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Superconductivity and Quantum Critical Behavior in Cr_{100-z}Os_z

100%
Prinsloo A., Sheppard C., Fernando P.
Acta Physica Polonica A
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2017
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vol. 131
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issue 4
1132-1134
EN
The magnetic phase diagrams of Cr alloys with group-8 metals, such as Re and Ru, show interesting properties. These include quantum critical and superconducting behavior. In order to extend these investigations this paper reports on the physical properties and the magnetic phase diagram of the Cr_{100-z}Os_{z} system, investigated in a concentration range of 2 ≤ z ≤ 30.6. Structural analysis indicate that samples with z<22 have a bcc structure, while those with z>22 include a cubic A15-type structure. Transport and magnetic property measurements indicate that the antiferromagnetic phase disappears at z ≈ 14 and that samples with concentrations z ≥ 12.2 show superconducting behavior. The magnetic phase diagram of Cr_{100-z}Os_{z} is constructed from the Néel and superconducting transition temperatures obtained from various measurements. The commensurate spin-density-wave to paramagnetic phase line decreases sharply for z>4. Fitting parameters of a power law fit to the commensurate spin-density-wave to paramagnetic phase line indicate possible quantum critical behavior. Present results suggest that this phase line terminates in a superconducting dome near a magnetic quantum critical point, similar to that observed in certain heavy fermion systems, but not previously reported for other Cr alloys.
2
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Effect of Fe Substitution on Structural and Magnetic Properties of NiCr₂O₄

100%
Mohanty P., Prinsloo A., Sheppard C., Roos W.
Acta Physica Polonica A
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2018
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vol. 133
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issue 3
574-577
EN
At room temperature, the normal oxide spinel NiCr₂O₄ is tetragonally distorted and crystallizes in the I4₁/amd space group due to cooperative Jahn-Teller ordering driven by the orbital degeneracy of tetrahedral Ni²⁺. The ferrimagnetic Curie temperature (T_{C}) for NiCr₂O₄ is 74 K. The magnetic moments of NiCr₂O₄ are composed of a ferrimagnetic (longitudinal) and an antiferromagnetic (transverse) component. Exchange interaction between the magnetic cations influences the overall magnetic properties of the compound. Present work focuses on the modification of structural and magnetic properties upon substituting Fe at Cr site in NiCr₂O₄ with the motivation of changing the magnetic exchange interaction. In order to do so, single phase Ni(Cr_{0.5}Fe_{0.5})₂O₄ samples were prepared by co-precipitation techniques, while controlling the pH of precipitation. Upon Fe substitution, crystal structure was not affected much contrary to the earlier reports. In order to determine the oxidation state of each elements X-ray photoelectron spectroscopy (XPS) was performed. T_{C} was found to increase dramatically above 300 K, confirmed both from temperature and field dependent dc-magnetization studies.
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Residual Stress in Cr₉₉Al₁ Polycrystalline Thin Films

76%
Mudau Z., Sheppard C., Prinsloo A., Venter A., Ntsoane T., Fullerton E.
Acta Physica Polonica A
|
2018
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vol. 133
|
issue 3
578-581
EN
The magnetic phase diagram of bulk Cr_{100-x}Al_{x} shows interesting behaviour close to the triple point concentration of x ≈2. Since the magnetic properties of Cr are influenced by dimensionality, stress and strain, this study focussed on the investigation of Cr₉₉Al₁ thin films prepared on fused silica substrates with thicknesses t varying from 29 to 452 nm using sputtering techniques. Resistance measurements covering the temperature range 2 to 400 K did not show any clear anomalies that could be indicative of changes in the magnetic ordering. X-ray diffraction (XRD) and atomic force microscopy (AFM) studies indicate the films are polycrystalline textured and that the 80 nm sample has the smallest grain size. In-plane stresses in these thin films were studied using the specialised XRD ın²ψ-method. The results show that the stress varies with film thickness. The 29 nm sample has stress in the order of 833 MPa and with increasing film thickness the stress reaches 1471 MPa for the 80 nm layer, where after it systematically reduces for the thicker coatings to 925 MPa for the 452 nm film. The highest stress for the Cr₉₉Al₁ thickness sample series is seen in the film with the smallest grain size.
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