Phase diagram of CeFeAs1−x_{1-x}Px_{x}O obtained from electric resistivity, magnetization, and specific heat measurements

We performed a systematic study on the properties of CeFeAs$_{1-x}$P$_{x}$O ($0\leq x\leq 1$) by electrical resistivity, magnetization and specific heat measurements. The c-axis lattice constant decreases significantly with increasing P content, suggesting a remarkable chemical pressure. The Fe-3d electrons show the enhanced metallic behavior upon P-doping and undergo a magnetic quantum phase transition around $x \approx 0.4$. Meanwhile, the Ce-4f electrons develop a ferromagnetic order near the same doping level. The ferromagnetic order is vanishingly small around $x=0.9$. The data suggest a heavy-fermion-like behavior as $x\geq 0.95$. No superconductivity is observed down to 2 K. Our results show the ferromagnetic ordered state as an intermediate phase intruding between the antiferromagnetic bad metal and the nonmagnetic heavy fermion metal and support the cerium-containing iron pnictides as a unique layered Kondo lattice system.Comment: 7 pages, 6 figures, text and figures revised, references added

Large Low Temperature Magnetoresistance and Magnetic Anomalies in Tb2_2PdSi3_3 and Dy2_2PdSi3_3

The results of heat-capacity, magnetic susceptibility, electrical resistivity and magnetoresistance $(\Delta \rho/\rho)$ measurements on the compounds Tb$_2$PdSi$_3$ and Dy$_2$PdSi$_3$, are reported. The results establish that these compounds undergo long-range magnetic ordering (presumably with a complex magnetic structure) below (Tc=) 23 and 8 K respectively. The $\Delta \rho/\rho$ is negative in the vicinity of Tc and the magnitude grows as Tc is approached from higher temperature as in the case of well-known giant magnetoresistance systems (La manganite based perovskites); this is attributed to the formation of some kind of magnetic polarons. The magnitude of magnetoresistance at low temperatures is quite large, for instance, about 30% in the presence of 60 kOe field at 5 K in the Dy sample.Comment: 4 pages, 3 figures, RevTe

Intriguing complex magnetism of Co in RECoAsO (RE=La, Nd and Sm)

We synthesized bulk polycrystalline samples of RECoAsO (RE=La, Nd and Sm) by solid state reaction route in an evacuated sealed quartz tube. All these compounds are crystallized in a tetragonal structure with space group P4/nmm. The Co, in these compounds is in itinerant ferromagnetic state with its paramagnetic moment above 1.5 microB and the same orders ferromagnetically (FM) with small saturation moment of around 0.20 microB below say 80K. This bulk intrinsic magnetism of Co changes dramatically when nonmagnetic La is changed by magnetic Sm and Nd. Although the itinerant ferromagnetism occurs below 80-100K with small saturation moment, typical anti-ferromagnetic (AFM) transitions (TN1, TN2) are observed at 57K and 45K for Sm and at 69K and 14K for Nd. The transition of Co spins from FM to AFM, for magnetic Sm and Nd in RECoAsO is both field and temperature dependent. For applied fields below 100Oe, both TN1 and TN2 are seen, with intermediate fields below 1-2kOe only TN1 and above say 5kOe the AFM transition is not observed. This is evidenced in isothermal magnetization (MH) plots as well. It is clear that Sm/Nd magnetic moments interact with the ordered Co spins in adjacent layer and thus transforms the FM ordering to AFM. All the studied compounds are metallic in nature, and their magneto-transport R(T)H follows the temperature and field dependent FM-AFM transition of ordered Co spins.Comment: 11 pages text + Figs: Queries to - [email protected] (www.freewebs.com/vpsawana

Coexistence of different magnetic moments in CeRuSn probed by polarized neutrons

We report on the spin densities in CeRuSn determined at elevated and at low temperatures using polarized neutron diffraction. At 285 K, where the CeRuSn crystal structure, commensurate with the CeCoAl type, contains two different crystallographic Ce sites, we observe that one Ce site is clearly more susceptible to the applied magnetic field whereas the other is hardly polarizable. This finding clearly documents that distnictly different local environment of the two Ce sites causes the Ce ions to split into magnetic Ce3+ and non-magnetic Ce(4-delta)+ valence states. With lowering the temperature, the crystal structure transforms to a structure incommensurately modulated along the c axis. This leads to new inequivalent crystallographic Ce sites resulting in a re-distribution of spin densities. Our analysis using the simplest structural approximant shows that in this metallic system Ce ions co-exist in different valence states. Localized 4f states that fulfill the third Hund's rule are found to be close to the ideal Ce3+ state (at sites with the largest Ce-Ru interatomic distances) whereas Ce(4-delta)+ valence states are found to be itinerant and situated at Ce sites with much shorter Ce-Ru distances. The similarity to the famous alpha-gamma transition in elemental cerium is discussed.Comment: 3 figures, 1 tabl

Synthesis, structural and transport properties of the hole-doped Superconductor Pr_{1-x}Sr_xFeAsO

Superconductivity was achieved in PrFeAsO by partially substituting Pr^{3+} with Sr^{2+}. The electrical transport properties and structure of this new superconductor Pr_{1-x}Sr_xFeAsO at different doping levels (x = 0.05$\sim$ 0.25) were investigated systematically. It was found that the lattice constants (a-axis and c-axis) increase monotonously with Sr or hole concentration. The superconducting transition temperature at about 16.3 K (95% $\rho_n$) was observed around the doping level of 0.20$\sim$ 0.25. A detailed investigation was carried out in the sample with doping level of x = 0.25. The domination of hole-like charge carriers in this material was confirmed by Hall effect measurements. The magnetoresistance (MR) behavior can be well described by a simple two-band model. The upper critical field of the sample with T_c = 16.3 K (x = 0.25) was estimated to be beyond 45 Tesla. Our results suggest that the hole-doped samples may have higher upper critical fields comparing to the electron-doped ones, due to the higher quasi-particle density of states at the Fermi level.Comment: 7 pages, 8 figures, 2 new figures and some contents adde

Dependence of the magnetic ordering temperature on hydorstatic pressure for the ternary intermetallic compounds GdAgMg, GdAuMg, EuAgMg, and EuAuMg

Quantum Matter and Optic

57Fe Mossbauer spectroscopy and magnetic measurements of oxygen deficient LaFeAsO

We report on the magnetic behavior of oxygen deficient LaFeAsO1-x (x-0.10) compound, prepared by one-step synthesis, which crystallizes in the tetragonal (S.G. P4/nmm) structure at room temperature. Resistivity measurements show a strong anomaly near 150 K, which is ascribed to the spin density wave (SDW) instability. On the other hand, dc magnetization data shows paramagnetic-like features down to 5 K, with an effective moment of 0.83 mB/Fe. 57Fe Mossbauer studies (MS) have been performed at 95 and 200 K. The spectra at both temperatures are composed of two sub-spectra. At 200 K the major one (88%), is almost a singlet, and corresponds to those Fe nuclei, which have two oxygen ions in their close vicinity. The minor one, with a large quadrupole splitting, corresponds to Fe nuclei, which have vacancies in their immediate neighborhood. The spectrum at 95 K, exhibits a broadened magnetic split major (84%) sub-spectrum and a very small magnetic splitting in the minor subspectrum. The relative intensities of the subspectra facilitate in estimating the actual amount of oxygen vacancies in the compound to be 7.0(5)%, instead of the nominal LaFeAsO0.90. These results, when compared with reported 57Fe MS of non-superconducting LaFeAsO and superconducting LaFeAsO0.9F0.1, confirm that the studied LaFeAsO0.93 is a superconductivity-magnetism crossover compound of the newly discovered Fe based superconducting family.Comment: 7 pages text + Figs : Comments/suggestions welcome ([email protected]

Distorted magnetic orders and electronic structures of tetragonal FeSe from first-principles

We use the state-of-the-arts density-functional-theory method to study various magnetic orders and their effects on the electronic structures of the FeSe. Our calculated results show that, for the spins of the single Fe layer, the striped antiferromagnetic orders with distortion are more favorable in total energy than the checkerboard antiferromagnetic orders with tetragonal symmetry, which is consistent with known experimental data, and the inter-layer magnetic interaction is very weak. We investigate the electronic structures and magnetic property of the distorted phases. We also present our calculated spin coupling constants and discuss the reduction of the Fe magnetic moment by quantum many-body effects. These results are useful to understand the structural, magnetic, and electronic properties of FeSe, and may have some helpful implications to other FeAs-based materials

To What Extent Iron-Pnictide New Superconductors Have Been Clarified: A Progress Report

In this review, the authors present a summary of experimental reports on newly discovered iron-based superconductors as they were known at the end of 2008. At the same time, this paper is intended to be useful for experimenters to know the current status of these superconductors. The authors introduce experimental results that reveal basic physical properties in the normal and superconducting states. The similarities and differences between iron-pnictide superconductors and other unconventional superconductors are also discussed.Comment: 20 pages, 32 figures. Open selec