Refine
Has Fulltext
- yes (2)
Is part of the Bibliography
- yes (2)
Year of publication
- 2014 (2) (remove)
Document Type
- Doctoral Thesis (2)
Language
- English (2)
Keywords
- Spintronik (2) (remove)
Institute
- Physikalisches Institut (2) (remove)
The present thesis “Hot spin carriers in cold semiconductors” investigates hot carrier effects in low-temperature photoinduced magneto-optical Kerr effect (MOKE) microscopy of electron spins in semiconductor heterostructures. Our studies reveal that the influence of hot photocarriers in magneto-optical pump-probe experiments is twofold.
First, it is commonly assumed that a measurement of the local Kerr rotation using an arbitrary probe wavelength maps the local electron spin polarization. This is the fundamental assumption that underlies the widely used two-color MOKE microscopy technique. Our continuous-wave (cw) spectroscopy experiments demonstrate that this assumption is not correct.
At low lattice temperatures the nonresonant spin excitation by the focused pump laser inevitably leads to a strong heating of the electron system. This heating, in turn, locally modifies the magneto-optical coefficient which links the experimentally observed Kerr rotation to the electron spin polarization. As a consequence, the spin-induced local Kerr rotation is augmented by spin-unrelated changes in the magneto-optical coefficient. A spatially resolved measurement of the Kerr rotation then does not correctly map the electron spin polarization profile.
We demonstrate different ways to overcome this limitation and to correctly measure the electron spin profile. For cw spectroscopy we show how the true local electron spin polarization can be obtained from a quantitative analysis of the full excitonic Kerr rotation spectrum. Alternatively, picosecond MOKE microscopy using a spectrally broad probe laser pulse mitigates hot-carrier effects on the magneto-optical spin detection and allows to directly observe the time-resolved expansion of optically excited electron spin packets in real-space.
Second, we show that hot photocarriers strongly modify the spin diffusion process. Owing to their high kinetic energy, hot carriers greatly enhance the electron spin diffusion coefficient with respect to the intrinsic value of the undisturbed system. Therefore, for steady-state excitation the spin diffusivity is strongly enhanced close to the pump spot center where hot electrons are present. Similarly, for short delays following pulsed excitation the high initial temperature of the electrons leads to a very fast initial expansion of the spin packet which gradually slows as the electrons cool down to the lattice temperature.
While few previous publications have recognized the possible influence of hot carriers on the electron spin transport properties, the present work is the first to directly observe and quantify such hot carrier contributions. We develop models which for steady-state and pulsed excitation quantitatively describe the experimentally observed electron spin diffusion. These models are capable of separating the intrinsic spin diffusivity from the hot electron contribution, and allow to obtain spin transport parameters of the undisturbed system.
We perform extensive cw and time-resolved spectroscopy studies of the lattice temperature dependence of the electron spin diffusion in bulk GaAs. Using our models we obtain a consistent set of parameters for the intrinsic temperature dependence of the electron spin diffusion coefficient and spin relaxation time and the hot carrier contributions which quantitatively describes all experimental observations. Our analysis unequivocally demonstrates that we have, as we believe for the first time, arrived at a coherent understanding of photoinduced low-temperature electron spin diffusion in bulk semiconductors.
Fabrication and characterization of CPP-GMR and spin-transfer torque induced magnetic switching
(2014)
Even though the unique magnetic behavior for ferromagnets has been known for thousands of years, explaining this interesting phenomenon only occurred in the 20th century. It was in 1920, with the discovery of electron spin, that a clear explanation of how ferromagnets achieve their unique magnetic properties came to light. The electron carries an intrinsic electric charge and intrinsic angular momentum. Use of this property in a device was achieved in 1998 when Fert and Gru¨nberg independently found that the resistance of FM/NM/FM trilayer depended on the angle between the magnetization of the two layers. This phenomena which is called giant magnetoresistance (GMR) brought spin transfer into mainstream. This new discovery created a brand new research fi called “spintronics” or “spin based electronics” which exploits the intrinsic spin of electron.
As expected spintronics delivered a new generation of magnetic devices which are currently used in magnetic disk drives and magnetic random access memories (MRAM). The potential advantages of spintronics devices are non-volatility, higher speed, increased data density and low power consumption. GMR devices are already used in industry as magnetic memories and read heads.
The quality of GMR devices can be increased by developing new magnetic materials and also by going down to nanoscale. The desired characteristic properties of these new materials are higher spin polarization, higher curie temperature and better spin filtering. Half-metals are a good candidate for these devices since they are expected to have high polarization. Some examples of half-metals are Half-Heusler alloy, full Heusler alloy and Perovskite or double Perovskite oxides. The devices discussed in this thesis have NiMnSb half-Heusler alloy and permalloy as the ferromagnetic layers separated by Cu as the nonmagnetic layer.
This dissertation includes mainly two parts, fabrication and characterization of nan- opillars. The layer stack used for the fabrication is Ru/Py/Cu/NiMnSb which is grown on an InP substrate with an (In,Ga)As buff by molecule beam epitaxy (MBE). A new method of fabrication using metal mask which has a higher yield of working samples over the previous method (using the resist mask) used in our group is discussed in detail. Also, the advantages of this new method and draw backs of the old method are explained thoroughly (in chapter 3).
The second part (chapters 4 and 5) is focused on electrical measurements and charac- terization of the nanopillar, specially with regard to GMR and spin-transfer torque (STT)
measurements. In chapter 4, the results of current perpendicular the plane giant mag- netoresistance (CPP-GMR) measurements at various temperatures and in-plane magnetic fi are presented. The dependence of CPP-GMR on bias current and shape anisotropy of the device are investigated. Results of these measurements show that the device has strong shape anisotropy.
The following chapter deals with spin-transfer torque induced magnetic switching measurements done on the device. Critical current densities are on the order of 106 A/cm2, which is one order of magnitude smaller than the current industry standards. Our results show that the two possible magnetic configurations of the nanopillar (parallel and anti-parallel) have a strong dependence on the applied in-plane magnetic fi Fi- nally, four magnetic fi regimes based on the stability of the magnetic configuration (P stable, AP stable, both P and AP stable, both P and AP unstable) are identified.