Training effect in specular spin valves

Abstract

Specular spin valves show an enhanced giant magnetoresistive ͑GMR͒ ratio due to specular reflection in nano-oxide layers ͑NOLs͒ formed by the partial oxidation of CoFe pinned and free layers. The oxides that form the ͑pinned layer͒ NOL were recently shown to antiferromagnetically order at T ϳ 175 K. Here, we study the training effect ͑TE͒ in MnIr/CoFe/NOL/CoFe/Cu/CoFe/NOL/Ta specular spin valves in the 300-15 K temperature range. The exchange bias direction between the MnIr and CoFe layers impressed during annealing is taken as the positive direction. The training effect is observed in antiferromagnetic ͑AFM͒/ferromagnetic ͑FM͒ exchange systems and related to the rearrangement of interfacial AFM spin structure with the number of hysteretic cycles performed ͑n͒, resulting in the decrease of the exchange field ͑H exch ͒. Here, in the studied specular spin valve, TE was only observed for T Ͻ 175 K and is thus related to the pinned layer NOL-AFM ordering and to the evolution of the corresponding spin structure with n. We show that FM spins that are strongly coupled to AFM domains do not align with the applied positive magnetic field ͑H͒, giving rise to a residual MR at H 0. Such nonsaturating MR will be related with a spin-glass-like behavior of the interfacial magnetism induced by the nano-oxide layer. The observed dependence of the training effect on the field cooling procedure is also likely associated with the existence of different spin configurations available in the magnetically disordered oxide. Furthermore, anomalous magnetoresistance cycles measured after cooling runs under −500 Oe are here related to induced NOL exchange bias/applied magnetic field misalignment. The temperature dependence of the training effect was obtained and fitted by using a recent theoretical model.

Exchange bias effect in alloys and compounds

“…The values of H E or M E obtained from the least square fit with λ satisfactorily interpret the experimental data using equation (2) for different cases such as layered structures [102,103,104,105,106,107,108,109,110,111], nanocomposites [112], bulk Heusler alloys [113,114] and oxide compounds [95,98,115,116,117,118,119]. Equation (2) can also satisfactorily interpret the shift of MR curves, exhibiting EB effect [99,120,121]. As seen in figure 2 and right inset of figure 3, the values (filled symbols) as obtained from the fit using equation (2) can satisfactorily match with all the experimental data.…”

Exchange bias effect and intragranular magnetoresistance in Nd0.84Sr0.16CoO3

“…TE is typically observed in the magnetic hysteresis loop [7] which is recently reported in the MR-H curve of only two multilayer films, exhibiting EB effect [26,27]. In figure 5 typical signature of TE is illustrated at 5 K up to 5 successive cycles (λ).…”

Temperature dependence of the training effect in electrodeposited Co/CoO nanotubes

“…Therefore, the gradual decrease of jH ex j with n would reflect the rearrangement of the spin structure of the AFM layer after each reversal of the FM layer magnetization. 2,5,6,40 Based on free energy considerations, C. Binek obtained the following recursive formula to describe TE: 6…”

“…1,2 This effect is seen as a shift of the FM hysteresis loop by an EB field (H ex ) measured from the origin when field cooling the sample from above the Neel temperature (T N ) of the AFM material. [1][2][3][4][5] In addition, when cycling the FM/AFM bilayer structure through consecutive hysteresis loops, a H ex decrease is often observed. This phenomenon, called training effect (TE), can be explained as arising from the partial loss of the AFM net magnetization, as its spin structure rearranges with each magnetization reversal of the FM layer.…”

“…This phenomenon, called training effect (TE), can be explained as arising from the partial loss of the AFM net magnetization, as its spin structure rearranges with each magnetization reversal of the FM layer. 2,[5][6][7][8] The TE plays a crucial role in the reliable performance of devices based on EB. One should also note that both the EB and its training effect strongly depend on temperature (T), disappearing above the so-called blocking temperature (T B ) of the AFM material.…”