Tailoring anisotropy and domain structure in amorphous TbCo thin films through combinatorial methods

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Frisk, A., Magnus, F., George, S., Arnalds, U B., Andersson, G. (2016)

Tailoring anisotropy and domain structure in amorphous TbCo thin films through combinatorial methods.

Journal of Physics D: Applied Physics, 49(3): 035005 http://dx.doi.org/10.1088/0022-3727/49/3/035005

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methods

Andreas Frisk

1 ‡

^{, Fridrik Magnus}

^1,2

^{, Sebastian George}

¹

^,

Unnar B. Arnalds

2

and Gabriella Andersson

1 1

DepartmentofPhysi sandAstronomy,UppsalaUniversity,Box516,SE-751

20Uppsala,Sweden

2

S ien eInstitute,UniversityofI eland,Dunhaga3,Reykjavik,IS-107,I eland

Abstra t.

Weapplyan in-plane external magneti eldduring growth of amorphous

TbCothinlmsandexaminetheee tsonthemagneti anisotropyanddomain

stru ture. A ombinatorialapproa hisemployedthroughoutthedepositionand

analysis to study a ontinuous range of ompositions between 7-95 at.% Tb.

Magnetometry measurements showthat allsamples havea strong out-of-plane

anisotropy,mu hlargerthananyin-plane omponents,regardlessofthepresen e

of a growth eld. However, magneti for e mi ros opy demonstrates that the

growtheld doesindeed havea largeee ton the magneti domainstru ture,

resultinginelongateddomainsalignedalongtheimprintingelddire tion. The

results show that the anisotropy an betuned inintri ate ways inamorphous

TbCo lms giving rise to unusual domain stru tures. Furthermorethe results

reveal that a ombinatorial approa h is highlyee tive for mappingout these

materialproperties.

PACSnumbers: 75.30.Gw,75.50.Kj,75.70.Ak,75.70.Kw,61.43.Dq

Keywords: magneti anisotropy,amorphousmagneti materials, magneti properties

ofthinlms,domainstru ture

‡

Correspondingauthor:andreas.friskphysi s.uu.se

1. Introdu tion

Thin lms of TbCo are well known for their

strong perpendi ular magneti anisotropy (PMA)[1 ,

2℄ whi h makes them of interest for a range of

magneti storage and spin-valve te hnologies.[3, 4℄

Furthermore, it has re ently been shown that all-

opti al magneti swit hing (AOS)[5℄ an be a hieved

in Tb(Co,Fe) lms,[6, 7℄ allowing the manipulation

of magneti domains on mu h shorter times ales

than is possible with magneti elds. AOS relies

on the ferrimagneti nature of the TbCo, where

the anti-aligned Tb and Co magneti sublatti es

ompensate ea h other at a given temperature,

resultinginzeronetmagnetization. The ompensation

temperature an also allow the generation of domain

walls using thermal or omposition gradients with

potential appli ations in domain wall memories.[8, 9℄

Both the ompensation temperature and PMA an

be engineeredby buildingheterostru tures ombining

TbCo with other materials.[10, 11℄ This allows great

exibility in tuning the magneti properties and an

even open up new possibilities su h as interlayer

ouplingthroughproximityindu edmagnetism.[12℄

Amorphouslmsareparti ularlyinterestinginthe

ontextofheterostru turesastheyformex eptionally

at and homogeneous layers[13, 14, 15℄ and dierent

materials anbe ombinedwithouthavingto onsider

dieren es in latti e onstants.[12℄ In addition, a

magneti anisotropy an be imprinted in amorphous

lmsinanarbitrarydire tionbyapplyingamagneti

eldduring growth.[16℄InSmCo(anotherrare-earth

transition metal ompound) it has for example been

shown that su h an imprinted anisotropy an be

very large [17℄ and lead to unusual magneti domain

stru tures.[18℄

Here we explore the imprinting of anisotropy in

amorphous TbCo thin lms and its ee t on domain

stru ture. We use a ombinatorial approa h[19, 20℄

whereby material is deposited from two separate

sour es on a large wafer under an angle, so that a

ontinuous omposition gradient is a hieved. This

allows us to map out the properties of a ontinuous

range of ompositions mu h more e iently than

with onventional methods. We nd that, while the

perpendi ular anisotropy faroutweighsthe imprinted

anisotropy,the domainstru tureis stillverysensitive

tothesmallimprintedanisotropy omponent.

2. Experimentaldetails

TbCo thin lms were deposited by a ombinatorial

te hniqueusing DC magnetron sputtering. Thesam-

ples were preparedin anultra-high va uum hamber,

with a maximum base pressure of

3 × 10 ⁻⁹

^{Torr, at}

roomtemperatureusing

99.999

^{%pureArasasputter-}

inggasat apressureof

1 × 10 ⁻³

^{Torr. Thelmswere}

depositedon

10

mmwidestrips utfromnaturallyox- idized3-in hSi(100)waferswhi hwerepre-heated, at

base pressure, to

650 ^◦

C for

20

min and ooled down toroom-temperaturebeforedeposition. Toensurethe

amorphi ity of the lms, a buerlayerof amorphous

Al ₈₀ Zr ₂₀

with a nominal thi kness of

3

nm was de- posited on the native oxide of the Si.[15℄ The ham-

ber geometry wassu h that Co and Tb targets were

positioned fa ing ea h other at opposite ends of the

substrate. Figure1(a)showsthetarget andsubstrate

onguration.

By o-sputtering from the Co and Tb targets,

with a non-rotating substrate, a omposition spread

a ross the sample was reated, as shown in g. 1(a).

In total, a ontinuous omposition range from 7 to

95 at.%Tb wasa hievedoverthree waferstripsea h

withanominallmthi knessof

50

nmatthe enterof ea hsli e. Toprote ttheTbColayerfromoxidationa

appinglayerof

3

nm

Al ₈₀ Zr ₂₀

wasdeposited. Boththe buerand appinglayerweredepositedwitharotating

substrate to ensure a homogeneous omposition and

thi kness. One set of lms was grown in a sample

holderwithtwopermanentmagnets reatinganearly

homogeneous stati in-plane magneti eld of about

130

^{mT, seeg.1(a),whereasanothersetoflmswas}

grownwithoutthis magneti eld. Themagneti eld

wasappliedinthedire tion

φ = 90 ^◦

^,perpendi ularto the omposition gradientdire tion whi h wedeneas

φ = 0 ^◦

^.

Rutherford ba ks attering (RBS) measurements

were performed at several points along the Tb-Co

gradient to determine the omposition. The yield of

ea h spe trum was normalized to the total ount of

the spe trum to enable omparison between dierent

RBS measurements. For ea h spe trum the peaks of

ea helementwereintegratedgivingtheyields

Y Tb

^and

Y Co

^{. By}simultaneouslysolvingthetwoequations

Y _Tb(Co) Y Tb + Y Co

= x _Tb(Co) Z _Tb(Co) ²

x Tb Z _Tb ² + x Co Z _Co ²

⁽¹⁾

Tb

Tb rich side

Magnet

MagnĞt

B _g S N

S N

( D)

0 10 20 30 40 50 60

Position from sample edge (mm) 0

10 20 30

x Tb (at.%)

(b)

F

Figure 1. (Color online) (a) A diagram of the target

onguration and substrateholder. Thesubstrateismounted

in between two permanent magnets produ ing a growth eld

B g ≈130

^{mT at the enter. The} omposition gradient is represented asa olourgradient. Theupperright ornershows

thepositionofthesubstrateholderwithrespe ttothetargets.

Theangle

φ

^{isalsodened. (b)A}representativemeasurement oftheTb on entrationgradienta rossasample. Themarkers

arethevaluesmeasuredwithRBSandthelineisthetusedto

determine ompositionsatintermediatepoints.

that

x Tb + x Co = 1

^{the elemental} on entrations

x Tb(Co)

^{in at.% were} determined. Here,

Z Tb(Co)

are the atomi numbers of ea h spe ies. The

omposition gradient was found to be almost linear

versus position asshown in g.1(b). A lineart was

therefore used to extrapolate the omposition along

theentiresamplelengthgivinga ompositiongradient

of

∆x Tb = 0.4

^

0.6

^{at.%/mm. This implies that the}

variation in on entration over the probed area of

ea hmeasurementpoint(less than

4

mmdiameter),is

1.5

^

2.4

^{at.%. The error bars in g. 1 represent this}

un ertainty, whi h is smaller than the experimental

un ertaintyofRBS.

X-ray ree tivity (XRR) and grazing in iden e

X-ray dira tion (GIXRD) were measured at several

pointsalongtheTb-CogradientwithCuK

α

^radiation

using a Bruker D8 Dis over in a parallel beam

geometry. A Göbel mirror was used on the in ident

sideaswellasbeam-shaperslitstolimitthemeasured

area. The ree ted/dira ted beam was measured

using aLynx EYE dete tor. For GIXRDan in ident

angle of

ω = 1 ^◦

^{was used. The probed area in ea h}

measurementwasabout

8

mm

× 10

^{mm ,withthelong}

dimensionperpendi ulartotheTb-Cogradient.Inthis

dire tion the Tb-Co omposition should be onstant,

seeg.1.

Both longitudinal and polar magneto-opti Kerr

ee t (L- and P-MOKE) measurements were used to

determine the magneti properties of the samples at

room temperature. The diameter of the laser spot

on the sample was about

1

^

2

^{mm. An in-plane or}

out-of-plane magneti eld was applied ( ontinuously

measuredwithaHallprobe) andmagnetizationloops

werere ordedatdierentpointsonthesamples,along

theTb-Co gradient. Tostudythein-plane anisotropy

the samples were also rotated around the azimuthal

angle

φ

^were

φ = 0 ^◦

orrespondstothedire tionwhere theeldisappliedparalleltothe ompositiongradient,

g.1.

Highereldmagneti hara terizationwas arried

outasafun tionoftemperatureand ompositionusing

a Cryogeni Ltd. vibrating sample magnetometer

(VSM). These measurements were performed on

leaved samples, no larger than

6.8

^{mm wide along}

the gradient dire tion. Magnetization loops with a

eld of up to 5 T applied both perpendi ular to

the plane and in the plane of the samples in the

temperature range 10 to 320K were measured. The

in-planemeasurementswereperformedat

φ = 90 ^◦

^,i.e.

the eld is applied perpendi ular to the omposition

gradient. The diamagneti ba kground from the

substratewassubtra tedbyalinearttothehigheld

parts of ea h magnetization s an. Magneti moments

were al ulated using the magneti lm thi knesses

extra tedfromXRR tting.

Magneti for emi ros opy(MFM)wasperformed

withaNanosurfMobileSatomi for emi ros opeand

MFM01 series tipsfrom NT-MDT. All measurements

weredonein phase ontrastmode.

3. Resultsand dis ussion

3.1. Stru turalProperties

SomeexamplesofGIXRDs ansareshownintheinset

in g. 2. These measurementsshow that all samples

upto about

x Tb = 80

^{at.%Tb are X-rayamorphous}

asseenbythepresen eofonlyonebroadlow-intensity

peaktypi alofamorphoussampleswithala koflong-

range atomi order.[21℄ The angular position of this

broad peak gives a measure of the average atomi

separationinthelm,whi hin reaseswithTb ontent

onsistentwiththelargerlatti eparameterofh p-Tb

(

3.60

^{Å) ompared to that of h p-Co (}

2.51

^{Å ). By}

inserting the average atomi spa ing and full-width

at half-maximum(FWHM) into the S herrer formula

[22℄ the orrelation length (sometimes referred to as

the grain size) an be estimated, as shown in the

main panelof g. 2. Forsmall Tb on entrationsthe

orrelation length is almost onstant at about

10

^Å

10 20 30 40 50 60 70 80 90 x Tb (at.%)

10 20 30 40

Correlation length (Å)

10 20 30 40 50 60 70 80

2
(°) 0

100 200 300 400 500 600 700 800

Intensity (CPS)

93 at.%

81 at.%

51 at.%

18 at.%

9 at.%

Tb

Figure 2. (Color online) X-ray orrelation length versusTb

omposition for the entire range studied. The inset shows

examplesofsomeGIXRDs ansfordierent ompositions(oset

for larity). Thebroad peakisanindi ationofan amorphous

stru ture. The s anfor the highest Tb on entration showsa

sharppeakwhi hisa signatureofat leastpartially rystalline

stru ture.

 ^

 ^

 ^

 ^





 

 

     





  

  !" _! #$ _%&!

'( )* +, -*

./

01234 5234 5234

./6 -

'( _! +, _" 6 %&!#"

'( )* +, -*

%234

%234

Figure3. (Coloronline)XRRs an(points)andatted urve

(solidline)for

x Tb = 46

at.%.Theinsetshowsthelayermodel usedtottheXRRdata.

anbe attributed to the hange in atomi separation

mentioned above. At approximately

x Tb = 80

at.%

there is a sudden in rease in the orrelation length

whi h anbeinterpretedasanonsetof rystallization.

GIXRDonthesamplesgrownwithoutaeldhavethe

sameappearan e and give the sameatomi spa ings,

FWHM and orrelation length as the orresponding

eld-grownsamples.

XRR measurements were used to determine the

thi knessandqualityofthelayeringinthesamples. A

representativeXRR s an an be seenin g. 3. Clear

interferen efringesarisingfrom the totalthi knessof

the lms an be observed up to

2θ = 6 ^◦

^{, attesting}

to their smoothness. By tting thedata to thelayer

model shown in the inset it is possible to extra t

-5 -4 -3 -2 -1 0 1 2 3 4 5

₀ H (T) -5

-3 -1 1 3 5

Magnetization (A/m)

10 ⁵

Out-of-

plane In-plane

Figure 4. (Color online) Room temperature in-plane and

out-of-plane hysteresis loops for

x Tb = 13

at.%, measured withVSM.Astrong out-of-planeanisotropy isobserved. The

slight dieren einsaturation magnetization anbeattributed

to sample alignment. The in-plane angle was

φ = 90 ^◦

^{, i.e.}

perpendi ulartothe ompositiongradient.

the layerthi knesses, densities and roughnesses. Low

surfa eroughnesses (root-mean-squared)in therange

0.61 nmare obtainedfor lmswithatotalthi kness

of 5257 nm whi h is similar to other amorphous

rare-earthtransition metal ompound lms.[17℄ As

for GIXRD, XRR showed no signi ant dieren es

between samplesgrown withand without amagneti

eld.

3.2. Magneti Properties

A ombination of MOKE and VSM measurements

in dierent geometries was used to map out the

magneti properties of the TbCo. Films with a

Tb on entration below

x Tb = 45 ± 3

^{at.% were}

ferrimagneti atroomtemperature,whi hisaslightly

larger omposition than the

38

at.% reported by Betz et al.[23℄ In this omposition range, the lms

have a strong out-of-plane anisotropy, as seen in the

hara teristi VSM measurementspresented in g. 4.

In the out-of-plane dire tion, the hysteresis loop is

squarewithalarge(temperaturedependent)remanent

magnetization whereas in the in-plane dire tion the

loopis smoothlyvarying with asmallremanen e and

largesaturationeld. Thisisobservedforlmsgrown

both with and without an applied magneti eld.

However, subtle dieren es are seen in the in-plane

magnetization for these two ases. Figure 5 shows

in-plane hysteresis loops along two perpendi ular

dire tions in the plane, for samples grown with and

without eld. Samples grown without eld [g. 5(a)℄

areisotropi intheplaneasseenbytheidenti alloops

along the two dire tions. In ontrast, for samples

grown in a eld [g. 5(b)℄ an opening is observed

-0.5 0.0 0.5

1.0 (a) No Growth Field

= 0°

= 90°

-800 -400 0 400 800

-0.5 0.0 0.5

(b) Growth Field

= 0°

= 90°

 ₀ H (mT)

Magnetization (arb. units)

Figure5.(Coloronline)In-planeminorloopsmeasuredinthe

L-MOKE geometry both parallel (

φ = 0 ^◦

^{) and} perpendi ular (

φ = 90 ^◦

^)tothe ompositiongradientfor(a)thesamplegrown without an external eld and (b) the sample grown inan in-

planemagneti eld along

φ = 90 ^◦

^{. The} omposition at the point measured was approximately

x _Tb = 10

at.% for both samples.

growtheldwhereasperpendi ulartothegrowtheld

thehysteresisisidenti alto that ofthesamplegrown

without a eld. This shows that, despite the out-of-

plane dire tion being the overall easy axis, there is

a omponent (of the easy axis) whi h lies along the

growth eld dire tion. This shows that the external

growth eld has indeed imprinted a small in-plane

anisotropy omponent.

Out-of-plane magnetization measurements by

VSMoverthetemperaturerange10320K onrmthe

ferrimagneti ordering in the lms. A ompensation

temperature

T comp

^{is observed where the oer ivity}

diverges and the magnetization is zero, as shown

in g. 6. This is due to the dierent temperature

dependen eofthemagnetizationoftheanti-alignedTb

and Co magneti sublatti es whi h an el ea h other

outat

T comp

^{. Within reasingTb ontent}

T comp

^isseen

to in rease and for ompositions above

21

at.% Tb,

T comp

^isaboveroomtemperature(see insetof g.6).

This value orresponds well with previouslyreported

values.[23,24℄

Theout-of-plane oer ivityisstronglydependent

on temperature and omposition as seen in g. 6.

For the omposition shown in the main graph

(

x Tb = 20.2

^{at.%) the oer ivity is larger at room}

temperature sin e it is lose to

T comp

^{. For the}

smaller omposition of

x Tb = 13

^{at.% [g. 4℄,}

150 200 250 300

T (K) 0

1 2 3 4

M r (A/m)

10

T _comp = 271 K

0 1 2 3 4

0 H c (T)

M r H c

16 17 18 19 20 21 22 23 x Tb (at.%) 0

100 200 300

T comp (K)

Figure 6. (Color online) The temperature dependen e of

the out-of-planeremanen e and oer ivity, for a sample pie e

with

x Tb = 20 .2

^{at.% atthe enter,as measuredwithVSM.}

The oer ivity has a singularity and diverges at

271

K while the remanen e goesto zeroat this ompensationtemperature,

T comp

^{.Theinsetshowsthemeasured}

T comp

^versus omposition.

the oer ivity is instead quite small. In this ase

the measurement is performed at room temperature

whi hforthis ompositionismu hhigherthan

T comp

^.

Generally,forall ompositionsthe oer ivityde reases

with in reasing temperature above

T comp

^{while for}

de reasing temperatures below

T comp H C

^initially

de reases, but eventuallyrea hes aminimum and for

evenlowertemperaturesin reasesslightlyon eagain.

Magneti for e mi ros opy was used to examine

themagneti domainstru ture ofthelmsforseveral

dierent ompositions as shown in g. 7, spe i ally

to ompare the samples grown with and without an

external eld. At around

7

at.% Tb (not shown in g. 7), the sample grown with an external eld

exhibits a similar labyrinthine domain stru ture to

that ofthesample grownin-eld at

8

at.%[g. 7( )℄.

ForaslightlyhigherTb on entrationaround

9

at.%, [g. 7(b) and (d)℄, there is a lear divergen e in the

domain stru tures between the two samples. For

the sample grown without an external eld, the

domains begin to align parallel to the omposition

gradient, while for the sample grown in-eld the

domains start to align parallel to the growth eld.

This reinfor es the idea that the growth eld does

indeed inuen e the zero-eld magnetization in the

sample, as was suggested by the L-MOKE results,

see g. 5. To be sure that this ee t was in fa t

relatedto thegrowtheld,severalMFM imageswere

measured for ea h sample and omposition. Before

ea h measurement, the sample was exposed to an

external in-plane eld of 700 mT (simulating an L-

MOKE s an) applied either parallel orperpendi ular

to the omposition gradient. However, the trends

dire tionofthemostre entlyappliedeld. Thisgives

strength to the assertionthat themagneti stru ture

ofthesamplesisimprintedduringthegrowthpro ess.

ForhigherTb on entrationsthedomainsgrowinsize.

This evolution is faster for the sample grown with

an externaleld than thesample grown without. At

11

^{at.%thedomainsoftheeld-grownsamplebe ome}

so large that several MFM s ans in dierent surfa e

lo ations failed to ontain any domain boundaries.

Hen e, these images exhibited little ontrast and are

thereforeomittedin g.7.

The origin of the PMA in Tb-Fe amorphous

lms has been shown to be related to dierent pair

orrelations between the Tb-Fe, Tb-Tb and Fe-Fe

atomi pairs in the plane and perpendi ular to the

plane[25℄. Thesedieren esareindu edbythebroken

symmetry at the interfa esof the lm during growth

and it is likely that the same applies to the origin

of the PMA in TbCo. The origins of eld indu ed

magneti anisotropy in amorphous materials are at

present less well understood. It is thought that the

magneti eld indu es hanges in the lo al atomi

onguration[26℄ in theform of alignmentof atomi

moment pairs via dipolar ee ts [27℄, alignment of

atomi lusters via lo al spin-orbit oupling (single-

ion anisotropy) [28℄, or dire tion dependent bonding

between atoms of dierent elements as des ribed

above [25℄. Imprinted anisotropy has also been

linkedwith thestrainindu edduringgrowththrough

magnetoelasti oupling [28℄. Although we annot

distinguish betweenthese me hanismshereit is lear

that the dire tional dependen e of pair orrelations

indu edbythelmsurfa esfaroutweighsthe hanges

inthelo al ongurationorthemagnetoelasti strain

indu edbythegrowtheld.

4. Con lusions

We have shown how ombinatorial methods are

valuable in mapping out various material properties

su h as amorphi ity and magneti anisotropy versus

omposition. We have furthermore shown that the

magneti properties of TbCo are very sensitive to

omposition and are ontinuously tunable, meaning

that the desired properties an be obtained by

arefullysele tingthe omposition. Thistunability,in

ombinationwiththeamorphousstru tureandsmooth

interfa es, makes amorphous TbCo lms ideal for

perpendi ularex hange oupledmultilayerstru tures.

Furthermore, these TbCo lms are anew exampleof

the possibilities asso iated with imprinting magneti

anisotropy in amorphous alloys. Even though TbCo

exhibits a strong intrinsi out-of-planeanisotropy for

all ompositions, imprinting an in-plane anisotropy

is still possible resulting in a tilt of the easy axis

away from the lm normal. The dire tion of this

tilt an be ontrolled by the growth eld and the

ompositiongradient. Themagneti domainstru ture

is strongly ae ted by the anisotropy and an thus

be ontrolled by manipulating the omposition and

in-plane growth eld. The possibility to ontrol the

orientation of the resulting elongated domains an

be useful in appli ations su h as magneti storage,

magneti logi ,andmagnoni devi es.

A knowledgments

ThisworkwasfundedbytheSwedishResear hCoun il

(VR), The Swedish Foundation for International

Cooperation in Resear h and Higher Edu ation

(STINT), and UBA a knowledges funding from the

I elandi Resear h Fund grant nr. 141518-051.

The authors thank the IBA group at the Tandem

laboratory at Uppsala University for their help with

RBSmeasurementsandanalysis.

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5
m

(c) (e) ^5
m

5
m

(f)

Grown in Field Grown without Field

G ro w th F ie ld

Composition Gradient Axis

Figure7. (Coloronline)Domainstru turesforthesamplegrowninanexternaleld(a)and(b),andgrownwithoutanexternal

eld( )-(f). The ompositionsare(a)8.5at.%,(b)9.2at.% ,( )8.5at.%,(d)9.3at.%,(e)11.5at.%,and(f)13.0at.%Tb,allwith

anun ertaintyof

±0.4

^{at.%. Darkandlightregions orrespondtoareaswherethesample}magnetizationpointsintooroutofthe sampleplane,respe tively. Field-grownsampleswith ompositions orrespondingto(e)and(f)donotshowanydomainboundaries

inthes alemeasuredwithMFM,and arehen eleftouthere. Theeld-grownsamplewith7at.%showsasimilarpatternto( ),

i.e.,withoutanyelongation. This ompositionwasnota essibleonthesamplegrownwithoutanexternaleld.

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