Selective Metal Exsolution in BaFe sub(2-y)M sub( y)(PO sub(4)) sub(2) (M = Co super(2+), Ni super(2+)) Solid Solutions

The 2D-lsing ferromagnetic phase BaFe super(2+)2 sub(()PO sub(4)) sub(2) shows exsolution of up to one-third of its iron content (giving BaFe super(3+) sub(1.33()PO sub(4)) sub(2)) under mild oxidation conditions, leading to nanosized Fe sub(2)O sub(3) exsolved clusters. Here we have prepared BaFe sub(2-y)M sub(y)(PO sub(4)) sub(2) (M = Co super(2+), Ni super(2+); y = 0, 0.5, 1, 1.5) solid solutions to investigate the feasibility and selectivity of metal exsolution in these mixed metallic systems. For all the compounds, after 600 [degrees]C thermal treatment in air, a complete oxidation of Fe super(2+) to Fe super(3+) leaves stable M super(2+) ions, as verified by super(57)Fe Mossbauer spectroscopy, TGA, TEM, microprobe, and XANES. The size of the nanometric alpha -Fe sub(2)O sub(3) clusters coating the main phase strongly depends on the y sub(M) metal concentration. For M-rich phases the iron diffusion is hampered so that a significant fraction of superparamagnetic alpha -Fe sub(2)O sub(3) particles (100% for BaFe sub(0.5-x)Co sub(1.5)(PO sub(4)) sub(2)) was detected even at 78 K. Although Ni super(2+) and Co super(2+) ions tend to block Fe diffusion, the crystal structure of BaFe sub(0.67)Co sub(1)(PO sub(4)) sub(2) demonstrates a fully ordered rearrangement of Fe super(3+) and Co super(2+) ions after Fe exsolution. The magnetic behaviors of the Fe-depleted materials are mostly dominated by antiferromagnetic exchange, while Co super(2+)-rich compounds show metamagnetic transitions reminiscent of the BaCo sub(2)(PO sub(4)) sub( 2) soft helicoidal magnet.