Browsing by Author "Sheppard, C.J"

Now showing 1 - 6 of 6
  • Thumbnail Image
    Item
    Effect of Mo content on the structural and physical properties of Cr100-xMox alloys
    (South African Institute of Physics, 2014) Chavan, A.U.; Prinsloo, A.R.E.; Sheppard, C.J; Muchono, Blessed
    Alloying Cr with Mo, which is isoelectric with it, shows an unexpected decrease in the Néel temperature (TN) with an increase in Mo concentration. This is attributed to a delocalization of the 3-d bands in Cr through the introduction of 4-d electrons of Mo. In the present investigation the effect of Mo concentration on the structural, magnetic and electrical properties of Cr is systematically studied. A series of Cr100-xMox alloys, with x = 0, 3, 7, 15 and 25, was prepared and the actual concentrations established using electron microprobe analyses. XRD studies confirm the bcc structure of these alloys as in pure Cr and indicate an increase in lattice constant with an increase in Mo concentration. The crystallite sizes calculated from these results for the Cr100-xMox alloys ranges between 15 and 30 nm. The physical properties of these alloys were investigated through magnetic susceptibility (χ), Seebeck coefficient (S), electrical resistivity (ρ) and Hall coefficient (RH) as function of temperature (T) measurements. TN values obtained from these measurements are comparable.
  • Thumbnail Image
    Item
    Magnetic behaviour of the Cr-Al alloy system around the triple point
    (South African Institute of Physics, 2011) Sheppard, C.J; Prinsloo, A.R.E.; Alberts, H.L.; Strydom, A.M.; Muchono, Blessed
    Electrical resistivity and specific heat measurements on a series of Cr1-xAlx (0 ≤ x ≤ 0.034) alloy single crystals are reported. The results indicate that the triple point, where the incommensurate and commensurate spin-density-wave phases coexist with the paramagnetic phase, is situated close to T = 0 K on the magnetic phase diagram.
  • Thumbnail Image
    Item
    Magnetic properties of the (Cr -xAlx) V alloy system
    (South African Institute of Physics, 2013) Muchono, Blessed; Sheppard, C.J; Prinsloo, A.R.E.; Alberts, H.L.
    Electrical resistivity (), Seebeck coefficient (S) and magnetic susceptibility () measurements as a function of temperature on the (Cr -xAlx) V alloy system, with 0 < x are reported. Néel temperatures (TN) obtained from all these measurements decrease with Al concentration, disappearing near x  1.5, again reappearing for x  . (T) and S(T) for samples with x  6.1 show weak anomalies making the determination of TN difficult. However, these anomalies are sharply defined in (T), proving that it is an important tool in probing antiferromagnetic in this system. The present results show that the addition of just 1 at.% V to the Cr -xAlx alloy system suppresses antiferromagnetism in the concentration range 5 ≤ x ≤ . This behaviour is similar to that observed for the (Cr -xAlx) 5Mo5 alloy system.
  • Thumbnail Image
    Item
    Magnetic susceptibility studies of the (Cr98.4Al1.6)100-x Mox alloy System
    (South African Institute of Physics, 2014) Muchono, Blessed; Sheppard, C.J; Prinsloo, A.R.E.; Alberts, H.L.
    The magnetic susceptibility as a function of temperature, 𝜒𝜒(𝑇𝑇), on a spin-densitywave antiferromagnetic (Cr98.4Al1.6)100-xMox alloy system in the concentration range 0 ≤ x ≤ 8.8 is reported in order to investigate the possibility of quantum critical behaviour in this alloy system. Néel temperatures, TN, obtained from 𝜒𝜒(𝑇𝑇) measurements decrease with Mo concentration and sharply tend towards 0 K at a critical concentration xc ≈ 4.5. Antiferromagnetism is suppressed to below 4 K in alloys with x ≳ 4.5. Alloys in the concentration range 0 ≤ x ≤ 3.0 depict an upturn in the 𝜒𝜒(𝑇𝑇) curves just above the Néel temperature. The upturn is attributed to local magnetic moments formed around the impurity atoms. The magnetic phase diagram of the alloy system points towards the existence of a quantum critical point at the critical concentration x = xc. The suggestion of quantum critical behaviour in this alloy system from previous electrical resistivity(𝜌𝜌), Seebeck coefficient (𝑆𝑆) and specific heat (𝐶𝐶p) measurements is corroborated in this study.
  • Thumbnail Image
    Item
    Spin-density-wave behaviour of the (Cr100-xAlx)95Mo5 alloy system
    (SAIP, 2011) Muchono, Blessed; Prinsloo, A.R.E.; Sheppard, C.J; Alberts, H.L.; Strydom, A.M.
    Electrical resistivity, Seebeck coefficient and specific heat measurements on a (Cr100-xAlx)95Mo5, 0 8.1 at.% Al alloy system are reported. The results indicate two possible quantum critical points in the magnetic phase diagram of this system. One is an incommensurate spin-density-wave – paramagnetic quantum critical point situated at 1.5 at.% Al and the other a paramagnetic – commensurate spin-density-wave critical point at 5 at.% Al. We forward experimental evidence that this system harbours two spin-density-wave related quantum critical points which presents an unusually rich case study for magnetic quantum criticality of the itinerant kind.
  • Thumbnail Image
    Item
    Thermal transport properties, magnetic susceptibility and neutron diffraction studies of the (Cr100-xAlx)95Mo5 alloy system
    (Elsevier, 2018) Muchono, B.; Sheppard, C.J; Venter, A.M.; Prinsloo, A.R.E.
    Previous electrical resistivity ðρÞ and specific heat ðCP Þ studies on the ternary (Cr100-xAlx)95Mo5 alloy system suggested that it harbours two quantum critical points (QCPs). This study reports comprehensive investigations of this alloy system through Seebeck coefficient ðSÞ, thermal conductivity ðκÞ, magnetic susceptibility ðχ Þ and neutron diffraction ðNDÞ measurements in the concentration range 0 x 8.6. The results of S and χ show that spin-density-wave (SDW) antiferromagnetism is suppressed to temperatures below 2 K for concentrations in the range 1.4 x 4.4. Plots of dS=dT in the limit as T→ 2 K depict two minima, i.e. just above x 1⁄4 1.4 and 4.4. This parameter has been used as a key indicator of quantum critical behaviour (QCB) in Cr alloys. Analyses against the Nordheim-Gorter relationship demonstrates a positive slope for the incommensurate (I) SDW alloys and a negative slope for the commensurate (C) SDW alloys. Extrapolations of these two slopes intercept at a concen- tration of 3.2 at.% Al indicating the occurrence of band structure modifications when Al is added into the Cr95Mo5 base alloy. The Lorenz number ðLÞ for the alloys with x 1⁄4 0 and 0.5 shows interesting anomalous behaviour associated with band structure effects and SDW magnetic ordering. ND measurements as a function of temper- ature confirm that alloys with x < 1.4 order in the incommensurate (I) SDW phase whilst alloys with x > 4.4 show commensurate (C) SDW order. Power law fits of the form TN ∝ð1:40 À xÞ0:35 Æ 0:05 for the ISDW to P phase transition and TN ∝ðx À 4:40Þ0:63 Æ 0:03 for the P to CSDW phase transition rendered the critical exponents 0:35 Æ 0:05 and 0:63 Æ 0:03 respectively. Overall the results of S, κ, χ and ND corroborate the existence of two QCPs at x % 1.4 and 4.4.