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Phan T.L.,Chungbuk National University | Nguyen V.D.,Vietnam Academy of Science and Technology | Ho T.A.,Chungbuk National University | Khiem N.V.,Hongduc University | And 4 more authors.
IEEE Transactions on Magnetics | Year: 2014

The magnetic properties of polycrystalline ceramic samples (Nd1-xYx)0.7Sr0.3MnO3 with x=0.21-0.35 were studied by means of dc magnetization and ac susceptibility measurements. Experimental results reveal a strong decrease of the ferromagnetic (FM)-paramagnetic phase-transition temperature (TC) from 97 to 65 K as increasing x from 0.21 to 0.35, respectively. There is magnetic inhomogeneity associated with short-range FM order. Particularly, the samples undergo a spin-glass (SG) phase transition at the so-called blocking temperature (TB) below TC , which shifts toward lower temperatures with increasing the applied field, Hex; TB to Tg (the SG phase-transition temperature) as Hex to 0. The existence of the SG behavior in these samples was also confirmed by frequency (f) dependences of the ac susceptibility. For the in-phase/real component, χ′(T) , it shows a frequency-dependent peak at the SG freezing temperature (Tf) ; Tf to Tg as f to 0. Dynamics of this process were analyzed by means of the slowing down scaling law, τ0 (Tf/Tg-1-zv , where τ0 and zv are the characteristic time and critical exponent, respectively. Fitting the experimental Tf(f) data to the scaling law gave the results of zv=10.1-12.3 and τ0=10-21-10-15~ s. These values are different from those expected for canonical SG systems with zv=10 and τ0=10-13~ s , revealing the cluster-SG behavior of ( Nd1-x Yx)0.7Sr0.3MnO3 samples. Notably, the increase in Y content leads to the shift of τ0 and zv values toward those of canonical SG systems, which is ascribed to an expansion of SG clusters. © 2015 IEEE. Source

Thanh T.D.,Chungbuk National University | Thanh T.D.,Vietnam Academy of Science and Technology | Bau L.V.,Hongduc University | Phan T.L.,Chungbuk National University | Yu S.C.,Chungbuk National University
IEEE Transactions on Magnetics | Year: 2014

We have investigated themagnetic properties and magnetocaloric effect in La0.7Sr0.3Mn1-xCoxO3 prepared by a conventional solid-state reaction method. Magnetic measurements versus temperature revealed that the Curie temperature (TC) decreased gradually with increasing Co content (x); TC values are about 360, 310, 296 and 280 K for x =0.0, 0.06, 0.08 and 0.1, respectively. Magnetic entropy change (ΔSm) of the samples under a magnetic field change of 10 kOe was calculated by using isothermal magnetization data.We have found its maximum (|ΔSmax|) achieved around TC, which is not changed much (∼ 1.5 J·kg -1·K-1) with increasing Co-doping concentration. If combining the samples with x =0.06, 0.08 and 0.10 for magnetic refrigeration, particularly, the temperature range can be used in between 266 K and 322 K, where |ΔSmax| values are stable at about 0.98 J·kg -1·K-1. The relative cooling power is accordingly about 55 J·kg-1 and comparable to that of other magnetocaloric alloys. These results suggest La0.7Sr0.3Mn 1-xCoxO3 compounds to be a potential candidate for magnetic refrigerators at room temperature. © 2013 IEEE. Source

Phong P.T.,Vietnam Academy of Science and Technology | Dai N.V.,Vietnam Academy of Science and Technology | Manh D.H.,Vietnam Academy of Science and Technology | Thanh T.D.,Vietnam Academy of Science and Technology | And 3 more authors.
Journal of Magnetism and Magnetic Materials | Year: 2010

The electrical transport properties and the magnetoresistance of La 0.7Ca0.3MnO3/La0.7Sr 0.2Ca0.1MnO3 composites are investigated as a function of sintering temperature. On the basis of an analysis by X-ray powder diffraction and scanning electron microscopy we suggest that raising the sintering temperature enhanced the interfacial reaction and creates interfacial phases at the boundaries of the La0.7Ca0.3MnO3 and La0.7Sr0.2Ca0.1MnO3. Results also show that in 3 kOe, and at the Curie temperature, the magnetoresistance value of 14% was observed for the composite sintered at 1300 °C. Based on the phenomenological equation for conductivity under a percolation approach, which depends on the phase segregation of ferromagnetic metallic clusters and paramagnetic insulating regions, we fitted the experimental resistivitytemperature data from 50300 K and find that the activation barrier decreases as temperature is increased. © 2010 Elsevier B.V. Source

Thanh T.D.,Vietnam Academy of Science and Technology | Phong P.T.,Vietnam Academy of Science and Technology | Dai N.V.,Vietnam Academy of Science and Technology | Manh D.H.,Vietnam Academy of Science and Technology | And 3 more authors.
Journal of Magnetism and Magnetic Materials | Year: 2011

We report magneto-transport and magnetic properties of (1x)La 0.7Ca0.3MnO3xAl2O3 composites synthesized through a solid-state reaction method combined with a high energy milling method. Most interestingly, the effective magnetic anisotropy is found to decrease with increase in the non-magnetic insulating Al2O3 phase fraction in the composites. In addition, we observed that the magnitude of low-field magnetoresistance arising from spin-polarized tunneling of conduction electrons, as well as that of high-field magnetoresistance, displays a CurieWeiss law-like behavior. Finally, we found that the temperature dependence of low and high-field magnetoresistance is controlled predominantly by the nature of temperature response of surface magnetization of the particles. © 2010 Elsevier B.V. All rights reserved. Source

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