Enhancement of Spin Polarization in a Transition Metal Oxide

Dec 20, 2010 - The highly integrated oxide nanodot diodes were constructed using nanoimprint lithography based on a Mo lift- off method in combination...
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Enhancement of Spin Polarization in a Transition Metal Oxide Ferromagnetic Nanodot Diode Satoru Yamanaka, Teruo Kanki, Tomoji Kawai, and Hidekazu Tanaka* The Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan ABSTRACT: Enhancement of spin polarization was observed in a transition metal oxide (Fe,Zn)3O4/Nb-SrTiO3 ferromagnetic nanodot Schottky diode. The highly integrated oxide nanodot diodes were constructed using nanoimprint lithography based on a Mo liftoff method in combination with a pulsed laser deposition technique. The junction magnetoresistance of diodes increased as diode size increased. The spin polarization estimated from the thermionic emission model is enhanced from P = 0.74 in a conventional film to P = 0.89 in a nanodot diode whose size is 300  300 nm2. The nanofabrication technique used here will enable us to construct superior transition metal oxide spintronic nanomaterial and nanodevices. KEYWORDS: Nanodot diode, nanoimprinting, ferromagnetic metal oxide, strongly correlated electron system, spin polarization

substrate.10-12 When a magnetic field is applied to a Fe3O4/ Nb-STO ferromagnetic diode, the exchange energy Δex increases to (Δex þ 2 μBB) in the ferromagnetic layer by a Zeeman splitting effect (μB, Bohr magnetron; B, magnetic field), and the majority (down) spin increases. As a result, the current increases under a magnetic field compared with the current under zero magnetic fields, as shown in Figure 1a. The (Fe3-xZnx)O4 material used in this research is a new class of tunable high TC ferromagnetic semiconductors2 with strongly correlated electrons.13 From the point of view of nanoscale functionalities, (Fe3-xZnx)O4 is also considered one of the transition metal oxide materials in which inhomogeneous ferromagnetic and antiferro/(para)magnetic domains coexist because it shows cluster glass behavior and its saturation magnetization decreases with increasing nonmagnetic Zn2þ concentration.14 Therefore, we have an opportunity to create an unusual behavior against a magnetic or electric field in nanosized spintronic devices using (Fe,Zn)3O4 in comparison with conventional thin film devices. For fabrication of the TMO nanodot diodes, we used the Mo lift-off nanoimprint lithography (NIL) technique in combination with the pulsed laser deposition (PLD) method, which allows us to construct the integrated epitaxial TMO nanoheterostructures we originally developed.15 NIL is a novel and low-cost nanolithography method for polymers and can be applied down to sub-10-nm regions and over large areas.16 An amorphous Mo nanomask fabricated via the NIL process is stable at high temperatures over 300 °C. Therefore, a Mo nanomask is used as the template for the lift-off process instead of a polymer resist, and high-quality epitaxial TMO films are deposited at high temperature onto it using PLD.17-19

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ransition metal oxides (TMOs) with strongly correlated electron systems attract a great deal of interest as one of the best candidates for novel spintronic devices1 due to their high TC ferromagnetism,2,3 perfect spin polarization,4 and metal-insulator transition against temperature or magnetic field.5 Recently, investigations concerning their nanoscale characteristics have received a great deal of attention in terms of both basic research and their potential use as advanced nanodevices. For example, perovskite (La,Pr,Ca)MnO 3 possesses nanoscale electronic phase separation between the ferromagnetic metal and charge ordering insulator at low temperatures, as observed by the scanning probe microscope6 and transmission electron microscope,7 and VO2 with a metal-insulator transition with increase of resistivity by the orders-of-magnitude at 340 K also exhibits this phenomenon between the metallic and insulating phases even at room temperature as observed by scanning near-field infrared microscopy.8 Interestingly, a field effect transistor (FET) device using (La,Ca)MnO3 film exhibited an anomalous colossal electric resistance change,9 and it is proposed that this result is caused by the inhomogeneous coexistence of the nanoscale phase separate domains. Therefore, much smaller nanosize heterostructured devices, such as nanoFET and nanodiode using TMOs, would be much more interesting as advanced electric/spintronic nanodevices with colossal response against an external field, but there are no reports in the area of epitaxial transition metal oxides due to the difficulty of their nanofabrication. In this paper, we report fabrication of a ferromagnetic (Fe,Zn)3O4/Nb doped SrTiO3 nanodot diode and its enhanced functionality at the nanoscale via Schottky junction magnetoresistance (MR). Spin injection via Schottky contacts (Schottky diode) is one of the most important techniques for spintronic device applications. In TMOs, this has been reported in heterostructures using metallic Fe3O4 or (Fe,Mn)3O4 degenerated semiconductor thin films grown on n-type semiconducting Nb doped SrTiO3 (Nb-STO) r 2010 American Chemical Society

Received: July 24, 2010 Revised: November 22, 2010 Published: December 20, 2010 343

dx.doi.org/10.1021/nl102601m | Nano Lett. 2011, 11, 343–347

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After SiO2 deposition, amorphous Mo (thickness, 30 nm) was deposited using magnetron sputtering (SC-701HMC Quick Coater, Sanyu Electron Co. Ltd.) in Ar gas at RT. The UV curable resist/lift-off resist and Mo/SiO2 deposited on them were removed with the lift-off process using acetone at 40 °C for an hour by ultrasonic treatment, and preparation of the Mo/SiO2 nanomask was completed.20 FZO thin films (thickness, 30 nm) were deposited on it using PLD (arc excimer: λ = 193 nm). The substrate temperature and O2 pressure were set to 320 °C and 1.0  10-6 mbar, respectively. The only Mo was dissolved in H2O2 after the PLD process. Finally, a Pt electrode was deposited on the FZO nanodot diode surrounded by an SiO2 insulator and an indium electrode was put on the Nb-STO substrate using rf sputtering. To make sure the nature of our nanostructures, we have checked by using the XRD 2θ-θ profile and the pole figure of the dot array samples of similar magnetic semiconductor oxide system of spinel (Fe,Mn)3O4 (FMO) using same nanoimprint and Mo lift-off process as for crystallography. The result indicated that excellent single crystalline and epitaxial nanostructures could be fabricated using this process as shown in ref 15. Furthermore, to make sure of electronic structure after the process of Mo lift-off, we have also conducted hard X-ray photoemission spectroscopy (HX-PES). As shown in ref 21, the HX-PES spectra of Fe:2p core levels and valence bands show that the ratio of Fe2þ and Fe3þ in each spectrum does not change in unprocessed and processed FMO films, indicating that Mo nanomask lift off does not give a significant change in the oxidation state. Moreover both valence band spectra also did not change. Therefore, we consider that the diffraction data and HX-PES data have proved that the Mo lift-off process itself does not give serious damage on the ferromagnetic and semiconductive spinel oxide nanodot array structure in their crystallography and electronic structure. Under these confirmations, I-V characteristics of the FZO/ Nb-STO nanodot diode were measured using a physical property measurement system (PPMS model 6000; Quantum Design Co. Ltd.) at several temperatures (50-300 K) and magnetic fields (0-8 T). Panels a-c of Figure 2 show I-V characteristics of the FZO (film, 1  1 μm2, 300  300 nm2)/Nb-STO ferromagnetic nanodot diode in the temperature range 50-300 K. It shows good rectifying properties and its asymmetric I-V characteristics survive over 300 K. According to the developed thermionic emission theory for ferromagnetic metal-semiconductor contacts by introducing the electron spin-split conduction band model, as proposed for Fe3O4/Nb-STO contact by Ziese et al.,10 the current (I(B)) can be described as follows

Figure 1. (a) Schematic illustration of an energy diagram at the interface of a Schottky diode. (b) SEM images of the FZO nanodot array structure on the Nb-STO substrate (300  300 nm2). (c) Schematic illustration of the FZO/Nb-STO nanodot diode structure.

We fabricated the integrated (Fe3-xZnx)O4 with x = 0.9 (FZO denoted hereafter) nanodot structures (dot size: 1  1 μm2, 500  500 nm2, 300  300 nm2) (Figure 1b), surrounded by SiO2 insulator20 on Nb-STO (100) substrate by applying NIL (Figure 1c) and using the Mo lift-off technique.15 First, the organic resist NXR-3030 (Nanonex Corp.), which is removed easily by acetone (called the lift-off resist), was spincoated on a 20  20 mm2 Nb-STO (100) substrate and baked at 150 °C for 10 min to vaporize a solvent included with NXR-3030. A UV curable organic resist (NXR-2030, Nanonex Corp.) was spin-coated on the lift-off resist/Nb-STO, and the quartz mold (NIM-OM02, NTT-AT Nanofabrication Corp.) was pressed into the UV curable resist with exposure to UV light (365 nm). We used NM-0401 (Meisyo Kiko Corp.) for the process of NIL with UV irradiation. The quartz mold was separated from the cured resist. The residual UV curable resist and lift-off resist beneath it were removed by an anisotropic reactive ion etch (RIE) with a mixture of CF4 and O2 (RIE-10NR, SAMCO). Secondary, amorphous SiO2 (thickness, 30 nm) was deposited by rf sputtering (SVC-700LRF, Sanyu Electron Co. Ltd.) in Ar and O2 gas at room temperature (RT) onto the resist pattern.



SAT 2 exp -

eφB kB T



IðBÞ ¼      Δex þ 2πB B eV cosh exp -1 2kB T nkB T

ð1Þ where S is the contact area, A* is the Richardson constant, V is the applied bias voltage, φB is the Schottky barrier height, Δex is the exchange energy in the ferromagnetic layer of FZO, μBB is the Zeeman splitting energy, n is the ideality factor for a Schottky diode, and kB is the Boltzmann constant. The ideality n for a Schottky diode based on the thermionic emission theory was investigated by plotting log{I(B = 0)/[1- exp(-eV/kBT)]} 344

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Figure 4. Magnetic field dependence of the junction magnetoresistance (I(B)/I(0)) of the FZO (300  300 nm2)/Nb-STO ferromagnetic diode at 100 K. Inset shows the magnetic field vs {I(B)/I(0)} - 1 estimated in negative bias region from -0.3 to -0.5 V for the calculation of spin polarization (P).

Figure 2. (a-c) I-V characteristics of the FZO/Nb-STO ferromagnetic nanodot diode in the temperature range 50-300 K under a magnetic field of 0 T {dot size: (a) film, (b) 1  1 μm2, and (c) 300  300 nm2}. (d-f) Temperature dependence of the ideality factor (n) {dot size: (d) film, (e) 1  1 μm2, and (f) 300  300 nm2}.

Figure 5. Size dependence of the spin polarization of FZO in the temperature range 50-200 K. Inset shows the temperature dependence of the spin polarization of FZO (300  300 nm2) and ferromagnetic materials (Fe 3O 4 , 10,11 (La,Ca)MnO 3 (LCMO),23 (La,Sr)MnO 3 (LSMO),24 CoFe,25 CoMnSi,25 CoMnAl,26 and Co27).

junction MR defined as I(B)/I(0) at 100 K is shown in Figure 3. A spin electron transfer from FZO to Nb-STO occurs in a reverse bias voltage and I(B)/I(0) is independent of bias voltage from eq 1. In Figure 3, the junction MR increased with decreasing dot size, indicating that the response against a magnetic field was enhanced with decreasing dot size. As for the origin of this phenomenon, modification of the magnetic properties of FZO is considered. Ziese et al. showed that the junction MR in the ferromagnetic metal/nonmagnetic semiconductor diode is dependent on spin polarization (P) of the ferromagnetic metal/degenerate semiconductor layer, and its P value can be estimated by10

Figure 3. Bias voltage dependence of the junction magnetoresistance (I(B = 8 T)/I(0)) of the FZO (300  300 nm2)/Nb-STO ferromagnetic diode in reverse bias voltage at 100 K.

against V over the whole voltage range.22 Figure 2d-f shows the temperature dependence of the ideality factor (n) of the FZO/ Nb-STO diode under no magnetic field for film with a dot size of 1  1 μm2 and 300  300 nm2. Values of n increased with increasing temperature for diodes of all sizes. According to Rhoderick et al., the acceptable value of the ideality factor is n < 1.1.23 Since the value of the ideality factor over 250 K becomes higher than 1.1 for each diode size, the thermionic emission model is applicable under 200 K for I-V characteristics of FZO/Nb-STO Schottky diodes. For estimation of the response of the FZO/Nb-STO diode (film, 1  1 μm2, 300  300 nm2) against a magnetic field, the

IðBÞ μ B ¼ 1þP B IðBÞ kB T

ð2Þ

where P is the spin polarization in a zero field (P = tanh(Δex/ 2kBT)) of FZO. Equation 2 is valid for the simplified parabolic semiconductor band structure. Figure 4 shows the magnetic field dependence of the junction MR of a FZO/Nb-STO ferromagnetic nanodot diode with a size of 300  300 nm2 at 100 K. 345

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Figure 6. Schematic illustrations of spin structures for (a) continuous film, (b) virtual state of divided nanodot structure with unstable antiferromagnetic domain, and (c) divided nanodot structure with expanded ferromagnetic domain.

With an increasing magnetic field from 0 to 8 T, the junction MR increased systematically from 1.00 (0 T) to 1.08 (8 T). According to eq 2, P was estimated by plotting the junction MR in negative bias region against B as shown in the inset of Figure 4. In this calculation, a P value of 0.89 ( 0.05 for FZO was obtained in a nanodot diode with a size of 300  300 nm2 at 100 K. Since this calculation can be applied when thermionic emission is dominant in I-V characteristics, P for FZO could be estimated under 200 K. Figure 5 shows the size dependence of P in the temperature range 50-200 K for diodes with sizes of film, 1  1 μm2 and 300  300 nm2. P increased with decreasing dot size. The inset of Figure 5 indicates that the P value of the FZO nanodot estimated from a Schottky diode is higher than that of other ferromagnetic materials obtained from trilayer TMR devices.10,11,24-28 As a possible origin of the enhancement of P in the nanodot, the coexistence of inhomogeneous phases such as ferromagnetic and antiferromagnetic (or paramagnetic) phases in FZO films is considered. In this case, eq 2 is modified as     IðBÞ μB B μB B ¼ R 1 þ PF þ ð1 - RÞ 1 þ PAF ð3Þ Ið0Þ kB T kB T

A physical interpretation for this phenomenon is proposed as follows. A single antiferromagnetic spin domain (or a single ferromagnetic spin domain) could be stable when it is larger than the characteristic correlation length of λAF(λF) owing to cooperative phenomena in a strongly correlated electron system. When dot size is larger than λF þ λAF, both domains can be stable and coexist (Figure 6a) as the spatial imaging experiments for manganites revealed existence of submicrometer size electric/ spin domain structures.6,7 Once the dot size is reduced to smaller size than λ F þ λ AF , both spin domains cannot be stable simultaneously. We consider that the ferromagnetic domain is more stable than the antiferromagnetic one because the ground state of Fe3O4 as mother material is ferro(ferri)magnetic. Namely an antiferromagnetic spin domain would be unstable in the smaller dot (Figure 6b) and then easily turned to a ferromagnetic one under the magnetic field applied in the I-V measurements as shown in Figure 6c, leading to enhancement of a ferromagnetic fraction. From the analogy of phase separation in manganites, it is considered that the domain size observed in the imaging experiments would correspond to the correlation length in this interpretation, so that an enhanced spin polarized fraction was observed at a nanodot diode whose size is smaller than the correlation length. In the future, it would be also necessary to discuss this model from the points of view such as spin domain size of FZO, nucleation of domain, and boundary energy in more detail. In summary, (Fe,Zn)3O4/Nb-STO ferromagnetic nanodot diodes were constructed (dot size: 1  1 μm2, 500  500 nm2, 300  300 nm2) using the NIL method based on a Mo lift-off technique combined with PLD. The I-V characteristics of the diodes were obtained in the temperature range 50-300 K and were dominated by the Schottky thermionic emission model under 200 K. The junction MR is enhanced by a decrease of the dot size down to 300  300 nm2, indicating that spin polarization of FZO is enhanced from P = 0.74 in a conventional film to P = 0.89 in a nanodot diode with a film size of 300  300 nm2. The nanofabrication technique used here will enable us to construct nano dot-sized heterostructured transition metal oxide spintronic devices, such as a nanodot TMR device with a half metallic ferromagnetic oxide like FZO, and it will open up superior functionalities to macrosize film devices.

where PF is the spin polarization of the ferromagnetic region, PAF is that of the antiferromagnetic (or paramagnetic) region, and R is the volume fraction of the ferromagnetic region in FZO films (0 < R e 1). When PAF = 0, eq 3 is transformed in the following manner IðBÞ μ B ¼ 1 þ ðRPF Þ B ð4Þ Ið0Þ kB T From eqs 2 and 4, P is simply described as P = RPF. Therefore, the enhancement of P estimated from the junction MR suggests the possibility of enhancement of the volume fraction of the highspin polarization (PF) region in the FZO ferromagnetic oxide semiconductor nanodot. This surprisingly indicates that the ferromagnetic high-spin polarization region is more stabilized in nanosized confined space than conventional thin film. This is important for functional oxide electronics because electrical and magnetic properties observed in thin films and bulks are sometime macroscopically averaged ones, and nanostructuring correlated oxides will give a superior property in nano-oxide device application. 346

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’ AUTHOR INFORMATION

(25) Ishii, Y.; Yamada, H.; Sato, H.; Akoh, H.; Ogawa, Y.; Kawasaki, M.; Tokura, Y. Appl. Phys. Lett. 2006, 89, No. 042509. (26) Sakuraba, Y.; Hattori, M.; Oogane, M.; Ando, Y.; Kato, H.; Sakuma, A.; Miyazaki, T.; Kubota, H. Appl. Phys. Lett. 2006, 88, No. 192508. (27) Oogane, M.; Sakuraba, Y.; Nakata, J.; Kubota, H.; Ando, Y.; Sakuma, A.; Miyazaki, T. J. Phys. D 2006, 39, 834. (28) Schmalhorst, J.; Br€uckl, H.; Justus, M.; Thomas, A.; Reiss, G.; Vieth, M.; Gieres, G.; Wecker, J. J. Appl. Phys. 2001, 89, 586.

Corresponding Author

*Telephone: þ81-6-6879-4280. Fax: þ81-6-6879-4283. E-mail: [email protected].

’ ACKNOWLEDGMENT This study was supported by a Grant-in-Aid for Young Scientists (S) (No. 21676001) from the Ministry of Education and Culture (MEXT) and the Mazuda Foundation, Research Foundation for the Electrotechnology of Chubu, Mitutoyo Association for Science and Technology. ’ REFERENCES (1) Ahn, C. H.; Triscone, J.-M.; Mannhart, J. Nature 2003, 424, 1015. (2) Takaobushi, J.; Tanaka, H.; Kawai, T.; Ueda, S.; Kim, J-J; Kobata, M.; Ikenaga, E.; Yabashi, M.; Kobayashi, K.; Nishino, Y.; Miwa, D.; Tamasaku, K.; Ishikawa, T. Appl. Phys. Lett. 2006, 89, No. 242507. (3) Ishikawa, M.; Tanaka, H.; Kawai, T. Appl. Phys. Lett. 2005, 86, No. 222504. (4) Park, J.-H.; Vescovo, E.; Kim, H.-J.; Kwon, C.; Ramesh, R.; Venkatesan, T. Nature 1998, 392, 794–796. (5) Urushibara, A.; Moritomo, Y.; Arima, T.; Asamitsu, A.; Kido, G.; Tokura, Y. Phys. Rev. B 1995, 51, 14103. (6) F€ath, M.; Freisem, S.; Menovsky, A. A.; Tomioka, Y.; Aarts, J.; Mydosh, J. A. Science 1999, 285, 1540. (7) Uehara, M.; Mori, S.; Chen, C. H.; Cheong, S.-W. Nature 1999, 399, 560–563. (8) Qazilbash, M. M.; Brehm, M.; Chae, B.-G.; Ho, P.-C.; Andreev, G. O.; Kim, B.-J.; Yun, S. J.; Balatsky, A. V.; Maple, M. B.; Keilmann, F.; Kim, H.-T.; Basov, D. N. Science 2007, 318, 1750–1753. (9) Wu, T.; Ogale, S. B.; Garrison, J. E.; Nagaraj, B.; Biswas, A.; Chen, Z.; Greene, R. L.; Ramesh, R.; Venkatesan, T.; Millis, A. J. Phys. Rev. Lett. 2001, 86, 26. (10) Ziese, M.; Kohler, U.; Bollero, A.; Hohne, R.; Esquinazi, P. Phys. Rev. B 2005, 71, No. 180406. (11) Kundaliya, D. C.; Ogale, S. B.; Fu, L. F.; Welz, S. J.; Higgins, J. S.; Langham, G.; Dhar, S.; Browning, N. D.; Venkatesan, T. J. Appl. Phys. 2006, 99, No. 08K304. (12) Satoh, I.; Takaobushi, J.; Tanaka, H.; Kawai, T. Solid State Commun. 2008, 147, 397. (13) Takaobushi, J.; Ishikawa, M.; Satoh, I.; Tanaka, H.; Kawai, T.; Ueda, S.; Kobayashi, K.; Kim, J.-J.; Kobata, M.; Ikenaga, E.; Yabashi, M.; Nishino, Y.; Miwa, D.; Tamasaku, K.; Ishikawa, T.; Takeda, Y.; Saito, Y. Phys. Rev. B 2007, 76, No. 205108. (14) Chikazumi, S. The physics of ferromagnetism; Syoukabo: Tokyo, 1978. (15) Suzuki, N.; Tanaka, H.; Yamanaka, S.; Kanai, M.; Lee, B. K.; Lee, H. Y.; Kawai, T. Small 2008, 4, 1661–1665. (16) Chou, S. Y.; Keimel, C.; Gu, J. Nature 2002, 417, 835–837. (17) Suzuki, N.; Tanaka, H.; Kawai, T. Adv. Mater. 2008, 20, 909–913. (18) Goto, K.; Tanaka, H.; Kawai, T. Nano Lett. 2009, 9, 1962–1966. (19) Goto, K.; Kanki, T.; Kawai, T.; Tanaka, H. Nano Lett. 2010, 10, 2772–2776. (20) Suzuki, N.; Tanaka, H.; Kawai, T. Jpn. J. Appl. Phys. 2009, 48, No. 116511. (21) Goto, K.; Tanaka, H.; Kawai., T. J. Appl. Phys. 2009, 105, No. 064301. (22) Missous, M.; Rhoderick, E. H.; Singer, K. E. J. Appl. Phys. 1986, 60, 7. (23) Rhoderick, E. H.; Williams, R. H. Metal Semiconductor Contacts, 2nd ed.; Oxford University Press: Oxford, 1988; p 3. (24) Jo, M. H.; Mathur, N. D.; Todd, N. K.; Blamire, M. G. Phys. Rev. B 2000, 61, R14905. 347

dx.doi.org/10.1021/nl102601m |Nano Lett. 2011, 11, 343–347