Optimization of interface condition for efficient spin ... - IEEE Xplore

Optimization of interface condition for efficient spin injection in Permalloy/Cu lateral spin valve 1

S. Yakata1,2 , Y. Ando1 and T. Kimura1,2 INAMORI Frontier Research Center, Kyushu University, 744 Motooka, Fukuoka, 819-0395, Japan 2 CREST, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi 332-0012, Japan

Abstract—We have investigated the cleaning condition of the Permalloy (Py) surface optimized for the efficient pure spin current injection in Py/Cu lateral spin valve structure. The efficiency of the spin injection is found to depend on the beam voltage and milling time strongly. The post annealing is also found to improve the spin injection efficiency. Index Terms—Spin injection, Pure spin current, Spin accumulation

I. I NTRODUCTION Electrical spin injection from ferromagnet (F) into nonmagnet (N) is an important technique for developing spintronics as well as spin-dependent transport phenomenon.[1] When a spin-polarized current flows across the interface between F and N, the spin accumulation is induced in the vicinity of the F-N junction because of a sudden change in the spindependent electrical conductivity.[2] Thus, one can induce the nonequilibrium magnetization in N. To achieve the electrical spin injection into N, most of the experiments have been carried out in the vertical configurations so called current perpendicular to plane (CPP) structure. This is because the traveling length for the spin current can be controlled by the spacer thickness.[3] However, the CPP configuration is under great disadvantages to make multi-terminal devices, wherefore one can only obtain limited information, about a series resistance of the magnetic multilayers. On the other hand, the laterally configured F/N hybrid structure is useful to realize functional multi-terminal devices because of their flexibilities. The problem is that the spurious magnetoresistance tends to smear intrinsic spindependent signals of interest in lateral structures. However, recent nanofabrication techniques enable us to observe spindependent transport phenomena even in lateral F/N hybrid structures. Especially, nonlocal detection technique is an effective method for extracting pure spin-related signal.[4]–[6] To achieve an efficient spin injection in the lateral structures, optimizing the interface condition between the F and N is indispensable. A clean F/N interface can be achieved by using a shadow evaporation technique.[7], [8] However, the device geometry is restricted by the undesired structures deposited during the process. This problem prevents the fabrication of the complex device geometry in lateral configurations. The spin dependent signal can be observed also in the lateral spin valve structures fabricated by repeating the lift-off process. In order to obtain the clean F/N interface, the Ar ion milling is performed. However, in the previous studies it has not

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Fig. 1. (a)SEM image of fabricated lateral spin-valve structure consisting of three Permalloy (Py) wires bridged by a Cu strip. (b)Nonlocal spin-valve signal measured at 77K.

been confirmed that the milling is suitable for the efficient spin injection and detection. In this study, to obtain the ideal F/N interface, we investigated the spin injection efficiency for various Ar milling conditions. II. E XPERIMENTAL PROCEDURE Lateral spin-valve structure consisting of three Permalloy (Py) wires bridged by a Cu strips has been fabricated by means of electron beam lithography and lift-off technique.   Figure 1 shows a scanning-electron-microscope (SEM) image of the lateral spin valve used in this study. First, we fabricated the Py wires 100 nm in width and 30 nm in thickness by e-gun evaporation with the base pressure less than 2 × 10−9 Torr. Here, Py1 has large pads connected to its edges while Py2 and Py3 have pointed edges and flat-end, respectively. This induces the difference in the switching field of each Py wire. Then, a Cu strip was fabricated across the Py wires by the lithographer and a Joule heating evaporator with a base pressure of 3 ×10−8 Torr. The interface between Py and Cu

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is cleaned by an Ar ion milling prior to the Cu deposition. We prepared the lateral spin valves with various ion milling conditions. The quality of interface has been evaluated by the nonlocal spin valve measurements. As shown in Fig. 1(a), when the electric current is injected from the Py1 into the top-side of the Cu wire, the nonlocal spin accumulation is induced in the bottom-side of the Cu wire. When one attaches another ferromagnet Py2 to the bottom-side of the Cu, the nonlocal spin accumulation is induced also in Py2 because of the continuity of the chemical potential at the interface. Since the conductivity of Py depends on the spin direction, the voltage difference is induced in between the Py2 and the bottom side of the Cu wires. The induced nonlocal voltage depends on the magnetization configurations. When the magnetizations Py1 and Py2 are in parallel (anti-parallel), the voltage becomes maximum (minimum) as in Fig. 1(b). The signal exhibits clear spin valve effect. In this way, by measuring the field dependence of the nonlocal voltage one can estimate the magnitude of the spin accumulation at the Py2/Cu interface.[5], [7], [9]

Fig. 2. Nonlocal spin-valve signal measured at R.T. as a function of Ar ion beam voltage

III. R ESULTS AND DISCUSSIONS First, we change the beam voltage of the Ar ion milling in the range from 400 V to 800 V. Figure 2 shows the nonlocal spin signals measured at room temperature as a function of the beam voltage. Here, the beam current and milling time are 40 mA and 20 s, respectively. The spin signal takes a maximum at 500 V. When the beam voltage is high, the Ar ion bombardments induce the defects in the Py surface.[10] The spin polarization should decrease. Therefore, the reduction of the spin signal with high beam voltage is due to the influence of the Ar ion bombardment. In the low voltage, the reduction of the spin signal is also observed. This is probably because the surface of the Py is not removed sufficiently. Consequently the spin injection efficiency is reduced indicating the spin flipping at the interface. The milling time is varied from 0 to 60 s, where the beam voltage and current are fixed at 500 V and 40 mA, respectively. As in Fig. 3, the spin signal takes a maximum with the milling time 10 s. The obtained time dependence can be understood by the mechanism similar to the beam voltage dependence. When the milling time is too long (> 20 s), the spin polarization decreases due to the Ar ion bombardment. Small spin signals at short milling time (> 10 s) is due to the insufficient cleaning of the Py surface. For increasing the spin polarization of the Py surface, the post annealing has been also performed for 1 hour at 300 o C in the base pressure of 4 × 10−6 Pa. As shown in Fig. 3, the spin signals in all devices are enhanced by the post annealing. Finally, we obtained the spin signal with 0.6 mΩ at RT and 1.4 mΩ at 77K. The improvements are caused by the increases of the spin polarization for the Py at interface and the spin diffusion length for the Cu wire. In fact, the electrical conductivity of the Cu increases with a factor of 1.5 by the post annealing. However, it should be noted that the post annealing condition is not properly optimized yet.

Fig. 3. The nonlocal spin-valve signal measured at R.T. as a function of milling time. The nonlocal spin-valve signal are enhanced with post annealing.

IV. C ONCLUSION We have fabricated lateral spin valves for various interface milling conditions. The spin signal does not change monotonically as functions of the beam voltage and milling time. We also found the post annealing for 1 hour at 300 o C improve the spin transport properties. The optimizing the milling yields the spin signal with. ACKNOWLEDGMENT This work was partly supported by a Grant-in-Aid for Scientific Research on Priority Area from the MEXT, Industrial Technology Research Grant Program from NEDO. R EFERENCES [1] For example, S. A. Wolf, D. D. Awschalom, R. A. Buhrman et al., “ Spintronics: A spin-based electronics vision for the future, ” Science, vol. 294, no. 5546, pp. 1488-1495, Nov 16, 2001. [2] P. C. Vanson, H. Vankempen, and P. Wyder, “ Boundary Resistance of the Ferromagnetic-Nonferromagnetic Metal Interface, ” Physical Review Letters, vol. 58, no. 21, pp. 2271-2273, May 25, 1987. [3] W. P. Pratt, Jr., S.F. Lee, J. M. Slaughter, R. Loloee, P. A. Schroeder, and J. Bass,“ Perpendicular Giant Magnetoresistances of Ag/Co Multilayers, ” Physical Review Letters, vol. 66, no. 23, pp. 3060-3063, Jun 10, 1991. [4] M. Johnson, and R. H. Silsbee, “ Interfacial Charge-Spin Coupling Injection and Detection of Spin Magnetization in Metals, ” Physical Review Letters, vol. 55, no. 17, pp. 1790-1793, 1985. [5] F. J. Jedema, A. T. Filip, and B. J. van Wees, “ Electrical spin injection and accumulation at room temperature in an all-metal mesoscopic spin valve, ” Nature, vol. 410, no. 6826, pp. 345-348, Mar 15, 2001.

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[6] T. Kimura, J. Hamrle, Y. Otani, K. Tsukagoshi and Y. Aoyagi, “ Spindependent boundary resistance in the lateral spin-valve structure, ” Applied Physics Letters, vol. 85, no. 16, pp. 3501-3503, Oct 18, 2004. [7] T. Kimura, T. Sato, and Y. Otani, “ Temperature evolution of spin relaxation in a NiFe/Cu lateral spin valve, ” Physical Review Letters, vol. 100, no. 6, pp. -, Feb 15, 2008. [8] S. O. Valenzuela, and M. Tinkham, “ Spin-polarized tunneling in roomtemperature mesoscopic spin valves, ” Applied Physics Letters, vol. 85, no. 24, pp. 5914-5916, Dec 13, 2004. [9] S. Takahashi, and S. Maekawa,“ Spin injection and detection in magnetic nanostructures, ”Physical Review B, vol. 67, no. 5, 052409, Feb 1, 2003. [10] C. Chappert, H. Bernas, J. Ferre, V. Kottler, J.-P. Jamet, Y. Chen, E. Cambril, T. Devolder, F. Rousseaux, V. Mathet, and H. Launois,“ Planar Patterned Magnetic Media Obtained by Ion Irradiation, ” Science, vol. 280, pp. 1919-1922, June 19, 1998.

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