APPLIED PHYSICS LETTERS 92, 173119 共2008兲

Crucial role of interdiffusion on magnetic properties of in situ formed
MgO / Fe3−␦O4 heterostructured nanowires
Aurelian Marcu,1 Takeshi Yanagida,2,a兲 Kazuki Nagashima,2 Keisuke Oka,2Hidekazu Tanaka,2 and Tomoji Kawai2
1

National Institute of Laser Plasma and Radiation Physics, Laser Department, P.O. Box MG-36,
Atomistilor 409, Buharest-Magurele, Romania
2
Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka Ibaraki,Osaka 567-0047, Japan共Received 23 March 2008; accepted 10 April 2008; published online 1 May 2008兲
Although a heterointerface in oxides plays a crucial role on the properties, such heterointerface
effects on oxide nanowires have not been well understood. Here, we demonstrate the crucial role ofatomic interdiffusion on the magnetic properties of in situ formed MgO / Fe3−␦O4 heterostructured
nanowires. The wide variation of nanowire heterointerfaces was found when varying the growth
atmosphere. The occurrence of the interdiffusion and the large interface area of nanowires strongly
affect the magnetism via forming mixed spinel structures at the heterointerface. Thus, it is crucial
to precisely control the atomic interdiffusion at the heterointerface for the desired properties ofheterostructured oxide nanowires. © 2008 American Institute of Physics.
关DOI: 10.1063/1.2918132兴
Transition metal oxide nanowires are potential candidates to incorporate the rich functionalities of the physicalproperties, including ferromagnetism, superconductivity, andferroelectricity, into nanowire-based devices.1–4 However,there is a fundamental limitation to fabricate nanowires ofvarious transition metal oxides due to the one-dimensional
crystal growth mechanism such as vapor-liquid-solidmechanism.5,6 Use of a core-shell heterostructured oxide
nanowire might overcome such critical issue, since the methodology essentially allows us to explore a wide range ofoxide materials.7–9 In oxide heterostructures, a heterointerface plays an important role on the physical properties.10–12
Therefore a well-defined heterointerface in core-shell heterostructured nanowires is strongly desired. Although the feasibility of ex situ formations of heterostructured oxide nanowires has been demonstrated,7–9 an in situ formation ofheterostructured nanowires without atmospheric exposure
would be one of powerful methods to create a well-definedoxide heterointerface. However, such in situ formations ofoxide heterostructured nanowires have not been reported sofar. We have reported the feasibility of MgO nanowires as ananoscale oxide substrate by using pulsed laser deposition
共PLD兲, which in principle allows us to form in situ core-shellheterostructured oxide nanowires.13–16 Ferrites, including
Fe3O4, ␣-Fe2O3 and ␥-Fe2O3, have been intensively investigated for the magnetic applications,17–22 and recently thespintronics applications2,3,23 and the nonvolatile memory24
have renewed the interests. Thus it would be an interesting
issue to fabricate the oxide heterostructured nanowires using
ferrite toward miniaturizing the ferrite-based device applications. Here, we report the in situ formation of MgO / Fe3−␦O4core-shell heterostructured oxide nanowires by PLD and theeffect of heterointerface on the magnetic properties. We
found a significant role of the atomic interdiffusion at theheterointerface on the magnetic properties of MgO / Fe3−␦O4heterostructured nanowires.
a兲Electronic mail: yanagi32@sanken.osaka-u.ac.jp.

MgO / Fe3−␦O4 core-shell heterostructured nanowires
were fabricated using in situ PLD method. Firstly, MgO
nanowires were grown on MgO 共100兲 single crystal substrate
by Au catalyst-assisted PLD, and the details can be seenelsewhere.13–16 Secondly, Fe3−␦O4 shell layer was deposited
onto the MgO nanowire without atmospheric exposure.
Fe2O3 pellet was used as the target, which was obtained bymilling Fe2O3 共99.9% pure兲 powder for 1 h and then sinteredat 1000 ° C for 24 h. The repetition rate, the laser energy, andthe substrate-to-target distance were 10 Hz, 15 mJ, and45 mm, respectively. The substrate temperature and oxygenpressure during the shell depositions were varied from room
temperature 共RT兲 to 800 ° C and 10−4 – 10 Pa, respectively.
Field emission scanning electron microscopy 共FESEM兲共JEOL JSM-6330FT兲 at an accelerating voltage of 30 kV
was used to characterize the macroscopic morphology. Priorto the FESEM observation, Pt was sputtered on the samples
to prevent the charging. The thickness of Pt was nearly 5 nm.
High-resolution transmission electron microscopy 共HRTEM兲
共JEOL JEM-3000F兲 coupled with energy dispersive spectroscopy 共EDS兲 was used to evaluate the microstructure at theinterface, and the composition of the fabricated nanowires.
Samples for HRTEM were prepared by placing a dropof the sample suspension on a copper microgrid 共JEOL
7801-11613兲. HRTEM measurements were performed at anaccelerating voltage of 300 kV. The magnetic properties ofheterostructured nanowires were evaluated by a superconducting quantum interference device.
Figure 1 shows the FESEM images of MgO / Fe3−␦O4heterostructured nanowires grown under 400 ° C substratetemperature and 10 Pa oxygen pressure. The smooth covering surface can be seen in the image. The variation of thelaser energy drastically changes the heterostructured nanowire morphology. Above the laser energy of 15 mJ, the preferential crystal growth of shell layers near the tips tends toappear as the laser energy tends to increase. The similar trend
was found when varying the distance between the target andthe substrate, indicating the significant role of the ablatedparticle flux on the shell covering formation process. The0003-6951/2008/92共17兲/173119/3/$23.00
92, 173119-1© 2008 American Institute of Physics
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173119-2

Marcu et al.

FIG. 1. FESEM images of MgO / Fe3−␦O4 core-shell nanowires grown under
400 ° C growth temperature and 10 Pa oxygen pressure.

preferential crystal growth at the tip when increasing the ablated particle flux can be understood in terms of the plumepenetration depth variation in between nanowires.25 Thus,
the dominant factor for the heterostructured nanowire formation using in situ PLD technique is rather dependent on theablated particle flux.
Although the macroscopic uniformity of heterostructured oxide nanowires can be achieved via controlling theablated particle flux, the magnetic properties might be alteredby varying the ambient atmosphere during the in situ formation of the heterostructures. Figure 2 shows the M-H datameasured at 300 K for two heterostructured nanowires
grown at 300 and 600 ° C growth temperatures under 10−4 Pa
oxygen pressures. The ferromagnetic behavior can be seen inboth samples. All fabricated heterostructured nanowires exhibited such ferromagnetic behavior. Figure 3 shows thevariation of the saturation magnetizations when varying theoxygen pressure and the growth temperature. According to athermodynamic law in spinel ferrites,26 the saturation magnetization should increase when the ambient atmospheretends to be reduced, i.e., decreasing the ambient oxygenpressure and increasing the growth temperature within therange of this study. This is simply due to an increase of Fe2+
ions within the spinel structure.26 Although such generalmagnetic trend can be seen in Fig. 3, there is a drastic reduction of the saturation magnetization around 300– 400 ° C.
This effect cannot be interpreted in terms of only thermodynamic law of ferrites.

Appl. Phys. Lett. 92, 173119 共2008兲

FIG. 3. 共Color online兲 Variation of saturation magnetizations when varying
the growth atmosphere including the growth temperature and the oxygenpressure.

The possible mechanism for the observed magnetic reduction might be due to the microstructure variation at theheterointerface. Figure 4 shows the microstructure variation
at the heterointerface when varying the ambient temperature
under 10−4 Pa oxygen pressure. The wide variation of theheterointerfaces can be seen in the HRTEM images. As thegrowth temperature it is increased, the shell layer at theheterointerface tends to show 共i兲 the polycrystallinelike
structure, 共ii兲 the single crystal core-shell heterostructures,
and 共iii兲 the mixed crystal structure without observable heterointerfaces. This trend was found to be similar even whenvarying the oxygen pressure. The variation from the polycrystallinelike structures to the well-defined core-shell interfaces below 300 ° C can be interpreted in terms of the enhancement of crystallization at the heterointerface. In themagnetic properties, the saturation magnetization increased
with increasing the growth temperature due to reduced atmosphere, i.e., increased Fe2+ ions. In addition, the results also
indicate that the spinel ferrite structure exists even for RT
grown heterostructured nanowires.
When further increasing the growth temperature above
300 ° C, the heterointerface between the core and the shelllayers tends to disappear, as shown in the HRTEM images.
At 800 ° C of the growth temperature, the heterointerface isno longer visible at all. These results may indicate the occurrence of the atomic interdiffusion at the heterointerfaces
formed above 300 ° C. In a spinel ferrite, Mg ions can beincorporated at mainly B site via forming MgFe2O4.27,28 The
saturation magnetization of MgFe2O4 is almost 1 bohr magnetron that is nearly one-fourth compared with Fe3O4.29,30
Thus, the formation of MgFe2O4 at the heterointerface canrigorously explain the observed reduction of the saturation

FIG. 4. HRTEM images of MgO / Fe3−␦O4 core-shell nanowires. 关共a兲–共d兲兴FIG. 2. 共Color online兲 M-H data of heterostructured nanowires grown underimages show the heterostructured nanowires grown at RT up to 800 ° C
10−4 Pa oxygen pressure with the varying grown temperature.
under 10−4 Pa oxygen pressure.
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173119-3

magnetization around 300– 400 ° C growth temperatures. Although the presence of MgFe2O4 structures at the heterointerface should be justified, the direct identification ofMgFe2O4 structures was unfortunately not feasible using ourHRTEM-EDS technique since the lattice constants of spinel
Fe3O4, ␥-Fe2O3, and MgFe2O4 are too close. The composition of mixed spinel structure at the heterointerface should be共Mg, Fe兲3O4 and differ from the nominal MgFe2O4, depending on the core-shell volume ratio. The space distribution ofinterdiffusion should also affect the magnetic properties,
which is dependent on the diffusion length of Mg with spinelferrites. It should be noted that the magnetic properties ofheterostructured nanowires grown at 500 ° C was indeed affected by the presence of the atomic interdiffusion, althoughthe HRTEM image shows relatively clear core-shell heterointerface. Therefore, above results highlight the significant
role of the atomic interdiffusion on the magnetic propertiesof oxide heterostructured nanowires. The reason why thephysical events at the heterointerface strongly dominate themeasured magnetic properties is due to the huge specific surface area of heterostructured nanowires. For example, we canroughly estimate that the interface area per unit area of theheterostructured nanowires is at least 104 times larger than
the thin film case. The interface effects must be much moresignificant on the measured physical properties when compared with thin films. In other words, the use of heterostructured nanowires offers a good tool to detect the physicalevents at the oxide heterointerface. Overall, this study demonstrates that the in situ formation of oxide heterostructured
nanowires and the precise control of heterointerfaces are important toward not only the fundamental investigations ofnanoscale properties but also the applications of oxide heterostructured nanowires.
In conclusion, we have demonstrated the crucial role ofinterdiffusion on the magnetic properties of MgO / Fe3−␦O4core-shell heterostructured nanowires in situ formed by
PLD. The wide variation of nanowire heterointerfaces wasfound when varying the ambient atmosphere. Especially thepresence of interdiffusion was demonstrated for growth temperature range above 300 ° C. The occurrence of the interdiffusion strongly affects the magnetic properties of heterostructured due to the huge interface area of heterostructured
nanowires. Thus, it is crucial to precisely control the atomicinterdiffusion at the oxide heterointerface for the desired oxide heterostructured nanowire properties.
1Appl. Phys. Lett. 92, 173119 共2008兲

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