Swiatkowska-Warkocka et al. Nanoscale Research Letters 2011, 6:226 http://www.nanoscalereslett.com/content/6/1/226 Page 4 of 7 Figure 3 X-ray diffraction patterns of raw magnetite and irradiated nanoparticles at various laser fluencies. Standard XRD peaks for Fe3O4 and FeO are plotted for reference. confining process of bubbles by melted droplets. Such bubbles may result from ultrasonic stirring during irra- diation. With laser fluence increase, the reducing reaction with the decomposed gases becomes significant. Partial surface melting of particles causes coalescence with close neighbours (in the intermediate fluence range) and/or formation of spherical composite particles (in the high fluence range), together with the reducing reaction by decomposed gas from ethyl acetate. Thus, a hundred nanometer-sized particles composed of magnetite and wustite nanoparticles grow with increased fluence. Magnetic properties of Fe3O4/FeO system with different Fe3O4 to FeO phase ratios In order to investigate the impact of different phase ratio of magnetite and wustite on the magnetic proper- ties of the final Fe3O4/FeO composite, the Fe3O4 nano- particles dispersed in ethyl acetate were irradiated with the fluence of 133, 166 and 177 mJ/pulse cm2 for 0.5, 1 and 2 h. The obtained particles are spherical with FeO volume fraction that varies from 20% to 85%. All Figure 4 XPS Fe 2p depth profile of the Fe3O4/FeO particles fabricated at 177 mJ/pulse cm2. particles have similar structures to those presented in Figure 1 with the fluence of 133 mJ/pulse cm2 or larger. Exchange coupling at the FM/AFM interface of the Fe3O4/FeO system is investigated by the zero field cooled (ZFC) and field cooled (FC) measurements of M (H). Figure 5 illustrates the FC (HFC = 50 kOe) and ZFC hysteresis loops at 5 K for a cycling field of ± 50 kOe of sample with 75% of wustite fraction (the ZFC and field cooled (FC) measurements of M(H) curves for particles with 20%, 45%, 60% and 85% of FeO are presented in supporting information on Figure S1 in Additional file 1). The interesting feature in the M(H) curves is that both the ZFC and FC loops remain open even in the 50 kOe field, known as the high field irreversibility, which could be interpreted as being due to the existence of the spin glass-like (SGL) phase [21,22]. According to the fig- ure, this system exhibits the properties of exchange bias system, with a horizontal shift along the field axis of the FC hysteresis loop with respect to the ZFC hysteresis loop. The loop shift is defined as an exchange bias field Hexch = |(H+ + H-)/2|, where H+ and H- are positive and negative coercive fields. The FC hysteresis loop is shifted with an exchange bias field of 1,960 Oe. The coercivity field given by Hc = (H+ - H-)/2 is also obtained for both the ZFC case with the value of 514 Oe and a