arXiv subjects

A. K. Ghosh

Publications and source records attributed to A. K. Ghosh.

At least 19 recordsLinked to original sources

arXiv2019

Wasp -- Waisted loop and Spin frustration in Dy$_{2-x}$Eu$_x$Ti$_2$O$_7$ Pyrochlore

The Raman spectroscopy and AC and DC magnetization of Dy$_{2-x}$Eu$_x$Ti$_2$O$_7$ have been investigated. In Raman Spectroscopy, the systematic shift in all phonon modes with Eu content in Dy$_{2-x}$Eu$_x$Ti$_2$O$_7$ confirms that Dy$^{3+}$ ion is substituted by Eu3+ ions. High concentration of Eu induces the dipolar exchange interactions and crystal-field interactions in Dy$_{2-x}$Eu$_x$Ti$_2$O$_7$. Rich Eu content samples (x=1.8 and 1.9) show the existence of wasp-waisted hysteresis loop and that can be attributed to the coexistence of dipolar field and anisotropy exchange interaction. AC susceptibility shows two single ion spin freezing transitions corresponding to Dy$^{3+}$ and Eu$^{3+}$ ions respectively in x = 1.5, 1.8, 1.9 samples.

cond-mat.mtrl-sci↗

arXiv2019

Study of Band structure, Transport and magnetic properties of BiFeO3-TbMnO3 composite

Magnetoelectric multiferroic composite of two types of multiferroic (Type I and II) consisting BiFeO3 and TbMnO3 is studied for enhanced magnetic and transport properties. A narrower band gap is estimated from the UV-visible absorption spectrum from that of BiFeO3 and TbMnO3. With known value of band gap, the band structure was estimated from the valence band x-ray photoemission spectra (XPS) and ultra violet photoemission spectra (UPS). The valence and conduction band was found at 1.0 eV and 0.45 eV above and below the Fermi level respectively. Thus the insulating behavior of the system is understood from the reconstruction of the energy bands at the interface which happens due to lattice mismatch of the two materials. The large coercivity and the increase on the magnetization value are understood to be due to superexchange interaction between different Mn ions (Mn2+, Mn3+ and Mn4+). From the composition study of EDXA and core level x-ray photoemission spectra oxygen vacancy was found which in turn creates the mixed valence state of Mn to maintain the charge neutrality.

cond-mat.mtrl-sci↗

arXiv2019

Spin freezing and Field induced transition in (Tb1-xEux)2Ti2O7 : A Magnetic Property study

The structural, magnetic and Raman effect have been investigated on (Tb1-xEux)2Ti2O7. From structural study it is clear that Eu substitutes the Tb in Tb2Ti2O7. Raman effect study indicates the existence of hardening due to phonon-phonon anharmonic interaction. From dc-magnetic measurement it is observed that in Eu rich samples contribution of dipolar interaction is significant. From ac magnetic measurement of (Tb1-xEux)2Ti2O7 a new single ion weak spin freezing ~ 33K at zero magnetic field is observed. On applying a field of 1T, all the compounds show a field induced transition (T*) which shifts towards higher temperature with increase of Eu content. This field induced transition corresponds to single moment saturation.

cond-mat.str-el↗

arXiv2019

Giant longitudinal negative magneto-resistance under perpendicular magnetic field in Bi$_{2-x}$Fe$_x$Se$_{3-x}$S$_x$ Topological insulators

The magnetic, magneto-transport and ARPES studies of Fe and S co-doped Bi2Se3 were investigated. With doping concentration magneto-resistance (MR) gradually decreases and for a certain doping concentration giant negative MR is observed which persists up to room temperature. Magnetic measurement indicates that the negative MR is observed when ferromagnetic ordering is induced with Fe doping. The magnetic ordering can be attributed with the RKKY interaction. Positive MR reappears with larger doping concentration which may be attributed to the decrease of FM ordering due to the turning off of the spin-orbit coupling leading to the destruction of non-trivial bulk state. This in-effect de-hybridizes the conduction band with the Fe spin. The ARPES data also indicates that above a critical doping concentration (x>0.09) the non-trivial bulk state is completely destroyed.

cond-mat.str-el↗

arXiv2019

Room temperature large spontaneous exchange bias in hard-soft antiferromagnetic composite BiFeO3-TbMnO3

We report the presence of giant spontaneous exchange bias (HSEB) in a hard and soft antiferromagnetic composite of BiFeO3-TbMnO3 (BFO-TMO in 7:3 and 8:2 ratio). The HSEB varies between 5-778Oe, but persists up to room temperature with a maximum near a spin reorientation transition temperature observed from magnetization vs. temperature measurement in Zero-field cooled (ZFC) and Field cooled (FC) modes. Isothermal remnant magnetization measurements at room temperature indicate the presence of an interfacial layer of a 2 dimensional dilute antiferromagnet in a field (2D DAFF). A stable value of the exchange bias has been observed via training effect measurements which signify the role of interfacial exchange coupling in the system. Based on the experimental results we explain the presence of the giant spontaneous exchange bias on the basis of a strong strain-mediated magnetoelectriccoupling induced exchange interaction and the creation of 2D DAFF layer at the interface. Theproperties of this layer are defined by canting and pinning of BFO spins at the interface with TMO due to Fe and Mn interaction. X-ray Magnetic Circular Dichroism (XMCD) confirms the presence of canted antiferromagnetic ordering of BiFeO3, charge transfer between Mn ions and different magnetically coupled layers which play vital role in getting the exchange bias.

cond-mat.str-el↗

arXiv2019

Probing the multi spin-phonon coupling and local B-site disorder in Pr2CoFeO6 by Raman spectroscopy and correlation with its electronic structure by X-ray photoemission spectroscopy

Electronic structure near Fermi level of Pr2CoFeO6 (at 300 K) was investigated by X-ray photoemission spectroscopy (XPS) technique. All three cations, i.e., Pr, Co and Fe were found to be trivalent in nature. XPS analysis also suggested the system to be insulating in nature. Moreover, Raman spectroscopy study indicated the random distribution of the B-site ions (Co/Fe) triggered by same charge states. In temperature-dependent Raman study, the relative heights of the two observed phonon modes exhibited anomalous behaviour near magnetic transition temperature TN~270 K, thus indicating towards interplay between spin and phonon in the system. Furthermore, clear anomalous softening was observed below TN which confirmed the existence of strong spin-phonon coupling occurring for at least two phonon modes of the system. The line width analysis of the phonon modes essentially ruled out the role of magnetostriction effect in the observed phonon anomaly. The investigation of the lattice parameter variation across TN (obtained from the temperature-dependent neutron diffraction measurements) further confirmed the existence of the spin-phonon coupling.

cond-mat.str-el↗

arXiv2018

Electronic structure by X-ray absorption spectroscopy and observation offield induced unusually slowspin relaxation from magnetic properties in pyrochlore Eu$_{2-x}$Fe$_x$Ti$_2$O$_7$

X-ray absorption spectroscopy (XAS) as well as x-ray magnetic circular dichroism (XMCD) and magnetization of hybrid pyrochlore Eu2-xFexTi2O7 were investigated, where the rare earth Eu (4f) was replaced with transition metal Fe (3d) to introduce competing 4f-3d interactions. It is confirmed that the valence states of Eu and Fe ions are formally trivalent while that of Ti ions are tetravalent (3d0). The analysis yielded that the tetravalent Ti ions occupy octahedral sites with distorted Oh symmetry which is triggered by the presence of vacant 8a anionic site adjacent to TiO6 octahedra. Further study with Fe doping revealed that it essentially reduces the octahedral distortion by introducing anionic disorder (migration of 48f oxygen ions to 8a site). Analysis of O K edge XAS spectra further confirmed the Fe substitution causing the systematic change in the ligand (O2-) coordination of the Ti4+ cations. On the other hand, a new field induced transition (with Fe doping) at low temperature T* (4 K<T*< 8 K) in ac susceptibility with unusually slow spin relaxation was observed. The transition shifted towards higher temperatures both with increasing applied field and Fe concentration. However, the single ion spin freezing (Tf ~35 K) appears to be suppressed with Fe substitution. Interestingly, small amount of Fe3+ ion substitution showed significant enhancement in the dc magnetization at lower temperatures (<100 K). Analysis further indicated rise of dipolar FM exchange interaction with Fe doping.

cond-mat.mtrl-sci↗

arXiv2018

B-site spin-state and anti-site disorder driven multiple-magnetic phases: Griffiths phase, re-entrant cluster glass and exchange bias in double perovskite Pr$_2$CoFeO$_6$

We report the comprehensive experimental results identifying the magnetic spin ordering and the magnetization dynamics of a double perovskite Pr2CoFeO6 by employing the (dc and ac) magnetization, powder neutron diffraction (NPD) and X-ray magnetic circular dichroism (XMCD) techniques. X-ray diffraction and neutron diffraction studies revealed that Pr2CoFeO6 adopts a B-site disordered orthorhombic structure with space group Pnma. Additionally, ab initio band structure calculations performed on this system suggested an insulating anti-ferromagnetic (Fe-Fe) ground state. Magnetometry study showed the system to possess a spectrum of interesting magnetic phases including long range antiferromagnetic (canted) spin ordering (TN ~269 K), Griffiths phase, re-entrant cluster glass (RCG) (TG~ 34 K) and exchange bias. However, the NPD study divulged the exhibition of a long range G-type (below TN ~269 K) of spin ordering by Fe spins. Spin dynamics study by ac susceptibility technique confirmed the system possessing long range ordering at higher temperatureundergoes a RCG transition at ~34 K. Existence of Griffiths phase was confirmed by non-analytic field variation of magnetization and Heisenberg type temporal spin relaxation above long range ordering temperature TN ~269 K. The anti-site disorder related to the B-sites (Co/Fe) is found to be the main driving force forthe observed multiple magnetic phases. Furthermore, the electronic structure probed by the X-ray absorption spectroscopy (XAS) study suggested a nominal valance state of +3 for both of the B-site ions (Co/Fe) which in turn triggered the anti-site disorder in the system. Magnetic, XRD, NPD and XAS analysis yielded a low spin state (LS) for the Co3+ ions. The random non-magnetic dilution of magnetic Fe3+ (HS) ions by Co3+ (LS) ions essentially played a crucial role in manifesting the magnetic properties of the system.

cond-mat.mtrl-sci↗

arXiv2018

Anomalous and Topological Hall effect in Cu doped Sb2Te3 Topological Insulator

The magneto-transport and magnetization measurements of Sb1.90Cu0.10Te3 were performed at different temperatures and different fields. Magneto-transport measurement at high field indicates the coexistence of both bulk and surface states. The magnetization shows the induced antiferromagnetic ordering with Cu doping and the observed quantum oscillation in it indicates that magnetization in Sb1.90Cu0.10Te3 is the bulk property. The non linearity in Hall data suggests the existence of anomalous and topological Hall effect. The anomalous and topological Hall effect (THE) from measured hall data of Cu doped Sb2Te3 topological insulator have been evaluated.

cond-mat.mes-hall↗

arXiv2017

Coexistence of surface and bulk state and negative magnetoresistance in Sulfur doped Bi2Se3

The magneto-transport properties in Sulfur doped Bi2Se3 are investigated. The magnetoresistance (MR) decreases with increase of S content and finally for 7% (i.e. y=0.21) S doping the magnetoresistance becomes negative. This negative MR is unusual as it is observed when magnetic field is applied with the perpendicular direction to the plane of the sample. The magneto-transport behavior shows the shubnikov-de hass (SdH) oscillation indicating the coexistence of both surface and bulk states. The negative MR has been attributed to the bulk conduction.

cond-mat.mtrl-sci↗

arXiv2017

High temperature spin-freezing transition in Pyrochlore Eu2Ti2O7: A new observation from ac-susceptibility

The magnetization (both dc and ac) of pyrochlore Eu2Ti2O7 have been investigated. Ac susceptibility measurements reveal a new unusual spin freezing below 35K for pure Eu2Ti2O7. Frequency dependence of these ac peak positions indicate slow spin relaxation near this temperature and it follows the Arrhenius formula suggesting a thermally activated relaxation process. The origin of this spin freezing has been attributed essentially to a single ion process which is associated to Eu3+ spin relaxation. Non-magnetic dilution by Y3+ ions also confirms the single ion freezing.

cond-mat.mtrl-sci↗

arXiv2017

Ground state properties of the bond alternating spin-$\frac{1}{2}$ anisotropic Heisenberg chain

Ground state properties, dispersion relations and scaling behaviour of spin gap of a bond alternating spin-$\frac{1}{2}$ anisotropic Heisenberg chain have been studied where the exchange interactions on alternate bonds are ferromagnetic (FM) and antiferromagnetic (AFM) in two separate cases. The resulting models separately represent nearest neighbour (NN) AFM-AFM and AFM-FM bond alternating chains. Ground state energy has been estimated analytically by using both bond operator and Jordan-Wigner representations and numerically by using exact diagonalization. Dispersion relations, spin gap and several ground state orders have been obtained. Dimer order and string orders are found to coexist in the ground state. Spin gap is found to develop as soon as the non-uniformity in alternating bond strength is introduced in the AFM-AFM chain which further remains non-zero for the AFM-FM chain. This spin gap along with the string orders attribute to the Haldane phase. The Haldane phase is found to exist in most of the anisotropic region similar to the isotropic point.

cond-mat.str-el↗

arXiv2016

Change-over of carrier type and magneto-transport property in Cu doped Bi2Te3 Topological Insulators

Structural, resistivity, thermoelectric power and magneto-transport properties of Cu doped Bi2Te3 topological insulators have been investigated. The occurrence of the tuning of charge carriers from n type to p type by Cu doping at Te sites of Bi2Te3 is observed both from Hall effect and thermoelectric power measurements. Carrier mobility decreases with the doping of Cu which provides evidence of the movement of Fermi level from bulk conduction band to the bulk valence band. Thermoelectric power also increaseswith doping of Cu.Moreover linear magnetoresistance (LMR) has been observed at high magnetic field in pure Bi2Te3 which is associated to the gapless topological surface states protected by time reversal symmetry (TRS), whereas doping of Cu breaks TRS and an opening of band gap occurs which quenches the LMR.

cond-mat.mtrl-sci↗

arXiv2016

Significant enhancement of compositional and superconducting homogeneity in Ti rather than Ta-doped Nb3Sn

Nb3Sn wires are now very close to their final optimization but despite its classical nature, detailed understanding of the role of Ta and Ti doping in the A15 is not fully understood. Long thought to be essentially equivalent in their influence on Hc2, they were interchangeably applied. Here we show that Ti produces significantly more homogeneous chemical and superconducting properties. Despite Ta-doped samples having a slightly higher Tc onset in zero-field, they always have a wider Tc-distribution. In particular, whereas the Ta-doped A15 has a Tc-distribution extending from 18 down to 5-6 K (the lowest expected Tc for the binary A15 phase), the Ti-doped samples have no A15 phase with Tc below 12 K. The much narrower Tc distribution in the Ti-doped samples has a positive effect on their in-field Tc-distribution too, leading to an extrapolated Hc2(0) 2 Tesla larger than the Ta-doped one. Ti-doping also appears to be very homogeneous even when the Sn content is reduced in order to inhibit breakdown of the diffusion barriers in very high Jc conductors. The enhanced homogeneity of the Ti-doped samples appears to result from its assistance of rapid diffusion of Sn into the filaments and by its incorporation into the A15 phase interchangeably with Sn on the Sn sites of the A15 phase.

cond-mat.supr-con↗

arXiv2015

Local structure surrounding V sites in Co doped ZnV2O4

Co doped ZnV2O4 has been investigated by Synchrotron X-ray diffraction, Magnetization measurement and Extended X-ray absorption fine structure (EXAFS) analysis. With Co doping in the Zn site the system moves towards the itinerant electron limit. From Synchrotron and magnetization measurement it is observed that there is an effect in bond lengths and lattice parameters around the magnetic transition temperature. The EXAFS study indicates that Co ion exists in the High spin state in Co doped ZnV2O4.

cond-mat.str-el↗

arXiv2014

Effect of dilution of both A- and B- sites on the multiferroic properties of spinal Mott insulators

The structural, magnetic, electrical and transport properties of FeV2O4, by doping Li and Cr ions respectively in A and B sites, have been studied. Dilution of A-site by Li doping increases the ferri-magnetic ordering temperature and decreases the ferroelectric transition temperature. This also decreases the V-V distances which in effect increases the A-V coupling. This increased A-V coupling dominates over the decrease in A-V coupling due to doping of non-magnetic Li. On the other hand, Cr doping increases the ferri-magnetic ordering temperature but does not alter the ferroelectric transition temperature which is due to the fact that the polarization origin to the presence of almost non-substituted regions.

cond-mat.str-el↗

arXiv2014

Effect of Li doping on magnetic and transport properties of CoV2O4 and FeV2O4

The structural, magnetic and transport properties have been studied of Li doped CoV2O4 and FeV2O4. Li doping increases the ferri-magnetic ordering temperature of both the samples but decreases the spin-glass transition temperature of CoV2O4. The Li-doping decreases the V-V distance which in effect increases the A-V coupling. Thus the increased A-V coupling dominate over the decrease in A-V coupling due to doping of non-magnetic Li.

cond-mat.str-el↗

arXiv2014

Chemical Pressure effect at the boundary of Mott insulator and itinerant electron limit of Spinel Vanadates

The chemical pressure effect on the structural, transport, magnetic and electronic properties (by measuring X-ray photoemission spectroscopy) of ZnV2O4 has been investigated by doping Mn and Co on the Zinc site of ZnV2O4. With Mn doping the V-V distance increases and with Co doping it decreases. The resistivity and thermoelectric power data indicate that as the V-V distance decreases the system moves towards Quantum Phase Transition. The transport data also indicate that the conduction is due to the small polaron hopping. The chemical pressure shows the non-monotonous behaviour of charge gap and activation energy. The XPS study also supports the observation that with decrease of the V-V separation the system moves towards Quantum Phase Transition. On the other hand when Ti is doped on the V-site of ZnV2O4 the metal-metal distance decreases and at the same time the TN also increases.

cond-mat.str-el↗