Browsing by Author "Debnath, JC"

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    Critical phenomena and estimation of the spontaneous magnetization by a magnetic entropy analysis in Mn(0.96)Nb(0.04)CoGe alloy
    (American Institute of Physics., 2013-06-21) Debnath, JC; Strydom, AM; Shamba, P; Wang, JL; Dou, SX
    Magnetic and magnetocaloric properties of the alloy Mn0.96Nb0.04CoGe have been investigated. According to the mean-field theory prediction, the relationship between Delta S-M proportional to (H/T-C)(2/3) has been confirmed in the temperature region near T-C for that system. To investigate the nature of the magnetic phase transition, a detailed critical exponent study has been performed. The critical components, gamma, beta, and delta determined using the Kouvel-Fisher method, the modified Arrott plot, as well as the critical isotherm analysis agree well. Moreover, these critical exponents are confirmed by the Widom scaling law and the validity of the calculated critical exponents was also confirmed by the scaling theory. The values deduced for the critical exponents are close to the theoretical prediction of the mean-field model values, thus indicating that long range interactions dominate the critical behavior in the Mn0.96Nb0.04CoGe system. It is also speculated that the competition between the localized Mn-Mn magnetic interactions should be responsible for the critical behavior in this system. Moreover, an excellent agreement is found between the spontaneous magnetization determined from the entropy change (-Delta S-M vs. M-2) and the classical extrapolation from the Arrott curves (H/M vs. M-2), thus confirming that the magnetic entropy change is a valid approach to estimate the spontaneous magnetization in this system. © 2013, American Institute of Physics.
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    Effects of Cr substitution on structural and magnetic properties in La0.7Pr0.3Fe11.4Si1.6 compound
    (AIP Scitation, 2014-04-03) Din, MFM; Wang, JL; Studer, AJ; Gu, QF; Zeng, R; Debnath, JC; Shamba, P; Kennedy, SJ; Dou, SX
    In an effort to explore the effect of substitution Fe by Cr in NaZn13-type La0.7Pr0.3Fe11.4−xCrxSi1.6 (x = 0, 0.06, 0.12, 0.26, and 0.34) compounds, the structure and magnetic properties have been investigated by high intensity of x-ray and neutron diffraction, scanning electron microscopy, specific heat, and magnetization measurement. It has been found that a substitution of Cr for Fe in this compounds leads to decrease in the lattice parameter a at room temperature but variation on Curie temperature (TC). While the first order nature of magnetic phase transition around TC does not change with increasing Cr content up to x = 0.34. High intensity x-ray and neutron diffraction study at variable temperatures for highest Cr concentration x = 0.34 confirmed the presence of strong magneto-volume effect around TC and indicated the direct evident of coexistence across magnetic transition as characteristic of first order nature. The values of −ΔSM around TC decrease from 17 J kg−1 K−1 for x = 0 to 12 J kg−1 K−1 for x = 0.06 and then increases with further increasing Cr content up to 17.5 J kg−1 K−1 for x = 0.34 under a change of 0–5 T magnetic field. Similar behavior on relative cooling power which is decrease from 390 J kg−1 for x = 0 to 365 J kg−1 for x = 0.06 at the beginning and then increases up to 400 J kg−1 for x = 0.34. From the point of this view with the highest Cr concentration (x = 0.34) exhibits favourable material candidate for magnetic refrigerator application therefore should inspire further study concerning on higher Cr concentration in this compound. © 2014, AIP Publishing LLC.
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    Effects of Cu substitution on structural and magnetic properties of La0.7Pr0.3Fe11.4Si1.6 compounds
    (Elsevier Ltd., 2013-05-01) Din, MFM; Wang, JL; Zeng, R; Shamba, P; Debnath, JC; Dou, SX
    The structure and magnetic properties of the La0.7Pr0.3Fe11.4−xCuxSi1.6 compounds have been investigated by X-ray diffraction, scanning electron microscopy and magnetisation measurements. Cu substitution for Fe in La0.7Pr0.3Fe11.4−xCuxSi1.6 (x = 0, 0.06, 0.12, 0.23, 0.34) leads to a reduction in hysteresis loss, a decrease in magnetic entropy change but an increase in Curie temperature (TC). The influences of annealing processes at different temperatures on TC, magnetic hysteresis, and the magnetocaloric effect (MCE) of La0.7Pr0.3Fe11.4Si1.6 are investigated in detail. It has been found that a short-time and high temperature annealing process has benefits for the formation of the NaZn13 types as phase compared to a long-time and low temperature annealing process. © 2013, Elsevier Ltd.
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    Investigation of the critical behavior in Mn[sub 0.94]Nb[sub 0.06]CoGe alloy by using the field dependence of magnetic entropy change
    (American Institute of Physics., 2013-03-07) Debnath, JC; Shamba, P; Strydom, AM; Wang, JL; Dou, SX
    The critical behaviour of Mn0.94Nb0.06CoGe alloy around the paramagnetic-ferromagnetic phase transition was studied based on the field dependence on magnetic entropy change. By using the obtained exponents, the modified Arrott plot is consistent with that by using conventional method. These critical exponents are confirmed by the Widom scaling relation. Based on these critical exponents, the magnetization, field and temperature data around Tc collapse into two curves obeying the single scaling equation M(H, epsilon) = epsilon(beta)f+/-(H/epsilon(beta+gamma)). The calculated critical exponents not only obey the scaling theory but also anastomose the deduced results from the Kouvel-Fisher method [J. S. Kouvel and M. E. Fisher, Phys. Rev. 136, A1626 (1964)]. The values deduced for the critical exponents in the Mn0.94Nb0.06CoGe alloy are close to the theoretical prediction of the mean-field model, indicating that the magnetic interactions are long range. This method eliminates the drawback due to utilization of multistep nonlinear fitting in a conventional manner. So it provides an alternative method to investigate the critical behaviour. © 2013 American Institute of Physics.
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    Magnetic properties in polycrystalline and single crystal Ca-doped LaCoO3
    (American Physical Society, 2011-04-01) Zeng, R; Debnath, JC; Chen, DP; Shamba, P; Wang, JL; Kennedy, SJ; Campbell, SJ; Silver, T; Dou, SX
    Polycrystalline (PC) and single crystalline (SC) Ca-doped LaCoO3 (LCCO) samples with the perovskite structure were synthesized by conventional solid-state reaction and the floating-zone growth method. We present the results of a comprehensive investigation of the magnetic properties of the LCCO system. Systematic measurements have been conducted on dc magnetization, ac susceptibility, exchange-bias, and the magnetocaloric effect. These findings suggest that complex structural phases, ferromagnetic (FM), and spin-glass/cluster-spin-glass (CSG), and their transitions exist in PC samples, while there is a much simpler magnetic phase in SC samples. It was also of interest to discover that the CSG induced a magnetic field memory effect and an exchange-bias-like effect, and that a large inverse irreversible magnetocaloric effect exists in this system.(C) 2011 American Institute of Physics. [doi:10.1063/1.3562518]
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    The magnetocaloric effect and critical behaviour of the Mn0.94Ti0.06CoGe alloy
    (IOP Publishing Ltd., 2013-02-06) Shamba, P; Wang, JL; Debnath, JC; Kennedy, SJ; Zeng, R; Din, MFM; Hong, F; Cheng, ZX; Studer, AJ; Dou, SX
    Structural, magnetic and magnetocaloric properties of the Mn(0.94)Ti(0.06)CoGe alloy have been investigated using x-ray diffraction, DC magnetization and neutron diffraction measurements. Two phase transitions have been detected, at T(str) = 235 K and T(C) = 270 K. A giant magnetocaloric effect has been obtained at around Tstr associated with a structural phase transition from the low temperature orthorhombic TiNiSi-type structure to the high temperature hexagonal Ni(2)In-type structure, which is confirmed by neutron study. In the vicinity of the structural transition, at T(str), the magnetic entropy change, -Delta S(M) reached a maximum value of 14.8 J kg(-1) K(-1) under a magnetic field of 5 T, which is much higher than that previously reported for the parent compound MnCoGe. To investigate the nature of the magnetic phase transition around T(C) = 270 K from the ferromagnetic to the paramagnetic state, we performed a detailed critical exponent study. The critical components gamma, beta and delta determined using the Kouvel-Fisher method, the modified Arrott plot and the critical isotherm analysis agree well. The values deduced for the critical exponents are close to the theoretical prediction from the mean-field model, indicating that the magnetic interactions are long range. On the basis of these critical exponents, the magnetization, field and temperature data around T(C) collapse onto two curves obeying the single scaling equation M(H, epsilon) = epsilon(beta)f +/- (H/epsilon(beta+gamma)). © 2013 IOP Publishing LTD
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    On the crystal structure and magnetic properties of the Mn0.94Ti0.06CoGe alloy
    (American Institute of Physics., 2013-05-07) Shamba, P; Wang, JL; Debnath, JC; Zeng, R; Hong, F; Cheng, ZX; Studer, AJ; Kennedy, SJ; Dou, SX
    Structural and magnetic properties of Mn0.94Ti0.06CoGe have been studied by a combination of bulk magnetisation and neutron diffraction measurements over the temperature range of 5 K-350 K. The crystal structural transition occurs at T-str (similar to 235 K) with a change in symmetry from the low temperature orthorhombic TiNiSi-type structure (space group Pnma) to the high temperature hexagonal Ni2In-type structure (space group P63/mmc) and the magnetic phase transition takes place around T-C = 270 K. It is found that the structural transition around T-str is incomplete and there is a co-existence of the orthorhombic and hexagonal structures between T-str and T-C (similar to 270 K). These results are discussed in connection with the magnetic and magnetocaloric behaviours in Mn0.94Ti0.06CoGe. © 2013, American Institute of Physics.
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    Reduction of hysteresis losses in the magnetic refrigerant La(0.8)Ce(0.2)Fe(11.4)Si(1.6) by the addition of boron
    (American Institute of Physics, 2011-04-01) Shamba, P; Debnath, JC; Zeng, R; Wang, JL; Campbell, SJ; Kennedy, SJ; Dou, SX
    In an effort to improve the magnetocaloric effects of the NaZn13-type La0.8Ce0.2Fe11.4Si1.6 compound, the effect of boron doping on the magnetic properties and magnetocaloric properties has been investigated. The magnetic entropy change (Delta S-M) for the La0.8Ce0.2Fe11.4Si1.6 compound, obtained for a field change of 0-5 T using the Maxwell relation exhibits a spike and appears to be overestimated and is thus corrected by using the Clausius-Clapeyron equation (CC). The Delta S-M determined from the CC equation is estimated to be 19.6 J kg(-1) K-1. However, large hysteretic losses which are detrimental to the magnetic refrigeration efficiency occur in the same temperature range. In this work, we report a significant reduction in hysteretic losses by doping the La0.8Ce0.2Fe11.4Si1.6 compound with a small amount of boron to obtain La0.8Ce0.2Fe11.4Si1.6Bx compounds. The hysteresis loss decreases from 131.5 to 8.1 J kg(-1) when x increases from 0 to 0.3, while Delta S-M, obtained for a field change of 0-5 T, varies from 19.6 to 15.9 J kg(-1) K-1. This also simultaneously shifts the T-C from 174 to 184 K and significantly improves the effective refrigerant capacity (RCeff) of the material from 164 to 305 J kg(-1).(C) 2011 American Institute of Physics. [doi: 10.1063/1.3565401]
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    Ti substitution for Mn in MnCoGe - the magnetism of Mn0.9Ti0.1CoGe
    (Elsevier Science, 2013-11-15) Wang, JL; Shamba, P; Hutchison, WD; Din, MFM; Debnath, JC; Avdeev, M; Zeng, R; Kennedy, SJ; Campbell, SJ; Dou, SX
    Bulk magnetization measurements (5–320 K; 0–8 T) reveal that below room temperature Mn0.9Ti0.1CoGe exhibits two magnetic phase transitions at ∼178 K and ∼280 K. Neutron diffraction measurements (3–350 K) confirm that the transition at ∼178 K is due to the structural change from the low-temperature orthorhombic TiNiSi-type structure (space group Pnma) to the higher temperature hexagonal Ni2In-type structure (space group P63/mmc), while the transition at ∼280 K originates from the transition from ferromagnetism to paramagnetism. The magnetocaloric behaviour of Mn0.9Ti0.1CoGe around Tstr ∼ 178 K and TC ∼ 280 K as determined via the magnetic field and temperature dependences of DC magnetisation are given by the maximum values of the magnetic entropy changes −Δ S M max = 6.6 J kg−1 K−1 around Tstr ∼ 178 K, and −Δ S M max = 4.2 J kg−1 K−1 around TC ∼ 280 K for a magnetic field change of ΔB = 0–8 T. Both structural entropy – due to the unit cell expansion of ∼4.04% – and magnetic entropy – due to an increase in the magnetic moment of ∼31% – are found to contribute significantly to the total entropy change around Tstr. Critical analysis of the transition around TC ∼ 280 K leads to exponents similar to values derived from a mean field theory, consistent with long-range ferromagnetic interactions. It was found that the field dependence of −Δ S M max can be expressed as −Δ S M max ∝ Bn with n = 1 for the structural transition around Tstr and n = 2/3 for the ferromagnetic transition around TC, thereby confirming the second order nature of this latter transition.© 2013, Elsevier Ltd.