Session B21: Single Molecule Magnets

Decoherence mechanisms in Mn3 single-molecule magnet

In spite of wide interest in the quantum nature of SMMs, decoherence effects that ultimately limit such behavior have yet to be fully understood. Recent investigations have shown that there are three main decoherence mechanisms present in SMMs: spins can couple locally (i) to phonons (phonon decoherence); (ii) to many nuclear spins (nuclear decoherence); and (iii) to each other via dipolar interactions (dipolar decoherence)[1]. We have recently uncovered quantum coherence in a Mn3 SMM by quenching decoherence due to dipole interaction between SMMs using a high frequency electron paramagnetic resonance and low temperature [2]. In this presentation, we will discuss temperature dependence of spin relaxation times and the decoherence mechanisms in the Mn3 SMM. [1] S. Takahashi et al., Nature 476, 76 (2011). [2] C. Abeywardana et al. (2015), submitted.   

Controlling electronic access to the spin excitations of a single molecule in a tunnel junction

Spintronic phenomena can be utilized to create new devices with applications in data storage and sensing. Scaling these down to the single molecule level requires controlling the properties of the current-carrying orbitals to enable access to spin states through phenomena such as inelastic electron tunneling. Here we show that the spintronic properties of a tunnel junction containing a single molecule can be controlled by their coupling to the local environment. For tunneling through iron phthalocyanine (FePc) on an insulating copper nitride (Cu$_2$N) monolayer above Cu(001), we find that spin transitions may be strongly excited depending on the binding site of the central Fe atom. Different interactions between the Fe and the underlying Cu or N atoms shift the Fe d-orbitals with respect to the Fermi energy, and control the relative strength of the spin excitations, an effect that can described in a simple co-tunneling model. This work demonstrates the importance of the atomic-scale environment in the development of single molecule spintronic devices.   

Inelastic Neutron Scattering and Magnetisation Investigation of an Exchange-Coupled Dy2 SMM

The strong spin orbit coupling and weak crystal field energies of simple exchange-coupled rare earth SMMs makes the precise evaluation of their magnetic properties nontrivial. Here we report a detailed investigation of the single molecule magnet hqH$_2$Dy$_2$(hq)$_4$(NO$_3$)$_3$MeOH. Inelastic neutron scattering is used to obtain direct access to several low energy crystal field excitations. The INS results display several features that are not found in earlier FIR absorption experiments [1], while other features found in the latter are absent. Based on the effective point charge model, numerical calculations are currently underway to resolve these apparent discrepancies using complementary magnetisation measurements to resolve the exchange between Dy ions. [1] E. M. Pineda et al. Nat. Commun. 5, 5243 (2014).   

Mechanisms of relaxation and spin decoherence in nanomagnets

Relaxation in spin systems is of great interest with respect to various possible applications like quantum information processing and storage, spintronics, and dynamic nuclear polarization (DNP). The implementation of high frequencies and fields is crucial in the study of systems with large zero-field splitting or large interactions, as for example molecular magnets and low dimensional magnetic materials. Here we will focus on the implementation of pulsed Electron Paramagnetic Resonance (ERP) at multiple frequencies of 10, 95, 120, 240, and 336 GHz, and the relaxation and decoherence processes as a function of magnetic field and temperature. Firstly, at higher frequencies the direct single-phonon spin-lattice relaxation (SLR) is considerably enhanced, and will more often than not be the dominant relaxation mechanism at low temperatures, and can be much faster than at lower fields and frequencies. In principle the measurement of the SLR rates as a function of the frequency provides a means to map the phonon density of states. Secondly, the high electron spin polarization at high fields has a strong influence on the spin fluctuations in relatively concentrated spin systems, and the contribution of the electron-electron dipolar interactions to the coherence rate can be partially quenched at low temperatures[1]. This not only allows the study of relatively concentrated spin systems by pulsed EPR (as for example magnetic nanoparticles and molecular magnets), it enables the separation of the contribution of the fluctuations of the electron spin system from other decoherence mechanisms. Besides choice of temperature and field, several strategies in sample design, pulse sequences, or clock transitions can be employed to extend the coherence time in nanomagnets. A review will be given of the decoherence mechanisms with an attempt at a quantitative comparison of experimental rates with theory. [1] Takahashi, S.; Hanson, R.; van Tol, J.; Sherwin, M.S. and Awschalom, D.D. \textit{Phys. Rev. Lett.,~}101, 047601 (2008)   

Evaluation of the exchange interaction and crystal fields in a prototype Dy$_{2}$ SMM

In order to gain an understanding of the INS and magnetization data obtained for Dy$_{2}$, the simplest member of a newly synthesized family of dysprosium-based molecular magnets [1], we report on calculations of the magnetic behavior of a Dy2 cluster with the formula [hqH$_{2}$][Dy$_{2}$(hq)$_{4}$(NO$_{3}$)$_{3}$]·MeOH. The molecular complex contains one high symmetry Dy(III) ion and one low symmetry Dy(III) ion. Our calculations suggest that exchange coupling between the two ions controls the behavior of the magnetization at low temperature, while the crystal field of the low symmetry Dy(III) ion controls the behavior at higher temperature. A point charge electrostatic model, based on crystallographic coordinates, provides a starting point for the determination of the crystal field [2]. Parameters in these calculations are adjusted to provide best fits to inelastic neutron scattering data (INS) and low temperature magnetometry [3]: the INS measurements access crystal field energies and low temperature magnetization probes the Dy-Dy exchange interaction. [1] E. M. Pineda, et al. Nat. Commun. 5, 5243 (2014). [2] J.J. Baldoví, et al J. Comput. Chem. 34 (22), 1961-1967, 2013. [3] N. F. Chilton, et al. J. Comput. Chem. 34, 1164-1175 (2013).   

The first single atom magnet

The prime feature of a magnet is to retain a significant fraction of its saturation magnetization in the absence of an external magnetic field. Realizing magnetic remanence in a single atom would allow storing and processing information in the smallest unit of matter. Here we show that individual rare-earth atoms on ultrathin insulating layers grown on non-magnetic metal substrates exhibit magnetic remanence and, therefore, are the first magnets formed by a single surface adsorbed atom. These magnets have a magnetic lifetime of 1500 s and a coercive field of 3.7 T at 10 K. In addition, their hysteresis loop remains open up to 30 K. This first example of a single atom magnet shows bistability at a temperature which is significantly higher than the best single molecule magnets reported so far. Its extraordinary stability is achieved by a suitable combination of magnetic ground state and adsorption site symmetry, and by decoupling the $4f$ spin from the underlying metal by a tunnel barrier.   

Giant exchange interaction in mixed lanthanides

Combining strong magnetic anisotropy with strong exchange interaction is a long standing goal in the design of quantum magnets. The lanthanide complexes, while exhibiting a very strong ionic anisotropy, usually display a weak exchange coupling, amounting to only few wavenumbers. Recently, an isostructural series of mixed Ln-R-Ln complexes with R the N$_2^{3-}$ radical have been reported, in which the exchange splitting is estimated to reach hundreds wavenumbers [1,2]. Here we apply a new methodology allowing to establish on the basis of DFT and {\it ab initio} calculations the microscopic mechanism governing the unusual exchange interaction in these compounds [3]. We find it to be basically kinetic and highly complex, involving non-negligible contributions up to seventh power of total momentum $\hat{\mathbf{J}}$ of each Ln site. The performed analysis also elucidates the origin of magnetization blocking in these compounds. Contrary to general expectations the latter is not always favored by strong exchange interaction. [1] J. D. Rinehart, M. Fang, W. J. Evans, and J. R. Long, Nat. Chem. {\bf 3}, 538 (2011). [2] J. D. Rinehart, M. Fang, W. J. Evans, and J. R. Long, J. Am. Chem. Soc. {\bf 133}, 14236 (2011). [3] V. Vieru, N. Iwahara, L. Ungur, and L. F. Chibotaru, arXiv:1509.02206.   

Visualizing Improved Spin Coupling in Large Magnetic Molecules

In an attempt to combine a high spin ground state and a large magnetic anisotropy in one molecule, triplesalen-based complexes are promising building blocks for a new generation of single molecule magnets (SMMs). The spin coupling in these molecules is based on the spin polarization effect, which requires a delocalized aromatic $\pi$-system in the central carbon ring of the complex. Unfortunately, chemical analysis indicates that this ring can change its configuration to [6]radialene, therefore causing a loss of aromaticity and weakening the magnetic coupling. We have employed a combination of scanning tunneling microscopy (STM) and spectroscopy (STS) to investigate single Cu$_3$-triplesalen and Cu$_3$-triplesalalen molecules, the latter being designed to show an enhanced intramolecular spin coupling. The large molecules were deposited in situ using the unconventional techniques pulse injection and rapid heating. A thorough structural and spectroscopic analysis allows us to discuss the electronic properties of the two complexes, with a special focus on the state of the central carbon ring. We find that even small changes in the ligand structure have a drastic influence on the intramolecular spin coupling, which opens the way for an improved rational design of future SMMs.   

Exchange coupling and anisotropy effects on the low temperature magnetization dynamics in rare-earth dioxolene complexes

The prelude of relevant magnetic coupling in $f$-element based complexes is actively pursued to improve the single-molecule magnetic features. However, a quantitative analysis of magnetic properties of exchange-coupled anisotropic rare-earth based complexes is often hampered owing to the comparable magnitude of the crystal field with the magnetic coupling. In this study, we investigated the properties of complexes containing different ligands with comparable molecular structures and ligand field strengths. Comparative low-temperature magnetic and EPR study of homologous Ln$^{\mathrm{III}}$Semiquinonate (LnSQ) and Ln$^{\mathrm{III}}$Tropolonate (LnTrp) complexes, where Ln $=$ Dy, Tb is investigated. Single-crystal EPR revealed that the direct exchange coupling in DySQ resulted in a highly anisotropic pseudo-triplet state. An out-of-phase susceptibility signal was observed for TbTrp only in the presence of an external magnetic field. Furthermore, the dynamics revealed slow relaxation of magnetization in the DySQ at low temperature which upon comparative study with the dynamics of the related DyTrp revealed a not so simple dependence on the crystal field effects of the coordination sphere of the lanthanide.   

Anomalous power dependence in the zero-field resonance for the molecular nanomagnet Cr$_7$Mn

We report electron-spin resonance studies of the paramagnetic ring [(CH$_3$)$_2$NH$_2$][Cr$_7$MnF$_8$((CH$_3$)$_3$CCOO)$_{16}$] ("Cr$_7$Mn"), a spin S=1 molecular nanomagnet with a large zero-field ground-state tunnel splitting of $\sim$4 GHz. We perform parallel-mode electron-spin-resonance (ESR) spectroscopy with loop-gap resonators (LGRs) with resonance frequencies of ~4-6 GHz. A crystal of Cr$_7$Mn is placed on the loop of the LGR with the sample's easy axis parallel to the field. We observe an ESR peak at zero dc field. With increasing radiation power, a pronounced dip develops in the center of the resonance peak, indicating a decoupling of the sample from the resonator with increased power. The onset of this decoupling depends on both the temperature and the applied power, with greater power required to observe the dip at higher temperatures. By pulsing the radiation, we can rule out that the dip is related to sample heating or saturation of the resonance. Power, temperature, and frequency dependence of the decoupling will be presented, and possible explanations will be discussed.   

Time-resolved Measurements of Spontaneous Magnetic Deflagration of \boldmath $\rm {Mn_{12}tBuAc}$

Magnetic deflagration in molecular magnets has been triggered by heat pulses [1,2] and acoustic waves [3,4]. In this work we report spontaneous magnetic deflagration (i.e. deflagration that occurs without an external trigger) in the axially symmetric single molecule magnet $\rm Mn_{12} tBuAc$. Magnetic hysteresis measurements show steps due to resonant quantum tunneling (RQT) below 1K, confirming the spin-Hamiltonian parameters for this material and previous results. Deflagration speeds measured with a newly constructed higher bandwidth (2MHz) setup will be presented as a function of transverse and longitudinal fields $\rm {H_x} \otimes {H_z}$ both on and off resonance. A large increase in front velocity near RQT steps is observed in experiments with swept transverse fields and will be discussed in light of models of deflagration. \\[0pt][1] S. McHugh {\it et al.} PRB {\bf 76}, 172410 (2007); [2] P. Subedi {\it et al.}, PRL {\bf 110}, 207203 (2013). [3] A. Hern\'andez-M\'inguez {\it et al.}, PRL {\bf 95}, 217205 (2005).   

ELECTRO-NUCLEAR CLOCK TRANSITIONS IN A Ho(III) MOLEULAR NANOMAGNET

One of the challenges in the field of quantum information processing involves protecting qubits against decoherence. The primary source of decoherence in spin qubits at low temperatures is the dipolar interaction, which can be minimized using so-called clock transitions [1]. Here, we report pulsed EPR studies of the Holmium Polyoxometalate, $[Na]_{9}[Ho_{x}Y_{1-x}(W_{5}O_{18})_{2}]$, where we observe electro-nuclear clock transitions that involve coupled dynamics of the electron and nuclear spins ($\Delta m_J=\pm8$ and $\Delta m_I=\pm1$). These transitions are formally forbidden in EPR. However, the symmetry of this molecule generates admixtures of the ground doublet ($m_J=\pm4$) through second order perturbation, and application of a transverse magnetic field mixes $m_{I}$ and $m_{I}\pm1$ states, allowing such transitions to occur in the vicinity of avoided level crossings. Pulsed EPR measurements on an x = 0.1 sample, were carried out at a temperature of 5 K at X-band. These experiments suggest an enhancement in the coherence time at these electro-nuclear clock transitions which is significant for applications in hybrid magnetic qubits, where manipulation of the nuclear spin is controlled by EPR pulses. [1] G. Wolfowicz, et al., Nature Nanotechnology 8, 561 (2013).   

A Crystal Field Approach to Orbitally Degenerate SMMs: Beyond the Spin-Only Hamiltonian

Single-Molecule Magnets (SMMs) with large magnetization reversal barriers are promising candidates for high-density information storage. Recently, a large uniaxial magnetic anisotropy was observed for a mononuclear trigonal bipyramidal (TBP) [Ni$^{\mathrm{II}}$Cl$_{\mathrm{3}}$(Me-abco)$_{\mathrm{2}}$] SMM [1]. High-field EPR studies analyzed on the basis of a spin-only Hamiltonian give \textbrokenbar D\textbrokenbar \textgreater 400 cm$^{\mathrm{-1}}$, which is close to the spin-orbit coupling parameter $\lambda =$ 668 cm$^{\mathrm{-1\thinspace \thinspace }}$for Ni$^{\mathrm{II}}$, suggesting an orbitally degenerate ground state. The spin-only description is ineffective in this limit, necessitating the development of a model that includes the orbital moment. Here we describe a phenomenological approach that takes into account a full description of crystal field, electron-electron repulsion and spin-orbit coupling effects on the ground state of a Ni$^{\mathrm{II}}$ ion in a TBP coordination geometry. The model is in good agreement with the high-field EPR experiments, validating its use for spectroscopic studies of orbitally degenerate molecular nanomagnets. [1] K. E. Marriott et al., Chem Sci (published Online)