The magnetocaloric effect (MCE) is the reversible entropy or temperature change of a magnetic material upon the change of external magnetic field.^[1–4] Most of the focus is on achieving high performance with giant magnetic cooling power on rigid inorganic materials. To realize the commercial magnetic refrigerators, vast improvements have been gone through in the search for new magnetic materials in terms of large MCE and relative cooling capacity. A giant MCE has been exhibited in the rigid inorganic materials with three-dimensional (3D) crystal structure such as pseudobinary alloys containing rare earth Gd₅(Si₂Ge₂), La(Fe_xSi_{1 − x})₁₃, perovskite oxides, sulfide spinels, hexaferrites, etc.^[5–12]

During recent decades, the versatility of the organic–inorganic hybrid materials has led to a huge expansion of the organic electronics.^[13–21] In particular, the organic–inorganic hybrid halide perovskite materials have attracted intense attention due to the extraordinary photonic and electronic properties.^[15–18] However, their potential of caloric effect originating from the ferroelectricity or magnetism has never been noticed, such as (CH₃NH₃)PbI₃,^[16–18] (C_nH_{2n + 1}NH₃)₂MCl₄ (M = Mn, Fe, Cu; n = 1,2,3,…),^[19–21] etc. To reveal the multifunctional performance of organic–inorganic hybrid materials and realize their large-scale application eventually, in this paper, the fundamental study on MCE in a prototype organic–inorganic hybrid with two-dimensional (2D) or quasi-2D structure is presented in (CH₃NH₃)₂CuCl₄ (MA₂CuCl₄).

The compound MA₂CuCl₄ crystallizes in the layered perovskite structure, consisting of staggered layers of corner-sharing CuCl₆ octahedra interleaved by alkylammonium cations (CH₃NH₃)⁺, as illustrated in Fig. 1(a). It has been proved to be a 2D Heisenberg planar ferromagnet.^[22–24] The magnetism comes from Cu ions with an ab plane ferromagnetic interaction between the magnetic ions.^[22] A very weak ferromagnetic coupling between the layers drives the system to a 3D long-range order at T_c ~ 8.9 K.^[23] The earlier magnetic studies in this system centered mainly in the behavior of the magnetic susceptibility^[23,24] and ferromagnetic resonance (FMR) studies.^[25,26] The objective of this paper is to report the studies of the magnetic entropy change with temperature and ab plane magnetic field. In addition, we measured the heat capacity under various ab plane magnetic fields to calculate the adiabatic temperature change. Eventually, a large magnetic entropy change near T_c is obtained. An abnormal adiabatic temperature change below T_c is also observed.

Fig. 1.

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	Fig. 1. (color online) (a) The crystal structure of the compound MA2CuCl4. (b) The temperature dependences of magnetization for MA2CuCl4 in zero-field-cooling (ZFC) at applied magnetic fields of 5, 10, 25, 100, 200 Oe and field-cooling (FC) at magnetic fields of 5, 10 Oe. The red dashed line indicates the Curie temperature Tc determined by the inflection point of low-field M–T curve.

Single crystals of MA₂CuCl₄ are naturally grown layer-by-layer along [001] direction by a solvothermal condition method. Yellow plate single crystals with a maximum size of 5 mm × 5 mm × 0.35 mm are obtained. Powder x-ray diffraction patterns with CuKα radiation revealed the single phase monoclinic perovskite structure with P2₁/a, which is in agreement with literature.^[27] The magnetization measurements were performed with a superconducting quantum interference device magnetometer (Quantum Design MPMS) with the magnetic field (B) applied parallel to the metal-halogen layers. The heat capacity was measured under the applied B of 0, 2, and 5 T using a physical property measurement system (Quantum Design).

Figure 1(b) shows the temperature dependence of magnetization under various magnetic fields. The Curie temperature T_c, which is defined as the inflection point in M–T curve at 5 Oe (1 Oe = 79.5775 A⋅m⁻¹), is 8.9 K, which is consistent with the published data.^[23,28] The zero field cooling (ZFC) and field cooling (FC) curves are superposed with each other at 5 Oe and 10 Oe, suggesting a typical ferromagnetic characteristic in ab plane of MA₂CuCl₄. We can also see the Curie temperature and magnetization increase significantly with the increasing magnetic field, implying a potential of large magnetic entropy change.

Isothermal magnetization curves were recorded in the temperature range of 5 K–30 K in the magnetic field up to 5 T [Fig. 2(a)]. The temperature step of 1 K is chosen for 5 K–30 K. The sweep rate of field is 40 Oe/s to ensure that M–B curves are recorded in an isothermal process. Figure 2(b) shows Arrott plots of magnetization in which neither the inflection point nor the negative slope is observed above T_c, suggesting the occurrence of a second-order magnetic transition in MA₂CuCl₄.

Fig. 2.

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	Fig. 2. (color online) (a) Magnetization as a function of applied field of MA2CuCl4 measured at 5 K–30 K. (b) The Arrott plots of MA2CuCl4.

From the thermodynamical theory, the entropy change generated by the variation of the magnetic field from 0 to H_max is given by

with Maxwell’s relation: 2 one obtains the following expression: 3 In order to evaluate the entropy change ΔS_H, some methods of numerical approximation to the integral in Eq. (3) are needed. One method is to use isothermal magnetization measurements. In the case of magnetization measurements at small discrete field and temperature intervals, ΔS_H can be approximated from Eq. (3) by 4 where M_i and M_{i + 1} are the experimental values of the magnetization at T_i and T_{i + 1}, respectively, under an applied magnetic field H_i. For our sample, the typical sample geometry is w = 5 mm, h = 3 mm, and t = 0.3 mm and the magnetic field B is applied along the w direction. The demagnetization factor can be roughly calculated to be N_w ≈ 2t/πw = 0.038, which is small enough that we can use B/μ₀ to estimate H.

The entropy changes associated with magnetic field variations have been calculated with Eq. (4). In Fig. 3(a), we plot the magnetic entropy change as a function of temperature for MA₂CuCl₄. As expected from Eq. (3), the peak of magnetic entropy change of MA₂CuCl₄ is at the Curie temperature where the variation of magnetization with the maximum entropy change ΔS_H is 1.72 J/kg⋅K and 4.98 J/kg⋅K for the field changes of 2 T and 5 T, respectively. The temperature showing the maximum ΔS_H under B = 1 T is 12.5 K, which is higher than the T_c = 8.9 K extracted from the magnetization data at low field (5 Oe), shifts to 17 K under B = 5 T. Moreover, the ΔS_H spans in a wide temperature range, and the full width of half maximum T approaches to 5 K and 18 K for the magnetic field changes from 0 T to 2 T and to 5 T, respectively. Consequently, relative cooling power (RCP),^[29] evaluated by RCP = |ΔS_{H max}| × δT, reaches 8.6 J/kg and 90 J/kg, respectively. According to thermodynamics, the adiabatic temperature change at an arbitrary temperature T₀ can be expressed as 5 where C_p is the specific heat. We further performed the measurements of the heat capacity in the fields of B = 0, 2, and 5 T, shown in Fig. 4(a). A thermal anomaly with an inflection slope in zero field corresponding to the magnetic transition is observed, as shown in Fig. 4(b). A broadened discrepancy appears between zero field and 2-T magnetic field above T_c and rounds off in the field of 5 T, indicating a second-order phase transition [see Fig. 4(a)]. If no magnetic transition has taken place at low temperature in MA₂CuCl₄, the low temperature heat capacity would change with decreasing temperature in the same way as that at higher temperature heat capacity (paramagnetic phase), as shown by the extrapolation line in Fig. 4(b). To further confirm the effect of magnetic transition on heat capacity, the relation of C_p/T³ versus T is plotted, as shown in the inset of Fig. 4(b). A cusp near T_c is clearly observed as indicated by the dashed line, implying the existence of a spin–lattice coupling in the magnetic transition process. Applying a magnetic field can suppress the specific heat below 18 K, which is in accordance with the exchange interaction temperature of nearly isolated quadratic ferromagnetic layers (ab plane) from paramagnetic state.

Fig. 3.

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	Fig. 3. (color online) (a) Entropy changes in MA2CuCl4 extracted from magnetization measurements with the magnetic field changes from 0 to 1, 2, 3, 4, and 5 T. (b) Temperature dependence of adiabatic temperature rise ΔTad in MA2CuCl4 induced by a magnetic field change of 2 T and 5 T.

Fig. 4.

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	Fig. 4. (color online) (a) Heat capacity of MA2CuCl4 measured under the fields of B = 0, 2, and 5 T. (b) Heat capacity in zero field and its fitting line (orange color) by the power formula (7) in a range of 2 K–12 K and the extrapolation (blue color) below Tc. The inset shows the temperature dependence of Cp/T3, the magenta dashed line is the Tc where a kink can be observed.

Based on Eq. (5), the adiabatic temperature rise (ΔT_ad) is calculated and presented in Fig. 3(b). The maximum ΔT_ad are about 1.3 K and 2.2 K for magnetic field changes of 2 T and 5 T, respectively. We can see the ΔT_ad below T_c increases with lowered temperature, indicating the existence of low dimensional heat capacity for MA₂CuCl₄ due to its layered structure. The C_p contribution in zero field is analyzed from crystal lattice and magnon by using the power law6

The large magnetic entropy change in MA₂CuCl₄ must have originated from the considerable increase of magnetization or T_c with magnetic fields. With the observation of a large magnetic entropy change and the increasing ΔT_ad in low temperature, one can conclude that a strong spin-lattice coupling in the magnetic ordering process and quasi-2D crystal structure would lead to magnetic entropy change near T_c and favors the MCE effect.

Though the maximum of entropy change is smaller than the most conspicuous magnetocaloric materials, MA₂CuCl₄ single crystalline and its derivatives are easy to be synthesized at room temperature and exhibits layered structure as well as potential multicaloric effect that is beneficial for the application in cryogen technique and alternative stimulus.^[20,30,31] Besides, since the Curie temperature of organic–inorganic hybrids is easy to modify by the replacement of organic functional group, a large magnetic entropy change may be tuned from low temperature to near room temperature, which is meritful for operating magnetic refrigeration at various temperatures.^[30]

In conclusion, MA₂CuCl₄ shows a large MCE with its second-order magnetic transition. The results obtained show a large magnetic entropy change near the ferromagnetic–paramagnetic transition and an increasing adiabatic temperature change below T_c. The large magnetocaloric effect in MA₂CuCl₄ suggests that organic–inorganic hybrid is a suitable candidate as working substance in magnetic refrigeration technology because of (i) large magnetic entropy change, (ii) soft magnetism and reversible MCE, (iii) wide temperature span of ΔS_H and large RCP, (iv) easy to fabricate and tune the magnetic transition temperature by modifying the organic functional group, and (v) potentially multicaloric effect to enhance the MCE.