Browsing by Author "Strydom, AM"

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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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    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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    Quantum critical behaviour in the (Cr97.8Si2.2)(100-y)Moy alloy system
    (American Institute of Physics, 2013-05-07) Sheppard, CJ; Prinsloo, ARE; Fernando, PR; Venter, AM; Strydom, AM; Peterson, VK
    The Cr-Si system offers an ideal reference alloy as a case study to follow the evolution of the well-known first-order magnetic transition in dilute Cr-Si by doping with Mo, which is iso-electronic with Cr. This alloy system, in addition, then also offers the opportunity to search for effects of quantum criticality (QC) associated with the process of driving the Neel transition temperature (T-N) to zero. Previous investigations on the ternary (Cr97.8Si2.2)(100-y)Mo-y alloy system (0 <= y <= 9) indicated possible quantum critical behaviour at a critical concentration y(c) approximate to 3.8. The present paper extends these studies to additional concentrations in the alloy series, as well as to explore key indicators of QC, such as Hall coefficient (R-H) and magnetic susceptibility (chi) measurements. Recent results indicate the possible existence of two critical points in the (Cr97.8Si2.2)(100-y)Mo-y alloy system. The first critical point at y(c1) is postulated to be related to the concentration where the first-order transition gives way to a second-order-like continuous magnetic phase transition. Behaviour normally associated with QC is observed at the second critical concentration y(c2), accompanying a transition to paramagnetism. © 2013 American Institute of Physics.