Spin structure of exchange biased heterostructures: Fe/MnF_1tn2 and Fe/FeF_1tn2 [Elektronische Ressource] / von Balaram Sahoo

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• Physik

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• Publié par: universitat_duisburg-essen
• Publié le: 01 janvier 2006
• Nombre de lectures: 30
• Langue: English
• Poids de l'ouvrage: 9 Mo

Extrait

Spin Structure of
Exchange Biased Heterostructures:
Fe/MnF and Fe/FeF2 2
Vom Fachbereich Physik der
Universit¨at Duisburg-Essen
zur Erlangung des akademischen Grades eines
Doktors der Naturwissenschaften
genehmigte Dissertation
von
Balaram Sahoo
aus
Nayagarh, Indien
Referent: Prof. Dr. Werner Keune
Korreferent: Prof. Dr. Gu¨nter Dumpich
Tag der mu¨ndlichen Pru¨fung: 18 Dezember 2006Abstract
57In this work, the Fe probe layer technique is used in order to investigate the
depth- and temperature-dependent Fe-layer spin structure of exchange biased Fe/MnF2
and Fe/FeF (pseudo-twinned)antiferromagnetic (AFM) systems by conversion electron2
M¨ossbauer spectroscopy (CEMS) and nuclear resonant scattering (NRS) of synchrotron
radiation.
57˚ ˚Two kinds of samples with a 10 A Fe probe layer directly at or 35 A away from the
interface, labeled as interface and center sample, respectively, were studied in this work.
Thespinstructurewasexplainedbyconsideringtwodifferentmodels, unidirectionaland
step-shaped distribution (fanning) model. The results obtained by CEMS for Fe/MnF2
suggests that, at 80 K, i.e., above T = 67 K of MnF , the remanent state Fe-layerN 2
spin structure of the two studied samples are slightly different due to their different
microstructure. Inthetemperaturerangefrom300Kto80K,theFe-layerspinstructure
doesnotchangejustbyzero-fieldcoolingthesampleinremanence. Byzero-fieldcooling
the samples in remanence to 18 K, i.e., below T , the Fe spins rotate towards the (±N
◦45 )- easy axes of MnF twins. This rotation results in the same spin structure for both2
the interface and center samples at 18 K. By field cooling the interface sample in a field
of 0.35 T to 18 K and measuring in remanence, a smaller rotation (or fanning angle) of
the Fe-spins in comparison to the case of zero-field cooling in remanence from 300 K to
18 K was observed. When the interface sample was zero-field cooled or field cooled to
18 K, and subsequently zero-field heated to 80 K (T > T ), the CEMS results indicateN
that the Fe-layer keeps the memory of its low temperature spin structure.
ForFe/FeF ,acontinuousnon-monotonicchangeoftheremanent-stateFespinstruc-2
turewasobservedbycoolingfrom300Kto18K.Thiseffectcanberelatedtothepeculiar
T-dependence of magnetic anisotropy of FeF and short-range-ordered magnetic corre-2
lations in the AFM induced by Fe above T = 78 K. The high temperature Fe spinN
structure of the two different samples (interface and center) is different due to their dif-
ferent microstructure, but at 18 K (T < T ) the spin structures of both samples are theN
same, and the Fe spins are oriented close to the easy axes of the FeF twins, similar to2
the case of Fe/MnF at 18 K.2
NRS of synchrotron radiation was used to investigate the temperature- and depth-
dependent Fe - layer spin structure during magnetization reversal in pseudo-twinned
57 56Fe/MnF . A Fe-probe layer was embedded in the Fe layer in a wedge-type man-2
57ner, so that the distance of the Fe layer from the Fe/MnF interface varies when the2
synchrotron beam is scanned from one end of the sample to the other end. A depth-
dependent Fe spin structure in an applied magnetic field (applied along the bisector of
the twin domains) was observed at 10 K, where the Fe spins closer to the interface are
not aligned along the field direction. During magnetization reversal the spins of the
top Fe layer rotate at a smaller field than the Fe spins closer to the interface. Upon
decreasing the field from the fully aligned state in a strong positive magnetic field, the
◦Fe spins coherently rotate up to the easy direction of MnF (at± 45 from the applied2
◦field), then ”jump” to the opposite direction of the easy axes (i.e., ∓ 45 ), and then
further rotate towards the negative applied field direction. The depth-dependence of the
spin structure in an applied field and the rotation via the jump disappear at 150 K, i.e.,
above T of MnF .N 2Contents
1 Introduction 1
2 Basics of experimental techniques 5
2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2 M¨ossbauer spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2.1 The M¨ossbauer effect . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2.2 Basics of M¨ossbauer spectroscopy . . . . . . . . . . . . . . . . . . 6
2.3 Conversion electron M¨ossbauer spectroscopy . . . . . . . . . . . . . . . . 8
2.4 M¨ossbauer spectroscopical parameters. . . . . . . . . . . . . . . . . . . . 11
2.4.1 M¨ossbauer linewidth and recoil-free events . . . . . . . . . . . . . 11
2.4.2 Chemical or isomer shift (δ) . . . . . . . . . . . . . . . . . . . . . 14
2.4.3 Second-order Doppler shift . . . . . . . . . . . . . . . . . . . . . . 15
2.4.4 Quadrupole splitting . . . . . . . . . . . . . . . . . . . . . . . . . 16
2.4.5 Magnetic hyperfine field . . . . . . . . . . . . . . . . . . . . . . . 20
2.4.6 Combined hyperfine interactions . . . . . . . . . . . . . . . . . . . 22
2.4.7 Calibration and least-squares fitting of the M¨ossbauer spectra . . 23
2.5 X-ray Diffraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
2.6 SQUID Magnetometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
3 Nuclear resonant scattering of synchrotron radiation 27
3.1 Synchrotron radiation: production and control . . . . . . . . . . . . . . . 27
3.2 Introduction to scattering techniques . . . . . . . . . . . . . . . . . . . . 31
3.3 NRS experimental setup . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
3.4 Nuclear resonant scattering: beat pattern . . . . . . . . . . . . . . . . . . 33
3.5 Simulation of the NRS time spectrum . . . . . . . . . . . . . . . . . . . . 34
4 Introduction to exchange bias 43
4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
4.1.1 Experimental observations of exchange bias . . . . . . . . . . . . 44
4.1.2 Theoretical Models explaining the observed Exchange Bias Effects 47
4.2 Exchange bias and spin structure . . . . . . . . . . . . . . . . . . . . . . 54
5 Magnetic Anisotropy and Hyperfine Interactions in MnF and FeF 572 2
5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
5.1.1 Crystallographic and magnetic structure of MnF and FeF . . . . 572 2
5.1.2 Anisotropies in MnF and FeF . . . . . . . . . . . . . . . . . . . 582 2
5.2 Sample preparation and characterization . . . . . . . . . . . . . . . . . . 62
5.3 Hyperfine interaction in FeF . . . . . . . . . . . . . . . . . . . . . . . . 662
iii CONTENTS
6 Fe Spin Structure in Exchange Biased Fe/MnF Bilayers 712
6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
6.2 Sample preparation and characterization . . . . . . . . . . . . . . . . . . 73
6.3 SQUID magnetometry: results . . . . . . . . . . . . . . . . . . . . . . . . 77
6.4 Conversion electron M¨ossbauer spectroscopy of Fe/MnF bilayers . . . . 792
6.4.1 CEMS measurement geometry . . . . . . . . . . . . . . . . . . . . 79
6.4.2 In-plane spin distribution models . . . . . . . . . . . . . . . . . . 81
6.4.3 Experimental details and CEMS results . . . . . . . . . . . . . . . 83
6.5 CEMS results: discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
6.5.1 Depth-dependent Fe spin structure at remanence . . . . . . . . . 89
6.5.2 Influence of the cooling field on the Fe spin structure . . . . . . . 93
6.5.3 Temperature dependence of the Fe spin structure . . . . . . . . . 94
6.5.4 Memory effect of the Fe spin structure . . . . . . . . . . . . . . . 96
6.6 Modeled angular Fe spin distributions in Fe/MnF . . . . . . . . . . . . . 972
6.6.1 Angular Fe spin distribution in remanence . . . . . . . . . . . . . 97
6.6.2 Angular Fe spin distribution after field cooling . . . . . . . . . . . 101
6.6.3 Memory of the angular Fe spin distribution. . . . . . . . . . . . . 102
6.7 Supplementary vector SQUID magnetometry: results and discussion . . 103
6.8 Conclusions for the Fe spin structure in Fe/MnF . . . . . . . . . . . . . 1062
7 Fe and FeF Spin Structure in Exchange Biased Fe/FeF Bilayers 1092 2
7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109
7.2 Sample Preparation and Characterization . . . . . . . . . . . . . . . . . . 111
7.3 CEMS Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115
7.3.1 Experimental details and the hyperfine parameters . . . . . . . . 115
7.3.2 Temperature and depth dependent Fe spin structure . . . . . . . 126
7.3.3 FeF -layer spin structure and the EFG components . . . . . . . . 1312
7.4 SQUID magnetometry results . . . . . . . . . . . . . . . . . . . . . . . . 131
7.5 Conclusions for the Fe spin structure in Fe/FeF . . . . . . . . . . . . . . 1352
8 Nuclear Resonant Scattering: Fe Spin Structure in Fe/MnF 1372
8.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137
8.2 Sample preparation and characterization . . . . . . . . . . . . . . . . . . 138
8.3 NRS experimental procedure . . . . . . . . . . . . . . . . . . . . . . . . . 142
8.4 NRS results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 143
8.4.1 Field dependent Fe spin rotation during magnetization reversal. . 143
8.4.2 Depth dependent Fe layer spin structure during magnetization re