High magnetic field scales and critical currents in SmFeAs(O,F) crystals: promising for applications

Superconducting technology provides most sensitive field detectors, promising implementations of qubits and high field magnets for medical imaging and for most powerful particle accelerators. Thus, with the discovery of new superconducting materials, such as the iron pnictides, exploring their potential for applications is one of the foremost tasks. Even if the critical temperature Tc is high, intrinsic electronic properties might render applications rather difficult, particularly if extreme electronic anisotropy prevents effective pinning of vortices and thus severely limits the critical current density, a problem well known for cuprates. While many questions concerning microscopic electronic properties of the iron pnictides have been successfully addressed and estimates point to a very high upper critical field, their application potential is less clarified. Thus we focus here on the critical currents, their anisotropy and the onset of electrical dissipation in high magnetic fields up to 65 T. Our detailed study of the transport properties of optimally doped SmFeAs(O,F) single crystals reveals a promising combination of high (>2 x 10^6 A/cm^2) and nearly isotropic critical current densities along all crystal directions. This favorable intragrain current transport in SmFeAs(O,F), which shows the highest Tc of 54 K at ambient pressure, is a crucial requirement for possible applications. Essential in these experiments are 4-probe measurements on Focused Ion Beam (FIB) cut single crystals with sub-\mu\m^2 cross-section, with current along and perpendicular to the crystallographic c-axis and very good signal-to-noise ratio (SNR) in pulsed magnetic fields. The pinning forces have been characterized by scaling the magnetically measured "peak effect"

Doping-Dependent Magnetism and Exchange Bias in CaMn1-xRexO3

International audienceMagnetic and structural properties of CaMn1-xRexO3 (0.02 <= x <= 0.1) have been investigated. Substitution of Re5+ ion for the Mn4+ site of CaMnO3 generates Mn3+ ions according to the chemical formula CaMn1-2x4+ Mnx3+Rex5+O3, accompanied by an increase of lattice parameters and unit-cell volume with increasing x. With increasing doping level x, the magnetic ground state evolves from an antiferromagnetic (AFM) with a weak ferromagnetic (FM) component, for x = 0.02 - 0.06, to the charge ordered C-type AFM state at x = 0.1. Spontaneous magnetization at T = 10 K increases quickly with increasing x, approaches the maximum value of 3.5 emu/g for x = 0.04, and then decreases rapidly to 0.2 emu/g for x = 0.1. Anomalous negative magnetization (NM) for x = 0.02 has been observed in the zero-field-cooled and field-cooled (FC) magnetization below the magnetic transition temperature. Exchange bias (EB) effect, manifested by horizontal shift in the hysteresis loops of FC samples, has also been observed. This effect is very small for x = 0.02, almost zeroes for 0.04, and monotonously increases with increasing x. The EB appears due to low-temperature phase separation into FM clusters and charge-ordered AFM phases. The effect of hydrostatic pressure for all samples revealed a significant increase of the FM phase volume under pressure, linked to both suppression of NM in x = 0.02 sample and reduction of the EB effect in all samples

Doping-Dependent Magnetism and Exchange Bias in CaMn1-xRexO3

Magnetic and structural properties of CaMn1-xRexO3 (0.02 <= x <= 0.1) have been investigated. Substitution of Re5+ ion for the Mn4+ site of CaMnO3 generates Mn3+ ions according to the chemical formula CaMn1-2x4+ Mnx3+Rex5+O3, accompanied by an increase of lattice parameters and unit-cell volume with increasing x. With increasing doping level x, the magnetic ground state evolves from an antiferromagnetic (AFM) with a weak ferromagnetic (FM) component, for x = 0.02 - 0.06, to the charge ordered C-type AFM state at x = 0.1. Spontaneous magnetization at T = 10 K increases quickly with increasing x, approaches the maximum value of 3.5 emu/g for x = 0.04, and then decreases rapidly to 0.2 emu/g for x = 0.1. Anomalous negative magnetization (NM) for x = 0.02 has been observed in the zero-field-cooled and field-cooled (FC) magnetization below the magnetic transition temperature. Exchange bias (EB) effect, manifested by horizontal shift in the hysteresis loops of FC samples, has also been observed. This effect is very small for x = 0.02, almost zeroes for 0.04, and monotonously increases with increasing x. The EB appears due to low-temperature phase separation into FM clusters and charge-ordered AFM phases. The effect of hydrostatic pressure for all samples revealed a significant increase of the FM phase volume under pressure, linked to both suppression of NM in x = 0.02 sample and reduction of the EB effect in all samples