Spin Hall Effect

Dr. Winfried Teizer

 

Objectives

The main objective of this project is to detect the recently postulated[i] Spin Hall Effect (SHE), a physical effect of fundamental importance, which allows the study of pure spin currents and the characterization of spin properties in materials.[ii] It is a topic of considerable pure and applied interest closely related to the Anomalous Hall Effect (AHE).[iii] The successful detection requires structures with the smallest dimensions in the nanometer range. In preliminary work, Teizer has determined the most likely route to detecting the SHE and designed a device for doing so. To implement the ideal device, nanolithography is needed. This project is funded through an Advanced Research Program grant of the Texas Higher Education Coordinating Board.

 

Methods


To understand the Spin Hall Effect we revert to the Anomalous Hall Effect (AHE). In ferromagnetic metals, the Hall resistivity empirically follows rH=RoB+4pRsM with B being the applied magnetic field, M the magnetization per unit volume and Ro and Rs the ordinary and anomalous Hall coefficient respectively. RoB describes the ordinary Hall effect present in all conductors and resulting from the Lorentz force. 4pRsM describes the anomalous contribution in ferromagnetic materials, which typically exceeds the ordinary Hall effect contribution. The microscopic origin of the AHE is controversial. Explanations like the side jump mechanism and skew scattering by impurities or phonons have been considered.43 However, the existence of the AHE is experimentally beyond doubt and indicates that electrons, carrying a spin (and a magnetic moment m), are subject to a transverse force F if they move in a longitudinal current (even in B=0!). Furthermore, electrons with opposite spin directions are subjected to a force in opposite directions. In a ferromagnetic material an applied magnetic field B produces a net magnetization, i.e. more carriers with spin aligned to the applied field than counter aligned. This imbalance of itinerant carrier spins leads to a spin and charge imbalance in the perpendicular direction, which gives rise to the anomalous Hall voltage. Typically, only the charge imbalance is detected. The spin imbalance is, however, a necessary condition for any spin dependent microscopic description of the AHE.

 

 

 

 

 

Figure 1: Spin Hall Effect. In a material without magnetization, the number of carriers with spin up equals the number of carriers with spin down. The same mechanism that leads to the AHE scatters spin up carriers preferentially to one side, while spin down carriers are scattered preferentially to the other side. The resulting spin imbalance leads to a spin potential VSH between opposing sides of the strip.

The central message of the Spin Hall Effect is that the spin imbalance even exists in a paramagnetic material. In a material without magnetization, the number of carriers with spin up balances the number of carriers with spin down (Figure 1). Consequently, by the same microscopic mechanism causing the AHE, the same number of carriers is scattered to one side of a current carrying strip than to the other and no charge imbalance between the different sides of the strip exists. There is, however, an imbalance of spins between the different sides of the strip, since spin up carriers are preferentially scattered to one side, while spin down carriers are preferentially scattered to the other. The resulting spin imbalance gives rise to a spin potential VSH, while the electronic potential is constant as one moves from one side of the strip to the opposite side (Figure 2). VSH is a consequence of same microscopic mechanism, causing to the AHE.

                                  

 

Figure 2. Slow wave electrooptic light modulator.

 

 

Teizer’s group has designed a device to detect the SHE. To manufacture this device, nanolithography (<30nm feature size) will be employed for pattern definition. Initially, Al will be chosen as device material, since prior work on its spin properties indicate a large spin diffusion length (450mm at T=4.3K). [iv] This is well above the device dimensions proposed here (~100mm total device dimension). If the effect has its predicted magnitude,[v] the detection will require standard low noise AC lock-in techniques. If the effect is much smaller than theoretically estimated, we plan to utilize low noise, low voltage measurement devices like SQUIDs or a low temperature amplifying system.[vi]

 

Impact

Discovering the postulated SHE and investigating its details will have several benefits for fundamental and applied physics. Fundamentally, this experiment would produce a pure spin current and would investigate the interrelations of spin and charge transport. Furthermore it would show that a skew scattering or side-jump mechanism exists even in a paramagnetic material and is thus a more general phenomenon. This would aid in the understanding of the AHE, one of the more significant outstanding issues in condensed matter physics. On the applied side, this project would provide information on the spin diffusion length and its dependence on material properties. The understanding of the material dependence of the spin diffusion length and spin-related scattering will aid technologists in the selection of materials for spintronics devices.


 

[i] J. E. Hirsch, Phys. Rev. Lett. 83, 1834 (1999).

[ii] S. Zhang, Phys. Rev. Lett. 85, 393 (2000).

[iii] C. M. Hurd, “The Hall Effect in Metals and Alloys”, Plenum, New York, 1973, Chapter 5.

[iv] M. Johnson and R. H. Silsbee, Phys. Rev. Lett. 55, 1790 (1985); Phys. Rev. B 37, 5312 (1988).

[v] J. E. Hirsch, Phys. Rev. Lett. 83, 1834 (1999)

 

[vi] .P. Dauguet et al., J. Appl. Phys. 79, 5823 (1996)