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Citation for the original published paper (version of record):

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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Tailoring anisotropy and domain stru ture in

amorphous TbCo thin �lms through ombinatorial

methods

Andreas Frisk

1 ‡, Fridrik Magnus

1,2, Sebastian George

1,

Unnar B. Arnalds

2and Gabriella Andersson

1

1Department of Physi s and Astronomy, Uppsala University, Box 516, SE-751

20 Uppsala, Sweden

2S ien e Institute, University of I eland, Dunhaga 3, Reykjavik, IS-107, I eland

Abstra t.

We apply an in-plane external magneti �eld during growth of amorphous

TbCo thin �lms and examine the e�e ts on the magneti anisotropy and domain

stru ture. A ombinatorial approa h is employed throughout the deposition and

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

Magnetometry measurements show that all samples have a strong out-of-plane

anisotropy, mu h larger than any in-plane omponents, regardless of the presen e

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

growth �eld does indeed have a large e�e t on the magneti domain stru ture,

resulting in elongated domains aligned along the imprinting �eld dire tion. The

results show that the anisotropy an be tuned in intri ate ways in amorphous

TbCo �lms giving rise to unusual domain stru tures. Furthermore the results

reveal that a ombinatorial approa h is highly e�e tive for mapping out these

material properties.

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

Keywords: magneti anisotropy, amorphous magneti materials, magneti properties

of thin �lms, domain stru ture

‡ Corresponding author: andreas.frisk�physi s.uu.se

Tailoring anisotropy and domain stru ture in amorphous TbCo thin �lms through ombinatorial methods 2

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,

resulting in zero net magnetization. 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 engineered by building heterostru 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

oupling through proximity indu ed magnetism.[12℄

Amorphous �lms are parti ularly interesting in the

ontext of heterostru tures as they form ex eptionally

�at and homogeneous layers[13, 14, 15℄ and di�erent

materials an be ombined without having to onsider

di�eren es in latti e onstants.[12℄ In addition, a

magneti anisotropy an be imprinted in amorphous

�lms in an arbitrary dire tion by applying a magneti

�eld during growth.[16℄ In SmCo (another rare-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 e�e 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 far outweighs the imprinted

anisotropy, the domain stru ture is still very sensitive

to the small imprinted anisotropy omponent.

2. Experimental details

TbCo thin �lms were deposited by a ombinatorial

te hnique using DC magnetron sputtering. The sam-

ples were prepared in an ultra-high va uum hamber,

with a maximum base pressure of 3× 10−9Torr, at

room temperature using 99.999% pure Ar as a sputter-

ing gas at a pressure of 1× 10−3Torr. The �lms were

deposited on 10mm wide strips ut from naturally ox-

idized 3-in h Si(100) wafers whi h were pre-heated, at

base pressure, to 650 ◦C for 20min and ooled down

to room-temperature before deposition. To ensure the

amorphi ity of the �lms, a bu�er layer of amorphous

Al80Zr20 with a nominal thi kness of 3nm was de-

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

ber geometry was su h that Co and Tb targets were

positioned fa ing ea h other at opposite ends of the

substrate. Figure 1(a) shows the target and substrate

on�guration.

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 was a hieved over three wafer strips ea h

with a nominal �lm thi kness of 50nm at the enter of

ea h sli e. To prote t the TbCo layer from oxidation a

apping layer of 3nm Al80Zr20 was deposited. Both the

bu�er and apping layer were deposited with a rotating

substrate to ensure a homogeneous omposition and

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

holder with two permanent magnets reating a nearly

homogeneous stati in-plane magneti �eld of about

130mT, see �g. 1(a), whereas another set of �lms was

grown without this magneti �eld. The magneti �eld

was applied in the dire tion φ = 90◦, perpendi ular to

the omposition gradient dire tion whi h we de�ne as

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 di�erent

RBS measurements. For ea h spe trum the peaks of

ea h element were integrated giving the yields YTb and

YCo. By simultaneously solving the two equations

YTb(Co)

YTb + YCo=

xTb(Co)Z2Tb(Co)

xTbZ2Tb + xCoZ

2Co

(1)

for ea h element Co and Tb together with the ondition

Tailoring anisotropy and domain stru ture in amorphous TbCo thin �lms through ombinatorial methods 3

Tb

Tb

rich side

Magnet

Magn t

Bg

S

N

S

N

( )

0 10 20 30 40 50 60

Position from sample edge (mm)

0

10

20

30

xT

b(a

t.%

)

(b)

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

on�guration and substrate holder. The substrate is mounted

in between two permanent magnets produ ing a growth �eld

Bg ≈130mT at the enter. The omposition gradient is

represented as a olour gradient. The upper right orner shows

the position of the substrate holder with respe t to the targets.

The angle φ is also de�ned. (b) A representative measurement

of the Tb on entration gradient a ross a sample. The markers

are the values measured with RBS and the line is the �t used to

determine ompositions at intermediate points.

that xTb + xCo = 1 the elemental on entrations

xTb(Co) in at.% were determined. Here, ZTb(Co)

are the atomi numbers of ea h spe ies. The

omposition gradient was found to be almost linear

versus position as shown in �g. 1(b). A linear �t was

therefore used to extrapolate the omposition along

the entire sample length giving a omposition gradient

of ∆xTb = 0.4�0.6 at.%/mm. This implies that the

variation in on entration over the probed area of

ea h measurement point (less than 4mm diameter), is

1.5�2.4 at.%. The error bars in �g. 1 represent this

un ertainty, whi h is smaller than the experimental

un ertainty of RBS.

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

X-ray di�ra tion (GIXRD) were measured at several

points along the Tb-Co gradient with Cu Kα radiation

using a Bruker D8 Dis over in a parallel beam

geometry. A Göbel mirror was used on the in ident

side as well as beam-shaper slits to limit the measured

area. The re�e ted/di�ra ted beam was measured

using a Lynx EYE dete tor. For GIXRD an in ident

angle of ω = 1◦ was used. The probed area in ea h

measurement was about 8mm× 10mm, with the long

dimension perpendi ular to the Tb-Co gradient. In this

dire tion the Tb-Co omposition should be onstant,

see �g. 1.

Both longitudinal and polar magneto-opti Kerr

e�e 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�2mm. An in-plane or

out-of-plane magneti �eld was applied ( ontinuously

measured with a Hall probe) and magnetization loops

were re orded at di�erent points on the samples, along

the Tb-Co gradient. To study the in-plane anisotropy

the samples were also rotated around the azimuthal

angle φ were φ = 0◦ orresponds to the dire tion where

the �eld is applied parallel to the omposition gradient,

�g. 1.

Higher �eld magneti hara terization was arried

out as a fun tion of temperature and omposition using

a Cryogeni Ltd. vibrating sample magnetometer

(VSM). These measurements were performed on

leaved samples, no larger than 6.8mm 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 320 K were measured. The

in-plane measurements were performed at φ = 90◦, i.e.

the �eld is applied perpendi ular to the omposition

gradient. The diamagneti ba kground from the

substrate was subtra ted by a linear �t to the high �eld

parts of ea h magnetization s an. Magneti moments

were al ulated using the magneti �lm thi knesses

extra ted from XRR �tting.

Magneti for e mi ros opy (MFM) was performed

with a Nanosurf Mobile S atomi for e mi ros ope and

MFM01 series tips from NT-MDT. All measurements

were done in phase ontrast mode.

3. Results and dis ussion

3.1. Stru tural Properties

Some examples of GIXRD s ans are shown in the inset

in �g. 2. These measurements show that all samples

up to about xTb = 80 at.% Tb are X-ray amorphous

as seen by the presen e of only one broad low-intensity

peak typi al of amorphous samples with a la k of long-

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

broad peak gives a measure of the average atomi

separation in the �lm, whi h in reases with Tb ontent

onsistent with the larger latti e parameter of h p-Tb

(3.60Å) ompared to that of h p-Co (2.51Å). 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 panel of �g. 2. For small Tb on entrations the

orrelation length is almost onstant at about 10Å

with only a slight in rease with Tb ontent, whi h

Tailoring anisotropy and domain stru ture in amorphous TbCo thin �lms through ombinatorial methods 4

10 20 30 40 50 60 70 80 90

xTb

(at.%)

10

20

30

40C

orr

ela

tion

len

gth

(Å

)

10 20 30 40 50 60 70 80

2 (°)

0

100

200

300

400

500

600

700

800

Inte

nsi

ty (

CP

S)

93 at.%81 at.%51 at.%18 at.% 9 at.%

Tb

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

omposition for the entire range studied. The inset shows

examples of some GIXRD s ans for di�erent ompositions (o�set

for larity). The broad peak is an indi ation of an amorphous

stru ture. The s an for the highest Tb on entration shows a

sharp peak whi h is a signature of at least partially rystalline

stru ture.

!"!#$%&!

'()*+,-*

./

01234

5234

5234

./6-

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

'()*+,-*

%234

%234

Figure 3. (Color online) XRR s an (points) and a �tted urve

(solid line) for xTb = 46 at.%. The inset shows the layer model

used to �t the XRR data.

an be attributed to the hange in atomi separation

mentioned above. At approximately xTb = 80 at.%

there is a sudden in rease in the orrelation length

whi h an be interpreted as an onset of rystallization.

GIXRD on the samples grown without a �eld have the

same appearan e and give the same atomi spa ings,

FWHM and orrelation length as the orresponding

�eld-grown samples.

XRR measurements were used to determine the

thi kness and quality of the layering in the samples. A

representative XRR s an an be seen in �g. 3. Clear

interferen e fringes arising from the total thi kness of

the �lms an be observed up to 2θ = 6◦, attesting

to their smoothness. By �tting the data to the layer

model shown in the inset it is possible to extra t

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

0H (T)

-5

-3

-1

1

3

5

Ma

gn

etiz

atio

n (

A/m

)

105

Out-of-plane

In-plane

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

out-of-plane hysteresis loops for xTb = 13 at.%, measured

with VSM. A strong out-of-plane anisotropy is observed. The

slight di�eren e in saturation magnetization an be attributed

to sample alignment. The in-plane angle was φ = 90◦, i.e.

perpendi ular to the omposition gradient.

the layer thi knesses, densities and roughnesses. Low

surfa e roughnesses (root-mean-squared) in the range

0.6�1 nm are obtained for �lms with a total thi kness

of 52�57 nm whi h is similar to other amorphous

rare-earth�transition metal ompound �lms.[17℄ As

for GIXRD, XRR showed no signi� ant di�eren es

between samples grown with and without a magneti

�eld.

3.2. Magneti Properties

A ombination of MOKE and VSM measurements

in di�erent geometries was used to map out the

magneti properties of the TbCo. Films with a

Tb on entration below xTb = 45± 3 at.% were

ferrimagneti at room temperature, whi h is a slightly

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 measurements presented in �g. 4.

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

square with a large (temperature dependent) remanent

magnetization whereas in the in-plane dire tion the

loop is smoothly varying with a small remanen e and

large saturation �eld. This is observed for �lms grown

both with and without an applied magneti �eld.

However, subtle di�eren 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)℄

are isotropi in the plane as seen by the identi al loops

along the two dire tions. In ontrast, for samples

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

in the hysteresis loop in the dire tion parallel to the

Tailoring anisotropy and domain stru ture in amorphous TbCo thin �lms through ombinatorial methods 5

-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°

0H (mT)

Ma

gn

etiz

atio

n (

arb

. u

nits

)

Figure 5. (Color online) In-plane minor loops measured in the

L-MOKE geometry both parallel (φ = 0◦) and perpendi ular

(φ = 90◦) to the omposition gradient for (a) the sample grown

without an external �eld and (b) the sample grown in an in-

plane magneti �eld along φ = 90◦. The omposition at the

point measured was approximately xTb = 10 at.% for both

samples.

growth �eld whereas perpendi ular to the growth �eld

the hysteresis is identi al to that of the sample grown

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

VSM over the temperature range 10�320 K on�rm the

ferrimagneti ordering in the �lms. A ompensation

temperature Tcomp is observed where the oer ivity

diverges and the magnetization is zero, as shown

in �g. 6. This is due to the di�erent temperature

dependen e of the magnetization of the anti-aligned Tb

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

out at Tcomp. With in reasing Tb ontent Tcomp is seen

to in rease and for ompositions above 21 at.% Tb,

Tcomp is above room temperature (see inset of �g. 6).

This value orresponds well with previously reported

values.[23, 24℄

The out-of-plane oer ivity is strongly dependent

on temperature and omposition as seen in �g. 6.

For the omposition shown in the main graph

(xTb = 20.2 at.%) the oer ivity is larger at room

temperature sin e it is lose to Tcomp. For the

smaller omposition of xTb = 13 at.% [�g. 4℄,

150 200 250 300

T (K)

0

1

2

3

4

Mr (

A/m

)

105

Tcomp

= 271 K

0

1

2

3

4

0H

c (

T)

Mr

Hc

16 17 18 19 20 21 22 23

xTb

(at.%)

0

100

200

300

Tco

mp (

K)

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

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

with xTb = 20.2 at.% at the enter, as measured with VSM.

The oer ivity has a singularity and diverges at 271K while

the remanen e goes to zero at this ompensation temperature,

Tcomp. The inset shows the measured Tcomp versus omposition.

the oer ivity is instead quite small. In this ase

the measurement is performed at room temperature

whi h for this omposition is mu h higher than Tcomp.

Generally, for all ompositions the oer ivity de reases

with in reasing temperature above Tcomp while for

de reasing temperatures below Tcomp HC initially

de reases, but eventually rea hes a minimum and for

even lower temperatures in reases slightly on e again.

Magneti for e mi ros opy was used to examine

the magneti domain stru ture of the �lms for several

di�erent 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 of the sample grown in-�eld at 8 at.% [�g. 7( )℄.

For a slightly higher Tb on entration around 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 in�uen e the zero-�eld magnetization in the

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

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

related to the growth �eld, several MFM images were

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 or perpendi ular

to the omposition gradient. However, the trends

des ribed above remained onstant regardless of the

Tailoring anisotropy and domain stru ture in amorphous TbCo thin �lms through ombinatorial methods 6

dire tion of the most re ently applied �eld. This gives

strength to the assertion that the magneti stru ture

of the samples is imprinted during the growth pro ess.

For higher Tb on entrations the domains grow in size.

This evolution is faster for the sample grown with

an external �eld than the sample grown without. At

11 at.% the domains of the �eld-grown sample be ome

so large that several MFM s ans in di�erent surfa e

lo ations failed to ontain any domain boundaries.

Hen e, these images exhibited little ontrast and are

therefore omitted in �g. 7.

The origin of the PMA in Tb-Fe amorphous

�lms has been shown to be related to di�erent pair

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

atomi pairs in the plane and perpendi ular to the

plane [25℄. These di�eren es are indu ed by the broken

symmetry at the interfa es of 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

on�guration [26℄ in the form of alignment of atomi

moment pairs via dipolar e�e ts [27℄, alignment of

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

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

between atoms of di�erent elements as des ribed

above [25℄. Imprinted anisotropy has also been

linked with the strain indu ed during growth through

magnetoelasti oupling [28℄. Although we annot

distinguish between these me hanisms here it is lear

that the dire tional dependen e of pair orrelations

indu ed by the �lm surfa es far outweighs the hanges

in the lo al on�guration or the magnetoelasti strain

indu ed by the growth �eld.

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

arefully sele ting the omposition. This tunability, in

ombination with the amorphous stru ture and smooth

interfa es, makes amorphous TbCo �lms ideal for

perpendi ular ex hange oupled multilayer stru tures.

Furthermore, these TbCo �lms are a new example of

the possibilities asso iated with imprinting magneti

anisotropy in amorphous alloys. Even though TbCo

exhibits a strong intrinsi out-of-plane anisotropy 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

omposition gradient. The magneti domain stru ture

is strongly a�e 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 , and magnoni devi es.

A knowledgments

This work was funded by the Swedish Resear h Coun 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

RBS measurements and analysis.

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404�409

Tailoring anisotropy and domain stru ture in amorphous TbCo thin �lms through ombinatorial methods 7

5 m(a)

5 m(b) 5 m(d)

5 m(c) 5 m(e)

5 m(f)

Grown in Field Grown without FieldG

row

th F

ield

Composition Gradient Axis

Figure 7. (Color online) Domain stru tures for the sample grown in an external �eld (a) and (b), and grown without an external

�eld ( )-(f). The ompositions are (a) 8.5 at.%, (b) 9.2 at.%, ( ) 8.5 at.%, (d) 9.3 at.%, (e) 11.5 at.%, and (f) 13.0 at.% Tb, all with

an un ertainty of ±0.4 at.%. Dark and light regions orrespond to areas where the sample magnetization points into or out of the

sample plane, respe tively. Field-grown samples with ompositions orresponding to (e) and (f) do not show any domain boundaries

in the s ale measured with MFM, and are hen e left out here. The �eld-grown sample with 7 at.% shows a similar pattern to ( ),

i.e., without any elongation. This omposition was not a essible on the sample grown without an external �eld.

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