Controlling exchange bias in Fe3O4/FeO composite particles prepared by pulsed laser irradiation
© Swiatkowska-Warkocka et al; licensee Springer. 2011
Received: 10 September 2010
Accepted: 16 March 2011
Published: 16 March 2011
Spherical iron oxide nanocomposite particles composed of magnetite and wustite have been successfully synthesized using a novel method of pulsed laser irradiation in ethyl acetate. Both the size and the composition of nanocomposite particles are controlled by laser irradiation condition. Through tuning the laser fluence, the Fe3O4/FeO phase ratio can be precisely controlled, and the magnetic properties of final products can also be regulated. This work presents a successful example of the fabrication of ferro (ferri) (FM)/antiferromagnetic (AFM) systems with high chemical stability. The results show this novel simple method as widely extendable to various FM/AFM nanocomposite systems.
Magnetic nanoparticles and hybrid magnetic nanostructures are of growing interest because of their technological applications in magnetic recording media, sensitive magnetic sensors and various biomedical applications such as drug delivery system, hyperthermia or magnetic resonance imaging . In order to fulfil the requirements of many of these applications, an accurate control over the coercivity is strongly required.
Exchange bias coupling at the ferro (ferri) (FM)/antiferromagnetic (AFM) interface has attracted considerable attention due to their applications in permanent magnet applications and high density recording media [2, 3]. The exchange bias effect is manifested by the shifting and broadening of a magnetic hysteresis loop of a sample cooled under an applied field [1, 4, 5]. Although the intrinsic origin of exchange bias effect is not yet understood fully, it is generally accepted that the interface exchange coupling between FM and AFM is the origin of the exchange bias .
Exchange bias has been extensively studied in bilayer and multilayer thin films [7, 8], nanoparticles with core/shell structure [9–11] and particles dispersed in matrix . However, to date, the report about how to control the exchange bias by changing the FM/AFM ratio is seldom.
So far, various experimental methods have been used to produce FM/AFM heterostructure particles, e.g. chemical and thermal decomposition [10, 13, 14], ball milling method , gas condensation and chemical vapour deposition [15, 16]. However, decomposition methods need the chemicals which often cannot be removed and remain as residual molecules on particle surfaces. Gas-phase methods require expensive and large-scale vacuum equipments. Such methods are generally effective for preparing particles with a narrow size distribution. However, most of these approaches are limited to synthesizing particles with a diameter smaller than 30 nm. Additionally, most of mentioned methods lead to the sintering of two phases and the poor quality of the interface. This has been attributed to the weak interfacial interaction between the FM and AFM phases in particles and results in a weak exchange bias. Therefore, the development of new synthetic techniques for FM/AFM particles with high exchange bias is still a target of current research.
In this study, we demonstrate a novel method for preparing submicrometer iron oxide nanocomposite spherical particles by pulsed laser irradiation in liquid (PLIL). In contrast to the pulsed laser ablation in liquid using a focused laser beam, which has been widely studied, PLIL irradiating source particles dispersed in liquid with an unfocused laser light which gives relatively mild reaction conditions [17, 18]. The present study demonstrates the easy control of size and composition of submicrometer spherical iron oxide particles by PLIL. Furthermore, we report the structural effect and the exchange bias effect in Fe3O4/FeO composites by PLIL method. Varying the phase ratio of magnetite and wustite, we can control the coercivity and exchange bias effect.
The magnetite nanoparticles were prepared by conventional 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. Magnetite particles were removed from the solution by using permanent magnet and were washed several times with deionized water. Finally, the magnetite nanoparticles 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 CuKα 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 maximum 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 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 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.
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.
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 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.
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.
In conclusion, the pulsed laser irradiation technique was demonstrated to be a simple method for preparing submicrometer iron oxide heterostructure spherical particles. Size and composition of obtained particles can be tuned in a controllable manner by only laser fluence. Additionally, obtained particles exhibit interesting magnetic properties, especially exchange bias interaction at the ferrimagnet-antiferromagnet interface. For 75% of AFM concentration, the H exch 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. H exch 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 generally 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 makes it possible to synthesize magnetic heterostructure particles with controllable size, composition and magnetic properties.
This work was supported by KAKENHI 2008734, and the magnetization measurements were conducted at the Nano-Processing Facility, supported by IBEC Innovation Platform, AIST.
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