Nanoscale patterning of CrPt3 magnetic thin films by using ion beam irradiation

Results in Physics 6 (2016) 186–188

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Nanoscale patterning of CrPt3 magnetic thin films by using ion beam irradiation Edi Suharyadi a,⇑, Daiki Oshima b, Takeshi Kato c, Satoshi Iwata b a

Department of Physics, Gadjah Mada University, Sekip Utara BLS 21, Yogyakarta 55281, Indonesia Institute of Materials and Systems for Sustainability, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8603, Japan c Department of Electrical Engineering and Computer Science, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8603, Japan b

a r t i c l e

i n f o

Article history: Received 17 November 2015 Accepted 23 March 2016 Available online 7 April 2016 Keywords: Ordered L12 alloy films CrPt3 Ion irradiation Planar patterned media

a b s t r a c t We have successfully fabricated planar patterned CrPt3 ordered L12 alloy films by Kr+ ion irradiation. Planar-patterned CrPt3 nanodots with various bit sizes from 200 nm to 50 nm were successfully fabricated by 30 keV Kr+ ion irradiation at a dose of 2  1014 ions/cm2, where e-beam lithography was used for creating the resist mask. We have confirmed that the nanofabrication process didn’t change the magnetic properties of CrPt3 ordered L12 alloy films. As-prepared film exhibited perpendicular hysteresis loop with the coercivity of 5.5 kOe. The typical perpendicular maze domain structure with the stripe structure was clearly seen in as-prepared CrPt3 film. Magnetic force microscopy (MFM) images of patterned CrPt3 nanodots indicated that each un-irradiated bit consists of localized perpendicular magnetic domain structures, which corresponds to perpendicular magnetization direction. Nanodots with bit size 680 nm show either dark or bright contrast, suggesting single domain structure. No magnetic contrast in irradiated space is due to the suppressing of the magnetization by Kr+ ion irradiation. Ó 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Introduction In recent years, planarization is one off crucial issue in bits patterned media (BPM), since the hard disk drive (HDD) heads are flying only a few nanometers above the media. Ion beam irradiation has been proposed as a new approach to pattern magnetic materials locally without etching magnetic materials, i.e., without altering the surface topography [1–4]. Using ion irradiation to alter the properties of magnetic films and multilayers has attracted increasing interest, since basic magnetic properties such as coercive field and magnetic anisotropy field can be locally and controllably changed to engineer the behavior of submicrometer scale magnetic devices. Ion irradiation also decreases the Curie temperature, so that it can define regions which are non-magnetic at room temperature [1,2]. Fabrication of ion-patterned magnetic multilayers into sub-micrometer scale is very important to be studied, especially, how isolated pattern magnetic domain structure can appear in ion-irradiation-patterned media. Ion beam irradiation has demonstrated the capability to pattern magnetic media directly [5,6]. Moreover, the use of ion ⇑ Corresponding author. E-mail address: [email protected] (E. Suharyadi).

irradiation patterning of a perpendicular magnetic film has been previously introduced to achieve a high density patterning of the media [7,8]. Recently, planar patterned Co/Pt and Co/Pd multilayers have been first reported, where Ga ion irradiation into the multilayers causes the change of the easy axis of their magnetic anisotropy from perpendicular to in-plane [1–4], [9,10]. However, when the patterned size becomes less than a few hundred nm, the adjacent magnetic bits tend to couple with each other due to the exchange coupling through in-plane magnetized spacing [9,10], which will limit the ultimate density of the media. Ion irradiated CrPt3 ordered alloy films by Kr+ ion irradiation with low ion doses have been also reported [11]. The CrPt3 films had been reported as L12type long-range order (LRO) alloy films [12,13], and ion irradiation was used to transform its structure from ferromagnetic L12 to nonmagnetic A1 phase, and it was assumed that there exists no exchange coupling between the bits [11]. In this paper, planarpatterned CrPt3 nanodots with various dot sizes fabricated by Kr+ ion irradiation are investigated. Here, Kr+ ion irradiationpatterned CrPt3 nanodots without modifications of the surface roughness using e-beam lithography technique, for creating the resist mask followed by ion irradiation are investigated for obtaining the magnetically localized patterns.

http://dx.doi.org/10.1016/j.rinp.2016.03.010 2211-3797/Ó 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

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magnetometer (AGM). Surface image and the etching depth of the irradiated area were studied by atomic force microscopy (AFM) and scanning electron microscope (SEM). A series of measurements were made using magnetic force microscopy (MFM) to study the magnetic domain structure of the individual dots.

Results and discussions

Fig. 1. (a) AFM and (b) MFM images of as-prepared CrPt3 ordered L12 alloy films and M–H loops of as-prepared films for (c) before and (d) after nanofabrication process.

Experimental methods L12 phase CrPt3 films were obtained by postannealing of Cr/Pt multilayers [12,14]. The [Cr (0.4 nm)/Pt (1.5–1.7 nm)]10 multilayers were prepared by alternating sputtering of Cr and Pt on thermally oxidized (500 nm SiO2) silicon substrates. The samples were annealed in vacuum (<3  10 4 Pa) at a temperature of 850 °C for 15 min. Planar-patterned CrPt3 nanodots with various dot/bit sizes (D) of 200–50 nm were fabricated by the following method. At first, CrPt3 film was covered by ZEP520A resist 30 nm in thickness using the spin coater technique. E-beam lithography technique was used for creating a residual mask to localize the irradiated region. JEOL electron beam (EB) system with an acceleration voltage of 50 kV and a typical beam current of 100 pA was used in this fabrication process. The resist in the e-beam exposed region was removed by dipping into Xylene. Then, uniform 30 KeV Kr+ ions with a dose of 2  1014 ions/cm2 were directly irradiated on all regions of the sample. As already reported, the magnetization of CrPt3 can be completely suppressed by Kr+ ion irradiation at 2  1014 ions/cm2 [11]. Finally, planar BPM was obtained by removing the residual resist using O2 plasma in a reactive ion etching (RIE) system. M–H loops of ion-irradiationpatterned films were measured by alternating a gradient

AFM image of as-prepared CrPt3 ordered L12 alloy film is shown in Fig. 1(a), with root means square (RMS) roughness of 0.64 nm and average surface grain size of 190 nm. The magnetic domain structure of the film, as measured by MFM, is shown in Fig. 1(b). Here, lift scan height during measurement was 30 nm. The typical perpendicular maze domain structure with a stripe domain structure is clearly seen in the MFM image. The stripe domain width is about 100 nm. M–H loops of as-prepared L12 alloy film before and after nanofabrication process are shown in Fig. 1(c) and (d), respectively. As-prepared exhibits a perpendicular hysteresis loop with the coercivity of 5.5 kOe, as shown in Fig. 1(c). Nanofabrication process did not change the magnetic properties of CrPt3 ordered L12 alloy films, as confirmed in Fig. 1(d). We also confirmed that the nanofabrication process doesn’t change the typical perpendicular maze domain structure. Fig. 2 shows SEM images of e-beam lithography exposed sample (after develop and before irradiation) with bit sizes of 200, 100 and 50 nm. Here, E-beam lithography technique was used for creating a residual mask The result shows that the residual mask is still existing upto a bit size of 50 nm. Fig. 3 shows AFM and MFM images of patterned CrPt3 ordered L12 alloy nanodots with various bit sizes. AFM and MFM images of patterned nanodots with a bit size of 200 nm is shown in Fig. 3(a)–(c) and, with root means square (RMS) roughness of 0.57 nm and an average surface grain size of about 200 nm. AFM profile bird’s view image clearly shows that there is no etching of the surface during the irradiation process, as shown in Fig. 3(a). The surface topography of patterned nanodots is almost identical to that of the as-annealed film. It is clear that Kr+ ion beam with energy of 30 KeV and a dose of 2  1014 ions/cm2 does not cause a significant etch un-masked region as shown in Fig. 3(b). MFM images taken at remanent state without applied magnetic field of patterned nanodots with bit sizes of 200 nm, 80 nm, 65 nm and 50 nm are shown in Fig. 3(c)–(f), respectively. For MFM measurement, the tip was magnetized in the z direction (perpendicular to the film plane) prior to the measurement and the magnetic contrast can be explained by an interaction between the perpendicular magnetized film and the MFM tip. Significant difference in contrast between un-irradiated bits and irradiated space is seen. MFM images of patterned nanodots indicate that each un-irradiated bit consists of perpendicular magnetization. No magnetic contrast in the irradiated space is due to the suppressing of the magnetization by Kr+ ion irradiation, as reported previously [11]. This localized

Fig. 2. SEM images of after e-beam lithography exposed sample (after develop and before irradiation) with bit sizes (D) of (a) 200 nm, (b) 100 nm and (c) 50 nm.

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Fig. 3. (a) AFM profile bird’s view, (b) AFM surface profile and (c) MFM images of CrPt3 nanodots with bit size (D) of 200 nm. MFM images of nanodots with bit sizes of (d) 80 nm, (e) 65 nm and (f) 50 nm.

magnetic structure is due to the atomic ordered L12 and disordered A1 phases in the CrPt3 film. The CrPt3 shows ferrimagnetism when it has an ordered L12 phase, and becomes paramagnetic when it has a disordered A1 phase and the ferrimagnetism of CrPt3 can be destroyed by a quite low dose of ion irradiation because of the phase change from ordered L12 to disordered A1 phases [11]. For patterned nanodots with a bit size of 200 nm, magnetic multi-domain structure is observed, as shown in Fig. 3(c). However, all the nanodots with bit sizes of 80 nm, 65 nm and 50 nm show uniformly bright or dark contrast, indicating that they consist of a single domain with perpendicular magnetization, as shown in Fig. 3(d)–(f), respectively. It can be observed that every nanodot shows a distinct switch (dark to bright) reinforcing the postulate that the nanodots have mostly single magnetic domain structure. Since the magnetization of the irradiated part (spacing) was completely suppressed, the exchange coupling between bits is expected to be suppressed. Conclusion Ion irradiation planar-patterned CrPt3 ordered L12 perpendicularly magnetized nanodots with various bit sizes from 200 nm to 50 nm without significant modifications of the surface roughness have been fabricated by irradiation ions through e-beam lithography made resist mask. AFM images showed that Kr+ ion beam with energy of 30 KeV and dose of 2  1014 ions/cm2 does not etch the CrPt3, and the surface roughness of patterned nanodots is still flat. MFM images showed that the planar patterned nanodots with sizes lower than 100 nm have either uniformly bright or dark, indicating that it contains a perpendicularly single-magnetic domain. At the ion-irradiation-patterned films, space between bits has no magnetic contrasts, as shown in MFM images, which indicates that localized magnetically pattern can be engineered by ion beam irradiation.

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