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 irradiation) is estimated to be 6 nm. Figure 1 indicates that spherical particles with smooth surfaces were formed after laser irradiation. Their spherical shape clearly indicates 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 systematic 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 (∝ d ³), while the absorption energy is proportional to the particle's cross section (∝ d ²). Thus, the minimum fluence to melt a particle is proportional to the diameter d = d ³/d ². 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 information 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 particles were observed.

X-ray diffraction patterns of particles before and after laser irradiation (Figure 3) revealed a gradual phase transformation from magnetite (Fe₃O₄) to wustite (FeO) with fluence increase. Starting raw nanoparticles were confirmed to be a pure magnetite phase. The sample after irradiation 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 (111) at 36.1 and (200) at 42.0 appears as shoulders of magnetite (222) at 37.1 and (400) at 43.1, respectively in the 66-mJ/pulse cm² result in Figure 3. Those wustite reflections grew and magnetite ones decreased with the irradiation fluence increase. Volume fractions of Fe₃O₄/FeO, calculated by ratio of highest intensity peaks from XRD data, are summarized in Figure 2.

Further information about the iron oxide formation during laser irradiation can be gained by analyzing XPS data. For the sake of simplicity, just the XPS Fe 2p depth profile of sample obtained after irradiation with fluence 177 mJ/pulse cm² is shown as being representative of the behaviour of all composite particles (Figure 4). All spectra for pre-sputtering and post-sputtering shows peaks positioned around 711 and 724 eV, which are typical core level spectra of Fe₃O₄ and around 710 and 723 eV, which are characteristic for Fe²⁺ ions in FeO 20 . These results are qualitatively consistent to the XRD measurements. On the basis of TEM and XPS results, we can suppose that composite particles obtained by pulsed laser irradiation reveal aggregated structures.

Morphology and composition of the particles obtained by laser irradiation suggest that Fe₃O₄ nanoparticles are melted to form a large spherical shape and reduced to form FeO phase. Temperature to melt iron oxide nanoparticles definitely induces the decomposition of surrounding ethyl acetate and possibly leads to the reduction of magnetite to wustite. Thermodynamic calculation was performed to investigate the possible thermal decomposition reaction of ethyl acetate and probable reducing reaction of magnetite. Gibbs free energy calculation of possible thermal decomposition reaction suggests that ethyl acetate can be thermodynamically decomposed at 1,600°C (the melting point of bulk magnetite) to methane, ethylene, carbon monoxide or hydrogen, and that these gases can reduce Fe₃O₄ to FeO.

The magnetite nanoparticles dispersed in ethyl acetate melt and formed spherical hollow particles by laser irradiation at low fluence. The formation of submicrometer hollow particles at low fluence may be related to the confining process of bubbles by melted droplets. Such bubbles may result from ultrasonic stirring during irradiation. 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.

In order to investigate the impact of different phase ratio of magnetite and wustite on the magnetic properties of the final Fe₃O₄/FeO composite, the Fe₃O₄ nanoparticles 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 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 (H _FC = 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 figure, 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 H _exch = |(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 H _c = (H ⁺ - H ^-)/2 is also obtained for both the ZFC case with the value of 514 Oe and a 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 vertical 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, H _c and H _exch as a function of FeO phase ratio in Fe₃O₄/FeO particles was investigated. Figure 6 shows the variation of H _c (after ZFC) and H _exch (after FC in 50 kOe) with the increase FeO fraction in particles at 5 K. It is clear that H _c increases with the increasing FeO fraction, while H _exch firstly increases and then reaches a maximum of 1,960 Oe for 75% of wustite fraction. Further increase of FeO concentration leads to the decrease of H _exch. With increasing FeO concentration, the AFM phase can supply enough force to pin the uncompensated spin in FM phase which leads to a much stronger exchange interaction and further increase in H _exch. With a further increase in FeO concentration (> 75%), the balance between FM and AFM is destroyed, and H _exch do not increase continuously with increasing FeO concentration. 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 area rapidly decreases entailing the exchange bias decrease

In order to explore the origin of H _exch, the temperature dependence of H _exch obtained from magnetic hysteresis 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 presented in Figure 7, H _exch decreases with the temperature increase and appears to vanish at about 190 K. This blocking temperature is equal to the Neel temperature (T_N ) of FeO (for FeO T_N = 198 K 24 ). At a higher temperature, the coupling between FM and AFM regions is weakened by thermal disturbance. As the temperature decreases, the exchange interaction between the above two types of regions becomes stronger, resulting in the loop shift which becomes more prominent at a lower temperature. However, coercivity H _c does not decrease to zero and its value approaches to the intrinsic H _c of particles without exchange bias.

Thus, the observed exchange bias effect can be explored on the exchange coupling between the interfacial FM phase and AFM (or SGL) phase, and AFM can play an important role in pinning the uncompensated interfacial moments.