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6CCVD Research

The Paramagnetic Meissner Effect (PME) in Metallic Superconductors

At a Glance

Section titled “At a Glance”
MetadataDetails
Publication Date2023-06-19
JournalMetals
AuthorsM.R. Koblischka, L. PƯst, Crosby-Soon Chang, Thomas Hauet, Anjela Koblischka‐Veneva
InstitutionsExtreme Light Infrastructure Beamlines, Centre National de la Recherche Scientifique
Citations5
AnalysisFull AI Review Included

Executive Summary

Section titled “Executive Summary”
  • PME Confirmation in S-Wave Metals: The Paramagnetic Meissner Effect (PME) is confirmed across a wide range of conventional s-wave metallic superconductors, including bulk Nb, Al mesoscopic disks, Pb, Ta, and MgB2, in various forms (bulk, films, nanowires, multilayers).
  • Extrinsic Origin Dominant: The PME in bulk metallic samples is primarily attributed to an extrinsic mechanism involving flux trapping, flux compression, and the stabilization of a Giant Vortex State at the sample surface, linked to surface superconductivity (Hc3).
  • Characteristic Temperatures: PME exhibits clear thermal fingerprints in m(T) curves, defined by the magnetization minimum (T1) and the maximum positive moment (Tp), which are crucial for constructing the superconducting phase diagram.
  • Surface Control: PME is highly sensitive to surface conditions: it can be completely eliminated by mechanical abrasion of the Nb disk surface and can be induced or enhanced by targeted Kr ion implantation (creating surface defects).
  • Intrinsic PME Proof: Low-Energy Muon Spin Spectroscopy (LE-”SR) provided direct experimental evidence for intrinsic PME (odd s-wave superconductivity) in Au/Ho/Nb trilayer hybrid systems, confirming theoretical predictions for broken time-reversal symmetry systems.
  • Measurement Techniques: Observation requires highly sensitive SQUID magnetometry, often utilizing a stationary sample technique and ultra-low magnetic fields (less than 1 mT) to avoid artifacts from trapped flux in the magnet system.

Technical Specifications

Section titled “Technical Specifications”
ParameterValueUnitContext
Niobium (Nb) Tc,onset9.20 to 9.28KBulk disks (99.98% purity, cold-rolled sheet).
Nb Disk Dimensions6.4 mm dia, 127 ”mmm, ”mStandard sample geometry for PME studies.
Nb Ginzburg-Landau Parameter (Îș)~0.8-1.0DimensionlessClose to the Type-I/Type-II boundary.
MgB2 Tc (Bulk)38.2KPolycrystalline bulk sample, highest Tc metallic SC.
Al Mesoscopic Disk Diameter1.0 to 2.5”mUsed to observe size quantization effects (Giant Vortex State).
Nb PME Field Range (Positive m)1 to 3mTMaximum applied field for observing positive magnetization (FC-C).
Nb PME Characteristic Slope (dTp/dBe)13.5K/TTemperature dependence of the paramagnetic peak (less than 60 mT).
Nb PME Shift Magnitude (mshift)1.33 x 10-9Am2Additional paramagnetic moment at 0.05 mT (Figure 8a).
Kr Ion Implantation Energy200keVUsed to induce PME in previously non-PME Nb disks.
Kr Ion Penetration Depth40 to 120nmEffective depth of defect creation influencing PME.
AC Susceptibility Frequency1HzFrequency required to observe PME signatures in Nb disks (vanishes at 10 Hz).
LE-”SR Trilayer Tc8.52KAu (27.5 nm)/Ho (4.5 nm)/Nb (150 nm) hybrid structure.

Key Methodologies

Section titled “Key Methodologies”
  1. Stationary SQUID Magnetometry: Measurements were primarily conducted using commercial SQUID magnetometers (Quantum Design MPMS) in a stationary sample mode. This technique, utilizing a low-field copper coil (H less than 1 mT), was crucial to eliminate spurious signals caused by trapped flux and field inhomogeneities inherent to moving sample measurements.
  2. Field-Cool (FC) Protocols: PME was characterized by measuring magnetization versus temperature (m(T)) using two modes:
    • FC-Cooling (FC-C): Cooling the sample from the normal state (T > Tc) down to low temperatures in a constant applied field.
    • FC-Warming (FC-W): Reversing the temperature sweep direction after reaching the minimum temperature.
  3. Magnetic Hysteresis Loop (MHL) Analysis: Detailed MHLs (m(H)) were measured in the critical temperature range (Tp less than T less than Tc) using the stationary SQUID setup to analyze the transition from conventional vortex pinning (Component C) to the PME-related magnetic moment (Component P).
  4. Surface Engineering: The influence of surface defects was tested by:
    • Abrasion: Mechanically abrading the surface of Nb disks, which resulted in the complete disappearance of the PME.
    • Ion Implantation: Bombarding non-PME Nb disks with 200 keV Kr ions (dose 6 x 1016/cm2) to induce surface defects and successfully create the PME.
  5. AC Susceptibility: AC measurements (real part χ’ and imaginary part χ”) were used to probe the dynamics of the PME. PME signatures were only observed at very low frequencies (1 Hz) and small DC fields (1 mT), indicating a slow, non-equilibrium process.
  6. Magnetic Imaging and Depth Profiling: Advanced spatial techniques were used to visualize flux structures:
    • Scanning SQUID Microscopy: Used to image vortex distributions in Bi-2212 and Nb disks at 4.2 K.
    • LE-”SR: Used on Au/Ho/Nb trilayers to obtain depth-resolved profiles of the local magnetic field (Bloc(z)), providing evidence for intrinsic PME in the Au layer via proximity effect.

Commercial Applications

Section titled “Commercial Applications”

The research on the Paramagnetic Meissner Effect (PME) in metallic superconductors has direct implications for several high-technology engineering fields:

  • Superconducting Spintronics: The demonstration of intrinsic PME in superconductor/ferromagnet hybrid systems (like Au/Ho/Nb trilayers) is foundational for designing superconducting spintronic devices. This includes spin valves, memory elements, and logic circuits that leverage the unique magnetic properties of odd s-wave superconductivity.
  • Quantum Computing and Sensing: The detailed study of the Giant Vortex State and flux quantization in mesoscopic Al and Nb disks is critical for developing stable and high-coherence superconducting qubits and resonators. Controlling vortex dynamics is essential for minimizing noise in quantum circuits.
  • Superconducting RF (SRF) Cavities: Niobium is the primary material for SRF cavities in particle accelerators. Understanding how surface defects and treatments (like ion implantation or abrasion) influence PME and Tc is vital for optimizing the surface impedance and maximizing the quality factor (Q) of these high-power components.
  • High-Field Magnet Stabilization: The extrinsic PME mechanism (flux compression/trapping) directly relates to magnetic stability in Type-II superconductors. This knowledge is crucial for engineering MgB2 tapes and Nb-based wires used in MRI machines, fusion reactors, and high-energy physics magnets, ensuring reliable operation and minimizing flux jump instabilities.
  • Advanced Materials Characterization: The methodologies developed (stationary SQUID, LE-”SR depth profiling) provide highly sensitive, non-invasive tools for quality control and characterization of thin-film and multilayer superconducting materials used in microelectronics and sensor manufacturing.
View Original Abstract

The experimental data in the literature concerning the Paramagnetic Meissner Effect (PME) or also called Wohlleben effect are reviewed with the emphasis on the PME exhibited by metallic, s-wave superconductors. The PME was observed in field-cool cooling (FC-C) and field-cool warming (FC-W) m(T)-measurements on Al, Nb, Pb, Ta, in compounds such as, e.g., NbSe2, In-Sn, ZrB12, and others, and also in MgB2, the metallic superconductor with the highest transition temperature. Furthermore, samples with different shapes such as crystals, polycrystals, thin films, bi- and multilayers, nanocomposites, nanowires, mesoscopic objects, and porous materials exhibited the PME. The characteristic features of the PME, found mainly in Nb disks, such as the characteristic temperatures T1 and Tp and the apparative details of the various magnetic measurement techniques applied to observe the PME, are discussed. We also show that PME can be observed with the magnetic field applied parallel and perpendicular to the sample surface, that PME can be removed by abrading the sample surface, and that PME can be introduced or enhanced by irradiation processes. The PME can be observed as well in magnetization loops (MHLs, m(H)) in a narrow temperature window Tp<Tc, which enables the construction of a phase diagram for a superconducting sample exhibiting the PME. We found that the Nb disks still exhibit the PME after more than 20 years, and we present the efforts of magnetic imaging techniques (scanning SQUID microscopy, magneto-optics, diamond nitrogen-vacancy (NV)-center magnetometry, and low-energy muon spin spectroscopy, (LE-ÎŒSR)). Various attempts to explain PME behavior are discussed in detail. In particular, magnetic measurements of mesoscopic Al disks brought out important details employing the models of a giant vortex state and flux compression. Thus, we consider these approaches and demagnetization effects as the base to understand the formation of the paramagnetic signals in most of the materials investigated. New developments and novel directions for further experimental and theoretical analysis are also outlined.

Tech Support

Section titled “Tech Support”

Original Source

Section titled “Original Source”

References

Section titled “References”
  1. 1933 - Ein neuer Effekt bei Eintritt der SupraleitfÀhigkeit [Crossref]

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