Thickness limit in perpendicular magnetic anisotropy L10 FePt(0 0 1) thin film

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Journal of Magnetism and Magnetic Materials 310 (2007) 2650–2652

Thickness limit in perpendicular magnetic

anisotropy L1

₀

FePt(0 0 1) thin ﬁlm

An-Cheng Sun

a

, Jen-Hwa Hsu

a,

, P.C. Kuo

b

, H.L. Huang

a

, H.C. Lu

c

, S.F. Wang

c

a_{Department of Physics and Center for Nanostorage Research, National Taiwan University, Taipei 106, Taiwan} b_{Institute of Materials Science and Engineering, National Taiwan University, Taipei 106, Taiwan} c

Department of Materials and Mineral Resources Engineering, National Taipei University of Technology, Taipei 106, Taiwan Available online 29 November 2006

Abstract

Perpendicular magnetic anisotropy L10FePt (0 0 1) ﬁlms with various thicknesses were prepared onto Pt(0 0 1)/Cr(0 0 2) bilayers. When

the thickness of FePt layer was smaller than 30 nm, a single orientation of L10FePt(0 0 1) texture was present, conﬁrming that the easy

axis was perpendicular to the ﬁlm plane. But as the thickness increased over 30 nm along with the L10FePt(0 0 1) orientation, the peak

corresponding to L10 FePt (1 1 1) orientation was also observed, which indicated the quality degradation of perpendicular magnetic

anisotropy. From our study, the critical thickness for growing perfect L10FePt(0 0 1) single orientation is around 30 nm.

Keywords: Magnetic anisotropy; L10ordered structure; Magnetic recording; Thickness effect; Epitaxial growth

In order to increase the magnetic recording density to 1 Tb/in2, perpendicular magnetic recording is believed to be one of the potential technologies to replace the current longitudinal magnetic recording in the near future. It is because, the volume of the recording bit in the perpendi-cular magnetic recording medium can be maintained well by increasing the medium thickness when the surface area of the recording bit is reduced [1]. However, it is still a question whether the film thickness of the perpendicular magnetic layer could be unlimitedly increased as the surface area of the recording bit is largely reduced. So far, most investigations focused on the processes to induce perpendicular magnetic anisotropy [2,3]. The thickness effects on perpendicular magnetic properties were seldom discussed in the past. Hence, it is important to understand how film thickness affects the magnetic properties and microstructures in perpendicular magnetic recording med-ia. In this investigation, the L10FePt (0 0 1)-thin films were

employed to determine the thickness effects on the magnetic properties and microstructures. The mechanism

of the thickness limitation on the recording layer was also examined.

Fe48Pt52(t)/Pt(3 nm)/Cr(70 nm) trilayer ﬁlms (t ¼ 10–

50 nm) were prepared on preheated 7059 glass substrate by conventional DC magnetron sputtering in a ultra-high vacuum sputtering system. The substrate was heated to 350 and 420 1C for depositing the Pt/Cr bilayer and FePt magnetic layer, respectively. Magnetic properties were measured by a vibrating sample magnetometer (VSM) at room temperature with a maximum applied ﬁeld of 1.2 T. The crystal structure and cross-sectional microstructures of the ﬁlms were investigated by X-ray diffraction (XRD) using Cu-Ka radiation and high-resolution transmission

electron microscopy (HRTEM).

Fig. 1 displayed the M–H loops of the FePt/Pt/Cr

trilayer ﬁlms with different thickness. The out-of-plane coercivity (Hc?) was increased from 2.5 to 5 kOe as t

increases from 10 to 30 nm. Then it declined to 3.5 kOe as t was further increased to 50 nm. The out-of-plane square-ness (S?)E1 when tp30 nm. S? was reduced as t was

larger than 30 nm. However, the in-plane coercivity (H_cJ) and squareness (S_J) were always increasing with the thickness of the FePt layer. The perpendicular magnetic

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anisotropy of the ﬁlm deteriorated gradually as t430 nm, and eventually H_cJwas found to be higher than Hc?when

t ¼ 50 nm.

Fig. 2 shows the XRD patterns of FePt/Pt/Cr trilayer

ﬁlms. The L10 FePt(0 0 1) texture was found when

tp30 nm, indicating that the magnetic easy axis of the L10 FePt layer was perpendicular to the ﬁlm plane. As t

exceeded 30 nm, a small L10 FePt(1 1 1) peak appeared,

revealing that the perpendicular magnetic anisotropy of FePt ﬁlm began to be destroyed. It suggests that the critical thickness for growing perfect L10FePt(0 0 1) orientation is

around 30 nm.

The cross-sectional HRTEM images of FePt/Pt/Cr trilayer ﬁlms are shown in Fig. 3, where the thicknesses of the FePt layer were (a) 30 nm and (b) 50 nm. As shown

in Fig. 3(a), the epitaxial directions in the FePt/Pt/Cr

trilayer ﬁlms started with [0 1 1] in the Cr underlayer and ended with ½¯1 1 2 in the FePt layer. There was only one ½¯1 1 2 direction in the FePt layer, indicating that a well epitaxial FePt(0 0 1) growth established in the whole FePt layer. In contrast as inFig. 3(b)with t ¼ 50 nm, the ½1 ¯1 2 was not the only observed epitaxial direction in the FePt layer. The arrow apparently switched from ½1 ¯1 2 to ½0 ¯1 0, revealing that two orientations (0 0 1) and (1 1 1) were present in the FePt layer. Therefore, the epitaxial growth of FePt(0 0 1) cannot last over a long distance in thicker FePt ﬁlms. When the FePt was epitaxially grown on the Pt lattice, both the in-plane a- and b-axis of FePt(0 0 1) was expanded by the Pt(0 0 1) plane. This is because the lattice

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Fig. 1. M–H loops of the FePt/Pt/Cr trilayer ﬁlms, where the thickness of FePt ﬁlms were (a) 10 nm, (b) 20 nm, (c) 30 nm and (d) 50 nm.

Fig. 2. X-ray diffraction patterns of FePt/Pt/Cr trilayer ﬁlms with various thickness of the FePt magnetic layer.

Fig. 3. Cross-sectional HRTEM images of FePt/Pt/Cr trilayer ﬁlm. The thicknesses of the FePt layer were (a) 30 nm and (b) 50 nm.

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constant of Pt(0 0 1) is 3.92 A˚ and is slightly larger than 3.86 A˚ of FePt(0 0 1). An increase in the elastic strain energy exists at the FePt(0 0 1)/Pt(0 0 1) interface and thus affects the formation of FePt(0 0 1). As the FePt layer was thinner than 30 nm, the L10FePt(0 0 1) formed due to the

effect of elastic strain energy in the FePt lattice. For the thicker FePt layer, the elastic strain energy effect from the Pt (0 0 1) lattice was substantially reduced and the FePt (1 1 1) plane formed[4].

In summary, the effects of FePt ﬁlm thickness on the L10

FePt(0 0 1) orientation and the magnetic properties of the ﬁlms were investigated. The L10 FePt(0 0 1) texture with

perpendicular magnetic anisotropy was found as the thickness of the FePt layer was smaller than 30 nm. When the FePt ﬁlm thickness was larger than 30 nm, the orientation switched from FePt (0 0 1) to FePt (1 1 1), leading to the degradation of the perpendicular magnetic

anisotropy. The upper limit of the magnetic FePt layer thickness with good perpendicular anisotropic behavior was determined to be about 30 nm.

This work was supported by the Ministry of Economic Affairs of Taiwan (Contract No. 94-EC-17-A-08-S1-0006).

References

[1] S. Suzuki, IEEE Trans. Magn. 20 (1984) 675.

[2] Y.K. Takahashi, K. Hono, T. Shima, K. Takanashi, J. Magn. Magn. Mater. 267 (2003) 248.

[3] T. Suzuki, K. Harada, N. Honda, K. Ouchi, J. Magn. Magn. Mater. 193 (1999) 85.

[4] P.D. Kim, I.A. Turpanov, S.V. Stolyar, R.S. Iskhakov, V.I. Yushkov, A. Ya. Beten’Kova, L.A. Li, E.V. Bondareva, T.N. Isaeve, M.M. Karpenko, Tech. Phys. 49 (2004) 431.

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