Bulk Susceptibility Measurements Research Articles

Determining Local Magnetic Susceptibility Tensors in Paramagnetic Lanthanide Crystalline Powders from Solid-State NMR Chemical Shift Anisotropies

Exploring magnetic properties at the molecular level is a challenge that has been met by developing many experimental and theoretical solutions, such as polarized neutron diffraction (PND), muon-spin rotation (μ-SR), electron paramagnetic resonance (EPR), SQUID-based magnetometry measurements, and advanced modeling on open-shell systems and relativistic calculations. These methods are powerful tools that shed light on the local magnetic response in specifically designed magnetic materials such as contrast agents, for MRI, molecular magnets, magnetic tags for biological NMR, etc. All of these methods have their advantages and disadvantages. In order to complement the possibilities offered by these methods, we propose a new tool that implements a new approach combining simulation and fitting for high-resolution solid-state NMR spectra of lanthanide-based paramagnetic species. This method relies on a rigorous acquisition thanks to short high-power adiabatic pulses (SHAP) of high-resolution solid-state NMR isotropic and anisotropic data on a powdered magnetic material. It is also based on an efficient modeling of this data thanks to a semiempirical model based on a parametrization of the local magnetism and the crystal structure provided by diffraction methods. The efficiency of the calculation relies on a thorough simplification of the electron-nucleus interactions (point-dipole interaction, no Fermi contact) which is validated by experimental analysis. By taking advantage of the efficient calculation possibilities offered by our method, we can compare a great number of simulated spectra to experimental data and find the best-matching local magnetic susceptibility tensor. This method was applied to a series of isostructural lanthanide oxalates which are used as a benchmark system for many analytical methods. We present the results of thorough solid-state NMR and extensive modeling of the hyperfine interaction (including up to 400 paramagnetic centers) that yield local magnetic susceptibility tensor measurements that are self-consistent as well as consistent with bulk susceptibility measurements.

Magnetic ordering of the distorted kagome antiferromagnet Y3Cu9(OH)18[Cl8 (OH)] prepared via optimal synthesis

Experimental studies of high-purity kagome-lattice antiferromagnets (KAFM) are of great importance in attempting to better understand the predicted enigmatic quantum spin-liquid ground state of the KAFM model. However, realizations of this model can rarely evade magnetic ordering at low temperatures due to various perturbations to its dominant isotropic exchange interactions. Such a situation is for example encountered due to sizable Dzyaloshinskii-Moriya magnetic anisotropy in YCu$_3$(OH)$_6$Cl$_3$, which stands out from other KAFM materials by its perfect crystal structure. We find evidence of magnetic ordering also in the distorted sibling compound Y$_3$Cu$_9$(OH)$_{18}$[Cl$_8$(OH)], which has recently been proposed to feature a spin-liquid ground state arising from a spatially anisotropic kagome lattice. Our findings are based on a combination of bulk susceptibility, specific heat, and magnetic torque measurements that disclose a N\'eel transition temperature of $T_N=11$~K in this material, which might feature a coexistence of magnetic order and persistent spin dynamics as previously found in YCu$_3$(OH)$_6$Cl$_3$. Contrary to previous studies of single crystals and powders containing impurity inclusions, we use high-purity single crystals of Y$_3$Cu$_9$(OH)$_{18}$[Cl$_8$(OH)] grown via an optimized hydrothermal synthesis route that minimizes such inclusions. This study thus demonstrates that the lack of magnetic ordering in less pure samples of the investigated compound does not originate from the reduced symmetry of spin lattice but is instead of extrinsic origin.

Quantum Oscillations in Flux-Grown SmB6 with Embedded Aluminum

SmB_{6} is a candidate topological Kondo insulator that displays surface conduction at low temperatures. Here, we perform torque magnetization measurements as a means to detect de Haas-van Alphen (dHvA) oscillations in SmB_{6} crystals grown by aluminum flux. We find that dHvA oscillations occur in single crystals containing embedded aluminum, originating from the flux used to synthesize SmB_{6}. Measurements on a sample with multiple, unconnected aluminum inclusions show that aluminum crystallizes in a preferred orientation within the SmB_{6} cubic lattice. The presence of aluminum is confirmed through bulk susceptibility measurements, but does not show a signature in transport measurements. We discuss the ramifications of our results.

Crystal field levels and magnetic anisotropy in the kagome compounds Nd3Sb3Mg2O14 , Nd3Sb3Zn2O14 , and Pr3Sb3Mg2O14

We report the crystal field levels of several newly-discovered rare-earth kagome compounds: $\rm{Nd_3Sb_3Mg_2O_{14}}$, $\rm{Nd_3Sb_3Zn_2O_{14}}$, and $\rm{Pr_3Sb_3Mg_2O_{14}}$. We determine the CEF Hamiltonian by fitting to neutron scattering data using a point-charge Hamiltonian as an intermediate fitting step. The fitted Hamiltonians accurately reproduce bulk susceptibility measurements, and the results indicate easy-axis ground state doublets for $\rm{Nd_3Sb_3Mg_2O_{14}}$ and $\rm{Nd_3Sb_3Zn_2O_{14}}$, and a singlet ground state for $\rm{Pr_3Sb_3Mg_2O_{14}}$. These results provide the groundwork for future investigations of these compounds and a template for CEF analysis of other low-symmetry materials.

Orphan Spins in the S=5/2 Antiferromagnet CaFe_{2}O_{4}.

CaFe_{2}O_{4} is an anisotropic S=5/2 antiferromagnet with two competing A (↑↑↓↓) and B (↑↓↑↓) magnetic order parameters separated by static antiphase boundaries at low temperatures. Neutron diffraction and bulk susceptibility measurements, show that the spins near these boundaries are weakly correlated and a carry an uncompensated ferromagnetic moment that can be tuned with a magnetic field. Spectroscopic measurements find these spins are bound with excitation energies less than the bulk magnetic spin waves and resemble the spectra from isolated spin clusters. Localized bound orphaned spins separate the two competing magnetic order parameters in CaFe_{2}O_{4}.

Observation of enhanced magnetic transition in Pbnm SmFeO3

Rare-earth orthoferrite RFeO3 materials have recently attracted great attention for their intriguing technological potential. Among these materials, SmFeO3 holds great promise, not only for its excellent physical properties (fast magnetic switching, spin reorientation, and magnetization reversal), but also for its potential ferroelectric properties, which have been recently under debate. Here, we focus our attention on the T-dependent Zero Field Cooled and Field Cooled magnetization properties of micrometer scale crystals of SmFeO3 obtained by annealing methods. We report the observation of an enhanced magnetic transition at a temperature of approximately 139 K. From literature bulk susceptibility measurements, it has been suggested that below the temperature of about 140 K, Sm3+ moments begin to order antiparallel to the Fe-moments due to antiferromagnetic f-d exchange interactions. We attribute the observed transition to compensation effects induced by the appearance of long range ordering in Sm3+ spins. The magnetic-nature of the observed transition is confirmed by additional temperature dependent XRD analysis, which did not show structural changes in the samples in the same temperature range (from 298 K to 100 K). Due to residual small fractions of ferromagnetic α-Fe from the sample-growth, possible interactions between the magnetic moment of α-Fe and the SmFeO3 crystals at the compensation temperature cannot be excluded and could be at the origin of the enhanced magnetic signal reported in this work.

Holocene cirque glacier activity in Rondane, southern Norway

Skriufonnen is a small cirque glacier (0.03km2) in the continental mountains of Rondane in southern Norway. At present, it is the only glacier in Rondane, and very little is known about Holocene glacier fluctuations in this region. Direct observations of the glacier began in 2002, since which time Skriufonnen has been in a state of strong decline. In order to provide a temporal context, past glacier fluctuations were reconstructed based on a series of short HTH gravity cores (n=8) and long piston cores (n=6) retrieved from three downstream lakes of Skriufonnen. The cores were analysed for selected magnetic properties (χbulk, ARM, SIRM, 77K/293K), organic content (LOI), and geochemical trace elements. Soil catchment samples (n=6) were collected along a transect running from the three lakes up to the present glacier terminus. Bulk susceptibility (χbulk) measurements show that the finest fractions systematically return the highest values and that ferromagnetic minerals are depleted with distance to the glacier front. This means that periods dominated by paramagnetic minerals indicate very little or no glacier activity, whereas intervals with more ferromagnetic minerals suggest increased glacier activity. The quantitative core analyses indicate that Skriufonnen existed prior to 10,200b2k (years before A.D. 2000) and disappeared ~10,000b2k. No glacier activity is recorded from c. 10,000b2k until the glacier reoccurred at the onset of the local Neoglacial period, c. 4000b2k. The glacier attained its maximum extent between 3200 and 2400b2k and during the end of the ‘Little Ice Age’ (LIA) c. A.D. 1800. Neoglacial fluctuations of Skriufonnen are in line with shifts in local summer temperatures and show a delayed Neoglacial inception compared to western Norway.

X-ray Magnetic Circular Dichroism Investigation of the Electron Transfer Phenomena Responsible for Magnetic Switching in a Cyanide-Bridged [CoFe] Chain

The cyanide-bridged [CoFe] one-dimensional chain, [Co(II)((R)-pabn)][Fe(III)(Tp)(CN)3](BF4)·MeOH·2H2O, where (R)-pabn = (R)-N2,N(2')-bis(pyridin-2-ylmethyl)-1,1'-binaphthyl-2,2'-diamine and Tp = hydrotris(pyrazolyl)borate, exhibits magnetic and electric bistabilities originating from an electron transfer coupled spin transition between Fe-CN-Co pairs. The use of L-edge X-ray absorption spectroscopy (XAS) in combination with L-edge X-ray magnetic circular dichroism (XMCD) is explored for the investigation of the electronic structure and magnetization of Co and Fe ions separately, in both diamagnetic and paramagnetic states. It has been established from susceptibility results that the switching between diamagnetic and paramagnetic phases emanates from electron transfer between low spin Fe(II) and Co(III), resulting in low spin Fe(III) (S = 1/2) and high spin Co(II) (S = 3/2). The XAS and XMCD results are consistent with the bulk susceptibility measurements, where greater detail regarding the charge transfer process is determined. The Fe-CN-Co electron transfer pathway is highlighted by a strongly XMCD dependent transition to a cyanide back bonding orbital, giving evidence for strong hybridization with Fe(III) t2g orbitals. In addition to thermally induced and photoinduced switching, [CoFe] is found to exhibit a switching by grinding induced dehydration. Analysis of XAS shows that on grinding diamagnetic [CoFe], 75% of metal ions lock into the magnetic Co(II)Fe(III) phase. Density functional theory calculations based on the [CoFe] crystal structure in the magnetic and nonmagnetic phases aid the spectroscopic results and provide a complementary insight into the electronic configuration of the [CoFe] 3d shells, quantifying the change in ligand field around Co and Fe centers on charge transfer.

Solid state NMR strategies for the structural characterization of new hybrid materials based on the intercalation of nitroxide radicals into CdPS 3

The radical cations 2-(3- N-butylpyridinium)-4,4,5,5-tetramethyl-4,5-dihydro-1 H-imidazol-1-oxyl-3- N-oxide ( m-BuPYNN) and 4-(ethylammonium)-2,2,6,6-tetramethylpiperidin-1-oxide (EATEP) are successfully intercalated into the layered host structure of CdPS 3 via ion exchange. The reaction proceeds either directly from ethanolic solutions of the radical iodide salt or via a two-stage process involving first the formation of an intermediate tetramethylammonium intercalate. The resulting materials, which are described by the compositional formula Cd 1− x PS 3{Rad} 2 x , are characterized by chemical analysis, X-ray powder diffraction, bulk susceptibility measurements and EPR spectroscopy. Modern single and double resonance solid state NMR techniques are introduced successfully to study the structural modifications of the host lattice and the details of the intermolecular guest/host interactions. 1H MAS-NMR spectra reveal substantial differences in the unpaired electron spin density distributions within the radical ions intercalated into the host lattice compared to those obtained for the pure radical ion salts, leading to different bulk magnetic properties. The results of 1H/ 31P double resonance experiments indicate that the orientation of the guest molecules is dominated by Columbic interactions between the radical cations and the negatively charged cadmium vacancies in the host lattice.

High-resolution neutron scattering study ofTb2Mo2O7: A geometrically frustrated spin glass

The low-temperature magnetic properties of ${\text{Tb}}_{2}{\text{Mo}}_{2}{\text{O}}_{7}$ have been studied with bulk susceptibility measurements and with elastic and high-resolution inelastic neutron scattering. This system is a spin glass with a freezing temperature ${T}_{g}\ensuremath{\sim}25\text{ }\text{K}$. A reverse Monte Carlo simulation of the neutron diffraction data shows weak ferromagnetic near-neighbor spatial correlations that do not extend beyond $\ensuremath{\lesssim}10\text{ }\text{\AA{}}$. Neutron measurements of the spin dynamics reveal a slowing down with decreasing temperature without an anomaly at the glass transition. A low-lying $Q$-independent mode is seen at $\ensuremath{\hbar}{\ensuremath{\omega}}_{0}\ensuremath{\sim}0.28\text{ }\text{meV}$. This dispersionless crystal electric field transition is measurable up to 60 K.

Neutron powder diffraction study of the layer organic–inorganic hybrid iron(II) methylphosphonate-hydrate, Fe[(CD 3PO 3)(D 2O)

The crystal and magnetic structures of the hybrid organic–inorganic layer compound Fe[(CD 3PO 3)(D 2O)] have been studied by neutron powder diffraction as a function of temperature down to 1.5 K. The neutron diffraction pattern recorded at 200 K shows that the fully deuterated compound crystallizes in one of the two known forms of the undeuterated Fe[(CH 3PO 3)(H 2O)]. The crystal structure is orthorhombic, space group Pmn2 1, with the following unit-cell parameters: a=5.7095(1) Å, b=8.8053(3) Å and c=4.7987(1) Å; Z=2. The crystal structure remains unchanged on cooling from 200 to 1.5 K. Moreover, at low temperature, Fe[(CD 3PO 3)(D 2O)] shows a commensurate magnetic structure ( k=(0,0,0)). As revealed by bulk susceptibility measurements on Fe[(CH 3PO 3)(H 2O)], the magnetic structure corresponds to a canted antiferromagnet with a critical temperature T N=25 K. Neutron powder diffraction reveals that below T N=23.5 K the iron magnetic moments in Fe[(CD 3PO 3)(D 2O)] are antiferromagnetically coupled and oriented along the b-axis, perpendicular to the inorganic layers. No ferromagnetic component is observable in the neutron powder diffraction experiment, due to its too small value (<0.1 μ B).

Electronic State ofPuCoGa5andNpCoGa5as Probed by Polarized Neutrons

By using single crystals and polarized neutrons, we have measured the orbital and spin components of the microscopic magnetization in the paramagnetic state of NpCoGa(5) and PuCoGa(5). The microscopic magnetization of NpCoGa(5) agrees with that observed in bulk susceptibility measurements and the magnetic moment has spin and orbital contributions as expected for intermediate coupling. In contrast, for PuCoGa(5), which is a superconductor with a high transition temperature, the microscopic magnetization in the paramagnetic state is small, temperature-independent, and significantly below the value found with bulk techniques at low temperatures. The orbital moment dominates the magnetization.

Influence of Ligand Geometry in Bimetallic Ytterbocene Complexes of Bridging Bis(bipyridyl) Ligands

Two new bimetallic complexes, [Cp*2Yb]2(μ-1,3-(2,2‘-bipyridyl)-5-tBu-C6H3) (1) and [Cp*2Yb]2(μ-1,4-(2,2‘-bipyridyl)-C6H4) (2), and their corresponding two-electron oxidation products [1]2+ and [2]2+ have been synthesized with the aim of determining the impact of the bridging ligand geometry on the electronic and magnetic properties of these materials. Electrochemistry, optical spectroscopy, and bulk susceptibility measurements all support a ground-state electronic configuration of the type [(f)13-(πa*)1-(πb*)1-(f)13]. Density functional theory calculations on the uncomplexed bridging ligands as doubly reduced species also indicate that the diradical electronic configuration is the lowest lying for both meta- and para-bis(bipyridyl) systems. The electrochemical and optical spectroscopic data indicate that the electronic coupling between the metal centers mediated by the diradical bridges is weak, as evidenced by the small separation of the metal-based redox couples and the similarity of the f−f transitions of the associated dicationic complexes ([1]2+ and [2]2+) relative to those of the monometallic [Cp*2Yb(bpy)]+ analogue. The magnetic susceptibility data show no evidence for exchange coupling between the paramagnetic metal centers in the neutral complexes, but do indicate weak exchange coupling between YbIII and ligand radical spins on each of the effectively independent halves of the bimetallic complexes. These findings are in contrast to those reported recently for CoIII/II dioxolene bimetallic complexes bridged by these same bis(bipyridyl) ligands. The difference is attributed in part to the dominant singlet diradical character of the bridging ligands in the ytterbocene complexes. These experimental and theoretical results are consistent with expectations for organic diradical spin orientations for meta versus para substituents across a phenylene linker, but this effect does not induce significant longer-range superexchange or electronic interactions between the metal centers in these systems.

Heavy fermions and quantum critical behavior in CeCu 6−xAu x ( x=0–0.2) and CeAl 3

Precision magnetoresistance (MR) Δ ρ / ρ ( H , T ) and Hall coefficient R H measurements have been carried out for archetypal Ce-based heavy fermion compounds—CeCu 6− x Au x ( x = 0 , 0.1 and 0.2) and CeAl 3 on high quality polycrystalline samples over a wide temperature range 1.8–300 K in magnetic fields up to 70 kOe. It was found the Brillouin–type component of MR Δ ρ / ρ ( H , T ) ∼ H 2 that can be consistently interpreted in the frameworks of a simple relation between resistivity and magnetization - Δ ρ / ρ ∼ M 2 obtained by K. Yosida (Phys. Rev. 107 (1957) 396). A local magnetic susceptibility χ loc ( T , H ) = ( 1 / H ( d ( Δ ρ / ρ ) / d H ) ) 1 / 2 deduced from the data obtained is compared with the data of bulk susceptibility measurements χ ( T , H ) . The study of anomalous Hall effect allows us to establish for the first time a complicated activation type behavior of R H and estimate the bound energies E a 1 , 2 of many body states at E F for CeAl 3 and CeCu 6 heavy fermion compounds. Microscopic parameters of charge carriers transport (effective masses, relaxation time) and localization radius a p 1 , 2 * of these heavy quasiparticles ( a p 1 * ( T > 5 0 K ) ∼ 1.5 A and a p 2 * ( T < 5 0 K ) ∼ 1 5 Å both for CeCu 6 and CeAl 3) were also estimated.

Magnetic properties of Ho0.1Sr0.9CoO3− perovskites (0.15⩽x⩽0.49)

Abstract The cobalt-based perovskite Ho 0.1 Sr 0.9 CoO 3− x ( x =0.15, 0.27, 0.31, 0.42, 0.49) has been studied by means of neutron diffraction as well as bulk susceptibility and magnetization measurements over the temperature range of 10– 300 K . It is found that the degree of oxygen deficiency not only strongly affects the magnetic ordering temperature, but also influences the magnetic and crystal structures.

Impurity-induced antiferromagnetic order in the Haldane-gap compoundPbNi2−xMgxV2O8(x=0.24)

An investigation of the effect of nonmagnetic Mg doping in the Haldane gap system ${\mathrm{PbNi}}_{2}{\mathrm{V}}_{2}{\mathrm{O}}_{8}$ is presented. Specifically, the magnetic properties of the quasi-one-dimensional compound ${\mathrm{PbNi}}_{2\ensuremath{-}x}{\mathrm{Mg}}_{x}{\mathrm{V}}_{2}{\mathrm{O}}_{8}$ $(x=0.24),$ located in the heavily doped region of the ${T}_{N},$ x phase diagram, have been examined using neutron powder diffraction, muon-spin relaxation $({\ensuremath{\mu}}^{+}\mathrm{SR})$ and magnetic susceptibility. Neutron-diffraction measurements show that antiferromagnetic long-range order (propagation vector $\mathbf{\ensuremath{\kappa}}=[000])$ is established below ${T}_{N}=3.2\mathrm{K}.$ The spins in the Ni sublattice lie either parallel or antiparallel to the c axis, which is the magnetic easy axis of the system. The spin configuration is antiferromagnetic both along the ${\mathrm{NiO}}_{6}$ chains, i.e., the c axis of the crystal structure, and within the ab-plane for the nearest-neighbor Ni atoms. The refined ordered moment at the ${\mathrm{Ni}}^{2+}$-site is $\ensuremath{\mu}=0.9(1){\ensuremath{\mu}}_{B}.$ To our knowledge, this is the first diffraction evidence of the antiferromagnetic ordering in this system. The static magnetic correlations of the doping-induced magnetic moments along the Ni chains were probed by zero-field ${\ensuremath{\mu}}^{+}\mathrm{SR}$ at $T&lt;{T}_{N}.$ The large number of detected spontaneous muon-precession frequencies $(0.5&lt;{\ensuremath{\nu}}_{\ensuremath{\mu}}&lt;3\mathrm{MHz})$ implies a nonuniform distribution in the magnitude of the ordered moments along the spiral ${\mathrm{NiO}}_{6}$ chains. Bulk susceptibility measurements reveal metamagnetic behavior below ${T}_{N},$ consistent with the picture of an antiferromagnet with magnetic anisotropy and competing exchange interactions.

Magnetism and the magnetic structure of the pillared perovskite, La5Re3MnO16

Abstract The new, pillared perovskite material, La5Re3MnO16, has been synthesized by a somewhat modified method than that published previously (J. Solid State Chem. 151 (2000) 31) in sufficient quantities for neutron diffraction experiments. Such experiments were carried out between 10 and 250 K at zero applied field. The crystal structure found at 250 K from refinement of powder neutron diffraction data is in excellent agreement with previous results obtained from a single crystal X-ray experiment. Bulk susceptibility measurements show a sharp spike at 161 K and a broad maximum at ∼100 K. Long range magnetic order was found below the critical temperature of ∼165 K with an ordering wave vector, k =(0 0 1 2 ) . The magnetic structure found consists of perovskite type layers containing Re5+ and Mn2+ moments coupled ferrimagnetically (Re antiparallel to Mn) within the layers and with antiferromagnetic interlayer coupling. The ordered moments at 10 K are 4.0(1)μB for Mn2+ and 1.0(1)μB for Re5+, not unreasonable values. The Re moments lie nearly along the c-axis while the Mn moments make a angle of ∼18° with respect to c. While most of the magnetic scattering can be accounted for by the above model, two weak reflections are seen which appear at 90 K and below and require k =( 1 2 1 2 1 2 ) .

NMR, EPR, and bulk susceptibility measurements of one-dimensionalSrNbO3.41

NMR, EPR, and bulk susceptibility measurements were performed in the perovskite-related layered compound ${\mathrm{SrNbO}}_{3.41}.$ We determined ${}^{93}\mathrm{Nb}$ NMR spectra, spin-spin relaxation rates ${1/T}_{2}$ and spin-lattice relaxation rates ${1/T}_{1}$ down to 600 mK. From the spin-lattice relaxation ${1/T}_{1}$ we identified an activated behavior ${1/T}_{1}\ensuremath{\propto}\mathrm{exp}(\ensuremath{-}{\ensuremath{\Delta}}_{\mathrm{NMR}}{/k}_{B}T)$ with a gap ${\ensuremath{\Delta}}_{\mathrm{NMR}}=6.5\mathrm{meV}$ for temperatures $Tg20\mathrm{K}.$ Below 20 K, the line width $\ensuremath{\Delta}H(T)$ from acceptor-EPR and ${}^{93}{(1/T}_{1})(T)$ both exhibit a temperature dependence propotional to $\mathrm{exp}[\ensuremath{-}{(T}_{0}{/T)}^{1/2}]$ which is indicative for a variable range hopping process. We discuss the possibility of charge-density-wave formation at low temperatures.

Single-ion versus dipolar origin of the magnetic anisotropy in iron(III)-oxo clusters: a case study.

A multitechnique approach has allowed the first experimental determination of single-ion anisotropies in a large iron(III)-oxo cluster, namely [NaFe6(OCH3)12(pmdbm)6ClO4 (1) in which Hpmdbm = 1,3-bis(4-methoxyphenyl)-1,3-propanedione. High-frequency EPR (HF-EPR). bulk susceptibility measurements, and high-field cantilever torque magnetometry (HF-CTM) have been applied to iron-doped samples of an isomorphous hexagallium(III) cluster [NaGa6(OCH3)12-(pmdbm)6]ClO4, whose synthesis and X-ray structure are also presented. HF-EPR at 240 GHz and susceptibility data have shown that the iron(III) ions have a hard-axis type anisotropy with DFe = 0.43(1) cm(-1) and EFe = 0.066(3) cm(-1) in the zero-field splitting (ZFS) Hamiltonian H = DFe[S2(z) - S(S + 1)/3] + Fe[S2(x) - S2(y)]. HF-CTM at 0.4 K has then been used to establish the orientation of the ZFS tensors with respect to the unique molecular axis of the cluster, Z. The hard magnetic axes of the iron(III) ions are found to be almost perpendicular to Z, so that the anisotropic components projected onto Z are negative, DFe(ZZ)= -0.164(4) cm(-1). Due to the dominant antiferromagnetic coupling, a negative DFe(ZZ) value determines a hard-axis molecular anisotropy in 1, as experimentally observed. By adding point-dipolar interactions between iron(III) spins, the calculated ZFS parameter of the triplet state, D1 = 4.70(9) cm(-1), is in excellent agreement with that determined by inelastic neutron scattering experiments at 2 K, D1 = 4.57(2) cm(-1). Iron-doped samples of a structurally related compound, the dimer [Ga2(OCH3)2(dbm)4] (Hdbm = dibenzoylmethane), have also been investigated by HF-EPR at 525 GHz. The single-ion anisotropy is of the hard-axis type as well, but the DFe parameter is significantly larger [DFe = 0.770(3) cm(-1). EFe = 0.090(3) cm(-1)]. We conclude that, although the ZFS tensors depend very unpredictably on the coordination environment of the metal ions, single-ion terms can contribute significantly to the magnetic anisotropy of iron(III)-oxo clusters, which are currently investigated as single-molecule magnets.

Characterization of the metastable magnetic phaseof LixNi1-xO using non-linear susceptibility

Metastable magnetism is observed in two important compounds (thosewith x = 0.35 and x = 0.5) of the LixNi1-xO series,with similar experimental features found in bulk susceptibilitymeasurements. For each of these two compositions, a frequencydependence in the first-order alternating-current (ac)susceptibility (χ1), a negative peak in the third-order acsusceptibility (χ3), and a field-cooled/zero-field-cooledbifurcation in the direct-current magnetization were seen. Wehave used χ3 to discern the nature of the metastabilityfor these samples. We find that for x = 0.35, χ3 becomescritical as a function of field (h), frequency (f), andtemperature (T), confirming the existence of a spin-glass-likephase. However, χ3 for x = 0.5 does not become criticalas a function of h and f, and shows a -T-3-dependence aspredicted by Wohlfarth's model of superparamagnetism. ThusLixNi1-xO shows magnetic phases of different types in twoimportant composition ranges which are identified using χ3.The results also indicate the importance of non-linearsusceptibility for distinguishing the two metastable phases by meansof bulk susceptibility measurements.