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Large negative magnetoresistance and ferromagnetism induced by interfacial spins in granular Cr/C films Z.W. Fan, P. Li, H.L. Bai n Tianjin Key Laboratory of Low Dimensional Materials Physics and Preparing Technology, Institute of Advanced Materials Physics, Faculty of Science, Tianjin University, Tianjin 300072, People's Republic of China
art ic l e i nf o
a b s t r a c t
Article history: Received 1 December 2013 Received in revised form 30 April 2014
Cr/C composite films with 7.9 and 8.5 at% Cr contents were fabricated by facing-target magnetron sputtering. Cr nanoparticles are embedded in amorphous carbon matrix, and Cr atoms bond with C atoms at the interface. The Cr/C films were found to exhibit ferromagnetism in the temperature range of 2–300 K, which is probably induced by uncompensated spins at the surface of Cr nanoparticles. Spindependent magnetoresistance (up to 19.5% at 90 kOe and 2 K) was observed in the Cr/C films. The negative MR originates from the alignment of the uncompensated spins at the interface between nanoparticles and matrix. The spin polarization which comes from the interfacial p–d hybridization was calculated to be 21% and 17% in the Cr/C films with 7.9% and 8.5% Cr, respectively. & 2014 Published by Elsevier B.V.
Keywords: Amorphous carbon Magnetoresistance Spin polarization Interfacial spin
1. Introduction Magnetic granular films have attracted considerable attention due to their unique magnetic and magnetotransport properties for several decades, and they are considered to be potential materials in spintronic applications [1–4]. Compared to traditional matrix materials, amorphous carbon (a-C) is nonmagnetic and promising material due to several advantages: tunable electrical properties [5], long spin diffusion length and relaxation time [5,6], and interesting interfacial phenomena [7,8]. Till now, various interesting magnetoresistance (MR) effects have been reported in magnetic material/a-C composite systems. Giant positive MR effects were observed in a-C:Co and a-C:Fe films at both room temperature and low temperature, which were explained by orbital Zeeman splitting, spin-dependent transport and spin blockage effect [9–12]. MnO/C composite coatings show unusual large positive magnetoconductance ( 28.8%) at 100 K [13]. An enormous negative MR below 30 K (about 103 at 3 K and 70 kOe) was reported in Gd-implanted a-C films [14,15]. Besides a-C based composite films, voltage-dependent positive MR was also observed in Cox–C1 x/Si, Fex–C1 x/Si, and Cr-DLC/Si junctions at room temperature [16–19]. All these MR results suggest abundant magnetotransport properties existing in a-C based composite systems, which have great potential for spintronic applications. However, the physical mechanisms of these MR effects are unclear. Systematic investigation
n
Corresponding author. E-mail address: [email protected] (H.L. Bai).
is thus needed to explore the underlying physics for FM/a-C materials. In our previous study, a large spin-dependent negative MR effect was found in both Ni/CNx and Co/C granular films [20–22]. It was revealed that the interface hybridization between metal and C atoms contributes to high spin polarization, which plays a key role in the magnetotransport properties of a-C based granular films [22]. However, more evidence is needed to exclude the influence on MR by the spin polarization of ferromagnetic nanoparticles in the ferromagnetic metal/carbon films. Different from ferromagnetic metal, the antiferromagnetic metal, such as Cr, etc., shows no spin polarization [23]. Studying the magnetotransport of Cr/C granular films is helpful to clarify the interfacial hybridization or metallic particles contribution to spin polarization in metal/carbon systems. If spin-dependent negative MR still exists in the Cr/C granular films, the conclusion that interface hybridization between 3d metal and C atoms contributes to high spin polarization can be strongly proved. Moreover, the subtle arrangement of the spins in the antiferromagnetic granular systems could alter the electron transport, leading to a novel MR effect referred as antiferromagnetic giant MR [24]. The uncompensated spins at the surface of antiferromagnetic nanoparticles were reported to be the origin of weak ferromagnetism. The interesting magnetotransport phenomenon induced by the uncompensated spins was also expected in antiferromagnetic granular systems. Therefore, studying the magnetotransport and magnetic properties of Cr/C granular systems is helpful to understand its physical mechanisms. In this paper, the Cr/C films show the inhomogeneous granular structures with Cr-rich nanoparticles embedded in a-C matrix.
http://dx.doi.org/10.1016/j.jmmm.2014.05.046 0304-8853/& 2014 Published by Elsevier B.V.
Please cite this article as: Z.W. Fan, et al., Journal of Magnetism and Magnetic Materials (2014), http://dx.doi.org/10.1016/j. jmmm.2014.05.046i
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Spin-dependent negative MR and low-temperature ferromagnetism were observed in the Cr/C films, both of which originated from interfacial uncompensated spins of Cr nanoparticles. The interfacial hybridization between Cr and C atoms rather than metallic nanoparticles contributes to the spin polarization.
paste. The direction of magnetic field was perpendicular to the current in the MR measurements.
3. Results and discussion 3.1. Microstructure and electrical transport property
2. Experiments 100-nm-thick Cr/C composite films were fabricated by a facingtarget magnetron sputtering apparatus using graphite targets (99.99%) on which Cr pieces were placed. The Cr content was adjusted by the area of Cr pieces. When base pressure was greater than 5.0 10 4 Pa, Ar gas was introduced into the chamber to bring the sputtering pressure to 0.5 Pa. The sputtering power and rate were 200 W and 1.7 nm/min, respectively. The substrates were kept at ambient temperature during the sputtering, and no post-annealing was applied. Different substrates, such as glass, cleaved NaCl, Si and kapton™, were used for structure characterizations and physical property measurements. The thickness of the films was measured using a Dektak 6M surface profiler. The Cr contents were measured by energy dispersive x-ray spectroscopy. The microstructure and chemical states of the films were analyzed by high-resolution transmission electron microscopy (HRTEM) and x-ray photoelectron spectroscopy (XPS), respectively. The films on the cleaved NaCl substrates were floated in purified water, and put on copper grids for HRTEM analyses. XPS spectra were recorded in a PHI 1600 spectrometer equipped with a spherical capacitor analyzer, using Mg Kα radiation (1253.6 eV) with a resolution of about 0.25 eV. In order to remove the contaminated surface layer, Ar ions with 2 keV energy, a current density of 1 mA mm 2 and an incidence angle of 451 were used to sputter the samples on the glass substrates. The magnetic properties of the samples on kapton™ substrates were measured by a superconducting quantum interference device. The electrical transport properties and MR measurements of the samples on the glass substrates were performed by a Quantum Design physical property measurement system in the temperature range of 2–300 K. Four-probe method was used for transport property measurements and the electrodes were air-drying silver
Fig. 1 presents the planar-view HRTEM image and corresponding selected area electron diffraction (SAED) pattern of the asdeposited Cr/C film with 7.9 at% Cr content. The contrast in HRTEM image suggests that the Cr/C film is an inhomogeneous film. The darker regions correspond to Cr-rich nanoparticles, and the lighter regions refer to disorder carbon matrix. The blurred contour of the Cr particles indicates the incomplete phase segregation between Cr and C. The distribution of Cr nanoparticles is uniform, and the diameter of Cr nanoparticles is 1.3 nm (the right-bottom inset of Fig. 1). The SAED pattern shows diffused rings, reflecting that the carbon matrix is amorphous. It was reported that carbide-like Cr clusters are formed in highly-doped a-C films ( 9 at% Cr), while metallic-like Cr nanoparticles are uniformly distributed in the carbon matrix when Cr doping level is low ( 6 at% Cr) [25,26]. As Cr contents of the Cr/C films in our work are between high and low doping levels, it is considered that the inner of nanoparticles is metallic Cr, and some Cr atoms bond with C atoms at the interface. In order to verify our assumption, the chemical states of C and Cr atoms were measured by XPS spectroscopy. Fig. 2(a) and (b) shows the C 1s and Cr 2p XPS spectra of the Cr/C film with 7.9% Cr content. The peak at 283.4 eV in the C 1s spectrum corresponds to chromium bonded carbon [26], while the two peaks at 576.4 and 585.5 eV in the Cr 2p spectrum correspond to carbon bonded chromium [27]. The Cr–C bonds indicate the existence of interfacial carbides at the interface. In C 1s spectrum, the two peaks at 284.5 and 285.6 eV correspond to the binding energy of C–sp2C and C–sp3C bonds in a-C matrix, respectively. The peak at 287.1 eV is characteristic of C–O bonds. The peaks at 574.5 and 583.8 eV in the Cr 2p spectrum correspond to the binding energy of metallic Cr [28]. Therefore, XPS results further demonstrate the microstructure of Cr-rich nanoparticles with metallic core, and some Cr atoms bond with C atoms at the interface.
Fig. 1. HRTEM image and the corresponding selected area electron diffraction pattern of the as-deposited Cr/C film with 7.9% Cr content. The right-bottom inset shows the distribution of particle sizes in HRTEM image.
Please cite this article as: Z.W. Fan, et al., Journal of Magnetism and Magnetic Materials (2014), http://dx.doi.org/10.1016/j. jmmm.2014.05.046i
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ln R–T 0.5 curves for the Cr/C films with Cr contents of 7.9% and 8.5%. ln R is proportional to T 0.5, indicating that the dominant conduction mechanism is tunneling.
3.2. Ferromagnetism
Fig. 2. (a) C 1s and (b) Cr 2p XPS spectra of the Cr/C film with 7.9% Cr content. Open circles: experimental data, black line: fitted curves, colored line: decomposed curves. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 4 shows the zero-field-cooling (ZFC) and field-cooling (FC) curves at 200 Oe for the Cr/C films with 7.9% and 8.5% Cr contents. ZFC (FC) curves were measured as temperature increases from 2 to 300 K after cooling down the samples with 0 Oe (or 200 Oe) magnetic field. In all samples, the FC and ZFC curves branch even at 300 K, indicating that the Cr/C films are ferromagnetic at room temperature. The ferromagnetism is also proved by the M–H curves measured at 300 K (the insets of Fig. 4). The coercivity of the Cr/C samples with 7.9% and 8.5% Cr was 79 and 50 Oe at 300 K, respectively. The magnetization in the FC curves changes little from 300 to 120 K, and then slightly decreases as temperature decreases. Fig. 5 shows the M–H curves measured at different temperatures for the Cr/C films with 7.9% and 8.5% Cr contents. From M–H curves, the Cr/C films show ferromagnetism in the whole temperature range of 2–300 K. The ferromagnetism in Cr/C films has little relationship to a-C matrix, because the pure carbon films, which are prepared under the same condition, show no ferromagnetic property. For the two samples with 7.9% and 8.5% Cr content, they have obviously different saturate magnetizations which give further evidence that the ferromagnetism does not come from carbon matrix or Cr nanoparticles. The magnetic properties of the Cr/C films were quite different from those of the ferromagnetic metal/C granular films, which exhibited superparamagnetism without coercivity at 300 K and ferromagnetism when the temperature was below the blocking temperature [31]. ZFC curves show no peak, precluding the
Fig. 3. R–T curves of the Cr/C films with 7.9% and 8.5% Cr contents in the temperature range of 2–300 K. The inset displays R vs T 0:5 curves for the asdeposited Cr/C films.
Fig. 3 exhibits the temperature dependence of resistance for the Cr/C films. The resistance shows strong temperature dependence especially at low temperatures, suggesting that the Cr/C films are in the insulating conduction regime. For insulating regime, resistivity can be analyzed by [29,30] ρðTÞ ¼ ρ0 exp ðT 0 =TÞn : ð1Þ The value of n can be used to judge the conduction mechanism. n ¼1 indicates thermal activation conduction, n ¼1/2 tunneling, and n ¼1/4 Mott variable range hopping. The inset of Fig. 3 shows
Fig. 4. Zero-field-cooling and field-cooling curves of the Cr/C films with Cr contents of (a) 7.9% and (b) 8.5% under the magnetic field of 200 Oe. The insets show the corresponding M–H curves measured at 300 K.
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Fig. 5. M–H curves of the Cr/C films with (a) 7.9% and (b) 8.5% Cr contents in the temperature range of 2–300 K. The inset shows the dependence of magnetization on Cr content at 50 kOe magnetic field.
existence of the ferromagnetic nanoparticles in the Cr/C films. It was reported that small antiferromagnetic nanoparticles might exhibit low-temperature ferromagnetism which comes from the uncompensated surface moments of nanoparticles [32,33]. The observed room-temperature magnetization of 19 emu/cm3 is rather small. If all the Cr atoms in the Cr/C films are fully ferromagnetic, only about 5% Cr atoms contribute to the magnetization. Therefore, the magnetization probably comes from uncompensated surface spins of Cr nanoparticles. Furthermore, as shown in the inset of Fig. 5, the magnetization of Cr/C films reduces when Cr content increases from 4.5% to 9.4%, suggesting that the contribution of uncompensated surface spins becomes less as surface area ratio decreases. 3.3. Large negative MR Fig. 6 presents the MR–H curves for the Cr/C films with 7.9% and 8.5% Cr in the temperature range of 2–20 K. The MR is defined as MR ¼
RðHÞ Rð0Þ 100%: Rð0Þ
ð2Þ
In Fig. 6, the large negative MR is observed at low temperatures, and the MR is nearly linear at high fields. The maximum value reaches 19.5% in the 7.9% Cr sample at 2 K and 90 kOe. The MR rapidly decreases as temperature increases (the insets of Fig. 6). When the temperature is above 30 K, no obvious MR is observed in the Cr/C films (not shown). Large negative MR effect was reported to exist in glassy carbon due to spin flip scattering by localized defect states [34]. However, in our previous study [22], we measured the low-temperature magnetoresistance of (Ti,Cu)/C films, which were prepared under the same condition, and no magnetoresistance was observed. This proves that the observed negative MR in Cr/C films is related to Cr
Fig. 6. MR–H curves measured at different temperatures for the Cr/C films with Cr contents of (a) 7.9% and (b) 8.5%. The insets show the temperature dependence of MR for the corresponding samples.
particles rather than amorphous carbon matrix. The as-deposited pure carbon films by facing-target sputtering are almost insulating. Therefore, the pure carbon films will not show negative magnetoresistance as in glassy carbon reported by Saxena and Bragg which is related to the anomalous behavior of the spin susceptibility on the metallic side of the metal–insulator transition [34]. Third, the dominate conduction mechanism in the Cr/C films is tunneling. This means the electrons mainly tunnel from one Cr particle to another. The effect of magnetic field on the electron conduction could be much smaller in carbon matrix than that between Cr nanoparticles. Therefore, the low-temperature magnetoresistance is believed to be related to Cr nanoparticles. A large negative MR (about 50%) was reported in Cr-doped hydrogenated amorphous diamond-like carbon (Cr-DLC) heterojunction devices with n-type silicon films [19]. The reported negative MR saturates at 300 Oe and shows weak field dependence, especially at higher magnetic fields [19]. The MR was speculated to originate from some magnetic ordering in the heterojunctions. The negative MR observed in our cases is different from that in Cr-DLC heterojunctions. Firstly, their results were obtained from the Cr-DLC heterojunctions with Si substrates, which suggests that the MR may be extrinsic properties of Cr-DLC films. Our negative MR is intrinsic properties of the Cr/C granular films because the MR was measured by the fourprobe method on insulating glass substrates. Secondly, compared to our unsaturated MR, their results show rapid saturation even at 50– 200 Oe, indicating the different mechanisms. TEM images and R–T curves indicate that our Cr/C films are inhomogeneous granular films with tunneling conduction mechanism, in which spin-dependent negative MR is likely to be observed [35]. The unsaturated MR at high fields and the strong temperature dependence further confirm the spin-dependent origin of MR [35]. This spin-dependent MR should not come from the spin alignments of the metallic Cr nanoparticles due to its antiferromagnetic nature below the Néel temperature of 308 K with no spin polarization. Therefore, the negative MR most probably originates from the alignment of the uncompensated
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spins at the interface between particles and matrix. This is also consistent with its weak saturation at high fields, because the interfacial spins are hard to align with the field direction due to large surface anisotropy and surface defect pinning. In the granular systems with uncorrelated magnetic grains, tunnel magnetoresistance (TMR) was often observed as the moments of ferromagnetic nanoparticles align parallel under applied magnetic field [35]. The field dependence of TMR is proportional to that of ðM=M s Þ2 [2,36]. The measured MR in the Cr/C films was considered to arise from the alignment of interfacial spins between Cr nanoparticles and a-C matrix. Therefore, MR should be proportional to the square of magnetization attributed to the uncompensated surface spins of Cr nanoparticles. Fig. 7 shows that the shape of field dependent MR is consistent with that of ðM=M s Þ2 . Ms is defined as the magnetization at 50 kOe. These results further prove that the interfacial spins play a key role on the magnetotransport and magnetic properties in the Cr/C films. For spin-dependent interfacial MR, the spin polarization can be estimated by fitting MR–T curves using a cotunneling model and temperature-dependent exponential decaying of spin polarization [22]. The temperature dependence of MR in the cotunneling model is MR ¼ 1 ð1 þm2 P 2 Þ j ;
ð3Þ
among which j is the order of cotunneling, m ¼ M=M s , P the spin polarization with exponential decaying [20,21] PðTÞ ¼ P 0 expð βT a Þ
ð4Þ
By Eqs. (3) and (4), the interfacial spin polarization P0 is estimated to be 21% and 17% for Cr/C films with Cr contents of 7.9% and 8.5%, respectively. The efficient spin polarization in the Cr/C films is most likely attributed to the interfacial hybridization between Cr 3d and C 2p electrons, because metallic Cr nanoparticles have no spin polarization. The interfacial p–d hybridization, which was proved by the existence of carbides in XPS result, may induce the interfacial spin-filtering effect at the interface, and lead to the high spin polarization [22]. The half-metallic electronic
5
Fig. 8. The temperature dependence of the magnetization (H¼ 50 kOe) for the Cr/C films with Cr contents of 7.9% and 8.5%. Open circles: experimental data, solid lines: fitted curves.
properties were predicted in Cr-doped wide-band semiconductors, such as Ge, SiC, GaN, etc. This unusual electronic property is commonly attributed to the coupling effects of the Cr 3d states with the host elements [37–39]. Our experimental results are consistent with their predictions that the interaction between Cr and nonmetal atoms can generate high spin polarization. The similar spin-dependent interfacial negative MR was also observed in Co/C films. The calculated spin polarization P0 was as high as 48%, which is larger than the obtained values of metal Co [22]. This higher spin polarization is considered to originate from interfacial hybridization between the metal and carbon atoms. This explanation can be further demonstrated in our Cr/C films, for the spin polarization can only come from the hybridization between Cr and C atoms. Therefore, the similar mechanism of negative MR in the Cr/C films and Co/C films shows that interface hybridization plays an important role in the spin polarization of magnetic metal/C systems. With the similar origin of interfacial spins, MR and magnetization both show rapid decreasing trend with increasing temperature. Fig. 8 shows the temperature dependence of magnetization measured at 50 kOe for the Cr/C films with 7.9% and 8.5% Cr contents. M–T curves can be fitted by MðTÞ ¼ M 0 expð βT a Þ, which is similar to the temperature behavior of spin polarization. It was reported that saturation magnetization induced by uncompensated surface moments varies linearly with temperature [33]. In the Cr/C films, the anomalous temperature dependence of magnetization and spin polarization at low temperatures may be related to the interfacial magnetic impurities [20,21] or disordered spin-glass-like moments [40].
4. Conclusion
Fig. 7. The field dependence of MR and ðM=M s Þ2 for the Cr/C films with Cr contents of (a) 7.9% and (b) 8.5%.
Cr/C composite films with 7.9 and 8.5 at% Cr contents were fabricated by facing-target sputtering. The films show the inhomogeneous granular structure with Cr-rich nanoparticles embedded in a-C matrix. The Cr/C films were found to exhibit ferromagnetism in the temperature range of 2–300 K, which was probably induced by uncompensated spins at the surface of Cr nanoparticles. The negative MR (up to 19.5% at 90 kOe and 2 K) was observed at low temperatures, and most probably originated from the alignment of the uncompensated spins at the interface between particles and matrix. The results further verify that the interfacial hybridization between 3d metal and carbon atoms contributes to efficient spin polarization in the metal/carbon films. The calculated spin polarizations of Cr/C films with 7.9% and 8.5% Cr contents were 21 and 17%, respectively. Our results show that interfacial spins and p–d hybridization between metal and C
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atoms plays an important role on the interesting magnetic and magnetotransport properties of the metal/carbon composite films. Acknowledgments This work was supported by National Natural Science Foundation of China (Grant no. 51072132) and Natural Science Foundation of Tianjin City (12JCYBJC11100). References [1] S. Honda, T. Okada, M. Nawate, M. Tokumoto, Phys. Rev. B 56 (1997) 14566. [2] S. Mitani, S. Takahashi, K. Takanashi, K. Yakushiji, S. Maekawa, H. Fujimori, Phys. Rev. Lett. 81 (1998) 2799. [3] J.H. Chi, S.H. Ge, C.M. Liu, H.P. Kunkel, X.Z. Zhou, G. Williams, J. Appl. Phys. 93 (2003) 6188. [4] A. Milner, A. Gerber, B. Groisman, M. Karpovsky, A. Gladkikh, Phys. Rev. Lett. 76 (1996) 475. [5] D.S. McClure, J. Chem. Phys. 20 (1952) 682. [6] I. Bergenti, V. Dediu, M. Preziosi, A. Riminucci, Philos. Trans. R. Soc. A 369 (2011) 3054. [7] S. Sanvito, Nat. Phys. 6 (2010) 562. [8] N. Atodiresei, J. Brede, P. Lazić, V. Caciuc, G. Hoffmann, R. Wiesendanger, S. Blügel, Phys. Rev. Lett. 105 (2010) 066601. [9] H.S. Hsu, P.Y. Chung, J.H. Zhang, S.J. Sun, H. Chou, H.C. Su, C.H. Lee, J. Chen, J.C.A. Huang, Appl. Phys. Lett. 97 (2010) 032503. [10] P. Tian, X. Zhang, Q.Z. Xue, Carbon 45 (2007) 1764. [11] C. Wan, X. Zhang, J. Vanacken, X. Gao, X. Zhang, L. Wu, X. Tan, H. Lin, V.V. Moshchalkov, J. Yuan, Diam. Relat. Mater. 20 (2011) 26. [12] R. Tang, M. Mizuguchi, H. Wang, R. Yu, K. Takanashi, IEEE Trans. Magn. 46 (2010) 2144. [13] A. Varade, S.A. Shivashankar, Carbon 49 (2011) 1401.
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Large negative magnetoresistance and ferromagnetism induced by interfacial spins in granular Cr/C films Z.W. Fan, P. Li, H.L. Bai n Tianjin Key Laboratory of Low Dimensional Materials Physics and Preparing Technology, Institute of Advanced Materials Physics, Faculty of Science, Tianjin University, Tianjin 300072, People's Republic of China
art ic l e i nf o
a b s t r a c t
Article history: Received 1 December 2013 Received in revised form 30 April 2014
Cr/C composite films with 7.9 and 8.5 at% Cr contents were fabricated by facing-target magnetron sputtering. Cr nanoparticles are embedded in amorphous carbon matrix, and Cr atoms bond with C atoms at the interface. The Cr/C films were found to exhibit ferromagnetism in the temperature range of 2–300 K, which is probably induced by uncompensated spins at the surface of Cr nanoparticles. Spindependent magnetoresistance (up to 19.5% at 90 kOe and 2 K) was observed in the Cr/C films. The negative MR originates from the alignment of the uncompensated spins at the interface between nanoparticles and matrix. The spin polarization which comes from the interfacial p–d hybridization was calculated to be 21% and 17% in the Cr/C films with 7.9% and 8.5% Cr, respectively. & 2014 Published by Elsevier B.V.
Keywords: Amorphous carbon Magnetoresistance Spin polarization Interfacial spin
1. Introduction Magnetic granular films have attracted considerable attention due to their unique magnetic and magnetotransport properties for several decades, and they are considered to be potential materials in spintronic applications [1–4]. Compared to traditional matrix materials, amorphous carbon (a-C) is nonmagnetic and promising material due to several advantages: tunable electrical properties [5], long spin diffusion length and relaxation time [5,6], and interesting interfacial phenomena [7,8]. Till now, various interesting magnetoresistance (MR) effects have been reported in magnetic material/a-C composite systems. Giant positive MR effects were observed in a-C:Co and a-C:Fe films at both room temperature and low temperature, which were explained by orbital Zeeman splitting, spin-dependent transport and spin blockage effect [9–12]. MnO/C composite coatings show unusual large positive magnetoconductance ( 28.8%) at 100 K [13]. An enormous negative MR below 30 K (about 103 at 3 K and 70 kOe) was reported in Gd-implanted a-C films [14,15]. Besides a-C based composite films, voltage-dependent positive MR was also observed in Cox–C1 x/Si, Fex–C1 x/Si, and Cr-DLC/Si junctions at room temperature [16–19]. All these MR results suggest abundant magnetotransport properties existing in a-C based composite systems, which have great potential for spintronic applications. However, the physical mechanisms of these MR effects are unclear. Systematic investigation
n
Corresponding author. E-mail address: [email protected] (H.L. Bai).
is thus needed to explore the underlying physics for FM/a-C materials. In our previous study, a large spin-dependent negative MR effect was found in both Ni/CNx and Co/C granular films [20–22]. It was revealed that the interface hybridization between metal and C atoms contributes to high spin polarization, which plays a key role in the magnetotransport properties of a-C based granular films [22]. However, more evidence is needed to exclude the influence on MR by the spin polarization of ferromagnetic nanoparticles in the ferromagnetic metal/carbon films. Different from ferromagnetic metal, the antiferromagnetic metal, such as Cr, etc., shows no spin polarization [23]. Studying the magnetotransport of Cr/C granular films is helpful to clarify the interfacial hybridization or metallic particles contribution to spin polarization in metal/carbon systems. If spin-dependent negative MR still exists in the Cr/C granular films, the conclusion that interface hybridization between 3d metal and C atoms contributes to high spin polarization can be strongly proved. Moreover, the subtle arrangement of the spins in the antiferromagnetic granular systems could alter the electron transport, leading to a novel MR effect referred as antiferromagnetic giant MR [24]. The uncompensated spins at the surface of antiferromagnetic nanoparticles were reported to be the origin of weak ferromagnetism. The interesting magnetotransport phenomenon induced by the uncompensated spins was also expected in antiferromagnetic granular systems. Therefore, studying the magnetotransport and magnetic properties of Cr/C granular systems is helpful to understand its physical mechanisms. In this paper, the Cr/C films show the inhomogeneous granular structures with Cr-rich nanoparticles embedded in a-C matrix.
http://dx.doi.org/10.1016/j.jmmm.2014.05.046 0304-8853/& 2014 Published by Elsevier B.V.
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Spin-dependent negative MR and low-temperature ferromagnetism were observed in the Cr/C films, both of which originated from interfacial uncompensated spins of Cr nanoparticles. The interfacial hybridization between Cr and C atoms rather than metallic nanoparticles contributes to the spin polarization.
paste. The direction of magnetic field was perpendicular to the current in the MR measurements.
3. Results and discussion 3.1. Microstructure and electrical transport property
2. Experiments 100-nm-thick Cr/C composite films were fabricated by a facingtarget magnetron sputtering apparatus using graphite targets (99.99%) on which Cr pieces were placed. The Cr content was adjusted by the area of Cr pieces. When base pressure was greater than 5.0 10 4 Pa, Ar gas was introduced into the chamber to bring the sputtering pressure to 0.5 Pa. The sputtering power and rate were 200 W and 1.7 nm/min, respectively. The substrates were kept at ambient temperature during the sputtering, and no post-annealing was applied. Different substrates, such as glass, cleaved NaCl, Si and kapton™, were used for structure characterizations and physical property measurements. The thickness of the films was measured using a Dektak 6M surface profiler. The Cr contents were measured by energy dispersive x-ray spectroscopy. The microstructure and chemical states of the films were analyzed by high-resolution transmission electron microscopy (HRTEM) and x-ray photoelectron spectroscopy (XPS), respectively. The films on the cleaved NaCl substrates were floated in purified water, and put on copper grids for HRTEM analyses. XPS spectra were recorded in a PHI 1600 spectrometer equipped with a spherical capacitor analyzer, using Mg Kα radiation (1253.6 eV) with a resolution of about 0.25 eV. In order to remove the contaminated surface layer, Ar ions with 2 keV energy, a current density of 1 mA mm 2 and an incidence angle of 451 were used to sputter the samples on the glass substrates. The magnetic properties of the samples on kapton™ substrates were measured by a superconducting quantum interference device. The electrical transport properties and MR measurements of the samples on the glass substrates were performed by a Quantum Design physical property measurement system in the temperature range of 2–300 K. Four-probe method was used for transport property measurements and the electrodes were air-drying silver
Fig. 1 presents the planar-view HRTEM image and corresponding selected area electron diffraction (SAED) pattern of the asdeposited Cr/C film with 7.9 at% Cr content. The contrast in HRTEM image suggests that the Cr/C film is an inhomogeneous film. The darker regions correspond to Cr-rich nanoparticles, and the lighter regions refer to disorder carbon matrix. The blurred contour of the Cr particles indicates the incomplete phase segregation between Cr and C. The distribution of Cr nanoparticles is uniform, and the diameter of Cr nanoparticles is 1.3 nm (the right-bottom inset of Fig. 1). The SAED pattern shows diffused rings, reflecting that the carbon matrix is amorphous. It was reported that carbide-like Cr clusters are formed in highly-doped a-C films ( 9 at% Cr), while metallic-like Cr nanoparticles are uniformly distributed in the carbon matrix when Cr doping level is low ( 6 at% Cr) [25,26]. As Cr contents of the Cr/C films in our work are between high and low doping levels, it is considered that the inner of nanoparticles is metallic Cr, and some Cr atoms bond with C atoms at the interface. In order to verify our assumption, the chemical states of C and Cr atoms were measured by XPS spectroscopy. Fig. 2(a) and (b) shows the C 1s and Cr 2p XPS spectra of the Cr/C film with 7.9% Cr content. The peak at 283.4 eV in the C 1s spectrum corresponds to chromium bonded carbon [26], while the two peaks at 576.4 and 585.5 eV in the Cr 2p spectrum correspond to carbon bonded chromium [27]. The Cr–C bonds indicate the existence of interfacial carbides at the interface. In C 1s spectrum, the two peaks at 284.5 and 285.6 eV correspond to the binding energy of C–sp2C and C–sp3C bonds in a-C matrix, respectively. The peak at 287.1 eV is characteristic of C–O bonds. The peaks at 574.5 and 583.8 eV in the Cr 2p spectrum correspond to the binding energy of metallic Cr [28]. Therefore, XPS results further demonstrate the microstructure of Cr-rich nanoparticles with metallic core, and some Cr atoms bond with C atoms at the interface.
Fig. 1. HRTEM image and the corresponding selected area electron diffraction pattern of the as-deposited Cr/C film with 7.9% Cr content. The right-bottom inset shows the distribution of particle sizes in HRTEM image.
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ln R–T 0.5 curves for the Cr/C films with Cr contents of 7.9% and 8.5%. ln R is proportional to T 0.5, indicating that the dominant conduction mechanism is tunneling.
3.2. Ferromagnetism
Fig. 2. (a) C 1s and (b) Cr 2p XPS spectra of the Cr/C film with 7.9% Cr content. Open circles: experimental data, black line: fitted curves, colored line: decomposed curves. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 4 shows the zero-field-cooling (ZFC) and field-cooling (FC) curves at 200 Oe for the Cr/C films with 7.9% and 8.5% Cr contents. ZFC (FC) curves were measured as temperature increases from 2 to 300 K after cooling down the samples with 0 Oe (or 200 Oe) magnetic field. In all samples, the FC and ZFC curves branch even at 300 K, indicating that the Cr/C films are ferromagnetic at room temperature. The ferromagnetism is also proved by the M–H curves measured at 300 K (the insets of Fig. 4). The coercivity of the Cr/C samples with 7.9% and 8.5% Cr was 79 and 50 Oe at 300 K, respectively. The magnetization in the FC curves changes little from 300 to 120 K, and then slightly decreases as temperature decreases. Fig. 5 shows the M–H curves measured at different temperatures for the Cr/C films with 7.9% and 8.5% Cr contents. From M–H curves, the Cr/C films show ferromagnetism in the whole temperature range of 2–300 K. The ferromagnetism in Cr/C films has little relationship to a-C matrix, because the pure carbon films, which are prepared under the same condition, show no ferromagnetic property. For the two samples with 7.9% and 8.5% Cr content, they have obviously different saturate magnetizations which give further evidence that the ferromagnetism does not come from carbon matrix or Cr nanoparticles. The magnetic properties of the Cr/C films were quite different from those of the ferromagnetic metal/C granular films, which exhibited superparamagnetism without coercivity at 300 K and ferromagnetism when the temperature was below the blocking temperature [31]. ZFC curves show no peak, precluding the
Fig. 3. R–T curves of the Cr/C films with 7.9% and 8.5% Cr contents in the temperature range of 2–300 K. The inset displays R vs T 0:5 curves for the asdeposited Cr/C films.
Fig. 3 exhibits the temperature dependence of resistance for the Cr/C films. The resistance shows strong temperature dependence especially at low temperatures, suggesting that the Cr/C films are in the insulating conduction regime. For insulating regime, resistivity can be analyzed by [29,30] ρðTÞ ¼ ρ0 exp ðT 0 =TÞn : ð1Þ The value of n can be used to judge the conduction mechanism. n ¼1 indicates thermal activation conduction, n ¼1/2 tunneling, and n ¼1/4 Mott variable range hopping. The inset of Fig. 3 shows
Fig. 4. Zero-field-cooling and field-cooling curves of the Cr/C films with Cr contents of (a) 7.9% and (b) 8.5% under the magnetic field of 200 Oe. The insets show the corresponding M–H curves measured at 300 K.
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Fig. 5. M–H curves of the Cr/C films with (a) 7.9% and (b) 8.5% Cr contents in the temperature range of 2–300 K. The inset shows the dependence of magnetization on Cr content at 50 kOe magnetic field.
existence of the ferromagnetic nanoparticles in the Cr/C films. It was reported that small antiferromagnetic nanoparticles might exhibit low-temperature ferromagnetism which comes from the uncompensated surface moments of nanoparticles [32,33]. The observed room-temperature magnetization of 19 emu/cm3 is rather small. If all the Cr atoms in the Cr/C films are fully ferromagnetic, only about 5% Cr atoms contribute to the magnetization. Therefore, the magnetization probably comes from uncompensated surface spins of Cr nanoparticles. Furthermore, as shown in the inset of Fig. 5, the magnetization of Cr/C films reduces when Cr content increases from 4.5% to 9.4%, suggesting that the contribution of uncompensated surface spins becomes less as surface area ratio decreases. 3.3. Large negative MR Fig. 6 presents the MR–H curves for the Cr/C films with 7.9% and 8.5% Cr in the temperature range of 2–20 K. The MR is defined as MR ¼
RðHÞ Rð0Þ 100%: Rð0Þ
ð2Þ
In Fig. 6, the large negative MR is observed at low temperatures, and the MR is nearly linear at high fields. The maximum value reaches 19.5% in the 7.9% Cr sample at 2 K and 90 kOe. The MR rapidly decreases as temperature increases (the insets of Fig. 6). When the temperature is above 30 K, no obvious MR is observed in the Cr/C films (not shown). Large negative MR effect was reported to exist in glassy carbon due to spin flip scattering by localized defect states [34]. However, in our previous study [22], we measured the low-temperature magnetoresistance of (Ti,Cu)/C films, which were prepared under the same condition, and no magnetoresistance was observed. This proves that the observed negative MR in Cr/C films is related to Cr
Fig. 6. MR–H curves measured at different temperatures for the Cr/C films with Cr contents of (a) 7.9% and (b) 8.5%. The insets show the temperature dependence of MR for the corresponding samples.
particles rather than amorphous carbon matrix. The as-deposited pure carbon films by facing-target sputtering are almost insulating. Therefore, the pure carbon films will not show negative magnetoresistance as in glassy carbon reported by Saxena and Bragg which is related to the anomalous behavior of the spin susceptibility on the metallic side of the metal–insulator transition [34]. Third, the dominate conduction mechanism in the Cr/C films is tunneling. This means the electrons mainly tunnel from one Cr particle to another. The effect of magnetic field on the electron conduction could be much smaller in carbon matrix than that between Cr nanoparticles. Therefore, the low-temperature magnetoresistance is believed to be related to Cr nanoparticles. A large negative MR (about 50%) was reported in Cr-doped hydrogenated amorphous diamond-like carbon (Cr-DLC) heterojunction devices with n-type silicon films [19]. The reported negative MR saturates at 300 Oe and shows weak field dependence, especially at higher magnetic fields [19]. The MR was speculated to originate from some magnetic ordering in the heterojunctions. The negative MR observed in our cases is different from that in Cr-DLC heterojunctions. Firstly, their results were obtained from the Cr-DLC heterojunctions with Si substrates, which suggests that the MR may be extrinsic properties of Cr-DLC films. Our negative MR is intrinsic properties of the Cr/C granular films because the MR was measured by the fourprobe method on insulating glass substrates. Secondly, compared to our unsaturated MR, their results show rapid saturation even at 50– 200 Oe, indicating the different mechanisms. TEM images and R–T curves indicate that our Cr/C films are inhomogeneous granular films with tunneling conduction mechanism, in which spin-dependent negative MR is likely to be observed [35]. The unsaturated MR at high fields and the strong temperature dependence further confirm the spin-dependent origin of MR [35]. This spin-dependent MR should not come from the spin alignments of the metallic Cr nanoparticles due to its antiferromagnetic nature below the Néel temperature of 308 K with no spin polarization. Therefore, the negative MR most probably originates from the alignment of the uncompensated
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spins at the interface between particles and matrix. This is also consistent with its weak saturation at high fields, because the interfacial spins are hard to align with the field direction due to large surface anisotropy and surface defect pinning. In the granular systems with uncorrelated magnetic grains, tunnel magnetoresistance (TMR) was often observed as the moments of ferromagnetic nanoparticles align parallel under applied magnetic field [35]. The field dependence of TMR is proportional to that of ðM=M s Þ2 [2,36]. The measured MR in the Cr/C films was considered to arise from the alignment of interfacial spins between Cr nanoparticles and a-C matrix. Therefore, MR should be proportional to the square of magnetization attributed to the uncompensated surface spins of Cr nanoparticles. Fig. 7 shows that the shape of field dependent MR is consistent with that of ðM=M s Þ2 . Ms is defined as the magnetization at 50 kOe. These results further prove that the interfacial spins play a key role on the magnetotransport and magnetic properties in the Cr/C films. For spin-dependent interfacial MR, the spin polarization can be estimated by fitting MR–T curves using a cotunneling model and temperature-dependent exponential decaying of spin polarization [22]. The temperature dependence of MR in the cotunneling model is MR ¼ 1 ð1 þm2 P 2 Þ j ;
ð3Þ
among which j is the order of cotunneling, m ¼ M=M s , P the spin polarization with exponential decaying [20,21] PðTÞ ¼ P 0 expð βT a Þ
ð4Þ
By Eqs. (3) and (4), the interfacial spin polarization P0 is estimated to be 21% and 17% for Cr/C films with Cr contents of 7.9% and 8.5%, respectively. The efficient spin polarization in the Cr/C films is most likely attributed to the interfacial hybridization between Cr 3d and C 2p electrons, because metallic Cr nanoparticles have no spin polarization. The interfacial p–d hybridization, which was proved by the existence of carbides in XPS result, may induce the interfacial spin-filtering effect at the interface, and lead to the high spin polarization [22]. The half-metallic electronic
5
Fig. 8. The temperature dependence of the magnetization (H¼ 50 kOe) for the Cr/C films with Cr contents of 7.9% and 8.5%. Open circles: experimental data, solid lines: fitted curves.
properties were predicted in Cr-doped wide-band semiconductors, such as Ge, SiC, GaN, etc. This unusual electronic property is commonly attributed to the coupling effects of the Cr 3d states with the host elements [37–39]. Our experimental results are consistent with their predictions that the interaction between Cr and nonmetal atoms can generate high spin polarization. The similar spin-dependent interfacial negative MR was also observed in Co/C films. The calculated spin polarization P0 was as high as 48%, which is larger than the obtained values of metal Co [22]. This higher spin polarization is considered to originate from interfacial hybridization between the metal and carbon atoms. This explanation can be further demonstrated in our Cr/C films, for the spin polarization can only come from the hybridization between Cr and C atoms. Therefore, the similar mechanism of negative MR in the Cr/C films and Co/C films shows that interface hybridization plays an important role in the spin polarization of magnetic metal/C systems. With the similar origin of interfacial spins, MR and magnetization both show rapid decreasing trend with increasing temperature. Fig. 8 shows the temperature dependence of magnetization measured at 50 kOe for the Cr/C films with 7.9% and 8.5% Cr contents. M–T curves can be fitted by MðTÞ ¼ M 0 expð βT a Þ, which is similar to the temperature behavior of spin polarization. It was reported that saturation magnetization induced by uncompensated surface moments varies linearly with temperature [33]. In the Cr/C films, the anomalous temperature dependence of magnetization and spin polarization at low temperatures may be related to the interfacial magnetic impurities [20,21] or disordered spin-glass-like moments [40].
4. Conclusion
Fig. 7. The field dependence of MR and ðM=M s Þ2 for the Cr/C films with Cr contents of (a) 7.9% and (b) 8.5%.
Cr/C composite films with 7.9 and 8.5 at% Cr contents were fabricated by facing-target sputtering. The films show the inhomogeneous granular structure with Cr-rich nanoparticles embedded in a-C matrix. The Cr/C films were found to exhibit ferromagnetism in the temperature range of 2–300 K, which was probably induced by uncompensated spins at the surface of Cr nanoparticles. The negative MR (up to 19.5% at 90 kOe and 2 K) was observed at low temperatures, and most probably originated from the alignment of the uncompensated spins at the interface between particles and matrix. The results further verify that the interfacial hybridization between 3d metal and carbon atoms contributes to efficient spin polarization in the metal/carbon films. The calculated spin polarizations of Cr/C films with 7.9% and 8.5% Cr contents were 21 and 17%, respectively. Our results show that interfacial spins and p–d hybridization between metal and C
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