Srinivasa Rao Singamaneni

Ferroelectric and ferromagnetic properties in BaTiO3 thin films on Si (100)

In this paper, we report on the epitaxial integration of room temperature lead-free ferroelectric BaTiO3 thin (∼1050 nm) films on Si (100) substrates by pulsed laser deposition technique through a domain matching epitaxy paradigm. We employed MgO and TiN as buffer layers to create BaTiO3/SrRuO3/MgO/TiN/Si (100) heterostructures. C-axis oriented and cube-on-cube epitaxial BaTiO3 is formed on Si (100) as evidenced by the in-plane and out-of-plane x-ray diffraction, and transmission electron microscopy. X-ray photoemission spectroscopic measurements show that Ti is in 4(+) state. Polarization hysteresis measurements together with Raman spectroscopy and temperature-dependent x-ray diffraction confirm the room temperature ferroelectric nature of BaTiO3. Furthermore, laser irradiation of BaTiO3 thin film is found to induce ferromagnetic-like behavior but affects adversely the ferroelectric characteristics. Laser irradiation induced ferromagnetic properties seem to originate from the creation of oxygen vacancies...

Magnetic properties of BaTiO3/La0.7Sr0.3MnO3 thin films integrated on Si(100)

Two-phase multiferroic heterostructures composed of room-temperature ferroelectric BaTiO3 (BTO) and ferromagnetic La0.7Sr0.3MnO3 (LSMO) epitaxial thin films were grown on technologically important substrate Si (100). Bilayers of BTO/LSMO thin films display ferromagnetic Curie transition temperatures of ∼350 K, close to the bulk value, which are independent of BTO films thickness in the range of 25–100 nm. Discontinuous magnetization jumps associated with BTO structural transitions were suppressed in M(T) curves, probably due to substrate clamping effect. Interestingly, at cryogenic temperatures, the BTO/LSMO structure with BTO layer thickness of 100 nm shows almost 2-fold higher magnetic coercive field, 3-fold reduction in saturation magnetization, and improved squareness compared to the sample without BTO. We believe that the strong in-plane spin pinning of the ferromagnetic layer induced by BTO layer at BTO/LSMO interface could cause such changes in magnetic properties. This work forms a significant ste...

Complete vertical M-H loop shift in La0.7Sr0.3MnO3/SrRuO3 thin film heterostructures

In the current work, we have epitaxially integrated La0.7Sr0.3MnO3/SrRuO3 (LSMO/SRO) BLs with the technologically important substrate Si (100) using pulsed laser deposition. Interestingly, at 4 K, under the magnetic field sweep of ±1500 Oe, a complete vertical M-H loop shift is observed in the sample prepared with 180 nm SRO thickness, which is unusual. This vertical shift persists even up to a field sweep range of ±6000 Oe, at which point the shift disappears and a symmetrical hysteresis loop centered at the origin is observed. In contrast, at the same temperature, under the same field sweep range, we observe a normal M-H loop (no or little vertical shift) from the sample with 45 nm SRO thickness. In both the cases, the LSMO thickness was held constant at ∼100 nm. It appears that SRO moment is frozen in place in the latter case, providing a clear demonstration of the effect that biasing layer (SRO) thickness can have on the magnetic characteristics of bilayer films. We attribute this vertical shift to th...

Intrinsic and extrinsic defects in a family of coal-derived graphene quantum dots

In this letter, we report on the high frequency (239.2 and 336 GHz) electron spin resonance (ESR) studies performed on graphene quantum dots (GQDs), prepared through a wet chemistry route from three types of coal: (a) bituminous, (b) anthracite, and (c) coke; and from non-coal derived GQDs. The microwave frequency-, power-, and temperature-dependent ESR spectra coupled with computer-aided simulations reveal four distinct magnetic defect centers. In bituminous- and anthracite-derived GQDs, we have identified two of them as intrinsic carbon-centered magnetic defect centers (a broad signal of peak to peak width = 697 (10−4 T), g = 2.0023; and a narrow signal of peak to peak width = 60 (10−4 T), g = 2.003). The third defect center is Mn2+ (6S5/2, 3d5) (signal width = 61 (10−4 T), g = 2.0023, Aiso = 93(10−4 T)), and the fourth defect is identified as Cu2+ (2D5/2, 3d9) (g⊥ = 2.048 and g‖ = 2.279), previously undetected. Coke-derived and non-coal derived GQDs show Mn2+ and two-carbon related signals, and no Cu2+...

Multifunctional epitaxial systems on silicon substrates

Multifunctional heterostructures can exhibit a wide range of functional properties, including colossal magneto-resistance, magnetocaloric, and multiferroic behavior, and can display interesting physical phenomena including spin and charge ordering and strong spin-orbit coupling. However, putting this functionality to work remains a challenge. To date, most of the work reported in the literature has dealt with heterostructures deposited onto closely lattice matched insulating substrates such as DyScO3, SrTiO3 (STO), or STO buffered Si(100) using concepts of lattice matching epitaxy (LME). However, strain in heterostructures grown by LME is typically not fully relaxed and the layers contain detrimental defects such as threading dislocations that can significantly degrade the physical properties of the films and adversely affect the device characteristics. In addition, most of the substrates are incompatible with existing CMOS-based technology, where Si (100) substrates dominate. This review discusses recent advances in the integration of multifunctional oxide and non-oxide materials onto silicon substrates. An alternative thin film growth approach, called “domain matching epitaxy,” is presented which identifies approaches for minimizing lattice strain and unwanted defects in large misfit systems (7%–25% and higher). This approach broadly allows for the integration of multifunctional materials onto silicon substrates, such that sensing, computation, and response functions can be combined to produce next generation “smart” devices. In general, pulsed laser deposition has been used to epitaxially grow these materials, although the concepts developed here can be extended to other deposition techniques, as well. It will be shown that TiN and yttria-stabilized zirconia template layers provide promising platforms for the integration of new functionality into silicon-based computer chips. This review paper reports on a number of thin-film heterostructure systems that span a variety of ferroelectric, multiferroic, magnetic, photocatalytic, and smart materials. Their properties have been extensively investigated and their functionality found to be comparable to films grown on single-crystal oxide substrates previously reported by researchers in this field. In addition, this review explores the utility of using laser processing to introduce stable defects in a controlled way and induce magnetism and engineer the optical and electrical properties of nonmagnetic oxides such as BaTiO3, VO2, NiO, and TiO2 as an alternative for incorporating additional magnetic and conducting layers into the structure. These significant materials advancements herald a flurry of exciting new advances in CMOS-compatible multifunctional devices.

Ferroelectric and magnetic properties of multiferroic BiFeO3-La0.7Sr0.3MnO3 heterostructures integrated with Si (100)

We report on the electrical, ferroelectric, and magnetic properties of BiFeO3 (BFO)-La0.7Sr0.3MnO3 heterostructures deposited epitaxially onto Si(100) substrates. Temperature dependent (200–350 K) current-voltage (I-V), switching spectroscopy piezo-response force microscopy (SSPFM), and temperature dependent (5–300 K) anisotropic magnetization measurements have been performed. The BFO (100-nm thick)-based device structures were fabricated with a 250 nm thick La0.7Sr0.3MnO3 bottom electrode and 200 μm circular top Pt electrodes. I-V measurements performed at various temperatures indicated that the devices retained their as-deposited characteristics and exhibited non-leaky behavior up to at least 50 cycles. The temperature-dependent measurements showed clear diode-like behavior and resistive (hysteretic) switching behaviour. Characteristic butterfly loops (of several cycles) were observed in the PFM amplitude signals of the BFO film. In addition, the phase signal indicated a clear (180°) switching behavior ...

Magnetic defects in chemically converted graphene nanoribbons: electron spin resonance investigation

Electronic spin transport properties of graphene nanoribbons (GNRs) are influenced by the presence of adatoms, adsorbates and edge functionalization. To improve the understanding of the factors that influence the spin properties of GNRs, local (element) spin-sensitive techniques such as electron spin resonance (ESR) spectroscopy are important for spintronics applications. Here, we present results of multi-frequency continuous wave (CW), pulse and hyperfine sublevel correlation (HYSCORE) ESR spectroscopy measurements performed on oxidatively unzipped graphene nanoribbons (GNRs), which were subsequently chemically converted (CCGNRs) with hydrazine. ESR spectra at 336 GHz reveal an isotropic ESR signal from the CCGNRs, of which the temperature dependence of its line width indicates the presence of localized unpaired electronic states. Upon functionalization of CCGNRs with 4-nitrobenzene diazonium tetrafluoroborate, the ESR signal is found to be 2 times narrower than that of pristine ribbons. NH3 adsorption/d...

Diamagnetism to ferromagnetism in Sr-substituted epitaxial BaTiO3 thin films

We report on the ferromagnetic-like behavior in otherwise diamagnetic BaTiO3 (BTO) thin films upon doping with non-magnetic element Sr having the composition Ba0.4Sr0.6TiO3 (BST). The epitaxial integration of BST (∼800 nm) thick films on Si (100) substrate was achieved using MgO (40 nm) and TiN (20 nm) as buffer layers to prepare BST/MgO/TiN/Si (100) heterostructure by pulsed laser deposition. The c-axis oriented and cube-on-cube epitaxial BST is formed on Si (100) as evidenced by the in-plane and out-of-plane X-ray diffraction. All the deposited films are relaxed through domain matching epitaxy paradigm as observed from X-ray diffraction pattern and A1TO3 mode (at 521.27 cm−1) of Raman spectra. As-deposited BST thin films reveal ferromagnetic-like properties, which persist up to 400 K. The magnetization decreases two-fold upon oxygen annealing. In contrast, as-deposited un-doped BTO films show diamagnetism. Electron spin resonance measurements reveal no evidence of external magnetic impurities. XRD and X...

Polarized neutron reflectivity studies on epitaxial BiFeO3/La0.7Sr0.3MnO3 heterostructure integrated with Si (100)

This work reports polarized neutron reflectivity (PNR) measurements performed using the Magnetism Reflectometer at Oak Ridge National Laboratory on epitaxial BiFeO3(BFO)/La0.7Sr0.3MnO3(LSMO)/SrTiO3(STO)/MgO/TiN heterostructure deposited on Si (100) substrates. By measuring the angular dependence of neutrons reflected from the sample, PNR can provide insights on interface magnetic spin structure, chemical composition and magnetic depth profiles with a nanometer resolution. Our first analysis of nuclear scattering length density (NSLD) and magnetic scattering length density (MSLD) depth profiles measured at 4 K have successfully reproduced most of the expected features of this heterostructure, such as the NSLD for the Si, TiN, MgO, STO, LSMO layers and remanent magnetization (2.28μB/Mn) of bulk LSMO. However, the SLD of the BFO is decreased by about 30% from the expected value. When 5 V was applied across the BFO/LSMO interface, we found that the magnetic moment of the LSMO layer could be varied by about 15...

Magneto-optical properties of BaTiO3/La0.76Sr0.24MnO3/BaTiO3 heterostructures

The magnetic properties of epitaxial BaTiO3/La0.76Sr0.24MnO3/BaTiO3 (BTO/LSMO/BTO) heterostructures have been studied using magneto-optic Kerr effect (MOKE) technique. Both longitudinal and polar MOKE were probed as a function of magnetic field and temperature (in the range between 80 and 320 K) for epitaxial films of BTO/LSMO/BTO and LSMO grown on TiO2-terminated SrTiO3 (001) substrates by pulsed laser deposition technique. The LSMO film without the BTO layers exhibits nearly square field-dependent MOKE hysteresis loops with low saturation fields below a bulk-like Curie temperature (TC) of ∼ 350K. In contrast, the film with the BTO layers exhibits a significantly suppressed TC of 155 K, accompanied by significantly enhanced coercive fields and perpendicular magnetic anisotropy.