Swiatkowska-Warkocka et al. Nanoscale Research Letters 2011, 6:226 http://www.nanoscalereslett.com/content/6/1/226 an unfocused laser light which gives relatively mild reac- tion conditions [17,18]. The present study demonstrates the easy control of size and composition of submicrom- eter spherical iron oxide particles by PLIL. Furthermore, we report the structural effect and the exchange bias effect in Fe3O4/FeO composites by PLIL method. Vary- ing the phase ratio of magnetite and wustite, we can control the coercivity and exchange bias effect. Experimental The magnetite nanoparticles were prepared by conven- tional co-precipitation from FeCl2 and FeCl3 at high values of pH. Iron salts were dissolved in water with a magnetic stirrer for 1 h. The pH value was increased by adding NaOH. The colour of the solution turned to black immediately, inducting magnetite formation. Mag- netite particles were removed from the solution by using permanent magnet and were washed several times with deionized water. Finally, the magnetite nanoparti- cles were dispersed in ethyl acetate and transferred to a sealed cell with quartz window to introduce laser light. The magnetite nanoparticles were stirred and irradiated for 1 h with the third harmonic (355 nm) of an Nd: YAG (yttrium aluminium garnet) laser operated at 30 Hz without focusing. Laser fluence varied from 33 to 177 mJ/pulse cm2. No evaporation of solvents was observed during irradiation. The formed iron oxide phases and composition were determined by a powder X-ray diffractometer (XRD) (Rigaku Ultima IV, Rigaku Corporation, Akishima, Tokyo, Japan) with CuKa radiation. The morphology of the obtained particles was observed by a field emission scanning electron microscope (FE-SEM) (Hitachi S4800, Hitachi High Technologies Japan Inc., Tokyo, Japan) and a transmission electron microscope (TEM) (JEOL JEM 2010, Tokyo, Japan). Average particle size was determined by measuring the diameters of 200 particles from SEM images. The size of spherical particles was simply defined from the diameter. Calculation of size of non-spherical particles was based on replacing a given particle with a sphere that has the same volume as a given particle. The chemical states of elements in the samples were confirmed by an X-ray photoelectron spectrometer (XPS) (PHI, Versa Probe, ULVAC-PHI, Inc., Chigasaki, Kanagawa, Japan). A highly sensitive superconducting quantum interference device (Quantum Design, MPMS, San Diego, CA, USA) magnetometer was employed to measure the magnetic properties of nanocomposite particles. Hysteresis measurements were recorded for dried samples of nanoparticles in a gelatin capsule. Hysteresis loops were obtained by using maxi- mum applied field up to 50 kOe at 5 and 300 K. The exchange bias properties of samples were investigated by measuring field cooled (FC) hysteresis loops in the Page 2 of 7 temperature range 5-300 K. In the FC procedure, the sample was cooled down from the initial temperature of 300 K to the measuring temperature T, under an applied field 50 kOe. Once T was reached, the field was set to 50 kOe and the measurement of the loop started. Results and discussion Fabrication and structural investigation of Fe3O4/FeO system obtained by PLIL method The size and shape of the particles obtained by laser irradiation in ethyl acetate were examined by FE-SEM (Figure 1, left). The average diameter of raw magnetite nanoparticles in the aggregates (Figure 1 before irradia- tion) is estimated to be 6 nm. Figure 1 indicates that spherical particles with smooth surfaces were formed after laser irradiation. Their spherical shape clearly indi- cates melt formation during the process, which suggests that the temperature of the particles is transiently increased over the melting point of iron oxide. A fluence increases from 33 to 177 mJ/pulse cm2 and shows a sys- tematic increase in the particle size from 150 to 460 nm (Figures 1 and 2). The relationship between particle size and fluence is simply explained by the thermal energy absorbed of laser light. The absorption cross section of particles with diameters larger than the irradiation laser wavelength is considered the same as the geometrical cross section. The minimum energy to melt a particle is proportional to the particle volume (∝ d3), while the absorption energy is proportional to the particle’s cross section (∝ d2). Thus, the minimum fluence to melt a particle is proportional to the diameter d = d3/d2. The relationship, however, is not so simple for particles with a diameter equal to or less than laser wavelength because of the complex dependence of the cross section on particle size [18,19]. TEM analyses provided more detailed structural infor- mation on the submicrometer spheres (Figure 1, right). Some of the particles formed at 33-66 mJ/pulse cm2 had hollow structures. In contrast, smaller particles ranging from 5 to 60 nm were embedded in the larger spherical particles at a fluence exceeding 100 mJ/pulse cm2. In the intermediate fluence range of 66-100 mJ/pulse cm2, particles with a merged structure of two primary parti- cles were observed. X-ray diffraction patterns of particles before and after laser irradiation (Figure 3) revealed a gradual phase trans- formation from magnetite (Fe3O4) to wustite (FeO) with fluence increase. Starting raw nanoparticles were con- firmed to be a pure magnetite phase. The sample after irra- diation at 33 mJ/pulse cm2 remained pure magnetite without chemical change, though the crystalline size increased judging from the reduced width and increased intensity of the reflections. Small wustite reflections of