C. J. Sheppard

Deposition of single-phase CuIn(Se,S)2 thin films from the sulfurization of selenized CuIn alloys

The reaction of dc sputtered metallic CuIn alloys to a reactive H2Se/Ar/H2S gaseous atmosphere is an attractive industrial production process to produce CuIn(Se,S)2 chalcopyrite absorber films for applications in photovoltaic modules. However, the obvious technological advantages of this deposition technology are overshadowed by growth-related anomalies such as the separation or at least partial separation of the ternary phases (i.e. CuInSe2 and CuInS2) during the high temperature selenization/sulphurization of the metallic alloy. This in turn prevents the effective band gap widening of the semiconductor alloys in order to achieve open-circuit voltages in excess of 600 mV, which is a critical prerequisite for the optimal performance of thin film solar modules. In this contribution, the material properties of homogeneous single-phase CuIn(Se,S)2 alloys are discussed, produced with a novel two-stage deposition process.

Evidence for a possible quantum critical point in a Cr-Si alloy doped with Mo

The magnetic behavior and possibility of quantum criticality in a (Cr0.98Si0.02)1−yMoy alloy system (0 ≤ y ≤ 0.09) is investigated through electrical resistivity, specific heat, and thermoelectric power measurements in a temperature range 2–350 K. Alloys with 0 ≤ y ≤ 0.011 depict first-order Neel transitions that give way to continuous Neel transitions for 0.011 < y ≤ 0.038. The Neel temperature, TN(y), seems to be suppressed down to 0 K at a critical concentration yc ≈ 0.04. The Sommerfeld coefficient of specific heat γ(y) peaks at y ≈ yc, showing a sharp decreasing trend on decreasing y to below yc. This behavior is reminiscent of that observed for γ of the prototypical Cr1−xVx quantum critical system, and indeed also portrays the physics of a quantum critical point in the present (Cr0.98Si0.02)1−yMoy system.

Possible quantum critical behaviour in the (Cr84Re16)100−yVy alloy system

Possible quantum critical behaviour in the (Cr84Re16)100−yVy alloy system, with 0 ≤ y < 14, was investigated using electrical resistivity (ρ), Seebeck coefficient (S), magnetic susceptibility (χ), Hall coefficient (RH), and specific heat (Cp) measurements as a function of temperature (T). Characteristics typically associated with a quantum critical point (QCP) can be observed in the low temperature behaviour of ρ(y), RH(y), χ(y), and γ(y); where γ is the Sommerfeld coefficient determined from least square fits to the low temperature specific heat measurements. Present results provide evidence of the existence of a putative QCP at a critical concentration of yc ≈ 10.5 for the (Cr84Re16)100−yVy alloy system.

Low temperature and magnetic field behaviour of the (Cr84Re16)89.6V10.4 alloy

Measurements of the temperature (T) dependence of the magnetic susceptibility (χ) and electrical resistance (R) on an antiferromagnetic (AFM) (Cr84Re16)89.6V10.4 alloy are reported in order to probe the existence of quantum critical behaviour (QCB) utilizing static magnetic fields (H) as a tuning parameter. The results indicate that an increase in H suppresses TN in such a way that it varies exponentially with increasing H. R(T) measurements show evidence of possible superconducting (SC) behaviour below 1 K at H = 0 T. These results therefore indicate the coexistence of the AFM and SC phases in the (Cr84Re16)89.6V10.4 alloy.

Structural and magnetic properties of (Co1–xNix)Cr2O4 (x = 0.5, 0.25) nanoparticles

Nanoparticles of (Co1–xNix)Cr2O4, with x = 0.5 and 0.25, were prepared utilizing the sol-gel technique, in order to investigate the effect of Ni substitution at the Co site. The crystal structure of the prepared samples was identified using X-ray diffraction. Transmission electron microscopy images indicate a non-uniform distribution in particle sizes. Temperature dependent magnetization measurements as a function of probing field demonstrate different magnetic transition temperatures to that of both the parent compounds. The magnetization as a function of applied magnetic field shows a wasp-waist like feature for (Co0.5Ni0.5)Cr2O4 nanoparticles measured at 10 K, which is absent in both NiCr2O4 and CoCr2O4. This feature diminished for other measurement temperatures below the Curie temperature and was also absent at all temperatures for the (Co0.75Ni0.25)Cr2O4 nanoparticles. X-ray photoemission spectroscopy results show that the Ni cations prefers the 3+ and Co the 2+ oxidation states, while that of Cr was...

Thermal simulation of magnetization reversals for size-distributed assemblies of core-shell exchange biased nanoparticles

A temperature dependent coherent magnetization reversal model is proposed for size-distributed assemblies of ferromagnetic nanoparticles and ferromagnetic-antiferromagnetic (AF) core-shell nanoparticles. The nanoparticles are assumed to be of uniaxial anisotropy and all aligned along their easy axis. The thermal dependence is included by considering thermal fluctuations, implemented via the Neel-Arrhenius theory. Thermal and angular dependence of magnetization reversal loops, coercive field, and exchange-bias field are obtained, showing that ferromagnetic-antiferromagnetic size-distributed exchange-coupled nanoparticles exhibit temperature-dependent asymmetric magnetization reversal. Also, non-monotonic evolutions of exchange-bias and coercive fields with temperature are demonstrated. The angular dependence of coercive field with temperature exhibits a complex behavior, with the presence of an apex, whose position and amplitude are strongly temperature-dependent. The angular dependence of exchange bias wi...

Temperature dependence of the exchange bias properties in polycrystalline BiFeO 3 /Ni 80 Fe 20

In this study, we report a temperature dependent analysis of the exchange bias properties in BFO/Py bilayers as a function of the BFO thickness. The temperature dependence of the exchange bias field (H_e) and the coercive field (H_c) are first presented. To understand exchange bias magnetization reversal and the magnetic anisotropies, we will then present an azimuthal study of H_e and H_c at 300 K and 77 K. Finally, results of a controlled field cooling protocol applied on all samples will be discussed to understand anisotropy energy distribution. The bilayers were grown by radio-frequency sputter deposition, with the following structure: Si/Pt(14 nm)/BiFeO₃(t_BFO)/Ni₈₀Fe₂₀(10 nm)/ Pt(10 nm), with t_BFO among 0 nm, 29 nm and 177 nm . To induce uniaxial anisotropy, a 300 Oe field H_dep was applied during the growth . The XRD analysis confirmed a single polycrystalline structure for the BFO layer.

Putative quantum criticality in the (Cr90Ir10)100−yVy alloy system

Quantum criticality (QC) in spin-density-wave antiferromagnetic Cr and Cr alloy systems is a topic of current interest. In the present study, V was used as a tuning parameter to drive the Neel transition temperature (TN) of the (Cr90Ir10)100−yVy alloy series with 0 ≤ y ≤ 14.3 to zero and search for effects of QC in the process. The magnetic properties and possible QC behaviour (QCB) in this alloy system were investigated through electrical resistivity (ρ), specific heat (Cp), and susceptibility (χ) measurements as a function of temperature (T), indicating that TN is suppressed to zero at a critical concentration yc ≈ 9. The Sommerfeld coefficient (γ) is considered a key indicator of QCB and a peak is observed in γ(y) at yc on decreasing y through this concentration, followed by a sharp decreasing trend. This behaviour is reminiscent of that observed for γ of the prototypical Cr100−xVx QC system and allows for the classification of yc in the (Cr90Ir10)100−yVy alloy system as a possible QC point.

Quantum critical behaviour in the (Cr97.8Si2.2)100−yMoy alloy system

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 (TN) to zero. Previous investigations on the ternary (Cr97.8Si2.2)100−yMoy alloy system (0 ≤ y ≤ 9) indicated possible quantum critical behaviour at a critical concentration yc ≈ 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 (RH) and magnetic susceptibility (χ) measurements. Recent results indicate the possible existence of two critical points in the (Cr97.8Si2.2)100−yMoy alloy system. The first critical point at yc1 is postulated to be related to the concentration where the first-order transition gives ...