Spin Injection and detection in copper spin valve structures

My PhD work was on spin injection and detection in spin valve structures using both transport measurements and magnetic force microscopy. I fabricated mesoscopic scale structures like the ones shown in the left by using standard electron beam lithography, lift-off, and metal deposition techniques, on an Si-SiO₂ substrate. This particular sample shows two Cobalt electrodes of different length connected with a thin vertical Copper line and four additional probing contacts. By applying an external field parallel to the ferromagnetic (Cobalt) electrodes, their magnetization state can be independently controlled (due to their difference in shape anisotropy).

A schematic representation of the above device is shown in the right. A typical transport experiment consists of injecting a current between T₁ and N₁ as shown (grounding N₁) and measuring the voltage between terminals T₂ and N₂ with a high input impedance device. Our measurements are "nonlocal" in the sense that there is no charge current between F₁ and F₂, so the voltage is due completely to electron spin diffusion (a spin current). The measured voltage depends on the magnetization states of the ferromagnetic electrodes: due to the current perpendicular to plane (CPP) magnetoresistance effect, the electrons can diffuse more easily between injector (F₁) and detector (F₂) when their magnetization states are parallel than when they are anti-parallel. The measured voltage also decreases (in magnitude) as  the separation between injector and detector increases due to spin relaxation, hence by measuring samples with varying lengths L it is possible to extract the spin relaxation length l of electrons in the nonmagnetic material (N, in our case Copper).

The figure on the left shows the typical parallel magnetic field dependence of the nonlocal resistance, defined as the ratio of the measured voltage to the applied current, at 4.2 K. The symmetry between parallel and anti-parallel magnetizations of the ferromagnets is clear.

If the magnetic field is applied perpendicular to the plane, the spin of the electrons precesses as they diffuse between injector and detector producing the typical bell shaped curve of the Hanle effect. From a single Hanle effect measurement it is possible to extract the spin relaxation length and the product of the "spin polarizations" (not exactly...) of the injector and detector.