Euan K. Brechin

Magnetization tunneling in single-molecule magnets

The quantum mechanical tunneling of the direction of magnetization is discussed for several examples of single-molecules magnets (SMM’s). SMM’s are molecules that function as nanomagnets. Magnetization tunneling is described for two crystallographically different forms of [Mn12O12(O2CC6H4-p-Me)16(H2O)4] solvate. The two Mn12 complexes are isomers that both differ in the positioning of the H2O and carboxylate ligands and also in the orientations of the Jahn–Teller elongation at the Mn III ions. The magnetization versus magnetic field hysteresis loop is quite different for the two isomeric Mn12 complexes. One Mn12 complex exhibits a magnetization hysteresis loop that is characteristic of considerably faster magnetization tunneling than in the other Mn12 isomer. The lower symmetry and greater rhombic zero-field splitting are the origin of the faster magnetization tunneling. Frequency-dependent ac magnetic susceptibility and dc magnetization decay data are presented to characterize the magnetization relaxation rate versus temperature responses of three mixed-valence Mn4 complexes. In all three cases, the Arrhenius plot of the logarithm of the magnetization relaxation rate versus the inverse absolute temperature shows a temperature-dependent region as well as a temperature-independent region. The temperature-independent magnetization rate is definitive evidence of magnetization tunneling in the lowest-energy zero-field component of the ground state.

New hexanuclear and octanuclear iron(III) oxide clusters: octahedral [Fe6O2]14+ species and core isomerism in [Fe8O4]16+ complexes

Abstract The synthesis, X-ray crystal structures and magnetic properties of two new classes of hexanuclear and octanuclear iron(III) clusters are reported. The reaction of hmpH (2-(hydroxymethyl)pyridine) with [Fe 3 O(O 2 CR) 6 (H 2 O) 3 ](NO 3 ) (R=Bu t , Ph) gives [Fe 6 O 2 (O 2 CR) 6 (hmp) 6 ](NO 3 ) 2 containing a Fe III 6 octahedron with two opposite faces bridged by μ 3 -O 2− ions. Similar reactions employing FeCl 3 /NaO 2 CR (R=Me, Ph) give [Fe 8 O 4 (O 2 CPh) 11 (hmp) 5 ] and [Fe 8 O 4 (O 2 CMe) 12 (hmp) 4 ]. Both contain a similar [Fe 8 (μ 3 -O) 4 ] core that can be described as four triangular [Fe 3 O] units joined together by edge or vertex sharing or, alternatively, as two [Fe 3 O] units bridged by a [Fe 2 O 2 ] rhombus; however, the dispositions of the [Fe 3 O] units about the [Fe 2 O 2 ] central unit differ in the two complexes and can be described as syn and anti core isomers. The Fe 6 and Fe 8 complexes have been found to have S =0 ground states when studied by variable-temperature, solid-state magnetic susceptibility measurements, consistent with the expected antiferromagnetic exchange interactions between the Fe(III) ions.

[Mn 18 ] 2+ and [Mn 21 ] 4+ single-molecule magnets

Abstract The synthesis and structural characterization of the two new Mn complexes [Mn18O14(O2CMe)18(hep)4(hepH)2(H2O)2](ClO4)2 (1) and [Mn21O16(O2CMe)16(hmp)6(hmpH)2(pic)2(py)(H2O)](ClO4)4 (3) are presented, together with a detailed study of their magnetic properties. Complex 1 possesses a ground-state spin of S=13, and the ground-state spin for 3 is estimated to be S=17/2 or 19/2. Both complexes 1 and 3 are new examples of single-molecule magnets (SMMs), displaying frequency-dependent out-of-phase AC signals, as well as magnetization vs. DC field hysteresis at temperatures below 1 K. Complex 1 straddles the classical/quantum interface by also displaying quantum tunneling of the magnetization (QTM).

Two new hexanuclear iron(III) complexes with S 5 ground states

The synthesis and magnetic properties of two new hexanuclear iron complexes [Fe6O2(OH)2(O2CR)10L2] (R = But (3), Me (4); LH = 2-(2-hydroxyethyl)pyridine (hepH) (3), 6-methyl-2-(hydroxymethyl)pyridine (Me-hmpH) (4)) are reported. Both compounds are prepared by treatment of [Fe3O(O2CR)6(H2O)3]+ with three equivalents of LH in MeCN. The X-ray crystal structure of 3·2CHCl3·2H2O is presented. It consists of a planar array of six Fe3+ ions comprising two [Fe3(μ3-O)] subunits that are related by an inversion centre and linked at two of their apices, each linkage consisting of one μ-hydroxo and two μ-carboxylato groups. DC magnetic susceptibility measurements at 1.0 and 0.10 Tesla in the 2.0–300 K range show an increase in the effective magnetic moment with decreasing temperature, corresponding to a high spin (S) ground state. The spin of the ground state was established by magnetization measurements in the 1.0–7.0 T field range and 1.7–4.0 K temperature range. Fitting of the reduced magnetization data by full matrix diagonalization, incorporating both axial and rhombic anisotropy, gave S = 5, g = 1.96, D = 0.46 cm−1 and |E| = 0.046 cm−1 for 3, and S = 5, g = 2.07, D = 0.27 cm−1 and |E| = 0 cm−1 for 4. Alternative fits with a negative ZFS were rejected based on their relative fitting error as well as on measurements of the magnetization relaxation behaviour of the complexes at very low temperature (≥0.04 K), where no hysteresis characteristic of a single-molecule magnet was observed.

Synthetic and magnetic studies of a dodecanuclear cobalt wheel

The synthesis, structural characterisation and preliminary magnetic studies of a Co12 wheel are reported; the magnetic investigations reveal that the electronic ground state has a spin S = 6, which corresponds to ferromagnetic interactions between the twelve Co(II) ions.

New routes to high nuclearity cages: dimerisation of a manganese triangle via solvothermal synthesis

Heating [Mn3O(O2CPh)6(py)2(H2O)] in MeCN at 100 degrees C for 12 hours in a sealed Teflon container produces the hexanuclear species [Mn6O2(O2CPh)12(py)2]: dimerisation of the triangle occurs via solvent loss and ligand rearrangement.

Magnetization tunneling in an enneanuclear manganese cage

The enneanuclear mixed-valent manganese cage [Mn 9 O 7 (OAc) 1 1 (thme)(py) 3 (H 2 O) 2 ] 1 {Mn 9 } possesses an S = 17/2 ground state as a result of an antiferromagnetic interaction between three ferromagnetically coupled Mn I V ions and a wheel of four Mn I I I and two Mn I I ions. AC magnetization measurements show a frequency-dependent out-of-phase signal at 3.4 K and 997 Hz, indicative of single-molecule magnetism behaviour. Data obtained from varying the frequency of oscillation of the AC field gives an effective energy barrier (U e f f ) for the reversal of magnetization of 27 K. Magnetic measurements of single crystals of 1 at low temperature show time- and temperature-dependent hysteresis loops which contain steps at regular intervals of field. DC and AC relaxation measurements show both the temperature-dependent and temperature-independent regions, the latter being definitive evidence of magnetization tunneling in the lowest energy zero-field split component of the ground state.

Muons as a probe of magnetism in molecule-based low dimensional magnets

We present the results of muon spin relaxation (μ+SR) studies on low dimensional molecular magnet systems. μ+SR measurements have been carried out on the Cu-based chain compounds CuX2(pyz) (where X = Br, Cl, NCS and pyz = pyrazine) as a function of temperature and applied longitudinal magnetic field. Oscillations in the time dependence of the muon polarization, characteristic of magnetic order at two distinct muon sites, are detected in both CuBr2(pyz) (below TN = 3.6(1) K) and CuCl2(pyz) (below TN = 3.2(2) K). No evidence of magnetic order is observed in Cu(NCS)2(pyz) down to 0.35 K. The results are discussed in terms of the estimated Cu–X–Cu and Cu–(pyz)–Cu exchange constants. The theory of μ+SR in high spin molecule (HSM) systems, which are effectively zero-dimensional magnets, is discussed and results are presented on [Ni12(chp)12(O2CMe)12(H2O)6(THF)6] (S = 12), [Mn9O7(OAc)11(thme)(py)3(H2O)2] (S = 17/2) and [Fe14(bta)6(O)6(OMe)18 Cl6] (). Measurements made in applied longitudinal magnetic fields on HSM materials at dilution refrigerator temperatures strongly suggest that dynamic local magnetic field fluctuations are responsible for the relaxation of the muon spin ensemble. Trends in temperature and field dependent behaviour in these systems, as probed by the muon, are discussed.

A new class of single-molecule magnet: (Mn9O7(OAc)11(thme)(py)3(H2O)2) with an S = 17/2 ground state

The reaction of [Mn3O(OAc)6(py)3] with 1,1,1-tris(hydroxymethyl)ethane (H3thme) gives the Mn(IV)3Mn(III)4Mn(II)2 complex [Mn9O7(OAc)11(thme)(py)3(H2O)2], which has an S = 17/2 ground state and displays strong out-of-phase signals in ac susceptibility studies that establish it as a new class of single-molecule magnet.

Dodecanuclear and octanuclear manganese rods

The reaction of the neutral triangle [Mn3O(O2CR)6(L)3] with 1,1,1-tris(hydroxymethyl)ethane (H3thme) affords novel dodecanuclear and octanuclear manganese complexes with unusual ladder-like cores built from edge-sharing triangles.