Kramers Doublet Research Articles

Multiphase competition in the quantum XY pyrochlore antiferromagnet CdYb2Se4 : Zero and applied magnetic field study

We study magnetic behaviour of the Yb$^{3+}$ ions on a frustrated pyrochlore lattice in the spinel {\CYS}. The crystal-electric field parameters deduced from high-energy inelastic neutron scattering reveal well-isolated ytterbium ground state doublet with a weakly Ising character. Magnetic order studied by powder neutron diffraction evolves from the $XY$-type antiferromagnetic $\Gamma_5$ state to a splayed ice-like ferromagnet (both with k=0) in applied magnetic field with $B_c$=3 T. Low-energy inelastic neutron scattering identifies weakly dispersive magnetic bands around 0.72 meV starting at $\mid\bf{Q}\mid$ = 1.1 \AA$^{-1}$~ at zero field, which diminish with field and vanish above 3 T. We explain the observed magnetic behaviour in framework of the nearest-neighbour anisotropic exchange model for effective $S=1/2$ Kramers doublets on the pyrochlore lattice. The estimated exchanges position the {\CYS} spinel close to the phase boundary between the $\Gamma_5$ and splayed ferromagnet states, similar to the Yb-pyrochlores suggesting an important role of the competition between these phases.

Frustrated Magnetism in a 2-D Ytterbium Fluoride.

Geometric frustration of magnetic ions combines with spin-orbit coupling and structural disorder to give rise to potentially, highly entangled magnetic states such as quantum spin liquids. While fluoride-based frustrated magnets are common in the d-block, in the f-block, fluoride-based frustrated magnets are extremely rare. Herein, we report the synthesis of KYb2F5SO4, a fluoride-based distorted triangular lattice antiferromagnet with no structural disorder but significant geometric distortion from an ideal triangular lattice. In KYb2F5SO4, no long-range ordering can be observed down to 0.10 K, and the low-temperature Curie-Weiss temperature θcw = -0.46(2) K yields a frustration parameter of greater than 4.6. The magnetic entropy released at low temperature indicates an effective spin-1/2 Kramer's doublet ground state. However, the low saturation field and incomplete recovery of magnetic entropy down to 0.10 K under zero-field imply weak quantum entanglement.

Edge-derived magnetisms in very thin non-doped Bi2Te3 nanomesh

Pristine topological insulators (TIs) with no carrier doping principally suffer from a lack of magnetic ordering. We create a nanomesh structure, a honeycomblike array of hexagonal nanopores with extremely high density, on a nondoped two-dimensional (2D) thin TI (Bi2Te3). We observe antiferromagnetism (AFM) in completely hydrogen-terminated nanomeshes, while it is eliminated by ferromagnetism (FM) in completely O-terminated nanomeshes. Interplay between the AFM and FM is observed in partially O-terminated nanomeshes. As a possible origin for the observed magnetisms, Kramers doublets, which may exist around nanopore edges, for AFM and those eliminated by FM spin alignment due to O-Te coupling along pore edges are discussed based on the structure and annealing dependence of the magnetisms. The nondoped 2D nanomesh formed on a TI will lead to new avenues of research on topological magnetisms and spintronics.

Lanthanide-Based Layer-Type Two-Dimensional Coordination Polymers Featuring Slow Magnetic Relaxation, Magnetocaloric Effect and Proton Conductivity.

Three lanthanide-based two-dimensional (2D) coordination polymers (CPs), [Ln(L)(H2 O)2 ]n , {H3 L=(HO)2 P(O)CH2 CO2 H; Ln=Dy3+ (CP 1), Er3+ (CP 2)} and [{Gd2 (L)2 (H2 O)3 }. H2 O]n , (CP 3) were hydrothermally synthesized using phosphonoacetic acid as a linker. Structural features revealed that the dinuclear Ln3+ nodes were present in the 2D sheet of CP 1 and CP 2 while in the case of CP 3, nodes were further connected to each other forming a chain-type arrangement throughout the network. The magnetic studies show field-induced slow magnetic relaxation property in CP 1 and CP 2 with Ueff values of 72 K (relaxation time, τ0 =3.05×10-7 s) and 38.42 K (relaxation time, τ0 =4.60×10-8 s) respectively. Ab-initio calculations suggest that the g tensor of Kramers doublet of the lanthanide ion (Dy3+ and Er3+ ) is strongly axial in nature which reflects in the slow magnetic relaxation behavior of both CPs. CP 3 exhibits a significant magnetocaloric effect with -ΔSm =49.29 J kg-1 K-1 , one of the highest value among the reported 2D CPs. Moreover, impedance analysis of all the CPs show high proton conductivity with values of 1.13×10-6 S cm-1 , 2.73×10-3 S cm-1 and 2, 6.27×10-6 S cm-1 for CPs 1-3, respectively, at high temperature (>75 °C) and maximum 95 % relative humidity (RH).

Insight into D6h Symmetry: Targeting Strong Axiality in Stable Dysprosium(III) Hexagonal Bipyramidal Single-Ion Magnets.

Following a novel synthetic strategy where the strong uniaxial ligand field generated by the Ph3SiO− (Ph3SiO−=anion of triphenylsilanol) and the 2,4‐di‐tBu‐PhO− (2,4‐di‐tBu‐PhO−=anion of 2,4‐di‐tertbutylphenol) ligands combined with the weak equatorial field of the ligand LN6, leads to [DyIII(LN6)(2,4‐di‐tBu‐PhO)2](PF6) (1), [DyIII(LN6)(Ph3SiO)2](PF6) (2) and [DyIII(LN6)(Ph3SiO)2](BPh4) (3) hexagonal bipyramidal dysprosium(III) single‐molecule magnets (SMMs) with high anisotropy barriers of U eff=973 K for 1, U eff=1080 K for 2 and U eff=1124 K for 3 under zero applied dc field. Ab initio calculations predict that the dominant magnetization reversal barrier of these complexes expands up to the 3rd Kramers doublet, thus revealing for the first time the exceptional uniaxial magnetic anisotropy that even the six equatorial donor atoms fail to negate, opening up the possibility to other higher‐order symmetry SMMs.

Insight into D6h Symmetry: Targeting Strong Axiality in Stable Dysprosium(III) Hexagonal Bipyramidal Single‐Ion Magnets

AbstractFollowing a novel synthetic strategy where the strong uniaxial ligand field generated by the Ph3SiO− (Ph3SiO−=anion of triphenylsilanol) and the 2,4‐di‐tBu‐PhO− (2,4‐di‐tBu‐PhO−=anion of 2,4‐di‐tertbutylphenol) ligands combined with the weak equatorial field of the ligand LN6, leads to [DyIII(LN6)(2,4‐di‐tBu‐PhO)2](PF6) (1), [DyIII(LN6)(Ph3SiO)2](PF6) (2) and [DyIII(LN6)(Ph3SiO)2](BPh4) (3) hexagonal bipyramidal dysprosium(III) single‐molecule magnets (SMMs) with high anisotropy barriers of Ueff=973 K for 1, Ueff=1080 K for 2 and Ueff=1124 K for 3 under zero applied dc field. Ab initio calculations predict that the dominant magnetization reversal barrier of these complexes expands up to the 3rd Kramers doublet, thus revealing for the first time the exceptional uniaxial magnetic anisotropy that even the six equatorial donor atoms fail to negate, opening up the possibility to other higher‐order symmetry SMMs.

Effects of geometry on spin-orbit Kramers states in semiconducting nanorings

The holonomic manipulation of spin-orbital degenerate states, encoded in the Kramers doublet of narrow semiconducting channels with spin-orbit interaction, is shown to be intimately intertwined with the geometrical shape of the nanostructures. The presence of doubly degenerate states is not sufficient to guarantee a non-trivial mixing by only changing the Rashba spin-orbit coupling. We demonstrate that in nanoscale quantum rings the combination of arbitrary inhomogeneous curvature and adiabatic variation of the spin-orbit amplitude, e.g., through electric-field gating, can be generally employed to get non-trivial combinations of the degenerate states. Shape symmetries of the nanostructure act to constrain the adiabatic quantum evolution. While for circular rings the geometric phase is not generated along a non-cyclic path in the parameters space, remarkably, for generic mirror-symmetric shape deformed rings the spin-orbit driving can lead to a series of dynamical quantum phase transitions. We explicitly show this occurrence and propose a route to detect such topological transitions by measuring a variation of the electron conductance into the semiconducting channel.

Single-Molecule Magnetism in Three Dy2 Complexes from the Use of a Pentadentate Schiff Base Ligand and Different Benzoates.

Three dinuclear dysprosium(III) complexes, [Dy2 L2 (O2 CPh)2 ]⋅2 MeOH (1), [Dy2 L2 {(2-NO2 )O2 CPh}2 ] (2), and [Dy2 L2 {(2-OH)O2 CPh}2 ]⋅MeOH⋅MeCN (3) (H2 L=N1 ,N3 -bis(4-chlorosalicyladehyde)diethylenetriamine), have been synthesized and structurally characterized. Complexes 1-3 possess similar Ln2 cores and differ in substituents at the benzyl rings of benzoates. Direct current (dc) magnetic susceptibility studies in the 2-300 K range showed weak antiferromagnetic interactions between two dysprosium(III) ions in 1-3. The alternating current (ac) magnetic susceptibility measurements indicated that they all exhibited SMM behavior. The strategic attachment of the -NO2 group (in 2) and the -OH functionality (in 3) on the skeleton of the benzoic acid led to subtle variations of the bond lengths and bond angles in the coordination environments of the central dysprosium(III) ions, consequently resulting in the enhancement of the energy barriers for 2 and 3. Complete-active-space self-consistent field (CASSCF) calculations were employed to rationalize the experimental outcomes. Theoretical calculations confirm the existence of antiferromagnetic interactions in 1-3, and the calculated dc magnetic susceptibility data agree well with those obtained experimentally. The computational results reveal more axial g tensors, as well as higher first excited Kramers doublets in 2 and 3; thus resulting in higher energy barriers in compounds 2 and 3.

Lattice dynamics, crystal-field excitations, and quadrupolar fluctuations of YbRu2Ge2

We report Raman-scattering results of YbRu$_2$Ge$_2$ single crystals to explore the phononic and crystal-field (CF) excitations. This heavy-fermion metal is suggested to enter a ferroquadrupolar (FQ) phase below T$_0$=10 K. The tetragonal CF potential splits the Yb$^{3+}$ $^2F_{7/2}$ ground multiplet into two $\Gamma_6$ and two $\Gamma_7$ Kramers doublets. We establish the following CF level scheme of the ground multiplet: a $\Gamma_6$ ground state, with the two $\Gamma_7$ states at 2 cm$^{-1}$, 95 cm$^{-1}$ and the other $\Gamma_6$ state at 239 cm$^{-1}$. The $\sim$2 cm$^{-1}$ separation between the CF ground and first excited states is in agreement with the previously proposed quasi-quartet CF ground state. The intensity of the lowest-energy CF transition remarkably increases on cooling, indicating a coupling of this CF excitation to the quadrupolar fluctuations above T$_0$. From symmetry analysis, we suggest that the FQ order has B$_{1g}$ symmetry. Moreover, temperature-dependent study of four Raman-active phonon modes shows that the intensities of the A$_{1g}$ and one E$_{g}$ modes increase significantly on cooling, which is explained by a near-resonant coupling between these two phonon modes and CF transitions.

Antiferromagnetic ordering and dipolar interactions of YbAlO3

In this paper we report low-temperature magnetic properties of the rare-earth perovskite material YbAlO$_3$. Results of elastic and inelastic neutron scattering experiment, magnetization measurements along with the crystalline electrical field (CEF) calculations suggest that the ground state of Yb moments is a strongly anisotropic Kramers doublet, and the moments are confined in the $ab$-plane, pointing at an angle of $\varphi = \pm 23.5^{\circ}$ to the $a$-axis. With temperature decreasing below $T_{\rm N}=0.88$ K, Yb moments order into the coplanar, but non-collinear antiferromagnetic (AFM) structure $AxGy$, where the moments are pointed along their easy-axes. In addition, we highlight the importance of the dipole-dipole interaction, which selects the type of magnetic ordering and may be crucial for understanding magnetic properties of other rare-earth orthorhombic perovskites. Further analysis of the broad diffuse neutron scattering shows that one-dimensional interaction along the $c$-axis is dominant, and suggests YbAlO$_3$ as a new member of one dimensional quantum magnets.

Crystal-field levels of Nd3+ in a new langasite compound Nd3CrGe3Be2O14

We report on the crystal-field (CF) levels of Nd3+ in Nd3CrGe3Be2O14, a representative of recently synthesized Be-containing langasites. They are the first in the family to contain two magnetic subsystems, 1D chromium and Kagome-like rare-earth ones. High-resolution broad-band temperature-dependent spectra allow us to find all five CF Kramers doublets of the ground 4I9/2 CF multiplet and 38 levels of the excited 4I11/2, 13/2, 15/2, 4F3/2, 4F5/2 + 2H9/2, 4F7/2 + 4S3/2 and 4F9/2 CF multiplets. These data form a basis for a future exploration of this interesting two-sublattice magnet with frustrated interactions. Optical transitions between Kramers doublets of Nd3+ indicate magnetic activity of 1D-chromium chains at T < 20 K.

Relaxing Kondo-screened Kramers doublets in CeRhSi3

CeRhSi$_{3}$ is a superconductor under pressure coexisting with a weakly antiferromagnetic phase characterized by a Bragg peak at $\vec{q}_{0}$=($\sim$ 0.2, 0, 0.5) (N. Aso et al. J. Magn. Magn. Mater. 310, 602 (2007)). The compound is also a heavy fermion material with a large specific heat coefficient $\gamma$=110 mJ $\cdot$ mol$^{-1}$ $\cdot$ K$^{-2}$ and a high Kondo temperature of $T_{K}$=50 K indicative that CeRhSi$_{3}$ is in a strongly Kondo screened state. We apply high resolution neutron spectroscopy to investigate the magnetic fluctuations in the normal phase, at ambient pressures, and at low temperatures. We measure a commensurate dynamic response centered around the $\vec{Q}$=(0, 0, 2) position that gradually evolves to H$\sim$ 0.2 with decreasing temperature and/or energy transfers. The response is broadened both in momentum and energy and not reminiscent of sharp spin wave excitations found in insulating magnets where the electrons are localized. We parameterize the excitation spectrum and temperature dependence using a heuristic model utilizing the random phase approximation to couple relaxing Ce$^{3+}$ ground state Kramers doublets with a Kondo-like dynamic response. With a Ruderman-Kittel-Kasuya-Yosida (RKKY) exchange interaction within the $ab$ plane and an increasing single site susceptibility, we can qualitatively reproduce the neutron spectroscopic results in CeRhSi$_{3}$ and namely the trade-off between scattering at commensurate and incommensurate positions. We suggest that the antiferromagnetic phase in CeRhSi$_{3}$ is driven by weakly correlated relaxing localized Kramers doublets and that CeRhSi$_{3}$ at ambient pressures is on the border between a Rudderman-Kittel-Yosida antiferromagnetic state and a Kondo screened phase where static magnetism is predominately absent.

Probing Magnetic Excitations in CoII Single-Molecule Magnets by Inelastic Neutron Scattering.

Co(acac)2(H2O)2 (1, acac = acetylacetonate), a transition metal complex ( ), displays field-induced slow magnetic relaxation as a single-molecule magnet. For 1 and its isotopologues Co(acac)2(D2O)2 (1-d 4 ) and Co(acac-d 7)2(D2O)2 (1-d 18 ) in approximately symmetry, zero-field splitting of the ground electronic state leads to two Kramers doublets (KDs): lower energy and higher energy states. This work employs inelastic neutron scattering (INS), a unique method to probe magnetic transitions, to probe different magnetic excitations in 1-d 4 and 1-d 18 . Direct-geometry, time-of-flight Disk-Chopper Spectrometer (DCS), with applied magnetic fields up to 10 T, has been used to study the intra-KD transition as a result of Zeeman splitting, , in 1-d 18 . This is a rare study of the excitation in transition metal complexes by INS. Indirect-geometry INS spectrometer VISION has been used to probe the inter-KD, ZFS transition, in both 1-d 4 and 1-d 18 , by variable-temperature (VT) properties of this excitation. The INS spectra measured on VISION also give phonon features of the complexes that are well described by periodic DFT phonon calculations.

Topological acoustic transports in chiral sonic crystals

&lt;sec&gt;Topological sound has enriched the way of implementing the sound manipulation, which can effectively suppress the backscattering due to topological protection. As an inherent longitudinal wave, sound wave has no " spin” and only supports longitudinal vibration. Creating the " pseudospin” degree of freedom is crucial to topological state for acoustic wave. In previous studies, a circulating fluid flow in the background field is introduced to break the reciprocity of wave propagation in an acoustic system, which still faces technically a challenge. On the other hand, acoustic analogues of quantum spin Hall state and valley Hall state are realized by relying on the Kramers doublet in the lattices with &lt;i&gt;C&lt;/i&gt;&lt;sub&gt;6&lt;/sub&gt; symmetry and the broken mirror symmetry or inversion symmetry, respectively. In these cases, the distributions of acoustic energy flux in the unit cells emulate the pseudospins. Based on the band inversion, the topological sound carrying pseudospin is implemented at the interface between topologically trivial and non-trivial sonic crystal. Because of the close relevance to the lattice symmetry, these pseudospin-based topological state in the time-reversal invariant system is sensitive to structural defects. &lt;/sec&gt;&lt;sec&gt;In this work, we investigate the topological sound in chiral sonic crystal consisting of resonant air tubes. The counterclockwise and clockwise length variation of air tube correspond to different topological phases. A defect meta-molecule is created at the symmetric interface, which supports resonant state in the band gap. The interface state occurs at the boundary between two opposite chiral sonic crystals. Owing to the resonant structure, we realize subwavelength topological sound transport with a subwavelength-transverse confinement. For the state carrying monopolar-mode symmetry, it is expected to preserve the mode symmetry under randomly introduced defects. As anticipated, the numerical results show that the topological sound has very strong robustness against various defects, such as the variation of positions and length of air tube. Finally, we utilize the field symmetry of topological sound in chiral sonic crystal to realize robust edge transport along soft or rigid boundary. Through the mirror symmetry operation of soft or rigid boundary, we construct an interface between the real lattice and its virtual image. The approach greatly reduces the dimension of sonic crystal device. Our work may conduce to the advances in topological acoustics, since the subwavelength-scale topological state promotes the applications of miniaturized acoustic devices.&lt;/sec&gt;

Fabrication, sintering, heat capacity, magnetic and magnetroresistivity properties of ytterbium sulfides

Abstract The preparation, sintering and low-temperature physical properties of four kinds of ytterbium sulfides (η-Yb2S3, e-Yb2S3, Yb3S4 and YbS) were systematically investigated. The suitable sintering processes were ensured for each ytterbium sulfide. SEM results suggested the sintered ytterbium sulfides had high dense. The measured susceptibility and magnetization of ytterbium sulfides suggested Yb was mainly trivalent in η-Yb2S3 and e-Yb2S3. Moreover, the magnetroresistivity of η-Yb2S3 and YbS shows semiconductor behavior. The resistivity of orthorhombic η-Yb2S3 is consistent with Kondo-like characteristic (logarithmic increase with decreasing temperature), while the ρ(T) behavior of YbS is contrast to the Kondo-like behavior. The contribution of electron, lattice and magnetism to heat capacity of ytterbium sulfides was analyzed. For η-Yb2S3, Yb3S4 and YbS, the crystal electron field (CEF) ground state is a Kramers doublet while for e-Yb2S3 it changed to quartet (Γ8). Chemical composition and crystal structure had strong influence on the heat capacity of lanthanide sesquisulfides.

Ab initio investigation of magnetic anisotropy in intermediate spin iron(iii) complexes.

Mononuclear Fe(iii) complexes commonly exist in high-spin or low-spin states, whereas their occurrence in the intermediate-spin state (S = 3/2) is scarce. The magnetic anisotropy in two trigonal-bipyramidal mononuclear Fe(iii) complexes, (1) and (2), in their intermediate-spin ground state has been examined by ab initio electronic structure calculations. The calculations successfully reproduce the experimental magnetic anisotropic barrier, U eff in 1 (81 cm-1) and 2 (42 cm-1), which is shown to arise due to thermally assisted quantum tunneling of magnetization from the second Kramer's doublets. The magnetic anisotropy in both the complexes is found to be significantly influenced by the axial ligands, while the equatorial ligands have negligible contribution. The large reduction in U eff of 2 has been shown to arise due to the phenyl groups, which results in the lifting of orbital degeneracy of e″ and e' frontier orbitals and leads to a net quenching of the orbital angular momentum of the metal center causing a diminished spin-orbit splitting in 2. While the crystal structure of 2 shows two phenyl rings out of plane to each other, the present study discovered another stable conformation of 2, where the two phenyl rings are in the same plane (2a). Unlike 2, the planarity of the two phenyl rings in 2a restores the degeneracy of the frontier orbitals, thereby increasing the spin-orbit splitting and a consequent rise in U eff from 42 to 80 cm-1 in 2a.

Cubic hastatic order in the two-channel Kondo-Heisenberg model

Materials with non-Kramers doublet ground states naturally manifest the two-channel Kondo effect, as the valence fluctuations are from a non-Kramers doublet ground state to an excited Kramers doublet. Here, the development of a heavy Fermi liquid requires a channel symmetry breaking spinorial hybridization that breaks both single and double time-reversal symmetry, and is known as hastatic order. Motivated by cubic Pr-based materials with $\Gamma_3$ non-Kramers ground state doublets, this paper provides a survey of cubic hastatic order using the simple two-channel Kondo-Heisenberg model. Hastatic order necessarily breaks time-reversal symmetry, but the spatial arrangement of the hybridization spinor can be either uniform (ferrohastatic) or break additional lattice symmetries (antiferrohastatic). The experimental signatures of both orders are presented in detail, and include tiny conduction electron magnetic moments. Interestingly, there can be several distinct antiferrohastatic orders with the same moment pattern that break different lattice symmetries, revealing a potential experimental route to detect the spinorial nature of the hybridization. We employ an SU(N) fermionic mean-field treatment on square and simple cubic lattices, and examine how the nature and stability of hastatic order varies as we vary the Heisenberg coupling, conduction electron density, band degeneracies, and apply both channel and spin symmetry breaking fields. We find that both ferrohastatic and several types of antiferrohastatic orders are stabilized in different regions of the mean-field phase diagram, and evolve differently in strain and magnetic fields.

Probing Magnetic Excitations in CoII Single‐Molecule Magnets by Inelastic Neutron Scattering

Co(acac)2(H2O)2 (1, acac = acetylacetonate), a transition metal complex (S = 3/2), displays field‐induced slow magnetic relaxation as a single‐molecule magnet. For 1 and its isotopologues Co(acac)2(D2O)2 (1‐d4) and Co(acac‐d7)2(D2O)2 (1‐d18) in approximately D4h symmetry, zero‐field splitting of the ground electronic state leads to two Kramers doublets (KDs): lower energy MS = ±1/2 (ϕ1,2) and higher energy MS = ±3/2 (ϕ3,4) states. This work employs inelastic neutron scattering (INS), a unique method to probe magnetic transitions, to probe different magnetic excitations in 1‐d4 and 1‐d18. Direct‐geometry, time‐of‐flight Disk‐Chopper Spectrometer (DCS), with applied magnetic fields up to 10 T, has been used to study the intra‐KD transition as a result of Zeeman splitting, MS = –1/2 (ϕ1) → MS = +1/2 (ϕ2), in 1‐d18. This is a rare study of the MS = –1/2 → MS = +1/2 excitation in transition metal complexes by INS. Indirect‐geometry INS spectrometer VISION has been used to probe the inter‐KD, ZFS transition, MS = ±1/2 (ϕ1,2) → MS = ±3/2 (ϕ3,4) in both 1‐d4 and 1‐d18, by variable‐temperature (VT) properties of this excitation. The INS spectra measured on VISION also give phonon features of the complexes that are well described by periodic DFT phonon calculations.

Ordered magnetism in the intrinsically decorated jeff=12α−CoV3O8

The antiferromagnetic mixed valence ternary oxide $\alpha$-CoV$_{3}$O$_{8}$ displays disorder on the Co$^{2+}$ site that is inherent to the $Ibam$ space group. The zero field structural and dynamic properties of $\alpha$-CoV$_{3}$O$_{8}$~have been investigated using a combination of neutron and x-ray diffraction, DC susceptibility, and neutron spectroscopy. The low temperature magnetic and structural properties are consistent with a random macroscopic distribution of Co$^{2+}$ over the 16$k$ metal sites. However, by applying the sum rules of neutron scattering we observe the collective magnetic excitations are parameterized with an ordered Co$^{2+}$ arrangement and critical scattering consistent with a three dimensional Ising universality class. The low energy spectrum is well-described by Co$^{2+}$ cations coupled $via$ a three dimensional network composed of competing ferromagnetic and stronger antiferromagnetic superexchange within the $ab$ plane and along $c$, respectively. While the extrapolated Weiss temperature is near zero, the 3D dimensionality results in long range antiferromagnetic order at $T\rm{_{N}}\sim$ 19 K. A crystal field analysis finds two bands of excitations separated in energy at $\hbar \omega$ $\sim$ 5 meV and 25 meV, consistent with a $j\rm{_{eff}}=\frac{1}{2}$ ground state with little mixing between spin-orbit split Kramers doublets. A comparison of our results to the random 3D Ising magnets and other compounds where spin-orbit coupling is present indicate that the presence of an orbital degree of freedom, in combination with strong crystal field effects and well-separated $j\rm{_{eff}}$ manifolds may play a key role in making the dynamics largely insensitive to disorder.

La3CrGe3Be2O14 and Nd3CrGe3Be2O14: New magnetic compounds of the langasite family

This communication focuses on recently discovered Ln3CrGe3Be2O14 (Ln = La–Sm) compounds as a new class of low-dimensional magnets. We conducted the first studies of their magnetic properties by means of the magnetic susceptibility, specific heat, and high-resolution optical spectroscopy measurements of the La and Nd representatives. In La3CrGe3Be2O14, which contains only one (chromium) magnetic system, the shortest exchange paths are within isolated chromium chains along the c axis. The compound demonstrates one-dimensional magnetic behavior, i.e., an absence of a long-range order, at least down to 0.4 K, and short-range magnetic correlations with the antiferromagnetic exchange integral along the chains estimated as JCr = 4.0 ± 0.5 K. In Nd3CrGe3Be2O14 containing two magnetic subsystems, a splitting of the neodymium Kramers doublets at low temperatures indicates magnetic activity due to both short-range correlations and possible magnetic ordering at 2 K.