Joseph H. Ross

A novel copper organophosphonate with a pore-like 3D framework and Cu–Cu magnetic ordering

H2O3PCH2NH2+CH2PO3H− was hydrothermally reacted with Cu(II) in aqueous solution at a 1 : 1 ratio, forming Cu3[NH2(CH2PO3)2]2 (1). Two kinds of coordination environments exist around the central copper ions: a distorted square planar Cu(1) ion and an octahedral Cu(2) ion. They are bridged with one terminal oxygen atom forming a 3D-coordination framework of layers sharing corners. Novel antiferromagnetic coupling between the two kinds of Cu units was observed at 15 K, followed by ferromagnetic ordering at 9 K.

Ferromagnetism in Fe-doped Ba6Ge25 chiral clathrate

We have synthesized a Ba6Ge25 clathrate, substituting 3 Ge per formula unit by Fe. This chiral clathrate has Ge sites forming a framework of closed cages and helical tunnel networks. Fe atoms randomly occupy these sites, and exhibit high-spin magnetic moments. A ferromagnetic transition is observed with Tc=170 K, the highest observed Tc for a magnetic clathrate. However, the magnetic phase is significantly disordered, and exhibits a transformation to a reentrant spin glass phase. This system has a number of features in common with other dilute magnetic semiconductors.

Magnetic entropy change of the layered perovskites La2−2xSr1+2xMn2O7

Magnetocaloric properties of perovskite-type manganese oxides with double Mn-O layers of composition La2−2xSr1+2xMn2O7 (x=0.33 and 0.4) have been investigated. A broad peak of magnetic entropy change (−ΔSM) was observed at the Curie temperature. The shape of −ΔSM is strongly dependent on the Sr concentration. In contrast to Ln1−xAxMnO3 perovskites, the distinct curvilinear shape of −ΔSM for perovskites with double Mn-O layers shows different magnetic mechanisms arising from magnetocrystalline anisotropy.

Thermomagnetic hysteresis effects in NiMn and NiMnPd thin films

dc magnetization measurements, for zero-field cooled (MZFC) and field-cooled (MFC) cases, have been carried out for flash-evaporated Pd-doped NiMn thin films. These included reentrant phases (Ni76−xPdx)Mn24, for 0⩽x⩽5, and Ni75Mn23Pd2, a pure spinglass phase. The studies were performed over the temperature range 3–300 K. Low-field magnetization measurements show the irreversibility effect (MZFC and MFC diverge) at temperatures below the Curie temperature Tc. In Ni75Mn23Pd2, MZFC falls below MFC, as usually observed. However, in reentrant compositions, MZFC crosses MFC upon warming into the ferromagnetic regime, where it stays above MFC at temperatures below Tc. This unusual behavior is attributed to a model of Imry and Ma in which, in a ferromagnet with antiferromagnetic impurities, the impurities can couple to the host ferromagnetic alignment and force the system to break into domains antiferromagnetically coupled to each other. Field-cooled hysteresis measurements indicate the uniaxial anisotropy in the...

High-field magneto-thermo-mechanical testing system for characterizing multiferroic bulk alloys

Multiferroic meta-magnetic shape memory alloys are well known for exhibiting large magnetic field induced actuation strains, giant magnetocaloric effects, magneto-resistance, and structural and magnetic glassy behaviors. Thus, they are candidates for improving modern day sensing, actuation, magneto-resistance, and solid-state refrigeration processes. Until now, however, experimental apparatuses have typically been able to probe a limited ferroic parameter space in these materials, i.e., only concurrent thermal and mechanical responses, or magnetic and thermal responses. To overcome this barrier and better understand the coupling of multiple fields on materials behavior, a magneto-thermo-mechanical characterization device has been designed and implemented. This device is capable of compressing a specimen at load levels up to 5300 N collinearly with applied fields up to 9 T between temperatures of -100 °C and 120 °C. Uniaxial stress, strain, temperature, magnetic field, and the volumetric average magnetization have been simultaneously measured under mixed loading conditions on a NiCoMnIn meta-magnetic shape memory alloy and a few selected results are presented here.

Effect of exchange bias on the electrical resistivity of Pd doped NiMn thin films: Two-channel Kondo system

Electrical resistivity measurements have been carried out for both flash-evaporated reentrant spin glasses (RSGs) (Ni76−xPdx)Mn24 and Ni74.5Mn23.5Pd2, as well as Ni75Mn23Pd2, a pure SG. These measurements were carried out at temperatures down to 4K. We observed a very deep resistivity minimum at about 75K for Ni74Mn24Pd2. It was found previously [Oner et al., J. Appl. Phys. 89, 7044 (2001)] that this sample shows the largest coercivity and exchange unidirectional anisotropy among these films. In addition, magnetization measurements show that this takes place just on the border of the RSG such that it could be handled as a superparamagnetic sample. Previously it was assumed that the exchange bias created in the sample between the domains plays the dominant role in the resistivity minimum. On the other hand, in order to account for the temperature dependence of the resistivity below the minimum we have analyzed these data using the Kondo, two-channel Kondo, weak localization, and Cochrane models for structu...

Magnetic and thermodynamic properties of Nd2Ni2Pb

We report magnetic and thermodynamic measurements for Nd2Ni2Pb, one of the recently discovered R2Ni2Pb family of layered intermetallics. A specific heat jump at 19 K corresponds to an antiferromagnetic transition involving crystal-field-split Nd levels. Magnetization measurements show this phase to have a canted structure, with a metamagnetic transition to an aligned phase occurring in H=3T at low temperatures. The single antiferromagnetic phase stands in contrast to the more complex magnetic structures and multiple transitions in the heavy-rare-earth members of this family.

Pseudogap and anharmonic phonon behavior in Ba8Ga16Ge30: An NMR study

We have performed (69)Ga, (71)Ga, and (137)Ba NMR on Ba8Ga16Ge30, a clathrate semiconductor which has been of considerable interest due to its large figure of merit for thermoelectric applications. In measurements from 4 K to 450 K, we used measurements on the two Ga nuclei to separate the magnetic and electric quadrupole hyperfine contributions and thereby gain information about the metallic and phonon behavior. The results show the presence of a pseudogap in the Ga electronic states within the conduction band, superposed upon a large Ba contribution to the conduction band. Meanwhile the phonon contributions to the Ga relaxation rates are large and increase more rapidly with temperature than typical semiconductors. These results provide evidence for enhanced anharmonicity of the propagative phonon modes over a wide range, providing experimental evidence for enhanced phonon-phonon scattering as a mechanism for the reduced thermal conductivity.

Electrical transport, heat capacity, and high-field magnetization study in intermetallic Ni2CeSn compound

Magnetization, heat capacity, and electrical resistivity measurements were performed on a Ni2CeSn compound (orthorhombic structure) in the temperature range of 2–300K. This compound is paramagnetic down to 6K. At higher temperatures above T=150K, the magnetic susceptibility obeys Curie–Weiss behavior yielding an effective magnetic moment μeff=2.56μB∕f.u., which is very close to that of the free Ce3+ ion (2.54μB) with a high negative Curie–Weiss temperature, ΘCW=−170K. As the temperature is decreased, the magnetic moment decreases gradually to 0.43μB∕f.u. at 4.2K. We also infer that, based on the high field (up to 23T) magnetization and the magnetic susceptibility data, a crystal-field splitting of cerium atoms becomes significant at temperatures below 150K. We used heat capacity and resistivity measurements to determine the crystal-field splitting of the Ce3+ magnetic sublevels. It is found that the ground magnetic state for the Ce3+ is a doublet of Jz=±1∕2 states, with a first excited quartet of Jz=±3∕2 ...

Magnetic phase transitions in R5NiPb3 (R=Ce, Nd, and Gd)

We report magnetic and thermodynamic measurements for R5NiPb3 (R=Ce, Nd, Gd). Gd5NiPb3 has a transition at 68K, apparently ferrimagnetic from specific heat and magnetization measurements. For Ce5NiPb3, a phase transition is observed in heat capacity at 48 K, with the field dependence of heat capacity and magnetization also indicating ferrimagnetic behavior. Ce5NiPb3 shows metamagnetism at low temperatures, behavior rather similar to Nd5NiPb3 with its previously reported magnetic ordering below 42 K. Using nonmagnetic La5NiPb3, we isolated the magnetic heat capacity for all three materials. The results include a relatively large linear-T term in the specific heat and integrated magnetic entropies consistently smaller than expected.