Marc Tibor Kelemen

Structural and magnetic properties of TbMn2Hx hydrides

Abstract Powder samples of TbMn 2 H x hydrides, with 0≤ x ≤4.3, are characterised by X-ray analysis and SQUID magnetometry for temperatures ranging between 2 and 375 K. We observed structural phase transformations as a function of the hydrogen concentration and temperature. In particular a spinodal decomposition for x

Structure and reactivity of paramagnetic cyclopentadienyl cobalt complexes with bulky alkyl substituents

Sodium tetraisopropylcyclopentadienide or lithium tri( tert -butyl)cyclopentadienide reacts with cobalt(II) chloride in THF to give the corresponding dimers [(C 5 HR 4 )Co(μ-Cl)] 2 ( 1a , R=CHMe 2 ) or [(C 5 H 2 R 3 )Co(μ-Cl)] 2 ( 1b , R=CMe 3 ) in good yield. The nitrosyl complexes [(C 5 HR 4 )Co(μ-NO)] 2 ( 2a ), [(C 5 HR 4 )Co(μ-Cl)(μ-NO)Co(C 5 HR 4 )] ( 3a ), and [(C 5 HR 4 )Co(NO)Cl] ( 4a ) (R=CHMe 2 ) or the tri( tert -butyl)cyclopentadienyl derivatives 2b , 3b , and 4b have been obtained from CoCl 2 , the respective cyclopentadienide, and nitric oxide. From the disproportionation of 1a or 1b with carbon monoxide, the salt [(C 5 HR 4 )Co(μ-Cl) 3 Co(C 5 HR 4 )] + 2 [Cl 2 Co(μ-Cl)] 2 2− ( 5a , R=CHMe 2 ) and the dicarbonyl complex [(C 5 HR 4 )Co(CO) 2 ] ( 6a , R=CHMe 2 ) or the tri( tert -butyl)cyclopentadienyl derivatives 5b and 6b have been isolated. Compounds 6a and 6b were converted to the carbonyl-bridged dimers [(C 5 HR 4 )Co(μ-CO)] 2 ( 7a , R=CHMe 2 ) or [(C 5 H 2 R 3 )Co(μ-CO)] 2 ( 7b , R=CMe 3 ) by UV irradiation. Compound 1a was cleaved with acetonitrile to yield the novel 17 valence electron cation [(C 5 HR 4 )Co(MeCN) 2 ] + ( 8 , R=CHMe 2 ), which in acetonitrile solution could be further oxidized to [(C 5 HR 4 )Co(MeCN) 3 ] 2+ ( 9 , R=CHMe 2 ) by electrochemical means or chemically with ferrocenium hexafluorophosphate. Compound 1a gave the neutral 17 VE complex [(C 5 HR 4 )Co(PMe 3 )Cl] ( 10 , R=CHMe 2 ) with trimethylphosphane, which could be converted to the methyl-substituted odd-electron compound [(C 5 HR 4 )Co(PMe 3 )CH 3 ] ( 11 , R=CHMe 2 ) by treatment with methyllithium. Hydrolysis of complex 11 proceeded with phosphane loss and resulted in formation of the hydroxo-bridged dimer [(C 5 HR 4 )Co(μ-OH)] 2 ( 12 , R=CHMe 2 ). Crystal structure analyses have been carried out for 1a , 3b , 5a , 7a , 7b , 8 , 10 , and 12 . For growth of single crystals tetraisopropylcyclopentadiene turned out to be a well-suited solvent.

Magnetic properties of new Fe6(triethanolaminate(3-))6 spin-clusters

Abstract We report the magnetic properties of three cyclic hexanuclear iron(III) spin-clusters ([Fe 6 (tea) 6 ] · 6MeOH ( 1 ), [NaFe 6 (tea) 6 ]ClO 4 · 2MeOH ( 2 ) and [LiFe 6 (tea) 6 ]ClO 4 · 2MeOH ( 3 ), (tea: triethanolaminate(3-)). The intra-molecular exchange coupling between the Fe III -spins is determined by measurements of the magnetic susceptibility in the range 2–300 K. The zero-field splitting of the lowest cluster spin states ( E/k B 200K) is studied by ESR (electron-spin resonance) at frequencies between 9.5 and 350 GHz. The level splitting is analyzed by the irreducible-tensor operator technique.

Magnetic structures of GdMn6Ge6 - a nuclear magnetic resonance analysis

The origin of the interesting magnetic properties of GdMn6 Ge6 in the low-temperature, low-field phase is a spiral arrangement of the magnetic moments. This has been established by pulsed nuclear magnetic resonance experiments. Spin-echo spectra taken in an external magnetic field, and the observed enhancement of the radio-frequency field, suggest the presence of a spiral structure. This information is crucial for a quantitative analysis of the complicated 55 Mn spectra. The anisotropy of the hyperfine field at the 55 Mn site is about 10% and the enhancement has been found to be highly anisotropic, as well. Comparing the temperature dependencies of the Zeeman splitting at the individual lattice sites, Mn has been identified as the source of the transferred hyperfine fields at the 73 Ge nuclei. An analysis of the 73 Ge and 155/157 Gd spectra yields the size and relative orientation of the hyperfine fields and electric field gradients at the nuclei in agreement with 155 Gd Mossbauer spectroscopy results.

Spiral spin structures of RMn6Ge6 (R=Gd, Tb, Dy)

Abstract The field dependences of the magnetic moment of several Gd X Y 1− X Mn 6 Ge 6 compounds have been analysed by a mean-field theory of several spin structures. The assumption of spiral spin structures at low temperatures in these compounds has been confirmed. In addition to this, the magnetic phase diagrams of TbMn 6 Ge 6 and DyMn 6 Ge 6 have been deduced from magnetisation measurements of powder samples. In contrast to GdMn 6 Ge 6 , the crystal fields of these compounds produce a variety of additional spin phases.

Ferromagnetism in carbon-doped Mn5Si3 films

The effect of carbon doping on the structural and magnetic properties of Mn5Si3Cx films is investigated for different substrate temperatures TS and concentrations x. Samples with x≈0.75 prepared at TS=650–750 K exhibit ferromagnetic order with enhanced ordering temperatures TC well above room temperature in contrast to the undoped antiferromagnetic Mn5Si3 compound. Structural analysis shows that C is incorporated interstitially in the hexagonal Mn5Si3-type structure with a lattice expansion with respect to the undoped compound. The enhanced ferromagnetic order is presumably not simply due to a change of the interatomic Mn–Mn distances but due to a change of the electronic structure and/or exchange interactions.

Characterisation of New Organic Ligand-Based Magnets: [MNTTP] [TCNE] and [MNTTP][TCNQ]

Abstract We analysed the temperature and field dependence of the magnetisation of the electron transfer salts meso-(Tetratolylporphinato)manganese(III)-tetracyanoethenide, [MnIIITTP]:: +[TCNE]− and meso-(Tetratolylporphinato)manganese(III)-7,7,8,8-tetracyano-p-quinodimethane [MnIIITTP]:: +[TCNQ]−. They show cooperative magnetic behaviour for temperatures below 10 K.

Ferromagnetism above room temperature in Mn–Si–C alloy films

Mn–Si–C alloy films are prepared by e‐beam coevaporation onto a Si substrate held at 600 °C. Ferromagnetism is observed below Tc=(360±5) K with SQUID magnetometry and magneto‐optical Kerr effect. This is the highest Curie temperature Tc yet observed for a Mn‐based alloy. Although the composition determined by Auger depth profiling varies appreciably for different films, their Tc is the same indicating that ferromagnetism is caused by an alloy of well‐defined composition independent of precipitations.

Magnetic properties of new charge-transfer complexes based on porphyrins

Tetra-p-tolyl and tetrathienyl substitution of manganese(III) porphyrins in charge-transfer compounds with organic acceptors like tetracyanoethylene is used to modify molecular packing of ferrimagnetically correlated chains in the solid. Ferri- and antiferromagnetically ordered compounds and also borderline situations are realized. The strong influence of the solvent used for crystal growth is exemplified.

NMR analysis and the magnetic properties of GdMn6Ge6

Abstract The magnetic phase diagram of GdMn6Ge6 was deduced from magnetisation measurements of powder samples. In agreement with the results of molecular field calculations it contains a paramagnetic phase, a ferrimagnetic phase where the magnetisation of ferromagnetically coupled Mn and Gd sublattices points in opposite directions, a spin flop phase and a phase where ferromagnetically ordered Gd and Mn planes are present, but the interaction between these planes results in a macroscopic magnetisation smaller than that in the ferri-phase. For the first time the zero-field magnetic resonance of 73Ge-nuclei has been observed at 77 K and 4.2 K. A separation of the Zeeman and quadrupolar contributions yields the hyperfine fields and the electric field gradients at the three inequivalent Ge sites. The enhancement observed indicates the presence of some net magnetisation in the low-temperature zero-field phase. Applying an external magnetic field, the sign of the hyperfine field at the Ge nuclei was determined.