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Register a new userEvolution of magnetism in Yb(Rh_(1-x)Co_x)2Si2
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We present a study of the evolution of magnetism from the quantum critical system YbRh2Si2 to the stable trivalent Yb system YbCo2Si2. Single crystals of Yb(Rh_(1-x)Co_x)2Si2 were grown for 0 < x < 1 and studied by means of magnetic susceptibility, electrical resistivity, and specific heat measurements, as well as photoemission spectroscopy. The results evidence a complex magnetic phase diagram, with a non-monotonic evolution of T_N and two successive transitions for some compositions resulting in two tricritical points. The strong similarity with the phase diagram of YbRh2Si2 under pressure indicates that Co substitution basically corresponds to the application of positive chemical pressure. Analysis of the data proves a strong reduction of the Kondo temperature T_K with increasing Co content, T_K becoming smaller than T_N for x ~ 0.5, implying a strong localization of the 4f electrons. Furthermore, low-temperature susceptibility data confirm a competition between ferromagnetic and antiferromagnetic exchange. The series Yb(Rh_(1-x)Co_x)2Si2 provides an excellent experimental opportunity to gain a deeper understanding of the magnetism at the quantum critical point in the vicinity of YbRh2Si2 where the antiferromagnetic phase disappears (T_N=>0).
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Yb(Rh1-xCox)2Si2 is a model system to address two challenging problems in the field of strongly correlated electron systems: The first is the intriguing competition between ferromagnetic (FM) and antiferromagnetic (AFM) order when approaching a magnetic quantum critical point (QCP). The second is the occurrence of magnetic order along a very hard crystalline electric field (CEF) direction, i.e. along the one with the smallest available magnetic moment. Here, we present a detailed study of the evolution of the magnetic order in this system from a FM state with moments along the very hard c direction at x = 0.27 towards the yet unknown magnetic state at x = 0. We first observe a transition towards an AFM canted state with decreasing x and then to a pure AFM state. This confirms that the QCP in YbRh2Si2 is AFM, but the phase diagram is very similar to those observed in some inherently FM systems like NbFe2 and CeRuPO, which suggests that the basic underlying instability might be FM. Despite the huge CEF anisotropy the ordered moment retains a component along the c-axis also in the AFM state. The huge CEF anisotropy in Yb(Rh1-xCox)2Si2 excludes that this hard-axis ordering originates from a competing exchange anisotropy as often proposed for other heavy-fermion systems. Instead, it points to an order-by-disorder based mechanism.
The magnetic properties of polycrystalline Tb(Co_{x}Ni_{1-x})_{2}B_{2}C (x=0.2,0.4,0.6,0.8) samples were probed by magnetization, specific heat, ac susceptibility, and resistivity techniques. For x{ eq}0.4, the obtained curves are consistent with the features expected for the corresponding magnetic modes, namely k_{1}=(0.55,0,0) at x=0; k_{2}=([nicefrac] icefrac{1}{2}</LaTeX>,0,[nicefrac]<LaTeX> icefrac{1}{2}) at x= 0.2; k_{3}=(0,0,[nicefrac] icefrac{1}{3}) at x= 0.6, and k_{4}=(0,0,0) at x= 0.8 and 1. For x=0.4, even though the neutron diffraction indicates a k_{2} mode, but with a reduced magnetic moment, the magnetization, the ac susceptibility, and resistivity indicate two magnetic events; furthermore, deviation from Curie-Weiss behavior is observed below 150 K for this sample. These features, together with the evolution of both magnetic moment and critical temperature, are attributed to an interplay between competing magnetic couplings; for the particular x=0.4 case, additional factors such as crystalline electric field effects may be in operation.
Soft-x-ray photoemission and absorption spectroscopies are employed to investigate the electronic structures of Sr_{1-x}Rh_{2}O_{4}. Similarly to the layered cobaltates such as Na_{1-x}CoO_{2}, a valence-band satellite feature (VBS) occurs at higher binding energy to the O 2p band. We find that the VBS resonates at the O 1s edge. Additionally, core absorption shows clear x dependence in the O 1s edge rather than in the Rh 3p edge. These results indicate that the holes in the initial state mainly have O 2p character presumably due to d-p rehybridizations affected by Sr^{2+} vacancy potentials. The resultant inhomogenous charge texture may have impact on the TE transport properties at low x.
We report on the evidence for the multiband electronic transport in $alpha$-YbAlB$_{4}$ and $alpha$-Yb$_{0.81(2)}$Sr$_{0.19(3)}$AlB$_{4}$. Multiband transport reveals itself below 10 K in both compounds via Hall effect measurements, whereas anisotropic magnetic ground state sets in below 3 K in $alpha$-Yb$_{0.81(2)}$Sr$_{0.19(3)}$AlB$_{4}$. Our results show that Sr$^{2+}$ substitution enhances conductivity, but does not change the quasiparticle mass of bands induced by heavy fermion hybridization.
We study the structural, magnetic, transport and electronic properties of LaCoO$_3$ with Sr/Nb co-substitution, i.e., La$_{(1-2x)}$Sr$_{2x}$Co$_{(1-x)}$Nb$_{x}$O$_3$ using x-ray and neutron diffraction, dc and ac-magnetization, neutron depolarization, dc-resistivity and photoemission measurements. The powder x-ray and neutron diffraction data were fitted well with the rhombohedral crystal symmetry (space group textit{R$bar{3}$c}) in Rietveld refinement analysis. The calculated effective magnetic moment ($approx$3.85~$mu_B$) and average spin ($approx$1.5) of Co ions from the analysis of magnetic susceptibility data are consistent with 3+ state of Co ions in intermediate-spin (IS) and high-spin (HS) states in the ratio of $approx$50:50, i.e., spin-state of Co$^{3+}$ is preserved at least up to $x=$ 0.1 sample. Interestingly, the magnetization values were significantly increased with respect to the $x=$ 0 sample, and the M-H curves show non-saturated behavior up to an applied maximum magnetic field of $pm$70 kOe. The ac-susceptibility data show a shift in the freezing temperature with excitation frequency and the detailed analysis confirm the slower dynamics and a non-zero value of the Vogel-Fulcher temperature T$_0$, which suggests for the cluster spin glass. The unusual magnetic behavior indicates the presence of complex magnetic interactions at low temperatures. The dc-resistivity measurements show the insulating nature in all the samples. However, relatively large density of states $approx$10$^{22}$ eV$^{-1}$cm$^{-3}$ and low activation energy $approx$130~meV are found in $x=$ 0.05 sample. Using x-ray photoemission spectroscopy, we study the core-level spectra of La 3$d$, Co 2$p$, Sr 3$d$, and Nb 3$d$ to confirm the valence state.
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