Investigations on advanced magnetic recording media 2

Chapter Ion Implantation in Magnetic Media: Effect of Mass of Ion Species CHAPTER 5 Ion Implantation in Magnetic Media : Effect of Mass of Ion Species 5.1 Introduction The previous chapter focused on the effect of energy and species of the implanted ions on the magnetic and structural properties. It was shown that lower energy implantation led to smaller lateral straggle from the TRIM calculations [195]. As the main objective of ion implantation technique is for fabricating patterned media and lateral straggle is one of the main issues that needs to be understood and controlled for extremely high areal densities, the results on lateral straggle from the previous chapter shed new light on the subject. It was further speculated from the previous study that another parameter which might play a significant role in controlling the lateral range and straggle is the mass of the implanted species, based on the ion-matter interaction principles. Thus, such a study might be useful for understanding the lateral straggle and identifying a suitable species. In this chapter, therefore, the effects of implantations of ion species of variable masses have been investigated on the magnetic and structural properties of the media. 5.2 Patterned Media Requirements As mentioned earlier, the focus of this thesis is on understanding and fabrication of ion-implantation-based patterned medium towards 10 Tbits/in2. At 10 Tbits/in2, each magnetic island would occupy an area of about 64 nm2, including the nm spacing as 125 Chapter Ion Implantation in Magnetic Media: Effect of Mass of Ion Species well. If granular medium is used, this area could accommodate grains of nm diameter. In the case of patterned medium, such a bit-cell could accommodate a nm island with a spacing of nm on all sides, as depicted in Figure 5-1. Figure 5-1 Schematic of granular media and BPM for 10 Tbits/in2. The first step to fabricate such a recording medium is to form resist patterns on the media. The densest patterns fabricated by electron beam lithography that has been reported so far is at about Tbits/in2 [196]. Although an areal density of about 10 Tbits/in2 has been claimed to be achieved using block copolymers, the results can only be achieved on single crystal substrates with a specific cut. Furthermore, the variable height of the wedges would lead to non-uniform pattern transfer [183]. Therefore, at this point of time, it is very difficult to fabricate patterns at 10 Tbits/in2. Studying the lateral straggle of ions at this areal density is equally important as seen in Figure 5-2. If the lateral straggle is not controlled the whole area may get implanted instead of only the unmasked region Figure 5-2 (c). A study on lateral straggle that has been reported in the literature is only on 78.6 nm patterns at a pitch of 213 nm [195]. It is very difficult to study lateral straggle at a very small scale such as 10 Tbits/in2. 126 Chapter Ion Implantation in Magnetic Media: Effect of Mass of Ion Species Figure 5-2 Schematic of magnetic islands of (a) ideal BPM and BPM fabricated by ion implantation with (b) minimal lateral straggle and (c) very high lateral straggle. We have used an innovative approach as an alternate route to study lateral straggle in patterned dots, by emulating similar effects in granular films. Figure 5-3 shows the in-plane TEM image of a conventional perpendicular recording medium in a granular structure. The magnetic grains are surrounded by non-magnetic grain boundaries. It can be observed that the grain boundary is much narrower (1-2 nm) than the bitboundary in 10 Tbits/in2 patterned media. Ion implantation in magnetic medium will cause ions to move horizontally from magnetic grains to grain boundaries or vice versa. Therefore, a study on such a thin film with 1-2 nm grain boundaries could shed light on the lateral straggle effects at densities such as 10 Tbits/in2 and above. A drawback that is associated with this technique is that there is no selective implantation, but it is thought that some preliminary understanding from such a study could shed light on lateral straggle and other issues. The effect of lateral straggle was studied from the measurement of exchange interaction in granular films and from the TRIM simulations. Exchange interactions could be studied from hysteresis loops to a certain extent. However, the first-order reversal curves (FORC) method can provide more details such as the type of interaction field and the magnetic phase, in addition to regular information obtainable from the hysteresis loops [149]–[151], [197]. The FORC contour is basically plotted with coercivity field in the x-direction and 127 Chapter Ion Implantation in Magnetic Media: Effect of Mass of Ion Species interaction field along the y-ordinate. The peak position along the x-axis in this contour shows the switching field or coercivity value and width refers to SFD. A positive interaction field refers to magnetostatic interactions and a movement towards negative values shows an increase in exchange coupling. From the MH loops, even though the slope does give an idea of the interactions present, it cannot separate the possibility of different interactions present. FORC has been explained in detail in Chapter 3. Therefore, we have studied FORC on ion-implanted thin films. Lateral range and straggle, which are essential for achieving high-density patterns, have also been studied for various ions species/masses using TRIM and have been correlated with the results obtained from FORC for the first time. Figure 5-3 Plan-view TEM of conventional PMR showing grain boundary ~1-2 nm. 5.3 Experimental Details Figure 5-4 shows the structure of granular Co69.6Cr7Pt17.4-(SiO2)6 media fabricated on the glass slides without any soft underlayer. No soft underlayer was deposited in this set of samples, with the purpose of obtaining magnetic signals solely from the recording layer. As a result, more information such as saturation magnetization and 128 Chapter Ion Implantation in Magnetic Media: Effect of Mass of Ion Species FORC contour plots can be obtained using force magnetometry. The media samples contained two ruthenium intermediate layers (Ru1 and Ru2) for inducing texture and grain segregation in the recording layer [198], [199]. A seed layer, Tantalum (Ta), was deposited to provide a smooth surface and desired hcp (0002) texture to the Ru1 and Ru2 intermediate layers. The samples were homogeneously implanted with helium (4He+), carbon (12C+), nitrogen (14N+), argon (40Ar+), cobalt (59Co+) and antimony (121Sb+) ions over the entire media surface by means of ion beam scanning. The sources for the ions were: He (gas), CO2 (gas), N2 (gas), Ar (gas), CoClx (solid; x=2 or 3) and SbO3 (solid) respectively. The interval between the doses was reduced in order to increase the number of data points by which the effect of ion dose could be clearly seen. The implantation doses chosen were 1014, 5×1014, 1015, 5× 1015, 1016 ions/cm2 and beyond up to 5×1016 ions/cm2. From the literature, it is known that the projected depth of implantation depends on the ion’s mass and energies. Ion implantation was done in the recording layer such that the maximum peak of implantation profile was in the middle of the recording layer and the ruthenium interlayer was affected the least as shown in Figure 5-5. This was done keeping in mind the application for patterned media. The TRIM ion profile is a well-established method and has been proven to correlate well with the secondary ion mass spectrometry (SIMS) ion profile but is shallower by 2-2.5 nm [200]. Also as previously mentioned, TRIM does not consider the morphology of the layers while calculating the various collisions. 129 Chapter Ion Implantation in Magnetic Media: Effect of Mass of Ion Species Figure 5-4 Schematic of media layer structure. The requirement of energy to implant the ions is calculated first, using the most comprehensive TRIM (Transport of Ions in Matter) program included in the SRIM (The Stopping and Range of Ions in Matter) software [111], [190]. In order to implant 4He+, 12C+, 14N+, 40Ar+, 59Co+ and 121Sb+ into the recording layer, the respective 0A Target Depth Glass substrate Tantalum Ru1 and Ru2 Recording Layer Carbon (ATOMS/ cm3) / (ATOMS/ cm2) energies were fixed to 2.5 keV, 5.8 keV, 6.5 keV, 13 keV, 17 keV and 37 keV. 470A Figure 5-5 Ion profile in the recording media target obtained by TRIM for carbon ions (similar profiles for other ions as well). 130 Chapter Ion Implantation in Magnetic Media: Effect of Mass of Ion Species Irradiated and as-deposited samples were tested for magnetic properties by a polar magneto-optic Kerr (MOKE) magnetometer and alternating gradient force magnetometer (AGFM). MOKE was primarily used for preliminary understanding of the hysteresis loops and the AGFM was used for obtaining magnetization and FORC contours. The crystallographic structure was evaluated by a Philips X-Pert X-ray diffractometer (XRD) using Cu Kα radiation (l.540Å). FORC studies were also conducted as it has been shown in the literature that a FORC diagram provides more detailed information than the standard hysteresis loops about the exchange interactions. It will be seen that the heavier the ion species, the higher would be the change in interaction field. The change in interaction field and lateral straggle showed a correlation. 5.4 Magnetic Properties 5.4.1 Hysteresis Loops In order to investigate the effect of ion implantation on the magnetic properties, hysteresis loop measurements were carried out (Figure 5-6). MH loops have been normalized to see clear differences in the change of the slope. 131 Chapter Ion Implantation in Magnetic Media: Effect of Mass of Ion Species He + Normalized Kerr rotation C+ N+ Ar + Co + Sb + Magnetic field (Oe) Figure 5-6 Kerr loops on the sample implanted with various doses and implanted species – 4He+, 12C+, 14N+, 40Ar+, 59Co+ and 121Sb+. 132 Chapter Ion Implantation in Magnetic Media: Effect of Mass of Ion Species Figure 5-6 shows the hysteresis loops of the samples as measured by MOKE. The reference samples show rectangular hysteresis loops with Hc of about 4000 Oe and Hn of about 2000 Oe. The implantation leads to a reduction in the Hc and Hn values as a function of implantation dose. When different species are compared, 4He+ shows the least change and 121Sb+ the most dramatic change. The implantation of species with higher mass appears to cause a more dramatic decrease of Hc and Hn. The loop slopes also show a change as a function of the ion dose. Figure 5-7 shows the SFD calculated by obtaining full width half maximum value of dM/dH obtained from the hysteresis loops as a function of dose for different species as explained in Chapter 3. Switching of magnetic grains at one magnetic field provides a delta peak. Hence, the width of dM/dH shows the variation in the switching field known as SFD. It can be noticed that the SFD kept on narrowing as the fluence increased. A narrow SFD is a possible indication of increase in exchange coupling. The loop looks very similar to the samples in the previous chapter, which were sputtered without Ru2 and where the grains were exchange coupled. It is quite likely that the implantation of 59Co + that occurs at the grain boundary causes a stronger exchange coupling that leads to such a change. However, it should be noted that saturation magnetization is also reduced for carbon, nitrogen, argon and antimony at high fluence values, as seen in Figure 5-9. 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Chow, “L10 Phase CoPt-TiO2/FePtTiO2 Exchange Coupled Media With Small Switching Field,” Magnetics, IEEE Transactions on, vol. 46, no. 6, pp. 1955 –1958, Jun. 2010. 213 [...]...Chapter 5 Ion Implantation in Magnetic Media: Effect of Mass of Ion Species 4000 Hc (Oe) 3000 20 00 1000 0 1014 5x1014 1015 5x1015 1016 5x1016 Ion Fluence (ions/cm2) Figure 5-8 Coercivity plotted as a function of fluence for all the implanted species – 4He+, 12C+, 14N+, 40Ar+, 59Co+ and 121 Sb+ 5.4 .2 Saturation Magnetization In patterned media, the irradiated regions are required to produce non -magnetic. .. calculations in this thesis is to explain theoretically the variation in saturation magnetization of CoCrPt-SiO2 media upon implantation with high doses of various ion species Saturation magnetization, Ms, is an intrinsic property of the material and is defined as the magnetic moment per unit volume The 147 Chapter 5 Ion Implantation in Magnetic Media: Effect of Mass of Ion Species magnetic moment depends on. .. further 141 Chapter 5 Ion Implantation in Magnetic Media: Effect of Mass of Ion Species He + C+ Inetensity (a u.) N+ Ar + Co + Sb + θ -2 (Degree) Figure 5-13 XRD θ -2 scan plotted as a function of fluence for all the implanted species – 4He+, 12C+, 14N+, 40Ar+, 59Co+ and 121 Sb+ 1 42 Chapter 5 Ion Implantation in Magnetic Media: Effect of Mass of Ion Species The disappearance of the Co (00 02) peak started appearing... i.e in magnetic grains Therefore for BPM fabrication using Co-based alloys, heavy ions should be a good choice 146 Chapter 5 Ion Implantation in Magnetic Media: Effect of Mass of Ion Species Ions grain Time Grain boundary (a) Light Ions (b) Heavy Ions Figure 5-16 Schematic of straggle/movement of ions in (a) light ion (b) heavy ion implantation with time 5.8 Density Functional Theory Calculations (DFT)... Implantation in Magnetic Media: Effect of Mass of Ion Species media was ~465 emu/cc Except for 4He+ and a 59Co+ ion, the decline in Ms was observed after 1015 ions/cm2 of fluencies depending on the ion species Antimony and argon showed drastic changes in Ms compared to other ions In the implantation involving these elements, Ms was reduced to ~ 0 emu/cc at a fluence of 1016 ions/cm2 500 Ms (emu/cc) 400 300 20 0... ions In fact, for heavier ions like 121 Sb+, the SFD of 139 Chapter 5 Ion Implantation in Magnetic Media: Effect of Mass of Ion Species the implanted sample is much narrower than samples implanted with lighter ions like the 4He+ ions Narrow SFD and shift towards Hu=0 in perpendicular magnetic recording media indicates the emergence of exchange interactions A narrow SFD may also be due to the reduction... Ion Fluence (ions/cm2) Figure 5-9 Ms plotted as a function of fluence for all the implanted species – 4He+, 12C+, 14N+, 40Ar+, 59Co+ and 121 Sb+ Saturation magnetization, Ms, is an intrinsic property of the material and is defined as magnetic moment per unit volume The magnetic moment depends on the electronic band structure; hence one of the reasons for changes in Ms could be due to the modification... properties and lateral straggles were investigated as a function of the mass of the ion species It was seen that saturation magnetization (Ms) was reduced to ~0 emu/cc at a fluence of 2 1016 ions/cm2 for antimony ( 121 Sb+) and 5×1016 ions/cm2 for argon (40Ar+), which could particularly be beneficial for patterned media application The mass of the implanted ion species played an extremely crucial role in reducing... source for the ions was SbO3 157 Chapter 6 Patterned Media Fabrication (solid) The implantation doses chosen were 1015, 5×1015, 1016 and 5×1016 ions/cm2 From the literature, it is known that the projected depth of implantation depends on the ion’s mass and energies Ion implantation was done in the recording layer such that the maximum peak of implantation profile was in the middle of the recording layer... due to atomic dilution or change in electronic structure of the FePt media as a result of implantation 159 Chapter 6 Patterned Media Fabrication As mentioned earlier, the reduction in Ms could also be partially due to the sputter etch of the FePt layer by Sb+ ions Our previous studies on Sb+ implantation have indicated that a dose of 2 1016 ions/cm2 is sufficient to reduce magnetization to zero in Co-based . Chapter 5 Ion Implantation in Magnetic Media: Effect of Mass of Ion Species 125 CHAPTER 5 5 Ion Implantation in Magnetic Media : Effect of Mass of Ion Species 5.1 Introduction The previous. x-direction and Chapter 5 Ion Implantation in Magnetic Media: Effect of Mass of Ion Species 128 interaction field along the y-ordinate. The peak position along the x-axis in this contour. Tbits/in 2 . Chapter 5 Ion Implantation in Magnetic Media: Effect of Mass of Ion Species 127 Figure 5 -2 Schematic of magnetic islands of (a) ideal BPM and BPM fabricated by ion implantation