Amplification of magnetoresistance and Hall effect of Fe 3 O 4 – SiO 2 – Si structure

In this paper, we report the magnetoresistance and the Hall effect in the Fe(3)O(4)-SiO(2)-Si structure. Single phase magnetite films were deposited on n-type silicon substrates using laser molecular beam epitaxy. When the temperature is increased beyond 230 K, the resistance drops rapidly because the conduction path starts to switch from the Fe(3)O(4) film to the inversion layer underneath the native SiO(2) via thermally assisted tunneling. A large negative magnetoresistance is observed at about 230 K, and this maximum shifts to higher temperature with increasing film thickness. Hall effect data of the structure show that the carriers are holes above the channel switching temperature. Our results confirm that the large magnetoresistance at similar to 230 K originates from the amplification of the magnetoresistance of the magnetite in the Fe(3)O(4)-SiO(2)-Si structure. (c) 2009 American Institute of Physics. [DOI: 10.1063/1.3065987]


I. INTRODUCTION
Half-metallic oxide Fe 3 O 4 films have long been investigated due to its speculated high spin polarization and high Curie temperature of 840 K, which is desirable for spin electronic device applications. 1One of the main device concepts involving Fe 3 O 4 is the magnetic tunnel junction ͑MTJ͒.Magnetic read heads based on MTJs have been on the market for several years.However, the MTJ uses transition-metal electrodes that have typical spin polarization of 40%.If 100% spin polarization of the tunneling electrons can be realized as expected, MTJs that show higher magnetoresistance ͑MR͒ and thus higher signal will find application in magnetic random access memory and magnetic read heads.However, Fe 3 O 4 based MTJs have so far shown small tunneling MR. [2][3][4][5] Recently, we observed the amplification of MR of magnetite in an Fe 3 O 4 -SiO 2 -Si structure with current-controlled channel switching of electron transport between the Fe 3 O 4 film and the inversion layer underneath the SiO 2 via thermally assisted tunneling. 6][9][10][11][12] The inversion layer is the basis for metal-oxide-semiconductor field-effect transistor ͑MOSFET͒, which provides a low resistive path for carrier transport along the surface of the Si substrate. 13The amplification of MR in this Fe 3 O 4 -SiO 2 -Si MOSFET structure may have useful applications.Clearer experimental evidence and understanding of the mechanism of the amplification of the MR are, however, still lacking.
In this paper, we report the MR and the Hall effect in the Fe 3 O 4 -SiO 2 -Si structure.A large negative MR is observed at about 230 K. Hall effect data of the structure confirm that the carriers are holes above the channel switching temperature, and suggest the large MR originates from the amplification of the MR of the magnetite in the Fe 3 O 4 -SiO 2 -Si structure.

II. EXPERIMENT
The Fe 3 O 4 thin films were grown by laser molecular beam epitaxy on n-type Si ͑100͒ substrates that have a 1.5 nm thick native SiO 2 layer on the surface.The target was ␣-Fe 2 O 3 that was prepared by pressing ␣ -Fe 2 O 3 powders.The films were prepared in a vacuum of 10 −7 Torr at substrate temperature of 350 °C. 14The pulsed excimer laser uses KrF ͑ = 248 nm͒ and produces a laser beam with an intensity of 1 -2 J / cm 2 and a repetition rate of 3 Hz.The deposition rate is about 1 nm/s and the film thickness is 35 nm.After deposition, the film and substrate were annealed for 30 min under the same condition.The resistivity and Hall effect were measured using a physical properties measurement system from Quantum Design.

III. RESULTS AND DISCUSSION
Figure 1 shows the temperature dependence of zero-field resistance of the Fe 3 O 4 -SiO 2 -Si structure measured in the plane of the Fe 3 O 4 film.Below 200 K, the resistance in-creases with decreasing temperature, which shows a typical behavior of thin Fe 3 O 4 film.However, the resistance drops sharply at about 230 K and becomes metallic at higher temperatures.Similar temperature dependence of resistance have been observed in an Fe 3 C -SiO 2 -Si structure and explained in terms of the current channel switching between the upper Fe 3 C film and the inversion layer due to the thermally assisted tunneling by Tang et al. 7 In Fe 3 O 4 -SiO 2 -Si, the carriers are electrons in the Fe 3 O 4 film and should be holes in the inversion layer on n-type Si substrate, thus the Hall effect is an effective tool and used to confirm the conducting channel switching effect.Figure 2 shows the Hall voltage for the Fe 3 O 4 -SiO 2 -Si structure at 350 K.It is well known that the carries of pure Fe 3 O 4 film are electrons with extraordinary Hall 15 and the Curie temperature of Fe 3 O 4 film is about 840 K. Thus, the extraordinary Hall effect for pure Fe 3 O 4 film is expected at 350 K.However, the measured result of Hall data is an ordinary Hall effect and the carriers are holes at 350 K.9][10][11][12] The channel switching in the Fe 3 O 4 -SiO 2 -Si structure exhibits bias voltage dependence that can be explained on the basis of a MOSFET.Here the ferromagnetic Fe 3 O 4 film is the gate. 6The carrier transport of the Fe 3 O 4 -SiO 2 -Si structure is strongly influenced by the voltage on the Fe 3 O 4 film.The inset in Fig. 1 shows the temperature dependence of the resistance at different measuring currents.Clearly the switching temperature increases with increasing measuring currents.This current-dependence switching temperature is due to the following.Higher current results in higher voltage on the Fe 3 O 4 film, which can attract electrons from the n-type Si substrate and reduce the hole conductance in the inversion layer.As a result, the carriers emitted cross the SiO 2 layer by thermal excitation will accumulate at the interface region of SiO 2 / Si.This charge accumulation can raises the effective barrier height and prevent the transfer of the carriers from the film to the inversion layer.Therefore, the current in the inversion layer reverts to the film at higher voltage and the current switching temperature is increased.This current-dependent switching temperature, combined with the Hall data, suggests that the inversion layer plays an important role in the transport in the Fe 3 O 4 -SiO 2 -Si structure.
A large low field negative MR was found in Fe 3 O 4 -SiO 2 -Si at the channel switching temperature, as shown in Fig. 3, where the resistance was measured in the plane of the Fe 3 O 4 film.The MR is defined as MR= ͓R ͑H͒ − R ͑0͒ ͔ / R ͑0͒ , where R ͑0͒ and R ͑H͒ are the measured resistance of the sample in zero field and external field H, respectively.The magnetic field was applied perpendicular to both the film and the current.At low temperature, the current only flows in the Fe 3 O 4 film and a small negative MR is observed.The MR value is much lower than the reported data on pressed Fe 3 O 4 powders and polycrystalline films ͑the maximum MR of Fe 3 O 4 powders is about 22.8% at 280 K and 14 T͒ but it is similar to the data of typical of thin Fe 3 O 4 films. 14,16Near the switching temperature of 230 K, the Fe 3 O 4 -SiO 2 -Si structure shows a greatly magnified low field negative MR, as shown in Fig. 3͑a͒.When a magnetic field is applied, the measured resistance of the film drops, which leads to a reduced voltage on the film and opens up the inversion layer, and a much enhanced low-field negative MR of Ϫ1.91% at 0.05 T is observed.The enhanced lowfield negative MR is the amplification of the negative MR of the Fe 3 O 4 film in this Fe 3 O 4 -SiO 2 -Si structure, similar to the amplification effect of a MOSFET. 6As discussed above, when a magnetic field is applied to the film, the resistance of the Fe 3 O 4 film is reduced, which means the voltage on the Fe 3 O 4 film ͑gate voltage͒ decreases.The conductance of the holes in the inversion layer increases accordingly.Thus the measured resistance is greatly reduced.
Figure 3͑b͒ shows the temperature dependence of MR data of Fe 3 O 4 -SiO 2 -Si with different Fe 3 O 4 film thicknesses measured at 0.05 T. The maximum MR value shifts to higher temperature with increasing film thickness.This is consistent with the change in the channel switching temperature of the Fe 3 O 4 -SiO 2 -Si structure for the 75 and 35 nm thick Fe 3 O 4 films, 280 and 230 K, respectively. 6The amplification effect is the strongest near the switching tempera- ture.At lower temperatures, the current runs mainly in the film, while it runs mainly in the inversion layer above the switching temperature.Therefore the amplification of the MR of the film is not as obvious.The value of the MR enhancement seems to increase with decreasing Fe 3 O 4 film thickness, which suggests that reducing the film thickness may enhance the amplification effect.Thinner films have higher resistance, so the carriers are emitted into the Si inversion layer at lower temperature, which corresponds to a lower channel switching temperature.

IV. CONCLUSIONS
In summary, we observed the current channel switching and greatly amplified low-field negative MR of Fe

FIG. 1 .
FIG. 1. ͑Color online͒ Temperature dependence of the zero field resistance of the Fe 3 O 4 -SiO 2 -Si structure.Inset shows the temperature dependence of resistance at different measuring currents.
3 O 4 film in a Fe 3 O 4 -SiO 2 -Si structure at the same temperature.Hall data imply that the inversion layer play an important role in the charge transport of the Fe 3 O 4 -SiO 2 -Si structure.The much enhanced low field negative MR achieved in the Fe 3 O 4 -SiO 2 -Si structure with ultrathin Fe 3 O 4 film may have useful application in spintronic devices.This work was supported by the National Natural Science Foundation of China ͑Grant Nos.10804024 and 50672019͒, the Natural Scientific Research Innovation Foundation in Harbin Institute of Technology ͑Grant No. HIT-.NSRIF.2008.03͒, the Program for New Century Excellent Talents in University ͑Grant No. NCET-06-335͒, NSF, and UW SER/MGF.