Mn12 Single-molecule Magnets Research Articles

Interaction of Langmuir-Blodgett films of Mn12 single molecule magnets with superconducting micro-tracks and nano-SQUIDs.

Molecular magnets with large spin moments are promising spintronic materials. In this report we study the feasibility of integrating these molecules into the field of superconducting spintronics which essentially deals with the mutual interactions of magnetic and superconducting systems. In this regard we have done two separate experiments using the widely studied single molecule magnet (SMM) Mn12-ac. By performing transport measurements on thin superconducting micro-tracks of Nb coated with a Langmuir-Blodgett film of the Mn12-ac SMM, we show that the SMM film significantly enhances the vortex activation energy near the transition temperature. The SMM can, therefore, help tuning the operating conditions of superconducting transition edge sensors. In a separate experiment, a Langmuir-Blodgett film of the SMM was grown onto a superconducting Nb nano-SQUID to look for local changes in magnetization arising from the magnetization tunneling phenomenon in the SMM. We observe random jumps in the voltage across the nano-SQUID corresponding to changes in the magnetization state of the SMM near the SQUID loops, which were not observed in the nano-SQUID without the SMM. These experiments show that the large spin moment and the discrete relaxation of magnetization in molecular magnets can be utilized to generate measurable signals in superconducting spintronic devices.

Electronegative ligands enhance charge transfer to Mn12 single-molecule magnets deposited on graphene

Combining single-molecule magnets (SMMs) and emergent two-dimensional substrates such as graphene may lead to device configurations that are promising for spintronics and quantum computing. However, to fully exploit the unique features of SMMs anchored to two-dimensional substrates, the choice of ligand attachments, which could affect the magnetic and electronic properties, is critical. In this work, we focus on hybrid junctions comprising CVD-grown graphene and [Mn12O12(O2CR)16(H2O)4](R=CH3,CHCl2) SMMs with different ligands. We find that [Mn12O12(O2CCH3)16(H2O)4] SMMs barely change the graphene’s conductivity, while [Mn12O12(O2CCHCl2)16(H2O)4] SMMs with more electronegative ligands, by means of charge transfer, remarkably modify the electronic transport in graphene as revealed by gate-voltage dependent magnetotransport measurements.

Berry-phase blockade and oscillations in single-molecule magnets Mn12 and Fe8 coupled to polarized leads

We have studied the Berry-phase interference in single-molecule magnets [Formula: see text] and [Formula: see text] coupled to polarized electrodes in the Coulomb blockade regime under external magnetic field. We show that for the oppositely fully polarized leads, the current is completely blockaded as a consequence of the topologically quenched tunnel splitting, which is called as Berry-phase blockade; whereas for the partially polarized leads, the current oscillates periodically with increasing external transverse magnetic field (Berry-phase oscillations). The reason is that different quantum spin tunneling trajectories can combine and give rise to either constructive or destructive interference effects.

Spin transport properties of single molecule magnet Mn(dmit)2 devices with phosphorene electrodes

Multiple-effect molecular devices have been constructed by single molecule magnet Mn(dmit)2 and phosphorene electrodes. First-principle quantum transport calculations show that the devices present excellent spin-filtering, spin-switching and negative differential resistance effects, which can be explained by the frontier molecular orbitals, the projected density of states and evolution of the transmission spectra. These excellent spin transport properties are better than the performance of the devices constructed by Au or graphene electrodes, which suggest that the one-dimensional devices with phosphorene electrodes are promising candidates for future applications of phosphorene-based multi-functional spintronics devices.

The Separation of the Mn12 Single-Molecule Magnets onto Spherical Silica Nanoparticles.

The Mn12 single-molecule magnets (SMMs) could be attached to the surface of spherical silica for the first time with a high probability. This allowed separation of the individual molecular magnets and direct microscopic observation of the SMMs. We described in detail how to fabricate such a composite material. The synthesis procedure proposed here is simple and efficient. We confirmed the efficiency of the method by transmission electron microscopy (TEM): single-molecule magnets were visible at the surface of a silica substrate. Based on TEM observation, we described how the molecules anchor to the surface of silica (the geometry of the magnetic molecule in regard to the surface of the substrate). The SQUID magnetometry showed that single-molecule magnet behaviour is kept intact after grafting. The attachment of the single-molecule magnets to the surface of silica allows to investigate their properties as separate molecules. This is particularly important in the analysis of magnetic properties such as magnetic states of the separated SMMs, their mutual interactions, and the influence of a silica support.

Unravelling the Spin Dynamics of Molecular Nanomagnets with Four‐Dimensional Inelastic Neutron Scattering

Molecular nanomagnets (MNMs) have attracted the attention of the scientific community as the rich physics behind their magnetic behavior make them ideal test‐beds for fundamental concepts in quantum mechanics. Sophisticated experiments and targeted research activities have also unveiled their potential for several technological applications. Inelastic neutron scattering is a powerful and widely used technique to investigate the properties of these systems. The new generation of spectrometers, equipped with arrays of position‐sensitive detectors, enable to efficiently measure the neutron cross‐sections as a function of energy and of the three components of the momentum transfer vector Q, in vast portions of the reciprocal space. Exploiting these capabilities together with the availability of sufficiently large single‐crystal samples of MNMs, it is now possible to obtain an unprecedented insight into the coherent spin dynamics of these molecular clusters. This is witnessed by several recent results that we present in this review. By using the benchmark system Cr8, it has been demonstrated that the richness of the four‐dimensional inelastic neutron scattering technique enables to extract dynamical correlation functions directly from the data. This technique has also been applied to the archetypical single‐molecule magnet Mn12 to unambiguously characterize its Spin Hamiltonian as well as to portray the entanglement between molecular qubits in (Cr7Ni)2.

Efficient spin-filtering, magnetoresistance and negative differential resistance effects of a one-dimensional single-molecule magnet Mn(dmit)2-based device with graphene nanoribbon electrodes

We present first-principle spin-dependent quantum transport calculations in a molecular device constructed by one single-molecule magnet Mn(dmit)2 and two graphene nanoribbon electrodes. Our results show that the device could generate perfect spin-filtering performance in a certain bias range both in the parallel configuration (PC) and the antiparallel configuration (APC). At the same time, a magnetoresistance effect, up to a high value of 103%, can be realized. Moreover, visible negative differential resistance phenomenon is obtained for the spin-up current of the PC. These results suggest that our one-dimensional molecular device is a promising candidate for multi-functional spintronics devices.

Controlled Dimerization of Mn12 Single-Molecule Magnets.

Controlled dimerization of Mn12 single-molecule magnets (SMMs) was achieved via a synthetic route involving a competition between bridging and terminal ligands, namely, diols and alcohols. The reaction using a 1:1 ratio of the competing ligands resulted in the isolation of a new family of covalently linked dimers of Mn12 SMMs. This is the first step toward the controlled growth of SMM oligomeric arrays.

Three-Dimensional (3-D) Ferromagnetic Network of Mn12 Single-Molecule Magnets: Subtle Environmental Effects and Switching to Antiferromagnetic.

A new member of the Mn12 family of single-molecule magnets (SMMs) has been prepared and found to be the first of this family to give a 3-D ferromagnetic network. [Mn12O12(O2CC6H4-p-F)16(H2O)4] (2) was prepared by carboxylate substitution on the acetate derivative with p-F-benzoic acid and crystallizes as 2·8MeCN in space group I4̅2m with extensive formation of intermolecular C-H···F hydrogen-bonding. The latter leads to a combination of ferromagnetic (F) and antiferromagnetic (AF) interactions and an overall F network that gives a χMT value at low T that is abnormally high for an S = 10 ground state. 2·8MeCN undergoes solvent loss under vacuum to 2, with a decrease in unit-cell volume of 17%, primarily due to a 13% decrease in the c-axis. The χMT vs T plot for 2 indicates a switch to a net AF network. Exposure to air causes hydration to 2·3H2O, a concomitant increase in unit cell volume, and a switch back to a F network. The same conversion of 2·8MeCN to 2·3H2O can also be accomplished in one step rather than two steps, by leaving crystals of the former exposed to air at ambient temperature and pressure for 10 days, giving the same magnetic plots. Interestingly, the desolvation/solvation processes cause Jahn-Teller isomerism to occur, but the ratio of the faster-relaxing isomer to the normal slowly relaxing one does not change monotonically. Single-crystal micro-SQUID studies on 2·8MeCN show the expected magnetization hysteresis loops for a SMM and a small exchange-bias from the intermolecular interactions that is unexpectedly AF. Since the micro-SQUID study only identifies interactions along the easy-axis (z-axis) of the crystal, this is readily rationalized as due to the Jz components of the intermolecular interactions in 2·8MeCN being net AF. The combined results offer useful insights into the degree of sensitivity of the magnetic properties to small environmental perturbations.

Systematic Investigation of Controlled Nanostructuring of Mn12 Single-Molecule Magnets Templated by Metal-Organic Frameworks.

This is the first systematic study exploring metal-organic frameworks (MOFs) as platforms for the controlled nanostructuring of molecular magnets. We report the incorporation of seven single-molecule magnets (SMMs) of general composition [Mn12O12(O2CR)16(OH2)4], with R = CF3 (1), (CH3)CCH2 (2), CH2Cl (3), CH2Br (4), CHCl2 (5), CH2But (6), and C6H5 (7), into the hexagonal channel pores of a mesoporous MOF host. The resulting nanostructured composites combine the key SMM properties with the functional properties of the MOF. Synchrotron-based powder diffraction with difference envelope density analysis, physisorption analysis (surface area and pore size distribution), and thermal analyses reveal that the well-ordered hexagonal structure of the host framework is preserved, and magnetic measurements indicate that slow relaxation of the magnetization, characteristic of the corresponding Mn12 derivative guests, occurs inside the MOF pores. Structural host-guest correlations including the bulkiness and polarity of peripheral SMM ligands are discussed as fundamental parameters influencing the global SMM@MOF loading capacities. These results demonstrate that employing MOFs as platforms for the nanostructuration of SMMs is not limited to a particular host-guest system but potentially applicable to a multitude of other molecular magnets. Such fundamental findings will assist in paving the way for the development of novel advanced spintronic devices.

Comparison of magnetic relaxation between Mn12-Ac and Mn12-tBuAc single-molecule magnets

The magnetic relaxation properties of two derivatives of Mn12 single-molecule magnets (SMMs), Mn12-Ac and Mn12-tBuAc, are investigated and compared in this paper. By a simplified “hole digging” method, the hyperfine fields turn out to be very similar in these two compounds (39 ± 5 mT for Mn12-Ac at −3.55 T and 32 ± 5 mT for Mn12-tBuAc at −3.2 T). The small difference of hyperfine fields received by the same magnetic cores Mn12O12 in these two SMMs shows that the hyperfine field is not the main source of the large difference of magnetic relaxation rates between Mn12-Ac and Mn12-tBuAc. The intentional inclined-field experiments show that a larger transverse component of the dipolar field in Mn12-Ac causes a faster magnetic relaxation rate only when the transverse component of the external applied field Hx exceeds 0.5 T. On the other hand, a larger longitudinal component of the dipolar field pushes more molecules out of tunneling, then decreases the relaxation rate. Therefore, the faster magnetic relaxation rates in Mn12-tBuAc result from its smaller . Furthermore, it is found that the hereditable characteristic of the “hole digging” effect in these two SMMs is relative to the dipolar field distribution.

A stochastic model for magnetic dynamics in single-molecule magnets

Hysteresis and magnetic relaxation curves were performed on double well potential systems with quantum tunneling possibility via stochastic simulations. Simulation results are compared with experimental ones using the Mn12 single-molecule magnet, allowing us to introduce time dependence in the model. Despite being a simple simulation model, it adequately reproduces the phenomenology of a thermally activated quantum tunneling and can be extended to other systems with different parameters. Assuming competition between the reversal modes, thermal (over) and tunneling (across) the anisotropy barrier, a separation of classical and quantum contributions to relaxation time can be obtained.

Rectifying, giant magnetoresistance, spin-filtering, newgative differential resistance, and switching effects in single-molecule magnet Mn(dmit)2-based molecular device with graphene nanoribbon electrodes

By applying the density functional theory and the nonequilibrium Green’s function formalism, we investigate the spin transport properties of a single-molecule magnet Mn(dmit)2 sandwiched between two ferromagnetic zigzag-edge graphene nanoribbon electrodes. The results show that the system can present large rectifying, giant magnetoresistance, spin-filtering and negative differential resistance effects with the help of magnetic field modulation. Moreover, an improved switching effect can also be realized by changing the orientation between planes of two dmit ligands. Therefore, the system will provide the possibilities for a multifunctional molecular device design.

Spin-dependent thermoelectronic transport of a single molecule magnet Mn(dmit)2.

We investigate spin-dependent thermoelectronic transport properties of a single molecule magnet Mn(dmit)2 sandwiched between two Au electrodes using first-principles density functional theory combined with nonequilibrium Green's function method. By applying a temperature difference between the two Au electrodes, spin-up and spin-down currents flowing in opposite directions can be induced due to asymmetric distribution of the spin-up and spin-down transmission spectra around the Fermi level. A pure spin current and 100% spin polarization are achieved by tuning back-gate voltage to the system. The spin caloritronics of the molecule with a perpendicular conformation is also explored, where the spin-down current is blocked strongly. These results suggest that Mn(dmit)2 is a promising material for spin caloritronic applications.

Observation of Zero-Point Quantum Fluctuations of a Single-Molecule Magnet through the Relaxation of its Nuclear Spin Bath

A single-molecule magnet placed in a magnetic field perpendicular to its anisotropy axis can be truncated to an effective two-level system, with easily tunable energy splitting. The quantum coherence of the molecular spin is largely determined by the dynamics of the surrounding nuclear spin bath. Here we report the measurement of the nuclear spin-lattice relaxation rate 1/T1n in a single crystal of the single-molecule magnet Mn12-ac, at T ≈ 30 mK in perpendicular fields B⊥ up to 9 T. The relaxation channel at B ≈ 0 is dominated by incoherent quantum tunneling of the Mn12-ac spin S, aided by the nuclear bath itself. However for B⊥>5 T we observe an increase of 1/T1n by several orders of magnitude up to the highest field, despite the fact that the molecular spin is in its quantum mechanical ground state. This striking observation is a consequence of the zero-point quantum fluctuations of S, which allow it to mediate the transfer of energy from the excited nuclear spin bath to the crystal lattice at much higher rates. Our experiment highlights the importance of quantum fluctuations in the interaction between an "effective two-level system" and its surrounding spin bath.

Inverse acoustic Faraday effect in single molecule magnets

A theoretical study into the effect of stationary magnetization induced in a paramagnetic medium by propagation of a circularly-polarized acoustic wave is reported. We termed this magnetoacoustic phenomenon “the inverse acoustic Faraday effect” for its obvious similarity to the inverse Faraday effect in electrodynamics (magnetooptics, in particular). A phenomenological relation is established between the inverse acoustic Faraday effect and the paramagnetic acoustic Faraday rotation widely known as the acoustic Faraday effect. This relation is demonstrated for single molecule magnets. We estimated the magnetization induced by the inverse acoustic Faraday effect in single molecule magnets Mn12–Ac, for the case of a hypersonic wave. This value proved sufficient for the effect to be observed experimentally.

Anomalous field sweep rate dependence of the tunnel relaxation in single-molecule magnet Mn4–Bet

We present a low-temperature magnetometry study of the behavior of the quantum tunneling of the magnetization (QTM) in a Mn4 single-molecule magnet (SMM) as a function of the rate at which a longitudinal magnetic field is swept across a QTM resonance. An initial decrease of the tunnel relaxation rate with increasing field sweeping rate is eventually replaced by an anomalous monotonic increase for moderately high sweep rates. This behavior is distinct from the drastic reversal of the total sample magnetization caused by thermal avalanches (“hot” process), which is commonly observed in other SMMs when increasing the sweep rate, usually due to poor thermal conduction or inadequate thermal anchoring of the sample. We associate the observations to a (“cold”) mechanism by which different collections of molecules within the crystal relax collectively without affecting the rest of the molecules.

A Mn4 single-molecule magnet with the defective-dicubane structure from the use of pyrenecarboxylic acid

The reaction between Mn(pc)2·4H2O (pcH is 1-pyrenecarboxylic acid), N-methyldiethanolamine (mdaH2), and NEt3 in a 2:2:3 molar ratio in CH2Cl2 or CHCl3 gives the tetranuclear complex [Mn4(pc)4(mda)2(mdaH)2], isolated as the ·2CH2Cl2 (3) and ·2CHCl3 (4) solvates, respectively. Only crystals of 4 were of sufficient quality for single-crystal X-ray crystallography. Complex 4 possesses a [MnII2MnIII2] core with the Mn ions arranged in a planar rhombus that can be described as two Mn3 triangles fused at one edge. Additional monoatomic bridging by mda2− or mdaH−μ- and μ3-O atoms on each edge and at the center of the Mn3 triangular units gives a defective-dicubane core structure. Ligation is completed by two bridging and two monodentate pc− groups, each of which is involved in π–π stacking interactions with those on neighboring Mn4 molecules to give a 2D network. Variable-temperature solid-state magnetic susceptibility studies of 3 and 4 in the temperature range 5.0–300K. Various fitting and simulation models were employed in analyzing the data for 3, which shows no evidence of significant intermolecular interactions, and it was concluded that it contains three symmetry-inequivalent exchange interactions, J=+2.34cm−1, J′=+7.70cm−1, J″=−1.33cm−1 and g=1.99, where J′ is the MnIII⋯MnIII interaction, and J and J″ are the MnII⋯MnIII interactions. This indicates an S=8 ground state with a very low-lying an S=9 excited state. This conclusion was supported by fits of reduced magnetization data, which gave an S=8 ground state with axial zero-field splitting parameter D=−0.19(1)cm−1, and g=1.94(1). Ac susceptibility studies on 3 show that it is a single-molecule magnet (SMM). Complex 4 shows different magnetic behavior from 3 due to the intermolecular interactions.

Significant charge transfer between a single-molecule magnet Mn12 and a Bi substrate

We investigate electronic and magnetic properties of a single-molecule magnet Mn12 adsorbed on Bi(111) without any linker molecules, using a first-principles method. This study is motivated by a scanning tunneling microscopy experiment on individual Mn12 molecules on a Bi substrate. We apply density-functional theory including spin–orbit coupling, on-site Coulomb repulsion U, and dipole corrections. With geometry relaxation, the Mn12 molecule remains slightly tilted relative to the surface such that its magnetic easy axis is 6° away from the axis normal to the surface. Upon adsorption, a gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the Mn12, is reduced to 0.43eV, compared to the corresponding gap of 1.07eV for an isolated Mn12. The total magnetic moment of the adsorbed Mn12 increases to 21μB. This is due to charge transfer from the Bi slab to the Mn12. The tilted geometry of the Mn12 allows to favor one of the outer Mn sites for charge transfer and magnetic moment change. Although bulk Bi is semimetal, there are surface states near the Fermi level, which facilitates significant charge transfer and a change in the magnetic moment.

Magnetization quantum tunneling and improper rotational symmetry

We examine the magnetization quantum tunneling (MQT) behavior expected for a single-molecule magnet (SMM) with improper rotational symmetry. The simplest possible realization is the [NiII(hmp)(dmb)Cl]4 cubane complex that crystallizes in the I41/a space group, resulting in S4 molecular point-group symmetry. A mapping is performed of the energy-level diagram obtained via exact diagonalization of a multi-spin Hamiltonian onto that of a giant-spin model which assumes ferromagnetic coupling and a spin S=4 ground state. The results are compared with a similar analysis for a C3 symmetric Mn3 SMM (S=6 ground state). In the even rotational case (Ni4), the time-reversal invariance associated with the spin–orbit interaction gives rise to a zero-field spin-Hamiltonian that possesses an additional mirror plane perpendicular to the S4 axis, which is not a symmetry element of the molecular point-group. This conclusion applies quite generally to any molecule with improper rotational symmetry (Sq, with q even), including the more widely studied Mn12 SMM. The combined Ni4 and Mn3 studies lead to some interesting predictions concerning MQT selection rules in molecules with even versus odd rotational symmetries. We conclude by considering a case with essentially no symmetry at all, by deliberately distorting the high-symmetry Ni4 molecule. In this case, finite gaps are found at all intersections in the energy-level diagram, indicating a complete absence of MQT selection rules.