3RD INTERNATIONAL CONGRESS ON TECHNOLOGY - ENGINEERING & SCIENCE - Kuala Lumpur - Malaysia (2017-02-09)

Magnetocaloric Effect And Ferromagnetic Interactions In Pr0.6sr0.4mno3 Nano-crystals
It is known that when the crystalline size (D) of manganites was reduced to the nanometer scale, some unusual physical properties will appear, such as super-paramagnetism, spin-glass or super-spin-glass behaviours, core-shell effects, low saturation magnetization (MS), and large coercivities (HC), ect. To further understand the influence of the reduction of the crystalline size on the magneticocaloric effect (MCE) and as well as the magnetic interactions of Pr0.6Sr0.4MnO3, in this work, we prepared three nano-crystalline samples with the crystalline size D = 39, 59, and 88 nm by using a combination of the solid-state reaction and the ball-milling methods, and investigated their critical properties. Our results showed that the Curie temperature (TC) decreases slightly from 291 K for D = 88 nm to 286 K for D = 39 nm, meanwhile the value of HC risen significantly. Namely, at 10 K, HC value increases linearly from 322 to 585 Oe as a function of the surface/volume ratio (D-1) suggesting that our nano-crystals are close to a multi-domain structure [1]. Besides, a linear reduction of MS value obtained at 10 K with D-1 was also observed, which confirms that the magnetization is actually effected by the particle surface. The nonmagnetic layer or spin disorder on the particle surface lead to the weakening of ferromagnetic (FM) double-exchange interaction strength of the Mn3+-Mn4+ pairs [2]. The MCE of nano-crystals has been assessed via the temperature and magnetic field dependences of magnetic entropy change (ΔSm), see Fig. 1. A maximum value of ΔSm (|ΔSmax|) appears on the -ΔSm(T) curve, corresponding to the ferromagnetic-paramagnetic (FM-PM) phase transition, and an applied field change (ΔH) increase enhances |ΔSm|. For ΔH = 30 kOe, the value of |ΔSmax| is found to be 3.0, 2.6, and 2.1 J/kg•K for D = 88, 59, and 39 nm, respectively, that decreases with decreasing D. Based on modified Arrott plot method and analyses the isothermal magnetization M(H, T) data around the FM-PM phase transitions, Fig. 2, we pointed out that Pr0.6Sr0.4MnO3 nano-crystals undergo a second-order phase transition with the critical exponents β = 0.486-0.495, γ = 1.009-1.042, and δ = 2.934-3.095, which are very close to those expected for a long-range FM order of the mean-field theory (β = 0.5, γ = 1.0, and δ = 3.0) [3]. These results prove that an existence of the long-range FM interaction in these samples. In other word, the FM interactions in Pr0.6Sr0.4MnO3 nano-crystals obey the mean-field theory. However, there is a small deviation of the critical exponents from those of the mean-field theory, and a decrease of β value when decreases D, which are related to a magnetic disorder on the surface of the nano-crystals. The nature of these observed phenomena will be discussed thoroughly based on the core-shell structure and phase transition theory.
Chi Linh Dinh,Dang Thanh Tran, Thi Thanh Pham, Thi Anh Xuan Chu, Duc Huyen, Yen Pham, Seong Cho Yu