4f Moments Research Articles

Anisotropic spin fluctuations in CeCoIn 5

CeCoIn 5 is an unconventional heavy fermion superconductor with a relatively large transition temperature T c = 2.3 K . NMR measurements of the spin lattice relaxation rate at the In(1) site reveal a significant anisotropy of the fluctuating hyperfine fields, which reverses below 30 K. These results suggest that two-dimensional fluctuations of the Ce 4f moments are relevant for the superconducting pairing.

Role of Ge in Bridging Ferromagnetism in the Giant MagnetocaloricGd5(Ge1−xSix)4Alloys

X-ray magnetic circular dichroism (XMCD) measurements and density functional theory (DFT) are used to study the electronic conduction states in Gd5(Ge(1-x)Si(x))4 materials through the first-order bond-breaking magnetostructural transition responsible for their giant magnetocaloric effect. Spin-dependent hybridization between Ge 4p and Gd 5d conduction states, which XMCD senses through the induced magnetic polarization in Ge ions, enables long-range Ruderman-Kittel-Kasuya-Yosida ferromagnetic interactions between Gd 4f moments in adjacent Gd slabs connected by Ge(Si) bonds. These interactions are strong below but weaken above the Ge(Si) bond-breaking transition that destroys 3D ferromagnetic order.

Compton Scattering Studies of 4f Ferromagnetism

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Muon spin resonance study on UCu1.5Sn2

We report muon spin relaxation measurements results on UCu1.5Sn2, which crystallizes in the CaBe2Ge2-type structure. Our analysis is consistent with collinear antiferromagnetic order that occurs below 108K, in agreement with previous reports. The Brillouin-like behavior of the temperature dependence of the magnetic response is consistent with localized 5f moments. The phase transition at TN is unusual. Critical slowing down for the paramagnetic U spin fluctuations is absent, a behavior usually observed in first-order transitions. In contrast, the temperature dependence of the local field generated by the U spins is a typical second-order feature.

Experimental investigation of the electronic structure ofGd5Ge2Si2 by photoemission and x-ray absorption spectroscopy

The electronic structure of the magnetic refrigerantGd5Ge2Si2 has been experimentally investigated by photoemission and x-ray absorption spectroscopy.The resonant photoemission and x-ray absorption measurements performed across theGd N4,5 and Gd M4,5 edges identify the position of Gd 4f multiplet lines, and assess the 4f occupancy(4f7) and the character of the states close to the Fermi edge. The presence ofGd 5d states in the valence band suggests that an indirect 5d exchangemechanism underlies the magnetic interactions between Gd 4f moments inGd5Ge2Si2. From 175 to 300 K the first 4 eV of the valence band and the Gd partial density of statesdo not display clear variations. A significant change is instead detected in thephotoemission spectra at higher binding energy, around 5.5 eV, likely associated to thevariation of the bonding and antibonding Ge(Si) s bands across the phase transition.

Magnetic Compton scattering study of the magnetocaloric materialGd7Pd3

The spin-dependent momentum density ofGd7Pd3 was probed by the magnetic Compton scattering technique with ellipticallypolarized synchrotron radiation. A contribution to the spin moment from Pd 4delectrons was observed, at 2 and 280 K, alongside a large Gd 4f moment and asmaller Gd 5d moment. The total spin moment, at 2 K, was determined as50.8 ± 0.7 μB (f.u.)−1. The Gd 4f contribution to the spin moment was determined as43.4 ± 1.8 μB (f.u.)−1, the Gd 5dmoment as 4.4 ± 0.7 μB (f.u.)−1 and the Pd 4d spin moment contribution as2.9 ± 1.1 μB (f.u.)−1, where f.u. represents a formula unit. At 280 K the total spin moment was27.3 ± 0.9 μB (f.u.)−1 with individual contributions determined as a Gd 4f spin moment of23.8 ± 1.1 μB (f.u.)−1, a Gd 5d contributionof 2.2 ± 0.5 μB (f.u.)−1 and a Pd 5dspin moment of 1.2 ± 0.6 μB (f.u.)−1.

NMR studies of the partially disordered state in a triangular antiferromagnetUNi4B

A triangular antiferromagnet UNi4B experiences a partially disordered state, in which two-thirds of U 5f moments order in a vertex-likestructure and one-third of U 5f moments remain paramagnetic. Magnetization and NMR measurementsof UNi4B have been performed to study the dynamical properties of U magnetic momentsin the partially disordered state. The value of the effective moment aboveTN derived from the fitting of the Curie–Weiss law suggests a significant crystal fieldsplitting and the Kondo effect. The Weiss temperature was also estimated to be−28 K, which suggests that the exchange interaction is antiferromagnetic.Furthermore, the exchange interaction estimated from the relaxation rateT1−1 above 50 K is 18 K, which is close to the Weiss temperature. These results suggest that the dominantinteractions between U moments are antiferromagnetic in the basal plane. The relaxation rateT1−1 decreasesabruptly below TN down to 15 K and is almost constant between 10 and 15 K. In the sametemperature range, the magnetic susceptibility still increases withdecreasing temperature. However, the effective moment estimated belowTN is almost the sameas that above TN. This shows that there are anomalous fluctuations belowTN which cannot be explained by the simple model.

Ferrimagnetism inEuFe4Sb12due to the Interplay off-Electron Moments and a Nearly Ferromagnetic Host

We combine x-ray magnetic circular dichroism spectroscopy at Fe L2,3 edges, at Eu M4,5 edges, x-ray absorption spectroscopy (XAS) investigation of Eu valence, and local spin density calculations, to show that the filled skutterudite Eu0.95Fe4Sb12 is a ferrimagnet in which the Fe 3d moment and the Eu2+ 4f moment are magnetically ordered with dominant antiferromagnetic coupling. From Eu L3 edge XAS, we find that about 13% of the Eu have a formal valence of 3+. We ascribe the origin of ferrimagnetism at a relatively high transition temperature TC of 85 K in Eu0.95Fe4Sb12 to f-electron interaction with the nearly ferromagnetic [Fe4Sb12]2.2- host lattice.

Geometrical frustration and spin-liquid behavior of the metallic pyrochlore antiferromagnet [formula omitted

DC susceptibility measurements on single crystals of the metallic pyrochlore antiferromagnet Pr 2 Ir 2 O 7 revealed no sign of magnetic long-range order down to 70 mK, despite an antiferromagnetic interaction of 20 K between 〈 1 1 1 〉 Ising-like Pr 4f moments. The underscreened moments due to Kondo coupling between 4f moments and Ir 5d-conduction electrons form the spin-liquid-like state below 1.7 K.

Variational Monte Carlo Study of Flat Band Ferromagnetism –Application to CeRh3B2–

A new mechanism for ferromagnetism in CeRh_3B_2 is proposed on the basis of variational Monte Carlo results. In a one-dimensional Anderson lattice where each 4f electron hybridizes with a ligand orbital between neighboring Ce sites, ferromagnetism is stabilized due to a nearly flat band which is a mixture of conduction and 4f electron states. Because of the strong spin-orbit interaction in 4f electron states, and of considerable amount of hybridization in the nearly flat band, the magnetic moments from 4f and conduction electrons tend to cancel each other. The resultant ferromagnetic moment becomes smaller as compared with the local 4f moment, and the Fermi surface in the ferromagnetic ground state is hardly affected by the presence of 4f electrons. These theoretical results are consistent with experimental observations in CeRh_3B_2 by neutron scattering and dHvA effects.

Compton scattering studies of 4f ferromagnetism

Ferromagnetism can be studied in Compton scattering experiments, which use high-energy, elliptically polarised synchrotron radiation. The basis of the method and its interpretation in terms of site-specific moments will be explained and illustrated by a series of examples of archetypal compounds. For example, the hexaborides exhibit a wide range of electronic properties. Data will be presented for the heavy Fermion system CeB 6 and the controversial ferromagnet Ca 1− x La x B 6. Undoped CeB 6 exhibits a delocalised spin moment, more characteristic of the 5d than the 4f orbital. Whereas the magnetism in the Ca 1− x La x B 6 system has been suggested to be an extrinsic effect arising from iron contamination, our data indicate that the magnetism is intrinsic and inconsistent with what would be expected for an iron 3d moment. This supports the proposal that this system is, indeed, a weak itinerant ferromagnet. In Ru 2SrGdCu 20 8, we observe a combination of Gd 4f, Gd 5d and Ru 4d moments and the 5d moment appears to flip orientation below the Néel temperature.

Magnetic-field-induced switching between ferroelectric phases in orthorhombic-distortion-controlled RMnO3

We have investigated the dielectric and magnetic properties of Eu0.595Y0.405MnO3 without the presence of the 4f magnetic moments of the rare-earth ions and have found two ferroelectric phases with polarization along the a and c axes in a zero magnetic field. A magnetic-field-induced switching from one to the other ferroelectric phase took place in which the direction of ferroelectric polarization changed from the a axis to the c axis by the application of magnetic fields parallel to the a axis. In contrast to the case of TbMnO3, in which the 4f moments of Tb3+ ions play an important role in such a ferroelectric phase switching, the magnetic-field-induced switching between ferroelectric phases in Eu0.595Y0.405MnO3 does not originate from the magnetic transition of the rare-earth 4f moments but from that of the Mn 3d spins.

Electronic and magnetic properties of CeAl 2 nanoparticles

We presented the X-ray magnetic circular dichroism (XMCD) and X-ray absorption spectroscopy (XAS) studies of heavy fermion compound CeAl 2 bulk and 8 nm nanoparticles, performed at the Ce M 4,5 - and L 3 - absorption edges. XMCD and XAS revealed that Ce in bulk CeAl 2 exhibits localized 4f 1 character with magnetic ordering. The Ce in nanoparticles, on the other hand, shows a small amount delocalized 4f 0 character with non-magnetic Kondo behavior. By applying general sum rules, an estimation of the orbital and spin contribution to those Ce 4f moments can be obtained. Our results also demonstrated that the magnetic behavior in CeAl 2 is very sensitive to the degree of localization of the 4f electrons.

Formula omitted] NMR study of [formula omitted

We have performed 11 B nuclear magnetic resonance (NMR) measurements on the single crystals of GdB 4 to investigate local electronic structure and 4f spin dynamics. 11 B NMR spectrum, Knight shift, spin–lattice and spin–spin relaxation rates, 1 / T 1 and 1 / T 2 , were measured down to 4 K at 8 T . 11 B NMR shift and linewidth are huge and strongly temperature-dependent due to the 4f moments. In addition, both are proportional to magnetic susceptibility, indicating that the hyperfine field at the boron site originates from the 4f spins of Gd. Below T N , the single broad resonance peak of 11 B NMR splits into several peaks reflecting the local magnetic fields due to antiferromagnetic spin arrangements. The spin–lattice relaxation rate, independent of temperature above T N , decreases significantly below T N confirming huge suppression of spin fluctuation.

11B NMR study of [formula omitted

We have performed 11B nuclear magnetic resonance (NMR) measurements on a single crystal of TbB 4 to investigate local electronic structure and 4f spin dynamics. 11B NMR spectrum, Knight shift, spin–lattice and spin–spin relaxation rates, 1 / T 1 and 1 / T 2 , were measured down to 5 K at 8 T. 11B NMR shift and linewidth are huge and strongly temperature-dependent due to the 4f moments. In addition, both are proportional to magnetic susceptibility, indicating that the hyperfine field at the boron site originates from the 4f spins of Tb. Below T N , the single broad resonance peak of 11B NMR splits into several peaks reflecting the local magnetic fields due to antiferromagnetic spin arrangements. The spin–lattice relaxation rate, independent of temperature above T N , decreases significantly below T N confirming huge suppression of spin dynamics.

Distinct order of Gd4fand Fe3dmoments coexisting inGdFe4Al8

Single crystals of flux-grown tetragonal GdFe4Al8 were characterized by thermodynamic, transport, and x-ray resonant magnetic scattering measurements. In addition to antiferromagnetic order at TN ~ 155 K, two low-temperature transitions at T1 ~ 21 K and T2 ~ 27 K were identified. The Fe moments order at TN with an incommensurate propagation vector (tau,tau,0) with tau varying between 0.06 and 0.14 as a function of temperature, and maintain this order over the entire T<TN range. The Gd 4f moments order below T2 with a ferromagnetic component mainly out of plane. Below T1, the ferromagnetic components are confined to the crystallographic plane. Remarkably, at low temperatures the Fe moments maintain the same modulation as at high temperatures, but the Gd 4f moments apparently do not follow this modulation. The magnetic phase diagrams for fields applied in [110] and [001] direction are presented and possible magnetic structures are discussed.

Combined local-density and dynamical mean field theory calculations for the compressed lanthanides Ce, Pr, and Nd

This paper reports calculations for compressed Ce (4f^1), Pr (4f^2), and Nd (4f^3) using a combination of the local-density approximation (LDA) and dynamical mean field theory (DMFT), or LDA+DMFT. The 4f moment, spectra, and the total energy among other properties are examined as functions of volume and atomic number for an assumed face-centered cubic (fcc) structure.

Spin-resolved Compton scattering study ofRuSr2GdCu2O8

The spin-polarized electron momentum distributions of the magnetic superconductorRuSr2GdCu2O8 in its various phases have been measured using the magnetic Compton scattering techniquewith elliptically polarized synchrotron radiation. A contribution to the spin moment frommagnetically ordered Ru 4d electrons was seen with a large induced Gd 4f moment below25 K. A small negatively polarized component is observed in the antiferromagnetic phase ofthe Gd sublattice, at 2 K. The momentum distribution of this was consistent with theexistence of a Gd 5d electron moment anti-parallel with respect to the now ordered Gd 4fmoment.

Magnetic field effects in UNi1∕3Ru2∕3Al

We report on the electronic properties of a UNi1∕3Ru2∕3Al single crystal as a function of temperature and magnetic field. Our specific-heat, magnetization, and resistivity data provide strong evidence that the uranium 5f moments order ferromagnetically below TC=74K. Magnetic measurements reveal that the c axis is the easy magnetization direction with a saturation moment of about 0.78μB∕U-atom. Our neutron diffraction studies on UNi1∕3Ru2∕3Al provide strong evidence that Ni occupies the 1b (and Ru the 2c) positions of the ZrNiAl structure. We argue that such preferential occupation of Ni and Ru atoms is responsible for the observed ferromagnetic ground state.

Neutron scattering study on 5f itinerant antiferromagnet UPdGa 5

The magnetic structure of UPdGa 5 was studied by means of neutron diffraction on a single-crystalline sample. We observed an appearance of antiferromagnetic peaks with the propagation vector q = ( 0 0 1 / 2 ) . The uranium 5f moment of 0.33 μ B / U parallel to the c-axis was found to order ferromagnetically in the basal c-plane and coupled antiferromagnetically along the c-axis.