[Show abstract][Hide abstract] ABSTRACT: The microstructure, crystal structure, and magnetic properties of low-temperature phase (LTP) Mn-Bi nanosheets, prepared by surfactant assistant high-energy ball milling (SA-HEBM) with oleylamine and oleic acid as the surfactant, were examined with scanning electron microscopy, X-ray diffraction, and vibrating sample magnetometer, respectively. Effect of ball-milling time on the coercivity of LTP Mn-Bi nanosheets was systematically investigated. Results show that the high energy ball milling time from tens of minutes to several hours results in the coercivity increase of Mn-Bi powders and peak values of 14.3 kOe around 10 h. LTP Mn-Bi nanosheets are characterized by an average thickness of tens of nanometers, an average diameter of ̃1.5 μm, and possess a relatively large aspect ratio, an ultra-high room temperature coercivity of 22.3 kOe, a significant geometrical and magnetic anisotropy, and a strong (00l) crystal texture. Magnetization and demagnetization behaviors reveal that wall pinning is the dominant coercivity mechanism in these LTP Mn-Bi nanosheets. The ultrafine grain refinement introduced by the SA-HEBM process contribute to the ultra-high coercivity of LTP Mn-Bi nanosheets and a large number of defects put a powerful pinning effect on the magnetic domain movement, simultaneously. Further magnetic measurement at 437 K shows that a high coercivity of 17.8 kOe and a strong positive temperature coefficient of coercivity existed in the bonded permanent magnet made by LTP Mn-Bi nanosheets.
Journal of Applied Physics 04/2014; 115(17). · 2.19 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Bonded La(Fe, Si)13 magnetic refrigeration materials have been prepared, and the microstructure, mechanical properties, and magnetocaloric effect (MCE) of bonded LaFe11.7Si1.3C0.2Hx have been investigated systematically. Bonded materials show porous architecture, and the mechanical properties increase with the increase of epoxy resin content, which could fill more pores and boundaries and thus enhance the binding force between different particles. Bonded LaFe11.7Si1.3C0.2H1.8 with 3 wt. % epoxy resin exhibits a compressive strength of 162 MPa, 35% higher than that of bulk compound. The mass magnetic entropy change (ΔSM) remains nearly unchanged while the volumetric ΔSM reduces due to the decrease of density in bonded materials. For a low magnetic field change of 2 T, the maximum ΔSM value of bonded LaFe11.7Si1.3C0.2H1.8 is ∼10.2 J/kg K and ∼54.7 mJ/cm3 K, which is larger than those of some magnetocaloric materials in the same temperature range. Enhanced mechanical properties and great MCE suggest that bonded La(Fe, Si)13-based materials could be promising candidates of magnetocaloric materials for practical applications of magnetic refrigeration.
[Show abstract][Hide abstract] ABSTRACT: La(Fe, Si)13-based compounds have been considered as promising candidates for magnetic refrigerants particularly near room temperature. Herein we review recent progress particularly in the study of the effects of interstitial H and/or C atoms on the magnetic and magnetocaloric properties of La(Fe, Si)13 compounds. By introducing H and/or C atoms, the Curie temperature T C increases notably with the increase of lattice expansion which makes the Fe 3 d band narrow and reduces the overlap of the Fe 3 d wave functions. The first-order itinerant-electron metamagnetic transition is conserved and the MCE still remains high after hydrogen absorption. In contrast, the characteristic of magnetic transition varies from first-order to second-order with the increase of C concentration, which leads to remarkable reduction of thermal and magnetic hysteresis. In addition, the introduction of interstitial C atoms promotes the formation of NaZn13-type (1:13) phase in La(Fe, Si)13 compounds, and thus reducing the annealing time significantly from 40 days for LaFe11.7Si1.3 to a week for LaFe11.7Si1.3C0.2. The pre-occupied interstitial C atoms may depress the rate of hydrogen absorption and release, which is favorable to the accurate control of hydrogen content. It is found that the reduction of particle size would greatly depress the hysteresis loss and improve the hydrogenation process. By the incorporation of both H and C atoms, large MCE without hysteresis loss can be obtained in La(Fe, Si)13 compounds around room temperature, for instance, La0.7Pr0.3Fe11.5Si1.5C0.2H1.2 exhibits a large |Δ S M| of 22.1 J/(kg·K) at T C = 321 K without hysteresis loss for a field change of 0-5 T.
[Show abstract][Hide abstract] ABSTRACT: Magnetocaloric effect (MCE) of RMn2 (R = Tb, Dy, Ho, Er) compounds are investigated. TbMn2 and DyMn2 crystallize in cubic Laves phase structure (C15 type), whereas HoMn2 and ErMn2 crystallize in hexagonal Laves phase structure (C14 type). For TbMn2 compound, the field-induced metamagnetic transition accompanying a spontaneous cell volume expansion is observed (inverse MCE), which leads to a large positive value (8.3 J kg−1 K−1) of magnetic entropy change around 36 K under the field change of 0–1 T, while the maximal values of magnetic entropy change (ΔSM) and the refrigerant capacity (RC) for other RMn2 (R = Dy, Ho, Er) compounds are −15.7, −18.4, −25.5 J kg−1 K−1 and 403.6, 404.3, 316.0 J kg−1 around their TC with negligible thermal and magnetic hysteresis loss for the field change of 0–5 T, respectively. The results suggest that RMn2 (R = Dy, Ho, Er) may be appropriate candidates for magnetic refrigerant working at low temperature region 10–80 K.
Journal of Alloys and Compounds 10/2013; 575:162–167. · 2.73 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The magnetocaloric effect (MCE) of RNi2Si2 (R=Dy, Ho, Er) compounds with the ThCr2Si2-type body-centered tetragonal structure are investigated. RNi2Si2 compounds are antiferromagnetic (AFM) with Néel temperature TN=6.5 K, 4.9 K, and 3.5 K, respectively. A field-induced metamagnetic transition from AFM-to-ferromagnetic (FM) state is found below TN, which leads to a large MCE around the TN. The maximal values of magnetic entropy change (ΔSM) for RNi2Si2 (R=Dy, Ho, Er) reach −6.9, −10.9, and −15.1 Jkg−1 K−1 and −21.3, −21.7, and −21.3 Jkg−1 K−1 without thermal and magnetic hysteresis losses for the field changes of 0–2 T and 0–5 T, respectively. The large ΔSM is associated with the field-induced first-order AFM–FM metamagnetic transition and low critical field. The excellent MCE under low field change without hysteresis loss suggests that RNi2Si2 (R=Dy, Ho, Er) can be an appropriate candidate for magnetic refrigerant in liquid helium temperature ranges.
Journal of Magnetism and Magnetic Materials 10/2013; 344:96–100. · 2.00 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The compound Mn1.1Fe0.9P0.76Ge0.24 has been studied using neutron powder diffraction (NPD), differential scanning calorimeter (DSC), and magnetic measurements, in order to clarify the nature of the magnetic and structural transition and measure the associated entropy change (ΔS). The strongly first order transition occurs from a paramagnetic (PM) to a ferromagnetic (FM) phase and can be induced either by temperature or by an applied magnetic field. Our investigations indicate that the two processes exhibit identical evolutions regarding the crystal and magnetic structures, indicating they should have the same entropy change. We, therefore, conclude that the ΔSDSC obtained by the DSC method (where the transition is temperature induced) is valid also for the magnetically induced transition, thus avoiding uncertainties connected with the magnetic measurements. We have obtained the ΔSDSC = 33.8 J/kg · K for this sample upon cooling, which would increase to 42.7 J/kg · K for a impurity-free and completely homogeneous sample. For comparison, the magnetic entropy changes (ΔSM) induced by magnetic field and calculated using the Maxwell relation yields a ΔSM = 46.5J/kg · K, 38% higher than ΔSDSC. These entropy results are compared and discussed.
Journal of Applied Physics 01/2013; 113(4). · 2.19 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The effect of hydrogenating process on the homogeneity of hydrogen absorption in the La0.8Ce0.2
(Fe1−xMnx)11.5Si1.5 compounds was investigated. It is attractive that the distribution of hydrogen atoms in the compounds becomes more uniform with increasing hydrogen pressure and annealing temperature. Homogeneous hydrides of La0.8Ce0.2(Fe0.985Mn0.015)11.5Si1.5 were obtained by annealing them at the temperature of 773 K and under the hydrogen atmosphere of 0.5 MPa. With changes of Mn content, the Curie temperatures (TC) of the hydrides of La0.8Ce0.2
(Fe1−xMnx)11.5Si1.5 can be adjusted in the room temperature range from 279 to 312 K. Large magnetic entropy changes due to the itinerant-electron metamagnetic transition are obtained in the room temperature range for the hydrides of La0.8Ce0.2(Fe1−xMnx)11.5Si1.5.
Journal of Applied Physics 03/2011; 109(7):07A910-07A910-3. · 2.19 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The exchange coupling and magnetic properties distributions in Co/Pd multilayer perpendicular magnetic anisotropy films with different magnetic properties are investigated using the experimental first order reversal curve (FORC) diagram with assistance of Landau–Lifshitz–Gilbert simulation. The simulated FORC diagrams of perpendicular magnetic anisotropy films with different exchange couplings and magnetic property distributions are quite different, which make FORC diagrams very powerful for characterizing perpendicular magnetic recording media.
Journal of Applied Physics 12/2009; · 2.19 Impact Factor