Tuning the exchange bias of soft metallic antiferromagnets by inserting nonmagnetic defects

Abstract

The effect of enhancing exchange bias (EB) by nonmagnetic defects in high-anisotropy insulating antiferromagnets (AFMs), such as CoO, is tested in low-anisotropy metallic AFMs, such as γ-Fe50Mn50. The temperature dependence of EB and coercivity of Ni80Fe20∕(Fe50Mn50)1−xCux ferromagnetic/antiferromagnetic (FM/AFM) bilayers, where x is the defect concentration in the AFM, is investigated. We show both by experiment and Monte Carlo simulations that the EB presents a maximum as a function of x. However, the increasing role of thermal relaxation of the AFM spin structure leads to a decrease of the FM/AFM blocking temperature with increasing x. The simulations indicate the occurrence of a fast thermal relaxation in the AFM at temperatures below the isolated spin blocking temperature. This result explains the experimental observation that with increasing x the temperature dependence of the FM/AFM coercivity changes from a monotonous decrease to a nonmonotonous dependence, exhibiting a maximum near the blocking temperature.

Exchange bias and training effect in Ni/Ag-doped NiO bilayers

“…6 The above-mentioned evolution of EB with x can be ascribed to the AF microstructure in the framework of the domain state (DS) model [7][8][9]17]. The presence of structural and compositional defects throughout the finite AF lattice leads to the formation of AF domains during the field cooling.…”

“…Recently, nonmagnetic substitutional elements have been used to tune the density of the uncompensated AF interfacial moments. For metallic polycrystalline AF materials, a maximum of EB field as a function of doped Cu concentration has been observed in CoFe/IrMn and NiFe/FeMn bilayers [7,8]. In expitaxial CoO/Co bilayers, EB field can be enhanced by two times when the AF CoO is diluted by Mg substitution [9].…”

Training and temperature effects of epitaxial and polycrystallineNi80Fe20∕Fe50Mn50ex

“…In contact with an FM layer, the irreversible DS magnetization gives rise to the EB effect. 8,10 Within this model the training effect is attributed to the rearrangement of the AFM domain structure, which results in a partial loss of the irreversible DS magnetization of the AFM layer during field cycling. 8,9,23 This has been observed as a vertical shift of the hysteresis loop of, e.g., Co/ CoO bilayers.…”

“…3͑a͔͒ main nucleation and wall propagation have been assigned in the vicinity of H 1 . 21 The DS model 8,10 for EB considers the presence of defects 22 throughout the finite AFM lattice, leading to the formation of AFM domains. Consequently, an irreversible DS magnetization develops in the AFM during the field cooling.…”

Exchange coupling between an amorphous ferromagnet and a crystalline antiferromagnet

“…The EB is a very complex phenomenon, associated with poorly understood properties at the FM/AFM interface. [6][7][8] Considerable theoretical efforts have been undertaken in order to understand the various aspects of the EB phenomena. [9][10][11][12][13][14] Although many GMR sensors comprise crystalline FM sense ͑free͒ layers, such as NiFe, CoFe, or Co, there are special applications which require very soft FM layers.…”