Large Magnetic Entropy Change Research Articles

Giant room temperature magnetocaloric response in a (MnNiSi)1−x(FeNiGa)x system

The coincidence of magnetic and structural transitions near room temperature is observed in (MnNiSi)1−x(FeNiGa)x (x = 0.16 and 0.17) systems, which leads to a coupled magnetostructural transition (MST) from a high-temperature paramagnetic Ni2In-type hexagonal phase to a low-temperature ferromagnetic TiNiSi-type orthorhombic phase associated with a substantial change in magnetization and a large change in structural unit cell volume, and thus, across MST, a giant magnetocaloric effect is obtained in these systems. The alloys with x = 0.16 and 0.17 are observed to show a giant isothermal magnetic entropy change (ΔSM) of about −26.2 and −63.2 J kg−1 K−1, accompanied with a large relative cooling power of about 220.1 and 264.5 J/kg, respectively, due to a magnetic field change (μ0ΔH) of 5 T only. Moreover, the material with x = 0.16 and 0.17 shows a large temperature average magnetic entropy change of about −21.64 and −34.4 J kg−1 K−1 over a temperature span of 10 K due to μ0ΔH ∼ 5 T. Thus, these low-cost materials with giant magnetocaloric responses are highly suitable to be used as magnetic refrigerants for room temperature solid-state-based cooling technology.

Unique layered microstructure and magnetocaloric effect in hot-rolled La-Fe-Co-Si plates

Aiming to improve the heat transfer efficiency in active magnetic regeneration, this work for the first time has proposed hot rolling as a near-net-shape and up-scaled processing method to fabricate La-Fe-Si-based magnetocaloric plates. The composition of LaFe11Co0.8Si1.2 alloys was rolled at 1273 K into 2 mm thickness with a reduction ratio of 31.0%. We observed a unique layered microstructure in the hot-rolled plate: there were apparent cracks and oxidation phenomenon occurred in the part close to the plate surface, but almost defects-free deformed microstructure in the inner part. Upon the homogenization annealing, the plate has a large magnetic entropy change of 6.1 J/kg K under an applied field change of 2 T around room temperature. Additionally, by using a self-developed infrared characterization technique we directly measured a reversible adiabatic temperature change of 1.6 K in 0–1.3 T field cycles.

Magnetostructural Transition and Magnetocaloric Effect with Negligible Magnetic Hysteresis in MnCoGe1.02−xGax Alloys

The behavior of magnetostructural transition and the magnetocaloric effect in the MnCoGe1.02−xGax (x = 0, 0.02, 0.04, 0.06) alloys are investigated in this study. The addition of Ga changes the crystal structure of MnCoGe1.02−xGax alloys at room temperature and reduces the phase transition temperatures with increasing Ga content. The coupling of magnetostructural transition and negligible magnetic hysteresis is observed in the Mn-Co-Ge-Ga alloy. At 305 K, under the action of a 5 T magnetic field, the MnCoGeGa0.02 alloy exhibits 23.47 J/kg∙K magnetic entropy change, and its refrigeration capacity reaches 387 J/kg. The large magnetic entropy change near room temperature and the high refrigeration capacity in the Mn-Co-Ge-Ga alloy make it a promising new type of refrigeration material.

Magnetic performance of amorphous manganese nanoparticles doped with rare earth elements

We studied magnetic and magnetocaloric response in nanoparticle manganite oxides (ABO3-type manganese oxides), which were chemically prepared by sol–gel method with different composition RMnO3 (R = Gd, Tb, Ce) (GMO, TMO, CMO). The structural characterization was performed by EDX (Energy-dispersive X-ray spectroscopy) technique, using TEM (Transmission electron microscopy) methods and XRD and confirmed the nanosized character and morphology of studied manganites. Despite the amorphous character of the samples RMnO3 (R = Gd, Tb), the relatively large magnetic entropy change − ΔSM(7 K) ∼15 J/kgK and − ΔSM (13 K) ∼8 J/kgK has been observed in the case of GMO and TMO, respectively. Sample CMO, despite of its crystalline character, exhibits the lowest magnetic entropy change and the magnetic properties are affected by Jahn-Teller distortion in Mn3O4 nanoparticles (NPs). All systems GMO, TMO, CMO exhibit the Arrott plots with linear behavior and positive slopes that is according to the Banerjee criterion a signature of the second order phase transition. This is in line with our conclusions on the presence of AFM-PM phase transitions in the systems that we drew from ZFC-FC data. Our results confirm that also amorphous manganites especially with Gd show feasible properties for potential magnetic refrigeration applications.

Magnetocaloric Effect of Two Gd-Based Frameworks

Magnetic refrigeration material is the key to adiabatic demagnetization refrigeration technology. In this work, two magnetic refrigerants, Gd5(C4O4)(HCOO)3(CO3)2(OH)6·2.5H2O (1) and Gd2(OH)4SO4 (2), were prepared through hydrothermal reaction. Magnetic study reveals that their magnetic entropy changes are 24.8 J kg−1 K−1 for 1 and 15.1 J kg−1 K−1 for 2 at 2 K and 2 T, respectively. The magnetic entropy changes of 1 and 2 at T = 2 K and ∆H = 2 T exceed most gadolinium hydroxyl compounds, indicating that magnetic refrigerants with large magnetic entropy changes at low magnetic fields can be obtained by introducing more weak magnetic exchange ligands to replace hydroxyl groups in gadolinium hydroxyl compounds.

The structural, magnetic and magnetocaloric properties of MnCrNiGeSi high-entropy alloy

The structural, magnetic and magnetocaloric properties of Mn20Cr14Ni33Ge25Si8 and Mn24Cr10Ni33Ge25Si8 high entropy alloys (HEAs) were investigated. The HEAs were prepared by arc melting method. The structural analysis indicates that the structure of HEAs is orthorhombic with Pnma space group. In magnetic measurements, the maximum saturation magnetization was found to be 56.7 A m2kg−1. The Curie temperature of HEAs is 361 and 387 K, respectively. The Curie temperature shifts to the high temperature, when 4% Mn is added, resulting the long-range orientation of the magnetic moments. The magnetic entropy change of Mn24Cr10Ni33Ge25Si8 is 3.6 Jkg−1K−1 at around 387 K. Consequently, Large magnetic entropy change is achieved with low hysteresis and soft magnetic property at high temperature.

Giant Rotational Magnetocaloric Effect in Ni(en)(H2O)4·2H2O: Experiment and Theory

An experimental study of the rotational magnetocaloric effect in Ni(en)(H2O)4SO4∙2H2O (en = ethylenediamine) single crystal is presented. The study was carried out at temperatures above 2 K and was associated with adiabatic crystal rotation between the easy plane and hard axis in magnetic fields up to 7 T. The magnetocaloric properties of the studied system were investigated by isothermal magnetization measurement. The experimental observations were completed with ab initio calculations of the anisotropy parameters. A large rotational magnetic entropy change ≈12 Jkg−1K−1 and ≈16.9 Jkg−1K−1 was achieved in 5 T and 7 T, respectively. The present study suggests a possible application of this material in low-temperature refrigeration since the adiabatic rotation of the single crystal in 7 T led to a cooldown of the sample from the initial temperature of 4.2 K down to 0.34 K. Finally, theoretical calculations show that S = 1 Ni(II)-based systems with easy-plane anisotropy can have better rotational magnetocaloric properties than costly materials containing rare-earth elements in their chemical structures.

Structural and magnetic properties of DyCrO3

In this contribution, the structural and magnetic properties of DyCrO3 are studied, along with the magnetocaloric effect in this compound. The susceptibility as a function of temperature, χ(T), indicates that DyCrO3 has a G-type antiferromagnetic behaviour with weak ferromagnetism below Néel temperature, TNCr, at 147.1 ± 0.1 K, attributed to the ordering of Cr moments. The Dy moments orders antiferromagnetically below the spin reorientation temperature TSR = 4.81 ± 0.04 K. The dependence of magnetization on the applied magnetic field, Mμ0H, shows a behaviour that corresponds to the χ(T) data. Arrott plots reflect the various magnetic orderings with a change in the gradient of the curves. For the first time, the magnetocaloric effect of sol-gel synthesized DyCrO3 is studied having an average particle size 215 ± 3 nm as obtained from transmission electron microscopy (TEM). Large magnetocaloric effects (MCE) are observed in the temperature range of 10 to 80 K for DyCrO3. The compound shows a relatively large magnetic entropy change (ΔSM) of 21 J.Kg−1.K−1 and relative cooling power (RCP) of 498 J.Kg−1 at 7 T and 10 K. Assuming the relations |ΔSmT,H|=a(H)n and RCP=CH1+1δ, with critical exponents n = 1 and δ → ∞ were obtained from the linearization, confirming the weak ferromagnetic behaviour.

Achievement of giant cryogenic refrigerant capacity in quinary rare-earths based high-entropy amorphous alloy

• Quinary rare-earths based high-entropy (HE) amorphous alloy was obtained. • Microstructure, magnetic properties and magnetocaloric effect were studied. • Er 20 Ho 20 Gd 20 Ni 20 Co 20 HE ribbons show excellent magnetocaloric performances. Magnetic refrigeration (MR) by utilizing the magnetocaloric (MC) effect is recognized as one of the most potential promising solid state environmentally friendly and high efficiency alternative method to the well-used state-of-the-art gas compression cooling technique. In this work, a systematic investigation of quinary equi-atomic rare-earths (RE) based Er 20 Ho 20 Gd 20 Ni 20 Co 20 high-entropy (HE) amorphous alloy in terms of the microstructure, magnetic and magnetocaloric (MC) properties have been reported. The Er 20 Ho 20 Gd 20 Ni 20 Co 20 exhibits promising glass forming ability with an undercooled liquid region of 72 K. Excellent cryogenic MC performances can be found in wide temperature from ∼25 and ∼75 K, close to H 2 and N 2 liquefaction, respectively. Apart from the largest magnetic entropy change (-Δ S M ) reaches 17.84 J/(kg K) with 0-7 T magnetic field change, corresponding refrigerant capacity (RC) attains a giant value of 1030 J/kg. The promising cryogenic MC performances together with the unique HE amorphous characterizations make the quinary Er 20 Ho 20 Gd 20 Ni 20 Co 20 HE amorphous alloy attractive for cryogenic MR applications.

Influence of Covalent Element B and Si Addition on Magnetocaloric Properties of Gd-Co-Fe-(B,Si) Amorphous Alloys

The effect of covalent element addition on the magnetic and magnetocaloric properties of ferrimagnetic Gd60Co20Fe20 amorphous alloy was studied. Particularly, the co-doping of B and Si promoted the magnetocaloric performance (with a larger magnetic entropy change |ΔSM| at higher working temperature) of the initial Gd60Co20Fe20 amorphous alloy. This is possibly ascribed to the modified magnetic transition behavior. Additionally, the broadened magnetocaloric effect with |ΔSM| of 2.25 J kg−1 K−1 at 222.5 K under a magnetic field change of 0–2 T and high relative cooling power of 396.0 J kg−1 were obtained for (Gd0.6Co0.2Fe0.2)95B2Si3. These properties reveal that the material could be a good candidate as a magnetic refrigerant suitable for active magnetic refrigerators.

Study of magnetic and magnetocaloric effect of REMnO3(RE=Dy,Eu)manganites

We have systematically investigated the magnetic and magnetocaloric effect (MCE) of REMnO3 (RE = Dy, Eu) manganites sintered by solid-state reaction method. The Neel temperature (TN), the maximum magnetic entropy change (-ΔSmmax) and the relative cooling power (RCP) of the sample were obtained by measuring the magnetization and heat capacity. Under the magnetic field change of 50 kOe, large -ΔSmmax of 8.93J/kg K and 9.31J/kg K are achieved around the TN of 14 K and 50 K for DyMnO3 and EuMnO3, respectively. It is proved by the slope of the Arrott plot and the universal phenomenological curve that the sample undergoes second order phase transition. The critical behavior of antiferromagnetic to paramagnetic phase transition is studied by modifying Arrott plot, which shows that the spin order of the sample decreases at the phase transition temperature and tends to change into shortrange order. The large magnetic entropy change calculated by Landau theory and experimental data indicates that the sample has a large magnetocaloric effect and has potential application in low temperature magnetic refrigeration materials.

Influence of Chemical Substitution and Light Element Insertion on the Magnetic Properties of Nanocrystalline Pr2Co7 Compound

It is well recognized that intermetallics based on rare-earth (R) and transition metals (T) display numerous interesting magnetic properties, leading to potential applications in different fields. The latest progress regarding magnetic properties and the magnetocaloric effect (MCE) in the nanostructured Pr2Co7 compound, as well as its carbides and hydrides, is reviewed in this paper. Some of this progress reveals remarkable MCE performance, which makes it attractive in the field of magnetic refrigeration at high temperatures. With the purpose of understanding the magnetic and magnetocaloric characteristics of these compounds, the crystal structure, microstructure, and magnetism are also brought into focus. The Pr2Co7 compound has interesting magnetic properties, such as a high Curie temperature TC and uniaxial magnetocrystalline anisotropy. It crystallizes in a hexagonal structure (2:7 H) of the Ce2Ni7 type and is stable at relatively low temperatures (Ta ≤ 1023 K), or it has a rhombohedral structure (2:7 R) of the Gd2Co7 type and is stable at high temperatures (Ta ≥ 1223 K). Studies of the magnetocaloric properties of the nanocrystalline Pr2Co7 compound have shown the existence of a large reversible magnetic entropy change (ΔSM) with a second-order magnetic transition. After its substitution, we showed that nanocrystalline Pr2Co7−xFex compounds that were annealed at Ta = 973 K crystallized in the 2:7 H structure similarly to the parent compound. The extended X-ray absorption fine-structure (EXAFS) spectra adjustments showed that Fe atoms preferably occupy the 12k site for x ≤ 1. The study of the magnetic properties of nanocrystalline Pr2Co7−xFex compounds revealed an increase in TC of about 26% for x = 0.5, as well as an improvement in the coercivity, Hc (12 kOe), and the (BH)max (9 MGOe) product. On the other hand, the insertion of C atoms into the Pr2Co7 cell led to a marked improvement in the TC value of 21.6%. The best magnetic properties were found for the Pr2Co7C0.25 compound annealed at 973 K, Hc = 10.3 kOe, and (BH)max = 11.5 MGOe. We studied the microstructure, hydrogenation, and magnetic properties of nanocrystalline Pr2Co7Hx hydrides. The crystal structure of the Pr2Co7 compound transformed from a hexagonal (P63/mmc) into an orthorhombic (Pbcn) and monoclinic (C2/c) structure during hydrogenation. The absorption of H by the Pr2Co7 compound led to an increase in the TC value from 600 K at x = 0 to 691 K at x = 3.75. The Pr2Co7H0.25 hydride had optimal magnetic properties: Hc = 6.1 KOe, (BH)max = 5.8 MGOe, and TC = 607 K. We tailored the mean field theory (MFT) and random magnetic anisotropy (RMA) methods to investigate the magnetic moments, exchange interactions, and magnetic anisotropy properties. The relationship between the microstructure and magnetic properties is discussed. The obtained results provide a fundamental reference for adapting the magnetic properties of the Pr2Co7, Pr2Co6.5Fe0.5, Pr2Co7C0.25, and Pr2Co7H0.25 compounds for potential permanent nanomagnets, high-density magnetic recording, and magnetic refrigeration applications.

Accelerated design for magnetocaloric performance in Mn-Fe-P-Si compounds using machine learning

• An effective method for predicting the magnetocaloric performance of Mn-Fe-P-Si alloys using machine learning is proposed. • Curie temperature, thermal hysteresis, and magnetic entropy change as a function of compositions are clearly displayed. • The composition window suitable for room temperature magnetic refrigeration is optimized to accelerate the alloy design. Magnetocaloric performance is of vital importance for Mn-Fe-P-Si alloys. However, when processes and compositions are considered, designing alloys with large magnetic entropy changes (Δ S m ), low thermal hysteresis (Δ T hys ), and Curie temperatures ( T C ) around room temperature become relatively complicated. In this study, we adopt machine learning methods to predict the magnetocaloric performance of Mn-Fe-P-Si compounds for the first time. To achieve this goal, 503, 465, and 660 data points for datasets with T C , Δ T hys , and Δ S m are collected, respectively. The collected datasets contain parameters of compositions, preparations, heat treatment, and magnetic field changes. We search for the optimal configuration using various methods and also compare their mean squared errors (MSE) and allowable errors. Evaluation results show that the performance of neural networks (NNs) is better than other methods. Therefore, we select NN to explore the T C , Δ T hys , and Δ S m values as a function of Mn, Si, metal/non-metal ratios, and B (Boron). We also propose to use the composition window with excellent magnetocaloric performance. These results not only help us gain deep insights into Mn-Fe-P-Si alloys but also accelerate the design process of alloys suitable for magnetocaloric materials. This work has the potential to solve the challenges and boost the research of Mn-Fe-P-Si alloys.

Large Magnetocaloric Effect in the Frustrated Antiferromagnet Euir2p2

We present a theoretical analysis of the magnetocaloric effect in the frustrated antiferromagnet \Eu. Monte Carlo simulations indicate a large magnetic entropy change $\Delta S_M^{\text{max}} \sim 5.14$ J kg$^{-1}$K$^{-1}$ at $T=2.45$K for a moderate change in the external magnetic field $\Delta B=1.2$ Tesla. The highest value of $\Delta S_M$ seems to be associated with the presence of a multicritical point in the magnetic field versus temperature phase diagram (at $B_m\sim 1.2$ Tesla and $T_m\sim 2.2$ K) but it persists with similar large values for a broad range of temperatures $T\lesssim T_m$. Single crystal \Eu~magnetization vs magnetic field experiments allowed us to obtain the magnetocaloric effect for $T>2$ K. The results are consistent with the theoretical analysis and show a large magnetocaloric effect for $T\sim 2.5$ K. Finally, we present a phenomenological Landau functional analysis that provides a simplified description of the phase diagram and of the magnetocaloric effect. Although the magnetic frustration in this system leads to a rich phase diagram, we found no clear indication that it plays a significant role enhancing the magneticaloric effect, other than by reducing the transition temperatures and fields.

Syntheses, structures, and magnetic properties of a series of Mn-M-Mn trinuclear complexes with different spin configurations.

A series of trinuclear complexes, [MnII2YIII(L)2(HL)2(NO3)3][YIII(NO3)5]·7H2O (1'), [MnII2GdIII(HL)4(NO3)4]2[MnII2GdIII(L)(HL)3(NO3)4][GdIII(NO3)5]4·2(o-Xy)·12H2O (2') and [MnII3(L)(HL)2(NO3)4](NO3)·1.25(p-Xy) (3'), were synthesized using a β-diketone ligand HL (HL = 1,3-bis(pyridin-2-yl)propane-1,3-dione). X-ray structural analyses revealed that each complex has a trinuclear core with an Mn(II)-M-Mn(II) arrangement (M = YIII (1), GdIII (2), and MnII (3)). In 1' with a diamagnetic Y(III) ion, negligible antiferromagnetic interactions between terminal Mn(II) ions are operative. On the other hand, 2' shows ferromagnetic interactions between Mn(II) and Gd(III) ions, affording a spin ground state of ST = 17/2. The homometallic Mn(II)3 complex of 3' has an ST = 5/2 spin ground state resulting from the antiferromagnetic interactions between neighboring Mn(II) ions. The maximum magnetic entropy change (-ΔSm) of 1'-3' was estimated to be 12.3, 24.8, and 8.0 J kg-1 K-1, respectively.

Sizable magnetic entropy change in bismuth-substituted La0.75Bi0.1Na0.15MnO3 manganite.

The bismuth (Bi)-substituted La0.85-xBixNa0.15MnO3 (x = 0 and x = 0.1) manganites have been investigated for achieving room temperature magnetic entropy change at low magnetizing intensities. A 10% Bi ion substitution at the La site has led to the relatively higher symmetry R3̄c rhombohedral structure showing improvement in Mn-O-Mn bond angles. The chemical additive Bi2O3 has assisted in grain growth to form densified specimens even at low sintering temperatures. The sample has a soft ferromagnetic nature with a single magnetic transition occurring at 275 K. Phenomenal improvement in magnetic sensitivity at low magnetic fields was achieved with TC close to room temperature. Nearly 100-500 percent increases in magnetic entropy change was realised over existing parent compounds and the entropy changes are greater than the prototype, Gd of active magnetic refrigeration. The magnetic interactions are governed by the Tri-Critical Mean Field model with emphasis on the coexistence of long-range and short-range interactions. The characteristics, viz. soft ferromagnet of second-order phase transition, abrupt phase change at TC, the resultant large magnetic entropy change of 4.4 J kg-1 K-1 and temperature averaged entropy change of 3.914 J kg-1 K-1 (10 K) at the low magnetic field of 1.5 T are expected to facilitate application prospects as a magnetic refrigerants for room temperature working range.

Enhanced magnetocaloric performance in nanocrystalline/amorphous Gd3Ni/Gd65Ni35 composite microwires

A novel class of nanocrystalline/amorphous Gd 3 Ni/Gd 65 Ni 35 composite microwires were created directly from melt-extraction through controlled solidification. X-ray diffraction (XRD), transmission electron microscopy (TEM), and selected area electron diffraction (SAED) confirmed the formation of a biphase nanocrystalline/amorphous structure in these wires. Magnetic and magnetocaloric experiments indicate a large magnetic entropy change (-Δ S M ~9.64 J/kg K), large refrigerant capacity ( RC ~742.1 J/kg), and large maximum adiabatic temperature change ( Δ T a d max ~5 K) around the Curie temperature of ~120 K for a field change of 5 T. These values are ~1.5 times larger relative to its bulk counterpart and are superior to other candidate materials being considered for active magnetic refrigeration in the liquid nitrogen temperature range. • Novel nanocrystalline/amorphous Gd 3 Ni/Gd 65 Ni 35 microwires. • Large magnetic entropy change and refrigerant capacity. • Magnetic refrigerators for nitrogen liquefaction.

Giant conventional and rotating magnetocaloric effects in TbScO3 single crystal

Magnetocaloric effect (MCE) shows great potential in refrigeration field and especially fundamental scientific research with cooling requirement. In this work, we studied the anisotropic magnetic and magnetocaloric properties of TbScO3 single crystal, a popular substrate used for depositing epitaxial ferroelectric films. Our results indicate a strong antiferromagnetic ordering below TN ~ 3.1 K in orthorhombic TbScO3 single crystal. Large magnetocrystalline anisotropy exists between the magnetic easy ab plane and the hard c axis, leading to a giant anisotropic MCE. In field changes of 20 and 50 kOe along the a axis, large magnetic entropy changes reach 15.25 and 23.71 J/kg K at 3 K and 7 K, respectively, while the magnetic entropy changes along the c axis are negligible. As a result, large rotating magnetic entropy changes of 15.27 and 23.63 J/kg K are obtained by rotating TbScO3 single crystal with constant fields of 20 and 50 kOe lying in the ac plane. The large anisotropic MCE of TbScO3 single crystal indicates this material as a promise refrigerant candidate for rotating magnetic refrigeration technique near the helium temperature.

Magnetic properties of the quasi two-dimensional centered honeycomb antiferromagnet GdInO3

The crystal structure and magnetic property of the single crystalline hexagonal rare-earth indium oxides ${\mathrm{GdInO}}_{3}$ have been studied by combing experiments and model calculations. The two inequivalent ${\mathrm{Gd}}^{3+}$ ions form the centered honeycomb lattice, which consists of honeycomb and triangular sublattices. The dc magnetic susceptibility and specific heat measurements suggest two antiferromagnetic phase transitions at ${T}_{\text{N1}}=2.3\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ and ${T}_{\text{N2}}=1.02\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. An inflection point is observed in the isothermal magnetization curve, which can be an indication of an up-up-down phase with a 1/3 magnetization plateau, further supported by our theoretical calculation. We also observe a large magnetic entropy change originated from the magnetic frustration in ${\mathrm{GdInO}}_{3}$. By considering a classical spin Hamiltonian, we establish the ground state phase diagram, which suggests that ${\mathrm{GdInO}}_{3}$ has a weak easy-axis anisotropy and is close to the equilateral triangular-lattice system. The theoretical ground-state phase diagram may be used as a reference in NMR, ESR, or $\ensuremath{\mu}\mathrm{SR}$ experiments in future.

Size-dependent magnetocaloric effect in GdVO4 nanoparticles

We investigated the magnetization and electron spin resonance (ESR) in nanocrystals of antiferromagnet GdVO4 with the grain size ranging from 30 nm to 5 µm. Our nanoparticles with an average size reduction from 5 µm to 300 nm exhibited enhanced paramagnetic-like behavior, in which the weakly coupled spins can be well aligned along the direction of an external magnetic field, resulting in a giant entropy change of -ΔSM = 45 J/kg K at 3 K for ΔH = 7 T. Moreover, with a further reduction of grain size to ~ 30 nm, the saturation magnetization together with the negative entropy change -ΔSM, max decreased dramatically. The aggregation effect is believed to be responsible for the decline in -ΔSM, max, in which small nanoparticles agglomerating to ‘large’ grains made them behave like bulk material. Finding the critical point of positive size effect on magnetocaloric effect offered a new way to achieve large magnetic entropy change.