钙钛矿氧化物Nd_（1-x）Sr_xCoO_3体系的磁性研究
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摘要
由于钙钛矿钴氧化物丰富的物理现象,近年来Ln1-xAxCoO3(Ln为三价稀土离子,A为二价碱土离子)体系被广泛地研究。本文通过传统的固相反应法合成了Nd1-xSrxCoO3 (0.1≤x≤0.5)系列多晶样品。通过对Nd1-x SrxCoO3系列样品的XRD测试和分析,表明用固相反应法制得的样品呈单相。本文对Nd1-xSrxCoO3(0.1≤x≤0.5)系列样品进行了直流磁化强度、磁滞回线、交流磁化率、弛豫现象以及交换偏置现象的测试,并对实验数据进行了分析。
研究发现由于Sr掺杂,使一部分Co~(3+)转变为Co~(4+),Co~(3+)和Co~(4+)的双交换作用形成了铁磁性团簇。当铁磁颗粒镶嵌在非磁基底上时,一个自旋无序的界面层或表面层形成。因此体系自发地相分离为铁磁团簇、非磁基底和自旋玻璃相。交流磁化率和弛豫现象的测试证实了低掺杂x≤0.2时,样品为自旋玻璃/团簇玻璃态。与锰氧化物不同的是:团簇间的相互作用和自旋玻璃相共同导致了体系的玻璃行为。同时对于x=0.3~0.5这几个样品,随着掺杂量的增加,Co~(4+)的数目增加,双交换作用增强,铁磁团簇的尺寸和数目增加,使体系在高掺杂时表现出了铁磁序特征。磁化曲线在高温区出现了铁磁有序态,又在更低的温度下发生了亚铁磁转变。亚铁磁转变的原因是由于Co~(3+)/Co~(4+)与Nd~(3+)磁矩的反向排列引起的。由于二者之间的耦合较弱,亚铁磁转变仅在低温出现。由磁滞回线的测试,可以看到磁化强度在很高的外场下仍未达到饱和,这与自旋玻璃的模型一致。在自旋玻璃体系中有限的铁磁团簇镶嵌在非磁基底中,磁滞回线不可能达到饱和。由于Nd~(3+)具有轨道角动量和自旋角动量,存在轨道-自旋耦合,提高了磁各向异性,因此样品的矫顽场较大。
因此从总的趋势上看,Nd1-xSrxCoO3体系存在清晰的相分离。本文得出了Nd1-xSrxCoO3体系的磁性相图,低掺杂时为自旋玻璃/团簇玻璃相,高掺杂时表现出铁磁序特征。
在带场冷却条件下,对低掺杂样品进行了磁滞回线的测试,发现相分离体系Nd1-xSrxCoO3中存在交换偏置现象。在低场条件下将样品冷却到低温(远低于冻结温度),这时磁滞回线出现水平方向和竖直方向的移动。与传统的交换偏置现象不同,Nd1-xSrxCoO3体系的交换偏置现象对外场有很强的依赖关系。外场对样品中共存相相对比例的影响是造成这种现象的原因。
Recently, Ln1-xAxCoO3 (Ln=rare earths) system has received attention, due to the rich phenomena of the perovskite cobaltites. In this paper, the rare earth cobaltite Nd1-xSrxCoO3 (0.1≤x≤0.5) samples were prepared using conventional solid state reaction method. The crystal structures of Nd1-xSrxCoO3 have been investigated. Powder X-ray diffraction patterns show that the samples are single. dc magnetization, ac susceptibility, magnetic relaxation and exchange bias of polycrystalline Nd1-xSrxCoO3 are investigated in detail.
After doping Sr, a proportional number of Co~(3+) is converted into Co~(4+) in the compound. The double-exchange interaction between Co~(3+) and Co~(4+) gives rise to ferromagnetic clusters. It has been well known that a spin-disordered interface/ surface layer is usually formed when a FM particle is embedded in a non-FM matrix or the magnetic particle size is small enough. Therefore, it is concluded that the phase-separated state in cobaltites consist of FM clusters and non-FM matrix, and spin glass-like regions. The presence of the typical features of spin/ cluster glass state in x≤0.2 samples was well revealed by ac susceptibility and magnetic relaxation. In contrary to the situation in manganites, spin glass phase and intercluster interactions contribute to the glassy behaviors. For the x=0.3~0.5 samples, the number and size of FM cluster increase sharply with increasing Sr content, which show ferromagnetic ordering. The magnetization show large ferromagnetic-type magnetizations at high temperature, however, the magnetization exhibit a decrease at low temperature, which comes from the contribution of magnetic Nd~(3+) ions. This behavior has been observed in Nd1-x SrxCoO3 samples, which was revealed to be the result of a spontaneously antiferromagnetic order of the Nd~(3+) ions. Due to the weak Nd-Co magnetic coupling, the decrease is only observed for some ferrimagnetic temperature. We note that according to the short-range-order ferromagnetic model the hysteresis loops should not saturate due to the finite number of FM clusters in the non-FM matrix. It should be pointed out that the coercive field for these samples is large, which is driven by a higher magnetic anisotropy induced through the L-S coupling of the Nd~(3+) ion.
In this context, Nd1-xSrxCoO3 has been found to exhibit a strong tendency toward magnetic phase separation. We present the rich magnetic phase diagram of Nd1-xSrxCoO3. At lower Sr doping (x≤0.2) the system exhibits SG/CG behavior, while at higher Sr doping the system presents ferromagnetic transition.
Moreover, the exchange bias effect has been observed for the first time when we measured hysteresis loops of the lower doping samples with FC processes cooling. The horizontal and vertical shift occurs when the samples were cooled down in the external magnetic field through freezing temperature. Furthermore, unlike conventional exchange bias phenomenon, exchange bias in Nd1-xSrxCoO3 is strongly dependent on magnetic field. The influence of applied field on relative proportion of coexisting phases may be responsible for this phenomenon.
研究发现由于Sr掺杂,使一部分Co~(3+)转变为Co~(4+),Co~(3+)和Co~(4+)的双交换作用形成了铁磁性团簇。当铁磁颗粒镶嵌在非磁基底上时,一个自旋无序的界面层或表面层形成。因此体系自发地相分离为铁磁团簇、非磁基底和自旋玻璃相。交流磁化率和弛豫现象的测试证实了低掺杂x≤0.2时,样品为自旋玻璃/团簇玻璃态。与锰氧化物不同的是:团簇间的相互作用和自旋玻璃相共同导致了体系的玻璃行为。同时对于x=0.3~0.5这几个样品,随着掺杂量的增加,Co~(4+)的数目增加,双交换作用增强,铁磁团簇的尺寸和数目增加,使体系在高掺杂时表现出了铁磁序特征。磁化曲线在高温区出现了铁磁有序态,又在更低的温度下发生了亚铁磁转变。亚铁磁转变的原因是由于Co~(3+)/Co~(4+)与Nd~(3+)磁矩的反向排列引起的。由于二者之间的耦合较弱,亚铁磁转变仅在低温出现。由磁滞回线的测试,可以看到磁化强度在很高的外场下仍未达到饱和,这与自旋玻璃的模型一致。在自旋玻璃体系中有限的铁磁团簇镶嵌在非磁基底中,磁滞回线不可能达到饱和。由于Nd~(3+)具有轨道角动量和自旋角动量,存在轨道-自旋耦合,提高了磁各向异性,因此样品的矫顽场较大。
因此从总的趋势上看,Nd1-xSrxCoO3体系存在清晰的相分离。本文得出了Nd1-xSrxCoO3体系的磁性相图,低掺杂时为自旋玻璃/团簇玻璃相,高掺杂时表现出铁磁序特征。
在带场冷却条件下,对低掺杂样品进行了磁滞回线的测试,发现相分离体系Nd1-xSrxCoO3中存在交换偏置现象。在低场条件下将样品冷却到低温(远低于冻结温度),这时磁滞回线出现水平方向和竖直方向的移动。与传统的交换偏置现象不同,Nd1-xSrxCoO3体系的交换偏置现象对外场有很强的依赖关系。外场对样品中共存相相对比例的影响是造成这种现象的原因。
Recently, Ln1-xAxCoO3 (Ln=rare earths) system has received attention, due to the rich phenomena of the perovskite cobaltites. In this paper, the rare earth cobaltite Nd1-xSrxCoO3 (0.1≤x≤0.5) samples were prepared using conventional solid state reaction method. The crystal structures of Nd1-xSrxCoO3 have been investigated. Powder X-ray diffraction patterns show that the samples are single. dc magnetization, ac susceptibility, magnetic relaxation and exchange bias of polycrystalline Nd1-xSrxCoO3 are investigated in detail.
After doping Sr, a proportional number of Co~(3+) is converted into Co~(4+) in the compound. The double-exchange interaction between Co~(3+) and Co~(4+) gives rise to ferromagnetic clusters. It has been well known that a spin-disordered interface/ surface layer is usually formed when a FM particle is embedded in a non-FM matrix or the magnetic particle size is small enough. Therefore, it is concluded that the phase-separated state in cobaltites consist of FM clusters and non-FM matrix, and spin glass-like regions. The presence of the typical features of spin/ cluster glass state in x≤0.2 samples was well revealed by ac susceptibility and magnetic relaxation. In contrary to the situation in manganites, spin glass phase and intercluster interactions contribute to the glassy behaviors. For the x=0.3~0.5 samples, the number and size of FM cluster increase sharply with increasing Sr content, which show ferromagnetic ordering. The magnetization show large ferromagnetic-type magnetizations at high temperature, however, the magnetization exhibit a decrease at low temperature, which comes from the contribution of magnetic Nd~(3+) ions. This behavior has been observed in Nd1-x SrxCoO3 samples, which was revealed to be the result of a spontaneously antiferromagnetic order of the Nd~(3+) ions. Due to the weak Nd-Co magnetic coupling, the decrease is only observed for some ferrimagnetic temperature. We note that according to the short-range-order ferromagnetic model the hysteresis loops should not saturate due to the finite number of FM clusters in the non-FM matrix. It should be pointed out that the coercive field for these samples is large, which is driven by a higher magnetic anisotropy induced through the L-S coupling of the Nd~(3+) ion.
In this context, Nd1-xSrxCoO3 has been found to exhibit a strong tendency toward magnetic phase separation. We present the rich magnetic phase diagram of Nd1-xSrxCoO3. At lower Sr doping (x≤0.2) the system exhibits SG/CG behavior, while at higher Sr doping the system presents ferromagnetic transition.
Moreover, the exchange bias effect has been observed for the first time when we measured hysteresis loops of the lower doping samples with FC processes cooling. The horizontal and vertical shift occurs when the samples were cooled down in the external magnetic field through freezing temperature. Furthermore, unlike conventional exchange bias phenomenon, exchange bias in Nd1-xSrxCoO3 is strongly dependent on magnetic field. The influence of applied field on relative proportion of coexisting phases may be responsible for this phenomenon.
引文
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2 J. A. Mydoson. Spin Glasses: an Experimental Introduction. Taglor and Francis, 1993:29~35
3 J. Wu, C. Leighton. Glassy Ferromagnetism and Magnetic Phase Separation in La1-xSrxCoO3. Phys. Rev. B. 2003, 67:174408-1~174408-10
4 Yan-kun Tang, Yong Sun and Zhao-hua Cheng. Exchange Bias Associated with Phase in the Perovskite Cobaltite La1-xSrxCoO3. Phys. Rev. B. 2006, 73:174419-1~174419-2
5 M. Gruyters. Spin-Glass-Like Behavior in CoO Nanoparticles and the Origin of Exchange Bias in Layered CoO/Ferromagnet Structures. Phys. Rev. Lett. 2005, 95:077204-1~077204-3
6 A. N. Dobrynin, D. N. levev, K. Temst, P. Lievens, J. Margueritat, J. Gonzalo, C. N. Afonso, S. Q. Zhou, A. Vantomme, E. Piscopiello and G. Van Tendeloo. Critical Size for Exchange Bias in Ferromagnetic-Antiferromagnetic Particles. Appl. Phys. Lett. 2005, 87:012501-1~012501-2
7 Lucia Del Bianco, Dino Fiorani, Alberto M. Testa, Ennio Bonetti and Luca Signorini. Field-Cooling Dependence of Exchange Bias in a Granular System of Fe Nanoparticles Embedded in an Fe Oxide Matrix. Phys. Rev. B. 2004, 70: 052401-1~052401-4
8 S. Mukherjee, R. Ranganthan, P. S. Anilkumar and P. A. Joy. Static and Dynamic Respond of Cluster Glass in La0.5Sr0.5CoO3. Phys. Rev. B. 1996, 54:9267~9268
9 D. N. H. Nam, R. Mathieu, P. Nordblod, N. V. Khiem and N. X. Phuc. Spin-Glass Dynamic of La0.95Sr0.05CoO3. Phys. Rev. B. 2000, 62:8989~8991
10 R. V. Helmolt, J. Wecker and B. Holzapfel. Giant Negative Magnetoresistance in Perovskite La3/2Ba1/3MnO3 Ferromagnetic Films. Phys. Rev. Lett. 1993, 71: 2331~2333
11 M. A. Se?arís-Rodríguez, J. B. Goodenough. Magnetic and Transport Properties of the System La1-xSrxCoO3-δ(0
13 C. Martin, A. Maignan, D. Pelloquin, N. Nguyen and B. Raveau. Magneto-resistance in the Oxygen Deficient LnBaCo2O5.4. Appl. Phys. Lett. 1997, 71: 1421~1422
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15 S. R. English, J. Wu and C. Leighton. Thermally Excited Spin Disorder Contri-bution to the Resistivity of LaCoO3. Phys. Rev. B. 2002, 65:220407-1~220407-2
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