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(BZT-BCT/LSMO/LAO) Devices for Multiferroic Applications

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(BZT-BCT/LSMO/LAO) Devices for Multiferroic Applications 1 Structural, Electronic, and Magnetic Analysis and Device Characterization of Ferroelectric-Ferromagnetic Heterostructure (BZT-BCT/LSMO/LAO) Devices for Multiferroic Applications Md Abdullah-Al Mamun1,2 Ariful Haque1,3 Anthony Pelton1, Bithi Paul1, and Kartik Ghosh1 Abstract— Ferroelectricity and ferromagnetism have devices, high sensitivity magnetic field sensors, actuators, etc been investigated in a lead-free 0.5Ba(Zr0.2Ti0.8)O3- [2], [3]. Over the years, researchers have been trying to 0.5(Ba0.7Ca0.3)TiO3(BZT-BCT)/La0.7Sr0.3MnO3(LSMO) combine suitable electrical and magnetic properties in a single heterostructure for multiferroic applications. The BZT-BCT thin film has been grown on LSMO/Lanthanum Aluminate, material to yield new MF materials with desirable performance LaAlO3 (LAO) by pulsed laser deposition (PLD). Prior to [4]. Until now, the most reported MF material is BiFeO3(BFO) that, the LSMO layer was deposited on a single-crystal LAO which displays the coexistence of spontaneous electric and substrate by PLD. The epitaxial growth of the (001) oriented magnetic ordering in the same phase at room temperature films was confirmed by X-ray Diffraction (XRD) analysis. The [2], [5]. However, the limitations of BFO, such as high small value of the full width at half maximum of the rocking curve peak (0.1°) performed about (002) plane of the BZT-BCT leakage current and antiferromagnetism at room temperature, film indicates an out-of-plane orientation of the film. The must be overcome for commercial applications. Researchers polarization switching behavior in the heterostructure device have investigated some other materials, including YMnO3[6], ∼ C cm2 was observed with a remnant polarization of 47 µ / TbMnO3[5], DyMnO3[7], and hexaferrites [8] with limited ∼ kV cm and a coercive field of 180 / at an applied voltage of success. Some researchers also doped rare earth (RE) elements 5V. The frequency-dependent relative dielectric constant varies in-between 5100 and 4900 in the frequency range from 1 kHz and transition metal ions into BFO to enhance the MF proper- to 50 kHz during the dielectric measurements of the fabricated ties [9]. But the RE element as a dopant degrades the ferroelec- device. The observed low value of the dielectric loss (0.02) tric properties [10]. In addition, there are several reports on MF confirms the outstanding quality of the ferroelectric device. A properties of composite materials or heterostructure systems M − H well-saturated room temperature curve, with a coercive such as BFO/CoFe [11] and BFO/CoFeB [12]. However, a field of ∼1200 A/m and a remnant magnetization of ∼110 kA/m, was observed in the LSMO/LAO system indicating the system has yet to be experimentally demonstrated that can be ferromagnetic behavior of the film. The temperature-dependent electrically and magnetically tunable. magnetization of the LSMO film exhibits a ferromagnetic In this study, we have used heterostructure of ferroelectric ∼ to paramagnetic transition at 360 K. These results on all lead (Pb)-free 0.5Ba(Zr0.2Ti0.8)O3-0.5(Ba0.7Ca0.3)TiO3(BZT- solid-state ferroelectric-ferromagnetic heterostructure using BCT) and ferromagnetic La Sr MnO (LSMO) to investi- BZT-BCT and LSMO open viable possibilities for multiferroic 0.7 0.3 3 applications. gate MF properties. BZT-xBCT is a solid solution of rhombo- hedral (R3m) BZT and tetragonal (P4mm) BCT near x = 0.5 Index Terms — Dielectric losses, Ferroelectric capacitor, Leak- with a morphotropic phase boundary (MPB). Fig. 1 shows age currents, Magnetoelectric coupling, Polarization switching. the unit cell structure of BZT-xBCT. Recently, Liu and Ren reported that the bulk ceramics of a Pb-free BZT-xBCT system I. INTRODUCTION has a high piezoelectric coefficient (d33 ∼ 620 pC/N) near the ULTIFERROIC (MF) materials simultaneously exhibit MPB [13]. Their reported piezoelectricity is comparable to that Mferroelectricity and ferromagnetism in the same phase, of Lead Zirconate Titanate (PZT) and even it is superior to that and they can provide desired magnetoelectric coupling of existing Pb-free systems. The improved dielectric, piezo- between the order parameters [1]. Materials with these kind of electric and ferroelectric properties are due to the MPB start- properties have applications in different solid-state fields such ing from a tetragonal-cubic-rhombohedral triple point, which as magnetic data storage, solid-state transformers, spintronic flattens the energy barrier for lattice distortion and polarization rotation from (001)T state to (111)R state [14]. On the other 1Department of Physics, Astronomy, and Materials Science, Missouri State hand, the ferromagnetic LSMO is a mixed-valence manganite, University, Springfield, MO 65897, USA which is an optimal source of fully spin-polarized carriers and 2 Department of Electrical Engineering, University of South Carolina it shows a rich physics of magnetic phases and transport mech- Columbia, SC 29208, USA 3Department of Materials Science and Engineering, North Carolina State anisms [15]. The LSMO shows high-spin polarization due to University, Raleigh, NC 27606, USA its half-metallic nature arising from conducting electrons [15]. Digital Object Identifier: 10.1109/TMAG.2018.2873513 1941-0069 c 2018 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications standards/publications/rights/index.html for more information. 2 of 248 nm, a pulse duration of 20 ns, was used with 10 Hz rep-rate for the deposition. Thin films were grown at different growth temperatures (600°C to 850°C) and different oxygen pressures (10 N/m2 to 10−2 N/m2). The best quality films were deposited at 800°C under 40 N/m2 oxygen pressure. The base pressure of the chamber was below 1×10−3 N/m2. After the desired number of PLD shots, the deposited film was cooled down to room temperature maintaining the oxygen pressure at 1.1×104 N/m2. In this study, the thicknesses of BZT-BCT and LSMO thin films were ∼ 95 nm and ∼ 89 nm, respectively. The thicknesses were measured by a profilometer (Veeco, Dektak 150). Fig. 1. The unit cell structure of BZT-xBCT (tetragonal). B. Sample Characterizations The PLD grown thin films were characterized by X-ray Diffractometer (Bruker, D8 Discover) using θ-2θ scan in the The high-spin feature makes LSMO reliable for applications range of 20° to 80° maintaining the Bragg−Brentano reflection in spin-dependent transport devices [16]. We used LAO as a geometry. A CuKα x-ray source with the wavelength of 1.54 substrate for this heterostructure system for its low dielectric Å was used as an excitation source. The vibrational properties loss [17] and small lattice mismatch with LSMO which is only of the samples were characterized by micro-Raman scattering around −2.1% [18]. In this study, the ferroelectric BZT-BCT (Horiba Labram Raman-PL System) with a 532 nm green thin film, grown by pulsed laser deposition (PLD), shows a laser. The measurements were performed in a back-scattering high degree of polarization with low leakage current while the geometry with 15 seconds exposure time, and 20 accumulation ferromagnetic LSMO continues to show its ferromagnetism cycles. The illuminated spot size was approximately 2.5 µm up to 360K. Combination of such magnificent properties in diameter on the thin film. We were aware of the possible of those individual layers makes this heterostructure system damage of the thin film due to the excitation source. During more directly amenable to fabricating high performance MF Raman measurements the laser energy was chosen in such devices for practical applications. The top ferroelectric thin a way that the sample did not get damaged due to the film is attributed to bring desirable changes in the underlayer overheating produced by the continuous solid-state laser beam. ferromagnetic properties. Details of the structural property cor- The data was collected through an NGS Labspec-5 software in relation of the BZT-BCT/LSMO/LAO heterostructure system the range of 100 cm−1 to 1000 cm−1. The data were analyzed and the device characterization will be subsequently discussed by Gaussian-Lorentzian peak fitting using Origin Pro 8.5.1. in the results and discussion section. Temperature and magnetic field dependent magnetization of the films were characterized by a Superconducting Quan- II. EXPERIMENTAL DETAILS tum Interference Device (SQUID) magnetometer (Quantum A. Sample Preparation Design, MPMS 5XL) interfaced through MultiVu software. The temperature was varied from 5K to 370K. The M − The ferroelectric BZT-BCT ceramic target was prepared by H hysteresis loop of the sample was observed by varying a standard solid-state reaction using high purity chemicals, i.e., magnetic field from -400 kA/m to +400 kA/m. The maximum BaCO (99.9%, Inframat Advanced Materials), CaO (99.95%, 3 sensitivity of the magnetometer was 10−12 Am2. Polarization Alfa Aesar), TiO (99.9%, Sigma-Aldrich), and ZrO (99.9%, 2 2 measurement was carried out by Sawyer-Tower technique. Au- Inframat Advanced Materials), in appropriate proportions. Ti top electrodes were deposited onto the thin film by thermal The agglomeration of the powder particles was prevented by evaporation using a physical mask. The diameter of the elec- adding Polyvinyl alcohol to the mixture. The final mixer was trodes was 100 to 200 µm. The relative dielectric constant and pressed with a ‘hydraulic press’ to form the target. After the loss tangent were measured using a precision LCZ meter that, the prepared target was calcined at 1350°C and then (Hewlett Packard, 428-4A). The data were analyzed through sintered at 1450°C in the air. On the other hand, a high Origin Pro 8.5.1 software. purity (99.9 %) dense LSMO target was purchased from Kurt J. Lesker Company. A chemical mechanical polished (CMP) III. RESULTS AND DISCUSSION highly oriented (001) LAO substrate was purchased from MTI Corporation.
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