Magnetic Order and Coupling Phenomena: A Study of Magnetic Structure and Magnetization Reversal Processes in Rare-Earth-Transition-Metal Based Alloys and Heterostructures PDF
Preview 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 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 For furthervolumes: http://www.springer.com/series/8790 Aims and Scope The series ‘‘Springer Theses’’ brings together a selection of the very best Ph.D. theses from around the world and across the physical sciences. Nominated and endorsed by two recognized specialists, each published volume has been selected for its scientific excellence and the high impact of its contents for the pertinent fieldofresearch.Forgreateraccessibilitytonon-specialists,thepublishedversions includeanextendedintroduction,aswellasaforewordbythestudent’ssupervisor explaining the special relevance of the work for the field. 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Theses are accepted into the series by invited nomination only and must fulfill all of the following criteria • They must be written in good English. • ThetopicshouldfallwithintheconfinesofChemistry,Physics,EarthSciences, Engineering andrelatedinterdisciplinaryfieldssuchasMaterials, Nanoscience, Chemical Engineering, Complex Systems and Biophysics. • The work reported in the thesis must represent a significant scientific advance. • Ifthethesisincludespreviouslypublishedmaterial,permissiontoreproducethis must be gained from the respective copyright holder. • They must have been examined and passed during the 12 months prior to nomination. • Each thesis should include a foreword by the supervisor outlining the signifi- cance of its content. • The theses should have a clearly defined structure including an introduction accessible to scientists not expert in that particular field. 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 Thisworkissubjecttocopyright.AllrightsarereservedbythePublisher,whetherthewholeorpartof the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation,broadcasting,reproductiononmicrofilmsorinanyotherphysicalway,andtransmissionor informationstorageandretrieval,electronicadaptation,computersoftware,orbysimilarordissimilar methodology now known or hereafter developed. Exempted from this legal reservation are brief excerpts in connection with reviews or scholarly analysis or material supplied specifically for the purposeofbeingenteredandexecutedonacomputersystem,forexclusiveusebythepurchaserofthe work. Duplication of this publication or parts thereof is permitted only under the provisions of theCopyright Law of the Publisher’s location, in its current version, and permission for use must always be obtained from Springer. Permissions for use may be obtained through RightsLink at the CopyrightClearanceCenter.ViolationsareliabletoprosecutionundertherespectiveCopyrightLaw. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publicationdoesnotimply,evenintheabsenceofaspecificstatement,thatsuchnamesareexempt fromtherelevantprotectivelawsandregulationsandthereforefreeforgeneraluse. While the advice and information in this book are believed to be true and accurate at the date of publication,neithertheauthorsnortheeditorsnorthepublishercanacceptanylegalresponsibilityfor anyerrorsoromissionsthatmaybemade.Thepublishermakesnowarranty,expressorimplied,with respecttothematerialcontainedherein. Printedonacid-freepaper SpringerispartofSpringerScience+BusinessMedia(www.springer.com) Information is a commodity. It can be traded, sold, and purchased. And in the end, credits are only as useful as the secrets they can buy. —D.B. 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.
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