arXiv subjects

Kalpataru Pradhan

Publications and source records attributed to Kalpataru Pradhan.

33 records · Page 2Linked to original sources

arXiv2020

Impact of Next-Nearest-Neighbor hopping on Ferromagnetism in Diluted Magnetic Semiconductors

Being a wide band gap system GaMnN attracted considerable interest after the discovery of highest reported ferromagnetic transition temperature $T_C$ $\sim$ 940 K among all diluted magnetic semiconductors. Later it become a debate due to the observation of either a ferromagnetic state with very low $T_C$ $\sim$ 8 K or sometimes no ferromagnetic state at all. We address these issues by calculating the ferromagnetic window, $T_C$ Vs $p$, within a $t-t'$ Kondo lattice model using a spin-fermion Monte-Carlo method on a simple cubic lattice. We exploit the next-nearest-neighbor hopping $t'$ to tune the degree of delocalization of the free carriers and show that carrier localization (delocalization) significantly widen (shrunken) the ferromagnetic window with a reduction (enhancement) of the optimum $T_C$. We connect our results with the experimental findings and try to understand the ambiguities in ferromagnetism in GaMnN.

cond-mat.str-el↗

arXiv2019

Effect of A-site ionic radius on metamagnetic transition in charge ordered $Sm_{0.5}(Ca_{0.5-y}Sr_{y})MnO_3$ compounds

We investigate the ultra-sharp jump in the isothermal magnetization and the resistivity in the polycrystalline $Sm_{0.5}(Ca_{0.5-y}Sr_{y})MnO_3$ $(y = 0, 0.1, 0.2, 0.25, 0.3, 0.5)$ compounds. The critical field $(H_{cr})$, required for the ultra-sharp jump, decreases with increase of `Sr' concentration, i.e. with increase of average A-site ionic radius $\langle r_A\rangle$. The magnetotransport data indicate that the phase separation increases with the increase of $\langle r_A\rangle$, i.e. with $y$. The dependency of $H_{cr}$ with magnetic field sweep rate reveals that the ultra-sharp jump from antiferromagnetic (AFM) state to the ferromagnetic (FM) state is of martensitic in nature. Our two-band double exchange model Hamiltonian calculations show that the `Sr' doping induces the ferromagnetic clusters in the antiferromagnetic insulating phase and in turn reduces the critical field. In the end we present a phenomenological picture obtained from our combined experimental and theoretical study.

cond-mat.mtrl-sci↗

arXiv2019

Designing multi-level resistance states for multi-bit storage using half doped manganites

Designing nonvolatile multi-level resistive devices is the necessity of time to go beyond traditional one-bit storage systems, thus enhancing the storage density. Here, we explore the electronic phase competition scenario to design multi-level resistance states using a half doped CE-type charge ordered insulating bulk manganite, $Sm_{0.5}Ca_{0.25}Sr_{0.25}MnO_3$ (SCSMO). By introducing electronic phase coexistence in a controllable manner in SCSMO, we show that the system can be stabilized into several metastable states, against thermal cycling, up to 62 K. As a result the magnetization (and the resistivity) remains unaltered during the thermal cycling. Monte Carlo calculations using two-band double exchange model, including super-exchange, electron-phonon coupling, and quenched disorder, show that the system freezes into a phase coexistence metastable state during the thermal cycling due to the chemical disorder in SCSMO. Using the obtained insights we outline a pathway by utilizing four reversible metastable resistance states to design a prototype multi-bit memory device.

physics.app-ph↗

arXiv2018

Huge magnetoresistance and ultra-sharp metamagnetic transition in polycrystalline ${Sm_{0.5}Ca_{0.25}Sr_{0.25}MnO_3}$

Large magnetoresistive materials are of immense interest for a number of spintronic applications by developing high density magnetic memory devices, magnetic sensors and magnetic switches. Colossal magnetoresistance, for which resistivity changes several order of magnitude (${\sim10^4 \%}$) in an external magnetic field, occurs mainly in phase separated oxide materials, namely manganites, due to the phase competition between the ferromagnetic metallic and the antiferromagnetic insulating regions. Can one further enhance the magnetoresistance by tuning the volume fraction of the two phases? In this work, we report a huge colossal magnetoresistance along with the ultra-sharp metamagnetic transition in half doped ${Sm_{0.5}Ca_{0.25}Sr_{0.25}MnO_3}$ manganite compound by suitably tuning the volume fraction of the competing phases. The obtained magnetoresistance value at 10 K is as large as $\sim10^{13}\%$ in a 30 kOe external magnetic field and $\sim10^{15}\%$ in 90 kOe external magnetic field and is several orders of magnitude higher than any other observed magnetoresistance value reported so far. Using model Hamiltonian calculations we have shown that the inhomogeneous disorder, deduced from tunneling electron microscopy, suppresses the CE-type phase and seeds the ferromagnetic metal in an external magnetic field.

cond-mat.mtrl-sci↗

arXiv2017

Nanoclustering phase competition induces the resistivity hump in colossal magnetoresistive manganites

Using a two-band double-exchange model with Jahn-Teller lattice distortions and super-exchange interactions, supplemented by quenched disorder, at electron density $n=0.65$, we explicitly demonstrate the coexistence of the $n$ = 1/2-type ($π, π$) charge-ordered and the ferromagnetic nanoclusters above the ferromagnetic transition temperature $T_{\rm c}$ in colossal magnetoresistive (CMR) manganites. The resistivity increases due to the enhancement of the volume fraction of the charge-ordered and the ferromagnetic nanoclusters with decreasing the temperature down to $T_{\rm c}$. The ferromagnetic nanoclusters start to grow and merge, and the volume fraction of the charge-ordered nanoclusters decreases below $T_{\rm c}$, leading to the sharp drop in the resistivity. By applying a small external magnetic field $h$, we show that the resistivity above $T_{\rm c}$ increases, as compared with the case when $h=0$, a fact which further confirms the coexistence of the charge-ordered and the ferromagnetic nanoclusters. In addition, we show that the volume fraction of the charge-ordered nanoclusters decreases with increasing the bandwidth and consequently the resistivity hump diminishes for large bandwidth manganites, in good qualitative agreement with experiments. The obtained insights from our calculations provide a complete pathway to understand the phase competition in CMR manganites.

cond-mat.str-el↗

arXiv2017

A new route to enhance the ferromagnetic transition temperature in diluted magnetic semiconductors

We investigate the magnetic and the transport properties of diluted magnetic semiconductors using a spin-fermion Monte-Carlo method on a 3D lattice in the intermediate coupling regime. The ferromagnetic transition temperature $T_c$ shows an optimization behavior, first increases and then decreases, with respect to the absolute carrier density $p_{abs}$ for a given magnetic impurity concentration $x$, as seen in the experiment. Our calculations also show an insulator-metal-insulator transition across the optimum $p_{abs}$ where the $T_c$ is maximum. Remarkably, the optimum $p_{abs}$ values lie in a narrow range around 0.11 for all $x$ values and the ferromagnetic $T_c$ increases with $x$. We explain our results using the polaron percolation mechanism and outline a new route to enhance the ferromagnetic transition temperature in experiments.

cond-mat.str-el↗

arXiv2015

Magnetic Reconstructions in B-site Doped Manganites

The magnetic nature of the B-site dopants controls the magnetic phases in B-site doped manganites RE$_{1-x}$AE$_{x}$Mn$_{1-η}$B$_η$O$_3$. Different B-site dopants of equal valence, doped into the same reference manganite, lead to different magnetic phases at low temperature, which can not be explained using the valence change scenario. We focus on trivalent B-site dopants in CE-CO-OO-I manganites at half-filling $x=0.50$ to study the role of magnetic interactions between the B-site dopants and the neighboring Mn-sites by using a two-orbital double-exchange model including super-exchange interactions, Jahn-Teller lattice distortions and substitutional disorder in two dimensions. We show that the magnetic reconstructions around the B-site dopants due to the modified double-exchange and super-exchange interactions control the phase competition in B-site doped manganites.

cond-mat.str-el↗

arXiv2013

Interfacial Magnetism in Manganite Superlattices

We use a two-orbital double-exchange model including Jahn-Teller lattice distortions, superexchange interactions, and long-range Coulomb (LRC) interactions to investigate the origin of magnetically disordered interfaces between ferromagnetic metallic (FM) and antiferromagnetic insulating (AFI) manganites in FM/AFI superlattices. The induced magnetic moment in the AFI layer varies non-monotonically with increasing AFI layer width as seen in the experiment. We provide a framework for understanding this non-monotonic behavior which has a one-to-one correspondence with the magnetization of the FM interface. The obtained insights provide a basis for improving the tunneling magnetoresistance in FM/AFI manganite superlattices by avoiding a magnetic dead layer (MDL) in the FM manganite.

cond-mat.str-el↗

arXiv2013

Electronic and Magnetic Reconstructions in Manganite Superlattices

We investigate the electronic reconstruction at the interface between ferromagnetic metallic (FM) and antiferromagnetic insulating (AFI) manganites in superlattices using a two-orbital double-exchange model including superexchange interactions, Jahn-Teller lattice distortions, and long range Coulomb interactions. The magnetic and the transport properties critically depend on the thickness of the AFI layers. We focus on superlattices where the constituent parent manganites have the same electron density n = 0.6. The induced ferromagnetic moment in the AFI layers decreases monotonically with increasing layer width, and the electron-density profile and the magnetic structure in the center of the AFI layer gradually return to the bulk limit. The width of the AFI layers and the charge-transfer profile at the interfaces control the magnitude of the magnetoresistance and the metal-insulator transition of the FM/AFI superlattices.

cond-mat.str-el↗

arXiv2009

Conductance Switching and Inhomogeneous Field Melting in the Charge Ordered Manganites

The field induced switching of conductance in the charge ordered half-doped manganites is controlled by the combination of metastability, an inhomogeneous high field state, and cation disorder. We study this non-equilibrium problem via real space Monte Carlo on a disordered strong coupling model appropriate to the manganites. We reproduce the variation of the switching fields with the mean ionic radius r_A and cation disorder σ_A, and demonstrate how the experimental features arise from the proximity of several phases in the Landau free energy landscape. Our prediction for the field melted state is consistent with a growing body of experimental evidence.

cond-mat.str-el↗

arXiv2009

Magnetic Order Beyond RKKY in the Classical Kondo Lattice

We study the Kondo lattice model of band electrons coupled to classical spins, in three dimensions, using a combination of variational calculation and Monte Carlo. We use the weak coupling `RKKY' window and the strong coupling regime as benchmarks, but focus on the physically relevant intermediate coupling regime. Even for modest electron-spin coupling the phase boundaries move away from the RKKY results, the non interacting Fermi surface no longer dictates magnetic order, and weak coupling `spiral' phases give way to collinear order. We use these results to revisit the classic problem of 4f magnetism and demonstrate how both electronic structure and coupling effects beyond RKKY control the magnetism in these materials.

cond-mat.str-el↗

arXiv2008

Exploiting B Site Disorder for Phase Control in the Manganites

Disorder on the active d element site is usually very disruptive for conduction and long range order in perovskite transition metal oxides. However, in the background of phase competition such `B site' dopants also act to promote one ordered phase at the expense of another. This occurs either through valence change of the transition metal or via creation of `defects' in the parent magnetic state. We provide a framework for understanding the complex variety of phenomena observed in B site doped manganites and identify the key parameters that control the physics. Using a spatially resolved analysis of B ions in various manganite phases we explain the existing data and predict new situations where highly polarisable phase separated states can be created.

cond-mat.str-el↗

arXiv2008

First-principles study of TMNan (TM= Cr, Mn, Fe, Co, Ni; n = 4-7) clusters

Geometry, electronic structure, and magnetic properties of TMNan (TM=Cr-Ni; n = 4-7) clusters are studied within a gradient corrected density functional theory (DFT) framework. Two complementary approaches, the first adapted to all-electron calculations on free clusters, and the second been on plane wave projector augmented wave (PAW) method within a supercell approach are used. Except for NiNan, the clusters in this series are found to retain the atomic moments of the TM atoms, and the magnetic moment presented an odd-even oscillation with respect to the number of Na atoms. The origin of these odd-even oscillations is explained from the nature of chemical bonding in these clusters. Differences and similarities between the chemical bonding and the magnetic properties of these clusters and the TMNan (TM = Sc, V and Ti; n = 4-6) clusters on one hand, and TM-doped Au and Ag clusters on the other hand, are discussed.

cond-mat.mtrl-sci↗

arXiv2007

First principles study of Sc, Ti and V doped Na$_n$(n =4, 5, 6) clusters: Enhanced magnetic moments

Theoretical studies on the geometry, electronic structure and spin multiplicity of Sc, Ti and V doped Na$_n$ (n = 4, 5, 6) clusters have been carried out within a gradient corrected density functional approach. Two complementary approaches including all-electron calculations on free clusters, and supercell calculations using planewave pseudopotential and projector augmented wave formalisms have been carried out. It is shown that spin magnetic moments of the transition metal atoms, the magnitude of host polarization, and the sign of the host polarization all change with the number of alkali atoms. In particular the transition metal atoms are shown to attain spin moments that are higher than their atomic values. The role of hybridization between the transition atom d-states and the alkali $sp$-states is highlighted to account for the evolutions in the spin moments and host polarization.

cond-mat.mtrl-sci↗

arXiv2007

The Distinct Effects of Homogeneous Weak Disorder and Dilute Strong Scatterers on Phase Competetion in the Manganites

We study the two orbital double-exchange model in two dimensions in the presence of antiferromagnetic (AF) superexchange, strong Jahn-Teller coupling, and substitutional disorder. At hole doping x=0.5 we explore the `bicritical' regime where the energy of a ferromagnetic metal and a charge and orbital ordered (CO-OO) CE state are closely balanced, and compare the impact of weak homogeneous disorder to that of a low density of strong scatterers. Even moderate homogeneous disorder suppresses the CE-CO-OO phase and leads to a glass with nanoscale correlations. Dilute strong scatterers of comparable strength, however, convert the CE-CO-OO phase to a phase separated state with ferromagnetic and AF-CO-OO clusters. We provide the first spatial description of these phenomena and compare our results in detail to experiments on the half-doped manganites.

cond-mat.str-el↗