Magnetic properties of nanocrystalline Fe 3 O 4 films

Nanocrystalline magnetite Fe3O4 films of about 180 nm thick have been deposited on Si(100) substrates by pulsed laser deposition. Zero-field-cooled magnetization shows clearly the Verwey transition near 120 K by an abrupt change, which is absent from the field-cooled magnetization. This is correlated to its hysteresis curves where the loops remain open until a high field of 2 T. The magnetization does not saturate in field 2 orders of magnitude higher than its coercive field. Such behaviors may result from the existence of antiphase domains. Antiphase boundaries inside the grains are clearly observed with transmission electron microscopy. Negative magnetoresistance of about 12% has been observed near 120 K in a field of 9 T.

Nanocrystalline magnetite Fe 3 O 4 films of about 180 nm thick have been deposited on Si͑100͒ substrates by pulsed laser deposition.Zero-field-cooled magnetization shows clearly the Verwey transition near 120 K by an abrupt change, which is absent from the field-cooled magnetization.This is correlated to its hysteresis curves where the loops remain open until a high field of 2 T. The magnetization does not saturate in field 2 orders of magnitude higher than its coercive field.Such behaviors may result from the existence of antiphase domains.Antiphase boundaries inside the grains are clearly observed with transmission electron microscopy.Negative magnetoresistance of about 12% has been observed near 120 K in a field of 9 T. © 2001 American Institute of Physics.͓DOI: 10.1063/1.1358350͔Pulsed laser deposition ͑PLD͒ is an attractive method for preparation of a variety of films. 1 It is a highly versatile tool for a range of thin films from nanocomposite to epitaxial growth. 2 A few of the characteristic features of PLD are stoichiometric transfer, growth from an energetic beam, reactive deposition, and its simplicity to operate.Highly nanocrystalline or cluster-assembled films have been produced by PLD.PLD has also been used to form other exotic hybrid or composite thin films containing normally incompatible materials that could be difficult to synthesize otherwise. 3here has been increased interest in the half-metallic magnetite Fe 3 O 4 as its highly spin polarized nature ͑supposedly ϳ100%͒ is desirable for tunneling magnetoresistance based device applications. 4PLD has been successfully used recently to grow Fe 3 O 4 films by several groups.Epitaxial Fe 3 O 4 films were prepared on MgO, 5 SrTiO 3 , and ␣-Al 2 O 3 6 substrates by the technique.Polycrystalline Fe 3 O 4 films have also been prepared using PLD 7 and other techniques, for example, reactive dc sputtering on Si substrate. 8The average grain size of these polycrystalline films is about a few micrometers.
Another interesting aspect of thin film Fe 3 O 4 is that the magnetization does not saturate in fields 2 orders of magnitude higher than its bulk anisotropy field. 9This may be due to the presence of antiphase boundaries in single crystal films, across which the exchange coupling is altered. 10So far, antiphase boundaries have been observed in sputtered and MBE epitaxial Fe 3 O 4 films grown on MgO substrates only. 11e have prepared nanocrystalline Fe 3 O 4 films of average grain size 50 nm on Si substrates by PLD and report in this article some of the important features found in the films.
Fe 3 O 4 films of about 180 nm thick were deposited on Si ͑100͒ substrates by PLD.The ␣-Fe 2 O 3 target used for laser ablation was prepared by pressing high purity ␣-Fe 2 O 3 powders ͑99.998%͒ into a pellet and sintered at 1000 °C for 2 h.
A focused beam of KrF excimer laser (ϭ248 nm) was used for the deposition.The repetition rate was 12 Hz and the Si substrate was heated to 350 °C.Prior to the deposition, an unfocused laser beam was rastered across the Si substrate in a vacuum of 3ϫ10 Ϫ6 Torr to clean the substrate surface.
Glancing angle x-ray diffraction data were collected with a Philips X'Pert diffractometer using Cu K␣ radiation.Figure 1 shows the diffraction pattern of the Fe 3 O 4 film.The lattice constant aϭ0.8392 (2) nm is close to the powder diffraction ͑Card No. 19-629͒ value.Transmission electron microscopy ͑TEM͒ was done with a JEOL Model 2010 TEM. Figure 2͑top͒ is a plane-view TEM image of the Fe 3 O 4 film.The average grain size is 50 nm.This is much smaller than those in the polycrystalline films mentioned earlier, which is in micrometers.
It is evident from the micrograph that within each single crystal grain it is not uniform.This is due to the presence of antiphase domains in the PLD nanocrystalline Fe 3 O 4 films.The TEM image made using ͑220͒ reflection shows antiphase domains within each single crystal grain, see Fig. 2͑bottom͒, where sharp contrast between the neighboring antiphase domains is seen. 12Our results suggest the antiphase domains also exist in Fe 3 O 4 films deposited on Si substrates, not exclusively on MgO substrates as originally thought. 11,12agnetization data obtained with a Quantum Design superconducting quantum interference device is shown in Fig. 3 as a function of temperature.A magnetic field of 500 Oe was used for both the zero-field-cooled ͑ZFC͒ and fieldcooled ͑FC͒ measurements.ZFC data clearly show the Verwey transition at about 120 K with the characteristic sharp drop.One of the unique features of the PLD nanocrystalline films is the absence of the sharp drop in the FC data below the Verwey transition in a field of 500 Oe.An abrupt drop is usually observed in FC runs in applied fields up to several kOe in bulk and epitaxial films. 9,13We will discuss this feature in conjunction with the magnetic hysteresis curves later.
The resistance of the sample measured with a standard four-probe method is also shown in Fig. 3.The change in the resistance at the Verwey transition is not as sharp as those observed in some single crystal samples and relatively thick epitaxial films, but is similar to the relatively thin ͑р50 nm͒ films. 7For comparison, the resistance of a bulk-like 660 nm thick Fe 3 O 4 film is also shown ͑data taken from Ref. 7͒.
Strain, size effects, and possible small departure from the precise Fe 3 O 4 stoichiometry might be responsible for the observed broadening. 14,15he temperature dependence of the coercivity H c and remanence M r of the Fe 3 O 4 nanocrystalline films is shown in Fig. 4. The remanence increases from 140 to 195 emu/cm 3 as temperature decreases from 300 to 5 K.A plateau forms just below the Verwey transition which is consistent with the single crystal data.14 The coercivity is 275 Oe at 300 K and increases very slowly from 300 to 120 K. Then it undergoes a rapid increase at the Verwey transition and reaches 750 Oe at 5 K.The abrupt increase in coercivity is associated with a change of anisotropy due to the Verwey transition where a structural change from cubic to monoclinic occurs.16,17 As mentioned earlier, the FC magnetization shows no anomaly at the Verwey transition in a field of 500 Oe.This is consistent with the hysteresis data shown in Fig. 5, inset, where the three curves measured at 100, 120, and 140 K, respectively, lie almost on top of each other at Hу500 Oe as the field decreases.An important point to make is that the magnetization does not saturate at 2 T and the hysteresis loop does not close until 2 T. Figure 5 shows the portion of the hysteresis curve from 0 to 2 T at 120 K. High saturation field has been observed in epitaxial films 9 and explained as the consequence of disturbed exchange coupling across antiphase boundaries.10 The open loop is unique to the PLD nanocrystalline films.This open loop and the lack of saturation at such high field suggest the existence of an additional anisotropy beyond the bulk value, and it is most likely caused by the presence of antiphase boundaries.
Magnetoresistance ͑MR͒ of the film was measured in a Quantum Design physical property measurement system up to a maximum magnetic field of 9 T. The magnetic field was applied perpendicular to the film.Figure 6 shows the MR ϭ͓R (H) ϪR (0) ͔/R (0) of the film at 90, 130, and 300 K.The MR is about 5% at 300 K and 12% near the Verwey transition.The data also indicate the MR exhibits a peak at the Verwey transition as the temperature is varied.These results are similar to those of the epitaxial films. 5,6oey et al. reported negative MR of pressed Fe 3 O 4 powders and polycrystalline films prepared by reactive dc sputtering. 8The grain sizes of the sputtered film were about 1-2 m.The MR ͑ϳ1.0% at room temperature and 0.5 T͒ was attributed to the intergranular transport of spin polarized electrons. 18On the other hand, comparison between epitaxial and polycrystalline films 7 suggests minimal contribution from the electron transport across grain boundaries.Our study does not address specifically whether the spindependent intergranular transport plays a dominant role in the observed negative MR, however the connection, if any, between the MR and possible existence of antiphase domains should be explored.It has been argued that the antiphase boundaries may become immobile magnetic domain walls. 10t is of interest to study the MR due to pinned domain walls as their thickness can become smaller than the spin diffusion length and give large MR.This research was supported by the Louisiana Board of Regents Support Fund ͑No.LEQSF͑2000-03͒-RD-B-10͒ and Sharp Laboratories of America.

FIG. 2 .
FIG. 2. ͑Top͒ A plane view of TEM of the nanocrystalline Fe 3 O 4 film.͑Bottom͒ TEM image made with 220 reflection that shows antiphase domains in a single crystal.