Spin Transition Phenomenon Research Articles

Three-way switching in a cyanide-bridged [CoFe] chain

Bistable compounds that exist in two interchangeable phases under identical conditions can act as switches under external stimuli. Among such switchable materials, coordination complexes have energy levels (or phases) that are determined by the electronic states of their constituent metal ions and ligands. They can exhibit multiple bistabilities and hold promise in the search for multifaceted materials that display different properties in different phases, accessible through the application of contrasting external stimuli. Molecular systems that exhibit both thermo- and photoinduced magnetic bistabilities are excellent candidates for such systems. Here we describe a cyanide-bridged [CoFe] one-dimensional chiral coordination polymer that displays both magnetic and electric bistabilities in the same temperature range. Both the electric and magnetic switching probably arise from the same electron-transfer coupled spin-transition phenomenon, which enables the reversible conversion between an insulating diamagnetic phase and either a semiconducting paramagnetic (thermoinduced) or a type of ferromagnetic single-chain magnet (photoinduced) state.

Spin Transition and Exchange Interaction: Janus Visions of Supramolecular Spin Coupling between Face‐to‐Face Verdazyl Radicals

Look both ways: Coordination bonding of verdazyl radicals to diamagnetic metal ions dramatically modifies the crystal packing of the electronic spin bearers. The face-to-face positioning of two radicals leads to a magnetic behavior that is more relevant to a S=0 to S=1 spin-transition phenomenon than to the usual exchange-interaction view.

Room-temperature spin-transition iron compounds

Since the discovery of the spin transition phenomenon in tris(N,N-dialkyldithiocarbamato) iron(III) complexes [1], numerous investigations have been devoted to this field of molecular magnetism. The spin transition phenomenon is probably the most spectacular example of bistability in molecular chemistry. However, it is a challenge to obtain spin transition materials when working under ambient conditions (e.g. room temperature and pressure), which would be highly advantageous for potential applications. So far, only some Fe(II) and Fe(III) molecular systems have shown temperature-induced spin transitions around and even above room temperature. Within this review we discuss the characteristics of this class of bistable compounds in detail and we try to draw more general conclusions regarding the integration, implementation and application of spin transition compounds as switching elements in hybrid molecular devices.

Mössbauer spectroscopy in molecular magnetism

The aim of this paper is to highlight some selected research activities on molecular magnetic materials using Mossbauer spectroscopy as a technique carried out in our laboratory in recent years. The first part of the present article is devoted to the studies of the various magnetic interactions, metal-to-metal electron-transfer phenomenon, glass transition occurring in molecular magnetic materials, whereas the second part deals with the iron(II) high spin (S = 2)–low spin (S = 0) transition phenomenon occurring in some isoxazole ligand based iron(II) compounds as examples with unusually complicated spin transition behaviour. Also, an example of a dinuclear a spin crossover compound of iron(II) is described, where Mossbauer spectroscopy has most convincingly unraveled the mechanism of the spin transition process. Finally, an example from our most recent studies of spin crossover materials exhibiting both thermal spin crossover and liquid crystalline properties in the same temperature interval near room temperature will be presented.

Realization of the mean-field universality class in spin-crossover materials

In spin-crossover materials, the volume of a molecule changes depending on whether it is in the high-spin (HS) or low-spin (LS) state. This change causes distortion of the lattice. Elastic interactions among these distortions play an important role for the cooperative properties of spin-transition phenomena. We find that the critical behavior caused by this elastic interaction belongs to the mean-field universality class, in which the critical exponents for the spontaneous magnetization and the susceptibility are $\ensuremath{\beta}=1∕2$ and $\ensuremath{\gamma}=1$, respectively. Furthermore, the spin-spin correlation function is a constant at long distances, and it does not show an exponential decay in contrast to short-range models. The value of the correlation function at long distances shows different size dependences: $O(1∕N)$, $O(1∕\sqrt{N})$, and constant for temperatures above, at, and below the critical temperature, respectively. The model does not exhibit clusters, even near the critical point. We also found that cluster growth is suppressed in the present model and that there is no critical opalescence in the coexistence region. During the relaxation process from a metastable state at the end of a hysteresis loop, nucleation phenomena are not observed, and spatially uniform configurations are maintained during the change of the fraction of HS and LS. These characteristics of the mean-field model are expected to be found not only in spin-crossover materials, but also generally in systems where elastic distortion mediates the interaction among local states.

Prussian Blue Analogue CsFe[Cr(CN)6] as a Matrix for the Fe(II) Spin-Crossover

The origin of the intriguing spin-transition behavior of the Prussian blue analogue cesium iron hexacyanochromate CsFe[Cr(CN)6] has been investigated by means of correlated ab initio CASPT2 calculations. Using the smallest transiting core [Fe(NC)6]4-, the relative importance of the local ligand field and the Madelung field generated by the rest of the crystal was estimated. It is shown that in the presence of a frozen-charge environment, the high-spin state lies lower in energy than the low-spin state, thus excluding the possibility of observing a spin transition. In contrast, the charge reorganization in the environment evaluated from unrestricted periodic Hartree-Fock calculations creates a prerequisite for the spin-transition phenomenon. The influence of the disorder in the cesium ions' positions on the spin transition has been examined as a possible stabilizing factor of the low-spin state of [Fe(NC)6]4-. It is concluded that this experimentally observed disorder cannot account solely for the unprecedented behavior of the CsFe[Cr(CN)6] compound.

Study of thermal spin crossover in[Fe(II)(isoxazole)6](BF4)2 with Mössbauer spectroscopy

57Fe Mössbauer spectroscopyof the mononuclear [Fe(II)(isoxazole)6](BF4)2 compound has been studied to reveal the thermal spin crossover of Fe(II) between low-spin(S = 0) andhigh-spin (S = 2) states. A temperature-dependent spin transition curve has been constructed with theleast-square fitted data obtained from the Mössbauer spectra measured at varioustemperatures in the 240–60 K range during the cooling and heating cycle. The compoundexhibits a temperature-dependent two-step spin transition phenomenon withTSCO (step1) = 92and TSCO (step 2) = 191 K. The compound has three high-spin Fe(II) sites at the highest temperatureof study; among them, two have slightly different coordination environments.These two Fe(II) sites are found to undergo a spin transition, while the thirdFe(II) site retains the high-spin state over the whole temperature range. Possiblereasons for the formation of the two steps in the spin transition curve are discussed.The observations made from the present study are in complete agreementwith those envisaged from earlier magnetic and structural studies made on[Fe(II)(isoxazole)6](BF4)2, but highlights the nature of the spin crossover mechanism.

Chemical pressure effect on CMR behavior of Sr substituted [formula omitted

A series of systematic substitution of isovalent Sr ion into the Ca site of La 0.67 Ca 0.33 - x Sr x MnO 3 system has been studied for different x values ranging from x = 0.00 , 0.033 , 0.066 , … , 0.33 . It is observed that at x = 0.066 , the system begins to exhibit multiphasic behavior with the onset of X-ray shoulder peak at 2 θ value of 32 . 96 ∘ . In this paper, we report low field SQUID magnetization behavior and electrotransport properties of the above system with x = 0.00 , 0.033 and 0.066. It is noted that larger cationic size ( Sr 2 + ) does not allow the system to attain low bandwidth, which is the important parameter to control the electric and magnetic properties including Curie temperature ( T c ) and metal insulator transition temperature ( T IM ) . The role of Sr 2 + ion substitution as larger radius with respect to Ca 2 + ion in the La 0.67 Ca 0.33 - x Sr x MnO 3 system can specifically be assigned to enhance the electron bandwidth ( W ), which promotes higher T c and T IM . In addition to this, an interesting spin glass cluster and transition phenomena are observed at low temperature region ( ≈ 50 K ) for x = 0.066 .

Assessment of the exchange-correlation functionals for the physical description of spin transition phenomena by density functional theory methods: All the same?

This study aims to assess present day density functionals in the description of spin crossover iron(II) complexes. Two recently synthesized spin crossover complexes were considered. Theoretical calculations were made using 53 of the most popular exchange-correlation density functionals with triple zeta plus polarization quality basis sets. The present work shows that even though different density functionals can lead to different energy gaps between spin states, most of them are very similar for these two compounds when a comparison between energy gaps is sought. The present work shows that even though different exchange correlations can lead to different energy gaps between spin states, the difference between these gaps calculated at different geometries and that calculated at a given reference geometry is surprisingly independent of the choice of functional. The reasons for the similarities and the differences among exchange and correlation functional combinations are discussed.

Magnetophoretic study of photo-induced spin transition of single crystalline particles of cobalt–iron Prussian blue analogues

The photo-induced spin transition of crystalline microparticle of Co–Fe Prussian blue analog was investigated bymeans of magnetophoresis. An apparatus to observe the photo-induced spin transition of a single microcrystal in suspension was constructed, and the magnetophoretic velocities before and after the irradiation with 532 nm pulse laser were observed. The photo-induced spin transition from a low-spin state to a high-spin state of the microparticle was observed from theabrupt increase of the magnetophoretic velocity after the irradiation. The threshold value for spin transition was estimated to be lower than 10 mJ cm−2 by this method, more precisely than that reported from SQUID magnetometer measurements. Moreover, the magnetic susceptibility of an individual particle after irradiation was calculated from the magnetophoretic velocity change. The magnetic susceptibility values determined by the present method were highly scattered. This was caused not only by the experimental error resulting from Brownian motion but also by the difference in the spin transition probability for individual particles. The effect of size on the spin transition phenomena was discussed.

Theoretical Design of Optical Switches Using the Spin Transition Phenomenon

The spin characteristics of octahedrically coordinated Fe(II) compounds are determined from first-principles quantum chemical calculations. Four novel Fe(II) spin transition materials are suggested for use in optical switching applications.

Possible Photoinduced Spin Transitions in Bis(phenylmethylenyl)[2.2]paracyclophanes. A Spin−Orbit Coupling Study

A possible mechanism for the spin transitions in various stacking conformations of bis(phenylmethylenyl)[2.2]paracyclophanes, which have close-lying lowest singlet, triplet, and quintet spin states, is theoretically investigated by using diphenylcarbene dimers as models. Spin−orbit coupling (SOC) matrix elements, which play an essential role in the spin transition phenomena, are calculated with the effective one-electron spin−orbit Hamiltonian. The SOC between the first excited singlet state and the first excited triplet state and that between the first excited triplet state and the lowest quintet state are strong. The SOC between the first excited quintet state and the first excited triplet state and that between the first excited triplet state and the lowest singlet state are also strong. These results demonstrate that the spin conversion between the low-spin singlet state and the high-spin quintet state can occur via the first excited intermediate-spin triplet state. We propose that possible photoinduced spin-crossover phenomena can be observed in these organic molecular systems.

Control of charge-transfer-induced spin transition temperature on cobalt-iron Prussian blue analogues.

The electronic and spin states of a series of Co-Fe Prussian blue analogues containing Na(+) ion in the lattice, Na(x)()Co(y)()Fe(CN)(6) x zH(2)O, strongly depended on the atomic composition ratio of Co to Fe (Co/Fe) and temperature. Compounds of Co/Fe = 1.5 and 1.15 consisted mostly of the Fe(III)(t(2g)(5)e(g)(0), LS, S = 1/2)-CN-Co(II)(t(2g)(5)e(g)(2), HS, S = 3/2) site and the Fe(II)(t(2g)(6)e(g)(0), LS, S = 0)-CN-Co(III)(t(2g)(6)e(g)(0), LS, S = 0) site, respectively, over the entire temperature region from 5 to 350 K. Conversely, compounds of Co/Fe = 1.37, 1.32, and 1.26 showed a change in their electronic and spin states depending on the temperature. These compounds consisted mainly of the Fe(III)-CN-Co(II) site (HT phase) around room temperature but turned to the state consisting mainly of the Fe(II)-CN-Co(III) site (LT phase) at low temperatures. This charge-transfer-induced spin transition (CTIST) phenomenon occurred reversibly with a large thermal hysteresis of about 40 K. The CTIST temperature (T(1/2) = (T(1/2) descending + T(1/2) ascending)/2) increased from 200 to 280 K with decreasing Co/Fe from 1.37 to 1.26. Furthermore, by light illumination at 5 K, the LT phase of compounds of Co/Fe = 1.37, 1.32, and 1.26 was converted to the HT phase, and the relaxation temperature from this photoproduced HT phase also strongly depended on the Co/Fe ratio; 145 K for Co/Fe = 1.37, 125 K for Co/Fe = 1.32, and 110 K for Co/Fe = 1.26. All these phenomena are explained by a simple model using potential energy curves of the LT and HT phases. The energy difference of two phases is determined by the ligand field strength around Co(II) ions, which can be controlled by Co/Fe.

The Neel point for spin-transition systems: toward a two-step transition

Abstract The spin-transition (ST) phenomenon may be described by an Ising Hamiltonian with a temperature dependent tranvserse field. This Hamiltonian is analyzed for “antiferromagnetic” or “heterophilic” coupling, both in the mean-field approximation (MFA) and with a Monte Carlo simulation (MCS). Both methods show that above a critical value of the coupling parameter a two-step spin transition is obtained. This two-step behavior corresponds to “antiferromagnetic” ordering. Then, a global phase diagram is proposed, valid for Ising magnetic and spin-transition systems, for both “ferromagnetic” and “antiferromagnetic” couplings.

Iron (II) - 1,2,4,- triazole spin transition molecular materials

The phenomenon of spin transition is probably one of the most spectacular examples of molecular bistability. Our main goal along this line concerns the design of molecular materials exhibiting abrupt spin transitions accompanied by thermochromic and large thermal hysteresis effects. The ideal situation, in terms of possible application of these compounds as active elements of memory devices, is realized when room temperature falls in the middle of the thermal hysteresis loop. In this context, we review our work concerning iron(II)-1,2,4-triazole compounds. We first introduce the idea that the cooperativity should be more pronounced in polymeric than in mononuclear compounds. Then we present some results concerning trinuclear iron(II)-1,2,4-triazole species. The heart of the paper is devoted to the polymeric compounds of formulae \[Fe(Htrz)$\_{2}$(trz)\](BF$\_{4}$) and \[Fe(Htrz)$\_{3}$\](BF$\_{4}$)$\_{2}\cdot $ H$\_{2}$O. We report first on the magnetic, optical and calorimetric, then on the structural properties of these compounds. Afterwards, we introduce the concept of spin transition molecular alloy, and emphasize that it is possible to fine tune the spin transition regime of these alloys through their chemical composition. For some alloys, room temperature falls within the thermal hysteresis loop. In the conclusion, the mechanism of cooperativity in the spin transition polymeric compounds is briefly discussed.

From Spin Transition to Display and Memory Devices

Abstract After a presentation of a user-friendly and reliable optical technique to characterize the spin transition phenomenon in iron II derivatives, this paper describes the first realization of a thermally addressed display. This new technology, still at the beginning of its development, is promising for further recording or display applications.

Magnetic and mössbauer characterization of the discontinuous high-spin ( 6A 1)⇄low-spin ( 2T 2) transition in solid bis(pyridoxal 4-phenylthiosemicarbazonato)iron(III) chloride

The iron(III) complexes [Fe(HPhthsa) 2]Cl, [Fe(HEtthsa) 2]Cl and [Fe(HMethsa) 2]Cl were synthesized and the phenomenon of discontinuous spin transition between the high-spin ( 6A 1, S=5/2) and the low-spin ( 2T 2, S=1/2) states in [Fe(HPhthsa) 2]Cl was confirmed by the measurement of temperature dependent magnetism and Mössbauer spectra. The tridendate ligands used were pyridoxal 4-phenylthiosemicarbazone (H 2Phthsa), pyridoxal 4-ethylthiosemicarbazone (H 2Etthsa), and pyridoxal 4-methylthiosemicarbazone (H 2Methsa). The magnetic moments for the spin-crossover complex show a thermal hysteresis of width δ T≈9 K with the transition centered at T c ↑≈299 K for increasing temperature and at T c ↓≈290 K for decreasing temperature. Variable temperature 57Fe Mössbauer results also provide evidence for the presence of a first order phase transition in this complex. The first order (cooperative) nature of the spin-crossover transformation most likely occurs through the extended coupling of ferric complexes via intermolecular H bonds. The EPR results indicate that the ground state is a well separated Kramers doublet in the low-spin complexes.

Spin transition in iron complexes induced by heat, pressure, light and nuclear decay

The phenomenon of temperature-dependent spin transition will be introduced and the numerous chemical and physical influences affecting the spin transition characteristics will be discussed. We shall mainly concentrate on the spin crossover system [Fe(2-pic)3]X2·Sol (2-pic=2-aminomethylpyridine; X=Cl, Br; Sol=C2H5OH, CH3OH) and demonstrate how the behaviour of the spin transition5T2g(Oh)⇌1A1g(Oh) is influenced by substituting the metalion, the non-coordinating anions X, the crystal solvent molecules Sol and by isotopic exchange with H/D and14N/15N. It will also be shown that the spin transition is very susceptible to pressure. A quantitative spin state conversion from low spin to high spin can also be achieved by illuminating the crystals of a spin crossover system at sufficiently low temperatures. The metastable quintet state can be trapped with practically infinite lifetimes. Several examples for this “Light-Induced Excited Spin State Trapping” (LIESST) will be given. Finally, the occurrence of short-lived anomalous spin quintet states following the57Co(EC)57Fe nuclear decay, which have been observed in the Mossbauer emission spectra of57Co-doped complex compounds, will be discussed with particular references to the LIESST effect.