- Email: [email protected]
Interface scattering and spin-dependent transport in granular magnetite
ARTICLE IN PRESS Journal of Magnetism and Magnetic Materials 321 (2009) 1909–1911
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Interface scattering and spin-dependent transport in granular magnetite Zhongtian Yang , Liya Zhu College of Science, China Three Gorges University, Yichang 443002, China
a r t i c l e in fo
abstract
Article history: Received 8 November 2008 Available online 7 December 2008
We have prepared nearly monodisperse Fe3O4 of 50 nm by a chemical route and investigated the electrical and magnetic transports of the composite granular system. A Verwey transition is observed in the vicinity of 113 K. Above and below the Verwey transition, the electrical transport is dominated by electron hopping behavior showing a good linear relation between resistance and T1/2. The magnetoresistance (MR) increases with the applied field and does not follow the magnetization to reach the saturation at 10 KOe field. This indicates that the MR is mainly arising from the spindependent scattering of electrons through the grain boundaries. The temperature dependence of MR shows it has the highest MR value near the Verwey transition. & 2008 Elsevier B.V. All rights reserved.
PACS: 75.47.m 72.25.Mk Keywords: Magnetite Spin-dependent transport Granular system
1. Introduction Recently, the magnetoresistance (MR) of magnetite Fe3O4 has attracted much attention due to its half-metallic nature and the highest Curie temperature of 860 K, which is considered to be the best candidate for applications of the spin electronic devices [1]. Variety forms of Magnetite has been extensively studied including crystal [2], pressed powders [3], epitaxial and polycrystal films [4], magnetic tunneling junctions [5] and some composite granular systems [6]. In most cases, the obtained MR values are much lower than that of theory prediction, especially at high temperature. The rapid decrease of the MR with temperature increasing was mostly attributed to the loss of spin polarization or increase of the higher order inelastic hopping of electrons through the localized states [1,6]. However, none of the proposed models can explain all the phenomena observed in the experiments. For granular system, the MR is mainly arising from the effect of spin-dependent interface scattering or tunneling through the grain boundaries. The former one has a nearly linear MR behavior in a large field range [3], while the later shows a quick decrease of MR in a very small field corresponding to a tunneling magnetoresistance (TMR) [6]. However, it is still amazing to observe the two different mechanisms in the pure magnetite, even in the same sample [7]. Indeed, defects (or impurities) and the surface states of the ferromagnetic grains are responsible for the result of the spin-dependent transport [1,6]. To better understand the mechanism of MR behavior in the pure magnetite sample, we have prepared nearly monodisperse Fe3O4 of 50 nm by a chemical
Corresponding author.
E-mail address: [email protected] (Z. Yang). 0304-8853/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2008.12.004
route and investigated the MR effect of the granular system. The results indicate that the MR is mainly arising from the spindependent scattering of electrons on the interfaces and exhibits temperature dependent.
2. Sample preparation and characterization The monodisperse magnetite of 50 nm was synthesized using a solvothermal reduction method described in detail elsewhere [8]. In brief, the required amount of 1,6-hexanediamine, anhydrous sodium acetate and FeCl3–6H2O were dispersed in 30 ml glycol and stirred vigorously at 50 1C to give a transparent solution, and then transferred into a Teflonlined autoclave and reacted at 190 1C for 6 h. The magnetite nanoparticles were then obtained and rinsed with ethanol to effectively remove the solvent and unbound 1,6-hexanediamine. Fig. 1 shows the X-ray diffraction pattern (Cu Ka) (XRD) of the as-prepared magnetite. All the peaks can be indexed in the Fe3O4 spinel structure. By the Rietveld analysis of the XRD data, the lattice parameter is calculated as a ¼ 0.8390 nm in comparison with the listed value of 0.8394 [9], indicating the sample is of single phase. The average size of the magnetite nanoparticles is about 50 nm determined by the transmission electronic microscopy (TEM) image (not shown here). Finally, the magnetite powder was cold pressed into pellet under pressure of 6 MPa and subsequently sintered in Ar atmosphere at 500 1C for 2 h. Fig. 2 shows the scanning electronic microscopy (SEM) image of the sintered magnetite sample. It can be seen that the magnetite particles almost maintain the previous size of 50 nm after sintered for 2 h.
ARTICLE IN PRESS 1910
Z. Yang, L. Zhu / Journal of Magnetism and Magnetic Materials 321 (2009) 1909–1911
Fig. 1. X-ray diffractions of the as-prepared monodisperse Fe3O4 nanoparticles (the average size is 50 nm).
Fig. 3. Temperature dependence of the resistivity, (a) is shown as r versus T and (b) is plotted as r versus T1/2. r is plotted in a logarithmic scale to show the Verwey transition.
Fig. 2. SEM image of the pressed magnetite after sintered at 500 1C for 2 h.
3. Measurements and analysis Measurements of the electrical and magnetic properties were performed on a commercial physical property measurement system (PPMS) and the magnetic property measurement system (MPMS). Electrical resistivity r was measured by a conventional four-probe method and a golden film was vaporized on the four terminal of the sample to assure good Ohmic contacts. Fig. 3(a) shows the temperature dependence of resistivity of the sample from 300 to 30 K. At 300 K in the absence of applied magnetic field, the resistivity is estimated to be 0.67 O cm and is about several orders of magnitude larger than those seen in the bulk single crystal. The much larger resistivity indicates that most of the resistance arises from the interfaces or the grain boundaries between the nanoparticles, as can be seen from the SEM result shown in Fig. 2. In addition, the extrinsic effect is large enough to obscure the intrinsic nature of the Verwey transition appearing in the vicinity of 113 K. In Fig. 3(b), the resistivity is plotted in a logarithmic scale versus 1/T1/2. Above or below the Verwey transition, it shows a good linear relation of rT1/2 indicating the granular nature of the sample (the solid line is a guide for eye sight) [3,6,7].
Fig. 4. The field dependence of the magnetization (left) and the magnetoresistance (right). The magnetization is measured at 100 K.
The MR measurement was performed using the four-probe technique by sweeping the applied field from 0 to 80 kOe. Fig. 4 shows the field dependence of MR at 300, 200 and 100 K, respectively. The MR is defined as (R0RH)/R0, where R0 is the resistance at zero field and RH is that measured in the applied field. Also plotted in the same figure is the field-dependent magnetization of the sample at 100 K. It can be seen that at 100 K, the MR does not follow the magnetization to reach the saturation
ARTICLE IN PRESS Z. Yang, L. Zhu / Journal of Magnetism and Magnetic Materials 321 (2009) 1909–1911
1911
several orders of magnitude at the transition temperature. In our sample, the MR ratio increases with the decrease of temperature above Verwey transition and then decreases below the transition. The drop of MR ratio below the Verwey point is probably attributed to the rapid increase in the resistivity of magnetite, which diminishes the spin-dependent scattering current [10].
4. Conclusion In this work, we have investigated the magnetoresistance of a granular sample, monodisperse Fe3O4 magnetite nanoparticles of 50 nm, compactly cold pressed and sintered at 500 1C. The results reveal that the MR is mainly arising from the spin-dependent scattering of electrons on the interfaces and shows the temperature dependent with the highest value near the Verwey transition.
Acknowledgment Fig. 5. The variety of the MR with temperature decreasing. The MR value shows a maximum in the vicinity of 120 K.
at the applied field of 10 KOe. This suggests that a simple directtunneling conductance mechanism cannot be a suitable explanation for such phenomena. In addition, no obvious transition is observed in the whole field range to distinguish the low-field MR and the high-field MR, which are found in composite granular system [6,10] In our case, the MR behavior is more like the spindependent scattering of electrons on the interfaces than the spindependent tunneling of electrons through the grain boundaries. In the high-field region, the MR value reaches to about 10% at 100 K, while decreases quickly to a small value of 3% at 300 K. The quick decrease of MR with temperature increasing is attributed to the spin-polarization loss during the increase of the inelastic-scattering process [1,6]. Fig. 5 shows the temperature dependence of MR ratio in an applied field of 10 KOe. In magnetite, there is a Verwey transition which is characterized by an increase of resistivity by about
This work is supported by the Excellent Youth Foundation of Hubei Scientific Committee (2006ABB036) and Natural Science Foundation of China Three Gorges University (2003C02). References [1] M. Ziese, Rep. Prog. Phys. 65 (2002) 143. [2] M. Ziese, H.J. Blythe, J. Phys.: Condens. Matter 12 (2000) 13. [3] J.M.D. Coey, A.E. Berkowitz, Ll. Balcells, F.F. Putris, F.T. Parker, Appl. Phys. Lett. 72 (1998) 734. [4] W. Eerenstein, T.T.M. Palstra, S.S. Saxena, T. Hibma, Phys. Rev. Lett. 88 (2002) 247204. [5] G. Hu, Y. Suzuki, Phys. Rev. Lett. 89 (2002) 276601. [6] J.F. Wang, J. Shi, D.C. Tian, H. Deng, Y.D. Li, P.Y. Song, C.P. Chen, Appl. Phys. Lett. 90 (2007) 213106. [7] P.Y. Song, J.F. Wang, C.P. Chen, H. Deng, Y.D. Li, J. Appl. Phys. 100 (2006) 044314. [8] L.Y. Wang, J. Bao, L. Wang, F. Zhang, Y.D. Li, Chem. Eur. J 12 (2006) 6341. [9] JCPDS-International Center for Diffraction Data, Card no. 79-0417 (unpublished). [10] W.D. Wang, L. Malkinski, J.K. Tang, J. Appl. Phys. 101 (2007) 09J504.
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Interface scattering and spin-dependent transport in granular magnetite Zhongtian Yang , Liya Zhu College of Science, China Three Gorges University, Yichang 443002, China
a r t i c l e in fo
abstract
Article history: Received 8 November 2008 Available online 7 December 2008
We have prepared nearly monodisperse Fe3O4 of 50 nm by a chemical route and investigated the electrical and magnetic transports of the composite granular system. A Verwey transition is observed in the vicinity of 113 K. Above and below the Verwey transition, the electrical transport is dominated by electron hopping behavior showing a good linear relation between resistance and T1/2. The magnetoresistance (MR) increases with the applied field and does not follow the magnetization to reach the saturation at 10 KOe field. This indicates that the MR is mainly arising from the spindependent scattering of electrons through the grain boundaries. The temperature dependence of MR shows it has the highest MR value near the Verwey transition. & 2008 Elsevier B.V. All rights reserved.
PACS: 75.47.m 72.25.Mk Keywords: Magnetite Spin-dependent transport Granular system
1. Introduction Recently, the magnetoresistance (MR) of magnetite Fe3O4 has attracted much attention due to its half-metallic nature and the highest Curie temperature of 860 K, which is considered to be the best candidate for applications of the spin electronic devices [1]. Variety forms of Magnetite has been extensively studied including crystal [2], pressed powders [3], epitaxial and polycrystal films [4], magnetic tunneling junctions [5] and some composite granular systems [6]. In most cases, the obtained MR values are much lower than that of theory prediction, especially at high temperature. The rapid decrease of the MR with temperature increasing was mostly attributed to the loss of spin polarization or increase of the higher order inelastic hopping of electrons through the localized states [1,6]. However, none of the proposed models can explain all the phenomena observed in the experiments. For granular system, the MR is mainly arising from the effect of spin-dependent interface scattering or tunneling through the grain boundaries. The former one has a nearly linear MR behavior in a large field range [3], while the later shows a quick decrease of MR in a very small field corresponding to a tunneling magnetoresistance (TMR) [6]. However, it is still amazing to observe the two different mechanisms in the pure magnetite, even in the same sample [7]. Indeed, defects (or impurities) and the surface states of the ferromagnetic grains are responsible for the result of the spin-dependent transport [1,6]. To better understand the mechanism of MR behavior in the pure magnetite sample, we have prepared nearly monodisperse Fe3O4 of 50 nm by a chemical
Corresponding author.
E-mail address: [email protected] (Z. Yang). 0304-8853/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2008.12.004
route and investigated the MR effect of the granular system. The results indicate that the MR is mainly arising from the spindependent scattering of electrons on the interfaces and exhibits temperature dependent.
2. Sample preparation and characterization The monodisperse magnetite of 50 nm was synthesized using a solvothermal reduction method described in detail elsewhere [8]. In brief, the required amount of 1,6-hexanediamine, anhydrous sodium acetate and FeCl3–6H2O were dispersed in 30 ml glycol and stirred vigorously at 50 1C to give a transparent solution, and then transferred into a Teflonlined autoclave and reacted at 190 1C for 6 h. The magnetite nanoparticles were then obtained and rinsed with ethanol to effectively remove the solvent and unbound 1,6-hexanediamine. Fig. 1 shows the X-ray diffraction pattern (Cu Ka) (XRD) of the as-prepared magnetite. All the peaks can be indexed in the Fe3O4 spinel structure. By the Rietveld analysis of the XRD data, the lattice parameter is calculated as a ¼ 0.8390 nm in comparison with the listed value of 0.8394 [9], indicating the sample is of single phase. The average size of the magnetite nanoparticles is about 50 nm determined by the transmission electronic microscopy (TEM) image (not shown here). Finally, the magnetite powder was cold pressed into pellet under pressure of 6 MPa and subsequently sintered in Ar atmosphere at 500 1C for 2 h. Fig. 2 shows the scanning electronic microscopy (SEM) image of the sintered magnetite sample. It can be seen that the magnetite particles almost maintain the previous size of 50 nm after sintered for 2 h.
ARTICLE IN PRESS 1910
Z. Yang, L. Zhu / Journal of Magnetism and Magnetic Materials 321 (2009) 1909–1911
Fig. 1. X-ray diffractions of the as-prepared monodisperse Fe3O4 nanoparticles (the average size is 50 nm).
Fig. 3. Temperature dependence of the resistivity, (a) is shown as r versus T and (b) is plotted as r versus T1/2. r is plotted in a logarithmic scale to show the Verwey transition.
Fig. 2. SEM image of the pressed magnetite after sintered at 500 1C for 2 h.
3. Measurements and analysis Measurements of the electrical and magnetic properties were performed on a commercial physical property measurement system (PPMS) and the magnetic property measurement system (MPMS). Electrical resistivity r was measured by a conventional four-probe method and a golden film was vaporized on the four terminal of the sample to assure good Ohmic contacts. Fig. 3(a) shows the temperature dependence of resistivity of the sample from 300 to 30 K. At 300 K in the absence of applied magnetic field, the resistivity is estimated to be 0.67 O cm and is about several orders of magnitude larger than those seen in the bulk single crystal. The much larger resistivity indicates that most of the resistance arises from the interfaces or the grain boundaries between the nanoparticles, as can be seen from the SEM result shown in Fig. 2. In addition, the extrinsic effect is large enough to obscure the intrinsic nature of the Verwey transition appearing in the vicinity of 113 K. In Fig. 3(b), the resistivity is plotted in a logarithmic scale versus 1/T1/2. Above or below the Verwey transition, it shows a good linear relation of rT1/2 indicating the granular nature of the sample (the solid line is a guide for eye sight) [3,6,7].
Fig. 4. The field dependence of the magnetization (left) and the magnetoresistance (right). The magnetization is measured at 100 K.
The MR measurement was performed using the four-probe technique by sweeping the applied field from 0 to 80 kOe. Fig. 4 shows the field dependence of MR at 300, 200 and 100 K, respectively. The MR is defined as (R0RH)/R0, where R0 is the resistance at zero field and RH is that measured in the applied field. Also plotted in the same figure is the field-dependent magnetization of the sample at 100 K. It can be seen that at 100 K, the MR does not follow the magnetization to reach the saturation
ARTICLE IN PRESS Z. Yang, L. Zhu / Journal of Magnetism and Magnetic Materials 321 (2009) 1909–1911
1911
several orders of magnitude at the transition temperature. In our sample, the MR ratio increases with the decrease of temperature above Verwey transition and then decreases below the transition. The drop of MR ratio below the Verwey point is probably attributed to the rapid increase in the resistivity of magnetite, which diminishes the spin-dependent scattering current [10].
4. Conclusion In this work, we have investigated the magnetoresistance of a granular sample, monodisperse Fe3O4 magnetite nanoparticles of 50 nm, compactly cold pressed and sintered at 500 1C. The results reveal that the MR is mainly arising from the spin-dependent scattering of electrons on the interfaces and shows the temperature dependent with the highest value near the Verwey transition.
Acknowledgment Fig. 5. The variety of the MR with temperature decreasing. The MR value shows a maximum in the vicinity of 120 K.
at the applied field of 10 KOe. This suggests that a simple directtunneling conductance mechanism cannot be a suitable explanation for such phenomena. In addition, no obvious transition is observed in the whole field range to distinguish the low-field MR and the high-field MR, which are found in composite granular system [6,10] In our case, the MR behavior is more like the spindependent scattering of electrons on the interfaces than the spindependent tunneling of electrons through the grain boundaries. In the high-field region, the MR value reaches to about 10% at 100 K, while decreases quickly to a small value of 3% at 300 K. The quick decrease of MR with temperature increasing is attributed to the spin-polarization loss during the increase of the inelastic-scattering process [1,6]. Fig. 5 shows the temperature dependence of MR ratio in an applied field of 10 KOe. In magnetite, there is a Verwey transition which is characterized by an increase of resistivity by about
This work is supported by the Excellent Youth Foundation of Hubei Scientific Committee (2006ABB036) and Natural Science Foundation of China Three Gorges University (2003C02). References [1] M. Ziese, Rep. Prog. Phys. 65 (2002) 143. [2] M. Ziese, H.J. Blythe, J. Phys.: Condens. Matter 12 (2000) 13. [3] J.M.D. Coey, A.E. Berkowitz, Ll. Balcells, F.F. Putris, F.T. Parker, Appl. Phys. Lett. 72 (1998) 734. [4] W. Eerenstein, T.T.M. Palstra, S.S. Saxena, T. Hibma, Phys. Rev. Lett. 88 (2002) 247204. [5] G. Hu, Y. Suzuki, Phys. Rev. Lett. 89 (2002) 276601. [6] J.F. Wang, J. Shi, D.C. Tian, H. Deng, Y.D. Li, P.Y. Song, C.P. Chen, Appl. Phys. Lett. 90 (2007) 213106. [7] P.Y. Song, J.F. Wang, C.P. Chen, H. Deng, Y.D. Li, J. Appl. Phys. 100 (2006) 044314. [8] L.Y. Wang, J. Bao, L. Wang, F. Zhang, Y.D. Li, Chem. Eur. J 12 (2006) 6341. [9] JCPDS-International Center for Diffraction Data, Card no. 79-0417 (unpublished). [10] W.D. Wang, L. Malkinski, J.K. Tang, J. Appl. Phys. 101 (2007) 09J504.