Swiatkowska-Warkocka et al. Nanoscale Research Letters 2011, 6:226 http://www.nanoscalereslett.com/content/6/1/226 considerably higher value of 1950 Oe for FC cases. The large coercivity and exchange bias indicates a strong magnetic interaction through the interface between magnetite and wustite. Additionally, a slight positive vertical shift along the magnetization axis is presented. In FM/AFM systems, vertical shifts are generally related to pinned uncompensated spins that exhibit either FM or AFM coupling at the interface [23]. The positive ver- tical shift in Figure 5 indicates a dominant FM coupling between the pinned uncompensated spins and the FM magnetization. To reveal the effect of the phase ratio of FM and AFM phase on the exchange bias of FM/AFM composites, Hc and Hexch as a function of FeO phase ratio in Fe3O4/ FeO particles was investigated. Figure 6 shows the varia- tion of Hc (after ZFC) and Hexch (after FC in 50 kOe) with the increase FeO fraction in particles at 5 K. It is clear that Hc increases with the increasing FeO fraction, while Hexch firstly increases and then reaches a maxi- mum of 1,960 Oe for 75% of wustite fraction. Further increase of FeO concentration leads to the decrease of Hexch. With increasing FeO concentration, the AFM phase can supply enough force to pin the uncompen- sated spin in FM phase which leads to a much stronger exchange interaction and further increase in Hexch. With a further increase in FeO concentration (> 75%), the balance between FM and AFM is destroyed, and Hexch do not increase continuously with increasing FeO con- centration. In this case, 75% is a critical concentration of AF phase in Fe3O4/FeO composites. This result can confirm granular structure of obtained particles. With increasing FeO phase the effective interface area increases affecting exchange bias. AFM is too high for 75% of FeO concentration, and the effective interface Page 5 of 7 area rapidly decreases entailing the exchange bias decrease In order to explore the origin of Hexch, the tempera- ture dependence of Hexch obtained from magnetic hys- teresis loops for the samples with 75% of FeO were also studied. The sample was first cooled down from 300 K to the measuring temperature under a magnetic field of 50 kOe, and then the loop was measured. This process was repeated for every measuring temperature. As pre- sented in Figure 7, Hexch decreases with the temperature increase and appears to vanish at about 190 K. This blocking temperature is equal to the Neel temperature (TN) of FeO (for FeO TN = 198 K [24]). At a higher temperature, the coupling between FM and AFM regions is weakened by thermal disturbance. As the tem- perature decreases, the exchange interaction between the above two types of regions becomes stronger, result- ing in the loop shift which becomes more prominent at a lower temperature. However, coercivity Hc does not decrease to zero and its value approaches to the intrin- sic Hc of particles without exchange bias. Thus, the observed exchange bias effect can be explored on the exchange coupling between the interfa- cial FM phase and AFM (or SGL) phase, and AFM can play an important role in pinning the uncompensated interfacial moments. Conclusions In conclusion, the pulsed laser irradiation technique was demonstrated to be a simple method for preparing sub- micrometer iron oxide heterostructure spherical parti- cles. Size and composition of obtained particles can be tuned in a controllable manner by only laser fluence. Additionally, obtained particles exhibit interesting Figure 5 Illustration of the FC and ZFC hysteresis loops. (a) Hysteresis loops of the Fe3O4/FeO particles fabricated at 177 mJ/pulse.cm2. FC means that the sample is cooled from 300 to 5 K in the 50 kOe field. (b) The magnification around origin of hysteresis loops of the Fe3O4/FeO particles fabricated at 177 mJ/pulse cm2.