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Journal of Magnetism and Magnetic Materials 310 (2007) e735e737

Magnetic behaviour of CoAlN thin films with various Co

concentrations

Takanobu Sato, Yasushi Endo, Yu Shiratsuchi, Masahiko Yamamoto

Department of Materials Science and Engineering, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan

Available online 27 November 2006

Abstract

We have investigated the magnetic behaviour of CoAlN (Al1xCoxN) thin films with various Co concentrations (x) of 0.060.25. Forall Co concentrations, the hysteresis are not observed in any magnetization curves at the temperatures of 10300K. For the Co

concentration x below 0.10, the magnetization is almost zero at the temperature above 50 K and increases slightly with decreasing

temperature below 50 K. In contrasts, for the Co concentrationx above 0.17, the peak temperature in zero-field-cooled magnetization

can be observed and its value increases from around 5 to 10 K with increasing Co concentration. On the basis of these results, it is found

that the room-temperature ferromagnetism is not observed at any Co concentrations. It is suggested that the magnetic behaviour at the

temperature more than 10 K changes from paramagnetism or spin-glass state to superparamagnetism or spin-glass state with increasing

Co concentration.

r 2006 Elsevier B.V. All rights reserved.

PACS: 75.50.Pp; 75.70.i

Keywords: Diluted magnetic semiconductor; AlN; Magnetic behaviour; FCZFC magnetization

Diluted magnetic semiconductors (DMSs) have attracted

much attention as materials with potential for application

in spin-dependent electronic devices. In particular, DMSs

with the high Curie temperature above room temperature

(RT) have been of much interest in the viewpoint of

fundamental physical topics as well as useful potential

applications. Recently, on the basis of theoretical predica-

tions[1], it is reported that several oxides and nitrides with

the magnetic element, such as CoZnO [2], CoTiO2 [3],

MnGaN [4] and CrAlN [5], have the high Curie

temperature greater than RT. Among them, CrAlN

shows the highest Curie temperature, and thus AlN mightbe the useful host semiconductor material for the room-

temperature ferromagnetic DMSs [5]. However, in

TMAlN (TM V, Co, Fe, Ni or Mn), their magnetic

properties have been scarcely reported, and it is still unclear

whether ferromagnetism can be observed at 300 K or not.

In this paper, we choose Co as a magnetic element, because

ferromagnetism has been observed at the temperature

above 300 K in the CoZnO[2] or CoTiO2[3]. In order to

clarify the magnetic behaviour of CoAlN (Al1xCoxN)

thin films with various Co concentrations (x) of 0.060.25,

we have investigated the temperature dependence of

magnetic properties in these films.

The 250-nm-thick Al1xCoxN thin films were fabricated

at RT onto the thermally oxidized Si (0 0 1) substrates by

reactive DC magnetron sputtering. The composition of

Al1xCoxN films was evaluated using X-ray photoelectron

spectroscopy (XPS). The details of the fabrication condi-

tion were described in Ref. [6]. The magnetic properties

were measured in the temperature range of 4300 K bymeans of SQUID magnetometer with a maximum field of

50 kOe. The film structure was characterized by X-ray

diffraction (XRD).

The magnetization curves at various temperatures of

Al1xCoxN films are shown in Fig. 1. For all Co

concentrations, no hysteresis can be observed in any

magnetization curves at the temperatures of 10300 K:

both remanent magnetization and coercivity are zero at all

temperatures. From these results, it is found that any

Al1xCoxN films with the Co concentration of 0.060.25

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0304-8853/$ - see front matterr 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.jmmm.2006.11.038

Corresponding author. Tel.: +81 6 68797488; fax: +81 6 68797522.

E-mail address: takanobu.sato@mat.eng.osaka-u.ac.jp (T. Sato).

http://www.elsevier.com/locate/jmmmhttp://localhost/var/www/apps/conversion/tmp/scratch_1/dx.doi.org/10.1016/j.jmmm.2006.11.038mailto:takanobu.sato@mat.eng.osaka-u.ac.jpmailto:takanobu.sato@mat.eng.osaka-u.ac.jphttp://localhost/var/www/apps/conversion/tmp/scratch_1/dx.doi.org/10.1016/j.jmmm.2006.11.038http://www.elsevier.com/locate/jmmm

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do not exhibit ferromagnetism in the temperature range of

10300 K. However, the shape of the magnetization curve

at each temperature depends markedly on the Co

concentration. Namely, when the Co concentration x is

below 0.10, the shape becomes linear in the range of

magnetic field up to 750 kOe at all temperatures. These

curves reveal that the magnetic behaviour is paramagnet-

ism or spin-glass state. In contrast, when the Co

concentration x is above 0.17, the shape remarkably

deviates from the linear shape in the low-temperature

region. These curves suggest that the magnetic behaviour is

superparamagnetism or spin-glass state. From these

results, it is considered that the magnetic behaviour

changes from paramagnetism or spin-glass state to super-paramagnetism or spin-glass state with increasing Co

concentration.

In order to clarify the change of the magnetic behaviour

in Al1xCoxN films, we investigated the temperature

dependence of the magnetization in these films. As shown

in Fig. 2, for the Co concentration x below 0.10, the

magnetizations are almost zero at the temperature above

50 K, and increase slightly with decreasing temperature in

the range below 50K. These can be explained by the

magnetic behaviour being paramagnetism or spin-glass

state. In contrast, for the Co concentration x above 0.17,

the magnetizations increase with decreasing temperature in

the range of 10300 K. These are not due to paramagnet-

ism. Thus, these results are consistent with the change of

the magnetization curve (Fig. 1).

In order to more precisely explore the magnetic

behaviour of Al1xCoxN films with Co concentration

above 0.17, we investigated the temperature dependence of

magnetization showing the field-cooled (FC) and zero-

field-cooled (ZFC) magnetization in these films. The peak

in ZFC magnetization is observed in Fig. 3. The peak

temperatures increase from 5 K to around 10K with

increasing Co concentration. From these results, it is

considered that the magnetic behaviour is superparamag-

netism at the temperature more than 10 K. However, the

behaviour shown here is also observed in the spin-glass.

Thus, the possibility of the spin-glass state cannot be

excluded completely.

In order to verify the origin of the change of the

magnetic behaviour, the film structure of Al1xCoxN films

was investigated. These films seem to have the Wurtzite-

type AlN single phase at all Co concentrations from XRD

profiles. However, the other investigations using transmis-

sion electron microscopy (TEM) imply that the structuraltransition, which can cause the change of the magnetic

behaviour, is observed. More detailed investigation of the

film structure is in progress now.

In summary, it is concluded that the room-temperature

ferromagnetism is not observed at any Co concentrations.

It is suggested that the magnetic behaviour changes from

paramagnetism or spin-glass state to superparamagnetism

or spin-glass state with increasing Co concentration at the

temperature more than 10 K.

This work was partly supported by a Grant-in-Aid for

Scientific Research (S), Encouragement of Young Scientists

ARTICLE IN PRESS

-50 -25 0 25 50

-80

-40

0

40

80

Magnetic Field, H / kOe

dx = 0.2510 K

50 K

300 K

300 K

-50 -25 0 25 50

-40

-20

0

20

40

Magnetization,

M/

G

c x = 0.17 10 K50 K

-20

-10

0

10

20bx = 0.10 10 K

50 K

300 K

-20

-10

0

10

20a x = 0.06 10 K

50 K

300 K

Fig. 1. Magnetization curves at various temperatures of Al1xCoxN films

with the Co concentration (x) of (a) x 0.06, (b) x 0.10, (c) x 0.17

and (d) x 0.25. The magnetic field is applied in the film plane.

0 100 200 3000

10

20

30

40

50

60

Temperature, T / K

Magnetiza

tion,

M/

G

: x = 0.06

: x = 0.10

: x = 0.17

: x = 0.25

Fig. 2. Magnetization as a function of temperature in the Al1xCoxN

films with the Co concentration (x) of 0.060.25. The magnetic field is

applied in the film plane and its value is fixed at 10 kOe.

0 50 1006

8

10

12 x = 0.25

0 50 1006

7

8

9

Magnetization,

M/

G x = 0.17

Temperature, T / K

200 300 200 3000 50 1006

8

10

12

0 50 1006

7

8

9

Magnetization,

M/

G

200 300 200 300

a b

Fig. 3. Field-cooled (closed circles) and zero-field-cooled (opened circles)

magnetization as a function of temperature in Al1xCoxN films with the

Co concentration (x) of (a) x 0.17 and (b) x 0.25. The magnetic fieldis applied in the film plane and its value is fixed at 0.1kOe.

T. Sato et al. / Journal of Magnetism and Magnetic Materials 310 (2007) e735e737e736

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(B) and Priority Assistance of the Formation of World-

wide Renowned Centers of ResearchThe 21st Century

COE Program (Project: Center of Excellence for Advanced

Structural and Functional Materials Design) from the

Japanese Ministry of Education, Culture, Sports, Science

and Technology.

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