Co multilayers by He+-ion irradiation

Nuclear Instruments and Methods in Physics Research B 267 (2009) 1608–1611

Contents lists available at ScienceDirect

Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb

Modification of the magnetic and the structural properties of Pt/Cr/Co multilayers by He+-ion irradiation J.K. Tripathi a, A. Kanjilal b, Parasmani Rajput c, A. Gupta c, T. Som a,* a

Institute of Physics, Sachivalaya Marg, Bhubaneswar 751 005, India Forschungszentrum Dresden Rossendorf, 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 31 January 2009 PACS: 61.80.Jh 75.70.Cn 75.50.Vv Keywords: Ion irradiation Pt/Cr/Co multilayer CoCrPt ternary alloy Coercivity

a b s t r a c t We report on the effects of 2 MeV He+-ion irradiation on the magnetic and structural properties of Pt/Cr/ Co multilayers. We observe He+-ion irradiation leads to mixing across the interfaces [Pt (2.5 nm)/Cr (0.8 nm)/Co (3.0 nm)]  6/Si multilayers. In addition, we observe Co–Cr–Pt phase formation at the highest fluence of 5.5  1016 ions cm 2. This is accompanied by an enhancement in the coercivity. Such enhancement in the coercivity is attributed to inhomogeneous alloying and a possible mixing-induced strain. High-resolution transmission electron microscopy confirms the formation of CoCrPt ternary alloy phase. These findings are explained in the light of ion beam induced recoil mixing and ionization events. Ó 2009 Elsevier B.V. All rights reserved.

1. Introduction On account of their reduced dimensions, the magnetic properties of ultrathin multilayers, often depend strongly on the surface and interface structures. In addition, chemical composition, crystallinity, grain sizes and their distribution also govern the magnetic behaviour. All the above structural properties can be modified by energetic ion irradiation [1]. Consequently, several extrinsic magnetic properties like coercivity, magnetic anisotropy, and magnetic exchange coupling can be tailored by ion irradiation over a highly localized region [1–8]. In addition, ion irradiation has also been shown to be an efficient tool for patterning magnetic thin films for information storage [9–13]. Majority of these studies show that magnetic multilayers are more sensitive to light-ion irradiation [1,2]. For instance, attention in recent years, has been put on lightion induced spin orientation transitions from perpendicular to the plane to in-plane for Co/Pt multilayers [2,7]. It has also been observed that 2 MeV He+ irradiation of Co/Pt multilayers leads to large decrease in the coercivity [5], while that leads to direct ordering in FePt films [14,15]. These investigations are of interests as the technique may be a way to produce magnetically patterned thin films suitable for perpendicular magnetic recording applications [9,16]. Presently, Co–Cr–Pt ternary alloy based media are in commonplace for high-density perpendicular magnetic recording media, * Corresponding author. E-mail address: [email protected] (T. Som). 0168-583X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2009.01.116

because of high perpendicular magnetic anisotropy, reasonably smaller grain sizes (and grain size distribution), low media noise, and good thermal stability than the perpendicular recording media of past [16–18]. At the same time Co–Pt media are being considered for ultrahigh-Density magnetic recording applications [19]. In this paper, we have studied 2 MeV He+-ion induced modifications of magnetic and structural properties of Pt/Cr/Co magnetic multilayer films. Complementary techniques like X-ray reflectivity (XRR), grazing incidence X-ray diffraction (GIXRD), and magnetooptical Kerr effect (MOKE) were used to address the ion induced changes. We present a direct evidence of interfacial mixing, based upon XRR, GIXRD and MOKE data. In order to find out the possible mechanism leading to such changes under MeV He+-ion irradiation, we make use of the TRIDYN_FZD simulation, which uses the binary collision approximation model for ballistic transport [20]. This code is based on the dynamic change of thickness and/or composition of multi-component targets during high-dose ion irradiation/implantation.

2. Experimental Ultra high vacuum e-beam evaporation technique was employed for sequential growth of [Pt (2.5 nm)/Cr (0.8 nm/Co (3.0 nm)]  6/Si ML samples on a native oxide covered Si(1 0 0) at room temperature (RT) under a base vacuum of 5.0  10 8 Torr. The deposition rate was maintained at 0.01 nm s 1. Pt was chosen

1609

J.K. Tripathi et al. / Nuclear Instruments and Methods in Physics Research B 267 (2009) 1608–1611

as the top layer to avoid any possible oxidation in the ML structure. The MLs were uniformly irradiated at RT by 2 MeV He+-ions in the fluence range of 1  1015–5.5  1016 ions cm 2 using a 3 MV Pelletron accelerator. He+-ion irradiation was performed at room temperature using an ion current of 20 nA. This helps to restrict the sample temperature to raise much beyond the room temperature during ion irradiation. In order to achieve uniform irradiation, an ion beam having a diameter of 2 mm was scanned over an area of 6  6 mm2, covering the whole sample. In addition, the samples were mounted on a high purity copper block with a conducting carbon tape. The projected ranges of He-ions in Pt, Cr and Co layers are 2.66, 3.2 and 3.02 lm, respectively. Therefore, in our case, He-ions will pass through the [Pt (2.5 nm)/Cr (0.8 nm)/Co (3.0 nm)]  6/Si ML stack and get buried deep into the Si substrate. Interface modification due to He-ion irradiation was studied by XRR measurements, whereas GIXRD measurements were performed (using the Cu Ka radiation of wavelength of 1.54 Å) for phase identification studies. MOKE measurements were performed in the longitudinal mode to study ion induced changes in the magnetic properties of the MLs. We also performed high-resolution transmission electron microscopy (HRTEM) on selective samples. 3. Results and discussion Fig. 1 shows the low-angle XRR spectra of the pristine and the irradiated Pt/Cr/Co ML samples. The pristine ML shows Bragg peaks up to the fourth order with a clear four (i.e., n 2, where n is total number of stacks) Kiessig fringes, which confirms that the interfaces of the ML are well defined. In order to determine changes in the micro-structural properties such as individual layer thicknesses, surface and interface roughness and electron density, due

Data Simulation

1x1019

5.5 x 1016ions cm-2

Reflectivity (a.u.)

1x1014 1 x 1016ions cm-2

1x109 1x104

1 x 1015ions cm-2

1x10-1 Pristine 1x10-6 10-11 0.01

0.02

0.03

0.04

qz (nm)-1 Fig. 1. Low angle XRR spectra obtained from the pristine and 2 MeV He+-ion irradiated Pt/Cr/Co multilayers to the fluences of 1  1015 and 5.5  1016 ions-cm 2.

to ion irradiation, the XRR spectra were fitted using Parratt’s formalism [21] and the results are tabulated in Table I. It is observed that there is a progressive increase in the rms roughness values of the Pt surface and other interfaces. Further, it is evident from Fig. 1 that for helium ion irradiation, at the lowest fluence, there is a shift in positions of both the first and the second order Bragg peaks towards the lower angle side indicating a dilation in the period of multilayers. It may be noted that amount of shift in the Bragg peaks is not the same and changes with increasing ion fluence. Such shift in the Bragg peaks towards lower angle corresponds to a multilayer period (d) dilation from 6.59 nm in the pristine sample to 6.75 nm in the irradiated samples. These observations (increase in the interface roughness) indicate ion induced mixing across the irradiated Pt/Cr/Co interfaces. However, from the presence of the two Bragg peaks, it is clear that the complete mixing does not take place even at the highest fluence. GIXRD measurements were performed for phase identification of the MLs before and after helium ion irradiation. The spectra shown in Fig. 2 were recorded at a grazing incidence of 0.5°. The XRD pattern of the pristine ML film exhibits the presence of fcc Pt(1 1 1) and the hcp Co(10.0) peaks. The presence of the Pt satellite peak indicates a strong structural coherence among the MLs. It is significant to note that the satellite peak disappears even for irradiation performed at the lowest fluence of 1  1015 ions-cm 2 (not shown here). Such a signature of disappearing Pt satellite peak indicates an intermixing among the constituent layers. Since the extent of mixing is not enough for the lower fluences, particularly for 1  1016 ions-cm 2 helium ion irradiated multilayers, no signature of mixed phase is detected in the corresponding XRD spectrum. Upon increasing the ion fluence to 5.5  1016 ions-cm 2, a new peak evolves corresponding to the 2h value of 42.7° which does not match with either of Co, Cr, Pt or any other known binary alloy involving them. On the other hand, it matches closely with the CoCrPt(10.0) reflection [22]. Thus, He+-ion irradiation of Pt/ Cr/Co ML samples at the fluence of 5.5  1016 ions-cm 2 leads to the formation of CoCrPt ternary alloy phase at RT. Phase formation has been further confirmed by HRTEM (shown in Fig. 5) images. To study the magnetic properties of the multilayer samples, MOKE measurements were performed. Fig. 3 presents the longitudinal MOKE (LMOKE) data corresponding to the pristine and the irradiated ML samples. Since MOKE cannot measure the absolute magnetization, all the LMOKE magnetization reversal loops were centered about the origin and normalized to unity. It is observed that there is a progressive increase in the MOKE hysteresis loop area with increasing ion fluence. In addition, the coercivity value increases from 74.2 Oe (pristine) to 81.5 Oe (1  1015 ions-cm 2) to 97.5 Oe (1  1016 ions-cm 2) to 103 Oe (5.5  1016 ions-cm 2). The increase in the coercivity may result due to the modification of physically and magnetically defined interfaces between the layers and grains. Usually, for magnetic 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 high coercivity [23,24]. However, it was shown recently by

Table 1 Results of fitting of the X-ray reflectivity data of pristine and irradiated Pt/Cr/Co 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%. Samples

Layer electron density,

Layer thickness (nm)

q (  103 nm 3) Pristine 1  1015 ions-cm 2 1  1016 ions-cm 2 5.5  1016 ions-cm

2

Pt

Cr

Co

Pt

Cr

Co

4.87 4.86 4.82 4.79

1.99 2.01 2.02 2.20

2.24 2.23 2.15 1.81

2.58 2.58 2.56 2.52

0.75 0.90 0.99 1.01

3.26 3.25 3.23 3.22

Multilayer period d (nm)

6.59 6.73 6.78 6.75

Roughness, r (nm) Surface

Pt on Cr interface

Cr on Co interface

Co on Pt interface

0.15 0.59 0.60 0.65

0.61 0.98 0.97 0.92

1.19 1.84 1.89 2.16

1.1 1.16 1.20 1.38

1610

J.K. Tripathi et al. / Nuclear Instruments and Methods in Physics Research B 267 (2009) 1608–1611

400

Pt (111)

350

Co (10.0)

CoCrPt (10.0)

Intensity (a.u.)

300

16

-2

16

-2

5.5x10 ions cm

Pt (111) Co (10.0)

250 200

1x10 ions cm

Pt (111)

150

Co (10.0)

100 Pt satellite

Pristine

50 0 36

38

40

42

44

46

48

2θ (deg.) Fig. 2. Representative GIXRD patterns obtained from the pristine and 2 MeV He+ion irradiated Pt/Cr/Co multilayers to the fluences of 1  1015 and 5.5  1016 ionscm 2.

Pristine 16 -2 5.5x10 ions cm

1.0

M/Ms

0.5

0.0

-0.5

-1.0 -200

-100

0

100

Fig. 5. High-resolution transmission electron microscopy images of Pt/Cr/Co MLs: (a) pristine and (b) after 2 MeV He+-ion irradiation to the fluence of 5.5  1016 ionscm 2.

200

Hex (Oe) Fig. 3. Representative longitudinal MOKE plots for the pristine and 2 MeV He+-ion irradiated Pt/Cr/Co multilayers to the fluence of 5.5  1016 ions-cm 2. Magnetization reversal loops corresponding to the intermediate fluences are not shown here in order to maintain the clarity.

Pt

Relative Concentration

1.0 0.5 0.0 1.0

Cr

0.5 0.0 1.0

Co

0.5 0.0

0

2

4

6

32

34

36

38

Depth (nm) Fig. 4. TRIDYN_FZD simulation results for 2 MeV helium ion irradiation of Pt/Cr/Co multilayers to the fluence of 5.5  1016 ions-cm 2.

Kleeman et al., that even at room temperature, helium ion irradiation actually can smoothen and accelerate domain wall motion by several orders of magnitude [6]. Thus, in the present case, the enhanced coercivity is likely due to mixing-induced strain and inhomogeneous alloying effects at the higher mixing rates. This is in good agreement with our XRR and GIXRD results that show the sig-

nature of progressively higher but incomplete mixing and the subsequent phase formation. Let us now try to understand the possible mechanism leading to the CoCrPt ternary alloy phase formation across the Pt/Cr/Co interfaces. In case of a head-on collision although the cross section is quite high, the average transferred energy to the atoms is quite low so that the depth of mixing is also low enough. This is quite evident from the GIXRD results because even at the highest fluence, Pt and Co peaks are clearly visible. TRIDYN_FZD simulation was performed for a fluence of 5.5  1016 ions-cm 2 to have a quantitative estimate for the numbers of displaced Pt, Cr and Co atoms. Fig. 4 presents the TRIDYN_FZD simulation results where we observe that a fraction of each three atomic species (viz. Pt, Cr and Co) is pushed into the neighboring layers. For instance, at the highest fluence (5.5  1016 ions-cm 2), on an average, there is a transfer of 3.1 at.% Pt within 0.8 nm Cr layers and 1.8 at.% Pt within 3.0 nm Co layers. Similarly, there is a transfer of 2.7 at.% Cr within 2.5 nm Pt layers and 2.2 at.% Cr within 3.0 nm Co layers. It also results transfer of 4.7 at.% Co within the Pt layers and 2.9 at.% Co within the Cr layers. These displaced Pt, Cr and Co atoms would cause the intermixing across the Pt/Cr/Co interfaces. This is quite consistent with our XRR data where an interface broadening is accompanied by a reduction in the electron density (as described earlier). 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 layer or the composition of the mixed patches. In general, it has been observed that the multilayers are much more sensitive to irradiation than expected on the basis of a near-

J.K. Tripathi et al. / Nuclear Instruments and Methods in Physics Research B 267 (2009) 1608–1611

est-neighbor coupling model and simple ballistic ion beam mixing and in particular, for lighter ions, the effect would be more prominent [5]. Thus, it would be reasonable to assume that thermodynamically preferred local cluster configurations of Co–Cr–Pt atoms, as suggested by Chappert et al. [2] for the Co/Pt system, are assisted to form without a large ballistic energy transfer. In addition, we have also obtained the simulated ionization events to find out that 2 MeV He+-ions deposit 65 times more energy into ionization events as they do into recoils. Thus, it is quite possible that in this case local relaxation of the interface order could possibly be stimulated by an additional mechanism based on energy deposited through electronic excitations. This ionization energy may get coupled to the lattice phonons and contribute to atomic rearrangements leading to interface mixing and subsequent phase formation [5]. 4. Conclusions In conclusion, we have shown that 2 MeV He+-ion irradiation of Pt/Cr/Co multilayers at room temperature leads to change in their magnetic and structural properties through ion beam mixing and subsequent CoCrPt ternary alloy phase formation across the interfaces. There is a systematic increase in the coercivity with increasing fluence which is attributed to inhomogeneous alloying across the Pt/Cr/Co ML interfaces and a possible mixing-induced strain. Changes in structural and magnetic properties have been well correlated by XRR, GIXRD, and MOKE measurements. HRTEM results confirm the formation of CoCrPt ternary alloy phase. Based on TRIDYN_FZD simulation, mixing has been attributed to recoil mixing and ionization events. 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 underway using different ML configurations and heavier ions (viz. Au and Ar ions). Acknowledgments The authors acknowledge the accelerator group members at IOP, S.R. Potdar and Anil Gom (UGC-DAE CSR, Indore) for their helps during this work.

1611

References [1] J. Fassbender, D. Ravelosona, Y. Samson, J. Phys. D: Appl. Phys. 37 (2004) R179. [2] 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. [3] B.D. Terris, L. Folks, D. Weller, J.E.E. Baglin, A.J. Kellock, H. Rothuizen, P. Vettiger, Appl. Phys. Lett. 75 (1999) 403; J.E.E. Baglin, A.J. Kellock, K.A. Hannibal, M.F. Toney, G. Kusinski, S. Lang, L. Folks, M.E. Best, B.D. Terris, J. Appl. Phys. 87 (2000) 5768. [4] T. Devolder, J. Ferre´, C. Chappert, H. Bernas, J.-P. Jamet, V. Mathet, Phys. Rev. B 64 (2001) 064415; J. Ferre´, T. Devolder, H. Bernas, J.-P. Jamet, V. Repain, M. Bauer, N. Ernier, C. Chappert, J. Phys. D: Appl. Phys. 36 (2003) 3103. [5] C.T. Rettner, S. Anders, J.E.E. Baglin, T. Thomson, B.D. Terris, Appl. Phys. Lett. 80 (2002) 279. [6] W. Kleemann, J. Rhensius, O. Petracic, J. Ferre´, J.-P. Jamet, H. Bernas, Phys. Rev. Lett. 99 (2007) 097203. [7] D. Weller, J.E.E. Baglin, A.J. Kellock, K.A. Hannibal, M.F. Tony, G. Kusinski, S. Lang, L. Folks, M.E. Best, B.D. Terris, J. Appl. Phys. 87 (2000) 5768. [8] J.K. Tripathi, Dileep Kumar, P. Malar, T. Osipowicz, V. Ganesan, A. Gupta, T. Som, Nucl. Instr. and Meth. Phys. Res. B 266 (2008) 1247; T. Som, S. Ghosh, J.K. Tripathi, R. Grötzschel, M. Mäder, V. Ganesan, A. Gupta, D. Kanjilal, Nucl. Instr. and Meth. Phys. Res. B 266 (2008) 1542; T. Som, S. Ghosh, M. Mäder, R. Grötzschel, S. Roy, D. Paramanik, A. Gupta, New J. Phys. 9 (2007) 164; S. Ghosh, M. Mäder, R. Grötzschel, A. Gupta, T. Som, Appl. Phys. Lett. 89 (2006) 104104. [9] J. Fassbender, J. McCord, J. Magn. Magn. Mater. 320 (2008) 579. [10] M.J. Bonder, N.D. Telling, P.J. Grundy, C.A. Faunce, T. Shen, V.M. Vishnyakov, J. Appl. Phys. 93 (2003) 7226. [11] G.J. Kusinski, K.M. Krishnsan, G. Denbeaux, G. Thomas, B.D. Terris, D. Weller, J. Appl. Phys. 91 (2002) 7541. [12] J. Lohau, A. Moser, C.T. Rettner, M.E. Best, B.D. Terris, Appl. Phys. Lett. 78 (2001) 990. [13] B.D. Terris, L. Folks, D. Weller, J.E.E. Baglin, A.J. Kellock, H. Rothuizen, P. Vettiger, Appl. Phys. Lett. 75 (1999) 403. [14] C.-H. Lai, C.-H. Yang, C.-C. Chiang, Appl. Phys. Lett. 83 (2003) 4550. [15] C.-H. Yang, C.-H. Lai, C.-C. Chiang, IEEE Trans. Magn. 40 (2004) 2519. [16] S.N. Piramanayagam, J. Appl. Phys. 102 (2007) 011301. [17] D. Weller, M.F. Doerner, Ann. Rev. Mater. Sci. 30 (2000) 611. [18] H.J. Richter, J. Phys. D: Appl. Phys. 40 (2007) R149. [19] D. Weller, A. Moser, L. Folks, M.E. Best, W. Lee, M.F. Toney, M. Schwickert, J. Thiele, M.F. Doerner, IEEE Trans. Magn. 36 (2000) 10. [20] W. Möller, W. Eckstein, J.P. Biersack, Comput. Phys. Commun. 51 (1988) 355. [21] L.G. Parrat, Phys. Rev. 95 (1954) 359. [22] Y. Xu, J.P. Wang, Y. Su, J. Phys. D: Appl. Phys. 33 (2000) 1460. [23] R.A. Ristau, K. Barmak, L.H. Lewis, K.R. Koffey, J.K. Howard, J. Appl. Phys. 86 (1999) 4527. [24] J.D. Livingston, J. Appl. Phys. 52 (1981) 2544.