Perpendicular exchange bias in antiferromagnetic-ferromagnetic nanostructures

Exchange bias effects have been induced along the perpendicular-to-film direction in nanostructures prepared by electron beam lithography, consisting of a ferromagnetic [Pt/Co] multilayer exchange coupled to an antiferromagnet (FeMn). As a general trend, the exchange bias field and the blocking temperature decrease, whereas the coercivity increases, as the size of the nanostructures is reduced.

During the last decades, the areal density of data storage devices has dramatically increased each year.This trend demands a miniaturization of the reading/writing magnetic data devices.Reading heads are typically composed of spin-valve or tunnel junction structures, in which antiferromagnetic ͑AFM͒/ ferromagnetic ͑FM͒ exchange biased bilayers constitute an essential part 1 ͑for recent reviews on exchange bias, see Refs.2-5͒.][8][9][10][11][12] However, so far, these studies have only been carried out in systems with an in-plane magnetic anisotropy.
4][15] Perpendicular exchange bias offers the possibility of preparing spin valves or tunnel junctions with perpendicular magnetization. 16 -18][21] In this letter, we demonstrate that it is possible to induce perpendicular exchange bias in nanostructures consisting of a ͓Pt/Co͔ multilayer ͑ML͒ exchange coupled to FeMn.Our results show that the magnitude of exchange bias decreases as the lateral size of the nanostructure is reduced.Moreover, the temperature at which exchange bias disappears during heating also decreases when the continuous film is patterned.
A continuous multilayer with the composition ͓Pt(20 Å)/Co(4 Å)͔ 5 /Pt(5 Å)/FeMn(130 Å)/Pt(20 Å) was deposited onto a thermally oxidized Si wafer by dc magnetron sputtering.From the sputtered multilayer, different types of nanostructures were fabricated by electron beam lithography ͑for a recent review on fabrication and properties of magnetic ordered arrays of nanostructures, see Ref. 22͒.Shown in Fig. 1 are the scanning electron microscopy images of ͑a͒ the 200ϫ200 nm 2 square dots, ͑b͒ the 200 nm ϫ1 m stripes and ͑c͒ the 200 nmϫ100 m wires.The continuous film and the nanostructures were field cooled under a field of 2.5 kOe, applied perpendicular to the film plane, from Tϭ600 K. Hysteresis loops were measured by polar Kerr effect.
Figure 2 shows the hysteresis loops of ͑a͒ the continuous film, ͑b͒ the 200-nm-wideϫ100-m-long wires, ͑c͒ the 200 nmϫ1 m stripes and ͑d͒ the 200ϫ200 nm 2 dots.All the hysteresis loops are rather square, with a remanence to saturation ratio, M R /M S , close to 1.In arrays of very small dots, it is rather common to observe a broad distribution of switching fields. 22This probably accounts for the slight tilt of the hysteresis loop observed in the 200ϫ200 nm 2 dots.The good squareness of the loops indicates that the perpendicular effective magnetic anisotropy is essentially maintained during the patterning process.Since the perpendicular anisotropy in ͓Pt/Co͔ ML arises from the interface anisotropy between the Pt and Co layers, 23 it is likely that the Pt/Co interfaces remain parallel to the thin film plane after the patterning.
The magnitudes of the coercivity, H C , and the exchange bias field, H E , for the continuous film and the different nanostructures investigated are shown in Table I.It is worth noting that in the 200ϫ200 nm 2 dots H C increases by a factor of 4 compared to the continuous film.On the contrary, H E decreases from 223 Oe ͑in the continuous film͒ to about 14 Oe ͑in the smallest dots͒.The enhancement of H C is commonly observed in patterned elements and is probably due to the role of the edges of the nanostuctures as barriers for the domain wall propagation.The decrease of H E as the size of the nanostructure is reduced has also been sometimes observed in AFM-FM patterned elements with in-plane magnetic anisotropy. 7,8,10This decrease can be attributed to the constraints imposed by the reduced dimensions of the nanostructures on the formation of antiferromagnetic domain walls.Indeed, the majority of models dealing with exchange bias relate the magnitude of H E to the formation of domains in the AFM layer. 24 -26It can be argued that the existence of AFM domain walls allows a small surplus of magnetization at the AFM/FM interface, which couples to the FM, resulting in the unidirectional anisotropy, i.e., exchange bias.In fact, taking into account the values of the magnetic stiffness for FeMn (A FeMn ϭ3ϫ10 Ϫ7 erg/cm) and its magnetic anisotropy (K FeMn ϭ1.3ϫ10 5 erg/cm 3 ), the domain wall width in FeMn, ␦ FeMn , can be estimated to be ␦ FeMn ϭ (A FeMn /K FeMn ) 1/2 ϳ50 nm. 24When the lateral dimensions of the nanostructures become about the same order of magnitude as the AFM domain wall width, it is likely that some AFM domain walls, instead of being able to completely form may be just partially developed inside the nanostructures.These partial AFM domain walls may be less effectively pinned than complete 180°AFM domain walls in continuous AFM-FM films, hence leading to the observed decrease of H E in the nanostructures.It should be noted that our results are opposite to a theoretical study 27 which predicted an H E enhancement in AFM/FM nanostructures with relatively large AFM anisotropies and an experimental study 28 where an H E enhancement was observed in FM dots on macroscopic AFM.
Furthermore, it is well known that FM particles tend to lose their magnetic order when their size becomes very small.A reduction of the Curie temperature in ultrafine FM particles has been experimentally reported 29 and theoretically interpreted. 30In addition, a decrease of the Ne ´el temperature has also been observed in ultrathin AFM layers or ultrafine AFM particles. 31,32By analogy, it could be argued that in AFM-FM systems, a reduction in the temperature at which exchange bias disappears heating ͑i.e., the so-called blocking temperature, T B ), may be expected if the continuous bilayer is patterned.Indeed, T B has been found to reduce in AFM-FM continous bilayers with very thin AFM layer thickness or very small AFM crystallites. 33,34The temperature dependence of H E is shown in Fig. 3͑a͒.As expected, in both the continuous film and the nanowires, H E decreases during heating due to the loss of magnetic order in the AFM.Nevertheless, H E vanishes in the nanowires at a lower temperature (T B ϭ540 K) than in the continuous films ͑where T B ϭ570 K).This can be ascribed to an increasing thermally induced unpinning of antiferromagnetic domain walls as the size of the nanostructure is reduced.Moreover, it is remarkable that the overall shapes of the H E vs T curves in the continuous films and the nanowires are quite different.Namely, although in the continuous films this dependence is roughly linear, in the nanowires H E decreases slowly for TϽ400 K and at a higher rate for larger temperatures.This probably indicates that the distribution of local blocking temperatures in the nanostructures is narrower than in the continuous film.In particular, it can be inferred that, during the patterning, those AFM-FM regions of the continuous films with lower local T B may become, in fact, essentially uncoupled.
Figure 3͑b͒ shows the dependence of H C on temperature for the continuous film and the nanowires.the coercivity also decreases during heating.Interestingly, although H C is larger in the nanowires at room temperature, the values of H C for the continuous film and the nanowires become similar for TϾ450 K and even slightly smaller in the nanostructures at higher temperatures, where exchange bias virtually vanishes.In fact, one finds that the difference H C (550 K) -H C (300 K) is equal to 200 Oe for the continuous film, whereas it is equal to 320 Oe for the nanowires.This is an indication that size effects on AFM/FM coupling are partially responsible for the large H C observed in the nanostructures.In addition, the decrease of H C with temperature is also associated with the characteristic loss of the perpendicular anisotropy in ͓Pt/Co͔ multilayers.From Fig. 3͑b͒ it seems that the FM anisotropy decreases more rapidly with temperature in the nanostructures than in the continuous films.Finally, although structural changes at the AFM-FM interface or in the bulk of the AFM due to the ion milling process have not been found to be significant in AFM-FM nanostructures with in-plane anisotropy fabricated by electron beam lithography, 7 some structural effects, particularly edge defects, should not be completely disregarded as partially responsible for the reduction observed in H E or T B or the increase of H C .
In conclusion, we demonstrate the possibility of inducing perpendicular exchange bias in nanostructures composed of a ͓Pt/Co͔ multilayer exchange coupled with an antiferromagnet ͑FeMn͒, fabricated by electron beam lithography.It is found that, although the patterned elements preserve the perpendicular magnetic anisotropy of the multilayer, the magnitude of the exchange bias field progressively decreases and the coercivity increases as the size of the nanostructure is reduced, probably due to the constraints imposed by the small lateral dimensions of the nanostructures on the formation of AFM domain walls.In addition, the blocking temperature of the AFM-FM system is also found to decrease after the patterning process.These phenomena are certainly important to take into account in the implementation of magnetic sensor nanodevices based on spin valves or tunnel junctions with perpendicular anisotropy.This work was supported by the European Community through NEXBIAS Grant No. HPRN-CT-2002-00296.

TABLE I .
Summary of the values of the exchange bias field, H E , and the coercivity, H C , evaluated at room temperature, for the continuous film and the different nanostructures investigated in our study.