Glassy magnetic freezing of interacting clusters in LK-99-family materials

Abstract

We report reproducible magnetization anomalies appearing below room temperature in copper-doped apatite materials belonging to the LK-99 family synthesized via hydrothermal methods. These anomalies are observed consistently across samples prepared under comparable conditions. Although the extracted Mydosh parameter lies within the range often associated with vortex-glass behavior in superconductors, a detailed analysis of DC magnetization, AC susceptibility, field dependence, and magnetic memory effects demonstrates that the observed phenomena are not related to superconductivity. Instead, the data are consistent with glassy magnetic freezing of interacting clusters. Compositional and structural analysis identifies covellite (CuS), an ubiquitous secondary phase in these intrinsically multiphase materials, as the primary origin of the observed behavior. Our results clarify the magnetic origin of LK-99-related anomalies and highlight the importance of phase complexity in interpreting apparent superconducting signatures in this materials family.

Figures (6)

• Figure 1: Temperature dependence of the magnetization measured in a copper-doped pyromorphite pellet (mass $\approx 98.5$ mg, diameter $\approx 4$ mm, thickness $\approx 1.5$ mm). Zero-field-cooled (ZFC) warming and subsequent field-cooled (FC) data are shown, illustrating an anomaly near room temperature. All other measured pellets have the same diameter.
• Figure 2: Temperature dependence of the magnetization showing bifurcation between zero-field-cooled (ZFC) and field-cooled (FC) curves for four representative samples synthesized under slightly different hydrothermal conditions. Panels (a)--(d) correspond to samples #a--#d listed in Table \ref{['tab:table1']}. Applied magnetic fields are indicated in each panel.
• Figure 3: Temperature dependence of the real ($\chi'$) and imaginary ($\chi"$) components of the AC magnetic susceptibility for the sample shown in Fig. \ref{['fig:fig2']}, measured at several excitation frequencies. The vertical line marks the temperature of the $\chi'$ maxima. Solid line is a guide to the eye.
• Figure 4: Temperature-dependent magnetization of the sample shown in Fig. \ref{['fig:fig2']}(d), measured under higher applied magnetic fields (Tesla scale). Zero-field-cooled warming and field-cooled data are shown for different field strengths, illustrating the evolution of the low-temperature anomaly with increasing field.
• Figure 5: Magnetization as a function of magnetic field measured during successive field sweeps between $\pm 5$ T, demonstrating magnetic memory and partial irreversibility. The absence of sharp hysteresis loops and the dependence on field history are characteristic of glassy magnetic behavior.