Spin Switching Behavior Research Articles

Triflate vs Acetonitrile: Understanding the Iron(II)-Based Coordination Chemistry of Tri(quinolin-8-yl)amine

In this study, a synthesis route of tri(quinolin-8-yl)amine (L), a recent member of the tetradentate tris(2-pyridylmethyl)amine (TPA) ligand family, is reported. With the neutral ligand L bound to an iron(II) center in κ4 mode, two cis-oriented coordination sites remain vacant. These can be occupied by coligands such as counterions and solvent molecules. How sensitive this equilibrium can be is most evident if both triflate anions and acetonitrile molecules are available. All three combinations─bis(triflato), bis(acetonitrile), and mixed coligand species─could be characterized by single-crystal X-ray diffraction (SCXRD), which is unique so far for this class of ligand. While at room temperature, the three compounds tend to crystallize concomitantly, the equilibrium can be shifted in favor of the bis(acetonitrile) species by lowering the crystallization temperature. Removed from their mother liquor, the latter is very sensitive to evaporation of the residual solvent, which was observed by powder X-ray diffraction (PXRD) and Mössbauer spectroscopy. The solution behavior of the triflate and acetonitrile species was studied in detail using time- and temperature-resolved UV/vis spectroscopy, Mössbauer spectroscopy of frozen solution, NMR spectroscopy, and magnetic susceptibility measurements. The results indicate a bis(acetonitrile) species in acetonitrile showing a temperature-dependent spin-switching behavior between high- and low-spin. In dichloromethane, the results reveal a high-spin bis(triflato) species. In pursuit of understanding the coordination environment equilibria of the [Fe(L)]2+ complex, a series of compounds with different coligands was prepared and analyzed with SCXRD. The crystal structures indicate that the spin state can be controlled by changing the coordination environment─all of the {N6}-coordinated complexes display geometries expected for low-spin species, while any other donor atom in the coligand position induces a shift to the high-spin state. This fundamental study sheds light on the coligand competition of triflate and acetonitrile, and the high number of crystal structures allows further insights into the influence of different coligands on the geometry and spin state of the complexes.

Influence of Coordinated Triflate Anions on the Solution Magnetic Properties of a Neutral Iron(II) Complex.

In an effort to probe the impacts of speciation on spin-state switching, the synthesis and unique solution-phase magnetic properties of [((TIPSC≡C)3tren)Fe(OTf)2] (1) are described. Analysis of the single-crystal X-ray diffraction data shows that the tris(iminoalkyne) ligand coordinates to the iron(II) center through all four nitrogen atoms, while the other two coordination sites are filled by the oxygen atoms from triflate anions. Solid-state variable-temperature (VT) magnetic studies show that 1 remains high-spin (HS) at all temperatures. In the presence of moderately strong coordinating solvents, solvent replaces the two bound triflate counteranions, as observed by 19F NMR spectroscopy and supported by conductivity measurements. VT solution measurements show 1 to be in the HS state when this solvent is oxygen-donating but low-spin (LS) with a nitrogen-donating solvent. In the noncoordinating solvent dichloromethane, both triflates are bound to the iron(II) center at room temperature, but upon cooling, 1 undergoes a coordination change, resulting in the loss of one triflate, as shown by 19F NMR. With the moderately coordinating solvent acetone, triflate dissociation upon cooling results in a spin-switching species with a T1/2 value of 171 K, characterized via 19F NMR, Evans' method, and solution magnetometry measurements. Solution magnetic measurements collected in structurally similar cyclopentanone suggest that the spin-state switching event is exclusive to the acetone environment, suggesting the influence of both the local coordination environment and aggregation. Additionally, a comparison of the solvodoynamic diameters via dynamic light scattering suggests that aggregation of 1 is significantly different in (CH3)2CO and (CD3)2CO, leading to the observation of spin-switching behavior in the former and fully HS behavior in the latter. This study highlights the sensitivity of solution magnetic properties to solvent choice.

Effect of Structure on the Spin Switching and Magnetic Bistability of Solid-State Aryl Dicyanomethyl Monoradicals and Diradicals.

Stable organic radicals with switchable spin states have applications in medicine, biology, and material science. An emerging class of such spin-switchable radicals is based on dicyanomethyl radicals, which are typically thermally and air-stable species that form weakly bonded (closed-shell singlet) dimers at a lower temperature that rupture into electron paramagnetic resonance-active diradicals at a higher temperature. However, thus far, the study of these dicyanomethyl radicals has focused on their solution-phase behavior. An understanding of how chemical structure affects the solid-state spin switching behavior for these radicals is unknown. Here, we examine the solid-state spin crossover behavior of 6 monoradicals and 10 tethered diradicals and demonstrate that these species also undergo spin switching in the solid state. We find that the susceptibility for solid-state spin switching for the intermolecular dimers is weakly correlated to the solution-phase Gibbs free energies of dimerization, but no apparent correlations are seen between the solution-state free energies for the intramolecular dimerization and the solid-state behavior. Furthermore, intramolecular diradical dimers have greatly enhanced temperature-responsive behavior compared to their intermolecular counterparts. Crystalline and amorphous powders of the same radicals feature similar spin switching behavior, but the crystalline materials have slower bond-rupture kinetics at higher temperatures, suggesting that solid-state packing effects are an important kinetic consideration. An interesting feature of these systems is that, upon cooling down to room temperature after heating, some radicals remain trapped in the solids, indicating magnetic bistability, while others partially or fully return to the diamagnetic dimers. This work provides insights into how chemical structure affects spin crossover in the solid state for this new class of air-stable radicals, the knowledge of importance for the construction of dynamically responsive solid-state materials, and organic spin crossover polymers.

Microwave Energy Drives "On-Off-On" Spin-Switch Behavior in Nitrogen-Doped Graphene.

The established application of graphene in organic/inorganic spin-valve spintronic assemblies is as a spin-transport channel for spin-polarized electrons injected from ferromagnetic substrates. To generate and control spin injection without such substrates, the graphene backbone must be imprinted with spin-polarized states and itinerant-like spins. Computations suggest that such states should emerge in graphene derivatives incorporating pyridinic nitrogen. The synthesis and electronic properties of nitrogen-doped graphene (N content: 9.8%), featuring both localized spin centers and spin-containing sites with itinerant electron properties, are reported. This material exhibits spin-switch behavior (on-off-on) controlled by microwave irradiation at X-band frequency. This phenomenon may enable the creation of novel types of switches, filters, and spintronic devices using sp2 -only 2D systems.

Magnetic and magnetotransport properties of epitaxial La0.7Sr0.3MnO3/SrIrO3/La0.7Sr0.3MnO3 spin valves

We report on the fabrication of epitaxial La0.7Sr0.3MnO3/SrIrO3/La0.7Sr0.3MnO3 (LSMO/SIO/LSMO) spin valve structures deposited on (0 0 1) (LaAlO3)0.3(Sr2TaAlO6)0.7 substrates (LSAT) by high pressure sputtering. We present the structural, magnetic and magnetotransport properties of trilayers. Magnetization measurements indicate indirect exchange coupling between LSMO layers across the SrIrO3 layer. An enhancement of saturation magnetic moment was observed for trilayers with one and six unit cell thick SrIrO3 spacing layers. Low field magnetoresistance measurements show standard spin switching behavior and inverse spin switching in epitaxial LSMO/SIO/LSMO spin valve structures depending on the orientation of magnetic field to the plane and magnetic field to the current.

Control of vortex state in cobalt nanorings with domain wall pinning centers

Magnetic rings at the mesoscopic scale exhibit new spin configuration states and switching behavior, which can be controlled via geometrical structure, material composition and applied field. Vortex states in magnetic nanorings ensure flux closure, which is necessary for low stray fields in high packing density in memory devices. We performed magnetoresistance measurements on cobalt nanoring devices and show that by attaching nanowires to the ring, the vortex state can be stabilized. When a square pad is attached to the free end of the wire, the domain wall nucleation field in the nanowire is reduced. In addition, the vortex state persists over a larger range of magnetic fields, and exists at all in-plane orientations of the magnetic field. These experimental findings are well supported by our micromagnetic simulations.

Study of Non-PGM ORR Catalyst Degradation Using Synchrotron Techniques

Several recent reports on the development of oxygen reduction reaction (ORR) non-platinum group metal (non-PGM) catalysts have shown an impressive progress towards high ORR activity [1-2]. By now, in RDE testing, these catalysts are quite routinely reaching E½ values of 0.8 V versus reversed hydrogen electrode [1]. The high initial performance notwithstanding, truly durable non-PGM catalysts, capable of maintaining ORR activity for hundreds or thousands of hours at low pH values of the polymer electrolyte fuel cell (PEFC), are yet to be developed. The development of such catalysts must involve a carefully thought-out experimental design and the use of reliable multiple techniques to provide answers to the quarter-of-a-century-old questions about the structure of the ORR active site(s) in metal-nitrogen-carbon (M-N-C) catalysts. According to some previous in-situ XAS studies [3-4], a vast majority of non-PGM catalysts consist of two forms of transition metal, iron coordinated by nitrogen species (Fe-Nx) and non-coordinated iron nanoparticles (FeNP). While the existence of these species has been detected using different methods, the in-situ XAS reviled a specific redox behavior accompanied with spin switching behavior of the Fe-Nx species when subjected to potential bias simulating PEFC environment [2-4]. There is still a large ambiguity, however, about the actual nature of the M-N-C active sites and their degradation modes. Among many challenges facing this research, it is important to take a closer look at other elements present in such catalysts, such as nitrogen and carbon. This is especially needed for non-PGM catalysts without nitrogen-coordinated Fe (e.g. FeNP) with very high activity [5]. Herein, we present a study of a Fe-based non-PGM catalyst derived from multiple nitrogen precursors. The study has been performed using synchrotron X-ray absorption techniques coupled with standard electrochemical methods, including RRDE and MEA testing. We look at the structures involving all elements in the catalyst by employing high-energy photons at Argonne National Laboratory (to monitor in-situ Fe-containing species) and low-energy photons at Stanford Synchrotron Lightsource to monitor carbon and nitrogen species (C and N K-edge), claimed to play an important part in the ORR active site sites. With better understanding of the structure-to-function relationship in Fe-N-C species as the main objective, we study the effect of durability cycling (Figure1) on the catalyst performance and structural changes involving all elements in Fe-N-C species. Figure 1

Hybrid Magnetic Superconductors Formed by TaS2 Layers and Spin Crossover Complexes

The restacking of charged TaS2 nanosheets with molecular counterparts has so far allowed for the combination of superconductivity with a manifold of other molecule-intrinsic properties. Yet, a hybrid compound that blends superconductivity with spin crossover switching has still not been reported. Here we continue to exploit the solid-state/molecule-based hybrid approach for the synthesis of a layered TaS2-based material that hosts Fe(2+) complexes with a spin switching behavior. The chemical design and synthetic aspects of the exfoliation/restacking approach are discussed, highlighting how the material can be conveniently obtained in the form of highly oriented easy-to-handle flakes. Finally, proof of the presence of both phenomena is provided by the use of a variety of physical characterization techniques. The likely sensitivity of the intercalated Fe(2+) complexes to external stimuli such as light opens the door for the study of synergistic effects between the superconductivity and the spin crossover switching at low temperatures.

Bias-current effect on inverse spin-switch effect in Permalloy/Nb/Permalloy pseudo spin-valves

The magnetoresistance of Permalloy/Nb/Permalloy pseudo spin-valves in the superconducting transition region have been measured in sweeping magnetic fields while varying bias current in order to investigate bias-current effect on the inverse spin-switch behavior. The magnetoresistance measured between two notches in a micron-size bar showed an ideal inverse spin-switch effect between two magnetic states of Permalloy layers, parallel and antiparallel domain state, at high enough bias-current densities exceeding 105 A/cm2, in which the antiparallel domain state returns to the normal whereas the parallel state remains superconducting.

Effect of Interface-Induced Exchange Fields on Cuprate-Manganite Spin Switches

We examine the anomalous inverse spin switch behavior in La0.7Ca0.3MnO3(LCMO)/YBa2Cu3O7-δ (YBCO)/LCMO trilayers by combined transport studies and polarized neutron reflectometry. Measuring magnetization profiles and magnetoresistance in an in-plane rotating magnetic field, we prove that, contrary to many accepted theoretical scenarios, the relative orientation between the two LCMO's magnetizations is not sufficient to determine the magnetoresistance. Rather the field dependence of magnetoresistance is explained by the interplay between the applied magnetic field and the (exponential tail of the) induced exchange field in YBCO, the latter originating from the electronic reconstruction at the LCMO/YBCO interfaces.

Cyclopentadienyl–benzene based sandwich molecular wires showing efficient spin filtering, negative differential resistance, and pressure induced electronic transitions

Using density functional theory, we investigate TM–cyclopentadienyl–benzene sandwich molecular wires (SMWs) which are composites of TM–cyclopentadienyl and TM–benzene wires (TM = transition metal (V, Fe)). All the SMWs are found to be highly stable ferromagnetic half-metals, showing spin switching behavior. Transport calculations show that finite size clusters display spin filter property when coupled with Au electrodes on either side. I–Vb characteristics of all systems confirm the spin filter property, with Au–BzVCpVBz–Au displaying exceptionally high performance. In addition to spin filtering, the Au–BzFeCpFeBz–Au system also shows negative differential resistance (NDR). Compression causes an abrupt reduction in magnetic moment and a transition to a metallic phase, while stretching causes an increase in magnetic moment. Half-metallicity is preserved for modest amounts of stretching and compression.

Domain Wall Pinning for Enhanced Inverse Spin-Switch Effect in Ferromagnet/Superconductor/Ferromagnet Trilayer

We studied the effect of domain wall pinning on the inverse spin-switch (ISS) behavior in pseudo spin-valve ferromagnet/superconductor/ferromagnet (F/S/F) trilayers. The deposited films were patterned to contain a micron-size triangular notch at their centers for the purpose of domain wall pinning by the notch. The magnetoresistance (MR) and resistance-temperature curves of the notch segment and the side segments next to the notch were simultaneously measured, and then compared against each other to investigate the role of the notch as a domain wall pinning site and then its influence on the ISS effect. In the superconducting transition state, the MR of the notch segment showed less variation in the antiparallel domain state in comparison with the side segments. In addition, the transition temperature difference between the on and off states of the ISS effect was found to be larger in the notch segment, which can be interpreted as enhanced ISS performance. These observations of more stable and the enhanced ISS effect in the notch segment was satisfactorily explained by the domain wall pinning due to the notch in F/S/F trilayers.

Unconventional Proximity Effect and Inverse Spin-Switch Behavior in a Model Manganite-Cuprate-Manganite Trilayer System

The proximity effect in a model manganite-cuprate system is investigated theoretically. We consider a situation in which spin-polarized electrons in manganite layers antiferromagnetically couple with electrons in cuprate layers as observed experimentally. The effect of the interfacial magnetic coupling is found to be much stronger than the injection of spin-polarized electrons into the cuprate region. As a result, the superconducting transition temperature depends on the thickness of the cuprate layer significantly. Since the magnetic coupling creates negative polarization, an applied magnetic field and the negative polarization compete, resulting in the inverse spin-switch behavior where the superconducting transition temperature is increased by applying a magnetic field.

Observation of a Transition From Inverse-Spin-Switch to Spin-Switch Behavior in Domain State of a Py/Nb/Py Trilayer

We study spin switch behavior of the six bridges of bare, or pseudo spin-valve Py/Nb/Py trilayer prepared on a single chip. Magnetization measurement of both longitudinal and transverse moment on a large companion sample reveals that a significant amount of transverse component exists in the antiparallel domain (AD) state, which is originated in stray magnetic field from domain walls. The stray magnetic field induces flux lines into the Nb layer and the motion of these flux lines under the force exerted by the bias current is the main origin for inverse spin switch effect observed in the AD state of most bridges. In addition to inverse spin switch effect, we observe a peculiar behavior of a transition from inverse spin switch to spin switch behavior with decreasing temperature in one of six bridges. In order to understand the extraordinary behavior, we first note that the flux pinning should be present in this particular sample to suppress the dissipation by flux motion, then we propose that when the domain structure incidentally matches underlying distribution of pinning sites, spin switch effect in the AD state can occur by domain wall superconductivity and/or proximity effect.

Dependence of Magnetoresistance and Superconducting Transition Temperature on Relative Orientation of Magnetization Easy Axis to Current Direction in a Py/Nb/Py Trilayer

The magnetoresistance (MR) and the superconducting transition temperature <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> as well as the longitudinal and the transverse magnetic moments are studied in order to find an underlying cause for the inverse spin switch behavior of bare Py/Nb/Py trilayers. Our measurements consistently show that the increase of MR and the decrease of <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> in the case of the antiparallel alignment between the magnetizations of the two Py layers (AP state) compared to those in the parallel alignment (P state), which is known as the inverse spin switch behavior, are largely due to stray field from the domain walls in the domain state that occurs during the AP state. The influence of stray field on the MR and the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> shift between the AP and P state is measured in the bridges with each axis oriented at various angles phi with respect to the magnetization easy axis. With increasing phi from 0deg to 90deg, <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> shift between the AP and P state is almost constant around 5 mK up to phi = 54deg, then becomes 11 mK when phi = 72deg, but is reduced to 2 mK for phi = 90deg . In addition, the effect by the stray field not originating in the domain walls is observed outside the domain states in the MR and in the transverse magnetic moment measurements.

Origin of the inverse spin-switch behavior in manganite/cuprate/manganite trilayers

We studied ferromagnet/superconductor/ferromagnet trilayers based on La_(0.7)Ca_(0.3)MnO_(3) manganite and YBa_(2)Cu_(3)O_(7−δ) (YBCO) high-T_(c) cuprate with magnetoresistance and magnetization measurements. We find an inverse superconducting spin-switch behavior, where superconductivity is favored for parallel alignment of the magnetization in the ferromagnetic layers. We argue that this inverse superconducting spin switch originates from the transmission of spin-polarized carriers into the superconductor. In this picture, the thickness dependence of the magnetoresistance yields the spin-diffusion length in YBCO as 13 nm. A comparison of bilayers and trilayers allows ruling out the effect of the stray fields of the domain structure of the ferromagnet as the source of the inverse superconducting spin switch.