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 Fe_1-xZnₓF₂(110)/Fe14nm/Ag35nm, $x=0.4.$ Its blocking temperature exhibits a significant enhancement with respect to the global ordering temperature TN=46.9K, 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 $0<~x<~1.$ 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.