Loop bifurcation and magnetization rotation in exchange-biasedNi∕FeF2

Abstract

Exchange biased Ni/ FeF 2 films have been investigated using vector coil vibrating sample magnetometry as a function of the cooling field strength H FC . In films with epitaxial FeF 2 , a loop bifurcation develops with increasing H FC as it divides into two subloops shifted oppositely from zero field by the same amount. The positively biased subloop grows in size with H FC until only a single positively shifted loop is found.Throughout this process, the negative/positive (sub)loop shift has maintained the same discrete value. This is in sharp contrast to films with twinned FeF 2 where the exchange field gradually changes with increasing H FC . The transverse magnetization shows clear correlations with the longitudinal sub-loops. Interestingly, over 85% of the Ni reverses its magnetization by rotation, either in one step or through two successive rotations. These results are due to the single crystal nature of the antiferromagnetic FeF 2 , which breaks down into two opposite regions of large domains.

Magnetization reversal of uncompensated Fe moments in exchange biased Ni∕FeF2 bilayers

“…This loop bifurcation is due to a coexistence of domains with uncompensated Fe moments pinned parallel and antiparallel to the cooling field. 10 From the occurrence of loop bifurcation and the fact that M positive bias ͑H͒ =−M negative bias ͑−H͒ for the Ni loops, we conclude that the same amount of moments pinned parallel to the cooling field for H cf = 0.55 T is pinned antiparallel to the cooling field in the case of H cf = 0.02 T. This leads us to a nominal coverage about 0.01 AL of pinned uncompensated moments in the Ni/ FeF 2 bilayer sample assuming an escape depth of 1.7 nm. 12 We employed XMLD to characterize the impact of external magnetic fields and the magnetization reversal in the Ni layer on the spin structure of the antiferromagnetically ordered FeF 2 in an experiment completely analog to that performed by Scholl et al on a NiO single-crystal exchange coupled to a thin Co layer.…”

“…10 The bilayer was capped withbiased by saturating the Ni magnetization at T = 150 K in external fields of 0.55 T applied along the FeF 2 easy axis, that is the ͓001͔ direction, and then cooling the sample in an external field ͑H cf ͒ below the FeF 2 Néel temperature of 78 K. All spectra and hysteresis loops were obtained with the eight pole magnet at ALS beamline 4.0.2 ͑see Ref. 11͒ at sample temperatures of 55 K to eliminate the influence of sample charging effects on the electron yield measurements.…”

Asymmetric Reversal in Inhomogeneous Magnetic Heterostructures

“…Vector vibrating sample magnetometry (VSM) of FeF 2 50 nm=Ni21 nm= Al7:6 nm gives simultaneously the in-plane longitudinal (parallel to the magnetic field) and transverse (perpendicular to the magnetic field) magnetic moments [18,19]. The magnetic field is applied along the FeF 2 easy axis [001] with a small misalignment that defines the sign of the transverse component during reversal [12,18].…”

Irreversibility of magnetization rotation in exchange biased Fe/epitaxial-FeF2 thin films

“…The sample was field cooled from 300 to 15 K in a 1 kOe field, a field sufficient to saturate the sample but still small enough not to induce a two-step reversal previously found in other exchange biased samples incorporating epitaxial FeF 2 . 18,19 With the cooling field applied in the sample plane along the AF spin axis direction, the sample became biased along this direction. 17 Care was taken to align the AF spin axis with the applied field by monitoring the transverse loop.…”

“…15,18,19 An epitaxial layer of 500 Å FeF 2 was first evaporated onto a single crystal ͑110͒ MgF 2 substrate held at 300°C. A layer of 300 Å Fe and a capping layer of 80 Å Al were subsequently grown at 150°C.…”