Unusual Magnetic Relaxation Research Articles

Unusual magnetic relaxation in a single-molecule magnet with toroidal magnetic moments

We report the synthesis and characterization of a single-molecule magnet composed of triangular clusters of dysprosium ions. The structural study shows that the symmetry changes from one polar point group (mm2) at room temperature to another polar point group (m) at low temperature. Magnetic studies and theory calculations illustrate that the vortex distribution of magnetic dipoles in the triangular dysprosium clusters forms a toroidal magnetic moment. Interestingly, the analysis of AC magnetic susceptibility reveals the coexistence of three distinct magnetic relaxation processes, corresponding to the Raman, Orbach, and QTM relaxation pathways, respectively. The sum of three modified Debye functions is successfully used to describe the multiple relaxation behavior.

Unusual magnetic relaxation behavior of hydrophilic colloids based on gadolinium(III) octabutoxyphthalocyaninate

The work introduces the original and efficient approach toward hydrophilization of Gd(III) complex with hydrophobic octa-2,3,9,10,16,17,23,24-n-butoxyphthalocyanine (1H2). It consists in the solvent-mediated self-assembly of the preliminary synthesized octa-2,3,9,10,16,17,23,24-n-butoxyphthalocyaninatogadolinium(III) acetate 1GdOAc into the colloid species followed by their hydrophilization through the polyelectrolyte deposition. Small-angle X-ray scattering, TEM, and DLS measurements of the aqueous colloids reveal the disk-like colloid species formation with the greatest size value about 40 nm. Deposition of polystyrene sulfonate layer onto the surface of nanocolloids does not prevent their partial aggregation in aqueous solutions. The phthalocyaninate ligand is the reason for specific electrochemical behavior of the colloids, which is affected by the nanoparticulate form of the complex. The magnetic behavior of the complex reveals it as pure paramagnetic, while magnetic relaxation behavior of the colloids points to some specificity. The r2/r1 ratio measured at 0.47 T is higher (2.6) than the ratios commonly reported for Gd(III) complexes, coming to 36.8 at 11.75 T. Thus, the synthesized colloids are more efficient as T2- than T1-contrasting agents at magnetic field strengths above 1.4 T.

Unusual Magnetic Behavior of a 2D Citrate‐Bridged Dysprosium(III) Coordination Polymer

AbstractA new lanthanide citrate coordination polymer [Dy(citrate)(H2O)]n (1) was hydrothermally synthesized and structurally characterized by elemental analyses, single‐crystal X‐ray diffraction, IR spectroscopy, thermogravimetric analysis, photoluminescence spectrum, and magnetic measurements. The structure of 1 exhibits a 2D layer structure, in which the Dy2O2 dimer serves as the building block. The photoluminescence spectrum shows blue luminescence in the solid state at room temperature. Owing to the special frustration structure, unusual magnetic relaxation was found in compound 1. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Directionally dependent magneto-optical response of(La0.4Pr0.6)1.2Sr1.8Mn2O7: Understanding the high-field ferromagnetic metallic state

We investigate the polarized magneto-optical response of ${({\mathrm{La}}_{0.4}{\mathrm{Pr}}_{0.6})}_{1.2}{\mathrm{Sr}}_{1.8}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}$ in order to reveal the microscopic nature of the states involved in the field-induced paramagnetic semiconductor to ferromagnetic metal transition. We find significant changes in the electronic structure of the title compound under magnetic fields due to the evolution of the Jahn-Teller-split ${\mathrm{Mn}}^{3+}\phantom{\rule{0.3em}{0ex}}{e}_{g}$ orbitals. Consequences of this field-induced modification of the $d$ orbitals include the substantial colossal magnetoresistence effect, unusual magnetic relaxation behavior, and a change in orbital occupation. Based upon the directional dependence of the magneto-optical spectra, we demonstrate that the high-field ferromagnetic state is homogeneous.

Structural and magnetic properties of two- and three-dimensional molecule-based magnets (cat)+[MIIMIII(C2O4)3

Abstract We discuss the different structural arrangements of NBu 4 [Fe II Cr III (C 2 O 4 ) 3 ] layered compounds in their racemic and enantiomeric forms and related magnetic properties. For [Mn II Fe III (C 2 O 4 ) 3 ] networks of dimensionalities 2 and 3 Mossbauer spectroscopy was applied to study the Fe III sublattice magnetization. Unusual magnetic relaxation phenomena below T N were observed for both 2D and 3D networks.

Structural and Magnetic Properties of Two-Dimensional Oxalate-Bridged Bimetallic Compounds

Single crystals of (cat)±[MnIIMIII(C2O4)3] ferromagnetic (MIII˭CrIII, (cat)± = (n-Pr)4, n-Bu(Ph)3P), and antiferromagnetic (MIII˭FeIII, (cat)±=(n-Bu)4N, (Ph)4P) compounds have been synthesized in order to further elucidate the correlations between their structural and magnetic properties. Single crystal X-ray structural as well as 57Fe Mössbauer studies are reported here. In all these compounds, assigned to a space group R3c. Z=6. alternating [MnIIMIII(C2O4)3]nn-2D honeycomb-like networks, comprise [MIII(C2O4)3]3− building units of both kinds of chirality. It has been established that previously reported crystal data for ((n-Bu)4N)[MnIIFeIII(C2O4) 3] (space group P63, Z=2)[1] represent a polymorph structure with similar metallo-oxalate layers containing [FeIII(C2O4)3]3− units of the same kind of chirality (Δ or Δ). An account for twinning effects in crystallization allow us to locate carbon atoms of the (cat)± in unit cell. Zero- and high-field 57Fe Mössbauer spectroscopy of the polycrystalline compounds as a function of temperature revealed that: (i) ((n-Pn)4N)[MnIIeIII(C2O4)3] is a weak ferromagnet rather than a ferrimagnet, (ii) most of the {MnIIFeIII} and {FeIIFeIII} compositions are planar (XY) magnets exhibiting unusual magnetic relaxation well below Tc , and (iii) the negative magnetization previously observed[2] in ((n-Pn)4N)[FeIIFeIII(C2O4)3] is certainly a result of strong magnetic anisotropy and an occurrence of a “magnetic compensation point”, i.e. a crossing in FeII and FeIII sub-lattice magnetization curves.

Characterization of porous carbons with high resolution α s-analysis and low temperature magnetic susceptibility

The effectiveness of integrated characterization of porous carbons with different approaches such as the high resolution α s-method for N 2 adsorption, X-ray diffraction, and magnetic susceptibility measurement is shown. The theoretical basis for the high resolution α s-analysis for N 2 adsorption using GCMC simulation and the background of magnetic susceptibility of carbon are given. The applicability of the integrated characterization to changes of ACF with heating at 1473–3173 K in Ar is shown. The activated mesocarbon microbead (a-MCMB) and activated carbon aerogel (a-CA) were examined by the integrated characterization method. a-MCMB showed an unusual magnetic relaxation below 10 K due to random magnetism. The random magnetism was associated with the mutually isolated network structure of nanographitic units. a-CA also showed random magnetism at low temperature, which was associated with the spin isolated nanographitic structure similar to a-MCMB.

Site-specific ligand variation in manganese–oxide cubane complexes, and unusual magnetic relaxation effects in [Mn4O4X(OAc)3(dbm)3](X = N3–, OCN–Hdbm = dibenzoylmethane)

Treatment of [Mr4O2(OAc)6(py)2(dbm)2]1(py = pyridine, Hdbm = dibenzoylmethane) in hot MeCN with Me3SiX (X = N3–, OCN –) yields [Mn4O3X(OAc)3(dbm)3] containing extremely rare η1, µ3-N3– or -OCN– groups; alternating-current magnetic susceptibility studies reveal unusual magnetic relaxation effects and indicate these species to be the newest and smallest examples yet of truly molecular magnets.

Effect of transport currents on the critical state of YBa2Cu3O7- delta thin films.

After preparation of the remanent critical state, the normal component of the local time-dependent magnet field ${\mathit{B}}_{\mathit{z}}$ near the surface of a ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ thin film was measured before, during, and after the application of a transport current. The results are shown to be consistent with new calculations of ${\mathit{B}}_{\mathit{z}}$ for the thin film geometry. The calculated behavior in a thin film is shown to be substantially different from that of the Bean critical state in a long slab in a parallel field. In slab geometry, starting from the remanent critical state, \ensuremath{\Vert}J(x)\ensuremath{\Vert} is always equal to ${\mathit{J}}_{\mathit{c}}$, whereas in thin films a continuous distribution of current densities J(x) is found. This feature results in a unique distribution of the magnetic field ${\mathit{B}}_{\mathit{z}}$ and unusual magnetic relaxation behavior in the presence of a transport current.

Magnetic bistability in a metal-ion cluster

MAGNETIC materials of mesoscopic dimensions (a few to many thousands of atoms) may exhibit novel and useful properties such as giant magnetostriction, magnetoresistivity and magnetocaloric effects1–4. Such materials also allow one to study the transition from molecular to bulk-like magnetic behaviour. One approach for preparing mesoscopic magnetic materials is to fragment bulk ferromagnets; a more controllable method is to take a 'bottom-up' approach, using chemistry to grow well defined clusters of metal ions5,6. Lis7 has described a twelve-ion manganese cluster in which eight of the Mn ions are in the +3 oxidation state (spin S=2) and four are in the +4 state (S=3/2). These ions are magnetically coupled to give an S=10 ground state8, giving rise to unusual magnetic relaxation properties8,9. Here we report that the magnetization of the Mn12 cluster is highly anisotropic and that the magnetization relaxation time becomes very long below a temperature of 4 K, giving rise to pronounced hysteresis. This behaviour is not, however, strictly analogous to that of a bulk ferromagnet, in which magnetization hysteresis results from the motion of domain walls. In principle, a bistable magnetic unit of this sort could act as a data storage device.

Order–disorder and magnetic clusters in some mixed-metal perovskites

The temperature dependence of the 57Fe and 119Sn Mossbauer spectra and the zero-field-cooled and field-cooled magnetic susceptibilities of SrLaFeSnO6, SrEuFeSnO6, BaLaFeSnO6 and CaLaFeSnO6 show the existence of unusual magnetic relaxation behaviour. In the case of CaLaFeSnO6 there is evidence for a non-random distribution of the Fe and Sn, although the other compounds are fully disordered. It is proposed that competition between the first- and second-nearest-neighbour superexchange interactions leads to the formation of magnetic clusters which can grow with decrease in temperature until long-range antiferromagnetic order is achieved.

Study of magnetic relaxation in the perovskite SrLaFeSnO6by Mössbauer spectroscopy

The temperature dependence of the 57Fe and 119Sn Mössbauer spectra of SrLaFeSnO6 has revealed the existence of unusual magnetic relaxation behaviour. The transferred hyperfine interactions in the Sn resonance at 4.2 K establish that the Fe and Sn cations are disordered. An explanation is developed in terms of the formation of magnetic clusters below ca. 220 K which increase in size with decreasing temperature until a long-range ordered G-type antiferromagnetic structure is achieved below ca. 38 K. This behaviour is intermediate between a true antiferromagnet and a spin-glass. A reinterpretation of the related SrFe1 –xSnxO3 –y system is given which suggests that compositions in the region of Sr2FeSnO5.71 may also exhibit spin-glass behaviour.