Large anomalous enhancement of perpendicular exchange bias by introduction of a nonmagnetic spacer between the ferromagnetic and antiferromagnetic layers

In (Pt/Co)n/FeMn multilayers, the magnitude of exchange bias, HE, can be considerably enhanced by placing an ultrathin nonmagnetic Pt spacer between the multilayer (ML) and the antiferromagnetic (AFM) layer. The bias is maximum for a spacer layer thickness, t, of a few angstroms and it decreases progressively as t is further increased. This bias enhancement is accompanied by an increase of coercivity, HC. This behavior is due to the role of the Pt spacer in enhancing the perpendicular effective anisotropy of the last Co layer in the ML, which has the effect of increasing the net ferromagnetic (FM)/AFM spin projection, thus leading to the HE and HC enhancements. The decrease of HE and HC for thicker spacer layers is due to the limited range of the FM–AFM proximity effect.


Large anomalous enhancement of perpendicular exchange bias by introduction of a nonmagnetic spacer between the ferromagnetic and antiferromagnetic layers
In (Pt/Co) n /FeMn multilayers, the magnitude of exchange bias, H E , can be considerably enhanced by placing an ultrathin nonmagnetic Pt spacer between the multilayer ͑ML͒ and the antiferromagnetic ͑AFM͒ layer.The bias is maximum for a spacer layer thickness, t, of a few angstroms and it decreases progressively as t is further increased.This bias enhancement is accompanied by an increase of coercivity, H C .This behavior is due to the role of the Pt spacer in enhancing the perpendicular effective anisotropy of the last Co layer in the ML, which has the effect of increasing the net ferromagnetic ͑FM͒/AFM spin projection, thus leading to the H E and H C enhancements.The decrease of H E and H C for thicker spacer layers is due to the limited range of the FM-AFM proximity effect.© 2003 American Institute of Physics.͓DOI: 10.1063/1.1619562͔During the last decades, much attention has been focused on exchange-coupled ferromagnetic ͑FM͒antiferromagnetic ͑AFM͒ bilayers, because of both their underlying physics and their applications. 1,2These materials exhibit a shift of the hysteresis loop ͑exchange bias, H E ) and an enhanced coercivity, H C , when they are field cooled through the blocking temperature of the AFM.In spite of extensive research, the physical mechanisms governing exchange bias still remain controversial. 3However, the majority of models elaborated so far consider that exchange bias mainly originates from a short-range FM-AFM interaction.Only a few studies have shown the existence of exchange bias across a nonmagnetic spacer located between the FM and the AFM layers. 4Nevertheless, up to now, these studies have always reported an exponential decrease of H E as a function of the spacer thickness. 4In this letter we demonstrate that by depositing a Pt spacer layer between a FM multilayer ͑ML͒ and an AFM it is possible to increase the magnitude of H E .
It is noteworthy that H E has been recently, to some extent, increased by diluting the AFM by nonmagnetic defects located in its bulk. 5Similar H E enhancements have been obtained by ion irradiating, in a controlled way, FM-AFM bilayers. 6All these results have been interpreted taking into account the role of impurities or defects as pinning centers for domain walls motion in the volume part of the AFM, [5][6][7] or considering local stress-induced increase of the AFM anisotropy. 8Nevertheless, at the FM-AFM interface, nonmagnetic impurities are usually assumed to reduce the exchange coupling constant, J INT , and consequently also H E . 4 -8urthermore, although in the past most exchange bias investigations have been carried out in thin films with inplane easy axis, it has been recently shown that it is also possible to induce exchange bias in multilayers exhibiting a perpendicular-to-film uniaxial anisotropy, e.g., Pt/Co ML. [9][10][11][12][13] These multilayered structures are very appealing from both fundamental and applied points of view.For in-stance, the FM uniaxial anisotropy in these systems ͑and also H C and the saturation magnetization, M S ) can easily be tuned by simply varying the number of ML repeats or the relative thickness of the Pt and Co layers. 11,12Perpendicular exchange bias also offers the possibility to prepare spin valves or tunnel junctions with perpendicular-to-plane magnetization 14,15 Different series of multilayers with the composition ͓Pt (20 Å)/Co (4 Å)͔ n /Pt (t)/FeMn (130 Å), with nϭ2, 3, and 5 and t between 0 and 20 Å, were deposited onto thermally oxidized Si wafers by dc magnetron sputtering and capped with a 20 Å Pt layer.Some other samples ͑described in the following paragraphs͒ were also prepared.The samples were cooled from 150 °C ͑i.e., above the blocking temperature͒ under a field of 2.5 kOe, applied perpendicular to the film plane.Structural characterization was performed by x-ray diffraction ͑XRD͒ and high-resolution transmission electron microscopy ͑HRTEM͒, while hysteresis loops were measured, perpendicular to the thin film direction, using the extraordinary Hall effect ͑EHE͒. 16RTEM images, together with XRD patterns, reveal that both the ͑Pt/Co͒ ML and FeMn are polycrystalline with a weak ͑111͒ texture.No significant differences in the microstructure were observed between samples with or without the Pt spacer at the FM-AFM interface.
Typical hysteresis loops of the (Pt/Co) n /Pt (t)/FeMn systems for nϭ2 and 5, measured at room temperature, are shown in Figs.1͑a͒ and 1͑b͒, respectively.The loops correspond to tϭ0, 2, 4, and 20 Å.The (Pt/Co) n /FeMn system exhibits a crossover from in-plane to perpendicular-to-plane easy axis when increasing the number of ͑Pt/Co͒ repeats, 11,15 due to the interplay between the perpendicular anisotropy induced by the Co/Pt interfaces and the large in-plane anisotropy induced at the Co/FeMn interface together with the shape anisotropy.A similar effect is observed for low number of repeats as the Pt spacer thickness increases.As can be seen in Fig. 1͑a͒, for nϭ2 the loops become more square as t increases.A similar trend is observed for nϭ3.However, for nϭ5 all the loops already exhibit a square shape, due to a strong perpendicular anisotropy.
The dependence of H E on the Pt spacer layer thickness, t, for nϭ2, 3, and 5 is shown in Fig. 2. It can be seen that H E increases significantly for low values of t, reaching a maximum at tϭ5, 2, and 1 Å, respectively.When t is further increased, H E progressively decreases.Moreover, the H E enhancement is more pronounced for lower values of n ͑for n ϭ2, the bias increases by a factor of 4, whereas for nϭ5 the enhancement is of about a factor of 2͒.The increase of H E is accompanied by an enhancement of H C .This can be seen in the inset of Fig. 2, where the dependence of H C on t is plotted for nϭ2, 3, and 5.The increase of H C vs n can be ascribed to the increase of the ML perpendicular anisotropy.
The initial increase in the strength of FM-AFM exchange coupling as the Pt spacer layer thickness is varied can be qualitatively understood in terms of a better alignment of the last Co layer magnetization along the perpendicular-tofilm direction.This is well evidenced in Fig. 1͑a͒, where, for nϭ2, the easy axis direction of the ML changes from being basically in-plane ͑for tϭ0 Å) to perpendicular to plane ͑for tϭ20 Å).For nϭ5, in spite of the square appearance of the hysteresis loops, it is reasonable to assume that some tilt in the Co magnetization could still be present, especially for the case where the Co is in direct contact with the AFM.In order to determine possible changes in the amplitude of the magnetization when t is varied, a series of spin-valve-like samples with the composition ͓Pt (20 Å) / Co (4 Å)͔ 3 / Pt (20 Å) / Cu (30 Å) / ͓Pt (20 Å) / Co (4 Å)͔ 5 /Pt (t)/FeMn (130 Å) were prepared, the signal from the unpinned bottom multilayer being used as an internal calibration.The hysteresis loops for tϭ0, 2, 5, and 7 Å are shown in Fig. 3.An enlargement of a part of these hysteresis loops ͓inset ͑a͔͒ clearly shows that the magnetization of the pinned layer increases with t.
Quantification of this effect can be obtained by evaluating, for each value of t, the ratio between the magnetization amplitude ͑at the remanent state͒ of the pinned layer and that of the free layer, and normalizing to the ratio corresponding to tϭ20 Å.This normalized ratio, R t , is plotted in inset ͑b͒ of Fig. 3 as a function of t.It can be seen that the magnetization of the pinned multilayer increases by about 20% when going from tϭ0 Å to tϭ20 Å.
Several authors have shown in the last years that exchange bias is roughly proportional to the FM-AFM spin projection at the interface. 17Their results indicate that H E is optimized when the FM and AFM easy axes are completely parallel to each other.In our case, during the perpendicular field cooling ͑FC͒ of as-deposited samples, the Co spins are basically aligned perpendicular to the film plane in the field of 2.5 kOe.As temperature is reduced to below the blocking temperature, the spins in the different FeMn crystallites orient along their easy axes closer to the FC direction.However, once at room temperature, when the field is removed, the Co spins, especially those of the last Co layer, tend to tilt at a certain angle towards in-plane, due to the induced easyplane anisotropy from the Co/FeMn interface.This is clearly seen for nϭ2 in Fig. 1͑a͒.Moreover, for nϭ5, one can see from Fig. 3͑b͒ that if the angular tilt in the remanent state occurred only in the last Co layer, the magnetization of this layer should lie in-plane in order to account for the 20% loss of the magnetization amplitude.However, in this case, since the unidirectional direction was set perpendicular-to-plane, one would observe zero bias.Conversely, if all Co layers in the ML were so strongly coupled that the relative angular tilt between them was zero, the twist of the overall remanent magnetization would be of around 35°for tϭ0 Å.However, according to the difference in the magnetization amplitude, the relative tilt between magnetizations for tϭ2 Å and t ϭ0°would be of the order of 5°, which is too small to account for a difference in bias of a factor of 2. Therefore, from these results one can infer that, for n sufficiently large, a kind of partial domain wall is formed across the ML, the tilt of the Co magnetization increasing progressively when approaching the FM/AFM interface.The actual value of H E could then be related to the energy stored in these partial domain walls in the FM, similar to what has been suggested by some recent exchange bias models. 18oreover, since FM-AFM exchange bias is mainly a short-range interaction, H E tends to exponentially decrease when increasing the nonmagnetic spacer layer thickness. 4ence, the curves presented in Fig. 2 are the result of the interplay between two opposing effects: the increase of H E due to the role of the Pt spacer in reorienting the Co spins to make them lie more perpendicular to the film plane and its exponential decrease, typical of FM-nonmagnetic-AFM heterostructures.0][21] It should be noted that the angular tilt of Co spins is probably more pronounced for lower number of Pt/Co repeats in the ML, where the perpendicular effective anisotropy is lower.This explains why the bias variation is markedly larger for lower values of n ͑see Fig. 2͒.
Further evidence for the previous interpretation is obtained when another nonmagnetic ͑NM͒ spacer is used.If, at the FM-AFM interface, Pt is substituted by another metal which does not favor an effective perpendicular anisotropy in the ML, no H E enhancement is observed.This was confirmed using Al, Ru, and Cu as spacers in ͓Pt (20 Å)/Co (4 Å)͔ 5 /NM (t)/FeMn (130 Å) samples.As can be seen in Fig. 4, H E decreases with t in all these series.
Finally, one should not completely disregard that some other factors ͑e.g., structural ones͒ could also have some influence on the H E enhancement.
In conclusion, we have demonstrated that the presence of a nonmagnetic ultrathin layer between a FM and an AFM layer ͓e.g., a Pt spacer layer between a ͑Pt/Co͒ ML and FeMn͔ can enhance the magnitudes of H E and H C .In this system, these enhancements are mainly governed by the role of the Pt/Co interface in increasing the net perpendicular magnetic moment of the last Co layer in the ML and the consequent FM-AFM spin projection at the interface.