Magnetic field perturbations to a soft X-ray activated Fe (II) molecular spin state transition

The X-ray induced spin crossover transition of an Fe(II) molecular thin film in the presence and absence of a magnetic field has been investigated. The thermal activation energy barrier in the soft X-ray activation of the spin crossover transition for [Fe{H2B(pz)2}2(bipy)] molecular thin films is reduced in the presence of an applied magnetic field, as measured through X-ray absorption spectroscopy at various temperatures. The influence of a 1.8 T magnetic field is sufficient to cause deviations from the expected exponential spin state transition behavior that is measured in the field free case. We find that orbital moment diminishes with increasing temperature, relative to the spin moment in the vicinity of room temperature.

The fact that the application of a magnetic field tends to relax [Fe{H2B(pz)2}2(bipy)] back to the low spin state after the soft X-ray excitation to the high spin state [12] suggests that an applied magnetic field may lower the activation barriers separating high spin and low spin states. A lowering of the free energy of the high spin state and a decrease in the activation barrier separating the high spin state from the low spin state, by application of a magnetic field, has also been suggested for Fe(phen)2(NCS)2 [11]. This magnetic field induced perturbation of the low spin to high spin state barrier is explored here for [Fe{H2B(pz)2}2(bipy)]. The influence of magnetic fields on the excitation and de-excitation in molecules has been observed before [13][14][15][16][17][18] and is not unique to spin crossover complexes. These latter studies are not of molecular systems but organics like anthracene and rubrene where this phenomenon arises due to the coupling of a molecule in the electronic singlet ground state with an adjacent molecule in the first excited singlet state to form a correlated triplet-pair state. In the case of [Fe{H2B(pz)2}2(bipy)], the excitation is accompanied by a spin state change [12,19], and perturbations to the spin state transition, by a magnetic field, is a step toward elucidating the origin of any magneto-electric coupling.

Magnetic Field Effects On The Excitation Spin Crossover
As previously reported [12], a soft X-ray irradiation excites [Fe{H2B(pz)2}2(bipy)] thin films on NiCo2O4(111) from the low spin state to the high spin state. Furthermore, an alternating magnetic field has been shown to perturb the [Fe{H2B(pz)2}2(bipy)] thin film spin state on NiCo2O4(111) [12]. Here we demonstrate how a static applied magnetic field subtly changes the characteristics of the spin state evolution induced by incident X-rays.
The spin state occupancy of [Fe{H2B(pz)2}2(bipy)] Fe(II) thin films may be ascertained from the X-ray absorption spectra (XAS) [12,[19][20][21][22][23][24][25][26][27]. In the low spin (LS) state of the [Fe{H2B(pz)2}2(bipy)] molecule, the 3d electrons occupy the t2g orbitals in pairs leaving the eg orbitals empty. The empty eg orbitals are generally observed in the Fe L3 edge (2p3/2) Xray absorption spectra as a major feature at a photon energy around 708.9 eV. In the high spin (HS) state configuration, the eg orbitals are partially populated while the t2g orbitals become partly depopulated, and thus contribute features in the XAS spectra at lower photon energies leading to a decrease of the peak intensity at 708.9 eV and an increase of the t2g shoulder around 706.8 eV. as shown in Figure 1. The asymmetry between the empty t2g and empty eg state peaks signatures is a measure of the fraction of molecules in the high spin state as has been shown experimentally [12,[19][20][21][22][23][24][25][26][27]. Such analysis of the XAS spectra has also been shown to be in agreement with theory as the spin state occupancy changes [20].
There is an evolution of the XAS spectra of [Fe{H2B(pz)2}2(bipy)] thin films, on NiCo2O4(111), that occurs with an incident X-ray fluence, as seen in Figure 1. From these spectra, there is clear evidence of an evolution of the average spin state, from a low spin state, towards larger fractions of high spin state with increasing incident X-ray fluence, , as reported elsewhere [12]. This soft X-ray excitation into the high spin state has been demonstrated to be reversable [12].
This evolution of the average spin state occupancy is perturbed by the presence of an external magnetic field. The XAS spectra indicate differences in the spin state occupancy for identical X-ray exposures where the only variable is the applied external magnetic field. Figure 1a shows spectra from the time evolution at 300 K, measured without any magnetic field, as well as measured with an applied field of 1.8 T. While the eg to t2g peak ratios for the initial state are identical, the XAS spectra, with and without an applied magnetic field, differ markedly after 3000 s of the X-ray exposure. Analysis of the relative intensity of the eg and t2g XAS features show that in the absence of an applied magnetic field, the [Fe{H2B(pz)2}2(bipy)] molecular thin film resides well more than 80% in the high spin state. In the presence of a 1.8 T out-of-plane applied magnetic field, however, the [Fe{H2B(pz)2}2(bipy)] molecular thin film, on NiCo2O4(111), resides in a mixed spin state with a relatively smaller high spin state fraction (i.e. <70% high spin), as indicated from the XAS spectra. Differences in the spin state occupancy are evident at other temperatures as well (Figure 1b and Supplementary Material). This is consistent with prior observations that the magnetic field has been seen to perturb the [Fe{H2B(pz)2}2(bipy)] thin film spin state on NiCo2O4(111) [12]. To clarify this soft X-ray induced spin state evolution of [Fe{H2B(pz)2}2(bipy)] molecular thin film on NiCo2O4(111), with and without an applied magnetic field, we extracted the occupancy evolution from series of spectra taken over the course of 6000 s (the data for 200 K and 300 K are plotted in Figure 2). All temperatures shown here are above the spin crossover temperature of the pure molecule which is generally observed to occur at about 160 K [20][21][22]26,[28][29][30]. Yet without an incident X-ray fluence, the [Fe{H2B(pz)2}2(bipy)] molecular thin films on NiCo2O4(111) are locked in the low spin state by the interface to the substrate [12]. The evolution towards the high spin state is the result of the X-ray fluence overcoming the tendency of the substrate to lock the [Fe{H2B(pz)2}2(bipy)] molecular thin film in the low spin state. In the presence of an applied magnetic field, the initial increase towards the high spin state occupancy is rapid (red shaded curves in Figure 2). We denote this initial increase in high spin state occupancy as regime I. This is followed by a slower and more linear increase in high spin state occupancy (the blue shaded curves in Figure 2). We denote this latter evolution toward the high spin state, in an applied magnetic field, as regime II. This evolution towards high spin state occupancy behavior, in an applied magnetic field, differs significantly from the exponential increase in high spin state occupancy observed in the absence of an applied magnetic field (black shaded curves in Figure 2).  (111), that accompany an incident soft X-ray fluence are plotted for a) 300 K and b) 200 K, in the absence of an applied magnetic field (black) as well as in the presence of a 1.8 T applied magnetic field (red and blue). Exponential fits to the high spin state conversion in the absence of an applied magnetic field are indicated.
The excitation to the high spin state, in the presence of the X-ray fluence and an applied magnetic field , is complcated by competing process, which are also evident as there are fluctuations in the evolution to the high spin state ( Figure 2, blue regions). These fluctuations, on the time scale of tens to 200 seconds in regime II (Figure 2), are both magnetic field and temperature dependent, thus unlikely to be instrumental, but are indicative of competing dynamic processes whose nature should be investigated but is outside ths cope of this work. Here we focus on the effects on a long timescale of tens of minutes to hours.

The Temperature Dependence of The Spin To Orbital Moment Ratio
A thermal component to the magnetic moment and the spin state transition must occur through the ligand field. This implies the orbital moment is more susceptible to temperature than the spin moment in the high spin state. To test this, we measured the high spin state spin to orbital moment of [Fe{H2B(pz)2}2(bipy)] thin films in the high spin state on a substrate where the influence of the substrate on the spin state of the [Fe{H2B(pz)2}2(bipy)] thin film is known to be minimal, i.e. graphite [25,26]. Temperatures in the vicinity of 300 K are well above the spin state transition of about 160 K [20][21][22]26,[28][29][30] as noted above, but thermal effects are seen in the orbital contribution to the local moments, as seen in Xray circular dichroism (XMCD) spectra at the Fe 2p core thresholds of [Fe{H2B(pz)2}2(bipy)] thin films on graphite, as seen in Figure 3.
The normalized XMCD data, for the Fe 2p3/2 and 2p1/2 (L3 and L2 edges), for [Fe{H2B(pz)2}2(bipy)] thin films on graphite, is shown in Fig. 3a for different temperatures. After subtracting a linear background, sum rules were applied to calculate the average spin moment based on the relative integrated XMCD signals, as shown in Figure 3b, following the procedure of Chen et al. [31]. The moments obtained from XMCD will be only a small fraction of the actual Fe(II) moments, largely because the [Fe{H2B(pz)2}2(bipy)] moments are not saturated [32], but also because the thin films contain both chiral isomers each of which respond differently to circularly polarized light, the light is elliptical not perfectly circularly polarized, there is likely a misalignment of light angular momentum vector with respect to the magnetic moment and there are Fresnel boundary conditions in play. Yet in the high spin state a local moment is likely the case. As seen in Figure 4a, the relative spin moment Sz (red), extracted from the XMCD spectra, is relatively insensitive to small changes in temperature while the relative orbital moment Lz (blue) decreases significantly with a small increase in temperature for the [Fe{H2B(pz)2}2(bipy)] on graphite sample. (a) The spectra were taken at 280 K (green), 290 K (red) and 300 K (blue). (b) The Fe 2p3/2 and 2p1/2 (L3 and L2) XMCD spectrum taken at 280 K (blue curve) compared to the integrated XMCD signal (red curve). The XMCD spectra are plotted as a percent of the total X-ray absorption spectra. The expected spin to orbital moment ratio should lie between 3 and 6, for an Fe(II) spin crossover complex, like [Fe{H2B(pz)2}2(bipy)], in the high spin state [33]. Estimating the ratio of the spin to orbital moment from the XMCD spectra as <Sz>/<Lz>= (9p -6q)/2q [31], with p and q determined from the integrated XMCD plots, as indicated in Figure 3, we find that the <Sz>/<Lz> from about 4.5 to nearly 7 (Figure 4b). This is reasonably consistent with expectations. More surprising is that the spin to orbital moment ratio <Sz>/<Lz> varies widely with small changes in temperature (Figure 4b) as a result of the changes in orbital moment with temperature ( Figure 4a). This indicates that the orbital moment of [Fe{H2B(pz)2}2(bipy)] is very soft with respect to temperature changes.

Discussion
It is clear that a static applied magnetic field has an influence on the X-ray induced transition from the low spin to high spin state for [Fe{H2B(pz)2}2(bipy)] molecular thin films on NiCo2O4(111). We note that, in the absence of an applied magnetic field, the observed exponential increase in high spin state occupancy for [Fe{H2B(pz)2}2(bipy)] molecular thin films on NiCo2O4(111), between 200 K and 300 K (black shaded curves in Figure  2), resembles the exponential increase in high spin state occupancy observed as a result of X-ray beam induce low spin state (LS) to high spin state (HS) transitions at low temperatures (5 K), seen for [Fe{H2B(pz)2}2(bipy)] molecular thin films on graphite [25,26]. This soft X-ray-induced excited spin state trapping (SOXIESST), seen for [Fe{H2B(pz)2}2(bipy)] molecular thin films on graphite at below 40 to 50 K, occurs because the graphite substrate does not lock the spin state of the [Fe{H2B(pz)2}2(bipy)] molecular thin films into the low spin state [25,26], while in contrast to the case graphite of the weakly interacting graphite subsrate, a spin state locking effect occurs for [Fe{H2B(pz)2}2(bipy)] molecular thin films on NiCo2O4(111) [12]. The temperature dependence of the spin state evolution provides an opportunity to estimate the thermal component of the activation barrier in this X-ray induced transition. Arrhenius rate plots with temperature ( Figure 5) were calculated based on the high spin occupancy results shown above, based on the initial rates of spin state. In the absence of an applied magnetic field, an activation barrier of 14.5+1.5 meV per molecule is calculated. The initial rates of [Fe{H2B(pz)2}2(bipy)] conversion to high spin state in an applied magnetic field (regime I, in Figure 2) suggests a somewhat lower activation energy of 7.2+2 meV. After a significant fraction of the film has already converted to the high spin state (regime II in Figure 2), the activation energy in an applied magnetic field is again restored to 15.9+2.7 meV, as indicated by the inset in Figure 5. A magnetic field dependence on the free energy and activation barrier has been suggested for the Fe(phen)2(NCS)2 spin state transition [11], so the effect of larger magnetic fields than exploited here is of considerable interest.
The decrease in the thermal component to the activation energy can also explain the conversion rate curves of Figure 2 for this X-ray induced transition. When the [Fe{H2B(pz)2}2(bipy)] high spin state occupancy increases enough, it is clear that X-ray induced photo-excitation to the high spin state of the small amount of material remaining in the low spin state begins to compete with the conversion of the large amount of film in the high spin state back to the low spin state favored by the [Fe{H2B(pz)2}2(bipy)] interaction with the NiCo2O4(111) substrate. This is certainly consistent with a nearly 50% decline in activation energy with the application of a magnetic field. Due to possible inhomogeneities, such as a distribution of crystalline grain sizes, the thermal energy barriers towards conversion to the high spin state or back conversion to the low spin state could be only lowered for part of the film, while either unchanged or retaining a different value for another part of the film.
The presence of this low spin state back-conversion prevents a full excitation into the high spin state when there is an applied magnetic field. Indeed, this enhanced back-conversion to the low spin state has been seen previously for [Fe{H2B(pz)2}2(bipy)] thin films, as a result of applied alternating magnetic field [12]. It is then the competition between excitation to the high spin state and the enhanced back conversion to the low spin state that then leads to the apparent increase in the thermal component of the activation energy. The appearance of two regimes in the soft X-ray fluence induced evolution (regime I and II in Figure 2) can be explained by expanding the model to include two separate conversion rates, as indicated in models of the spin crossover plotted in Figure 6. This difference in [Fe{H2B(pz)2}2(bipy)] spin state conversion rates could either be induced by heterogeneity or by a different behavior of the film depending on the proximity to the substrate or vacuum interface, as it has been observed before [21]. Figure 6. A simulation of the evolution of the high spin state occupancy in the [Fe{H2B(pz)2}2(bipy)] molecular thin film assuming a simple excitation rate uniformly throughout the film (blue) and an inhomogeneous excitation process where 40 % of the film is activated with one constant rate and 60% of the film at a lower rate that is 10% of the higher rate (orange).
A single molecule could gain = ⋅ energy for a moment change under the presence of a magnetic field, but with a moment change of ∆m=4 µB and an applied field H=1.8 T, this gain would be 0.42 meV at most. In fact, because of finite temperature effects, leading to moment canting, the actual energy in the vicinity of room temperature would be much smaller. A magnetic field dependent change on an energy scale of several meV is well above that level and can only be explained by cooperative coupling of several molecules. This suggests that the spin crossover occurs collectively with a cluster of molecules, consistent with previous work [21,23] which indicates spin-state domains in Fe{H2B(pz)2}2(bipy)] thin films.
These activation energy values of 14.5 to 15.9 meV per molecule are close to the 14 to 18 meV value of the thermal activation energy component of the X-ray fluence induced transition to the high spin state for [Fe{H2B(pz)2}2(bipy)] locked in the low spin state with zwitterion additions [27]. This similarity suggests that the energetics involved in locking of [Fe{H2B(pz)2}2(bipy)] in the low spin state by zwitterion additions or by a NiCo2O4(111) substrate are comparable.
The presence of thermal activation barriers does imply that the ligand field of [Fe{H2B(pz)2}2(bipy)] is subject to temperature variations. The thermal effects on the ligand field is supported by the significal changes in the relative orbital moment, with temperature as validated by the XMCD measurements. As noted above, spin to orbital moment ratio <Sz>/<Lz> for [Fe{H2B(pz)2}2(bipy)] thin films on graphite varies widely with small changes in temperature (Figure 4b). For such a wide variation of <Sz>/<Lz> to occur, in response to small temperature changes, the ligand field for [Fe{H2B(pz)2}2(bipy)] must be sensitive to temperature. This sensitivity of the ligand field, evident in the wide variation in <Lz> with small changes in temperature, also provides some explanation as to why there is a thermal component of the activation energy barrier in the soft X-ray activation of the spin crossover transition for [Fe{H2B(pz)2}2(bipy)] molecular thin films.
Magnetic anisotropy is very characteristic of local moment molecular systems [34], yet the effect of an anisotropy barrier resulting from the substrate [19,21] or cooperative effects [21,23] cannot be excluded by any data presented here data. The fact that this ligand field is both temperature sensitive and that there are known cooperative effects [21,23,25,26] indicates that a magnetic anisotropy is a very real possibility.

Materials and Methods
The [Fe{H2B(pz)2}2(bipy)] molecule was synthesized as described elsewhere [20], and characterized as noted elsewhere [12,[19][20][21][22][23]28]. A schematic diagram of [Fe{H2B(pz)2}2(bipy)] is shown in Figure 1. The X-ray absorption spectroscopy measurements of [Fe{H2B(pz)2}2(bipy)] thin films were performed at the Advanced Light Source at Lawrence Berkeley National Laboratory on the bending magnet beamline 6.3.1 at a reduced storage ring current of 15 mA using circularly polarized, negative helicity photons. Spectra were taken both in the absence of an applied magnetic field as well as in the presence of a 1.8 T applied magnetic field, with the field applied perpendicular to the surface of the film. The incident photon flux was in the region of 10 11 photons s -1 0.1%BW -1 . The sample temperature was allowed to equilibrate for 2 hours at each temperature before the measurement ensuring not only a stable temperature but also negligible positional drift with respect to the ~80 µm diameter of the x-ray beam along the cryostat direction (the Xray beam spot size is is 180 µm x 80 at the Advanced Light Source beamline 6.3.1). The total electron yield mode was used to measure the absorption across the Fe 2p3/2 (L3) edge, as was done in previous studies [12,[19][20][21][22][23][24] that have also confirmed that the x-ray induced excitation into the high spin is reversible.
To get into the high spin state, a soft X-ray fluence has been seen to be effective, and as noted previously, this is seen for [Fe{H2B(pz)2}2(bipy)] on NiCo2O4(111) [12]. The experiments were repeated at different temperatures on a fresh spot of the [Fe{H2B(pz)2}2(bipy)] thin film on NiCo2O4(111), to ensure otherwise equivalent starting conditions for each measurement. In addition, control experiments with a swapped order of in-field and field free measurement conditions and various stabilizations time of the electromagnet were made to exclude thermal drift.
Additional X-ray absorption spectroscopy (XAS) and magnetic circular dichroism (XMCD) characterization of [Fe{H2B(pz)2}2(bipy)] thin films were performed at Advanced Photon Source beamline 4-ID-C at Argonne National Laboratory [37,38]. The spot size is roughly 1 mm at Advanced Photon Source beamline 4-ID-C. For these XMCD measurements, the samples consisted of nominal ~30 nm of [Fe{H2B(pz)2}2(bipy)] thin films on a HOPG substrates, again prepared using thermal evaporation. Graphite, as a substrate, is better suited for studies of magnetic field perturbations of [Fe{H2B(pz)2}2(bipy)] in the high spin state than NiCo2O4(111), as graphite is known as a substrate where [Fe{H2B(pz)2}2(bipy)] undergoes a complete thermal-and light-induced spin transition even in the monolayer limit [25,26], very much like the powder, so substrate effects on the [Fe{H2B(pz)2}2(bipy)] thin film are thus minimal. Measurements of the Fe L2,3-edge XMCD spectra were taken in total electron yield mode with a 2 T magnetic field applied nearly perpendicular to the sample surface. In order to eliminate some of the artifacts, the final spectrum was obtained from the difference of two spectra taken at + 2 T magnetic field. To study the temperature effects, XMCD data was obtained at several temperatures near room temperature, T = {280 K, 290 K, 300 K}.

Conclusions
There is now clear evidence that a moderate, static magnetic field can affect the soft X-ray induced conversion of [Fe{H2B(pz)2}2(bipy)] thin film spin state, locked in the low spin state by a NiCo2O4(111) substrate. The changes in the activation energy and in the soft X-ray photo-excitation enhanced spin crossover transition indicates an influence of the magnetic field on the thermal component of the spin-crossover transition. In the presence of a large magnetic field (2T) the ligand field is seen to be sensitive to variations in temperature as the relative orbital moment changes dramatically with small changes in temperature. This is consistent with a temperature dependence of the the soft X-ray induced conversion of [Fe{H2B(pz)2}2(bipy)] thin film spin state. Deepening the understanding of this relationship between magnetic, electric, and optical factors of the spin-crossover transition is prerequisite for a thermodynamic basis for multiferroic behavior and magneto-electric coupling. .
Supplementary Materials: Figure S1 contains X-ray absorption spectra taken at 250 K with and without the presence of an applied 1.8 T magnetic field. Reference X-ray absorption spectra of the [Fe{H2B(pz)2}2(bipy)] low spin and high spin states are shown in Figure S2. The evolution of the high spin state at 250 K, with and without the presence of an applied 1.8 T magnetic field, is shown in Figure S3. Data Availability Statement: The data presented in this study arises from the sources cited or are available on request from the corresponding authors.

Conflicts of Interest:
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
Appendix A Figure S1. The time evolution of the X-ray absorption spectra with and without an applied 1.8T magnetic field of [Fe{H2B(pz)2}2(bipy)], under a constant incident X-ray fluence at 250 K. The spectra in blue are indicative of the low spin state, while the spectra in red are largely indicative of the high spin state. Figure S2. Higher resolution X-ray absorption spectra showing an example of reference spectra for the high spin state and low spin state of [Fe{H2B(pz)2}2(bipy)] above and below the accepted spin transition temperature in the region of 160 K. The reference spectra provide a facile mechanism for ascertaining spin state occupancy. Figure S3. The evolution of the high spin state occupancy in the [Fe{H2B(pz)2}2(bipy)] molecular thin film that accompany an incident soft X-ray fluence at 250 K, in the absence of an applied magnetic field (black) as well as in the presence of a 1.8 T applied magnetic field (red and blue). Exponential fit to the high spin state conversion in the field-free case is indicated as a thin solid black line.