Ferromagnetic (Ga,Mn)As Layers and ... - OPUS Würzburg

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12 1. Introduction the standard III-V semiconductor GaAs with magnetic Mn acceptors [Ohno 98]. The strong spin-orbit mediated coupling of magnetic and semiconducting properties in this material [Diet 00] gives rise to many novel transport-related phenomena. Previously reported device concepts include strong anisotropic magnetoresistance (AMR) [Baxt 02], planar Hall effect [Tang 03], tunneling AMR (TAMR) [Goul 04, Rüst 05, Papp 06] and Coulomb blockade AMR [Wund 06]. To date, the low ferromagnetic transition temperature limits the use of FS to laboratory applications. For (Ga,Mn)As, the highest obtained Curie temperature is 180 K [Olej 08]. Even though the Curie temperature in this material may never reach room temperature, the insight gained about the aforementioned phenomena is expected to apply to any FS material with strong spin-orbit coupling. Promising material research is ongoing worldwide in the search for alternate room temperature FS. Most previous demonstrations have been based on structures that have the same magnetic properties, inherited from the unstructured (Ga,Mn)As layer, throughout the device. Recent improvements in lithographic capabilities have opened the way for nanoscale structural patterning of (Ga,Mn)As layers. With this achievement, it has become possible to access locally induced strain relaxation as a means of influencing the magnetic anisotropy properties of the material. This greatly enhances the scope of possible device paradigms, as it allows for devices where the functional element involves transport between regions with different, independently engineered magnetic anisotropy properties. The objective of this thesis is twofold. The main focus lies on presenting a comprehensive study on the technique of local anisotropy control by strain engineering. Also, the manufacturing of (Ga,Mn)As samples by molecular beam epitaxy (MBE) will be described in detail, including a discussion of all factors which need to be considered in the epitaxial growth of this material system. The outline of this thesis is as follows. Chapter 2 encompasses a general introduction into the (Ga,Mn)As material system, beginning with the basic concepts of doping and growth-induced lattice strain. The occurrence of ferromagnetism and the magnetic anisotropy properties are explained. Furthermore, an overview of the factors playing a role in k · p band structure calculations in this material is provided. The chapter closes with a remark on the influence of annealing on the material system. Chapter 3 focuses on the epitaxial growth of (Ga,Mn)As, with a detailed examination of the relevant growth parameters and their influence on the material properties. The most important in-situ (RHEED) and ex-situ (HRXRD) characterization techniques are addressed. Chapter 4 supplies the groundwork needed to understand the structural effect of lithographic nanopatterning of (Ga,Mn)As layers on the crystal. A set of formulas is derived for a 3D finite element simulation. Using the results of such simulations, we examine the influence of several geometrical parameters on the strain relaxation of a stripe structure, either aligned along the [100] or [1¯10] crystal direction. Chapter 5 contains the characterization of a series of stripe samples, beginning with the structural characterization by HRXRD and GIXRD. The magnetic properties are
13 investigated with SQUID and magnetotransport measurements, revealing the large impact of anisotropic strain on the magnetic properties of the stripes. Finally, we present a theoretical model based on k·p calculations and magnetostatic energy considerations to explain the influence of strain on the magnetic anisotropy of (Ga,Mn)As. Chapter 6 closes the gap between fundamental studies on strain relaxation in simple stripe structures and device application. Two orthogonal nanobars are electrically coupled via a constriction. The resulting device can act as a nonvolatile memory storage by making use of the relative magnetization states in the stripes. We investigate the strain distribution in the constriction region with simulations and discuss its influence on the constriction properties.
Page 1 and 2: Ferromagnetic (Ga,Mn)As Layers and
Page 3 and 4: Contents Zusammenfassung 5 Summary
Page 5 and 6: Zusammenfassung Ferromagnetische Ha
Page 7 and 8: Anisotropiekontrolle in (Ga,Mn)As i
Page 9 and 10: Summary The great promise of ferrom
Page 11: Chapter 1 Introduction The event co
Page 15 and 16: Chapter 2 The (Ga,Mn)As Material Sy
Page 17 and 18: 2.1. Ferromagnetism in (Ga,Mn)As 17
Page 19 and 20: 2.2. Band Structure Calculations in
Page 21 and 22: 2.3. Magnetic Anisotropy 21 growth
Page 23 and 24: 2.3. Magnetic Anisotropy 23 E n e r
Page 25 and 26: 2.3. Magnetic Anisotropy 25 Figure
Page 27 and 28: Chapter 3 MBE Growth of Ferromagnet
Page 29 and 30: 3.2. Epitaxial Growth of (Ga,Mn)As
Page 31 and 32: 3.3. Crystal Defects 31 lead to a d
Page 33 and 34: 3.4. RHEED 33 Figure 3.5: (a) Ewald
Page 35 and 36: 3.5. X-Ray Diffraction 35 1 0 7 1 0
Page 37 and 38: 3.5. X-Ray Diffraction 37 between t
Page 39 and 40: 3.5. X-Ray Diffraction 39 lattice d
Page 41 and 42: 3.5. X-Ray Diffraction 41 substrate
Page 43 and 44: Chapter 4 Finite Element Simulation
Page 45 and 46: 4.1. Derivation of the Equation Sys
Page 47 and 48: 4.2. The FlexPDE Software 47 4.2 Th
Page 49 and 50: 4.3. Simulation Results - Physical
Page 55 and 56: 4.4. Simulation Results - (In,Ga)As
Page 57 and 58: 4.5. Stripes Along [1¯10] 57 Figur
Page 59 and 60: 4.5. Stripes Along [1¯10] 59 in-pl
Page 61 and 62: Chapter 5 Local Anisotropy Control
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5.1. Patterning and Structural Char
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5.2. Magnetic Characterization 73 p
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5.2. Magnetic Characterization 75 1
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5.2. Magnetic Characterization 77 8
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5.3. Shape Anisotropy 79 ∼ 0.2 k
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5.4. A Model for Anisotropy Orienta
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5.4. A Model for Anisotropy Orienta
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Chapter 6 Device Application As sta
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6.2. The Role of the Constriction 8
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Chapter 7 Conclusion and Outlook Th
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Appendix A Band Structure Hamiltoni
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Appendix B Sample FlexPDE Input Fil
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99 BOUNDARIES s u r f a c e ’ s u
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Bibliography [Abol 01] M. Abolfath,
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103 [McGu 75] T. R. McGuire and R.
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Publications • J. Wenisch, C. Gou
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Acknowledgements There is a number
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Ferromagnetic (Ga,Mn)As Layers and ... - OPUS Würzburg

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