Publications

Abstract: The use of graphene in spintronic devices depend, on its ability to convert a spin current into an electric charge current. We have systematically investigated the spin pumping induced spin-to-charge current conversion at the Graphene/FM interface and the effect of interface modification through high spin orbit coupling (SOC) material (Pt) as an interlayer (IL) of varying thicknesses by using broadband FMR spectroscopy. The spin mixing conductance was enhanced from 1.66×10^18 m^−2 to 2.72×10^18 m^−2 where as the spin current density was enhanced from 0.135 ±0.003 to 0.242 ± 0.004 MA/m^2 at the Graphene/FM interface due to the interface modification using high SOC material Pt as an interlayer. We observe the value of spin current to charge current conversion efficiency ≈0.292×10^−3 nm for the Graphene/FM interface. These findings support the idea that Graphene in contact with high SOC material (Pt) be a potential candidate for spintronic applications, specifically for spin-torque-based memory applications. 

Abstract: High-symmetry rocksalt type tetragonal CuO (T-CuO) does not exist in bulk but can be synthesized via thin film epitaxy limited to a few unit cells (3, 4) thick and above which it relaxes to its bulk tenorite structure. Direct probe into magnetic properties of T-CuO layer has been a challenge because of its ultrathin limit. Here, we demonstrate the interfacial magnetic coupling between ultrathin T-CuO and ferromagnetic (La0.7Sr0.3MnO_3) layers in an epitaxial CuO/La0.7Sr0.3MnO_3 bilayer grown on (001)-oriented SrTiO3. We observe a positive exchange bias shift of ∼30 Oe at 2 K in CuO/La0.7Sr0.3MnO_3 bilayer. The observation of positive exchange bias indicates that there exists antiferromagnetic exchange coupling between Mn and Cu moments at the interface. Notably, the exchange bias vanishes at 5 K and it is discussed in view of the proposed spin structure revealed from low-energy muon spin rotation and x-ray magnetic circular dichroism study [Phys. Rev. B 103, 224429 (2021)]. Furthermore, an enhanced Gilbert damping, linewidth broadening and larger inhomogeneous 4πMeff  value from in-plane ferromagnetic resonance measurements, are the direct consequence of antiferromagnetic exchange coupling at the CuO/La0.7Sr0.3MnO_3 interface. Combining both static and dynamic magnetic characterization, we establish an understanding of interfacial exchange coupling in CuO/La0.7Sr0.3MnO_3 bilayer. 

Abstract: Controlled spin transport in magnetic stacks is required to realize pure spin current-driven logic and memory devices. The control over the generation and detection of the pure spin current is achieved by tuning the spin to charge conversion efficiency of the heavy metal interfacing with ferromagnets. Here, we demonstrate the direct tunability of spin angular momentum transfer and thereby spin pumping, in CoFeB/Pt stack, with interfacial magnetic anisotropy. The ultra-low thickness of CoFeB thin film tilts the magnetic easy axis from in-plane to out-of-plane due to surface anisotropy. The Ferromagnetic resonance measurements are performed to investigate the magnetic anisotropy and spin pumping in CoFeB/Pt stacks. We clearly observe tunable spin pumping effect in the CoFeB/Pt stacks with varying CoFeB thicknesses. The spin current density, with varying ferromagnetic layer thickness, is found to increase from 0.11 to 0.24 MA/m2, with increasing in-plane anisotropy field. Such interfacial anisotropy-controlled generation of pure spin current can potentially lead to next-generation anisotropic spin current-controlled spintronic devices. 

Abstract: We report experimental evidence of emergent broken symmetry Fermi liquid state in an isolated single crystalline nanorod of 

Pr2Ir2O7. We find clear signature of the onset of the Fermi liquid behavior at low temperature marked by the sign inversion of magnetoresistance from negative at high temperature, characteristic of incoherent Kondo scattering, to positive as well as a T^2 dependence of resistivity at low temperature. A resistive anomaly is observed, which is accompanied by thermal hysteresis in the presence of magnetic field, suggesting itinerant metamagnetism. The observed high field negative magnetoresistance with quadratic field dependence at low temperature, which is most likely due to suppression of itinerant spin fluctuation, and the irreversibility of the magneto-resistive properties in the Fermi liquid regime suggest existence of an unusual state with broken spin rotation and time reversal symmetry, hallmark of `hastatic' order. The major features of such temperature dependence of resistivity and magnetoresistance can be explained in a phenomenological model incorporating two distinct hybridization channels, which is physically consistent with the possibility of the formation of the `hastatic' Fermi liquid phase. 

Abstract: The stability of thin liquid films on a surface can be controlled by using external stimuli, such as an electric field, temperature, or light, by manipulating the total excess free energy of the system. It has been previously shown that thin lubricating films on slippery surfaces can be destabilized via the spinodal mechanism using an external electric field, which returns to the original stable configuration upon the electric field. However, the role of the frequency of the applied ac electric field is not clear, which is the main topic of study in this report. When an ac electric field of fixed voltage and varying frequency is applied across thin lubricating films of slippery surfaces, a different dewetting behavior is observed. Characteristic length and time scales of dewetting depend strongly on the frequency of the applied voltage, which is primarily due to the change in the dielectric behavior of the lubricating fluid. In addition, the interplay of various time scales involved in the dewetting process also depends on the frequency. 

Abstract: The subtle interplay of band topology and symmetry broken phase, induced by electron correlations, has immense contemporary relevance and potentially offers novel physical insights. Here, we provide evidence of possible charge density wave (CDW) in bulk Y2Ir2O7 for T<10K, and the Weyl semimetal (WSM) phase at higher temperatures. We observe the following characteristic properties of the CDW phase: (i) current induced nonlinear conductivity with negative differential resistance at low temperature, (ii) low-frequency Debye-like dielectric relaxation at low temperature with a large dielectric constant ∼10^, and (iii) an anomaly in the temperature dependence of the thermal expansion coefficient. The WSM phase at higher temperature is analyzed using the dc and ac transport measurements, which show an inductive response at low frequencies. More interestingly, we show that by reducing the crystallite size, the low-temperature CDW phase can be eliminated leading to the restoration of the WSM phase. 

Abstract: We investigate the effect of partial replacement of extended 5d Ir4+ sites by localized 3d Cr3+ moments on the magnetocaloric properties of Y2Ir2O7 (YIO) pyrochlore iridates. We find that Y2Ir2–xCrxO7 (YICO) undergoes a cluster glass transition, possibly due to a Ruderman–Kittel–Kasuya–Yosida (RKKY)-like interaction between localized Cr3+ moments occupying random sites in the pyrochlore network, mediated by 5d Ir conduction electrons. The coexistence of ferromagnetic and antiferromagnetic clusters gives rise to the conventional and inverse magnetocaloric effect (MCE). We observe significant enhancement of conventional as well as inverse MCE with substitution. Although the values of the conventional MCE and inverse MCE in substituted iridates are not large, the effect spans a giant working temperature window, thus leading to orders of magnitude enhancement of cooling power, the value being comparable to that of standard magnetocaloric materials. 

Abstract: We present experimental evidence of incoherent Kondo scattering as the source of resistivity minima in bulk polycrystalline and nanocrystalline Pr2Ir2O7. The temperature dependence of thermopower shows a positive maximum at high temperature followed by a negative minimum at low temperature, with the sign inversion occurring at a much higher temperature than TK. Moreover, we observe little correlation between TK and intersite coupling strength given by ∣θCW∣. We describe the temperature dependence of thermopower and resistivity within the framework of crystal field excitation in a Kondo lattice. 

Abstract: We investigate the interplay of Kondo and Ruderman-Kittel-Kasuya-Yosida coupling in the presence of disorder by chemically substituting local moment Cr3+ at the Ir sublattice in the metallic Pr2Ir2O7. We find evidence of non-Fermi liquid behavior in the transport and thermodynamic measurements at low temperature. Specifically, the magnetic susceptibility exhibits power law divergence at T=0. The nonanalytic temperature and magnetic-field dependence of magnetic susceptibility and the associated scaling suggest the existence of a two-fluid system consisting of a Kondo-screened paramagnetic metal coexisting with magnetically ordered rare regions dominated by interimpurity interaction, similar to the quantum critical Griffiths phase. 

Abstract: We report the structural, magnetic, and electrical transport properties of Y_2Ir_2−𝑥Cr_𝑥O7 pyrochlore iridates. Chemical doping leads to the order of magnitude enhancement of electrical conductivity. The introduction of Cr^3+ at the Ir^4+ site tends to distort the Ir–O_6 octahedra and weakens antiferromagnetic correlation. The x-ray photoemission spectroscopy measurements suggest the coexistence of Ir^4+ and Ir^5+ valence states in the Y_2Ir_(2−x)Cr_xO_7 compounds. The concentration of  Ir^5+ is enhanced with Cr doping, leading to weak ferromagnetism and enhanced electrical conductivity. A cluster-glass-like transition is also observed at low temperatures with Cr doping, possibly due to competing ferromagnetic and antiferromagnetic interaction. 

Abstract: We find that the long range magnetic ordering is absent and electrical conduction suppressed in 

Y1.7Bi0.3Ir2O7/YSZ(1 0 0) thin film prepared by pulsed laser deposition. The sharp down-turn of inverse magnetic susceptibility X^-1(T) from the conventional Curie-Weiss behavior below  T∗∼168K suggests an inhomogeneous ferromagnetic Griffiths like phase. The transport and magnetic properties are explained on the basis of the coexistence of mixed oxidation states of Ir, (i.e. Ir^4+ and Ir^3+) leading to non-magnetic defects and reduction in t2g density of states at the Fermi level. 

Abstract: We report the systematic study of structural, magnetic, and electrical transport properties of  Y_(2−𝑥)Bi_𝑥Ir_2O_7 (𝑥=0.0, 0.1, 0.2, and 0.3) pyrochlore iridates. Chemical doping enhances electrical conductivity and antiferromagnetic correlation substantially. The replacement of a nonmagnetic Y^3+ion with nonmagnetic Bi^3+ in Y_2Ir_2O_7 tends to reduce the octahedral distortion, thus enhancing the antiferromagnetic correlation. Raman spectroscopy shows that the Ir–O bond contracts slightly, and the R–O′ bond turns longer as disorder and phononic oscillation are reduced by Bi doping, leading to wider 𝑡2𝑔 bands, which enhances the electrical conductivity. Additionally, the enhancement in electrical conductivity and antiferromagnetic correlation with Bi^3+ doping is attributed to the hybridization between the Y^3+ (4𝑝)/Bi^3+(6𝑠/6𝑝) orbital with the Ir^4+(5𝑑)orbital as a result of enhancement in the Ir–O–Ir bond angle and contraction in the Ir–O bond length. 

We discuss the particle size driven tunability of the coexistence of ferromagnetism and ferroelectricity in  Pr0.67Ca0.33MnO3 (PCMO) with the help of x-ray diffraction, magnetization, impedance spectroscopy, and remanent polarization measurements. The remanent polarization measurements using the “positive up negative down” method clearly prove the existence of ferroelectricity in PCMO with phase separation between Zener polaron (ZP) ordered P21nm and disordered Pbnm structures. We also find that the ferroelectric response is enhanced in nanocrystalline samples so long as ZP ordering is not destroyed while the long-range antiferromagnetic ordering at low temperature in bulk system is replaced by ferromagnetic correlations in nanoparticles. The conclusion—that by reducing the crystallite size it might be possible to make ferromagnetism and ferroelectricity coexist near room temperature—should be generally applicable to all ZP ordered manganites.