Multiferroic based microwave devices - Advanced Materials Letters

Research Article

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Advanced Materials Proceedings

Multiferroic based microwave devices Vinay Sharma*, Priyanka Rani, Bijoy K. Kunar Special Centre for Nanoscience, Jawaharlal Nehru University, Delhi, 110067, India *

Corresponding author, E-mail: [email protected]; Tel: +91 9212757501

Received: 31 March 2016, Revised: 01 August 2016 and Accepted: 03 August 2016 DOI: 10.5185/amp.2016/1xx www.vbripress.com/amp

Abstract Recently, there has been significant interest on the fundamental science and technological applications of complex oxides and multiferroics. Low-power multiferroic have potential to fabricate and characterize frequency tunable, compatible with MMIC Technology, small light-weight for hand-held operation, cost-effective, high-frequency (>10GHz), devices for next generation communication devices and military applications. Multiferroic materials consists of both magnetic and ferroelectric phase and they offer the possibility of magneto-electric (ME) coupling. The purpose of this research is to show strong magnetic field dependent frequency tuning of multiferroics (Nickel doped BFO – BiFe1-xNixO3) based devices over a broad frequency band. We have shown here the magnetic field control of ferromagnetic resonance (FMR) field/frequency from C to Ku band frequencies. Nanoparticles of BiFe 1xNixO3 (x=0.025 & 0.05) were prepared by auto combustion method. The XRD study confirms the formation of pure phase Bismuth Ferrite Nanoparticles. Ferromagnetism of un-doped BFO was enhanced by Ni substitution. Microwave characterization was done in co-planar waveguide (CPW) geometry both in field sweep and frequency sweep mode. BiFe1-xNixO3 nanoparticles were deposited using electrophoretic deposition method (EPD) on top of CPW to do the FMR experiments. The operating frequency of the device was tuned by application of magnetic field (H) over a wide range (5 to 20 GHz) with a field up to 8 kOe. Copyright © 2016 VBRI Press Keywords: Multiferroic nanoparticle, ferromagnetic resonance, auto combustion method, super-exchange interaction, kittel equation.

Introduction Multiferroic nature in a single material is of great interest due to its simultaneous control by electric and magnetic field. Many oxides, fluorides and their alloys are currently showing multiferroicity [1]. But for device applications more emphasis has been laid on room temperature Multiferroic [2, 3]. Very few room temperature multiferroics are available and hence magneto electric devices developed so far are very rare or under research. The most effective candidate in this list is BiFeO3(BFO) which is a multiferroic at room temperature and have the Neel temperature of 653K and ferroelectric curie temperature of 1100K [4,5]. BFO is antiferromagnetic with G-type ordering [5]. BFO structure is already studied in very detail and classified as rhombohedral in Space group R3c [6]. Many research groups are working on its different applications in wide range of areas. Kundys et.al studied the magneto-photo-striction in BFO single crystal [7], Zhao et.al used BFO as a nano-capacitor [8], Nayek et.al showed BFO as electro-optic material [9], Yang et.al showed BFO as a photovoltaic material [10], Bibes et.al used BFO as MERAM device [11]. Instead of all above reports it is worth mentioning that a very few reports have been devoted to its application in High frequency or radio

Copyright © 2016 VBRI Press

frequency (RF) devices. Recently Yong li et.al showed ‘A’ site doping of BiFeO3 (ABO3) with La and Nd and studied its electromagnetic waves absorbing properties [12, 13]. Still there is a lack of detailed analysis in electromagnetic wave propagation in BFO and RF device applications of doped BFO Nanoparticles. In BFO, ‘A’ site is mainly responsible for ferroelectric property and ‘B’ site is devoted to weak ferromagnetism. Hence, instead of ‘A’ site doping we have doped ‘B’ site with Nickel (Ni) to increase its magnetic property which leads to its high frequency applications. ‘B’ site doped BFO Nanoparticles also exhibit high efficiency towards RF wave absorption because of its good dielectric and magnetic properties. Kang et.al found that BFO nanoparticles (NPs) showed excellent EM wave attenuation due to its good EM matching [14]. Here we demonstrate the BiFe(1-x)NixO3(BFNO) NPs with x=0.025 and 0.05 as good RF waves absorber and also worked as a microwave frequency filters with magnetic field tunability. BFNO NPs were synthesized by auto combustion route with high crystalline nature and size. X-ray diffraction (XRD) confirms the pure phase polycrystalline nature of NPs and also the shifting in characteristic peak confirms the efficient doping of Ni at ‘B’ site. Transmission Electron Microscopy (TEM) images show crystalline and uniformly distributed NPs with change in ‘d’

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spacing as Ni doping increases. Vibrating Sample Magnetometer (VSM) characterization confirms the increase in magnetic properties of NPs with the increase of Ni doping. For RF applications, we have done magnetic field dependent ferromagnetic resonance (FMR) spectroscopy both in frequency and field sweep mode. Electrophoretically deposited BFNO nanoparticles coated CPW was designed as a device under test (DUT). It comes out to be very cheap and is easy to handle. BFNO NPs were successfully deposited on signal line of coplanar waveguide (CPW) and connected to a Vector Network Analyzer (VNA) to check the systematic changes in S-parameters. We mainly concentrate on S21 parameter in field sweep and frequency sweep mode in a wide frequency range. In field sweep mode S21 was measured at a constant frequency (5 to 32 GHz) and sweeping the magnetic field from 0 to 16 kOe. In device application like a microwave filter, S21 was swept from 5 to 25 GHz at a constant magnetic field and tunability of the filter was observed to be from 5 to 20 GHz. Thus it is shown here that the device can be operated in a wide frequency range by tuning the magnetic field.

The mixture was again stirred for 30 mins at 60 oC. After getting a dark brownish gel without any precipitate, mixture was initially dried at 120 0C to remove the moisture and then the dried powder was ignited at 250oC for fusion. Glycine was added as a fuel in the mixture to fuse metal salts with each other. After complete ignition the dried powder was crushed and calcined at 600oC for 2 hours in air to get crystalline BFNO NPs. Fig. 1 shows the complete scheme and chemical reactions involved in the synthesis process.

Experimental

Bi(NH2CH2COO)3 +Fe(NH2CH2COO)3 +Ni(NH2CH2COO)2

Chemical Reactions Following Chemical reactions are involved during synthesis of BFNO NPs:Bi3+ + NO3-

Bi(NO3)3.5H2O Fe(NO3)3.9H2O

Fe3+ + NO3-

On Hydrolysis

Ni(NO3)2.6H2O

Ni2+ + NO3-

(NH2CH2COOH)

NH2CH2COO- + H+

(Glycine)

Bi3+ +(1-x)Fe3+ +(x)Ni2+ +NH2CH2COO-

Heat at 60o C

(Metal acetates)

Metal Acetates

Materials

Synthesis Method Auto combustion route was employed for the synthesis of BFNO NPs. All the precursors were mixed in an Alumina crucible without using any solvent and stirred with a spatula till all of them make a dark brownish gel at 60oC. After dissociation of all the hydrated salts, Glycine was added in the mixture in the ratio of 2:1 with metal salts.

Dried at 120o C

Bi2O3 + Fe2O3 + NiO

Metal Oxides

Ignite at 250o C BiFe(1-x)NixO3

Bi2O3 + Fe2O3 + NiO

High purity A.R grade materials i.e Bi(NO3)3.5H2O(99.5%), Fe(NO3)3.9H2O(99.5%) and Ni(NO3)2.6H2O(99%) (Sigma Aldrich, USA) were mixed in the ratio of 1:(1-x):x respectively, where x=0,0.025,0.05.

Bi(NH2CH2COO)3 + Fe(NH2CH2COO)3 + Ni(NH2CH2COO)3+ H2O

Metal Oxides

Calcine at 600o C

BFNO NPs

Device Designing For Microwave characterization, electrophoretic deposition of BFNO NPs was done on the signal line of the CPW. At first, 20mg of NPs was sonicated in 20 ml ethanol for 30 mins and then 100µl of PVA and 100µl of Mg ions solution (5mg/ml) was added in the mixture. A 60 Volts DC was applied to the 2 electrode system with Pt used as counter electrode and copper coated FR-4 substrate made CPW as working electrode. Electrophoresis was done for 10 mins to completely fill the signal line of CPW. 50Ω matched end launch connectors from Southwest Microwave, (1.85mm; 67GHz) were connected to the CPW. Then the device was connected to the Vector Network Analyzer (VNA) to perform the FMR measurements. Characterization Techniques

Fig.1. Schematic of auto combustion route for synthesis of BFNO NPs and chemical reactions involved.


We have used various techniques vide; (i) XRD (Miniflex Rigaku) was done to check the crystallinity nature of BFNO NPs. (ii) Vibrating Sample Magnetometry (VSM) (PPMS, Cryogenics Ltd.) was employed to check their magnetic properties. (iii) The morphology and microstructure characterization were done using Transmission Electron Microscopy (JEOL, 2100F Japan). (iv) Magnetic nature of NPs was examined using Physical Property measurement system (PPMS, Cryogenics Pvt. Ltd.). (v) Microwave characterization was done in 2 ports measurements

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using a Vector Network Analyzer (Keysight Tech. Model No - N5224A) in transmission mode.

Results and discussion XRD was done using Rigaku Miniflex diffractometer at room temperature. Fig. 2 shows the XRD graph of BFO and BFNO NPs. It is found that BFO crystallites in pure phase are rhombohedral perovskite with R3c space group and lattice parameters a=b=5.5744 Å and c=13.8568 Å. Peaks at (104) and (110) are clearly separated in the BFO sample as shown in XRD (inset to Fig.2). On increasing the Ni concentration, all the doublets appear merging to give a single peak. This is clearly visible in BiFe1-xNixO3 (x=0, 0.025, 0.05) [15]. These results correspond to the lattice distortion in rhombohedral structure of BFO with the increase in the substituted element concentration. This leads to change from rhombohedral to orthorhombic crystal phase transition. As shown in the insert, nickel doped BFO peaks shift towards left with respect to BFO, which indicates the increment in lattice parameters. Compared with the lattice constant of BiFeO3 prepared under the normal calcination process, lattice parameters of normal calcined BFNO sample were found to be increased. This may be due to the large ionic radius of Ni2+ (∼0.74 Å) in comparison with Fe3+ (∼0.69 Å) [16]. Due to the slight variation of atomic radius, strain would be generated in the matrix which may be the cause for slight reduction of crystallite size in Ni-doped sample. The crystallite size has been calculated using Scherrer’s formula by the Gaussian fits to the observed maximum intensity X-ray diffraction peak. The development of lattice strain inside the lattice due to ionic size mismatch between Ni2+(0.70) and Fe3+(0.645) lead to local structural disorder. This reduces the rate of nucleation and hence decreases the crystallite size [17]. Addition of Ni2+ leads to suppression of oxygen vacancy leading to lowering of conductivity in the sample. The polarization effect contributes towards better ferroelectricity in the doped sample [18].

Fig.2: XRD plots of BFNO NPs. Inset in figure shows the shift in characteristic peak (highest intensity peak) and size distribution with Ni concentration.


Advanced Materials Proceedings Fig. 3 shows the TEM image of BFNO NPs which confirms the highly polycrystalline nature of the particles. As the ionic radii of Ni2+ (∼0.74 Å) is higher than the Fe3+ (∼0.69 Å), it leads to the strain generated in the perovskite system and hence there is an increase in the lattice parameter as shown in HRTEM image in Fig.3 [19]. SAED pattern clearly shows the highly polycrystalline nature of NPs with rhombohedral phase formation. With increase of Ni Concentration, the pattern shifts more toward orthorhombic phase. (a)

(b)

(c)

(d)

(e)

(f)

Fig.3: (A, C and E) shows the HR-TEM images of BFNO NPs with different Ni concentration and (B, D and F) shows SAED pattern of respective NPs. [A, B (BiFeO3); C, D (BiFe0.975Ni0.025O3); E, F (BiFe0.95Ni0.05O3)]

Magnetic characterization was done using VSM setup at room temperature. Fig. 4 clearly shows the ferromagnetism in the magnetization (M) vs. magnetic field (H)(M-H) curves at room temperature.

Fig.4 Magnetization (M) vs. Magnetic field (H); M-H curve at room temperature of BFO and BFNO NPs.

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As can be seen Ni2+ concentration increases the magnetization value in BFO (from 0.306 emu/gm for BFO to 2.707 emu/gm for x=5% BFNO), approximately an order of magnitude. There was almost negligible moments observed in BFO, which is due to the canted antiferromagnetic spins in BFO. As Ni doping increases in BFO we observed larger moments which is due to the release of cycloid spin structure in BFO. This indicates the formation of magnetic secondary phases in BFNO system like Bi25FeO40 [20]. Substitution of Ni2+ ions leads to the super-exchange interaction of Ni and Fe atoms and hence increases the saturation magnetization [21]. The secondary magnetic phases can be easily seen in XRD pattern of BFNO which are more ferromagnetic than BFO. Table 1 shows the ensemble of magnetic properties of BFO and BFNO NPs.

Advanced Materials Proceedings higher for these NPs. Fig. 6 shows ensemble of resonance field (Hr) and FMR line width (ΔH) data. Both Hr and ΔH clearly shows the increasing behavior with increasing frequency.

Table 1. Saturation Magnetisation (Ms), Remanence ratio (Mr) and Coercive field (Hc) of BFO and BFNO nanoparticles.

Nanoparticles BiFeO3 BiFe0.975Ni0.025O3 BiFe0.95Ni0.05O3

Ms(emu/gm) 0.3 2.32 2.7

Mr(emu/gm) 0.03 0.45 0.6

Hc(Oe) 37.3 115.5 125.6

The main concern in the magnetization of BFO is towards the d-orbitals, when a Fe3+ ion is present at the centre of an oxygen octahedron, the degenerated orbital states get split down to t2g triplet and eg doublet. It is expected that magnetic moment of a Fe3+ ion is interacting with six neighboring local spin via the exchange interaction. This type of effective super exchange interaction strengthens the magnetization in BFO even though it has G-type antiferromagnetic order [22]. Microwave characterization was done using FMR spectroscopy. BFNO NPs were deposited on CPW device as stated earlier and connected to the VNA to do the microwave measurements both in frequency and field sweep mode. Room temperature fixed frequency FMR measurements were performed in field sweep mode. In these experiments we fix the microwave frequency from the VNA and sweep the applied dc magnetic field through a microprocessor controlled electro-magnet power supply from zero to 16 kOe magnetic field. The S21 parameters for each dc magnetic field were recorded simultaneously from the VNA and gauss meter, respectively, using a PC recorder programme. Fig. 5 shows the FMR field sweep data for BFNO NPs with x=0.025 and 0.05. The exact resonance field (Hr) and field linewidth (ΔH) was obtained from Lorentzian fits to the S21 experimental data. These graphs clearly states that as the operating frequency increases the resonance field (Hr) increases linearly, which is accordance with the kittel equation [23]. It is observed that all the spectra have symmetrical and reasonably narrow linewidth, especially below 15 GHz frequency range. Above 15 GHz, there is asymmetric behaviour of the FMR spectra at higher field side. As can be seen from Fig. 5, microwave absorption is high in x=0.05 NPs (~ 16 dB at 32 GHz), because magnetization value is


Fig.5 Field sweep FMR graphs for BFNO NPs with x=2.5% and 5%.

The FMR resonance field increases with the increase in operating frequency, for both BFNO particles. At a fixed frequency, say 20 GHz, the 2.5% Ni substituted BFNO NPs resonate at slightly lower field than the 5% Ni substituted BFNO NPs. This is because 2.5% BFNO NPs has slightly lower saturation magnetization than the 5% BFNO NPs. Considering spherical shape of the nanoparticles, the ferromagnetic resonance field is given by; (1) where,  denotes the Larmor precession frequency,  is the gyromagnetic ratio. The effective field, Heff is given by: (2) Heff have several contributions, namely the demagnetizing field (HD), the anisotropy field (HK), and the interaction field (Hint). The solid lines in Fig. 6 (top panel) show the theoretical fittings to eq. (1). The trends in the change in resonance field versus frequencies for different x values are similar to that as observed in the experiment. The resonance field showed a linear dependence with applied frequency. This linear dependence suggests that the susceptibility and hence the magnetization of the particles are sufficiently low. It is observed that the internal field which includes the anisotropy field increases from 0.18 kOe for x=0.025 Ni content to 0.23 kOe for x = 0.05 Ni content in BFNO. In this case the effective field includes only the demagnetization field. The value of saturation magnetization obtained from the fits are very close to VSM data and are also observed to increase with

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increase in Ni content in the BFO NPs. The value of gyromagnetic ration is almost same (little higher for x=0.025) for both the NPs samples.

Advanced Materials Proceedings the whole frequency band. The stop-band suppression is > -10 dB/cm in the high field range (> 5 kOe).

Conclusion In this research we have successfully synthesized BFNO NPs with x=0, 0.025, 0.05 using auto combustion method. XRD and TEM characterization confirms the pure phase polycrystalline nature of NPs. XRD graph suggest that as Ni concentration increases the BFO shifts from rhombohedral to orthorhombic phase and lattice parameter increases linearly. M-H loop shows the enhanced magnetic properties in BFNO NPs as compared to BFO, which are due to super exchange interaction between Ni and Fe atoms. Microwave characterization of BFNO NPs was governed by FMR spectroscopy. The application of BFNO NPs was demonstrated with magnetic field tunability. A microwave frequency band-stop filter has been fabricated using BFNO NPs that can be tuned over 5 to 20 GHz frequency range. Microwave characterization shows that BFNO NPs are good candidate for electro-magnetic wave absorption.

Fig. 6 Change in FMR line width (ΔH) (lower panel) and resonance field (Hr) (top panel) with microwave frequency.

In frequency sweep mode the EPD deposited CPW worked as microwave band-stop filter which can easily be tuned with magnetic field. Fig. 7(a) shows the frequency swept FMR curves for BFNO deposited NPs, in which resonance frequency increases with increase in magnetic field (Fig. 7(b)). Frequency swept FMR experiments were done for x=5% BFNO NPs. The figure clearly shows that the device can very well work as a tunable band-stop filter with magnetic field tuning. A microwave signal propagating in the CPW based device interact with the magnetic NPs and transfer electromagnetic energy to them, thus attenuating the signal at FMR. The attenuation increased from -2 dB to -8 dB when the magnetic fields increase from 0.76 kOe to 6.5 kOe. The tunability of the device is shown in Fig. 7(b).

Acknowledgements We acknowledge the AIRF, JNU for providing the VSM facilty for doing magnetization measurement. We are also thakful to SPS, JNU for providing the facility of XRD. One of the author (VS) is thankful to UGC India for providing financial assistance.

Author’s contributions P.R synthesized the NPs. V.S done all the characterization of the samples and writes the manuscript. B.K.K helps in analyzing the magnetic and microwave results. Authors have no competing financial interests. References 1. 2. 3. 4. 5. 6.

(a) (b)

7. 8. 9. 10. 11. 12. 13. Fig7. (a) Frequency sweep FMR curves & (b) change in resonance frequency with magnetic field for x= 5% Ni.

As seen above (Fig. 7(b)), the operating frequency was tuned by application of magnetic field (H) over a wide range (5 to 20 GHz) with a field up to 6.5 kOe. These result in a tunability of 2.5 GHz/kOe with a band-width >3 GHz. The pass-band insertion loss is higher (~ 3 dB) and return Loss (S11) is > -12 dB over


14. 15.

16. 17. 18.

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