Enhanced exchange bias effects in a nanopatterned system consisting of two perpendicularly coupled ferromagnets

The effect of patterning on the shift of the hysteresis loop HE and coercivity HC in a system composed of two perpendicularly exchange-coupled ferromagnets NiFe sputtered onto a Pt /Co multilayer is investigated in long stripes and square dots. Setting the exchange bias coupling along the stripes results in a threefold increase of HE compared to the continuous films. HC increases dramatically when the coupling is set perpendicular to the stripes and also in the dots. Magnetic force microscopy studies and micromagnetic simulations suggest that differences in the number and orientation of the magnetic domains can account for the observed effects. © 2008 American Institute of Physics. DOI: 10.1063/1.2833124

][8][9][10][11] The heating-cooling procedure often used to induce H E can lead to undesirable structural effects ͑e.g., interdiffusion͒, which may in turn modify the magnetic behavior of the system.][14][15][16][17] Recently, a novel approach for field-induced EB, based on multilayer structures consisting of two exchange coupled FM materials with mutually orthogonal easy axes has been proposed, Permalloy, NiFe ͑Ni 81 Fe 19 ͒, with in-plane anisotropy, deposited onto a ͓Pt/ Co͔ multilayer ͑ML͒, with perpendicular-to-plane anisotropy. 12The procedure consists in in-plane saturation of the whole system followed by recording of the in-plane hysteresis loop with a maximum applied field strong enough to saturate the NiFe layer but not the ͓Pt/ Co͔ ML.Intuitively, a perpendicular coupling should not result in any EB, similar to what is observed in AFM based systems. 18However, in-plane saturation of the system results in the creation of Néel-type flux closure caps in the ͓Pt/ Co͔ ML with unequal sizes, i.e., enlarged domains parallel to the saturation direction. 19The coupling between the NiFe magnetic moments and those of the ͓Pt/ Co͔ ML through the top flux closure caps generated at the interface provides the bias.
In this letter, we study the effect of reducing the lateral size ͑i.e., patterning͒ on the loop shift and coercivity of a ͓Pt/ Co͔ -NiFe system.
Si wafers were prepatterned by e-beam lithography and reactive ion etching to form 1 ϫ 1 mm 2 arrays of Si stripes with lateral sizes of 200 nmϫ 1 mm, height of 200 nm, and spacing between stripes ͑trenches͒ of 200 nm.The inset of Fig. 1͑a͒ shows a detail of this array.Square dots with lateral sizes, height, and periodicity of 200 nm were also prepared on identical Si wafers.Pt 20 nm / ͓Co 0.6 nm / Pt 1.8 nm ͔ 6 / Co 0.2 nm / NiFe 2.5 nm / Cu 2 nm / Pt 2 nm ͓͑Pt/ Co͔ -NiFe͒ ML were dc-magnetron sputtered on the prepatterned wafers thereby coating the patterned stripes ͑or dots͒ and the trenches between them.
The loop shift H E was induced by applying an in-plane magnetic field H 0 = 11 kOe and subsequently measuring the in-plane hysteresis loop, at room temperature, with a smaller maximum applied field, H hyst,max , ranging from 1.5 to 4.25 kOe ͑sufficient to saturate the NiFe layer but not the ͓Pt/ Co͔ ML͒, by longitudinal magneto-optical-Kerr effect ͑MOKE͒.The magnetic domain structures in the remanent state were imaged by magnetic force microscopy ͑MFM͒ with a low magnetic moment tip. 20Micromagnetic simulations were carried out using a Landau-Lifshitz-Gilbert micromagnetic solver. 21The ͓Pt/ Co͔ -NiFe system was considered as a NiFe layer exchange-coupled to a ͓Pt/ Co͔ ML structure represented in three layers.The values of H E and H C remain approximately constant for H hyst,max Ͻ 2500 Oe both in the stripes and the continuous film.The observed decrease ͑increase͒ of H E ͑H C ͒ with H hyst,max can be understood considering the progressive dragging of the net inplane magnetic moment of the ͓Pt/ Co͔ ML and the extra energy required to switch the in-plane magnetic moment arising from the ͓Pt/ Co͔ ML. 12 The H E values experience a more pronounced drop for the nanostructures than for the continuous part of the film for H hyst,max Ͼ 2500 Oe which might be due to the more weakly coordinated spins located at the edges of the top of the stripes. 6For large enough H hyst,max values, the moments in the ͓Pt/ Co͔ closure domains will reverse with the applied field, resulting in a zero net in-plane magnetic moment ͑i.e., no uncompensated spins͒, consequently the loop shift will vanish, as observed in Fig. 1͑a͒.
To understand the difference in the H E values between the stripes and the continuous films and bearing in mind that magnetic domains play a crucial role in EB, 3,7,23,24 MFM images at remanence were recorded, after applying H 0 , for the continuous films ͓Fig.2͑a͔͒ and the stripes ͑H 0 parallel to the stripes͒ ͓Fig.2͑b͔͒.Magnetic signal is measured from both the top of the nanostructures and the trenches between them since the deposition is carried out on prepatterned Si substrates.Note that these images mainly show the magnetic contrast stemming from the stripe domains in the ͓Pt/ Co͔ ML, which, in this system, play a role analogous to the AFM domains in AFM-FM bilayers.The magnetic domains are significantly reduced in the patterned structure. 10Since EB is due to the induced uncompensated moments, smaller domains ͑and the concomitant increase in the number of domain walls͒ lead to the enhanced H E values in the patterned stripes compared to the continuous ML. 7,23 A drastically different behavior is observed in the case of applying H 0 and H hyst,max in-plane, perpendicular to the stripes.The corresponding hysteresis loop with H hyst,max = 3200 Oe is shown in Fig. 3͑a͒.Identical measurement procedure was applied to an array of 200 nm dots resulting in a similar hysteresis loop, as also shown in Fig. 3͑a͒.In both cases, the loops exhibit a shift along the magnetic field axis and a dramatic enhancement of coercivity ͑from H C Ϸ 85 Oe in the continuous film to H C Ͼ 1600 Oe for the nanostructures͒.Note that both loops are very similar due to the identical dimension ͑200 nm͒ of the direction along which the external magnetic field is applied.The MFM images at remanence ͑after H 0 ͒ show that a wavy domain pattern tends to form in the stripes at remanence ͓Fig.3͑b͔͒, while comparable magnetic configurations exhibiting a dipolar contrast indicative of a single flux closure cap state form in the square dots ͓Fig.3͑c͔͒.
Additional insight into the understanding of the magnetic behavior of the system can be obtained from micromagnetic simulations.Simulations of the stripes ͑Fig.4͒ reveal that the magnetic configuration at remanence is constituted by fragmented domains orientated preferentially out of plane.The general character of the domain patterns after saturation parallel and perpendicular to the stripes is in good agreement with the MFM images shown in Figs. 2 and  3, respectively.The simulations also provide information of the evolution of the magnetic structure when following a complete hysteresis loop, as shown for the case H hyst,max = 3000 Oe.The character of the PtCo domain configuration changes little as a function of field when measuring along the stripes ͓Fig.4͑a͔͒ and, concomitantly, the simulated loop exhibits a pronounced shift along the magnetic field axis, of the same order of magnitude as in the loop measured experimentally ͑see Ref. 22͒.The simulation for the case of the saturated and measured applying in-plane fields perpendicular to them ͓Fig.4͑b͔͒, however, shows an evolution of the magnetization as a function of field.A multidomain configuration is displayed at H hyst,max .Such magnetic structure sustained during magnetization reversal would be favourable to obtain an enlarged H E .However, the simulations show a rapid transformation to a more stable dipolar configuration as the field is reduced that remains throughout the remainder of the hysteresis loop.The domain wall shape becomes wavy at remanence, with many domain walls parallel to the measurement direction, in excellent agreement with the MFM image ͓Fig.3͑b͔͒.Similar magnetic configuration is observed for the case of a simulated dot at remanence ͓Fig.4͑c͔͒.The simulations ͑see Ref. 22͒ suggest that the H C enhancement results from rotation of the domain wall from being perpendicular to the applied field ͑at saturation͒ to being roughly parallel to the applied field direction ͑for H Ϸ H C ͒.Such partial rotation would occur so as to avoid having an energetically unfavourable flux closure domain pattern in the ͓Pt/ Co͔ ML.
The simulated hysteresis loops, 22 despite achieving only qualitative agreement with the experiment, due to the complexity of the system, showed a decrease in H E with decreasing the number of domain walls perpendicular to the measuring direction.Domain walls oriented parallel to the field, in contrast, will have flux closure perpendicular to the measurement direction and contribute little to the exchange bias, being mainly responsible for the H C enhancement.This result, coupled with the decreased domain size observed by MFM, supports the idea that the large H E in these nanostructures in comparison to the continuous film is related to the increased ratio between uncompensated and compensated spins.
In conclusion, a large increase in the loop shift with respect to continuous films has been achieved for a nanostructured ͓Pt/ Co͔ -NiFe system.As demonstrated by MFM studies, the domain sizes decrease upon patterning the system.This effect results in an increased ratio between uncompensated and compensated spins in the nanostructures which is responsible for the observed loop shift enhancement.Micromagnetic simulations have shown the key role played by the number of unequal closure domains, oriented perpendicular to the measuring field on increasing the loop shift.

Figures
Figures 1͑a͒ and 1͑b͒show the dependence of H E and the coercivity H , respectively, on the maximum field H hyst,max applied along the stripes.Representative hysteresis loops for the case H hyst,max = 2100 Oe are shown in the inset of Fig.1͑b͒.For low H hyst,max values the bias field in the stripes ͑H E Ϸ 180 Oe͒ is much larger than for the continuous film ͑H E Ϸ 60 Oe͒, in contrast to what is usually observed in patterned conventional EB systems.7The values of H E and H C remain approximately constant for H hyst,max Ͻ 2500 Oe both in the stripes and the continuous film.The observed decrease ͑increase͒ of H E ͑H C ͒ with H hyst,max can be understood considering the progressive dragging of the net inplane magnetic moment of the ͓Pt/ Co͔ ML and the extra energy required to switch the in-plane magnetic moment arising from the ͓Pt/ Co͔ ML.12The H E values experience a more pronounced drop for the nanostructures than for the continuous part of the film for H hyst,max Ͼ 2500 Oe which might be due to the more weakly coordinated spins located at the edges of the top of the stripes.6For large enough H hyst,max values, the moments in the ͓Pt/ Co͔ closure domains will reverse with the applied field, resulting in a zero net in-plane magnetic moment ͑i.e., no uncompensated spins͒, consequently the loop shift will vanish, as observed in Fig.1͑a͒.To understand the difference in the H E values between the stripes and the continuous films and bearing in mind that magnetic domains play a crucial role in EB,3,7,23,24 MFM images at remanence were recorded, after applying H 0 , for the continuous films ͓Fig.2͑a͔͒ and the stripes ͑H 0 parallel to the stripes͒ ͓Fig.2͑b͔͒.Magnetic signal is measured from both the top of the nanostructures and the trenches between them since the deposition is carried out on prepatterned Si substrates.Note that these images mainly show the magnetic contrast stemming from the stripe domains in the ͓Pt/ Co͔

FIG. 1 .
FIG. 1. Evolution of ͑a͒ the exchange bias field H E and ͑b͒ the coercivity H C with the maximum in-plane field H hyst,max after applying H 0 along the stripes ͑full symbols͒ and the continuous film ͑empty symbols͒.Also shown are a SEM image showing a detail of the array of stripes ͑upper inset͒, and hysteresis loops for both the stripes and the continuous film ͑bottom inset͒.

Financial
support from the 2005SGR-00401, the MAT-2007-66302-C02, and the HF2006-0197 research projects is acknowledged.A.B. acknowledges also support by the Spanish Ministry of Education and Science.Work at the Center for Nanoscale Materials was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.