Spin Dynamics in Spintronic Devices

Spintronics - spin based electronics involves controlling and manipulating the electron’s spin degree of freedom in solid-state systems. Recent advances in manipulating spin-polarized electron currents in atomically engineered magnetic heterostructures enable entirely new classes of sensor, memory and logic devices with all the advantages of scaling and low-power. 

One key component of many such devices is the spin-transfer torque based magnetic tunneling junction (STT-MTJ) - a sandwich of thin layers of metallic ferromagnetic electrodes separated by a tunneling barrier. Its energy efficiency depends directly on the Gilbert damping, namely, the losses of spin angular momentum to the lattice. In this talk, I will start by presenting a new insightful method for reliably extracting the Gilbert damping parameter in 2-3 atoms thick ferromagnetic heterostructures from time-resolved free induction decay measurements. 

I will then show that under certain conditions of applied magnetic field, the spins in the magnetic layer may be found in a new phase where the potential barrier acting on the spins is completely suppressed. Consequently, “free” unconstrained motion of spins result, which may serve as a sensitive probe for very weak torques associated with proximate spin currents. 

Using a combination of femtosecond laser pulses and microwave excitations in a novel spectroscopy technique, the inner workings of the non-adiabatic regime are explored. Rabi nutations in a ferromagnetic system were shown for the first time and will be described. Furthermore, spin-mode locking as well as the ability to overcome the inhomogeneous broadening and sense coherences which represent more closely those of individual spins will be demonstrated. 

Finally, I will show how to use time-resolved spectroscopy methods to directly detect the spin-currents induced in a spin Hall device without the need to rely on the anisotropic magneto resistance (AMR). This measurement is a first step towards phase coherent manipulation of ferromagnetic qubits using spin currents without the complications of a driving electromagnetic field.