Kiran Singh

Evidence for magneto-electric and spin–lattice coupling in PbFe0.5Nb0.5O3 through structural and magneto-electric studies

Neutron diffraction (ND) studies were carried out on polycrystalline single-phase multiferroic Pb(Fe0.5Nb0.5)O3 (PFN) in the temperature range of 290–2xa0K to understand the structural and magnetic properties as a function of temperature. ND data were refined using the Rietveld refinement method for both crystallographic and magnetic structures. The structure at room temperature was found to be monoclinic, in Cm space group. No structural transition was observed till 2xa0K. At low temperatures (i.e., from Txa0<xa0TN; TNxa0=xa0155xa0K), an additional peak appears at scattering vector, Qxa0=xa01.35xa0Å−1, indicating the onset of antiferromagnetic ordering. The magnetic structure was found to be commensurate with the crystallographic structure and could be refined using the propagation vector, kxa0=xa0[0.125, 0.5, and 0.5]. Magnetization, ferroelectric P–E loops, and dielectric measurements on PFN reveal a strong anomaly at the antiferromagnetic transition temperature (TN) indicating the magneto-electric coupling. The refined temperature-dependent structural parameters such as unit cell volume and monoclinic distortion angle (β) reveal pronounced anomalies at the magnetic ordering temperature (TN), which indicates strong spin–lattice coupling. An anomaly in lattice volume was observed with a small negative thermal expansion below and a large thermal expansion above the TN, respectively. It shows the occurrence of isostructural phase transition accompanying the magnetic ordering below TN ~155xa0K, leading to significant change in ionic polarization, octahedral tilt angle, and lattice strain around TN. We have used refined atomic positional coordinates from the nuclear and magnetic structures, to obtain ionic polarization. These detailed studies confirm the magneto-electric and spin–lattice coupling in PFN across TN.

A reentrant phenomenon in magnetic and dielectric properties of Dy2BaNiO5 and an intriguing influence of external magnetic field

We report that the spin-chain compound Dy2BaNiO5, recently proven by us to exhibit magnetoelectric coupling below its Néel temperature (TN) of 58 K, exhibits strong frequency-dependent behavior in ac magnetic susceptibility and complex dielectric properties at low temperatures (<10u2009K), mimicking the reentrant multiglass phenomenon. Such a behavior is not known among undoped compounds. A new finding in the field of multiferroics is that the characteristic magnetic feature at low temperatures moves towards higher temperatures in the presence of a magnetic field (H), whereas the corresponding dielectric feature shifts towards lower temperatures with H, unlike the situation near TN. This observation indicates that the alignment of spins by external magnetic fields tends to inhibit glassy-like slow electric-dipole dynamics, at least in this system, possibly arising from peculiarities in the magnetic structure.

Low-temperature high magnetic field powder x-ray diffraction setup for field-induced structural phase transition studies from 2 to 300 K and at 0 to 8-T field

A low-temperature and high magnetic field powder x-ray diffractometer (XRD) has been developed at UGC-DAE CSR (UGC: University Grant Commission, DAE: Department of Atomic Energy, and CSR: Consortium for scientific research), Indore, India. The setup has been developed around an 18 kW rotating anode x-ray source delivering Cu-Kα x-rays coming from a vertical line source. It works in a symmetric θ-2θ parallel beam geometry. It consists of a liquid helium cryostat with an 8 T split-pair Nb-Ti superconducting magnet comprising two x-ray windows each covering an angular range of 65°. This is mounted on a non-magnetic type heavy duty goniometer equipped with all necessary motions along with data collection accessories. The incident x-ray beam has been made parallel using a parabolic multilayer mirror. The scattered x-ray is detected using a NaI detector through a 0.1° acceptance solar collimator. To control the motions of the goniometer, a computer programme has been developed. The wide-angle scattering data can be collected in a range of 2°-115° of 2θ with a resolution of ∼0.1°. The whole setup is tightly shielded for the scattered x-rays using a lead hutch. The functioning of the goniometer and the artifacts arising possibly due to the effect of stray magnetic field on the goniometer motions, on the x-ray source, and on the detector have been characterized by collecting powder XRD data of a National Institute of Standards and Technology certified standard reference material LaB6 (SRM-660b) and Si powder in zero-field and in-field conditions. Occurrence of field induced structural-phase transitions has been demonstrated on various samples like Pr0.5Sr0.5MnO3, Nd0.49Sr0.51MnO3-δ and La0.175Pr0.45Ca0.375MnO3 by collecting data in zero field cool and field cool conditions.

Evidence for spin glass state of NdCo1−xNixO3 (x = 0.3−0.5)

Low-temperature magnetic properties of single phase NdCo1−xNixO3(xu2009=u20090.3−0.5) have been studied using ac and dc magnetic susceptibility measurements. Nickel substituted samples have been found to exhibit a different magnetic state at low temperature as compared to pristine NdCoO3. The temperature dependent dc magnetization M (T) revealed the presence of a sharp cusp occurring at characteristic temperatures TP, for xu2009=u20090.3, 0.4, 0.5. Below TP, clear effect of magnetic field can be seen in M (T) curves and TP decreases with increasing magnetic field as well as Ni substitution content. The isothermal magnetization measurements at low temperatures shows small unsaturated hysteresis loop at lowest temperature (10u2009K). The ac susceptibility results show a clear frequency dependent feature. These results are analyzed to distinguish superparamagnetic and spin glass behavior by using Neel-Arrhennius, Vogel-Fulcher law, and power law fitting. This analysis ruled out the superparamagnet like state and suggests the pr...

Dielectric and magnetodielectric behavior of polycrystalline Ca3CoRhO6, a geometrically frustrated magnetic system

We have investigated dielectric andmagnetodielectric properties of the geometrically frustrated spin -chain compound Ca3CoRhO6, which is known to undergo ‘partially disordered antiferromagnetic’ transition around 90u2005K (T1) followed by another glassy magnetic transition around 30u2005K (T2). We have observed dielectric glass behavior around 90u2005K, at the magnetic transition, which indicates that this behavior is predominantly magnetic in its origin. The magnetic field dependent dielectric constant in the magnetically ordered state tracks the features in isothermal magnetization thereby establishingmagnetodielectric coupling in this compound.

Low temperature structural and transport studies of La0.175Pr0.45Ca0.375MnO3-δ

The temperature (T) dependent x-ray diffraction (XRD) and resistivity measurements of La0.175Pr0.45Ca0.375MnO3-δ (LPCMO) have been performed down to 2K to understand the structural and transport properties. From room temperature down to 220K, LPCMO exists in orthorhombic phase with Pnma structure and at T~220K, it transforms to charge ordered (CO) monoclinic phase with P21/m structure and remains as it is down to 2K. The CO phase is evident from the occurrence of weak but well defined superlattice peaks in the XRD pattern. This structural transformation is of first order in nature as evident from the phase coexistence across the transition region. These results thus clearly illustrate that LPCMO undergoes a first order structural phase transition from charge disordered orthorhombic phase to CO monoclinic phase at ~220K, consistent with temperature dependent resistivity results. Our structural analysis of T dependent XRD data using Rietveld refinement infers that below 220K, LPCMO forms commensurate CO mon...

Effect of HgO Addition on the Resistivity Behaviour of La0.75Ca0.25MnO3

We have reported the effect of HgO addition on electrical resistivity of La0.75Ca0.25MnO3 manganites. The samples have been synthesised by conventional solid state reaction method. The pellets of these samples were sintered at 1200°u2009C in ambient atmosphere. These samples were characterized by X‐Ray diffraction technique and low temperature four probe resistivity measurement using closed cycle helium cryostat. From XRD analysis, these samples were found to have single phase orthorhombic (Pnma) crystal structure. From the resistivity vs temperature behaviour of these samples, it has been found that the resistivity of HgO added La0.75Ca0.25MnO3 decreases considerably in comparison to La0.75Ca0.25MnO3. It has also been observed that HgO addition decreases the metal—insulator transition from 242 K to 232 K which is due to cationic disorder created by HgO in La0.75Ca0.25MnO3.