CoCrPt/Ti Perpendicular Media onto Nanostructured Polymer Templates

Copyright © 2012 American Scientific Publishers All rights reserved Printed in the United States of America

Journal of Nanoscience and Nanotechnology Vol. 12, 1–5, 2012

CoCrPt/Ti Perpendicular Media onto Nanostructured Polymer Templates W. O. Rosa1 ∗ , D. Navas2 , A. Asenjo3 , V. M. Prida1 , B. Hernando1 , and M. Vázquez3 1

Depto. de Fisica, Universidad de Oviedo, Calvo Sotelo s/n, 33007 - Oviedo, Spain 2 Depto. de Química-Física, Universidad del País Vasco, 48940 - Bilbao, Spain 3 Instituto de Ciencia de Materiales de Madrid, CSIC, Cantoblanco, 28049 - Madrid, Spain The fabrication and the study of the magnetic properties of CoCrPt/Ti nanostructures produced by sputtering onto ordered polymer templates are reported here. Samples exhibit a significant outof-plane component of the magnetization higher than for planar films, and it is stronger for the thicker CoCrPt films, and for nanostructured films with the shorter period ordering. The shape of the polymeric templates plays an important role for the determination of magnetic easy-axis. Magnetic Force Microscopy images of the samples show a single magnetic domain structure with high outof-plane anisotropy for the samples with longer ordering (480 nm period).

Keywords: Perpendicular Magnetic Media, Nanostructured Polymer, Aluminium Anodization, Magnetic Force Microscopy.

1. INTRODUCTION

∗

Author to whom correspondence should be addressed.

J. Nanosci. Nanotechnol. 2012, Vol. 12, No. xx

1533-4880/2012/12/001/005

doi:10.1166/jnn.2012.4909

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RESEARCH ARTICLE

The control of anisotropy and shape in nanostructures is crucial to optimize high-density magnetic recording media. Hexagonal closed-packed (hcp) structure CoCrPt thin films have been intensively investigated for high-density longitudinal recording media.1 High coercivity and low noise media are necessary criteria for achieving high-density recording. CoCrPt films with a Cr underlayer are one of the most frequently employed permanent magnets structures for this purpose.2–4 Despite the technological developments, basic research is still necessary to understand and predict the optimal system behavior, including magnetization reversal mechanism, exchange coupling or magnetotransport effects. Co-based alloys containing small amount of nonmagnetic elements have also been reported to be effective in reducing the inter-grain coupling and so, enhancing the coercivity. For example, studies on CoCrPt granular films prepared by Co-sputtering CoCrPt alloy and on non-magnetic compounds such as SiO2 were reported,5 showing a coercivity as high as about 6000 Oe in the CoCrPt+SiO2 (5 vol%) system. In this same way, Ilievsky et al.6 have investigated the thermal behavior of CoCrPt magnetic nanoparticles ensemble with collinear uniaxial anisotropy, which presents a maximum at finite temperature when the applied field is perpendicular to the easy

axis and lower than anisotropy field, displaying a second order phase transition. CoCrPt-Ti has been also receiving important attention because this composition presents a high out-of-plane anisotropy and coercive field. This kind of alloy has been studied in thin films shape,7 nanogranular films8 and also in line and ring shapes.9 Concerning the specific case of polymer substrates, there are recent works reporting the properties of CoCrPt alloys deposited onto such substrates. Nguyen et al.10 have researched about oblique sputtered CoCrPt/CoCrMn onto columnar polyethylene terephthalate (PET) observing a clearly out-of-plane anisotropy from VSM and torque magnetometer measurements. Lee et al.11 have fabricated a CoCrPt thin film onto flexible polymer (kapton) comparing with other substrates and they have studied the recording properties of this type of materials. Additionally, they reported a large transition width (195 nm) for these CoCrPt thin films indicating the existence of significant exchange coupling between the grains. Fujiura et al.12 have also investigated the importance of flexible polymer substrates analyzing CoCrPt-SiO2 thin films deposited on them. In their case, high coercive field (4270 Oe) and remanence (0,78) are reported. In the present work, we combine the use of nanostructured Al with controlled geometry as template for the replication of its ordering into polymeric nanohill arrays, as reported elsewhere.13–15 Such polymer, in this case PMMA, is used to fabricate magnetic CoCrPt/Ti nanostructures on top of it, providing a strong out-of-plane

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CoCrPt/Ti Perpendicular Media onto Nanostructured Polymer Templates

magnetization combined with hexagonal symmetry from the nanostructured PMMA matrix.

RESEARCH ARTICLE

2. PREPARATION OF CoCrPt NANOSTRUCTURED FILMS The samples have been produced using one-step Aluminum anodization process as is reported elsewhere.13 The anodization process is done using three different acid solutions, i.e., sulfuric, oxalic and phosphoric acids in distinct concentrations. After 24 h of anodization we obtain a nanostructured Al surface at the bottom of a disordered porous anodic alumina membrane. After removal of the alumina membrane by chemical etching, we obtain the nanostructured Al surface, which will be subsequently used as a template to imprint its nanostructured shape on a dissolved poly (methyl methacrylate) – PMMA polymer. The PMMA was dissolved in chloroform and then dropped into the Al surface carefully to recover the entire surface. After the dissolvent complete evaporation, we obtain a nanostructured PMMA surface. Note that this process is not destructive, so that we are able to use the same Al template for subsequent experiences that enable the preparation of additional samples reproducing the same ordering. PMMA is used as a substrate for the sputtering growth of magnetic nanostructured thin films. A 5 nm thick Ti layer and then either a 10 or 20 nm thick layer of Co66 Cr22 Pt12 (CoCrPt) film were deposited sequentially on PMMA templates by rf sputtering. The Ar (99.999% pure) sputtering gas pressure was 2 mTorr, the base pressure was below 2 × 10−8 Torr, and the rf power was 300 W for 5-cmdiameter targets16 17 The deposition rates were 1.9 Å/sec for CoCrPt and 0.8 Å/sec for Ti. Finally, we obtain a magnetic thin film reproducing the nanohill-like nanostructure of the Al precursor and the PMMA polymer. The samples produced are label by the templates name, sulfuric, oxalic and phosphoric, for which the periodicity or wavelength of the nanostructure takes values of 65 nm, 105 nm and 480 nm, respectively.

Fig. 1.

2

For comparative reasons, planar thin films of the same composition and thicknesses have been prepared. These thin films were deposited onto a flat PMMA substrate with 5 nm r.m.s. roughness.

3. PERPENDICULAR MAGNETIC ANISOTROPY AND ITS DEPENDENCE ON NANOSTRUCTURE Magnetic hysteresis loops have been measured with a VSM magnetometer under a maximum applied field of 1.8 T as a function of the angle between applied field and the plane of the films. Figure 1 shows the comparison between hysteresis loops of non-structured planar 10 and 20 nm thin films magnetized along the in-plane and out-of-plane directions. A significant out-of-plane remanence values are observed in both cases which denotes a main out-of-plane anisotropy easy axis, particularly for the 20 nm thick film. Figures 2(a and b) show the comparison between planar and nanostructured samples. As observed, the effect introduced by the nanostructures is to increase both the coercive field and the remanence, denoting an enhancement of the out-of-plane anisotropy in the nanostructured films. The highest coercive field and remanence values are observed for films prepared onto the sulfuric template with 65 nm nanohill periodicity. Figures 3(a and b) show the angular dependence of coercivity between in-plane (0 ) and out-of-plane (90 ) orientations of the applied field, for non-structured and nanostructured CoCrPt films with different thickness (10 and 20 nm). One can observe that for 10 nm thick CoCrPt film, the easy axis is out-of-plane as deduced from the highest coercivity and remanence at 90 orientation. For the 20 nm thick films, one deduces again an overall out-of-plane anisotropy from the angular dependence of remanence. Nevertheless, coercivity shows a much more reduced variation with a non-clear trend for different samples suggesting a more complex magnetization mechanism than for the previous sample. Comparing these results with

In-plane (IP) and out-of-plane (OOP) hysteresis loops for CoCrPt planar thin films: (a) 10 nm and (b) 20 nm.

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Fig. 2.

CoCrPt/Ti Perpendicular Media onto Nanostructured Polymer Templates

Out-of-plane hysteresis loops for (a) 10 nm and (b) 20 nm of CoCrPt thin films.

Fig. 3.

microscope allowing topology (AFM) and magnetic (MFM) imaging of the surface. In this case, magnetic images correspond to the out-of-plane component of magnetic moments at the imaged surface which correspond to the overall easy magnetization direction. Figure 4 shows the topology and magnetic images of the 20 nm thick CoCrPt non-structured PMMA. For this planar CoCrPt thin film we observe a magnetic domain structure typical of an out-of-plane easy magnetization direction. Moreover, the typical length or average size of the magnetic domains is of around 190 nm, in agreement with the value reported by Lee et al.11 Figure 5 shows the AFM and MFM images of the phosphoric samples with 480 nm periodicity, 10 and 20 nm

(a, b) Coercivity and (c, d) remance variation as a function of applied field angle.

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the obtained by Fujiura et al.12 one concludes that the large coercive field obtained for our samples (500 Oe) is still lower, altough with higher remanence to saturation magnetization ratio close to 0.9. Concerning the role of the periodicity or wavelength of the nanostructured films, one deduces, in general, an increase of the out-of-plane anisotropy as the periodicity decreases. Moreover, the hemispherical shape of the nanohills at the nanostructured PMMA templates induces a distribution of the magnetization easy-axis which seemingly contains an important out-of-plane direction. Complementary information about the magnetic anisotropy has been gained by magnetic force imaging of the samples. That has been performed in a Nanotec

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CoCrPt/Ti Perpendicular Media onto Nanostructured Polymer Templates

Fig. 4.

(a) AFM and (b) MFM images of CoCrPt sputtered onto a non-structured PMMA substrate.

In the case of the 10 nm thick film, we first observe a reduced contrast, while in addition it seems that some nanohills contain more than one contrast evidencing a non single domain structure denoting a somehow reduced

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thick. For the 20 nm thick sample, we are able to observe that each nanohill corresponds roughly to a single domain, magnetized either along up or down the perpendicular orientation as deduced by the color contrast of each nanohill.

Fig. 5.

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AFM (left) and MFM (right) images for the 10 nm (a, b) and 20 nm (c, d) thick CoCrPt films.

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CoCrPt/Ti Perpendicular Media onto Nanostructured Polymer Templates

out-of-plane anisotropy in agreement with what is deduced from the hysteresis loops analysis. For samples with smaller periodicity, the sulfuric and oxalic samples (not shown here), we have observed that each magnetic domain contains several nanohills, therefore many CoCrPt/Ti nanohills are required to compose one magnetic domain in the out-of-plane direction. In particular, for the nanostructure prepared with sulfuric and oxalic baths each domain contains around 25 and 13 nanohills, respectively.

4. CONCLUSIONS

Acknowledgments: Authors wish to thank to FEDER, Spanish MICINN and FICyT fundings for providing financial support through research Projects N MAT200913108-C02-01, MAT2010-20798-C05-01, MAT201020798-C05-04 and FC09-IB09-131. Dr. W. O. Rosa also thanks the scientific support from FICyT under research grant FC-10-COF10-04. The Brazilian funding agency CAPES is also recognized.

References and Notes 1. Q. Leng, M. Mao, L. Miloslavsky, B. Simion, C.-Y. Hung, C. Qian, M. Miller, R. Basi, H. C. Tong, J. Wang, and H. Hegde, J. Appl. Phys. 85, 5843 (1999). 2. A. Tsoukatos, S. Gupta, and D. Marx, J. Appl. Phys. 79, 5018 (1996). 3. M. Xiao, A. J. Devasahayam, and M. H. Kryder, IEEE Trans. Magn. 34, 1945 (1998). 4. G. Choe, S. Funada, A. Tsoukatos, and S. Gupta, J. Appl. Phys. 81, 4894 (1997). 5. Y. Xu, J. P. Wang, and Y. Su, J. Phys. D: Appl. Phys. 33, 1460 (2000). 6. F. Ilievski, A. Cuchillo, W. Nunes, M. Knobel, C. A. Ross, and P. Vargas, App. Phys. Lett. 95, 202503 (2009). 7. A. G. Roy, S. Jeong, and D. E. Laughlin, IEEE Trans Magn. 38, 2018 (2002). 8. H. Y. Sun, J. Hu, Z. F. Su, J. L. Xu, and S. Z. Feng, IEEE Trans Magn. 42, 1782 (2006). 9. D. Navas, C. Nam, D. Velazquez, and C. A. Ross, Phys. Rev. B 81, 224439 (2010). 10. L. T. Nguyen, F. D. Tichelaar, and J. C. Lodder, J. Magn. Mag. Mater. 290, 1294 (2005). 11. H.-S. Lee, D. E. Laughlin, and J. A. Bain, J. App. Phys. 93, 7785 (2003). 12. H. Fujiura and S. Nakagawa, J. Magn. Mag. Mater. 310, 2659 (2007). 13. W. O. Rosa, M. Jaafar, A. Asenjo, and M. Vázquez, Nanotechnology 20, 075301 (2009). 14. W. O. Rosa, M. Jaafar, A. Asenjo, and M. Vázquez, J. App. Phys. 105, 07C108 (2009). 15. W. O. Rosa, L. Martínez, M. Jaafar, A. Asenjo, and M. Vázquez, J. App. Phys. 106, 103906 (2009). 16. F. Ilievski, J. C. Perkinson, and C. A. Ross, J. Appl. Phys. 101, 09D116 (2007). 17. F. Ilievski, C. A. Ross, and G. J. Vancso, J. Appl. Phys. 103, 07C520 (2008).

Received: 1 December 2010. Accepted: 1 May 2011.

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Ordered CoCrPt/Ti nanostructures were prepared using ordered polymer templates with two particular thicknesses and three periodicities of the nanostructure. They display an out-of-plane magnetic anisotropy stronger than in the case of planar films. Such anisotropy increases as the periodicity of the nanostructure decreases. This feature is related with the dipolar interaction generated between the different nanostructures, where the periodicity plays a very important role. This feature arises from the fact that due to the hemispherical shape of the template, the CoCrPt/Ti layer is preferentially deposited on top than to its side, providing an asymmetric deposition ruled by the template shape, creating a local anisotropy. Accordingly, coercivity and remanence shows higher values for out-of-plane loops. Complementary studies performed by MFM reveal a dependence of the size of domains between planar and nanostructured films as well as with the periodicity of the nanostructure. This is in correspondence with the strength of the out-of-plane anisotropy of each particular film. As all the samples studied, 10 and 20 nm CoCrPt thick, present this out-of-plane component, MFM measurements were done and reveal that in the case of samples with smallest diameters (sulphuric and oxalic template), the size of magnetic domains is bigger than the nanostructures themselves, therefore many nanostructures are needed to characterize one magnetic domain, meanwhile the samples that have biggest parameters (phosphoric template) present

a single-domain state, having a brighter MFM contrast for 20 nm thick sample.