Browsing by Subject "Spin detection"
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Item Dynamic Detection of Spin Accumulation by Ferromagnetic Resonance(2016-11) Liu, ChangjiangThis thesis describes dynamical phenomena occurring in ferromagnet (FM)/n-GaAs heterostructures, while the ferromagnet is driven to resonance (FMR). The relevant work was published in Appl. Phys. Lett. 105, 212401 (2014) and Nature Communications 7, 10296 (2016), respectively. In a FM/n-GaAs heterostructure, strong spin-orbit coupling at the FM/n-GaAs interface leads to a tunneling anisotropic magnetoresistance (TAMR) effect, which makes the resistance across the FM/n-GaAs interface depend on the orientation of the magnetization relative to the crystalline axes. When the FM is driven on resonance, the magnetization starts to precess around an effective magnetic field with an elliptical trajectory. Because of the TAMR effect, the onset of FMR modulates the tunneling resistance across the FM/n-GaAs interface, enabling electrical detection of FMR. I demonstrate that through the TAMR mechanism, the FMR precession cone angles can be characterized quantitatively. This study shows that the TAMR effect plays a predominant role in producing a dc voltage in FM/n-GaAs heterostructures at FMR, which has to be taken into account when studying the FMR-induced phenomena in similar systems. When a large forward bias current is applied across the FM/n-GaAs interface, a spin accumulation can be generated in the n-GaAs. I show that, in this spin accumulation regime, FMR can be used to detect the spin accumulation. This technique utilizes the fact that at FMR, the magnetization can precess at a frequency faster than the electron spin decay rate in the n-GaAs. The electrical signal from a FM, in measuring the electrochemical potential of a spin accumulation in the n-GaAs, depends on the relative angle between the magnetization vector in the FM and the spin accumulation in the semiconductor. Traditionally, to detect the spin accumulation, one applies a perpendicular magnetic field to let the spins in the semiconductor precess (the Hanle effect) or an in-plane magnetic field to switch the direction of the magnetizations (spin valve measurement). Neither one works well at room temperature in n-GaAs due to the significant decrease of spin lifetime as temperature increases. In my experiment, the magnetization is forced to precess at the FMR frequency, which is faster than the decay rate of spins in the n-GaAs. Once spins are injected from the precessing magnetization of the FM into the n-GaAs, they precess at a much lower frequency due to the different effective magnetic fields and electron g-factors of FM and n GaAs. This difference in precession frequency creates a phase angle between the magnetization vector in the FM and the spins in the n-GaAs, which enables an effective detection of spin accumulation. My modeling shows that this FMR technique provides a new way to determine the spin lifetime by measuring the FMR frequency dependence of the spin signal, which is verified experimentally, and the spin lifetime is measured at room temperature. The FMR-spin detection technique developed in this work can be applied to other systems in which spin lifetimes are short, such as metals and perhaps topological insulators.Item Spin polarized charge carrier injection, transport, and detection in organic semiconductors.(2011-05) Yunus, MohammadIn this thesis we explore spin polarized charge carrier injection, transport, and detection in organic semiconductors. Device structures considered have one or more ferromagnetic contacts to the organic semiconductor, and the condition for which charge carrier injection from ferromagnetic contacts is strongly spin polarized is discussed. Spin injection into semiconductors can be greatly enhanced if the injection mechanism is spin selective, such as is the case for tunnelling from ferromagnetic contacts. By contrast, if the carrier injection is by thermionic emission or another process that does not depend on spin, the injection is only weakly spin polarized. To discuss spin transport and spin detection, we consider a unipolar organic spin valve consisting of an organic semiconductor layer sandwiched between two ferromagnetic contacts. The polarizations of the magnetic contacts can be parallel or anti-parallel. Spin and charge carrier transport in the organic semiconductor is described by spin dependent transport equations in drift-diffusion approximation and the spin detection process is through magneto-resistance. We discuss the impact of various degrees of spin relaxation in organic semiconductors on the spatial variation of the spin current and its effect on magneto-resistance. The spatial profile of the spin current inside the organic semiconductor depends not only on the spin diffusion length but also on the alignment of the contact polarizations. However, the magneto-resistance decreases strongly with decreasing spin diffusion length. Electron tunnelling from a ferromagnetic contact can have significant spin dependence because the spatial part of the electron wave function is different for the majority and minority spin states of the ferromagnetic contacts. The tunnelling process occurs from the ferromagnetic contact through an insulating layer into the organic semiconductor. The insulating layer is modeled first as an ohmic layer with spin dependent contact resistances. The effectiveness of spin dependent contact resistances on spin polarized injection and magneto-resistance is examined on the basis of a simple analytical model. We then model the insulating layer as a tunnel barrier with spin dependent rate equations. Both majority and minority spin electrons of the ferromagnetic contact tunnel through the insulating layer into the localized molecular states of the organic semiconductor at the semiconductor/insulator interface. Tunnelling matrix elements and transition rates of the two spin types are calculated using a Transfer Hamiltonian approach. The transition rates are thus spin dependent and used in rate equations to calculate the injected (extracted) current for carriers of either spin direction. We explore the various aspects of the ferromagnetic contacts, the thickness and barrier height of the insulating layer, and the energy of the localized molecular states on spin injection and magneto-resistance. Consistent with the experimental data, the spin injection from ferromagnetic contacts can be either positive or negative, and the magneto-resistance decreases strongly with the applied bias across the device.