Oleg Petracic

Lateral length scales in exchange bias

When a ferromagnet is in proximity to an antiferromagnet, lateral length scales such as the respective magnetic domain sizes drastically affect the exchange bias. Bilayers of FeF2 and either Ni, Co or Fe are studied using SQUID and spatially resolved MOKE. When the antiferromagnetic domains are larger than or comparable to the ferromagnetic domains, a local, non-averaging exchange bias is observed. This gives rise to unusual and tunable magnetic hysteresis curves.

Dynamic phase transitions in ferroic systems with pinned domain walls

The creep and relaxation of domain walls under an ac electric field that are observed in an ideal model system, periodically poled superionic KTiOPO4 (KTP), appear to occur in different regimes that are separated by a dynamic phase transition at a well-defined frequency, f m = 0.003 Hz, at temperature T = 233 K. The power-law dispersion of the creep susceptibility, χ ∝ 1 + (iωτ)− β (with β ≈ 0.4), and the large nonlinearity encountered for f<f m is contrasted with the Cole–Cole-type relaxational dispersion, χ ∝ [1 + (iωτ)1− α −1 (with α ≈ 0.3), for f > f m. Similar creep-to-relaxation transitions are observed at low frequencies in other ferroic systems with weak disorder: the multidomained uniaxial relaxor Sr0.69Ba0.31Nb2O6 (SBN), the quantum ferroelectric domain state of SrTi18O3 (STO18) and the superferromagnetic nanoparticle system [(Co80Fe20(1.4 nm)/Al2O3(3 nm)]10, which appear to belong to the same dynamical universality class.

Asymmetric Reversal in Inhomogeneous Magnetic Heterostructures

Asymmetric magnetization reversal is an unusual phenomenon in antiferromagnet/ferromagnet (AF/FM) exchange biased bilayers. We investigated this phenomenon in a simple model system experimentally and by simulation assuming inhomogeneously distributed interfacial AF moments. The results suggest that the observed asymmetry originates from the intrinsic broken symmetry of the system, which results in local incomplete domain walls parallel to the interface in reversal to negative saturation of the FM. The magneto-optical Kerr effect unambiguously confirms such an asymmetric reversal and a depth-dependent FM domain wall in accord with the magnetometry and simulations.

Magnetization depth dependence in exchange biased thin films

The depth dependence of the magnetization has been studied in antiferromagnet/ferromagnet (AF/FM) exchange coupled systems. Results from vector magnetometry and magneto-optical Kerr effect probing both the AF/FM and FM/air interfaces demonstrate the existence of a magnetization depth profile in FeF2∕FM (FM=Fe, Ni, and Py) bilayers, contrary to the assumptions of most exchange bias models. The appearance of asymmetrical hysteresis loops below the AF Neel temperature (TN) is explained by the creation of spring-like walls parallel to the AF/FM interface and the existence of incomplete domain walls. Changes in the reversal mechanism above TN have also been discussed.

Reversal behavior of exchange-biased submicron dots

Nanostructured Fe dots were prepared on antiferromagnetic FeF2 thin films and investigated by magneto-optical Kerr effect (MOKE). We studied the influence of dot sizes on the magnetic hysteresis and compared the result with both continuous thin film bilayers and nanostructured Fe∕FeF2 pillars. Hysteresis loops were measured at temperatures below and above (10 and 90K, respectively) the Neel temperature of the antiferromagnet. A vortex state is found for dots of 300nm diameter, where the exchange bias field is reduced compared to larger dot system and the continuous bilayer. Micromagnetic simulations including the interaction with the antiferromagnet show qualitatively similar behavior.

Loop bifurcation and magnetization rotation in exchange-biased Ni/FeF2

Exchange-biased Ni/FeF2 films have been investigated using vector coil vibrating-sample magnetometry as a function of the cooling field strength HFC. In films with epitaxial FeF2, a loop bifurcation develops with increasing HFC as it divides into two sub-loops shifted oppositely from zero field by the same amount. The positively biased sub-loop grows in size with HFC until only a single positively shifted loop is found. Throughout this process, the negative and positive subloop shifts maintain the same discrete value. This is in sharp contrast to films with twinned FeF2 where the exchange field gradually changes with increasing HFC. The transverse magnetization shows clear correlations with the longitudinal subloops. 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 FeF2, which breaks down into two opposite regions of large domains.

Magnetic Structure in Exchange-coupled Antiferromagnet-Ferromagnet Thin Films

In this work we investigated the magnetic structure of the AF (FeF2) and the FM (Fe, Ni, Permalloy) in thin films grown on MgF2. For this purpose we prepared two sets of samples. The in-depth magnetization reversal of the FM was studied by magneto-optical Kerr effect (MOKE). Hysteresis loops for FeF2/Py was measured. The difference between both loops is an unambiguous demonstration of the existence of domain structures parallel to the AF-FM interface. The reversibility of this structure has also been studied and extended to ferromagnetic materials with higher magnetocrystalline anisotropy (Ni and Fe).