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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 tetragonal compound Mn$_{1.4}$PtSn with the $D_{2d}$ symmetry recently attracted attention as the first known material that hosts magnetic antiskyrmions, which differ from the so far known skyrmions by their internal structure. The latter have been found in a number of magnets with the chiral crystal structure. In previous works, the existence of antiskyrmions in Mn$_{1.4}$PtSn was unambiguously demonstrated in real space by means of Lorentz transmission electron microscopy on thin-plate samples ($\sim$100~nm thick). In the present study, we used small-angle neutron scattering and magnetic force microscopy to perform reciprocal- and real-space imaging of the magnetic texture of bulk Mn$_{1.4}$PtSn single-crystals at different temperatures and in applied magnetic field. We found that the magnetic texture in the bulk differs significantly from that of thin-plate samples. Instead of spin helices or an antiskyrmion lattice, we observe an anisotropic fractal magnetic pattern of closure domains in zero field above the spin-reorientation transition temperature, which transforms into a set of bubble domains in high field. Below the spin-reorientation transition temperature the strong in-plane anisotropy as well as the fractal self-affinity in zero field is gradually lost, while the formation of bubble domains in high field remains robust. The results of our study highlight the importance of dipole-dipole interactions in thin-plate samples for the stabilization of antiskyrmions and identify criteria which should guide the search for potential (anti)skyrmion host materials. Moreover, they provide consistent interpretations of the previously reported magnetotransport anomalies of the bulk crystals.

International audience; The compound La2−2xSr1+2xMn2O7, x = 0.30–0.40, consists of bilayers of ferromagnetic metallic MnO2 sheets that are separated by insulating layers. The materials show colossal magnetoresistance—a reduction in resistivity of up to two orders of magnitude in a field of 7 T—at their three-dimensional ordering temperatures, TC = 90–126 K, and are the layered analogues of the widely studied pseudo-cubic perovskite manganites, R1−xAxMnO3 (R = rare earth, A = Ca, Sr, Ba, Pb). Two distinct short-range orderings—antiferromagnetic fluctuations and correlated polarons, which are related to the magnetic and the lattice degrees of freedom respectively—have previously been discovered in La2−2xSr1+2xMn2O7, x = 0.40, and have each been qualitatively connected to the resistivity. Here, in a comprehensive study as a function of both temperature and magnetic field for the different hole-concentrations per Mn site of x = 0.30 and 0.35, we show that antiferromagnetic fluctuations also appear at temperatures just above TC, and that the intensities of both the antiferromagnetic fluctuations and polaron correlations closely track the resistivity. In particular, for x = 0.35 we show that there is a simple scaling relation between the intensities of the antiferromagnetic fluctuations and the in-plane resistivity that applies for the temperatures and magnetic fields used in the experiments. The results show that antiferromagnetic fluctuations are a common feature of La2−2xSr1+2xMn2O7 with ferromagnetic bilayers, and that there is a close connection between the antiferromagnetic fluctuations and polarons in these materials.

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 study the evolution of helical magnetism in MnGe chiral magnet upon partial substitution of Mn for $3d$-Co and $4d$-Rh ions. At high doping levels, we observe spin helices with very long periods---more than ten times larger than in the pure compound---and sizable ordered moments. This behavior calls for a change in the energy balance of interactions leading to the stabilization of the observed magnetic structures. Strikingly, neutron scattering unambiguously shows a double periodicity in the observed spectra at $x=0.5$ and g0.2 for Co- and Rh-doping, respectively. In analogy with observations made in smectic liquid crystals, we suggest that it may reveal the presence of magnetic ``twist grain boundary'' phases, involving a dense short-range correlated network of magnetic screw dislocations. The dislocation cores are here tentatively described as smooth textures, made of nonradial double-core skyrmions.

Prussian blue analogues (PBA) ferromagnetic nanoparticles Cs I x Ni II [Cr III (CN)6 ] z ·3(H2O) embedded in CTA+ (cetyltrimethylammonium) matrix have been investigated by magnetometry and magnetic small-angle neutron scattering (SANS). Choosing particle sizes (diameter D = 4.8 and 8.6 nm) well below the single-domain radius and comparable volume fraction of particle, we show that the expected superparamagnetic regime for weakly anisotropic isolated magnetic particles is drastically affected due to the interplay of surface/volume anisotropies and dipolar interactions. For the smallest particles (D = 4.8 nm), magnetocrystalline anisotropy is enhanced by surface spins and drives the system into a regime of ferromagnetically correlated clusters characterized by a temperature-dependent magnetic correlation length L mag which is experimentally accessible using magnetic SANS. For D = 8.6 nm particles, a superparamagnetic regime is recovered in a wide temperature range. We propose a model of interacting single-domain particles with axial anisotropy that accounts quantitatively for the observed behaviors in both magnetic regimes.

We present in this paper the structural and magnetic properties of high aspect ratio Co nanoparticles (~10) at high temperatures (up to 623 K) using in situ X ray diffraction (XRD) and SQUID characterizations. We show that the anisotropic shapes, the structural and texture properties are preserved up to 500 K. The coercivity can be modelled by u0Hc=2(Kmc+Kshape)/Ms with Kmc the magnetocrystalline anisotropy constant, Kshape the shape anisotropy constant and Ms the saturation magnetization. Hc decreases linearly when the temperature is increased due to the loss of the Co magnetocrystalline anisotropy contribution. At 500K, 50% of the room temperature coercivity is preserved corresponding to the shape anisotropy contribution only. We show that the coercivity drop is reversible in the range 300 - 500 K in good agreement with the absence of particle alteration. Above 525 K, the magnetic properties are irreversibly altered either by sintering or by oxidation., 8 pages, 7 figures, submitted to Journal of Solid State Chemistry

International audience; no abstract

International audience; We have determined by polarized neutron diffraction (PND) the low-temperature molecular magnetic susceptibility tensor of the anisotropic low-spin complex PPh4 [Fe(III) (Tp)(CN)3 ]⋅H2 O. We found the existence of a pronounced molecular easy magnetization axis, almost parallel to the C3 pseudo-axis of the molecule, which also corresponds to a trigonal elongation direction of the octahedral coordination sphere of the Fe(III) ion. The PND results are coherent with electron paramagnetic resonance (EPR) spectroscopy, magnetometry, and ab initio investigations. Through this particular example, we demonstrate the capabilities of PND to provide a unique, direct, and straightforward picture of the magnetic anisotropy and susceptibility tensors, offering a clear-cut way to establish magneto-structural correlations in paramagnetic molecular complexes

High-quality monodisperse metallic cobalt nanorods are obtained by the reduction of carboxylate salts of CoII in 1,2-butanediol using a rapid, simple, and solid-template-free procedure. In this polyol process, particle shape can be controlled via the growth rate, which depends on three parameters: i) the nature of the cobalt carboxylate, ii) the temperature ramp, and iii) the basicity of the medium. Cobalt in the hexagonally close-packed phase favored the growth of anisotropic particles. Magnetic measurements of the cobalt nanorods indicate they are ferromagnetic at room temperature. They have a very high coercivity of 9.0 kOe at 140 K, much higher than that observed for wires prepared with solid templates. This can be attributed to their small mean diameter and high crystallinity.

Behaviour of magnetic excitations in the Damon–Eshbach (DE) and backward volume (BV) geometries in nano-composite (SiO2)100−xCox (50% at

Cobalt nanorods and wires were prepared by reduction of a cobalt salt in a liquid polyol. These particles crystallize with the hcp structure and the growth axis is parallel to the crystallographic c-axis. The kinetic control of the growth allows to vary the mean diameter of the rods and their aspect ratio. Dumbbell like shape particles consisting of a central rod with two conical tips were also obtained. Magnetization curves of oriented wires present very high coercivity (up to 9 kOe) resulting from both a high shape anisotropy and the high magnetocrystalline anisotropy of the hcp cobalt. Micromagnetic simulations showed that the magnetization reversal is shape dependent. The conical tips of the dumbbell particles strongly contribute to the coercivity decrease and must be precluded for permanent magnet applications.

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 report on the magnetic properties of the enneanuclear mixed-valent single-molecule magnet [Mn9O7(OAc)11(thme)(py)3(H2O)2] (1). Using inelastic neutron scattering (INS) spectroscopy, we have confirmed the S = 17/2 cluster ground state of 1. Furthermore, we have determined accurately the anisotropy parameters of the zero field split ground state. The cluster 1 exhibits a strong negative axial anisotropy D [ S ˆ z 2 - S ( S + 1 ) / 3 ] where D = −0.249(5) cm−1. It is necessary to include an axial higher order term ( B 4 0 ) in the effective spin Hamiltonian to account for the experimental data.

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.

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.

International audience; The implementation of nano-objects in numerous emerging applications often demands their integration in macroscopic devices. Here we present the bottom-up epitaxial solution growth of high-density arrays of vertical 5 nm diameter single-crystalline metallic cobalt nanowires on wafer-scale crystalline metal surfaces. The nanowires form regular hexagonal arrays on unpatterned metallic films. These hybrid heterostructures present an important perpendicular magnetic anisotropy and pave the way to a high density magnetic recording device, with capacities above 10 Terabits/in2. This method bypasses the need of assembling and orientating free colloidal nanocrystals on surfaces. Its generalization to other materials opens new perspectives toward many applications.

Polarized small-angle neutron scattering (PSANS) experimental results obtained on arrays of ferromagnetic Co nanowires ($\phi\approx13$ nm) embedded in self-organized alumina (Al$_{2}$O$_{3}$) porous matrices are reported. The triangular array of aligned nanowires is investigated as a function of the external magnetic field with a view to determine experimentally the real space magnetization $\vec{M}(\vec{r})$ distribution inside the material during the magnetic hysteresis cycle. The observation of field-dependentSANSintensities allows us to characterize the influence of magnetostatic fields. The PSANS experimental data are compared to magnetostatic simulations. These results evidence that PSANS is a technique able to address real-space magnetization distributions in nanostructured magnetic systems. We show that beyond structural information (shape of the objects, two-dimensional organization) already accessible with nonpolarized SANS, using polarized neutrons as the incident beam provides information on the magnetic form factor and stray fields \textgreek{m}0Hd distribution in between nanowires., Comment: 13 pages, 10 figures, submitted to Phys. Rev. B

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.

International audience; Polarized SANS results obtained on ferromagnetic Co nanowires embedded in alumina (Al2O3) porous matrices are reported. The triangular arrays of aligned nanowires are investigated as a function of the external magnetic field in order to determine experimentally the real space magnetization M' ('r) distribution inside the material during the magnetic hys- teresis cycle. The observation of field-dependent SANS intensities allows us to characterize the influence of stray fields. Micromagnetic calculations are compared to the SANSPOL experimental data. These results show that SANSPOL is a technique able to address real-space magnetization distributions in magnetic nanosystems.

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.

The molecular spin cluster V15 is a polyoxovanadate with 15 spins s=1/2 but total spin ground state S=1/2 due to strong antiferromagnetic (AFM) interactions. Below 1 K, the relaxation of the magnetisation exhibits a very peculiar behaviour in connection of the multi-spin character of the cluster and the ground state is composed of two non-degenerate S=1/2 Kramers doublets separated by a gap of about 0.3 K. The mechanism leading to such gap formation in the ground state has been elucidated using inelastic neutrons scattering (INS) under magnetic field. The observation and characterisation of several field-dependent INS transitions enable a comprehensive understanding of the low-energy quantum spin states. The energy gap between the two Kramers doublets is caused by slight deviations from the trigonal symmetry (symmetry lowering) of the cluster. The INS data are successfully mapped onto an S=1/2 AFM Heisenberg triangle with scalene distortion and a quantitative description of the wavefunction mixing (entanglement) within the ground state is derived. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

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 investigate the role of dipolar interactions on the magnetic properties of nanowires aggregates. Micromagnetic simulations show that dipolar interactions between wires are not detrimental to the high coercivity properties of magnetic nanowires composites even in very dense aggregates. This is confirmed by experimental magnetization measurements and Henkel plots which show that the dipolar interactions are small. Indeed, we show that misalignment of the nanowires in aggregates leads to a coercivity reduction of only 30%. Direct dipolar interactions between nanowires, even as close as 2 nm, have small effects (maximum coercivity reduction of ~15%) and are very sensitive to the detailed geometrical arrangement of wires. These results strenghten the potential of magnetic composite materials based on elongated single domain particles for the fabrication of permanent magnetic materials., Comment: 7 pages, 8 figures, submitted to Journal of Applied Physics

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.

We explore the behavior of periodic arrays of magnetic nanowires by micromagnetic simulations using the Nmag modeling package. A large number of modeling studies on such arrays of nanowires have been performed using finite size models. We show that these finite size micromagnetic descriptions can only be used in specific situations. We perform a systematic study of more or less dense 1D and 2D arrays of nanowires using either finite size or infinite size models and we show that finite size models fail to capture some of the features of real infinite systems. We show that that the mean field model scaled to the system porosity is valid. This work can be used as a basis to the extension of micromagnetic calculations of the magnetization dynamics in arrays of nanowires., Comment: 8 pages, 11 figures, submitted to Journal of Applied Physics

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.

The magnetic properties of Co (=15 nm, =130nm) nanowires are reported. In oxidized wires, we measure large exchange bias fields of the order of 0.1 T below T ~ 100 K. The onset of the exchange bias, between the ferromagnetic core and the anti-ferromagnetic CoO shell, is accompanied by a coercivity drop of 0.2 T which leads to a minimum in coercivity at $\sim100$ K. Magnetization relaxation measurements show a temperature dependence of the magnetic viscosity S which is consistent with a volume distribution of the CoO grains at the surface. We propose that the superparamagnetic fluctuations of the anti-ferromagnetic CoO shell play a key role in the flipping of the nanowire magnetization and explain the coercivity drop. This is supported by micromagnetic simulations. This behavior is specific to the geometry of a 1D system which possesses a large shape anisotropy and was not previously observed in 0D (spheres) or 2D (thin films) systems which have a high degree of symmetry and low coercivities. This study underlines the importance of the AFM super-paramagnetic fluctuations in the exchange bias mechanism., Comment: 10 pages, 10 figures, submitted to Phys. Rev. B

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.

The ground-state properties of a Co3II moiety encapsulated in a polyoxometalate anion were investigated by combining measurements of specific heat, magnetic susceptibility, and low-temperature magnetization with a detailed inelastic neutron scattering (INS) study on a fully deuterated polycrystalline sample of Na12[Co3W(D2O)2(ZnW9O34)2].40D2O (Co3). The ferromagnetic Co3O14 cluster core consists of three octahedrally oxo-coordinated CoII ions. According to the single-ion anisotropy and spin-orbit coupling of the octahedral CoII ions, the appropriate exchange Hamiltonian to describe the ground-state properties of the Co3 spin cluster is anisotropic and is expressed as H = -2 sigma a = x,y,z (Ja12 S1a S2a + Ja23 S2a S3a), where Ja are the components of the exchange interactions between the CoII ions. To reproduce the INS data, different orientations of the two anisotropic J tensors must be considered, and the following conditions had to be introduced: Jx12 = Jy23, Jy12 = Jx23, Jz12 = Jz23. This result was correlated with the molecular symmetry of the complex. The following set of parameters was obtained: Jx12 = Jy23 = 1.37, Jy12 = Jx23 = 0.218, and Jz12 = Jz23 = 1.24 meV. This set also reproduces in a satisfactory manner the specific heat, susceptibility, and magnetization properties of Co3.

(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.

Magnetic nanowires could be the building bricks in the fabrication of composite magnetic materials because of their large intrinsic shape anisotropy. We investigate the relation between the detailed shape of a magnetic nanowire and its magnetic coercivity. We have performed three-dimensional micromagnetic simulations on various types of nanowires synthesized during the past few years such as cylinders, dumbbells, or diabolos. The calculations, performed on individual model objects, show that the wire tip plays a key role in the reversal mechanism and on the magnitude of the coercive field and that the aspect ratio plays a much lesser role. Ellipsoidal or cylindrical shapes favor a coherent rotation of the magnetization and thus large coercive fields. Complex tip shapes act as nucleation points and significantly reduce the coercive field. Thus, in order to optimize the shape of magnetic nanowires for permanent magnet applications, the focus should be put on the detailed shape of the wire tips and thus on the growth mechanism rather than on the aspect ratio. The numerical modeling results on individual wires are compared with the experimental data obtained on various types of wires synthesized by soft chemistry methods.

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