Pinned synthetic ferrimagnets with perpendicular anisotropy and tuneable exchange bias

Pinned synthetic ferrimagnets (syFerri) with perpendicular-to-plane magnetic anisotropy, of the form AP1/Ru/AP2/FeMn [where AP1 and AP2 are (Co/Pt) multilayers], have been prepared and characterized. The magnitudes of the exchange bias fields of both AP1 and AP2 can be tuned at room temperature by simply varying the relative number of (Co/Pt) repeats in each multilayer. This effect can be quantitatively interpreted by considering the different energy contributions involved during magnetization reversal. Moreover, from the values of these fields, the characteristic parameters of the system (i.e., coupling strength through the Ru and AP2/FeMn pinning energy), can be evaluated. Interestingly, an extended plateau with a virtually constant magnetization is observed around zero field when the number of Co/Pt repeats in AP1 is equal or larger than in AP2. This is very appealing for field sensor or memories applications using spin valves or tunnel junctions with perpendicular anisotropy, since it offers a large dynamic range over which the magnetic configuration of the syFerri remains stable.

Pinned synthetic ferrimagnets ͑syFerri͒ with perpendicular-to-plane magnetic anisotropy, of the form AP 1 /Ru/AP 2 /FeMn ͓where AP 1 and AP 2 are ͑Co/Pt͒ multilayers͔, have been prepared and characterized.The magnitudes of the exchange bias fields of both AP 1 and AP 2 can be tuned at room temperature by simply varying the relative number of ͑Co/Pt͒ repeats in each multilayer.This effect can be quantitatively interpreted by considering the different energy contributions involved during magnetization reversal.Moreover, from the values of these fields, the characteristic parameters of the system ͑i.e., coupling strength through the Ru and AP 2 /FeMn pinning energy͒, can be evaluated.Interestingly, an extended plateau with a virtually constant magnetization is observed around zero field when the number of Co/Pt repeats in AP 1 is equal or larger than in AP 2 .This is very appealing for field sensor or memories applications using spin valves or tunnel junctions with perpendicular anisotropy, since it offers a large dynamic range over which the magnetic configuration of the syFerri remains stable.© 2003 American Institute of Physics.͓DOI: 10.1063/1.1606495͔Recently, in spin-valve based read heads for high density magnetic recording media, 1 the replacement of the traditional antiferromagnetic ͑AFM͒ exchange layer by synthetic antiferromagnets ͑syAFM͒ has been found to be very appealing, since syAFM considerably enhance the thermal and magnetic stability of these sensors and reduce the magnetostatic field created by the pinned layer on the free layer in submicronic devices. 2 Typically, a syAFM consists of two ferromagnetic ͑FM͒ layers separated by a nonmagnetic spacer, such as Ru ͑6 -10 Å thick͒, which couples them antiferromagnetically due to the Ruderman-Kittel-͑Kasuya͒-Yosida ͓RK͑K͒Y͔ interaction. 3ll syAFM-based spin valves elaborated so far exhibit an in-plane magnetic anisotropy.Just recently, spin valves and tunnel junctions with perpendicular-to-plane anisotropy have also been developed. 4However, they do not make use of syAFM.Studies on syAFM-like systems with perpendicular anisotropy are very scarce 5 and the effect of varying the relative thickness of the two FM layers in contact with Ru has only been systematically investigated in in-plane syAFM. 6Nevertheless, a few studies on the physics of antiferromagnetically coupled multilayers with perpendicular anisotropy have been reported. 7,8n this letter, we demonstrate the feasability to grow pinned synthetic ferrimagnets ͑syFerri͒ with perpendicular anisotropy, exhibiting tuneable exchange bias at room temperature.The systems consist of two antiferromagnetically coupled ͑Pt/Co͒ multilayers ͑ML͒, denoted as AP 1 and AP 2 , in which AP 2 is, in turn, exchange biased with an AFM. 9 The switching fields of both AP 1 and AP 2 can be easily tuned by simply varying the relative number of ͑Pt/Co͒ repeats in each multilayer.
A series of pinned synthetic ferrimagnets with the com-position Pt ͑50 Å͒/͓Co ͑5 Å͒/Pt(20Å)] n /Co(5 Å͒/Ru͑7 Å͒/ ͓Co͑5 Å͒/Pt͑20 Å)] 3 /Co(5 Å͒/FeMn͑130 Å͒/Pt͑20 Å͒, i.e., AP 1 /Ru/AP 2 /FeMn, with nϭ1, 2, 3, 4, and 5, were deposited onto thermally oxidized Si substrates by dc magnetron sputtering.The AP 1 multilayer contains a variable number of Co layers, i.e., n AP1 ϭnϩ1, whereas AP 2 contains always 4 Co layers.All depositions were performed at room temperature.A stray field perpendicular to plane is known to exist over the substrate area in the sputtering unit.Hysteresis loops of as-deposited samples were recorded using extraordinary Hall effect, with magnetic field applied perpendicular to the film direction. 10igure 1 shows the hysteresis loops of the pinned syFerri with n AP1 ϭ2, 3, 4, and 6.The magnetic configurations of AP 1 and AP 2 multilayers are indicated by the thin and thick arrows, respectively.It can be seen that when the field is reduced from positive to negative saturation, the magnetization of AP 1 (M AP1 ) always switches before that of AP 2 , M AP2 , at a positive field denoted by H E,AP1 ϩ .A broad plateau is then observed around zero field, over which the two multilayers are in antiparallel magnetic configuration.When the field is further reduced, the magnetization of the AP 2 layer (M AP2 ) finally switches at a negative field H E,AP2 ϩ .Interestingly, for n AP1 ϭ2, the switching of M AP2 is accompanied by a backswitching ͑i.e., spin-flip͒ of M AP1 , due to the antiferromagnetic RK͑K͒Y interaction through Ru.Finally, M AP1 switches down again at a slightly larger negative field.
When increasing the field from negative to positive saturation, significant differences in the magnetization reversal process are observed depending on the value of n AP1 .
͑1͒ For n AP1 ϭ2 and 3, M AP1 switches first at a characteristic negative field ϩ is observed.As the field is further increased, M AP2 switches.In the case n AP1 ϭ2, this switching is again accompanied by a backswitching of M AP1 .In this case, another transition occurs at a larger posi-tive field corresponding to the reversal of M AP1 .Note that the backswitching of M AP1 is not observed for n AP1 ϭ3 because it would cost too much Zeeman energy.
͑2͒ For n AP1 у4, M AP2 switches first.This results in an extended plateau with constant magnetization around zero field.We point out that in these cases, there is only one stable remanent state of the system in zero field.This is a situation of practical interest for device applications in order to use such a synthetic pinned layer as a reference layer in spin valves or tunnel junctions with perpendicular magnetization.At a larger positive field, the magnetization M AP1 reverses towards positive saturation.
The differences in the magnetization curves of the different systems can be understood as the result, in each sy-Ferri, of an interplay between the Zeeman energy, the exchange energy across the Ru spacer and the pinning energy with the FeMn.The purpose of the following calculations is to relate the values of the AP 1 and AP 2 switching fields with the coupling energy per unit area through the Ru spacer (A Ru ) and the coupling energy per unit area with the antiferromagnetic layer (A pinning ).These calculations assume that the is always able to reach its lowest minimum, i.e., as a first step, we do not try to describe the irreversibility associated with the switching of M AP1 and M AP2 .
Let us first consider the switching of AP 1 when decreasing field from positive saturation.This switching occurs at a positive field when the gain in coupling energy due to the antiferromagnetic coupling through the Ru spacer balances the cost in Zeeman energy due to the negative orientation of M AP1 in the positive field.If t Co is the thickness of the individual Co layers and Ms Co its saturation magnetization, then the bias field of AP 1 , H E,AP1 ͑evaluated as the average of the two switching fields in the AP 1 minor loop͒, is given by: This expression predicts a linear variation of H E,AP1 vs 1/n AP1 , provided that A Ru is constant.If one plots H E,AP1 as a function of 1/n AP1 , a linear dependence of H E,AP1 is experimentally observed, as shown in Fig. 2. Assuming Ms Co ϭ1450 emu/cm 3 ͑bulk value͒, the slope of H E,AP1 (1/n AP1 ) in Fig. 2 allows us to determine A Ru .One obtains A Ru ϭ0.134 erg/cm 2 .2][13] The coercivity of AP 1 can be also easily determined from the AP 1 minor loops.The coercive field increases with n AP1 , as commonly observed in ͑Pt/Co͒ multilayers, from 95 Oe for n AP1 ϭ2 to 200 Oe for n AP1 ϭ6.This is due to an increase of the overall perpendicular anisotropy in AP 1 with the number of Pt/Co interfaces, as well as to the increase of structural defects, which may act as pinning centers for domain wall propagation.
The switching of M AP2 at negative fields occurs when the Zeeman energy brought by the applied field exceeds the cost in energy to overcome both the antiferromagnetic coupling through the Ru spacer and the exchange bias energy with FeMn.The associated coercivity can be clearly seen for n AP1 ϭ4 and 6 from the width of the AP 2 minor loops ͓Figs.1͑c͒ and 1͑d͔͒.Note that the coercivity of AP 2 is larger than for AP 1 for the same number of repeats due to the exchange biasing of AP 2 with FeMn. 9 The bias field of AP 2 , H E,AP2 , can be evaluated by considering the shift of the AP 2 minor loops for n AP1 ϭ4 and 6.One finds H E,AP2 to be around 810 Oe.H E,AP2 is then related to the coupling constants by the expression This relationship allows to calculate A pinning from the knowledge of H E,AP2 .Namely A pinning ϭ0.068 erg/cm 2 .
The values of H E,AP2 in the syFerri are significantly larger than the ones for Pt/Co ML exchange biased with FeMn ͑i.e., about 810 Oe, compared 150 Oe in simple ML ϩFeMn structures͒.
It is also worthwhile to illustrate why the layers switch the way they do and not in the reverse order.Note that, when decreasing field from positive saturation, if M AP2 switched before M AP1 , Eqs. ͑1͒ and ͑2͒ would no longer govern the reversal.Instead, the hypothetical bias field of AP 2 , H E,AP2 (hypo) would be given by the following equation: Comparing the values of H E,AP1 and H E,AP2 (hypo) ͓Eqs.͑1͒ and ͑3͔͒, it is found that M AP2 would reverse before M AP1 only if Taking the previously determined values of A Ru and A pinning one finds that Eq. ͑4͒ is never fulfilled, which explains why M AP1 always switches first when the field is decreased from positive saturation.
Analogously, when the field is increased from negative saturation, it can be shown that M AP2 reverses before M AP1 only if Condition ͑5͒ is fulfilled for n AP1 ϭ3, 4, 5, and 6 and, consistently, it is experimentally observed that M AP2 reverses before M AP1 for n AP1 ϭ4, 5, and 6.Similarly, Eq. ͑5͒ is not fulfilled for n AP1 ϭ2 in agreement with the experimental observation that M AP1 switches first.For n AP1 ϭ3, although inequality ͑5͒ is fulfilled, M AP1 switches before M AP2 ͓see Fig. 1͑c͔͒.This is because the coercive field in AP 2 is always larger than in AP 1 .
In conclusion, it has been shown that pinned synthetic ferrimagnets with perpendicular anisotropy, can be prepared.The bias fields of both ML constituting the syFerri can be tuned at room temperature by varying the relative number of Co/Pt repeats comprised in each ML and their values are determined by the exchange constants A Ru and A pinning .However, an extended plateau with constant magnetization around zero field and a single stable remanent state is obtained when the condition n AP1 ϾA Ru /(A Ru ϩA pinning ) n AP2 is fulfilled.These properties are particularly useful to implement spin valves or tunnel junctions with perpendicular anisotropy and enhanced magnetic stability.

FIG. 1 .
FIG. 1. Normalized Hall amplitude hysteresis loops of syFerri of the form AP 1 /Ru/AP 2 /FeMn, with n AP1 ϭ2, 3, 4, and 6 ͑where n AP1 is the number of Co layers in the AP 1 multilayer͒.The thin and thick arrows indicate the magnetization state of multilayers AP 1 and AP 2 in the syFerri, respectively.