Discovery of Co-existing Non-collinear Spin Textures in D2d Heusler Compounds

147 Pages·2022·6.865 MB·English

by  Jagannath Jena

Springer Theses
Recognizing Outstanding Ph.D. Research
Jagannath Jena
Discovery
of Co-existing
Non-collinear Spin
Textures in D
2d
Heusler Compounds
Springer Theses
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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
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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