Spin crossover (SCO) complexes have been shown to exhibit giant mechanocaloric effects. Due to the change of magnetization at the spin crossover transition, they are also expected to show magnetocaloric effects. However, experimental studies on the magnetocaloric properties in SCOs are scarce. Here, we have studied the magnetocaloric response in the SCO complex [Fe(L)2](BF4)2, [L = 2,6-di(pyrazol-1-yl)pyridine] using pulsed magnetic fields. We show that applying a magnetic field can induce a partial transformation from the low spin to the high spin state. We directly measure the adiabatic temperature change of the transformation for different initial sample temperatures and magnetic fields and compare them with calculations using the Clausius–Clapeyron equation. While we found a large change in entropy of 70 J kg−1 K−1 at 50 T, the corresponding temperature change of 1.5 K is small due to the weak dependence of the transformation temperature on the magnetic field. Our study enhances the knowledge of caloric effects in SCO complexes, which so far have mainly focused on mechanocaloric studies.
I. INTRODUCTION
Many metal–organic complexes exhibit a spin crossover (SCO) phase transition from a high spin (HS) to a low spin (LS) phase, which can be triggered by diverse external stimuli such as temperature, pressure, and light irradiation. In these materials, the crystal field splitting of the metallic ion d-orbitals is of the order of the thermal energy (kBT). At high temperatures, the complex is in a HS state with a net magnetic moment at the metallic ions, resulting from the distribution of electrons within d orbitals according to first Hund’s rule. However, at low temperatures, first Hund’s rule is broken, and the complex is in a LS state.1 The switching between HS and LS states gives rise to many technological applications of these complexes.2
Because the SCO transition involves a considerable latent heat and a relative volume change of the unit cell, it was suggested that SCO complexes would exhibit giant mechanocaloric properties when subjected to mechanical stresses.3 The occurrence of giant barocaloric effects has recently been demonstrated for a variety of SCO complexes4–6 subjected to hydrostatic pressure, and very recently, elastocaloric effects associated with the SCO transition have been reported for SCO/polymer films subjected to uniaxial stress.7
At the SCO transition, there is a significant change in the magnetic properties, with the complex being paramagnetic in the HS state and diamagnetic in the LS state. It is, therefore, expected that a magnetic field will affect the SCO transition and also that SCO materials exhibit a magnetocaloric effect (MCE). There are few works on the effect of the magnetic field on the SCO transition,8–13 which report small effects of the magnetic field on the SCO temperature and on the relative fraction of material in the HS state. With regard to MCE, theoretical studies14 predict isothermal entropy changes (ΔS = 3.04 J K−1 kg−1 for 10 T of magnetic field), but there is a lack of experimental data.
In this work, we present a detailed study of the magnetocaloric properties of a prototype SCO complex, [Fe(L)2](BF4)2, [L = 2,6-di(pyrazol-1-yl)pyridine], which has been shown to exhibit giant mechanocaloric properties.4,7 We have used diverse experimental techniques to determine the isothermal entropy (ΔS) and adiabatic temperature (ΔT) changes resulting from the application of high magnetic fields (up to 50 T).
II. EXPERIMENTAL DETAILS
A. Sample preparation
The ligand [L] [L = 2,6-di(pyrazol-1-yl)pyridine] was synthesized in a two step procedure.15 A mass of 6.80 g (0.10 mol) of pyrazole was dissolved in diethylene glycol dimethyl ether (50 cm3) in a two-necked round bottom flask at room temperature. A mass of 2.30 g (0.10 mol) of metallic sodium was added to this solution in small pieces under an argon atmosphere. To obtain sodium pyrazolate, the solution was stirred for 3–4 h. A 7.4 g (0.05 mol) mass of 2,6-dichloropyridine was added, and the solution was heated under reflux for 12 h and then left to cool down to room temperature. Cold distilled water (300 cm3) was added, and the solid that formed was filtered by suction and then purified with a hot methanol–water mixture. This gave bright white crystals. The yield was 72% with a melting point of 136–138 °C.
To synthesize the complex [Fe(L)2](BF4)2, Fe(BF4)2 · 6H2O (0.338 g, 0.001 mol) was dissolved in MeOH (25 ml). In another flask, the ligand (0.422 g, 0.002 mol) was added to 40 ml of MeOH. Both solutions were combined and stirred while heated for 5 min. The product was a crystalline yellow complex, which was filtered and dried in air. The yield was 72%. The calculated elemental analysis for [Fe(L)2](BF4)2 was C: 40.51, H: 2.78, N: 21.47, and Fe: 8.56; and the measured analysis was C: 39.82, H: 2.61, N: 21.07, and Fe: 8.59.
B. Calorimetric and magnetic measurements
Differential scanning calorimetry (DSC) was performed in a TA instruments Q2000 calorimeter on 19.7 mg of powder at scanning rates of 10 K min−1.
Magnetic measurements for low fields were performed with a Superconducting Quantum Interference Design (SQUID) magnetic property measurement system (MPMS). A mass of mg was added to a polycarbonate capsule and placed inside a vertical plastic straw. Isofield magnetization was measured with a 2 K min−1 temperature scanning rate at selected values of an applied magnetic field in the range of 0–7 T. Measurements at high magnetic fields were performed at the Dresden High Magnetic Field Laboratory. For the pulsed high field measurements, about 100 mg of powder were compressed into two cuboids of around 3 × 2 × 1 mm3. A thermocouple was sandwiched in between the cuboids and fixed with silver epoxy. This sandwich was then inserted into a sample holder and secured with GE Varnish. A heater surrounds the sample to control the temperature. The sample was then placed in a pulse-field coil placed in a tank with liquid nitrogen. Further details of the experimental setup can be found in Ref. 16. Before each measurement, the sample was brought to 220 K to ensure that it was completely in the low spin state. Then it was heated to the desired temperature before pulsing first a negative and then a positive magnetic pulse. The magnetic field was measured using a pickup coil. The real field values were a few percent higher than indicated in the results. However, for the sake of clarity, the maximum fields were rounded to integer values. The absolute values of the thermocouple were recalibrated to fit the transformation temperature of the sample measured in static magnetic fields.
III. RESULTS AND DISCUSSION
Figure 1 shows the calorimetric measurement of the SCO complex. On cooling, the transition starts at = 259 K and finishes at = 254 K, while on heating it starts at = 258 K and finishes at = 263 K. The spin crossover transformation proceeds with a hysteresis of ∼4 K, as previously reported for this SCO complex.17 Magnetic measurements of the SCO sample are summarized in Fig. 2. The magnetization as a function of temperature in different fields [Fig. 2(a)] shows a negligible magnetic moment below the spin crossover transition temperature where the sample is in the low spin state. The zoom-in for low temperatures [inset in Fig. 2(a)] confirms that the magnetization is negative at low temperatures, and thus that the SCO is diamagnetic in a low spin state. At the SCO transition temperature, the magnetization increases abruptly. The transition temperatures from magnetization data are in agreement with those derived from DSC. For the measured fields between 1 and 7 T, the transition temperature (determined as the inflection points in the magnetization curves) does not depend upon the magnetic field, within experimental errors. Figure 2(b) shows the isothermal magnetization measurements for selected values of temperature within the SCO transition range. In the HS state, the sample exhibits paramagnetic behavior.
Transformation of the SCO sample measured with differential scanning calorimetry (DSC) on heating (upper curve, red) and cooling (lower curve, blue).
Transformation of the SCO sample measured with differential scanning calorimetry (DSC) on heating (upper curve, red) and cooling (lower curve, blue).
(a) Transformation of the SCO sample in magnetic measurements at different magnetic fields. The inset is a magnification of the same plot at temperatures below the transformation. (b) Isothermal magnetization measurements as a function of magnetic field for temperatures 221–255 K measured in 2 K steps. Lines are linear fits to the measurements.
(a) Transformation of the SCO sample in magnetic measurements at different magnetic fields. The inset is a magnification of the same plot at temperatures below the transformation. (b) Isothermal magnetization measurements as a function of magnetic field for temperatures 221–255 K measured in 2 K steps. Lines are linear fits to the measurements.
We performed measurements of the temperature change in pulsed magnetic fields for different initial sample temperatures for magnetic fields of 20, 30, and 50 T (Fig. 3). It is worth noticing that a magnetic field stabilizes the high temperature HS and promotes the transition from LS to HS, equivalent to heating. For that reason, before each measurement, the sample was cooled to 220 K to ensure that it was in the low spin state, and it was then heated to the desired measurement sample temperature. At each temperature, a measurement with a negative and a positive magnetic field pulse was performed. The curves in Fig. 3 show the average of the measurement with positive and negative fields, as this eliminates parasitic electromotive force signals induced in the thermocouple by the pulsed magnetic field.16 For each pulse, the magnetic field rises quickly within 15 ms and then decreases. The entire pulse duration is about 0.1 s, and an example of the pulse shape is depicted in the inset of Fig. 3(a) for a 50 T field. Each curve in Fig. 3 shows how the temperature of the sample changes with rising and decreasing pulsed magnetic fields for maximum applied fields of 20 T [Fig. 3(a)], 30 T [Fig. 3(b)], and 50 T [Fig. 3(c)]. The arrows in Fig. 3(a) exemplarily illustrate the increase and decrease in the field. The measurement curves of the sample temperature change with the magnetic field pulse have three characteristic shapes depending on the initial sample temperature. The first curve shape occurs for temperatures well below the transition temperature , e.g., the gray curve in Fig. 3(b): there, no temperature change is visible within the scatter of the measurement because the sample remains in the LS state. The second characteristic shape occurs for temperatures above the transition temperature , where ΔT rises with increasing magnetic field and then decreases with decreasing field [e.g., the green curve in Fig. 3(b) and the light blue curve in Fig. 3(c)]. This behavior stems from the conventional magnetocaloric effect of the paramagnetic sample at high temperatures, in the HS state. The third characteristic curve shape occurs when the initial temperature of the sample is between the transformation temperatures and or only slightly lower than . Examples correspond to Figs. 3(a) and 3(b) (orange curve at T = 260.8 K) and Fig. 3(c) (dark blue, green, and violet curves for T = 258.1, 259.8, and 260.9 K, respectively). In these cases, the large magnetic field induces the partial transformation of the sample from the LS to the HS state. This leads to a decrease in the sample temperature, which can be measured as a |ΔTad|, which is exemplarily marked in Fig. 3. This behavior corresponds to the inverse magnetocaloric effect associated with the SCO transition with increasing magnetic field. When the field decreases, the sample heats and recovers its initial temperature. The |ΔTad| value was extracted for each measurement as shown in Fig. 3(a), and it depends on the initial temperature of the sample, as illustrated in Fig. 4(a) by dots corresponding to the measurements at 20, 30, and 50 T. |ΔTad| first gradually rises the closer the sample temperature is to the transformation temperature until it suddenly drops once the transformation temperature is reached. Then, the sample is already in the high spin state, and no transformation can be induced by a magnetic field pulse. Comparing the measurements for different maximum values of the magnetic field shows that the induced |ΔTad| is larger for higher values of the magnetic field.
Temperature change during a magnetic field pulse of (a) 20 T, (b) 30 T, and (c) 50 T at different sample temperatures. Arrows in (a) represent the increasing and decreasing field. The inset in (a) shows the change of magnetic field during the pulse over time.
Temperature change during a magnetic field pulse of (a) 20 T, (b) 30 T, and (c) 50 T at different sample temperatures. Arrows in (a) represent the increasing and decreasing field. The inset in (a) shows the change of magnetic field during the pulse over time.
(a) Measured values of adiabatic temperature change at selected magnetic field values extracted from the pulse field measurements (solid symbols) and adiabatic temperature change at selected magnetic field values calculated from temperature-dependent entropy curves (solid lines). (b) Temperature-dependent entropy curves at selected values of magnetic fields calculated from the DSC measurement and considering the field-induced shift in the transition (see the text for details). Vertical and horizontal arrows indicate the field-induced isothermal entropy and adiabatic temperature changes, respectively. (c) Isothermal entropy change as a function of temperature. The inset in (c) is an enlargement of a restricted temperature region showing the calculated curves for 1, 3, 5, and 7 T, together with dots, representing the entropy calculated from M(H) [Fig. 2(b)] curves via the Maxwell relation.
(a) Measured values of adiabatic temperature change at selected magnetic field values extracted from the pulse field measurements (solid symbols) and adiabatic temperature change at selected magnetic field values calculated from temperature-dependent entropy curves (solid lines). (b) Temperature-dependent entropy curves at selected values of magnetic fields calculated from the DSC measurement and considering the field-induced shift in the transition (see the text for details). Vertical and horizontal arrows indicate the field-induced isothermal entropy and adiabatic temperature changes, respectively. (c) Isothermal entropy change as a function of temperature. The inset in (c) is an enlargement of a restricted temperature region showing the calculated curves for 1, 3, 5, and 7 T, together with dots, representing the entropy calculated from M(H) [Fig. 2(b)] curves via the Maxwell relation.
A dependence of the kinetics of the SCO transition on the magnetic field sweep rate has been reported for other SCO complexes.13 In our experiments, such a dependence is also observed, as the strong temperature drop of the sample in Fig. 3 occurs at higher magnetic field values as the maximum applied field increases (which implies a larger field rate).
We have computed ΔSt by proper integration of the calorimetric data. No significant differences have been found for cooling and heating runs, with an average value of ΔSt = 92 J kg−1 K−1. In SCO compounds, the entropy is mostly of vibrational origin,18 and it is not expected that ΔSt significantly depends on the magnetic field.
We have computed the saturation magnetization from magnetization data (Fig. 2), and the value is 39.5 A m2 kg−1.
Integration of Eq. (1) gives the shift of the transition temperature δT(H) for a given magnetic field. It is found that |δT(H)| increases quadratically with H, as also reported for a different SCO compound.11
In Fig. 4(a), we plot the maximum values in temperature change measured in the pulsed field. The measured values of |ΔTad| are about half of the calculated values. This difference is to be expected from the measurement setup: on the one hand, the thermal delay between the sample and the thermocouple leads to an underestimation of the magnetocaloric effect, and on the other hand, the presence of inert mass (silver paste, varnish, etc.) also reduces the measured ΔT values. Still, we can show that the measured values of temperature change are comparable with the expected values from the calculation.
We have found large ΔS values at high magnetic fields (ΔS = 70 J kg−1 K−1 for 50 T), but |ΔTad| exhibits only small values (|ΔTad| in the range 1–2 K). Notably, the measured |ΔTad| is much lower than . Such a discrepancy is due to the weak dependence of the transition temperature on the magnetic field. When the transition temperature is weakly sensitive to the magnetic field, the adiabatic temperature change is approximately ,21 which is consistent with our direct |ΔTad| measurements.
IV. CONCLUSION
We have measured the MCE effect in a prototype SCO complex. It is found that MCE is inverse, in accordance with the magnetic field stabilizing the high temperature HS state. The isothermal entropy change for a 50 T applied magnetic field is large (ΔS = 70 J kg−1 K−1), but the weak sensitivity of the transition temperature to the magnetic field gives rise to very low adiabatic temperature changes, with ΔT ∼ 1.5 K for a 50 T magnetic field. Present results provide extensive fundamental knowledge of a technologically relevant material. From a technological point of view, the low values found for the MCE temperature changes make this material of limited interest for magnetocaloric cooling, in contrast to its excellent barocaloric and elastocaloric recently reported properties.
ACKNOWLEDGMENTS
We acknowledge the financial support from MCIN/AEI/10.13039/501100011033 (Spain) under Grants Nos. PCI2022-132957 (EU, MAT ERA.Net Program) and PID2020-113549RB-I00/AEI and from AGAUR (Catalonia) under Project No. 2021SGR00328. E.S.-T. acknowledges the support from Grant No. IJC2020-043957-I funded by MICIU/AEI/10.13039/501100011033 and by the European Union NextGeneration/PRTR. The authors acknowledge the funding from HLD at HZDR, a member of the European Magnetic Field Laboratory (EMFL); from Deutsche Forschungsgemeinschaft (DFG) through BEsT (Project-ID 456263705); and from the EU Horizon 2020 project ISABEL (Grant Agreement No. 871106). We acknowledge the Scientific and Technological Research Council of Turkey (TÜBÏTAK), Grant No: MFAG-121F151, and the Ankara University Scientific Research Projects Coordination Unit (AÜ-BAP) Grant No. ADEP FBA-2022-2660.
AUTHOR DECLARATIONS
Conflict of Interest
The authors have no conflicts to disclose.
Author Contributions
Klara Lünser: Conceptualization (equal); Data curation (equal); Formal analysis (lead); Investigation (lead); Visualization (equal); Writing – original draft (equal). Catalina Salazar Mejía: Data curation (equal); Investigation (lead); Resources (equal); Writing – review & editing (equal). Tino Gottschall: Conceptualization (equal); Project administration (equal); Supervision (equal); Writing – review & editing (equal). Kübra Gürpinar: Investigation (equal); Writing – review & editing (equal). Orhan Atakol: Supervision (equal); Writing – review & editing (equal). Eyüp Kavak: Investigation (equal); Writing – review & editing (equal). Baris Emre: Funding acquisition (equal); Resources (equal); Supervision (equal); Writing – review & editing (equal). Enric Stern-Taulats: Conceptualization (equal); Investigation (equal); Visualization (equal); Writing – review & editing (equal). Lluís Mañosa: Conceptualization (lead); Funding acquisition (equal); Project administration (equal); Supervision (equal); Writing – original draft (lead); Writing – review & editing (equal).
DATA AVAILABILITY
The data that support the findings of this study are available from the corresponding author upon reasonable request.