The phenomena of strongly correlated electrons have been at the forefront of contemporary research in condensed matter physics for more than four decades. Over the years this study field has remained ...resourceful in providing a wealth of new physics and has compelled new thinking in many aspects of our understanding of the behavior of electrons in metals. Underpinning the vigor with which the field has kept on reinventing itself has been an ever-broadening materials class that convey the fascinating physics of correlated electrons in one guise or another. One particular group of compounds stem from the combination of the chemical elements cerium and ruthenium. The element cerium has been one of the most profitable starting points from which to synthesize compounds that show interesting and often anomalous magnetic, structural and correlated electron behavior. Among the key ingredients for characterizing these compounds is the nearest-neighbor Ce−Ru separation in the crystal lattice. Several examples are found in which this distance is notably shorter than the sum of the Ce and Ru covalent bonding radii, and this leads to hybridization of the normally well-localized 4f−electron orbital of Ce, with degenerate conduction electron bands. In this way an unstable or non-integer valence is produced on the Ce ion, which is responsible for many correlated electron features and a variety of magnetic phenomena that range from long-range order, through metamagnetism and phenomena at the edge of magnetism, to quantum criticality when order is suppressed to the absolute zero of temperature. Here we review results of a selected number of cerium-ruthenium compounds as examples with extraordinary physical properties. We highlight the commonalities of strongly correlated electron behavior in these compounds, in an effort to guide and expand the materials base for future studies.
•Ce in combination with Ru hosts a wide variety of unusual physical properties.•Ce–Ru compounds often have short bonding distances, with unusual magnetic phenomena.•The manuscript collects and discusses the known Ce–Ru compounds.
•Synthesis and crystal structure of R3Cu (R = Ce, Pr) compounds.•Frustrated antiferromagnets.•Spin-glass behavior and metamagnetism.•Heavy fermion state.
We report the synthesis, crystal structure, ...and thermodynamic properties of the two binary laves phases Ce3Cu and Pr3Cu, which crystallize in the tetragonal I4/mmm (space group: 139). We have investigated the compounds using dc magnetic susceptibility χdc(T), ac magnetic susceptibility χac(T), isothermal magnetization M(B) and specific heat Cp(T) measurements. Both compounds show features of strong geometrical frustration, which is driven by the arrangement of the rare-earth ions on triangular lattice sites. Ce3Cu shows a phase transition at TN = 5 K and its M(B) at 2 K reveals the presence of a metamagnetic transition near 2 T. Pr3Cu, on the other hand, shows a spin-glass behaviour with a freezing temperature of Tf1 ≃ 12 K and with an irreversible behavior in zero-field-cooled (ZFC) and field-cooled (FC) χdc(T) below about 13 K. The real part of χac(T) reveals a frequency dependence of Tf. The result, however shows the presence of another frequency-dependent phase below Tf2 ≃ 9.35 K, a feature which signals a re-entrant spin-glass behaviour in the compound. Furthermore, the frequency dependence of Tf1 follows the Vogel-Fulcher law, with an activation energy Ea∕kB = 5.3 K. This observation along with the estimated relative shift in Tf1 per decade in frequency δTf = 0.0105, irreversible behaviour in ZFC-FC χdc(T) and a slow decay in thermo-remnant magnetization provide supports for the formation of a cluster spin-glass in Pr3Cu. In addition, we observe the enhancement of the Sommerfeld coefficient, γ in both compounds with values of 0.1892(5) J/(molCe K2) and 0.5078(1) J/(molPr K2). The estimation of the Sommerfeld-Wilson ratio WR yield values of 0.576 and 1.17 for Ce3Cu and Pr3Cu, respectively.
Here we report the magnetic, electronic and thermal transport properties of the heavy-fermion semimetal Pr3Os4Ge13 with a cage-like structure by means of magnetic susceptibility, χ(T), isothermal ...magnetization, M(B, T), electrical resistivity, ρ(B, T), Hall coefficient, RH(T), specific heat, Cp(T), thermal conductivity, κ(T) and thermoelectric power, S(T). ρ(T) and RH(T) show semimetallic features in a manner that mimics a thermal activated behaviour with an activation energy, Δ/kB = 6.5 K indicating the opening of a small energy gap in the material. At low temperatures, a Sommerfeld coefficient, γ = 128 mJ (mol−1 K−2) observed indicates a mass enhancement of the quasiparticles at low temperatures which bears witness to a heavy fermion state in Pr3Os4Ge13. A Wilson-Sommerfeld ratio, WR and the dimensionless ratio, S/γT of 1.01 and 0.62 ± 0.018 observed, respectively, are in good agreement with the Fermi liquid scenario. S(T) and RH(T) reveal hole dominated transport (p-type material) with a relatively large room temperature value of S(T) = 32.85 ± 0.98 μV K−1. A room temperature low value of κ(T) = 1.61 W K−1 m−1 leads to a thermoelectric figure of merit, ZT = 0.03 ± 0.001 which is comparable to values achieved in several clathrates around the same temperature. Features from S(T) and ρ(T) favour the realization of a higher ZT value at elevated temperatures.
•Synthesis and crystal structure of Eu2Cu5Ni5 phase.•Divalent electronic state of Eu.•Magnetic phase transitions and spin reorientation.•Electron dominated transport mechanism.•Negative ...magnetoresistance.
We report the synthesis and physical properties of a new ternary Eu2Cu5Ni5 phase which crystallizes in the orthorhombic space group Pmmm, No. 47, Z = 1, with lattice parameters; a = 4.015(4), b = 4.921(2) and c = 8.621(3) Å. Analysis from magnetic susceptibility χdc(T) results reveal an effective magnetic moment μeff = 7.8 μB/Eu that is comparable to the theoretical value of 7.93 μB for a divalent Eu2+ ion. The Eu2+ electronic state of 4f7 is well localized in this phase and the results preclude both the Cu and Ni sub-lattices from bearing a magnetic moment. Two long-range phase transitions pertaining to the Eu2+ moments are observed at TC1 = 72 K and TC2 = 47.2 K, followed by another anomaly at TSR = 22 K, attributed to a spin re-orientation of Eu moments. The Seebeck coefficient S(T) attains a value of − 16.7 μV/K at room temperature (RT) and indicates that the electrons are the dominant charge carriers. Evidence from the electrical transport reveals a metallic behavior with the magnetotransport showing a negative magnetoresistance below RT.
•A polycrystalline compound Tb2Rh3Ge has been prepared by arc-melting.•Crystal structure and magnetic properties of Tb2Rh3Ge have been studied.•Tb2Rh3Ge shows a second-order ...ferromagnetic–paramagnetic phase transition.•Magnetocaloric effect of Tb2Rh3Ge has been studied from isothermal magnetization.
We report the structural, and magnetic properties as well as magnetocaloric effect of a polycrystalline compound of Tb2Rh3Ge. This compound crystallizes with Mg2Ni3Si-type of rhombohedral Laves phases (space group R3‾m, hR18). The magnetic properties and magnetocaloric effect of Tb2Rh3Ge are explored through dc-magnetization measurements. Temperature dependence of magnetization revealed that the compound exhibits ferromagnetic behavior with TC=56 K. The field dependence of magnetization indicates that Tb2Rh3Ge is a soft ferromagnet. The obtained isothermal magnetic entropy changes (ΔSm) and refrigeration capacity (relative cooling power) for a change of magnetic field 0–9 T are 12.74 J kg−1K−1 and 497(680) J/kg respectively. The Arrott plots and universal curve of normalized ΔSm indicate that this compound undergoes a second order ferromagnetic phase transition.
•Observation of large negative magnetoresistance.•Hole dominated transport properties with features of temperature driven instabilities.•Formation of heavy fermion state at low temperatures.
We ...report the magnetotransport and thermodynamic properties of PrPtSi which crystallizes with the noncentrosymmetric LaPtSi-type structure (space group: I41md). Signatures from specific heat and magnetic susceptibility reveal an anomaly at T* ≃ 5.5 K which is attributed to crystal field effects. We find a negative magnetoresistance whose magnitude grows with decreasing temperature, with a value of −17.4% at 1.9 K and in 7 T. Results from the electronic-derived specific heat C4f reveal an upturn in C4f(T)/T at low temperature and traces a −logT dependence, attaining values of 1.1 J/(mol K2) and 3.6 J/(mol K2) at 0.38 K in zero and 4 T, respectively.
•Polycrystalline Gd2Rh3Ge and Er2Rh3Ge compounds successfully synthesized by arc-melting process.•Gd2Rh3Ge and Er2Rh3Ge compounds show second order ferromagnetic phase transition with their ...corresponding transition temperature.•Large magnetocaloric effect with large relative cooling power is obtained in Gd2Rh3Ge and Er2Rh3Ge compounds.
Magnetic phase transitions in two Mg2Ni3Si-type of Gd2Rh3Ge and Er2Rh3Ge compounds (space group R3‾m, hR18) belonging to rhombohedral Laves phases are studied in this paper. Structural investigations using X-ray diffraction and subsequent Rietveld analysis confirm the rhombohedral R3‾m Laves phase for both compounds. Magnetization as a function of temperature, M(T), identifies the ferromagnetic transition temperature in both samples with TC = 64 K and 20 K for Gd2Rh3Ge and Er2Rh3Ge respectively which is supported by the specific heat, Cp(T). Arrott plots, universal scaling curves by normalizing magnetic entropy changes (ΔSm) and field dependent ΔSm revealed that both compounds undergo second order ferromagnetic phase transitions at TC. Significant magnetocaloric effect (MCE) is observed for both compounds, attaining a value of 12 J kg−1 K−1 at T = 64 K, H = 7 T for Gd2Rh3Ge and 11 J kg−1 K−1 at T = 20 K, H = 7 T for Er2Rh3Ge. The adiabatic temperature change is 5.9 K for Gd2Rh3Ge and 10.5 K for Er2Rh3Ge for H = 7 T.
We report the crystal structure as well as the magnetic, electronic and transport properties of PrCu2Au3 which crystallizes in the MgCu4Sn-type structure with the cubic space group F4¯3m (No. 216). ...In this structure, the Pr3+ ion sits on a lattice site with the cubic Td symmetry which dictates the crystal electric field (CEF) splitting of the Hund’s 3H4 ground multiplet into four levels. The sample has been investigated by measurements of temperature dependences of specific heat, Cp(T), electrical resistivity, ρ(T), magnetic susceptibility, χ(T), isothermal magnetization, M(B,T), thermal conductivity, κT(T) and thermoelectric power, S(T). χ(T) follows a Curie-Weiss behavior above 100 K with an effective magnetic moment, μeff = 3.3 μB/Pr which is comparable to a calculated value of 3.58 μB/Pr for a free Pr3+ ion and a Weiss temperature, θCW = 15.70 K indicating a dominant ferromagnetic interaction. From the analysis of M(B,T) and the 4f-electron derived specific heat (C4f(T)) results, the CEF ground state is a Γ5 magnetic triplet and the CEF energy scheme is presented. The Sommerfeld coefficient, γ estimated from the low-temperature C4f(T) is 304 mJ/(mol K2) indicating a mass enhancement of the conduction electrons at low temperatures. Further analysis of the heavy fermion properties based on the Sommerfeld-Wilson ratio supports the formation of a heavy fermion state in PrCu2Au3. However, the absence of an enhancement in S/T at low temperatures possibly suggests a weak correlation between the conduction electrons and the 4f-electrons at the Fermi energy. The thermal transport properties suggest that PrCu2Au3 is likely a compensated metal.
•Synthesis and crystal structure of PrCu2Au3.•Formation of a heavy fermion state.•Crystalline electric field effects.
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Targeted delivery of carmustine (BCNU), an efficient brain tumor therapeutic, has been challenged with bioavailability issues due to the Blood Brain Barrier (BBB). The currently ...effective delivery approach is by implants at the site of the tumor, but this is highly invasive. The intranasal route, which is non-invasive and bypasses the BBB, may be alternative route for delivering BCNU to the brain. In this work, polyvinyl alcohol/polyethyleneimine/fIuorecein isothiocyanate complex (Polyplex) coated iron-oxide nanoparticles (Magnetite) were synthesized employing co-precipitation, epoxidation and EDC/NHS coupling reactions. The Polyplex coated magnetite (Nano-co-Plex) was loaded with BCNU for potential magnetically targeted delivery to the brain following intranasal administration. The Nano-co-Plex was characterized employing Thermogravimetric analysis (TGA), Superconducting Quantum Interference Device (SQUID) magnetometry, Fourier Transform Infrared Spectroscopy (FTIR), Nuclear Magnetic Resonance (NMR), X-ray Diffractometry (XRD), Transmission Electron Microscopy (TEM) and Zetasize analysis. Results revealed superparamagnetic hexagonally shaped “core-shell” nanoparticles with cell labeling attributes, of size ranging between 30–50nm, and a zeta potential value of +32±2mV. The Nano-co-Plex synthesized was found to possess high degree of crystallinity with 32% Polyplex coating. The loading and release studies indicated a time-dependent loading with maximum loading capacity of 176.82μg BCNU/mg of the carrier and maximum release of 75.8% of the loaded BCNU. Cytotoxicity of the BCNU-loaded Nano-co-Plex displayed superiority over the conventional BCNU towards human glioblastoma (HG) cells. Cell studies revealed enhanced uptake and internalization of BCNU-loaded Nano-co-plex in HG cells in the presence of an external magnetic field. These Nano-co-Plexes may be ideal as an intranasal magnetic drug targeting device for BCNU delivery.
In this article, we report the modification of the electronic and magnetic properties of few-layered graphene (FLG) nanoflakes by nitrogen functionalization carried out using radio-frequency ...plasma-enhanced chemical vapor deposition (rf-PECVD) and electron cyclotron resonance (ECR) plasma processes. Even though the rf-PECVD N2 treatment led to higher N-doping levels in the FLG (4.06 atomic %) as compared to the ECR process (2.18 atomic %), the ferromagnetic behavior of the ECR FLG (118.62 × 10–4 emu/g) was significantly higher than that of the rf-PECVD FLG (0.39 × 10–4 emu/g) and pristine graphene (3.47 × 10–4 emu/g). Although both plasma processes introduce electron-donating N atoms into the graphene structure, distinct dominant nitrogen bonding configurations (pyridinic, pyrrolic) were observed for the two FLG types. Whereas the ECR plasma introduced more sp2-type nitrogen moieties, the rf-PECVD process led to the formation of sp3-coordinated nitrogen functionalities, as confirmed through Raman measurements. The samples were further characterized using X-ray absorption near-edge spectroscopy (XANES), and X-ray and ultraviolet photoelectron spectroscopies revealed an increased electronic density of states and a significantly higher concentration of pyrrolic groups in the rf-PECVD samples. Because of the formation of reactive edge structures and pyridinic nitrogen moieties, the ECR-functionalized FLG samples exhibited highest saturation magnetization behavior with the lowest field hysteretic features. In comparison, the rf-PECVD samples displayed the lowest saturation magnetization owing to the disappearance of magnetic edge states and formation of stable nonradical-type defects in the pyrrole type structures. Our experimental results thus provide new evidence regarding the control of the magnetic and electronic properties of few-layered graphene nanoflakes through control of the plasma-processing route.
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