In a recent Letter Hammar et al. claim the observation of injection of a
spin-polarized current in a two-dimensional electron gas (2DEG). This is an
important observation, since, despite considerable effort of several groups,
all attempts to realize spin-injection into a 2DEG using purely electrical
measurements have failed sofar. However, in my opinion the claim made is not
correct, and the observed behaviour can be explained by a combination of a
magneto resistance (Hall) effect (e.g. generated by the fringe magnetic fields
present at the edges of the ferromagnetic electrode), with a {\it
spin-independent} rectification effect due to the presence of a metal-
semiconductor junction.Comment: accepted for PRL, 1 pag

B. J. van Wees

M. Buttiker

M. Johnson

P. R. Hammar

English

arXiv

Crossref

Physical Review Letters

Comment on “Observation of Spin Injection at a Ferromagnet-Semiconductor Interface”

van Wees, B.J.

ARTS repository - University of Groningen

   University of GroningenComment on “Observation of Spin Injection at a Ferromagnet-Semiconductor Interface”van Wees, B.J.Published in:Physical Review LettersIMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2000Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):van Wees, B. J. (2000). Comment on “Observation of Spin Injection at a Ferromagnet-SemiconductorInterface”. Physical Review Letters, 84(21), 5023-5023.CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 10-09-2023VOLUME 84, NUMBER 21 P H Y S I C A L R E V I E W L E T T E R S 22 MAY 2000Comment on “Observation of Spin Injectionat a Ferromagnet-Semiconductor Interface”In a recent Letter, Hammar et al. [1] claim the obser-vation of injection of a spin-polarized current in a two-dimensional electron gas (2DEG). This is an importantobservation, since, despite considerable effort of severalgroups, all attempts to realize spin injection into a 2DEGusing purely electrical measurements have failed so far.However, in my opinion, the claim made in [1] is notcorrect, and the observed behavior can be explained bya combination of a magnetoresistance (Hall) effect with aspin-independent rectification effect due to the presence ofa metal-semiconductor junction.The interpretation of the data depends crucially on thetheoretical description formulated in [2]. A 2DEG is con-sidered, connected to a ferromagnetic electrode. The keyingredient is that the electron spin is conserved in the2DEG, but the spin-orbit interaction of the Rashba-typeinduces an asymmetry between electrons moving in a par-ticular direction with different spin directions. As a resultrectification is predicted, which depends on the directionM of the magnetization of the ferromagnetic electrode.However, at low currents linear response theory requiresthat V 2I  2V I. Rectification can only occur forcurrents I beyond the linear transport regime, under theconditions that the transport properties of the electrons areenergy dependent and that the current in both directions iscarried by electrons with different energies. However, noenergy scale which determines the onset of rectificationis discussed in [2], where the rectification depends on thedirection of the current only. Therefore the theory of [2]cannot be correct, and it is not possible to detect spininjection in this way.Turning attention to the experiment, it should be notedfirst that the authors test the prediction of spin-dependentcurrent rectification by performing a four-terminal mea-surement. They reverse the current direction by inter-changing one current and one voltage lead. This, however,is not a critical test of the theory, since [2] predicts a changein resistance when the current direction is reversed, butthe current and voltage leads themselves are not changed.The reciprocity theorem for multiterminal measurements[3] states that (in the linear transport regime) the resis-tance should be invariant under interchange of the currentand voltage leads, accompanied by a reversal of the mag-netic field B. Although in the experiment only one currentand voltage probe is interchanged, the result should be al-most equivalent to interchanging both pairs, since the otherpair is connected by the low resistance ferromagnetic elec-trode, and interchanging this pair should make little differ-ence on the measured resistance. Therefore the observed0031-90070084(21)5023(1)$15.00behavior is consistent with the reciprocity theorem, andthe change in resistance can possibly be caused by a Halleffect due to the reversal of the magnetic field (e.g., result-ing from the fringe fields at the edges of the ferromagneticelectrode). An estimate of the effect on the resistance isdifficult, however, due to the uncertainty where the currentfrom the 2DEG actually enters the ferromagnet.The second important aspect is that the authors studytransport through a semiconductor-metal system, whichcontains a barrier. Judged from the measured resistance,which is low and comparable to the resistance of the 2DEGitself, the barrier is not very effective. However, on thevoltage scale of 0.1 V which the authors use, some sortof rectification might occur, leading to V 2I fi 2V I.This effect will not depend on the direction of the magne-tization M, since the electron spin is not relevant for recti-fication in metal-semiconductor junctions. In other words,V IM  V I2M .The presented data therefore do not exclude thatthe observed effects are due to a combination of a(magnetization-independent) rectification, combined witha magnetoresistance (Hall) effect. It is, however, possibleto distinguish between this explanation and that of theauthors. To support their case they should show that therectification behavior observed in the V I characteristicschanges sign, when the magnetization M is reversed.In other words, for a fixed configuration of current andvoltage leads, they should show explicitly that the relationV IM  2V 2I2M is obeyed, for the full current range.In particular, this implies that when M switches directionwhen the coercive field is exceeded, and a change DR isobserved for positive I, an opposite change 2DR shouldbe observed at negative I . This behavior can then clearlybe distinguished from a Hall effect, which predicts thatthe change in resistance DR will have the same sign forboth current directions.B. J. van WeesDepartment of Applied Physicsand Materials Science CentreUniversity of Groningen9747 AG Groningen, The NetherlandsReceived 23 July 1999PACS numbers: 75.25.+z, 73.40.Qv, 73.50.–h, 73.61.–r[1] P. R. Hammar et al., Phys. Rev. Lett. 83, 203 (1999).[2] M. Johnson, Phys. Rev. B 58, 9635 (1998).[3] M. Buttiker, Phys. Rev. Lett. 57, 1761 (1986). This paperalso discusses the Onsager-Casimir reciprocity relations,which apply to the general case of multiterminal resistancemeasurements.© 2000 The American Physical Society 5023

Proceedings - University of Groningen

   University of GroningenComment on “Observation of Spin Injection at a Ferromagnet-Semiconductor Interface”van Wees, B.J.Published in:Physical Review LettersIMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2000Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):van Wees, B. J. (2000). Comment on “Observation of Spin Injection at a Ferromagnet-SemiconductorInterface”. Physical Review Letters, 84(21), 5023-5023.CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 29-10-2022VOLUME 84, NUMBER 21 P H Y S I C A L R E V I E W L E T T E R S 22 MAY 2000Comment on “Observation of Spin Injectionat a Ferromagnet-Semiconductor Interface”In a recent Letter, Hammar et al. [1] claim the obser-vation of injection of a spin-polarized current in a two-dimensional electron gas (2DEG). This is an importantobservation, since, despite considerable effort of severalgroups, all attempts to realize spin injection into a 2DEGusing purely electrical measurements have failed so far.However, in my opinion, the claim made in [1] is notcorrect, and the observed behavior can be explained bya combination of a magnetoresistance (Hall) effect with aspin-independent rectification effect due to the presence ofa metal-semiconductor junction.The interpretation of the data depends crucially on thetheoretical description formulated in [2]. A 2DEG is con-sidered, connected to a ferromagnetic electrode. The keyingredient is that the electron spin is conserved in the2DEG, but the spin-orbit interaction of the Rashba-typeinduces an asymmetry between electrons moving in a par-ticular direction with different spin directions. As a resultrectification is predicted, which depends on the directionM of the magnetization of the ferromagnetic electrode.However, at low currents linear response theory requiresthat V 2I  2V I. Rectification can only occur forcurrents I beyond the linear transport regime, under theconditions that the transport properties of the electrons areenergy dependent and that the current in both directions iscarried by electrons with different energies. However, noenergy scale which determines the onset of rectificationis discussed in [2], where the rectification depends on thedirection of the current only. Therefore the theory of [2]cannot be correct, and it is not possible to detect spininjection in this way.Turning attention to the experiment, it should be notedfirst that the authors test the prediction of spin-dependentcurrent rectification by performing a four-terminal mea-surement. They reverse the current direction by inter-changing one current and one voltage lead. This, however,is not a critical test of the theory, since [2] predicts a changein resistance when the current direction is reversed, butthe current and voltage leads themselves are not changed.The reciprocity theorem for multiterminal measurements[3] states that (in the linear transport regime) the resis-tance should be invariant under interchange of the currentand voltage leads, accompanied by a reversal of the mag-netic field B. Although in the experiment only one currentand voltage probe is interchanged, the result should be al-most equivalent to interchanging both pairs, since the otherpair is connected by the low resistance ferromagnetic elec-trode, and interchanging this pair should make little differ-ence on the measured resistance. Therefore the observed0031-90070084(21)5023(1)$15.00behavior is consistent with the reciprocity theorem, andthe change in resistance can possibly be caused by a Halleffect due to the reversal of the magnetic field (e.g., result-ing from the fringe fields at the edges of the ferromagneticelectrode). An estimate of the effect on the resistance isdifficult, however, due to the uncertainty where the currentfrom the 2DEG actually enters the ferromagnet.The second important aspect is that the authors studytransport through a semiconductor-metal system, whichcontains a barrier. Judged from the measured resistance,which is low and comparable to the resistance of the 2DEGitself, the barrier is not very effective. However, on thevoltage scale of 0.1 V which the authors use, some sortof rectification might occur, leading to V 2I fi 2V I.This effect will not depend on the direction of the magne-tization M, since the electron spin is not relevant for recti-fication in metal-semiconductor junctions. In other words,V IM  V I2M .The presented data therefore do not exclude thatthe observed effects are due to a combination of a(magnetization-independent) rectification, combined witha magnetoresistance (Hall) effect. It is, however, possibleto distinguish between this explanation and that of theauthors. To support their case they should show that therectification behavior observed in the V I characteristicschanges sign, when the magnetization M is reversed.In other words, for a fixed configuration of current andvoltage leads, they should show explicitly that the relationV IM  2V 2I2M is obeyed, for the full current range.In particular, this implies that when M switches directionwhen the coercive field is exceeded, and a change DR isobserved for positive I, an opposite change 2DR shouldbe observed at negative I . This behavior can then clearlybe distinguished from a Hall effect, which predicts thatthe change in resistance DR will have the same sign forboth current directions.B. J. van WeesDepartment of Applied Physicsand Materials Science CentreUniversity of Groningen9747 AG Groningen, The NetherlandsReceived 23 July 1999PACS numbers: 75.25.+z, 73.40.Qv, 73.50.–h, 73.61.–r[1] P. R. Hammar et al., Phys. Rev. Lett. 83, 203 (1999).[2] M. Johnson, Phys. Rev. B 58, 9635 (1998).[3] M. Buttiker, Phys. Rev. Lett. 57, 1761 (1986). This paperalso discusses the Onsager-Casimir reciprocity relations,which apply to the general case of multiterminal resistancemeasurements.© 2000 The American Physical Society 5023

   University of GroningenComment on “Observation of Spin Injection at a Ferromagnet-Semiconductor Interface”van Wees, B.J.Published in:Physical Review LettersIMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2000Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):van Wees, B. J. (2000). Comment on “Observation of Spin Injection at a Ferromagnet-SemiconductorInterface”. Physical Review Letters, 84(21), 5023-5023.CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 31-10-2023VOLUME 84, NUMBER 21 P H Y S I C A L R E V I E W L E T T E R S 22 MAY 2000Comment on “Observation of Spin Injectionat a Ferromagnet-Semiconductor Interface”In a recent Letter, Hammar et al. [1] claim the obser-vation of injection of a spin-polarized current in a two-dimensional electron gas (2DEG). This is an importantobservation, since, despite considerable effort of severalgroups, all attempts to realize spin injection into a 2DEGusing purely electrical measurements have failed so far.However, in my opinion, the claim made in [1] is notcorrect, and the observed behavior can be explained bya combination of a magnetoresistance (Hall) effect with aspin-independent rectification effect due to the presence ofa metal-semiconductor junction.The interpretation of the data depends crucially on thetheoretical description formulated in [2]. A 2DEG is con-sidered, connected to a ferromagnetic electrode. The keyingredient is that the electron spin is conserved in the2DEG, but the spin-orbit interaction of the Rashba-typeinduces an asymmetry between electrons moving in a par-ticular direction with different spin directions. As a resultrectification is predicted, which depends on the directionM of the magnetization of the ferromagnetic electrode.However, at low currents linear response theory requiresthat V 2I  2V I. Rectification can only occur forcurrents I beyond the linear transport regime, under theconditions that the transport properties of the electrons areenergy dependent and that the current in both directions iscarried by electrons with different energies. However, noenergy scale which determines the onset of rectificationis discussed in [2], where the rectification depends on thedirection of the current only. Therefore the theory of [2]cannot be correct, and it is not possible to detect spininjection in this way.Turning attention to the experiment, it should be notedfirst that the authors test the prediction of spin-dependentcurrent rectification by performing a four-terminal mea-surement. They reverse the current direction by inter-changing one current and one voltage lead. This, however,is not a critical test of the theory, since [2] predicts a changein resistance when the current direction is reversed, butthe current and voltage leads themselves are not changed.The reciprocity theorem for multiterminal measurements[3] states that (in the linear transport regime) the resis-tance should be invariant under interchange of the currentand voltage leads, accompanied by a reversal of the mag-netic field B. Although in the experiment only one currentand voltage probe is interchanged, the result should be al-most equivalent to interchanging both pairs, since the otherpair is connected by the low resistance ferromagnetic elec-trode, and interchanging this pair should make little differ-ence on the measured resistance. Therefore the observed0031-90070084(21)5023(1)$15.00behavior is consistent with the reciprocity theorem, andthe change in resistance can possibly be caused by a Halleffect due to the reversal of the magnetic field (e.g., result-ing from the fringe fields at the edges of the ferromagneticelectrode). An estimate of the effect on the resistance isdifficult, however, due to the uncertainty where the currentfrom the 2DEG actually enters the ferromagnet.The second important aspect is that the authors studytransport through a semiconductor-metal system, whichcontains a barrier. Judged from the measured resistance,which is low and comparable to the resistance of the 2DEGitself, the barrier is not very effective. However, on thevoltage scale of 0.1 V which the authors use, some sortof rectification might occur, leading to V 2I fi 2V I.This effect will not depend on the direction of the magne-tization M, since the electron spin is not relevant for recti-fication in metal-semiconductor junctions. In other words,V IM  V I2M .The presented data therefore do not exclude thatthe observed effects are due to a combination of a(magnetization-independent) rectification, combined witha magnetoresistance (Hall) effect. It is, however, possibleto distinguish between this explanation and that of theauthors. To support their case they should show that therectification behavior observed in the V I characteristicschanges sign, when the magnetization M is reversed.In other words, for a fixed configuration of current andvoltage leads, they should show explicitly that the relationV IM  2V 2I2M is obeyed, for the full current range.In particular, this implies that when M switches directionwhen the coercive field is exceeded, and a change DR isobserved for positive I, an opposite change 2DR shouldbe observed at negative I . This behavior can then clearlybe distinguished from a Hall effect, which predicts thatthe change in resistance DR will have the same sign forboth current directions.B. J. van WeesDepartment of Applied Physicsand Materials Science CentreUniversity of Groningen9747 AG Groningen, The NetherlandsReceived 23 July 1999PACS numbers: 75.25.+z, 73.40.Qv, 73.50.–h, 73.61.–r[1] P. R. Hammar et al., Phys. Rev. Lett. 83, 203 (1999).[2] M. Johnson, Phys. Rev. B 58, 9635 (1998).[3] M. Buttiker, Phys. Rev. Lett. 57, 1761 (1986). This paperalso discusses the Onsager-Casimir reciprocity relations,which apply to the general case of multiterminal resistancemeasurements.© 2000 The American Physical Society 5023

   University of GroningenComment on “Observation of Spin Injection at a Ferromagnet-Semiconductor Interface”van Wees, B.J.Published in:Physical Review LettersIMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2000Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):van Wees, B. J. (2000). Comment on “Observation of Spin Injection at a Ferromagnet-SemiconductorInterface”. Physical Review Letters, 84(21), 5023-5023.CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 21-06-2022VOLUME 84, NUMBER 21 P H Y S I C A L R E V I E W L E T T E R S 22 MAY 2000Comment on “Observation of Spin Injectionat a Ferromagnet-Semiconductor Interface”In a recent Letter, Hammar et al. [1] claim the obser-vation of injection of a spin-polarized current in a two-dimensional electron gas (2DEG). This is an importantobservation, since, despite considerable effort of severalgroups, all attempts to realize spin injection into a 2DEGusing purely electrical measurements have failed so far.However, in my opinion, the claim made in [1] is notcorrect, and the observed behavior can be explained bya combination of a magnetoresistance (Hall) effect with aspin-independent rectification effect due to the presence ofa metal-semiconductor junction.The interpretation of the data depends crucially on thetheoretical description formulated in [2]. A 2DEG is con-sidered, connected to a ferromagnetic electrode. The keyingredient is that the electron spin is conserved in the2DEG, but the spin-orbit interaction of the Rashba-typeinduces an asymmetry between electrons moving in a par-ticular direction with different spin directions. As a resultrectification is predicted, which depends on the directionM of the magnetization of the ferromagnetic electrode.However, at low currents linear response theory requiresthat V 2I  2V I. Rectification can only occur forcurrents I beyond the linear transport regime, under theconditions that the transport properties of the electrons areenergy dependent and that the current in both directions iscarried by electrons with different energies. However, noenergy scale which determines the onset of rectificationis discussed in [2], where the rectification depends on thedirection of the current only. Therefore the theory of [2]cannot be correct, and it is not possible to detect spininjection in this way.Turning attention to the experiment, it should be notedfirst that the authors test the prediction of spin-dependentcurrent rectification by performing a four-terminal mea-surement. They reverse the current direction by inter-changing one current and one voltage lead. This, however,is not a critical test of the theory, since [2] predicts a changein resistance when the current direction is reversed, butthe current and voltage leads themselves are not changed.The reciprocity theorem for multiterminal measurements[3] states that (in the linear transport regime) the resis-tance should be invariant under interchange of the currentand voltage leads, accompanied by a reversal of the mag-netic field B. Although in the experiment only one currentand voltage probe is interchanged, the result should be al-most equivalent to interchanging both pairs, since the otherpair is connected by the low resistance ferromagnetic elec-trode, and interchanging this pair should make little differ-ence on the measured resistance. Therefore the observed0031-90070084(21)5023(1)$15.00behavior is consistent with the reciprocity theorem, andthe change in resistance can possibly be caused by a Halleffect due to the reversal of the magnetic field (e.g., result-ing from the fringe fields at the edges of the ferromagneticelectrode). An estimate of the effect on the resistance isdifficult, however, due to the uncertainty where the currentfrom the 2DEG actually enters the ferromagnet.The second important aspect is that the authors studytransport through a semiconductor-metal system, whichcontains a barrier. Judged from the measured resistance,which is low and comparable to the resistance of the 2DEGitself, the barrier is not very effective. However, on thevoltage scale of 0.1 V which the authors use, some sortof rectification might occur, leading to V 2I fi 2V I.This effect will not depend on the direction of the magne-tization M, since the electron spin is not relevant for recti-fication in metal-semiconductor junctions. In other words,V IM  V I2M .The presented data therefore do not exclude thatthe observed effects are due to a combination of a(magnetization-independent) rectification, combined witha magnetoresistance (Hall) effect. It is, however, possibleto distinguish between this explanation and that of theauthors. To support their case they should show that therectification behavior observed in the V I characteristicschanges sign, when the magnetization M is reversed.In other words, for a fixed configuration of current andvoltage leads, they should show explicitly that the relationV IM  2V 2I2M is obeyed, for the full current range.In particular, this implies that when M switches directionwhen the coercive field is exceeded, and a change DR isobserved for positive I, an opposite change 2DR shouldbe observed at negative I . This behavior can then clearlybe distinguished from a Hall effect, which predicts thatthe change in resistance DR will have the same sign forboth current directions.B. J. van WeesDepartment of Applied Physicsand Materials Science CentreUniversity of Groningen9747 AG Groningen, The NetherlandsReceived 23 July 1999PACS numbers: 75.25.+z, 73.40.Qv, 73.50.–h, 73.61.–r[1] P. R. Hammar et al., Phys. Rev. Lett. 83, 203 (1999).[2] M. Johnson, Phys. Rev. B 58, 9635 (1998).[3] M. Buttiker, Phys. Rev. Lett. 57, 1761 (1986). This paperalso discusses the Onsager-Casimir reciprocity relations,which apply to the general case of multiterminal resistancemeasurements.© 2000 The American Physical Society 5023

University of Groningen

   University of GroningenComment on “Observation of Spin Injection at a Ferromagnet-Semiconductor Interface”van Wees, B.J.Published in:Physical Review LettersIMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2000Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):van Wees, B. J. (2000). Comment on “Observation of Spin Injection at a Ferromagnet-SemiconductorInterface”. Physical Review Letters, 84(21), 5023-5023.CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 13-11-2019VOLUME 84, NUMBER 21 P H Y S I C A L R E V I E W L E T T E R S 22 MAY 2000Comment on “Observation of Spin Injectionat a Ferromagnet-Semiconductor Interface”In a recent Letter, Hammar et al. [1] claim the obser-vation of injection of a spin-polarized current in a two-dimensional electron gas (2DEG). This is an importantobservation, since, despite considerable effort of severalgroups, all attempts to realize spin injection into a 2DEGusing purely electrical measurements have failed so far.However, in my opinion, the claim made in [1] is notcorrect, and the observed behavior can be explained bya combination of a magnetoresistance (Hall) effect with aspin-independent rectification effect due to the presence ofa metal-semiconductor junction.The interpretation of the data depends crucially on thetheoretical description formulated in [2]. A 2DEG is con-sidered, connected to a ferromagnetic electrode. The keyingredient is that the electron spin is conserved in the2DEG, but the spin-orbit interaction of the Rashba-typeinduces an asymmetry between electrons moving in a par-ticular direction with different spin directions. As a resultrectification is predicted, which depends on the directionM of the magnetization of the ferromagnetic electrode.However, at low currents linear response theory requiresthat V 2I  2V I. Rectification can only occur forcurrents I beyond the linear transport regime, under theconditions that the transport properties of the electrons areenergy dependent and that the current in both directions iscarried by electrons with different energies. However, noenergy scale which determines the onset of rectificationis discussed in [2], where the rectification depends on thedirection of the current only. Therefore the theory of [2]cannot be correct, and it is not possible to detect spininjection in this way.Turning attention to the experiment, it should be notedfirst that the authors test the prediction of spin-dependentcurrent rectification by performing a four-terminal mea-surement. They reverse the current direction by inter-changing one current and one voltage lead. This, however,is not a critical test of the theory, since [2] predicts a changein resistance when the current direction is reversed, butthe current and voltage leads themselves are not changed.The reciprocity theorem for multiterminal measurements[3] states that (in the linear transport regime) the resis-tance should be invariant under interchange of the currentand voltage leads, accompanied by a reversal of the mag-netic field B. Although in the experiment only one currentand voltage probe is interchanged, the result should be al-most equivalent to interchanging both pairs, since the otherpair is connected by the low resistance ferromagnetic elec-trode, and interchanging this pair should make little differ-ence on the measured resistance. Therefore the observed0031-90070084(21)5023(1)$15.00behavior is consistent with the reciprocity theorem, andthe change in resistance can possibly be caused by a Halleffect due to the reversal of the magnetic field (e.g., result-ing from the fringe fields at the edges of the ferromagneticelectrode). An estimate of the effect on the resistance isdifficult, however, due to the uncertainty where the currentfrom the 2DEG actually enters the ferromagnet.The second important aspect is that the authors studytransport through a semiconductor-metal system, whichcontains a barrier. Judged from the measured resistance,which is low and comparable to the resistance of the 2DEGitself, the barrier is not very effective. However, on thevoltage scale of 0.1 V which the authors use, some sortof rectification might occur, leading to V 2I ﬁ 2V I.This effect will not depend on the direction of the magne-tization M, since the electron spin is not relevant for recti-fication in metal-semiconductor junctions. In other words,V IM  V I2M .The presented data therefore do not exclude thatthe observed effects are due to a combination of a(magnetization-independent) rectification, combined witha magnetoresistance (Hall) effect. It is, however, possibleto distinguish between this explanation and that of theauthors. To support their case they should show that therectification behavior observed in the V I characteristicschanges sign, when the magnetization M is reversed.In other words, for a fixed configuration of current andvoltage leads, they should show explicitly that the relationV IM  2V 2I2M is obeyed, for the full current range.In particular, this implies that when M switches directionwhen the coercive field is exceeded, and a change DR isobserved for positive I, an opposite change 2DR shouldbe observed at negative I . This behavior can then clearlybe distinguished from a Hall effect, which predicts thatthe change in resistance DR will have the same sign forboth current directions.B. J. van WeesDepartment of Applied Physicsand Materials Science CentreUniversity of Groningen9747 AG Groningen, The NetherlandsReceived 23 July 1999PACS numbers: 75.25.+z, 73.40.Qv, 73.50.–h, 73.61.–r[1] P. R. Hammar et al., Phys. Rev. Lett. 83, 203 (1999).[2] M. Johnson, Phys. Rev. B 58, 9635 (1998).[3] M. Buttiker, Phys. Rev. Lett. 57, 1761 (1986). This paperalso discusses the Onsager-Casimir reciprocity relations,which apply to the general case of multiterminal resistancemeasurements.© 2000 The American Physical Society 5023

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   University of GroningenComment on “Observation of Spin Injection at a Ferromagnet-Semiconductor Interface”van Wees, B.J.Published in:Physical Review LettersIMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2000Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):van Wees, B. J. (2000). Comment on “Observation of Spin Injection at a Ferromagnet-SemiconductorInterface”. Physical Review Letters, 84(21), 5023-5023.CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 28-12-2022VOLUME 84, NUMBER 21 P H Y S I C A L R E V I E W L E T T E R S 22 MAY 2000Comment on “Observation of Spin Injectionat a Ferromagnet-Semiconductor Interface”In a recent Letter, Hammar et al. [1] claim the obser-vation of injection of a spin-polarized current in a two-dimensional electron gas (2DEG). This is an importantobservation, since, despite considerable effort of severalgroups, all attempts to realize spin injection into a 2DEGusing purely electrical measurements have failed so far.However, in my opinion, the claim made in [1] is notcorrect, and the observed behavior can be explained bya combination of a magnetoresistance (Hall) effect with aspin-independent rectification effect due to the presence ofa metal-semiconductor junction.The interpretation of the data depends crucially on thetheoretical description formulated in [2]. A 2DEG is con-sidered, connected to a ferromagnetic electrode. The keyingredient is that the electron spin is conserved in the2DEG, but the spin-orbit interaction of the Rashba-typeinduces an asymmetry between electrons moving in a par-ticular direction with different spin directions. As a resultrectification is predicted, which depends on the directionM of the magnetization of the ferromagnetic electrode.However, at low currents linear response theory requiresthat V 2I  2V I. Rectification can only occur forcurrents I beyond the linear transport regime, under theconditions that the transport properties of the electrons areenergy dependent and that the current in both directions iscarried by electrons with different energies. However, noenergy scale which determines the onset of rectificationis discussed in [2], where the rectification depends on thedirection of the current only. Therefore the theory of [2]cannot be correct, and it is not possible to detect spininjection in this way.Turning attention to the experiment, it should be notedfirst that the authors test the prediction of spin-dependentcurrent rectification by performing a four-terminal mea-surement. They reverse the current direction by inter-changing one current and one voltage lead. This, however,is not a critical test of the theory, since [2] predicts a changein resistance when the current direction is reversed, butthe current and voltage leads themselves are not changed.The reciprocity theorem for multiterminal measurements[3] states that (in the linear transport regime) the resis-tance should be invariant under interchange of the currentand voltage leads, accompanied by a reversal of the mag-netic field B. Although in the experiment only one currentand voltage probe is interchanged, the result should be al-most equivalent to interchanging both pairs, since the otherpair is connected by the low resistance ferromagnetic elec-trode, and interchanging this pair should make little differ-ence on the measured resistance. Therefore the observed0031-90070084(21)5023(1)$15.00behavior is consistent with the reciprocity theorem, andthe change in resistance can possibly be caused by a Halleffect due to the reversal of the magnetic field (e.g., result-ing from the fringe fields at the edges of the ferromagneticelectrode). An estimate of the effect on the resistance isdifficult, however, due to the uncertainty where the currentfrom the 2DEG actually enters the ferromagnet.The second important aspect is that the authors studytransport through a semiconductor-metal system, whichcontains a barrier. Judged from the measured resistance,which is low and comparable to the resistance of the 2DEGitself, the barrier is not very effective. However, on thevoltage scale of 0.1 V which the authors use, some sortof rectification might occur, leading to V 2I fi 2V I.This effect will not depend on the direction of the magne-tization M, since the electron spin is not relevant for recti-fication in metal-semiconductor junctions. In other words,V IM  V I2M .The presented data therefore do not exclude thatthe observed effects are due to a combination of a(magnetization-independent) rectification, combined witha magnetoresistance (Hall) effect. It is, however, possibleto distinguish between this explanation and that of theauthors. To support their case they should show that therectification behavior observed in the V I characteristicschanges sign, when the magnetization M is reversed.In other words, for a fixed configuration of current andvoltage leads, they should show explicitly that the relationV IM  2V 2I2M is obeyed, for the full current range.In particular, this implies that when M switches directionwhen the coercive field is exceeded, and a change DR isobserved for positive I, an opposite change 2DR shouldbe observed at negative I . This behavior can then clearlybe distinguished from a Hall effect, which predicts thatthe change in resistance DR will have the same sign forboth current directions.B. J. van WeesDepartment of Applied Physicsand Materials Science CentreUniversity of Groningen9747 AG Groningen, The NetherlandsReceived 23 July 1999PACS numbers: 75.25.+z, 73.40.Qv, 73.50.–h, 73.61.–r[1] P. R. Hammar et al., Phys. Rev. Lett. 83, 203 (1999).[2] M. Johnson, Phys. Rev. B 58, 9635 (1998).[3] M. Buttiker, Phys. Rev. Lett. 57, 1761 (1986). This paperalso discusses the Onsager-Casimir reciprocity relations,which apply to the general case of multiterminal resistancemeasurements.© 2000 The American Physical Society 5023

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Comment on "Observation of Spin Injection at a Ferromagnet-Semiconductor Interface, by P.R. Hammar et al

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