Session Y23: Single Molecule Magnets

Pulsed-Radiation-Induced Magnetization Relaxation in Single-Molecule Magnets

Millimeter-wave radiation induces large dips in the magnetization of a single crystal of the Fe$_8$ single-molecule magnet (SMM) when the radiation is on resonance with transitions between energy levels. In our recent studies, we pulsed the radiation with the goal of determining T$_1$, the lifetime of the first excited state. We found that during a 0.2-ms pulse of intense radiation the spin system and the lattice are driven out of thermal equilibrium. Experiments at shorter time scales, carried out with the use of an inductive thin-film pick-up loop, revealed a surprisingly long relaxation time for magnetization on the order of $\sim$ 10 $\mu$s. A poor signal-to-noise (S/N) ratio required averaging of $\sim$ 4 $\times$ 10$^5$ individual traces to obtain acceptable data. Incorporating a superconducting interference device (SQUID) as a low-noise voltmeter into our experimental setup improves the S/N ratio, allowing us to explore the origin of the observed long relaxation time. The results of these experiments on Fe$_8$ as well as other SMMs will be presented.   

Control of quantum magnetic deflagration in Mn12 acetate.

Magnetic avalanches in Mn12-ac involve spin tunneling through an energy barrier which is controlled by the applied magnetic field. The reversal proceeds through the propagation of a narrow magnetic reversal front at constant velocity through the crystal [1]. In this contribution, we demonstrate that the ignition of the deflagration can be controlled in a deterministic way using surface acoustic waves (SAWs). For that purpose, the Mn12 crystal was mounted on the surface of a piezoelectric LiNbO3 substrate containing an interdigital transducer (IDT) for the excitation of SAWs. In the super-paramagnetic regime (above 3 K), the Mn12 magnetization shows pronounced changes when the IDT is excited at its resonant frequencies, thus proving that the crystals can be used as very sensitive acoustic detectors. At low temperatures (below 3 K), both the ignition rate and the velocity of the deflagration front present peaks for the values of the magnetic fields that bring the spin levels on both sides of the tunneling barrier into resonance, thus demonstrating the occurrence of both quantum magnetic ignition and quantum magnetic deflagration [2]. [1] Y. Suzuki et. al., Phys. Rev. Lett. 95, 147201 (2005). [2] A. Hern\'{a}ndez-M\'{\i}nguez et. al., Phys. Rev. Lett. 95, 217205 (2005).   

Pressure-dependence of the zero-field splittings for the Fe$_{8}$ single-molecule magnet

We present a study of the pressure-dependent electron paramagnetic resonance (EPR) spectrum for the Fe$_{8}$ single-molecule magnet (SMM). The biaxial [Fe$_{8}$O$_{2}$(OH)$_{12}$(tacn)$_{6}$]Br$_{8}\cdot $9H$_{2}$O (Fe$_{8})$ SMM has recently been studied extensively because its low-temperature magnetization dynamics are dominated by quantum tunneling of its spin S~=~10 magnetic moment through a sizeable anisotropy barrier. To date, chemical methods have usually been employed in order to control the magnetic quantum tunneling (MQT) behavior of a SMM, e.g. by varying the magnetic ions in the molecular core, or the ligand/solvent environment. The advantage of this approach is that many different SMMs can be realized in this way, with widely varying MQT behavior. However, controllable variation of MQT is difficult. As an alternative approach for manipulation of the MQT, we have recently studied the effect of physical pressure on the Fe$_{8}$ SMM. In this presentation, we show the pressure dependence of the zero-field splittings of Fe$_{8}$, as studied by an angle and pressure-dependent high-frequency EPR technique.   

Alignment of Mn$_{12}$-acetate in Suspension

The magnetization of Mn$_{12}$-acetate single molecule magnets has been studied in an oriented Mn$_{12}$-acetate suspension that, unlike in prior work [1, 2], exceeds the solution saturation. We observe magnetic properties of the frozen suspension similar to large oriented single crystals, specifically several sharp steps in the low temperature hysteresis loop, indicating alignment. The surface morphology of a film made from this suspension, which was studied by atomic force microscopy, indicates micron-size crystals are likely the main source of the magnetization signal. The greater the external magnetic field during alignment, the sharper the steps in the low temperature hysteresis loops. Experimental data showed that $\sim $5000 Oe was sufficient to orient the micro crystals in the organic solvent to a degree comparable to a single crystal. \newline [1] D. M. Seo et al., J. Mag. Magn. Mater. in press (2005), doi:10.1016/j.jmmm.2005.06.005 [2] K. Kim et al., Appl. Phys. Lett. \textbf{85,} 3872 (2004).   

V, C, and N soft x-ray absorption and MCD of molecular magnet V[TCNE]$_{x\sim 2}$ films

CVD films of V[TCNE]$_{x\sim 2}$ are magnetic room temperature and of interest as prototypes in functional organic magnetic systems. In addition to potential technological interest, fundamental questions regarding the electronic structure and spin distribution in V[TCNE]$_{x\sim 2}$ films remain, and motivate these measurements of x-ray absorption (XAS) and magnetic circular dichroism (MCD) spectra at the vanadium L, carbon K, and nitrogen K edges. XAS spectra reveal strong multiplet splitting at the V edge and strong $\pi \ast $ features at the C and N edges. The registry of these features at different edges indicates a distinct molecular orbital structure involving all constituents. That this molecular orbital structure supports magnetism is indicated by vanadium MCD spectrum and by the loss of specific XAS and MCD features for oxidized samples. Results are interpreted in the context of prior neutron scattering [1] and EXAFS [2] studies, multiplet calculations of V XAS and MCD, and established C and N XAS features. \newline [1] A. Zheludev, et al., J. Am. Chem. Soc. 116, 7243 (1994). \newline [2] D. Haskel, et al., Phys. Rev. B 70, 054422 (2004).   

Understanding the Gap in Polyoxovanadate Molecular-Based Magnets

We report a joint experimental and theoretical investigation of the transport gap, optical properties, and electronic structure of two chemically similar, inhomogeneously mixed-valent polyoxavanadate molecule-based magnets. We attribute the substantial gap in [NHEt$_3$]$_4$[V$_{8}$$^{IV}$V$_{4}$$^{V}$As$_8$O$_{40}$(H$_2 $O))]$^.$H$_2$O to weak $p$-$d$ hybridization and a large on-site Coulomb repulsion ($U$ = 5 eV). The reduced gap in [NHEt$_3$]$_3$[V$_{6}$$^{IV}$V$_{6}$$^{V}$As$_8$O$_{40}$(HCO$_2 $)]$\cdot$2H$_2$O is associated with a smaller value of $U$, at least from a molecular point of view, although the transport properties also reflect subtle organization of the molecular structure and the difference between direct and indirect intermolecular charge transfer. A detailed analysis of the vibrational response supports the important role of local molecular distortion and hydrogen bonding in the intramolecular and intermolecular charge transport in [NHEt$_3$]$_4$[V$_{8}$$^{IV}$V$_{4}$$^{V}$As$_8$O$_{40}$(H$_2 $O))]$^.$H$_2$O. This work is supported by PRF and the U.S. Department of Energy.   

High-frequency Infrared Studies of Manganese-based Single-molecule Magnets

High-resolution far-infrared transmission studies of Mn$_{12}$ single crystals (both aligned crystal assemblies and randomly oriented samples) have been carried out as a function of temperature and magnetic field over a wide frequency region (7 - 100 cm$^{-1}$). Several absorption lines corresponding to different transitions within the S = 10 manifold can be observed as a function of temperature. Our previous low frequency studies have shown that the sum of absorption coefficients of these absorption lines does not seem to conserve as a function of temperature. The new high- frequency measurements indicate that the oscillator strength is recovered at higher frequencies with the appearance of new absorption bands. The origin and the frequency dependence of these new absorption bands will be discussed.   

Field-dependent magnetic parameters in $\{$Ni$_4$Mo$_{12}$$\}$: Magnetostriction at the molecular level?

We present the optical and magneto-optical properties (0 - 32 T) of Mo$_{12}^V$O$_{30}$($\mu$$_2$-OH)$_{10}$H$_2$\{Ni$^{II}$(H$_2$O) $_3$\}$_4$, a magnetic molecule with antiferromagnetically coupled tetrahedral Ni$^{II}$ in a diamagnetic molybdenum matrix. A magnetochromic effect, centered at $\sim$1.9 eV, is observed at 4.2 K, and it is attributed to a change in the Ni $d \rightarrow d$ on-site excitation. The low-temperature magnetization exhibits steps at irregular field intervals, a result that cannot be explained using a Heisenberg model even if it is augmented by magnetic anisotropy and biquadratic terms. Field-dependent exchange parameter, however, provides the best fit to magnetization, suggesting that the molecular structure (and thus the interactions between spins) may be changing with applied magnetic field. The magneto-optical response of Mo$_{12}^V$O$_{30}$($\mu$$_2$-OH)$_{10}$H$_2$\{Ni$^{II}$(H$_2$O) $_3$\}$_4$ supports a small change in the NiO$_6$ coordination geometry and the associated electronic single-ion properties.   

Numerical Analysis of the EPR Spectrum of a Ni$_{4}$ Single-Molecule Magnet through Direct Diagonalization of the Four-Spin Hamiltonian

EPR studies have established the Giant Spin (GS) Hamiltonian parameters, $D$, $B_{4}^{0}$ and $B_{4}^{4}$, for members of the [Ni(hmp)(ROH)X]$_{4}$ (R = Me, Et, etc., and X = Cl and Br) family of single-molecule magnets.$^{1}$ Four $S$ = 1 Ni$^{II}$ ions, aligned on corners of a cubic core, couple ferromagnetically creating a spin $S$~=~4 ground state. Experiments on an isostructural Ni/Zn alloy established single-ion $d_{i}$ and $e_{i}$ parameters, as well as the orientations of the local magnetic axes.$^{1}$ A numerical model utilizing matrix diagonalization has simulated EPR spectra for the coupled $S$~=~1 Ni$^{II}$ ions using parameters from the Ni/Zn studies. Fourth order anisotropy parameters in the giant spin model arise from the isotropic Heisenberg coupling, \textit{JS}$_{1}$.$S_{2}$, and quadratic single-ion anisotropy in the four-spin Hamiltonian. Heisenberg coupling causes higher energy states to influence the $S$ = 4 ground state addressed in the GS model. Matching the lowest nine energies of the four-spin model to those of the GS model allows direct spectroscopic determination of $J$. $^{1}$E.-C. Yang et al., Inorg. Chem. \textbf{44}, 3827-3836 (2005).   

Magnetic Quantum Tunneling in a Mn$_{12}$ Single-Molecule Magnet Measured With High Frequency Electron Paramagnetic Resonance

The low temperature spin dynamics of the single-molecule magnet [Mn$_{12}$O$_{12}$(CH$_{3}$COOH)$_{16}$(H$_{2}$O)$_{4}$]$\cdot $ 2CH$_{3}$COOH$\cdot $4H$_{2}$O, were studied using High Frequency Electron Paramagnetic Resonance (HFEPR) in order to demonstrate magnetic quantum tunneling between resonant spin projection states. We prepare the spins such that they populate only one side of the axial potential energy barrier and, using a magnetic field, we cause tunneling of the magnetic moment between resonant spin projection states. We then use HFEPR to monitor the populations on each side of the potential energy barrier. We show that, in addition to measuring the ensemble average of the relaxation, these HFEPR experiments demonstrate that one can separately monitor the relaxation from different parts of the inhomogeneous distribution of spin environments. This technique, therefore, provides an alternative method for performing hole-digging experiments for measuring spin relaxation dynamics.   

Acoustomagnetic pulse experiments in LiNbO3/Mn12 hybrids

We report here that single crystals of molecular magnets like Mn12 and Fe8 mounted on the surface of piezoelectric LiNbO3 can be used as very sensitive detectors for surface acoustic waves (SAW). These SAWs are generated by sending microwave pulses in the time range between 1 $\mu $s and several tens of ms to an interdigital transducer mounted on the LiNbO3. Our experiments were carried at low temperatures and in the presence of external magnetic fields, and the analysis of the magnetization variations was done using an SQUID magnetometer with time resolution of 1 $\mu $s that allows us to study the quantum properties of these nanomagnets. The results obtained show that the variation of magnetization depends: 1) on the intensity and quality factor of the SAW at the different frequencies, 2) on whether the temperature of the experiment is above or below the magnetic blocking temperature of the molecular clusters. Furthermore, the heat released by the scattering of the coherent phonons forming the SAW contributes to the generation of the so-called magnetic avalanches. J.M. Hernandez et. al., App. Phys. Lett., (accepted).   

Magnetism in Metal Clusters

We have measured the magnetic moments of clusters of Cobalt, Niobium, and Chromium, ranging in size from $<$20 to 200 atoms. Improvements to our cluster source have allowed us to produce smaller and colder ($\sim $60K) clusters than we were able to study in previous work and to study each cluster size individually. We will present measured values for the magnetic moments of these clusters as functions of size, temperature, and applied field. We have also investigated superparamagnetic behaviors of these clusters, looking for deviations from that behavior. This presentation is based upon work supported by the National Science Foundation under Grant No. DMR-0405203.   

STM differential conductance of a pair of magnetic adatoms

Competition between screening of local moments through the Kondo effect and magnetic ordering of those moments through direct or induced (RKKY) exchange interactions is a key feature of heavy-fermion systems and of metals containing dilute magnetic impurities. It should be possible to probe this competition in its purest form through scanning tunneling microscopy (STM) studies of pairs of magnetic adatoms on metallic surfaces. However, in order to interpret the STM differential conductance through a pair of nearby adatoms, it is necessary to understand the effects of Fano-like interference between different tunneling paths from the STM tip into the substrate. We report preliminary results of numerical renormalization-group calculations of the impurity spectral function and the differential tunneling conductance for the two-impurity Anderson model, and compare our results with those for the well-studied one-impurity case.   

Electronic Structure of Magnetic Titanocene Dimer Molecules at a Metal Surface Measured by Scanning Tunneling Microscopy

The titanocene dimer ([Cp$_{2}$TiCl]$_{2}$, where Cp = C$_{5}$H$_{5}$ ) is an interesting magnetic molecule because it incorporates two spin-1/2 Ti atoms in an antiferromagnetic configuration. We have used cryogenic scanning tunneling microscopy to study the local electronic properties of titanocene dimer molecules adsorbed onto metal surfaces. Ordered patterns of titanocene dimmers have been observed for submonolayer coverage on Au(111). Scanning tunneling spectroscopy of the molecules shows sharp features near the Fermi energy that may be magnetic in origin.