Neutron-scattering studies on CeM2Ge2(M=Ag, Au, and Ru)

Fantastic n = 4: Ce5Co4+xGe13-ySny of the An+1MnX3n+1 homologous series

Ce-based intermetallics are of interest due to the potential to study the interplay of localized magnetic moments and conduction electrons. Our work on Ce-based germanides led to the identification of a new homologous series A_n+1M_nX_3n+1 (A = rare earth, M = transition metal, X = tetrels, and n = 1-6). This work presents the single-crystal growth, structure determination, and anisotropic magnetic properties of the n = 4 member of the Ce_n+1Co_nGe_3n+1 homologous series. Ce₅Co_4+xGe_13-ySn_y consists of three Ce sites, three Co sites, seven Ge sites, and two Sn sites, and the crystal structure is best modeled in the orthorhombic space group Cmmm where a = 4.3031(8) Å, b = 45.608(13) Å, and c = 4.3264(8) Å, which is in close agreement with the previously reported Sn-free analog where a = 4.265(1) Å, b = 45.175(9) Å, and c = 4.293(3) Å. Anisotropic magnetic measurements show Kondo-like behavior and three magnetic transitions at 6, 4.9, and 2.4 K for Ce₅Co_4+xGe_13-ySn_y.

Structural Model and Spin-Glass Magnetism of the Ce3Au13Ge4 Quasicrystalline Approximant

In a search for unconventional heavy-Fermion compounds with the localized 4f moments distributed quasiperiodically instead of a conventional distribution on a regular, translationally periodic lattice, we have successfully synthesized a stable Ce₃Au₁₃Ge₄ Tsai-type 1/1 quasicrystalline approximant of the off-stoichiometric composition Ce_3+xAu_13+yGe_4+z (x = 0.17, y = 0.49, z = 1.08) and determined its structural model. The structure is body-centered-cubic (bcc), with space group Im3̅, unit cell parameter a = 14.874(3) Å, and Pearson symbol cI174, and can be described as a bcc packing of partially interpenetrating multishell rhombic triacontahedral clusters. The cerium sublattice, corresponding to the magnetic sublattice, consists of a bcc packing of Ce icosahedra with an additional Ce atom in a partially occupied site (occupation 0.7) at the center of each icosahedron. The measurements of its magnetic properties and the specific heat have demonstrated that it is a regular intermetallic compound with no resemblance to heavy-Fermion systems. The partially occupied Ce2 site in the center of each Ce1 icosahedron, the mixed-occupied Au/Ge ligand sites between the Ce2 and Ce1 atoms, and the random compositional fluctuations due to nonstoichiometry of the investigated Ce_3+xAu_13+yGe_4+z alloy introduce randomness into the Ce magnetic sublattice, which causes a distribution of the indirect-exchange antiferromagnetic interactions between the spins. Together with the geometric frustration of the triangularly distributed Ce moments, this leads to a spin-glass phase below the spin freezing temperature T_f ≈ 0.28 K.

A stable Ce₃Au₁₃Ge₄ Tsai-type 1/1 quasicrystalline approximant was synthesized, and its structural model and magnetic properties were determined. Randomness and geometric frustration within the Ce sublattice lead to a spin-glass phase below 0.28 K.

Direct hybridization gap from intersite and onsite electronic interactions in CeAg2Ge2

Electronic and crystal structure studies are presented to describe the role of intersite and onsite interactions for antiferromagnetic ordering in CeAg2Ge2. The crystal structure showed a prominent magnetovolume effect with anomalous negative thermal expansion at low temperature as a consequence of itinerant electron magnetism. The direct hybridization gap with a V-shaped band observed in the angle resolved photoemission data at room temperature, indicates that spin polarized quasiparticle states exist in the gapped region. Valence band broadening and enhanced localization effects at low temperature indicate strong hybridization of the valence orbitals of Ce atoms with the near neighbor Ge atoms. We find that the intersite interaction between the Ce atoms at high temperature stabilizes the onsite interaction at low temperature that leads to the spin density wave type antiferromagnetism in CeAg2Ge2.

Quantum critical behavior in Ce(Fe0.76Ru0.24)2Ge2

Systems with embedded magnetic ions that exhibit a competition between magnetic order and disorder down to absolute zero can display unusual low-temperature behaviors of the resistivity, susceptibility, and specific heat. Moreover, the dynamic response of such a system can display hyperscaling behavior in which the relaxation back to equilibrium when an amount of energy E is given to the system at temperature T only depends on the ratio E / T . Ce(Fe0.755Ru0.245)2Ge2 is a system that displays these behaviors. We show that these complex behaviors are rooted in a fragmentation of the magnetic lattice upon cooling caused by a distribution of local Kondo screening temperatures, and that the hyperscaling behavior can be attributed to the flipping of the total magnetic moment of magnetic clusters that spontaneously form and order upon cooling. We present our arguments based on the review of two-decades worth of neutron scattering and transport data on this system, augmented with new polarized and unpolarized neutron scattering experiments.

Ising-type Magnetic Anisotropy in CePd2As2

We investigated the anisotropic magnetic properties of CePd₂As₂ by magnetic, thermal and electrical transport studies. X-ray diffraction confirmed the tetragonal ThCr₂Si₂-type structure and the high-quality of the single crystals. Magnetisation and magnetic susceptibility data taken along the different crystallographic directions evidence a huge crystalline electric field (CEF) induced Ising-type magneto-crystalline anisotropy with a large c-axis moment and a small in-plane moment at low temperature. A detailed CEF analysis based on the magnetic susceptibility data indicates an almost pure |±5/2〉 CEF ground-state doublet with the dominantly |±3/2〉 and the |±1/2〉 doublets at 290 K and 330 K, respectively. At low temperature, we observe a uniaxial antiferromagnetic (AFM) transition at T_N = 14.7 K with the crystallographic c-direction being the magnetic easy-axis. The magnetic entropy gain up to T_N reaches almost R ln 2 indicating localised 4 f-electron magnetism without significant Kondo-type interactions. Below T_N, the application of a magnetic field along the c-axis induces a metamagnetic transition from the AFM to a field-polarised phase at μ₀H_c0 = 0.95 T, exhibiting a text-book example of a spin-flip transition as anticipated for an Ising-type AFM.

Role of valence fluctuations in the superconductivity of Ce122 compounds

Pressure dependence of the Ce valence in CeCu(2)Ge(2) has been measured up to 24 GPa at 300 K and to 17 GPa at 18-20 K using x-ray absorption spectroscopy in the partial fluorescence yield. A smooth increase of the Ce valence with pressure is observed across the two superconducting (SC) regions without any noticeable irregularity. The chemical pressure dependence of the Ce valence was also measured in Ce(Cu(1-x)Ni(x))(2)Si(2) at 20 K. A very weak, monotonic increase of the valence with x was observed, without any significant change in the two SC regions. Within experimental uncertainties, our results show no evidence for the valence transition with an abrupt change in the valence state near the SC II region, challenging the valence-fluctuation mediated superconductivity model in these compounds at high pressure and low temperature.

Ferromagnetism in the Kondo-lattice compound CePd2P2

We report physical properties of CePd2P2 crystallizing in the tetragonal ThCr2Si2-type structure (space group I4/mmm). Dc-magnetic susceptibility, magnetization, specific heat, electrical resistivity and magnetoresistance measurements establish a ferromagnetic ordering below the Curie temperature TC = 28.4 ± 0.2 K. Critical analysis of isothermal and isofield magnetization yields critical exponents of β = 0.405 ± 0.005, γ = 1.11 ± 0.05 and δ = 3.74 ± 0.04. The ordered state is characterized by saturation moment Ms ∼ 0.98μB and magnon energy gap Δ/kB ∼25–35 K. The studied properties reflect a competing influence of the Kondo and crystalline electric field (CEF) interactions. The strength of the Kondo effect is assigned by a low-temperature Kondo scale TK ∼19 ± 10 K and a high-temperature Kondo scale TK ~ H 117 } 10 K. A model of the inelastic scattering of the conduction electrons with an exchanged CEF energy ΔCEF was applied to the magnetic resistivity. An average value ΔCEF = 260 ± 30 K is consistent in the relationships with TK and TK H. We argue that the CePd2P2 compound appears to be a new ferromagnetic Kondo-lattice among the Ce-based intermetallics.

Anisotropic magnetic properties and crystal electric field studies on CePd2Ge2 single crystal

The anisotropic magnetic properties of the antiferromagnetic compound CePd2Ge2, crystallizing in the tetragonal crystal structure have been investigated in detail on a single crystal grown by the Czochralski method. From the electrical transport, magnetization and heat capacity data, the Néel temperature is confirmed to be 5.1 K. Anisotropic behaviour of the magnetization and resistivity is observed along the two principal crystallographic directions-namely, [100] and [001]. The isothermal magnetization measured in the magnetically ordered state at 2 K exhibits a spin reorientation at 13.5 T for the field applied along the [100] direction, whereas the magnetization is linear along the [001] direction attaining a value of 0.94 μ(B)/Ce at 14 T. The reduced value of the magnetization is attributed to the crystalline electric field (CEF) effects. A sharp jump in the specific heat at the magnetic ordering temperature is observed. After subtracting the phononic contribution, the jump in the heat capacity amounts to 12.5 J K(-1)mol(-1) which is the expected value for a spin ½ system. From the CEF analysis of the magnetization data the excited crystal field split energy levels were estimated to be at 120 K and 230 K respectively, which quantitatively explains the observed Schottky anomaly in the heat capacity. A magnetic phase diagram has been constructed based on the field dependence of magnetic susceptibility and the heat capacity data.

Three-dimensional bulk fermiology of CeRu2Ge2 in the paramagnetic phase by soft x-ray hnu-dependent (700-860 eV) ARPES

By virtue of the soft x-ray angle-resolved photoelectron spectroscopy, the three-dimensional bulk fermiology has been successfully performed for a strongly correlated Ce compound, ferromagnet CeRu2Ge2 in the paramagnetic phase. A clear difference of the Fermi surface topology from either band calculation or de Haas-van Alphen results in the ferromagnetic phase is observed and interpreted by considering the difference of the 4f contribution to the Fermi surfaces in the paramagnetic phase.