Excitations in a four leg antiferromagnetic Heisenberg spin tube

Inelastic neutron scattering is used to investigate magnetic excitations in the quasi one dimensional quantum spin liquid system Cu2Cl4 D8C4SO2. Contrary to previously conjectured models that relied on bond alternating nearest neighbor interactions in the spin chains, the dominant interactions are actually next nearest neighbor in chain antiferromagnetic couplings. The appropriate Heisenberg Hamiltonian is equivalent to that of a S 1 2 4 leg spin tube with almost perfect one dimensionality and no bond alternation. A partial geometric frustration of rung interactions induces a small incommensurability of short range spin correlations

Crystals for neutron scattering studies of quantum magnetism

We review a strategy for targeted synthesis of large single crystal samples of prototype quantum magnets for inelastic neutron scattering experiments. Four case studies of organic copper halogenide S=1/2 systems are presented. They are meant to illustrate that exciting experimental results pertaining to forefront many-body quantum physics can be obtained on samples grown using very simple techniques, standard laboratory equipment, and almost no experience in in advanced crystal growth techniques.Comment: 16 pages, 10 figure

Controlling Magnetic Ordering in Ca1 xEuxCo2As2 by Chemical Compression

To investigate the interplay between electronic structure and itinerant magnetism, Ca$_{1–x}$Eu$_x$Co$_2$As$_2$ solid solutions (x = 0, 0.1, 0.3, 0.4, 0.5, 0.6, 0.65, 0.7, 0.9, 1.0) were prepared by reactions between constituent elements in molten Bi. All of the samples crystallize in the ThCr$_2$Si$_2$ structure type. The crystal structure refinement revealed the formation of Co vacancies, the concentration of which decreases as the Eu content increases. The Eu site exhibits mixed valence in all samples. X-ray absorption near-edge structure spectroscopy revealed that the average Eu oxidation state decreases from +2.17 at 0 < x ≤ 0.6 to +2.14 at x ≥ 0.65. The same borderline behavior is observed in magnetic properties. The substitution of Eu for Ca causes the transition from the antiferromagnetic (AFM) ordering of Co moments in CaCo$_2$As$_2$ to ferromagnetic (FM) ordering of Co moments in Ca1–xEuxCo$_2$As$_2$ with 0.1 ≤ x ≤ 0.6. At higher Eu content, AFM ordering of Eu moments is observed, whereas the Co sublattice exhibits only paramagnetic behavior. Single-crystal neutron diffraction studies revealed that both Co and Eu sublattices order FM in Ca$_{0.5}$Eu$_{0.5}$Co$_2$As$_2$ with the magnetic moments aligned along the tetragonal c axis. In the AFM phases with x ≥ 0.65, only Eu moments are ordered in a helical spin structure defined by an incommensurate propagation vector k = [00q], with the moment lying in the ab plane. The changes in magnetic behavior are well-justified by the analysis of the electronic density of states and crystal orbital Hamilton population

Lattice dynamics of Ni-Mn-Al Heusler alloys

International audienceWe have studied the lattice dynamics of a Ni54Mn23Al23 (at.%) Heusler single-crystalline alloy by means of neutron scattering and ultrasonic techniques. Results show the existence of a number of precursor phenomena. We have found an anomaly (dip) in the low TA2 phonon branch at the wave number ξ0 ≈ 0.33 (in reciprocal lattice units) that becomes more pronounced (phonon softening) with decreasing temperature. We have also observed softening of the associated shear elastic constant (C') with decreasing temperature. Ultrasonic measurements under applied magnetic field, both isothermally and varying the temperature show that the values of elastic constants depend on magnetic order thus evidencing magnetoelastic couplin

A Transition from Localized to Strongly Correlated Electron Behavior and Mixed Valence Driven by Physical or Chemical Pressure in ACo2As2 A Eu and Ca

We demonstrate that the action of physical pressure, chemical compression, or aliovalent substitution in ACo$_2$As$_2$ (A = Eu and Ca) has a general consequence of causing these antiferromagnetic materials to become ferromagnets. In all cases, the mixed valence triggered at the electropositive A site results in the increase of the Co 3d density of states at the Fermi level. Remarkably, the dramatic alteration of magnetic behavior results from the very minor (<0.15 electron) change in the population of the 3d orbitals. The mixed valence state of Eu observed in the high-pressure (HP) form of EuCo$_2$As$_2$ exhibits a remarkable stability, achieving the average oxidation state of +2.25 at 12.6 GPa. In the case of CaCo$_2$As$_2$, substituting even 10% of Eu or La into the Ca site causes ferromagnetic ordering of Co moments. Similar to HP-EuCo$_2$As$_2$, the itinerant 3d ferromagnetism emerges from electronic doping into the Co layer because of chemical compression of Eu sites in Ca$_{0.9}$Eu$_{0.1}$Co$_{1.91}$As$_2$ or direct electron doping in Ca$_{0.85}$La$_{0.15}$Co$_{1.89}$As$_2$. The results reported herein demonstrate the general possibility of amplifying minor localized electronic effects to achieve major changes in material’s properties via involvement of strongly correlated electrons

Dynamics of quantum spin liquid and spin solid phases in IPA CuCl3 under an applied magnetic field studied with neutron scattering

Inelastic and elastic neutron scattering is used to study spin correlations in the quasi one dimensional quantum antiferromagnet IPA CuCl3 in strong applied magnetic fields. A condensation of magnons and commensurate transverse long range ordering is observe at a critical field Hc 9.5 T. The field dependencies of the energies and polarizations of all magnon branches are investigated both below and above the transition point. Their dispersion is measured across the entire one dimensional Brillouin zone in magnetic fields up to 14 T. The critical wave vector of magnon spectrum truncation [Masuda et al., Phys. Rev. Lett. 96, 047210 2006 ] is found to shift from hc[approximate]0.35 at H lt;Hc to hc 0.25 for H gt;Hc. A drastic reduction of magnon bandwidths in the ordered phase [Garlea et al., Phys. Rev. Lett. 98, 167202 2007 ] is observed and studied in detail. New features of the spectrum, presumably related to this bandwidth collapse, are observed just above the transition fiel

Neutron spectroscopy of molecular nanomagnets

This short overview gives an account on the use of neutron spectroscopy for the examination of molecular nanomagnets, systems constructed by crystalline arrangement of finite size clusters (usually with regular form) of interacting moment carrying atoms - magnetic molecules. Opposed to extended magnetic systems with bands of collective excitations such as spin-waves the molecular nanomagnets are entities with local properties, each magnetic molecule possessing a finite number of energy levels that can be solved exactly for small enough systems. In essence, the number of states remains finite despite growing rapidly with increasing number of magnetic centers and the value of the spin quantum number. Increasingly large numbers of states and more complex exchange networks lead to the need for approximative treatments, the validity of which can be checked with neutron spectroscopy. Molecular nanomagnets provide interesting examples of physics and magnetochemistry, illustrated here with a few examples that highlight the power of neutron spectroscopy for precise investigation of the energy level structure and spatial configuration of the magnetic exchange parameters.</p