Fabrication and Static Magnetic Properties of Novel One-and Two

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2-D MCs for future spintronic applications. For special reviews on magnonics, see references [16]–[20]. In fact, with advances in nanofabrication techniques, it is ...
IEEE TRANSACTIONS ON MAGNETICS, VOL. 47, NO. 6, JUNE 2011

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Fabrication and Static Magnetic Properties of Novel One- and Two-Dimensional Bi-Component Magnonic Crystals A. O. Adeyeye, S. Jain, and Y. Ren Information Storage Materials Laboratory, Department of Electrical and Computer Engineering, National University of Singapore, 117576 Singapore We have fabricated using advanced electron beam lithography, periodic arrays of lateral one-dimensional (1-D) and two-dimensional (2-D) bi-component exchange and magnetostatically coupled magnonic crystals (MC) consisting of alternating cobalt (Co) and Permalloy (Py) nanowires (NWs) and nanodots (NDs), lying side by side. For 1-D MCs, the width of the Co NWs is fixed at 160 nm while the width of the Py NWs is varied from 160 to 800 nm. We observed two distinct switching steps corresponding to the reversal of Py and Co NWs in the array, in contrast to single step switching of homogeneous NWs. For 2-D MCs, the diameter of both Co and Py dots were kept constant at 400 nm. We found that strong dipolar fields from neighboring Co dots strongly influence the reversal process of Py dots resulting in the tuneability of vortex nucleation and annihilations fields. Index Terms—Magnetic nanowires, magnetic superlattice, magnonic crystals.

I. INTRODUCTION

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ERIODIC layered magnetic structures have been studied for more than a decade since the discovery of the giant magneto-resistive effect in the three layer system consisting of magnetic and nonmagnetic layers [1]. Recently, there has been a growing interest aimed at the fundamental understanding of periodic magnetic composites [2]–[5], which have been conceived as the magnetic counterpart of a photonic crystal, and consist of at least two magnetic materials, with spin waves acting as the information carrier. Such periodic magnetic composites can be referred to as “magnonic crystals” (MC) with unique properties that are not found in homogenous magnetic structures. The simplest forms of MCs are periodic arrays of neighboring magnetic nanowires (NWs) (one-dimensional MCs) [6]–[9], a periodic array of holes in a ferromagnetic (FM) thin-film (two-dimensional MC) [10]–[12], or a three-dimensional arrangement of magnetic nanostructures. It has been shown theoretically [13], [14], and experimentally [7]–[9], that the spin wave spectrum of this composite structure exhibits frequency forbidden regions where spin waves are not allowed to propagate, and the energy gaps are found to be sensitive to the exchange contrast between the constituent materials along with the magnetization contrast. Recently, it has been shown theoretically [15], that in 1-D bi-component MCs, the observed band gaps are due to the localization of the spin waves in the permalloy (Py) nanowires. This suggests that the spin wave spectrum is sensitive to the static magnetic properties of FM layers. It is therefore crucial to understand the magnetization reversal process in both1-D and 2-D MCs for future spintronic applications. For special reviews on magnonics, see references [16]–[20]. In fact, with advances in nanofabrication techniques, it is now possible to synthesize such high quality MCs with precisely controlled lateral dimensions [21]–[23]. Manuscript received September 30, 2010; accepted December 25, 2010. Date of current version May 25, 2011. Corresponding author: A. Adeyeye (e-mail: [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMAG.2010.2103554

In this work, we present a systematic study of the magnetic properties of 1-D and 2-D MCs consisting of lithographically defined periodic arrays of lateral bi-component exchange coupled and magnetostatically coupled alternating cobalt (Co) and Py NWs and nanodots (NDs) lying side by side. We have investigated the tuneability of the magnetic properties of the bi-component MCs by varying the width of the Py NWs from 160 nm to 800 nm for a constant Co NW width of 160 nm. The magnetic properties of the MCs are found to be markedly different from the homogeneous NWs consisitng of Co and Py NWs of identical geometrical parameters due to the strong coupling between the alternating Co and Py NWs constituting the MCs. Similarly, 2-D MCs consisting Co and Py dots show distinct reversal mechanism when compared to homogeneous ND arrays. II. EXPERIMENTAL METHODS AND MODELING MCs consisting of periodic arrays of alternating FM NWs and NDs made from two distinct materials (Co and Py) were fabricated on oxidized Si(001) substrates using advanced multi-level electron beam lithography (EBL). For 1-D array, the first NW array of width in the range 160 nm to 800 nm, length , and fixed gap was defined on polymethyl methacrylate (PMMA) resist. Appropriate alignment marks needed for the second stage of the lithography process were also defined at the same time. A 35 nm thick Py film was then deposited by dc magnetron sputtering in a process pressure , followed by of 5 mTorr, with a base pressure of ultrasonic assisted lift-off process. In order to incorporate the Co NW array, another layer of PMMA resist was spun on the same sample. The alignment marks designed during the first EBL stage were then used in exposing the second set of NWs in the gaps between the neighboring Py NWs. The growth process was repeated for the deposition of 35 nm thick Co NWs using dc magnetron sputtering followed by ultrasonic assisted lift-off process. The schematics of the described process are shown in Fig. 1(a). For exchange coupled MCs, the NWs are in direct contact (the inter-wire spacing between the Co and Py NWs is ). For magnetostatically coupled MCs, however, there is a spacing between the NWs. Shown in Fig. 1(b) and (c) are the representative scanning electron microscope

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IEEE TRANSACTIONS ON MAGNETICS, VOL. 47, NO. 6, JUNE 2011

M -H loops for field applied along the easy axis

( = 0 ) for array of Co NWs of width W = 160 nm, inter-wire separation g = 400 nm; arrays of Py NWs of width W = 400 nm, g = 160 nm, Fig. 3.

(a) Experimental

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and (b) experimental - loops of 1-D periodic array of exchange and magnetostatically coupled magnonic crystals from the structures shown in (b & c) consisting of alternating Py and Co NWs of widths , respectively. The peak positions of the differentiated half loops indicate the coercive fields of Co and Py NWs.

400 nm

Fig. 1. (a) Schematic of the fabrication process for bi-component MC structures using multi-level electron beam lithography process, metallization and lift-off processes. Scanning electron microscope images of (b) 1-D exchange , with and (c) 1-D coupled MC of magnetostatically coupled MC of , with .

s = 50 nm

W = 160 nm W = 400 nm s = 0 W = 160 nm W = 400 nm

W = 160 nm W Py =

micromagnetic modeling with LLG micromagnetic software using periodic boundary conditions [24]. Standard parameters were used to characterize the properties of Py , saturation mo(exchange constant , anisotropy ) and Co ment , saturation moment (exchange constant , anisotropy ) layers. The structures were discretized into 10 nm 10 nm cells for which the direction of magnetocrystalline anisotropy was randomly oriented to simulate the polycrystalline nature of the films. III. RESULTS AND DISCUSSION A. 1-D Magnonic Crystals

Fig. 2. (a) Schematic representation of 2-D MC comprised of alternating Co and Py NDs in 2-D arrangement. Scanning electron microscope images for (b) single layer Py NDs, (c) exchange coupled Py and Co NDs placed adjacent to each other, and (d) magnetostatically coupled NDs with a spacing of 80 nm between them.

(SEM) images of 1-D periodic array of exchange coupled MC and Py NWs consisting of Co NWs of width , and magnetostatically coupled NWs of width of similar dimensions with . The 2-D MCs were also fabricated using two-step electron beam lithographic process shown in Fig. 1, with the nanowires patterns replaced with dot arrays. The diameter of both the Co and Py dots were kept constant at 400 nm. Fig. 2(a) shows the schematic of a typical 2-D MC consisting of Co and Py NDs. For exchange coupled 2-D MCs, the adjacent Co and Py dots are in direct contact whereas for magnetostatically coupled array, there is a gap of 80 nm between the NDs. Shown in Fig. 2(b)–(d) are the corresponding SEM images of a homogenous Py ND array, exchange coupled 2-D MCs, and magnetostatically coupled 2-D MCs. The collective in-plane magnetic hysteresis curves were characterized using a focused magneto-optics Kerr effect in the (MOKE) setup with a laser spot size of about 5 longitudinal geometry at room temperature. We have also facilitated our understanding of the reversal process using

Shown in Fig. 3(a) are - loops for arrays of 35 nm thick , inter-wire spacing Co NWs of width , and arrays of 35 nm thick Py NWs of width , for the field applied along the NW easy axis . The NWs display a near rectangular - loops with a coercive field of 570 Oe for Co NWs and 160 Oe for the Py NWs. Details of the reversal mechanism of homogenous NWs are described in [25]. As expected, the coercive field for 160 nm wide Co NWs is much larger than that of the 400 nm wide Py NWs. The corresponding - loops for the exchange and magnetostatically coupled MCs consisting of alternating , and NWs made from Py and Co of widths are shown in Fig. 3(b). Compared with the homogenous NWs, the - loops are markedly different exhibiting distinct two-step switching, corresponding to the magnetization reversal of Py NWs at a low field and Co NWs at a high field. For the magnetostatically coupled MCs, the loop can be understood by considering dipolar interactions between the bi-component NWs caused by the magnetic field of the poles at the ends of the NWs. In addition to external applied due field, each wire feels the influence of dipolar fields is the field of wire to the presence of the other wire, where over the wire and is given by [26]

(1)

ADEYEYE et al.: FABRICATION AND STATIC MAGNETIC PROPERTIES OF NOVEL ONE- AND TWO-DIMENSIONAL BI-COMPONENT MAGNONIC CRYSTALS

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W

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Fig. 4. (a) Simulated - loops for field applied along the easy axis ( = 0 ) for array of Co NWs of width = 160 nm, inter-wire separation = 400 nm; arrays of Py NWs of width = 400 nm, = 160 nm, and 1-D magnetostatically coupled MC similar to the structure shown in Fig. 1(c). S are shown as insets. The corresponding magnetic spin states marked as S (b) The demagnetizing energy obtained from LLG is also shown for the three NW arrays as function of external magnetic field.

s

W

0

s

where is a geometric factor which is a function of inter-wire is the magnetization of the th wire. For the spacing and simplest case, we can consider two bi-component NWs (one Co NW and one Py NW) lying side by side with a very small spacing between them where the mutual dependence is produced by the dipolar interaction through the functions and . At saturation field, both Py and Co NWs are parallel to the field direction. When the reverse field is greater than the switching field of Py NWs but less than the switching field of Co NWs, a state of anti-parallel alignment of the magnetization is attained. The reversal of the Py NWs leads to modification of the total magnetic field strength sensed by the Co NWs whose magnetization has not yet reversed. This state persists until the reserve field is greater than the switching field of the Co NWs. The corresponding switching fields of individual NWs in the bi-component NWs is lower than the homogenous NWs of identical dimensions due to the strong coupling. We have modeled the magnetization reversal of magnetostatically coupled MC (consisting of a 35 nm thick Co NWs of width , sandwiched between a 35 nm Py NWs of width ) using periodic boundary conditions and compared our results with homogeneous NWs. Shown in Fig. 4(a) are the simulated - loops for homogenous Co and Py NWs and magnetostatically coupled MCs corresponding to the experimental - loops shown in Fig. 3(a) and (b) respectively. The detailed features of the experimental - loops including the 2-step switching for the magnetostatically coupled MCs can be clearly reproduced in the simulated results. We have captured snapshots of the magnetic states at positions , and on the simulated - loop of the coupled MC as shown as an

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W

Fig. 5. (a)–(d) Experimental - loops of 1-D periodic array of exchange and = 160 nm) magnetostatically coupled MCs of constant Co NWs width ( as a function of Py NWs width in the range from 160 to 800 nm. (e) A plot of the coercive field of Py NWs extracted from Fig. 3(a)–(d) for both exchange and for fixed of magnetostatically coupled arrays, as a function of the 160 nm.

W

W

inset. The magnetization states and correspond to magnetic states when both the Co and Py NWs are aligned along the field directions. The magnetic state at position corresponds to the relative anti-parallel alignment of magnetization between the two different arrays of NWs. This magnetic spin configuration was not observed in the simulated homogenous NWs. We have also extracted the demagnetizing energy for both the homogenous and bi-component MCs from the simulated data as shown in Fig. 4(b). It is obvious that demagnetizing energy play a key role in the magnetization reversal process. The demagnetizing energy of homogenous NWs is much larger than the magnetostatically coupled MCs due to the strong dipolar coupling which in turn affects the switching fields. It was difficult to model the reversal of the exchange coupled MC due to the problem associated with determining the exchange parameters at the interface between the bi-component NWs. The tuneability of the static magnetic properties of both exchange and magnetostatically coupled MCs were systematically investigated by varying the varying the width of Py NWs from 160 to 800 nm while keeping the width of Co NWs fixed at . Shown in Fig. 5(a)–(d) are the representative - loops for 1-D exchange and magnetostatically coupled MCs of fixed Co NWs width and varied Py NWs width . As

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Fig. 6. Experimental - loops for (a) exchange coupled NDs and (b) magnetostatically coupled NDs along with their single layer NDs of Py and Co. The simulated spin states at remanence for both types of 2-D MCs are shown in (c) and (d). The simulated spin states for single Py dots at remanence are also shown in (e) and (f).

sponding - loops for the magnetostatically coupled MCs for 2-D MCs is comare shown in Fig. 6(b). Interestingly, parable in magnitude to that of Py NDs. The difference between the static magnetic properties of 2-D exchange and magnetostatically coupled MCs can be attributed to the type of coupling mechanism between the two NDS. One is dominated by the exchange interactions between the dots, as they are in direct contact with each other, and the second arrangement is dominated by strong dipolar coupling between neighboring dots. The result is tuneability in switching fields for the individual NDs, as well as in the overall reversal mechanism. We have also modeled the reversal mechanism in the 2-D exchange and magnetostatically coupled MCs and compared the remanent spin configurations adopted by each element constituting the MCs. Shown in Fig. 6(c), (d) are the simulated spin configurations at remanence for both 2-D exchange coupled and magnetostatically coupled MCs. In order to study how the presence of Co dots in direct contact (exchange) and close proximity (magnetostatic) affect the spin state of the Py dots in the MCs, we have also simulated the spin configuration using the same mask but with the Co dots removed, thereby corresponding to homogenous Py dots. Shown in Fig. 6(e), (f) are the corresponding spin configurations at remanence for the homogenous Py dots. It is clearly evident that the spin configurations of Py dots in the MC are strongly dominated by the stray fields from neighboring Co dots. This prevents nucleation of vortex core in Py dots resulting in complex “S” shape spin states. This is in contrast to homogeneous Py NDs where all NDs are in stable vortex state at remanence. IV. CONCLUSION

clearly shown, the magnetic properties of the MCs can be tuned by changing the geometrical parameters such as the width of the Py NWs. In addition, the relative contribution of the two individual NWs to the overall magnetization reversal changes drastically due to relative changes in the volume of the materials. We have extracted the coercive field of Py NWs and plotted it as a function of for the both exchange coupled and magnetostatically coupled MCs as shown in Fig. 5(e). As increases, the coercive field of the Py NWs decreases rapidly and approaches the minimum value for . This is also in agreement with the micromagnetic simulations on similar structures (results not shown here). We also note that the coercive field of exchange coupled MCs are much lower than the magnetostatically coupled MCs of identical NW widths. B. 2-D Magnonic Crystals In this section, we have investigated the magnetic properties of both exchange and magnetostatically coupled 2-D MCs. Shown in Fig. 6(a) are the representative - loops for 2-D exchange coupled MCs along with the corresponding - loops for homogenous Py and Co ND arrays fabricated under similar conditions. It is evident that the reversal process of the 2-D MCs is markedly different from homogenous Co and Py NDs. for Py in MC is higher in magniThe nucleation field tude as compared to homogeneous Co and Py dots. Similarly, for the exchange coupled MCs is in the annihilation field between the for homogenous Py and Co dots. The corre-

Highly ordered exchange and magnetostatically coupled bi-component MCs consisting of alternating Co and Py NWs and NDs lying side by side have been fabricated and characterized using focused magneto-optic Kerr effects. The magnetic properties of the bi-component crystals are found to be markedly different from homogeneous 1-D NWs and 2-D arrays of NDs of identical geometrical parameters due to the strong coupling. Our experimental results of bi-component MCs are in good agreement with micromagnetic simulations. ACKNOWLEDGMENT This work was supported by the National Research Foundation, Singapore under Grant NRF-G-CRP 2007-05. REFERENCES [1] M. N. Baibich, J. M. Broto, A. Fert, F. N. Vandau, F. Petroff, P. Eitenne, G. Creuzet, A. Friederich, and J. Chazelas, “Giant magnetoresistance of (001)Fe/(001)Cr magnetic superlattices,” Phys. Rev. Lett., vol. 61, p. 2472, 1988. [2] S. A. Nikitov, P. Tailhades, and C. S. Tsai, “Spin waves in periodic magnetic nanostructurre-magnonic crystals,” J. Magn. Magn. Mater., vol. 236, p. 320, 2001. [3] V. V. Kruglyak and R. J. Hicken, “Magnonics: Experiments to prove the concept,” J. Magn. Magn. Mater., vol. 306, p. 191, 2006. [4] M. Krawczyk and H. Puszkarski, “Plane-wave theory of three-dimensional magnonic crystals,” Phys. Rev. B, vol. 77, p. 054437, 2008. [5] K.-S. Lee, D.-S. Han, and S.-K. Kim, “Physical origin and generic control of magnonic band gaps of dipole-exchange spin waves in widthmodulated nanostrip waveguides,” Phys. Rev. Lett., vol. 102, p. 127202, 2009.

ADEYEYE et al.: FABRICATION AND STATIC MAGNETIC PROPERTIES OF NOVEL ONE- AND TWO-DIMENSIONAL BI-COMPONENT MAGNONIC CRYSTALS

[6] G. Gubbiotti, S. Tacchi, G. Carlotti, N. Singh, S. Goolaup, A. O. Adeyeye, and M. Kostylev, “Collective spin modes in monodimensional magnonic crystals consisting of dipolarly coupled nanowires,” Appl. Phys. Lett., vol. 90, p. 092503, 2007. [7] M. Kostylev, P. Schrader, R. L. Stamps, G. Gubbiotti, G. Carlotti, A. O. Adeyeye, S. Goolaup, and N. Singh, “Partial frequency band gap in one-dimensional magnonic crystals,” Appl. Phys. Lett., vol. 92, no. 13, p. 132504, 2008. [8] A. V. Chumak, A. A. Serga, B. Hillebrands, and M. P. Kostylev, “Scattering of backward spin waves in a one-dimensional magnonic crystal,” Appl. Phys. Lett., vol. 93, p. 022508, 2008. [9] Z. K. Wang, V. L. Zhang, H. S. Lim, S. C. Ng, M. H. Kuok, S. Jain, and A. O. Adeyeye, “Observation of frequency band gaps in a onedimensional nanostructured magnonic crystal,” Appl. Phys. Lett., vol. 94, p. 083112, 2009. [10] S. L. Vysotskii, S. A. Nikitov, and Y. A. Filimonov, “Magnetostatic spin waves in two-dimensional periodic structures,” J. Exp. Theor. Phys., vol. 101, p. 547, 2005. [11] C. C. Wang, A. O. Adeyeye, and N. Singh, “Magnetic antidot nanostructures: Effect of lattice geometry,” J. Inst. Phys. Nanotechnol., vol. 17, p. 1629, 2006. [12] S. Neusser, B. Botters, M. Becherer, D. Schmitt-Landsiedel, and D. Grundler, “Spin-wave localization between nearest and next-nearest neighboring holes in an antidot lattice,” Appl. Phys. Lett., vol. 93, p. 122501, 2008. [13] H. Al-Wahsh, A. Akjouj, B. Djafari-Rouhani, J. O. Vasseur, L. Dobrzynski, and P. A. Deymier, “Large magnonic band gaps and defect modes in one-dimensional comblike structures,” Phys. Rev. B, vol. 59, p. 8709, 1999. [14] J. O. Vasseur, L. Dobrzynski, B. DjafariRouhani, and H. Puszkarski, “Magnon band structure of periodic composites,” Phys. Rev. B, vol. 54, no. 2, pp. 1043–1049, Jul. 1, 1996.

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[15] M. L. Sokolovskyy and M. Krawczyk, unpublished. [16] V. V. Kruglyak, S. O. Demokritov, and J. Grundler, “Magnonics,” J. Phys. D: Appl. Phys., vol. 43, p. 264001, 2010. [17] A. A. Serga, A. V. Chumak, and B. Hillebrands, “YIG magnonics,” J. Phys. D: Appl. Phys., vol. 43, p. 264002, 2010. [18] G. Gubbiotti, S. Tacchi, M. Madami, G. Carlotti, A. O. Adeyeye, and M. Kostylev, “Brillouin light scattering studies of planar metallic magnonic crystals,” J. Phys. D: Appl. Phys., vol. 43, p. 264003, 2010. [19] S-K. Kim, “Micromagnetic computer simulations of spin waves in nanometre-scale patterned magnetic elements,” J. Phys. D: Appl. Phys., vol. 43, p. 264004, 2010. [20] A. Khitun, M. Bao, and K. L. Wang, “Magnonic logic circuits,” J. Phys. D: Appl. Phys., vol. 43, p. 264005, 2010. [21] A. O. Adeyeye and N. Singh, “Large area patterned magnetic nanostructures,” J. Phys. D: Appl. Phys., vol. 41, p. 153001, 2008. [22] J. Fassbender, D. Ravelosona, and Y. Samson, “Tailoring magnetism by light-ion irradiation,” J Phys D Appl Phys, vol. 37, no. 16, pp. R179–R196, Aug. 21, 2004. [23] J. I. Martin, J. Nogues, K. Liu, J. L. Vicent, and I. K. Schuller, “Ordered magnetic nanostructures: Fabrication and properties,” J. Magn. Magn. Mater., vol. 256, p. 449, 2003. [24] M. R. Scheinfein, LLG Micromagnetics Simulator [Online]. Available: http://llgmicro.home.mindspring.com [25] S. Goolaup, N. Singh, A. O. Adeyeye, V. Ng, and M. B. A. Jalil, “Transition from coherent rotation to curling mode reversal process in ferromagnetic nanowires,” Eur. Phys. J. B, vol. 44, p. 259, 2005. [26] J. Velázquez, C. García, M. Vázquez, and A. Hernando, “Dynamic magnetostatic interaction between amorphous ferromagnetic wires,” Phys. Rev. B, vol. 54, no. 14, pp. 9903–9911, Oct. 1996.