Using exchange bias to extend the temperature range of square loop behavior in † Pt / Co ‡ multilayers with perpendicular anisotropy

The temperature dependence of the magnetic properties of [Pt∕Co] multilayers (ML), exhibiting perpendicular anisotropy, with and without exchange biasing with an antiferromagnet (AFM) has been investigated. Upon heating, a loss of the out-of-plane anisotropy and, consequently, of the remanence to saturation ratio is observed in these systems. However, such effect occurs at higher temperatures in the [Pt∕Co] ML exchange coupled to the AFM than for the unbiased ML. This is attributed to the additional anisotropy induced to the ML by the ferromagnetic-antiferromagnetic exchange coupling.

The temperature dependence of the magnetic properties of ͓Pt/ Co͔ multilayers ͑ML͒, exhibiting perpendicular anisotropy, with and without exchange biasing with an antiferromagnet ͑AFM͒ has been investigated.Upon heating, a loss of the out-of-plane anisotropy and, consequently, of the remanence to saturation ratio is observed in these systems.However, such effect occurs at higher temperatures in the ͓Pt/ Co͔ ML exchange coupled to the AFM than for the unbiased ML.This is attributed to the additional anisotropy induced to the ML by the ferromagnetic-antiferromagnetic exchange coupling.© 2005 American Institute of Physics.͓DOI: 10.1063/1.2139840͔Exchange interacting ferromagnetic ͑FM͒antiferromagnetic ͑AFM͒ materials exhibit a shift in the hysteresis loop ͑i.e., exchange bias͒ and an enhancement of coercivity when they are field cooled through the blocking temperature, T B , of the AFM. 1 The shift of the hysteresis loops has been technologically exploited in devices based on spin valves or tunnel junctions. 2Additionally, the H C enhancement has been proposed to improve the hard magnetic properties of permanent magnetic materials. 3Recently, the coupling of FM nanoparticles embedded in an AFM matrix has been shown to increase their superparamagnetic blocking temperature. 4sually, exchange bias is observed in FM-AFM bilayers with in-plane anisotropy.][7][8][9][10] Most of the studied systems exhibiting out-ofplane exchange bias are based on ͓Pt/ Co͔ or ͓Pd/ Co͔ multilayers, ML, ͑as FM͒ coupled to an AFM.9]11 Moreover, these ML have potential applications in magneto-optic storage technology. 12,13However, one of the features commonly observed in multilayers with perpendicular anisotropy is that, upon heating, a twostage transition towards in-plane anisotropy occurs. 14,15First, at a temperature called T * , the out-of-plane uniaxial anisotropy decreases, giving rise to multidomain structures ͑with out-of-plane domains͒, which results in a complete loss of squareness, i.e., remanence to saturation ratio, M R / M S =0.5][16][17][18][19][20][21] This loss of squareness can be a limiting factor for the application of ML with out-ofplane anisotropy.
In this letter we demonstrate that it is possible to extend the temperature range where ͓Pt/ Co͔ ML maintain M R / M S = 1 by exchange coupling them, along the perpendicular direction, with an AFM with high T B ͑IrMn and FeMn͒.This effect is linked to the uniaxial anisotropy, induced by the FM-AFM interface coupling, resulting also in coercivity enhancement, rather than to the unidirectional anisotropy which generates the loop shift.
Note that the ultrathin Pt layer between the ͓Pt/ Co͔ ML and the AFM was introduced to enhance the perpendicular orientation of the ͓Pt/ Co͔ ML and, consequently, the magnitude of exchange bias. 7Moreover, the presence of the interface Pt layer ensures that the top Co layer in both MLs is in contact with a Pt layer, rather than in direct contact with the AFM layer.This avoids the possible effects that different capping layers have on the out-of-plane anisotropy of ML. 22 The Pt layer has to be chosen thin enough not to deteriorate the exchange bias, 7 but thick enough to elude the effects of the thickness of the capping layers on the out-of-plane anisotropy.Since such effects saturate for t Capping = 0.5 nm, 22 this thickness was chosen for the interface Pt layer.
Shown in Fig. 1 are the hysteresis loops of the ͓Pt/ Co͔ and ͓Pt/ Co͔ / IrMn systems, measured at T = 300 and T = 375 K.At room temperature, the loop corresponding to ͓Pt/ Co͔ / IrMn is shifted along the magnetic field axis by an amount H E =90 Oe ͑where H E designates the hysteresis loop shift͒.Moreover, the loops exhibit a square shape, with M R / M S =1 ͓note that the squareness ratio is evaluated after recentering for the hysteresis loop shift͔, confirming that both systems exhibit a perpendicular effective magnetic anisotropy.Remarkably, the coercivity of the exchange coupled ͓Pt/ Co͔ / IrMn system is significantly larger than for the unbiased ͓Pt/ Co͔ ML, i.e., the presence of the AFM also induces a coercivity enhancement.At T = 375 K, although the ͓Pt/ Co͔ / IrMn system still preserves a large squareness ratio, the uncoupled ͓Pt/ Co͔ ML exhibits a virtually zero remanent magnetization.This suggests that at high temperatures the out-of-plane effective magnetic anisotropy is exceedingly weak to maintain the M R / M S = 1 ratio.The central constriction observed in the hysteresis loop of Fig. 1͑b͒, at T = 375 K is typical of the instability of the single-domain configuration ͑i.e., the formation of out-of-plane domains͒, 14,17 probably indicating that for the uncoupled ML The temperature dependence of H C for both ͓Pt/ Co͔ and ͓Pt/ Co͔ / IrMn systems and the loop shift, H E , for ͓Pt/ Co͔ / IrMn, after perpendicular field cooling, is shown in Fig. 2͑a͒ and its inset, respectively.H E progressively decreases with temperature vanishing at about T B ϳ 400 K.As expected, the ͓Pt/ Co͔ ML without AFM exhibits no exchange bias.The coercivity also decreases with temperature in both systems.In the unbiased ͓Pt/ Co͔ ML the reduction of H C is due to the decrease of perpendicular anisotropy with temperature.In the ML/ IrMn system the reduction of the coercivity originates both from the weakening of the out-ofplane anisotropy and from the decrease of the strength of the FM-AFM exchange interactions with temperature.This exchange coupling reduction is due to the decrease of anisotropy in IrMn.Fig. 2͑a͒ also reveals that H C vanishes at T = 380 K for the unbiased ͓Pt/ Co͔ ML and at T = 410 K for the exchange biased ͓Pt/ Co͔ / IrMn system.The loss of coercivity indicates that the out-of-plane anisotropy is exceedingly weak to keep the out-of-plane homogeneous state.Hence, the loss of out-of-plane anisotropy occurs at a lower temperature for the single ͓Pt/ Co͔ ML without AFM.This is more clearly evidenced in Fig. 2͑b͒, where the temperature dependence of M R / M S is shown.The transition is found to occur at approximately 20 K higher temperature for ͓Pt/ Co͔ / IrMn.In fact, in multilayered structures with perpendicular anisotropy, T * and T Reo can be tailored by varying the thickness of the different layers conforming the ML or the number of Pt/ Co repeats. 16,18,20,21However, since in our case both samples were deposited in the same run, the thicknesses of the Co, t Co , and Pt, t Pt , layers are the same.The inset of Fig. 2͑b͒ shows the temperature dependence of M S ͑T͒ / M S ͑300 K͒.It can be observed that, for both systems, M S ͑T͒ / M S ͑300 K͒ vanishes at T = 420 K, which is the Curie temperature, T C , of the systems.Similar to T * and T Reo , T C also depends on t Co and t Pt . 18,23Hence, the fact that the temperature dependence of M S ͑T͒ is the same for both systems is a confirmation that t Co and t Pt in the two samples do not differ.Therefore, the enhancement of the M R / M S = 1 temperature range is attributed to the exchange coupling between the ͓Pt/ Co͔ ML and IrMn.It is worth noting that the ML/ IrMn system after zero field cooling exhibits no loop shift, although the coercivity enhancement with respect to the single ML remains unchanged ͑not shown͒.Remarkably, the temperature dependence of the coercivity, M R / M S and M S of the field cooled and zero field cooled ML/ IrMn layers remains exactly the same.Hence, the enhancement of the transition temperature is not associated with the loop shift itself but, rather, to the coercivity enhancement induced by the AFM-FM coupling.
This enhancement of the transition temperature is exacerbated in exchange biased ͓Pt/ Co͔ multilayers with thinner Co layers provided that the T B of the AFM is sufficiently high ͑e.g., 13-nm-thick FeMn, where T B ϳ 400 K͒.This is illustrated in Fig. 3, where the temperature dependence of M R / M S is plotted for both ͓Pt/ Co͔ and ͓Pt/ Co͔ / FeMn.Note that since t Co is smaller ͑0.3 nm͒ the loss of remanence for the unbiased ͓Pt/ Co͔ ML occurs at lower temperatures, i.e., ϳ330 K ͑Fig.3͒ than for the ͓Pt/ Co͔ ML with t Co = 0.4 nm ͑Fig.2͒.The larger difference between T B and the temperature of the loss of remanence for the unbiased ML results in a larger enhancement of the range of out-of-plane stability.The presence of the AFM also increases the temperature range for which H C Ͼ 0 ͑inset of Fig. 3͒.However, the transitions for FeMn are somewhat more rounded than for IrMn.This could be ascribed to the different spin structure of the AFMs when field cooled out of plane.
It has already been reported that exchange AFM-FM coupling can be used to enhance the Néel temperature of the AFM phase 24 or even the Curie temperature of the FM phase. 25The former was explained by using a mean field model in which two disparate ordering temperatures approach each other as a function of the relative thickness of the layers.Moreover, exchange bias effects can induce a 30fold increase in the superparamagnetic blocking temperature of FM nanoparticles. 4 This has been explained by the extra anisotropy provided to the FM by the interface FM-AFM exchange coupling. 4,26It is well known that in FM-AFM systems the interface coupling induces a unidirectional anisotropy, which causes the loop shift and extra uniaxial ͑or higher order͒ anisotropies, which result in a coercivity enhancement. 27Hence, the enhancement of the transition temperature at which M R / M S becomes 0 in ͓Pt/ Co͔/AFM can be interpreted by considering that the AFM induces an additional out-of-plane anisotropy to the ML after cooling.The extra anisotropy makes the ML more stable against thermal fluctuations.Hence, provided that the exchange bias blocking temperature in the ͓Pt/ Co͔ / IrMn or ͓Pt/ Co͔ / FeMn systems is higher than T * and T Reo of ͓Pt/ Co͔ ML, the FM-AFM coupling extends the temperature range in which the FM preserves a perpendicular effective orientation.This finding may have important implications for perpendicular recording media or spin valves or tunnel junctions with outof-plane anisotropy since it would extend the temperature range of applicability.
In conclusion, it has been shown that the temperature at which the perpendicular magnetic anisotropy of a ͓Pt/ Co͔ ML loses its M R / M S = 1 state can be enhanced by exchange coupling the ML along the perpendicular to film direction with an AFM with T B higher than T * and T Reo of the single ͓Co/ Pt͔ ML.The phenomenon can be understood considering that the AFM induces an extra-anisotropy in the FM, along the perpendicular to film direction, which persists at temperatures higher than T * and T Reo .