过渡金属掺杂氧化物的磁性和交换偏置效应研究
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摘要
过渡金属掺杂氧化物的磁性和交换偏置研究是当前自旋电子学领域的热门课题。其中,对其磁性和交换偏置的调控研究显得尤为重要,它将为研制新一代自旋电子器件奠定物理基础。本论文中选择铁磁金属(Ni, Fe)与非磁性金属(Cu)掺杂反铁磁(CuO,NiO)基体和La0.4Sr0.6TiO3-δ稀磁氧化物,研究金属离子掺杂对铁氧体嵌入反铁磁基体复合材料体系交换偏置效应和La0.4Sr0.6TiO3-δ稀磁氧化物室温磁性的影响,为进一步研究宏观调控这两类材料的磁性和交换偏置效应提供实验数据和理论指导。主要研究内容和结果包括以下几个方面：
1.为了更好的研究掺杂对铁磁／反铁磁系统交换偏置效应的影响,我们用化学共沉淀法和溶胶凝胶法制备了Cu1-xFexO纳米颗粒复合样品。研究不同制备方法对复合颗粒体系的磁性和交换偏置效应的影响,从而为以后的工作探索合适的制备工艺。XRD结果表明,对于x=0.05的样品,化学共沉淀法制备的样品中,没有发现有CuFe2O4相,而溶胶凝胶法制备的样品则发现有CuFe2O4旧和其它杂相的存在。磁性的测量显示,溶胶凝胶法制备的样品中由于产生了CuFe2O4阳并主导了样品中的磁性质,即其磁性强于共沉淀法制备的样品。对于同一种方法制备的样品,不同掺杂浓度对样品的磁性影响也很显著。如用化学沉淀法制备的样品,随着Fe掺杂量的增多,样品的磁化强度增加,x>0.05时,Ms急剧增加,说明这时候样品中出现了第二相CuFe2O4颗粒。两种方法对复合体系交换偏置效应的影响不同,与溶胶凝胶法相比,化学共沉淀法有利于获得相对较大的交换偏置场。
2.在上一工作的基础上,我们选择共沉淀法结合相偏析的原理制备了NiFe2O4/Ni1-xCuxO(0≤X≤0.1)块材复合材料,并对其磁性和交换偏置效应进行了研究。结果显示,Cu掺杂NiO纳米复合材料的交换偏置场和矫顽力随着掺杂量的增加而减小,说明在体系中,Cu掺杂并没有增加材料的交换偏置效应,反而减弱了。这说明非磁性离子Cu尽管掺杂到了NiO基体中,但是没有形成磁畴,并没有增加表面的未补偿自旋数目。同时非磁性离子也造成了反铁磁自旋钉扎能力的减弱,所以交换偏置场随着Cu掺杂量的增多而逐渐减小。
3.采用非均相沉淀法并结合相偏析的原理制备了MFe2O4(M=Cu, Ni)/ Cu1-xNixO(x= 0,0.03,0.06 and 0.1)纳米复合样品。微观结构和磁性测量结果表明,掺入的磁性离子Ni在样品中有两种存在形式,即多数掺入到了CuO的晶格中取代了Cu的位置,少数与Fe反应形成了NiFe2O4颗粒,并主导了样品中的磁行为。研究发现,随着Ni掺杂量的增加,样品的交换偏置场逐渐增加大,表明由于Ni的掺杂在反铁磁CuO形成了畴态,增加了表面的未补偿自旋,同时增加了反铁磁对铁磁自旋的钉扎能力,因此交换偏置场增加。样品x=0.1的交换偏置急剧增加的原因则与样品中出现了较大各向异性能的NiO相有关。同时,矫顽力随着掺杂量的变化趋势也说明了这一点。
4.采用溶胶凝胶法结合后期的热处理制备了金属Ni掺杂氧化物半导体La0.4Sr0.6Ti1-xNixO3-δ(x=0.01，0.02,0.04和0.08)的块材体系。XRD结果表明,当x≥0.04时,样品中出现了NiO和一些Ti—Ni化合物,而x<0.04时,样品中没有发现任何杂相。我们选定固定组分x=0.02,重点研究了掺杂前后样品的室温磁性比较,以及烧结气氛和温度对于样品的磁性的影响。研究结果表明,掺杂Ni的样品在室温下表现出良好的铁磁性,没有掺杂的样品则没有磁性。对于同一温度烧结的样品,在氩气中烧结的样品的室温饱和强度是在空气中烧结样品的近20倍。对于在氩气中烧结的样品,随着烧结温度的升高,样品的磁性先增加而后减弱,说明烧结温度对样品的室温磁性有重要的影响。对样品室温铁磁性的起因,我们运用束缚极化子理论进行了解释,认为是由于样品中的氧空位诱导的室温磁性。
The magnetic properties and exchange bias in oxides doped by thansition metals are hot project of current spintronics. The studies on the regulation of exchange bias effect and magnetism in oxides become particularly important, for which lay the foundations for developing practical applications of spintronic devices. In the thesis, our studies focus on the ferromagnetic metal (Ni, Cu) and non-magnetic metal (Cu) doped in antiferromagnetic base (CuO, NiO) and La0.4Sr0.6TiO3-δdiluted magnetic oxides (DMOs). The effects of transition metal doping on the exchange bias effect in composites system which composed of ferrites embedded in antiferromagnetic base and the room-temperature magnetism in La0.4Sr0.6TiO3-δdiluted magnetic oxides (DMOs) are studied. The studies provide experimental and theoretic basis for the further research of regulation of the room-temperature magnetism and exchange bias in these two types of materials. The main contents of this thesis can be summarized as follows:
1. To better study the effects of the doping on the exchange bias in the FM/AFM system, the Cu1-xFexO nano-particle composites were synthesized by chemical co-precipitation and gol-gel method. The effects of different methods on the magnetism and exchange bias in compound particle system have been studied, and the right preparation processes for the future works have been explored. The XRD results indicate that the CuFe2O4 and other phases are detected in the sample synthesized by sol-gel method, while there is no the second phase in the sample synthesized by chemical co-precipitation method for the composition of x=0.05.The magnetic measurement results imply that the magnetism in samples by sol-gel method is larger than that by co-precipitation method due to the emergence of CuFe2O4 which dominate the magnetism in the sample. For the samples by the same method, the effect of different doping content on magnetism is much notable. For the samples propared by co-precipitation method, with the increase of Fe doping content, the magnetization increases, and when x>0.05, the Ms increase sharply because of the emergence of CuFe2O4 particles. These two methods have different impacts on the exchange bias of the composites. The chemical co-precipitation method is in favor of producing large exchange bias field.
2. On the basis of previous works, a series of NiFe2O4/Ni1-xCuxO (0≤x≤0.1) bulk composites were synthesized by the chemical co-precipitation method to study the magnetic property and exchange bias. In the Cu doping nanocomposites, the exchange bias field and coercivity decrease with the increase of Cu doping implying the Cu doping did not increase exchange bias effect in this system. It indicates that although the non-magnetic Cu ions are doped in NiO matrix, the number of uncompensated spins is not enhanced at the interface due to the absence of domain state. Meanwhile, the pinned action force of the antiferromagnetic material is weakened by the non-magnetic Cu ions, thus the exchange bias field decreases.
3. The MFe2O4(M=Cu, Ni)/Cu1-xNixO(x=0,0.03,0.06 and 0.1)nanocomposites were synthesized by the non-equal precipitation method. The results of microstructure and magnetic measurement show that there are two existed forms for the doping magnetic ions in the samples, most of which are doped in CuO matrix substituting for Cu ions, a few react with Fe to form into NiFe2O4 particles dominating the magnetic actions in the samples. With the increase of Ni doping content the exchange bias field increases gradually, implying that the domain state is formed in AFM, the uncompensated spins are increased, simultaneously the pinned action force of the antiferromagnetic material is enhanced too. The origin of the sharply increase of exchange bias field in the sample with x=0.10 is related to the multiphase such as NiO. Furthermore, the movement trend of coercivity dependent of the doping content also prove the phenomena.
4. A series of La0.4Sr0.6Ti1-xNixO3-δ(x=0.01,0.02,0.04, and 0.08) bulk samples were synthesized by the sol-gel method. The XRD results indicate that there are NiO and Ti-Ni compounds in the samples with x>0.04 in contrast with no any other phase in samples with x<0.04. For the sample with x=0.02,the comparison of room-temperature magnetism in undoped and doped samples was especially studied while the effect of sintering ambience and sintering temperature on the magnetism was also studied. The results show that, the samples doped Ni exhibit good ferromagnetism and the others exhibit no ferromagnetism at room-temperature. The magnetization of the samples sintered in argon ambience is a factor of twenty larger than the samples sintered in air for the same sintering temperature. For the samples sintered in argon, with the increase of sintering temperature the magnetism first increases, then decreases, implying that the sintering temperature has an large impact on the magnetism of the samples. As to the origin of room-temperature ferromagnetism, in the framework of bound magnetic polaron theory, the oxygen defects in the samples induce the ferromagnetism.
1.为了更好的研究掺杂对铁磁／反铁磁系统交换偏置效应的影响,我们用化学共沉淀法和溶胶凝胶法制备了Cu1-xFexO纳米颗粒复合样品。研究不同制备方法对复合颗粒体系的磁性和交换偏置效应的影响,从而为以后的工作探索合适的制备工艺。XRD结果表明,对于x=0.05的样品,化学共沉淀法制备的样品中,没有发现有CuFe2O4相,而溶胶凝胶法制备的样品则发现有CuFe2O4旧和其它杂相的存在。磁性的测量显示,溶胶凝胶法制备的样品中由于产生了CuFe2O4阳并主导了样品中的磁性质,即其磁性强于共沉淀法制备的样品。对于同一种方法制备的样品,不同掺杂浓度对样品的磁性影响也很显著。如用化学沉淀法制备的样品,随着Fe掺杂量的增多,样品的磁化强度增加,x>0.05时,Ms急剧增加,说明这时候样品中出现了第二相CuFe2O4颗粒。两种方法对复合体系交换偏置效应的影响不同,与溶胶凝胶法相比,化学共沉淀法有利于获得相对较大的交换偏置场。
2.在上一工作的基础上,我们选择共沉淀法结合相偏析的原理制备了NiFe2O4/Ni1-xCuxO(0≤X≤0.1)块材复合材料,并对其磁性和交换偏置效应进行了研究。结果显示,Cu掺杂NiO纳米复合材料的交换偏置场和矫顽力随着掺杂量的增加而减小,说明在体系中,Cu掺杂并没有增加材料的交换偏置效应,反而减弱了。这说明非磁性离子Cu尽管掺杂到了NiO基体中,但是没有形成磁畴,并没有增加表面的未补偿自旋数目。同时非磁性离子也造成了反铁磁自旋钉扎能力的减弱,所以交换偏置场随着Cu掺杂量的增多而逐渐减小。
3.采用非均相沉淀法并结合相偏析的原理制备了MFe2O4(M=Cu, Ni)/ Cu1-xNixO(x= 0,0.03,0.06 and 0.1)纳米复合样品。微观结构和磁性测量结果表明,掺入的磁性离子Ni在样品中有两种存在形式,即多数掺入到了CuO的晶格中取代了Cu的位置,少数与Fe反应形成了NiFe2O4颗粒,并主导了样品中的磁行为。研究发现,随着Ni掺杂量的增加,样品的交换偏置场逐渐增加大,表明由于Ni的掺杂在反铁磁CuO形成了畴态,增加了表面的未补偿自旋,同时增加了反铁磁对铁磁自旋的钉扎能力,因此交换偏置场增加。样品x=0.1的交换偏置急剧增加的原因则与样品中出现了较大各向异性能的NiO相有关。同时,矫顽力随着掺杂量的变化趋势也说明了这一点。
4.采用溶胶凝胶法结合后期的热处理制备了金属Ni掺杂氧化物半导体La0.4Sr0.6Ti1-xNixO3-δ(x=0.01，0.02,0.04和0.08)的块材体系。XRD结果表明,当x≥0.04时,样品中出现了NiO和一些Ti—Ni化合物,而x<0.04时,样品中没有发现任何杂相。我们选定固定组分x=0.02,重点研究了掺杂前后样品的室温磁性比较,以及烧结气氛和温度对于样品的磁性的影响。研究结果表明,掺杂Ni的样品在室温下表现出良好的铁磁性,没有掺杂的样品则没有磁性。对于同一温度烧结的样品,在氩气中烧结的样品的室温饱和强度是在空气中烧结样品的近20倍。对于在氩气中烧结的样品,随着烧结温度的升高,样品的磁性先增加而后减弱,说明烧结温度对样品的室温磁性有重要的影响。对样品室温铁磁性的起因,我们运用束缚极化子理论进行了解释,认为是由于样品中的氧空位诱导的室温磁性。
The magnetic properties and exchange bias in oxides doped by thansition metals are hot project of current spintronics. The studies on the regulation of exchange bias effect and magnetism in oxides become particularly important, for which lay the foundations for developing practical applications of spintronic devices. In the thesis, our studies focus on the ferromagnetic metal (Ni, Cu) and non-magnetic metal (Cu) doped in antiferromagnetic base (CuO, NiO) and La0.4Sr0.6TiO3-δdiluted magnetic oxides (DMOs). The effects of transition metal doping on the exchange bias effect in composites system which composed of ferrites embedded in antiferromagnetic base and the room-temperature magnetism in La0.4Sr0.6TiO3-δdiluted magnetic oxides (DMOs) are studied. The studies provide experimental and theoretic basis for the further research of regulation of the room-temperature magnetism and exchange bias in these two types of materials. The main contents of this thesis can be summarized as follows:
1. To better study the effects of the doping on the exchange bias in the FM/AFM system, the Cu1-xFexO nano-particle composites were synthesized by chemical co-precipitation and gol-gel method. The effects of different methods on the magnetism and exchange bias in compound particle system have been studied, and the right preparation processes for the future works have been explored. The XRD results indicate that the CuFe2O4 and other phases are detected in the sample synthesized by sol-gel method, while there is no the second phase in the sample synthesized by chemical co-precipitation method for the composition of x=0.05.The magnetic measurement results imply that the magnetism in samples by sol-gel method is larger than that by co-precipitation method due to the emergence of CuFe2O4 which dominate the magnetism in the sample. For the samples by the same method, the effect of different doping content on magnetism is much notable. For the samples propared by co-precipitation method, with the increase of Fe doping content, the magnetization increases, and when x>0.05, the Ms increase sharply because of the emergence of CuFe2O4 particles. These two methods have different impacts on the exchange bias of the composites. The chemical co-precipitation method is in favor of producing large exchange bias field.
2. On the basis of previous works, a series of NiFe2O4/Ni1-xCuxO (0≤x≤0.1) bulk composites were synthesized by the chemical co-precipitation method to study the magnetic property and exchange bias. In the Cu doping nanocomposites, the exchange bias field and coercivity decrease with the increase of Cu doping implying the Cu doping did not increase exchange bias effect in this system. It indicates that although the non-magnetic Cu ions are doped in NiO matrix, the number of uncompensated spins is not enhanced at the interface due to the absence of domain state. Meanwhile, the pinned action force of the antiferromagnetic material is weakened by the non-magnetic Cu ions, thus the exchange bias field decreases.
3. The MFe2O4(M=Cu, Ni)/Cu1-xNixO(x=0,0.03,0.06 and 0.1)nanocomposites were synthesized by the non-equal precipitation method. The results of microstructure and magnetic measurement show that there are two existed forms for the doping magnetic ions in the samples, most of which are doped in CuO matrix substituting for Cu ions, a few react with Fe to form into NiFe2O4 particles dominating the magnetic actions in the samples. With the increase of Ni doping content the exchange bias field increases gradually, implying that the domain state is formed in AFM, the uncompensated spins are increased, simultaneously the pinned action force of the antiferromagnetic material is enhanced too. The origin of the sharply increase of exchange bias field in the sample with x=0.10 is related to the multiphase such as NiO. Furthermore, the movement trend of coercivity dependent of the doping content also prove the phenomena.
4. A series of La0.4Sr0.6Ti1-xNixO3-δ(x=0.01,0.02,0.04, and 0.08) bulk samples were synthesized by the sol-gel method. The XRD results indicate that there are NiO and Ti-Ni compounds in the samples with x>0.04 in contrast with no any other phase in samples with x<0.04. For the sample with x=0.02,the comparison of room-temperature magnetism in undoped and doped samples was especially studied while the effect of sintering ambience and sintering temperature on the magnetism was also studied. The results show that, the samples doped Ni exhibit good ferromagnetism and the others exhibit no ferromagnetism at room-temperature. The magnetization of the samples sintered in argon ambience is a factor of twenty larger than the samples sintered in air for the same sintering temperature. For the samples sintered in argon, with the increase of sintering temperature the magnetism first increases, then decreases, implying that the sintering temperature has an large impact on the magnetism of the samples. As to the origin of room-temperature ferromagnetism, in the framework of bound magnetic polaron theory, the oxygen defects in the samples induce the ferromagnetism.
引文
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