Spontaneous Magnetic Moment of Pt in Perpendicular Magnetic Recording Media The hard disk drive (HDD) that stores a large amount of information for computers and networks is becoming a crucial device for information technology. The essential figure of merit for the HDD is the areal density of the recorded bits on a disk, which contributes to large capacity, small size, and low cost simultaneously. At present, the areal density exceed 100 Gbits per square inch. This density corresponds to a bit area of only 80 × 80 nm 2 . However, the areal density increase is approaching saturation due to the physical limitation of the thermal decay of recorded bits. As element permanent magnets that represent recorded bits become smaller and smaller with the areal density increase, thermal energy overwhelms the anisotropy energy that maintains the direction of recorded magnetization. Perpendicular magnetic recording [1] is the key technology to overcome this limit. The Co/ Pt multilayer film described here is one of the most promising candidate disk material for perpendicular recording, which was originally developed for magneto- optical recording [2]. It has a layered structure consisting of alternative deposited Co and Pt layers whose thicknesses are both on the subnanometer scale. The film exhibits a strong perpendicular magnetic anisotropy caused by interfacial anisotropy between the Co and Pt layers. This high anisotropy is advantageous for high-density recording disks to ensure a good thermal stability of the recorded bits. In this experiment, the study of the magnetic behavior of Pt was carried out. The Co/Pt multilayer exhibits a very high anisotropy. In particular, the magnetism of Pt at the interface of Co was studied. The addition of Pt to Co based alloy enhances magnetic anisotropy. Therefore, Pt is widely used for disks. On the other hand, Pt addition sometimes increases media noise. Since media noise depends on film microstructure, Pt atoms may modify the magnetic grain structure. The atomic magnetic behavior of Pt is interesting from this viewpoint, as well. Samples are prepared by sputtering deposition. The film structure is shown in Fig 1. The number of Co/Pt deposition pair s is usually more than ten; however in this experiment, one Pt film and Co film were layered. Co 15 nm in thickness was deposited without any additive elements on the Ti seed layer, followed by the deposition of Pt films of various thicknesses. The Pt thicknesses were 0.2, 0.5, 1 and 2 nm. No passivation layer was deposited on it. Co Ti-underlayer Substrate (Glass) Pt 0 -2 nm Fig. 1. Schematic of layer structure of prepared samples of Co/Pt layered disk. Fig. 2. XMCD spectra at the Pt L 3 edge of Pt in Co/Pt films of different Pt thicknesses, compared with spin- averaged XAS spectra. Applied fields are in-plane (upper) and perpendicular (lower). X-ray Energy (keV) X-ray Energy (keV) Pt L 3 -edge Pt L 3 -edge XMCD XAS XAS XMCD 108 fit to m orb + m orb m orb / m spin m orb m spin Fig. 3. Spontaneous magnetic moment of Pt atoms as a function of thickness of the Co interface. Morb (top of the bars) and Mspin (bottom of the bars) are orbit and spin moments, respectively. The black exponential decay line is the fit curve to the measurement. Hiroaki Muraoka a and Motohiro Suzuki b (a) Research Institute of Electrical Communication, Tohoku University (b) SPring-8 / JASRI E-mail: muraoka@riec.tohoku.ac.jp References [1] S. Iwasaki: IEEE Trans. Magn. 20 (1984) 657. [2] W.B. Zeper et al. : IEEE Trans. Magn. 25 (1989) 3 764. X MC D e x periments on the Pt L 2, 3 edges were carried out in the fluorescence mode at beamline BL39XU . Monochromatic X -rays were incident on a Co /Pt sample perpendicularly to the film plane. A magnetic field was applied in the perpendicular direction to the sample plane so as to measure the perpendicular ‘ hysteresis loop ’ . The ma x imum field was 2 0 k O e to attain saturation magnetization. The M s of Co is appro x imately 142 0 emu/cm 3 , therefore the field of 2 0 k O e is greater than the demagnetization field, 4 π M s , that was sufficient to magnetically saturate the sample. Applied in-plane field was also measured to compare the two hysteresis loops, and to observe magnetic anisotropy. The intensities of fluorescent X - rays were detected using a silicon drift chamber, which confirms that total count rates as high as 1 0 5 cps and the efficient separation of Pt L α 1 and L α 2 lines from the elastic scattering mainly originate from the substrate. F igure 2 shows the observed X MC D spectra of Co/Pt films of different Pt thicknesses for in-plane (upper) and perpendicular (lower) applied fields. The spin-averaged X AS spectra of the samples are compared in the bottom of each figure. The X MC D a n d X A S a m p l i t u d e s w e r e s c a l e d p e r u n i t P t thickness. The two sets of spectra are almost identical, which means no magnetic anisotropy of the magnetism of Pt is present. The Pt sample of 0 .2 nm thickness shows the largest X MC D amplitude, which is 2 0% of that in the case of the X AS edge jump. X MC D amplitude decreases with increasing Pt thickness, indicating that Pt magnetization decreases with the distance from the interface with Co. With the help of the magneto-optical sum rules, we estimated the magnetic moment of the Pt 5 d electrons as a function of distance from the Co/Pt interface as shown in F ig. 3 . The amount of magnetic moment per hole e x hibits an almost e x ponential decay, in which the ma x imum number is 0 .4 μ B / hole for a Pt atom in contact with Co atoms at the interface. This spontaneous moment is close to that of N i. The change in Pt moment m( x ) with Pt thickness x was well reproduced by an e x ponential function, m( x ) = m 0 * e x p(- x /d), with a characteristic thickness d = 0 .4 nm, which characterizes the effective range of hybridization between Co and Pt. The relatively large magnetic moment of Pt will affect the macroscopic m a g n e t i c c h a r a c t e r i s t i c s s u c h a s s a t u r a t i o n magnetization, M s , of the Co/Pt multi-layered film. F rom the viewpoint of the nanostructure of CoCrPt alloy films, the moment of Pt can adversely affect the 0.5 0.4 0.3 0.2 0.1 0.0 2.0 1.5 1.0 0.5 0.0 0.20 0.15 0.10 0.05 0.00 Pt thickness (nm) Pt moment ( μ B / hole) m orb / m spin magnetic isolation of CoCr gains. Since the Pt atoms in CoCr are not localized in grains, the polarized Pt atoms at grain boundaries magnetically connect the grains. The X MC D measurement contributes to the nanotechnology of magnetic disks. 109
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