Magnetic Order and Coupling Phenomena: A Study of Magnetic Structure and Magnetization Reversal Processes in Rare-Earth-Transition-Metal Based Alloys and Heterostructures

136 Pages·2014·5.27 MB·English

by  Christian Schubert (auth.)

Springer Theses
Recognizing Outstanding Ph.D. Research
Christian Schubert
Magnetic Order
and Coupling
Phenomena
A Study of Magnetic Structure
and Magnetization Reversal Processes
in Rare-Earth–Transition-Metal Based
Alloys and Heterostructures
Springer Theses
Recognizing Outstanding Ph.D. Research
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Christian Schubert
Magnetic Order and
Coupling Phenomena
A Study of Magnetic Structure
and Magnetization Reversal Processes
in Rare-Earth–Transition-Metal Based
Alloys and Heterostructures
Doctoral Thesis accepted by
Chemnitz University of Technology, Germany
123
Author Supervisor
Dr. Christian Schubert Prof.Manfred Albrecht
Experimental Physics IV Experimental Physics IV
Instituteof Physics Instituteof Physics
Universityof Augsburg Universityof Augsburg
Augsburg Augsburg
Germany Germany
ISSN 2190-5053 ISSN 2190-5061 (electronic)
ISBN 978-3-319-07105-3 ISBN 978-3-319-07106-0 (eBook)
DOI 10.1007/978-3-319-07106-0
Springer ChamHeidelberg New YorkDordrecht London
LibraryofCongressControlNumber:2014939393
(cid:2)SpringerInternationalPublishingSwitzerland2014
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Supervisor’s Foreword
Since the discovery of a unidirectional magnetic exchange anisotropy in Co/CoO
nanoparticlesin1956byMeiklejohnandBean[1],causingashiftofthemagnetic
hysteresisloop,muchattentionhasbeenpaidtotheunderstandingofthisso-called
exchange-bias effect. But even more attention was triggered by the pioneering
workoftheNobelPrizelaureatesPeterGrünbergandAlbertFert,whodiscovered
the giant magnetoresistance effect, which led to the application in magnetic field
sensorsusingtheexchange-bias effect forpinningthemagneticreference layerin
spin valve systems. Exchange-bias is most frequently observed in antiferromag-
netic/ferromagnetic bilayers due to the interfacial exchange coupling of frozen
uncompensated spins [2]. But in recent years a variety of different material sys-
tems have been investigated, including ferrimagnetic bilayers [3] employing
amorphous rare-earth-transition-metal alloys. The magnetic coupling in such
systems consists of two types of pair interactions, an antiparallel magnetic
exchange coupling between the rare-earth and transition-metal moments and a
parallel magnetic exchangecouplingof the transition-metal moments themselves.
Taking advantage of the tunability of the exchange interaction between the fer-
rimagnetic layers, the shift of the hysteresis loop can even be reversed. In com-
bination with a hard magnetic layer, a large exchange-bias effect has also been
reported [4].
In this regard, the goal of this thesis was to investigate the magnetic structure
andreversalmechanismofsingleamorphousferrimagneticFe–Tballoythinfilms
and ferromagnetic [Co/Pt] multilayers with perpendicular magnetic anisotropy in
comparisontoexchangecoupledheterostructures.Withthiscomprehensivestudy,
Christian Schubert was able to provide a deeper insight into the magnetization
reversalprocess,themagneticmomentconfiguration,andtheroleoftheinterfacial
domain wall, occurring in exchange-biased Fe–Tb/[Co/Pt] heterostructures. Fur-
thermore, this study was supported by element-specific X-ray magnetic circular
dichroismmeasurements,revealingtheinteractionbetweenthemomentsoftheFe
and Tb sublattices as a function of external magnetic field and temperature. The
understanding of the peculiar magnetic coupling phenomena in rare-earth-transi-
tion-metal alloy-based exchange-bias systems presented in this thesis opens up
interesting paths for spintronic applications.
Augsburg, March 2014 Prof. Manfred Albrecht
vii
viii Supervisor’sForeword
References
1.W.H.Meiklejohn,C.P.Bean,Phys.Rev.102,1413(1956)
2.F.Radu,H.Zabel,SpringerTractsMod.Phys.227,97(2008)
3.F.Raduetal.,Nat.Commun.3,715(2012)
4.S.Romeretal.,Appl.Phys.Lett.101,222404(2012)
Abstract
Theamorphousstructureandmagneticexchangecouplingbetweenrare-earthand
transition-metal elements give rise for peculiar magnetic order (e.g., spero-,
aspero-, and sperimagnetism) in thin amorphous alloy films. The sperimagnetic
configuration with its non-collinear ferrimagnetic spin structure occurs in
amorphous Fe–Tb alloy films with perpendicular magnetic anisotropy. Within
this study, thin amorphous Fe–Tb alloy films were investigated according to their
structuralandmagneticproperties.Besidestoichiometry-dependentchangesofthe
net magnetization and coercivity, strong variations in the reversal process for Fe
and Tb dominated alloy films were found mainly in the vicinity of the
compensation point. Even for fully compensated magnetic films a magnetization
reversal driven by a spin-flop transition is observed. This originates most likely
from an orientational moment distribution existing in the Fe and Tb sublattices,
which allows an interaction with high external magnetic fields. Furthermore,
arrays of percolated Fe–Tb nanodots with pillar diameters of 30 nm and a period
of 60 nm became analyzed. Despite the exchange interaction to the trench
material,thenanodotsrevealasingledomainmagnetizationstateandreverseviaa
more coherent rotation process as deduced from in-field magnetic force
microscopy imagingandangular-dependentmagneto-opticalKerr effect measure-
ments.Contrarytothis,thereversalofthecontinuoustrenchmaterialisdominated
by domain wall motion and the coercive field becomes enhanced due to pinning
effects caused by the nanodot array.
In the second part of this work the exchange interaction and magnetization
reversal processes in exchange-biased heterostructures consisting of amorphous
ferrimagnetic Fe–Tb alloy films and ferromagnetic Co/Pt multilayers were
investigated. The dependence of the interfacial exchange coupling on the
stoichiometry and thickness of the Fe–Tb layer as well as the number of
repetitions in the Co/Pt multilayers were analyzed. The net magnetization and
effective magnetic anisotropy of the Fe–Tb alloy films have an influence on the
exchangeenergyperunitarea.Alargeexchange-biasfieldupto8 kOeisfoundto
beaccompaniedbyaninterfacialdomainwallasprobedbyelement-specificX-ray
magnetic circular dichroism absorption measurements. This domain wall exhibits
a total thickness between 3 and 4 nm and affects strongly the magnetization
reversal in the heterostructure. Additionally, a novel kind of training effect was
observed,wheretheexchange-biasfieldincreasesbyabout3.5 %fromthefirstto
ix
x Abstract
thesecondfieldcycleat10 K.Finally,theinterlayerexchangecouplingthrougha
PtspacerlayerinFe–Tb/Pt/[Co/Pt] heterostructureswasprobed.Highresolution
10
transmission electron microscopy revealed continuous Pt layers with thicknesses
ofmorethan0.8 nm,which provide only indirectexchangecoupling betweenthe
Fe–Tb film and the Co/Pt multilayer due to spin-polarization of the Pt. Thinner
spacer layers cover partially the Co/Pt multilayers and the nucleation of a 3-nm-
thick interfacial domain wall provides an exchange-bias field similar to
heterostructures without spacer layer.