Dynamic detection of electron spin accumulation

137 Pages·2016·11.81 MB·English

by  Changjiang Liu

Dynamic Detection of Spin Accumulation by
Ferromagnetic Resonance
ADISSERTATION
SUBMITTEDTOTHEFACULTYOFTHEGRADUATESCHOOL
OFTHEUNIVERSITYOFMINNESOTA
BY
ChangjiangLiu
INPARTIALFULFILLMENTOFTHEREQUIREMENTS
FORTHEDEGREEOF
DoctorofPhilosophy
PaulCrowell
November,2016
(cid:13)c Changjiang Liu 2016
ALL RIGHTS RESERVED
Acknowledgments
I would like to first thank my advisor, Paul Crowell, who accepted me to work in his lab in 2011.
At that time, I had little experience in condensed matter experiments. Paul taught me how to start
an experimental project from scratch and guided me through the process. He sometimes helped
troubleshootmyexperimentalsetupforhourstilllatenight. Paul’sbroadknowledgeasacondensed
matterexperimentalistaswellashispatiencewhileteachingmehavegreatlybenefitedmeoverthe
course of my Ph.D. study. I appreciate Paul’s hard working and sincere attitude towards science,
andIamgreatlyindebtedtohimforhisguidancewithhighstandards.
IwouldliketothankmycollaboratorProfessorChrisPalmstrømatUCSB,whohasledawon-
derfulteamgrowinghighqualitysampleswhichIuseinmyresearch. Inparticular,Iwouldliketo
thankSahelPatel,TonyMcFaddenandMihirPendharkarinPalmstrøm’sgroup,withouttheirhard
workingmyPh.D.workwouldnothavebeenpossible.
The environment in Paul’s lab has always been intellectually vibrant and supportive to each
other. I would like to thank Chad Geppert and Kevin Christie, who were senior graduate students
when I joined Paul’s group. During the first two years, Chad and Kevin gave me tremendous help
in setting up experiments and doing sample fabrications in the cleanroom. Without their hands-
on guidance, my Ph.D. career would not have been smooth. I would also like to thank Timothy
Peterson and Gordon Stecklein, who have been great lab partners. Tim and Gordon have provided
invaluable feedback for nearly all of my work. Other group members, Andrew Galkiewicz, Dima
SpivakandJustinWattshavealsobeengreattoworkwith.
Iworkedwithanumberofextremelytalentedundergrads,includingREUstudentsIakovBoyko,
i
AlexBoldinandPhillipDang,andundergradsKendraBergstedtandDavidBoschfromthisdepart-
ment. It was a great pleasure to instruct them and to do research with them together. They made
importantcontributionstovariousprojectsduringmyPh.D.study.
I would like to thank staff at the Minnesota Nanofabrication Center, including Mark Fisher,
KevinRoberts,TerryBroughandLagevonDissen. AnytimeIrequestaserviceorsomethinggoes
wronginthecleanroom,theyaretherereadytohelp.
IwouldliketothankProfessorOriolValls,whowasmyacademicadvisorduringmyfirstyear.
Oriolinstructedmehowtosurviveasagraduatestudent. IwouldalsoliketothankProfessorRobert
Lysak,Yong-ZhongQian,VukMandicandBorisShklovskii,fromwhomIhavelearnedalotinthe
coursestheytaught,includingclassicalphysics,quantummechanicsandsolidstatephysics.
IwouldliketothankBobOsburnandSusanOsburnandtheirfamilymembers. Bobpickedme
upattheairportwhenIfirstcametotheU.S.. Sincethen,BobandSusanhaveinvitedmetojoinhis
familyduringholidayseveryyear. Theytakemeaspartoftheirfamily. ThankyouBobandSusan.
Fellow graduate students in this department, Qianhui Shi, Sheng Tian, Yangmu Li, Xiaoyu
Wang, Yang Tang, Jiaming Zheng and Chun Chen, have made my graduate life enjoyable and a
richerexperience.
I was fortunate to get to know Haoyu Zhu, who is at the University of Idaho. During the final
year of my Ph.D. career, Haoyu has been kind, understanding and has given me a lot of mental
support. ThankyouHaoyu.
Finally, I am deeply grateful to have my father Huazhong Liu, mother Xinju Wei and my little
sisterJiangxiaoLiu. TheyhavealwaysbeensupportivetowhateverIpursueinmycareer,andhave
giventheirbesttome.
ii
Dedication
Tomyparents,towhomIowemostofmysuccess.
iii
Abstract
This thesis describes dynamical phenomena occurring in ferromagnet (FM)/n-GaAs heterostruc-
tures, while the ferromagnet is driven to resonance (FMR). The relevant work was published in
Appl. Phys. Lett. 105,212401(2014)andNatureCommunications7,10296(2016),respectively.
In a FM/n-GaAs heterostructure, strong spin-orbit coupling at the FM/n-GaAs interface leads
to a tunneling anisotropic magnetoresistance (TAMR) effect, which makes the resistance across
the FM/n-GaAs interface depend on the orientation of the magnetization relative to the crystalline
axes. When the FM is driven on resonance, the magnetization starts to precess around an effec-
tive magnetic field with an elliptical trajectory. Because of the TAMR effect, the onset of FMR
modulatesthetunnelingresistanceacrosstheFM/n-GaAsinterface,enablingelectricaldetectionof
FMR. I demonstrate that through the TAMR mechanism, the FMR precession cone angles can be
characterized quantitatively. This study shows that the TAMR effect plays a predominant role in
producingadcvoltageinFM/n-GaAsheterostructuresatFMR,whichhastobetakenintoaccount
whenstudyingtheFMR-inducedphenomenainsimilarsystems.
WhenalargeforwardbiascurrentisappliedacrosstheFM/n-GaAsinterface,aspinaccumula-
tioncanbegeneratedinthen-GaAs. Ishowthat,inthisspinaccumulationregime,FMRcanbeused
todetectthespinaccumulation. ThistechniqueutilizesthefactthatatFMR,themagnetizationcan
precess at a frequency faster than the electron spin decay rate in the n-GaAs. The electrical signal
from a FM, in measuring the electrochemical potential of a spin accumulation in the n-GaAs, de-
pendsontherelativeanglebetweenthemagnetizationvectorintheFMandthespinaccumulationin
thesemiconductor. Traditionally,todetectthespinaccumulation,oneappliesaperpendicularmag-
iv
netic field to let the spins in the semiconductor precess (the Hanle effect) or an in-plane magnetic
field to switch the direction of the magnetizations (spin valve measurement). Neither one works
wellatroomtemperatureinn-GaAsduetothesignificantdecreaseofspinlifetimeastemperature
increases. Inmyexperiment,themagnetizationisforcedtoprecessattheFMRfrequency,whichis
fasterthanthedecayrateofspinsinthen-GaAs. Oncespinsareinjectedfromtheprecessingmag-
netization of the FM into the n-GaAs, they precess at a much lower frequency due to the different
effective magnetic fields and electron g-factors of FM and n-GaAs. This difference in precession
frequency creates a phase angle between the magnetization vector in the FM and the spins in the
n-GaAs, which enables an effective detection of spin accumulation. My modeling shows that this
FMRtechniqueprovidesanewwaytodeterminethespinlifetimebymeasuringtheFMRfrequency
dependence of the spin signal, which is verified experimentally, and the spin lifetime is measured
at room temperature. The FMR-spin detection technique developed in this work can be applied to
othersystemsinwhichspinlifetimesareshort,suchasmetalsandperhapstopologicalinsulators.
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Contents
Acknowledgments i
Dedication iii
Abstract iv
ListofTables ix
ListofFigures x
ListofSymbols xv
1 Introduction–briefoverviewoftechniquesusedinspintronicsresearch 1
1.1 ElectricalspininjectionusingFM/NMheterostructures . . . . . . . . . . . . . . . 2
1.2 DetectionofspinaccumulationbyaFM . . . . . . . . . . . . . . . . . . . . . . . 10
1.2.1 Thenon-localspinvalvemeasurement . . . . . . . . . . . . . . . . . . . . 11
1.2.2 Thenon-localHanlemeasurement . . . . . . . . . . . . . . . . . . . . . . 14
1.2.3 The3-terminalHanlemeasurement . . . . . . . . . . . . . . . . . . . . . 19
1.3 Spinpumpingusingferromagneticresonance . . . . . . . . . . . . . . . . . . . . 22
1.3.1 FerromagneticresonanceandtheLandau–Lifshitz–Gilbert(LLG)equation 23
1.3.2 Spintransfertorque . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
1.3.3 Thespinpumpingeffect . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
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1.3.4 Electricaldetectionofspincurrentproducedbyspinpumping . . . . . . . 38
1.3.4.1 ElectricaldetectionofspinpumpinginFM/NMbilayersystems . 38
1.3.4.2 SpinpumpinginFM/SCbilayersystems . . . . . . . . . . . . . 43
1.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
2 Heusleralloys,devicefabrication,andmeasurementdetails 47
2.1 Heusleralloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
2.2 Samplegrowth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
2.2.1 InterfacetunnelbarrierinFM/n-GaAsheterostructures . . . . . . . . . . . 50
2.3 Devicefabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
2.3.1 FMRMeasurementdetails . . . . . . . . . . . . . . . . . . . . . . . . . . 55
3 TunnelinganisotropicmagnetoresistanceinFM/GaAsheterostructures 58
3.1 TheTAMReffectandmeasurementresults . . . . . . . . . . . . . . . . . . . . . 58
3.1.1 TheoreticalconsiderationsoftheTAMReffect . . . . . . . . . . . . . . . 58
3.1.2 TAMReffectmeasurements . . . . . . . . . . . . . . . . . . . . . . . . . 60
3.2 DCvoltagepeakproducedbytheTAMReffectatFMR . . . . . . . . . . . . . . . 66
3.2.1 TheFMRexperimentresults . . . . . . . . . . . . . . . . . . . . . . . . . 66
3.2.2 Quantitativemodelingandangledependencemeasurement . . . . . . . . . 69
3.3 ComparisonbetweentheFMRpeakandtheTAMReffect . . . . . . . . . . . . . . 73
3.3.1 Interfacebiasvoltagedependence . . . . . . . . . . . . . . . . . . . . . . 73
3.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
4 Dynamicdetectionofspinaccumulationusingferromagneticresonance 78
4.1 Physicalpictureandroomtemperaturemeasurementresults. . . . . . . . . . . . . 78
4.2 Obliquefieldmeasurementatlowtemperature . . . . . . . . . . . . . . . . . . . . 81
4.3 Comparisonwithtraditionalmeasurementresults . . . . . . . . . . . . . . . . . . 86
4.3.1 Biasdependence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
4.3.2 Temperaturedependence . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
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4.4 Physicalmodelingandpredictions . . . . . . . . . . . . . . . . . . . . . . . . . . 88
4.4.1 Frequencydependencemeasurement . . . . . . . . . . . . . . . . . . . . 93
4.4.2 SpinrelaxationinGaAs . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
4.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
References 103
A ChargetransportmeasurementonHeuslerthinfilms 111
A.1 Resistivityasfunctionoftemperature . . . . . . . . . . . . . . . . . . . . . . . . 111
A.2 Negativemagneoresistane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115
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