phys. stat. sol. (c) 4, No. 12, 4569–4572 (2007) / DOI 10.1002/pssc.200777322

© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Influence of film thickness on the soft magnetic properties

of sputtered Co-Fe-Hf-O films

Anh-Tuan Le1,5

, Nguyen Duy Ha2, Manh-Huong Phan

3, Kwang-Eun Lee

1,

Chong-Oh Kim1, Heebok Lee

4, and Seong-Cho Yu

5, *

1 Research Center for Advanced Magnetic Materials (ReCAMM), Chungnam National University,

Daejeon 305-764, South Korea 2 The Kamerlingh Onnes Laboratory, Leiden University, P.O. Box 9504, 2300 RA, Leiden,

The Netherlands 3 Advanced Composites Centre for Innovation and Science, University of Bristol, Queen's Building,

Bristol BS8 1TR, England 4 Department of Physics Education, Kongju National University, Kongju 314-701, South Korea 5 BK 21 Physics Program and Department of Physics, Chungbuk National University, Cheongju 361-763,

South Korea

Received 7 May 2007, revised 23 October 2007, accepted 28 October 2007

Published online 18 December 2007

PACS 75.70.Ak, 85.70.Kh

Influence of film thickness on the soft magnetic properties of Co19.35Fe53.28Hf7.92O19.35 films, which were

deposited on Si(100) substrates using the oxygen reactive RF-sputtering method, has been thoroughly in-

vestigated. It is found that as the film thickness (t) increases from 50 to 430 nm, the easy-axis coercivity

(HCE) strongly decreases from 26.2 to 1.5 Oe, while the hard-axis anisotropy field (HKH) significantly in-

creases from 68.7 to 84.8 Oe and the saturation magnetization (4πMs) tends to increase from 16.8 to 19.7

kG. As t exceeds 430 nm, HKH gradually decreases, while both HCE and 4πMs remain almost unchanged. It

reveals that a strong phase separation appears to occur in films with t > 430 nm, resulting in the decrease

of HKH.

© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction

Rapid developments in telecommunication and electronic devices have led to an increasing demand for

further miniaturization and higher operating frequency of magnetic devices [1]. Metallic thin films have

long been used for technological applications [2], but they are unable to work in the high-frequency

regime (~GHz) due to their low electrical resistivity [3]. In this context, the recent development of soft

ferromagnetic oxide thin films is of practical importance in high-frequency magnetic device applications,

because these materials can show a good high-frequency performance into the gigahertz range, owing to

their high electrical resistivity, which is approximately ten times higher than that of conventional metal-

lic films [4–6]. Recently, we successfully produced Co19.35Fe53.28Hf7.92O19.35 films with a novel laminate

nanostructure and showing the excellent magnetic properties, such as large saturation magnetization,

4πMs ∼ 19.86 kG, small easy-axis coercivity, Hc ∼ 1.5 Oe, large hard-axis anisotropy field Hk ~ 84 Oe [6].

The excellent magnetic properties of this film in addition to a high electrical resistivity of ρ ~ 3569

µΩcm make it ideal for use in high-frequency applications. It is believed that the formation of a laminate

* Corresponding author: e-mail: tuancnu@cnu.ac.kr, scyu@chungbuk.ac.kr, Phone: +82 43 2612269, Fax: +82 43 2756415

4570 Anh-Tuan Le et al.: Influence of film thickness on properties of Co-Fe-Hf-O

© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.pss-c.com

nanostructure results in the excellent magnetic properties of this film [5, 6]. Nonetheless, the mechanism

that is responsible for this film’s nanostructural growth still remains an open question.

In order to further clarify this, in the present work, we have thoroughly investigated the soft magnetic

properties of Co19.35Fe53.28Hf7.92O19.35 films with varying thicknesses in the range of 50–2500 nm. The

results obtained indicate that varying film thickness significantly changes the laminate nanostructure and

therefore modifies the soft magnetic properties. The films with a thickness of 300–600 nm have been

found to show the excellent soft magnetic properties of small easy-axis coercivity (HCE ~ 1.5–2.4Oe),

large hard-axis anisotropy field (HKH ~ 70.54–84.87 Oe) and large saturation magnetization (4πMs ~

17.37–19.74 kG), which are attractive candidate materials for high-frequency micromagnetic device

applications

2 Experimental details

Co19.35Fe53.28Hf7.92O19.35 films with varying thickness in the range of 50–2500 nm were deposited by

reactive rf-sputtering using an Ar+O2 atmosphere with a base pressure of less than 2.0×10-7 Torr, onto

Si(100) substrates at ambient temperature, and in a dc magnetic field of 100 Oe to induce an in-plane

uniaxial anisotropy. The films were fabricated from a composite target. Hf chips were placed very close

to the outer plasma ring on the Co30Fe70 alloy target surface. The structure of all films prepared was

examined by X-ray diffraction (XRD) and high-resolution transmission electron microscopy (TEM). The

composition of the prepared films was analyzed by Auger Electro Spectroscopy (AES) and Rutherford

Back-scattering Spectroscopy (RBS). In addition, local micro-compositions were determined in-situ by

Energy Dispersive x-ray spectroscopy (EDS) plug-in modules of TEM. The thickness of films were

measured using α-step. Magnetic hysteresis loops were measured using a vibrating sample magnetometer

(VSM), along the easy and hard axes of magnetization. Note that the films were measured in plane paral-

lel (perpendicular) to the direction of the applied magnetic field. Electrical resistivity was measured by a

standard four-probe method.

3 Results and discussion

Figure 1 displays the magnetic hysteresis loops measured along the easy and hard directions for the sam-

ples with different film thicknesses. The magnetic parameters extracted (e.g. saturation magnetization,

4πMs, easy-axis coercivity, HcE, and hard-axis anisotropy field, HkH) are plotted as a function of film

thickness in Fig. 2.

Fig. 1 Magnetic hystersis loops of Co19.35Fe53.28Hf7.92O19.35 films with different thicknesses (t = 120, 430, 520

and 1200 nm)

phys. stat. sol. (c) 4, No. 12 (2007) 4571

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It can be seen from Fig. 1 that the easy-axis hysteresis loop exhibits high coercive squareness, indicating

that the magnetization reversal results mainly from irreversible domain wall motion across the film,

whereas the magnetization reversal along the hard axis is caused by rotation of domain magnetization

[7]. As the film thickness increases from 50 to 430 nm, HcE decreases strongly, while HkH increases sig-

nificantly (Fig. 2). The 4πMs tends to increase in the range of t = 50–430 nm. The softest magnetic prop-

erty is achieved in the film with t = 430 nm. That is why this film has been found to show the best high-

frequency performance among the samples investigated [5,6]. With further increase of film thickness (t >

430 nm), however, the HkH starts to decrease, while the HcE and the 4πMs remain almost unchanged (see

Fig. 2). These results suggest considerable variations in the laminate nanostructure as the film thickness

exceeds 430 nm.

To elucidate this intriguing feature, we have studied the influence of film thickness on the laminate

nanostructure of a Co19.35Fe53.28Hf7.92O19.35 film, using XRD patterns and high-resolution TEM images. It

is found that the structure of a Co19.35Fe53.28Hf7.92O19.35 film consists of α-Fe(Co)-rich bcc nanograins

embedded in a HfO2-rich amorphous matrix. The XRD results (not shown here) reveal that as the film

thickness increases, the ratio of the HfO2 (111) and α-Fe(Co) (110) peak height increases, indicating the

development of a HfO2-rich amorphous phase at the expense of the α-Fe(Co)-rich nanocrystalline phase.

This is consistent with what is seen from cross-sectional TEM images in Fig. 3. It has been found that the

laminate nanostructure is improved as the film thickness increases up to 430 nm, whereas this nanostruc-

ture is significantly modified in thicker films (t > 430 nm). As can be seen from Fig. 3, the laminate

nanostructure is strongly modified in the t = 480 nm sample when compared to the t = 250 nm sample. A

strong phase separation appears to occur in films with t > 430 nm, where the HfO2-rich amorphous phase

strongly develops and becomes dominant over the α-Fe(Co)-rich nanocrystalline phase.

In terms of the structural analyses, it is reasonable to conclude that for the 50 ≤ t ≤ 430 nm films, the

decrease of HCE is attributed to the decrease in size of α-Fe(Co) nanograins, while the increase of HKH is

caused by the increase of crystalline anisotropy and the formed laminate nanostructure [6]. The drastic

decrease of HKH for t > 430 nm films can arise mainly from the strong development of the HfO2-rich

Fig. 3 Cross-sectional TEM images of

Co19.35Fe53.28Hf7.92O19.35 films with different thicknesses

(a) 250 nm and (b) 480 nm.

Fig. 2 The film-thickness dependences of the

easy-axis coercivity (HCE), hard-axis anisotropy

field (HKH) and saturation magnetization (4πMs) for

a Co19.35Fe53.28Hf7.92O19.35 thin film. The marked

pattern shows the good magnetic properties of the

films for high-frequency applications.

4572 Anh-Tuan Le et al.: Influence of film thickness on properties of Co-Fe-Hf-O

© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.pss-c.com

amorphous phase and the strongly modified laminate nanostructure. In fact, the strong phase separation

for t > 430 nm films [Fig. 3b] can also be seen from the variation in the M-H curve (Fig. 1). Furthermore,

since the size of α-Fe(Co) nanograins only slightly varies in films with t > 430 nm, the HCE remains al-

most unchanged (Fig. 2). These results indicate that the presence of a novel laminate nanostructure re-

sults in the excellent soft magnetic properties and any variation in this nanostructure due to changing

film thickness can significantly modify the magnetic nature.

It is very interesting to note, in the present work, that the films with a thickness of 300–600 nm exhibit

the excellent soft magnetic properties, including small easy-axis coercivity (HCE ~ 1.5–2.4Oe), large

hard-axis anisotropy field (HKH ~ 70.54–84.87 Oe) and large saturation magnetization (4πMs ~ 17.37–

19.74 kG). Therefore, these films are very attractive candidate materials for high-frequency micromag-

netic device applications [1].

4 Conclusions

The influence of film thickness on the soft magnetic properties of Co19.35Fe53.28Hf7.92O19.35 films has been

studied. The change of film thickness significantly modifies the laminate nanostructure and hence the

soft magnetic properties. The films of 300–600 nm thickness showing the excellent soft magnetic proper-

ties are attractive candidate materials for high-frequency micromagnetic device applications.

Acknowledgements The support by the Brain Korea 21 project in 2007 at Chungbuk National University and

the Research Center for Advanced Magnetic Materials (ReCAMM) at Chungnam National University is gratefully

acknowledged.

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