Observation of magnetically hard grain boundaries in double-perovskite Sr₂FeMoO₆

Unusual low-temperature magneto-resistance (MR) of ferromagnetic Sr₂FeMoO₆ 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 Sr₂FeMoO₆ 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 ∼ 5 nm) and the total fluorescence yield (TFY) mode (probing depth ∼ 100 nm). At 20 K, the magnetic coercivity detected in the TEY mode (H_c,TEY) was several times larger than that in the TFY mode(H_c,TFY), indicating harder ferromagnetism of the grain boundaries than that of the grains. At room temperature, the grain boundary magnetism became soft H_c,TEYand H_c,TFYand were nearly the same. From the line-shape analysis of the XAS and XMCD spectra, we found that in the grain boundary region the ferromagnetic component is dominated by Fe²⁺ or well-screened signals, while the non-magnetic component is dominated by Fe³⁺ or poorly screened signals.