Chan-Soo Jee

Unusual magnetic and lattice transformation in UNiSn, a possible half‐metallic ferromagnetic system

UNiSn, as well as certain other ternary actinide compounds, displays very anomalous behavior in the temperature dependence of the electrical resistivity, ρ(T). In UNiSn this unusual ρ(T) dependence has been attributed to half‐metallic behavior with a semiconducting gap in the minority spin band and metallic behavior in the majority spin band with a ferromagnetic transition in the vicinity of 50 K. We have studied UNiSn using 119Sn Mossbauer spectroscopy and observed that annealing procedures influence the microscopic properties of the sample in a profound way. As‐cast samples are nonmagnetic. Annealing in the neighbor of 800 °C results in the formation of a magnetic component. The conversion to a magnetic phase is incomplete even after annealing for 210 days at 800 °C. The magnetic component goes through two transformations: (1) a region below approximately 60 K where there is a coexistence and gradual transformation of paramagnetic to magnetically ordered material and (2) a region below 43 K where the ma...

Magnetic to nonmagnetic transition in a highly hybridized f‐electron system: U(Ga1−xGex)3

UGa3 is an antiferromagnetic system with a Neel temperature TN =67 K and UGe3 is a weakly paramagnetic system with a nearly temperature‐independent magnetic susceptibility and a slightly enhanced low‐temperature electronic specific heat coefficient of 20 mJ/mole K2. The room‐temperature lattice constant and the temperature dependence of the magnetic susceptibility, electrical resistivity, and low‐temperature specific heat have been measured for the pseudobinary alloy system, U(Ga1−xGex)3. TN is rapidly depressed with increasing x and appears to go to zero at x≂0.18. The magnetic susceptibility remains nearly temperature independent across the series and neither the magnetic susceptibility nor low‐temperature specific heat display any significant enhancement in the paramagnetic regime.

Thermodynamic and pressure dependent magnetic characteristics of PrBa2Cu3O7

Abstract The temperature dependence of the specific heat and magnetic susceptibility of PrBa 2 Cu 3 O 7 indicates that it is nonsuperconducting for T > 1.0 K but magnetically orders in the vicinity of 17 K. The entropy removal associated with this magnetic transition is 5 J/mol K and the isothermal magnetization at T = 4.2 K to 23 T shows no evidence of saturation.

Pressure and field dependence of the magnetic transition in GdBa2Cu3O7-x

Abstract The influence of hydrostatic pressure and of magnetic field strenght is presented for the low temperature antiferromagnetic ordering temperature ( T N =2.3 K) of GdBa 2 Cu 3 O 7- x . Data are presented for both superconducting and normal samples, the superconducting sample having a sharp 95 K transition and the oxygen-depleted normal sample being a semiconductor. For both systems the Neel temperatures, extrapolated to zero measuring field, are identical: T N = (2.33±0.03) K. The effect of pressure is to raise the transition temperature slightly for both samples, d T N /d P =+0.03 K/kbar for the superconducting sample and +0.04 K/kbar for the normal sample. The temperature dependence of the heat capacity made in several fixed external magnetic fields and the isothermal magnetization for T T N provide a measure of the antiferromagnetic-paramagnetic phase boundary, which shows T N approaching T =0 K at about 2.5 T.

119Sn Mössbauer study of the evolution of the magnetic state in U(Sn,Pb)3

USn3 is a highly enhanced paramagnet that is very near a magnetic instability, and UPb3 has an antiferromagnetic transition at 31.3 K. As Sn is substituted for Pb in U(Sn1−xPbx)3, the Neel temperature, TN, initially increases to 38 K at x=0.7, then decreases, and appears to go to zero for x≲0.1. The temperature dependence of the magnetic susceptibility for 0≤x≤1.0 and 119Sn Mossbauer spectra for 0≤x≤0.98 have been measured. The Mossbauer spectra indicate that the antiferromagnetic structure for the Pb‐rich pseudobinary alloys is not type I as previously reported. As x decreases, at least two, and possibly four, distinct Sn hyperfine field sites are found. The saturation hyperfine fields at the Sn sites do not show as pronounced a variation with concentration as is seen in TN. For larger x the saturation hyperfine fields are independent of x, which suggests that the U magnetic moments are nearly constant for the Pb‐rich pseudobinary alloys, and then drop rapidly to zero at the magnetic instability. The fra...

Light heavy-fermions in Zn and Ni doped 123 Cu-O superconductors

Abstract The temperature dependence of the specific heat C(T) has been measured for YBa2(Cu1−xMx)3O7 (YBCO) with M = Zn and Ni and 1.2 λ LT = C(T) T at T = 1.5 K reaching values of 100–150 mJ⧸mol K2 for modest Zn and Ni doping. This γLT dependency for Zn doped YBCO has been associated with a localization of d-holes within the CuO2 planes caused by a disorder induced disruption of hybridization within the planes.

Epr Spectroscopy on Zn-Substituted YBa 2 Cu 3 O 7

The temperature and concentration dependence of the electron paramagnetic resonance spectra of YBa 2 (Cu 1−x Zn x ) 3 O 7 has been measured for 0≤x≤0.16. Zn substituted on the Cu-site cause a rapid nearly linear depression of the superconducting transition temperature T c with T c going to zero in the vicinity of x=0.10. Only weak EPR spectra due to ≤1% of the Cu-ions are seen in the superconducting phase of YBa 2 Cu 3 O 7 . These spectra are most likely attributed to Cu-ions in the vicinity of grain boundaries and do not represent a bulk response. However, a strong asymmetric resonance is observed in the Zn-substituted samples with x>0.1. The EPR results obtained for YBa 2 (Cu 1−x Zn x ) 3 O 7 are discussed in terms of a possible localization of d-electrons.

Hybridization effects — The evolution from non-magnetic to magnetic behavior in uranium-based systems☆

Abstract Hybridization of the f electrons with the conduction electrons and f electrons on adjacent ions drives the rich non-magnetic to magnetic behavior observed within the lighter actinides and intermetallic compounds containing these elements. Within and near this transition region, many of the very challenging problems associated with narrow band phenomena occur, e.g. strong spin fluctuations, spin glass behavior, itinerate magnetism, Kondo effect and heavy fermion behavior. Measurements of the temperature dependence of the specific heat, electrical resistivity and magnetic susceptibility are presented for several uranium-based pseudobinary alloy systems which display a substantial variation in f-f overlap and/or f conduction electron hybridization. Such studies clearly display the role hybridization plays in the very interesting and varied features reported for these systems.

Evidence for d-Electron Localization in YBa 2 (Cu 1−x Zn x ) 3 O 7 for 0<x<0.16

Zn substitution for Cu in YBa 2 Cu 3 O 7 rapidly reduces the superconducting transition temperature, T c . Superconductivity is quenched between x=0.08 (T =30 K) and x=0.10. The normal state paramagnetism grows with Zn substu-tition, presumably due to increased localization on the Cu sublattice. Susceptibility studies of oxygen depleted (nonsuperconducting) Zn-substituted samples support this. Strong non-linear isothermal magnetization suggesting an internal magnetic field is found at T=4.2 K in samples with Zn concentration near to the critical value for suppression of superconductivity. The results are discussed in terms of increased localization of d-electrons on the Cu sites with increasing Zn concentration, which is consistent with recent EPR data.