Magnetic tunneling junctions with the Heusler compound Co_1tn2Cr_1tn0_1tn._1tn6Fe_1tn0_1tn._1tn4Al [Elektronische Ressource] / von Andrés Conca Parra

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• Physik

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• Publié par: johannes_gutenberg-universitat_mainz
• Publié le: 01 janvier 2008
• Nombre de lectures: 18
• Langue: English
• Poids de l'ouvrage: 7 Mo

Extrait

Magnetic tunneling junctions with the
Heusler compound Co Cr Fe Al2 0.6 0.4
Dissertation
zur Erlangung des Grades
Doktor der Naturwissenschaften (Dr.rer.nat.)
am Fachbereich Physik
der Johannes Gutenberg-Universit¨at Mainz
von
Andr´es Conca Parra
geboren in Biar, Alicante, Spanien
Mainz, 2007Tag der mu¨ndliche Pru¨fung: 20.07.2007Keep an open mind, but no so open that your brain falls out.
Richard P.FeynmanivContents
1 Theoretical background of TMR 5
1.1 Interfaces and barrier influence . . . . . . . . . . . . . . . . . . . 5
1.1.1 Diffusive and coherent regime . . . . . . . . . . . . . . . . 5
1.1.2 Relation between SP of the tunneling current and SP of
electrodes. Validity of Julli`ere model . . . . . . . . . . . . 7
1.1.3 Interfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
1.2 Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
1.3 Applications of magnetic tunneling junctions. . . . . . . . . . . . 11
2 Half metallic Heusler compounds 17
2.1 General considerations . . . . . . . . . . . . . . . . . . . . . . . . 17
2.1.1 Origin of the gap in heusler alloys . . . . . . . . . . . . . . 18
2.2 Co CrAl and Co Cr Fe Al . . . . . . . . . . . . . . . . . . . . 202 2 0.6 0.4
2.2.1 Disorder effects . . . . . . . . . . . . . . . . . . . . . . . . 21
2.2.2 Spin-orbit interaction and SP . . . . . . . . . . . . . . . . 22
2.2.3 Is the surface/interface also half-metallic? . . . . . . . . . 23
2.2.4 Temperature dependence of the SP in half-metals . . . . . 24
3 Preparation of the samples 27
3.1 Deposition system . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
3.2 Deposition of the samples . . . . . . . . . . . . . . . . . . . . . . 28
3.2.1 Substrates preparation . . . . . . . . . . . . . . . . . . . . 28
3.2.2 Cr and Fe buffer layer deposition . . . . . . . . . . . . . . 29
3.2.3 Co Cr Fe Al and MTJ’s deposition . . . . . . . . . . . . 302 0.6 0.4
3.3 Preparation of mesa structures . . . . . . . . . . . . . . . . . . . . 33
3.3.1 Etching angle dependence . . . . . . . . . . . . . . . . . . 37
vvi CONTENTS
4 CCFA thin films 39
4.1 Crystallographic properties . . . . . . . . . . . . . . . . . . . . . . 39
4.1.1 General considerations . . . . . . . . . . . . . . . . . . . . 39
4.1.2 Polycrystalline samples . . . . . . . . . . . . . . . . . . . . 40
4.1.3 Epitaxial films with buffer layer . . . . . . . . . . . . . . . 43
4.1.4 Epitaxial films without buffer layer . . . . . . . . . . . . . 49
4.1.5 Estimation of the degree of disorder . . . . . . . . . . . . 54
4.2 Magnetic properties . . . . . . . . . . . . . . . . . . . . . . . . . . 57
4.2.1 Polycrystalline samples . . . . . . . . . . . . . . . . . . . 57
4.2.2 Epitaxial samples . . . . . . . . . . . . . . . . . . . . . . . 58
4.2.3 XMCD and XAS results . . . . . . . . . . . . . . . . . . . 61
4.3 Transport properties . . . . . . . . . . . . . . . . . . . . . . . . . 63
4.4 Surface ordering and topology . . . . . . . . . . . . . . . . . . . . 65
5 Magnetic tunneling junctions 75
5.1 Exchange bias . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
5.2 TMR of polycrystalline samples . . . . . . . . . . . . . . . . . . . 78
5.3 TMR in epitaxial samples . . . . . . . . . . . . . . . . . . . . . . 81
5.3.1 Annealing temperature dependence . . . . . . . . . . . . . 81
5.3.2 Correlation between surface order and topology and TMR 84
5.3.3 Surface tailoring . . . . . . . . . . . . . . . . . . . . . . . 86
5.3.4 Oxidation time dependence . . . . . . . . . . . . . . . . . 88
5.3.5 TEM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
6 Tunneling spectroscopy 91
6.1 Measurements of the differential conductivity. . . . . . . . . . . . 92
6.2 Interpretation of the dI/dV (V ) and ... . . . . . . . . . . . . 101BIAS
6.2.1 Zero bias anomaly . . . . . . . . . . . . . . . . . . . . . . 101
6.2.2 Change in the TMR sign . . . . . . . . . . . . . . . . . . . 103
6.2.3 Additional structures . . . . . . . . . . . . . . . . . . . . 106
6.3 IET measurements . . . . . . . . . . . . . . . . . . . . . . . . . . 110
A 121
A.1 Patterning of mesa structures . . . . . . . . . . . . . . . . . . . . 121
A.2 Sputtering and oxidation parameters . . . . . . . . . . . . . . . . 123
Bibliography 125List of Figures
1.1 Calculationsofthek -dependentconductanceoftheminoritychan-k
nel of MTJ’s with ordered ZnSe barrier . . . . . . . . . . . . . . . 6
1.2 Calculationsofthek -dependentconductanceoftheminoritychan-k
nel of MTJ’s with ordered ZnSe barrier . . . . . . . . . . . . . . . 8
1.3 Simple model of a tunneling barrier . . . . . . . . . . . . . . . . . 10
1.4 Simple scheme of a MRAM array. . . . . . . . . . . . . . . . . . . 12
1.5 Realization of a AND and a OR gate with a MTJ. . . . . . . . . . 14
2.1 L2 structure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181
2.2 DOS of Co CrAl. . . . . . . . . . . . . . . . . . . . . . . . . . . . 192
2.3 Calculation of the band structure for Co CrAl (from [58]) . . . . . 202
2.4 SP of Co CrAl versus Fe content. . . . . . . . . . . . . . . . . . . 212
2.5 Calculation of the band structure for Co Cr Fe Al . . . . . . . 232 0.6 0.4
3.1 Deposition system. . . . . . . . . . . . . . . . . . . . . . . . . . . 28
3.2 Exchange biased Co film. . . . . . . . . . . . . . . . . . . . . . . . 31
3.3 Structure of a MTJ stack. . . . . . . . . . . . . . . . . . . . . . . 33
3.4 Realization of a mesa structure. . . . . . . . . . . . . . . . . . . . 34
3.5 Picture of a mesa structure with a diameter of 200μm. . . . . . . 36
3.6 Resistance versus temperature fortwo mesa structures etched per-
opendicular and with an angle of 70 respect to the normal to the
film surface. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
4.1 PowderCell simulation of a θ/2θ scan with the Bragg-Bentano ge-
ometryforthedifferentkindsofpossiblestructuresofCo Cr Fe Al2 0.6 0.4
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
viiviii LIST OF FIGURES
4.2 Top: TEM image of a polycrystalline sample deposited on Al O2 3
at room temperature. The circles mark crystallites with different
crystallineorientation. Bottom: Electrondiffractionpatternofthe
same sample. Reflections corresponding to the B2 structure can
be identified. Both figures are courtesy of Christian Herbort. . . . 41
4.3 Schematic representation of the epitaxial relationship of a CCFA
thin film on an Fe or Cr buffer layer on a MgO (100) substrate. . 43
4.4 θ/2θ scan and rocking curve of the (400) reflection of a 100 nm
o ˚thick CCFA film deposited at 100 C on a 50Athick Cr buffer
layer on a MgO (100) substrate. . . . . . . . . . . . . . . . . . . . 44
4.5 θ/2θ scan and rocking curve of the (400) reflection of a 100 nm
o ˚thick CCFA film deposited at 100 C on a 100Athick Fe buffer
layer on a MgO (100) substrate. . . . . . . . . . . . . . . . . . . . 45
4.6 X-rayφ-scan ofthe (220)equivalent reflections ofa CCFA film de-
oposited at 100 C (filled symbols) and of the MgO substrate (open
symbols, peaks labeled in italics). A Fe buffer layer was used in
this sample. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
4.7 X-rayφ-scan ofthe (220)equivalent reflections ofa CCFA film de-
oposited at 100 C (filled symbols) and of the MgO substrate (open
symbols, peaks labeled in italics). A Fe buffer layer was used in
this sample. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
4.8 θ/2θscanandrockingcurveofa100nmthickCCFAfilmdeposited
o oat 100 C on a MgAl O (100) substrate and annealed at 600 C . 502 4
4.9 θ/2θscanandrockingcurveofa100nmthickCCFAfilmdeposited
o o¯at 100 C on an Al O (1120) substrate and annealed at 600 C . . 512 3
4.10 Schematic representation of the epitaxial relation between CCFA
¯and Al O (1120) substrates. . . . . . . . . . . . . . . . . . . . . . 532 3
4.11 Schematic explanation of the origin of geometrical effects in mea-
sured scattering intensities in thin film diffraction experiments. . . 55
4.12 Hysteresisloopsat300and4KofapolycrystallineCo Cr Fe Al2 0.6 0.4
o othin film deposited on Al O at 300 C and annealed at 450 C for2 3
60min. The thickness of the film is 100nm. . . . . . . . . . . . . . 57LIST OF FIGURES ix
4.13 Top: Hysteresiscurvesat300and4Kofa100nmthickCCFAsam-
ople deposited at 100 C on Fe buffer layer on MgO (100) without
annealing. Bottom: Dependence of the magnetization at 20mT
on the temperature. The grey line is a fit with a power law with
β = 1.72 (see text). . . . . . . . . . . . . . . . . . . . . . . . . . . 59
4.14 θ/2θ scan and rocking curve of the (400) reflection of a 100 nm
o ˚thick CCFA film deposited at 100 C on a 50Athick Cr buffer
layer on a MgO (100) substrate. . . . . . . . . . . . . . . . . . . . 60
4.15 ResistanceversustemperaturemeasuredforaCo Cr Fe Althin2 0.6 0.4
film with a thickness of 100nm deposited on a MgAl O (100)2 4
substrate. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
4.16 Specific resistivity at 4K versus magnetic field measured for a
Co Cr Fe Al thin film with a thickness of 100nm deposited on2 0.6 0.4
a MgAl O (100) substrate. . . . . . . . . . . . . . . . . . . . . . 642 4
4.17 Schematic representation of a RHEED experiment. For a 2D lat-
tice, the reciprocal lattice degenerates in infinite rods perpendic-
ular to the film surface (black lines). Th