Session H13: Focus Session: Low-Dimensional and Molecular Magnetism - Magnetism and Transport of Isolated Molecular Magnets - Molecular spintronics

Manipulating Molecular Kondo Effect by Chemical Reactions

Motivated by spintronic applications, the control of Kondo effect arising from spin exchange interaction between isolated spins and conduction electrons of non-magnetic metals, has been explored. A method to control the molecular Kondo effect is demonstrated via chemical reactions. A spontaneous binding between molecules was exploited to control the molecular Kondo effect on Au(111). The Kondo effect was switched back on using local scanning tunneling microscope manipulation. This method relies on the hybridized pairing of unpaired spins two molecules, as supported by our density functional theory calculation results.   

Electronic readout of a single nuclear spin using a molecular spin transistor

Quantum control of individual spins in condensed matter devices is an emerging field with a wide range of applications ranging from nanospintronics to quantum computing [1,2]. The electron, with its spin and orbital degrees of freedom, is conventionally used as carrier of the quantum information in the devices proposed so far. However, electrons exhibit a strong coupling to the environment leading to reduced relaxation and coherence times. Indeed quantum coherence and stable entanglement of electron spins are extremely difficult to achieve. We propose a new approach using the nuclear spin of an individual metal atom embedded in a single-molecule magnet (SMM). In order to perform the readout of the nuclear spin, the quantum tunneling of the magnetization (QTM) of the magnetic moment of the SMM in a transitor-like set-up is electronically detected. Long spin lifetimes of an individual nuclear spin were observed and the relaxation characteristics were studied. The manipulation of the nuclear spin state of individual atoms embedded in magnetic molecules opens a completely new world, where quantum logic may be integrated.\\[4pt] [1] L. Bogani, W. Wernsdorfer, Nature Mat. 7, 179 (2008).\\[0pt] [2] M. Urdampilleta, S. Klyatskaya, J.P. Cleuziou, M. Ruben, W. Wernsdorfer, Nature Mat. 10, 502 (2011).   

DFT calculations of the charged states of N@C60 and {\{}Fe4{\}} single molecule magnets investigated in tunneling spectroscopy

For device applications of single molecule magnets (SMMs) in high-density information storage and quantum-state control it is essential that the magnetic properties of the molecules remain stable under the influence of metallic contacts or surface environment. Recent tunneling experiments [1, 2] on N@C60 and {\{}Fe4{\}} SMM have shown that these molecules preserve their magnetic characteristics when they are used as the central island of single-electron transistors. Although quantum spin models have been used extensively to study theoretically tunneling spectroscopy of SMMs, it has been shown recently that the orbital degrees of freedom, which is absent in spin models, can significantly affect the tunneling conductance [3]. In this work we present first-principles calculations of the neutral and charged states of N@C60 and {\{}Fe4{\}} SMMs, and discuss a strategy to include their properties into a theory of quantum transport. We also present results of the magnetic anisotropy for the different charge states of Fe4 and discuss their relevance for experiments [2] in the sequential tunneling and cotunnelling regimes. \\[4pt] [1]. N. Roch et al., Phys. Rev. B 83, 081407 (2011). \\[0pt] [2]. A.S. Zyazin et al., Nano Lett. 10, 3307 (2010). \\[0pt] [3]. L. Michalak et al., Phys. Rev. Lett. 104, 017202 (2010).   

Beller Lectureship: Single Molecular Magnets on Conductive Surfaces

For more than a decade molecules showing magnetic bistability, generally known as Single Molecule Magnets, have represented a playground to study quantum effects that appear in magnetism when the nanometer scale is attained. The field is now mature to investigate more complex nanostructures where the interplay between transport and magnetism at the molecular scale can be exploited. The first step along this direction requires to organize SMMs on surfaces rather than as bulk phases. Pursuing this apparently straight forward task has already encountered many difficulties because of the complex nature and fragility of SMMs and of the peculiar origin of their magnetic bistability. Also robust SMMs based on lanthanide ions and phthalocyanine loose most of their SMM properties when the crystalline phase is abandoned. Thanks to a collaboration with Prof. Cornia in Modena, Italy, and Prof. Sainctavit in Paris, France, we have carried out a very low temperature synchrotron investigation showing that a polynuclear SMM cluster, based on a propeller-like tetranuclear iron(III) core, Fe4, retains the typical hysteresis also when the molecules are chemically grafted to a gold surface. The tailoring of the anchoring ligand has allowed the control of the orientation of the molecules on the substrate and has given the possibility to observe the resonant quantum tunneling of the magnetization. Preliminary investigations on Fe4 SMMs thermally evaporated in UHV conditions on a conducting ferromagnetic oxide like Lanthanium Strontium Manganite, have shown an unprecedented phenomenon. While the reported studies of paramagnetic molecules on magnetic substrates have in general shown a sizeable magnetic interaction with the substrate but no evidences of SMM behavior, in our investigation the magnetic hysteresis of Fe4 exceeds in coercive field that of the substrate, recorded at the Mn L3 edge. More interestingly, the zero field step of the hysteresis, typical of quantum tunneling of magnetization that characterizes Fe4 SMMs, disappears when deposited on LSMO, opening the perspective of a novel hybrid magnetism at the nanoscale.   

Theoretical study of hysteresis in electron transport through spin-crossover molecules

Recent advances in nanoscale molecular systems stimulate experimental studies of electron transport across molecular junctions formed by single molecules or nanoparticles bridged between electrodes, or molecular monolayers adsorbed onto surfaces, using three-terminal set-ups or scanning tunneling microscope. Among them, spin-crossover molecular systems draw attention due to their unusual coupling between spin degrees of freedom and external stimuli. Spin magnetic moments of these molecular systems increase with increasing temperature or pressure, or shining light, and their magnetization shows hysteresis behavior with temperature, pressure, or light. Recent transport measurements across nanoparticles made of such spin-crossover molecules reveal hysteresis behavior in current-voltage characteristics, driven by voltage at a given temperature. In this talk, we present our work on understanding of hysteresis in electron transport through a nanoparticle consisting of Fe-based spin-crossover molecules, using a model-Hamiltonian approach and insight obtained from density functional theory.   

Individual Magnetic Molecules on Ultrathin Insulating Surfaces

Single molecule magnets have attracted ample interest because of their exciting magnetic and quantum properties. Recent studies have demonstrated that some of these molecules can be evaporated on surfaces without losing their magnetic properties [M. Mannini \textit{et al}., \textit{Nature} 468, 417, (2010)]. This remarkable progress enhances the chances of real world applications for these molecules. We present STM imaging and spectroscopy data on iron phthalocyanine molecules deposited on Cu(100) and on a Cu$_{2}$N ultrathin insulating surface. These molecules have been shown to display a large magnetic anisotropy on another thin insulating surface, oxidized Cu(110) [N. Tsukahara \textit{et al.}, \textit{Phys. Rev. Lett.} 102, 167203 (2009)]. By using a combination of elastic and inelastic electron tunnelling spectroscopy, we investigate the binding of the molecules to the surface and the impact that the surface has on their electronic and magnetic properties.   

Interfacial Electronic and Magnetic Coupling in Organic-metal System Studied by Scanning Tunneling Microscopy

Organic materials have drawn much attention in spintronics studies because of their tunable properties by functional groups and potential to achieve molecular magnets. An important factor influencing these properties is the interfacial effect. In organic-metal systems, different interfaces lead to strong modulation of electronic structures and even magnetic behaviors like spin coupling. In our study, Mn-phthalocyanine (MnPc) deposited on Cu(111) surface have been measured by scanning tunneling microscopy (STM) and spectroscopy (STS) at 4.5 K. With different deposition amount, MnPc are adsorbed as isolated molecules or in an ordered assembled structure. From STS curves, assembled MnPc possess a broadened state near the onset of Cu(111) surface state comparing to islolated ones. According to ab initio calculation, distance between the central Mn atom and the substrate in assembled molecules is reduced due to intermolecular interaction and affects the electronic structures. Magnetic behaviors of MnPc on ferromagnetic metal substrate are further investigated by spin-polarized STM (SP-STM). Spin contrast of isolated molecules on Co nanoislands on Cu(111) is found near the Fermi level in STS maps, which is considered to be ferromagnetic coupling between MnPc and Co islands.   

The Kondo effect in molecular magnets from first principles

When a magnetic molecule is deposited on a metallic substrate or attached to metallic contacts its magnetic moment may actually be screened by the conduction electrons due to the Kondo effect. In view of possible applications of molecular magnets such as nanoscale spintronics and magnetic storage devices, it is important to being able to predict whether the Kondo effect will take place or not in a given system. Also one would like to understand in detail how the Kondo effect emerges in a given situation and how it is controlled by the various parameters such as the molecular conformation and the type of substrate. Using a recently developed ab initio approach for molecular devices [1,2] that explicitly takes into account the strong electronic correlations that give rise to the Kondo effect, we have calculated the electronic structure and transport properties of different magnetic molecules coupled to nanocontacts [3] and surfaces [4]. Our calculations shed light on the complex nature of the Kondo effect in molecular-scale devices. [1] D. Jacob {\it et al.}, PRL {\bf 103}, 016803 (2009); [2] D. Jacob {\it et al.}, PRB {\bf 82}, 195115 (2010); [3] M. Karolak {\it et al.}, PRL {\bf 107}, 146604 (2011); [4] K. J. Franke {\it et al.}, Science {\bf 20} 940 (2011)   

Electronic transport in Co-based valence tautomeric conjugated polymers

Using first principle density functional theory (DFT) methods combined with maximally localized Wannier function (MLWF), real-space basis sets and a Green's function transport scheme within the Landauer ballistic transport regime, we investigated the electronic structures and electronic transport properties of a Co-based valence tautomeric (VT) conjugated backbone polymeric system. We found that GGA+U induced high-spin structure not in satisfactorily agreement with realistic circumstances from the computed Co projected density of states (PDOS). So we instead employed constrained magnetization calculations to induce the low-spin to high-spin magnetic transition computationally. Transport calculations showed that the high-spin structure is two orders of magnitude more conductive than the low-spin structure, thus supporting the vision that this kind of Co-based VT polymer can function as basis for switchable molecular spintronic devices. Finally, we will briefly discuss the chemisorption of this VT system on metallic substrates the spin transport properties of metal-molecule-metal configurations.   

Dynamical magnetic anisotropy in spin--1 molecular systems

We study electronic transport through a deformable spin-1 molecular system in a break junction setup, under the influence of a local vibrational mode. Our study shows that the magnetic anisotropy, which arises due to stretching along the transport axis[Science 328 1370 (2010)], is renormalized by the interactions with vibrations. The coupling induces additional spin--asymmetric hybridizations that contribute to the net molecular anisotropy. We show that the low temperature physics of such device can be described by an anisotropic Kondo model ($J_{\perp} > J_{\parallel}$), with a magnetic anisotropy term, $A_{Net}S_z^2$, negative at zero stretching. A quantum phase transition (QPT) is explored by stretching the molecule, driving $A_{Net}$ into positive values, and changing the character of the device from a non--Fermi--liquid (NFL) to a Fermi liquid (FL) ground state. This transition can be directly observed through the zero--bias conductance, which we find to be finite for negative anisotropy, zero for positive anisotropy, and to reach the unitary limit at $A_{Net} \approx 0$. At that point, an underscreened spin-1 Kondo ground state appears due to the restitution of the spin-1 triplet degeneracy.   

Hot electron spin transport in C$_{60}$ fullerene

Carbon-based molecular materials are interesting for spin transport application mainly due to their small sources of spin relaxation [1]. However, spin coherence lengths reported in many molecular films do not exceed a few tens of nanometers [2]. In this work we will present results showing how hot spin-polarized electrons injected well above the Fermi level in C$_{60}$ fullerene films travel coherently for hundreds of nanometers. We fabricated hot-electron vertical transistors, in which the current created across an Al/Al$_{2}$O$_{3}$ junction is polarized by a metallic Co/Cu/Py spin valve trilayer and subsequently injected in the molecular thin film. This geometry allows us to determine the energy level alignment at each interface between different materials. Moreover, the collector magnetocurrent excess 85{\%}, even for C$_{60}$ films thicknesses of 300 nm. We believe these results show the importance of hot spin-polarized electron injection and propagation in molecular materials. [1] V. Dediu, L.E. Hueso, I. Bergenti, C. Taliani, Nature Mater. 8, 707 (2009) [2] M. Gobbi, F. Golmar, R. Llopis, F. Casanova, L.E. Hueso, Adv. Mater. 23, 1609 (2011)   

A Copper Nitride Nanotemplate for Individual Magnetic Molecules

Molecular magnets hint at a promising future in information technology applications because of their interesting quantum and magnetic properties in the bulk. If these molecules are to be useful for device applications, it may be necessary to place them on surfaces, and work has begun to concentrate in this area. However, the practical issues of isolating and accessing these molecules have yet to be resolved. We present scanning tunnelling microscopy (STM) data on FePc and (DyPc$_{2}$) deposited on a (Cu$_{2}$N-Cu(100)) surface. (Cu$_{2}$N-Cu(100)) forms a quasi-periodic lattice, and has been shown to force porphyrin molecules to sit at specific sites (D. Ecija et al., Appl. Phys. Lett. 92, 223117 (2008)). As with the porphyrins, we find that these molecules sit at the intersection of the copper lines. This spatial separation restrains any potential dipolar or exchange interaction between the molecules, and it allows for individual, independent spins to be addressed. Using data taken with an STM we investigate the properties of these molecules, and furthermore show that the molecules on this surface are immobile up to room temperature.