Dielectric and magnetic properties of YIG/PMMA nanocomposites

Yittrium iron garnet, Y3Fe5O12 (YIG), is a material used widely in electronic devices for the microwave region as well as the magnetic bubble domain-type memories. Yittrium iron garnet (Y3Fe5O12) was produced by mechanochemical synthesis from Y2O3 and Fe2O3 with particle size of around 150 nm. PMMA/YIG composite films were prepared by solution casting method at diffferent concentration (i.e. 10%, 20% and 40%) of YIG filler. Dielectric permitivity of composite materials were studied over a wide a range of frequency and temperature as a function of filler concentration. The electrical properties of composites were explained by in terms of molecular mobility and interfacial polarization. 1. Intoduction Composite systems consisting of an insulating matrix and randomly dispersed fine inorganic particles have generated significant research interest, mostly, due to their electrical and electromagnetic performance [1-3]. The primary applications of these polymer composites refer to electromagnetic interference (EMI) shielding, radio frequency interference (RFI) shielding and electrostatic dissipation of charges (ESD). Composites consisting of inorganic particles embedded in a polymer matrix have attracted more and more attentions due to integrative advantages of the two constituents. Introducing high dielectric constant (ε) ceramic particles into polymer matrix will result in high ε composite with the processibility and flexibility similar to polymers [4–8]. Such kind of composites could contribute to the promising candidates for dielectric materials in flexible capacitors with high performance and as a microwave absorber. The dielectric properties of polymer/ceramic composites are influenced not only by the dielectric constant of the polymer and the ceramic, but also by the dispersion and loading of the ceramic in the polymer matrix. Using higher dielectric constant particles and polymer matrix, and increasing the loading of particle fillers can increase the effective dielectric constant of a composite. As well, the morphological and structural optimization of particles will enhance their contribution to dielectric constant improvement [9]. Though lifting the filler content is a choice, however, too high loading of fillers would lead to poor quality of the composite. Polymers and polymer matrix composites are basically electrical insulators, due to their low concentration of free charge carriers. Thus their electrical response is, mainly, associated with relaxation phenomena occurring under the influence of ac field. The observed relaxation processes are related to dipolar orientation effects or space charge migration [10, 11]. Molecular mobility and interfacial polarization are regarded as the origin of International Conference On Superconductivity and Magnetism (ICSM2008) IOP Publishing Journal of Physics: Conference Series 153 (2009) 012061 doi:10.1088/1742-6596/153/1/012061 c © 2009 IOP Publishing Ltd 1 dielectric effects. At sufficient high temperatures, in the vicinity of the glass transition temperature, large segments of the polymer chains are able to move trying to follow the alternation of the field, while at lower temperatures polar side groups are contributing to the electrical performance of the system. Interfacial polarization is the result of the heterogeneity of the system, such as mobile charges accumulated at the fillers–polymer matrix interface, form large dipoles. Recently, adding conductive fillers (with infinitively large ε) into polymer was reported on preparation of high ε composites [12– 17], but the insulating properties of the composites will be reduced in this case. So, the inorganic fillers with super high ε should be the key consideration in obtaining higher ε composites with integrative properties needed in practical applications. The ferromagnetic garnet, yttrium iron garnet (YIG), is a well-known material, and it has been widely used in electronic devices, such as circulators and phase shifters for microwave and magneto-optical devices [18, 19]. Additionally, YIG has controllable saturation magnetization, low dielectric loss tangent (tan δ) in microwave regions, and small line-width (∆H) in ferromagnetic resonance [20]. Oscillations and waves of magnetization or spin waves in yttrium–iron garnet (YIG) ferrite films and mono-crystals have very interesting and useful properties such as a high Q-factor, a wide variety of dispersion laws dependent on the orientation of bias magnetic fields, and a possibility to tune their frequencies in the range of several GHz by changing the magnitude of the bias magnetic field. Another important property of ferrite films is low (of the order of several microwatts) power threshold of nonlinear spin wave processes. The nonlinear properties of ferrite films can be used for the development of novel microwave signal processing devices based on the parametric interaction of spin wave packets propagating in the films with localized electromagnetic fields of the microwave pumping. Excellent features of the ferrite/polymer composites, such as sharply reduced dielectric loss compared to in the bulk ferrites, while uninfluenced microwave absorption properties owing to the domination of natural ferromagnetic resonance absorption in the loss mechanism of the ferrite absorber materials [21, 22], make them quite attractive for applications not only as inductive and capacitive materials but also as microwave absorber materials. In this work, Yittrium iron garnet, Y3Fe5O12 (YIG) is synthesized using mechanochemical synthesis method and the polymethyl methacryalate polymer-based composites consisting of YIG nanoparticle fillers with varying concentrations were reported, aiming to investigate how nano fillers affects the electrical properties of such composites which could be explained by in terms of molecular mobility and interfacial polarization. 2. Experimental procedure YIG ceramic was produced from Y2O3 and Fe2O3 by mechanochemical method. The synthesis was carried out in a Fritsch Pulverisette 5 Planetary high energy ball milling system. The following milling conditions were used: stainless steel vessel with a volume of 500 cm 3 , stainless steel balls with diameter of 10 mm, ball-to-powder weight ratio was 40:1, air atmosphere, rotation speed of discs with vials was 320 min −1 and milling time was 8 h. Polymethyl methacrylate (PMMA) was used as the polymer matrix. Polymer-based nanocomposites were prepared by directly mixing YIG nanoparticles and PMMA together, by solution casting method. Tetrahydrofuran (THF) was used as a solvent to dissolve the polymer. The mixture of polymers and nanofillers were stirred with different volume fraction of YIG filler (i.e. 10%, 20% and 40%) at room temperature using magnetic stirrer at high speed of around 700 rpm until all polymer dissolved. After obtaining a homogeneous mixture, the mixture was poured into a clean glass trough. The solvent was evaporated at room temperature (25 0 C+ 1 0 C) to get thin films of polymer nanocomposites which were then dried in vacuum oven at 30 0 C. Powder X-ray diffraction analysis (Rigaku, Cu Ka, 2°/min) was used to determine the crystalline phases of milled powder. The particleshape and morphologies of YIG were investigated by SEM (JEOL-5910LV) after coating with gold. The particle sizes of milled powder were measured using Malvern particle size analyzer (Nano ZS Zeta Sizer). High frequency magnetic properties of the YIG particles were studied using Bruker MMX X-band ESR spectrometer. A small sample was located at the center of x-band cavity and field derivative FMR spectrum was recorded as a function of external International Conference On Superconductivity and Magnetism (ICSM2008) IOP Publishing Journal of Physics: Conference Series 153 (2009) 012061 doi:10.1088/1742-6596/153/1/012061