Characterization of Nanocrystalline Permalloy Thin Films ... - CiteSeerX

IEEE TRANSACTIONS ON MAGNETICS, VOL. 44, NO. 11, NOVEMBER 2008

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Characterization of Nanocrystalline Permalloy Thin Films Obtained by Nitrogen IBAD P. Prieto1 , J. Camarero2 , J. F. Marco3 , E. Jiménez2 , J. M. Benayas1 , and J. M. Sanz1 Departamento de Física Aplicada and Instituto de Ciencia de Materiales “Nicolas Cabrera,” Universidad Autónoma de Madrid, Cantoblanco, 28049-Madrid, Spain Departamento de Física de la Materia Condensada and Instituto de Ciencia de Materiales "Nicolas Cabrera" and Instituto Madrileño de Estudios Avanzados en Nanociencia IMDEA-Nanociencia, Campus Universidad Autónoma de Madrid, 28049 Madrid, Spain Instituto de Química Física Rocasolano-CSIC, c/serrano 119, E-28006 Madrid, Spain We report on the structural changes and the corresponding magnetic effects induced by nitrogen ion beam assisted deposition (IBAD) of Fe20 Ni80 thin films. The films have been prepared by dual ion beam sputtering using a controlled mixture of Ar+ and N+ 2 ions in the ion beam used to assist the deposition. The structure, composition, and magnetic properties of the films have been studied by X-ray diffraction, Mössbauer spectroscopy, resonant Rutherford backscattering spectroscopy (RBS), and vectorial Kerr magnetometry. It has been observed that the presence of Ar+ ions in the assistant beam induces the formation of a nanocrystalline structure. It has also been observed the expected dependence of the coercivity with the crystallite size in the ferromagnetic samples. However, the presence of small amount of paramagnetic -FeNi3 N, as determined by Mössbauer spectroscopy, leads to a significant increase of the coercivity. Finally, it is also observed that the reduction of the average crystallite size as well as the presence of small amount of paramagnetic -FeNi3 N causes an important change in the magnetization reversal mechanism. Index Terms—Magnetization reversal, Mössbauer spectroscopy, soft magnetic films.

I. INTRODUCTION iFe alloys are widely used in magnetic recording media and sensor industry. In particular, Permalloy (Fe Ni ) is well known due to its wide used in the magnetic recording industry because of its high permeability, low coercivity, and small magnetic anisotropy. Nevertheless, to improve the performance of the Permalloy-based devices, a further optimization of its magnetic properties is usually required, e.g., higher ferromagnetic resonance frequency (FMR) [1] for RF integrated circuits applications, or higher saturation magnetic flux density for applications in recording heads. In fact, several efforts have been performed to optimize the magnetic properties of Permalloy thin films by patterning the films [1] or by adding a third element to the FeNi alloy, for example Co [2]. It is also well known that the incorporation of nitrogen by sputtering processes on metals induces the formation of amorphous or fine-grained structures [3]. In fact, iron nitride compounds have been extensively studied because of their excellent magnetic properties in combination with a significantly improved corrosion and wear resistance over pure iron [4]. The incorporation of small amounts of nitrogen results in the formation of a ferromagnetic compound ( -Fe N) with higher magnetic induction and lower coercivity than that of pure -Fe [5], usually associated with a reduction of the average grain size. Several works [6]–[8] have been dedicated to explore the structural and magnetic properties of the Fe–Ni–N system in order to improve the thermal stability but also the magnetic and other

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Digital Object Identifier 10.1109/TMAG.2008.2002483 Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org.

properties of the iron nitrides [6], [7]. Recently, R. Gupta et al. [8] have reported the fabrication of nanocrystalline Fe Ni thin films using nitrogen reactive magnetron sputtering and have studied the mechanisms that induce their nanocrystallization and amorphization. The aim of this work is to explore the structural and magnetic effects induced in Permalloy thin films, deposited by ion-beam sputtering assisted by nitrogen ion bombardment, after the incorporation of nitrogen either as an interstitial atom within the Permalloy lattice of by forming a nitride. II. EXPERIMENTAL Nanocrystalline Fe Ni –N films have been deposited on Si (100) substrates at 200 C using a dual ion-beam sputtering system. The films were obtained by ion beam sputtering of a Fe Ni target under simultaneous bombardment of the growing film with a controlled mixture of low energy argon and nitrogen ions. For sputtering we have used an Ar beam of 500 eV and a total current density of 10 mA. The deposition rate 0.02 nm/s, as determined by a quartz crystal monitor. was The assisting ion beam was composed of a controlled mixture of Ar and N ions of 55 eV energy and a current density of 0.045 mA/cm . The structure of the films was analyzed using X-ray diffraction (XRD) at a grazing incidence of 1 with respect to the surface plane using CuK radiation ( 0.15408 nm) in a Siemens-5000 diffractometer. The linewidth at half maximum of the diffraction peak, B, has been used to estimate the crystallite size of the specimen in the direction perpendicular to the /Bcos plane of the films using the Scherrer relationship , where d is the diameter in angstroms, is the Bragg angle and is the wavelength of the X-rays [9]. The average composition of the thin films has been studied in terms of resonant RBS (i.e., Rutherford backscattering spec-

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Fig. 2. Average grain sizes as a function of the composition of the assistant beam. Fig. 1. Grazing incidence XRD diffraction patterns of Fe Ni thin films obtained by dual ion beam deposition with different concentrations of N ions in the assistant beam.

troscopy) using an incident beam of 3.7 MeV 4He ions. Integral Conversion Electron Mössbauer Spectroscopy (ICEMS) has also been used to characterize the films. ICEMS spectra were obtained using a Co(Rh) source and a Parallel Plate Avalanche Counter (PPAC). The spectra were recorded at room temperature in the constant acceleration mode. All the spectra were computer fitted and the isomer shifts are referred to the centroid of the spectrum of -Fe at room temperature. The magnetic anisotropy and the magnetization reversal process of the films were studied by Vectorial Kerr magnetometry at room temperature. The incident light on the sample was p-polarized whereas the two orthogonal components of the reflected light were simultaneously detected to measure the in-plane parallel components of the magnetiza(M ) and perpendicular tion with respect to the direction of the applied magnetic field [10]. III. RESULTS AND DISCUSSION Fig. 1 shows the XRD patterns of a nonassisted Fe Ni thin films and several Fe Ni thin film assisted during the deposition with different concentrations of %N ions in the assistant beam, as labeled in the figure. The nonassisted Permalloy shows reflections assigned to (111), (200), and (220) planes of the face-centered cubic Permalloy phase. As the N ions assistance increases up to 50%, the diffraction peaks start to broaden but not significant shifts are observed. For N concentrations 75%, a complete change of the structure is noticed and the (111), (200), and (220) reflections of fcc -Ni FeN are now observed in the corresponding XRD diffraction pattern. Fig. 1 also shows that ion bombardment during growth induces a (111) preferential orientation as compared with the nonassisted film, which shows a clear preferential growth of the (220) planes. The ratio of the integral intensity of the (111) to that of the (220) diffraction peaks varies from 0.15 for the nonassisted film up to 0.75 for the 50% N assisted film. When -Ni FeN is completely formed the intensities ratio reaches a value of 3.2.

Fig. 3. (a) Room temperature ICEMS spectra recorded from the different films, as labeled. (b) Corresponding hyperfine field distributions.

The broadening of the X-ray diffraction is a clear indication of the nanocrystalline nature of the films. Fig. 2 shows the estimated values of the crystallite sizes, according to the Scherrer equation, as a function of the N assistance. Fig. 2 shows that the largest crystallites correspond to the nonassisted and the 100% N assisted films indicating that not only the incorporation of nitrogen in the film favors the growth of smaller crystallites. Fig. 3(a) shows the ICEM spectra recorded from the nonassisted Permalloy as well as those of the films grown with N concentrations of 33, 50, and 100% in the assistant beam. The spectrum recorded from the nonassisted Permalloy showed a sextet with broad lines. Fitting this spectrum with a discrete sextet did not produce satisfactory results. A much better fit was obtained when a hyperfine magnetic field distribution was used. The distribution obtained (Fig. 3(b)) was quite narrow

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PRIETO et al.: CHARACTERIZATION OF NANOCRYSTALLINE PERMALLOY THIN FILMS OBTAINED BY NITROGEN IBAD

and symmetric. The maximum of the distribution is located at 27.5 T, the isomer shift, i.e., , is nearly zero and the quadrupole shift, i.e., 2 , is small (0.01 mms ); this is consistent with previously reported data [11]. The spectrum recorded from the 33% N assisted film is very similar, although, and interestingly, the distribution was slightly narrower and the maximum of the distribution shifted to smaller values (27.1 T). The spectrum recorded from the sample 50% N assisted film shows, in addition to the magnetic sextet pattern, a small paramagnetic signal in the center of the spectrum which was simulated by including a quadrupole doublet in the fit (13% of the total spectral area). The maximum of the hyperfine magnetic field distribution shifted to lower field values (26.0 T) and the parameters of 0.25 mm and quadrupole the doublet (i.e., isomer shift 1.16 mm ) are similar to those shown by the nonsplitting magnetic -(Fe Ni ) N phase [7], [12]. The small decrease in the magnitude of the hyperfine magnetic field might be associated with the presence of some interstitial nitrogen atoms within the Permalloy lattice. In fact, the nitrogen concentration in the 50% N assisted Permalloy is 12% while in the 33% N assisted Permalloy remains lower than the detection limit, i.e., 5%, as determined by resonant RBS. Finally, the spectrum recorded from the 100% N assisted film is composed by a relatively sharp doublet with Mössbauer and quadrupole splitting parameters, i.e., isomer shift of 0.29 mm and 1.23 mm , respectively, which are very close to those characteristic of -(Fe Ni ) N as it has been already reported [7], [12]. The only presence of this doublet indicates that the distribution of Fe and Ni atoms in the -(Fe Ni ) N lattice is not random and that Fe atoms occupy only the sites in the center of the faces of the fcc cubic structure while Ni atoms occupy the corner sites. If the sites in the corner of the fcc cube were occupied by Fe atoms then a singlet should be also observed in the corresponding Mössbauer spectrum, which is not the case. At this point, it should be mentioned that the nitrogen atomic concentration in this film is 30% at., i.e., much higher than the expected concentration, i.e., 20%at., for the -(Fe Ni ) N phase. Fig. 4 shows the easy axis M-H loops for nonassisted Permalloy as well as for the films assisted with mixtures of Ar and N ions. The coercive field at the easy axis varies from a value of 0.50 mT for the non-assisted Permalloy to 0.19 mT for the film assisted with 25% N and 0.36 mT for the 33% N assisted film. However, when the concentration of N ions in the assistant beam moves up to 50%, the film increases its coercivity up to values of 1.5 mT. The dependence of the coercive field on the concentration of N ions in the assistant beam has been depicted in the inset of Fig. 4. The large value of the coercive field, shown by the film with a nitrogen content of 12% at. (i.e., labeled as 50% N ), cannot be explained by the crystallite sizes and is probably related to the small amount of paramagnetic nitride -(Fe Ni ) N found to be present by ICEMS in that film that also modify the magnetization reversal mechanism in that film. The angular dependence of the reduced remanence, i.e., , and coercive field, i.e., , provide information of the magnetic anisotropy as well as the magnetization reversal modes. Fig. 5(a) and (b) shows the variation of the reduced

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Fig. 4. In plane hysteresis loop at the easy axis for the longitudinal component of different Permalloy films as labelled.

M M

Fig. 5. Reduced remanence = (b) and coercive field (b) as a function of the angle between the field direction and the easy-axis direction for the different films.

remanence and coercive field, respectively, as a function of the applied field orientation for the nonassisted film and those for films assisted with 33% and 50% N ions setting


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the easy axis. Both coercivity as well as the reduced remanence are sensitive to the orientation of the field and are symmetrical with respect to field orientation of 180 for the three samples studied in Fig. 5. For the nonassisted Permalloy, the reduced dependence expected in remanence follows the typical well-defined uniaxial anisotropy magnetic systems included also in the figure as a solid line. The N assisted films also show uniaxial anisotropy but not as well-defined as the nonassisted film with an opening at the hard axis. Fig. 5(b) shows different magnetization reversal behaviors depending on the angle. For fields close to hard axis, i.e., 90 , the magnetization is reversing via a rotation process as indicated with shown by the three films. Howby the decrease of ever, as the field is tilted away from the hard axis, the angular dependence of the coercivity is completely different for the three films. In the case of the nonassisted film, the application of magnetic fields at angles 75 produce a magnetization reversal caused by domain-wall nucleation and/or unpinning following law predicted by this magnetization mechanism the reversal [13], instead of coherent rotation processes. This dependence of the coercive field completely changes for the 50% N assisted film. In this case, as the field is tilted away from the hard axis, the coercive field increase reaching the highest , i.e., easy axis. This angular dependence of the value at coercive field is consistent with the coherent rotation reversal mode based on the Stoner–Wolhfarth model [14], shown also in Fig. 5(b) (solid line). The angular dependence of the coercivity for the 33% N assisted film seems to be an intermediate step between the two different reversal mechanisms. In fact, the magnetization reversal in the nonassisted Permalloy is the expected behavior of a thin film due to domain-wall nucleation. This behavior starts to break when the crystalline size decreases as expected in nanogranular systems 13% [15]. For the 50% N assisted film the presence of of paramagnetic nitride -(Fe Ni ) N, as determined by Mössbauer spectroscopy, could decoupled ferromagnetic grains and the coherent rotation reversal process, expected for single particles, is more favorable than other magnetization reversal mechanisms [15]. IV. CONCLUSSION Nanocrystalline Fe Ni thin films have been prepared by dual in-beam sputtering. Interesting magnetic and structural effects have been observed as the proportion of N ions present in the assistant beam is varied. When compared with nonassisted Permalloy films, those assisted with low N concentration are softer and nanocrystalline Fe Ni –N films. However, the increase of the nitrogen content in the film, due to the increase of

N ion concentration in the assistance up to 50%, produce the formation of a small amount of paramagnetic nitride -(Fe Ni ) N as well as an increment of the coercivity. The films assisted with 75 and 100% of N ions, are formed only by the paramagnetic –Ni FeN phase with crystallite sizes of 13 and 24 nm, respectively. The ferromagnetic films show in-plane magnetic uniaxial anisotropy. The magnetization reversal mechanisms are studied in terms of the angular dependence of the coercive field with the applied field direction. We have observed an evolution from domain-wall nucleation process to coherent rotation process as the nitrogen content increases in the films. ACKNOWLEDGMENT This work has been funded by the CAM-UAM (CCG07UAM/MAT-1871) and the MEC (MAT2006-08158). J. Camarero and E. Jiménez acknowledge support through projects MAT2006-13470 and CSD2007-00010. J. F. Marco acknowledges support through project S-0505/MAT/0194. REFERENCES [1] Y. Zhuang, M. Vroubel, B. Rejaei, J. N. Burghartz, and K. Attenboroung, J. Appl. Phys., vol. 99, p. 08C705, 2006. [2] X. Liu and G. Zangari, IEEE Trans. Magn., vol. 37, p. 1764, 2001. [3] H. Naganuma, R. Nkatani, Y. Endo, Y. Kawamura, and M. Yamamoto, Surface and Technology of Advanced Materials, vol. 5, p. 101, 2004. [4] L. de Wit, T. Weber, J. S. Custer, and F. W. Saris, Phys. Rev. Lett., vol. 72, p. 3835, 1994. [5] T. Koyano, T. Nomiyama, N. Kahoh, H. Numata, T. Ohba, E. Kita, and H. Ohtsuka, J. Appl. Phys., vol. 100, p. 033906, 2006. [6] P. Prieto, K. R. Pirota, J. M. Sanz, E. Jimenez, J. Camarero, F. Maccherozzi, and F. G. Panaccione, Appl. Phys. Lett., vol. 90, p. 032505, 2007. [7] X. G. Diao, R. B. Scorzelli, and H. R. Rechenberg, J. Magnetism and Magnetic Mater., vol. 218, p. 81, 2000. [8] R. Gupta and M. Gupta, Phys. Rev. B, vol. 72, p. 024202, 2005. [9] B. D. Cullity, Elements of X-ray Diffraction. Reading, M.A.: Addison-Wesley, 1978. [10] J. Camarero, J. Sort, A. Hoffmann, J. M. García-Martín, B. Dieny, R. Miranda, and J. Nogués, Phys. Rev. Lett., vol. 95, p. 057204, 2005. [11] P. Auric, S. Bonat, and B. Rodmaq, J. Phys: Condens. Mater., vol. 10, pp. 3755–3768, 1998. [12] D. L. Williamson and M. Kopcewicz, Hyperfine Interact., vol. 80, pp. 1043–1050, 1993. [13] S. Chikazumi, Physics of Magnetism. New York: Wiley, 1964, pp. 281–291. [14] E. C. Stoner and E. P. Wohlfarth, Philos. Trans. Roy. Soc. London, vol. 240, p. 599, 1948. [15] J. Zhou, R. Skomski, A. Kashyap, K. D. Sorge, Y. Sui, M. Daniil, L. Gao, M. L. Yan, S. H. Liou, R. D. Kirby, and D. J. Sellmyer, J. Magnetism and Magnetic Mater., vol. 290, p. 227, 2005.

Manuscript received March 08, 2008. Current Version published December 17, 2008. Corresponding author: P. Prieto (e-mail: [email protected]).
