Structure and magnetic properties of a new single-molecule magnet [Mn11Fe1O12 (CH3COO)16(H2O)4].2CH3COOH.4H2O

A new single-molecule magnet [Mn₁₁Fe₁O₁₂ (CH₃COO)₁₆(H₂O)₄]?2CH₃COOH?4H₂O (Mn₁₁Fe₁) has been synthesized.The structure has been studied by the single crystal x-ray diffraction. The difference of Jahn--Teller distortion between Fe³⁺ and Mn³⁺ ion reveals that Fe³⁺ ion substitutes for Mn³⁺ ion on the Mn(3) sites in the Mn₁₂ skeleton. The temperature dependence of the magnetization gives a blocking temperature T_B=1.9K for Mn₁₁Fe₁. Based on the magnetization process analysis of the crystal at T=2K, we suggest that Mn₁₁Fe₁ has the ground state with a total spin S= 11/2.     

Transport through artificial single-molecule magnets: Spin-pair state sequential tunneling and Kondo effects

The transport properties of an artificial single-molecule magnet based on a CdTe quantum dot doped with a single Mn+2 ion(S=5/2) are investigated by the non-equilibrium Green function method.We consider a minimal model where the Mn-hole exchange coupling is strongly anisotropic so that spin-flip is suppressed and the impurity spin S and a hole spin s entering the quantum dot are coupled into spin pair states with(2S+1) sublevels.In the sequential tunneling regime,the differential conductance exhibits(2S+1) possible peaks,corresponding to resonance tunneling via(2S+1) sublevels.At low temperature,Kondo physics dominates transport and(2S+1) Kondo peaks occur in the local density of states and conductance.These peaks originate from the spin-singlet state formed by the holes in the leads and on the dot via higher-order processes and are related to the parallel and antiparallel spin pair states.     

Berechnung der streuspektren schwach ionisierter plasmen mit hilfe zweizeitiger wahrscheinlichkeitsdichten

ESR linewidth measurements have been performed on the quasi-two-dimensional Heisenberg magnet (C₂H₅NH₃)₂MnCl₄ at room temperature in the X-, K-, Q- and V-band. As expected from theory a logarithmic frequency dependence of the magic angle linewidth has been determined, which represents a characteristic feature of spin diffusion in a two-dimensional lattice. The spin diffusion constant is evaluated to be D/a² = (1700±250) rad GHz.     

EPR and NMR characterization of theS=9 excited state and spin density distribution in the single-molecule magnet Fe8Br8: Implications to theS=10 model and magnetization tunneling pathways

High-sensitivity, variable-frequency, high-field electron paramagnetic resonance (EPR) measurements and nuclear magnetic resonance (NMR) measurements are reported for theS=10 single-molecule magnet Fe₈Br₈. We find that theS=10 state is nested with its first excited state, withS=9, located at only 24±2 K above. Also reported are some preliminary⁸¹Br NMR measurements of the unpaired electron spin density on the Br^? sites. The results provide new insight and benchmarks for improved electronic and magnetic structural calculations and macroscopic tunneling pathways of this class of single-molecule magnets.     

Control of Charge Transfer Phase Transition in Iron Mixed-Valence System (n-C n H2n+1)4N[FeIIFeIII(dto)3] (n=3–6; dto = C2O2S2)

The spin flop behaviour of low anisotropy Mn ions within the mixed halide (⁵⁴Mn)Mn(Br _x Cl_1−x )₂·4H₂O is shown to be interpolative with that of the two terminal compounds, in striking contrast with the dynamics of nuclear spin lattice relaxation.     

Mixed spin- and spin- Ising models with random nearest-neighbour interactions

Using the effective field theory with correlations, we study mixed spin?3/2 and spin?1/2 Ising models with random bonds and crystal-field interactions on the honeycomb lattice. The nearest-neighbour couplings J_ij are taken as random variables with distribution P(J_ij) = pδ(J_ij ? J)+(1 ? p)δ(J_ij ? αJ), where J > 0 and |α| ≤ 1. In a certain range of negative values of α, the phase diagrams exhibit re-entrant behaviour. In detail, we investigate separately two kinds of disorder: Bond dilution (α = 0) and random ±J interactions (α = ?1). In both cases, the influence of the an-isotropy on the phase diagrams shows some new outstanding features.     

Submillimeter-wave ESR measurements of low-dimensional quantum spin systems

Temperature-dependent electron spin resonance (ESR) measurements of two low-dimensional quantum spin systems, theS = 1/2 spin-ladder system Cu₂(1,4-diazacycloheptane)₂Cl₄ (Cu₂(C₅H₁₂N₂)₂Cl₄) and the gaplessS = 1 bond-alternating one-dimensional antiferromagnettrans-Ni(333-tet)(N₃)(ClO₄), have been performed. X-band electron spin resonance (ESR) measurements of single-crystal Cu₂(C₅H₁₂N₂)₂Cl₄ show the increase of linewidth and anisotropicg-shifts below 8 K similar to those known for one-dimensional antiferromagnet. On the other hand, Ni(333-tet)(N₃)(ClO₄) has broad line-width and ESR has been observed for the first time by our high-field ESR. Its linewidth increases as the temperature is decreased, while theg-shift seems to be isotropic and theg-value decreases as the temperature is decreased. Thisg-shift can be connected to the quantum fluctuation of the system.     

Berichtigung

The Fe compounds (Et₄N)₂{[((TSP) (TSPH)Fe]₂suc}³) and K[Fe(TSP) (TSPH)prop] · 3H₂O³) were characterized by means of ⁵⁷Fe Moessbauer spectroscopy and magnetic measurements. In the temperature region from 300 to 390 K the Fe(III) of both compounds undergoes a discontinuous transition from low spin state to high spin state, returning only slowly into low spin state after cooling to room temperature. This process causes a hysteresis behaviour of the magnetic values. The spin crossover is connected with a complex isomerization. Moessbauer spectra of the compounds show a significant asymmetry, which can be explained by relaxation effects according to Blume's theory.     

Pressure-induced changes in the magnetic properties of the single crystal of Mn3 single-molecule magnet

The magnetic measurements of the single crystal of Mn3 single-molecule magnet under high pressure at T=2 K have been performed.We find both the antiferromagnetic intermolecular coupling parameter J and the effective energy barrier to vary compared with the measurements at low pressure.The increase of|J|is estimated to be 12%at 0.7 GPa when compared with that of0 GPa,whereas the effective energy barrier becomes smaller with increasing pressure.Our results demonstrate that the intermolecular interaction of single-molecule magnet can be changed by pressure.Compared with the normal magnetic alloy,the effect of pressure on the magnetic properties of Mn3 is much more prominent,which implies that Mn3 may have great potential in magnetic multifunctional material.     

Level structure of 99Rh at high spins

High spins states in ⁹⁹Rh were populated via the ⁶⁶Zn(³⁷Cl, 2p2n)⁹⁹Rh reaction at an incident beam energy of 130 MeV. Seventeen new transitions have been observed in the present study and the level scheme has now been extended up to a spin of J∼ 25ħ and an excitation energy of about E _x∼ 10 MeV. The observation of a positive parity E2 cascade based on the 9/2⁺ isomeric level is suggestive of collective behaviour in this nucleus up to high spins. Spherical shell model (within restricted model space) and Cranked shell model calculations were performed to obtain an insight into the observed level structure. The new collective band observed up to a spin of J∼ 25ħ is suggested to be based on (πg _9/2 ³) ⊗ (νg _7/2 ²) quasi-particle excitations. Received: 12 July 1999 / Revised version: 14 September 1999     

Low-energy electronic spin excitations between filling factors ν=1 and studied by optically detected nuclear magnetic resonance

We report measurements of the spin relaxation time (T_1n) for nuclei in the potential well confining a high-mobility two-dimensional electron system at a single GaAs–GaAlAs heterojunction. At low temperatures nuclear spin relaxation is dominated by electron–nuclear spin scattering: we find that T_1n displays sharp maxima at incompressible states throughout the hierarchy of the fractional quantum Hall effect. This behaviour is consistent with the existence of low-energy spin excitations only where the electron system is compressible. Our measurements also provide evidence for a gap in the spin excitation spectrum at .     

Cooperativity in a spin transition ferrous polymer: Interacting domain model, thermodynamic, optical and EPR study

A new thermodynamic model is proposed in order to account for the high spin low spin conversion in metal-organic polymers. The model, based on the idea that the spin conversion occurs in interacting domains of like-spin metal ions, allows to explain most of the important features of various types of spin conversion. The sine qua non condition of the existence of spin transitions with hysteresis is obtained. In the case of very large cooperativity, the model predicts unusual behaviour of the spin conversion system due to a low-temperature metastable high spin state. Existence of such a state is interesting in the context of the light induced excited spin state trapping recently observed in some ferrous compounds. The model is applied to interpret the spin transition in polycrystalline ferrous polymer [Fe _1-y Cu _y (Htrz)₂ trz] (BF ₄) with y = 0.00, 0.01 and 0.10, detected by differential scanning calorimetry, optical reflectivity and electron paramagnetic resonance. The domain size and the interaction energy between the domains are estimated as, respectively, n = 11 and for the y = 0 compound. As the copper content is growing, n and tend to decrease, resulting in transformations of the shape of hysteresis loop which becomes less steep, narrows and shifts to lower temperatures. The electron paramagnetic resonance gives further evidence of the presence of like-spin domains. Received 27 November 1998 and Received in final form 19 April 1999     

Relaxor behaviour in PZN-PT-BT ferroelectric ceramics

The relaxor behaviour of the (Pb_0.8Ba_0.2)[(Zn_1/3Nb_2/3)_0.7Ti_0.3]O₃ ferroelectric ceramic is presented. A strong dispersion of the maximum of the dielectric permittivity (ε′) below the transition temperature (T_m) is observed, which shift towards higher temperatures with increasing frequency. There is a strong deviation from the Curie-Weiss law. The results are fitted by using the Volger-Fulcher relationship, showing typical behaviour of a spin glass system. The hysteresis loops suggest relaxor behaviour.     

Anomalous pressure dependence of the Lamb-Mössbauer factor in a spin crossover system

The dependence of the f-factor on temperature and on pressure up to 1500 bar has been measured in the spin crossover compound [Fe(2-pic-ND ₂ ) ₃ ]cl ₂ ·EtOD. In the transition region around T _c =146.7 K the pressure dependence of f shows an anomalous increase, which can be explained on the basis of a theoretical model for spin transitions developed by us.     

The 4 A 2→2 E transitions of 2[Cr(en)3Cl3]KCl. 6H2O in strong magnetic fields

The Zeeman effect of the R absorption lines for the pure compound 2[Cr(en)₃Cl₃]KCl.6H₂O has been measured photographically, using a strong pulsed magnet with fields of up to 200 000 gauss at 77 K. The results indicate g _∥ (⁴ A ₂) = 1·82 , g _⊥(⁴ A ₂) = 1·90 with g _∥(ē) = -1·82 and g _∥ (2ā) = 1·82. The uncertainties are of the order of ± 0·05.     

Magnetic properties of defects in spin-gap magnets

The magnetic properties of defects were studied in spin-gap magnets such as spin-Peierls magnet CuGeO₃, Haldane magnet PbNi₂V₂O₈, and charge-ordered ladder magnet NaV₂O₅. Doping of these systems with nonmagnetic impurities leads to additional magnetic degrees of freedom, which manifest themselves at low temperatures, where the intrinsic magnetic susceptibility of a spin-gap system is close to zero. Magnetic susceptibility appears due to the local destruction of the singlet ground state as a result of impurity-induced breakage of spin chains. Antiferromagnetically correlated areas arise near the spin-chain breaks. The sizes of these areas and the effective spin of these specific spin clusters are estimated. The order parameter and its spatially modulated depth are determined for impurity-induced magnetically ordered phases. The magnetic properties of defects for the NaV₂O₅ ladder structure are explained in the model of electrons “hopping” near the chain break. The hopping degree of freedom effectively influences the total spin of a spin-chain fragment and magnetization of the system.     

Inelastic low-temperature transport through a quantum dot with a Mn ion

Using the nonequilibrium Hubbard operator Green's function technique, we study the inelastic low-temperature quantum transport through an artificial single-molecule magnet coupled to a single phonon mode. For a minimal model based on CdTe quantum dot doped with a single Mn²⁺ ion (S=5/2), the calculated results show that in the presence of hole–phonon coupling, in addition to main Kondo-like peaks associated with (2S+1) sublevels of spin pair states, satellite Kondo-like peaks originating from emitting phonons appear in the local density of states and differential conductance. Moreover, the number of these phonon-induced Kondo-like peaks depends on the parity of the local large spin, i.e., S=integer or half-integer. It is expected that the intrinsic properties of artificial single-molecule magnets can be obtained by detecting these transport characteristics.     

Hyperfine interaction in a quantum spin chain

From an electron spin resonance measurement on a single crystal sample of theS=1 linear chain Heisenberg antiferromagnet Ni(C₃H₁₀N₂)₂NO₂ClO₄ (NINO) containing a small amount of Cu impurity atoms, we have observed two sets of four hyperfine lines, one of which has almost three times larger field splitting than the other. The hyperfine lines are well explained as arising from the hyperfine interaction between the Cu nuclear spin andthe Cu electron spin which interact with theS=1/2 degrees of freedom induced at the Ni sites by the quantum effect. A large anisotropy in the hyperfine constant is observed andanalyzed using a ligand field theory with covalency effects.