多铁垒隧道结的自旋输运
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摘要
本论文重点研究了多铁垒隧道结中的自旋输运性质,包括自旋过滤效应、隧穿磁电阻效应和隧穿电致电阻效应。我们详细分析了势垒的多铁性,包括铁磁性和铁电性,以及势垒的逆压电性对隧道结自旋输运的作用机制。在此基础上,建立了相应的理论模型,详细研究了势垒的多铁性和逆压电性对隧道结自旋输运性质可能产生的作用和影响,探索了增强机制和相应的调控手段。
我们的具体工作主要有以下几方面:
1.利用单相多铁垒隧道结提高半导体的自旋注入效率并实现多重调控
我们分析了铁磁垒和铁电垒隧道结中产生自旋过滤效应的机制,发现它们分别属于两种不同的类型:势垒高度的自旋相关性和势垒厚度的自旋相关性。由于铁磁-铁电型单相多铁垒同时具有铁磁性和铁电性,因此,如果选用合适的电极与多铁垒相结合,就有可能在一个隧道结中同时实现这两种自旋过滤机制。基于这样的考虑,我们建立了一种新的多铁垒隧道结模型,即DMS-MF-NS模型,其中DMS为稀磁半导体电极,MF为铁磁-铁电型单相多铁垒,NS为非磁半导体电极。该模型的关键是DMS电极与多铁垒的结合,从而实现了在一个隧道结中同时存在两种自旋过滤机制。当这两种机制同时发生作用时,就可以使自旋过滤效应得到增强,从而可以利用这一模型提高对半导体的自旋注入效率。同时,通过外场对多铁垒中磁矩和电极化取向的控制,可以实现注入自旋态的四重控制。
2.利用铁磁-铁电型两相复合多铁垒隧道结增强自旋输运效应
现有的天然单相多铁材料很难同时具备较强的铁磁性和铁电性,而且,它们的居里温度都较低,室温下多铁性很弱甚至消失,从而使多铁垒隧道结的应用前景受到很大限制。这就促使人们通过其他途径来寻找更强的多铁材料,如通过技术设计将一层铁磁材料和一层铁电材料进行复合,构成铁磁-铁电型两相复合多铁材料。对于这种以铁磁-铁电型复合多铁材料为势垒的多铁垒隧道结中的自旋输运性质,理论的研究还很缺乏。为此,我们建立了一种复合多铁垒隧道结模型,即M1-FE-FM-M2模型,其中FE表示一层铁电垒,FM表示一层铁磁垒,M1和M2分别为两个金属性电极。通过这一模型,我们研究了复合多铁垒隧道结中自旋输运的主要特性。结果表明,由强铁磁层与强铁电层进行复合而构建的多铁性较强的复合多铁垒隧道结中,存在着比单相多铁垒隧道结中强得多的自旋过滤效应、隧穿磁电阻效应和隧穿电致电阻效应,并且在这种隧道结中,对自旋输运效应的调控手段也可以更加多样化。
3.外加偏压下势垒的逆压电性对隧道结自旋输运性质的作用与影响
研究表明,不仅大部分铁电材料具有逆压电性,而且许多多铁材料也具有明显的逆压电性。因此,在外加偏压的情况下,多铁垒中的逆压电性将不可避免地对隧道结中的电子隧穿行为产生影响。目前,关于铁电垒隧道结中的逆压电性对电子隧穿特性的影响已经有人从理论上进行了较为详细的研究,但对于多铁垒隧道结的相关研究仍是空白。为此,我们分别建立了单相多铁垒隧道结和复合多铁垒隧道结模型,对其中势垒的逆压电性可能对自旋输运产生的作用和影响进行了详细研究和分析。结果表明,势垒的逆压电性将对自旋过滤效应、隧穿磁电阻效应和隧穿电致电阻效应产生明显的增强或削弱作用,具体取决于势垒中的电极化相对于外电场的取向。并且,当势垒的逆压电性很强时,隧穿磁电阻效应和隧穿电致电阻效应均会出现不同于传统隧道结的偏压特性,而这一偏压特性对于自旋电子学器件的设计和应用是十分有利的。
The purpose of this work is to study the spin transport properties, including spin filtering (SF), tunneling magnetoresistance (TMR) and tunneling electroresistance (TER) effects, in the tunnel junctions with multiferroic barriers. The mechanisms responsible for the effects of multiferroism, including ferromagnetism and ferroelectricity, and converse piezoelectricity in the barrier on the spin transport properties have been analyzed in detail. Several theoretical models of tunnel junctions have been set up to study the spin transport properties under the influence of multiferroism and converse piezoelectricity in the barrier. The mechanisms and methods to enhance, control and adjust these effects have also been investigated.
Our main work and results are listed bellow:
1. Multiple switching of spin polarization injected into a semiconductor by a multiferroic tunnel junction
A method for switching between multiple spin polarization of the electric current injected into a semiconductor is proposed, based on injecting spins from a diluted magnetic semiconductor through a multiferroic tunnel barrier. We show that it is important to combine a diluted magnetic semiconductor electrode with a multiferroic barrier to realize the coexistence of two spin filtering mechanisms and multiple switching of spin polarization on a wide range. The reversal of either electric polarization or magnetization in a multiferroic barrier results in a sizable change in the spin polarization of the injected current both in magnitude and in sign, thereby providing a four-state electrical control of spin polarization. The electroresistance and electromagnetoresistance effects can be also realized in this structure. Our investigations may stimulate experimental studies of the multiferroic tunnel junctions and offer a new route towards spin injection into the semiconductors.
2. Tunnel junctions with a ferroelectric-ferromagnetic composite barrier A theoretical model for a tunnel junction with a ferroelectric-ferromagnetic (insulator) composite barrier separating two metallic electrodes is proposed. By using free electron direct quantum tunneling method and transfer matrix formalism, taking into account screening of polarization charges in metallic electrodes and dielectric response in ferromagnetic barrier, we investigate the SF, TMR and TER effects in the junction. It is shown that the large SF effect, hence TMR and TER effects, can be achieved. Eight resistive states in the junction can also be realized by the reversal of electric polarization in the ferroelectric layer and magnetization either in the ferromagnetic layer or in the electrodes.
3. Converse piezoelectric effect on the electron tunneling across the multiferroic junctions
Converse piezoelectric effect on the electron tunneling across the tunnel junctions with a single-phase or ferroelectric-ferromagnetic two-phase composite multiferroic barrier is investigated theoretically. It is found that the SF, TMR and TER are enhanced or reduced due to the presence of the strain caused by the converse piezoelectricity in the barrier when the electric polarization is oriented antiparallel or parallel to the applied field. The TMR and TER can even increase with the increasing applied voltage when the converse piezoelectric effect is very strong in the barrier, which is totally different from the voltage dependence in the junction with the barrier with nonpiezoelectricity. The investigations offer a new route towards controllable spin transport.
我们的具体工作主要有以下几方面:
1.利用单相多铁垒隧道结提高半导体的自旋注入效率并实现多重调控
我们分析了铁磁垒和铁电垒隧道结中产生自旋过滤效应的机制,发现它们分别属于两种不同的类型:势垒高度的自旋相关性和势垒厚度的自旋相关性。由于铁磁-铁电型单相多铁垒同时具有铁磁性和铁电性,因此,如果选用合适的电极与多铁垒相结合,就有可能在一个隧道结中同时实现这两种自旋过滤机制。基于这样的考虑,我们建立了一种新的多铁垒隧道结模型,即DMS-MF-NS模型,其中DMS为稀磁半导体电极,MF为铁磁-铁电型单相多铁垒,NS为非磁半导体电极。该模型的关键是DMS电极与多铁垒的结合,从而实现了在一个隧道结中同时存在两种自旋过滤机制。当这两种机制同时发生作用时,就可以使自旋过滤效应得到增强,从而可以利用这一模型提高对半导体的自旋注入效率。同时,通过外场对多铁垒中磁矩和电极化取向的控制,可以实现注入自旋态的四重控制。
2.利用铁磁-铁电型两相复合多铁垒隧道结增强自旋输运效应
现有的天然单相多铁材料很难同时具备较强的铁磁性和铁电性,而且,它们的居里温度都较低,室温下多铁性很弱甚至消失,从而使多铁垒隧道结的应用前景受到很大限制。这就促使人们通过其他途径来寻找更强的多铁材料,如通过技术设计将一层铁磁材料和一层铁电材料进行复合,构成铁磁-铁电型两相复合多铁材料。对于这种以铁磁-铁电型复合多铁材料为势垒的多铁垒隧道结中的自旋输运性质,理论的研究还很缺乏。为此,我们建立了一种复合多铁垒隧道结模型,即M1-FE-FM-M2模型,其中FE表示一层铁电垒,FM表示一层铁磁垒,M1和M2分别为两个金属性电极。通过这一模型,我们研究了复合多铁垒隧道结中自旋输运的主要特性。结果表明,由强铁磁层与强铁电层进行复合而构建的多铁性较强的复合多铁垒隧道结中,存在着比单相多铁垒隧道结中强得多的自旋过滤效应、隧穿磁电阻效应和隧穿电致电阻效应,并且在这种隧道结中,对自旋输运效应的调控手段也可以更加多样化。
3.外加偏压下势垒的逆压电性对隧道结自旋输运性质的作用与影响
研究表明,不仅大部分铁电材料具有逆压电性,而且许多多铁材料也具有明显的逆压电性。因此,在外加偏压的情况下,多铁垒中的逆压电性将不可避免地对隧道结中的电子隧穿行为产生影响。目前,关于铁电垒隧道结中的逆压电性对电子隧穿特性的影响已经有人从理论上进行了较为详细的研究,但对于多铁垒隧道结的相关研究仍是空白。为此,我们分别建立了单相多铁垒隧道结和复合多铁垒隧道结模型,对其中势垒的逆压电性可能对自旋输运产生的作用和影响进行了详细研究和分析。结果表明,势垒的逆压电性将对自旋过滤效应、隧穿磁电阻效应和隧穿电致电阻效应产生明显的增强或削弱作用,具体取决于势垒中的电极化相对于外电场的取向。并且,当势垒的逆压电性很强时,隧穿磁电阻效应和隧穿电致电阻效应均会出现不同于传统隧道结的偏压特性,而这一偏压特性对于自旋电子学器件的设计和应用是十分有利的。
The purpose of this work is to study the spin transport properties, including spin filtering (SF), tunneling magnetoresistance (TMR) and tunneling electroresistance (TER) effects, in the tunnel junctions with multiferroic barriers. The mechanisms responsible for the effects of multiferroism, including ferromagnetism and ferroelectricity, and converse piezoelectricity in the barrier on the spin transport properties have been analyzed in detail. Several theoretical models of tunnel junctions have been set up to study the spin transport properties under the influence of multiferroism and converse piezoelectricity in the barrier. The mechanisms and methods to enhance, control and adjust these effects have also been investigated.
Our main work and results are listed bellow:
1. Multiple switching of spin polarization injected into a semiconductor by a multiferroic tunnel junction
A method for switching between multiple spin polarization of the electric current injected into a semiconductor is proposed, based on injecting spins from a diluted magnetic semiconductor through a multiferroic tunnel barrier. We show that it is important to combine a diluted magnetic semiconductor electrode with a multiferroic barrier to realize the coexistence of two spin filtering mechanisms and multiple switching of spin polarization on a wide range. The reversal of either electric polarization or magnetization in a multiferroic barrier results in a sizable change in the spin polarization of the injected current both in magnitude and in sign, thereby providing a four-state electrical control of spin polarization. The electroresistance and electromagnetoresistance effects can be also realized in this structure. Our investigations may stimulate experimental studies of the multiferroic tunnel junctions and offer a new route towards spin injection into the semiconductors.
2. Tunnel junctions with a ferroelectric-ferromagnetic composite barrier A theoretical model for a tunnel junction with a ferroelectric-ferromagnetic (insulator) composite barrier separating two metallic electrodes is proposed. By using free electron direct quantum tunneling method and transfer matrix formalism, taking into account screening of polarization charges in metallic electrodes and dielectric response in ferromagnetic barrier, we investigate the SF, TMR and TER effects in the junction. It is shown that the large SF effect, hence TMR and TER effects, can be achieved. Eight resistive states in the junction can also be realized by the reversal of electric polarization in the ferroelectric layer and magnetization either in the ferromagnetic layer or in the electrodes.
3. Converse piezoelectric effect on the electron tunneling across the multiferroic junctions
Converse piezoelectric effect on the electron tunneling across the tunnel junctions with a single-phase or ferroelectric-ferromagnetic two-phase composite multiferroic barrier is investigated theoretically. It is found that the SF, TMR and TER are enhanced or reduced due to the presence of the strain caused by the converse piezoelectricity in the barrier when the electric polarization is oriented antiparallel or parallel to the applied field. The TMR and TER can even increase with the increasing applied voltage when the converse piezoelectric effect is very strong in the barrier, which is totally different from the voltage dependence in the junction with the barrier with nonpiezoelectricity. The investigations offer a new route towards controllable spin transport.
引文
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[1] N. A. Spaldin and M. Fiebig, Science, 2005, 309: 391.
[2] Y. Tsymbal and H. Kohlshedt, Science, 2006, 313: 181.
[3] H. Kohlstedt, N. A. Pertsev, J. Rodrguez Contreras, and R. Waser, Phys. Rev. B, 2005, 72: 125341.
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[1] N. A. Hill, J. Phys. Chem. B, 2000, 104: 6694.
[2] N. A. Spaldin and M. Fiebig, Science, 2005, 309: 391.
[3] J. Wang and Z. Y. Li, Appl. Phys. Lett., 2008, 93: 112501.
[4] M. Y. Zhuravlev, R. F. Sabirianov, S. S. Jaswal, and E. Y. Tsymbal, Phys. Rev. Lett., 2005, 94: 246802.
[5] M. Ye. Zhuravlev, S. S. Jaswal, E. Y. Tsymbal, and R. F. Sabirianov, Appl. Phys. Lett., 2005, 87: 222114.
[6] N.W. Ashcroft and N. D. Mermin, Solid State Physics, (Saunders College Publishing, New York, 1976).
[7] C. B. Duke, Tunneling in Solids (Academic, New York, 1969).
[8] J. Rodriguez Contreras, H. Kohlstedt, U. Poppe, R. Waser, C. Buchal, and N. A. Pertsev, Appl. Phys. Lett., 2003, 83: 4595.
[9] G. A. Samara, in Solid State Physics, edited by H. Ehrenreich and F. Spaepen (Academic, San Diego, 2001), 56: 239.
[10] M. Gajek, M. Bibes, A. Barthelemy, K. Bouzehouane, S. Fusil, M. Varela, J. Fontcuberta, and A. Fert, Phys. Rev. B, 2005, 72: 020406(R).
[11] S. Ju, T.-Y. Cai, G.-Y. Guo, and Z.-Y. Li, Phys. Rev. B, 2007, 75: 064419.
[1] N. A. Spaldin and M. Fiebig, Science, 2005, 309: 391.
[2] Y. Tsymbal and H. Kohlshedt, Science, 2006, 313: 181.
[3] H. Kohlstedt, N. A. Pertsev, J. Rodrguez Contreras, and R. Waser, Phys. Rev. B, 2005, 72: 125341.
[4] J. Wang, J. B. Neaton, H. Zheng, V. Nagarajan, S. B. Ogale, B. Liu, D. Viehland, V. Vaithyanathan, D. G. Schlom, U. V. Waghmare, N. A. Spaldin, K. M. Rabe, M. Wuttig, R. Ramesh, Science, 2003, 299: 1719.
[5] S. Y. Yang, F. Zavaliche, L. Mohaddes-Ardabili, V. Vaithyanathan, D. G. Schlom, Y. J. Lee, Y. H. Chu, M. P. Cruz, Q. Zhan, T. Zhao, and R. Ramesh, Appl. Phys. Lett., 2005, 87: 102903
[6] Gary W. Pabst, Lane W. Martin, Ying-Hao Chu, and R. Ramesh. Appl. Phys. Lett., 2007, 90: 072902.
[7] A. Z. Simos, A. H. M. Gonzalez, L. S. Cavalcantea, C. S. Riccardi, E. Longo, and J. A. Varela, J. Appl. Phys., 2007, 101: 074108.
[8] R. Y. Zheng, X. S. Gao, Z. H. Zhou, and J. Wang, J. Appl. Phys., 2007, 101: 054104.
[9] M. Gajek, M. Bibes, S. Fusil, K. Bouzehouane, J. Fontcuberta, A. Barthelemy, and A. Fert, Nat. Mater., 2007, 6: 296.
[10] M. Ye. Zhuravlev, R. F. Sabirianov, S. S. Jaswal, and E. Y. Tsymbal, Phys. Rev. Lett., 2005, 94: 246802.
[11] M. Ye. Zhuravlev, S. S. Jaswal, E. Y. Tsymbal, and R. F. Sabirianov, Appl. Phys. Lett., 2005, 87: 222114.
[12] M. Abramowitz and I. A. Stegun, Handbook of Mathematical Functions (Dover, New York, 1965).
[13] K. H. Gundlach, Solid-State Electron., 1966, 9: 949.
[14] J. C. Slonczewski, Phys. Rev. B, 1989, 39: 6995.
[15] C. B. Duke, Tunneling in Solids (Academic, New York, 1969).
[16] E. Burstein, Tunneling Phenomena in Solids (Plenum, New York, 1969).
[17] Claude Ederer and Nicola A. Spaldin, Phys. Rev. Lett., 2005, 95: 257601.
[1] N. A. Spaldin and M. Fiebig, Science, 2005, 309: 391.
[2] J. Wang and Z. Y. Li, Appl. Phys. Lett., 2008, 93: 112501.
[3] M. Y. Zhuravlev, R. F. Sabirianov, S. S. Jaswal, and E. Y. Tsymbal, Phys. Rev. Lett., 2005, 94: 246802.
[4] M. Ye. Zhuravlev, S. S. Jaswal, E. Y. Tsymbal, and R. F. Sabirianov, Appl. Phys. Lett., 2005, 87: 222114.
[5] H. Kohlstedt, N. A. Pertsev, J. Rodrguez Contreras, and R. Waser, Phys. Rev. B, 2005, 72: 125341.
[6] A. Z. Simos, A. H. M. Gonzalez, L. S. Cavalcantea, C. S. Riccardi, E. Longo, and J. A. Varela, J. Appl. Phys., 2007, 101: 074108.
[7] R. Tsu and L. Esaki, Appl. Phys. Lett., 1973, 22: 562.
[8] J. Rodriguez Contreras, H. Kohlstedt, U. Poppe, R. Waser, C. Buchal, and N. A. Pertsev, Appl. Phys. Lett., 2003, 83: 4595.
[9] G. A. Samara, in Solid State Physics, edited by H. Ehrenreich and F. Spaepen (Academic, San Diego, 2001), 56: 239.
[10] M. Gajek, M. Bibes, A. Barthelemy, K. Bouzehouane, S. Fusil, M. Varela, J. Fontcuberta, and A. Fert, Phys. Rev. B, 2005, 72: 020406(R).
[11] S. Ju, T.-Y. Cai, G.-Y. Guo, and Z.-Y. Li, Phys. Rev. B, 2007, 75: 064419.
[12] Yuhuan Xu, Ferroelectric Materials and their Applications, (North-Holland, 1991).