Directly probing spin dynamics in a molecular magnet with femtosecond time-resolution

Ultrafast and persistent photoinduced phase transition at room temperature monitored by streaming powder diffraction

Ultrafast photoinduced phase transitions at room temperature, driven by a single laser shot and persisting long after stimuli, represent emerging routes for ultrafast control over materials' properties. Time-resolved studies provide fundamental mechanistic insight into far-from-equilibrium electronic and structural dynamics. Here we study the photoinduced phase transformation of the Rb0.94Mn0.94Co0.06[Fe(CN)6]0.98 material, designed to exhibit a 75 K wide thermal hysteresis around room temperature between MnIIIFeII tetragonal and MnIIFeIII cubic phases. We developed a specific powder sample streaming technique to monitor by ultrafast X-ray diffraction the structural and symmetry changes. We show that the photoinduced polarons expand the lattice, while the tetragonal-to-cubic photoinduced phase transition occurs within 100 ps above threshold fluence. These results are rationalized within the framework of the Landau theory of phase transition as an elastically-driven and cooperative process. We foresee broad applications of the streaming powder technique to study non-reversible and ultrafast dynamics.

Ultrafast Jahn-Teller Photoswitching in Cobalt Single-Ion Magnets

Single-ion magnets (SIMs) constitute the ultimate size limit in the quest for miniaturizing magnetic materials. Several bottlenecks currently hindering breakthroughs in quantum information and communication technologies could be alleviated by new generations of SIMs displaying multifunctionality. Here, ultrafast optical absorption spectroscopy and X-ray emission spectroscopy are employed to track the photoinduced spin-state switching of the prototypical complex [Co(terpy)2 ]2+ (terpy = 2,2':6',2″-terpyridine) in solution phase. The combined measurements and their analysis supported by density functional theory (DFT), time-dependent-DFT (TD-DFT) and multireference quantum chemistry calculations reveal that the complex undergoes a spin-state transition from a tetragonally elongated doublet state to a tetragonally compressed quartet state on the femtosecond timescale, i.e., it sustains ultrafast Jahn-Teller (JT) photoswitching between two different spin multiplicities. Adding new Co-based complexes as possible contenders in the search for JT photoswitching SIMs will greatly widen the possibilities for implementing magnetic multifunctionality and eventually controlling ultrafast magnetization with optical photons.

Laser-Controlled Implementation of Controlled-NOT, Hadamard, SWAP, and Pauli Gates as Well as Generation of Bell States in a 3d-4f Molecular Magnet

Using high-level ab initio many-body theory, we theoretically propose that the Dy and the Ni atoms in the [Dy₂Ni₂(L)₄(NO₃)₂(DMF)₂] real molecular magnet as well as in its core, that is, the [Dy₂Ni₂O₆] system, act as two-level qubit systems. Despite their spatial proximity we can individually control each qubit in this highly correlated real magnetic system through specially designed laser-pulse combinations. This allows us to prepare any desired two-qubit state and to build several classical and quantum logic gates, such as the two-qubit (binary) CNOT gate with three distinct laser pulses. Other quantum logic gates include the single-qubit (unary) quantum X, Y, and Z Pauli gates; the Hadamard gate (which necessitates the coherent quantum superposition of two many-body electronic states); and the SWAP gate (which plays an important role in Shor's algorithm for integer factorization). Finally, by sequentially using the achieved CNOT and Hadamard gates we are able to obtain the maximally entangled Bell states, for example, (12)(|00⟩ + |11⟩).

Out-of-equilibrium dynamics driven by photoinduced charge transfer in CsCoFe Prussian Blue Analogue nanocrystals

In this paper we study the out-of-equilibrium dynamics associated with photoinduced charge-transfer (CT) in cyanide-bridged Co-Fe Prussian blue analogue nanocrystals. In these coordination networks, the structural trapping of the photoinduced...

A femtosecond magnetic circular dichroism spectrometer

We describe the development of a broadband magneto-optical spectrometer with femtosecond temporal resolution. The absorption spectrometer is based on a white-light supercontinuum (∼320 to 750 nm) using shot-to-shot temporal and spectral referencing at 1 kHz. Static and transient absorption spectra using circularly polarized light are collected in a magnetic field. The difference spectra with respect to the external field direction give the static and transient magneto-optical Faraday rotation (magnetic optical rotary dispersion) and ellipticity (magnetic circular dichroism) spectra. An achromatic quarter-wave plate is used, and the impact of the deviation from ideal retardance on the spectra is discussed. Results from solution-based and thin-film samples are used to demonstrate the performance and wide applicability of the instrument. The sensitivities for the static and time-resolved data were found to be 5 and 0.4 mdeg, respectively. The method presents a simple way to measure magneto-optical spectra using a transient absorption spectrometer and an electromagnet.

Strongly Red-Emissive Molecular Ruby [Cr(bpmp)2]3+ Surpasses [Ru(bpy)3]2

Gaining chemical control over the thermodynamics and kinetics of photoexcited states is paramount to an efficient and sustainable utilization of photoactive transition metal complexes in a plethora of technologies. In contrast to energies of charge transfer states described by spatially separated orbitals, the energies of spin-flip states cannot straightforwardly be predicted as Pauli repulsion and the nephelauxetic effect play key roles. Guided by multireference quantum chemical calculations, we report a novel highly luminescent spin-flip emitter with a quantum chemically predicted blue-shifted luminescence. The spin-flip emission band of the chromium complex [Cr(bpmp)₂]³⁺ (bpmp = 2,6-bis(2-pyridylmethyl)pyridine) shifted to higher energy from ca. 780 nm observed for known highly emissive chromium(III) complexes to 709 nm. The photoluminescence quantum yields climb to 20%, and very long excited state lifetimes in the millisecond range are achieved at room temperature in acidic D₂O solution. Partial ligand deuteration increases the quantum yield to 25%. The high excited state energy of [Cr(bpmp)₂]³⁺ and its facile reduction to [Cr(bpmp)₂]²⁺ result in a high excited state redox potential. The ligand's methylene bridge acts as a Brønsted acid quenching the luminescence at high pH. Combined with a pH-insensitive chromium(III) emitter, ratiometric optical pH sensing is achieved with single wavelength excitation. The photophysical and ground state properties (quantum yield, lifetime, redox potential, and acid/base) of this spin-flip complex incorporating an earth-abundant metal surpass those of the classical precious metal [Ru(α-diimine)₃]²⁺ charge transfer complexes, which are commonly employed in optical sensing and photo(redox) catalysis, underlining the bright future of these molecular ruby analogues.

Exploring Ultrafast Photoswitching Pathways in RbMnFe Prussian Blue Analogue

We study by femtosecond optical pump-probe spectroscopy the photoinduced charge transfer (CT) in the RbMnFe Prussian blue analogue. Previous studies evidenced the local nature of the photoinduced Mn^III Fe^II → Mn^II Fe^III process, occurring within less than 1 ps. Here we show experimentally that two photoswitching pathways exist, depending on the excitation pump wavelength, which is confirmed by band structure calculations. Photoexcitation of α spins corresponds to the Mn(d-d) band, which drives reverse Jahn-Teller distortion through the population of antibonding Mn-N orbitals, and induces CT within ≈190 fs. The process launches coherent lattice torsion during the self-trapping of the CT small-polaron. Photoexcitation of β spins drives intervalence Fe→Mn CT towards non-bonding states and results in a slower dynamic.

Single Mn Atom Doping in Chiral Sensitive Assembled Gold Clusters to Molecular Magnet

Chiral stirred optical and magnetic properties, through the doping of assembled ultrasmall metal clusters (AMCs), are promising discernment to rivet the molecule-like quantum devices. Here, the single manganese (Mn) atom doping and assembly of the gold cluster (Au₈), leading to the chirality driven magnetism, has been achieved through a ligand-mediated growth. The X-ray absorption near edge structure and electron paramagnetic resonance studies corroborate the tetrahedral coordinated local structure of Mn dopant in the Au host. The optical and vibrational circular dichroic analysis affirms the modulation of chirality (negative to positive) in the presence of the Mn. A distinct ferromagnetic hysteresis loop at 300 K shows Mn ridden chiral sensitive ferromagnetism in contrary to the ligand influenced superparamagnetic undoped AMCs. The spin-polarized density functional theory level of calculations reveal the partial overlapping of spin-up and -down density of states in the doped AMCs, attributing to the ferromagnetic nature as like a molecular magnet suitable for the opto-spintronics application.

Ultrafast photoinduced dynamics in Prussian blue analogues

A review on ultrafast photoinduced processes in molecule-based magnets with an emphasis on Prussian blue analogues.

Resonant Pumping of d-d Crystal Field Electronic Transitions as a Mechanism of Ultrafast Optical Control of the Exchange Interactions in Iron Oxides

The microscopic origin of ultrafast modification of the ratio between the symmetric (J) and antisymmetric (D) exchange interaction in antiferromagnetic iron oxides is revealed, using femtosecond laser excitation as a pump and terahertz emission spectroscopy as a probe. By tuning the photon energy of the laser pump pulse we show that the effect of light on the D/J ratio in two archetypical iron oxides FeBO_{3} and ErFeO_{3} is maximized when the photon energy is in resonance with a spin and parity forbidden d-d transition between the crystal-field split states of Fe^{3+} ions. The experimental findings are supported by a multielectron model, which accounts for the resonant absorption of photons by Fe^{3+} ions. Our results reveal the importance of the parity and spin-change forbidden, and therefore often underestimated, d-d transitions in ultrafast optical control of magnetism.

Spontaneous Magnetization and Optical Activity in the Chiral Series {(L-proline)nV[Cr(CN)6]x} (0 < n < 3)

The incorporation of the natural amino acid L-proline in the synthesis to vanadium-chromium Prussian blue derivatives results in materials exhibiting magnetic ordering including chiral magnetic centers. Although the amorphous nature of these materials makes difficult to assess the structural features of these proline-containing compounds, magnetic and spectroscopic data confirms their multifunctionality. They exhibit high-temperature magnetic ordering (Tc < 255 K) and a circular dichroic signal, representing the molecule-based chiral magnets with the highest ordering temperatures reported to date. In addition, the presence of chiral L-proline (or D-proline) has additional benefits, including higher redox stability and the appearance of magnetic hysteresis. The latter was not observed in the parent compounds, the series of room temperature molecule-based magnets V[Cr(CN)6]x.

Vibrational coherences in manganese single-molecule magnets after ultrafast photoexcitation

Magnetic recording using femtosecond laser pulses has recently been achieved in some dielectric media, showing potential for ultrafast data storage applications. Single-molecule magnets (SMMs) are metal complexes with two degenerate magnetic ground states and are promising for increasing storage density, but remain unexplored using ultrafast techniques. Here we have explored the dynamics occurring after photoexcitation of a trinuclear µ₃-oxo-bridged Mn(III)-based SMM, whose magnetic anisotropy is closely related to the Jahn-Teller distortion. Ultrafast transient absorption spectroscopy in solution reveals oscillations superimposed on the decay traces due to a vibrational wavepacket. Based on complementary measurements and calculations on the monomer Mn(acac)₃, we conclude that the wavepacket motion in the trinuclear SMM is constrained along the Jahn-Teller axis due to the µ₃-oxo and µ-oxime bridges. Our results provide new possibilities for optical control of the magnetization in SMMs on femtosecond timescales and open up new molecular-design challenges to control the wavepacket motion in the excited state of polynuclear transition-metal complexes.

Site-Selective Photoswitching of Two Distinct Magnetic Chromophores in a Propeller-Like Molecule To Achieve Four Different Magnetic States

Magnetic photoswitching is a highly important but relatively rare phenomenon for enabling optical writing/reading of the magnetic state of a molecule. In this work, an unprecedented site-selective double photoswitching is reported from the assembly of two different "photomagnetic chromophores" into a single hexanuclear molecule: namely, a spin-crossover Fe(II) center exhibiting light-induced excited spin state trapping (LIESST) and a photochemically active octacyanometalate(IV) unit. Four different magnetization levels are accessible through the appropriate combination of violet/red light and temperature, results that highlight the potential of photomagnetic molecules as future molecular memory cells.

RGB Magnetophotonic Crystals for High-contrast Magnetooptical Spatial Light Modulators

Magnetooptical spatial light modulators (MOSLMs) are photonic devices that encode information in photonic waveforms by changing their amplitude and phase using magnetooptical Faraday or Kerr rotation. Despite the progress on both MO materials and switching methods, significant improvements on materials engineering and SLM design are needed for demonstrating low-power, multicolor, analog and high-contrast MOSLM devices. In this study, we present design rules and example designs for a high-contrast and large figure-of-merit MOSLM using three-color magnetophotonic crystals (MPC). We demonstrate for the first time, a three-defect MPC capable of simultaneously enhancing Faraday rotation, and high-contrast modulation at three fundamental wavelengths of red, green and blue (RGB) within the same pixel. We show using 2D finite-difference time-domain simulations that bismuth-substituted yttrium iron garnet films are promising for low-loss and high Faraday rotation MOSLM device in the visible band. Faraday rotation and loss spectra as well as figure-of-merit values are calculated for different magnetophotonic crystals of the form (H/L)^p/(D/L)^q/(H/L)^p. After an optimization of layer thicknesses and MPC configuration, Faraday rotation values were found to be between 20–55° for losses below 20 dB in an overall thickness less than 1.5 µm including three submicron garnet defect layers. The experimental demonstration of our proposed 3-color MOSLM devices can enable bistable photonic projectors, holographic displays, indoor visible light communication devices, photonic beamforming for 5 G telecommunications and beyond.

Ultrafast magnetodynamics with free-electron lasers

The study of ultrafast magnetodynamics has entered a new era thanks to the groundbreaking technological advances in free-electron laser (FEL) light sources. The advent of these light sources has made possible unprecedented experimental schemes for time-resolved x-ray magneto-optic spectroscopies, which are now paving the road for exploring the ultimate limits of out-of-equilibrium magnetic phenomena. In particular, these studies will provide insights into elementary mechanisms governing spin and orbital dynamics, therefore contributing to the development of ultrafast devices for relevant magnetic technologies. This topical review focuses on recent advancement in the study of non-equilibrium magnetic phenomena from the perspective of time-resolved extreme ultra violet (EUV) and soft x-ray spectroscopies at FELs with highlights of some important experimental results.

Electron spin fluctuation in intense laser fields

Spin fluctuation dynamics of diatomic and monoatomic ions interacting with ultrashort intense laser pulses is studied by solving the time-dependent Dirac equation.