Prahallad Padhan

Strong anisotropy is observed in the magnetization and magnetoresistance (MR) of a series of La0.7Sr0.3MnO3 (LSMO) films grown on (001) oriented Si using pulsed sputtering plasma. Magnetic anisotropy (MA) of LSMO films is in the order of 106 erg/cm3, comparable to or larger than that of the other heterostructures. Analysis using the Stoner-Wohlfarth model and the density functional theory (DFT) indicates that the crystalline and induced anisotropies are almost compensating with each other, and observed MA is of the same order of shape anisotropy. The rise in MA with the LSMO thickness is attributed to the shape anisotropy and interfacial spin reordering. These LSMO films exhibit positive and negative MR, even though the LSMO is well known for colossal negative MR. The 12.4 nm thick LSMO film shows a maximum ∼7 % in-plane low-field positive MR, which increases to ∼12 % for 20.0 nm thick LSMO, but the variation of out-of-plane positive MR is ∼ ± 0.8 %. The maximum in-plane positive MR occurs at ∼ ± 0.2 kG (switching field), which corresponds to the coercive field. In contrast, the out-of-plane positive MR switching field is relatively higher, ∼ ± 5 kG, 25 times larger than the in-plane switching field. The anisotropy in positive MR and its switching field is due to the strong MA, modulated by the charge transfer-induced interfacial exchange coupling strength, confirmed using the DFT. The MA and anisotropy in MR with the dual sign of MR are intriguing phenomena; their tunability could pave the way for advanced technology in magnetic devices.

Dispersed nanostructures of nickel (Ni) have been synthesized by thermal decomposition of nickel-oleate in the presence of 1-octadecene with controlled synthesis temperature. The evolution of face-centered-cubic (fcc) phase with increasing synthesis temperature from 320 to 365°C leads to structural phase transformation of nickel nanostructures from hexagonal-close-packed (hcp) to fcc through mixed phases. The saturation magnetization (MS) of pure fcc and hcp is ∼37 and ∼0.67emu/g, respectively. The quenched M S of hcp Ni nanostructure compared to that of the fcc Ni indicates the presence of frustrated or canted spins in it. As the fcc phase fraction increases the MS increases, but the observed MS is significantly larger than the theoretical MS calculated by considering the contribution solely from the pure hcp and fcc Ni. This enhanced MS indicates the presence of exchange coupling between the phases and nanoparticles. © 2014 AIP Publishing LLC.

Magnetocaloric effect in (111)-oriented La0.7Sr0.3MnO3-SrRuO3 (LSMO-SRO) superlattices grown with both the stacking orders by reversing the individual layer thickness on (111)-oriented SrTiO3(STO) substrates using the pulsed laser deposition technique has been studied. Pseudomorphic growth with 0.64% in-plane tensile strain in [11 unit cell (u.c.)SRO/3u.c.LSMO]×15 superlattice is favourable for a larger change in entropy (ΔSM) as compared to relaxed growth with in-plane compressive strain in [11u.c.LSMO/3u.c.SRO]×15 superlattice. The reduction of ΔSM in [11u.c.LSMO/3u.c.SRO]×15 could be due to the orientation-dependent in-phase and out-of-phase tilt of the unit cell between ±1° along the 〈103〉pc of the {103}pc, which softens the exchange coupling and leads to the faster alignment of the magnetization near the Curie temperature (TC). Stabilization of the orthorhombic phase of LSMO in the superlattices with both stacking orders is evidenced from the existence of anomaly around the TC of LSMO and SRO in the temperature-dependent phonon frequency shifts. Reduction in symmetry of LSMO from the rhombohedral to orthorhombic structure modulates the Mn-O-Mn bond length and angles, which induces the spin reorientations and hence, modifies the electronic and magnetic properties in these LSMO-SRO superlattices. The ΔSM of these superlattices suggest that the strain, magnitude of the magnetic field, volume and magnetization of the ferromagnet can control the magnetocaloric effect. These results will be useful for designing the magnetic entropy based devices to improve renewable energy systems.

The double perovskite oxides with the general formula A2B'B″O6, where B' and B″ sites are occupied alternately by different transition-metal cations, exhibit a wide range of functional properties and are being investigated for a variety of applications. The most striking feature of a number of these compounds is the presence of more than one functional property in the same material. Recent advances in the synthesis of such materials in the form of thin films, with unit-cell-level control, suggest that it may be possible to additionally tailor their properties. In addition to artificial structuring, thin films provide an excellent opportunity to control the strain and chemical heterogeneity in order to reveal completely new or enhanced properties, which are absent in the bulk. It is also possible to overcome the natural preference for disorder or low-dimensional ordering in some of the materials by suitable control of the growth parameters. In this chapter, the magnetic/multifunctional double perovskite oxides are introduced with a brief overview. The important requirements for synthesizing high-quality oxide thin films of these materials using the pulsed laser deposition technique are then discussed. This is followed by characterization of their morphology, surface, electronic structure, transport, and magnetic properties. In the concluding section, potential applications and future scope for thin films of this fascinating class of complex oxides that remain largely unexplored are provided.

A strong perpendicular magnetocrystalline anisotropy (PMA) in antiferromagnetically coupled SrRuO 3 (17 uc (unit cell))/PrMnO 3 (n uc) superlattices effectively reconstructs the interfacial spin ordering. The occurrence of significant anisotropic interfacial antiferromagnetic coupling between the Ru and Mn ions is systematically tuned by varying the PrMnO 3 layer thickness in ultrathin scale from 3 to 12 uc, which is associated with a rise in PMA energy from 0.28 × 10 6 to 1.60 × 10 6 erg/cm 3 . The analysis using the Stoner-Wohlfarth model and density functional theory confirm that the exchange anisotropy is the major contribution to the PMA. The superlattices with PrMnO 3 layer thickness ≥7 uc exhibit the tunneling-like transport of Ru 4d electrons, which is rather expected in the stronger antiferromagnetically coupled superlattices with thinner PrMnO 3 layer. Tunneling-like transport at thicker spacer layer in the SrRuO 3 -PrMnO 3 superlattice system is an unique feature of two ferromagnet-based superlattices. Our investigations show that the technologically important interfacial magnetic coupling, PMA, and tunneling magnetoresistance could be achieved in a periodically stacked bilayer and can be precisely manipulated by the size effect in ultrathin scale.

The breaking of orthorhombic to tetragonal crystal symmetry is realized by increasing the PrMnO3 layer thickness in the superlattices consisting two ferromagnets, SrRuO3 and PrMnO3. The octahedral rotation pattern is a+c−c− and a0a0c− type for the superlattices with orthorhombic and tetragonal phase, respectively, inferred in the simulated projected density of states. The 15% reduction in dz2 orbital occupancy due to the a0a0c− type octahedral rotation compared to that of the a+c−c− type suggests the presence of stronger antiferromagnetic (AFM) coupling. The larger orbital overlapping leads to a stronger spin–orbit coupling, associated with a shift of 42.8% of the minor in-plane field cooled (FC) magnetic hysteresis loop(M(H)) along the magnetization axis in orthorhombic superlattices. While, minor in-plane FC M(H) shifts along the field axis due to the strong AFM coupling in tetragonal superlattices. In field-dependent magnetoresistance, the rotation of spins in the antiferromagnetically coupled interfacial layers is detected as a unique anomaly, which is stronger in the superlattices for the biased spins and tetragonal symmetry than the pinned spins and orthorhombic symmetry. The results demonstrate that the tuning of interfacial exchange coupling and spin-dependent transport by controlling structural distortion could be used as a tool in fabricating modern spintronics-based devices.

Quantum corrections to conductivity in the ferromagnetic La0.7Sr0.3MnO3 (LSMO) and SrRuO3 (SRO) thin films depend on the structural mismatches and interfaces accommodating ions and their spins. Here, by making interfaces of LSMO and SRO in the form of artificial superlattices, we achieve positive magnetoresistance (MR) and weak antilocalization (WAL), although the individual component shows negative MR and weak localization (WL). The [20 unit cell (u.c.) LSMO/3 u.c. SRO]×15 superlattice stabilizes in tetragonal symmetry associated with the rhombohedral and orthorhombic structures and demonstrates the occurrence of the single magnon scattering process. The low-field MR of the superlattice fit to the Hikami-Larkin-Nagaoka expression yields 595 Å phase coherence length (lφ) with WAL of carriers. As the SRO layer thickness in the superlattice increases to 5 u.c., the value of lφ = 292 Å decreases, and positive MR increases confirm the manifestation of WAL by SRO. The orthorhombic symmetry of the SRO is preserved in the [20 u.c. SRO/3 u.c. LSMO]×15 superlattice, which shows the existence of locally cooperative bond-length fluctuations and conduction due to the scattering of the electron by the Fermi liquid electrons, bond length, and spin fluctuations. However, as the LSMO layer thickness in the superlattice is increased to 5 u.c., the WL effect suppresses WAL at the low field. The spin-orbit coupling associated with magnetic anisotropy, i.e., spin and bond length fluctuations, modifies the WL in the superlattices and leads to WAL, thereby achieving positive MR.

A ferromagnetic 120 Å thick La 0.7 Sr 0.3 M n O 3 (LSMO) film grown on (001) Si using the sputtering deposition technique demonstrates a large positive in-plane magnetoresistance (MR) at 10 K, in the field window of ± 0.084 kG to ± 0.405 kG, although the bulk LSMO exhibits negative MR. Around the coercive field (∼ 179 G), the positive MR becomes ∼ 11 %. The positive MR of the LSMO thin film is explained by the charge transfer driven localized strong antiferromagnetic coupling at the Si - LSMO interface, which favors the reduction of the Curie temperature T C of LSMO compared to that of its bulk value. The construction of the interface on the top surface of LSMO with ZnO thin films further reduces T C ∼ 30 K and the positive MR decreases to ∼ 1 % for 45 ° oriented in-plane current with the in-plane field. The coupling through Mn - O - Zn at the LSMO - ZnO interface preserves the charge state, and the weak exchange coupling at the (La / Sr) O - ZnO interface reduces the spin-dependent scattering process under the field and thereby, the negative MR. The reduced T C and in-plane low-field MR at 10 K of a series of Si / LSMO / ZnO are the same irrespective of the ZnO thickness, which confirms their interfacial origin. The presence of interfacial spin disorder at the Si - LSMO interface is further confirmed from the increase in resistance at low temperatures, which is explained by the Kondo like effect and quantum interference effect. Our investigations show that the technologically important interfacial magnetic coupling and magnetoresistance could be achieved and manipulated by the selective interfacial exchange coupling.

The functional properties of oxide heterostructures depend on the interfaces accommodating ions, their spins, and structural mismatches. Here, by stabilizing tetragonal symmetry, we achieve the in-plane antiferromagnetic (AFM) ordering and dual-exchange bias in the superlattices consisting of two ferromagnets SrRuO3 (SRO) and PrMnO3 (PMO). The tetragonal symmetry of this superlattice system achieved after the octahedral rotations yield an elongation of the c-axis parameter with Ru-O-Mn bond angle close to 180°, induces an interfacial antiferromagnetic ordering, which is suppressed as the ferromagnetic (FM) ordering in the PMO layer increases. The 0.1 T in-plane cooling field (Hcool) leads to the shift (ca. -0.04 T) of minor hysteresis loop along the negative field axis due to the presence of -0.87 erg/cm2 AFM interfacial exchange coupling energy density (ERu,Mn) at 20 K. The exchange bias field (HEB) switches from negative to positive value with the increase in Hcool. For 5 T Hcool, the HEB is positive, but the ERu,Mn is -1.25 erg/cm2 for n ≤ 8 (n = number of unit cells of PMO) and 1.52 erg/cm2 for n ≥ 8. The HEB and its switching from negative to positive with the increase in Hcool are explained by the interplay of strong antiferromagnetic coupling energy and Zeeman energy at the interfaces. The results demonstrate that the SRO-PMO superlattice could be a model system for the investigation of the interfacial exchange coupling in functional oxides.

The band gap (Eg) engineering and Dirac point tuning of the (0001) surface of 8 QLs (quintuple layers) thick Bi2Se3slab are explored using the first-principles density functional theory calculations by varying the strain. The strain on the Bi2Se3slab primarily varies the bandwidth, modifies the pz- orbital population of Bi and moves the Dirac point of the (0001) surface of Bi2Se3. The Dirac cone feature of the (0001) surface of Bi2Se3is preserved for the entire range of the biaxial strain. However, around 5% tensile uniaxial strain and even lower value of volume conservation strain annihilate the Dirac cone, which causes the loss of topological (0001) surface states of Bi2Se3. The biaxial strain provides ease in achieving the Dirac cone at the Fermi energy (EF) than the uniaxial and volume conservation strains. Interestingly, the transition from directEgto indirectEgstate of the (0001) surface of Bi2Se3is observed in the volume conservation strain-dependentEg. The strain on Bi2Se3, significantly modifies the conduction band of Se2 atoms nearEFcompared to Bi and Se1, and plays a vital role in the conduction of the (0001) surface of Bi2Se3. The atomic cohesive energy of the Bi2Se3slab is very close to that of (0001) oriented nanocrystals extracted from the Raman spectra. The strain-dependent cohesive energy indicates that at a higher value of strain, the uniaxial and volume conservation strain provides better stability than that of the biaxial strain (0001) oriented growth of the Bi2Se3nanocrystals. Our study establishes the relationship between the strained lattice and electronic structures of Bi2Se3, and more generally demonstrates the tuning of the Dirac point with the mechanical strain.