Nanocrystalline multiferroic BiFeO3 thin films made by room temperature sputtering and thermal annealing, and formation of an iron oxide-induced exchange bias
Introduction
Substantial research effort is being focused on multiferroic materials that simultaneously display more than one ferroic order ((anti-)ferromagnetic, ferroelectric, and ferroelastic) because this can result in a coupling between the electric and magnetic fields, and hence an intrinsic magneto-electric effect [1], [2]. The appearance of magneto-electric coupling can lead to new devices such as magneto-electric RAM [3], [4], zero power magnetic sensors [2], [5], [6], and electrostatically tunable inductors [7], [8]. Considerable attention has focused on BiFeO3 because it was one of the first compounds to be shown to be multiferroic at room temperature [2]. The magnetic and electric dipole ordering temperatures are also far above room temperature where the Néel temperature is 643 K and the ferroelectric ordering temperature is 1103 K [2]. It displays G-type antiferromagnetic order and there is canting of the magnetic moments and a spin cycloid structure with a period of 62 nm [9]. The bulk compound has a small net magnetic moment when averaged over the spin cycloid period [10]. The measured magnetic moment in bulk BiFeO3 can be significantly enhanced by partial substitution of different ions for Bi or Fe [11] that may be due to suppression of the spin cycloid. An exchange bias, Bex, has been reported in thin film multilayers that use BiFeO3 as a pinning layer next to a ferromagnetic layer [2], [12], [13] where it can be electrically switched [2], [11], which is particularly useful for magneto-electric RAM. Bex can occur when an antiferromagnetic phase is adjacent to a ferromagnetic phase and the sample is cooled through the Néel temperature in the presence of an applied magnetic field [14], [15], [16].
A number of studies on BiFeO3 nanoparticles and nanoceramics have reported that the saturation moment per Fe, ms,Fe, is enhanced when the nanoparticle size is reduced [10], [17], [18], [19], [20] and it can reach 0.41 μB/Fe [17], where μB is the Bohr magneton. This is far greater than the bulk value of 0.02 μB/Fe [10]. It has been suggested that this occurs because the nanoparticles are smaller than the spin cycloid period, there is a suppression or modification of the spin cycloid structure, and there are uncompensated surface spins that lead to a ferromagnetic shell [10], [17], [18], [19], [21]. The small nanoparticle size can also lead to enhanced magneto-electric [19] and multiferroic properties [22]. An exchange bias has also been reported that is absent in bulk BiFeO3 [10], [17], [19] where it is either increases [17], [19] or decreases as the nanoparticle size is reduced [10]. It is believed to arise in BiFeO3 from a ferromagnetic shell induced in part by uncompensated surface spins [10], [17], [19]. Interestingly, there have been a few studies that do not report a significant enhancement of the saturation moment per Fe or the appearance of Bex [23], [24] although a small amount of Eu doping leads to a large enhancement of the saturation moment [24]. This suggests that the appearance of an enhanced moment and exchange bias depends on the nanoparticle preparation method. This is consistent with previous reports on other nanoparticle ferrites and related magnetic systems where the preparation method affects the morphology and magnetic properties [25], [26]. It would be interesting to see if small BiFeO3 nanoparticles can be made in thin films that might be more practical than nanoparticle powders for application purposes.
In the report we present the results from structural, vibrational and magnetic measurements on thin films that were made by ion beam sputtering of a BiFeO3 target at room temperature followed by annealing in an oxygen atmosphere. We show that annealing leads to the formation of BiFeO3 nanoparticles. There is a large saturation magnetic moment and an exchange bias that is predominantly due to magnetic iron oxides.
Section snippets
Methods
An Ar+ ion beam sputtering system [27] was used to deposit thin films at room temperature using a bulk BiFeO3 rotatable target in a high vacuum (∼2 × 10−5 Pa) and with an anode voltage of 20 kV. The films were deposited onto a 100 nm thick SiO2 film on a crystalline Si (100) substrate. They were annealed in a 100% oxygen atmosphere for 15 min at 500 °C. Cross-sectional transmission electron microscope (TEM) images were taken using a Tecnai TF20 with an acceleration voltage of 200 kV. The
Results and discussion
Fig. 1 shows the RBS spectra from an as-made film and a 500 °C annealed film. In both cases the peaks from Bi and Fe can clearly be seen as well as the Si edges and lower energy peaks from oxygen in the sputtered film and the SiO2 film. Two Si edges are evident in the RBS data where the higher energy edge is due to Si in the SiO2 layer and the lower energy edge is from Si in the substrate. The only significant difference in the RBS spectra before and after annealing is that the Bi peak
Conclusions
In conclusion, nanocrystalline BiFeO3 films have been made by room temperature sputtering and thermal annealing at 500 °C in an oxygen atmosphere. XRD measurements before annealing show the presence of nanocrystalline Bi, and other unidentified BiFexOy phases. SAED shows that there is also some magnetite or maghemite as well as some FeO. The Raman data are consistent with the presence of Bi and β-Bi2O3 as well as magnetite, maghemite, and hematite. Annealing in an oxygen atmosphere at 500 °C
Acknowledgments
We acknowledge funding support from the New Zealand Ministry of Business, Innovation and Employment (C0X01206, C05X1404) and the MacDiarmid Institute for Advanced Materials and Nanotechnology.
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