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Keywords:

  • zinc ferrite;
  • pulsed laser deposition;
  • ferrimagnetic materials;
  • magnetic hysteresis;
  • electrical conduction;
  • anomalous Hall effect

Abstract

  1. Top of page
  2. Abstract
  3. Acknowledgements
  4. References
Thumbnail image of graphical abstract

The electrical conductivity of zinc ferrite (ZFO) thin films can be tuned over 7 orders of magnitude. The conductivity is thermally induced with two activation energies around 40–55 meV and 70–130 meV. Doping of the ZFO films with 1 at% Si or Ge further increases the conductivity. The magnetic hysteresis shows coercive fields of about 0.01 T and high saturation magnetization up to 300 emu/cm3 at 300 K. The electronic Hall mobility and carrier concentration at high magnetic fields are about 0.07 cm2/V s and 1.2 × 1020 cm–3 at room temperature, respectively, which indicates a hopping dominated conductivity. The out-of-plane orientation of the ZFO films fabricated by pulsed laser deposition is (001) on SrTiO3(001) and (111) on SrTiO3(111).

The resistivity of ZnFe2O4 thin films on SrTiO3 is tunable over 7 orders of magnitude by the PLD growth parameters and doping with group IV elements.

(© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Within the current search for spintronic materials with spin polarized electronic conductivity, the spinel-type ferrite materials gain more and more interest due to their comparably high intrinsic magnetic moments that are based on strong antiferromagnetic superexchange interaction between A and B ions. In case of normal spinels as for example ZnFe2O4 (ZFO), additional disorder on the tetrahedral (A–Zn2+) and octahedral (B–Fe3+) lattice sites leads to effective ferrimagnetism.

Suzuki 1, 2 further pointed out that ferrite thin films show strong magnetic coupling, high resistivity, and low-loss characteristics at high frequencies, making them suitable for flux guides and sensors in thin film recording heads. ZFO thin films deposited on fused quartz show a grain size dependent spontaneous magnetization 3. Thin Fe3–xZnx O4 films with 0 ≤ x ≤ 0.9 show spin polarized carriers at 300 K, as confirmed by photoemission spectroscopy 4. Semitransparent and conducting ZFO films with Curie temperature above 600 K were demonstrated on MgAl2O4 substrates 5. A photomagnetic effect up to 8.1% due to HeNe illumination was observed in (MnZnFe)3O4 films 6. These properties indicate the potential of Zn-ferrite films for room-temperature spintronic applications 7, 8.

In this Letter, we present the high tuneability of conductivity of ZFO thin films on SrTiO3 (STO) substrates. Further increase of conductivity was achieved for the first time with Si and Ge doping of ZFO, making it a promising magnetic semiconducting material for use in field effect transistors or magnetic tunnel junctions. Undoped ZFO films and ones doped with 1 at% Si or Ge were grown by pulsed laser deposition (PLD).

Details on our PLD method can be found in 9. Stoichiometric PLD targets were prepared from 5N ZnO and Fe2O3 powders together with the dopant oxide. ZFO films were grown on STO(100) and (111) substrates with oxygen partial pressures p (O2) from 5 × 10–5 mbar to 0.016 mbar. The substrate temperature was controlled from room temperature to about 700 °C. The chemical composition of the ZFO films was determined by energy-dispersive X-ray spectrometry. At the highest growth temperature above 600 °C and lowest oxygen partial pressure below 10–4 mbar the Zn/Fe ratio decreases from the stoichiometric value around 0.5 due to the lower Zn sticking coefficient. Typical film thickness is around 200 nm, and lateral grain size around 150 nm. Conductivity and Hall-effect was measured in a square Van der Pauw geometry.

Figure 1 shows the wide-angle X-ray diffraction (XRD) patterns of ZFO thin films grown on STO(001) and STO(111). Although the lattice mismatch of ZFO to STO is larger compared to MgO or MgAl2O4 substrates 5, the XRD film peaks confirm a single ZFO phase (except ZFO(111) at 0.016 mbar) and a preferential out-of-plane film orientation in (001) or (111) direction. As the inset in Fig. 1(a) shows, the out-of-plane lattice constants decrease from the ZFO bulk value of 8.44 Å to about 8.33 Å for lowest oxygen partial pressure. In addition, with decreasing substrate temperature the out-of-plane lattice constant increases up to 8.62 Å. XRD φ -scans of the (202) reflections (not shown here) demonstrate the perfect in-plane alignment of ZFO and STO without rotation domains.

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Figure 1. XRD 2Θ–ω patterns (Cu Kα) of ZFO thin films grown at about 700 °C on STO(001) (a) and STO(111) (b). The insets show the oxygen partial pressure dependence of the ZFO(008) and (444) peak posi- tions and widths. The films show a dominating (001) (a) or (111) (b) out-of-plane orientation.

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Figure 2 shows a typical SQUID hysteresis loop of a ZFO film grown at low oxygen partial pressure, clearly revealing the ferrimagnetic behaviour. The magnetization is mainly in-plane and a decreasing coercive field for decreasing substrate temperature was observed. The saturation magnetization decreases with increasing oxygen partial pressure (Fig. 2). For p (O2) < 10–3 mbar typical values of saturation magnetization and coercive field at 300 K are 100 to 300 emu/cm3 and 0.01 to 0.02 T, respectively. At 10 K, the coercive field increases up to 0.05 T, as Fig. 2 shows. A correlation between saturation magnetization and conductivity was not found in contrast to Ref. 8. Ferrimagnetic order is certainly due to A–B-site disorder with Fe3+ ions occupying A-sites, especially at low oxygen partial pressures and high substrate temperatures, when the Zn/Fe ratio falls below its stoichiometric value 3–5. Further, Fe A-site interstitials and oxygen vacancies might play a role 10.

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Figure 2. Typical SQUID hysteresis loop of a ZFO film on STO(001) (650 °C, 2 ×10–3 mbar). The inset shows the field cooled M (T) curves of another film grown at 5 × 10–5 mbar with higher magnetization. The Curie temperature of this ZFO sample is expected to be well above 800 K. Magnetic field was applied parallel to the film. The diamagnetic slope was determined at 300 K and 1 T and subtracted from the data.

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The conductivity at 300 K can be controlled by the PLD growth conditions over 7 orders of magnitude (see abstract figure), giving the possibility to grow semiconducting ZFO. The substrate temperature has the dominating effect, samples grown at high temperatures are always highly insulating. With decreasing substrate temperature and oxygen partial pressure the conductivity increases. Doping of the ZFO with Si or Ge further reduces the resistivity.

Figure 3 shows the Arrhenius plots of the electronic conductivity of 5 different ZFO samples which give clear evidence of the thermal activation of conductivity. Only few publications have addressed the thermal activation of the conductivity in ZFO yielding the activation energy EA. ZFO films grown on Al2O3(0001) and MgAl2O4 gave EA values of 67 to 130 meV 7 and 90 to 130 meV 5, respectively. In both studies the accessible conductivities span a range of 3 orders of magnitude, namely the highest 3 orders measured in this work. It is obvious from the data presented here, that a single activation energy describes the conductivity sufficiently in this region. However, in the present study the measured range of conductivities could be enhanced by 2 orders of magnitude. We found that a model with two activation energies according to σ = σ01 exp (–EA1/kBT) + σ02 exp (–EA2/kBT) is required to model the complete dataset. Table 1 presents the numerical results of the fits plotted in Fig. 3.

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Figure 3. Electrical conductivity σ of various ZFO films on STO. The ZFO films were grown at about 500 °C, and the doped ZFO films at 6 × 10–5 mbar. The solid lines are model fits with two activation energies, see Table 1.

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Table 1.  Thermal activation of electronic conductivity of undoped and doped ZFO thin films on STO, grown at low temperature and various oxygen partial pressures, see Fig. 3.
  1. * The samples refer to the graphs in Fig. 3.

sample*EA1 (meV) σ01 (Sm–1)EA2 (meV) σ02 (Sm–1)
ZFO 2 × 10–3 mbar55 1130 180

ZFO 3 × 10–4 mbar

ZFO 6 × 10–5 mbar

ZFO:Ge

ZFO:Si

40 22

49 9

52 78

48 290

71 200

85 200

88 660

120 1450

The transport mechanism in ZFO is not well understood. Since the ferrites in general are strongly correlated systems, charge transport occurs by a hopping mechanism mediated by charge fluctuations preferentially on the octahedral sites 11. Vacancies might act as traps for charge carriers 11 and possibly the activation energies observed might be attributed to two different vacancy configurations.

Figure 4 shows a typical Hall-effect curve; both anomalous and normal Hall constant are negative, i.e. electron conduction prevails. From the high field part of the Hall resistivity curve, n and µH are derived from the slope, assuming that the magnetization has completely saturated. The extracted carrier density is n = 1.2 × 1020 cm–3, and the Hall mobility μH = 0.07 cm2/V s. Giving the low mobility and high carrier concentration, electron hopping between Fe2+ and Fe3+ions on octahedral B sites seems the most likely transport mechanism. The formation of Fe2+ is clearly promoted by the low oxygen pressure during PLD growth 9. In addition, a high defect density, i.e. grain boundaries 7, influences the magnetic properties 10. One of the very rare high-field reference Hall data are n = 1.6 × 1021 cm–3 and µH = 0.15 cm2/V s for Fe2.5Zn0.5O44.

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Figure 4. Hall resistivity versus magnetic field of a ZFO:Ge film grown on STO(001) at 400 °C and 6 × 10–5 mbar.

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By demonstrating the highly tuneable conductivity of undoped and Si or Ge doped zinc ferrite thin films on SrTiO3 substrates we pave the way for application of ZFO films both as magnetic semiconducting electrode in advanced all-oxide tunnel junctions and field effect transistors, but also as insulating barrier in spin filters.

Acknowledgements

  1. Top of page
  2. Abstract
  3. Acknowledgements
  4. References

We are indebted to Gabriele Ramm for PLD target synthesis and to Annette Setzer for SQUID measurements. We thank the Deutsche Forschungsgemeinschaft for support within SFB 762 “Functionality of oxide interfaces”. Kerstin Brachwitz is also supported by Leipzig School of Natural Sciences BuildMoNa (GS 185).

References

  1. Top of page
  2. Abstract
  3. Acknowledgements
  4. References