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15 May 2003

Volume 93, Issue 10, pp. 5855-8792

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ac-response of magnetic moments of diluted Ho3+ ions

R. Giraud, A. M. Tkachuk, and B. Barbara

J. Appl. Phys. 93, 7077 (2003); http://dx.doi.org/10.1063/1.1558230 (3 pages) | Cited 1 time

Online Publication Date: 9 May 2003

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Low-temperature dynamics of mesoscopic magnetic moments of a 0.1% at Ho3+-doped LiYF4 single crystal is probed by ac-susceptibility measurements above T=1.75 K. In the temperature and frequency range studied (1.75 K⩽T⩽5 K,10 Hz⩽f⩽1500 Hz), the results clearly show that both thermally activated quantum tunneling of a nearly isolated magnetic moment and spin–spin cross relaxations occur. This interpretation is strongly supported by the calculated electro-nuclear Zeeman diagram either of a single ion or of a pair of diluted ions. © 2003 American Institute of Physics.
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75.20.Hr Local moment in compounds and alloys; Kondo effect, valence fluctuations, heavy fermions
75.45.+j Macroscopic quantum phenomena in magnetic systems
75.30.Cr Saturation moments and magnetic susceptibilities
75.40.Gb Dynamic properties (dynamic susceptibility, spin waves, spin diffusion, dynamic scaling, etc.)
71.70.Ej Spin-orbit coupling, Zeeman and Stark splitting, Jahn-Teller effect

Electronic structure and exchange interactions in V15 magnetic molecules: LDA+U results

D. W. Boukhvalov, V. V. Dobrovitski, M. I. Katsnelson, A. I. Lichtenstein, B. N. Harmon, and P. Kögerler

J. Appl. Phys. 93, 7080 (2003); http://dx.doi.org/10.1063/1.1540035 (3 pages) | Cited 10 times

Online Publication Date: 9 May 2003

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Single-molecule magnets V15 (K6[V15As6O42(H2O)]⋅8H2O) have attracted a great deal of attention recently. These magnets are promising systems for studying such fundamental problems as spin tunneling, spin coherence, and low-temperature spin relaxation. To understand in detail the internal magnetic and electronic structure, and the intramolecular interactions responsible for the formation and low-energy excitations in V15 molecules, we have performed electronic structure calculations using the local spin density approximation with on-site repulsion paramter U (so-called LSDA+U) approach. The calculated values of magnetic moments and charge states of vanadium ions agree well with experiments confirming the V4+ state of vanadium ions. We found that for U∼4–5 eV, good description of known properties of V15 molecule can be achieved. Comparing the results of the band-structure calculations with the experiments, we have found that the LDA+U description of the V15 magnet is in agreement with experimental data. © 2003 American Institute of Physics.
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75.10.Lp Band and itinerant models
75.50.Xx Molecular magnets
71.15.Mb Density functional theory, local density approximation, gradient and other corrections
75.30.Et Exchange and superexchange interactions
75.50.Ee Antiferromagnetics
31.15.E- Density-functional theory
71.20.Ps Other inorganic compounds
71.45.Gm Exchange, correlation, dielectric and magnetic response functions, plasmons
71.70.Gm Exchange interactions
75.25.-j Spin arrangements in magnetically ordered materials (including neutron and spin-polarized electron studies, synchrotron-source x-ray scattering, etc.)

Heisenberg model of a {Cr8}-cubane magnetic molecule

Marshall Luban, Paul Kögerler, Lance L. Miller, and Richard E. P. Winpenny

J. Appl. Phys. 93, 7083 (2003); http://dx.doi.org/10.1063/1.1558231 (3 pages) | Cited 4 times

Online Publication Date: 9 May 2003

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A Heisenberg model of eight CrIII paramagnetic centers (spins s=3/2) at the vertices of a cube with four distinct exchange interactions is found to provide a reasonably accurate description of the magnetic susceptibility of the cubane-type magnetic molecule [Cr8O4(O2CC6H5)16]={Cr8} from 2–290 K for an external field of 0.5 T. We find that two exchange bonds are antiferromagnetic (13, 24 K) and two are ferromagnetic (5, 13.5 K), with an accuracy of approximately 1 K. The determination of the four exchange constants is greatly facilitated using the exact high-temperature expansion of the weak-field susceptibility, effectively reducing the number of unknown parameters to two. We have calculated the thermodynamic properties of the system and these can be compared with the results of future experiments. At temperatures below 0.5 K sharp increases are expected in the magnetization versus external magnetic field at approximately 6 and 12 T and higher fields due to level crossings. Inelastic neutron scattering could check our predictions for the low-lying magnetic energy levels. © 2003 American Institute of Physics.
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75.30.Cr Saturation moments and magnetic susceptibilities
75.50.Xx Molecular magnets
75.10.Jm Quantized spin models, including quantum spin frustration
75.30.Et Exchange and superexchange interactions

An Fe4 cluster with a slowly relaxing paramagnetic excited state: [Fe4O(OH)5(tacn)4]I7⋅2.5 H2O

D. Zipse, K. A. Abboud, and N. S. Dalal

J. Appl. Phys. 93, 7086 (2003); http://dx.doi.org/10.1063/1.1540036 (3 pages) | Cited 1 time

Online Publication Date: 9 May 2003

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We report the synthesis, x-ray structure, and magnetic susceptibility of a magnetic compound [Fe4O(OH)5(tacn)4]I7⋅2.5 H2O (Fe4), where tacn=1, 4, 7 triazacyclononane. Fe4 crystallizes in the monoclinic space group, P2(1)/c, with cell parameters a=18.7604(9) Å, b=12.5840(6) Å, c=24.064(1) Å, α=90°, β=108.783(1)°, and γ=90°. Dc susceptibility, χdc, experiments show the magnetic moment of Fe4 saturates at approximately 0.2 μB, giving a spin ground state of S=0. However, on application of dc fields, a peak develops in χdc, which is ascribed to the presence of a low lying excited magnetic state. Interestingly, ac susceptibility experiments show frequency dependence of χ″ and thermally activated magnetization switching dynamics, with a potential energy barrier of 26 K. The origin of these observations still remains unexplained. © 2003 American Institute of Physics.
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75.30.Cr Saturation moments and magnetic susceptibilities
75.50.Xx Molecular magnets
61.66.Hq Organic compounds
75.60.Jk Magnetization reversal mechanisms

Magnetic studies of crystal-engineered molecular nanostructures (invited)

H. Srikanth, R. Hajndl, B. Moulton, and and M. J. Zaworotko

J. Appl. Phys. 93, 7089 (2003); http://dx.doi.org/10.1063/1.1540037 (3 pages) | Cited 16 times

Online Publication Date: 9 May 2003

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Magnetic studies of dimeric copper complexes reveal interesting and predictable cooperative responses governed by the underlying topological lattice configurations. Temperature dependent susceptibility in several compounds measured with a physical property measurement system indicates predominantly antiferromagnetic exchange coupling. Both intra- and interdimer interactions are found to be important and the data could be fit well with a modified Bleaney–Bowers model from which these interaction parameters are extracted. Crystal engineering methods have been used to generate open and closed framework molecular nanostructures. A Kagome lattice configuration with the Cu(II) dimers arranged using triangular symmetry yields distinct hysteresis loops that are consistent with the occurrence of weak ferromagnetism. © 2003 American Institute of Physics.
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75.50.Xx Molecular magnets
75.75.-c Magnetic properties of nanostructures
81.07.Nb Molecular nanostructures
75.30.Cr Saturation moments and magnetic susceptibilities
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.50.Ee Antiferromagnetics
75.30.Et Exchange and superexchange interactions

Raman and infrared modes of the single molecule magnet Fe8Br8 and analogs

J. M. North and N. S. Dalal

J. Appl. Phys. 93, 7092 (2003); http://dx.doi.org/10.1063/1.1543856 (3 pages) | Cited 2 times

Online Publication Date: 9 May 2003

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We present Raman and infrared data for the S=10 single-molecule magnet (SMM) Fe8Br8, 17O-labeled Fe8Br8 and their analogs, Fe8Br6.4(ClO4))1.6 and Fe8Br4(ClO4))4, over a range of 100–1600 cm−1. The Raman modes were assigned through group theoretical analysis of smaller model compounds. These results could help understand the structural basis of the SMM behavior of these compounds. Additionally, Raman scattering appears to have high potential as an analytical technique for the identification of SMM analogs.© 2003 American Institute of Physics.
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78.30.Jw Organic compounds, polymers
63.20.D- Phonon states and bands, normal modes, and phonon dispersion
75.50.Xx Molecular magnets

More evidence for a distribution of tunnel splittings in Mn12–acetate

K. M. Mertes, Yoko Suzuki, M. P. Sarachik, Y. Myasoedov, H. Shtrikman, E. Zeldov, E. M. Rumberger, D. N. Hendrickson, and G. Christou

J. Appl. Phys. 93, 7095 (2003); http://dx.doi.org/10.1063/1.1543857 (3 pages) | Cited 1 time

Online Publication Date: 9 May 2003

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In magnetic fields applied parallel to the anisotropy axis, the magnetization of Mn12 has been measured in response to a field that is swept back and forth across the resonances corresponding to steps N=4,5,…9. The fraction of molecules remaining in the metastable well after each sweep through the resonance is inconsistent with expectations for an ensemble of identical molecules. The data are consistent instead with the presence of a broad distribution of tunnel splittings. A very good fit is obtained for a Gaussian distribution of the second-order anisotropy tunneling parameter XE=−ln(∣E∣/2D). We show that dipolar shuffling is a small effect and is not responsible for the nonexponential relaxation. © 2003 American Institute of Physics.
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75.50.Xx Molecular magnets
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.30.Gw Magnetic anisotropy

Magnetic properties and spin dynamics in magnetic molecule {Mn3}

B. J. Suh, D. Procissi, J. K. Jung, S. Bud’ko, W. S. Jeon, Y. J. Kim, and D.-Y. Jung

J. Appl. Phys. 93, 7098 (2003); http://dx.doi.org/10.1063/1.1555875 (3 pages) | Cited 1 time

Online Publication Date: 9 May 2003

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We report dc magnetization and nuclear magnetic resonance (NMR) measurements in a trinuclear manganese compound, {Mn3}, with formula [Mn3O(O2CCH3)6(C5H5N)3]⋅C5H5N. The magnetic properties are characterized by two antiferromagnetic (AF) coupling constants, J=−7.9 K and J′=−27.6 K. 1H NMR linewidth strongly depends on both the magnetic field and temperature, and is quantitatively explained by the dipolar interaction between proton nuclei and Mn ion spins. The strong enhancement of T1−1 at low temperatures is ascribed to the slowing down of magnetic fluctuations resulting from building up of AF correlations. From the T dependence of T1−1 at low T, we obtained the gap ΔNMR=19 K, in qualitative agreement with the gap Δ≅12 K obtained from susceptibility χ. © 2003 American Institute of Physics.
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75.50.Xx Molecular magnets
75.40.Gb Dynamic properties (dynamic susceptibility, spin waves, spin diffusion, dynamic scaling, etc.)
76.60.-k Nuclear magnetic resonance and relaxation
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.30.Cr Saturation moments and magnetic susceptibilities

Ferromagnetic CuII–CuII exchange interactions in a polyoxomolybdate-based nanocluster

Paul Kögerler and Achim Müller

J. Appl. Phys. 93, 7101 (2003); http://dx.doi.org/10.1063/1.1555892 (2 pages) | Cited 11 times

Online Publication Date: 9 May 2003

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We present initial magnetic properties and a simple Heisenberg model for a recently isolated new member of the {Mo57M6}-type family, a nanometer-sized polyoxomolybdate cluster anion comprising a triangular Cu6II prism. The exchange interactions between the six s=1/2 spin centers are weak compared to the exchange found for other members of the {Mo57M6} cluster family with M=FeII, FeIII, and VIV but display ferromagnetic exchange interactions of CuII–O–MoVI–O–CuII pairs, i.e., within two equilateral Cu3II triangles. The exchange interaction between the two triangles is antiferromagnetic. The observed temperature dependence of χT is described well by a Heisenberg model that employs a ferromagnetic intratriangle exchange constant J1=1.9 K and antiferromagnetic intertriangle coupling of J2=−1.4 K that eventually results in an S=0 ground state. © 2003 American Institute of Physics.
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75.50.Tt Fine-particle systems; nanocrystalline materials
75.50.Xx Molecular magnets
75.30.Et Exchange and superexchange interactions
61.46.-w Structure of nanoscale materials
75.10.Jm Quantized spin models, including quantum spin frustration

Dynamical Ising-like model for the two-step spin-crossover systems

K. Boukheddaden, J. Linares, E. Codjovi, F. Varret, V. Niel, and J. A. Real

J. Appl. Phys. 93, 7103 (2003); http://dx.doi.org/10.1063/1.1540038 (3 pages) | Cited 12 times

Online Publication Date: 9 May 2003

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In order to reproduce the two-step relaxation observed experimentally in spin-crossover systems, we investigate analytically the static and the dynamic properties of a two-sublattice Ising-like Hamiltonian. The formalism is based on a stochastic master equation approach. It is solved in the mean-field approximation, and yields two coupled differential equations that correspond to the HS fractions of the sublattices A and B. © 2003 American Institute of Physics.
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75.10.Hk Classical spin models
75.40.-s Critical-point effects, specific heats, short-range order
75.30.Wx Spin crossover
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
75.10.Dg Crystal-field theory and spin Hamiltonians
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