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We investigate the effect of Mn substitution in Fe$_2$Ti$_{1-x}$Mn$_x$Sn on electronic structure, magnetic and electrical transport properties. The spin-polarized density of states calculations using density-functional theory (DFT), yields a half-metallic ground state in Mn-rich compositions. Localized magnetic moments at Mn sites that interact through the cloud of conduction electrons formed by Fe and Ti atoms are also predicted. Electrical resistivity and magneto-transport measurements reveal a Kondo-like ground state at low temperatures and a peculiar linear negative temperature coefficient of resistance in the high-temperature regime with a predominant electron-phonon scattering mechanism. Analysis of room temperature powder X-ray diffraction data reveals a highly ordered L2$_1$ structure and reduction of antisite disorder, upon Mn substitution. The temperature-dependent magnetization measurements reveal distinct features indicative of weak anisotropy in the system. Isothermal magnetization measured as a function of the applied field helps identify the unique magnetic ground state of the half-metallic Fe$_2$Ti$_{1-x}$Mn$_x$Sn composition as a ferromagnet with a wandering axis, that distinctively orients in the direction of applied field. Our findings thus provide a new perspective for studying the mechanism of half-metallicity and associated magnetic order in Heuslers., Comment: 11 pages, 12 figures

This work proposes a hybrid Brain Computer Interface system for the automation of a wheelchair for the disabled. Herein a working prototype of a BCI-based wheelchair is detailed that can navigate inside a typical home environment with minimum structural modification and without any visual obstruction and discomfort to the user. The prototype is based on a combined mechanism of steady-state visually evoked potential and eye blinks. To elicit SSVEP, LEDs flickering at 13Hz and 15Hz were used to select the left and right direction, respectively, and EEG data was recorded. In addition, the occurrence of three continuous blinks was used as an indicator for stopping an ongoing action. The wavelet packet denoising method was applied, followed by feature extraction methods such as Wavelet Packet Decomposition and Canonical Correlation Analysis over narrowband reconstructed EEG signals. Bayesian optimization was used to obtain 5 fold cross-validations to optimize the hyperparameters of the Support Vector Machine. The resulting new model was tested and the average cross-validation accuracy 89.65% + 6.6% (SD) and testing accuracy 83.53% + 8.59% (SD) were obtained. The wheelchair was controlled by RaspberryPi through WiFi. The developed prototype demonstrated an average of 86.97% success rate for all trials with 4.015s for each command execution. The prototype can be used efficiently in a home environment without causing any discomfort to the user., Comment: Accepted to 2021 IEEE INTERNATIONAL CONFERENCE ON SYSTEMS, MAN, AND CYBERNETICS (SMC)

We present here a detailed investigation into the magnetic ordering of full Heusler alloy Ga$_2$MnCo using dc, ac magnetization measurements, neutron diffraction and neutron depolarization experiments. Crystal structure at room temperature was first confirmed to be L2$_1$ using the highly intense synchrotron X-ray diffraction (XRD) technique. Temperature dependent magnetization reveals that Ga$_2$MnCo enters a ferromagnetic (FM) state at $T_C = $154 K, characterized by a sharp increase in magnetization and a plateau-like region hereafter. As the temperature is decreased further, a sharp drop in magnetization is observed at $T_f$ = 50 K, hinting towards an antiferromagnetic (AFM) phase change. Neutron diffraction (ND) recorded over the range of temperature from 6 to 300 K, provides combined information regarding crystal as well as magnetic structure. Accordingly, an increase in the intensity of the ND pattern is seen at 150 K, signaling onset of long range FM order. However, there is no sign of appearance of superlattice reflections corresponding to the AFM phase, in the patterns recorded below 50 K. An unusual discontinuity in the unit cell volume is seen around $T_f$ indicating a coupling of this second transition with the contraction of the lattice. Attempts to unravel this interesting magnetic behaviour using ac susceptibility measurements lead to the existence of glassy magnetism below $T_f$. Systematic analysis of the susceptibility results along with neutron depolarization measurement, identifies the low temperature phase as a reentrant cluster glass., Comment: 8 pages, 7 figures, published in Phys. Rev. B

The present paper seeks to investigate effect of Mn doping in superconducting FeSe. It is found that over the entire doping range in Fe$_{1-x}$Mn$_x$Se ($0 \le x \le 1$), Mn does not substitute Fe in the superconducting tetragonal phase. Instead two impurity phases, NiAs type hexagonal phase and NaCl type cubic phase grow with increasing Mn content. Initially, hexagonal phase has a higher content than the cubic phase but beyond $x$ = 0.5, the cubic phase grows rapidly and for $x \ge 0.8$, the sample is monophasic with cubic NaCl type structure. The superconducting tetragonal phase content steadily decreases with increasing Mn concentration and completely disappears beyond $x = 0.5$. The premise that Mn never replaces Fe in the superconducting phase is further strengthened by observation of a sharp drop in AC susceptibility akin to superconducting transition at the T$_c$ of FeSe up to $x$ = 0.5. EXAFS studies at the Fe K edge also show that the Fe has a four coordinated tetragonal local structure in all compositions below $x = 0.5$, similar to that in FeSe and it gradually changes to a six coordinated one as is expected for a NaCl type cubic phase for $x \ge 0.5$.

Magnetism in Ni-Mn-Z (Z = Ga,In,Sn,Sb) Heusler alloys has so far been predominantly attributed to Rudermann-Kittel-Kasuya-Yoshida type interactions between Mn atoms. We investigate magnetic interactions in one such alloy, Ni$_{2}$Mn$_{1.4}$In$_{0.6}$ and attempt to explain the origin of antiferromagnetic (AFM) interactions that coexist with ferromagnetic ones. Through the combination of x-ray absorption spectroscopy and x-ray magnetic circular dichroism (XMCD), we find that Ni plays an important role along with Mn in the overall magnetism. A significant hybridization that develops between Mn and Ni orbitals results in a small antiferromagnetic moment at Ni sites. The shift in the XMCD hysteresis loops in the martensitic phase suggests that antiferromagnetism results from superexchange like interactions between Mn atoms mediated by Ni., Comment: Main paper 6 pages + supplementary material

Extended X-ray Absorption Fine Structure (EXAFS) studies on Ni$_{50}$Mn$_{25+x}$In$_{25-x}$ have been carried out at Ni and Mn K edge as a function of temperature. Thermal evolution of nearest neighbor Ni-Mn and Mn-Mn bond distances in the martensitic phase give a clear evidence of a close relation between structural and magnetic degrees of freedom in these alloys. In particular, the study highlights the role of Ni 3d - Mn 3d hybridization in the magnetism of martensitic phase of these alloys., Comment: Accepted for publication in EPL

Effect of Co, Ru and Cu substitution at B and B' sites on the magnetic and transport properties of LaSrCoRuO$_6$ have been investigated. All the doped compositions crystallize in the monoclinic structure in the space group $P2_1/n$ indicating a double perovskite structure. While the magnetization and conductivity increase in Co and Ru doped compounds, antiferromagnetism is seen to strengthen in the Cu doped samples. These results are explained on the basis of a competition between linear Co-O-Ru-O-Co and perpendicular Co-O-O-Co antiferromagnetic interactions and due to formation of Ru-O-Ru ferromagnetic networks.

The perovskite solid solutions of the type La$_{2x}$Sr$_{2-2x}$Co$_{2x}$Ru$_{2-2x}$O$_{6}$ with 0.25 $\leq$ x $ \leq $ 0.75 have been investigated for their structural, magnetic and transport properties. All the compounds crystallize in double perovskite structure. The magnetization measurements indicate a complex magnetic ground state with strong competition between ferromagnetic and antiferromagnetic interactions. Resistivity of the compounds is in confirmation with hopping conduction behaviour though differences are noted especially for $x$ = 0.4 and 0.6. Most importantly, low field (50Oe) magnetization measurements display negative magnetization during the zero field cooled cycle. X-ray photoelectron spectroscopy measurements indicate presence of Co$^{2+}$/Co$^{3+}$ and Ru$^{4+}$/Ru$^{5+}$ redox couples in all compositions except $x$ = 0.5. Presence of magnetic ions like Ru$^{4+}$ and Co$^{3+}$ gives rise to additional ferromagnetic (Ru-rich) and antiferromagnetic sublattices and also explains the observed negative magnetization., Comment: Accepted for publication in J. Magn. Magn. Mater

We report on the electronic structure of the perovskite oxide CaCrO3 using valence-band, core-level, and Cr 2p - 3d resonant photoemission spectroscopy (PES). Despite its antiferromagnetic order, a clear Fermi edge characteristic of a metal with dominant Cr 3d character is observed in the valence band spectrum. The Cr 3d single particle density of states are spread over 2 eV, with the photoemission spectral weight distributed in two peaks centered at ~ 1.2 eV and 0.2 eV below EF, suggestive of the coherent and incoherent states resulting from strong electron-electron correlations. Resonant PES across the Cr 2p - 3d threshold identifies a 'two-hole' correlation satellite and yields an on-site Coulomb energy U ~4.8 eV. The metallic DOS at EF is also reflected through the presence of a well-screened feature at low binding energy side of the Cr 2p core-level spectrum. X-ray absorption spectroscopy (XAS) at Cr L3,2 and O K edges exhibit small temperature dependent changes that point towards a small change in Cr-O hybridization. The multiplet splitting in Cr 2p core level spectrum as well as the spectral shape of the Cr XAS can be reproduced using cluster model calculations which favour a negative value for charge transfer energy between the Cr 3d and O 2p states. The overall results indicate that CaCrO3 is a strongly hybridized antiferromagnetic metal, lying in the regime intermediate to Mott-Hubbard and charge-transfer systems.

This paper reports effect of thermally induced disorder on the magnetic properties of LaSrCoRuO6 double perovskite. While the ordered sample is antiferromagnetic, the disordered sample exhibits negative values of magnetization measured in low applied fields. Isothermal magnetization on this sample shows hysteresis due to presence of ferromagnetic interactions. Based on neutron diffraction and X-ray Absorption Fine Structure (XAFS) studies, these results have been interpreted to be due disorder in site occupancy of Co and Ru leading to octahedral distortions and formation of Ru-O-Ru ferromagnetic linkages. Below 150K these ferromagnetic Ru spins polarize the Co spins in a direction opposite to that of the applied field resulting in observed negative magnetization.

The local structural distortions arising as a consequence of increasing Mn content in Ni_2Mn_1+xIn_1-x (x=0, 0.3, 0.4, 0.5 and 0.6) and its effect on martensitic transformation have been studied using Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy. Using the room temperature EXAFS at the Ni and Mn K-edges in the above compositions, the changes associated with respect to the local structure of these absorbing atoms are compared. It is seen that in the alloys exhibiting martensitic transformation ($x \ge 0.4$) there is a significant difference between the Ni-In and Ni-Mn bond lengths even in the austenitic phase indicating atomic volume to be the main factor in inducing martensitic transformation in Ni-Mn-In Heusler alloys., Comment: 8 pages, 2 figures

We investigate the electronic structure of Chromium Nitride (CrN) across the first-order magneto-structural transition at T_N ~ 286 K. Resonant photoemission spectroscopy shows a gap in the 3d partial density of states at the Fermi level and an On-site Coulomb energy U ~ 4.5 eV, indicating strong electron-electron correlations. Bulk-sensitive high resolution (6 meV) laser photoemission reveals a clear Fermi edge indicating an antiferromagnetic metal below T_N. Hard x-ray Cr 2p core-level spectra show T-dependent changes across T_N which originate from screening due to coherent states as substantiated by cluster model calculations using the experimentally observed U. The electrical resistivity confirms an insulator above T_N (E_g ~ 70 meV) which becomes a disordered metal below T_N. The results indicate CrN transforms from a correlated insulator to an antiferromagnetic metal, coupled to the magneto-structural transition., Comment: Submitted to Physical Review Letters (February 2010) 11 pages, 3 figures in the main text, 1 Supplementary Information

We present here a comprehensive investigation of the magnetic ordering in Ni50Mn35In15 composition. A concomitant first order martensitic transition and the magnetic ordering occurring in this off-stoichiometric Heusler compound at room temperature signifies the multifunctional character of this magnetic shape memory alloy. Unusual features are observed in the dependence of the magnetization on temperature that can be ascribed to a frustrated magnetic order. It is compelling to ascribe these features to the cluster type description that may arise due to inhomogeneity in the distribution of magnetic atoms. However, evidences are presented from our ac susceptibility, electrical resistivity and dc magnetization studies that there exists a competing ferromagnetic and antiferromagnetic order within crystal structure of this system. We show that excess Mn atoms that substitute the In atoms have a crucial bearing on the magnetic order of this compound. These excess Mn atoms are antiferromagnetically aligned to the other Mn, which explains the peculiar dependence of magnetization on temperature., Comment: Accepted in J. Phys. D.:Appl. Physics

Martensitic and magnetic properties of ferromagnetic shape memory alloys are known to depend up on structural modulations and associated changes in the Fermi surface. These modulations although periodic and spanning over multiple unit cells, involve movement of atoms typically of the order of 0.01A. Therefore X-ray Absorption Fine Structure (XAFS) is an ideal tool to map both, local atomic movements and changes in density of states (DOS) due to changing hybridization as the system transforms from austenitic to martensitic phase. This paper presents a compilation of our XAFS studies on the Ni-Mn based shape memory alloys. A complete description of the changes in local structure around the constituent metal ions in the following alloy compositions: Ni2+xMn1-xGa, Ni2Mn1.4Sn0.6 and Ni2Mn1.4In0.6 in the austenitic and martensitic phases have been obtained. The results give the new experimental evidence for the crucial hybridization component that influences and leads to structural transition in these Ni-Mn based Heusler alloys., Comment: Invited Talk at International Conference on Ferromagnetic Shape Memory Alloys, Kolkata, India

We have studied the effect of magnetic field on a non-stoichiometric Heusler alloy Ni$_{50}$Mn$_{35}$In$_{15}$ that undergoes a martensitic as well as a magnetic transition near room temperature. Temperature dependent magnetization measurements demonstrate the influence of magnetic field on the structural phase transition temperature. From the study of magnetization as a function of applied field, we show the occurrence of inverse-magnetocaloric effect associated with this magneto-structural transition. The magnetic entropy change attains a value as high as 25 J/kg-K (at 5 T field) at room temperature as the alloy transforms from the austenitic to martensitic phase with a concomitant magnetic ordering.

Heusler alloys that undergo martensitic transformation in ferromagnetic state are of increasing scientific and technological interest. These alloys show large magnetic field induced strains upon martensitic phase change thus making it a potential candidate for magneto-mechanical actuation. The crystal structure of martensite is an important factor that affects both the magnetic anisotropy and mechanical properties of such materials. Moreover, the local chemical arrangement of constituent atoms is vital in determining the overall physical properties. Ni$_{50}$Mn$_{35}$In$_{15}$ is one such ferromagnetic shape memory alloy that displays exotic properties like large magnetoresistance at moderate field values. In this work, we present the extended x-ray absorption fine-structure measurements (EXAFS) on the bulk Ni$_{50}$Mn$_{35}$In$_{15}$ which reveal the local structural change that occurs upon phase transformation. The change in the bond lengths between different atomic species helps in understanding the type of hybridization which is an important factor in driving such Ni-Mn based systems towards martensitic transformation.

Extended x-ray absorption fine-structure (EXAFS) measurements have been carried out on Ni$_{50}$Mn$_{35}$Sn$_{15}$ in the austenitic and martensitic phase. The temperatures associated with structural and magnetic phase transformations are obtained from magnetization measurements. The system orders ferromagnetically below 319 K while the structural phase transition occurs at T$_M$ $\sim$ 200 K. EXAFS measurements have been carried out at Mn and Ni K-edges and changes associated with respect to the local structure of these absorbing atoms are compared. Significant changes in the nearest neighbour interactions are seen as the system transits to the low temperature martensitic phase. EXAFS provides evidence for local structural distortion even in the cubic phase that is not seen from the x-ray diffraction studies. The results obtained are understood in terms of changing hybridization between the constituent atoms that influence the electronic structure at the Fermi level, associated with the austenitic-to-martensitic transition., Comment: 20 pages, 9 figures

The local atomic structure of Ni$_{2+x}$Mn$_{1-x}$Ga with 0 $\le$ $x$ $\le$ 0.16 alloys was explored using Mn and Ga K-edge Extended X-ray Absorption Fine Structure (EXAFS) measurement. Inorder to study the atomic re-arrangements that occur upon martensitic transformation, room temperature and low temperature EXAFS were recorded. The changes occurring in the L2$_1$ unit cell and the bond lengths obtained from the analysis enables us to determine the modulation amplitudes over which the constituent atoms move giving rise to shuffling of the atomic planes in the modulated structure. The EXAFS analysis also suggests the changes in hybridization of Ga-$p$ and Ni-$d$ orbitals associated with the local symmetry breaking upon undergoing martensitic transition., Comment: Accepted for publication in Physical Review B

The local atomic structure of Ni-Mn-Ga alloys was explored using Mn and Ga K-edge extended X-ray fine structure (EXAFS) measurement. The changes occuring in the L1$_0$ sub-cell of the martensitic unit cell and the bond lengths obtained from the analysis enables us to propose a scheme for the structural distortions responsible for the modulations and shuffling of the atomic planes in the martensitic phase of Ni-Mn-Ga system. The EXAFS analysis also suggests the changes in hybridization of Ga-$p$ and Ni-$d$ orbitals associated with the local symmetry breaking upon undergoing martensitic transition., Comment: 9 pages, 4 figures

The thermoelectric power (TEP) measurements have been carried out to investigate the changes in the electronic structure associated with the intermartensitic and martensitic transitions in Ni$_{2+x}$Mn$_{1-x}$Ga (0 $\le$ x $\le$ 0.19). The samples have been characterized by a.c. magnetic susceptibility measurements. The correlation between the TEP and the microstructural changes sensitive to the increasing Ni content in Ni$_{2+x}$Mn$_{1-x}$Ga has been investigated. The changes in the density of states as reflected in the temperature variation of the TEP are consistent with the conclusions drawn from the Jahn-Teller mechanism of lattice distortion., Comment: 11 pages, 5 figures; Accepted for publication in J. Phys. Condens. Matter (2006)

Investigation of transport and magnetic properties of MnTe at low temperatures sInvestigation of transport and magnetic properties of MnTe at low temperatures showed anomalies like negative coefficient of resistance below 100K and a sharp rise in susceptibility at around 83K similar to a ferromagnetic transition. Low temperature powder neutron diffraction experiments were therefore carried out to understand the underlying phenomena responsible for such anomalous behavior. Our study indicates that the rise in susceptibility at low temperatures is due to strengthening of ferromagnetic interaction within the plane over the inter plane antiferromagnetic interactions., Comment: Appearing in J. Magn. Magn. Mater

In this paper, we report results of the first studies on the thermoelectric power (TEP) of the magnetic heusler alloy Ni$_{2.19}$Mn$_{0.81}$Ga. We explain the observed temperature dependence of the TEP in terms of the crystal field (CF) splitting and compare the observed behavior to that of the stoichiometric system Ni$_2$MnGa. The resistivity as a function of temperature of the two systems serves to define the structural transition temperature, T$_M$, which is the transition from the high temperature austenitic phase to low temperatures the martensitic phase. Occurrence of magnetic (Curie-Weiss) and the martensitic transition at almost the same temperature in Ni$_{2.19}$Mn$_{0.81}$Ga has been explained from TEP to be due to changes in the density of states (DOS) at the Fermi level., Comment: 12 pages, 4 figures, Accepted in Physical Review B vol 70, Issue 13

Complex queries are becoming commonplace, with the growing use of decision support systems. These complex queries often have a lot of common sub-expressions, either within a single query, or across multiple such queries run as a batch. Multi-query optimization aims at exploiting common sub-expressions to reduce evaluation cost. Multi-query optimization has hither-to been viewed as impractical, since earlier algorithms were exhaustive, and explore a doubly exponential search space. In this paper we demonstrate that multi-query optimization using heuristics is practical, and provides significant benefits. We propose three cost-based heuristic algorithms: Volcano-SH and Volcano-RU, which are based on simple modifications to the Volcano search strategy, and a greedy heuristic. Our greedy heuristic incorporates novel optimizations that improve efficiency greatly. Our algorithms are designed to be easily added to existing optimizers. We present a performance study comparing the algorithms, using workloads consisting of queries from the TPC-D benchmark. The study shows that our algorithms provide significant benefits over traditional optimization, at a very acceptable overhead in optimization time.