As and Epitaxial-Growth MnSi Thin Films - OPUS Würzburg

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40 3. Electric Control of Magnetization in Thin (Ga,Mn)As Layers _ [010] 2.0 _ [110] [100] [110] [010] __ 1.8 R/R(Vg = -0.5V) 1.6 1.4 1.2 1.0 0 45 90 135 180 Angle of Magnetization Fig. 3.19: Conductance fluctuations with respect to angle of magnetization. All applied field set at a saturation value of 300mT. The gate voltages are from -0.5V to +1V at 5mV steps. We see a concentration of the largest resistance fluctuations along [ 110]. ¯ Fromthemagneticmeasurements, wesee thatthelargestructureshows monotonicbehavior consistent with the results from [Ohno 00, Chib 08] and even [Owen 09]. For smaller structures however, the mesoscopic interactions again becomes more evident as the hopping interactions become stronger with increasing positive gate voltage. Increasing the range of the gate voltage will be of interest for future experiments, driving the material closer to the MIT and will be addressed in the next chapter. 3.3 Reproducible Conductance Fluctuations For the smaller structures, reproducible resistance fluctuations begin to appear in our gating measurements. Shown in Figure 3.19 is the resistance versus voltage with values normalized by the resistance at the lowest gate voltage (V g = -0.5V DC). The gate voltage range is from -0.5V to +1V (5mV steps) with the resistance increasing with increased positive voltage. The applied field is 300mT along different angles. The fluctuations show a concentration of high peaks along the [ 110] ¯ directions. Figure 3.20 shows that the resistance curves are reproducible with respect to field direction, also showing dependence of the fluctuations to the crystalline anisotropy. From Figure 3.9 and 3.10, we see no strong trend between the magnetoresistance peaks, but the uniaxial contribution is greater along this crystal direction for the Hall bar struc-
3.3. Reproducible Conductance Fluctuations 41 8x10 5 [110] 7x10 5 __ [110] __ Resistance ( ) 6x10 5 5x10 5 __ [010] [010] 4x10 5 -0.50 -0.25 0.00 0.25 0.50 0.75 1.00 Gate Voltage (V) Fig. 3.20: Resistance with respect to gate voltage curves along different crystal directions. The conductance fluctuations appear to be reproducible and even with field direction. ture (Figure 3.8). Thus these fluctuations are not completely random but related to the spin-orbit interaction which defines the crystalline anisotropy. The increased resistance fluctuations along the hard axis may be possibly explained as the system being driven into a state with a smaller number of dominant hopping states by the field as the magnetization switches to a uniaxial hard axis [110]. This is possible, as in [Papp 06] a junction was switched into the MIT using an applied field, driving the system to open an Efros- Shklovskii gap.[Efro 75] We also note that our sample is already in the vicinity of the hopping conductance regime (Figure 3.4). Looking more closely to the behavior of these fluctuations at different crystal directions, we examine the normalized resistance fluctuations along the easy and hard axis magnetization directions. We normalize with the [100] curve to remove the linear electric field contribution inthebackground.[Raik 87] (Figure3.21)It can beseen that thefluctuations follow the same trend for each crystal direction shown in Figure 3.12. The fluctuations along the uniaxial hard axes are observed to increase their deviation as the material becomes more metallic, with [110] becoming a harder axis than [1¯10]. This is consistent with previous observations in both the Hall bar (see Figure 3.8) and Corbino structures (Figure 3.12) that as the electric field (i.e. gating voltage) is increased, the ratio between resistances [110]/[1¯10] for the uniaxial hard axes decrease. We can see that the driving the magnetization to the hard axes also affects the interactions in the system, possibly allowing either a small number of hopping paths to dominate conduction or driving the system closer to the MIT by manipulating the anisotropic behavior of the impurity wave-
Page 1 and 2: Characterization of Novel Magnetic
Page 3 and 4: Contents Zusammenfassung 4 Summary
Page 5 and 6: Zusammenfassung Um einerseits ein f
Page 7 and 8: Zusammenfassung 3 auf die anomale t
Page 9 and 10: Summary The study of magnetic phase
Page 11 and 12: Summary 7 bandstructure affecting t
Page 13 and 14: Chapter 1 Introduction Spin electro
Page 15 and 16: 11 states in the MnSi. Chapter 9 fi
Page 17 and 18: Chapter 2 Diluted Ferromagnetic Sem
Page 19 and 20: 2.1. Basic properties of (Ga,Mn)As
Page 21 and 22: 2.3. Electrical Control of Magnetiz
Page 23 and 24: 2.4. (Ga, Mn)As and the Metal-Insul
Page 25 and 26: 2.4. (Ga, Mn)As and the Metal-Insul
Page 27 and 28: 2.5. Summary 23 2.5 Summary A brief
Page 29 and 30: Chapter 3 Electric Control of Magne
Page 31 and 32: 3.2. Electrical control of magnetiz
Page 43: 3.2. Electrical control of magnetiz
Page 47 and 48: 3.4. Summary 43 changed significant
Page 49 and 50: Chapter 4 Epitaxial lift-off of (Ga
Page 51 and 52: 4.2. Test Barrier Material 47 Fig.
Page 53 and 54: 4.4. Magnetotransport in ELO-proces
Page 55 and 56: 4.4. Magnetotransport in ELO-proces
Page 57 and 58: 4.5. Electrical gating of ELO-proce
Page 59 and 60: Chapter 5 Basic properties of Ferro
Page 61 and 62: 5.1. Bulk properties of MnSi 57 (a)
Page 63 and 64: 5.2. Expitaxially-grown MnSi Thin F
Page 65 and 66: 5.2. Expitaxially-grown MnSi Thin F
Page 67 and 68: 5.3. Skyrmions, Chirality and Magne
Page 69 and 70: 5.3. Skyrmions, Chirality and Magne
Page 71 and 72: 5.4. Summary 67 tions. MnSi provide
Page 73 and 74: Chapter 6 Epitaxial Growth of MnSi
Page 75 and 76: 6.2. Material Characterization 71 F
Page 77 and 78: 6.2. Material Characterization 73 M
Page 79 and 80: 6.3. Device Fabrication 75 Fig. 6.5
Page 81 and 82: 6.4. Temperature-dependent measurem
Page 83 and 84: 6.5. Summary 79 expected for the he
Page 85 and 86: Chapter 7 Magnetotransport with H
Page 87 and 88: 7.1. Hall Measurements 83 0 J || [-
Page 89 and 90: 7.1. Hall Measurements 85 0.40 R xy
Page 91 and 92: 7.1. Hall Measurements 87 is two or
Page 93 and 94: 7.1. Hall Measurements 89 6 J || [1
Page 95 and 96:
7.1. Hall Measurements 91 0.02 0.00
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7.1. Hall Measurements 93 0.100 -55
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7.1. Hall Measurements 95 0.15 R xy
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7.1. Hall Measurements 97 Magnetore
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7.2. Summary 99 T c . The sign of t
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Chapter 8 Magnetotransport with In-
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8.1. Magnetotransport Measurements
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Page 111 and 112:
Page 113 and 114:
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8.2. Summary 115 8.2 Summary Low ma
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Chapter 9 Conclusions & Outlook In
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Appendix A Fabrication of Four-Term
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A.2. Optimized final process 121 Fi
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A.2. Optimized final process 123 95
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Appendix B Epitaxial lift-off techn
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127 Evap 2 Wire Rinse in water 1min
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Bibliography [Abra 96] E. Abrahams
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131 [Edmo 03] K. Edmonds. Journal o
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133 [Ishi 77] Y. Ishikawa, G. Shira
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135 [Mudu 05] P. K. Muduli, K.-J. F
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137 [Rich 10] A. Richardella, P. Ro
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139 [Vila 07] L. Vila, R. Giraud, L
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Acknowledgements First ofall I woul
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As and Epitaxial-Growth MnSi Thin Films - OPUS Würzburg

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