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Co multilayers
Nuclear Instruments and Methods in Physics Research B 272 (2012) 96–99
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Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Effect of low energy He+-ion irradiation on structural and magnetic properties of thin Pt/Cr/Co multilayers J.K. Tripathi a,1, M.O. Liedke b, T. Strache b, S.N. Sarangi a, R. Grötzschel b, A. Gupta c, T. Som a,⇑ a
Institute of Physics, Sachivalaya Marg, Bhubaneswar 751 005, India Forschungszentrum Dresden-Rossendorf, D-01314 Dresden, Germany c UGC-DAE Consortium for Scientific Research, Khandwa Road, Indore 452 017, India b
a r t i c l e
i n f o
Article history: Available online 2 February 2011 Keywords: Ion irradiation Pt/Cr/Co multilayer CoCrPt ternary alloy Coercivity
a b s t r a c t In this work, we report on the changes in structural and magnetic properties of [Pt (0.7 nm)/Cr (x nm)/Co (0.5 nm)]15/Si, x = 0.1 and 0.2 nm, due to 10 keV He+-ion irradiation at room temperature in the fluence range of 2 1015–5 1016 ions-cm 2. Enhancement in the coercivity values with a fairly square magnetization reversal loop (for both the multilayers), upon irradiation to the fluence of 5 1016 ions-cm 2 was observed. Above finding is discussed in the realm of ion beam mixing, leading to the CoCrPt ternary alloy phase formation, after low-energy He+-ion irradiation. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Ultra-thin (
0168-583X/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2011.01.040
by 2 MeV Ar+-ions or keV Ga+-ions [9]. The existing reports on Co/Pt system help to understand the nature of structural changes responsible for changes in magnetic properties. However, to the best of our knowledge, no attempt has been made to tune structural and magnetic properties of ultrathin Pt/Cr/Co MLs by low-energy light-ion (e.g., He+-ions) irradiation. Co–Cr–Pt based materials are considered to be important magnetic data storage materials. In this report, we focus mainly on the modifications occurring in the structural and the magnetic properties of Co/Cr/Pt MLs under 10 keV He+-ion irradiation at room temperature (RT). Complementary studies of X-ray reflectivity (XRR), grazing incidence X-ray diffraction (GIXRD), and magneto-optical Kerr effect (MOKE) techniques have been used. In addition, to find out the possible mechanism leading to such changes under keV He+-ion irradiation, TRIDYN_FZD simulations, which use the binary collision approximation model for ballistic transport, were performed [12]. This code explains the dynamic change in thickness and/or composition of multi-component targets during high-dose ion irradiation/ implantation.
2. Experimental Ultra high vacuum (UHV) e-beam evaporation technique was employed for sequential growth of [Pt (0.7 nm)/Cr (x nm)/Co (0.5 nm)]15/Si (x = 0.1 and 0.2 nm) ML samples on a native oxide covered Si (1 0 0) at RT under a base vacuum of 5.0 10 8 Torr and a deposition rate of 0.01 nm s 1. The MLs were uniformly irradiated at RT by 10 keV He+-ions in the fluence range of 2 1015–5 1016 ions-cm 2. The projected range of 10 keV He+ions in [Pt (0.7 nm)/Cr (x nm)/Co (0.5 nm)]15/Si (x = 0.1 and
J.K. Tripathi et al. / Nuclear Instruments and Methods in Physics Research B 272 (2012) 96–99
0.2 nm) are 80.5 and 78.4 nm, respectively. Therefore, in our case, He+-ion will pass through the ML stacks and get buried deep into the Si substrate. Interface mixing was studied by XRR measurements whereas for phase identification GIXRD studies were performed. MOKE measurements have been done in the longitudinal configuration in order to study the changes in magnetic properties of Pt/Cr/Co ML samples before and after helium ion irradiation. 3. Results and discussion Fig. 1 shows low-angle XRR spectra of pristine as well as irradiated [Pt (0.7 nm)/Cr (0.2 nm)/Co (0.5 nm)]15/Si ML samples. ML shows the ‘‘Bragg peak’’ along with thirteen (i.e., n 2, where n is the total number of stacks) subsidiary maxima (Kiessig fringes) for the pristine samples. This confirms well-defined interface structures indicating a quite smooth growth of ML samples. In order to determine the changes that occurred in the micro-structural parameters (e.g., thickness, electron densities, and individual layer roughness) due to He+-ion irradiation, the XRR spectra were fitted using Parratt’s formalism [13], and results are tabulated in the Table 1. A critical look into the XRR spectra (as well as in Table 1 also) suggests that the periodicity is very close to the nominal ones. The slight difference between the fitted and the experimental spectra (Fig. 1) may be understood as follows. The individual layer thickness for such ultrathin MLs is very small and therefore some of the layers (e.g., Cr) may not be continuous. Nevertheless, the simulations clearly show a slight increase in the surface/interface roughness values even for the fluence of 2 1015 ions-cm 2 (Table 1) signaling that mixing starts at the early stage of irradiation. This leads to a significant reduction in the ‘‘Bragg-peak’’ area (65% as compared to the pristine one) for the highest fluence irradiation (in both the MLs). Thus the observations assert a sufficient degree of ion beam mixing across the irradiated Pt/Cr/Co ML interfaces. GIXRD measurements were carried out for the phase identification, with minimum contribution from the substrate, in both irra-
Fig. 1. Low angle XRR spectra of pristine and 10 keV He+-ion irradiated [Pt (0.7 nm)/Cr (0.2 nm)/Co (0.5 nm)]15/Si multilayer.
97
diated and pristine [Pt (0.7 nm)/Cr (0.1 nm)/Co (0.5 nm)]15/Si, ML samples at the grazing incidence of 0.5°. The GIXRD pattern of pristine ML sample shows Bragg peaks of fcc Pt (1 1 1) and the hcp Co (1 0 0) at about 36.5° and 41.2°, respectively (Fig. 2). We do not see any diffraction peak corresponding to the Cr layers. This is due to the very small quantity of Cr in the Pt/Cr/Co stacking. Fig. 2 also shows the peak shift, particularly for fcc Pt (1 1 1), towards a higher angle side after ion irradiation. This behavior might be explained by the mixing of Pt into the neighboring Cr and Co layers and vice versa (that slightly reduces the average lattice constant of them) [14]. At the highest fluence (5 1016 ions-cm 2) Pt and Co signals become significantly weaker and a new peak appears. The peak appearing at 2h = 48.3° corresponds to the CoCrPt (1 0 1) reflection [15]. In addition, the presence of fcc Pt (1 1 1) and hcp Co (1 0 0), after irradiation to the highest fluence, suggests that mixing is incomplete. Similar trend is observed for the ML stack having Cr layer thickness of 0.2 nm and irradiated under similar conditions (figure not shown). These observations match well with our previously discussed XRR results (Fig. 1). To study the magnetic properties of the ML samples, longitudinal MOKE (LMOKE) measurements were performed. Fig. 3 depicts the LMOKE data corresponding to the pristine and the irradiated ML samples. Since MOKE cannot measure the absolute magnetization, all the hysteresis loops were centered with respect to zero of the Kerr signal and normalized to unity. It may be noted that, such a choice of having ultra-thin Cr layers with a high number of multilayer stacks gives rise to a situation where the easy-axis of magnetization in both the pristine ML stacks is already oriented in out-of-plane orientation (Fig. 3). We find a successive enhancement and saturation of the coercivity value (and hence in the LMOKE hysteresis curve area also) with increasing ion fluence (Fig. 3a and b). For instance, coercivity value increases from 9.5 Oe (2 1015 ions-cm 2) to 35.5 Oe (1 1016 ions-cm 2) and 36 Oe (5 1016 ions-cm 2) for the ML having the Cr layer thickness of 0.1 nm whereas for the ML having the Cr thickness of 0.2 nm it varies from 7 Oe (pristine) to 27 Oe (2 1015 ionscm 2) and 27.5 Oe (1 1016 ions-cm 2). The increase in the coercivity may be attributed to the formation of CoCrPt ternary alloy phase due to ion beam mixing at RT and related effects. Usually, for magnetic polycrystalline materials grain boundaries, phase boundaries, line defects, and defect clusters are known to form pinning sites that impede the movement of magnetic domain walls leading to a larger coercivity [16,17]. However, it was shown recently by Kleeman et al., that even at RT, helium ion irradiation actually can smoothen and accelerate domain wall motion by several orders of magnitude [18]. Thus, in the present case, the enhanced coercivity is likely due to clustering or mixing-induced strains and inhomogeneous alloying effects [19]. This is in good agreement with previously discussed XRR (Fig. 1) and GIXRD (Fig. 2) results where interface mixing was observed. In addition, there is a reduction in the saturation magnetic field due to mixing (and so the reduction in the relative in-plane anisotropy also) (Fig. 3). Furthermore, the M–H loop squareness, S, defined as the ratio of the remanent magnetization (Mr) to the saturation magnetization (Ms), is increased significantly due to ion beam irradiation. Fairly square magnetization reversal loops, under light-ion irradiation, confirm that the easy-axis of magnetization is reoriented from the out-of-plane to the in-plane orientation. It is noteworthy that, a very small change of the interface anisotropy is sufficient to cause such ‘‘in-plane’’ reorientation in Co/Pt system as well [8]. Let us now try to find out the possible mechanism leading to ion beam mixing across the ML interfaces. In order to do this TRIDYN_FZD simulations [12] are performed for different ion fluences. One such simulated elemental profile (at the highest fluence) is shown in Fig. 4. For the TRIDYN_FZD simulations, the sample has been modeled by a stack of fifteen [Pt (0.7 nm)/Cr (0.1 nm)/Co
98
J.K. Tripathi et al. / Nuclear Instruments and Methods in Physics Research B 272 (2012) 96–99
Table 1 Results of fitting of the X-ray reflectivity data of pristine and irradiated [Pt (0.7 nm)/Cr (0.2 nm)/Co (0.5 nm)]15/Si multilayer samples. Typical error bars for layer thicknesses and interface roughnesses are 0.1 nm while that on the electron density of mixed layers (Pt, Cr and Co) is 5%. [Pt (0.7 nm)/Cr (0.2 nm)/Co (0.5 nm)]15/Si ML
Pristine 2 1015 ions-cm 5 1016 ions-cm
2 2
Layer electron density q (103 nm 3)
Layer thickness (nm)
Pt
Cr
Co
Pt
Cr
Co
4.87 4.87 4.44
1.99 1.77 2.11
2.24 2.24 2.08
0.68 0.68 0.66
0.18 0.19 0.08
0.48 0.45 0.58
Multilayer period d (nm)
1.34 1.32 1.32
Roughness r (nm) Surface
Pt on Cr
Cr on Co
0.38 0.45 0.46
0.55 0.45 0.55
0.30 0.01 0.01
Fig. 2. Representative GIXRD patterns of pristine and 10 keV He+-ion irradiated [Pt (0.7 nm)/Cr (0.1 nm)/Co (0.5 nm)]15/Si multilayer.
(0.5 nm)] layers on a silicon substrate. Abrupt interfaces between the respective layers are assumed (shown by the dotted lines in Fig. 4). In this case, we have used 10 keV helium ions which strike the ML stacks normal to the sample surface. Figure shows the resulting compositional profiles for the highest fluence (5 1016 cm 2). The broadening in the profiles of Pt, Cr, and Co layers can easily be seen in the figure. Accordingly the interfaces between the individual layers become blurred. The figure shows a significant fraction of each three atomic species (viz. Pt, Cr, and Co) is pushed (due to ion irradiation) into the neighboring layers due to interface mixing across the interfaces. For instance (Fig. 4), on an average, there is a transfer of 34.3 at.% Pt within 0.1 nm thick Cr layers and 35.9 at.% Pt within 0.5 nm thick Co layers. Similarly, there is a transfer of 15.0 at.% Cr within 0.7 nm Pt layers and 26.7 at.% Cr within 0.5 nm Co layers. It also results in a transfer of 22.5 at.% Co within the Pt layers and 39.3 at.% Co within the Cr layers. Similar, results have also been observed for the MLs having Cr thickness of 0.2 nm (Figure not shown). These displaced Pt, Cr, and Co atoms would cause intermixing across the Pt/Cr/Co interfaces. This is quite consistent with our XRR (where a significant change in the electron density and roughness values is observed) and GIXRD data (where a reduction in the peak intensity of Co and Pt is observed which is followed by the formation of
Fig. 3. Longitudinal MOKE plots of pristine and 10 keV He+-ion irradiated [Pt (0.7 nm)/Cr (x nm)/Co (0.5 nm)]15/Si multilayer (a) x = 0.1 nm and (b) x = 0.2 nm.
CoCrPt ternary alloy phase). However, since the complete mixing could not be achieved under the present experimental conditions, it is difficult to extract the exact amount of the mixed layers or the composition of the mixed patches.
4. Conclusions In conclusion, we have shown that 10 keV He+-ion irradiation of [Pt (0.7 nm)/Cr (x nm)/Co (0.5 nm)]15/Si (x = 0.1 and 0.2 nm) multilayers at room temperature leads to a change in magnetic and structural properties through interface mixing and subsequent CoCrPt ternary alloy phase formation across the interfaces. Increase in the coercivity, with fairly square magnetization curve, is attributed to inhomogeneous alloying across the Pt/Cr/Co ML interfaces and a possible mixing-induced strain. Changes in the structural and the magnetic properties are well correlated by XRR, GIXRD, and MOKE measurements. In order to explore the possibility of tuning the magnetic properties of the Pt/Cr/Co multilayer system in a more controlled manner, further experiments are
J.K. Tripathi et al. / Nuclear Instruments and Methods in Physics Research B 272 (2012) 96–99
99
Germany) for providing the TRIDYN_FZD code and helpful discussion. References
Fig. 4. TRIDYN_FZD simulation results for 10 keV He+-ion irradiation of [Pt (0.7 nm)/Cr (0.1 nm)/Co (0.5 nm)]15/Si multilayer for the fluence of 5 1016 ions-cm 2.
underway where irradiation is being carried out at variable temperatures. Acknowledgments The authors acknowledge the accelerator group members at Forschungszentrum Dresden-Rossendorf, Germany, and S.R. Potdar (UGC-DAE CSR, Indore) for their helps during this work. Thanks are also due to M. Posselt (Forschungszentrum Dresden-Rossendorf,
[1] E. Chunsheng, J. Rantschler, S. Zhang, S. Khizroev, T.R. Lee, D. Litvinov, J. Appl. Phys. 103 (2008) 07B510. [2] C.W. Su, C.S. Shern, Y.D. Yao, J. Appl. Phys. 95 (2004) 6568. [3] M.-T. Lina, C.C. Kuoa, J.W. Hoa, Y.E. Wub, H.Y. Herb, C.S. Shernb, H.L. Huang, Appl. Surf. Sci. 169-170 (2001) 231. [4] P. He, W.A. McGahan, J.A. Woollam, F. Sequeda, T. McDaniel, H. Do, J. Appl. Phys. 69 (1991) 4021. [5] Y. Ochiai, S. Hashimoto, K. Aso, Proceedings of the Magnetics Conference, 1989 (Digests of Intermag ’92, International GB6, 1989). [6] J. Fassbender, D. Ravelosona, Y. Samson, J. Phys. D: Appl. Phys. 37 (2004) R179. [7] J. Fassbender, J. McCord, J. Magn. Magn. Mater. 320 (2008) 579. [8] C. Chappert, H. Bernas, J. Ferre´, V. Kottler, J.-P. Jamet, Y. Chen, E. Cambril, T. Devolder, F. Rousseaux, V. Mathet, H. Launois, Science 280 (1998) 1919. [9] C.T. Rettner, S. Anders, J.E.E. Baglin, T. Thomson, B.D. Terris, Appl. Phys. Lett. 80 (2002) 279. [10] T. Som, S. Ghosh, M. Mäder, R. Grötzschel, S. Roy, D. Paramanik, A. Gupta, New J. Phys. 9 (2007) 164. [11] S. Ghosh, M. Mäder, R. Grötzschel, A. Gupta, T. Som, Appl. Phys. Lett. 89 (2006) 04104. [12] W. Möller, W. Eckstein, J.P. Biersack, Comput. Phys. Commun. 51 (1988) 355. [13] G. Parratt, Phys. Rev. 95 (1954) 359. [14] M. Cai, T. Veres, S. Roorda, F. Schiettekatte, R.W. Cochrane, J. Appl. Phys. 95 (2004) 1996. [15] Y. Xu, J.P. Wang, Y. Su, J. Phys. D: Appl. Phys. 33 (2000) 1460. [16] R.A. Ristau, K. Barmak, L.H. Lewis, K.R. Koffey, J.K. Howard, J. Appl. Phys. 86 (1999) 4527. [17] J.D. Livingston, J. Appl. Phys. 52 (1981) 2544. [18] W. Kleemann, J. Rhensius, O. Petracic, J. Ferre´, J.-P. Jamet, H. Bernas, Phys. Rev. Lett. 99 (2007) 097203. [19] J.K. Tripathi, A. Kanjilal, P. Rajput, A. Gupta, T. Som, Nucl. Instr. Meth. Phys. Res. B 267 (2009) 1608.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Effect of low energy He+-ion irradiation on structural and magnetic properties of thin Pt/Cr/Co multilayers J.K. Tripathi a,1, M.O. Liedke b, T. Strache b, S.N. Sarangi a, R. Grötzschel b, A. Gupta c, T. Som a,⇑ a
Institute of Physics, Sachivalaya Marg, Bhubaneswar 751 005, India Forschungszentrum Dresden-Rossendorf, D-01314 Dresden, Germany c UGC-DAE Consortium for Scientific Research, Khandwa Road, Indore 452 017, India b
a r t i c l e
i n f o
Article history: Available online 2 February 2011 Keywords: Ion irradiation Pt/Cr/Co multilayer CoCrPt ternary alloy Coercivity
a b s t r a c t In this work, we report on the changes in structural and magnetic properties of [Pt (0.7 nm)/Cr (x nm)/Co (0.5 nm)]15/Si, x = 0.1 and 0.2 nm, due to 10 keV He+-ion irradiation at room temperature in the fluence range of 2 1015–5 1016 ions-cm 2. Enhancement in the coercivity values with a fairly square magnetization reversal loop (for both the multilayers), upon irradiation to the fluence of 5 1016 ions-cm 2 was observed. Above finding is discussed in the realm of ion beam mixing, leading to the CoCrPt ternary alloy phase formation, after low-energy He+-ion irradiation. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Ultra-thin (
0168-583X/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2011.01.040
by 2 MeV Ar+-ions or keV Ga+-ions [9]. The existing reports on Co/Pt system help to understand the nature of structural changes responsible for changes in magnetic properties. However, to the best of our knowledge, no attempt has been made to tune structural and magnetic properties of ultrathin Pt/Cr/Co MLs by low-energy light-ion (e.g., He+-ions) irradiation. Co–Cr–Pt based materials are considered to be important magnetic data storage materials. In this report, we focus mainly on the modifications occurring in the structural and the magnetic properties of Co/Cr/Pt MLs under 10 keV He+-ion irradiation at room temperature (RT). Complementary studies of X-ray reflectivity (XRR), grazing incidence X-ray diffraction (GIXRD), and magneto-optical Kerr effect (MOKE) techniques have been used. In addition, to find out the possible mechanism leading to such changes under keV He+-ion irradiation, TRIDYN_FZD simulations, which use the binary collision approximation model for ballistic transport, were performed [12]. This code explains the dynamic change in thickness and/or composition of multi-component targets during high-dose ion irradiation/ implantation.
2. Experimental Ultra high vacuum (UHV) e-beam evaporation technique was employed for sequential growth of [Pt (0.7 nm)/Cr (x nm)/Co (0.5 nm)]15/Si (x = 0.1 and 0.2 nm) ML samples on a native oxide covered Si (1 0 0) at RT under a base vacuum of 5.0 10 8 Torr and a deposition rate of 0.01 nm s 1. The MLs were uniformly irradiated at RT by 10 keV He+-ions in the fluence range of 2 1015–5 1016 ions-cm 2. The projected range of 10 keV He+ions in [Pt (0.7 nm)/Cr (x nm)/Co (0.5 nm)]15/Si (x = 0.1 and
J.K. Tripathi et al. / Nuclear Instruments and Methods in Physics Research B 272 (2012) 96–99
0.2 nm) are 80.5 and 78.4 nm, respectively. Therefore, in our case, He+-ion will pass through the ML stacks and get buried deep into the Si substrate. Interface mixing was studied by XRR measurements whereas for phase identification GIXRD studies were performed. MOKE measurements have been done in the longitudinal configuration in order to study the changes in magnetic properties of Pt/Cr/Co ML samples before and after helium ion irradiation. 3. Results and discussion Fig. 1 shows low-angle XRR spectra of pristine as well as irradiated [Pt (0.7 nm)/Cr (0.2 nm)/Co (0.5 nm)]15/Si ML samples. ML shows the ‘‘Bragg peak’’ along with thirteen (i.e., n 2, where n is the total number of stacks) subsidiary maxima (Kiessig fringes) for the pristine samples. This confirms well-defined interface structures indicating a quite smooth growth of ML samples. In order to determine the changes that occurred in the micro-structural parameters (e.g., thickness, electron densities, and individual layer roughness) due to He+-ion irradiation, the XRR spectra were fitted using Parratt’s formalism [13], and results are tabulated in the Table 1. A critical look into the XRR spectra (as well as in Table 1 also) suggests that the periodicity is very close to the nominal ones. The slight difference between the fitted and the experimental spectra (Fig. 1) may be understood as follows. The individual layer thickness for such ultrathin MLs is very small and therefore some of the layers (e.g., Cr) may not be continuous. Nevertheless, the simulations clearly show a slight increase in the surface/interface roughness values even for the fluence of 2 1015 ions-cm 2 (Table 1) signaling that mixing starts at the early stage of irradiation. This leads to a significant reduction in the ‘‘Bragg-peak’’ area (65% as compared to the pristine one) for the highest fluence irradiation (in both the MLs). Thus the observations assert a sufficient degree of ion beam mixing across the irradiated Pt/Cr/Co ML interfaces. GIXRD measurements were carried out for the phase identification, with minimum contribution from the substrate, in both irra-
Fig. 1. Low angle XRR spectra of pristine and 10 keV He+-ion irradiated [Pt (0.7 nm)/Cr (0.2 nm)/Co (0.5 nm)]15/Si multilayer.
97
diated and pristine [Pt (0.7 nm)/Cr (0.1 nm)/Co (0.5 nm)]15/Si, ML samples at the grazing incidence of 0.5°. The GIXRD pattern of pristine ML sample shows Bragg peaks of fcc Pt (1 1 1) and the hcp Co (1 0 0) at about 36.5° and 41.2°, respectively (Fig. 2). We do not see any diffraction peak corresponding to the Cr layers. This is due to the very small quantity of Cr in the Pt/Cr/Co stacking. Fig. 2 also shows the peak shift, particularly for fcc Pt (1 1 1), towards a higher angle side after ion irradiation. This behavior might be explained by the mixing of Pt into the neighboring Cr and Co layers and vice versa (that slightly reduces the average lattice constant of them) [14]. At the highest fluence (5 1016 ions-cm 2) Pt and Co signals become significantly weaker and a new peak appears. The peak appearing at 2h = 48.3° corresponds to the CoCrPt (1 0 1) reflection [15]. In addition, the presence of fcc Pt (1 1 1) and hcp Co (1 0 0), after irradiation to the highest fluence, suggests that mixing is incomplete. Similar trend is observed for the ML stack having Cr layer thickness of 0.2 nm and irradiated under similar conditions (figure not shown). These observations match well with our previously discussed XRR results (Fig. 1). To study the magnetic properties of the ML samples, longitudinal MOKE (LMOKE) measurements were performed. Fig. 3 depicts the LMOKE data corresponding to the pristine and the irradiated ML samples. Since MOKE cannot measure the absolute magnetization, all the hysteresis loops were centered with respect to zero of the Kerr signal and normalized to unity. It may be noted that, such a choice of having ultra-thin Cr layers with a high number of multilayer stacks gives rise to a situation where the easy-axis of magnetization in both the pristine ML stacks is already oriented in out-of-plane orientation (Fig. 3). We find a successive enhancement and saturation of the coercivity value (and hence in the LMOKE hysteresis curve area also) with increasing ion fluence (Fig. 3a and b). For instance, coercivity value increases from 9.5 Oe (2 1015 ions-cm 2) to 35.5 Oe (1 1016 ions-cm 2) and 36 Oe (5 1016 ions-cm 2) for the ML having the Cr layer thickness of 0.1 nm whereas for the ML having the Cr thickness of 0.2 nm it varies from 7 Oe (pristine) to 27 Oe (2 1015 ionscm 2) and 27.5 Oe (1 1016 ions-cm 2). The increase in the coercivity may be attributed to the formation of CoCrPt ternary alloy phase due to ion beam mixing at RT and related effects. Usually, for magnetic polycrystalline materials grain boundaries, phase boundaries, line defects, and defect clusters are known to form pinning sites that impede the movement of magnetic domain walls leading to a larger coercivity [16,17]. However, it was shown recently by Kleeman et al., that even at RT, helium ion irradiation actually can smoothen and accelerate domain wall motion by several orders of magnitude [18]. Thus, in the present case, the enhanced coercivity is likely due to clustering or mixing-induced strains and inhomogeneous alloying effects [19]. This is in good agreement with previously discussed XRR (Fig. 1) and GIXRD (Fig. 2) results where interface mixing was observed. In addition, there is a reduction in the saturation magnetic field due to mixing (and so the reduction in the relative in-plane anisotropy also) (Fig. 3). Furthermore, the M–H loop squareness, S, defined as the ratio of the remanent magnetization (Mr) to the saturation magnetization (Ms), is increased significantly due to ion beam irradiation. Fairly square magnetization reversal loops, under light-ion irradiation, confirm that the easy-axis of magnetization is reoriented from the out-of-plane to the in-plane orientation. It is noteworthy that, a very small change of the interface anisotropy is sufficient to cause such ‘‘in-plane’’ reorientation in Co/Pt system as well [8]. Let us now try to find out the possible mechanism leading to ion beam mixing across the ML interfaces. In order to do this TRIDYN_FZD simulations [12] are performed for different ion fluences. One such simulated elemental profile (at the highest fluence) is shown in Fig. 4. For the TRIDYN_FZD simulations, the sample has been modeled by a stack of fifteen [Pt (0.7 nm)/Cr (0.1 nm)/Co
98
J.K. Tripathi et al. / Nuclear Instruments and Methods in Physics Research B 272 (2012) 96–99
Table 1 Results of fitting of the X-ray reflectivity data of pristine and irradiated [Pt (0.7 nm)/Cr (0.2 nm)/Co (0.5 nm)]15/Si multilayer samples. Typical error bars for layer thicknesses and interface roughnesses are 0.1 nm while that on the electron density of mixed layers (Pt, Cr and Co) is 5%. [Pt (0.7 nm)/Cr (0.2 nm)/Co (0.5 nm)]15/Si ML
Pristine 2 1015 ions-cm 5 1016 ions-cm
2 2
Layer electron density q (103 nm 3)
Layer thickness (nm)
Pt
Cr
Co
Pt
Cr
Co
4.87 4.87 4.44
1.99 1.77 2.11
2.24 2.24 2.08
0.68 0.68 0.66
0.18 0.19 0.08
0.48 0.45 0.58
Multilayer period d (nm)
1.34 1.32 1.32
Roughness r (nm) Surface
Pt on Cr
Cr on Co
0.38 0.45 0.46
0.55 0.45 0.55
0.30 0.01 0.01
Fig. 2. Representative GIXRD patterns of pristine and 10 keV He+-ion irradiated [Pt (0.7 nm)/Cr (0.1 nm)/Co (0.5 nm)]15/Si multilayer.
(0.5 nm)] layers on a silicon substrate. Abrupt interfaces between the respective layers are assumed (shown by the dotted lines in Fig. 4). In this case, we have used 10 keV helium ions which strike the ML stacks normal to the sample surface. Figure shows the resulting compositional profiles for the highest fluence (5 1016 cm 2). The broadening in the profiles of Pt, Cr, and Co layers can easily be seen in the figure. Accordingly the interfaces between the individual layers become blurred. The figure shows a significant fraction of each three atomic species (viz. Pt, Cr, and Co) is pushed (due to ion irradiation) into the neighboring layers due to interface mixing across the interfaces. For instance (Fig. 4), on an average, there is a transfer of 34.3 at.% Pt within 0.1 nm thick Cr layers and 35.9 at.% Pt within 0.5 nm thick Co layers. Similarly, there is a transfer of 15.0 at.% Cr within 0.7 nm Pt layers and 26.7 at.% Cr within 0.5 nm Co layers. It also results in a transfer of 22.5 at.% Co within the Pt layers and 39.3 at.% Co within the Cr layers. Similar, results have also been observed for the MLs having Cr thickness of 0.2 nm (Figure not shown). These displaced Pt, Cr, and Co atoms would cause intermixing across the Pt/Cr/Co interfaces. This is quite consistent with our XRR (where a significant change in the electron density and roughness values is observed) and GIXRD data (where a reduction in the peak intensity of Co and Pt is observed which is followed by the formation of
Fig. 3. Longitudinal MOKE plots of pristine and 10 keV He+-ion irradiated [Pt (0.7 nm)/Cr (x nm)/Co (0.5 nm)]15/Si multilayer (a) x = 0.1 nm and (b) x = 0.2 nm.
CoCrPt ternary alloy phase). However, since the complete mixing could not be achieved under the present experimental conditions, it is difficult to extract the exact amount of the mixed layers or the composition of the mixed patches.
4. Conclusions In conclusion, we have shown that 10 keV He+-ion irradiation of [Pt (0.7 nm)/Cr (x nm)/Co (0.5 nm)]15/Si (x = 0.1 and 0.2 nm) multilayers at room temperature leads to a change in magnetic and structural properties through interface mixing and subsequent CoCrPt ternary alloy phase formation across the interfaces. Increase in the coercivity, with fairly square magnetization curve, is attributed to inhomogeneous alloying across the Pt/Cr/Co ML interfaces and a possible mixing-induced strain. Changes in the structural and the magnetic properties are well correlated by XRR, GIXRD, and MOKE measurements. In order to explore the possibility of tuning the magnetic properties of the Pt/Cr/Co multilayer system in a more controlled manner, further experiments are
J.K. Tripathi et al. / Nuclear Instruments and Methods in Physics Research B 272 (2012) 96–99
99
Germany) for providing the TRIDYN_FZD code and helpful discussion. References
Fig. 4. TRIDYN_FZD simulation results for 10 keV He+-ion irradiation of [Pt (0.7 nm)/Cr (0.1 nm)/Co (0.5 nm)]15/Si multilayer for the fluence of 5 1016 ions-cm 2.
underway where irradiation is being carried out at variable temperatures. Acknowledgments The authors acknowledge the accelerator group members at Forschungszentrum Dresden-Rossendorf, Germany, and S.R. Potdar (UGC-DAE CSR, Indore) for their helps during this work. Thanks are also due to M. Posselt (Forschungszentrum Dresden-Rossendorf,
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