NANO LETTERS
Spin-Filter Effect in Magnetite Nanowire
2006 Vol. 6, No. 6 1087-1091
Zhi-Min Liao,† Ya-Dong Li,‡ Jun Xu,† Jing-Min Zhang,† Ke Xia,§ and Da-Peng Yu*,† State Key Laboratory for Mesoscopic Physics, and Electron Microscopy Laboratory, School of Physics, Peking UniVersity, Beijing 100871, People’s Republic of China, Department of Chemistry, Tsing Hua UniVersity, Beijing, People’s Republic of China, and Institute of Physics, Chinese Academy of Sciences, Beijing 10080, People’s Republic of China Received November 8, 2005; Revised Manuscript Received March 23, 2006
ABSTRACT Spin-dependent electron transport in individual magnetite (Fe3O4) nanowires contacted with normal metallic electrodes was investigated. Such a configured device demonstrated a spin-filter effect, that is, only the minority spin carriers can transport through the magnetite nanowire due to its negative spin polarization. An anomalous positive magnetoresistance ∼7.5% is observed at room temperature. Moreover, the magnetoresistance can be controlled via bias voltage.
One-dimensional nanowires have demonstrated their potential as building blocks for nanoelectronics.1 Room-temperature ferromagnets also have attracted considerable attention for their potential application in spintronics.2 Therefore, magnetic nanowire is the ideal candidate for the intersectional applications in nanoelectronics and spintronics, which may be used as nanoscale ferromagnetic building blocks. Magnetite (Fe3O4) nanowire is promising for novel spintronic devices utilizing the spin degree of freedom of the electron, because magnetite is predicted to be a half-metallic ferromagnet with full spin polarization (only minority spin carriers at the Fermi level)3 and with Curie temperature ∼858 K. Moreover, spin transport in magnetite nanowire is a fundamental problem in mesoscopic physics, and unique properties arise from the inherent shape anisotropy and the low dimensionality. Though magnetite nanotubes4 and nanowires5 have been fabricated, synthesis of long aspect ratio and single-crystalline magnetite nanowires is still challenging. Although the transport properties of MgO/Fe3O4 core-shell nanowire have been studied,6 the physical properties of a single magnetite nanowire are still far from well understood. To the best of our knowledge, the transport properties of pure individual single-crystalline magnetite nanowires have not been reported yet. In this Letter, we report the electron transport in individual single-crystalline magnetite nanowires with diameter about 30 nm, which can act as a spin filter. * To whom correspondence may be addressed. E-mail:
[email protected]. Tel: +86/10-62759474. Fax: +86/10-62751615. † State Key Laboratory for Mesoscopic Physics, and Electron Microscopy Laboratory, School of Physics, Peking University. ‡ Department of Chemistry, Tsing Hua University. § Institute of Physics, Chinese Academy of Sciences. 10.1021/nl052199p CCC: $33.50 Published on Web 04/28/2006
© 2006 American Chemical Society
The magnetite nanowires were fabricated by a controlled hydrothermal conversion route, and details of the experimental setup were described elsewhere.7 The morphology of the magnetite nanowire was analyzed using scanning electron microscopy (SEM), which is shown in Figure 1a. The nanowires have a mean diameter of ∼30 nm and length up to a few micrometers, which makes the fabrication of electrodes on a single nanowire very easy. The microstructure and composition of the magnetite nanowires were carried out using field-emission high-resolution transmission electron microscopy (HRTEM, Tecnai F30). Figure 1b demonstrates the HRTEM image of a single magnetite nanowire, and it is clear that the nanowire is a monocrystal, as confirmed by fast Fourier transform of the HRTEM image shown in the inset. An amorphous layer is occasionally observed sheathing the nanowires; however, statistic HRTEM analysis on a large number of individual nanowires revealed that most (>90%) of the nanowires have a clear atomic surface without an amorphous sheathing layer. The composition of the nanowires is analyzed carefully via line scan along the radial direction by energy-dispersive X-ray spectroscopy (EDS) and electron energy loss spectroscopy (EELS), confirming that the whole nanowire is composed of elements Fe and O. The magnetic properties of the Fe3O4 nanowires were characterized using a superconducting quantum interference magnetometer (MPMS, Quantum Design). The temperature dependence of magnetization of the magnetite nanowires was measured by applying a magnetic field of 200 Oe at the zerofield-cooled (ZFC) condition. As shown in Figure 2a, the magnetization increases sharply with the increase of the temperature near the Verwey transition. The Verwey transi-
Figure 2. (a) Magnetization as a function of temperature with magnetic field 200 Oe under ZFC conditions. (b) Magnetization hysteresis loop at 300 K for the magnetite nanowires.
Figure 1. (a) SEM image of magnetite nanowire. (b) HRTEM image of an individual single-crystalline magnetite nanowire, with the inset being the corresponding fast Fourier transform image.
tion8 is a special character of magnetite, and its observation further confirms that our sample is Fe3O4. The Verwey transition temperature is around 119 K and marked with an arrow in Figure 2a, which is well consistent with the previous report.9 The magnetization was measured as a function of magnetic field strength at 300 K, and it is presented in Figure 2b. A hysteresis loop is observed, indicating that the magnetite nanowire is ferromagnetic at room temperature. To prepare a spin filter device using a single magnetite nanowire, the magnetite nanowires were dispersed onto a Si substrate with a 500 nm thick SiO2 layer and predefined marks for alignment. The position of the nanowires was located with reference to the marks, and the nanoelectrodes (10 nm Ti and 50 nm Au) were deposited directly onto the selected individual magnetite nanowire using a standard electron-beam lithography technique. Paramagnetic metal Ti was deposited first to have direct contact with the magnetite nanowire. Figure 3a shows a SEM image that demonstrates the deposited electrodes on top of a single magnetite nanowire. The electron transport in this system was carried out using a physical property measurement system (PPMS, 1088
Quantum Design) under two-probe configuration. Figure 3b shows the current-voltage characteristics of the magnetite nanowire device at temperatures between 80 and 300 K. The I-V curve exhibits good rectifying behavior, clearly indicating the current is restricted to flow toward one direction. Several mechanisms can be held responsible for such rectifying behavior. The most probable is the formation of a Schottky barrier due to the different work functions between the metal electrode and the magnetite nanowire. At a low bias voltage, very small current flows in the device, and the electrical properties are dominated by the barriers. As the bias voltage increases, the barrier height decreases and the electrons pass directly through the barrier without tunneling. At large bias voltage, as shown in Figure 3c the almost linear I-V curves are observed and the electron transport is mainly from the magnetite nanowire itself. In this situation, a voltage fall on the barriers tends to saturate and the differential dV/ dI reflects the nanowire resistance. From the linear fit in Figure 3c, we obtained the slope of the line, and the estimated resistivity of the magnetite nanowire at 300 K is ∼2.4 × 10-4 Ω m, close to 1.9 × 10-4 Ω m resistivity measured in bulk magnetite crystal.10 The asymmetric I-V curve may originate from many factors. Image force and fixed fastness with the electrodes may affect the barrier heights. Differences in effective contacting areas also can result in the asymmetric I-V curve even for the same barrier heights. More detailed work is needed to clarify such asymmetric I-V curves in the future. The magnetoresistance (MR) measurements were conducted with the magnetic field applied perpendicular to the Nano Lett., Vol. 6, No. 6, 2006
Figure 3. (a) SEM image of electrodes on a single magnetite nanowire. (b) The corresponding I-V characteristics at various temperatures for the device. (c) The I-V curves are almost linear at large voltage.
substrate and a constant voltage bias. The MR curves at 300, 250, and 120 K are plotted in Figure 4a, respectively. The MR is defined as MR ) [R(H) - R(0)]/R(0), where R(H) and R(0) are the resistance at an applied field and zero field, respectively. It is visible that the MR is positive and reaches 15% at 8 T and 120 K. As the magnetic field swept initially from 0 to 8 T, the initial part of the MR first decreases slightly, which is usually related to grain boundary or antiphase domain boundaries in magnetite.9-11 The MR increases then after reaching a minimum at about 1 T at 120 K, which is close to the saturated magnetization field Hs in the M-H measurement. When the magnetic field falls back from 8 T to zero, the MR monotonically decreases and reaches to a smaller value than the starting point at 0 T. The observed hysteretic property indicates that the MR is related to the magnetization process of the magnetite nanowire. After the initial sweep of the magnetic field, the MR data become Nano Lett., Vol. 6, No. 6, 2006
well reproducible as the field is swept back and forward. For the MR repeated part, it gradually increases as the applied magnetic field increases and there is no a saturation signal. It is very important to note that a large anomalous positive MR is observed in our samples. Usually a negative MR effect is observed in polycrystalline Fe3O4 and compacted powder, and epitaxial thin films, which is attributed to field-induced alignment of the magnetization of adjacent grains or antiphase domains.9-11 The observed positive MR is very different from the usual negative MR in magnetite. We attribute such anomalous positive MR to the nanocontact barrier and the high spin polarization in the magnetite nanowire device. Our system can be treated as a ferromagnetic nanowire sandwiched between two normal metal electrodes with contact barrier. The configuration of the device is depicted in Figure 4b. Magnetite has a cubic inverse spinel structure, whose chemical formula can be written as Fe3+A[Fe2+,Fe3+]BO2-4, where the Fe cations occupy interstices of a face-centered cubic closed packed frame of oxygen ions and A and B refer to the tetrahedral sites and octahedral sites, respectively. As shown in Figure 4c, its conduction stems from the hopping of minority spin electrons between Fe2+ and Fe3+ ions in the B sites; that is, the conducting carrier has a spin orientation opposite to the magnetization of the Fe3O4 nanowire. The magnetite is negative spin polarization,12 and only the spin-down electrons can transport through the magnetite nanowire. Hence, the magnetite nanowire acts here as an efficient spin filter on a traversing current; that is, the transverse component of spin angular momentum which is filtered out of the current must be absorbed by the magnetite nanowire. With the helpful depiction in Figure 4b, we try to understand the origin of the observed positive MR. Considering the resistors in series, the total resistance can be written as Rtotal ) RLC + RRC + Rnw, where RLC, RRC, and Rnw are the resistances of the left contact, the right contact ,and the nanowire, respectively. For the left contact, the spin-up electrons in the contact interface “see” a high potential barrier for the high polarization of spin-down electrons in the magnetite and experience an intensive scattering in the interface.13 Assuming nonpolarization in the left contact area, which is reasonable for the normal metal electrode and randomly oriented spin supplied through the current flowing, ∼50% of the carriers (spin-up electrons) can hardly pass through the magnetite nanowire, which makes the left contact resistance increase, in good agreement with the experimental data. The intensive spin-dependent scattering in the left contact interface results in the magnetite nanowire acting as a spin filter on a traversing current; that is, only the spindown electrons can transport through the magnetite nanowire. This explanation is consistent with that by Orozco et al.,13 who reported an almost linear increasing positive MR in TiN/ Fe3O4 superlattice with the magnetic field up to 8.5 T. Generalizing the above analyses, the positive MR is related to the properties of the contact, including the contact resistances, the different scattering probabilities of two type spins at the left contact interface that depends on the two 1089
Figure 4. (a) Magnetoresistance curves with temperature 300, 250, and 120 K, respectively. (b) The schematic picture of the device. (c) The ferromagnetic order of magnetite, the minority is the transport carrier. (d) Bias dependence of MR for the magnetite nanowire based device.
spin subbands different mismatch between the metal electrode and the magnetite. Additionally, the MR decreases with the increase of temperature, because the thermal fluctuations (phonons) can flip the spin and destroy the spin polarization of the magnetite nanowire. The high spin polarization and the reduced dimensionality in the magnetite nanowire allow us to perform sensitive dc bias voltage dependent MR measurements. The data were extracted from [R(V, 8 T) - R(V, 0 T)]/R(V, 0 T). Figure 4d shows that the variations of MR with the bias voltage at temperatures 300, 250, and 180 K, respectively. The MR decreases first and then increases with increasing voltage. To explain the bias voltage dependent MR, we focus on the variations of the spin torque resistance and the barrier resistance with the bias. As a spin polarized current transfers through the magnetite nanowire, the transverse component of the spin angular momentum is absorbed by the nanowire and a spin torque produced.14 In our system, the resistance associated with the spin torque also could be regarded as the strong spin-dependent scattering at the left contact interface as mentioned above, for more general, we defined it as Rtorque. Thus, the total resistance of the system is Rtotal 1090
) Rtorque + Rcontact + Rnw, where the Rcontact is the resistance of the contact barrier. The MR can be written as MR )
R(V, 8 T) - R(V, 0 T) ) R(V, 0 T) Rtorque(V, 8 T) + Rcontact(V, 8 T) + Rnw(8 T) Rcontact(V, 0 T) + Rnw(0 T)
- 1 (1)
At low bias voltage, the resistance is dominated by the contact barrier for the electron tunneling. According to the calculations of Zhang at al.,15 as the bias voltage increases, the total resistance decreases greatly; however, Rcontact(V, 8 T) decreases faster than Rcontact(V, 0 T), which results the MR decreasing with increasing voltage. At large voltage, the total resistance is dominated by the magnetite nanowire itself for the contact barrier decreasing and the ratio of Rcontact(8 T)/Rcontact(0 T) approaches saturation. As the voltage increases (that is, the current increases), more electrons pass through the nanowire and experience a spin torque. As a result, the Rtorque increases with the voltage increase. Hence, MR increases with the increase of voltage at large bias according to eq 1. Comparing Figure 4d with Figure 3b, one finds that the minimum MR value occurs at the voltage point Nano Lett., Vol. 6, No. 6, 2006
where current starts to increase abruptly in the I-V curve, which is consistent with our analysis.
beam along with its MR results. This material is available free of charge via the Internet at http://pubs.acs.org.
In conclusion, spin-dependent electron transport in an individual magnetite nanowire was investigated in detail. An anomalous large positive MR is observed even at room temperature. This suggests that magnetite nanowire can behave as a spin filter device for maintaining the high spin polarization, where it selects just one type of spin polarization passing through the interior of the magnetite nanowire. A voltage bias dependence of MR is demonstrated, which supports a voltage-controlled spin filter.
References
Acknowledgment. This project is financially supported by the National Natural Science Foundation of China (Grant Nos. 50025206, 50472024, 20151002), and national 973 projects (No. 2002CB613505, MOST). D. P. Yu is supported by the Cheung Kong Scholar Program, Ministry of Education, P. R. China, and the project from Engineering Research Institute, Peking University. Supporting Information Available: The detailed HRTEM, EDX, and EELS of the magnetite nanowire, the M-H curve at 120 K, and the device processed by a focused ion
Nano Lett., Vol. 6, No. 6, 2006
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