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Register a new userCommon effect of chemical and external pressures on the magnetic properties of $R$CoPO ($R$ = La, Pr, Nd, Sm). II
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The direct correspondence between Co band ferromagnetism and structural parameters is investigated in the pnictide oxides $R$CoPO for different rare-earth ions ($R$ = La, Pr, Nd, Sm) by means of muon-spin spectroscopy and {it ab-initio} calculations, complementing our results published previously [G. Prando {it et al.}, {it Phys. Rev. B} {bf 87}, 064401 (2013)]. Both the transition temperature to the ferromagnetic phase $T_{_{textrm{C}}}$ and the volume of the crystallographic unit cell $V$ are found to be conveniently tuned by the $R$ ionic radius and/or external pressure. A linear correlation between $T_{_{textrm{C}}}$ and $V$ is reported and {it ab-initio} calculations unambiguously demonstrate a full equivalence of chemical and external pressures. As such, $R$ ions are shown to be influencing the ferromagnetic phase only via the induced structural shrinkage without involving any active role from the electronic $f$ degrees of freedom, which are only giving a sizeable magnetic contribution at much lower temperatures.
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We report a detailed investigation of RECoPO (RE = La, Pr) and LaCoAsO materials performed by means of muon spin spectroscopy. Zero-field measurements show that the electrons localized on the Pr$^{3+}$ ions do not play any role in the static magnetic properties of the compounds. Magnetism at the local level is indeed fully dominated by the weakly-itinerant ferromagnetism from the Co sublattice only. The increase of the chemical pressure triggered by the different ionic radii of La$^{3+}$ and Pr$^{3+}$, on the other hand, plays a crucial role in enhancing the value of the magnetic critical temperature and can be mimicked by the application of external hydrostatic pressure up to 24 kbar. A sharp discontinuity in the local magnetic field at the muon site in LaCoPO at around 5 kbar suggests a sizeable modification in the band structure of the material upon increasing pressure. This scenario is qualitatively supported by emph{ab-initio} density-functional theory calculations.
We have investigated the temperature dependence of the magnetic susceptibility $chi(T)$ of rare-earth cobaltites RCoO$_3$ (R= La, Pr, Nd, Sm, Eu) in the temperature range $4.2-300$ K and also the influence of hydrostatic pressure up to 2 kbar on their susceptibility at fixed temperatures $T=78 $ and 300 K. The specific dependence $chi(T)$ observed in LaCoO$_3$ and the anomalously large pressure effect (d ln $chi$/d$Psim -100$ Mbar$^{-1}$ for $T = 78$ K) are analyzed in the framework of a two-level model with energy levels difference $Delta$. The ground state of the system is assumed to be nonmagnetic with the zero spin of Co$^{3+}$ ions, and magnetism at a finite temperature is determined by the excited magnetic spin state. The results of the analysis, supplemented by theoretical calculations of the electronic structure of LaCoO$_3$, indicate a significant increase in $Delta$ with a decrease in the unit cell volume under the hydrostatic pressure. In the series of RCoO$_3$ (R= Pr, Nd, Sm, Eu) compounds, the volume of crystal cell decreases monotonically due to a decrease in the radius of R$^{3+}$ ions. This leads to an increase in the relative energy $Delta$ of the excited state (the chemical pressure effect), which manifests itself in a decrease in the contribution of cobalt ions to the magnetic susceptibility at a fixed temperature, and also in a decrease in the hydrostatic pressure effect on the susceptibility of RCoO$_3$ compounds, which we have observed at $T=300$ K.
We have studied the thermal conductivity $kappa$ on single crystalline samples of the antiferromagnetic monolayer cuprates R$_2$CuO$_4$ with R = La, Pr, Nd, Sm, Eu, and Gd. For a heat current within the CuO$_2$ planes, i.e. for $kappa_{ab}$ we find high-temperature anomalies around 250 K in all samples. In contrast, the thermal conductivity $kappa_c$ perpendicular to the CuO$_2$ planes, which we measured for R = La, Pr, and Gd, shows a conventional temperature dependence as expected for a purely phononic thermal conductivity. This qualitative anisotropy of $kappa_i$ and the anomalous temperature dependence of $kappa_{ab}$ give evidence for a significant magnetic contribution $kappa_{mag}$ to the heat transport within the CuO$_2$ planes. Our results suggest, that a large magnetic contribution to the heat current is a common feature of single-layer cuprates. We find that $kappa_{mag}$ is hardly affected by structural instabilities, whereas already weak charge carrier doping causes a strong suppression of $kappa_{mag}$.
We report on structural and superconducting properties of La(3-x)R(x)Ni2B2N3 where La is substituted by the magnetic rare-earth elements Ce, Pr, Nd. The compounds Pr3Ni2B2N3 and Nd3Ni2B2N3 are characterized for the first time. Powder X-ray diffraction confirmed all samples R3Ni2B2N3 with R = La, Ce, Pr, Nd and their solid solutions to crystallize in the body centered tetragonal La3Ni2B2N3 structure type. Superconducting and magnetic properties of La(3-x)R(x)Ni2B2N3 were studied by resistivity, specific heat and susceptibility measurements. While La3Ni2B2N3 has a superconducting transition temperature Tc ~ 14 K, substitution of La by Ce, Pr, and Nd leads to magnetic pair breaking and, thus, to a gradual suppression of superconductivity. Pr3Ni2B2N3 exibits no long range magnetic order down to 2 K, Nd3Ni2B2N3 shows ferrimagnetic ordering below T_C = 17 K and a spin reorientation transition to a nearly antiferromagnetic state at 10 K.
We investigate the effect of external pressure on magnetic order in undoped LnFeAsO (Ln = La, Ce, Pr, La) by using muon-spin relaxation measurements and ab-initio calculations. Both magnetic transition temperature $T_m$ and Fe magnetic moment decrease with external pressure. The effect is observed to be lanthanide dependent with the strongest response for Ln = La and the weakest for Ln = Sm. The trend is qualitatively in agreement with our DFT calculations. The same calculations allow us to assign a value of 0.68(2) $mu_B$ to the Fe moment, obtained from an accurate determination of the muon sites. Our data further show that the magnetic lanthanide order transitions do not follow the simple trend of Fe, possibly as a consequence of the different $f$-electron overlap.
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