Presentation Information
[PC5-03-INV]Vortex dynamics, magnetic irreversibility, and the critical state in ferromagnetic superconductor EuFe2(As1-xPx)2
*Joseph Alec Wilcox1, William Robert Fern1, Lukas Schneider2, Estefani Marchiori2, Vadim Plastovets3, Alexandre Buzdin3, Pardis Sahafi6, Andrew Jordan6, Raffi Budakian6, Tong Ren4, Ivan Veshchunov4, Tsuyoshi Tamegai4, Sven Friedemann5, Martino Poggio2, Simon John Bending1 (1. University of Bath (UK), 2. University of Basel (Switzerland), 3. University of Bordeaux (France), 4. The University of Tokyo (Japan), 5. University of Bristol (UK), 6. University of Waterloo (Canada))
Keywords:
ferromagnetic superconductor,critical current,vortex pinning,iron-based superconductor,critical state
EuFe2(As1−xPx)2 is remarkable amongst iron-based superconductors in that it exhibits coexisting ferromagnetism over a wide temperature range ΔT ≈ 19 K. Superconductivity appears as a dome in the phase diagram for P-composition in the range of 0.15 < x < 0.3, and with a maximum Tc ≈ 25 K.
Magnetic force microscopy (MFM) measurements established that the ferromagnetism manifests as two distinct, uniaxial stripe domain structures. The first, the domain Meissner state (DMS), appears in a small window below TFM and exhibits a narrow domain width renormalised by the effect of Meissner screening currents. The second, the domain vortex state (DVS), appears at lower temperatures and is characterised by a wider domain width, and the spontaneous nucleation of vortices and antivortices that saturate the domains.
Our recent investigation of bulk magnetic irreversibility and vortex creep behaviour found both to be strongly influenced by the onset of ferromagnetism. The narrow domains of the DMS enhance vortex pinning due to the formation of vortex polarons, whereby the magnetic field associated with a vortex causes a localised polarisation of the ferromagnetic domains, reducing the total energy compared to a free vortex. This effect also leads to a short-range attractive vortex-vortex potential, enabling the unusual stabilisation of pairs and chains of vortices. Follow-up bulk magnetic measurements within the DVS have now revealed a field-dependence of the critical current that is strongly correlated with the magnetic saturation of the ferromagnetic domains. Here the effect of vortex polaron formation is greatly weakened, and we suggest this low-field enhancement of Jc is instead due to an activated vortex- antivortex annihilation process over a Bean-Livingston barrier near the ferromagnetic domain walls.
Utilising an array of microscopic Hall sensors, we have performed a systematic, spatially-resolved study of the local magnetic induction in a sample with optimal P-composition x ∼ 0.2 such that Tc > TFM. In the superconducting state and above TFM we find the critical state to be well described by the Brandt-Indenbom model for weak-pinning type II superconductors. On entering the DMS, the flux front no longer develops smoothly with field, instead exhibiting chaotic and erratic changes in magnetic flux ΔΦ > Φ0 that we associate with the motion of multi-quanta flux bundles and chains. At lower temperatures, within the DVS, the response becomes smooth once more but is no longer Brandt-like. Instead, we find evidence for the formation of a dome of flux that concentrates at the sample centre, due to strong Meissner edge currents.
Magnetic force microscopy (MFM) measurements established that the ferromagnetism manifests as two distinct, uniaxial stripe domain structures. The first, the domain Meissner state (DMS), appears in a small window below TFM and exhibits a narrow domain width renormalised by the effect of Meissner screening currents. The second, the domain vortex state (DVS), appears at lower temperatures and is characterised by a wider domain width, and the spontaneous nucleation of vortices and antivortices that saturate the domains.
Our recent investigation of bulk magnetic irreversibility and vortex creep behaviour found both to be strongly influenced by the onset of ferromagnetism. The narrow domains of the DMS enhance vortex pinning due to the formation of vortex polarons, whereby the magnetic field associated with a vortex causes a localised polarisation of the ferromagnetic domains, reducing the total energy compared to a free vortex. This effect also leads to a short-range attractive vortex-vortex potential, enabling the unusual stabilisation of pairs and chains of vortices. Follow-up bulk magnetic measurements within the DVS have now revealed a field-dependence of the critical current that is strongly correlated with the magnetic saturation of the ferromagnetic domains. Here the effect of vortex polaron formation is greatly weakened, and we suggest this low-field enhancement of Jc is instead due to an activated vortex- antivortex annihilation process over a Bean-Livingston barrier near the ferromagnetic domain walls.
Utilising an array of microscopic Hall sensors, we have performed a systematic, spatially-resolved study of the local magnetic induction in a sample with optimal P-composition x ∼ 0.2 such that Tc > TFM. In the superconducting state and above TFM we find the critical state to be well described by the Brandt-Indenbom model for weak-pinning type II superconductors. On entering the DMS, the flux front no longer develops smoothly with field, instead exhibiting chaotic and erratic changes in magnetic flux ΔΦ > Φ0 that we associate with the motion of multi-quanta flux bundles and chains. At lower temperatures, within the DVS, the response becomes smooth once more but is no longer Brandt-like. Instead, we find evidence for the formation of a dome of flux that concentrates at the sample centre, due to strong Meissner edge currents.