YBCO及其复合体系的超导和磁性研究
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摘要
最近,许多文献报导了小尺度下超导材料具有室温铁磁性,这在传统超导理论框架下几乎是不可以理解的,甚至大量的实验已经证实铁磁性在某些条件下可以与超导性能共存。大量研究工作已经表明铁磁性是低维无机纳米材料的一般属性,而且提出了缺陷诱导的铁磁性机理。但对于低维超导体系的磁性问题的研究还不够成熟,在超导体系中磁性的起源是什么?更关键的是磁性在低温下能否与超导态共存?超导机制是否与磁性存在关联等一系列问题还悬而未决。正是受这些问题的驱动,我们探究低维超导体系中的超导和磁性问题。本文的主要研究内容和结果如下：
1.YBa2Cu3O7-δ的室温铁磁性问题
我们采用柠檬酸盐热分解法制备了超导YBa2Cu3O7-δ体系。通过磁性测试揭示我们制备的样品具有室温铁磁性。低温磁性结果表明样品具有超导电性。低温ESR谱线上出现了象征超导的低场共振峰,进一步确认超导电性的存在。变场ZFC-FC曲线的结果表明超导的转变温度约为92K,并随着磁场的增大,超导转变温度略有下降。在氩气气氛下的低温热处理实验表明,热处理后样品的磁性有了明显的提高,而超导性能有所下降,这说明样品的磁性源于氧缺陷。另外,低温下的FC曲线上翘,暗示低温下铁磁和超导态的共存。
2. Y2BaCuO5的室温铁磁性问题
采用柠檬酸盐热分解法制备了绿色相的Y2BaCuO5颗粒体系。形貌测试表明样品颗粒约为几百纳米,形成了颗粒链。VSM测试表明在不同退火温度下所得样品均具有室温铁磁性,并且磁性随着退火温度的升高逐渐减小。EDS和前驱体磁性结果说明样品不存在任何铁磁污染,样品的铁磁性是本征的。在ZFC-FC曲线上我们观察到了两个磁性相变点的存在,11K为样品的反铁磁相变点,110K处为自旋玻璃转变温度。同样,为了研究样品中磁性来源,我们在氩气气氛中对样品进行了低温热处理,处理后的样品磁性有明显的提高,而且随着氩气流量的增加,磁性进一步提高。这些结果说明Y2BaCuO5颗粒的磁性来源于样品中的氧缺陷。
3. Y2O3内米颗粒的室温铁磁性问题
我们采用甘氨酸盐热分解法制备了Y203纳米颗粒。磁性测试结果表明Y203纳米颗粒具有室温铁磁性。XPS测试结果揭示样品中不存在其它铁磁污染,证明样品的铁磁性是本征属性。在真空中热处理和压片处理后,样品的铁磁性都有所提高,而空气中烧结使铁磁性降低,这些结果暗示样品的磁性与氧缺陷存在联系。高分辨氧峰位XPS谱的多峰拟合结果进一步证实了氧缺陷诱导的铁磁性机理。
4.YBa2Cu3O7-δ-Y2BaCuO5复合体系的室温铁磁性问题
在该部分研究工作中,我们分别采用水热法和柠檬酸盐热分解法制备了YBa2Cu3O7-δ-Y2BaCuO5复合体系。我们先采用水热法制备了YBa2Cu3O7-δ-Y2BaCuO5复合体系。SEM结果揭示样品由微米量级的球状物构成。磁性测试结果表明各复合体系都具有铁磁性。低温磁性和ESR的测试都说明样品具有超导电性,且超导临界温度约为93K。XPS和EDS结果说明样品中不存在磁性污染,样品的铁磁性是本征的。在氩气中的低温热处理实验揭示样品磁性可能与两相界面有关,在高分辨TEM中也清晰可见两相界面的存在。为了进一步揭示两相界面与磁性的关系,我们采用柠檬酸盐热分解法制备了不同两相比例的YBa2Cu3O7-8-Y2BaCuO5复合体系。磁性测试结果表明复合体系的磁性要明显强于对应的单相体系。氩气中的低温退火实验也揭示复合体系磁性机理不同于单相体系,应该与两相界面存在关联。在HRTEM图中也观测到了两相界面的存在。这两部分工作都揭示了(?)Ba2Cu3O7-δ-Y2BaCuO5复合体系中磁性来源于两相界面。
In recent years, room temperature ferromagnetism has been reported in small-scale superconducting materials. Within the BCS theory region, it is difficult to understand these. Furthermore, numerous experiments have revealed ferromagnetism can coexist with superconducting state. A great number of works have also shown that ferromagnetism is an intrinsic nature of nonorganic nanoparticles materials, and a defect-induced-ferromagnetism mechanism is established. But for low-dimensional superconducting systems, the origination of ferromagnetism is open question. A key problem is whether ferromagnetism can coexist with superconductivity at low temperature. It remains controversy that superconductivity is based on magnetism. Inspired by these, we try to explore the mechanism of ferromagnetism in low-dimensional superconductors. The main content and results is as follows:
1. Room temperature ferromagnetism in YBa2Cu3O7-δ particles
In this section, we employed a citrate pyrolysis technique to prepare superconducting YBa2Cu3O7-δ system. Magnetic measurement revealed room temperature ferromagnetism in the samples. Magnetic data at low temperature indicated the existence of superconductivity. Electron spin resonance spectra at low temperatures further confirmed that there is a transition from the normal to the superconducting state. The ZFC-FC curves at various applied magnetic fields showed that the superconducting transition temperature is92K, and it decreases with increasing of the magnetic field. Post-heating treatment at argon gas displayed that the ferromagnetism is enhanced but the superconductivity suppressed, suggesting that the ferromagnetism maybe originate from the oxygen defects. In addition, the upturn of FC curve at low temperature indicated the coexistence of ferromagnetism and superconductivity.
2. Room temperature ferromagnetism in Y2BaCuO5particles
Green phaseY2BaCuO5particles were prepared via a citrate pyrolysis technique. Scanning electron microscopy and transmission electron microscopy observation showed the formation of a particle chain Y2BaCuO5phase with the size of hundreds of nanometers. Magnetic measurements showed that all annealed samples exhibit room temperature ferromagnetism and the saturation magnetization decreases with increasing sintering temperature. The magnetic data of the precursor and the EDS result showed that there is not any magnetic contamination. So the ferromagnetism is intrinsic nature. Two magnetic phase transitions located at about11and110K were revealed at the ZFC-FC curves. The peak at about11K was associated with antiferromagnetic phase transition and a spin-glass transition at about110K was confirmed. Similarly, post-heating treatment under argon atmosphere was also performed on the samples. After heating treatment, the ferromagnetism increased greatly. And it was further enhanced as the flow rate increased. These results suggested that the magnetism of Y2BaCuO5particles is attributed to the surface oxygen defects.
3. Room temperature ferromagnetism in Y2O3nanoparticles
We adopted a glycine-nitrate method to synthesize nanoparticles of Y2O3. Magnetic measurement showed the existence of room temperature ferromagnetism. X-ray photoelectron spectroscopy results indicated that there is not any magnetic contamination and then the observed ferromagnetism is intrinsic. After post-heated at vacuum and pressed, an obvious increase of the ferromagnetism was observed. But for the sample heated in the air, the ferromagnetism decreased. These results suggested that the ferromagnetism is responsible for oxygen defects. By analyzing the multicomponent fitting to the O1s peaks for these samples, the correlation between the ferromagnetism and oxygen defects in Y2O3nanoparticles is established.
4. Room temperature ferromagnetism in YBa2Cu3O7-δ-Y2BaCuO5composites
In this section, we prepared YBa2Cu3O7-δ-Y2BaCuO5composites by a citric acid complexing-hydrothermal synthesis coupled method and a citrate pyrolysis technique. Firstly, the composite system was obtained via the citric acid complexing-hydrothermal synthesis coupled method. Scanning electron microscopy observation showed the formation of microspheres. Magnetic measurement showed room temperature ferromagnetism in these composites. Electron spin resonance spectra and magnetic data at low temperature confirmed the superconducting behavior. Also the superconducting transition located at93K was revealed by ZFC-FC curves. X-ray photoelectron spectroscopy and EDS results indicated that there is not magnetic contamination, suggesting the intrinsic nature of the ferromagnetism. Post-heating treatment at argon gas showed that the observed ferromagnetism should not be attributed to oxygen defects and be associated to the interface at the composites, which was observed in HRTEM. In addition, we employed the citrate pyrolysis technique to prepare the YBa2Cu3O7-δ-Y2BaCuO5composites. Magnetic measurement revealed that the ferromagnetism of the composites is greater than that of their pure phases. Also post-heating treatment at argon gas confirmed that the ferromagnetism results from oxygen defects and is attributed to the interface at the composites. A clear interface in the composite is observed in the HRTEM picture. These work revealed that the ferromagnetism in the YBa2Cu3O7-δ-Y2BaCuO5composites originates from the interface between the two phases.
1.YBa2Cu3O7-δ的室温铁磁性问题
我们采用柠檬酸盐热分解法制备了超导YBa2Cu3O7-δ体系。通过磁性测试揭示我们制备的样品具有室温铁磁性。低温磁性结果表明样品具有超导电性。低温ESR谱线上出现了象征超导的低场共振峰,进一步确认超导电性的存在。变场ZFC-FC曲线的结果表明超导的转变温度约为92K,并随着磁场的增大,超导转变温度略有下降。在氩气气氛下的低温热处理实验表明,热处理后样品的磁性有了明显的提高,而超导性能有所下降,这说明样品的磁性源于氧缺陷。另外,低温下的FC曲线上翘,暗示低温下铁磁和超导态的共存。
2. Y2BaCuO5的室温铁磁性问题
采用柠檬酸盐热分解法制备了绿色相的Y2BaCuO5颗粒体系。形貌测试表明样品颗粒约为几百纳米,形成了颗粒链。VSM测试表明在不同退火温度下所得样品均具有室温铁磁性,并且磁性随着退火温度的升高逐渐减小。EDS和前驱体磁性结果说明样品不存在任何铁磁污染,样品的铁磁性是本征的。在ZFC-FC曲线上我们观察到了两个磁性相变点的存在,11K为样品的反铁磁相变点,110K处为自旋玻璃转变温度。同样,为了研究样品中磁性来源,我们在氩气气氛中对样品进行了低温热处理,处理后的样品磁性有明显的提高,而且随着氩气流量的增加,磁性进一步提高。这些结果说明Y2BaCuO5颗粒的磁性来源于样品中的氧缺陷。
3. Y2O3内米颗粒的室温铁磁性问题
我们采用甘氨酸盐热分解法制备了Y203纳米颗粒。磁性测试结果表明Y203纳米颗粒具有室温铁磁性。XPS测试结果揭示样品中不存在其它铁磁污染,证明样品的铁磁性是本征属性。在真空中热处理和压片处理后,样品的铁磁性都有所提高,而空气中烧结使铁磁性降低,这些结果暗示样品的磁性与氧缺陷存在联系。高分辨氧峰位XPS谱的多峰拟合结果进一步证实了氧缺陷诱导的铁磁性机理。
4.YBa2Cu3O7-δ-Y2BaCuO5复合体系的室温铁磁性问题
在该部分研究工作中,我们分别采用水热法和柠檬酸盐热分解法制备了YBa2Cu3O7-δ-Y2BaCuO5复合体系。我们先采用水热法制备了YBa2Cu3O7-δ-Y2BaCuO5复合体系。SEM结果揭示样品由微米量级的球状物构成。磁性测试结果表明各复合体系都具有铁磁性。低温磁性和ESR的测试都说明样品具有超导电性,且超导临界温度约为93K。XPS和EDS结果说明样品中不存在磁性污染,样品的铁磁性是本征的。在氩气中的低温热处理实验揭示样品磁性可能与两相界面有关,在高分辨TEM中也清晰可见两相界面的存在。为了进一步揭示两相界面与磁性的关系,我们采用柠檬酸盐热分解法制备了不同两相比例的YBa2Cu3O7-8-Y2BaCuO5复合体系。磁性测试结果表明复合体系的磁性要明显强于对应的单相体系。氩气中的低温退火实验也揭示复合体系磁性机理不同于单相体系,应该与两相界面存在关联。在HRTEM图中也观测到了两相界面的存在。这两部分工作都揭示了(?)Ba2Cu3O7-δ-Y2BaCuO5复合体系中磁性来源于两相界面。
In recent years, room temperature ferromagnetism has been reported in small-scale superconducting materials. Within the BCS theory region, it is difficult to understand these. Furthermore, numerous experiments have revealed ferromagnetism can coexist with superconducting state. A great number of works have also shown that ferromagnetism is an intrinsic nature of nonorganic nanoparticles materials, and a defect-induced-ferromagnetism mechanism is established. But for low-dimensional superconducting systems, the origination of ferromagnetism is open question. A key problem is whether ferromagnetism can coexist with superconductivity at low temperature. It remains controversy that superconductivity is based on magnetism. Inspired by these, we try to explore the mechanism of ferromagnetism in low-dimensional superconductors. The main content and results is as follows:
1. Room temperature ferromagnetism in YBa2Cu3O7-δ particles
In this section, we employed a citrate pyrolysis technique to prepare superconducting YBa2Cu3O7-δ system. Magnetic measurement revealed room temperature ferromagnetism in the samples. Magnetic data at low temperature indicated the existence of superconductivity. Electron spin resonance spectra at low temperatures further confirmed that there is a transition from the normal to the superconducting state. The ZFC-FC curves at various applied magnetic fields showed that the superconducting transition temperature is92K, and it decreases with increasing of the magnetic field. Post-heating treatment at argon gas displayed that the ferromagnetism is enhanced but the superconductivity suppressed, suggesting that the ferromagnetism maybe originate from the oxygen defects. In addition, the upturn of FC curve at low temperature indicated the coexistence of ferromagnetism and superconductivity.
2. Room temperature ferromagnetism in Y2BaCuO5particles
Green phaseY2BaCuO5particles were prepared via a citrate pyrolysis technique. Scanning electron microscopy and transmission electron microscopy observation showed the formation of a particle chain Y2BaCuO5phase with the size of hundreds of nanometers. Magnetic measurements showed that all annealed samples exhibit room temperature ferromagnetism and the saturation magnetization decreases with increasing sintering temperature. The magnetic data of the precursor and the EDS result showed that there is not any magnetic contamination. So the ferromagnetism is intrinsic nature. Two magnetic phase transitions located at about11and110K were revealed at the ZFC-FC curves. The peak at about11K was associated with antiferromagnetic phase transition and a spin-glass transition at about110K was confirmed. Similarly, post-heating treatment under argon atmosphere was also performed on the samples. After heating treatment, the ferromagnetism increased greatly. And it was further enhanced as the flow rate increased. These results suggested that the magnetism of Y2BaCuO5particles is attributed to the surface oxygen defects.
3. Room temperature ferromagnetism in Y2O3nanoparticles
We adopted a glycine-nitrate method to synthesize nanoparticles of Y2O3. Magnetic measurement showed the existence of room temperature ferromagnetism. X-ray photoelectron spectroscopy results indicated that there is not any magnetic contamination and then the observed ferromagnetism is intrinsic. After post-heated at vacuum and pressed, an obvious increase of the ferromagnetism was observed. But for the sample heated in the air, the ferromagnetism decreased. These results suggested that the ferromagnetism is responsible for oxygen defects. By analyzing the multicomponent fitting to the O1s peaks for these samples, the correlation between the ferromagnetism and oxygen defects in Y2O3nanoparticles is established.
4. Room temperature ferromagnetism in YBa2Cu3O7-δ-Y2BaCuO5composites
In this section, we prepared YBa2Cu3O7-δ-Y2BaCuO5composites by a citric acid complexing-hydrothermal synthesis coupled method and a citrate pyrolysis technique. Firstly, the composite system was obtained via the citric acid complexing-hydrothermal synthesis coupled method. Scanning electron microscopy observation showed the formation of microspheres. Magnetic measurement showed room temperature ferromagnetism in these composites. Electron spin resonance spectra and magnetic data at low temperature confirmed the superconducting behavior. Also the superconducting transition located at93K was revealed by ZFC-FC curves. X-ray photoelectron spectroscopy and EDS results indicated that there is not magnetic contamination, suggesting the intrinsic nature of the ferromagnetism. Post-heating treatment at argon gas showed that the observed ferromagnetism should not be attributed to oxygen defects and be associated to the interface at the composites, which was observed in HRTEM. In addition, we employed the citrate pyrolysis technique to prepare the YBa2Cu3O7-δ-Y2BaCuO5composites. Magnetic measurement revealed that the ferromagnetism of the composites is greater than that of their pure phases. Also post-heating treatment at argon gas confirmed that the ferromagnetism results from oxygen defects and is attributed to the interface at the composites. A clear interface in the composite is observed in the HRTEM picture. These work revealed that the ferromagnetism in the YBa2Cu3O7-δ-Y2BaCuO5composites originates from the interface between the two phases.
引文
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