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15 May 2003

Volume 93, Issue 10, pp. 5855-8792

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High-frequency electron paramagnetic resonance investigations of tetranuclear nickel-based single-molecule magnets

R. S. Edwards, S. Maccagnano, E.-C. Yang, S. Hill, W. Wernsdorfer, D. Hendrickson, and G. Christou

J. Appl. Phys. 93, 7807 (2003); http://dx.doi.org/10.1063/1.1540050 (3 pages) | Cited 27 times

Online Publication Date: 9 May 2003

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We report preliminary high-frequency electron paramagnetic resonance (EPR) investigations for several tetranuclear nickel complexes which exhibit single-molecule magnetism, including low-temperature (below ∼1 K) hysteresis loops and resonant magnetic quantum tunneling. The combination of a cavity perturbation technique and a split-coil magnet facilitates high-sensitivity, multifrequency (40 to 200+ GHz), angle dependent single-crystal EPR measurements. The data confirm the expected S=4 ground states, and a negative magnetocrystalline anisotropy for each member in the series. An unusual splitting of the easy-axis EPR peaks is observed, which may be interpreted in terms of distinct Ni4 species within the crystals. Overall, however, the trends associated with the splitting, as well as the EPR linewidths and shapes, suggest that intermolecular exchange interactions are important. Indeed, differences between the EPR spectra obtained for different complexes correlate nicely with the expected strength of exchange interactions, as determined both from intermolecular contact distances and from independent hysteresis measurements. © 2003 American Institute of Physics.
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75.50.Xx Molecular magnets
76.30.Fc Iron group (3d) ions and impurities (Ti-Cu)
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.45.+j Macroscopic quantum phenomena in magnetic systems
75.30.Et Exchange and superexchange interactions
75.30.Gw Magnetic anisotropy

Unconventional Heisenberg spin triangle in magnetic molecule {V15}: A proton nuclear magnetic resonance study

D. Procissi, B. J. Suh, J. K. Jung, P. Kögerler, R. Vincent, and F. Borsa

J. Appl. Phys. 93, 7810 (2003); http://dx.doi.org/10.1063/1.1540051 (3 pages) | Cited 11 times

Online Publication Date: 9 May 2003

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We present results of 1H nuclear magnetic resonance (NMR) and relaxation in polyoxovanadate cluster {V15} with formula K6[V15As6O42]⋅9(H2O). The data in {V15} are compared with the published data in {V6} with the formula [CN3H6]4Na2[H4{V3L}2P4O4]⋅14H2O, which contains two independent spin triangles. Temperature T and field H dependence of the NMR linewidth are well explained by the magnetic dipolar interaction between the proton nuclei and vanadium ion spins in both compounds. T and H dependence of proton relaxation rate T1−1 show great differences in the two compounds. The difference indicates a completely different spin dynamics for the two Heisenberg spin-triangle systems whereby in {V15} the fluctuations of the VIV (s=1/2) in the central triangle are dominated by the interlayer coupling with the VIV spins in the two upper and lower hexagons. © 2003 American Institute of Physics.
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75.50.Xx Molecular magnets
76.60.Es Relaxation effects
75.40.Gb Dynamic properties (dynamic susceptibility, spin waves, spin diffusion, dynamic scaling, etc.)
75.10.Jm Quantized spin models, including quantum spin frustration
75.30.Et Exchange and superexchange interactions

Tunneling splitting of magnetic levels in Fe8 detected by 1H NMR cross relaxation

Y. Furukawa, K. Aizawa, K. Kumagai, R. Ullu, A. Lascialfari, and F. Borsa

J. Appl. Phys. 93, 7813 (2003); http://dx.doi.org/10.1063/1.1540052 (3 pages) | Cited 2 times

Online Publication Date: 9 May 2003

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Measurements of proton NMR and the spin lattice relaxation rate 1/T1 in the octanuclear iron (III) cluster [Fe8(N3C6H15)6O2(OH)12]⋅[Br8⋅9H2O], in short Fe8, have been performed at 1.5 K in a powder sample aligned along the main anisotropy z axis, as a function of a transverse magnetic field (i.e., perpendicular to the main easy axis z). A big enhancement of 1/T1 is observed over a wide range of fields (2.5–5 T), which can be attributed to the tunneling dynamics; in fact, when the tunneling splitting of the pairwise degenerate m=±10 states of the Fe8 molecule becomes equal to the proton Larmor frequency a very effective spin lattice relaxation channel for the nuclei is opened. The experimental results are explained satisfactorily by considering the distribution of tunneling splitting resulting from the distribution of the angles in the hard xy plane for the aligned powder, and the results of the direct diagonalization of the model Hamiltonian. © 2003 American Institute of Physics.
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75.50.Xx Molecular magnets
76.60.Es Relaxation effects
75.45.+j Macroscopic quantum phenomena in magnetic systems
75.50.Dd Nonmetallic ferromagnetic materials
71.70.Jp Nuclear states and interactions

Magnetic behavior of iron-oxoclusters prepared in an organosilica sol–gel matrix

I. S. Dubenko, M. S. Rao, S. Roy, B. C. Dave, and N. Ali

J. Appl. Phys. 93, 7816 (2003); http://dx.doi.org/10.1063/1.1555971 (3 pages)

Online Publication Date: 9 May 2003

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The crystal structure and magnetization of nanoscale enTMOS–Fe2O3 sol–gel composites with weight iron concentration x, varying from 0.003 to 0.065, have been studied by the transmission electron microscopy technique and a superconducting quantum interference device magnetometer. The clusters are crystallized in a hexagonal crystal structure. All the samples demonstrate a superparamagnetic behavior with antiferomagnetic cluster–cluster coupling at low temperature. The effective paramagnetic moment, μeff, has been found to vary in the range from 5.9 (S=5/2) to 2.5 μB per iron ion. The concentration dependence of the μeff shows a minimum for x∼0.01. At a low iron concentration x<0.01, μeff is practically independent of x and equals about 6 μB per Fe ion. The concentration interval 0.01<x<0.07 is characterized by a monotonical increase of μeff from 2.5 to about 3 μB per Fe ion. Thus, an abrupt variation of μeff (about two times) is observed at x≈0.01. It has been shown that such behavior can be caused by competition between the uncoupled “surface” and antiferromagnetically coupled “bulk” Fe magnetic moments. © 2003 American Institute of Physics.
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81.07.Pr Organic-inorganic hybrid nanostructures
75.50.Tt Fine-particle systems; nanocrystalline materials
61.46.-w Structure of nanoscale materials
75.20.Ck Nonmetals
68.37.Lp Transmission electron microscopy (TEM)
81.10.Dn Growth from solutions
81.10.Fq Growth from melts; zone melting and refining
81.15.Lm Liquid phase epitaxy; deposition from liquid phases (melts, solutions, and surface layers on liquids)

Structure and magnetic properties of a neutral dimeric copper (II) complex of N-(2-hydroxybenzyl)glycinamide ligand

Xiaobai Wang, Jun Ding, John D. Ranford, and Jagadese J. Vittal

J. Appl. Phys. 93, 7819 (2003); http://dx.doi.org/10.1063/1.1556217 (3 pages) | Cited 3 times

Online Publication Date: 9 May 2003

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A copper (II) complex ([Cu2(sglym)2(NO3)2]) of tridentate reduced Schiff base ligands, namely, (N-(2-hydroxybenzyl)-glycinamide (Hsglym) was synthesized and characterized in the solid state by single crystal x-ray diffraction techniques. The two Cu(II) atoms are bridged by two phenolate oxygen atoms in the neutral dimers and the geometry of Cu(II) can be described as a distorted square pyramid. The Cu–Cu distances are 3.020(2) Å. The magnetic properties of the complex were studied at variable temperatures. Transition of spin state (from spin zero to spin one) was observed due to thermal excitation. This transition of spin state observed in the temperature range 100–400 K was accompanied by magnetic viscosity. © 2003 American Institute of Physics.
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61.66.Hq Organic compounds
75.50.Xx Molecular magnets
75.20.Ck Nonmetals
75.60.Lr Magnetic aftereffects

Effective spin Hamiltonian for magnetic clusters in presence of S mixing

S. Carretta, E. Liviotti, N. Magnani, and G. Amoretti

J. Appl. Phys. 93, 7822 (2003); http://dx.doi.org/10.1063/1.1540053 (3 pages) | Cited 5 times

Online Publication Date: 9 May 2003

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A quite general and straightforward procedure to study the contributions to the high-order anisotropy terms due to the mixing between states with different spin S in magnetic clusters has been presented. This procedure is based on a perturbational approach and consists of adding to the effective spin Hamiltonian some terms depending on the anisotropic interactions causing the mixing. These terms are written as functions of some new operators which depends on the total spin S. The present approach has been applied to study the behavior of the cluster Cr8. © 2003 American Institute of Physics.
Show PACS
75.50.Xx Molecular magnets
75.10.Dg Crystal-field theory and spin Hamiltonians
75.30.Gw Magnetic anisotropy
75.30.Et Exchange and superexchange interactions
75.50.Ee Antiferromagnetics
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