Magnetization reversal of Co/Pd multilayers on nanoporous templates
© Huang et al; licensee Springer. 2012
Received: 13 September 2011
Accepted: 5 January 2012
Published: 5 January 2012
By making use of an e-beam deposition system, the [Co(2 Å)/Pd(10 Å)]15 multilayers were prepared on a Si(100) substrate and anodized aluminum oxide [AAO] templates with average pore diameters of around 185, 95, and 40 nm. The mechanism of magnetization reversal of the Co/Pd multilayers was investigated. Wall motion was observed on the Co/Pd multilayers grown on the Si substrate. A combination of wall motion and domain rotation was found in the sample grown on the AAO template with a 185-nm pore diameter. For the samples grown on the AAO templates with pore diameters of around 95 and 40 nm, the reversal mechanism was dominated by domain rotation. The rotational reversal was mainly contributed from the underlying nanoporous AAO templates that provided an additional pinning effect.
PACS: 75.30.Gw, magnetic anisotropy; 78.67.Rb, nanoporous materials; 75.60.Jk, magnetization reversal mechanisms.
KeywordsCo/Pd porous anodized aluminum oxide magnetization reversal
In the last decade, patterned magnetic nanostructures have undergone a rapid development because of their potential applications in future high-density magnetic recording media [1–4]. Nowadays, the FePt film with L10 phase has received significant attention owing to its excellent magnetic properties, presenting large values of the saturation magnetization (approximately 1,100 emu/cm3), the perpendicular magnetocrystalline anisotropy constant (approximately 108 erg/cm3), the coercivity, and the high environmental stability [5–7]. Another choice for the perpendicular recording is ferromagnetic/non-ferromagnetic multilayers. For example, Co/Pt and Co/Pd are also candidates for the perpendicular recording media of the next generation. As the areal density of today's hard disk drives keeps approaching to the limit of current technology, exploring new techniques to obtain higher and higher density is inevitable [8–10]. There have been significant research efforts focusing on the patterned magnetic recording medium with its density larger than 1 Tbit/in2. In addition, the percolated recording media has been investigated because of its high thermal stability and low-medium noise [11–13]. As a result, the magnetic nanomaterial fabricated through nanoporous anodized aluminum oxide [AAO] is a good candidate in the pursuit of patterned recording media.
In this article, we fabricated the Co/Pd multilayers on a Si(100) substrate and on nanoporous AAO templates with different pore diameters using an electron beam evaporator. We attempt to study the magnetization reversal and domain structures of the Co/Pd multilayers on nanoporous templates.
Before film deposition, the Si(100) substrate was cleaned in acetone and alcohol solutions using an ultrasonic bath. Three types of nanoporous AAO templates were purchased from Whatman International Ltd (Maidstone, UK) and have different pore diameters: 185, 95, and 40 nm. Prior to the growth of Co/Pd multilayers, a 10-nm-thick Pd buffer layer was grown on the Si substrate and AAO templates. The [Co(2 Å)/Pd(10 Å)]15 multilayers were then deposited on the Pd buffer layer by an e-beam system with a growth pressure of around 5 × 10-8 Torr. During the deposition, the temperature of the substrates and the deposition rate were kept at 150°C and 0.1 Å/s, respectively.
The surface morphology of AAO templates was examined using a scanning electron microscope [SEM] (JSM-6390, JEOL, Tokyo, Japan). The magnetic hysteresis loops were measured at room temperature using a vibrating sample magnetometer (7407, Lake Shore, Westerville, OH, USA) with an applied field up to 1 T. The domain structures were imaged using a magnetic force microscope [MFM] (XE-100, Park Systems, Suwon, South Korea) under a lift mode operation. The magnetic tip used for the MFM measurements was purchased from Nanosensors Inc. (Neuchatel, Switzerland) with a catalog number of ppp-MFMR. The MFM tip radius of curvature was less than 50 nm. It guaranteed a magnetic resolution better than 50 nm. Because the AAO templates were too thin (thickness of approximately 60 μm) for handling, they were first stuck on glass substrates prior to the above measurements.
Results and discussion
There was no doubt that the first magnetization reversal in Figure 3b, c, d was corresponding to the domain nucleation. The plots of normalized coercivity versus the out-of-plane angle suggested that the second magnetization reversal was a magnetization rotational process. Figure 3b, c, d showed that the magnetization in the second reversal process kept rotating till saturation. As indicated by the open arrows in Figure 3b, c, d, the saturation field of the Co/Pd multilayers increased as the size (or density) of nanopores of the AAO templates was getting smaller (or higher). Therefore, the magnetization rotation that took place in the Co/Pd multilayers can be realized by the pinning effect provided by the underlying AAO nanopores.
We systematically investigated the magnetization reversal of [Co(2 Å)/Pd(10 Å)]15 multilayers on the AAO templates with different pore sizes. The magnetization reversal, domain size, and saturation field of the Co/Pd multilayers were strongly influenced by the pore diameter (or density) of underlying AAO templates. The underlying AAO template provided a pinning effect that results in a rotational reversal and a limitation of domain growth. This study demonstrated the magnetization reversal of a percolated recording media with perpendicular anisotropy mainly dominated by the rotational mechanism.
This work was supported by the National Science Council of Taiwan through grant numbers 96-2119-M-390-001, 97-2112-M-390-003, and 98-2112-M-390-003.
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