Growth rate dependence of the extrinsic magnetic properties of electrodeposited CoPt films

ARTICLE IN PRESS Journal of Magnetism and Magnetic Materials 322 (2010) 1576–1580

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Growth rate dependence of the extrinsic magnetic properties of electrodeposited CoPt films M. Ghidini a,, A. Lodi-Rizzini a, C. Pernechele a, M. Solzi a, R. Pellicelli a, G. Zangari b, P. Vavassori c,d a

Department of Physics, University of Parma, Parco Area delle Scienze 7/A, 43100 Parma, Italy Materials Science and Engineering and CESE, University of Virginia, Charlottesville, USA c Department of Physics, University of Ferrara, Via G. Saragat, I-44100 Ferrara, Italy d CIC nanoGUNE Consolider, Mikeletegi Pasealekua 56, E-2009 San Sebastian, Spain b

a r t i c l e in f o

a b s t r a c t

Available online 9 September 2009

The magnetic properties and the magnetization process of electrodeposited thick films of Co-rich CoPt alloys are studied with particular emphasis on the effects of growth rate, controlled by varying the plating current density, and of lateral confinement, analyzing patterned micro-cylinders. We find that varying the plating current density has virtually no effect on the composition of the samples, and hence on the intrinsic magnetic properties, a substantial increase of both coercivity and squareness is obtained when the current is raised. The films are fine-grained, oriented polycrystals with typical grain sizes in the range 50–150 nm, depending on the growth rate. The complex magnetization process is studied in detail by Magnetic Force Microscopy and shown to be governed by interaction domains. It is shown that further improvement of the squareness can be obtained by exploiting the lateral confinement in patterned samples. & 2009 Elsevier B.V. All rights reserved.

Keywords: Hard Magnetic Materials Magnetic Force Microscopy Magnetization Reversal

1. Introduction Both hard magnetic films and nanostructures are of interest in the race towards low noise, ultra-high-density, magnetic recording media, and for the development of micro-magnets for microelectro-mechanical systems (MAG-MEMS) [1]. However, for obvious reasons, the thickness ranges involved in the two cases are extremely different: while nanometer thick films are required for magnetic recording media, films in the micron range are required for MAG-MEMS. In the latter case, one difficulty is that the optimum materials performance must be conjugated with its integration into the overall processing of the micro-system [2]. In fact, good hard magnetic properties are normally obtained in high anisotropy crystals, in which domain wall motion is inhibited by defects and inhomogeneities. The combination of high magnetocrystalline anisotropy with a suitable microstructure can be realized with relative ease in thin magnetic films, but is hard to achieve when growing films with thickness of several microns, as needed in micro-actuators. Moreover, the synthesis of high anisotropy phases such as those in NdFeB, Co–Sm compounds, and the high anisotropy phases of Fe–Pt and Co–Pt would in most cases require post-deposition annealing at relatively high temperatures. This circumstance raises an issue regarding the

compatibility of thermal treatments with the overall fabrication route. It has been demonstrated that Co–Pt films (Pt  20 at%) with good hard properties, high coercivity and strong perpendicular anisotropy, could be directly obtained in the as-deposited state by electrodeposition using various substrates [3,4] and over a wide range of thickness (5 nm–2 mm) [5]. The method yields polycrystalline films with a highly oriented hexagonal-close-packed (hcp) structure with no need for post-deposition treatments, which implies that patterning procedures for nano-structuring would require only additive processes. Electrodeposition can maintain hard magnetic properties at high thickness since suitable electrolyte chemistries in fact can induce precipitation of non-magnetic phases at grain boundaries [6], which inhibit grain growth and break exchange coupling among grains. Suitable electrolyte Co–Pt alloys with high anisotropy and with thickness of up to 10 mm have in fact been grown by electrodeposition [7]. In this paper, we analyze the effect of the growth rate and of lateral confinement on the magnetic properties and processes of Co–Pt films. The growth rate is controlled by varying the plating current density.

2. Experiment  Corresponding author.

E-mail address: massimo.ghidini@fis.unipr.it (M. Ghidini). 0304-8853/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2009.09.011

Co-rich Co–Pt films were prepared by electrodeposition onto Si(0 11)/Cu(111)/Ru(0 0 0 1) templates, using an alkaline

ARTICLE IN PRESS M. Ghidini et al. / Journal of Magnetism and Magnetic Materials 322 (2010) 1576–1580

citrate–glycinate electrolyte [3,4]. The (0 0 0 1)-oriented Ru was prepared by sequential sputtering of 200 nm Cu and 20 nm Ru onto hydrogen-terminated Si(0 11). Plating current densities in the range 10–100 mA/cm2 were utilized. We report results concerning films of constant thickness of 1 mm deposited at: 10 mA/cm2 (sample A), 50 mA/cm2 (sample B), and 100 mA/cm2 (sample C). Patterned micro-magnets in the form of cylinders, with a diameter of 1 mm, were deposited at 50 mA/cm2 by deposition onto pre-patterned substrates. The sample structure and morphology have been studied by X-ray diffraction and Atomic Force Microscopy (AFM). Magnetic properties and processes were studied by means of a Quantum Design MPMSXL-5 SQUID magnetometer. Magnetic Force Microscopy in Lift Mode was used to image both the virgin magnetic states and the easy-axis magnetization processes. Typically, a lift height of 50 nm was used. Tips covered with a 40-nm-thick CoCr layer for standard applications were mounted on a Nanoscope III (Digital Instruments) microscope. The frequency shift of the tip, due to vertical magnetic force gradients, was detected by the phase detection method.

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In fact, such measurement configuration yields almost completely reversible loops, with negligible remanence and coercivity. From the linear portion of the loop, we have extrapolated the value of the first-order anisotropy constant, K1 = 0.35 M J/m3. The spontaneous magnetization was measured to be 900 emu/cm3 (0.9 MA/m), within experimental error, in all samples. The onset of irreversible demagnetization processes, associated with irreversible domain displacements, takes place in all the films at reasonably well-defined critical fields in the first quadrant. The processes are characterized by a quasi-linear demagnetization rate, which appears to be the same in all cases, resulting in characteristic sheared loops, with almost parallel branches. However, the behavior of samples A and C presents some additional features with respect to sample B. In the latter case, the first departure from saturation coincides with the onset of the irreversible processes, yielding a quite simple loop. In samples A and C, demagnetization develops in a more complex way. In sample A the first departure from saturation, which takes place at about 0.56 MA/m (7 kOe) as in sample B, is followed by a steep drop of the magnetization, after which a further change in slope sets the demagnetization rate equal to the one of the samples C

3. Results Fig. 1 summarizes the effects of the deposition current density and lateral confinement on the magnetic properties of the samples under study. The hysteresis loops, measured for samples A, B, and C with the field applied perpendicular to the film plane, display significant modifications. In particular, coercivity is observed to double from 0.16 MA/m (2 kOe) to 0.32 MA/m (4 kOe) when the deposition current density is varied from 10 to 50 mA/cm2, accompanied by a similar increase in remanence, which varies from a minimum of 0.17 to a maximum of 0.38 (inset of Fig. 1), while only slight positive variations take place when current density is increased further to 100 mA/cm2. A further strong increase in remanence can be obtained by reducing the demagnetizing field of the film, in the patterned sample. A value of S =0.67 is obtained, indicating that magnetostatic interactions between dots are relatively small. It is worth noting that no significant modification of the loops measured along the hard direction (field applied parallel to the film plane) is observed.

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Fig. 1. Hysteresis loops measured with the field applied perpendicular to the film plane for the samples: A (crosses), B (black circles), C (open triangles), and for the patterned sample deposited at 50 mA/cm2 (open circles). Hysteresis loop measured with the field in-plane for sample B. Inset: current density dependence of coercivity (open circles) and squareness (closed circles).

Fig. 2. (a, c, e) AFM imaging of the samples A, B, and C, respectively. Z-scales are 0–80 nm for sample A and 0–50 nm for samples B and C, respectively; (b, d, f) MFM imaging of the virgin states for the samples A, B, and C, respectively. All scan areas are 5  5 mm2.

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and B. On the other hand, in sample C a progressive change in slope is observed around a higher critical field, followed by a quasi-linear, reversible, decrease down to the second critical field, at which dominant irreversible demagnetization processes start. It is worth noting that this critical field is substantially lower (0.4 MA/m, i.e., 5 kOe) than in samples A and B. This process is reminiscent to what observed at varying angles with an external field [5] in films of intermediate thickness, and therefore can be thought to be associated with an increased amount of disalignment present in sample C. We conjecture that the initial quasi-linear decrease is associated with the nucleation of a surface closure domain pattern. As a matter of fact, irreversible domain wall displacement takes place at the same rate in all cases, indicating that very similar reversal mechanisms are in place. Coercivity is lower in sample A because of the enhanced drop of magnetization at the critical field, a circumstance which is very probably due to the fact that in a fraction of the grains the nucleated non-uniform magnetization configuration can more easily develop than in the other samples. Typically, this could beascribed to the presence of a fraction of grains with sizes bigger than the single domain critical size. In summary, inspection of the results of the basic magnetic characterization of the samples shows that current density has an effect only on extrinsic magnetic properties, while it substantially leaves unchanged the intrinsic magnetic properties as spontaneous magnetization and magneto-crystalline anisotropy energy density. It can therefore be inferred that while the increasing growth rate has an effect on the microstructure and its defect density, it does not affect significantly either the alloy composi-

tion or the immediate crystalline environment of the magnetic atoms, as confirmed by X-ray diffraction and EDS [8]. Fig. 2(a, c, e) reports the morphology of the samples A, B, and C as observed by AFM in tapping mode. The apparent size of the grains diminishes for increasing the current density. Grain size is in the range of 120–150 nm for sample A, 60–80 nm for sample B, and 40–60 nm for sample C. It should be noted that sample A displays coarser grains, with an apparently broader grain size distribution function, with a small fraction of the grains with sizes bigger than the estimated single domain critical size (see next paragraph). The observation of a coarser grain size in sample A supports the ideas discussed above concerning its reversal process. Fig. 2(b, d, f) reports results of MFM imaging as obtained in the as-deposited samples. The magnetic domain structures are complex, involving bubbles, with a tendency to coalesce, islands and stripes with some disorder. All images reveal, after inspection of the pixels histograms, that the domain structure is described by a continuum of gray levels, indicating that contrast is irregular and strongly varying within these domains, a circumstance which would be hardly compatible with the magnetization pointing up or down, perpendicular to the film surface. Moreover, it should be noted that the features of contrast in samples B and C appear to be substantially larger than in the case of sample A, especially when one compares the size of the bubbles and the width of the stripes. Given that the samples have equal thickness and display the same intrinsic magnetic parameters, we ascribe this effect to the establishment of interaction domains [9]. This effect, usually witnessed by irregular magnetic microstructures, with many

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Fig. 3. Virgin magnetization curves for increasing maximum fields for the samples deposited at 10 mA/cm2 (sample A), 50 mA/cm2 (sample B), and 100 mA/cm2 (sample C).

ARTICLE IN PRESS M. Ghidini et al. / Journal of Magnetism and Magnetic Materials 322 (2010) 1576–1580

similarities to the present ones, is known to occur in fine-grained magnets, when the size of the grains reaches the single domain limit, and the fine grains constituting the material become strongly correlated via exchange or magnetostatic interactions. It is worth noting that interactions domains are also known to occur in elongated single domain magnets, in which exchange interaction between grains can be absolutely ruled out [10]. In fact, the single domain critical diameter for the present films can be pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi estimated by the formula Dc ¼ 36 AK1 =m0 Ms2 [11], yielding about 120 nm, a value which is slightly above or comparable with grain sizes as determined by AFM. Fig. 3 reports virgin loops for the samples. The initial magnetic states of each curve were prepared by the same classical degaussing process, which consisted in applying fields of logarithmically decreasing amplitude and

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alternatively opposite directions starting at 2.4 MA/m (30 kOe) (the factor of attenuation at each cycle was 0.9). The magnetization process appears to be qualitatively similar in the three samples, with negligible susceptibility until a critical field, of about the same magnitude as the coercive field, is reached, marking the onset of irreversible processes. This implies that domain walls are not free to move, as in finegrained magnets [12]. We have imaged the remanent states obtained after following the virgin magnetization curves at increasing maximum fields. Fig. 4(a, c, e) reports the images obtained on the demagnetized state, and after application of 0.48 and 1.44 MA/m fields, respectively (in this case 20  20 mm2 scan areas are employed). We have found that no evident modification of the domain structure could be observed for small and intermediate fields. In the image obtained after application of a 1.44 MA/m field, the magnetic contrast increases strongly and appears to be more concentrated in more well-defined irregular regions. A striking feature of the process is that the pixel histogram does not change significantly with respect to the demagnetized state: the domain structure is again described by continuum of gray levels and the area occupied by dark and clear domains remain the same, while a strong unbalance would be expected in view of the 40% remanence observed by SQUID. This observation is consistent with the distinctive feature of interaction domains, which is to have an elongated shape along the easy axis of the material and to expand primarily along it, while lateral movements are generally small and irregular [13]. This seems to be the case in the present films, in which easy axis is normal to image plane and domain wall movements take place in the bulk of the film, as made evident by the strong increase of magnetic contrast observed in Fig. 4e. A meaningful comparison can be made with the magnetization process of the patterned sample, grown under the same conditions as sample B and thus displaying the same morphology. Fig. 4(b, d, f) report the evolution of magnetic contrast from the ac demagnetized state along the virgin curve by applying increasing maximum fields (scan areas 5  5 mm). The images taken after applying fields of 0.24 and 0.72 MA/m are reported. A clear evolution of the domain structure inside each plot is observed, with the dark domains oriented along to the field expanding. After application of 0.72 MA/m, which is above the saturation field of the patterned samples (see Fig. 1), the dots have been almost completely magnetized and appear to be single domain, with the exception of the borders, where non-uniform magnetic configuration are still visible. Consistently, strong stray field gradients set up in the void region between the cylinders, as shown by the strong positive phase shifts detected, opposite to the one associated with the dots. It can be concluded that, the mobility of interaction domains is enhanced, yielding higher remanence, when the demagnetizing field of the film is strongly reduced due to lateral confinement, suggesting an obvious strategy for further optimization of the extrinsic properties of CoPt films. References

Fig. 4. Left: MFM imaging of the film deposited at 50 mA/cm2 (sample B) as obtained after ac demagnetization (a), ac demagnetization and application of a 0.48 MA/m (6 kOe) perpendicular field (c) and ac demagnetization and application of a 1.44 MA/m (18 kOe) perpendicular field. All scan area 20  20 mm2, phase shifts are represented on a scale 0–51 for all images. Right: MFM imaging of the remanent states of the patterned sample (cylinders deposited at 50 mA/cm2) as obtained after ac demagnetization (a), ac demagnetization and application of a 0.24 MA/m (3 kOe) perpendicular field (c) and ac demagnetization and application of a 0.72 MA/m (9 kOe) perpendicular field.

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[7] O. Berkh, Y.u. Rosenberg, Y. Shacham-Diamand, E. Gileadi, Electrochem SolidState Lett 11 (2008) D38–D41. [8] G. Zangari, unpublished. [9] W. Rave, D. Eckert, R. Schafer, B. Gebel, K.H. Muller, IEEE Trans. Magn. 32 (1996) 4362. [10] D.J. Craik, E.D. Isaac, Proc. Phys. Soc. 76 (1960) 160.

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