Manuscript

T. Magn. Soc. Jpn. (Special Issues)., 1, 26-29 (2017)

Click here to download Manuscript MAKUTA_manu._final2..pdf<Paper>Size dependence of magnetic properties for
L 1 0 -MnGa circular dots
H. Makuta, Y. Takahashi, T. Shima, and M. Doi

Graduate School of Engineering, Tohoku Gakuin University, 1-13-1 Chuo, Tagajo-shi, Miyagi 985-8537, Japan

L10-MnGa circular dot arrays have been microfabricated from continuous films. The thin films were preparedusing an alternate deposition method with a magnetron sputtering system. Improvement of degree of long-rangeorder (S) and saturation magnetization (Ms), for the L10-MnGa thin film was confirmed when repetition number (n)was increased to 10. The thin film with n = 10 exhibited Ms = 439 kA/m and Ku = 1.1 MJ/m3. The film with n = 10 was
microfabricated into circular dot arrays by electron beam (EB) lithography and Ar ion milling. The critical
single-domain size was determined to be 140 nmφ by observation of magnetic domain using a magnetic force
microscope, and the exchange stiffness constant (Aconst.) was then estimated to be 1.1 × 10-11 J/m from the critical
single-domain size.
Keywords: L10-MnGa thin film, circular dot arrays, microfabrication, critical single-domain size, exchange stiffness
constant
1. Introduction

L10-MnGa alloy attracts much attention as one ofmaterial exhibiting high uniaxial crystalline magnet
anisotropy without including rare earth elements ornoble metals. The L10 structure is corresponding to γ1
and γ2 -phase in the Mn-Ga binary phase diagram1), it isthermodynamically stable in range of approximately 64~ 67 (γ1-phase) and 58 ~ 61 (γ2-phase) at. % Mn. In thinfilm case, saturation magnetization Ms ≈ 600 kA/m anduniaxial magnetic anisotropy Ku ≈ 1.5 × 106 J/m3 havebeen reported in Mn54Ga46 (at. %) sputtered thin film2).
Such high Ku of L10-MnGa above 1 MJ/m is satisfying
requirement to maintain the magnetization direction
even in a nanoscale magnet. Therefore, a lot of studies
about the L10-MnGa and the application such as the thinfilms on various substrates3)-6), microparticles withnanocrystal7), spintronics devises8)-9), and a bit patternedmedia10) have been performed. However, there are few
reports about magnetic properties for the L10-MnGa in
nanoscale. Understanding of nanoscale magnetic
properties should be required for the design guidelines
for the various applications.
In this study, L10-MnGa circular dot arrays have been
microfabricated from the thin films. A relation between
the magnetic properties and the diameter of the dots has
also been investigated. The critical single-domain size
was determined by observation of magnetic domainusing a magnetic force microscope; exchange stiffness
constant (Aconst.) was then estimated from the critical
single-domain size.
2. Experimental procedure
The L10-MnGa thin films were prepared by alternate
deposition method with radio-frequency magnetron
sputtering system. Base pressure of the depositionchamber was less than 1 × 10-5 Pa. [Mn/MnGa]n (n:repetition number) multilayers were deposited on a Cr26(5 nm) buffered MgO (001) single crystal substrate. The
stacks were then capped by a Cr layer (10 nm). n wasvaried as followed: n = 5, 10, and 15, whereas totalthickness of the [Mn/MnGa]n multilayer was fixed at 20nm. The Cr buffer layer was deposited at room
temperature (R.T.), and annealed at 700ºC for 30 min.
The [Mn/MnGa]n multilayers were deposited at 100ºC byusing a Mn and Mn40Ga60 alloy target for the Mn andMnGa layer respectively, and then post-annealing wereapplied at 400ºC for 60 min to promote crystallization
L10 ordering of the [Mn/MnGa]n multilayers.
Composition of the L10-MnGa film can be controlled bychange of thickness ratio for the Mn and MnGa layer inthe multilayer. The composition was fixed at Mn58Ga42(at. %), in this case thicknesses of the Mn and MnGa
layer were approximately [Mn (0.97 nm)/MnGa (3.0nm)]5, [Mn (0.49 nm)/MnGa (1.5 nm)]10, and [Mn (0.32nm)/MnGa (1.0 nm)]15. The L10-MnGa dots weremicrofabricated from the continuous films through theuse of electron beam (EB) lithography with anegative-type EB resist and Ar ion milling. After themilling process, the dots were capped by an Au layer (5nm) without exposure to atmosphere to protect side ofthe dots from oxidation. Diameter of the dot (D) wasreduced from 1000 nmφ to 140 nmφ. The crystalline
structures of the thin films were identified by 2θ-θ scansof x-ray diffraction (XRD) with Cu-Kα radiation. The
magnetic properties for the films were measured with asuper conducting quantum interference device (SQUID)
magnetometer. The shapes of the dots were observed
using an atomic force microscope (AFM). The magnetic
properties for the dots were characterized using themagneto-optical Kerr effect (µ-MOKE) measurement
system with the polar configuration. Magnetic domain
structures were observed using a magnetic force
microscope (MFM). All the measurements wereperformed at R.T..

Transaction of the Magnetics Society of Japan (Special Issues) Vol.1, No.1, 2017

003

112Cr 200002

MgO sub.
001

Intensity (log. scale)

L10-MnGa(a) n = 5S = 0.71(b) n = 10
S = 0.75

20Fig. 130405060
2 Theta (deg.)

250(a) n = 50

Ms= 364 kA/m
3Ku = 1.3 MJ/m

-250

Magnetization (kA/m)

80

XRD patterns for L10-MnGa thin films withrepetition number (n) = (a) 5, (b) 10, and (c)15.
500-500
500250(b) n = 100

Ms= 439 kA/m
3
Ku = 1.1 MJ/m-250-500
500
2500-250-500-6Fig. 270(c) n = 15
S = 0.7390(c) n = 15
Ms= 422 kA/m
3
Ku = 1.0 MJ/m
-4-202
4
Magnetic ﬁeld (MA/m)

6

Magnetization curves for L10-MnGa thin films
with repetition number (n) = (a) 5, (b) 10, and (c)15. Solid circles and dashed lines denote
out-of-plane and in-plane curves respectively.
3.

Results and discussion

First, in order to find optimum n of the [Mn/MnGa]nmultilayer, investigate the effect of n on crystalline
structure and magnetic properties for the thin film. Fig.
1 shows XRD patterns for the L10-MnGa films with n = 5(a), 10 (b), and 15 (c). Fundamental (002) peaks andsuperlattice (001), (003) peaks of L10-MnGa wereobserved in n = 5 ~ 15. It indicating that the L10-MnGa
films were successfully obtained from the [Mn/MnGa]nmultilayer after post-annealing. A (112) peak was also
clearly observed in n = 5. It implies incompleteorientation along [001] direction (i.e., easy axis ofFig. 3

AFM plane view and cross-sectional images
for L10-MnGa dots with D = (a) 1000 nmφ,(b) 500 nmφ, and (c) 140 nmφ.

magnetization) of the MnGa layer. In case of n = 10 and15, the (112) peak intensity was reduced compared to thefilm with n = 5, indicating improve of [001] orientation of
the MnGa layer with n increase. The degree oflong-range order (S) was estimated from following Eq.
(1):S = √[I(001)/I(002)]meas./√[I(001)/I(002)]calc.,(1)
where I(001) and I(002) are integrated intensity of (001) and(002) peaks and [I(001)/I(002)]meas. and [I(001)/I(002)]calc. are themeasured and calculated peak intensity ratio,respectively. When n was increased to 10 from 5, S wasslightly increased to 0.75 from 0.71. It can be interpreted
as a result that increase of interfaces between the Mnand MnGa layers in the multilayer allowed easy
crystallization and L10 ordering.
Fig. 2 shows magnetization curves for the L10-MnGa
films with n = 5 (a), 10 (b), and 15 (c). Solid circles anddashed lines denote the out-of-plane and in-plane curves
respectively. Ku was estimated from estimated Eq. (2):Ku = µ0Ms × Hk/2 + µ0Ms2/2,(2)where µ0 is space permeability, and Hk is anisotropy field.
µ0Ms2/2 is correction of the demagnetizing energy. Hk ofthe films were estimated to be 5.2 MA/m for n = 5, 3.5
MA/m for n = 10 and 15. Ms was increased to Ms = 439
kA/m from 364 kA/m, Ku was decreased to 1.1 MJ/m3
from 1.3 MJ/m3 when n was increased to 10 from 5. Incase of n = 15, remarkable changes in Ms and Ku were
not observed.
The L10-MnGa film of n = 10 with maximum S and Ms
was microfabricated into the L10-MnGa circular dot
arrays. Fig. 3 shows representative AFM plane view andcross-sectional images for the L10-MnGa dots with D =1000 nmφ (a), 500 nmφ (b), and 140 nmφ (c). The dots withwell-defined circular shapes were observed for each D inthe AFM images.
Fig. 4 shows MOKE curves for the as-deposited
L10-MnGa continuous film (a) and the as-patterned dotswith D = 1000 nmφ (b), 700 nmφ (c), 500 nmφ (d), 300 nmφTransaction of the Magnetics Society of Japan (Special Issues) Vol.1, No.1, 201727

0.03 (a)0.00

Kerr rotational angle (deg.)

-0.03
0.03

Conti. ﬁlm
Hc =255 kA/m(b) D = 1000 nmφ(e) D = 300 nmφ0.00
-0.030.03

Hc = 509 kA/m

Hc = 326 kA/m
(f) D = 200 nmφ(c) D = 700 nmφ0.00
-0.030.03

Hc = 565 kA/m

Hc = 374 kA/m(d) D = 500 nmφ(g) D = 140 nmφ

0.00

-500

Coercivity, Hc (kA/m)-0.03

Fig. 40600
40020000Hc = 406 kA/m

500-500
Magnetic ﬁeld (kA/m)

0Hc = 589 kA/m

500(h)

MFM images of L10-MnGa (a) continuous film
and dots with D = (b) 1000 nmφ, (c) 500 nmφ, (d)200 nmφ, and (e) 140 nmφ in initial state. Brightand dark contrast denotes the upward anddownward magnetization state, respectively.

Value of conti. ﬁlm

500Diameter, D (nm)1000

MOKE curves for L10-MnGa (a) continuous film
and dots with D = (b) 1000 nmφ, (c) 700 nmφ, (d)500 nmφ, (e) 300 nmφ, (f) 200 nmφ, and (g) 140nmφ. (h) Coercivity (Hc) as a function of diameter(D) for the dots.

(e), 200 nmφ (f), and 140 nmφ (g); coercivity (Hc) as afunction of D for the dots is shown in (h). When thecontinuous film with Hc = 255 kA/m was microfabricatedinto the dots with D = 1000 nmφ, it exhibits Hc = 326
kA/m. Hc was more increased with decrease of D, Hc =589 kA/m was confirmed in D = 140 nmφ.
In order to determine the critical single-domain size of
the L10-MnGa thin film, observation of the magnetic
domain by using a MFM was carried out. Fig. 5 shows
MFM images of the as-deposited L10-MnGa continuousfilm (a) and the as-patterned dots which weremagnetically initial state with D = 1000 nmφ (b), 500nmφ (c), 200 nmφ (d), and 140 nmφ (e). The bright anddark contrast denotes the upward and downward
magnetization state respectively. Multiple-domain
structure was observed for the continuous film and thedots with D = 300 nmφ whereas few dots withsingle-domain structure was observed among the dotswith double-domain structure in D = 200 nmφ. When D
was decreased to 140 nmφ, single-domain structure wasobserved for most of the dots. It suggests that critical
single-domain size of the L10-MnGa dot is D = 140 nmφ.
Aconst. of the L10-MnGa dot was estimated using thecritical single-domain size. Here, domain wall energy
and magnetostatic energy in a dot with critical diameter

28

Fig. 5Fig. 6 Schematic illustration of a circular dot withdouble-domain structure.
is discussed. Fig. 6 is a schematic illustration of acircular dot with double-domain structure. Domain wallenergy of the dot (γ) is given byγ = 2rctπ√Αconst.Κu,(3)
where rc is the critical single-domain radius and t is
thickness of the dot. On the other hand, if the dot has
single-domain structure, magnetostatic energy of the dot(Um) is given by
Um = µ0Nπrc2tMs2/2,(4)where N is demagnetizing factor of the out-of-plane
direction. In the critical case,γ = U m.
(5)
Therefor, the following Eq. (6) is satisfied:
2rctπ√Αconst.Κu =µ0Nπrc2tMs2/2.
(6)
From Eq. (6), Aconst. is described asAconst. = rc2N2µ02Ms4/16Κu.
(7)
Using the rc = 70 nm, N = 0.81, Ms = 439 kA/m, and Κu =1.1 MJ/m3, Aconst. is estimated to be 1.1 × 10-11 J/m.
Magnetic
properties
for
therepresentative
ferromagnetic materials including the L10-MnGa issummarized in Table 1. The Aconst. of the L10-MnGa iscomparable to that of the other listed materials.

Transaction of the Magnetics Society of Japan (Special Issues) Vol.1, No.1, 2017

Table 1 Magnetic properties for the representative ferromagnetic materials.
Materialµ0Ms (T)

Ku (J/m3)

2rc (nm)Aconst. (J/m)

Ref.

Fe

2.15

≈ 4.2 × 104

12

2.0 × 10-1111)

Co

1.80

≈ 5.3 × 105

70

1.3 × 10-1111)

L10-FePt

1.45≈ 6.6 × 106

340

1.6 × 10-1112) - 14)

Nd-Fe-B

1.60≈ 4.5 × 106

210

0.8 × 10-1115)

L10-MnGa

0.55≈ 1.1 × 106

140

1.1 × 10-11—

As presented above, Hc, critical single-domain size,
and Aconst. of the L10-MnGa dots were demonstrated.
However, it must be note that the dots wereas-patterned; consequently, deterioration of themagnetic properties due to the damage of the dots
during the milling process must be considered.
Post-annealing of the dots and investigation of magnetic
properties for the dots are should be required.
4. Summary
In this study, L10-MnGa circular dot arrays have been
microfabricated from the thin films. A relation between
the magnetic properties and the diameter of the dots has
also been investigated. The L10-MnGa thin films wereprepared by alternate deposition method. Improvementof S, and Ms for the L10-MnGa thin film was confirmed
when n of the [Mn/MnGa]n multilayer before
post-annealing was increased to 10 from 5. The film withn = 10 exhibits Ms = 439 kA/m and Ku = 1.1 MJ/m3. Thefilm with n = 10 was microfabricated into the circular
dot arrays. The critical single-domain size wasdetermined to be 140 nmφ by observation of magnetic
domain using a magnetic force microscope, and Aconst.
was then estimated to be 1.1 × 10-11 J/m from the critical
single-domain size.
Acknowledgments
This study was performed at the Hi-tech ResearchCenter of Tohoku Gakuin University, and supported by“Collaborative Research Based on Industrial Demand”
program from Japan Science and Technology Agency.

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Received Oct. 11, 2016; Accepted Dec. 13, 2016

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