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Register a new userItinerant and localized magnetization dynamics in antiferromagnetic Ho
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Using femtosecond time-resolved resonant magnetic x-ray diffraction at the Ho L3 absorption edge, we investigate the demagnetization dynamics in antiferromagnetically ordered metallic Ho after femtosecond optical excitation. Tuning the x-ray energy to the electric dipole (E1, 2p -> 5d) or quadrupole (E2, 2p -> 4f) transition allows us to selectively and independently study the spin dynamics of the itinerant 5d and localized 4f electronic subsystems via the suppression of the magnetic (2 1 3-tau ) satellite peak. We find demagnetization timescales very similar to ferromagnetic 4f systems, suggesting that the loss of magnetic order occurs via a similar spin-flip process in both cases. The simultaneous demagnetization of both subsystems demonstrates strong intra-atomic 4f-5d exchange coupling. In addition, an ultrafast lattice contraction due to the release of magnetostriction leads to a transient shift of the magnetic satellite peak.
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In heavy-fermion compounds, f electrons show both itinerant and localized behaviour depending on the external conditions, and the hybridization between localized f electrons and itinerant conduction bands gives rise to their exotic properties like heavy-fermions, magnetic orders and unconventional superconductivity. Duo to the risk of handling radioactive actinide materials, the direct experimental evidence of the band structure evolution across the localized-itinerant and magnetic transitions for 5f electrons is lacking. Here, by using angle-resolved photoelectron spectroscopy, we revealed the dual nature (localized vs itinerant) and the development of two different kinds of heavy quasi-particle bands of 5f electrons in antiferromagnetic (AFM) USb2. Partially opened energy gaps were observed on one quasi-particle 5f band cross the AFM transition around 203 K, indicating that the magnetic orders in USb2 are of spin density wave (SDW) type similar to Cr. The localized 5f electrons and itinerant conduction bands hybridize to form another heavy quasi-particle band at about 120 K, and then open hybridization gaps at even lower temperature. Our results provide direct spectral demonstration of the localized-itinerant transition, hybridization and SDW transition of 5f electrons for uranium-based materials.
Electron correlations tend to generate local magnetic moments that usually order if the lattices are not too frustrated. The hexagonal compound SrRu$_2$O$_6$ has a relatively high Neel temperature but small local moments, which seem to be at odds with the nominal valence of Ru$^{5+}$ in the $t_{2g}^3$ configuration. Here, we investigate the electronic property of SrRu$_2$O$_6$ using density functional theory (DFT) combined with dynamical-mean-field theory (DMFT). We find that the strong hybridization between Ru $d$ and O $p$ states results in a Ru valence that is closer to $+4$, leading to the small ordered moment $sim1.2mu_B$. While this is consistent with a DFT prediction, correlation effects are found to play a significant role. The local moment per Ru site remains finite $sim2.3mu_B$ in the whole temperature range investigated. Due to the lower symmetry, the $t_{2g}$ manifold is split and the quasiparticle weight is renormalized significantly in the $a_{1g}$ state, while the renormalization in $e_g$ states is about a factor of 2--3 weaker. Our theoretical Neel temperature $sim700$~K is in reasonable agreement with experimental observations. SrRu$_2$O$_6$ is a unique system in which localized and itinerant electrons coexist with the proximity to an orbitally-selective Mott transition within the $t_{2g}$ sector.
Magnetic properties of polycrystalline CaMn1-xRuxO3 (x = 0 - 0.5) samples were investigated in the temperature range 4.2 - 250 K, under external magnetic field up to 15 kOe and under hydrostatic pressure up to 12 kbar. Transport properties of the samples with x = 0.1, 0.2, 0.4, 0.5 were also investigated under pressure up to 10 kbar. For x up to 0.4, the pressure was found to suppress ferromagnetic correlations and to increase the resistivity, while for x = 0.5 to act in the opposite way. While long ferromagnetic order is completely suppressed, in small clusters ferromagnetic correlations probably survive under pressure, as was revealed for CaMn0.9Ru0.1O3. The pressure effect on the magnetic interactions and on the volume of ferromagnetic phase was found to depend strongly on the Ru-content, and absolute value of the pressure coefficient of spontaneous magnetization was found to decrease practically linearly with increasing x in the range 0.1 < x < 0.5. The experimental data are discussed in the frame of proposed energy-level diagram, which includes magneto-impurity states at low and moderate Ru-doping and mixed-valence states of Ru presented by a strongly-correlated t2g-like band at heavy Ru-doping. An impact of disorder introduced by Ru-doping on the energy diagram and on derived magnetic interactions is discussed. Predictions of the model regarding the pressure effects on conductivity and temperature scales characteristic for magnetic interactions are in reasonable agreement with experiment.
Neutron diffraction and magnetic susceptibility studies of a polycrystalline SrCr$_2$As$_2$ sample reveal that this compound is an itinerant G-type antiferromagnet below the N${rm acute{e}}$el temperature $T_{textrm N}$ = 590(5) K with the Cr magnetic moments aligned along the tetragonal $c$ axis. The system remains tetragonal to the lowest measured temperature ($sim$12 K). The lattice parameter ratio $c/a$ and the magnetic moment saturate at about the same temperature below $sim$ 200 K, indicating a possible magnetoelastic coupling. The ordered moment, $mu=1.9(1)~mu_{rm B}$/Cr, measured at $T = 12$ K, is significantly reduced compared to its localized value ($4~mu_{rm B}$/Cr) due to the itinerant character brought about by the hybridization between the Cr $3d$ and As $4p$ orbitals.
The weak itinerant ferromagnet UIr is studied by magnetization and magnetostriction measurements. Critical behavior, which surprisingly extends up to several Tesla, is observed at the Curie temperature $T_Csimeq45$ K and is analyzed using Arrott and Maxwell relations. Critical exponents are found that do not match with any of the well-known universality classes. The low-temperature magnetization $M_ssimeq0.5$ $mu_B cong const.$ below 3 T rises towards higher fields and converges asymptotically around 50 T with the magnetization at $T_C$. From the magnetostriction and magnetization data, we extract the uniaxial pressure dependences of $T_C$, using a new method presented here, and of $M_s$. These results should serve as a basis for understanding spin fluctuations in anisotropic itinerant ferromagnets.
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