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iron-oxide films
ARTICLE IN PRESS
Physica B 400 (2007) 185–189 www.elsevier.com/locate/physb
Magnetic transport properties in iron/iron-oxide films S.L. Rena, B. Youa,, J. Dua, X.J. Baia, J. Zhanga, W. Zhanga, A. Hua, B. Zhangb, X.X. Zhangb a National Laboratory of Solid State Microstructures and Department of Physics, Nanjing University, Nanjing 210093, PR China Department of Physics & Chemistry and Institute of Nano Science and Technology, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong
b
Received 8 May 2007; received in revised form 11 July 2007; accepted 13 July 2007
Abstract Iron/iron-oxide granular films were fabricated using reactive dc magnetron sputtering. Their structural, magnetic and transport properties were systematically studied. XPS and TEM confirmed the coexistence of Fe, FeO and Fe2O3. A metal–insulator transition was observed with the increasing of the oxygen component in the film. The temperature dependencies of longitudinal resistivity rxx and anomalous Hall resistivity rxy were discussed. We found the enhancement of rxy and investigated the scaling law between anomalous Hall coefficient Rs and rxx. In all the samples, Rs was found to be proportional to rxx when rxx is small, which indicated the skew scattering is dominant. r 2007 Elsevier B.V. All rights reserved. PACS: 75.70.Ak; 72.15.Gd Keywords: Sputtering; Magnetic properties and measurements; Metal–insulator transition; Localization
1. Introduction Transition metal compounds have rich structures of phases due to their spin and orbital fluctuations. A great deal of research focuses on understanding properties of magnetic granular films. In these granular films, with ferromagnetic (FM) nano-particles embedded in the nonmagnetic matrix, some novel transport properties have been exhibited, such as giant magnetoresistance (GMR) effect [1,2] and giant Hall effect (GHE) [3,4]. GMR effect in granular film was first discovered in Co–Cu system, and has been observed in many other FM–non-magnetic metallic immiscible systems [5]. It is now believed that the mechanism of GMR is based on spin-dependent scattering of conduction electrons at particle–matrix interface. In addition, the matrix can be also non-metallic. Although the resistivity becomes high and spin-dependent scattering effect can be negligible, large magnetoresistance is also Corresponding author. Tel.: +86 25 83592762; fax: +86 25 83595535.
E-mail address: [email protected] (B. You). 0921-4526/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2007.07.005
obtained, which is attributed to spin-dependent tunneling instead [6]. On the other hand, due to the structural and magnetic disorder, the Hall resistivity in granular films can be four orders of magnitude greater than that of the relevant pure metal films [3,4], which is the so-called GHE. During the past several years, iron oxide thin films have been extensively investigated for their unusual magnetic properties [7–9]. In these studies, iron oxide thin films are prepared by a variety of methods, including reactive rf sputtering from iron targets in mixed oxygen and argon gases. The structural and magnetic properties of the films were found to strongly depend on the oxygen flow rate and the thickness of the film. Recent studies on iron/iron-oxide fine particles show both coercivity enhancement and exchange bias effect at low temperatures, which is consistent with core-shell particle morphology [10]. However, very few attentions are paid on the magnetic transport property of such systems, such as Hall and magnetic resistivity. In this paper, granular iron/iron-oxide films are fabricated and their electronic and magnetic properties have been studied. The metal–insulator transition (MIT)
ARTICLE IN PRESS S.L. Ren et al. / Physica B 400 (2007) 185–189
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appears with the increase of oxygen component in these samples. The mechanism of the transport property is discussed along with the magnetic property.
2. Experiment and measurements The granular iron/iron-oxide films are fabricated by reactive dc sputtering with a pure Fe target at room temperature. The base pressure of the chamber is lower than 1 105 Pa and the pressure of argon during sputtering is maintained at about 0.5 Pa. The flow rate of oxygen is varied, and the ratio of oxygen gas and argon are 1:400, 4:400 and 10:400 for samples S01, S02 and S03, respectively. The deposition rates are kept at 0.6 nm/s. Both glass and Kapton substrates are used so that the transport and magnetic properties can be separately measured. Pure iron film is also prepared for comparison. The thickness of the films is measured by a Dektak 3 surface profiler. The average thickness of film is about 400 nm. The structure of films is studied by X-ray diffraction (XRD) and transmission electron microscopy (TEM). X-ray photoelectron spectroscopy (XPS) is used to analyze the composition of the films. The magnetic measurements are carried out by a commercial superconducting quantum interference device (SQUID) magnetometer. Electronic transport measurements are performed in physical properties measurement systems (PPMS). The conventional fourprobe method is used to measure the longitudinal and the Hall resistivity.
3. Results and discussion 3.1. Structural properties X-ray diffraction spectrum is employed to determine the crystal structure. The diffraction patterns identify the existence of iron phases although the signal is very weak. The peak at 2y ¼ 44.81 is consistent with the (1 1 0) orientation of bulk cubic center (bcc) iron, exhibiting the texture structure of iron formed. No distinguished peaks of iron oxide are found in all the films. The absence of the iron oxide pattern is probably due to the small grain size and poorly crystallized at the high deposition rate. TEM morphology is taken to further investigate the microstructure of the film. Fig. 1 shows bright field micrographs and a selected area diffraction pattern of sample S02. A discontinuous distribution of grains can be seen from the bright field image, which indicates that Fe particles with size about 10 nm are dispersed in the oxide matrix. From selected area electron diffraction patterns, bcc a-Fe, FeO and a-Fe2O3 are recognized. This is not consistent with the results of Kim and Oliveria [7], which showed that for low oxygen flow rate, Fe2O3 phase was never found in the samples deposited at room temperature. In order to confirm the valence state of iron in our sample, we utilize XPS technique which is very sensitive to detect different chemical species in the sample. In the XPS spectra, a characteristic peak for Fe 2p3/2 located at 706.6 eV can be clearly observed. With increasing the oxygen flow rate during deposition, the Fe 2p3/2 peak becomes asymmetric
Fig. 1. Bright field TEM image and selected area electron diffraction patterns of sample S02.
ARTICLE IN PRESS S.L. Ren et al. / Physica B 400 (2007) 185–189
broadening at the higher energy edge. By means of peak decomposition, we confirm the existence of Fe2+ 2p3/2 peak at 709.0 eV and Fe3+ 2p3/2 peak at 711.3 eV. We fit the XPS results and evaluate the atom ratio of Fe, Fe2+ and Fe3+ which is shown in Table 1. The results of TEM and XPS indicate a granular-type Fe/FeOx structure is formed in the deposited samples.
3.2. Magnetic property Magnetic properties are performed in SQUID with the film deposited on the Kapton. First, the samples are cooled from 300 to 10 K with a magnetic field at 20,000 Oe applied parallel to the film plane. Then M–H loops are measured at different temperatures with magnetic field between 74 T. After field-cooling, the loops become asymmetrical and shift to negative field axis. This is called exchange bias effect which was first discovered by Meiklejohn and Bean [11]. The exchange bias effect is originated from the exchange coupling at ferromagnet/antiferromagnet interface. The shift of the M–H loops is consistent with the result in Fe–FeOx granular system, since FeO and a-Fe2O3 are antiferromagnetic at low temperature. The exchange bias field HE is defined by the shift of hysteresis loop away from zero field. Fig. 2 shows the dependence of HE on the
Table 1 The atom percentage (%) of Fe, Fe2+ and Fe3+ of samples S01, S02 and S03 obtained from XPS fitting
S01 S02 S03
Fe
Fe2+
Fe3+
61 44 20
18 30 27
21 26 53
800 S03
700
S02
temperature. HE tends to decrease at high temperature and eventually disappears at the blocking temperature of about 30 and 150 K for S01 and S02, respectively. The disappearance of HE at low temperature in S01 and S02 can be understood as the finite-size effect. According to the finite-size effects theory [12], for the small antiferromagnet grain, the Neel temperature should be lower than that of bulk one. Therefore, the blocking temperature decreases correspondingly. Moreover, the anisotropy energy of antiferromagnet can be written as KV, where K is the anisotropy constant and V is the volume of grain. For smaller grain size, the anisotropy energy is lower. Therefore, the spins of FeO and Fe2O3 at Fe/FeOx interfaces are easier to rotate along with the Fe particles. This can also lead to reduction of the blocking temperature. 3.3. Transport property The change of oxygen component can be shown from the change of longitudinal resistivity rxx. Fig. 3 plots temperature dependence of rxx(T)/rxx(300 K) for pure iron film and iron–iron oxide granular films. Sample S01 shows a positive temperature coefficient of resistivity (TCR), indicating a dirty metallic behavior since its resistivity is 30 times larger than that of pure iron. In addition, the change amplitude defined as rxx ð300 KÞ rxx ð10 KÞ =rxx ð300 KÞ is very small. For sample S01, it is only 0.11, less than that of pure iron at 0.5. It is known that the temperature dependence of resistivity in FM metals is determined by electron–phonon scattering and electron–magnon scattering. Nevertheless, structural disorder should also be taken into account. We speculate that with the increase of oxygen, the iron oxide grains form gradually and the temperature-independent boundary scattering increases. Therefore, the decrease of change amplitude in sample S01 may be caused by the enhancement of boundary scattering. With the increase of oxygen component, a negative TCR is observed in the whole temperature range for sample S03, which indicates the system is now an insulator. We fit the resistivity dependence on the temperature and find in the
S01
500
2.4
400
2.0
ρxx(T)/ρxx(300K)
HE (Oe)
600
187
300 200 100
S03 S02 S01 iron
1.6 1.2 0.8 0.4
0 0
20
40
60
80 100 120 140 160 180 200 T (K)
Fig. 2. The dependence of HE on the temperature of Fe/FeOx after cooled in the 20,000 Oe field.
0
100
200
300
Temperature (K) Fig. 3. The temperature dependence of normalized longitudinal resistivity defined as rxx(T)/rxx(300 K) of samples S01, S02, S03 and pure iron.
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temperature independent, the weak temperature dependence of resistivity suggests that rbc is dominant in the transport process. The sign change of TCR and a maximum appearing at 150 K can be understood as a result of competition between the electron scattering and the hopping conduction.
1.26 S03
logρxx
1.12
0.98
3.4. Hall effects 0.84 0.06
0.12
0.18
0.24
The Hall effect indicates important magnetic transport property in the FM granular films. Generally, the Hall resistivity in FM materials can be expressed as
0.30
T-1/2 Fig. 4. The logarithm of longitudinal resistivity rxx versus T1/2 for S03. The solid lines represent the linear fit results.
whole temperature range, the logarithm of rxx is inversely proportional to T1/2, which is shown in Fig. 4. The dependence of rxx ðTÞ / exp½ðT 0 =TÞ1=2 can be generally expressed although the constant T0 might vary in different temperature regions due to the different slopes obtained from Fig. 4. This relationship is consistent with the results observed in some granular systems [13–15] and implies that the hopping conduction is the dominant mechanism of transport for the sample with larger oxygen component. For the sample S02, the temperature dependence of resistivity becomes complicated. With temperature decreasing from 300 K, the resistivity first increases linearly with a negative TCR, and approaches the maximum at 150 K, manifesting an insulator behavior. As the temperature is decreased further, the resistivity drops linearly exhibiting a metal behavior. However, the temperature dependence of resistivity is weak since the relative change of r is less than 0.12. Owing to the moderate oxygen content in the sample, the mechanism of resistivity is complicated. The phonon scattering and magnon scattering in sample S01 and the hopping conduction in sample S03 may coexist in sample S02, and the temperature dependence of r can be expressed as rðTÞ ¼ r0 þ rbc þ rph þ rm þ rh
p T0 ¼ r0 þ rbc þ AT þ BT þ C exp , T n
2
ð1Þ
where r0 is the residual resistivity, and rbc, rph, rm and rh are the resistivies corresponding to the boundary scattering, phonon scattering, magnon scattering and hopping conduction respectively. A, B and C are the constants independent of temperature. In the light of classic conductivity theory, rph can be written as rph ¼ ATn, and n ¼ 1 at high temperature or n ¼ 5 at low temperature [16]. For the FM metals, the magnon scattering usually leads to a T2 dependence of rm [17]. rh ¼ C exp½ðT 0 =TÞp has been predicted from the theory of hopping conductivity in granular metals. According to sample S03, p ¼ 1/2 is obtained, although p ¼ 1/4 was also predicted by Mott’s theory [18]. Since the boundary scattering is almost
rxy ¼ R0 H þ Rs 4pM,
(2)
where the first term is the ordinary Hall resistivity proportional to the external field and the second term is the anomalous Hall resistivity proportional to the magnetization of the materials. R0 and Rs are the ordinary and anomalous Hall coefficients in Eq. (2), respectively. H and M are the external field and the magnetization of materials. The anomalous Hall effect (AHE) is originated from the asymmetry scattering of the polarized electrons via spin–orbit interaction, and the anomalous Hall coefficient can be expressed as Rs ¼ arxx þ br2xx ,
(3)
where the former term is ascribed to the skew scattering process, and the later one is associated with the side jump mechanism. AHE is more important in granular films and the Hall resistivity can be much larger than that of the pure FM metal [3]. We have measured the Hall resistivity of Fe/FeOx samples from 10 to 300 K, each 30 K, with field between 74 T. The saturated anomalous Hall resistivity rxys is obtained by a linear extrapolation of the data at high fields to H ¼ 0. In sample S01 and S03, rxys increases with temperature monotonously. rxys also increases with the oxygen, from 1.3 mO cm of sample S01 to 4 mO cm of sample S03 at 300 K. The relation between Rs and rxx is plotted in Fig. 5. It exhibits that Rs is proportional to rxx in both samples when rxx is small, which implies that the skew scattering mechanism might be dominant. When rxx increases the linear relationship of Rs and rxx is deviated. It should be pointed out that both the skew scattering and side jump are only suitable to the FM alloys. In contrast to granular alloy with metallic conductivity, the mechanism in the insulator region has not been completely revealed. However, recent studies on iron granular film show that the tunneling resistances between the grains dominate the longitudinal transport, while the Hall transport is still controlled by scattering processes within the iron grain. The scaling law can be interpreted within skew scattering model in terms with weak localization [19]. The results of sample of S01 and S03 are consistent with this viewpoint. Combined with the granular form and strong disorder reflect from the resistivity, the relation of Rs and rxx can be understood.
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4.8 1.5
0.9
4.2
3.6
0.6
Rs (arb.units)
Rs (arb.units)
S03
S01
1.2
3.0
0.3 800
840 880 ρxx (μΩ cm)
920
7200
10800 14400 ρxx (μΩ cm)
2.4 18000
Fig. 5. The anomalous Hall coefficient Rs versus the longitudinal resistivity rxx. The solid lines are the linear fit results. (a) Sample S01, and (b) sample S03.
4. Conclusion
References
Magnetic and transport properties of Fe/FeOx granular films are systematically studied. Large exchange bias effect due to Fe–FeO interfacial coupling is observed. The temperature dependence of resistivity becomes complicated and the metal–insulator transition occurs with increasing the oxygen atoms in the films. Our discussion suggests that various conduction mechanisms such as phonon scattering, magnon scattering, boundary scattering, and tunneling process should be considered. The competition among these mechanisms results in the complexity of temperature dependence of resistivity. The AHE is enhanced in the granular films. The present work indicates that the microstructures, magnetic property and the transport process should be taken into account as a whole to understand the unusual behaviors of magnetic transports in Fe/FeOx thin films.
[1] 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. [2] J.Q. Xiao, J.S. Jiang, C.L. Chien, Phys. Rev. Lett. 68 (1992) 3749. [3] A.B. Pakhomov, X. Yan, B. Zhao, Appl. Phys. Lett. 67 (1995) 3497. [4] X.N. Jing, N. Wang, A.B. Pakhomov, K.K. Fung, X. Yan, Phys. Rev. B 53 (1996) 14032. [5] J.Q. Wang, G. Xiao, Phys. Rev. B 49 (1996) 3982. [6] H. Fujimori, S. Mitani, S. Ohnuma, Mater. Sci. Eng. B 31 (1995) 219. [7] Y.K. Kim, M. Oliveria, J. Appl. Phys. 75 (1994) 431. [8] J.K. Lin, J.M. Sivertsen, J.H. Judy, J. Magn. Magn. Mater. 54 (1986) 1659. [9] S. Dhara, G.L. Malhotra, A.C. Rastogi, B.K. Das, Thin Solid Films 209 (1992) 116. [10] C. Baker, S.I. Shah, S.K. Hasanain, J. Magn. Magn. Mater. 280 (2004) 412. [11] W.H. Meiklejohn, C.P. Bean, Phys. Rev. 102 (1956) 1413. [12] T. Ambrose, C.L. Chien, Phys. Rev. Lett. 76 (1996) 1743. [13] J.L. Gittleman, Y. Goldstein, S. Bozowski, Phys. Rev. B 5 (1972) 3609. [14] H.R. Zeller, Phys. Rev. Lett. 28 (1972) 1452. [15] P. Sheng, B. Abeles, Y. Arie, Phys. Rev. Lett. 31 (1973) 44. [16] J. Callaway, Quantum Theory of the Solid State, second ed., Academic Press, New York, 1991. [17] I.A. Campbell, A. Fert, Transport properties of ferromagnets, in: E.P. Wohlfarth (Ed.), Ferromagnetic Materials, North Holland Publishing Co., 1982. [18] N.F. Mott, Philos. Mag. 19 (1969) 835. [19] P. Mitra, A.F. Hebard, K.A. Muttalib, P. Woelfle, Cond-mat/ 0606215.
Acknowledgments This work is supported by National Basic Research Program of China (No. 2007CB925104), the National Science Foundation of China (No. 10474038, 10574065) and the Hong Kong RGC Grants (No. 605704 and 605605).
Physica B 400 (2007) 185–189 www.elsevier.com/locate/physb
Magnetic transport properties in iron/iron-oxide films S.L. Rena, B. Youa,, J. Dua, X.J. Baia, J. Zhanga, W. Zhanga, A. Hua, B. Zhangb, X.X. Zhangb a National Laboratory of Solid State Microstructures and Department of Physics, Nanjing University, Nanjing 210093, PR China Department of Physics & Chemistry and Institute of Nano Science and Technology, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong
b
Received 8 May 2007; received in revised form 11 July 2007; accepted 13 July 2007
Abstract Iron/iron-oxide granular films were fabricated using reactive dc magnetron sputtering. Their structural, magnetic and transport properties were systematically studied. XPS and TEM confirmed the coexistence of Fe, FeO and Fe2O3. A metal–insulator transition was observed with the increasing of the oxygen component in the film. The temperature dependencies of longitudinal resistivity rxx and anomalous Hall resistivity rxy were discussed. We found the enhancement of rxy and investigated the scaling law between anomalous Hall coefficient Rs and rxx. In all the samples, Rs was found to be proportional to rxx when rxx is small, which indicated the skew scattering is dominant. r 2007 Elsevier B.V. All rights reserved. PACS: 75.70.Ak; 72.15.Gd Keywords: Sputtering; Magnetic properties and measurements; Metal–insulator transition; Localization
1. Introduction Transition metal compounds have rich structures of phases due to their spin and orbital fluctuations. A great deal of research focuses on understanding properties of magnetic granular films. In these granular films, with ferromagnetic (FM) nano-particles embedded in the nonmagnetic matrix, some novel transport properties have been exhibited, such as giant magnetoresistance (GMR) effect [1,2] and giant Hall effect (GHE) [3,4]. GMR effect in granular film was first discovered in Co–Cu system, and has been observed in many other FM–non-magnetic metallic immiscible systems [5]. It is now believed that the mechanism of GMR is based on spin-dependent scattering of conduction electrons at particle–matrix interface. In addition, the matrix can be also non-metallic. Although the resistivity becomes high and spin-dependent scattering effect can be negligible, large magnetoresistance is also Corresponding author. Tel.: +86 25 83592762; fax: +86 25 83595535.
E-mail address: [email protected] (B. You). 0921-4526/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2007.07.005
obtained, which is attributed to spin-dependent tunneling instead [6]. On the other hand, due to the structural and magnetic disorder, the Hall resistivity in granular films can be four orders of magnitude greater than that of the relevant pure metal films [3,4], which is the so-called GHE. During the past several years, iron oxide thin films have been extensively investigated for their unusual magnetic properties [7–9]. In these studies, iron oxide thin films are prepared by a variety of methods, including reactive rf sputtering from iron targets in mixed oxygen and argon gases. The structural and magnetic properties of the films were found to strongly depend on the oxygen flow rate and the thickness of the film. Recent studies on iron/iron-oxide fine particles show both coercivity enhancement and exchange bias effect at low temperatures, which is consistent with core-shell particle morphology [10]. However, very few attentions are paid on the magnetic transport property of such systems, such as Hall and magnetic resistivity. In this paper, granular iron/iron-oxide films are fabricated and their electronic and magnetic properties have been studied. The metal–insulator transition (MIT)
ARTICLE IN PRESS S.L. Ren et al. / Physica B 400 (2007) 185–189
186
appears with the increase of oxygen component in these samples. The mechanism of the transport property is discussed along with the magnetic property.
2. Experiment and measurements The granular iron/iron-oxide films are fabricated by reactive dc sputtering with a pure Fe target at room temperature. The base pressure of the chamber is lower than 1 105 Pa and the pressure of argon during sputtering is maintained at about 0.5 Pa. The flow rate of oxygen is varied, and the ratio of oxygen gas and argon are 1:400, 4:400 and 10:400 for samples S01, S02 and S03, respectively. The deposition rates are kept at 0.6 nm/s. Both glass and Kapton substrates are used so that the transport and magnetic properties can be separately measured. Pure iron film is also prepared for comparison. The thickness of the films is measured by a Dektak 3 surface profiler. The average thickness of film is about 400 nm. The structure of films is studied by X-ray diffraction (XRD) and transmission electron microscopy (TEM). X-ray photoelectron spectroscopy (XPS) is used to analyze the composition of the films. The magnetic measurements are carried out by a commercial superconducting quantum interference device (SQUID) magnetometer. Electronic transport measurements are performed in physical properties measurement systems (PPMS). The conventional fourprobe method is used to measure the longitudinal and the Hall resistivity.
3. Results and discussion 3.1. Structural properties X-ray diffraction spectrum is employed to determine the crystal structure. The diffraction patterns identify the existence of iron phases although the signal is very weak. The peak at 2y ¼ 44.81 is consistent with the (1 1 0) orientation of bulk cubic center (bcc) iron, exhibiting the texture structure of iron formed. No distinguished peaks of iron oxide are found in all the films. The absence of the iron oxide pattern is probably due to the small grain size and poorly crystallized at the high deposition rate. TEM morphology is taken to further investigate the microstructure of the film. Fig. 1 shows bright field micrographs and a selected area diffraction pattern of sample S02. A discontinuous distribution of grains can be seen from the bright field image, which indicates that Fe particles with size about 10 nm are dispersed in the oxide matrix. From selected area electron diffraction patterns, bcc a-Fe, FeO and a-Fe2O3 are recognized. This is not consistent with the results of Kim and Oliveria [7], which showed that for low oxygen flow rate, Fe2O3 phase was never found in the samples deposited at room temperature. In order to confirm the valence state of iron in our sample, we utilize XPS technique which is very sensitive to detect different chemical species in the sample. In the XPS spectra, a characteristic peak for Fe 2p3/2 located at 706.6 eV can be clearly observed. With increasing the oxygen flow rate during deposition, the Fe 2p3/2 peak becomes asymmetric
Fig. 1. Bright field TEM image and selected area electron diffraction patterns of sample S02.
ARTICLE IN PRESS S.L. Ren et al. / Physica B 400 (2007) 185–189
broadening at the higher energy edge. By means of peak decomposition, we confirm the existence of Fe2+ 2p3/2 peak at 709.0 eV and Fe3+ 2p3/2 peak at 711.3 eV. We fit the XPS results and evaluate the atom ratio of Fe, Fe2+ and Fe3+ which is shown in Table 1. The results of TEM and XPS indicate a granular-type Fe/FeOx structure is formed in the deposited samples.
3.2. Magnetic property Magnetic properties are performed in SQUID with the film deposited on the Kapton. First, the samples are cooled from 300 to 10 K with a magnetic field at 20,000 Oe applied parallel to the film plane. Then M–H loops are measured at different temperatures with magnetic field between 74 T. After field-cooling, the loops become asymmetrical and shift to negative field axis. This is called exchange bias effect which was first discovered by Meiklejohn and Bean [11]. The exchange bias effect is originated from the exchange coupling at ferromagnet/antiferromagnet interface. The shift of the M–H loops is consistent with the result in Fe–FeOx granular system, since FeO and a-Fe2O3 are antiferromagnetic at low temperature. The exchange bias field HE is defined by the shift of hysteresis loop away from zero field. Fig. 2 shows the dependence of HE on the
Table 1 The atom percentage (%) of Fe, Fe2+ and Fe3+ of samples S01, S02 and S03 obtained from XPS fitting
S01 S02 S03
Fe
Fe2+
Fe3+
61 44 20
18 30 27
21 26 53
800 S03
700
S02
temperature. HE tends to decrease at high temperature and eventually disappears at the blocking temperature of about 30 and 150 K for S01 and S02, respectively. The disappearance of HE at low temperature in S01 and S02 can be understood as the finite-size effect. According to the finite-size effects theory [12], for the small antiferromagnet grain, the Neel temperature should be lower than that of bulk one. Therefore, the blocking temperature decreases correspondingly. Moreover, the anisotropy energy of antiferromagnet can be written as KV, where K is the anisotropy constant and V is the volume of grain. For smaller grain size, the anisotropy energy is lower. Therefore, the spins of FeO and Fe2O3 at Fe/FeOx interfaces are easier to rotate along with the Fe particles. This can also lead to reduction of the blocking temperature. 3.3. Transport property The change of oxygen component can be shown from the change of longitudinal resistivity rxx. Fig. 3 plots temperature dependence of rxx(T)/rxx(300 K) for pure iron film and iron–iron oxide granular films. Sample S01 shows a positive temperature coefficient of resistivity (TCR), indicating a dirty metallic behavior since its resistivity is 30 times larger than that of pure iron. In addition, the change amplitude defined as rxx ð300 KÞ rxx ð10 KÞ =rxx ð300 KÞ is very small. For sample S01, it is only 0.11, less than that of pure iron at 0.5. It is known that the temperature dependence of resistivity in FM metals is determined by electron–phonon scattering and electron–magnon scattering. Nevertheless, structural disorder should also be taken into account. We speculate that with the increase of oxygen, the iron oxide grains form gradually and the temperature-independent boundary scattering increases. Therefore, the decrease of change amplitude in sample S01 may be caused by the enhancement of boundary scattering. With the increase of oxygen component, a negative TCR is observed in the whole temperature range for sample S03, which indicates the system is now an insulator. We fit the resistivity dependence on the temperature and find in the
S01
500
2.4
400
2.0
ρxx(T)/ρxx(300K)
HE (Oe)
600
187
300 200 100
S03 S02 S01 iron
1.6 1.2 0.8 0.4
0 0
20
40
60
80 100 120 140 160 180 200 T (K)
Fig. 2. The dependence of HE on the temperature of Fe/FeOx after cooled in the 20,000 Oe field.
0
100
200
300
Temperature (K) Fig. 3. The temperature dependence of normalized longitudinal resistivity defined as rxx(T)/rxx(300 K) of samples S01, S02, S03 and pure iron.
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temperature independent, the weak temperature dependence of resistivity suggests that rbc is dominant in the transport process. The sign change of TCR and a maximum appearing at 150 K can be understood as a result of competition between the electron scattering and the hopping conduction.
1.26 S03
logρxx
1.12
0.98
3.4. Hall effects 0.84 0.06
0.12
0.18
0.24
The Hall effect indicates important magnetic transport property in the FM granular films. Generally, the Hall resistivity in FM materials can be expressed as
0.30
T-1/2 Fig. 4. The logarithm of longitudinal resistivity rxx versus T1/2 for S03. The solid lines represent the linear fit results.
whole temperature range, the logarithm of rxx is inversely proportional to T1/2, which is shown in Fig. 4. The dependence of rxx ðTÞ / exp½ðT 0 =TÞ1=2 can be generally expressed although the constant T0 might vary in different temperature regions due to the different slopes obtained from Fig. 4. This relationship is consistent with the results observed in some granular systems [13–15] and implies that the hopping conduction is the dominant mechanism of transport for the sample with larger oxygen component. For the sample S02, the temperature dependence of resistivity becomes complicated. With temperature decreasing from 300 K, the resistivity first increases linearly with a negative TCR, and approaches the maximum at 150 K, manifesting an insulator behavior. As the temperature is decreased further, the resistivity drops linearly exhibiting a metal behavior. However, the temperature dependence of resistivity is weak since the relative change of r is less than 0.12. Owing to the moderate oxygen content in the sample, the mechanism of resistivity is complicated. The phonon scattering and magnon scattering in sample S01 and the hopping conduction in sample S03 may coexist in sample S02, and the temperature dependence of r can be expressed as rðTÞ ¼ r0 þ rbc þ rph þ rm þ rh
p T0 ¼ r0 þ rbc þ AT þ BT þ C exp , T n
2
ð1Þ
where r0 is the residual resistivity, and rbc, rph, rm and rh are the resistivies corresponding to the boundary scattering, phonon scattering, magnon scattering and hopping conduction respectively. A, B and C are the constants independent of temperature. In the light of classic conductivity theory, rph can be written as rph ¼ ATn, and n ¼ 1 at high temperature or n ¼ 5 at low temperature [16]. For the FM metals, the magnon scattering usually leads to a T2 dependence of rm [17]. rh ¼ C exp½ðT 0 =TÞp has been predicted from the theory of hopping conductivity in granular metals. According to sample S03, p ¼ 1/2 is obtained, although p ¼ 1/4 was also predicted by Mott’s theory [18]. Since the boundary scattering is almost
rxy ¼ R0 H þ Rs 4pM,
(2)
where the first term is the ordinary Hall resistivity proportional to the external field and the second term is the anomalous Hall resistivity proportional to the magnetization of the materials. R0 and Rs are the ordinary and anomalous Hall coefficients in Eq. (2), respectively. H and M are the external field and the magnetization of materials. The anomalous Hall effect (AHE) is originated from the asymmetry scattering of the polarized electrons via spin–orbit interaction, and the anomalous Hall coefficient can be expressed as Rs ¼ arxx þ br2xx ,
(3)
where the former term is ascribed to the skew scattering process, and the later one is associated with the side jump mechanism. AHE is more important in granular films and the Hall resistivity can be much larger than that of the pure FM metal [3]. We have measured the Hall resistivity of Fe/FeOx samples from 10 to 300 K, each 30 K, with field between 74 T. The saturated anomalous Hall resistivity rxys is obtained by a linear extrapolation of the data at high fields to H ¼ 0. In sample S01 and S03, rxys increases with temperature monotonously. rxys also increases with the oxygen, from 1.3 mO cm of sample S01 to 4 mO cm of sample S03 at 300 K. The relation between Rs and rxx is plotted in Fig. 5. It exhibits that Rs is proportional to rxx in both samples when rxx is small, which implies that the skew scattering mechanism might be dominant. When rxx increases the linear relationship of Rs and rxx is deviated. It should be pointed out that both the skew scattering and side jump are only suitable to the FM alloys. In contrast to granular alloy with metallic conductivity, the mechanism in the insulator region has not been completely revealed. However, recent studies on iron granular film show that the tunneling resistances between the grains dominate the longitudinal transport, while the Hall transport is still controlled by scattering processes within the iron grain. The scaling law can be interpreted within skew scattering model in terms with weak localization [19]. The results of sample of S01 and S03 are consistent with this viewpoint. Combined with the granular form and strong disorder reflect from the resistivity, the relation of Rs and rxx can be understood.
ARTICLE IN PRESS S.L. Ren et al. / Physica B 400 (2007) 185–189
189
4.8 1.5
0.9
4.2
3.6
0.6
Rs (arb.units)
Rs (arb.units)
S03
S01
1.2
3.0
0.3 800
840 880 ρxx (μΩ cm)
920
7200
10800 14400 ρxx (μΩ cm)
2.4 18000
Fig. 5. The anomalous Hall coefficient Rs versus the longitudinal resistivity rxx. The solid lines are the linear fit results. (a) Sample S01, and (b) sample S03.
4. Conclusion
References
Magnetic and transport properties of Fe/FeOx granular films are systematically studied. Large exchange bias effect due to Fe–FeO interfacial coupling is observed. The temperature dependence of resistivity becomes complicated and the metal–insulator transition occurs with increasing the oxygen atoms in the films. Our discussion suggests that various conduction mechanisms such as phonon scattering, magnon scattering, boundary scattering, and tunneling process should be considered. The competition among these mechanisms results in the complexity of temperature dependence of resistivity. The AHE is enhanced in the granular films. The present work indicates that the microstructures, magnetic property and the transport process should be taken into account as a whole to understand the unusual behaviors of magnetic transports in Fe/FeOx thin films.
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Acknowledgments This work is supported by National Basic Research Program of China (No. 2007CB925104), the National Science Foundation of China (No. 10474038, 10574065) and the Hong Kong RGC Grants (No. 605704 and 605605).