MnO_3 have been investigated. The low temperatures long-range charge order (CO) is suppressed and the CO temperature shift to lower temperatures with increasing Fe doping. The ferromagnetic fraction increases and resistivity decreases with increasing Fe doping for x≤0.05. However, both of them change with contrary trend. This complex behavior is interpreted in terms of the competition between geometric effect (size of the dopant) and the effect of electronic configuration of the dopant. The geometric effect is dominant for low Fe doping, and the weakening of the DE by Fe doping is dominant for high Fe doping in Pr_(0.75)Na_(0.25)Mn_(1-x)Fe_xO_3. This novel competitive mechanism explains the interaction in strongly correlated manganites. The step-like metamagnetic transition at 2 K for the samples with x≤5% is observed and the origin is ascribed to the phase transition from COOAF phase to FM phase in a phase separation scenario. And no step-like metamagnetic transition at 2 K is observed for the samples with x≥0.1 because COOAF phase is suppressed by further Fe doping.
The ground state of Pr0.75Na0.25Mn0.9Fe0.1O3 is studied detailedly in order to investigate the ground state of the samples without step-like metamagnetic transition at 2 K. The analysis of data shows that there exist correlated ferromagnetic clusters embedded in an antiferromagnetic insulative matrix. The growth of ferromagnetic clusters with increasing magnetic field was examined. The results indicate that the ferromagnetic clusters are far from filling up the whole sample, even up to H = 5 T. This result contributes to directly support the phase separation scenario that is one of the leading theories to explain the physics of manganites. The sample maintains an insulating state at temperatures below 65 K even for H = 5 T and the transport property is natural for magnetic semiconductors at temperatures above 65 K.
A detailed study of step-like metamagnetic transitions at 2 K in polycrystalline Pr0.75Na0.25Mn1-xFexO3 (0≤x≤0.30) is presented. The data shows that there are three phases in this sample series: COOAF、FM、AFII (antiferromagnet II). The fraction of AFII with larger antiferromagnetic exchange interaction than COOAF is proved to be the effective factor of critical fields (Hc) for the first time. A model of anisotropy barrier has been proposed to explain the extremely sharp step-like transitions (width < 0.01 T) in polycrystalline manganites and related phenomenon. Both step-like metamagnetic transition at 2 K (below the spin blocking temperature TB) and gradually changed metamagnetic transition at temperatures above TB originate from COOAF phase. And the different characteristic of the spins at different temperatures is responsible for the difference between them.
引文
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