Spontaneous ferromagnetic spin ordering at the surface of La$_2$CuO$_4$

TL;DR

This study reveals that high purity La$_2$CuO$_4$ nanoparticles exhibit spontaneous ferromagnetic spin clusters at their surfaces, with magnetic properties varying with particle size and temperature, indicating surface-driven magnetic phenomena.

Contribution

The paper demonstrates the formation of spontaneous ferromagnetic surface clusters in La$_2$CuO$_4$ nanoparticles and explores their temperature dependence and potential origins, advancing understanding of surface magnetism in cuprates.

Findings

Ferromagnetic spin clusters form at nanoparticle surfaces.

Hysteresis and irreversibility observed from 5 to 350 K.

Surface ferromagnetism decreases with increasing temperature.

Abstract

Magnetic properties of high purity stoichiometric La$_2$CuO$_4$ nanoparticles are systematically investigated as a function of particle size. Ferromagnetic single-domain spin clusters are shown to spontaneously form at the surface of fine grains as well as paramagnetic defects. Hysteresis loops and thermomagnetic irreversibility are observed in a wide temperature range $5 - 350$ K with the remnant moment and coercivity gradually decreasing with increasing temperature. Possible origins of the spontaneous surface ferromagnetic clusters and the relation of our data to the appearance of unusual magnetic phenomena and phase separation of doped cuprates are discussed.