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Magnetic characteristics of CoPd and FePd antidot arrays on nanoperforated Al2O3 templates
Journal of Magnetism and Magnetic Materials ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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Magnetic characteristics of CoPd and FePd antidot arrays on nanoperforated Al2O3 templates A. Maximenko a,b,n, J. Fedotova b, M. Marszałek a, A. Zarzycki a, Y. Zabila a a b
The Henryk Niewodniczanski Institute of Nuclear Physics Polish Academy of Sciences, Radzikowskiego Str. 152, 31-342 Krakow, Poland Research Institute for Nuclear Problems of Belarusian State University, Bobruiskaya Str. 11, 220030 Minsk, Belarus
art ic l e i nf o
a b s t r a c t
Article history: Received 22 June 2015 Received in revised form 5 August 2015 Accepted 13 August 2015
Hard magnetic antidot arrays show promising results in context of designing of percolated perpendicular media. In this work the technology of magnetic FePd and CoPd antidot arrays fabrication is presented and correlation between surface morphology, structure and magnetic properties is discussed. CoPd and FePd antidot arrays were fabricated by deposition of Co/Pd and Fe/Pd multilayers (MLs) on porous anodic aluminum oxide templates with bowl-shape cell structure with inclined intercellular regions. FePd ordered L10 structure was obtained by successive vacuum annealing at elevated temperatures (530 °C) and confirmed by XRD analysis. Systematic analysis of magnetization curves evidenced perpendicular magnetic anisotropy of CoPd antidot arrays, while FePd antidot arrays revealed isotropic magnetic anisotropy with increased out-of-plane magnetic contribution. MFM images of antidots showed more complicated contrast, with alternating magnetic dots oriented parallel and antiparallel to tip magnetization moment. & 2015 Elsevier B.V. All rights reserved.
Keywords: Antidots Percolated perpendicular media Co/Pd multilayers Ordered L10 FePd alloy Perpendicular magnetic anisotropy Porous anodic aluminum oxide templates
1. Introduction Nowadays there are a few novel recording concepts to increase the storage capacity of magnetic recording media. One of the most promising solutions for recording devices is the concept of bit patterned media which extends the areal density beyond 1 Tbit/in2 but requires the bit size and spacing of 12 nm which is a real challenge. This concept also meets problem related to superparamagnetic limit [1–3]. On the other hand an alternative concept of recording media, called percolated perpendicular media (PPM [1,4]), does not require such small grains. PPM consists of magnetic film with densely distributed nonmagnetic defects acting as pinning sites for magnetic domain walls (ordered arrays of antidots). Among many efforts to fabricate PPM a simple way is to deposit magnetic films on porous templates with ordered nanopore distribution (like anodized Al2O3, ZrO2, etc.). In this case a film with lateral arrays of holes (antidots) will be produced. The holes in such structure act as pinning sites and allow tuning the parameters of the magnetic films. CoPd and FePd magnetic films with intrinsic perpendicular n Corresponding author at: The Henryk Niewodniczanski Institute of Nuclear Physics Polish Academy of Sciences, Radzikowskiego Str. 152, 31-342 Krakow, Poland E-mail address: [email protected] (A. Maximenko).
magnetic anisotropy (PMA) are among the best candidates for application in designing of PPM. Perpendicular magnetic anisotropy in Co/Pd MLs originates from the interfacial anisotropy of the alloy-like structure [5,6] and demonstrates pronounced coercive field HC (over 2.1 kOe) [7,8]. Also, ferromagnetic L10 ordered FePd alloy is highly expected as forthcoming high-density recording material because it possesses a large uniaxial magneto-crystalline anisotropy of K 108 erg/cm3 [9–11]. The lattice parameters of bulk L10 are a ¼3.855 Å and c¼ 3.714 Å with small lattice distortion (c/a¼ 0.963) [12]. The ordered stoichiometric FePd L10 alloy is formed when the atomic concentration of Fe:Pd lies in the range (40–50):(60–50) [10].One of the simplest methods for FePd alloy formation is deposition of Fe and Pd multilayered thin films [9]. To obtain the FePd L10 phase (at room temperature the FePd alloy always crystallizes in A1 structure) the diffusion of the Fe and Pd atoms is necessary. It can be achieved by annealing of as-deposited films. For example, L10-ordered alloy could be manufactured from FePd MLs by conventional long thermal annealing at temperatures of about 450–600 °C [13–15]. Only a few papers focused on FePd antidot arrays and porous magnetic Co/Pd MLs with perpendicular anisotropy fabricated on porous templates are published up to now [16,17]. However, in these researches Co/Pd films were deposited on commercially produced anodized Al2O3 (AAO) templates with poorly ordered pores and large pore size distribution leading to the disordered antidot arrays. To improve magnetic performance of Co/Pd porous MLs uniform distribution of nanopores is required [1,4,18].
http://dx.doi.org/10.1016/j.jmmm.2015.08.057 0304-8853/& 2015 Elsevier B.V. All rights reserved.
Please cite this article as: A. Maximenko, et al., Journal of Magnetism and Magnetic Materials (2015), http://dx.doi.org/10.1016/j. jmmm.2015.08.057i
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With respect to these issues, main objective of our work was to study the structure and magnetic properties of Co/Pd MLs and FePd L10 alloys fabricated on laboratory prepared AAO templates with improved ordering of the pores.
2. Experimental The procedure consists of two main processes: fabrication of nanoporous anodic alumina substrates with different pore diameter DP and subsequent deposition of magnetic layers on it. Nanoporous anodic alumina templates(2 cm 2 cm) were fabricated by two-step anodization of high purity unpolished aluminum foil (99.999%, Laurand Associates, Inc.) with thickness of 500 μm. To remove imperfections of crystalline Al lattice and increase its grain size facilitating self-organization process of the pores during anodization. the foil was annealed in air at temperature of 560 °C for 24 h. After that Al foil was chemically polished to attain a desired 10 nm surface smoothness. Two-step anodization in 0.2 M phosphoric acid at U¼190 V and T ¼0 °C is used for fabrication of the ordered nanoporous Al2O3 templates. In this process, thick aluminum oxide film obtained after the first long anodization is stripped away by treatment in solution of 1.8 wt% CrO3 þ6 wt% H3PO4 for 15 min at 75 °C and followed by subsequent re-anodization. The non-epitaxial Pd2 nm/[Fe4 nm/Pd6 nm]x4/Fe4 nm/Pd4 nm and Pd10 nm/[Co0.3 nm/Pd0.55 nm]15/Pd2 nm MLs were deposited on the AAO templates by thermal evaporation in ultra high vacuum at pressures below 10 9 mbar. Reference MLs of the same composition were fabricated on Si/SiO2 wafers. The film thickness was controlled during evaporation with a quartz thickness monitor. After deposition the FePd ML samples were annealed at 530 °C in vacuum ( 10 6 mbar) during 15 min with the slow heating rate 10 °C/min. The thickness of Fe and Pd layers corresponded to 45:55 atomic composition of an alloy which promotes L10 structure formation [10,11]. Surface characterization of AAO templates and MLs was performed by Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM). Top-view SEM images were obtained by Scanning Electron Microscope FEI Quanta 3D FEG. X-Ray Reflectivity (XRR) and X-Ray Diffraction (XRD) in θ–2θ geometry were performed by applying two-circle laboratory diffractometer (Panalyical X'Pert Pro) equipped with PW3050/60 vertical goniometer and PW3383/00 x-ray tube (2.2 kW, λCu(Kα) ¼1.54056 Å). XRR made on continuous reference MLs was used for verification of thickness and roughness of the films. XRR results were fitted using X'Pert Reflectivity software, while XRD patterns were analyzed using single line profile method. The hysteresis loops were measured on MPMS XL SQUID (Quantum Design) with magnetic field (up to 4 T) parallel and perpendicular to sample surface (inplane geometry H// and out-of-plane geometry H⊥, respectively). The values of multilayer magnetization were corrected by subtraction of the contribution from the diamagnetic substrates. Topography analysis by Atomic Force Microscopy (AFM) and magnetic domain imaging by Magnetic Force Microscopy (MFM) were performed with XE-120 Scanning Probe Microscope (SPM) of Park System using MFMR cantilevers (NanoWorld AG) covered with 40 nm thick hard magnetic cobalt alloy and with permanent tip magnetization direction along the tip axis. The radius of tip curvature was 50 nm. SPM scans were performed at ambient conditions. Before MFM scans samples were demagnetized by applying a diminishing alternating magnetic field along the easy axis of magnetization. In this study the MFM images were acquired after topography measurements with tip at a constant 200 nm height.
3. Results and discussion 3.1. Morphology and phase composition The sketch of AAO cross-section illustrating typical bowl-shape cell structure with inclined intercellular region and one pore in the center is shown in Fig. 1. Due to imperfect round shape of nanopores the dimensions of pores were characterized with two parameters, Feret's diameter DF and equivalent diameter DE. DF is the longest distance between any two points along the selection boundary, while DE is calculated from the mean area of the pores using the assumption of perfect circular pores. Estimated from SEM images values of morphological features in the initial AAO template and as deposited CoPd and FePd MLs are summarized in Table 1. SEM images of FePd MLs deposited onto the AAO template reveal porous films with perfectly ordered hexagonal arrangement of nanopores. The size of fabricated antidots is equal to initial pore diameter of used AAO templates. Combination of imaged obtained by second electrons (sensitive to topography) and by back-scattered electrons (sensitive to chemical composition) revealed very homogeneous structure of arrays of FePd antidots. The same distribution of the material was observed on SEM images (not shown here) for CoPd antidots prepared by deposition of CoPd MLs over AAO template. Modification of the morphology of Fe/Pd antidots after annealing at 530 °C in comparison to as deposited MLs is illustrated by SEM images shown in Fig. 2. SEM image evidences the melting of MLs, the material is moved in the direction of cell borders leading to antidot diameter increase (Table 1), and some material is left on the borders of the pores. The XRR patterns of as deposited Fe/Pd MLs (Fig. 3) showed typical Kiessig fringes arising from the finite thickness of the films, and Bragg peaks at θ ¼0.64° and θ ¼1.43° (Fig. 3), originating from a periodicity of the bilayers characteristic for the well-ordered thin MLs. The XRR patterns of as deposited MLs were fitted by Parrat algorithm [19] based on the model of nominal composition of MLs, while samples annealed at 530 °C, fitted with the same algorithm, assumed the intermixing of MLs. During the fitting procedure interface roughness R and thickness t of the layers were fitted independently. The obtained fit parameters are collected in Table 2. For Fe and Pd layers the table does not show the R and t values of
Fig. 1. The schematic image of bowl-like cell structure of porous AAO template. The black area corresponds to pores surrounded by bowl-like shapes with protrusions in the place of contact between different cells.
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Table 1 Nanopore diameters DE and DF for AAO template, FePd and Co/Pd MLs deposited on AAO, annealed FePd MLs. Sample
DE, nm
DF, nm
AAO template FePd and Co/Pd as deposited antidots FePd antidots annealed at 530 °C
177.5 7 4.2 174.5 7 4.7 237.4 7 8.2
186.3 73.8 185.0 74.8 267.3 710.9
each layer separately, but presents mean values averaged over four repetitions of the layers. Taking into account that the accuracy of thickness measurement by quartz microbalance is never better than 5% a good agreement with nominal values of thickness was obtained. For the samples annealed at 530 °C we assumed that the whole stack of Fe/Pd films consists of FePd alloy with thickness equal to the total thickness of deposited multilayered system. Results of XRR pattern fits reveal significant increase of the R values for all interfaces, approximately 20% of the Fe and Pd layers thickness evidencing pronounced intermixing at the interfaces. XRR analysis of continuous Si/SiO2//Pd/[Co0.3 nm/Pd0.55 nm]/Pd MLs (not shown here) revealed that R values of Co and Pd interfaces are generally larger than the nominal thickness of films. This should be associated with intermixing of Co and Pd layers during the deposition resulting in the formation of CoxPd1 x quasi-alloy [5,6]. Therefore we assumed that the whole CoPd multilayer constitutes CoxPd1 x quasi-alloy with thickness equals to the thickness of 15 deposited CoPd bilayers. Parameters t, ρ and R extracted from fitted XRR patterns are collected in Table 3. XRD patterns of as deposited FePd continuous films and antidots are shown in Fig. 4a. XRD pattern of continuous film shows three reflections associated with the Si/SiO2 substrate, at 2θ ¼ 26.76° and 32.86° (Si) and at 2θ ¼ 33.90° (SiO2). Two broadened peaks observed at 2θ ¼39.5–40.0° and at 2θ ¼40.5–41.0° were fitted with a pseudo-Voigt function and associated with (111) Pd layers [20] and (111) Fe/Pd multilayers [21]. Reflection at 2θ ¼ 43.58° is assigned to pure Fe [22]. XRD patterns of as deposited FePd antidots are basically characterized with reflections similar to those of the flat FePd reference film. The additional peaks at 2θ ¼ 38.38° and 2θ ¼44.68° are associated with Al foil substrate. After annealing at 530 °C XRD patterns of both reference film and FePd antidots (shown in Fig. 4b) exhibit the appearance of several reflections at angles summarized in the Table 4. These peaks are identified as the reflections from (001), (110), (111), (200) and (002) planes of face-centered-tetragonal (fct) L10 FePd alloy [9,12]. The presence of the (001) reflection, prohibited in the fcc disordered alloy, indicates that the unit cell of the alloy formed during the annealing changed the symmetry and became
Fig. 3. The XRR patterns of the flat as deposited and annealed at 530 °C Fe/Pd MLs. The curves are shifted vertically for clarity.
Table 2 Thickness t, interface roughness R and density ρ for XRR patterns of reference continuous and annealed at 530 °C Fe/Pd films fitted with Parrat algorithm. Layer
ρ, g/cm3
t, nm
R, nm
As deposited Pd o Fe4 o Pd4 Fe Pd
12.38 7.80 11.77 7.64 11.72
2.0 4.1 5.8 3.8 4.0
0.9 1.1 0.8 1.2 1.1
Annealed 530 °C FePd
10.39
46.5
1.5
Table 3 Thickness t, interface roughness R and density ρ for XRR patterns of reference continuous CoPd films fitted with Parrat algorithm (for tPd ¼ 0.55 nm total thickness of multilayer 12.75 nm). Layer
ρ, g/cm3
t, nm
R, nm
Pd CoPd Pd
11.78 10.89 11.88
10.9 12.5 2.3
0.5 0.5 1.4
Fig. 2. SEM images of FePd MLs deposited on AAO template: (a) as deposited Fe/Pd MLs and (b) Fe/Pd MLs after annealing at 530 °C.
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Fig.5. XRD patters of continuous Si//SiO2//[Co0.3 nm/Pd0.55 nm]/Pd (a) and porous Al2O3//Pd/[Co0.3 nm/Pd0.55 nm]/Pd and (b) films.
Table 5 Structural data obtained by X-ray diffraction analysis of CoPd continuous and porous films: a – lattice parameter, Dcoh – the coherent scattering length corresponding to grain size. Film
a (Å)
Dcoh (nm)
Fig. 4. XRD patterns of as-deposited (a) and annealed at 530 °C (b) Fe/Pd multilayers deposited on the Si/SiO2 substrate and on AAO.
Table 4 FePd peaks positions (2θ) and structural data obtained by X-ray diffraction analysis of reference and porous films annealed at 530 °C. a and c-lattice parameters, c/a – lattice distortion, the mean coherence lengths oDcoh 4 obtained from the XRD patterns for the flat films and antidots of FePd alloy annealed at 530 °C.
(001) (110) (111) (200) (002) a, Å c, Å c/a oDcoh 4, nm a
Reference film
Antidots
24.357 0.06 – 41.23 70.06 47.417 0.06 49.747 0.08 3.831 70.005 3.711 70.015 0.97a 18.5 7 3.8
24.150 70.06 32.711 7 0.07 40.94 70.06 46.777 0.06 49.43 7 0.08 3.885 70.005 3.696 70.015 0.95 17.7 7 3.9
The value of c/a for bulk FePd alloy [23] is 0.97.
tetragonal. This is also confirmed by the separation of the (200) and (002) peaks. Calculated lattice constants a, c and the distortion c/a of fct crystalline lattice of FePd alloy are summarized in Table 4. Large values of c/a both for reference and porous films indicate large distortion of the crystallographic cell, which together with superstructure peaks in diffraction pattern support the formation of L10 phase after annealing at 530 °C. Experimental XRD patterns of continuous and porous CoPd MLs are presented in Fig. 5. The patterns of continuous MLs show only broad overlapped reflections that were deconvoluted by pseudoVoigt functions into two contributions with position of peaks at 2θ ¼ 40° for Pd buffer/cap, and 2θ ¼ 40.7–41.1° for CoPd MLs. Lattice parameters for both components are calculated based on approach proposed in [24] assuming the formation of CoxPd1 x
3.8777 0.002 3.888 7 0.005
Dcoh (nm)
CoxPd100 x
Pd Continuous Porous
a (Å)
7.17 0.5 9.5 7 1.4
3.8017 0.002 3.808 7 0.005
12.2 71.6 11.4 73.0
quasi-alloy with cubic fcc structure [24]. The obtained values (see Table 5) well correspond to (111) peak of Pd [18] and (111) peak of CoxPd1 x quasi-alloy [24–26]. Similar lattice constants a calculated for continuous and porous films indicate very close composition of CoxPd1 x quasi-alloy. From the width of Pd, FePd and CoPd reflections in XRD patterns of continuous and porous films the coherent scattering length Dcoh was estimated with Scherrer equation [27]. It is seen that oDcoh 4 for porous FePd and CoPd is 17–18 nm and 8–12 nm, respectively. One can see that the coherent scattering length for antidots does not change significantly in comparison to continuous films. 3.2. Magnetic properties Fig. 6 shows magnetization loops M(H) for continuous and porous CoPd ML film and FePd ML film annealed at 530 °C. The values of coercive field (HC), remanent magnetization (MR) and saturation magnetization (MS) (or magnetization at Hmax ¼ 2.0 kOe) determined from these curves are summarized in Table 6. The magnetization loops of CoPd and FePd systems were normalized to the mass of Co and Fe, respectively. Almost perfect rectangular shape of М(Н) dependence for reference CoPd films for perpendicular geometry shows that easy axis of magnetization is along the normal to the surface with squareness of the loop MR/MS ¼0.97. Additionally, the film shows very large perpendicular coercive field H⊥C equal to2.2 kOe, demonstrating its prominent PMA. Noticeably, PMA is conserved in CoPd antidots, but with reduced values of MR/MS (up to 0.5) and coercive field HC of approximately 1.2 kOe. Some deterioration of PMA is also confirmed by M(H) curves for continuous and nanoporous CoPd films in hard magnetization direction which demonstrated enhanced contribution of in-plane magnetic anisotropy at the expense of PMA. It can be explained by hemispherical distribution of the material on the U-shaped [28] surface of AAO templates with advanced morphology.
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Fig. 6. Magnetization loops M(H) for reference and porous CoPd MLs (a) and FePd MLs after annealing at 530° C (b). Table 6 Magnetic properties of CoPd films and FePd alloy films annealed at 530 °C. HC – coercive field, MR – remanent magnetization, МS – saturation magnetization. Samples
MS, emu/g
HC, kOe H//
MR/MS
HC, kOe H⊥
MR/MS
Continuous CoPd CoPd antidots Continuous FePd FePd antidots
235 210 260 325
0.36 0.83 2.30 2.40
0.13 0.26 0.67 0.52
2.20 1.20 1.80 2.00
0.97 0.50 0.24 0.45
Magnetization loops of FePd alloy annealed at 530 °C showed very different behavior (see Fig. 7). The reference film has in plane easy axis, but HC is about 2.0 kOe, whereas antidot arrays show isotropic magnetization distribution with HC value very close to this of continuous films. Such change from in-plane magnetic anisotropy for flat film towards isotropic magnetic properties occurring for antidot arrays could be associated with advanced morphology of the antidot surface. Homogeneous deposition of FePd film on U-shaped cells of AAO templates enhances contribution of magnetic moments locally oriented in perpendicular
Fig. 7. MFM images of reference CoPd (a), CoPd antidots (b), and morphology image of CoPd antidots (c); MFM images of annealed reference FePd film (d), annealed FePd antidots (e), and morphology image of annealed FePd antidots (f).
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direction with respect to the film surface. Subsequent annealing at 530 °C promotes increase of HC due to formation of L10 phase seen by XRD. At the same time M(H) curve of annealed FePd film is characterized with high in-plane magnetization contribution most likely due to little degree of ordering and lack of (001) texture preferable for clear PMA in FePd alloys [9,12,29,30]. The MFM images of reference and nanoporous CoPd film show distinct differences (Fig. 7a–c). The contrast in reference image has maze-like typical domain pattern with an average size of domains of a few hundred nm. In contrary, the pattern for porous sample shows the diminished domains which surround the pores as seen from comparison of topography and phase images. The dark spots in MFM image are pores exhibiting the enhanced signal due to the presence of stray fields coming from small domains distributed around the pores. In this case magnetic domains of antidots reflect the lines of the pores. The reference FePd film (Fig. 7d) displays the magnetic domains of similar character as for CoPd film but of bigger size. The corresponding MFM image for nanoporous FePd film exhibits clusters of dark and bright spots of the size corresponding to the pore size separated by the areas of mild contrast. It was already demonstrated by hysteresis loop measurements that in FePd alloy the magnetization distribution is almost isotropic which is reflected in MFM image. The in-plane component of magnetization makes contribution to the areas characterized by weak contrast while the out-of-plane component is focused on pores either in up direction or in down direction. In this case the complex morphology of nanoporous template resulted in appearance of magnetization component perpendicular to the plane.
4. Summary CoPd and FePd multilayered antidot arrays with perfect hexagonal ordering were fabricated by deposition of films on AAO templates with DE ¼ 180 nm. Phase analysis of as-deposited continuous and porous CoPd MLs indicated formation of fcc CoxPd1 x quasi-alloy with coherence length Dcoh ¼12 nm. Annealing at 530 °C of continuous and porous FePd MLs resulted in the formation of fct L10 FePd alloy with preferable (111) orientation. Porous CoPd MLs revealed conservation of PMA with HC ¼1.2 kOe and MR/MS ¼0.5, slightly deteriorated with respect to continuous CoPd film due to advanced (U-shaped) surface morphology of CoPd antidot arrays. Arrays of FePd antidots after annealing at 530 °C revealed isotropic magnetic properties contrary to the in-plane magnetic anisotropy of flat FePd film. Such transformation could be associated with advanced morphology of FePd MLs deposited on the U-shaped cells of AAO template which favors local alignment of magnetic moments in perpendicular direction with respect to the film surface. The presented method of antidot fabrication allows enhancement of PMA for hard magnetic films with in-plane anisotropy and refinement of their magnetic domains into alternating magnetic dots with upward and downward directions for applications in PPM.
Acknowledgments Authors gratefully acknowledge Prof. F. Krok and Dr. Benedykt R. Jany from Jagellonian University for the SEM images of FePd samples. We also acknowledge the support from the National Science Centre (NCN, Poland) grant 2014/13/N/ST8/00731 and Belarusian State program “Functional materials”, project 2.4.8.
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Magnetic characteristics of CoPd and FePd antidot arrays on nanoperforated Al2O3 templates A. Maximenko a,b,n, J. Fedotova b, M. Marszałek a, A. Zarzycki a, Y. Zabila a a b
The Henryk Niewodniczanski Institute of Nuclear Physics Polish Academy of Sciences, Radzikowskiego Str. 152, 31-342 Krakow, Poland Research Institute for Nuclear Problems of Belarusian State University, Bobruiskaya Str. 11, 220030 Minsk, Belarus
art ic l e i nf o
a b s t r a c t
Article history: Received 22 June 2015 Received in revised form 5 August 2015 Accepted 13 August 2015
Hard magnetic antidot arrays show promising results in context of designing of percolated perpendicular media. In this work the technology of magnetic FePd and CoPd antidot arrays fabrication is presented and correlation between surface morphology, structure and magnetic properties is discussed. CoPd and FePd antidot arrays were fabricated by deposition of Co/Pd and Fe/Pd multilayers (MLs) on porous anodic aluminum oxide templates with bowl-shape cell structure with inclined intercellular regions. FePd ordered L10 structure was obtained by successive vacuum annealing at elevated temperatures (530 °C) and confirmed by XRD analysis. Systematic analysis of magnetization curves evidenced perpendicular magnetic anisotropy of CoPd antidot arrays, while FePd antidot arrays revealed isotropic magnetic anisotropy with increased out-of-plane magnetic contribution. MFM images of antidots showed more complicated contrast, with alternating magnetic dots oriented parallel and antiparallel to tip magnetization moment. & 2015 Elsevier B.V. All rights reserved.
Keywords: Antidots Percolated perpendicular media Co/Pd multilayers Ordered L10 FePd alloy Perpendicular magnetic anisotropy Porous anodic aluminum oxide templates
1. Introduction Nowadays there are a few novel recording concepts to increase the storage capacity of magnetic recording media. One of the most promising solutions for recording devices is the concept of bit patterned media which extends the areal density beyond 1 Tbit/in2 but requires the bit size and spacing of 12 nm which is a real challenge. This concept also meets problem related to superparamagnetic limit [1–3]. On the other hand an alternative concept of recording media, called percolated perpendicular media (PPM [1,4]), does not require such small grains. PPM consists of magnetic film with densely distributed nonmagnetic defects acting as pinning sites for magnetic domain walls (ordered arrays of antidots). Among many efforts to fabricate PPM a simple way is to deposit magnetic films on porous templates with ordered nanopore distribution (like anodized Al2O3, ZrO2, etc.). In this case a film with lateral arrays of holes (antidots) will be produced. The holes in such structure act as pinning sites and allow tuning the parameters of the magnetic films. CoPd and FePd magnetic films with intrinsic perpendicular n Corresponding author at: The Henryk Niewodniczanski Institute of Nuclear Physics Polish Academy of Sciences, Radzikowskiego Str. 152, 31-342 Krakow, Poland E-mail address: [email protected] (A. Maximenko).
magnetic anisotropy (PMA) are among the best candidates for application in designing of PPM. Perpendicular magnetic anisotropy in Co/Pd MLs originates from the interfacial anisotropy of the alloy-like structure [5,6] and demonstrates pronounced coercive field HC (over 2.1 kOe) [7,8]. Also, ferromagnetic L10 ordered FePd alloy is highly expected as forthcoming high-density recording material because it possesses a large uniaxial magneto-crystalline anisotropy of K 108 erg/cm3 [9–11]. The lattice parameters of bulk L10 are a ¼3.855 Å and c¼ 3.714 Å with small lattice distortion (c/a¼ 0.963) [12]. The ordered stoichiometric FePd L10 alloy is formed when the atomic concentration of Fe:Pd lies in the range (40–50):(60–50) [10].One of the simplest methods for FePd alloy formation is deposition of Fe and Pd multilayered thin films [9]. To obtain the FePd L10 phase (at room temperature the FePd alloy always crystallizes in A1 structure) the diffusion of the Fe and Pd atoms is necessary. It can be achieved by annealing of as-deposited films. For example, L10-ordered alloy could be manufactured from FePd MLs by conventional long thermal annealing at temperatures of about 450–600 °C [13–15]. Only a few papers focused on FePd antidot arrays and porous magnetic Co/Pd MLs with perpendicular anisotropy fabricated on porous templates are published up to now [16,17]. However, in these researches Co/Pd films were deposited on commercially produced anodized Al2O3 (AAO) templates with poorly ordered pores and large pore size distribution leading to the disordered antidot arrays. To improve magnetic performance of Co/Pd porous MLs uniform distribution of nanopores is required [1,4,18].
http://dx.doi.org/10.1016/j.jmmm.2015.08.057 0304-8853/& 2015 Elsevier B.V. All rights reserved.
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With respect to these issues, main objective of our work was to study the structure and magnetic properties of Co/Pd MLs and FePd L10 alloys fabricated on laboratory prepared AAO templates with improved ordering of the pores.
2. Experimental The procedure consists of two main processes: fabrication of nanoporous anodic alumina substrates with different pore diameter DP and subsequent deposition of magnetic layers on it. Nanoporous anodic alumina templates(2 cm 2 cm) were fabricated by two-step anodization of high purity unpolished aluminum foil (99.999%, Laurand Associates, Inc.) with thickness of 500 μm. To remove imperfections of crystalline Al lattice and increase its grain size facilitating self-organization process of the pores during anodization. the foil was annealed in air at temperature of 560 °C for 24 h. After that Al foil was chemically polished to attain a desired 10 nm surface smoothness. Two-step anodization in 0.2 M phosphoric acid at U¼190 V and T ¼0 °C is used for fabrication of the ordered nanoporous Al2O3 templates. In this process, thick aluminum oxide film obtained after the first long anodization is stripped away by treatment in solution of 1.8 wt% CrO3 þ6 wt% H3PO4 for 15 min at 75 °C and followed by subsequent re-anodization. The non-epitaxial Pd2 nm/[Fe4 nm/Pd6 nm]x4/Fe4 nm/Pd4 nm and Pd10 nm/[Co0.3 nm/Pd0.55 nm]15/Pd2 nm MLs were deposited on the AAO templates by thermal evaporation in ultra high vacuum at pressures below 10 9 mbar. Reference MLs of the same composition were fabricated on Si/SiO2 wafers. The film thickness was controlled during evaporation with a quartz thickness monitor. After deposition the FePd ML samples were annealed at 530 °C in vacuum ( 10 6 mbar) during 15 min with the slow heating rate 10 °C/min. The thickness of Fe and Pd layers corresponded to 45:55 atomic composition of an alloy which promotes L10 structure formation [10,11]. Surface characterization of AAO templates and MLs was performed by Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM). Top-view SEM images were obtained by Scanning Electron Microscope FEI Quanta 3D FEG. X-Ray Reflectivity (XRR) and X-Ray Diffraction (XRD) in θ–2θ geometry were performed by applying two-circle laboratory diffractometer (Panalyical X'Pert Pro) equipped with PW3050/60 vertical goniometer and PW3383/00 x-ray tube (2.2 kW, λCu(Kα) ¼1.54056 Å). XRR made on continuous reference MLs was used for verification of thickness and roughness of the films. XRR results were fitted using X'Pert Reflectivity software, while XRD patterns were analyzed using single line profile method. The hysteresis loops were measured on MPMS XL SQUID (Quantum Design) with magnetic field (up to 4 T) parallel and perpendicular to sample surface (inplane geometry H// and out-of-plane geometry H⊥, respectively). The values of multilayer magnetization were corrected by subtraction of the contribution from the diamagnetic substrates. Topography analysis by Atomic Force Microscopy (AFM) and magnetic domain imaging by Magnetic Force Microscopy (MFM) were performed with XE-120 Scanning Probe Microscope (SPM) of Park System using MFMR cantilevers (NanoWorld AG) covered with 40 nm thick hard magnetic cobalt alloy and with permanent tip magnetization direction along the tip axis. The radius of tip curvature was 50 nm. SPM scans were performed at ambient conditions. Before MFM scans samples were demagnetized by applying a diminishing alternating magnetic field along the easy axis of magnetization. In this study the MFM images were acquired after topography measurements with tip at a constant 200 nm height.
3. Results and discussion 3.1. Morphology and phase composition The sketch of AAO cross-section illustrating typical bowl-shape cell structure with inclined intercellular region and one pore in the center is shown in Fig. 1. Due to imperfect round shape of nanopores the dimensions of pores were characterized with two parameters, Feret's diameter DF and equivalent diameter DE. DF is the longest distance between any two points along the selection boundary, while DE is calculated from the mean area of the pores using the assumption of perfect circular pores. Estimated from SEM images values of morphological features in the initial AAO template and as deposited CoPd and FePd MLs are summarized in Table 1. SEM images of FePd MLs deposited onto the AAO template reveal porous films with perfectly ordered hexagonal arrangement of nanopores. The size of fabricated antidots is equal to initial pore diameter of used AAO templates. Combination of imaged obtained by second electrons (sensitive to topography) and by back-scattered electrons (sensitive to chemical composition) revealed very homogeneous structure of arrays of FePd antidots. The same distribution of the material was observed on SEM images (not shown here) for CoPd antidots prepared by deposition of CoPd MLs over AAO template. Modification of the morphology of Fe/Pd antidots after annealing at 530 °C in comparison to as deposited MLs is illustrated by SEM images shown in Fig. 2. SEM image evidences the melting of MLs, the material is moved in the direction of cell borders leading to antidot diameter increase (Table 1), and some material is left on the borders of the pores. The XRR patterns of as deposited Fe/Pd MLs (Fig. 3) showed typical Kiessig fringes arising from the finite thickness of the films, and Bragg peaks at θ ¼0.64° and θ ¼1.43° (Fig. 3), originating from a periodicity of the bilayers characteristic for the well-ordered thin MLs. The XRR patterns of as deposited MLs were fitted by Parrat algorithm [19] based on the model of nominal composition of MLs, while samples annealed at 530 °C, fitted with the same algorithm, assumed the intermixing of MLs. During the fitting procedure interface roughness R and thickness t of the layers were fitted independently. The obtained fit parameters are collected in Table 2. For Fe and Pd layers the table does not show the R and t values of
Fig. 1. The schematic image of bowl-like cell structure of porous AAO template. The black area corresponds to pores surrounded by bowl-like shapes with protrusions in the place of contact between different cells.
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Table 1 Nanopore diameters DE and DF for AAO template, FePd and Co/Pd MLs deposited on AAO, annealed FePd MLs. Sample
DE, nm
DF, nm
AAO template FePd and Co/Pd as deposited antidots FePd antidots annealed at 530 °C
177.5 7 4.2 174.5 7 4.7 237.4 7 8.2
186.3 73.8 185.0 74.8 267.3 710.9
each layer separately, but presents mean values averaged over four repetitions of the layers. Taking into account that the accuracy of thickness measurement by quartz microbalance is never better than 5% a good agreement with nominal values of thickness was obtained. For the samples annealed at 530 °C we assumed that the whole stack of Fe/Pd films consists of FePd alloy with thickness equal to the total thickness of deposited multilayered system. Results of XRR pattern fits reveal significant increase of the R values for all interfaces, approximately 20% of the Fe and Pd layers thickness evidencing pronounced intermixing at the interfaces. XRR analysis of continuous Si/SiO2//Pd/[Co0.3 nm/Pd0.55 nm]/Pd MLs (not shown here) revealed that R values of Co and Pd interfaces are generally larger than the nominal thickness of films. This should be associated with intermixing of Co and Pd layers during the deposition resulting in the formation of CoxPd1 x quasi-alloy [5,6]. Therefore we assumed that the whole CoPd multilayer constitutes CoxPd1 x quasi-alloy with thickness equals to the thickness of 15 deposited CoPd bilayers. Parameters t, ρ and R extracted from fitted XRR patterns are collected in Table 3. XRD patterns of as deposited FePd continuous films and antidots are shown in Fig. 4a. XRD pattern of continuous film shows three reflections associated with the Si/SiO2 substrate, at 2θ ¼ 26.76° and 32.86° (Si) and at 2θ ¼ 33.90° (SiO2). Two broadened peaks observed at 2θ ¼39.5–40.0° and at 2θ ¼40.5–41.0° were fitted with a pseudo-Voigt function and associated with (111) Pd layers [20] and (111) Fe/Pd multilayers [21]. Reflection at 2θ ¼ 43.58° is assigned to pure Fe [22]. XRD patterns of as deposited FePd antidots are basically characterized with reflections similar to those of the flat FePd reference film. The additional peaks at 2θ ¼ 38.38° and 2θ ¼44.68° are associated with Al foil substrate. After annealing at 530 °C XRD patterns of both reference film and FePd antidots (shown in Fig. 4b) exhibit the appearance of several reflections at angles summarized in the Table 4. These peaks are identified as the reflections from (001), (110), (111), (200) and (002) planes of face-centered-tetragonal (fct) L10 FePd alloy [9,12]. The presence of the (001) reflection, prohibited in the fcc disordered alloy, indicates that the unit cell of the alloy formed during the annealing changed the symmetry and became
Fig. 3. The XRR patterns of the flat as deposited and annealed at 530 °C Fe/Pd MLs. The curves are shifted vertically for clarity.
Table 2 Thickness t, interface roughness R and density ρ for XRR patterns of reference continuous and annealed at 530 °C Fe/Pd films fitted with Parrat algorithm. Layer
ρ, g/cm3
t, nm
R, nm
As deposited Pd o Fe4 o Pd4 Fe Pd
12.38 7.80 11.77 7.64 11.72
2.0 4.1 5.8 3.8 4.0
0.9 1.1 0.8 1.2 1.1
Annealed 530 °C FePd
10.39
46.5
1.5
Table 3 Thickness t, interface roughness R and density ρ for XRR patterns of reference continuous CoPd films fitted with Parrat algorithm (for tPd ¼ 0.55 nm total thickness of multilayer 12.75 nm). Layer
ρ, g/cm3
t, nm
R, nm
Pd CoPd Pd
11.78 10.89 11.88
10.9 12.5 2.3
0.5 0.5 1.4
Fig. 2. SEM images of FePd MLs deposited on AAO template: (a) as deposited Fe/Pd MLs and (b) Fe/Pd MLs after annealing at 530 °C.
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Fig.5. XRD patters of continuous Si//SiO2//[Co0.3 nm/Pd0.55 nm]/Pd (a) and porous Al2O3//Pd/[Co0.3 nm/Pd0.55 nm]/Pd and (b) films.
Table 5 Structural data obtained by X-ray diffraction analysis of CoPd continuous and porous films: a – lattice parameter, Dcoh – the coherent scattering length corresponding to grain size. Film
a (Å)
Dcoh (nm)
Fig. 4. XRD patterns of as-deposited (a) and annealed at 530 °C (b) Fe/Pd multilayers deposited on the Si/SiO2 substrate and on AAO.
Table 4 FePd peaks positions (2θ) and structural data obtained by X-ray diffraction analysis of reference and porous films annealed at 530 °C. a and c-lattice parameters, c/a – lattice distortion, the mean coherence lengths oDcoh 4 obtained from the XRD patterns for the flat films and antidots of FePd alloy annealed at 530 °C.
(001) (110) (111) (200) (002) a, Å c, Å c/a oDcoh 4, nm a
Reference film
Antidots
24.357 0.06 – 41.23 70.06 47.417 0.06 49.747 0.08 3.831 70.005 3.711 70.015 0.97a 18.5 7 3.8
24.150 70.06 32.711 7 0.07 40.94 70.06 46.777 0.06 49.43 7 0.08 3.885 70.005 3.696 70.015 0.95 17.7 7 3.9
The value of c/a for bulk FePd alloy [23] is 0.97.
tetragonal. This is also confirmed by the separation of the (200) and (002) peaks. Calculated lattice constants a, c and the distortion c/a of fct crystalline lattice of FePd alloy are summarized in Table 4. Large values of c/a both for reference and porous films indicate large distortion of the crystallographic cell, which together with superstructure peaks in diffraction pattern support the formation of L10 phase after annealing at 530 °C. Experimental XRD patterns of continuous and porous CoPd MLs are presented in Fig. 5. The patterns of continuous MLs show only broad overlapped reflections that were deconvoluted by pseudoVoigt functions into two contributions with position of peaks at 2θ ¼ 40° for Pd buffer/cap, and 2θ ¼ 40.7–41.1° for CoPd MLs. Lattice parameters for both components are calculated based on approach proposed in [24] assuming the formation of CoxPd1 x
3.8777 0.002 3.888 7 0.005
Dcoh (nm)
CoxPd100 x
Pd Continuous Porous
a (Å)
7.17 0.5 9.5 7 1.4
3.8017 0.002 3.808 7 0.005
12.2 71.6 11.4 73.0
quasi-alloy with cubic fcc structure [24]. The obtained values (see Table 5) well correspond to (111) peak of Pd [18] and (111) peak of CoxPd1 x quasi-alloy [24–26]. Similar lattice constants a calculated for continuous and porous films indicate very close composition of CoxPd1 x quasi-alloy. From the width of Pd, FePd and CoPd reflections in XRD patterns of continuous and porous films the coherent scattering length Dcoh was estimated with Scherrer equation [27]. It is seen that oDcoh 4 for porous FePd and CoPd is 17–18 nm and 8–12 nm, respectively. One can see that the coherent scattering length for antidots does not change significantly in comparison to continuous films. 3.2. Magnetic properties Fig. 6 shows magnetization loops M(H) for continuous and porous CoPd ML film and FePd ML film annealed at 530 °C. The values of coercive field (HC), remanent magnetization (MR) and saturation magnetization (MS) (or magnetization at Hmax ¼ 2.0 kOe) determined from these curves are summarized in Table 6. The magnetization loops of CoPd and FePd systems were normalized to the mass of Co and Fe, respectively. Almost perfect rectangular shape of М(Н) dependence for reference CoPd films for perpendicular geometry shows that easy axis of magnetization is along the normal to the surface with squareness of the loop MR/MS ¼0.97. Additionally, the film shows very large perpendicular coercive field H⊥C equal to2.2 kOe, demonstrating its prominent PMA. Noticeably, PMA is conserved in CoPd antidots, but with reduced values of MR/MS (up to 0.5) and coercive field HC of approximately 1.2 kOe. Some deterioration of PMA is also confirmed by M(H) curves for continuous and nanoporous CoPd films in hard magnetization direction which demonstrated enhanced contribution of in-plane magnetic anisotropy at the expense of PMA. It can be explained by hemispherical distribution of the material on the U-shaped [28] surface of AAO templates with advanced morphology.
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Fig. 6. Magnetization loops M(H) for reference and porous CoPd MLs (a) and FePd MLs after annealing at 530° C (b). Table 6 Magnetic properties of CoPd films and FePd alloy films annealed at 530 °C. HC – coercive field, MR – remanent magnetization, МS – saturation magnetization. Samples
MS, emu/g
HC, kOe H//
MR/MS
HC, kOe H⊥
MR/MS
Continuous CoPd CoPd antidots Continuous FePd FePd antidots
235 210 260 325
0.36 0.83 2.30 2.40
0.13 0.26 0.67 0.52
2.20 1.20 1.80 2.00
0.97 0.50 0.24 0.45
Magnetization loops of FePd alloy annealed at 530 °C showed very different behavior (see Fig. 7). The reference film has in plane easy axis, but HC is about 2.0 kOe, whereas antidot arrays show isotropic magnetization distribution with HC value very close to this of continuous films. Such change from in-plane magnetic anisotropy for flat film towards isotropic magnetic properties occurring for antidot arrays could be associated with advanced morphology of the antidot surface. Homogeneous deposition of FePd film on U-shaped cells of AAO templates enhances contribution of magnetic moments locally oriented in perpendicular
Fig. 7. MFM images of reference CoPd (a), CoPd antidots (b), and morphology image of CoPd antidots (c); MFM images of annealed reference FePd film (d), annealed FePd antidots (e), and morphology image of annealed FePd antidots (f).
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direction with respect to the film surface. Subsequent annealing at 530 °C promotes increase of HC due to formation of L10 phase seen by XRD. At the same time M(H) curve of annealed FePd film is characterized with high in-plane magnetization contribution most likely due to little degree of ordering and lack of (001) texture preferable for clear PMA in FePd alloys [9,12,29,30]. The MFM images of reference and nanoporous CoPd film show distinct differences (Fig. 7a–c). The contrast in reference image has maze-like typical domain pattern with an average size of domains of a few hundred nm. In contrary, the pattern for porous sample shows the diminished domains which surround the pores as seen from comparison of topography and phase images. The dark spots in MFM image are pores exhibiting the enhanced signal due to the presence of stray fields coming from small domains distributed around the pores. In this case magnetic domains of antidots reflect the lines of the pores. The reference FePd film (Fig. 7d) displays the magnetic domains of similar character as for CoPd film but of bigger size. The corresponding MFM image for nanoporous FePd film exhibits clusters of dark and bright spots of the size corresponding to the pore size separated by the areas of mild contrast. It was already demonstrated by hysteresis loop measurements that in FePd alloy the magnetization distribution is almost isotropic which is reflected in MFM image. The in-plane component of magnetization makes contribution to the areas characterized by weak contrast while the out-of-plane component is focused on pores either in up direction or in down direction. In this case the complex morphology of nanoporous template resulted in appearance of magnetization component perpendicular to the plane.
4. Summary CoPd and FePd multilayered antidot arrays with perfect hexagonal ordering were fabricated by deposition of films on AAO templates with DE ¼ 180 nm. Phase analysis of as-deposited continuous and porous CoPd MLs indicated formation of fcc CoxPd1 x quasi-alloy with coherence length Dcoh ¼12 nm. Annealing at 530 °C of continuous and porous FePd MLs resulted in the formation of fct L10 FePd alloy with preferable (111) orientation. Porous CoPd MLs revealed conservation of PMA with HC ¼1.2 kOe and MR/MS ¼0.5, slightly deteriorated with respect to continuous CoPd film due to advanced (U-shaped) surface morphology of CoPd antidot arrays. Arrays of FePd antidots after annealing at 530 °C revealed isotropic magnetic properties contrary to the in-plane magnetic anisotropy of flat FePd film. Such transformation could be associated with advanced morphology of FePd MLs deposited on the U-shaped cells of AAO template which favors local alignment of magnetic moments in perpendicular direction with respect to the film surface. The presented method of antidot fabrication allows enhancement of PMA for hard magnetic films with in-plane anisotropy and refinement of their magnetic domains into alternating magnetic dots with upward and downward directions for applications in PPM.
Acknowledgments Authors gratefully acknowledge Prof. F. Krok and Dr. Benedykt R. Jany from Jagellonian University for the SEM images of FePd samples. We also acknowledge the support from the National Science Centre (NCN, Poland) grant 2014/13/N/ST8/00731 and Belarusian State program “Functional materials”, project 2.4.8.
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Please cite this article as: A. Maximenko, et al., Journal of Magnetism and Magnetic Materials (2015), http://dx.doi.org/10.1016/j. jmmm.2015.08.057i