Abstract
L 10 phase FePt thin films deposited on MgO (100) single-crystal substrate with good (001) texture were investigated. Epitaxial growth of the FePt (001) films was observed with a deposition temperature of 400 °C. With ultrathin Ag intermediate layers deposited between FePt layers, the film structures changed from an interconnection network to an isolated-island character. The perpendicular coercivity of the FePt film dramatically increased from 6.5 to 32.5 kOe. The improvement of the magnetic properties may be attributed to the formation of island structures by the additive Ag in the FePt films.
| Original language | English (US) |
|---|---|
| Article number | 052503 |
| Pages (from-to) | 1-3 |
| Number of pages | 3 |
| Journal | Applied Physics Letters |
| Volume | 88 |
| Issue number | 5 |
| DOIs | |
| State | Published - 2006 |
Bibliographical note
Funding Information:Zhao Z. L. Data Storage Institute , 5 Engineering Drive 1, DSI Building, Singapore 117608, Department of Material Sciences, National University of Singapore , Singapore 119620, and Nanoscience and Nanotechnology Initiative, National University of Singapore , Singapore 117576 Chen J. S. a) Data Storage Institute , 5 Engineering Drive 1, DSI Building, Singapore 117608 Ding J. Yi J. B. Liu B. H. Department of Material Sciences, National University of Singapore , Singapore 119620 Wang J. P. Department of Electrical and Computer Engineering, The Center for Micromagnetics and Information Technologies (MINT), University of Minnesota , Minnesota 55455 a) Author to whom correspondence should be addressed; electronic mail: [email protected] 30 01 2006 88 5 052503 09 05 2005 01 12 2005 01 02 2006 2006-02-01T15:53:23 2006 American Institute of Physics 0003-6951/2006/88(5)/052503/3/ $23.00 L 1 0 phase FePt thin films deposited on MgO (100) single-crystal substrate with good (001) texture were investigated. Epitaxial growth of the FePt (001) films was observed with a deposition temperature of 400 °C. With ultrathin Ag intermediate layers deposited between FePt layers, the film structures changed from an interconnection network to an isolated-island character. The perpendicular coercivity of the FePt film dramatically increased from 6.5 to 32.5 kOe. The improvement of the magnetic properties may be attributed to the formation of island structures by the additive Ag in the FePt films. With the advance in the magnetic recording media, the recording bit becomes smaller and smaller, which requires reducing the grain size to maintain the signal-to-noise ratio. However, with the reduction in grain size, a superparamagnetic limitation will be encountered. In order to delay the onset of superparamagnetism, extremely high magnetocrystalline anisotropy materials are required. 1,2 The L 1 0 -ordered iron-platinum (FePt) with a large magnetic anisotropy has received great attention because of its potential for ultrahigh-density recording media. 3–5 The epitaxial growth of FePt films with the c axis normal to the film plane was investigated by conventional deposition techniques, such as sputtering and molecular-beam epitaxy. 6–9 In most cases, however, the coercivity realized in highly ordered FePt films was less than 10 kOe. 9–11 Recently, Shima et al. 12 presented the FePt thin film with coercivity as high as 70 kOe. The formation of an islandlike structure and the special structure-related single-domain magnetization reversal mechanism is the possible reason for the extremely high coercivity. However, the fabrication temperature is as high as 780 °C, which is formidable for most of the labs and industry process. To reduce the ordered phase transformation, different methods, such as additive elements doping 8,13 and ion bombardment, 14,15 have been proposed to reduce the ordering temperature of the FePt equiatomic alloy. As an additive element, Ag was intensively studied to improve the magnetic properties of FePt alloy. 8,16,17 In this letter, we report that an FePt thin film with high coercivity of 32.5 kOe was obtained at a relatively low deposition temperature of 400 °C when ultrathin nonmagnetic Ag layers were employed to induce the formation of the islandlike structure. FePt thin films were deposited in a high-vacuum sputtering system (base pressure ∼ 5 × 10 − 8 Torr ) using an FePt alloy target with FePt composition ratio of around 1 : 1 and Ag target with purity higher than 99.99%. High-purity argon of 10 mTorr was fed during sputtering. The single-crystal (100) MgO substrates were attached to a rotating substrate holder, and were heated to 400 °C prior to the deposition. The nominal total thickness of the FePt film was fixed as 15 nm and the typical growth rate for FePt was 0.06 nm ∕ s . Up to three Ag layers with a thickness of 0.25 nm were deposited between FePt layers. The film structures and the perpendicular coercivity values of FePt films with different film structures are listed in Table I . Actually, from the observation of transmission electron microscopy (TEM), the thin Ag layers did not form continuous layers to present multilayer structure. The thickness of the film layers was calculated from deposition rates using a surface profilometer. Crystallographic texture of the films was studied at room temperature using Cu K α x-ray diffractometry (XRD). Magnetic properties of the films were measured with a superconducting quantum interference device (SQUID) with a maximum applied field of 70 kOe. The film mircrostructure was studied by a JOEL JEM 3010 TEM at 300 kV. The out-of-plane coercivity values of the FePt films with different numbers of Ag intermediate layers are listed in Table I . For the FePt film without a Ag layer deposited, the perpendicular coercivity is 6.5 kOe (Structure I). The coercivity increases to 32.5 kOe when two Ag layers were deposited. The following analysis will be mainly focused on the two samples with Structures I and III, in order to investigate the effect of Ag intermediate layers. Figure 1 shows the θ - 2 θ XRD patterns of the FePt films on MgO (100) single-crystal substrates (Structures I and III). The 2 θ range from 42° to 44° is removed from the curve because the intensity of the single-crystal MgO (100) is too strong compared to those of the FePt peaks. No peaks except the face-centered-tetragonal (fct) (001), (002), and (003) peaks are observed in the XRD pattern. This indicates that the film is highly oriented in the (001) direction (out-of-plane). The 6.8% of the lattice mismatch between MgO and FePt provides appropriate strain for the growth of the ordered FePt phase, 18 as is also illustrated by the dislocations in the interface of FePt film and MgO substrate [seen in Fig. 3(b) ]. With two Ag layers deposited between FePt layers, no significant change for the diffraction peaks is observed. These results also suggest that the (001) orientation of the FePt film is not affected by the insertion of the two ultrathin Ag intermediate layers. Additionally, the slight (111) Ag peak is observed in the pattern of the film with Structure III, suggesting that at least some free Ag is present. The ordering parameter is proportional to ( I 001 I 002 ) , where I 001 and I 002 are the integrated intensity of FePt(001) and (002) peaks. Here, we use the ( I 001 I 002 ) to qualitatively characterize the ordering degree. ( I 001 I 002 ) of Structures I and III is 1.04 and 1.06, respectively. This indicates that the ordering of FePt film is slightly increased with the additional of Ag layer. The inset in Fig. 1 is the rocking curves of the FePt fct (001) peak. The full width at half maximum (FWHM) of the rocking curve is about2.6 ° in film with Structure I. Although the growth of the FePt might be interrupted by the deposition of the two Ag intermediate layers, the FWHM of Structure III is nearly the same as that of Structure I. This again confirms that the crystallographic texture of the film is not influenced by the insertion of the Ag intermediate layers. Figures 2(a) and 2(b) show the plane-view TEM bright-field images of the FePt films (Structures I and III, respectively). A large amount of Moiré fringes are observed in the both images as a result of the double diffraction from the FePt grains and MgO substrate, which might signal the epitaxial growth of the FePt film on MgO substrate. Without Ag intermediate layers inserted, the morphology shows a continuous structure in Fig 2(a) . On the other hand, the morphology becomes an island structure with Ag intermediate layers inserted. It indicates that the continuous lateral growth of the FePt film is interrupted by the insertion of the Ag intermediate layers and results in the formation of island structure. The particle is not uniform and the particle size varies from several tens of nanometers to about 100 nm and the separate gap is about 10–20 nm. There are also some small particles with a particle size of several nanometers to about ten nanometers in the separate gap of the large particles. Since the Ag signals were observed in the XRD patterns of the FePt film with Ag intermediate layers, the small particles in the gap are suspected to be Ag particles. Figures 2(c) and 2(d) show the bright- and dark-field in-plane images of the FePt film with Structure III. By comparing the bright- and dark-field images, the particles in Fig. 2(c) were observed to be composed of small grains with a grain size about approximately 20 nanometers. Figure 3 shows a high-resolution (HRTEM) image of the FePt film (a) with Structure III and the fast Fourier transform (FFT) filtered image (b) of Fig. 3(a) . The HRTEM image shows clear interface between the FePt film and the MgO substrate. The inset of Fig. 3(a) is the FFT diffractogram of the corresponding HRTEM image, confirming the epitaxial growth of the FePt (200) on MgO (200). The reconstruction image in Fig. 3(b) clearly shows the epitaxial growth of the FePt (200) planes with a d spacing of 1.9Å on a MgO substrate. The dislocations are marked with circles in the image. Dislocations are observed to locate at the interface of the film and substrate. Since it has been reported that Ag does not form a solid solution phase with FePt, 8,16,17 it is reasonable to speculate that most of Ag should stay between FePt particles. Figure 4 shows the hysteresis loops for the FePt films grown on a MgO substrate measured by SQUID with maximum applied field of 70 kOe. As illustrated in the XRD patterns (Fig. 1 ), the magnetic easy axis of the tetragonal FePt is normal to the film plane. The out-of-plane coercivity for the film with Structure I is 6.5 kOe, while in-plane coercivity is about 1 kOe [seen in Fig. 4 (a)]. With the ultrathin Ag layers inserted between FePt layers, the out-of-plane coercivity of the film with Structure III dramatically increases to 32.5 kOe [seen in Fig. 4 (b)]. The anisotropy fields estimated by extrapolating the hysteresis loops are about 70 kOe and 85 kOe for films with Structures I and III, respectively. Using H k = 2 K u ∕ M s , K u is calculated to be 3.7 × 10 7 and 4.3 × 10 7 erg ∕ cc for Structures I and III, respectively. The drastic increase in the coercivity and anisotropy field should be attributed to the insertion of the ultrathin Ag layers because no other fabrication condition has been changed except for the film structure. While the squareness S (the ratio of magnetic remanence, M r to the saturation magnetization, M s , large S is an important factor in data storage) of the hysteresis loops is nearly the same for the films with Structures I and III, the coercivity squareness S * , ( Δ H ∕ H c ) decreases from 0.66 in the film with Structure I to 0.38 in the film with Structure III. The decrease of the S * suggests that the exchange coupling decrease with Ag intermediate layers inserted. In Ref. 12 , the extremely high coercivity is attributed to the formation of the island structure of FePt film at a very high temperature. The formation of the island structure might have realized the exchange decoupling between the FePt particles and even led to the change of magnetization reversal mechanism of the FePt films. 12,19 Additionally, because domain-wall propagation is inhibited in islands, the large increase in the coercivity could be mostly due to the distribution of nucleation fields in the FePt films as observed for patterned media. 20 The change of the surface morphology was also observed, along with the growth of Fe on a single-crystal MgO by Boubeta et al. 21 In Ref. 20 , with the increase in temperature from 400 to 700 °C, the morphology of the Fe film changed from dendrites to islands due to the large surface-diffusion at a high temperature. In our work, the deposition of the Ag should have impeded the formation of FePt interconnection network and promoted the formation of island morphology at relatively low temperatures. The large increase in coercivity should be attributed to the realization of the noninteraction mechanism of isolated-island structure in film with Structure III. 12,19 The noninteraction between particles also explains the small coercivity squareness S * of the hysteresis loop in film with Structure III [Fig 4 (b)]. In conclusion, the magnetic and microstructural properties of FePt thin films on MgO (100) substrate prepared by magnetron sputtering and the effect of the Ag additive layers have been studied. A MgO single-crystal substrate leads to the formation of the (001) easy-axis orientation in FePt films. The deposition of Ag intermediate layers results in the formation of an island structure with a crystalline size of 10–20 nm at the relatively low temperature of 400 °C. The unique structure results in a high perpendicular coercivity over 30 kOe. However, the distribution of the particle size from ten to several tens nanometers is required to be uniform for the magnetic recording application. The authors would like to thank Dr Y. N. Liu of School of Mechanical Engineering, University of Western Australia, for the SQUID measurement. One of the authors (Z. L. Z.) would like to thank the National University of Singapore (NUS) and Data Storage Institute (DSI) scholarship for the research project. Table I. FePt film structures and perpendicular coercivity ( H c ⊥ ) for FePt thin films with different structures. Structures Film structures H c ⊥ ( kOe ) I MgO ∕ FePt 15 nm 6.5 II MgO ∕ FePt 7.5 ∕ Ag 0.25 ∕ FePt 7.5 nm 11.0 III MgO ∕ FePt 5 ∕ Ag 0.25 ∕ FePt 5 ∕ Ag 0.25 ∕ FePt 5 nm 32.5 IV MgO ∕ FePt 4 ∕ Ag 0.25 ∕ FePt 4 ∕ Ag 0.25 ∕ FePt 4 ∕ Ag 0.25 ∕ FePt 3 nm 24.0 FIG. 1. θ − 2 θ XRD scans of the FePt films with Structures I and III. The 2 θ range from 41° to 44° is cut because the intensity of the single crystal MgO (100) is too strong compare to that of the FePt peaks. Slight (111) Ag peaks are observed in the films with Ag intermediate layers. The inset shows the rocking curve of the FePt (002) peak. The FWHM is nearly the same for the two samples. FIG. 3. (a) HRTEM image of the FePt film with Structure III. The inset is the FFT image of the corresponding HRTEM image in Fig. 2(a) . (b) The reconstruction image using the FFT filtering technique of the corresponding image. The dislocations are marked with circles. FIG. 2. Plan-view TEM images of FePt thin films with Structures I (a) and III (b). (c) and (d) The bright- and dark-field images of an FePt film with Structure III. FIG. 4. Magnetization curves for the two films (Structures I and III) measured by SQUID with maximum applied field of 70 kOe. The square spots represent the out-of-plane magnetization curves and the circle spots represent the in-plane magnetization curves. The solid line is drawn for guiding the eyes.