Präparation und Untersuchung von Submikrometer-Strukturen auf der Basis magnetischer Dünnschichten [Elektronische Ressource] / von Nicole Auth

Sujets

• Physik

Informations

• Publié par: johannes_gutenberg-universitat_mainz
• Publié le: 01 janvier 2005
• Nombre de lectures: 15
• Langue: English
• Poids de l'ouvrage: 7 Mo

Extrait

Pr¨aparation und Untersuchung von
Submikrometer Strukturen auf der
Basis magnetischer Du¨nnschichten
Dissertation
zur Erlangung des Grades
Doktor der Naturwissenschaften (Dr. rer. nat.)
am Fachbereich Physik
der Johannes Gutenberg-Universit¨at Mainz
von
Nicole Auth
geb. in Mainz
Mainz, 2005
D 77Tag der Einreichung: 24.03.2005
Tag der mu¨ndlichen Pru¨fung: 29.06.2005Preparation and study
of magnetic thin film based
sub-micron structures
Ph.D. thesis
by
Nicole Auth
Mainz, Germany 2005Diese Arbeit entstand mit Hilfe vonMikT X 1.20dund der Bearbeitungsklasse book.cls. Fu¨rE
das Erstellen des Inhaltsverzeichnisses wurde BibT X mit der Stilparameterdatei apsrev.bstE
(American Physical Society Version 4 von RevT X) verwendet. Das Seitenlayout basiert aufE
der Layoutdatei fancy.sty.Contents
Introduction 1
1 Epitaxial Sr FeMoO thin films: preparation and properties 52 6
1.1 Sr FeMoO basics - consolidated findings . . . . . . . . . . . . . . 52 6
1.1.1 Crystal structure and magnetism . . . . . . . . . . . . . . 6
1.1.2 Saturation magnetisation and antisite disorder . . . . . . . 6
1.1.3 Magnetotransport and sample texture. . . . . . . . . . . . 9
1.2 Preparation and structural characterisation . . . . . . . . . . . . . 11
1.2.1 Pulsed laser deposition of Sr FeMoO films . . . . . . . . . 112 6
1.2.2 Samples on SrTiO (001)-oriented substrates . . . . . . . . 143
1.2.3 Samples on SrTiO with Ba Sr TiO buffer layer . . . . 183 0.4 0.6 3
1.2.4 Samples on DyScO (110)-oriented substrates . . . . . . . 223
1.3 Interplay of structure and magnetism . . . . . . . . . . . . . . . . 24
1.3.1 Strain and ASD by x-ray diffractometry . . . . . . . . . . 24
1.3.2 Magnetisation measurements and data treatment . . . . . 27
1.3.3 Antisite disorder limited saturation magnetisation and the
role of strain. . . . . . . . . . . . . . . . . . . . . . . . . . 30
2 Magnetotransport in Sr FeMoO thin films 332 6
2.1 Temperature dependent residual resistivity . . . . . . . . . . . . . 33
2.1.1 Thin film series A, B and C . . . . . . . . . . . . . . . . . 34
2.1.2 Thin films on DyScO and Ba Sr TiO . . . . . . . . . 353 0.4 0.6 3
2.1.3 Universal temperature dependence and resistivity minimum 36
2.2 Magnetoresistance . . . . . . . . . . . . . . . . . . . . . . . . . . 41
2.2.1 Magnetoresistance in thin film series B and C . . . . . . . 43
2.2.2 Magnetoresistance in highly epitaxial and textured thin films 45
3 Investigating the spin polarisation of magnetoresistive materials
by point contact spectroscopy 47
3.1 Experimental methods for probing the spin polarisation . . . . . . 47
iii CONTENTS
3.1.1 Spin-polarised photoemission . . . . . . . . . . . . . . . . 48
3.1.2 The Meservey-Tedrow technique . . . . . . . . . . . . . . . 49
3.1.3 Point Contact Andreev Reflection . . . . . . . . . . . . . . 51
3.2 Theoretical basis of Andreev reflection in metal/superconductor
junctions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
3.2.1 BCS theory of superconductivity . . . . . . . . . . . . . . 54
3.2.2 BTK theory of transparent normal metal/BCS supercon-
ductor junctions . . . . . . . . . . . . . . . . . . . . . . . . 57
3.2.3 Mazins generalisation of the BTK approach . . . . . . . . 61
3.2.4 MATLAB simulation with the Mazin model . . . . . . . . 63
3.3 Experimental setup and test measurements . . . . . . . . . . . . . 66
3.3.1 Experimental setup for PCAR studies . . . . . . . . . . . . 66
3.3.2 Test measurements on Cu-Pb junctions . . . . . . . . . . . 68
3.4 PCAR analysis of potential half-metals with high resistivities . . . 70
3.4.1 Analysis of Sr FeMoO -Sn spectra . . . . . . . . . . . . . 712 6
3.4.2 Analysis of Co Cr Fe Al-Sn spectra . . . . . . . . . . . 742 0.6 0.4
3.5 Concluding discussion on practical limits of the PCAR method . . 76
4 Nanopatterning by Focussed Ion Beam Etching and Electron
Beam Lithography 79
4.1 Basic setup of EBL and FIB systems . . . . . . . . . . . . . . . . 80
4.2 Electron beam lithography . . . . . . . . . . . . . . . . . . . . . . 83
4.2.1 The electron resist and pre-exposure steps . . . . . . . . . 83
4.2.2 Resolution limits and proximity effect . . . . . . . . . . . . 85
4.3 Arrays of magnetic structures with submicron dimensions . . . . . 88
4.3.1 Micromagnetism . . . . . . . . . . . . . . . . . . . . . . . 88
4.3.2 Line and dot arrays fabricated by EBL . . . . . . . . . . . 90
4.3.3 MOKE study of the remagnetisation process . . . . . . . . 93
4.4 Nanofabrication with a focussed ion beam . . . . . . . . . . . . . 95
4.4.1 Performance characteristics of the SEM/FIB facility . . . . 96
4.5 Small area TMR devices . . . . . . . . . . . . . . . . . . . . . . . 98
4.5.1 Pre-structuring by optical lithography . . . . . . . . . . . 98
4.5.2 Definition of the mesa structure by FIB etching . . . . . . 99
4.5.3 Finalising steps of mesa fabrication . . . . . . . . . . . . . 101
Summary 103
A PCAR simulation routines 107
A.1 MATLAB program for ballistic contacts . . . . . . . . . . . . . . 107
A.2 MATLAB program for diffusive contacts . . . . . . . . . . . . . . 108CONTENTS iii
B FIB operation manual 113
B.1 Sample holder and preparatory steps . . . . . . . . . . . . . . . . 113
B.2 SEM operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
B.3 Starting the FIB unit . . . . . . . . . . . . . . . . . . . . . . . . . 115
B.4 FIB operation with the Elphy Plus software . . . . . . . . . . . . 116
B.5 System shut-down . . . . . . . . . . . . . . . . . . . . . . . . . . . 118
Bibliography 119
List of Publications 127List of Figures
1 Principle of the TMR effect. . . . . . . . . . . . . . . . . . . . . . 2
1.1 Graphical representation of the Sr FeMoO crystal structure. . . . 72 6
1.2 Pseudo double-exchange mechanism. . . . . . . . . . . . . . . . . 7
1.3 Relation between cationic ordering and saturation magnetisation
for Sr FeMoO bulk samples prepared at various temperatures. . . 82 6
1.4 ResistanceversustemperaturebehaviourofSr FeMoO bulksingle2 6
crystals. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
1.5 Photo of the Pulsed Laser Deposition setup. . . . . . . . . . . . . 12
1.6 Schematic illustration of the Pulsed Laser Deposition method. . . 13
1.7 Target preparation routine. . . . . . . . . . . . . . . . . . . . . . 14
1.8 AFM micrograph of an annealed SrTiO surface. . . . . . . . . . . 153
1.9 Sr FeMoO thin film with SrMoO impurity phase. . . . . . . . . 162 6 4
1.10 Influence of the deposition rate on the film quality. . . . . . . . . 16
1.11 AFM (STM) micrographs of typical samples from series B and C. 17
1.12 Preparationparametersandx-rayanalysisofBa Sr TiO target0.4 0.6 3
material. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
1.13 AFM picture and 2-circle x-ray diffraction pattern of an epitaxial
Ba Sr TiO sample. . . . . . . . . . . . . . . . . . . . . . . . . 200.4 0.6 3
1.14 Sr FeMoO on a textured Ba Sr TiO buffer layer. . . . . . . . 212 6 0.4 0.6 3
1.15 DyScO crystal structure and lattice constants. . . . . . . . . . . 223
1.16 AFMpicture and2-circlex-raydiffractionpatternofaSr FeMoO2 6
thin film on DyScO .. . . . . . . . . . . . . . . . . . . . . . . . . 233
1.17 Setup of the 2-circle x-ray diffractometer. . . . . . . . . . . . . . . 24
1.18 Setup of the 2- and 4-circle x-ray diffractometers. . . . . . . . . . 25
1.19 2-circle x-ray diffraction pattern of a epitaxial Ba Sr TiO layer. 250.4 0.6 3
1.20 Evolution of the c-axis for sample series B and C. . . . . . . . . . 26
1.21 Determination of the Sr FeMoO and SrTiO contributions in the2 6 3
4-circle diffractogram.. . . . . . . . . . . . . . . . . . . . . . . . . 27
1.22 Refinement of magnetisation data. . . . . . . . . . . . . . . . . . . 28
vvi LIST OF FIGURES
1.23 Typical magnetisation curves for sample series A, B and C. . . . . 29
1.24 Saturation magnetisation versus antisite disorder. . . . . . . . . . 31
1.25 Influenceofstrainonthesaturationmagnetisationandtheantisite
disorder. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
2.1 Resistivity versus temperature in sample series A, B and C. . . . 34
2.2 Resistivity versus temperature in samples DyScO . . . . . . . . . 353
2.3 Resistivity versus temperature in samples on Ba Sr TiO . . . . 360.4 0.6 3
2.4 Analysis of single-magnon scattering as a possible contribution to
the resistance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
2.5 Analysis of the temperature dependent resistivity with a model of
small-polaron transport. . . . . . . . . . . . . . . . . . . . . . . . 38
2.6 Magnetoresistance versus magnetic field for bulk polycrystals and
epitaxial thin films. . . . . . . . . . . . . . . . . . . . . . . . . . . 41
2.7 Derivative of MR versus magnetic field for bulk polycrystals and
epitaxial thin films. . . . . . . . . . . . . . . . . . . . . . . . . . . 42
2.8 Antisite boundaries as a source of low field magnetoresistance. . . 43
2.9 Temperature dependent magnetoresistance in series B and C. . . . 44
2.10 Magnetoresistance and derivative versus magnetic field for epitax-
ial thin films of series B and C. . . . . . . . . . . . . . . . . . . . 44
2.11 TemperaturedependentmagnetoresistanceinSr FeMoO thinfilms2 6
on DyScO and on SrTiO with a Ba Sr TiO buffer layer. . . . 453 3 0.4 0.6 3
2.12 Magnetoresi