Session F19: Magnetic Clusters and Molecular Magnets II

Thermally induced spin emergent effect on ultrafast spin dynamics in Ni4

It has already been shown that coupling of spins to an external heat bath can lead to interesting spin dynamics, e.g., the Spin Seebeck [1] and the Nernst effect. Here, using state-of-the-art ab initio quantum chemical calculations we present a new spin emergent effect, in which the combination of a laser pulse and a thermal bath gives rise to a transient spin-flip scenario [2] on a structurally distorted Ni₄, not possible otherwise.

The thermal coupling is described using the Lindblad-superoperator formalism. Furthermore, the Fourier analysis of the resulting spin and charge relaxation reveals two different regimes. We compare the time regimes of charge and spin and discuss the origin of their differences. Additionally, we investigate the effects of using two thermal baths on a model system [3]. It turns out that for an asymmetric coupling the population of the ground state triplet can be inverted with respect to the thermally equilibrated ones, thus the magnetization changes sign. Finally, we find that the Lamb-shift creates fluctuations in the spin values.

[1] G. Lefkidis and S. A. Reyes, Phys. Rev. B 94, 1444333 (2016)
[2] C. Li, J. Liu, S. Zhang, G. Lefkidis, and W. Hübner, Carbon 87, 153 (2015)
[3] G. Lefkidis, S. Sold, and W. Hübner, J. Magn. Magn. Mater. 432, 276 (2017)   

Locking and Unlocking of the Molecular Spin Crossover Transition

The electronic state of Fe(II) spin crossover complex [Fe{H₂B(pz)₂}₂(bipy)] (pz = pyrazol-1-yl, bipy = 2,2′-bipyridine) has been investigated by X-ray absorption spectroscopy (XAS). Locking in a largely low-spin-state configuration over a temperature range that includes temperatures well above the thermal spin crossover temperature of 160 K, to well above room temperature, has been found in both nanometer thin films (by substrate interactions) and powder (by mixing with strongly dipolar zwitterions). The XAS clearly shows the change of iron L₂ edge spectra, typically associated with thermal induced spin crossover. Remarkably, incident X-ray fluences then restore the complex moiety to a high spin state at temperatures of 200 K to above room temperature, well above the spin crossover transition temperature for the pristine powder, and this is an activated process, as will be discussed in this presentation. Relaxation back to the initial locked state can be accomplished by heating slightly above room temperature.   

Charge Transport in Molecular Devices via DFT+DMFT

As we are entering the nano-world, the modeling of electronic and spintronic devices is facing some great challenges. One has indeed to describe on the same footing material-specific properties, quantum behaviors and strong correlation effects in and out-of equilibrium. In this context, new opportunities are offered by extending Dynamical Mean-Field Theory (DMFT) to transport problems and by combining it with the non-equilibrium Green's function formalism. This method can accurately describe Kondo physics, Coulomb blockade as well as finite-bias effects, such as spin excitations, in molecular devices. After discussing some details of our implementation, we will present several applications to the transport properties of molecules comprising a few radical centers with a variety of magnetic ground states.
  

High Spin Cycles: Topping the Spin Record for a Single Molecule verging on Quantum Criticality

Theory predicts a number of interesting quantum critical phenomena for low-dimensional magnetic systems, where the ground state and thus low-temperature properties of a material change drastically upon even a small variation of an appropriate external parameter. Here we report a mixed 3d/4f cyclic coordination cluster, Gd₁₀Fe₁₀, that turns out to be very near or even at such a quantum critical point. The molecule forms a nano-torus with alternating gadolinium and iron ions with a nearest neighbor Fe-Gd coupling and a frustrating next-nearest neighbor Fe-Fe coupling. Such a spin arrangement corresponds to a cyclic delta or saw-tooth chain, which can exhibit a variety of frustration effects, among them giant magnetization jumps as well as macroscopic degeneracies of the ground state with profound caloric consequences. The present cluster is situated on the ferromagnetic side of the Quantum Critical Point with a ground state spin of S=60, which makes it simultaneously the magnetic molecule with the largest total spin ever observed for a magnetic molecule.
The present discovery is a lucky example where a magnetic material exists that corresponds to theory predictions. In addition, Gd₁₀Fe₁₀ constitutes a massive challenge for theory since the Hilbert space dimension assumes the staggering value of 2.7*10¹⁵.   

Evidence of Bose-Einstein condensation in a quantum magnet formed by free radical tetramers

Several experimental realizations of quantum magnets showing Bose-Einstein condensates (BEC) have been reported in the literature. The typical scenario invokes a ground state described by pairs of localized spins forming singlets. An external magnetic field acts as an effective chemical potential for triplet excitations that can subsequently form the BEC. The T versus H phase diagrams typically display a ``dome'' structure bounded by two critical fields and a field-dependent critical temperature.

In this work, we present experimental and theoretical evidence of Bose-Einstein condensation in an organic crystal formed of weakly interacting S=1/2 tetramers. We show that the physics can be described in terms of a fully rotational invariant system of quantum spins without frustration. Two BEC phases exist: at low fields, only the edge spins of each tetramer contribute to the condensate, while at high-fields, the order is determined by the two central spins. These unusual BECs are separated by a quantum state with half of the spins forming dimerized pairs, and the other half aligned in the direction of the field.

  

Spin-flip spectroscopy in phthalocyanine based molecular magnetic tunnel junctions

Molecular spintronics is an effervescent field of research, which aims at combining spin physics and molecular nano-objects. We have investigated spin-dependent transport through metallo-phthalocyanine molecules. Cobalt phthalocyanine (CoPc) and manganese phthalocyanine (MnPc) molecules have been integrated in magnetic tunnel junctions to act as tunnel barriers. By changing only the central metal atom, very different magnetoresistance behaviours are revealed. For CoPc molecules, a tunnelling magnetoresistance (TMR) effect coexists together with a tunnelling anisotropic magnetoresistance (TAMR) [1]. For MnPc based MTJs, anisotropic MR effects dominate with no direct evidence of TMR or clear conventional TAMR effect. These differences reflect different transport mechanisms and tunnelling paths through the molecules between the electrodes. For MnPc MTJs, inelastic tunnelling with spin-flip events through the Mn atoms chain formed by the MnPc layer stacking occurs. These results show that metallo-organic molecules could be used as a template to connect magnetic atomic chains or even a single magnetic atom in a solid state device.
[1] C. Barraud et al., Phys. Rev. Lett. 114, 206603 (2015)   

Temperature Dependence of Nonlinear Susceptibilities in an Infinite range Interaction model

We present thermodynamic properties of a model with a variable N number of particles, an infinite range antiferromagnetic exchange interaction J and under the influence of an external magnetic field B . We have calculated the magnetization m (B,T,N), more specifically, the temperature dependent linear susceptibility χ ₁(T) and nonlinear susceptibilities₎ χ₃ (T) and χ₅ (T). For an even number of particles the susceptibilities show maxima in their temperature dependence. For an odd number of particles there is an additional free spin response that dominates at low temperatures. In magnetization, for odd number of particles, there is a step at B = 0, followed by steps at critical fields B_c = 3J/2γ, 5J/2γ… (2n+1)J/2γ. Thus small clusters respond with metamagnetism in an otherwise isotropic spin space, while the largest clusters show no metamagnetism.   

Molecular scale dynamics of light-induced spin crossover in a two-dimensional layer

Spin crossover molecules show the unique ability to switch between two spin-states when submitted to external stimuli such as temperature, light or voltage. If controlled at the molecular scale, such switches would be of great interest for the development of molecular devices in electronics and spintronics. If the behavior of such molecules is well documented in bulk, little is known on their properties at the molecular scale and in direct contact with an electrode. We focus our study on Fe^II pyrazolyl borate molecules adsorbed on an Au(111) substrate. The combination of scanning tunneling microscopy measurements and ab initio calculations allows us discriminating between both states by local vibrational spectroscopy. We thus evidence the formation of a long range ordered mixed spin-state phase and confirm their magnetic properties by x-ray absorption spectroscopy. We also show that a single layer of spin-crossover molecules in contact with a metallic surface displays light-induced collective processes between two ordered mixed spin-state phases with two distinct time scales dynamics [1]. The thermal spin crossover is also evidence with the opening of a hysteresis. These results open a way to molecular scale control of two-dimensional spin crossover layers.

[1] K. Bairagi et al., Nat. Comm., 2016, 7, 12212.   

Enabling the Study of Anisotropy-driven Quantum Dynamics of Single-Molecule Magnet Spins at 100mK

We will present the results of an ongoing experimental project to allow pulse EPR studies (spin echo) of condensed samples of single-molecule magnets and single-atom magnets (non-diluted crystals) at temperatures at or below 100mK. The aim is to eliminate dephasing due to dipolar fluctuations by freezing the spin state of all molecules in the crystal in the ground state without the need of applying strong magnetic fields. We expect that these conditions would allow us to study the quantum dynamics of the spins as governed by the intrinsic molecular magnetic anisotropy, which should give rise to non-well defined Rabi oscillations of the spin state, including metastable precessional spin states.   

All-spin-based ultrafast nano-logic elements with a Ni4 cluster

Using high-level ab initio equation-of-motion coupled-cluster calculations, we present two new elementary laser-induced spin manipulation processes on Ni₄, namely spin bifurcation and spin merging, in which the spin density gets transferred from one to two magnetic centers, and vice versa. With these functional intramolecular processes at hand we are able to construct an all-spin-based OR gate, which, contrary to previously suggested nanologic elements [1], does not need any information carriers except the spins. The necessary spin-density localization is guaranteed by the non-perfect geometry of the cluster [2]. The spin-transfer scenarios allow us to also build a cyclic spin-SHIFT register.

We also exploit the Ni₄ cluster to derive two quantum-information operations, not possible classically. The one is the which-path information effect, in which the phase of the final spin state after two consecutive laser-induced transfers can reveal the exact path travelled by the spin density [3], and the other the universal pure-quantum SQUARE-ROOT-OF-NOT gate.

[1] W. Hübner, S. Kersten, and G. Lefkidis, Phys. Rev. B 79, 184431 (2009)
[2] D. Chaudhuri, H. P. Xiang, G. Lefkidis, and W. Hübner, Phys. Rev. B 90, 245113 (2014)
[3] D. Chaudhuri, G. Lefkidis, and W. Hübner, Phys. Rev. B (accepted)   

Probing the Exchange Interaction Between Two Magnetic Molecules in 3D Space

The exchange interaction is responsible for magnetic ordering in materials and is the principal means by which two nearby spins interact. Previous scanning tunneling microscopy studies of magnetic atoms on surfaces have explored the effects of exchange on their spin states, but in these experiments, the localized spins can only be positioned at fixed lattice adsorption sites in the two-dimensional surface plane. Here we use inelastic electron tunneling spectroscopy to probe the exchange interaction between the spins of two magnetic molecules by attaching one to a STM tip and positioning it in 3D space above another molecule adsorbed on a surface. We find that the exchange coupling requires re-diagonalization of the two-molecule spin Hamiltonian and depends sensitively on their relative positions. Our results demonstrate the ability to characterize and map the exchange coupling strength between spin densities in three dimensions and provide new opportunities to probe the coupling between quantum states with sub-Angstrom resolution.