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, 4859–4863, 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.

by Publishing Technology University of anisotropy Rochesterfield, displaying a second axisto:and lower than 1. INTRODUCTION Delivered IP: 128.151.244.46 On: Thu, 06 Nov 2014 15:26:30

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order phase transition. CoCrPt-Ti has been also receiving American Publishers The control of anisotropy and shape Copyright: in nanostructures is Scientific important attention because this composition presents a crucial to optimize high-density magnetic recording media. high out-of-plane anisotropy and coercive field. This kind Hexagonal closed-packed (hcp) structure CoCrPt thin films of alloy has been studied in thin films shape,7 nanogranular have been intensively investigated for high-density longifilms8 and also in line and ring shapes.9 tudinal recording media.1 High coercivity and low noise Concerning the specific case of polymer substrates, media are necessary criteria for achieving high-density there are recent works reporting the properties of CoCrPt recording. CoCrPt films with a Cr underlayer are one of alloys deposited onto such substrates. Nguyen et al.10 the most frequently employed permanent magnets struchave researched about oblique sputtered CoCrPt/CoCrMn tures for this purpose.2–4 Despite the technological develonto columnar polyethylene terephthalate (PET) observing opments, basic research is still necessary to understand and a clearly out-of-plane anisotropy from VSM and torque predict the optimal system behavior, including magnetizamagnetometer measurements. Lee et al.11 have fabricated tion reversal mechanism, exchange coupling or magnetoa CoCrPt thin film onto flexible polymer (kapton) compartransport effects. ing with other substrates and they have studied the recordCo-based alloys containing small amount of noning properties of this type of materials. Additionally, they magnetic elements have also been reported to be effective reported a large transition width (195 nm) for these CoCrPt in reducing the inter-grain coupling and so, enhancing thin films indicating the existence of significant exchange the coercivity. For example, studies on CoCrPt granucoupling between the grains. Fujiura et al.12 have also lar films prepared by Co-sputtering CoCrPt alloy and on investigated the importance of flexible polymer substrates non-magnetic compounds such as SiO2 were reported,5 analyzing CoCrPt-SiO2 thin films deposited on them. In showing a coercivity as high as about 6000 Oe in the their case, high coercive field (4270 Oe) and remanence CoCrPt+SiO2 (5 vol%) system. In this same way, Ilievsky (0,78) are reported. et al.6 have investigated the thermal behavior of CoCrPt In the present work, we combine the use of nanosmagnetic nanoparticles ensemble with collinear uniaxial tructured Al with controlled geometry as template for the anisotropy, which presents a maximum at finite temperareplication of its ordering into polymeric nanohill arrays, ture when the applied field is perpendicular to the easy as reported elsewhere.13–15 Such polymer, in this case PMMA, is used to fabricate magnetic CoCrPt/Ti nano∗ structures on top of it, providing a strong out-of-plane Author to whom correspondence should be addressed.

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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

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

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 soluMagnetic hysteresis loops have been measured with a tions, i.e., sulfuric, oxalic and phosphoric acids in disVSM magnetometer under a maximum applied field of tinct concentrations. After 24 h of anodization we obtain 1.8 T as a function of the angle between applied field a nanostructured Al surface at the bottom of a disorand the plane of the films. Figure 1 shows the comparidered porous anodic alumina membrane. After removal son between hysteresis loops of non-structured planar 10 of the alumina membrane by chemical etching, we obtain and 20 nm thin films magnetized along the in-plane and the nanostructured Al surface, which will be subsequently out-of-plane directions. A significant out-of-plane remaused as a template to imprint its nanostructured shape on nence values are observed in both cases which denotes a a dissolved poly (methyl methacrylate) – PMMA polymer. main out-of-plane anisotropy easy axis, particularly for the The PMMA was dissolved in chloroform and then dropped 20 nm thick film. into the Al surface carefully to recover the entire surface. Figures 2(a and b) show the comparison between plaAfter the dissolvent complete evaporation, we obtain a nar and nanostructured samples. As observed, the effect nanostructured PMMA surface. Note that this process is introduced by the nanostructures is to increase both the not destructive, so that we are able to use the same Al temcoercive field and the remanence, denoting an enhanceplate for subsequent experiences that enable the preparament of the out-of-plane anisotropy in the nanostructured tion of additional samples reproducing the same ordering. films. The highest coercive field and remanence values are PMMA is used as a substrate for the sputtering growth of observed for films prepared onto the sulfuric template with magnetic nanostructured thin films. A 5 nm thick Ti layer 65 to: nmUniversity nanohill periodicity. Delivered by Publishing Technology of Rochester and then either a 10 or 20 nm thick layer of Co66 Cr22On: Pt12 Thu, 06 Figures 3(a and b) show the angular dependence of IP: 128.151.244.46 Nov 2014 15:26:30 (CoCrPt) film were deposited sequentially on PMMA tem- Scientific Copyright: American coercivityPublishers between in-plane (0 ) and out-of-plane (90 ) plates by rf sputtering. The Ar (99.999% pure) sputtering orientations of the applied field, for non-structured and gas pressure was 2 mTorr, the base pressure was below nanostructured CoCrPt films with different thickness (10 2 × 10−8 Torr, and the rf power was 300 W for 5-cmand 20 nm). One can observe that for 10 nm thick CoCrPt diameter targets16 17 The deposition rates were 1.9 Å/sec film, the easy axis is out-of-plane as deduced from the for CoCrPt and 0.8 Å/sec for Ti. highest coercivity and remanence at 90 orientation. For Finally, we obtain a magnetic thin film reproducing the 20 nm thick films, one deduces again an overall the nanohill-like nanostructure of the Al precursor and the out-of-plane anisotropy from the angular dependence of PMMA polymer. The samples produced are label by the remanence. Nevertheless, coercivity shows a much more templates name, sulfuric, oxalic and phosphoric, for which reduced variation with a non-clear trend for different samthe periodicity or wavelength of the nanostructure takes ples suggesting a more complex magnetization mechanism than for the previous sample. Comparing these results with values of 65 nm, 105 nm and 480 nm, respectively.

Fig. 1.

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

J. Nanosci. Nanotechnol. 12, 4859–4863, 2012

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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.

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

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microscope allowing topology (AFM) and magnetic the obtained by Fujiura et al.12 one concludes that the large (MFM) imaging of the surface. In this case, magnetic coercive field obtained for our samples (500 Oe) is still images correspond to the out-of-plane component of maglower, altough with higher remanence to saturation magnetic moments at the imaged surface which correspond to netization ratio close to 0.9. the overall easy magnetization direction. Concerning the role of the periodicity or wavelength Figure 4 shows the topology and magnetic images of the of the nanostructured films, one deduces, in general, an 20 nm thick CoCrPt non-structured PMMA. For this planar increase of the out-of-plane anisotropy as the periodicCoCrPt thin film we observe a magnetic domain structure ity decreases. Moreover, the hemispherical shape of the typical of an out-of-plane easy magnetization direction. nanohills at the nanostructured PMMA templates induces Moreover, the typical length or average size of the maga distribution of the magnetization easy-axis which seemnetic domains is of around 190 nm, in agreement with the ingly contains an important out-of-plane direction. value reported by Lee et al.11 Complementary information about the magnetic Delivered by Publishing Technology to: University of Rochester Figure shows the AFM and MFM images of the phosanisotropy has been gained by IP: magnetic force imaging 128.151.244.46 On: Thu, 06 Nov 52014 15:26:30 phoric samples with of the samples. That has been performed in a Nanotec Copyright: American Scientific Publishers 480 nm periodicity, 10 and 20 nm

Rosa et al.

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.

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.

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

RESEARCH ARTICLE

Delivered by Publishing Technology to: University of Rochester IP: 128.151.244.46 On: Thu, 06 Nov 2014 15:26:30 Copyright: American Scientific Publishers

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

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

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

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