Marc Drillon

Quasi-2D XY Magnetic Properties and Slow Relaxation in a Body Centered Metal Organic Network of [Co4] Clusters

Octahedral Co(2+) centers have been connected by mu(3)-OH and mu(2)-OH(2) units forming [Co(4)] clusters which are linked by pyrazine forming a two-dimensional network. The two-dimensional layers are bridged by oxybisbenzoate (OBA) ligands giving rise to a three-dimensional structure. The [Co(4)] clusters bond with the pyrazine and the OBA results in a body-centered arrangement of the clusters, which has been observed for the first time. Magnetic studies reveal a noncollinear frustrated spin structure of the bitriangular cluster, resulting in a net magnetic moment of 1.4 microB per cluster. For T > 32 K, the correlation length of the cluster moments shows a stretched-exponential temperature dependence typical of a Berezinskii-Kosterlitz-Thouless model, which points to a quasi-2D XY behavior. At lower temperature and down to 14 K, the compound behaves as a soft ferromagnet and a slow relaxation is observed, with an energy barrier of ca. 500 K. Then, on further cooling, a hysteretic behavior takes place with a coercive field that reaches 5 T at 4 K. The slow relaxation is assigned to the creation/annihilation of vortex-antivortex pairs, which are the elementary excitations of a 2D XY spin system.

Copper(II) and cobalt(II) dicarboxylate-based layered magnets: influence of π electron ligands on the long range magnetic ordering

Abstract We discuss the structure and magnetic properties of multilayer compounds M 2 (OH) 2 A· z H 2 O, where M=Co(II) or Cu(II) and A is a dicarboxylate anion. They consist of metal hydroxide-based layers connected through difunctional species (A), which, depending on their nature (alkane-, alkene-dioate or terephthalate) act as pillars or electronic connectors. Either metamagnetism or ferromagnetism is observed, with a huge coercive field of 5.9 T at 4 K in the case of the cobalt(II) hydroxyterephthalate. The aim is to show that π electron species connecting ferromagnetic layers may induce unexpected magnetic properties. High-frequency EPR experiments (95 GHz) and magnetic measurements are reported, giving information on the local structure and the exchange couplings.

The Quest for Nanoscale Magnets: The example of [Mn12] Single Molecule Magnets

Recent advances on the organization and characterization of [Mn12] single molecule magnets (SMMs) on a surface or in 3D are reviewed. By using nonconventional techniques such as X-ray magnetic circular dichroism (XMCD) and scanning tunneling microscopy (STM), it is shown that [Mn12]-based SMMs deposited on a surface lose their SMM behavior, even though the molecules seem to be structurally undamaged. A new approach is reported to get high-density information-storage devices, based on the 3D assembling of SMMs in a liquid crystalline phase. The 3D nanostructure exhibits the anisotropic character of the SMMs, thus opening the way to address micrometric volumes by two photon absorption using the pump-probe technique. We present recent developments such as µ-SQUID, magneto-optical Kerr effect (MOKE), or magneto-optical circular dichroism (MOCD), which enable the characterization of SMM nanostructures with exceptional sensitivity. Further, the spin-polarized version of the STM under ultrahigh vacuum is shown to be the key tool for addressing not only single molecule magnets, but also magnetic nano-objects.

Hybrid organic–inorganic multilayer materials: influence of π electrons as magnetic media in a series of bridged-layer compounds M2(OH)4−xAx/2 (M=Cu(II) or Co(II), A=dicarboxylate anion)

Abstract We report the preparation, structure and magnetic properties of a series of multilayer copper(II) hydroxy-dicarboxylates. These are prepared by anion-exchange, starting from copper(II) hydroxy-acetate Cu 2 (OH) 3 (OAc)·H 2 O. The acetate ion is substituted for n- methylene dicarboxylates O 2 CXCO 2 2− , with X=(CH 2 ) n and n =1 to 8. Similar exchange reactions are carried out with unsaturated dicarboxylate anions (X=(CH) n , with n =2, 4 and X=C 4 H 6 ). The compounds, of general formulation Cu 2 (OH) 4− x (XC 2 O 4 ) x /2 . · z H 2 O, exhibit a layered structure with a step-like variation of the basal spacing, according to n parity. IR spectroscopy indicates that the carboxylate functions are linked to the metal ions, each aliphatic chain bridging adjacent hydroxide layers. Only compounds with n =2, 4 and 8 show a net magnetic moment within copper(II) layers. For the short chain anions, an antiferromagnetic order occurs at low temperature, with a metamagnetic transition in low field, except for the unsaturated dicarboxylate Cu 2 (OH) 1.92 (C 4 H 6 C 2 O 4 ) 1.04 ·0.22H 2 O which exhibits a long-range ferromagnetic order at T C =13 K. For the long-chain anion, X=C 8 H 16 , a ferrimagnetic 3d order is stabilized below T C =17 K. On the basis of structural and magnetic findings, a comparison is made between the saturated and unsaturated aliphatic chain compounds. This study provides new insight on the influence of π electrons on the ordering between magnetic layers. Finally, preliminary results for the cobalt(II) trans -hexenedioate analog indicate that a 3d ferromagnet is obtained below T C =56.5 K, thus emphasizing the key role of the in-plane spin–spin correlations on the ordering temperature.

Hybrid organic-inorganic layered compounds prepared by anion exchange reaction: correlation between structure and magnetic properties

The synthesis, structure and magnetic properties of a series of hybrid organic-inorganic copper(II) compounds are reported. Anion exchange reactions were used to prepare layered materials made of copper(II)-based inorganic layers separated by long chain anions (n-alkyl sulfates or n-alkyl carboxylates). Contrary to classical intercalated compounds, the exchanged anion is coordinated to the metal ion. Different packing modes of the n-alkyl chains (mono- or bi-layers) are evidenced, depending on the bridging functions. The magnetic properties have been related to the nature of the anionic spacer and the basal spacing. When ferromagnetic in-plane interactions dominate, 3D ferromagnetic ordering is usually observed for large spacing, due to a dipolar coupling effect. For unsaturated aliphatic chains, π electrons are shown to reinforce the interlayer exchange coupling, giving a ferromagnetic ground-state.

Spin-Frustrated Complex, [FeIIFeIII(trans-1,4-cyclohexanedicarboxylate)1.5]∞: Interplay between Single-Chain Magnetic Behavior and Magnetic Ordering

A three-dimensional mixed-valent iron(II,III) trans-1,4-cyclohexanedicarboxylate, 1,4-chdc, coordination polymer, [Fe(II)Fe(III)(mu(4)-O)(1,4-chdc)(1.5)](infinity), 1, has been synthesized hydrothermally by mixing iron powder and 1,4-chdcH(2) and investigated by X-ray diffraction, dc and ac magnetic susceptibility, and iron-57 Mossbauer spectroscopy over a wide range of temperatures. Single-crystal X-ray diffraction studies of 1 at 90(2), 293(2), and 473(2) K reveal a tetrahedral [Fe(II)(2)(mu(4)-O)Fe(III)(2)(mu(4)-O)](6+) mixed-spin-chain structure with no change in the P1 space group but with subtle changes in the Fe-O and Fe...Fe distances with increasing temperature. These changes are associated with the electron delocalization observed by Mossbauer spectroscopy above 225 K. Magnetic studies reveal three different magnetic regimes in 1 between 2 and 320 K. Above 36 K 1 is a one-dimensional ferrimagnetic-like complex with frustration arising from competing exchange interactions between the iron(II) and iron(III) ions in the chains. Between 36 and 25 K the interchain interactions are non-negligible and 1 undergoes three-dimensional ordering at 32.16 K but with some residual fluctuations. Below 25 K the residual fluctuations slow and eventually freeze below 15 K; the small net moment of 0.22 mu(B) per mole of 1 observed below 15 K may be attributed to a non-collinear or canted spin structure of the spins of the four iron ions in the [Fe(II)(2)(mu(4)-O)Fe(III)(2)(mu(4)-O)](6+) chains. Below 32 K the Mossbauer spectra of 1 exhibit sharp sextets for both the iron(III) and iron(II) ions and are consistent with either a static long-range or a short-range magnetic ground state or a slow relaxation between two canted magnetic states that are indistinguishable at the observed spectral resolution. The 85 and 155 K spectra reveal no electron delocalization and correspond solely to fixed valence iron(II) and iron(III). Between 225 and 310 K the spectra reveal the onset of electron delocalization such that, at 295 to 310 K, 25, 25, and 50% of the iron in 1 is present as iron(II), iron(III), and iron(II/III) ions, respectively. The absence of any spectral line broadening associated with this electron delocalization and the coexistence of four doublets between 225 and 310 K indicate that the delocalization occurs through electron tunneling via vibronic coupling. The sudden increase in the tunneling rate beginning above about 260 K and the presence of a cusp in the magnetic susceptibility centered at about 275 K strongly suggest the existence of a charge order/disorder transition whose nature and order are discussed.

Long range ferromagnetism in tunable cobalt(II) layered compounds up to 25 Å apart

Abstract We discuss the structure-magnetic property correlations for a series of layered cobalt(II) compounds, derived from the parent host Co(OH) 2 . Intercalation takes place by exchange of OH − by organic or inorganic anions resulting in interlayer spacing between 4.65 and 25.4 A. For all the compounds, ferromagnetic in-plane interaction dominate the magnetic behavior at high temperature, but long range order occurs at low temperature; 3d antiferromagnetic order is observed for small basal spacing and ferromagnetic for large spacing. The nature of the magnetic interaction is discussed briefly.

An approach to chiral magnets using α-hydroxycarboxylates

Carboxylate-bridged complexes of transition metals, MII = MnII, FeII, CoII, NiII, CuII, were synthesized by reaction of MII salts with (R)-mandelic acid ((R)-2-hydroxyphenylacetic acid), methoxymandelic acid (racemic) (2-methoxyphenylacetic acid) and (R)-malic acid ((R)-2-hydroxybutanedioic acid) under hydrothermal conditions. A wide variety of structures has been obtained, with various dimensionalities: 3D networks when the malic diacid is used, 2D networks with the mandelic acid, 1D isolated chains organised in planes for the Co and Cu compounds with MeOmandelic acid and monomers for the Ni analogue. These complexes have been fully characterized structurally and magnetically. During the reaction process, the (R)-mandelic ligand undergoes a racemisation reaction but the compounds obtained with (R)-malic acid are chiral and exhibit metal-centred circular dichroism. Extensive magnetic characterization of all compounds indicates rather weak coupling interactions between paramagnetic centres linked through carboxylate bridges. Curie-like paramagnetic, antiferromagnetic, ferromagnetic or weak ferromagnetic behaviour was observed and is discussed on the basis of the structural features. The bimetallic compounds Mn0.63Co0.37(R)-malate monohydrate (12) and Mn0.79Ni0.21(R)-malate monohydrate (13) are new examples of chiral magnets.

Spacing-dependent dipolar interactions in dendronized magnetic iron oxide nanoparticle 2D arrays and powders

Self-assembly of nanoparticles (NPs) into tailored structures is a promising strategy for the production and design of materials with new functions. In this work, 2D arrays of iron oxide NPs with interparticle distances tuned by grafting fatty acids and dendritic molecules at the NPs surface have been obtained over large areas with high density using the Langmuir-Blodgett technique. The anchoring agent of molecules and the Janus structure of NPs are shown to be key parameters driving the deposition. Finally the influence of interparticle distance on the collective magnetic properties in powders and in monolayers is clearly demonstrated by DC and AC SQUID measurements. The blocking temperature T(B) increases as the interparticle distance decreases, which is consistent with the fact that dipolar interactions are responsible for this increase. Dipolar interactions are found to be stronger for particles assembled in thin films compared to powdered samples and may be described by using the Vogel Fulcher model.

Recent Experimental and Theoretical Studies of Molecular and Layered Metal-Radical Based Magnets

Abstract We report in this paper the preparation and magnetic properties of layered metal(II) hydroxynitrates and hydroxyacetates which display, according to the metal ion, either ferro (Co(II) and Ni(II)) or antiferromagnetic (Cu(II)) intralayer interactions. These compounds are shown to be suitable host lattices for organic species. 2,6- and 2,5- pyridine-based nitronyl-nitroxide biradicals have been synthesized and their magnetic properties investigated. They respectively display ferro- and antiferromagnetic behaviors. X-rays studies show an angle of 88° (in the 2,6 case) and 0° (in the 2,5 case) between both radicals. The synthesis of a new imino-nitroxide benzoic acid radical, characterized by classical methods and X-rays diffraction, and preliminary results of exchange reaction with copper(II) basic salts are also discussed.