IEEE TRANSACTIONS ON MAGNETICS, VOL. 38, NO. 4, JULY 2002
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Measurements and Modeling of Soft Underlayer Materials for Perpendicular Magnetic Recording Chunghee Chang, Martin Plumer, Charles Brucker, Jianping Chen, Rajiv Ranjan, Johannes van Ek, Jun Yu, Duane Karns, Yukiko Kubota, Ganping Ju, and Dieter Weller
Abstract—Measurements and modeling of soft magnetic underlayer (SUL) materials for perpendicular magnetic recording application are carried out. The process dependent magnetic properties of FeCoB, CoZrNb, and FeAlN SUL materials on glass and aluminum disk substrates are studied and correlated with spin-stand noise performance. The SUL-induced dc noise amplitude approaches the electronic noise floor for certain material combinations, e.g., FeCoB or CoZrNb on glass, when care is taken to relieve stress-induced perpendicular anisotropy by thermal annealing. Landau–Lifshitz–Gilbert micromagnetics, finite-element method calculations, and a micromagnetic recording model show that write field amplitude, write field gradient, and readback waveform are only slightly impacted by SUL moment in the 1–2 T range. Much more important are the head-to-SUL distance and the write head saturation moment. These results suggest that extremely high SUL moment may not be necessary, which can be leveraged to meet other key practical requirements such as corrosion resistance and manufacturability. Index Terms—Finite-element method, Landau–Lifshitz– Gilbert, noise, perpendicular magnetic recording, soft magnetic underlayer, stripe domains.
I. INTRODUCTION
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HE chief advantage of double layer perpendicular recording media is the ability to write relatively thick, high anisotropy media utilizing the “deep-gap” field between a pole-type recording head and soft magnetic underlayer (SUL). Theoretical studies predict that this should extend the onset of thermal instability, i.e., the superparamagnetic limit, to areal densities a factor of two to five beyond the longitudinal limit, thought to be about 100 Gb/in [1], [2]. Thermal stability is crudely determined by the “stability ratio” of anisotropy energy , where is the anisotropy constant and is an effective , where is magnetic grain volume, to thermal energy Boltzman’s constant and is the absolute temperature [3]. A ratio in generally accepted minimum criterion for the perpendicular media has not yet been arrived at, but it is likely that the value for perpendicular media will be somewhat higher than that for longitudinal media due to geometry induced demagnetizing fields for the longer bit lengths. Thermal stability is enhanced by increasing , which implies higher coercivity, and/or increasing , by increasing film Manuscript received December 23, 2001; revised February 21, 2002. C. Chang, C. Brucker, J. Chen, and R. Ranjan are with Seagate Technology, Fremont, CA 94538 USA (e-mail:
[email protected]). M. Plumer and J. van Ek are with Seagate Technology, Bloomington, MN 55435 USA. J. Yu, D. Karns, Y. Kubota, G. Ju, and D. Weller are with Seagate Technology, Pittsburgh, PA 15203 USA. Publisher Item Identifier S 0018-9464(02)05672-8.
thickness, but either of these in turn place greater demands on the pole-SUL recording head structure for write field amplitude and gradient. This has stimulated an intensive search for SUL materials with high moment [4] to maximize the pole-SUL effectiveness. In the work reported here, SUL materials with moment ranging from 1.4 to 2.0 T are fabricated and characterized, and Landau–Lifshitz–Gilbert (LLG) micromagnetics and finite element modeling (FEM) is used to show that write head performance is only slightly impacted by SUL moment in the 1–2 T range. These results suggest that extremely high SUL moment is in fact not necessary (although high moment in the head material is), thus broadening the range of SUL materials that can be considered in view of other practical requirements that must be met for application in a low-cost, high-density, high-performance disk drive. II. EXPERIMENTAL SETUP A series of FeCoB and CoZrNb films with various thicknesses was deposited from 7-in diameter targets on either glass or aluminum substrates using dc-magnetron sputtering. 3-nmthick Ti was used as an adhesion layer. The sputtering power was varied from 1 to 4 kW, and the deposition pressure was varied between 3 and 6 mtorr. The structures were optionally post-annealed in the sputter system for 8 s using different annealing powers. Films were capped with a 3.5-nm hybrid carbon layer for wear and corrosion protection. FeAlN films were deposited using a FeAl target and reactive Ar N dc sputtering. The N partial pressure was adjusted for about 5 at.% content in the film and the total pressure was between 5 and 15 mtorr. Microstructure was characterized using X-ray diffraction (XRD). An in-plane magnetooptical Kerr effect (MOKE) device and a vibrating sample magnetometer (VSM) were used to obtain easy and hard axis hysteresis loops by applying the magnetic field along either radial or circumferential directions of the disk. Magnetic domain structures were observed by magnetic force microscopy (MFM) using a conventional Digital Instruments (DI 5000) MFM with standard vertically magnetized pyramidal tips. Spin-stand measurements were performed on a Guzik 2585 A/1701 A test spin-stand to quantify the amount of SUL-only readback noise as a function of sputtering and annealing conditions. The SUL readback noise was obtained in the following manner. A large band of the medium, 4000 in, was dc erased. The time-domain readback signals were captured for 0.5 ms with a sampling rate of 1 Gs/s. These time domain readback
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Fig. 1. Hysteresis loops, MFM image, and dc noise (upper) and electronic noise floor (lower) power spectra for an As-deposited FeCoB film.
signals were converted to the frequency domain and further to the spatial frequency domain, allowing the noise spectrum to be easily examined. At a media speed of 14 m/s, the conversion to the spatial frequency domain allowed spatial frequencies of up to 1800 kfci to be examined with a maximum resolution 0.007 kfci. The SUL readback noise was obtained by integrating the spatial frequency domain and normalizing by 600 kfci. The excess SUL readback noise was obtained by subtracting the integrated electronic noise from the integrated SUL readback noise. Noise sources from the SUL, the interaction between SUL and the recording layer, and the recording layer were distinguished by comparing spin-stand measurements of media with and without (i.e., SUL only) recording layers. III. EXPERIMENTAL RESULTS AND DISCUSSION A. As-Deposited FeCoB Films Fig. 1 shows representative data for an As-deposited FeCoB film. The in-plane hysteresis loops show nearly isotropic behavior with coercivities of 24 and 36 Oe in the radial and circumferential direction, respectively. The film saturates at about 160 Oe. The shape of the loops, and in particular the distinct slope between remanence and saturation, is indicative of perpendicular anisotropy. The anisotropy field is smaller than the shape (about 2 T) but greater than a certain critanisotropy and leads to a unique stripe domain pattern [5]. ical value Indeed, stripe domains are observed by MFM (Fig. 1), where bright-to-dark contrast arises from the magnetization canted up or down out of the film plane. The characteristic period of the domains is measured to be about 340 nm. As expected, this SUL exhibits considerable readback noise, as shown in the spin-stand results. The dominant feature in the SUL-induced dc noise spectrum is a steep shoulder rising above the electronic noise floor at about 150 kfci, with a broad peak around 120 kfci (Fig. 1). This noise is attributed to fringe magnetic fields emanating from the SUL stripe domains (the stripe domain period of 340 nm corresponds to linear densities between 0 and 150 kfci, depending on stripe orientation with respect to the track direction). The excess SUL readback noise is quantified as about 7.9 dB for this case.
Fig. 2. Period of stripe domains, measured from MFM images, for As-deposited FeCoB films on glass substrates.
The known mechanisms capable of producing perpendicular anisotropy in these films include magnetocrystalline anisotropy, microshape anisotropy, and magnetoelastic anisotropy [6]. As-deposited films are amorphous, judging from XRD; hence, the perpendicular anisotropy should be due to magnetoelastic effects. It has been shown that FeCoB films have a high positive 10 [7]. At 3 mtorr magnetostriction coefficient sputtering pressure, the stress inside the film is compressive and of the order of 300 Mpa [7]. This gives rise to a perpendicular 10 erg/cm . This is much smaller anisotropy 10 erg/cm . Bethan the magnetostatic energy yond a critical thickness the magnetization starts to oscillate out of the plane in a periodic manner to reduce the perpendicular anisotropy energies without paying a penalty in magnetostatic . The critical thickness, given by energy as large as [8], is estimated to be about 140 nm, assuming 10 erg/cm. The stripe the exchange stiffness constant domain width is also predicted to be 140 nm. Experimental results are consistent with these predictions. A series of As-deposited FeCoB films on glass substrates were observed by MFM and the stripe domain period was measured for various film thicknesses, as shown in Fig. 2. Stripe domains were not observed for the films thinner than 120 nm, while they were observed for films thicker than 140 nm. The domain period varies from 280 to 350 nm, with a tendency to increase as the film thickness increases from 140 to 200 nm. This tendency implies that the average compressive stress decreases with increasing thickness for As-deposited films. B. Annealing Effect of FeCoB SULs After annealing, the SULs exhibit good soft magnetic properties. In Fig. 3, the film annealed at 4.2 kW for 8 s, for which the estimated peak temperature is 400 C, shows an easy axis along the radial direction with a coercivity of about 0.8 Oe. The circumferential hard axis direction shows an anisotropy field of about 50 Oe. The MFM image is featureless in this case, suggesting a SUL with very low noise. This is verified by the spin-stand result. The dc noise is essentially down to the electronic noise floor. Thermal annealing apparently reduces compressive stress inside the film, thus reducing the perpendicular
CHANG et al.: SUL MATERIALS FOR PERPENDICULAR MAGNETIC RECORDING
Fig. 3. Hysteresis loops, MFM image, and power spectra for a FeCoB film annealed at 4.2 kW for 8 s.
Fig. 4. Dependence of coercivity annealing power.
H
and in-plane anisotropy field
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Fig. 5. Excess dc noise for samples at different annealing powers. Inset is an MFM image of a film annealed at 5 kW.
on the
magnetic anisotropy and eliminating the driving force for stripe domains. Furthermore, the induced radial magnetic anisotropy aligns the magnetization and no other domain structure is observed. The observed radial magnetic anisotropy in these films is dependent upon the annealing conditions. Results are summarized in Fig. 4. With increasing annealing power, the radial anisotropy increases initially for low heating (3–4 kW for 8 s), and then levels off at higher heating power (4.5 and 5 kW for 8 s). The Oe. With radial anisotropy develmaximum value is oping, the coercivity drops off significantly from about 30 Oe to about 1 Oe for both radial and circumferential directions. The spin-stand results demonstrate different responses to the annealing power (Fig. 5). Below 3 – 4 kW, excess dc noise associated with stripe domains is still observed, though it is reduced from 8 dB to near zero with increasing heating power. Between 4 and 5 kW, the SULs show very little noise, which essentially drops to zero, while at 5 kW and above, the dc noise increases again. The increasing noise level at higher annealing power is related to the onset of crystallization in these films. X-ray -2 scans show diffraction peaks associated with bcc structures in the films with very high annealing power. Crystallization results in local magnetocrystalline anisotropy varying in direction and magnitude. As a consequence, the magnetization is not uniform and induces ripple structures as observed in
Fig. 6. Coercivity of FeCoB films as a function of film thickness on glass (triangles) and aluminum (circles) substrates with (open symbol) and without (solid symbol) annealing.
MFM (inset to Fig. 5). The stray fields emanating from these magnetic ripple structures cause the observed noise. C. Effect of Substrate on FeCoB SUL Coercivities for FeCoB films on glass and aluminum substrates are shown in Fig. 6 for As-deposited and annealed films. The coercivity behavior of As-deposited films on glass substrates follows the tendency of stripe domain width shown in Fig. 2. Coercivity is low until stripe domains form at the critical thickness. However, coercivities are low in the entire measured thickness range once the films are annealed. The coercivity of As-deposited films on aluminum substrates shows behavior similar to that on glass substrates, but the critical thickness is smaller at about 120 nm. Annealing of the films on aluminum substrates does not, however, result in good magnetic properties as for glass. In fact, the critical thickness becomes even smaller, between about 80 and 120 nm, which implies an increase in compressive stress. The different stress effects inferred for glass versus aluminum are attributed to differential thermal expansion effects between film and substrate. Noise amplitude spectra of double layer media with 200-nmthick FeCoB SUL deposited on aluminum and glass substrates
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Fig. 7. Noise amplitude spectra for double-layer media with FeCoB SUL deposited and annealed on aluminum and glass substrates.
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Fig. 9. Noise spectra (electronic, virgin, ac-erase, and dc-erase) of 200-nm-thick CoZrNb SUL.
IV. MODELING EFFECTS OF THE SOFT UNDERLAYER ON THE WRITE FIELD
Fig. 8. Hysteresis loops of CoZrNb soft underlayer films measured in radial (solid symbol) and circumferential (open symbol) directions.
are shown in Fig. 7. The SUL-induced dc noise amplitude approaches the electronic noise floor for the example on glass substrate, while that on aluminum substrate starts rising significantly above the electronic noise level at about 150 kfci, with a peak around 130 kfci. As before in the case of As-deposited films on glass, this peak is attributed to SUL stripe domains.
D. CoZrNb and FeAl Soft Underlayers Representative hysteresis loops and noise spectra for a 1.4-T CoZrNb SUL are shown in Figs. 8 and 9. Coercivities of the film are below 1 Oe and no signs of stripe domains are observed in the MFM images or noise spectrum. CoZrNb films show wider margins for sputter process and annealing conditions compared to FeCoB. The greater processing margins for CoZrNb are likely attributable to the lower magnetostriction of this material [9]. In contrast to CoZrNb films, 2-T FeAlN film properties were found to be extremely sensitive to process conditions, especially N partial pressure, total pressure, and thickness. This sensitivity to processing is consistent with other work on FeAlN [10] and the related material FeTaN [11]. Although radial anisotropy could be induced in our As-deposited films, annealing studies were not extensively pursued.
LLG micromagnetics, FEM calculations, and a micromagnetic recording model are used to demonstrate the relative insensitivity of the writer head field to SUL saturation moment. The influence of changing the head-to-SUL distance, referred to here as head-to-keeper spacing (HKS), is also examined and found to have a more pronounced impact on the recording process. The micromagnetic method was utilized to explicitly verify the notion that it is not possible to saturate a SUL when the pole tip and the SUL have the same saturation magnetization. Simple magnetostatic arguments reveal that, to lowest order and within a realistic range of HKS values, saturation of the SUL would ) be twice the require that the moment of the pole tip ( ) [12]. The saturation of the SUL moment of the SUL ( . This funcan be safely excluded unless damental observation relaxes restrictions on the choice for materials for a SUL. Calculations were performed using the stray field from a long uniformly magnetized bar magnet with T (and a 0.25 0.25 m cross-section) as the head field, nm. and a micromagnetic model of an SUL with or The saturation moment of the SUL was either T. The results in Fig. 10 show that the use of the lower moment material in the SUL reduces the field at the record layer by only 15%. In addition, the field gradient over a large range of hypothetical medium coercivity values is not affected by the lower moment. These results are corroborated by a full FEM calculation of both the writer and SUL. Fig. 11 shows results for the center, at a distance 25 nm from the air bearing track head field, surface (ABS) from a single-pole type writer design (4-turn coil) with a track width of 0.25 m in the presence of a SUL 45 nm from the ABS. There is little impact (5%) in reducing the moment of the SUL from 2.0 to 1.0 T. Changing the top pole material from 2.4 to 1.6 T, however, has a significant impact. The micromagnetic recording model (MRM) [13] was used to further examine the effects of changing the SUL and top pole materials. A FEM calculated head field was applied to a LLG model of granular media to record transitions. The field from these transitions was then used in a micromagnetic model of a GMR spin valve device for playback. For this study, isolated transitions were recorded on a medium
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Fig. 10. Micromagnetic results for the field in the medium in the down-track direction. Solid and dotted curves correspond to SUL moments of 1.0 and 2.0 30 nm. T, respectively.
Fig. 12. MRM calculated differentiated playback voltage pulsewidth as a function of increasing write current for the three write pole/SUL materials combinations (as in Fig. 11).
Fig. 11. FEM results showing the field in the medium as a function of write current for three different pole and SUL materials combinations.
Fig. 13. FEM calculated field in the medium as a function of write current for three values of head-to-SUL separation.
with uniform 13 13 20 nm grains on a square lattice, emu/cc, kOe, and an exchange energy of 10 erg/cm to give a coercivity of 3.6 kOe. Fig. 12 shows the results for the pulsewidth of the differentiated , signal as a function of increasing head field strength, is the field in the medium taken from Fig. 11 and Hc where is the medium coercivity. There is little impact on the readback waveform as a consequence of changing the SUL moment. The use of a 1.6-T pole material in place of 2.4 T, however, increases the pulsewidth significantly at the higher values of write current. Note the overall increase in pulsewidth as write current increases due to a degradation in field gradient. We also consider the effect of changing the head-to-SUL (“keeper”) separation, HKS. For this study, a write pole width of 0.20 m was used. The pole material had a moment of 2.4 T and the SUL had a moment of 2.0 T. FEM calculated results for as a function of write current are shown in Fig. 13 for three , 40 and 45 nm. At the highest different values of
write current (50 mA zero-to-peak), increases significantly from 11.9 to 12.6 to 13.9 kOe as HKS decreases. It has been observed that a potential issue with regard to perpendicular recording is medium switching speed. Related to this is the fact that with a SUL present, the head field is directed almost entirely perpendicular to the ABS, i.e., along the medium magnetization easy axis. Faster switching occurs if there is a significant component of the head field in a direction [14]. For longiperpendicular to the medium magnetization tudinal recording with a gapped head, the write field impinges the medium at about a 45 angle [13]. Fig. 14 shows the effect of HKS on the direction of the write field, where is perpendicular to the ABS, as calculated by the FEM. Increasing HKS serves to enhance the longitudinal component but at the expense of a reduced overall magnitude. It is not clear which effect is more important for faster medium-grain switching. The MRM was also used to examine the effect of reducing m. HKS on playback pulsewidth in the case of a
HKS =
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ment to realize noise-free performance has been demonstrated. A general conclusion that can be drawn from this study is that the sputter processing challenges in fabricating noise-free SUL materials are greater the higher the moment, at least for the materials considered here. Theoretically, the saturation moment of the SUL has been shown to have a very little impact on the effectiveness of the pole-SUL recording head structure for writing perpendicular transitions. If extremely high moment is not in fact necessary, SUL material choices can be leveraged to meet other important practical requirements. In addition to low noise, these include corrosion resistance for environmental stability, suitability as a seed layer template for subsequently deposited layers, high-frequency permeability, mechanical toughness for crash resistance, and manufacturability. Further spinstand studies with full media recording structures are in progress to evaluate the predictions of the model. Fig. 14. Ratio of longitudinal and perpendicular components of the FEM is a calculated head field (evaluated near the trailing edge where maximum) as a function of head-to-SUL separation.
H
ACKNOWLEDGMENT The authors are grateful to Dr. G. Rauch for many useful discussions during the course of this work and for a critical reading of the manuscript. The authors would like to thank B. Van Ho, B. Arquero, and D. Massey for their expertise in fabricating the disk samples used for this study. REFERENCES
Fig. 15. MRM calculated differentiated playback voltage pulsewidth as a function of increasing write current for two values of HKS (as in Fig. 4).
Here, the medium had uniform grains with dimensions emu/cc, kOe with 10 10 14 nm , 10 erg/cm giving kOe. The results shown in Fig. 15 indicate up to a 25% increase in pulsewidth as HKS increases from 35 to 45 nm. A conclusion of these model results is that the SUL moment is not at all a big factor in limiting the head field for perpendicular recording. More important are the head-to-SUL separation, write-head saturation moment, and prevention of head saturation effects, as a means to obtain sufficient switching field as well as good playback pulsewidths. V. CONCLUSION Experimentally, FeCoB, CoZrNb, and FeAlSi SUL materials have been fabricated with moment ranging from 1.4 to 2.0 T, and the control of stripe domain structure by proper stress manage-
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