In this thesis spin dynamics in (Zn,Mn)Se/(Zn,Be)Se and (Cd,Mn)Te/(Cd,Mg)Te DMS quantum well heterostructures with a type-I band alignment are studied, where the carriers are quantum confined. Especially the important role of free carriers in heating of the Mn-system, by its interaction with photoexcited carriers with excess kinetic energy, and in the cooling of the Mn-system in the presence of cold background carriers, provided by modulation doping, is established.
The studies are separated in three chapters. In the fourth chapter of this thesis, new results on energy and spin transfer between free carriers and Mn-ion system are presented. Contributions of direct heating of the Mn-system by photocarriers and indirect heating via nonequilibrium phonons are distinguished and their competition is discussed. In the fifth chapter dynamics of spin-lattice relaxation of magnetic Mn-ions in DMS QW heterostructures is investigated and new experimental studies on (Zn,Mn)Se/(Zn,Be)Se heterostructures are shown.
Crucial for spintronic devices is the ability to tune the spin relaxation time precisely, as the spin relaxation time is important in double respects. On the one hand spin polarization must be conserved over long times and distances, if the spin shall be processed or stored in a region, which is spatial separated from the spin-injector. Especially for the possibility of utilizing spins as quantum bits for quantum information processing, long spin polarization is needed. On the other hand short spin relaxation time is needed for fast switching between different spin-states. For instance semiconductor lasers can be switched off extremely fast by reorientation of spin. This very relevant topic is devoted the sixth chapter, before the thesis is summarized in the last chapter. Especially for one of the biggest drawbacks for precise tuning, that the magnetization dynamics in DMS cannot be controlled separately from the static magnetization, solutions via electric field control of the magnetization dynamics or via the technological concept of “digital alloying” are presented.precise tuning, that the magnetization dynamics in DMS cannot be controlled separately from the static magnetization, solutions via electric field control of the magnetization dynamics or via the technological concept of “digital alloying” are presented.
Table of Contents
Introduction
1 II-VI diluted magnetic semiconductors
1.1 Crystal structure of (Cd,Mn)Te and (Zn,Mn)Se
1.2 Band structure of (Cd,Mn)Te and (Zn,Mn)Se
1.2.1 Band structure of zincblende semiconductors
1.2.2 Band structure of zincblende semiconductors containing manganese
1.3 Magnetic properties
1.3.1 Basic principles of magnetism
1.3.1.1 Larmor Diamagnetism
1.3.1.2 Paramagnetism
1.3.1.3 Heisenberg model
1.3.1.4 Ferromagnetism
1.3.1.5 Ferrimagnetism
1.3.1.6 Antiferromagnetism
1.3.2 Magnetic effects of free electrons
1.3.3 Magnetic properties of (Cd,Mn)Te and (Zn,Mn)Se without Mn-Mn interaction
1.3.4 Exchange Interactions
1.3.4.1 sp-d exchange interaction
1.3.4.2 d-d exchange interaction
1.3.4.3 Magnetic properties of (Cd,Mn)Te and (Zn,Mn)Se with Mn Mn interactions
1.3.4.4 Giant Zeeman-splitting
1.4 Quantum well heterostructures
1.4.1 Single-particle states in quantum wells
1.4.2 Spin-orbit-splitting in quantum wells
1.4.3 Heterostructures in magnetic field
1.4.4 Density of states in quantum wells
1.4.5 Selection rules and polarization degree in quantum wells
1.4.6 Parabolic and half-parabolic quantum wells
1.5 Excitons
1.5.1 Free exciton
1.5.2 Interaction of excitons with Mn2+-ions
1.5.3 Quasi-two-dimensional excitons in quantum wells
1.5.4 Quasi-two-dimensional excitons in magnetic field
1.5.5 Trions
2 Magnetization dynamics
2.1 Spin and energy transfer
2.1.1 Coupled systems in diluted magnetic semiconductors
2.1.2 Theoretical formulation of spin and energy transfer
2.1.3 Manganese spin temperature in stationary condition
2.2 Mechanisms for spin relaxation
2.2.1 D’yakonov-Perel mechanism
2.2.2 Elliott-Yafet mechanism
2.2.3 Bir-Aronov-Pikus mechanism
2.2.4 Hyperfine-interaction mechanism
2.2.5 Spin relaxation in excitons
2.3 Spin lattice relaxation
2.4 Spin diffusion
3 Experimental technique
3.1 Optical detection of Mn spin temperature
3.2 Heating of the Mn spin system
3.2.1 Heating by laser light
3.2.2 Heating by electric current
3.2.3 Heating by phonons
3.3 Time-resolved measurements
3.4 Experimental setup
4 Interaction between carriers and Mn-spin system
4.1 Twofold dynamic impact for Mn heating
4.2 Direct energy and spin transfer
4.3 Competition between direct and indirect energy and spin transfer
4.4 Influence of excitation density
4.5 Distinction between direct and indirect heating of the Mn system
4.5.1 Steady-state optical excitation
4.5.2 Long pulses with low and moderate excitation densities
4.5.3 Short pulses with high excitation densities
5 Spin-lattice relaxation
5.1 Dependence of the spin-lattice relaxation on the Mn content
5.2 Effect of free carriers in doped structures
6 Control of spin-lattice relaxation
6.1 Electric field control of 2DEG
6.2 Engineering of spin-lattice relaxation by digital growth
6.3 Spin-lattice relaxation in parabolic and half-parabolic quantum wells
6.4 Acceleration of spin-lattice relaxation by spin diffusion
A Samples
A.1 Preparation of the samples
A.1.1 Molecular beam epitaxy
A.1.2 Quantum well heterostructures
A.1.3 Structure with electric contacts
A.1.4 Digital growth technique
A.2 Tables of samples
A.3 Lattice and electronic properties
B Measurement and treatment of the experimental data
B.1 Giant Zeeman shift
B.2 Spin-lattice relaxation time
Research Objectives and Core Topics
This doctoral thesis investigates the magnetization dynamics in Diluted Magnetic Semiconductor (DMS) heterostructures based on (Cd,Mn)Te and (Zn,Mn)Se, focusing on the interplay between magnetic ions, free carriers, and the lattice. The central research goal is to understand and control the spin and energy transfer mechanisms, particularly the spin-lattice relaxation (SLR) and magnetization dynamics, in order to overcome the limitations where static and dynamic properties are intrinsically coupled. The thesis explores how external inputs, such as electric fields and customized digital growth profiles, can be used to independently tune these magnetic properties for potential spintronic applications.
- Spin-dynamics and energy transfer in DMS systems under non-equilibrium conditions.
- Distinction between direct carrier-mediated heating and indirect phonon-mediated heating of the Mn-spin system.
- Investigation of spin-lattice relaxation mechanisms and the impact of Mn concentration and free carrier density (2DEG).
- Technological control of spin dynamics through digital alloying and electric field-effect structures.
- Experimental development of time-resolved magneto-optical techniques for studying magnetization dynamics on nanosecond timescales.
Auszug aus dem Buch
1.1 Crystal structure of (Cd,Mn)Te and (Zn,Mn)Se
Solid states in crystalline phase have a spatial symmetry, arising from the continuous arrangement of the atoms in the crystal lattice. The materials, investigated in this thesis, are based on the II-VI binary compound-semiconductors CdTe and ZnSe. The latter have the propensity to crystallize in a variety of polymorphic modifications. In general, structures for II-VI semiconductors are hexagonal wurtzite and cubic zincblende (sphalerite) [Ave67]. CdTe and ZnSe possess under normal conditions the zincblende structure [Yeh92].
In the ternary materials (Zn,Mn)Se, (Zn,Be)Se and (Cd,Mn)Te and (Cd,Mg)Te a small amount of the Zn2+-ions and Cd2+-ions, respectively, is exchanged by likewise bivalent Be2+-, Mg2+- and Mn2+-ions, respectively. Bulk crystals of BeSe exhibit the zincblende structure [Wyc63], of MgTe the wurtzite structure [Kle51, Kuh71, Par71, Zac27], of MnSe the rocksalt (NaCl) structure [Dur89] and of MnTe the hexagonal NiAs structure [Oft27]. Therefore, the crystal structure of the ternary materials is dependant on the exchanged amount of cations. The corresponding composition ranges and the upper limits for successful incorporation of the magnetic Mn-ions are given in table 1.1. It is remarkable that such high values of x for the Mn-ions in ternary alloys can be reached, although the crystal structures of MnSe and MnTe are neither zincblende nor wurtzite.
Summary of Chapters
II-VI diluted magnetic semiconductors: Provides an overview of structural, electronic, and magnetic properties, focusing on II-VI host materials and manganese-doped ternary alloys.
Magnetization dynamics: Establishes the theoretical framework for spin and energy transfer between Mn-ions, free carriers, and the lattice, introducing governing spin-relaxation mechanisms.
Experimental technique: Describes the time-resolved photoluminescence (PL) techniques and the experimental setups used to investigate magnetization dynamics and Mn-spin temperature.
Interaction between carriers and Mn-spin system: Presents results on energy transfer pathways, distinguishing between direct carrier heating and indirect phonon-assisted heating.
Spin-lattice relaxation: Analyzes the dependency of spin-lattice relaxation (SLR) times on Mn concentration and the influence of a 2DEG on these relaxation dynamics.
Control of spin-lattice relaxation: Demonstrates methods to independently control SLR and static magnetization through electric field gate control of 2DEG and digital alloying growth techniques.
Keywords
Diluted Magnetic Semiconductors, Spintronics, Magnetization Dynamics, Spin-Lattice Relaxation, Quantum Well Heterostructures, (Cd,Mn)Te, (Zn,Mn)Se, Giant Zeeman-Splitting, Photoluminescence, Energy Transfer, Digital Alloying, 2DEG, Spin Diffusion, Spin-Spin Interaction
Frequently Asked Questions
What is the fundamental scope of this work?
The thesis explores the magnetization dynamics in II-VI diluted magnetic semiconductor (DMS) heterostructures, specifically investigating how to decouple and independently control static and dynamic magnetic properties for spintronic applications.
What are the primary thematic fields covered?
The core themes include spin and energy transfer between magnetic Mn-ions, free carriers, and phonons, as well as the experimental control of spin-lattice relaxation (SLR) using modulation doping and digital growth techniques.
What is the primary research goal?
The objective is to understand the interplay between the Mn-spin system and other subsystems to overcome current technological constraints, specifically the coupling of static and dynamic magnetic properties, which limits the optimization of spin-based devices.
Which scientific methods are utilized?
The work employs time-resolved photoluminescence (PL) spectroscopy, magneto-optical measurements in external magnetic fields, and numerical modeling to analyze relaxation dynamics and temperature-dependent magnetic responses.
What is treated in the main body?
The main body details the theoretical basis of spin dynamics, experimental setups for time-resolved measurement, and provides extensive results on energy transfer mechanisms and methods for controlling spin-lattice relaxation in quantum well structures.
Which keywords best characterize the work?
The work is defined by terms such as Diluted Magnetic Semiconductors, Spintronics, Magnetization Dynamics, Spin-Lattice Relaxation, Quantum Wells, and Digital Alloying.
How do free carriers influence the spin-lattice relaxation?
Free carriers, specifically a two-dimensional electron gas (2DEG), act as an efficient bypass channel for energy transfer from the Mn-spin system to the lattice, thereby significantly accelerating the spin-lattice relaxation rate.
What is the role of the digital growth approach?
The digital growth technique is used to engineer inhomogeneous Mn-ion profiles, allowing for the decoupling of static magnetization (from paramagnetic spins) and magnetization dynamics (influenced by clusters and spin diffusion), thus providing a new degree of freedom for spin engineering.
- Quote paper
- Dr. rer.nat. Dipl.-Phys. Dipl.-Kfm. Martin Kneip (Author), 2008, Magnetization Dynamics in Diluted Magnetic Semiconductor Heterostructures, Munich, GRIN Verlag, https://www.grin.com/document/122287
