The partial (up to 7%) substitution of Cd for Zn in the Yb-based heavy-fermion material YbFe2Zn20 is known to induce a slight (similar to 20%) reduction of the Sommerfeld specific-heat coefficient gamma and a huge (up to two orders of magnitude) reduction of the T-2 resistivity coefficient A, corresponding to a drastic and unexpected reduction of the Kadowaki-Woods ratio A/gamma(2). Here, Yb L-3-edge x-ray absorption spectroscopy shows that the Yb valence state is close to 3+ for all x, whereas x-ray diffraction reveals that Cd replaces the Zn ions only at the 16c site of the Fd (3) over barm cubic structure, leaving the 48f and 96g sites with full Zn occupation. Ab initio electronic structure calculations in pure and Cd-doped materials, carried out without considering correlations, show multiple conduction bands with only minor modifications of the band dispersions near the Fermi level and therefore do not explain the resistivity drop introduced by Cd substitution. We propose that the site-selective Cd substitution introduces light conduction bands with a substantial contribution of Cd(16c) 5p levels that have weak coupling to the Yb3+ 4f moments. These light fermions coexist with heavy fermions that originate from other conduction bands with larger participation of Zn(96g) 4p levels that remain strongly coupled with the Yb3+ local moments.

The competing or cooperative character between different degrees of freedom lead to different ground states in strongly-correlated systems.Even simple systems, such as pure rare earth compounds, present multiple phase transitions with complex magnetic structures in some of them, resulting from a strong interplay between magnetic dipolar interaction and temperature dependence of crystal field parameters [1].So, it is expected that some rare-earth-based compounds also have rich magnetic phase diagrams.One example is the series of intermetallic compounds RNiSi3 (R = rare earth), which shows anisotropic antiferromagnetic ground states evolving with R [2-4].The microscopic magnetic structures must be determined to rationalize such rich behavior.Here, resonant X-ray magnetic diffraction experiments are performed on single crystals of GdNiSi3, TbNiSi3 and HoNiSi3 at zero field.The primitive magnetic unit cell matches the chemical cell below the Néel temperatures TN = 22.2, 33.2 K, for Gd-and Tb-based compounds, respectively.The magnetic structure is determined to be the same for both compounds (magnetic space group Cmmm′) and could be fully described by a single one-dimensional irreducible representation of the Cmmm space group.It features ferromagnetic ac planes that are stacked in an antiferromagnetic + -+ -pattern, with the rare-earth magnetic moments pointing along the a direction [5].For HoNiSi3, the situation is more complicated, since this compound show two welldefined λ-shape anomalies at TN1 = 6.3K and TN2 = 10.4K. Additionally, different components of the total magnetic moment order at different temperatures.The a component orders at TN2, and after further cooling above TN1, the c component orders.For this compound, our results show that at temperatures between TN1 and TN2 (phase II), the ordered magnetic moment points along the a-axis, while below TN1 (phase I), the ordered magnetic moments have components both along with a and c.Remarkably, while at phase II the possible magnetic structure is the same as found in GdNiSi3 and TbNiSi3, at phase I two irreducible representations are needed to account the total magnetic moment direction.In this phase, the magnetic structure is consistent with C2'/m magnetic structure.Lastly, those magnetic structures contrasts with the + --+ stacking and moment direction along the b axis previously reported for YbNiSi3 [6].This indicates a sign reversal of the coupling constant between second-neighbor R planes as R is varied from Gd, Tb and Ho to Yb.The long b lattice parameter of GdNiSi3 and TbNiSi3 shows a magnetoelastic expansion upon cooling below TN, pointing to the conclusion that the + -+ -stacking is stabilized under lattice expansion.A competition between distinct magnetic stacking patterns with similar exchange energies tuned by the size of R sets the stage for the magnetic ground state instability observed along this series.

Polarized Raman scattering in magnetoelectric LiNiPO4 shows a sharp resonant peak at 37 cm(-1) in the magnetically ordered phase, originating from a one-magnon excitation of Ni2+ S = 1 localized moments at the zone center. Also, a broad component with maximum intensity at similar to 65 cm(-1) is observed and successfully modeled in terms of light scattering from two-magnon excitations within the framework of the Fleury-Loudon theory using five relevant exchange parameters, providing an independent experimental confirmation of their values previously obtained with inelastic neutron scattering data in this material. An additional peak at 58 cm(-1), already reported in previous works, shows no detectable Zeeman splitting for magnetic fields up to 6 T along the crystallographic a and c directions, excluding one-magnon scattering as a possible assignment for this peak. The possible nature of this excitation is discussed.

The delicate balance between spin-orbit coupling, Coulomb repulsion, and crystalline electric field interactions observed in Ir-based oxides is usually manifested as exotic magnetic behavior. Here we investigate the evolution of the exchange coupling between Co and Ir for partial La substitution by Ca in ${\mathrm{La}}_{2}{\mathrm{CoIrO}}_{6}$. A great advantage of the use of ${\mathrm{Ca}}^{2+}$ as a replacement for ${\mathrm{La}}^{3+}$ is the similarity of their ionic radii. Thus, the observed magnetic changes can more easily be associated with electronic variations. A thorough investigation of the structural, electronic, and magnetic properties of the ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{CoIrO}}_{6}$ system was carried out by means of synchrotron x-ray powder diffraction, muon spin rotation and relaxation $(\ensuremath{\mu}\mathrm{SR})$, AC and DC magnetization, x-ray absorption spectroscopy (XAS), x-ray magnetic circular dichroism, Raman spectroscopy, electrical resistivity, and dielectric permittivity. Our XAS results show that up to 25% Ca substitution at the La site results in the emergence of ${\mathrm{Co}}^{3+}$, possibly in a high-spin state, while the introduction of a larger amount of Ca leads to an increase in the Ir valence. The competing magnetic interactions resulting from the mixed valences lead to the coexistence of a magnetically ordered and an emerging spin-glass (SG) state for the doped samples. Our $\ensuremath{\mu}\mathrm{SR}$ results indicate that for ${\mathrm{La}}_{2}{\mathrm{CoIrO}}_{6}$ a nearly constant fraction of a paramagnetic (PM) phase persists down to low temperatures, possibly related to the presence of a small amount of ${\mathrm{Ir}}^{3+}$ and to the antisite disorder at Co/Ir sites. For doped compounds the PM phase freezes below 30 K, but there is still some dynamics associated with the SG. The dielectric data obtained for the parent compound and the one with 25% Ca doping indicate a possible magnetodielectric effect, which is discussed in terms of the electron hopping between the transition-metal ions, the antisite disorder at Co/Ir sites, and the distorted crystalline structure.

The series of intermetallic compounds RNiSi3 (R = rare earth) shows interesting magnetic properties evolving with R and metamagnetic transitions under applied magnetic field for some of the compounds. The microscopic magnetic structures must be determined to rationalize such rich behavior. Here, resonant x-ray magnetic diffraction experiments are performed on single crystals of GdNiSi3 and TbNiSi3 at zero field. The primitive magnetic unit cell matches the chemical cell below the Neel temperatures T-N = 22.2 and 33.2 K, respectively. The magnetic structure is determined to be the same for both compounds (magnetic space group Cmmm'). It features ferromagnetic ac planes that are stacked in an antiferromagnetic + - + - pattern, with the rare-earth magnetic moments pointing along the (a) over arrow direction, which contrasts with the + - - + stacking and moment direction along the (b) over arrow axis previously reported for YbNiSi3. This indicates a sign reversal of the coupling constant between second-neighbor R planes as R is varied from Gd and Tb to Yb. The long b lattice parameter of GdNiSi3 and TbNiSi3 shows a magnetoelastic expansion upon cooling below T-N, pointing to the conclusion that the + - + - stacking is stabilized under lattice expansion. A competition between distinct magnetic stacking patterns with similar exchange energies tuned by the size of R sets the stage for the magnetic ground state instability observed along this series.

The mixed-valent homometallic ludwigite (Co-2(2+) Co3+)O2BO3 is investigated above the ferrimagnetic ordering temperature T-c = 43 K through structural, thermal, magnetic, electric, and spectroscopic probes. X-ray absorption at the Co L-2,L-3 edges is consistent with the coexistence of Co2+ and Co (3+ )ions, as expected by the sample stoichiometry. Magnetic susceptibility shows a relatively large net paramagnetic moment per Co3+ ion above room temperature, p = 4.87, indicating that the Co3+ ions are not in a pure low-spin configuration at high temperatures, also showing a non-Curie-Weiss behavior below 300 K. Electrical conductivity and differential scanning calorimetry measurements on single crystals indicate two phase transitions at similar to 475 and similar to 495 K. X-ray powder diffraction shows substantial lattice parameter anomalies below 500 K. These results indicate phase transitions associated with changes in the Co oxidation state in each of its four crystallographic sites. Such transitions are possibly dictated by a competition between (i) an ordered ground state with all Co3+ ions occupying the same crystallographic site and (ii) either partially or totally charge-disordered states that are favored at high temperatures due to their higher entropy.

In the zero-field-cooled exchange bias (ZEB) effect, the unidirectional magnetic anisotropy is set at low temperatures even when the system is cooled in the absence of an external magnetic field. La1.5Sr0.5CoMnO6 stands out as presenting the largest ZEB reported so far, while for La1.5Ca0.5CoMnO6 the exchange bias field (HEB) is one order of magnitude smaller. Here we show that La1.5Ba0.5CoMnO6 also exhibits a pronounced shift of its magnetic hysteresis loop, with an intermediate H-EB value with respect to Ca- and Sr-doped samples. To figure out the microscopic mechanisms responsible for this phenomenon, these compounds were investigated by means of synchrotron x-ray powder diffraction, Raman spectroscopy, muon spin rotation and relaxation, ac and dc magnetization, x-ray absorption spectroscopy (XAS), and x-ray magnetic circular dichroism (XMCD). The parent compound La2CoMnO6 was also studied for comparison as a reference of a non-ZEB material. Our results show that the Ba-, Ca-, and Sr-doped samples present a small amount of phase segregation, and that the ZEB effect is strongly correlated to the system's structure. We also observed that mixed valence states Co2+/Co3+ and Mn4+/Mn3+ are already present at the La2CoMnO6 parent compound, and that Ba2+/Ca2+/Sr2+ partial substitution at the La3+ site leads to a large increase of Co average valence, with a subtle augmentation of Mn formal valence. Estimates of the Co and Mn valences from the L-edge XAS indicate the presence of oxygen vacancies in all samples (0.05 <= delta <= 0.1). Our XMCD results show a great decrease of Co moment for the doped compounds, and they indicate that the shift of the hysteresis curves for these samples is related to uncompensated antiferromagnetic coupling between Co and Mn.

The “isospin-phonon” coupling in Sr2IrO4 is investigated by temperature dependent phonon Raman scattering. Anomalous behavior in the frequency of all studied optical phonons is observed below the magnetic transition temperature TN ∼ 240 K. The strongest effect is detected for the A1g mode at 272 cm−1 associated with the modulation of the Ir–O–Ir bond angle. Additionally, the A1g mode at 560 cm−1 shows a Fano asymmetric lineshape sensitive to TN, supporting the existence of low energy (∼70 meV) electronic excitations that are renormalized by the magnetic order. These results reveal the characteristics of the interaction between the crystal lattice and electronic degrees of freedom in this “spin-orbit” entangled Mott insulator.

In the zero-field-cooled exchange bias (ZEB) effect the unidirectional magnetic anisotropy is set at low temperatures even when the system is cooled in the absence of external magnetic field. La$_{1.5}$Sr$_{0.5}$CoMnO$_{6}$ stands out as presenting the largest ZEB reported so far, while for La$_{1.5}$Ca$_{0.5}$CoMnO$_{6}$ the exchange bias field ($H_{EB}$) is one order of magnitude smaller. Here we show that La$_{1.5}$Ba$_{0.5}$CoMnO$_{6}$ also exhibits a pronounced shift of its magnetic hysteresis loop, with intermediate $H_{EB}$ value in respect to Ca- and Sr-doped samples. In order to figure out the microscopic mechanisms responsible for this phenomena, these compounds were investigated by means of synchrotron X-ray powder diffraction, Raman spectroscopy, muon spin rotation and relaxation, AC and DC magnetization, X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD). The parent compound La$_{2}$CoMnO$_{6}$ was also studied for comparison, as a reference of a non-ZEB material. Our results show that the Ba-, Ca- and Sr-doped samples present a small amount of phase segregation, and that the ZEB effect is strongly correlated to the system's structure. We also observed that mixed valence states Co$^{2+}$/Co$^{3+}$ and Mn$^{4+}$/Mn$^{3+}$ are already present at the La$_{2}$CoMnO$_{6}$ parent compound, and that Ba$^{2+}$/Ca$^{2+}$/Sr$^{2+}$ partial substitution at La$^{3+}$ site leads to a large increase of Co average valence, with a subtle augmentation of Mn formal valence. Estimates of the Co and Mn valences from the $L$-edge XAS indicate the presence of oxygen vacancies in all samples (0.05$\leq \delta \leq$0.1). Our XMCD results show a great decrease of Co moment for the doped compounds, and indicate that the shift of the hysteresis curves for these samples is related to uncompensated antiferromagnetic coupling between Co and Mn.

The Heavy Fermion compound YbFe2Zn20 was doped with Cd atoms. This compound adopts the complex cubic CeCr2Al20 - type structure with space group