Self-Enhancement of Rotating Magnetocaloric Effect in Anisotropic

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Self-Enhancement of Rotating Magnetocaloric Effect in Anisotropic Two-Dimensional (2D) Cyanido-Bridged MnII−NbIV Molecular Ferrimagnet Piotr Konieczny,*,† Łukasz Michalski,†,‡ Robert Podgajny,*,§ Szymon Chorazy,§ Robert Pełka,† Dominik Czernia,†,‡ Szymon Buda,§ Jacek Mlynarski,§ Barbara Sieklucka,§ and Tadeusz Wasiutyński† †

Institute of Nuclear Physics PAN, Radzikowskiego 152, 31-342 Kraków, Poland Faculty of Physics and Applied Computer Science, AGH University of Science and Technology, al. Mickiewicza 30, 30-059 Kraków, Poland § Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Kraków, Poland ‡

S Supporting Information *

ABSTRACT: The rotating magnetocaloric effect (RMCE) is a new issue in the field of magnetic refrigeration. We have explored this subject on the two-dimensional (2D) enantiopure {[MnII(R-mpm)2]2[NbIV(CN)8]}·4H2O (where mpm = α-methyl-2pyridinemethanol) coordination ferrimagnet. In this study, the magnetic and magnetocaloric properties of single crystals were investigated along the bc//H easy plane and the a*//H hard axis. The observed small easy plane anisotropy is due to the dipole− dipole interactions. For fields higher than 0.5 T, no significant difference in the magnetocaloric effect between both geometries was noticed. The maximal magnetic entropy change for conventional effect was observed at 32 K and the magnetic field change μ0ΔH = 5.0 T attaining the value of ∼5 J mol−1 K−1. The obtained maximal value of −ΔSm is comparable to previously reported results for polycrystalline octacyanidoniobate-based bimetallic coordination polymers. A substantial anisotropy of magnetocaloric effect between the easy plane and hard axis appears in low fields. This includes the presence of inverse magnetocaloric effect only for the a*//H direction. The difference between both geometries was used to study the rotating magnetocaloric effect. We show that the inverse part of magnetocaloric effect can be used to enhance the rotating magnetic entropy change up to 51%. This finding is of key importance for searching efficient materials for RMCE.

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nets9,10,12,14−18 or luminescent magnets,19 but also gave the opportunity of nontrivial interplay between magnetized matter and light irradiation, exploiting the second-order magnetooptical or magneto-chiral effects.12,14,17,18 One of the novel emerging potential applications of magnetic coordination polymers is the magnetocaloric effect (MCE), which is a magneto-thermodynamic process in which a change of magnetic field is used to alter the temperature of a magnetic material.20 Magnetic refrigeration based on MCE is considered to be an efficient and environmentally friendly alternative to the conventional gas compression−expansion refrigerators.21,22 To

INTRODUCTION Magnetic coordination networks have drawn strong interest since the 1990s, yielding the long-range ordered magnets with critical temperature (Tc) values above room temperature.1−3 Parallel to this, a new generation of so-called “multifunctional magnetic molecular materials” have emerged, relying on the additional functionalization of molecular spin carriers or networks with other key features (e.g., structural and electronic nonrigidity, host−guest behavior, noncentrosymmetry, chirality, luminescence, among others). Such compilation not only led to the interesting examples of reversible magneto-structural dynamic behavior (e.g., porous magnets, magnetic sponges, charge-transfer complexes, spin crossover magnets photomagnets), 4−13 noncentrosymmetric and chiral mag© XXXX American Chemical Society

Received: December 3, 2016

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DOI: 10.1021/acs.inorgchem.6b02941 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

the applied magnetic field. The variable temperature dc magnetic susceptibility under an applied field of 500 Oe was measured over a temperature range of 2−300 K. The isothermal magnetization was detected at 2.0 K in the field range from −7 T to +7 T. To calculate the MCE, isothermal magnetization curves were collected up to fields of 5.0 T in the temperature range of 2−80 K. The measurements of both components of the ac susceptibility (χac) were carried out as a function of temperature for 120.0 Hz and Hac = 3.0 Oe.

realize commercial refrigerators based on MCE, the scientific community concentrates on searching materials with large MCE, in specified temperature regions. These regions are determined by the type of materials, going from the long-range magnetic ordering TC coordination network23−25 toward lowtemperature isotropic Gd3+-based coordination clusters.26−30 Indeed, one of the simplest examples of the latter sort, Gd(HCO2)3,31,32 exhibits MCE of a magnitude comparable to gadolinium gallium garnet (Gd3Ga5O12), which is known to be a suitable refrigerant material at temperatures of 0.05 T, we can observe that the maximum shifts to lower temperatures from ∼22 K for 0.05 T down to ∼10 K for 0.2 T. This is a direct effect of the inverse MCE detected for the a*// H geometry. The highest −ΔSR value is equal to 0.17 J mol−1 K−1 at μ0H = 0.2 T and T = 10 K. Figure 7 shows a comparison of the RMCE in μ0H = 0.12 T and the conventional MCE for μ0ΔH = 0.12 T. Above 18.0 K (labeled as Te), the magnetic entropy changes for the bc//H and a*//H geometries have the same sign; therefore, the RMCE cannot exceed the −ΔSm value detected for the easy plane. Below Te, the presence of the hard axis induces the inverse MCE, which enhances the RMCE. In this instance, this leads to an excess of up to 42%, compared to the conventional easy plane MCE. To fully understand this effect, let us separately consider the RMCE and the conventional MCE for easy plane for a

Figure 3. Temperature dependence of zero-field-cooled (solid line) and field-cooled (open symbols) magnetization for H = 50 Oe. The data colored by red and blue corresponds to the bc//H and a*//H geometries, respectively.

latter interactions are rather weak; however, the small distance between the high spin ions (the shortest distance between MnII ions is 7.47 Å) and the collective character of the plane (relatively strong intramolecular interaction provoking the 2D spin−spin correlations with the correlation length increasing exponentially with decreasing temperature) can lead to a considerable coupling between the adjacent square grids of Mn2Nb units. Therefore, a phase transition to a 3D magnetic ordered state of relatively high critical temperature of tens of Kelvin is possible. Moreover, the dipole−dipole interactions are well-known to be anisotropic, which makes the in-plane orientation of the isotropic moments of the MnII and NbIV ions more preferable. Evaluation of the MCE. The MCE has been determined by the indirect method from isothermal magnetization measurements recorded in the temperature range of 2−80 K for bc//H and a*//H orientations (Figures S7 and S8 in the Supporting Information). The magnetic entropy change (ΔSm) was calculated from magnetization data with the use of the Maxwell equation, ΔSm(T , ΔH ) =

∫0

Hmax

⎛ ∂M(T , H ) ⎞ ⎜ ⎟ dH ⎝ ⎠H ∂T

Figure 4 displays the temperature dependence of ΔSm for 1 in both geometries where the field is changed from 0 to μ0Hmax = 0.5, 1.0, 3.0, and 5.0 T. As can be observed, there is almost no difference between the magnetic entropy change for bc//H and

Figure 4. Temperature dependence of the magnetic entropy change of 1 for the field parallel to the easy plane (bc//H, red squares) and parallel to the hard axis (a*//H, blue circles). The values depicted in the figure correspond to the μ0Hmax, which is the end point of field change: from 0 to μ0Hmax. Solid lines are guides for the eyes. D

DOI: 10.1021/acs.inorgchem.6b02941 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 5. Magnetic entropy change of 1, as a function of temperature for μ0ΔH from 0 to 0.02, 0.05, 0.1, 0.14, 0.2, and 0.3 T. The results for the a*//H geometry are marked by blue circles, whereas those for the bc//H orientation are indicated by red squares. The vertical scales are the same for the side-by-side plots. Solid lines are guides for the eyes.

Figure 6. Magnetic entropy change due to rotation of single crystals of 1 around the b-axis by 90°. Solid lines are guides for the eyes.

Figure 8. Schematic comparison of MCE and RMCE below Te. Sm‑C1, Sm‑C2, and Sm‑C3 stands for entropy in conventional MCE for zero field, H1 field, and H2 field, respectively. Sm‑R1, Sm‑R2, and Sm‑R3 corresponds to entropy in RMCE at field HR = H2.

causes a reorganization of the existing domains, so that the domains with magnetic moments that align with the external field take a relatively larger space than those with moments that are askew, with respect to the field (see Figures 8b and 8c). This leads to a further reduction of magnetic entropy, but the related difference −ΔSm will be relatively small, because the initial state was already ordered. For the RMCE, the sample is always in a constant field and only the orientation of the crystals, with respect to the direction of applied field, is changing (see Figures 8d, 8e, and 8f). In the first step, the crystals are in the a*//H geometry (Figure 8d). In this case, the easy plane anisotropy must be taken into account. In fact, the field is pointing perpendicular to the bc-plane, and, therefore, the magnetic moments are tugged from the easy plane. For certain fields (not too high), this leads to an increase of the entropy (Sm‑R1 > Sm‑C1). By rotating the crystals by 90° around the c-axis, the monocrystalline sample is turned to the easy plane orientation (Figure 8f). This is the same final point as for

Figure 7. Comparison of the magnetic entropy change by applying magnetic μ0ΔH = 0.12 T for bc//H (−ΔSm‑bc, the easy plane, denoted by red diamonds), a*//H (−ΔSm‑a*, the hard axis, denoted by blue circles), and due to rotation of the crystals in the field of 0.12 T (−ΔSR, denoted by cyan triangles). The filled green area depicts the excess part over the conventional −ΔSm value. Solid lines are guides for the eyes.

temperature below Te. In the case of the MCE for the bc//H geometry, the sample is initially in zero field. However, Te < Tc; therefore, the sample is in a long-range ordered state and there are domains of aligned magnetic moments throughout the entire sample. Therefore, the magnetic entropy in zero field is reduced, compared to a disordered state under the same conditions (see Figure 8a). Switching on the external field E

DOI: 10.1021/acs.inorgchem.6b02941 Inorg. Chem. XXXX, XXX, XXX−XXX

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the conventional MCE case with the same magnetic entropy (entropy is the state function, Sm‑C3 = Sm‑R3). Yet, the magnetic entropy change is bigger in the second case, because the initial entropy in the RMCE was higher (−ΔSm‑R = −[Sm‑R3 − Sm‑R1], −ΔSm‑C = −[Sm‑C3 − Sm‑C1]; −ΔSm‑R > −ΔSm‑C). All of the −ΔSm values shown in Figures 5, 6, and 7 are small, but a far-reaching conclusion is the possibility, for certain compounds with inverse MCE, to reach higher values of MCE by rotating the sample, than by changing the field in a conventional way.

CONCLUSIONS We have studied the bulk magnetic anisotropy and the magnetocaloric effect (MCE) of the 2D cyanido-bridged molecular ferrimagnet {[Mn II (R-mpm) 2 ] 2 [Nb IV (CN) 8 ]}· 4H2O. The single-crystal measurements reveal that 1 has a small easy-plane anisotropy with a domination of intralayer superexchange interaction over dipole−dipole, through hydrogen-bond and parallel π−π couplings. The MCE study shows that 1 has a comparable magnetic entropy change to previously reported polycrystalline samples of octacyanidoniobate magnetic coordination compounds. A detailed study of MCE in low fields has shown the difference between −ΔSm in the easy plane (bc//H) and the hard axis (a*//H). This inequality was used as a starting point to study the rotating magnetocaloric effect (RMCE) of 1. It has been proved that the inverse part of MCE can enhance the RMCE, reaching higher values of entropy change than in the conventional case. For 1, the excess MCE due to rotation and the inverse effect has attained up to 51% (Figure S11 in the Supporting Information) more than the conventional nonrotating easy plane MCE for the same field. Searching for compounds with a significant inverse MCE can be a breakthrough for the development of the RMCE. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02941. Magnetic data (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: (4812) 662 8019. E-mail: [email protected] (P. Konieczny). *Tel.: (4812) 6632051. E-mail: [email protected] (R. Podgajny). ORCID

Piotr Konieczny: 0000-0002-0024-9557 Łukasz Michalski: 0000-0002-4050-7506 Dominik Czernia: 0000-0003-3201-3765 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Polish National Science Center (Grant No. DEC-2013/11/N/ST8/01267). F

DOI: 10.1021/acs.inorgchem.6b02941 Inorg. Chem. XXXX, XXX, XXX−XXX

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