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We study the evolution of the low-temperature field-induced magnetic defects observed under an applied magnetic field in a series of frustrated amorphous ferromagnets (Fe$_{1-x}$Mn$_{x}$)$_{75}$P$_{16}$B$_{3}$Al$_{3}$ (a-FeMn). Combining small-angle neutron scattering and Monte Carlo simulations, we show that the morphology of these defects resemble that of quasi-bidimensional spin vortices. They are observed in the reentrant spin-glass (RSG) phase, up to the critical concentration $x_{\rm C} \approx 0.36$ which separates the RSG and "true" spin glass (SG) within the low temperature part of the magnetic phase diagram of a-FeMn. These vortices systematically decrease in size with increasing magnetic field or decreasing the average exchange interaction, and they finally disappear in the SG sample ($x = 0.41$), being replaced by field-induced correlations over finite length scales. We argue that the study of these nanoscopic defects could be used to probe the nature of the critical line between the RSG and SG phases., Comment: 23 pages, 14 figures, includes supplement

The lack of inversion symmetry in the crystal lattice of magnetic materials gives rise to complex noncollinear spin orders through interactions of a relativistic nature, resulting in interesting physical phenomena, such as emergent electromagnetism. Studies of cubic chiral magnets revealed a universal magnetic phase diagram composed of helical spiral, conical spiral, and skyrmion crystal phases. We report a remarkable deviation from this universal behavior. By combining neutron diffraction with magnetization measurements, we observe a new multidomain state in Cu2OSeO3. Just below the upper critical field at which the conical spiral state disappears, the spiral wave vector rotates away from the magnetic field direction. This transition gives rise to large magnetic fluctuations. We clarify the physical origin of the new state and discuss its multiferroic properties.

Reentrant spin glasses are frustrated disordered ferromagnets developing vortex-like textures under an applied magnetic field. Our study of a Ni$_{0.81}$Mn$_{0.19}$ single crystal by small angle neutron scattering clarifies their internal structure and shows that these textures are randomly distributed. Spin components transverse to the magnetic field rotate over length scales of 3-15 nm in the explored field range, decreasing as field increases according to a scaling law. Monte-Carlo simulations reveal that the internal structure of the vortices is strongly distorted and differs from that assumed for "frustrated" skyrmions, built upon a competition between symmetric exchange interactions. Isolated vortices have small non-integer topological charge. The vortices keep an anisotropic shape on a 3 dimensional lattice, recalling "croutons" in a "ferromagnetic soup". Their size and number can be tuned independently by the magnetic field and concentration x (or heat treatment), respectively. This opens an original route to understand and control the influence of quenched disorder in systems hosting non trivial spin textures., Comment: 23 pages, 15 figures, includes supplement

Magnetic skyrmions are nano-sized topologically protected spin textures with particle-like properties. They can form lattices perpendicular to the magnetic field and the orientation of these skyrmion lattices with respect to the crystallographic lattice is governed by spin-orbit coupling. By performing small angle neutron scattering measurements, we investigate the coupling between the crystallographic and skyrmion lattices in both Cu$_2$OSeO$_3$ and the archetype chiral magnet MnSi. The results reveal that the orientation of the skyrmion lattice is primarily determined by the magnetic field direction with respect to the crystallographic lattice. In addition, it is also influenced by the magnetic history of the sample which can induce metastable lattices. Kinetic measurements show that these metastable skyrmion lattices may or may not relax to their equilibrium positions under macroscopic relaxation times. Furthermore, multidomain lattices may form when two or more equivalent crystallographic directions are favored by spin-orbit coupling and oriented perpendicular to the magnetic field., Comment: Supplementory movie 4 is not uploaded due to size restrictions but is available upon request

We feature our recent work in the field of single-molecule magnets (SMMs) using inelastic neutron scattering (INS). The term "pressure" in the title has a triple meaning. First, there is the expectation from research-funding agencies and the public to make some significant steps towards applications. Second, the synthesis of new compounds and the applications of physical techniques for their understanding are being pushed to their limits. And third, by applying hydrostatic pressure, valuable insight into the mechanisms behind the SMM phenomena can be gained. Examples from our research have been taken to illustrate these points. After a brief introduction to the technique, the strength of INS for the accurate determination of exchange and anisotropy interactions in SMMs is highlighted. We hope to demonstrate that, by pushing INS measurements to their limits, i.e., using the best available neutron sources and instrumentation, combined with high quality samples, new insights into the relevant physical processes in SMMs can be gained.

We have obtained new inelastic neutron scattering (INS) data for the molecular magnet Mn 12 -acetate which exhibit at least six magnetic peaks in the energy range 5–35 meV. These are compared with a microscopic Heisenberg model for the 12 quantum spins localised on the Mn ions, coupled by four inequivalent magnetic exchange constants. A fit to the magnetic susceptibility under the constraint that the spin of the ground state be S = 10 yields two dominant exchange constants of very similar value, J 1 ≈ J 2 ≈ 65 K ( ≈ 5.5 meV), and two smaller exchange constants J 3 and J 4 . We compute the low-lying excitations by exact numerical diagonalisation and demonstrate that the parameters determined from the ground state and susceptibility fit provide qualitative agreement with the excitations observed by INS.

Molecular magnets are crystalline materials made up of interacting magnetic centres within each molecule. Each such “spin-cluster” is magnetically well isolated from its neighbours due to the surrounding ligands. The resulting magnetic properties are governed by exchange interactions between neighbouring spins and magneto-crystalline anisotropy. We present a brief overview of the salient features observed in three very different molecular magnets (Mn 4 , Ni 12 and V 15 ) where magnetic frustration plays a crucial role. It is demonstrated that Inelastic Neutron Scattering (INS) is an excellent technique to elucidate complex behaviour associated with geometrically frustrated molecular magnets.

We report on the magnetic properties of the tetranuclear metallic cluster [Mn4O2(O2CPh)6(dpm)2]. Using susceptibility, magnetization and inelastic neutron scattering (INS) spectroscopy we have determined the exchange and anisotropy parameters. The four Mn3+ (S=2) form a tetranuclear cluster with butterfly-type geometry. The two core Mn3+ ions are strongly antiferromagnetically coupled with Jb=−4.8±0.1 meV. The outer Mn3+ ions are loosely coupled to the core spins via two different weak interactions, ferromagnetic and antiferromagnetic, respectively: Jw1=+0.1±0.01 meV and Jw2=−0.1±0.01 meV. In addition, the octahedrally coordinated outer Mn3+ ions experience a large single-ion anisotropy DS2z with D=−0.47±0.01 meV due to a strong distortion of the MnO6 octahedra. The Lande g-factor is g=1.94±0.03. Frustration effects and competitive interactions lead to remarkable magnetic properties which cannot be unambiguously interpreted without the additional information from INS.

The properties of the spin state manifold of the tetranuclear cluster Ni[(OH)(2)Cr(bispictn)](3)]I(5).5H(2)O (bispictn = N,N'-bis(2-pyridylmethyl)-1,3-propanediamine) are investigated by combining magnetic susceptibility and magnetization measurements with an inelastic neutron scattering (INS) study on an undeuterated sample of Ni[(OH)(2)Cr(bispictn)](3)]I(5).5H(2)O. The temperature dependence of the magnetic susceptibility indicates an S = (1)/(2) ground state, which requires antiferromagnetic interactions both between Cr(3+) and Ni(2+) ions and among the Cr(3+) ions. INS reveals potential single-ion anisotropies to be negligibly small and enables an accurate determination of the exchange parameters. The best fit to the experimental energy level diagram is obtained by an isotropic spin Hamiltonian H = J(CrNi)(S(1)().S(4)() + S(2)().S(4)() + S(3)().S(4)()) + J(CrCr)(S(1)().S(2)() + S(1)().S(3)() + S(2)().S(3)()) with J(CrNi) = 1.47 cm(-)(1) and J(CrCr) = 1.25 cm(-)(1). With this model, the experimental intensities of the observed INS transitions as well as the temperature dependence of the magnetic data are reproduced. The resulting overall antiferromagnetic exchange is rationalized in terms of orbital exchange pathways and compared to the situation in oxalato-bridged clusters.

We report an Inelastic Neutron Scattering (INS) study of the fully deuterated molecular compound K6[V15As6O42].9(D2O) (V15). Due to geometrical frustration, the essential physics at low temperatures of the V15 cluster containing 15 coupled V4+ (S=1/2) is determined by three weakly coupled spin-1/2 on a triangle. The INS spectra at low-energy allow us to directly determine the effective exchange coupling J0 = 0.211 meV within the triangle and the gap 2 \Delta = 0.035(2)meV between the two spin-1/2 doublets of the ground state. Results are discussed in terms of deviations from trigonal symmetry and Dzyaloshinskii-Moriya (DM) interactions., Comment: 8 pages, 4 figures. to appear in Europhysics Letters

We have investigated by inelastic neutron scattering (INS), neutron diffraction, andmagnetometry the magnetic properties of the mononuclear complex [Mn3+(pyrol)3(tren)] in both high-spin (5E, HS, S = 2) and low-spin (3T1, LS, S = 1) states. The system presents a spin transition (ST) around 47 K with a small hysteresis width (TST,↑ = 47.5 K and TST,↓ = 46 K) characteristic of an efficient collective transition process. In the HS state, the INS spectrum at 56 K and zero magnetic field is accounted for by a zero-field splitting with D = −5.73(3) cm−1 and |E| = +0.47(2) cm−1 which may be the result of a dynamic Jahn-Teller effect reported in the literature.In the LS state, a single magnetic peak at 4.87 meV is observed, still at zero field. Despite the existence of an unquenched orbital moment (L = 1) in the ground 3T1 state, we argue that it may be described by a genuine S = 1 spin Hamiltonian owing to the existence of a strong trigonal distortion of the Mn3+ coordination octahedron. The observed peak corresponds to a transition M = +1 within the S = 1 ground state split by a large single-ion anisotropy term D = +39.3 cm−1. A full spin-Hamiltonian model is proposed based on these first INS results obtained in a thermal ST molecular magnetic system.

International audience; The magnetic anisotropy is a prerequisite for a metal complex to behave as a single-molecule magnet (SMM). Unfortunately, today we do not fully understand the relationships between the local structural parameters and the magnetic anisotropy that results at the molecular level. This is an issue that has become recursive in this area. Out of the synthesis work which is still important, but generates a multiplication of SMMs with frustrating properties, there are various studies to understand these relationships among which most are theoretical studies. In this context, we believe that polarized neutron diffraction (PND) can provide an experimental and complementary point of view to these theoretical studies. PND is indeed well known to allow an accurate determination of the spin density in magnetic compounds and in the field of molecular magnetism it has provided unique information on the pathways and the nature of intra- or intermolecular magnetic coupling [1]. In the case of highly anisotropic paramagnetic materials, where local magnetic moments cannot be aligned by an external magnetic field, that is more tricky, but a method based on local magnetic susceptibility tensor, has been recently developed that allows now analysing the data in this case and obtaining the magnetization distribution [2]. This approach was first used for inorganic compounds. Our idea has been to use this approach to go beyond the reconstruction of spin density to study the magnetic anisotropy in molecular systems. In this paper, we present the results of such an approach applied for the first time to metal complexes that are simple mono and dinuclear cobalt(II) complexes.

Specific heat ($C_V$) measurements in the spin-1/2 Cu$_2$(C$_2$H$_{12}$N$_2$)$_2$Cl$_4$ system under a magnetic field up to $H=8.25 T$ are reported and compared to the results of numerical calculations based on the 2-leg antiferromagnetic Heisenberg ladder. While the temperature dependences of both the susceptibility and the low field specific heat are accurately reproduced by this model, deviations are observed below the critical field $H_{C1}$ at which the spin gap closes. In this Quantum High Field phase, the contribution of the low-energy quantum fluctuations are stronger than in the Heisenberg ladder model. We argue that this enhancement can be attributed to dynamical lattice fluctuations. Finally, we show that such a Heisenberg ladder, for $H>H_{C1}$, is unstable, when coupled to the 3D lattice, against a lattice distortion. These results provide an alternative explanation for the observed low temperature ($T_C\sim 0.5K$ -- $0.8K$) phase (previously interpreted as a 3D magnetic ordering) as a new type of incommensurate gapped state.

In a magnetic field, spin-ladders undergo two zero-temperature phase transitions at the critical fields Hc1 and Hc2. An experimental review of static and dynamical properties of spin-ladders close to these critical points is presented. The scaling functions, universal to all quantum critical points in one-dimension, are extracted from (a) the thermodynamic quantities (magnetization) and (b) the dynamical functions (NMR relaxation). A simple mapping of strongly coupled spin ladders in a magnetic field on the exactly solvable XXZ model enables to make detailed fits and gives an overall understanding of a broad class of quantum magnets in their gapless phase (between Hc1 and Hc2). In this phase, the low temperature divergence of the NMR relaxation demonstrates its Luttinger liquid nature as well as the novel quantum critical regime at higher temperature. The general behaviour close these quantum critical points can be tied to known models of quantum magnetism.

We present an extensive NMR study of the spin-1/2 antiferromagnetic Heisenberg ladder $\mathrm{Cu}{}_{2}({\mathrm{C}}_{5}{\mathrm{H}}_{12}{\mathrm{N}}_{2}{)}_{2}\mathrm{Cl}{}_{4}$ in a magnetic field range 4.5--16.7 T. By measuring the proton NMR relaxation rate $1/{T}_{1}$ and varying the magnetic field around the critical field ${H}_{c1}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}\ensuremath{\Delta}/g{\ensuremath{\mu}}_{B}\ensuremath{\approx}7.5\mathrm{T}$, we have studied the transition from a gapped spin liquid ground state to a gapless magnetic regime which can be described as a Luttinger liquid. We identify an intermediate regime $T\ensuremath{\ge}|{H\ensuremath{-}H}_{c1}|$, where the spin dynamics is (possibly) only controlled by the $T\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0$ critical point ${H}_{c1}$.

The ${}^{1}\mathrm{H}$ hyperfine shift $K$ and NMR relaxation rate ${T}_{1}^{\ensuremath{-}1}$ have been measured as a function of temperature in the $S\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1/2$ Heisenberg antiferromagnetic ladder compound ${\mathrm{Cu}}_{2}({\mathrm{C}}_{5}{\mathrm{H}}_{12}{\mathrm{N}}_{2}{)}_{2}{\mathrm{Cl}}_{4}$. The presence of a spin gap $\ensuremath{\Delta}\ensuremath{\simeq}{J}_{\ensuremath{\perp}}{\ensuremath{-}J}_{\ensuremath{\parallel}}$ in this strongly coupled ladder ( ${J}_{\ensuremath{\parallel}}l{J}_{\ensuremath{\perp}}$) is supported by the $K$ and ${T}_{1}^{\ensuremath{-}1}$ results. By comparing ${T}_{1}^{\ensuremath{-}1}$ at two different ${}^{1}\mathrm{H}$ sites, we infer the evolution of the spectral functions ${S}_{z}(q,{\ensuremath{\omega}}_{n})$ and ${S}_{\ensuremath{\perp}}(q,{\ensuremath{\omega}}_{n})$. When the gap is significantly reduced by the magnetic field, two different channels of nuclear relaxation, specific to gapped antiferromagnets, are identified and are in agreement with theoretical predictions.

We have designed and constructed a torsional oscillator magnetometer for use in magnetic fields up to 25 T. The anisotropic component of the magnetization is detected by measuring the shift in the resonant frequency of the oscillator, which is fabricated from a single‐crystal silicon wafer using micromachining techniques. The frequencies for the oscillators described here are between 100 Hz and 1 kHz and can be measured with a resolution of order 1 part in 108 in a 10 s averaging time, allowing for the detection of magnetic moments of 2×10−11 J/T at 1 T. Our oscillators are optimized for experiments on GaAs‐AlGaAs heterostructures, but the method is suitable for any sample with an anisotropic susceptibility. We have applied the technique to two systems, a quasi‐one‐dimensional spin chain and a two‐dimensional electron gas.

We investigate the calculation of the magnetic form factors of nano-objects with complex geometrical shapes and non homogeneous magnetization distributions. We describe a numerical procedure which allows to calculate the 3D magnetic form factor of nano-objects from realistic magnetization distributions obtained by micromagnetic calculations. This is illustrated in the canonical cases of spheres, rods and platelets. This work is a first step towards a 3D vectorial reconstruction of the magnetization at the nanometric scale using neutron scattering techniques., Comment: 7 pages, 5 figures. To appear in Physics Procedia

We report the measurements of low temperature spin waves in the bilayer colossal magnetoresistive manganite La 2−2 x Sr 1+2 x Mn 2 O 7 with hole doping x =0.35. The dispersion relations for acoustic and optic branches are reasonably well described by a Heisenberg Hamiltonian with nearest neighbour exchange only. The coupling constants are 2 SJ ∥ =9.3±0.1 meV and 2 SJ ⊥ =5.7±0.2 meV within and between planes respectively. The ratio J ⊥ / J ∥ =0.61 is larger than the corresponding value of ∼0.3 for x =0.4. This is explained by greater d 3 z 2−r 2 orbital occupancy as the hole doping is reduced. The acoustic spin wave damping is proportional to the spin wave energy in the long wavelength limit. The damping is much larger than expected from magnon–magnon or magnon–electron scattering.

Dynamic magnetic properties of arrays of Ni nanorods with a low aspect ratio have been investigated. It has been shown that the spectra of spin-wave resonances localized on nanorods with a low aspect ratio typically feature the presence of zones with high density of states resulting in a characteristic two-peak pattern of Stokes and anti-Stokes lines of magneto-optical (MO) Brillouin light scattering with pronounced Stokes--anti-Stokes (S-AS) asymmetry. A simple theoretical model based on the analysis of the elliptic character of the polarization of the optical wave interacting with a dipole magnetostatic wave has been proposed. It has been shown that the S-AS asymmetry is due entirely to the asymmetry of the MO interaction efficiency with respect to time reversal of the magnetic precession in a magnon.

A detailed investigation of the unusual dynamics of the magnetization of ${({\mathrm{Et}}_{4}\mathrm{N})}_{3}{\mathrm{Fe}}_{2}{\mathrm{F}}_{9}$ $({\mathrm{Fe}}_{2})$, containing isolated ${[{\mathrm{Fe}}_{2}{\mathrm{F}}_{9}]}^{3\ensuremath{-}}$ dimers, is presented and discussed. ${\mathrm{Fe}}_{2}$ possesses an $S=5$ ground state with an energy barrier of $2.40\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ due to an axial anisotropy. Poor thermal contact between sample and bath leads to a phonon bottleneck situation, giving rise to butterfly-shaped hysteresis loops below $5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ concomitant with slow decay of the magnetization for magnetic fields ${H}_{z}$ applied along the $\mathrm{Fe}\mathrm{Fe}$ axis. The butterfly curves are reproduced using a microscopic model based on the interaction of the spins with resonant phonons. The phonon bottleneck allows for the observation of resonant quantum tunneling of the magnetization at $1.8\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, far above the blocking temperature for spin-phonon relaxation. The latter relaxation is probed by ac magnetic susceptibility experiments at various temperatures and bias fields ${H}_{\mathrm{DC}}$. At ${H}_{\mathrm{DC}}=0$, no out-of-phase signal is detected, indicating that at $T\ensuremath{\geqslant}1.8\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ ${\mathrm{Fe}}_{2}$ does not behave as a single-molecule magnet. At ${H}_{\mathrm{DC}}=1\phantom{\rule{0.3em}{0ex}}\mathrm{kG}$, relaxation is observed, occurring over the barrier of the thermally accessible $S=4$ first excited state that forms a combined system with the $S=5$ state.

A high resolution inelastic neutron scattering (INS) study of fully deuterated Mn$_{12}$-acetate provides the most accurate spin Hamiltonian parameters for this prototype single molecule magnet so far. The Mn$_{12}$-clusters deviate from axial symmetry, a non-zero rhombic term in the model Hamiltonian leading to excellent agreement with observed positions and intensities of the INS peaks. The following parameter set provides the best agreement with the experimental data: $D=-0.0570(1)$ meV, $B_{4}^0=-2.78(7)\cdot 10^{-6}$ meV, $B_{4}^4=-3.2(6)\cdot 10^{-6}$ meV and $\mid$\textit{E}$\mid =6.8(15)\cdot 10^{-4}$ meV. Crystal dislocations are not the likely cause of the symmetry lowering. Rather, this study lends strong support to a recently proposed model, which is based on the presence of several molecular isomers with distinct spin Hamiltonian parameters., 4 pages, 4 figures

The anisotropy splitting in the ground state of the single-molecule magnet ${\mathrm{Mn}}_{4}{\mathrm{O}}_{3}\mathrm{Br}{(\mathrm{O}\mathrm{Ac})}_{3}{(\mathrm{dbm})}_{3}$ is studied by inelastic neutron scattering as a function of hydrostatic pressure. This allows a tuning of the anisotropy and thus the energy barrier for slow magnetization relaxation at low temperatures. The value of the negative axial anisotropy parameter ${D}_{\text{cluster}}$ changes from $\ensuremath{-}0.0627(1)\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ at ambient to $\ensuremath{-}0.0603(3)\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ at $12\phantom{\rule{0.3em}{0ex}}\mathrm{kbar}$ pressure, and in the same pressure range the height of the energy barrier between up and down spins is reduced from $1.260(5)\phantom{\rule{0.3em}{0ex}}\mathrm{meV}\phantom{\rule{0.5em}{0ex}}\text{to}\phantom{\rule{0.5em}{0ex}}1.213(9)\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$. Since the $\mathrm{Mn}\mathrm{Br}$ bond is significantly softer and thus more compressible than the $\mathrm{Mn}\mathrm{O}$ bonds, pressure induces a tilt of the single ion ${\mathrm{Mn}}^{3+}$ anisotropy axes, resulting in the net reduction of the axial cluster anisotropy.

We perform inelastic neutron scattering measurements on the molecular nanomagnet ${\mathrm{Mn}}_{12}$-acetate to measure the excitation spectrum up to $45\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ $(500\phantom{\rule{0.3em}{0ex}}\mathrm{K})$. We isolate magnetic excitations in two groups at $5--6.5\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ $(60--75\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ and $8--10.5\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ $(95--120\phantom{\rule{0.3em}{0ex}}\mathrm{K})$, with higher levels appearing only at $27\phantom{\rule{0.3em}{0ex}}\mathrm{meV}\phantom{\rule{0.2em}{0ex}}(310\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ and $31\phantom{\rule{0.3em}{0ex}}\mathrm{meV}\phantom{\rule{0.2em}{0ex}}(360\phantom{\rule{0.3em}{0ex}}\mathrm{K})$. From a detailed characterization of the transition peaks we show that all of the low-energy modes appear to be separate $S=9$ excitations above the $S=10$ ground state, with the peak at $27\phantom{\rule{0.3em}{0ex}}\mathrm{meV}\phantom{\rule{0.2em}{0ex}}(310\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ corresponding to the first $S=11$ excitation. We consider a general model for the four exchange interaction parameters of the molecule. The static susceptibility is computed by high-temperature series expansion and the energy spectrum, matrix elements, and ground-state spin configuration by exact diagonalization. The theoretical results are matched with experimental observation by inclusion of cluster anisotropy parameters, revealing strong constraints on possible parameter sets. We conclude that only a model with dominant exchange couplings ${J}_{1}\ensuremath{\sim}{J}_{2}\ensuremath{\sim}5.5\phantom{\rule{0.3em}{0ex}}\mathrm{meV}\phantom{\rule{0.2em}{0ex}}(65\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ and small couplings ${J}_{3}\ensuremath{\sim}{J}_{4}\ensuremath{\sim}0.6\phantom{\rule{0.3em}{0ex}}\mathrm{meV}\phantom{\rule{0.3em}{0ex}}(7\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ is consistent with the experimental data.

The tetramer-based compound α-MnMoO 4 exhibits four prominent peaks in the inelastic neutron scattering (INS) spectrum between 0.5 and 2.0 meV below 10 K. They are assigned to magnetic excitations of the (Mn 2 + ) 4 rhombus shaped cluster, with resulting values of the exchange parameters J= +0.051 meV and J' = -0.019 meV along the edges and the short diagonal, respectively. The interactions within the tetramer are treated exactly in an isotropic quantum mechanical model leading to an S = 10 cluster ground state. The weaker antiferromagnetic (AFM) intercluster interactions, J i n t = -4.5×10 3 meV, are treated in a molecular-field model below the AFM transition temperature T N = 10.7 K. INS and susceptibility are in quantitative agreement with this approach.

(Et4N)3Fe2F9 exhibits a butterfly--shaped hysteresis below 5 K when the magnetic field is parallel to the threefold axis, in accordance with a very slow magnetization relaxation in the timescale of minutes. This is attributed to an energy barrier Delta=2.40 K resulting from the S=5 dimer ground state of [Fe2F9]^{3-} and a negative axial anisotropy. The relaxation partly occurs via thermally assisted quantum tunneling. These features of a single-molecule magnet are observable at temperatures comparable to the barrier height, due to an extremely inefficient energy exchange between the spin system and the phonons. The butterfly shape of the hysteresis arises from a phonon avalanche effect., 18 pages, 5 eps figures, latex (elsart)

We have used inelastic neutron scattering to measure the spin wave dispersion throughout the Brillouin zone of the bilayer manganite La 1.2 Sr 1.8 Mn 2 O 7 . Due to the bilayer structure, the spin-wave spectrum consist of two modes, acoustic and optical. In the ferromagnetic phase ( T ≪ T C =121 K), the dispersion relations are well described by a two-dimensional bilayer Heisenberg Hamiltonian with nearest neighbour interactions only. The effective magnetic exchange between adjacent Mn ions in the plane, J ‖ S =10.3±0.2 meV, is approximately 4 times larger than that between the planes of the bilayer J ⊥ S =2.5±0.25 meV. This large anisotropy between ferromagnetic exchanges within and across the bilayer might be associated with a strong planar orbital polarisation of the e g band electrons which would favour a larger double-exchange mechanism within the plane.

CHCupCl is a two-leg spin-ladder where the Cu++ ions (S= 1 2 ) are coupled by superexchange. Its magnetic properties change dramatically as a function of a magnetic field. Below the critical field Hc1, there is a singlet ground state separated from the triplet excited state by an energy gap. Between Hc1 and Hc2, several magnetic phases have been observed. Above 1 K, dynamical studies reveal a disordered quantum critical phase. At lower temperatures, a 3-D ordered phase has been observed in specific heat and NMR studies. New NMR results on the 3D-ordered and quantum critical phases are presented in this short report.

We report an inelastic neutron scattering (INS) study under a magnetic field on the frustrated molecular spin cluster $V_{15}$. Several field-dependent transitions are observed and provide a comprehensive understanding of the low-energy quantum spin states. The energy gap $2 \Delta_{0}\approx 27(3)\mu$eV between the two lowest $S=1/2$ Kramers doublets is unambiguously attributed to a symmetry lowering of the cluster. The INS data are mapped onto an S=1/2 Antiferromagnetic Heisenberg triangle with scalene distortion. A quantitative description of the wavefunction mixing within the ground state is derived., Comment: To be published in Europhysics Letters 7 pages, 3 figures