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TPD granular films
Journal of Alloys and Compounds 477 (2009) 32–35
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Tunneling magnetoresistance effect in Co/TPD granular films P. Sheng a,b , G. Ni a,b,d,∗ , J.F. Yin a,b , Y. Zhang a,b , Z.Y. Tang c , S.M. Zhou a,b , Q.Y. Jin a,b a
Department of Optics Science and Engineering, Fudan University, Shanghai 200433, PR China State Key Laboratory for Advanced Photonic Materials and Devices, Fudan University, Shanghai 200433, PR China c Department of Material Science and Engineering, Fudan University, Shanghai 200433, PR China d National Laboratory of Solid State Microstructures, Nanjing University, Nanjing, Jiangsu 210093, PR China b
a r t i c l e
i n f o
Article history: Received 12 May 2008 Received in revised form 21 October 2008 Accepted 24 October 2008 Available online 6 December 2008 Keywords: Composite materials Magnetic films and multilayers Magnetoresistance Tunneling
a b s t r a c t A series of Cox (TPD)1−x granular film samples were fabricated by co-evaporating technique, where Co nanoparticles are dispersed in TPD molecules matrix. The HRTEM images show typical microstructural characteristic of granular films. Magnetization, electrical conduction, and magnetoresistance (MR) of Cox (TPD)1−x granular films, have been investigated over a wide temperature range and Co volume fraction. The negative MR have been observed in these samples, reaching −1.75% (x = 36 vol.%, at 4.2 K, 10 kOe). The mechanism of magneto-transport behaviors has been discussed. It is suggested that the negative MR results from tunneling magnetoresistance (TMR) effect. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Recently, there has been growing interest in magnetoelectronic effects in organic semiconductors (OSCs). Because OSCs consist mainly of carbon and hydrogen, they provide longer spin relaxation time than which is achievable by conventional spintronics materials, such as metals and inorganic semiconductors [1]. Hence, the introduction of OSCs has led to novel developments in the field of spintronics [2–4]. Recent observations of magnetoresistance (MR) effects in OSCs open up the potential of these materials for spin-conserved transport. In 2004, Xiong et al., reported a maximum GMR ratio of 40% in a Tris-(8hydroxyquinoline) aluminum (III) (Alq3 ) based spin-valves at 11 K [3]. Recently, a large magnetoresistance effect has been observed in non-magnetic organic light-emitting diodes (OLEDs) devices, named organic magnetoresistance (OMAR) [4]. However, the mechanism of these novel phenomena in OSCs is currently not very clear. Until now, most of the researches are focused on junction structure with multilayer films in the field of organic spintronics. As we know, inorganic granular films, consisting of single-domain ferromagnetic particles embedded in an immiscible inorganic medium, have been extensively studied in the past two decades
∗ Corresponding author at: Department of Optics Science and Engineering, Fudan University, 220 Handan Road, Shanghai 200433, PR China. Fax: +86 21 55664192. E-mail address: [email protected] (G. Ni). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.10.123
[5,6]. Thus, it is very interesting that what will happen if the inorganic matrix is replaced by organic semiconductor materials in granular films. Recently, magnetoresistance effect has been also found in Co/C60 nanocomposites films [7], showing efficient spin polarized injection and transport can be realized in ferromagnetic-organic semiconductors (FM/OSCs) granular films. In the present work, we investigated the microstructures, magnetic and transport properties of FM/OSCs compound nanometer granular films, in which ferromagnetic Co nanoparticles dispersed in N,N -bis(3-methylphenyl)-1,1 -biphenyl-4,4 diamine (TPD) matrix. TPD is one of the most well-investigated OSCs, as a kind of hole transport materials which is popularly used in organic lightemitting diodes (OLEDs). The molecular structure of TPD is shown in Fig. 1. 2. Experimental A series of Co-TPD granular films were fabricated by coevaporation technique on glass substrates in a vacuum chamber with a base pressure of about 3 × 10−4 Pa. TPD and Co are deposited by using a resistance-heated tantalum crucible and BN beam source evaporator, respectively. The different volume ratio of TPD and Co in Co-TPD granular films are obtained by regulating the evaporation rate of the two materials. The evaporation rate of TPD were settled at 0.1–0.5 Å/s and that of Co were settled at 0.1–0.8 Å/s. The thicknesses of the films are about 350 nm. The value of evaporation rate and the film thickness are determined by a quartz crystal thickness monitor (Sigma SQM-160). The microstructures of granular film samples were investigated by high resolution transmission electron microscopy (HRTEM, Jeol JEM-2010). The magnetic properties of these samples were measured using a vibrating sample magnetometer (VSM, Lakeshore 7300) at room temperature. Magnetoresistance (MR) and resistance of the samples at different temperatures (from 4.2 to 300 K) were measured using physical properties measurement system (PPMS, Quantum Design,
P. Sheng et al. / Journal of Alloys and Compounds 477 (2009) 32–35
33
Fig. 1. The molecular structure of TPD.
Inc.) with a dc four-terminal configuration. The MR values are calculated according to MR = / = (H − 0 )/0 , where H and 0 are the resistivities in the applied field and initial zero field, respectively. The applied field was perpendicular to the plane of films.
3. Results and discussion Fig. 2 shows the HRTEM image of as-deposited Co0.36 (TPD)0.64 granular film sample and the corresponding electron diffraction pattern. It indicates the typical microstructural characteristic of granular films, where the randomly oriented granular Co particles are isolated by the TPD molecules. The particle size is approximately 2–5 nm. The brightly imaging regions are amorphous TPD, and the darkly imaging regions with a lattice fringe contrast are the metallic Co particles. The corresponding electron diffraction pattern revealed a polycrystalline structure of cobalt granules, three diffraction rings can be observed, which are close to the diffraction rings of cobalt with hexagonal close-packed (hcp) structure. The magnetic properties of samples were investigated with vibrating sample magnetometer (VSM). Fig. 3 presents the hysteresis loops of Co0.36 (TPD)0.64 granular film sample. The M–H curves shows no residual magnetization at zero magnetic field and no hysteresis curve, which indicates that the Co granules in the sample shows typical superparamagnetic nature. We also notice that there are no obvious differences between the two hysteresis loops when the magnetic field is applied parallel and perpendicular to the plane of film. This is a key feature of granular systems distinctively different from that in multilayer, indicating that the Co granules are almost spherical and there is no strong coupling between neighboring magnetic granules. The VSM results are consistent with the microstructural characteristic of granular films as shown in the above HRTEM image. The magnetoresistance curves are shown in Fig. 4 for Co0.36 (TPD)0.64 granular film sample at three different temperatures (T = 4.2, 80 and 300 K). At room temperature, a small negative magnetoresistance has been obtained, about −0.3% in the applied field of 10 kOe. As the temperature decreases, the MR ratio increases
Fig. 3. Magnetic hysteresis loops of Co0.36 (TPD)0.64 granular film sample.
Fig. 4. Magnetoresistance curves of Co0.36 (TPD)0.64 granular film sample at different temperatures (T = 4.2, 80, and 300 K).
accordingly, reaches about −0.6% at 80 K and −1.75% at 4.2 K, respectively. As we notice, the butterfly-like peaks have been observed in the MR curve near zero field at 4.2 K, indicates that the magnetization properties change from the superparamagnetism to
Fig. 2. HRTEM image of Co0.36 (TPD)0.64 granular film sample and the corresponding electron diffraction pattern.
34
P. Sheng et al. / Journal of Alloys and Compounds 477 (2009) 32–35
the ferromagnetism at 4.2 K. It is suggested that the maximum of the resistance takes place at the coercive field, which corresponds to the state of maximum disorder in the orientation of the neighboring magnetic particle moments. The lowest resistivity is realized at the saturation field where all the particles are ferromagnetically aligned. Thus, it is suggested that the spin-related scattering or spin-dependent tunneling of carriers between neighboring Co clusters may result in the negative MR in the granular system. In Fig. 5, temperature dependence of magnetoresistance of the Cox (TPD)1−x granular films with different Co fraction (x = 0.23–0.58) is shown. Both of these samples show small negative magnetoresistance effect at room temperature, and with the decrease of temperature, the MR ratio increase as shown before in Fig. 4, and reach largest values at 4.2 K. As shown in the inset of Fig. 5, the concentration dependence of MR (x = 0.23–0.58) is presented at three different temperatures (T = 4.2, 80, and 300 K). The optimal value of MR appears at x = 0.36 sample, which is a little below percolation threshold point. In order to understand the intrinsic mechanism of transport properties in the granular system, temperature dependence of the resistivity for different x samples has been investigated, shown in Fig. 6(a), where temperature axis is on a logarithmic scale. With the temperature decreases, the resistivity of the sample increases nonlinearly, showing negative temperature coefficient of resistance (TCR). It indicates its thermal activated conducting or tunneling conducting characteristics behavior. For disordered metal or metal/insulator granular system, Helamn and Abeles drew a conclusion that the temperature dependence of resistivity can be described as following [8],
sE 1/2 c
= a exp 2
kT
(1)
where Ec means the electrostatic charging energy of metal granules, and k stand for a constant that depend only on the volume fraction and Boltzmann’s constant, respectively. s is the tunnel-barrier thickness. As shown in Fig. 6(b), a logarithm of R plot is found well linearly proportional to T−1/2 . This agreement suggests that the electric transport in Cox (TPD)1−x granular films is mainly due to a tunneling mechanism of carriers across the barrier between neighboring Co granules through the organic semiconductor bar-
Fig. 5. Temperature dependence of magnetoresistance of the Cox (TPD)1−x granular films with different Co fraction (x = 0.23–0.58), the concentration dependence of MR at three different temperatures (T = 4.2, 80, and 300 K) is shown in the inset.
Fig. 6. (a) Temperature dependence of the resistivity for different x samples and (b) plotted in log − T−1/2 relation.
riers, resulting the negative tunneling magnetoresistance in this granular system. The cobalt granules with their anisotropic easyaxes orient randomly without an applied magnetic field, and the magnetic moments of the magnetic granules in the hybrid film have no preferred orientation. When an external magnetic field is applied, the magnetic moments of the cobalt particles will tend to align in parallel with the field direction. With the rise of applied magnetic field, the resistance of the film decreases, due to that the probability of electron tunneling process increases when the magnetization of the cobalt granules are in a parallel state [9]. In the tunnel-type magnetic nanostructures, the charge transport is caused by tunneling through insulating barriers. MR value is mostly relevant to the spin polarization of magnetic metal, the distance of neighbor granules and the granular size distribution. The enhancement of MR at low temperatures shown in Fig. 5 can be interpreted by the effect of higher-order tunneling in Coulomb blockade regime of granular systems [10]. The concentration dependence of TMR in Co-TPD granular film samples can be also understood as following. For larger x samples, the inter-granule barrier is not large enough for the carriers to tunnel, due to the partly overlap of electron wave functions in adjacent metal granules. For smaller x samples, due to the large inter-granule resistance, the tunneling probability is reduced, thus results in the reduced TMR. Thus, the optimal MR appears in the granular samples near and below the percolation point. As we also can see clearly in Fig. 6(b), with the rise of x, the slope of curve decreases, which indicates higher activation energy in smaller x samples due to the longer distance between adjacent Co granules. The results are consistent with the above microstructure characteristic of the granular
P. Sheng et al. / Journal of Alloys and Compounds 477 (2009) 32–35
film samples, and are also similar to inorganic M–I granular films [11,12]. 4. Conclusions In conclusion, granular films composed of Co granules embedded in TPD matrix were prepared by coevaporation. The HRTEM images and magnetic properties show typical microstructural characteristic of granular films. Negative magnetoresistance were observed, the concentration and temperature dependence of magnetoresistance and resistivity were investigated. It is suggested that the negative magnetoresistance is originated from the spin-dependent tunneling process between the neighboring ferromagnetic Co granules. Acknowledgment This work was supported by National Natural Science Foundation of China under Grant 60501002.
35
References [1] S. Pramanik, C.G. Stefanitai, S. Patibandla, S. Bandyopadhyay, K. Garre, N. Harth, M. Cahay, Nature Nanotech. 2 (2007) 216–219. [2] V. Dediu, M. Murgia, F.C. Matacotta, C. Taliani, S. Barbanera, Solid State Comm. 122 (2002) 181–184. [3] Z.H. Xiong, D. Wu, Z.V. Vardeny, J. Shi, Nature 427 (2004) 821–824. [4] Ö. Mermer, G. Veeraraghavan, T.L. Francis, Y. Sheng, D.T. Nguyen, M. Wohlgenannt, A. Köhler, M.K. Al-Suti, M.S. Khan, Phys. Rev. B 72 (2005) 205202. [5] A.E. Berkowitz, J.R. Mitchell, M.J. Carey, A.P. Young, S. Zhang, F.E. Spada, F.T. Parker, A. Hutten, G. Thomas, Phys. Rev. Lett. 68 (1992) 3745–3748. [6] J. Moodera, L.R. Kinder, T.M. Wang, R. Meservey, Phys. Rev. Lett. 74 (1995) 3273–3276. [7] S. Miwa, M. Shiraishi, S. Tanabe, M. Mizuguchi, T. Shinjo, Y. Suzuki, Phys. Rev. B 76 (2007) 214414. [8] J.S. Helamn, B. Abeles, Phys. Rev. Lett. 37 (1976) 1429–1432. [9] H.Y. Kwong, Y.W. Wong, K.H. Wong, J. Appl. Phys. 102 (2007) 114303. [10] S. Mitani, S. Takahashi, K. Takanashi, K. Yakushiji, S. Maekawa, H. Fujimori, Phys. Rev. Lett. 81 (1998) 2799–2802. [11] H. Fujimori, S. Mitani, S. Ohnuma, Mater. Sci. Eng. B 31 (1995) 219–223. [12] A. Milner, A. Gerber, B. Groisman, M. Karpovsky, A. Gladkikh, Phys. Rev. Lett. 76 (1996) 475–478.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Tunneling magnetoresistance effect in Co/TPD granular films P. Sheng a,b , G. Ni a,b,d,∗ , J.F. Yin a,b , Y. Zhang a,b , Z.Y. Tang c , S.M. Zhou a,b , Q.Y. Jin a,b a
Department of Optics Science and Engineering, Fudan University, Shanghai 200433, PR China State Key Laboratory for Advanced Photonic Materials and Devices, Fudan University, Shanghai 200433, PR China c Department of Material Science and Engineering, Fudan University, Shanghai 200433, PR China d National Laboratory of Solid State Microstructures, Nanjing University, Nanjing, Jiangsu 210093, PR China b
a r t i c l e
i n f o
Article history: Received 12 May 2008 Received in revised form 21 October 2008 Accepted 24 October 2008 Available online 6 December 2008 Keywords: Composite materials Magnetic films and multilayers Magnetoresistance Tunneling
a b s t r a c t A series of Cox (TPD)1−x granular film samples were fabricated by co-evaporating technique, where Co nanoparticles are dispersed in TPD molecules matrix. The HRTEM images show typical microstructural characteristic of granular films. Magnetization, electrical conduction, and magnetoresistance (MR) of Cox (TPD)1−x granular films, have been investigated over a wide temperature range and Co volume fraction. The negative MR have been observed in these samples, reaching −1.75% (x = 36 vol.%, at 4.2 K, 10 kOe). The mechanism of magneto-transport behaviors has been discussed. It is suggested that the negative MR results from tunneling magnetoresistance (TMR) effect. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Recently, there has been growing interest in magnetoelectronic effects in organic semiconductors (OSCs). Because OSCs consist mainly of carbon and hydrogen, they provide longer spin relaxation time than which is achievable by conventional spintronics materials, such as metals and inorganic semiconductors [1]. Hence, the introduction of OSCs has led to novel developments in the field of spintronics [2–4]. Recent observations of magnetoresistance (MR) effects in OSCs open up the potential of these materials for spin-conserved transport. In 2004, Xiong et al., reported a maximum GMR ratio of 40% in a Tris-(8hydroxyquinoline) aluminum (III) (Alq3 ) based spin-valves at 11 K [3]. Recently, a large magnetoresistance effect has been observed in non-magnetic organic light-emitting diodes (OLEDs) devices, named organic magnetoresistance (OMAR) [4]. However, the mechanism of these novel phenomena in OSCs is currently not very clear. Until now, most of the researches are focused on junction structure with multilayer films in the field of organic spintronics. As we know, inorganic granular films, consisting of single-domain ferromagnetic particles embedded in an immiscible inorganic medium, have been extensively studied in the past two decades
∗ Corresponding author at: Department of Optics Science and Engineering, Fudan University, 220 Handan Road, Shanghai 200433, PR China. Fax: +86 21 55664192. E-mail address: [email protected] (G. Ni). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.10.123
[5,6]. Thus, it is very interesting that what will happen if the inorganic matrix is replaced by organic semiconductor materials in granular films. Recently, magnetoresistance effect has been also found in Co/C60 nanocomposites films [7], showing efficient spin polarized injection and transport can be realized in ferromagnetic-organic semiconductors (FM/OSCs) granular films. In the present work, we investigated the microstructures, magnetic and transport properties of FM/OSCs compound nanometer granular films, in which ferromagnetic Co nanoparticles dispersed in N,N -bis(3-methylphenyl)-1,1 -biphenyl-4,4 diamine (TPD) matrix. TPD is one of the most well-investigated OSCs, as a kind of hole transport materials which is popularly used in organic lightemitting diodes (OLEDs). The molecular structure of TPD is shown in Fig. 1. 2. Experimental A series of Co-TPD granular films were fabricated by coevaporation technique on glass substrates in a vacuum chamber with a base pressure of about 3 × 10−4 Pa. TPD and Co are deposited by using a resistance-heated tantalum crucible and BN beam source evaporator, respectively. The different volume ratio of TPD and Co in Co-TPD granular films are obtained by regulating the evaporation rate of the two materials. The evaporation rate of TPD were settled at 0.1–0.5 Å/s and that of Co were settled at 0.1–0.8 Å/s. The thicknesses of the films are about 350 nm. The value of evaporation rate and the film thickness are determined by a quartz crystal thickness monitor (Sigma SQM-160). The microstructures of granular film samples were investigated by high resolution transmission electron microscopy (HRTEM, Jeol JEM-2010). The magnetic properties of these samples were measured using a vibrating sample magnetometer (VSM, Lakeshore 7300) at room temperature. Magnetoresistance (MR) and resistance of the samples at different temperatures (from 4.2 to 300 K) were measured using physical properties measurement system (PPMS, Quantum Design,
P. Sheng et al. / Journal of Alloys and Compounds 477 (2009) 32–35
33
Fig. 1. The molecular structure of TPD.
Inc.) with a dc four-terminal configuration. The MR values are calculated according to MR = / = (H − 0 )/0 , where H and 0 are the resistivities in the applied field and initial zero field, respectively. The applied field was perpendicular to the plane of films.
3. Results and discussion Fig. 2 shows the HRTEM image of as-deposited Co0.36 (TPD)0.64 granular film sample and the corresponding electron diffraction pattern. It indicates the typical microstructural characteristic of granular films, where the randomly oriented granular Co particles are isolated by the TPD molecules. The particle size is approximately 2–5 nm. The brightly imaging regions are amorphous TPD, and the darkly imaging regions with a lattice fringe contrast are the metallic Co particles. The corresponding electron diffraction pattern revealed a polycrystalline structure of cobalt granules, three diffraction rings can be observed, which are close to the diffraction rings of cobalt with hexagonal close-packed (hcp) structure. The magnetic properties of samples were investigated with vibrating sample magnetometer (VSM). Fig. 3 presents the hysteresis loops of Co0.36 (TPD)0.64 granular film sample. The M–H curves shows no residual magnetization at zero magnetic field and no hysteresis curve, which indicates that the Co granules in the sample shows typical superparamagnetic nature. We also notice that there are no obvious differences between the two hysteresis loops when the magnetic field is applied parallel and perpendicular to the plane of film. This is a key feature of granular systems distinctively different from that in multilayer, indicating that the Co granules are almost spherical and there is no strong coupling between neighboring magnetic granules. The VSM results are consistent with the microstructural characteristic of granular films as shown in the above HRTEM image. The magnetoresistance curves are shown in Fig. 4 for Co0.36 (TPD)0.64 granular film sample at three different temperatures (T = 4.2, 80 and 300 K). At room temperature, a small negative magnetoresistance has been obtained, about −0.3% in the applied field of 10 kOe. As the temperature decreases, the MR ratio increases
Fig. 3. Magnetic hysteresis loops of Co0.36 (TPD)0.64 granular film sample.
Fig. 4. Magnetoresistance curves of Co0.36 (TPD)0.64 granular film sample at different temperatures (T = 4.2, 80, and 300 K).
accordingly, reaches about −0.6% at 80 K and −1.75% at 4.2 K, respectively. As we notice, the butterfly-like peaks have been observed in the MR curve near zero field at 4.2 K, indicates that the magnetization properties change from the superparamagnetism to
Fig. 2. HRTEM image of Co0.36 (TPD)0.64 granular film sample and the corresponding electron diffraction pattern.
34
P. Sheng et al. / Journal of Alloys and Compounds 477 (2009) 32–35
the ferromagnetism at 4.2 K. It is suggested that the maximum of the resistance takes place at the coercive field, which corresponds to the state of maximum disorder in the orientation of the neighboring magnetic particle moments. The lowest resistivity is realized at the saturation field where all the particles are ferromagnetically aligned. Thus, it is suggested that the spin-related scattering or spin-dependent tunneling of carriers between neighboring Co clusters may result in the negative MR in the granular system. In Fig. 5, temperature dependence of magnetoresistance of the Cox (TPD)1−x granular films with different Co fraction (x = 0.23–0.58) is shown. Both of these samples show small negative magnetoresistance effect at room temperature, and with the decrease of temperature, the MR ratio increase as shown before in Fig. 4, and reach largest values at 4.2 K. As shown in the inset of Fig. 5, the concentration dependence of MR (x = 0.23–0.58) is presented at three different temperatures (T = 4.2, 80, and 300 K). The optimal value of MR appears at x = 0.36 sample, which is a little below percolation threshold point. In order to understand the intrinsic mechanism of transport properties in the granular system, temperature dependence of the resistivity for different x samples has been investigated, shown in Fig. 6(a), where temperature axis is on a logarithmic scale. With the temperature decreases, the resistivity of the sample increases nonlinearly, showing negative temperature coefficient of resistance (TCR). It indicates its thermal activated conducting or tunneling conducting characteristics behavior. For disordered metal or metal/insulator granular system, Helamn and Abeles drew a conclusion that the temperature dependence of resistivity can be described as following [8],
sE 1/2 c
= a exp 2
kT
(1)
where Ec means the electrostatic charging energy of metal granules, and k stand for a constant that depend only on the volume fraction and Boltzmann’s constant, respectively. s is the tunnel-barrier thickness. As shown in Fig. 6(b), a logarithm of R plot is found well linearly proportional to T−1/2 . This agreement suggests that the electric transport in Cox (TPD)1−x granular films is mainly due to a tunneling mechanism of carriers across the barrier between neighboring Co granules through the organic semiconductor bar-
Fig. 5. Temperature dependence of magnetoresistance of the Cox (TPD)1−x granular films with different Co fraction (x = 0.23–0.58), the concentration dependence of MR at three different temperatures (T = 4.2, 80, and 300 K) is shown in the inset.
Fig. 6. (a) Temperature dependence of the resistivity for different x samples and (b) plotted in log − T−1/2 relation.
riers, resulting the negative tunneling magnetoresistance in this granular system. The cobalt granules with their anisotropic easyaxes orient randomly without an applied magnetic field, and the magnetic moments of the magnetic granules in the hybrid film have no preferred orientation. When an external magnetic field is applied, the magnetic moments of the cobalt particles will tend to align in parallel with the field direction. With the rise of applied magnetic field, the resistance of the film decreases, due to that the probability of electron tunneling process increases when the magnetization of the cobalt granules are in a parallel state [9]. In the tunnel-type magnetic nanostructures, the charge transport is caused by tunneling through insulating barriers. MR value is mostly relevant to the spin polarization of magnetic metal, the distance of neighbor granules and the granular size distribution. The enhancement of MR at low temperatures shown in Fig. 5 can be interpreted by the effect of higher-order tunneling in Coulomb blockade regime of granular systems [10]. The concentration dependence of TMR in Co-TPD granular film samples can be also understood as following. For larger x samples, the inter-granule barrier is not large enough for the carriers to tunnel, due to the partly overlap of electron wave functions in adjacent metal granules. For smaller x samples, due to the large inter-granule resistance, the tunneling probability is reduced, thus results in the reduced TMR. Thus, the optimal MR appears in the granular samples near and below the percolation point. As we also can see clearly in Fig. 6(b), with the rise of x, the slope of curve decreases, which indicates higher activation energy in smaller x samples due to the longer distance between adjacent Co granules. The results are consistent with the above microstructure characteristic of the granular
P. Sheng et al. / Journal of Alloys and Compounds 477 (2009) 32–35
film samples, and are also similar to inorganic M–I granular films [11,12]. 4. Conclusions In conclusion, granular films composed of Co granules embedded in TPD matrix were prepared by coevaporation. The HRTEM images and magnetic properties show typical microstructural characteristic of granular films. Negative magnetoresistance were observed, the concentration and temperature dependence of magnetoresistance and resistivity were investigated. It is suggested that the negative magnetoresistance is originated from the spin-dependent tunneling process between the neighboring ferromagnetic Co granules. Acknowledgment This work was supported by National Natural Science Foundation of China under Grant 60501002.
35
References [1] S. Pramanik, C.G. Stefanitai, S. Patibandla, S. Bandyopadhyay, K. Garre, N. Harth, M. Cahay, Nature Nanotech. 2 (2007) 216–219. [2] V. Dediu, M. Murgia, F.C. Matacotta, C. Taliani, S. Barbanera, Solid State Comm. 122 (2002) 181–184. [3] Z.H. Xiong, D. Wu, Z.V. Vardeny, J. Shi, Nature 427 (2004) 821–824. [4] Ö. Mermer, G. Veeraraghavan, T.L. Francis, Y. Sheng, D.T. Nguyen, M. Wohlgenannt, A. Köhler, M.K. Al-Suti, M.S. Khan, Phys. Rev. B 72 (2005) 205202. [5] A.E. Berkowitz, J.R. Mitchell, M.J. Carey, A.P. Young, S. Zhang, F.E. Spada, F.T. Parker, A. Hutten, G. Thomas, Phys. Rev. Lett. 68 (1992) 3745–3748. [6] J. Moodera, L.R. Kinder, T.M. Wang, R. Meservey, Phys. Rev. Lett. 74 (1995) 3273–3276. [7] S. Miwa, M. Shiraishi, S. Tanabe, M. Mizuguchi, T. Shinjo, Y. Suzuki, Phys. Rev. B 76 (2007) 214414. [8] J.S. Helamn, B. Abeles, Phys. Rev. Lett. 37 (1976) 1429–1432. [9] H.Y. Kwong, Y.W. Wong, K.H. Wong, J. Appl. Phys. 102 (2007) 114303. [10] S. Mitani, S. Takahashi, K. Takanashi, K. Yakushiji, S. Maekawa, H. Fujimori, Phys. Rev. Lett. 81 (1998) 2799–2802. [11] H. Fujimori, S. Mitani, S. Ohnuma, Mater. Sci. Eng. B 31 (1995) 219–223. [12] A. Milner, A. Gerber, B. Groisman, M. Karpovsky, A. Gladkikh, Phys. Rev. Lett. 76 (1996) 475–478.