PhD Defense: Injection, transport and manipulation of pure spin currents in metallic lateral spin valves
CIC nanoGUNE Seminars
- Speaker
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Estitxu Villamor, Nanodevices Group, CIC nanoGUNE
- When
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2014/12/15
13:00 - Place
- Carlos Santamaria Building
- Add to calendar
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iCal
Thesis supervised by **Dr. Felix Casanova**
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Spintronics is a rapidly growing field that aims at using and manipulating not
only the charge, but also the spin of the electron, as an alternative to
conventional electronics. The objective of this field is to provide an answer
to the constant miniaturization of silicon-based transistors, which cannot
continue indefinitely due to increasing power dissipation and the emergence of
quantum fluctuations when fewer atoms are involved. After the success of
spintronics applications such as the hard-disk read head or the magnetic
random access memory, based on the giant magnetoresistance effect in magnetic
multilayers, the goal is to develop a second generation of spintronic devices,
in which pure spin currents can be created and manipulated. These devices
would provide a faster data processing, due to the added degree of freedom of
the spin, together with a lower power consumption and heat dissipation related
to the absence of charge transport. In such second generation of spintronic
devices, achieving spin transport over long distances is crucial, for which
materials with a long spin-diffusion length are needed.
In this thesis, we study the three key parameters for an optimum performance
of these spintronic devices: spin injection, transport and manipulation. With
this purpose, we use metallic lateral spin valves (LSVs), nanoscaled devices
consisting of two ferromagnetic (FM) electrodes bridged by a non-magnetic (NM)
channel, which, by using a non-local geometry, allow the electrical creation
of pure spin currents as well as their transport over a long distance, easily
enabling their manipulation.
After proving the reproducible performance of our devices due to a nanofabri-
cation based on two consecutive electron-beam lithography processes, where the
FM/NM interface quality is carefully optimized, the spin-injection properties
of Co and Py are studied and compared by obtaining the spin polarization of
both materials as a function of temperature in Co/Cu and Py/Cu LSVs with
trans- parent interfaces. The spin polarization of Py is higher than that of
the Co, and it presents a clear variation with temperature, which is in good
agreement with the two-channel model if the detected correction factor of ∼
2 is applied.
The spin transport properties of Cu are also studied as a function of temper-
ature. On the one hand, by using Co and Py electrodes, we show that the used
FM metal does not affect the spin transport properties of Cu. On the other
hand, the resistivity and the spin-diffusion length of Cu are obtained as a
function of temperature and the dimensions of the Cu channel, identifying
different sources of spin scattering. Whereas the spin-flip mechanisms coming
from phonons and defects (which are dominated by the grain boundaries rather
than the surface) are both independent of the temperature and the dimensions
of the Cu channel, the temperature dependence of the spin relaxation in Cu
arises from the contribution of the phonons to the momentum relaxation.
The most widely employed spin manipulation technique in LSVs is the Hanle
effect, which is based on the precession of spins under an out-of-plane
magnetic field. In this thesis, we use the Hanle effect as a tool for spin
manipulation and also for obtaining information regarding the spin-injection
and spin-transport properties of the LSVs. Devices with transparent interfaces
and with a non-zero interface resistance are compared. Whereas the spin
currents are effectively mod- ulated in LSVs with a non-zero interface
resistance and a reliable information is obtained, in the case of LSVs with
transparent interfaces interference effects are observed, which cannot be
explained with the current theory.
Finally, a novel method for spin manipulation is proposed by magnetically
gating pure spin currents. This is possible with the fabrication of LSVs on
top of a ferromagnetic insulator (FMI) and taking advantage of the concept of
the NM/FMI spin-mixing interface conductance. A theory is proposed in order to
explain a modulation of the pure spin currents, which is experimentally proven
in Co/Cu LSVs fabricated on top of YIG. The value of the spin-mixing
conductance of the Cu/YIG interface is obtained for the first time and clues
for improving such value, as well as the spin current modulation itself, are
presented. ****
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