news & views SPIN ORBITRONICS
Charges ride the spin wave
An alternating charge current pumped by the precessing magnetization of a ferromagnet demonstrates the direct conversion of magnons into charge currents via relativistic spin–orbit coupling.
Timo Kuschel and Günter Reiss which are important for both fundamental science and applications. These effects include the various magnetoresistance effects and Hall effects, in which the longitudinal or transverse resistances of a device vary as a function of the magnetization or an external magnetic field4. Furthermore, there are spin-dependent transport effects that are driven by, or generate, thermal gradients. These are studied by the field of ‘spin caloritronics’5 and include the spin Seebeck effect 6 and its reciprocal counterpart, the spin Peltier effect 7, which have been demonstrated recently. Several transport phenomena, and their inverse effects, have been reported in materials with large spin–orbit coupling and/or net magnetic moment. For example, in the spin Hall effect a longitudinal charge current gives rise to a transverse spin current 8,9, whereas in the inverse spin Hall effect a pure spin current induces a transverse charge current 10. It is interesting to note that the inverse spin Hall effect from spin orbitronics is commonly used
ion act ter n i it rb –o
s nic tro rbi o n pi
to detect spin currents in spin caloritronic devices. The relationship of the different sub-fields of spintronics — spin electronics, spin caloritronics and spin orbitronics — is illustrated in Fig. 1. Brataas and colleagues — who are based at the Norwegian University of Science and Technology, University of Cambridge, University of Copenhagen, Institute of Physics in Prague, University of California, Los Angeles, and Japan Science and Technology Agency — used a 40-μm-long, compressively strained (Ga,Mn)As bar on GaAs, which has no crystal inversion symmetry, as is required for the occurrence of SOT effects. A microwave current with a frequency of 7 GHz flows through the bar, and excites magnetization precession via SOT when the amplitude of an applied magnetic field is swept through resonance. The detection method employed by the researchers can discriminate between the a.c. and d.c. components of the voltage generated in the (Ga,Mn)As bar. Brataas and colleagues detected an alternating voltage
Spin cal ori tro n
Spin current
ics
S
Sp in
Magnetic field / magnetization Sp
Electric field
ele
i
n
et ics
E
E le
a Cl
s ic al
s
lectrons carry charge and spin. Therefore, moving electrons can create both charge currents and spin currents. Pure charge currents are the basis of electronics and are well understood. More recently, the creation, manipulation and use of spin currents has become a focus of research. Charge and spin degrees of freedom are tied to each other by the spin–orbit coupling, which is a relativistic effect that links the spin and orbital angular momentum of the electrons. Charge currents can thus act on the spin of the electrons, and create spin currents and/or spin accumulation, allowing electrical manipulation of the magnetic state of matter. One prominent example of such manipulation is the spin– orbit torque1 (SOT), where a charge current induces a net spin moment that can exert a torque on the magnetization. For each action of a charge current on the electron spin, there is a counterpart: that is, a flow of spins can generate a charge current. Now, in Nature Nanotechnology, Arne Brataas and colleagues report the inverse effect of SOT — a flow of net magnetic moment (spins) induced by a precessing magnetization that produces a charge current or an open circuit voltage2. This effect is called magnonic charge pumping. The generation and manipulation of spins by a charge current was discussed by Dyakonov and Perel as early as 19713. They demonstrated that the spin–orbit coupling energy term in the Hamiltonian opens the possibility to orient the spins of the electrons by driving a charge current through a material. This fundamental fact was not exploited for many years, probably because spin detection is tedious, and the electronics based on charge currents is easier to implement. However, more recently, considerable attention has been drawn to the field of ‘spin orbitronics’ — the study of coupled charge and spin transport phenomena — as it became clear that the link between spin and motion could be used in the design of ever-smaller devices with low power consumption. The field of spin orbitronics adds to the investigation of other spin electronic effects,
tra ns po rt
ctr oni cs
ctr oni
rm The
cs
om
a
Temperature gradient
gn
o rm The
c el e
t ri
cs
Charge current
Figure 1 | The scientific areas where charge and spin transport driven by either electric field or temperature gradient meet.
NATURE NANOTECHNOLOGY | ADVANCE ONLINE PUBLICATION | www.nature.com/naturenanotechnology
© 2014 Macmillan Publishers Limited. All rights reserved
1
news & views harge pumping Magnonic c
Spin–orbit torque
Figure 2 | The new magnonic charge pumping describes conversion of magnetization precession into charge current, as the inverse counterpart of the spin-orbit torque.
generated by the magnetization precession. They compared the experimental results with theoretical calculations based on the Onsager reciprocity to show the inverse character of magnonic spin pumping compared with SOT (Fig. 2). While the variety of spin phenomena is a fruitful playground for scientists, the multiplicity of effects and their simultaneous occurrence in one experiment can significantly hamper the interpretation of experimental results. For example, a lively discussion was triggered in the spin caloritronic community by the possible occurrence of parasitic effects in transverse spin currents generated by a temperature gradient, calling for the thorough determination of concurring effects, in particular the Nernst and spin Seebeck effects11,12. A similar need could arise for
2
the results on magnonic charge pumping by Brataas and colleagues. Their experimental set-up involves a microwave current driven through a (Ga,Mn)As sample that drives a precession of the magnetization. Both the electric current and the magnetization precession, however, could cause heat flow, which in turn can generate spin and charge currents. In addition, the different magnetic fields involved in the experiment could induce Hall and Nernst voltages. While the article presents a detailed and carefully performed study including discussion of parasitic effects, such additional charge-, spin- and heat-based effects have to be excluded to provide further evidence for pure magnonic charge pumping. Further work in the sub-fields of spintronics is also likely to lead to the discovery of new phenomena such as
the spin Nernst effect, or the generation of a temperature gradient by an electric field via spin–orbit coupling. Moreoever, applications of spin currents and spin accumulation driven by an electric field have the potential to provide nanoscale and highly energy-efficient solutions for future data-storage, -handling and -processing by combining magnetic materials with strongly spin–orbit-coupled metals. Exciting developments can be expected in the next few years. ❐ Timo Kuschel and Günter Reiss are in the Department of Physics, Center of Spintronic Materials and Devices, University of Bielefeld, PO Box 100131, D-33501 Bielefeld, Germany. e-mail: [email protected] References
1. Manchon, A. & Zhang, S. Phys. Rev. B 78, 212405 (2008). 2. Ciccarelli, C. et al. Nature Nanotech. http://dx.doi.org/10.1038/ nnano.2014.252 (2014). 3. Dyakonov, M. I. & Perel, V. I. Sov. Phys. JETP Lett. 13, 467 (1971). 4. Wolf, S. et al. Science 294, 1488–1495 (2001). 5. Bauer, G. E. W., Saitoh, E. & van Wees, B. J. Nature Mater. 11, 391–399 (2012). 6. Uchida, K. et al. Appl. Phys. Lett. 97, 172505 (2010). 7. Flipse, J. et al. Phys. Rev. Lett. 113, 027601 (2014). 8. Hirsch, J. E. Phys. Rev. Lett. 83, 1834–1837 (1999). 9. Kato, Y. K., Myers, R. C., Gossard, A. C. & Awschalom, D. D. Science 306, 1910–1913 (2004). 10. Saitoh, E. et al. Appl. Phys. Lett. 88, 182509 (2006). 11. Meier, D. et al. Phys. Rev. B 88, 184425 (2013). 12. Schmid, M. et al. Phys. Rev. Lett. 111, 187201 (2013).
Published online: 10 November 2014
NATURE NANOTECHNOLOGY | ADVANCE ONLINE PUBLICATION | www.nature.com/naturenanotechnology
© 2014 Macmillan Publishers Limited. All rights reserved
Charges ride the spin wave
An alternating charge current pumped by the precessing magnetization of a ferromagnet demonstrates the direct conversion of magnons into charge currents via relativistic spin–orbit coupling.
Timo Kuschel and Günter Reiss which are important for both fundamental science and applications. These effects include the various magnetoresistance effects and Hall effects, in which the longitudinal or transverse resistances of a device vary as a function of the magnetization or an external magnetic field4. Furthermore, there are spin-dependent transport effects that are driven by, or generate, thermal gradients. These are studied by the field of ‘spin caloritronics’5 and include the spin Seebeck effect 6 and its reciprocal counterpart, the spin Peltier effect 7, which have been demonstrated recently. Several transport phenomena, and their inverse effects, have been reported in materials with large spin–orbit coupling and/or net magnetic moment. For example, in the spin Hall effect a longitudinal charge current gives rise to a transverse spin current 8,9, whereas in the inverse spin Hall effect a pure spin current induces a transverse charge current 10. It is interesting to note that the inverse spin Hall effect from spin orbitronics is commonly used
ion act ter n i it rb –o
s nic tro rbi o n pi
to detect spin currents in spin caloritronic devices. The relationship of the different sub-fields of spintronics — spin electronics, spin caloritronics and spin orbitronics — is illustrated in Fig. 1. Brataas and colleagues — who are based at the Norwegian University of Science and Technology, University of Cambridge, University of Copenhagen, Institute of Physics in Prague, University of California, Los Angeles, and Japan Science and Technology Agency — used a 40-μm-long, compressively strained (Ga,Mn)As bar on GaAs, which has no crystal inversion symmetry, as is required for the occurrence of SOT effects. A microwave current with a frequency of 7 GHz flows through the bar, and excites magnetization precession via SOT when the amplitude of an applied magnetic field is swept through resonance. The detection method employed by the researchers can discriminate between the a.c. and d.c. components of the voltage generated in the (Ga,Mn)As bar. Brataas and colleagues detected an alternating voltage
Spin cal ori tro n
Spin current
ics
S
Sp in
Magnetic field / magnetization Sp
Electric field
ele
i
n
et ics
E
E le
a Cl
s ic al
s
lectrons carry charge and spin. Therefore, moving electrons can create both charge currents and spin currents. Pure charge currents are the basis of electronics and are well understood. More recently, the creation, manipulation and use of spin currents has become a focus of research. Charge and spin degrees of freedom are tied to each other by the spin–orbit coupling, which is a relativistic effect that links the spin and orbital angular momentum of the electrons. Charge currents can thus act on the spin of the electrons, and create spin currents and/or spin accumulation, allowing electrical manipulation of the magnetic state of matter. One prominent example of such manipulation is the spin– orbit torque1 (SOT), where a charge current induces a net spin moment that can exert a torque on the magnetization. For each action of a charge current on the electron spin, there is a counterpart: that is, a flow of spins can generate a charge current. Now, in Nature Nanotechnology, Arne Brataas and colleagues report the inverse effect of SOT — a flow of net magnetic moment (spins) induced by a precessing magnetization that produces a charge current or an open circuit voltage2. This effect is called magnonic charge pumping. The generation and manipulation of spins by a charge current was discussed by Dyakonov and Perel as early as 19713. They demonstrated that the spin–orbit coupling energy term in the Hamiltonian opens the possibility to orient the spins of the electrons by driving a charge current through a material. This fundamental fact was not exploited for many years, probably because spin detection is tedious, and the electronics based on charge currents is easier to implement. However, more recently, considerable attention has been drawn to the field of ‘spin orbitronics’ — the study of coupled charge and spin transport phenomena — as it became clear that the link between spin and motion could be used in the design of ever-smaller devices with low power consumption. The field of spin orbitronics adds to the investigation of other spin electronic effects,
tra ns po rt
ctr oni cs
ctr oni
rm The
cs
om
a
Temperature gradient
gn
o rm The
c el e
t ri
cs
Charge current
Figure 1 | The scientific areas where charge and spin transport driven by either electric field or temperature gradient meet.
NATURE NANOTECHNOLOGY | ADVANCE ONLINE PUBLICATION | www.nature.com/naturenanotechnology
© 2014 Macmillan Publishers Limited. All rights reserved
1
news & views harge pumping Magnonic c
Spin–orbit torque
Figure 2 | The new magnonic charge pumping describes conversion of magnetization precession into charge current, as the inverse counterpart of the spin-orbit torque.
generated by the magnetization precession. They compared the experimental results with theoretical calculations based on the Onsager reciprocity to show the inverse character of magnonic spin pumping compared with SOT (Fig. 2). While the variety of spin phenomena is a fruitful playground for scientists, the multiplicity of effects and their simultaneous occurrence in one experiment can significantly hamper the interpretation of experimental results. For example, a lively discussion was triggered in the spin caloritronic community by the possible occurrence of parasitic effects in transverse spin currents generated by a temperature gradient, calling for the thorough determination of concurring effects, in particular the Nernst and spin Seebeck effects11,12. A similar need could arise for
2
the results on magnonic charge pumping by Brataas and colleagues. Their experimental set-up involves a microwave current driven through a (Ga,Mn)As sample that drives a precession of the magnetization. Both the electric current and the magnetization precession, however, could cause heat flow, which in turn can generate spin and charge currents. In addition, the different magnetic fields involved in the experiment could induce Hall and Nernst voltages. While the article presents a detailed and carefully performed study including discussion of parasitic effects, such additional charge-, spin- and heat-based effects have to be excluded to provide further evidence for pure magnonic charge pumping. Further work in the sub-fields of spintronics is also likely to lead to the discovery of new phenomena such as
the spin Nernst effect, or the generation of a temperature gradient by an electric field via spin–orbit coupling. Moreoever, applications of spin currents and spin accumulation driven by an electric field have the potential to provide nanoscale and highly energy-efficient solutions for future data-storage, -handling and -processing by combining magnetic materials with strongly spin–orbit-coupled metals. Exciting developments can be expected in the next few years. ❐ Timo Kuschel and Günter Reiss are in the Department of Physics, Center of Spintronic Materials and Devices, University of Bielefeld, PO Box 100131, D-33501 Bielefeld, Germany. e-mail: [email protected] References
1. Manchon, A. & Zhang, S. Phys. Rev. B 78, 212405 (2008). 2. Ciccarelli, C. et al. Nature Nanotech. http://dx.doi.org/10.1038/ nnano.2014.252 (2014). 3. Dyakonov, M. I. & Perel, V. I. Sov. Phys. JETP Lett. 13, 467 (1971). 4. Wolf, S. et al. Science 294, 1488–1495 (2001). 5. Bauer, G. E. W., Saitoh, E. & van Wees, B. J. Nature Mater. 11, 391–399 (2012). 6. Uchida, K. et al. Appl. Phys. Lett. 97, 172505 (2010). 7. Flipse, J. et al. Phys. Rev. Lett. 113, 027601 (2014). 8. Hirsch, J. E. Phys. Rev. Lett. 83, 1834–1837 (1999). 9. Kato, Y. K., Myers, R. C., Gossard, A. C. & Awschalom, D. D. Science 306, 1910–1913 (2004). 10. Saitoh, E. et al. Appl. Phys. Lett. 88, 182509 (2006). 11. Meier, D. et al. Phys. Rev. B 88, 184425 (2013). 12. Schmid, M. et al. Phys. Rev. Lett. 111, 187201 (2013).
Published online: 10 November 2014
NATURE NANOTECHNOLOGY | ADVANCE ONLINE PUBLICATION | www.nature.com/naturenanotechnology
© 2014 Macmillan Publishers Limited. All rights reserved
