S. K. Dhar

Superconductivity at 12 K in YNiB system

We have observed a superconducting transition at 12 K in the system YNiB (nominal composition: YNi4B). Sharp drop in resistivity (it does not go to zero) and a diamagnetic susceptibility below 12 K clearly indicate a superconducting transition. Possibility of a magnetic phase transition at this temperature is ruled out by our specific heat measurements. Superconducting fraction of our samples, as estimated from Meissner effect measurements, is about 2%.

Growth of epitaxial and polycrystalline thin films of the electron doped system La1−xCexMnO3 through pulsed laser deposition

The polycrystalline La1−xCexMnO3 manganites do not exist in single phase in bulk under the preparation conditions so far employed, but their polycrystalline and epitaxial films deposited by the pulsed laser deposition (PLD) technique form readily in single phase. The cerium oxide (CeO2) remains partially unreacted when the bulk sample is prepared through the solid state reaction route. The resistivity of the bulk La0.7Ce0.3MnO3 sample shows a broad metal insulator transition (MIT) clearly resolved into two peaks, suggesting the presence of a second (impurity) phase, which is identified as unreacted CeO2 by the intensity analysis of the x-ray diffraction (XRD) data. However, when prepared as thin films by PLD, La0.7Ce0.3MnO3 forms in single phase, as corroborated by the uniqueness and sharpness of the MIT peak and also by the XRD patterns of the polycrystalline films. We also performed a detailed study of the epitaxial films by a high-resolution XRD system with a four-circle goniometer and did not find any...

Strong itinerant magnetism in ternary boride CeRh3B2

Reports a high-temperature itinerant ferromagnet CeRh3B2 which orders magnetically at 115K. The magnetic ordering seems to be intrinsic and presumably arises from the Rh d-band. Similar compounds of La, Pr and Nd do not show any magnetic ordering down to 77K.

Magnetic structure of EuFe2As2 determined by single-crystal neutron diffraction

Among various parent compounds of iron pnictide superconductors, EuFe2As2 stands out due to the presence of both spin density wave of Fe and antiferromagnetic ordering (AFM) of the localized Eu2+ moment. Single crystal neutron diffraction studies have been carried out to determine the magnetic structure of this compound and to investigate the coupling of two magnetic sublattices. Long range AFM ordering of Fe and Eu spins was observed below 190 K and 19 K, respectively. The ordering of Fe2+ moments is associated with the wave vector k = (1,0,1) and it takes place at the same temperature as the tetragonal to orthorhombic structural phase transition, which indicates the strong coupling between structural and magnetic components. The ordering of Eu moment is associated with the wave vector k = (0,0,1). While both Fe and Eu spins are aligned along the long a axis as experimentally determined, our studies suggest a weak coupling between the Fe and Eu magnetism.

Magnetic structure ofEuFe2As2determined by single-crystal neutron diffraction

Among various parent compounds of iron pnictide superconductors, EuFe2As2 stands out due to the presence of both spin density wave of Fe and antiferromagnetic ordering (AFM) of the localized Eu2+ moment. Single crystal neutron diffraction studies have been carried out to determine the magnetic structure of this compound and to investigate the coupling of two magnetic sublattices. Long range AFM ordering of Fe and Eu spins was observed below 190 K and 19 K, respectively. The ordering of Fe2+ moments is associated with the wave vector k = (1,0,1) and it takes place at the same temperature as the tetragonal to orthorhombic structural phase transition, which indicates the strong coupling between structural and magnetic components. The ordering of Eu moment is associated with the wave vector k = (0,0,1). While both Fe and Eu spins are aligned along the long a axis as experimentally determined, our studies suggest a weak coupling between the Fe and Eu magnetism.

Superconductivity and magnetism in quaternary F (F Y or f elements) transition metal borocarbides and magnetic properties of RNi4B (R Y or rare earth)

Abstract Our recent pioneering discovery of bulk superconductivity (SC) in YNiBC at an elevated temperature ( T c ≈ 12 K) has triggered intense research activity in borocarbide materials containing 4f and 5f elements. This was an offshoot of our studies of RNi 4 B (R  Y or rare earth) series of materials. Replacing Ni by Pd produces a superconducting phase in YPdBC with record T c (≈ 23 K) in bulk intermetallics. Here we present the results of some of our investigations in borocarbide materials. Our specific heat studies suggest that YNi 2 B 2 C is a strong-coupling type II superconductor. The temperature dependence of H c2 ( T ) is rather unusual. μ-SR measurements indicate the possibility that Ni atoms carry a magnetic moment. SC and magnetism coexist in RNi 2 B 2 C (R  Ho, Er or Tm). CeNi 2 B 2 C is found to be a mixed-valence material. NdNi 2 B 2 C and GdNi 2 B 2 C, which we have been able to synthesize as a single phase, do not superconduct but order magnetically at T m ≈ 4.5 K and T m ≈ 19 K respectively. We have observed SC in ThNiBC and ThPdBC systems. There are at least two superconducting transitions in the Pd-containing systems ThPdBC and YPdBC. YT 2 B 2 C (T  Co, Ir, Rh or Os) crystallize in a YNi 2 B 2 C-type structure but do not superconduct down to 4.2 K. Substitution studies show that Th (non-magnetic) and Gd (magnetic) suppress T c by nearly the same extent. We also summarize our results on RNi 4 B series of materials which show remarkable physical properties.

Magnetic behavior of RRh3B2 (R = La to Gd) ternary borides☆

Abstract Magnetization of the rare earth ternary borides RRh 3 B 2 (R = La to Gd), which crystallize in the hexagonal CeCo 3 B 2 type structure, has been measured in the temperature range 4.2 to 300 K. The Curie temperatures have been determined from the low field ac susceptibility measurements. All except LaRh 3 B 2 order magnetically. The Gd and Nd compounds have Curie temperatures ( T c ) of 91 K and 6 ; 7 F 0 ) and cerium has a valence greater than 3. In spite of this, CeRh 3 B 2 and EuRh 3 B 2 exhibit magnetic ordering with T c s of 108 and 40 K and saturation moments of 0.38 and 0.55μ B at 4.2 K, respectively. The magnetism in these two compounds is presumed to be of the itinerant type. The compound SmRh 3 B 2 has a very small moment at 4.2 K in 20 kOe field (0.046μ B ) and exhibits a peculiar temperature dependence of magnetization.

Anomalous suppression of Tc and moderate heavy fermion behaviour in YbNi2B2C

Abstract We have synthesized single phase material YbNi 2 B 2 C. Results of our X-ray diffraction and L III -edge studies are consistent with (nearly) trivalent state of Yb-ions in this material. Superconductivity is not observed down to 2 K in our resistivity and magnetic susceptibility studies. This is anomalous as, with Yb-ions in 3+ state, one expects T c ∼ 12K in this material on the basis of the systematic of T c in other magnetic superconductors belonging to the series R Ni 2 B 2 C ( R , a rare earth ion). Resistivity is relatively temperature independent in the temperature range 50K–300K. However, below 50K there is a large reduction of resistivity. The Sommerfeld coefficient, γ, obtained from the heat capacity data in the temperature range 12K–25K, is ∼200 mJ/mol.K 2 which suggests the formation of a moderately heavy fermion state in YbNi 2 B 2 C at low temperatures. Suppression of superconductivity constitutes an independent evidence of anomalous state of Yb-ions in YbNi 2 B 2 C

Bor on addition to RPd3 compounds (R = rare earth)

Abstract It is observed that boron can be alloyed with RPd 3 (R = rare earth) compounds to yield alloys of the type RPd 3 B x , where x typically ranges from 0 to 1. Addition of boron does not change the structure type but merely results in lattice expansion. The most likely position for boron is the body centered position. In CePd 3 and EuPd 3 addition of boron causes a change in the valence state of the rare earth ion.

Magnetic properties of RNi/sub 2/B/sub 2/C (R=Pr, Sm, Dy and Tb)

We report here the results of magnetization and heat capacity measurements on RNi/sub 2/B/sub 2/C (R=Pr, Sm, Dy and Tb). Sm, Dy and Th compounds order magnetically at /spl ap/10 K, 11 K and 18 K respectively. Dy and Tb compounds show field induced magnetic transitions. In the new compound, PrNi/sub 2/B/sub 2/C, magnetic susceptibility measured under zero field cooled and field cooled conditions show a significant difference.