Discovery of Co-existing Non-collinear Spin Textures in D2d Heusler Compounds PDF
Preview Discovery of Co-existing Non-collinear Spin Textures in D2d Heusler Compounds
Springer Theses Recognizing Outstanding Ph.D. Research Jagannath Jena Discovery of Co-existing Non-collinear Spin Textures in D 2d Heusler Compounds Springer Theses Recognizing Outstanding Ph.D. Research AimsandScope 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 foritsscientificexcellenceandthehighimpactofitscontentsforthepertinentfield of research. For greater accessibility to non-specialists, the published versions includeanextendedintroduction,aswellasaforewordbythestudent’ssupervisor explainingthespecialrelevanceoftheworkforthefield.Asawhole,theserieswill provide a valuable resource both for newcomers to the research fields described, and for other scientists seeking detailed background information on special questions. 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Moreinformationaboutthisseriesathttps://link.springer.com/bookseries/8790 Jagannath Jena Discovery of Co-existing Non-collinear Spin Textures in D Heusler Compounds 2d Doctoral Thesis accepted by Martin-Luther-Universität Halle-Wittenberg, Halle, Germany Author Supervisor Dr.JagannathJena Prof.StuartS.P.Parkin MaxPlanckInstituteofMicrostructure MaxPlanckInstituteofMicrostructure Physics Physics Halle,Germany Halle,Germany ISSN 2190-5053 ISSN 2190-5061 (electronic) SpringerTheses ISBN 978-3-031-03909-6 ISBN 978-3-031-03910-2 (eBook) https://doi.org/10.1007/978-3-031-03910-2 ©TheEditor(s)(ifapplicable)andTheAuthor(s),underexclusivelicensetoSpringerNature SwitzerlandAG2022 Thisworkissubjecttocopyright.AllrightsaresolelyandexclusivelylicensedbythePublisher,whether thewholeorpartofthematerialisconcerned,specificallytherightsoftranslation,reprinting,reuse ofillustrations,recitation,broadcasting,reproductiononmicrofilmsorinanyotherphysicalway,and transmissionorinformationstorageandretrieval,electronicadaptation,computersoftware,orbysimilar ordissimilarmethodologynowknownorhereafterdeveloped. Theuseofgeneraldescriptivenames,registerednames,trademarks,servicemarks,etc.inthispublication doesnotimply,evenintheabsenceofaspecificstatement,thatsuchnamesareexemptfromtherelevant protectivelawsandregulationsandthereforefreeforgeneraluse. Thepublisher,theauthors,andtheeditorsaresafetoassumethattheadviceandinformationinthisbook arebelievedtobetrueandaccurateatthedateofpublication.Neitherthepublishernortheauthorsor theeditorsgiveawarranty,expressedorimplied,withrespecttothematerialcontainedhereinorforany errorsoromissionsthatmayhavebeenmade.Thepublisherremainsneutralwithregardtojurisdictional claimsinpublishedmapsandinstitutionalaffiliations. ThisSpringerimprintispublishedbytheregisteredcompanySpringerNatureSwitzerlandAG Theregisteredcompanyaddressis:Gewerbestrasse11,6330Cham,Switzerland Dedicatedtomysister,Kati,for herunconditional love Supervisor’s Foreword IampleasedtowriteafewcommentsregardingthePh.D.thesisofJagannathJena. Jagannath joined my department at the Max Planck Institute for Microstructure Physics inNovember 2016, shortlyafter Ijoined theMaxPlanck Society. Hewas my first Ph.D. student whose research was focused on non-collinear spin textures and their novel properties using Lorentz transmission electron microscopy (TEM) techniques.EventhoughhewasnewtoTEM,Iwasimpressedbyhisenthusiasmto learn and apply this sophisticated experimental technique to explore non-collinear spintexturesinavarietyofmaterials.Inthisregard,Jagannathisatrueexperimen- talist.Helargelytaughthimselfseveraladvancedtransmissionelectronmicroscopy imaging techniques as well as the focused ion beam milling techniques which are neededtopreparetheultra-thin,electrontransparentlamellae,requiredfortheTEM studies. In recent years, there has been a surge of interest in non-collinear chiral spin texturessuchasBlochskyrmions(inB20compounds)andNéelskyrmions(inC nv andmultilayersystems).Theiremergentelectromagneticpropertiesresultingfrom their topological structures make them highly interesting candidates for spintronic deviceapplications.In2017,mygroupobservedforthefirsttimeamultichiralspin texture, namely an antiskyrmion, in a non-centrosymmetric D inverse tetragonal 2d Heuslercompound.Thespintextureofanantiskyrmioniscomplexandcoupledto theunderlyingcrystalstructureviaaDzyaloshinskii–Moriyainteractionthatwhose strength and sign depend on the direction within the tetragonal basal plane. This leads to a magnetization that rotates in a chiral fashion from a direction parallel to the tetragonal axis outside the object to the opposite direction in the interior of the object within a plane perpendicular to (Bloch-like) or parallel to (Néel-like) to the object’s radius depending on the in-plane orientation. Jagannath’s research vii viii Supervisor’sForeword is concerned with the discovery of several novel spin textures and the investiga- tion of their physical properties in several D compounds. These materials had 2d been hardly studied as compared to B20 compounds, Cnv, and multilayer systems priortoJagannath’swork.Thefirsttwochaptersofthethesisprovideaconcisebut thorough explanation of magnetic skyrmions. The experimental methods provide detailsonidentifyingthesingle-crystallinegrainsofapolycrystallinesample,how tomakesingle-crystallinethinspecimensandnano-tracks,andfinallydescriptions of different imaging techniques performed via transmission electron microscopy (TEM). These techniques and experimental methods are provided in a clear and understandablemannerforanyonewhowishestoconductresearchinexperimental nano-magnetismandtransmissionelectronmicroscopeimaging. Much of Jagannath’s research for his Ph.D. thesis was dedicated to a single compoundinwhichhediscoveredseveralunanticipatedandnovelspintextures.This work was published in three papers in high-impact journals during the thesis, and twootherpapersthatwerepublishedafterthethesiswascompleted.Morerecently, he investigated a second compound from the same Heusler family of compounds, andtherehealsouncoveredsomehighlyinterestingresults.Whatsurprisedmemost about Jagannath was his careful and methodical approach to his experiments and hisinterestinnotonlyindiscoveringnovelspintexturesbutinunderstandingtheir origin. Jagannath succeeded in carrying out at least twice as much work as one wouldtypicallyexpectforaPh.D.thesis;nevertheless,thisworkisjustabouthalf of his output from the past four years. Moreover, Jagannath also made significant contributionstoseveralotherprojects. In summary, Jagannath has been highly prolific during his Ph.D. He identi- fied several novel spin textures that include: elliptical Bloch skyrmions, fractional Blochskyrmions,fractionalBlochantiskyrmions,squareantiskyrmions,elongated (anti)skyrmions which had not previously been observed. These textures can be accountedforbyacompetitionbetweenlong-rangedipole–dipoleinteractionsand ashort-rangechiralvectorexchange.Moreover,asJagannathdiscoveredthisallows forthestabilizationofseveralspintexturesinthesamecompound.TheD Heusler 2d compoundisthefirstnon-centrosymmetricmagneticcompoundinwhichmultiple non-collinearchiralspintexturescanco-exist.Thesenon-trivialtextureshavegreat physicalrelevance,intermsofbothfundamentalphysicsandapplications.Elliptical Blochskyrmionsandantiskyrmions,forexample,couldbeusedasdistinctdatabits for‘0’and‘1’s,respectively,allowingforamagneticracetrackmemorydevicethatis lesssusceptibletodiffusionandrepulsionbetweenbitsthaninamoreconventional racetrackdevicethatusesthepresenceorabsenceofasinglespintexturedobject. Jagannathhasproventobeanexcellentteamplayer.Itisreallyamazingtome thatJagannathcouldlearnfromscratchadvancedtransmissionelectronmicroscopy techniquesandhasbecomeahighlycompetentmicroscopist,especiallywithrespect Supervisor’sForeword ix tomagneticimagingusingLorentztransmissionelectronmicroscopy.Jagannathwas apleasuretoworkwithandIenjoyedverymuchbeinghisPh.D.advisor.Iamsure thatJagannathhasaverysuccessfulcareeraheadofhim. October2021 Prof.StuartS.P.Parkin MaxPlanckInstituteofMicrostructurePhysics Halle,Germany Abstract Non-collinearspintexturesareofgreatinterestbothfundamentallyandtechnolog- ically. One of the most interesting such textures is a skyrmion, which is a vortex- likelocalizedtopologicalspintexturethatpossessesnumerousinterestingfeatures. Its exotic properties include robustness against impurities and external perturba- tions,highmobility,longlifetimesandalowthresholdcurrentformovement.These makemagneticskyrmionsfundamentallyinterestingandattractivefortechnological applicationssuchashigh-densitymemorydevices,especiallyinaracetrackstorage deviceandforneuromorphiccomputing.Overthelastdecade,extensivestudieshave beencarriedoutconcerningtwotypesofisotropictopologicalspintextures:Bloch skyrmionsandNéelskyrmionsthathavechiralboundarieswithhelicoidandcycloid propagatingwalls,respectively.TheyhavebeenobservedinthebulksystemwithB20 compoundsandthosewithC crystalsymmetry,respectively.Recently,adifferent 3v kindofanisotropicspintexturetermedan‘antiskyrmion’(composedofalternating chiral boundaries of helicoids and cycloids) has been observed in a ferromagnetic inversetetragonalHeuslercompoundMn Pt Pd Sn withD symmetry.Inthis 1.4 0.9 0.1 2d thesis,anunanticipateddiscoveryofanovelspintexturenamelyan‘ellipticalBloch Skyrmion’ having a topological charge –1 has been found in the same D system 2d whose crystalline symmetry supports antiskyrmions with topological charge +1. Thesimultaneousexistenceofthesedistinctnano-objectswithoppositetopological charges depends on the temperature and field history. These spin textures are also found in Mn Rh Ir Sn, a low saturation magnetization ferrimagnetic inverse 2 0.95 0.05 tetragonal Heusler compound, thereby opening a path toward fully zero moment spin textures. In addition to its fundamental importance, the findings presented in thisthesispermitanadvancedversionofthemagneticracetrackdatastoragedevice that could be more feasible than conventional racetracks. Instead of utilizing the presence or absence of skyrmions as binary bits, a sequence of skyrmions can be readas‘1’bitsandantiskyrmionsas‘0’bitsthatismorerobustagainstthermaldrift, andthenecessityofmaintainingdefineddistancesamongthedatabitsisnotneeded. xi
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