Observation of magnetically hard grain boundaries in double-perovskite Sr$_{2}$FeMoO$_{6}$

TL;DR

This study reveals that grain boundaries in Sr$_{2}$FeMoO$_{6}$ are magnetically harder than grains at low temperatures, with distinct magnetic behaviors detected via advanced x-ray spectroscopy techniques, highlighting their role in magneto-resistance.

Contribution

The paper demonstrates the magnetic hardness difference between grain boundaries and grains in Sr$_{2}$FeMoO$_{6}$ using XAS and XMCD, providing insight into their magnetic properties at different temperatures.

Findings

Grain boundaries exhibit higher coercivity than grains at 20 K.

Magnetism at grain boundaries becomes soft at room temperature.

Fe$^{2+}$ dominates the ferromagnetic component in grain boundaries.

Abstract

Unusual low temperature magneto-resistance (MR) of ferromagnetic Sr$_{2}$FeMoO$_{6}$ polycrystals has been attributed to magnetically hard grain boundaries which act as spin valves. We detected the different magnetic hysteresis curves for the grains and the grain boundaries of polycrystalline Sr$_{2}$FeMoO$_{6}$ by utilizing the different probing depths of the different detection modes of x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD), namely, the total electron yield (TEY) mode (probing depth $\sim$5 nm) and the total fluorescence yield (TFY) mode (probing depth $\sim$100 nm). At 20 K, the magnetic coercivity detected in the TEY mode ($H_{\rm c,TEY}$) was several times larger than that in the TFY mode ($H_{\rm c,TFY}$), indicating harder ferromagnetism of the grain boundaries than that of the grains. At room temperature, the grain boundary magnetism became soft and $H_{\rm c,TEY}$ and $H_{\rm c,TFY}$ were nearly the same. From line-shape analysis of the XAS and XMCD spectra, we found that in the grain boundary region the ferromagnetic component is dominated by Fe$^{2+}$ or well-screened signals while the non-magnetic component is dominated by Fe$^{3+}$ or poorly-screened signals.