Dilution-induced enhancement of the blocking temperature in exchange-bias heterosystems

The temperature dependence of the exchange bias field is investigated by superconducting quantum interference device magnetometry in Fe1-xZnxF2(110)/Fe14 nm/Ag35 nm, x = 0.4. Its blocking temperature exhibits a significant enhancement with respect to the global ordering temperature T-N = 46.9 K, of the bulk antiferromagnet Fe0.6Zn0.4F2. The enhancement is attributed to fluctuations of the diamagnetic dilution which creates clusters on all length scales having a Zn dilution of 0less than or equal toxless than or equal to1. While the infinite clusters give rise to the well-known Griffiths phase, finite clusters also provoke a local enhancement of the exchange bias. The temperature dependence of the integral exchange bias effect is modeled by averaging all local contributions of the antiferromagnetic surface magnetization which exhibit a surface critical behavior.