Giant Reversible Magnetocaloric Effect in Flower-like bCo ( OH ) 2 Hierarchical Superstructures Self-assembled by Nanosheets

A facile hydrothermal strategy is proposed to synthesize flower-like b-Co(OH)2 hierarchical microspherical superstructures with a diameter of 0.5-1.5 μm, which are self-assembled by b-Co(OH)2 nanosheets with the average thickness ranging between 20 and 40 nm. The magnetocaloric effect associated with magnetic phase transitions in Co(OH)2 superstructures has been investigated. A sign change in the magnetocaloric effect is induced by a magnetic field, which is related to a filed-induced transition from the antiferromagnetic to the ferromagnetic state below the Néel temperature. The large reversible magnetic-entropy change –ΔSm (13.4 J/kg K at 15 K for a field change of 5 T) indicates that flower-like Co(OH)2 superstructures is a potential candidate for application in magnetic refrigeration in the low-temperature range.


Introduction
Currently, there is a great deal of interest in utilizing the magnetocaloric effect (MCE) as an alternate technology for refrigeration, replacing the common gas-compression expansion technology, due to higher efficiency and environmental concerns [1][2][3][4][5][6][7][8][9][10][11] .An ideal material for magnetic refrigeration should be composed of relatively inexpensive raw materials, have a high MCE demonstrated by a high change in magnetic entropy (ΔS M ) and a high adiabatic temperature change, and have little or no thermal/magnetic hysteresis 1,8 .The giant MCE, closely associated with the first-order magnetic transition (FOMT), has been observed in different systems 1 .Unfortunately, the FOMT usually leads to considerable thermal and magnetic irreversibility which is disadvantageous for application.Therefore, much attention has been recently focused on finding new materials with a large MCE and a small thermal/magnetic hysteresis.A giant MCE has been observed in antiferromagnetic (AFM) systems, originating from a field-induced transition from a collinear AFM to a triangular AFM [(or ferromagnetic (FM)] state 1,11 .Furthermore, as the thermal/ magnetic hysteresis is quite small for AFM systems, compared with giant-MCE ferromagnetic (FM) materials, they may be more suitable for application on the aspect of refrigerant efficiency and energy conservation.In the past decade, there has been a growing interest in the investigation of superstructures selfassembled by nanosheets due to the interesting properties they possess, owing to their high anisotropy and thinness 7,12.A few investigations have been focused on the preparation and magnetic properties of cobalt hydroxide b-Co(OH) 2 [1,7]   .In this work, we synthesize the flower-like Co(OH) 2 hierarchical superstructures self-assembled by nanosheets and study the magnetic and magnetocaloric properties of the superstructures at low temperatures.A giant negative magnetic-entropy change is found together with a fieldinduced MCE conversion (the MCE changes its sign in the applied magnetic field).

Experimental Section
Analytically pure reagents were used in this experiment.A mixture was prepared by mixing water, ethanol and glycol amine in 20:1:10 volume ration, and then 0.2 mmol Co(NO 3 ) 2 •6H 2 O was dissolved in the 40 mL above mixture under stirring.This solution was stirred for 30 min, after which it was transferred into a 50-mL Telfon-lined stainless tell autoclave.The autoclave was sealed and maintained at 180 °C for 12 h and then cooled to room temperature naturally.The products obtained after hydrothermall treatment were centrifuged, washed with distilled water and ethanol several times and finally dried in vacuum at 60 °C for 4h.The as-prepared sample was characterized by x-ray diffraction (XRD, Bruker D8) and scanning electron microscopy (SEM, JEOL-6300 F) and transmission electron microscopy (TEM, JEOL JEM-2010).The magnetization measurements were carried out using a superconducting quantum interference device (SQUID, Quantum Design MPMS XL-7).The MCE is characterized by the isothermal magnetic-entropy change.By using the Maxwell relation M T , the magnetic-entropy change can be represented as

S M S T B S T B S T dB dB B B
(1)

Results and Discussion
The XRD pattern of the as-synthesized flower-like Co(OH) 2 hierarchical supersturcutures is illustrated in Figure 1a.Five obvious diffraction peaks can be easily identified for the (001), (100), ( 011 The temperature dependences of the magnetic susceptibility and the inverse magnetic susceptibility in a magnetic field of 0.01 T are shown in Figure 2a.A sign of AFM transition at Néel temperature T N ≈ 10 K can be found from the 1/χ-T curve, which is lower than the previously reported values of 11 and 12.3 K in References 1,8 , respectively.A decrease in T N is observed in many nanosized AFM systems as the grain size decreased 1,7 .The magnetic hysteresis loop at 5 K in an applied field of 5 T is shown in Figure 2b.It can be noticed that the coercivity of 0.014 T is extremely small and the remanent magnetization is close to zero.Furthermore, there is nearly zero magnetic hysteresis in the transition field.Gd 5 (Ge 1-x Si x ) 4 as a typical giant-MCE material has the magnetic hysteresis of about 1 T near the magnetic-transition temperature 10 .The small magnetic hysteresis of b-Co(OH) 2 is advantageous for application.In addition, it should be noted that the M-H loop shows step-type line (field-induced AFM to FM behavior) at 5 K [7] .
Figure 2c shows the isothermal magnetization curves of the b-Co(OH) 2 superstructures , which are plotted as a function of the applied magnetic field (ranging from B = 0 to 5 T) between 4 and 28 K with ΔT = 2 K. Below T N , the magnetization increases gradually with the applied field in the low-field range and then jumps at a critical field but remains unsaturated at 5 T. The step in the magnetization curves indicates a clear field-induced AFM to FM phase transition 1 .
A large MCE is expected around T N where the magnetization rapidly changes with varying temperature.The isothermal entropy change is derived from the magnetization data in Figure 2c according to Equation 1.The curves of -ΔS m versus T are given in Figure 2d.It can be seen that, for small magnetic-field changes, -ΔS m is negative below T N , whereas it changes to small positive values with increasing temperature 1 .Usually, the inverse MCE is observed in first-order magnetic transitions such as AFM/FI, AFM/FM, or collinear AFM/triangular AFM 1,11 .The inverse MCE has also been reported in AFM/PM transition systems, in which the applied field results in a further spin-disordered state near the transition temperature, which increases the configurational entropy 1,13,14 .The inset in Figure 2d presents -ΔS m versus ΔB at 9 K, where a minimum value of -1.62 J/kg K of -ΔS m is found for ΔB=1.7 T. The applied field destroys the antiparallel alignment of the spin moments, and the spin disorder will result in a negative -ΔS m , becoming more negative with increasing applied field.However, the value of -ΔS m increases with further increasing magnetic field due to the field-induced transition from the AFM to the FM state and becomes positive at 2.7 T. The field-induced AFM to FM transition is responsible for the conversion from the inverse to the conventional MCE in b-Co(OH) 2 .The maximum of -ΔS m of 13.4 J/kg K at K for ΔB = 5T, which is comparable with giant MCE reported for La-Fe-Si, MnAs and Mn-Fe-P based alloy 15 .The slope of the curve in Figure 2d is relatively small, and the smooth variation in -ΔS m with temperature is more useful than a sharp one, which is another property that makes it a promising magnetic refrigerant.

Conclusions
In summary, we propose an efficient synthetic strategy to synthesize the flower-like Co(OH) 2 hierarchical superstructures, which are self-assembled by Co(OH) 2 nanosheets.This material displays a large magnetic entropy change of 13.4 J/kg K almost without hysteresis at 15 K in a magnetic field change of 5 T, thereby indicating the suitability of this material for use in magnetic refrigeration.
), (012) and (110) planes of the hexagonal b-Co(OH) 2 crystalline structure, respectively.No peaks of any other phases or impurities are detected, suggesting high purity of the as-prepared pink Co(OH) 2 sample.Figure 1b, c demonstrate the SEM images of the Co(OH) 2 samples with different magnifications.The as-synthesized Co(OH) 2 sample presents a uniform flowerlike microspheres with a diameter of 0.5-1.5µm, as seen from Figure 1b.Furthermore, the magnified SEM images (Figure 1c) demonstrate that three-dimensional flowerlike microspheres are self-assembled by lots of Co(OH) 2 nanosheets building blocks with the average thickness ranging between 20 and 40 nm.It is worthwhile noting that the as-obtained flower-like Co(OH) 2 superstructures cannot be destroyed and broken into the individual Co(OH) 2 nanosheets even after subjecting long-time ultrasonication.In addition, the TEM image in Figure 1d clearly shows that the d-spacing of 0.236 nm corresponds to the lattice plane {011} of b-Co(OH) 2 .

Figure 2 .
Figure 2. (a) Temperature dependence of the magnetic susceptibility and the inverse magnetic susceptibility of Co(OH) 2 measured at an applied magnetic field of 0.01 T. (b) Magnetic hysteresis loop of at 5 K in applied fields up to 5 T. Inset: the hysteresis loop at 5 K in low fields.(c) Magnetic isotherms of Co(OH) 2 measured between 4 and 28 K with a temperature step of 2 K.(d) Negative magnetic-entropy change -ΔS m verse temperature T curves for different magnetic field changes (ΔB).Inset: -ΔS m as a function of ΔB at 9 K.