J. Magn. Soc. Jpn., 41, 99-107 (2017)<Paper>

Structure Analysis of Fe-Co and Fe-Co-B Alloy Thin Films
Formed on MgO(001) Substrate
Kana Serizawa1, Mitsuru Ohtake1,2, Masaaki Futamoto1, Fumiyoshi Kirino3, and Nobuyuki Inaba4
Faculty of Science and Engineering, Chuo University, 1-13-27 Kasuga, Bunkyo-ku, Tokyo 112-8551, Japan
2Faculty of Engineering, Kogakuin University, 2655-1 Nakano, Hachioji, Tokyo 192-0015, Japan1

3Graduate School of Fine Arts, Tokyo University of the Arts, 12-8 Ueno-Koen, Taito-ku, Tokyo 110-8714, Japan
4Faculty of Engineering, Yamagata University, 4-3-16 Jyonan, Yonezawa, Yamagata 992-8510, Japan

Fe-Co and Fe-Co-B alloy films of 40 nm thickness are prepared by sputtering on MgO(001) single-crystal
substrates using alloy targets of (Fe0.7Co0.3)100–xBx (x = 0, 5, 10, 15 at. %) by varying the substrate temperature fromroom temperature (RT) to 600 °C. The film structure is investigated by reflection high-energy electron diffraction
and out-of-plane, in-plane, and pole-figure X-ray diffractions. Preparation condition of epitaxial film is found todepend on the B content and the substrate temperature. Epitaxial (Fe0.7Co0.3)100–xBx films are obtained at theinvestigated temperatures for the B contents lower than 5 at. %, whereas films with the B contents of 10 and 15
at. % grow epitaxially at temperatures higher than 400 and 600 °C, respectively. Single-crystal films of bcc(001)orientation are formed at temperatures higher than RT, 200, 400, and 600 °C for the B contents of 0, 5, 10, and 15
at. %, respectively. As the substrate temperature decreases, the epitaxial films with B contents of 5–15 at. % tend to
involve four types of bcc(122) variant whose orientations are rotated around the film normal by 90° each other. The(Fe0.7Co0.3)90B10 film deposited at RT and the (Fe0.7Co0.3)85B15 films deposited at temperatures from RT to 200 °C arefound to be consisting of bcc polycrystal and amorphous, respectively. The structure is thus determined to vary inthe order of bcc(001) single-crystal => bcc(001) and bcc(122) epitaxial crystals => bcc polycrystal => amorphous withincreasing the B content and with decreasing the substrate temperature. The lattices of single-crystal Fe-Co (x = 0
at. %) and Fe-Co-B (x = 5–15 at. %) films are respectively expanded along the in-plane and the perpendicular
directions. The single-crystal Fe-Co films show in-plane magnetic anisotropies with the easy magnetization
directions of bcc[100] and bcc[010], which is reflecting the magnetocrystalline anisotropy of bulk bcc-Fe70Co30 alloy.
On the contrary, the single-crystal Fe-Co-B films show almost isotropic in-plane magnetic properties and weak
perpendicular anisotropies, which is possibly caused by an influence of lattice deformation along the perpendicular
direction.
Key words: Fe-Co alloy, Fe-Co-B alloy, thin film, epitaxial growth, MgO single-crystal substrate, lattice strain,
magnetic anisotropy

1.

Introduction

Magnetic tunnel junctions consisting of tunnel
barrier and ferromagnetic electrode layers have beenstudied for applications to tunnel magnetoresistance(TMR) sensors and magnetoresistive random accessmemory devices. In order to achieve high TMR ratios,
highly-oriented
polycrystalline1)
orepitaxial
single-crystal2) MgO layer of (001) orientation is usefulas the barrier layer.
Fe-Co and Fe-Co-B alloys with bcc structure are
typical soft magnetic materials with high saturation
magnetizations and have been frequently used as theelectrode material. However, the crystallographic
structure of Fe-Co-B film is affected by the B content3,4)
andtheformation
conditionlikeannealing
temperature5–8). Fe-Co-B film with a higher B content
deposited at a lower substrate temperature tends toinvolve amorphous. The magnetic properties are also
delicately influenced by the structure. Furthermore, it
has been reported that perpendicular magnetic
anisotropy is induced in Fe-Co and Fe-Co-B alloy films

when the crystal lattice is strained along theperpendicular direction9–16).
In order to investigate the basic structural andmagnetic properties, it is useful to employ epitaxial
films, since the crystallographic orientation can becontrolled by the substrate orientation. However, there
are few reports on the preparation of Fe-Co-B films on
single-crystal substrates by systematically varying theformation
condition.
In
thepresent
study,(Fe0.7Co0.3)100–xBx films are prepared on MgO(001)substrates. The influences of B content and substratetemperature on the structure and the magnetic
properties are investigated.
2.

Experimental Procedure

A radio-frequency (RF) magnetron sputteringsystem equipped with a reflection high-energy electron
diffraction (RHEED) facility was used for film formation.
The base pressures were lower than 4 × 10–7 Pa.
Polished MgO(001) substrates were heated at 600 °C for
1 hour to obtain clean surfaces. (Fe0.7Co0.3)100–xBx (x =Journal of the Magnetics Society of Japan Vol.41, No.5, 201799

B content (at. %)
0

5

10

15

(a-4)

(b-4)

(c-4)

(d-4)

(a-3)

(b-3)

(c-3)

(d-3)

(a-2)

(b-2)

(c-2)

(d-2)

(a-1)

(b-1)

(c-1)

(d-1)

Substrate temperature (ºC)

600

400

200

RT

_

_

_

_

_

(

222B

022B

022C

_

211C

011B 011C

422C

033B
222C

222B

222C

211B

No reflections

033C

004A

224B

222B

022B
022C
002A
222C
112A
112A
211C
211B
011B 011C
_

222C

224B

_

_
222B

033C

_

033B

_

112A

422C

_

_

114A

_

002A

(b) bcc(122)[011]B, [011]C (c) bcc(122)[411]D, [411]E (d) bcc(001)A + (122)B–E
_

112A

004A

_

114A

__

(a) bcc(001)A_(Fig. 1 RHEED patterns observed for (Fe0.7Co0.3)100–xBx films with the B contents of (a) 0, (b) 5, (c) 10, and (d) 15
at. % deposited on MgO(001) substrates at (a-1)–(d-1) RT, (a-2)–(d-2) 200 °C, (a-3)–(d-3) 400 °C, and (a-4)–(d-4)
600 °C. The incident electron beam is parallel to MgO[100].

Fig. 2 Schematic diagrams of RHEED patterns simulated for (a) bcc(001) and [(b), (c)] bcc(122) surfaces. Schematic
diagram of _(d) is drawn
by overlapping_(a)__ and _(b). The incident electron beam is parallel to (a) bcc(001)[110], (b)bcc(122)[011] and [011], or (c) bcc(122)[411] and [411].
0–15 at. %) alloy targets of 3 inch diameter were
employed. The distance between target and substrate
was fixed at 150 mm. The Ar gas pressure was kept
constant at 0.67 Pa. The RF powers for alloy targets
with the B contents of 0, 5, 10, and 15 at. % wererespectively adjusted at 57, 65, 60, and 58 W, where thedeposition rates were 0.02 nm/s. (Fe0.7Co0.3)100–xBx filmsof 40 nm thickness were deposited on the substrates attemperatures ranging from RT to 600 °C. In the present
study, the film compositions were considered to be
similar to the target compositions.
The surface structure was studied by RHEED. The
structural properties were investigated by 2θ/ω-scanout-of-plane, 2θχ/φ-scan in-plane, and pole-figure X-ray
diffractions (XRDs) with Cu-Kα radiation (λ = 0.15418
nm). The surface morphology was observed by atomic
force microscopy (AFM). The magnetization curves were
measured at RT by vibrating sample magnetometry.

100

3.

Results and Discussion

3.1

Crystal structure and orientation
Figure 1(a) shows the RHEED patterns of Fe70Co30(x = 0) films deposited on MgO(001) substrates atdifferent temperatures observed by making the incident
electron beam parallel to MgO[100]. Diffraction patterns
corresponding to bcc(001) single-crystal surface arerecognized, as shown in the simulated pattern of Fig.
2(a). The epitaxial orientation relationship isdetermined asbcc-(Fe0.7Co0.3)100–xBx(001)[110] || MgO(001)[100].

(type A)The Fe70Co30(001) lattice is rotated around the film
normal by 45° with respect to the MgO(001) lattice, as
shown in Fig. 3. In this configuration, the mismatch
between bcc-Fe 7 0 Co 3 0 (001) crystal and MgO(001)

Journal of the Magnetics Society of Japan Vol.41, No.5, 2017

[411](a)__(Fe0.7Co0.3)100–xBx
MgO

_

[110]

[100]

[100]

_

[110]
[010]
(Fe0.7Co0.3)100–xBx[001] MgO[001]
–4.2%

–4.2%

[011]

_

[011]

__

substrate is calculated by using the lattice constants ofbulk Fe70Co30 (aFe70Co30 = 0.2858 nm17)) and MgO (aMgO =0.4217 nm18)) crystals to be

_

[411]
(Fe0.7Co0.3)100–xBx[122]B[411](Fe0.7Co0.3)100–xBx[122]C

(1)

[100]

[011]

(b)

[011]
(Fe0.7Co0.3)100–xBx[122]E

[011]

19.5˚
(122)

(011
)

(001)

(Fe0.7Co0.3)100–xBx[122]C
[011]

( 011

)

[100]

[011]

19.5˚
(122)

[001]
[010]
MgO[100]_

are interpreted to be formed. Figure 2(c) shows theschematic diagram of RHEED pattern simulated for thetwo bcc(122) variants. When the_incident
electronbeam__
is parallel to MgO[100] (|| bcc[411] and bcc[411]), there[411](Fe0.7Co0.3)100–xBx[122]B_

When the atomic arrangements of MgO(001) and
Fe-Co-B(122) surfaces, which are with four- and one-foldsymmetries, are considered, bcc(122) variants with theorientation relationships of__bcc-(Fe0.7Co0.3)100–xBx(122)[411] || MgO(001)[100],(type D)_bcc-(Fe0.7Co0.3)100–xBx(122)[411] || MgO(001)[100],(type E)_

which suggests a possibility that the bcc-Fe70Co30
crystal lattice is expanded along the in-plane direction.
The RHEED patterns observed for (Fe0.7Co0.3)95B5
films deposited at 200–600 °C, (Fe0.7Co0.3)90B10 films
deposited at 400–600 °C, and an (Fe0.7Co0.3)85B15 film
deposited at 600 °C are respectively shown in Figs.
1(b-2)–(b-4), (c-3)–(c-4), and (d-4). Diffraction patterns
from bcc(001) surface are recognized, similar to the caseof Fe70Co30 films. (Fe0.7Co0.3)100–xBx single-crystal filmsof bcc(001) orientation are also obtained for thecompositional range of x = 5–15 at. % by using theelevated substrate temperatures.
Figures 1(b-1), (c-2), and (d-3) show the RHEED
patterns observed for an (Fe0.7Co0.3)95B5 film deposited
at RT, an (Fe0.7Co0.3)90B10 film deposited at 200 °C, andan (Fe0.7Co0.3)85B15 film deposited at 400 °C, respectively.
Diffraction patterns from bcc(122) surface [Fig. 2(b)] areobserved in addition to those from bcc(001) surface, as
shown in Fig. 2(d). The bcc(122) pattern is analyzed to
be an overlap of reflections from two variants, as shownby the indices with subscripts, B and C. The
crystallographic
orientation
relationships
aredetermined as_bcc-(Fe0.7Co0.3)100–xBx(122)[011] || MgO(001)[100],(type B)_bcc-(Fe0.7Co0.3)100–xBx(122)[011] || MgO(001)[100].
(type C)_[( 2 aFe70Co30 – aMgO) / aMgO] × 100 = –4.2%,

[010]

_

Fig. 3 Planer view schematic diagram of epitaxial
orientation relationship of bcc-(Fe0.7Co0.3)100–xBx crystal
lattice on MgO(001) substrate.

MgO[001]_–4.2%(Fe0.7Co0.3)100–xBx
MgO

[011]
(Fe0.7Co0.3)100–xBx[122]D

Fig. 4 (a) Planer and (b) cross-sectional view schematic
diagrams of epitaxial orientation relationships ofbcc-(Fe0.7Co0.3)100–xBx(001)crystalandfourbcc-(Fe0.7Co0.3)100–xBx(122)variantson
MgO(001)
substrate.

are no reflections in the diffraction pattern. Therefore,
the Fe-Co-B films are considered to be consisting ofbcc(001) crystal and four bcc(122) variants. The
structure is similar to the cases of Fe/GaAs(001)19) and
Fe50Co50/SrTiO3(001)20) systems. Figure 4 shows theplaner and cross-sectional view schematic diagrams oforientation relationships of bcc(001) crystal and fourbcc(122) variants. The orientations of bcc(122) variants
are rotated around the film normal _by 90° each
_ other.
The lattice misfit values along bcc[011] and bcc[411] arerespectively calculated to be[( 2 aFe70Co30 – aMgO) / aMgO] × 100 = –4.2%,(2)[(3 2 aFe70Co30 – 3aMgO) / 3aMgO] × 100 = –4.2%.

(3)

The misfit values do not differ depending on thein-plane direction and are the same with the latticemismatch at the bcc(001)/MgO(001) interface. In order
to confirm the orientation relationships, a pole-figure
XRD measurement was carried out. Figure 5(a) shows
the pole-figure XRD pattern of the (Fe0.7Co0.3)85B15 film
deposited at 400 °C measured by fixing the diffraction

Journal of the Magnetics Society of Japan Vol.41, No.5, 2017

101

–90°
–120°

600

–60°
15°

–150°

α = 0°

45°

90°
72°
±180°

β = 0°

Substrate temperature (ºC)(a)

bcc(001)
400

200

bcc(001)+ bcc(122)

RT

150°

30°

High

Enlarged view

(101)

(011)

(110)

(101)

_

_

(101)
(011)

–60°

(110)

(110) α = 0°15° (110) –30°
45°

90° 72°

(011)

±180°

Low

_

_

(110)

–150°

(110)

(110)

(110)

(101)

(011)

β = 0°

(101)
_

_

(101)

(011)

_

_

120°

(011)
(101)

(011)

_

(110)

(101)

90°

(110)

30°

_

_

(110)

_

150°

(110)

60°

Fig. 5 (a) Pole-figure XRD pattern of (Fe0.7Co0.3)85B15
film deposited at 400 °C measured by fixing thediffraction angle, 2θB, at 44.5°. The intensity is shownin logarithmic scale. (b) Schematic diagram obtained byoverlapping diffraction patterns simulated for thebcc-(Fe0.7Co0.3)100–xBx(122) variants grown in theorientation relationships determined by RHEED.
angle, 2θB, at 44.5°, where bcc{110} reflections are
expected to be detectable. Figure 5(b) shows theschematic diagram of pole-figure pattern calculated forthe four bcc(122) variants. The experimental pole-figure
pattern is in agreement with the simulated result. The
pole-figure
XRD
confirms
thecrystallographic
orientation relationships determined by RHEED.
Figure 1(c-1) shows the RHEED pattern observed
for an (Fe0.7Co0.3)90B10 film deposited at RT. A ring-like
diffraction pattern is observed. A polycrystalline(Fe0.7Co0.3)90B10 film is formed. Figures 1(d-1) and (d-2)
show the RHEED patterns observed for (Fe0.7Co0.3)85B15
films deposited at RT and 200 °C, respectively. Halodiffraction patterns are recognized. Amorphous(Fe0.7Co0.3)85B15 films are formed.
Figure 6 summarizes the structure and

102

5

10
B content (at. %)15

Fig. 6
Phase diagram of (Fe0.7Co0.3)100–xBx film
deposited on MgO(001) substrate.

90°

–90°
_

(110)

High

90°

_

–120°

72°

_

(b)

60°

_

120°

Low

0

bcc
poly- Amorphous
crystalcrystallographic orientation of (Fe0.7Co0.3)100–xBx film
deposited on MgO(001) substrate. The structure varies
in the order from bcc(001) single-crystal, bcc(001) andbcc(122) crystals, bcc polycrystal, and to amorphouswith increasing the B content and with decreasing thesubstrate temperature.
3.2

Lattice strain
Figure 7 shows the out-of-plane and in-plane XRD
patterns measured for the (Fe0.7Co0.3)100–xBx films withdifferent B contents deposited at 200 °C. Here, thescatteringvector of in-plane XRD is parallel to_
MgO[110]. As shown in the data of Fig. 6, these four
films are consisting of bcc(001) single-crystals (Bcontent: 0, 5 at. %), mixture of bcc(001) and bcc(122)crystals (B content: 10 at. %), and amorphous (Bcontent: 15 at. %), respectively. Strong out-of-planebcc(002) and in-plane bcc(200) reflections are observed
for the single-crystal Fe70Co30 and (Fe0.7Co0.3)95B5 films[Figs. 7(a), (b)]. In the XRD patterns measured for the(Fe0.7Co0.3)90B10 film consisting of bcc(001) and bcc(122)variants [Fig. 7(c)], weak out-of-plane and in-planereflections from bcc(001) variant are recognized, whilereflections from bcc(122) variants are absent, since thebcc(122) reflection is forbidden. The present study
shows that a combination of RHEED or pole-figure XRD
with conventional out-of-plane and in-plane XRDs iseffective to determine whether an Fe-Co-B film involvesbcc(122) crystals in addition to bcc(001) crystal. No
reflections from bcc crystals are observed in theout-of-plane and in-plane patterns measured for theamorphous (Fe0.7Co0.3)85B15 film [Fig. 7(d)].
Figure 8 shows the out-of-plane and in-planeorientation dispersions, Δθ50 and Δθχ50, measured forsingle-crystal (Fe0.7Co0.3)100–xBx films. Here, the Δθ50 andΔθχ50 values are respectively the full widths at half
maximum of ω- and φ-scan rocking curves measured by
setting the diffraction angles of 2θ and 2θχ at the peakangles of bcc(002) and bcc(200) reflections. The Δθ50 andΔθχ 50 values decrease not only with increasing the

Journal of the Magnetics Society of Japan Vol.41, No.5, 2017

Kβ

x = 10(c-2)

bcc(200)bcc(200)bcc(002)bcc(002)(b-2)

bcc(200)

WL
Kβ

MgO(220)WL

MgO(004)

bcc(002)

MgO(002)

WL
Kβ
Kβ
WL

x=5

WL

Intensity (arb. unit)

x=0
(a-2)

(b-1)

(c-1)

_

(a-1)

Kβ

x = 15
(d-2)

(d-1)
20

40

60
2θ (deg.)8040

100 2060
2θχ (deg.)

80

100

Fig. 7 (a-1)–(d-1) Out-of-plane and (a-2)–(d-2) in-plane XRD patterns measured for (Fe0.7Co0.3)100–xBx films with the
B contents of (a) 0, (b) 5, (c) 10, and (d) 15 at. % deposited on MgO(001)_ substrates at 200 °C. The in-plane XRD
patterns are measured by making the scattering vector parallel to MgO[110]. The intensity is shown in logarithmic
scale.

a = 2[λ / 2sin(θχ)bcc(200)],(4)

c = 2[λ / 2sin(θ)bcc(002)].

(5)

The errors are estimated by using the relationships of(Δa) / 2 = ± {a [cot(θχ)bcc(200)] Δ(θχ)bcc(200)} / 2,

(6)(Δc) / 2 = ± {c [cot(θ)bcc(002)] Δ(θ)bcc(002)} / 2.

(7)

For the film deposited at RT, the a value (0.2896 nm) islarger than the c value (0.2836 nm). The in-plane andout-of-plane lattices are respectively expanded andcontracted in accommodation of the lattice mismatch of–4.2% existing at the Fe70Co30/MgO interface. With
increasing the substrate temperature from RT to 200 °C,
the c/a value is approaching to 1. For the substratetemperature range of 200–600 °C, the c/a value isalmost kept constant at 1. The lattice deformation isΔθ50 (deg.)

6.0

(a)

4.0

5 at. %

10 at. %15 at. %2.0
0 at. %

0
6.0

Δθχ50 (deg.)

substrate temperature but also with decreasing the B
content. The crystallographic quality is improved for an(Fe0.7Co0.3)100–xBx film with a lower B content deposited
at a higher substrate temperature. The result seems to
be related with the structural variation of bcc(001)single-crystal => bcc(001) and bcc(122) crystals => bcc
polycrystal => amorphous.
Figure 9 shows the substrate temperaturedependences of in-plane and out-of-plane lattice
parameters, a and c, and the ratio, c/a, of single-crystal
Fe70Co30 film. Here, the a and the c values arerespectively calculated from the peak angles of bcc(002)
and bcc(200) reflections by using the relations of(b)
5 at. %

4.0
0 at. %

2.0

0

RT

10 at. %15 at. %

400
600
200
Substrate temperature (ºC)

Fig. 8 Substrate temperature dependences of (a) Δθ50
and (b) Δθχ50 values measured for single-crystal(Fe0.7Co0.3)100–xBx films with different B contents.

relaxed by using temperatures higher than 200 °C.
Figure 10 shows the lattice parameters of
single-crystal (Fe0.7Co0.3)100–xBx (x = 5–15 at. %) films.

Journal of the Magnetics Society of Japan Vol.41, No.5, 2017103

0.300

0.300

(a)

0.295

a (nm)

0.290
0.285

0.285

0.280
0.300

0.280
0.300

(b)

c (nm)

0.290

5 at. %
10 at. %(b)
5 at. %

10 at. %

0.290

0.285

0.285

0.280

0.280

(c)

1.04

15 at. %(c)

5 at. %

1.04

10 at. %1.02

15 at. %

1.00

1.00

0.98

0.98

(a)

2

(nm)

200 nm

0

(b)

6

(nm)

200 nm

0

(c)

)
01

bcc[001]

)
01

0

1)

(nm)

(1

[100]

(1

[010]
35

0
(1

Surface morphology and magnetic properties
Figure 11 shows the AFM image observed for
Fe70Co30 films deposited at 200–600 °C. The film growthon MgO substrate seems to follow Volmer-Weber 21)(island-like growth) mode. The film deposited at 200 °C

Fig. 10 Lattice parameters, (a) a, (b) c, and (c) c/a,
measured for single-crystal (Fe0.7Co0.3)100–xBx (x = 5–15
at. %) films.

_

For the (Fe0.7Co0.3)95B5 film deposited at 200 °C, the avalue (0.2818 nm) is smaller than the c value (0.2935nm). The in-plane and out-of-plane lattices arerespectively contracted and expanded, though there
exists the mismatch with minus sign at thefilm/substrate interface. The crystal lattice is deformed
along the perpendicular direction by addition of B atoms.
With increasing the substrate temperature from 200 to
600 °C, the c/a value of (Fe0.7Co0.3)95B5 film decreases
from 1.042 to 1.000. The lattice strain decreases withincreasing the substrate temperature, similar to thecase of Fe70Co30 film. The c/a value (1.019) of(Fe0.7Co0.3)90B10 film deposited at 400 °C is larger thanthat (1.004) of (Fe0.7Co0.3)95B5 film deposited at 400 °C.
A higher B content is apparently enhancing the lattice
deformation.

200
400
600
Substrate temperature (ºC)_

Fig. 9 Lattice parameters, (a) a, (b) c, and (c) c/a,
measured for single-crystal Fe70Co30 films.

0

1)

200
400
600
Substrate temperature (ºC)

(0
1

0

1)

c/a

c/a

1.02

104

15 at. %

0.295

0.295
c (nm)

0.290

(0
1

a (nm)

0.295

3.3

(a)

200 nmFig. 11
AFM images observed for Fe70Co30 films
deposited on MgO(001) substrates at (a) 200, (b) 400,and (c) 600 °C.

Journal of the Magnetics Society of Japan Vol.41, No.5, 2017

Ra (nm)

100

(b)

10

15 at. %10 at. %1

0 at. %
5 at. %

0.1
0

200
400
600
Substrate temperature ( ºC)
Fig. 12 Substrate temperature dependences of Ravalues measured for (Fe0.7Co0.3)100–xBx films withdifferent B contents.
has a flat surface with the arithmetical mean roughness,
R a , of 0.2 nm [Fig. 11(a)]. When the substratetemperature is increased up to 400 °C, an island-likesurface involving side bcc{110} facets is recognized [Fig.
11(b)]. The orientations of facets were estimated from

Normalized magnetization

1 (a)the cross-sectional profiles of the AFM images (notshown here). With further increasing the substratetemperature to 600 °C, the island size drasticallyincreases [Fig. 11(c)]. Migration and clustering ofdeposited atoms are promoted by employing a higher
substrate temperature. Figure 12 summarizes thesubstrate temperature dependences of Ra values
measured for (Fe0.7Co0.3)100–xBx films. Similar tendencies
are observed, even though the B contents are different.
Figure 13 shows the in-plane magnetization curvesof single-crystal Fe70Co30 films deposited at different
temperatures measured by applying the magnetic fieldalong bcc[100] or bcc[110]. The films are easily
magnetized when the magnetic field is applied alongbcc[100], while the magnetization curves measured
along bcc[110] saturate at higher magnetic fields. There
were no clear differences in the hysteresis loops
measured along bcc[100]_ and bcc[010] and measured
along bcc[110] and bcc[110] (not shown here). Therefore,
the films show four-fold symmetries in in-planemagnetic anisotropies, which are reflecting the(b)x = 0 at. %Ts = 400 ºC

x = 0 at. %Ts = 200 ºC(c)
x = 0 at. %Ts = 600 ºC

H // bcc[100]H // bcc[110]

0

–1
–2

–1

0

1

2 –2

–1
0
1
Magnetic field (kOe)

2 –2

–1

0

1

2

Fig. 13 In-plane magnetization curves measured for single-crystal Fe70Co30 films formed at (a) 200, (b) 400, and (c)600 °C. The magnetic field is applied along bcc[100] or bcc[110].

(b)c/a = 1.019
x = 10 at. %Ts = 400 ºC

Normalized magnetization

1 (a)c/a = 1.004
x = 5 at. %Ts = 400 ºC(c)c/a = 1.042
x = 5 at. %Ts = 200 ºC

H // bcc[100]H // bcc[110]

0

–1
–2

–1

0

1

2 –2

–1
0
1
Magnetic field (kOe)

2 –2

–1

0

1

2

Fig. 14 In-plane magnetization curves measured for single-crystal (Fe0.7Co0.3)100–xBx films with the c/a ratios of (a)1.004, (b) 1.019, (c) 1.042. The magnetic field is applied along bcc[100] or bcc[110].

Journal of the Magnetics Society of Japan Vol.41, No.5, 2017

105

(b)
c/a = 1.019
x = 10 at. %Ts = 400 ºC

Normalized magnetization

1 (a)c/a = 1.004
x = 5 at. %Ts = 400 ºC(c)c/a = 1.042
x = 5 at. %Ts = 200 ºC

H // In-plane
H // Out-of-plane

0

–1
–10

–5

0

5

10 –10–5
05Magnetic field (kOe)

10 –10

–5

0

5

10

Fig. 15 Out-of-plane and in-plane magnetization curves measured for single-crystal (Fe0.7Co0.3)100–xBx films with the
c/a ratios of (a) 1.004, (b) 1.019, and (c) 1.042.
magnetocrystalline anisotropy of bulk Fe70Co30 alloy
with the easy magnetization axes of bcc<100>. The
in-plane anisotropy decreases as the substratetemperature increases. The magnetic anisotropy seems
to be influenced not only by the magnetocrystalline
anisotropy but also by the shape anisotropy caused by
the surface undulation, which is more enhanced byusing a higher substrate temperature. With increasing
the substrate temperature from 200 to 600 °C, thecoercivity of hysteresis curve measured along bcc[100]increases from 2.5 to 150 Oe. Domain wall motion seems
to be suppressed by the crevasses existing between
islands, as shown in the AFM image of Fig. 11(c).
Figure 14 shows the in-plane magnetization curvesof single-crystal (Fe0.7Co0.3)100–xBx films with the c/a
ratios of 1.004, 1.019, and 1.042 whose B contents are 5,
10, and 5 at. % and whose formation temperatures are400, 400, and 200 °C, respectively. Although the films
are composed of bcc(001) single-crystal, almost isotropic
in-plane curves are observed. As the c/a ratio increases
from 1.004 to 1.042, the coercivity increases from 30 to
320 Oe, whereas the remnant magnetization, Mr/Ms,
decreases from 0.923 to 0.718. The result indicates thatperpendicular magnetic anisotropies have been also
induced in the strained (Fe0.7Co0.3)100–xBx single-crystal
films, similar to the cases of previous studies9–16). Figure
15 shows the out-of-plane and in-plane hysteresis loops
measured for the (Fe0.7Co0.3)100–xBx films. The films
show weak perpendicular magnetic anisotropies. The
perpendicular magnetic anisotropy seems to have beenslightly enhanced with increasing the c/a ratio. The
magnetic properties are apparently influenced by thelattice deformation.
4.

Conclusion(Fe0.7Co0.3)100–xBx alloy films are prepared by
sputter deposition on MgO(001) substrates by varying
the B content from 0 to 15 at. % and by varying thesubstrate temperature from RT to 600 °C. The detailed

106

structural properties are investigated by RHEED and
XRD. Single-crystal (Fe0.7Co0.3)100–xBx films with the B
contents of 0, 5, 10, and 15 at. % are formed attemperatures higher than RT, 200 °C, 400 °C, and
600 °C, respectively. The crystallographic orientation
relationshipisbcc-(Fe0.7Co0.3)100–xBx(001)[110]||MgO(001)[100]. As the B content increases and thesubstrate temperature decreases, the structure varies in
the order of bcc(001) single-crystal => a mixture ofbcc(001) and bcc(122) crystals => polycrystal =>amorphous. The lattice of single-crystal Fe70Co30 film is
expanded along the in-plane direction in accommodationof the mismatch existing at the Fe70Co30(001)/MgO(001)interface. On the contrary, the lattices of single-crystal(Fe0.7Co0.3)100–xBx (x = 5–15 at. %) films are expanded
along the perpendicular direction. The Fe70Co30 films
show in-plane magnetic anisotropies reflecting themagnetocrystalline anisotropy of bulk Fe70Co30 crystal,
whereas the (Fe0.7Co0.3)100–xBx films show weak
perpendicular magnetic anisotropies, which are possibly
caused by an influence of lattice strain along theperpendicular direction.
Acknowledgement A part of this work was supported
by Chuo University Grant for Special Research.

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Received Apr. 4, 2017; Accepted Jun. 10, 2017

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