Swiatkowska-Warkocka et al. Nanoscale Research Letters 2011, 6:226 http://www.nanoscalereslett.com/content/6/1/226 Page 6 of 7 Figure 6 Variation of Hc and Hexch. Coercivity Hc and exchange bias Hexch of Fe3O4/FeO composite particles as a function of relative fraction of FeO in particles measured at 5 K. magnetic properties, especially exchange bias interaction at the ferrimagnet-antiferromagnet interface. For 75% of AFM concentration, the Hexch can reach the maximum value 1,960 Oe, at 5 K after field cooling. The reason is that the FM and AFM phase reach to the balance; the value of pinning force of AFM phase, which can play a significant role in pinning the uncompensated interfacial moments, is maximum. Hexch decreases with increasing temperature and approach zero at 190 K. The exchange bias originates from the exchange coupling between the interfacial FM phase and AFM phase. Although gener- ally core/shell structures have been considered to explain a large exchange bias field, we have developed a new type of nanocomposite system with a large exchange bias field composed of ferrimagnetic Fe3O4 and antiferromagnetic FeO by pulsed laser irradiation of colloidal nanoparticles. In contrast with common chemical methods, pulsed laser irradiation in liquid is very simple, low-cost, and contamination-free. Hence, we believe that our method Figure 7 Magnetic hysteresis loops for the samples with 75% of FeO. Coercivity Hc and exchange bias Hexch of Fe3O4/FeO composite particles as a function of temperature. makes it possible to synthesize magnetic heterostructure particles with controllable size, composition and mag- netic properties. Additional material Additional file 1: supporting information. Fig. S1 Magnetization vs. field loop measured at 5 K under ZFC and FC conditions Fe3O4/FeO composite particles with different fraction of FeO. Acknowledgements This work was supported by KAKENHI 2008734, and the magnetization measurements were conducted at the Nano-Processing Facility, supported by IBEC Innovation Platform, AIST. Authors’ contributions ZS-W conducted most of the experiments and performed data analysis. KK supported the magnetic property measurement and contributed the data interpretation. HW supported to construct the formation mechanism by providing relating data of similar systems. YK helped the most of operation and data interpretation of analytical equipments used. NK conceived basic idea of this technique and supported the organization of this paper. Competing interests The authors declare that they have no competing interests. Received: 10 September 2010 Accepted: 16 March 2011 Published: 16 March 2011 References 1. Nogues J, Sort J, Langlais V, Skumryev V, Surinach S, Munoz JS, Baro MD: Exchange bias in nanostructures. Phys Rep 2005, 422:65-117. 2. Sort J, Nogues J, Surinach S, Munoz JS, Baro MD, Chappel E, Dupont F, Chouteau G: Coercivity and squareness enhancement in ball-milled hard magnetic-antiferromagnetic composites. Appl Phys Lett 2001, 79:1142-1144. 3. Gangopadhyay S, Hadjipanayis GC, Sorensen CM, Klabunde KJ: Magnetic properties of ultrafine Co particles. IEEE Trans Magn 1992, 28:3174-3176. 4. Meikleohn WH, Bean CP: New magnetic anisotropy. Phys Rev 1956, 102:1413-1414. 5. Berkowitz AE, Takano K: Exchange anisotropy-a review. J Magn Magn Mater 1999, 200:552-570. 6. Tang Y-K, Sun Y, Heng Z-H: Cooling field dependence of exchange bias in phase-separated La0.88Sr0.12CoO3. J Appl Phys 2006, 100:023914. 7. Binek C, He X, Polisetty S: Temperature dependence of the training effect in a Co/CoO exchange-bias layer. Phys Rev B 2005, 72:054408. 8. Yi JB, Ding J: Exchange coupling in CoO-Co bilayer. J Magn Magn Mater 2006, 303:e160-e164. 9. Salazar-Alvarez G, Sort J, Surinach S, Baro MD, Nogues J: Synthesis and size-dependent exchange bias in inverted core-shell MnO/Mn3O4 nanoparticles. J Am Chem Soc 2007, 129:9102-9108. 10. Kavich DW, Dickerson JH, Mahajan SV, Hasan SA, Park JH: Exchange bias of singly inverted FeO/Fe3O4 core-shell nanocrystals. Phys Rev B 2008, 78:174414. 11. Hajra P, Basu S, Dutta S, Brahma P, Chakravorty D: Exchange bias in ferrimagnetic-antiferromagnetic nanocomposite produced by mechanical attrition. J Magn Magn Mater 2009, 321:2269-2275. 12. Fiorani D, Del Bianco L, Testa AM: Glassy dynamics in an exchange bias nanogranular system: Fe/FeOx. J Magn Magn Mater 2006, 300:179-184. 13. Amara D, Felner I, Nowik I, Margel S: Synthesis and characterization of Fe and Fe3O4 nanoparticles by thermal decomposition of triiron dodecacarbonyl. Colloid Surf A-Physicochem Eng Asp 2009, 339:106-110. 14. Ali-zade RA: Magnetite nanoparticles with an antiferromagnetic surface layer. Inorg Mater 2006, 42:1215-1221. 15. Gangopadhyay S, Hadjipanayis GC, Dale B, Sorensen CM, Klabunde KJ, Papaefthymiou V, Kosticas A: Magnetic properties of ultrafine iron particles. Phys Rev B 1992, 45:9778-8787.