Interfacial exchange coupling and magnetization reversal in perpendicular [Co/Ni]N/TbCo composite structures

Interfacial exchange coupling and magnetization reversal characteristics in the perpendicular heterostructures consisting of an amorphous ferrimagnetic (FI) TbxCo100–x alloy layer exchange-coupled with a ferromagnetic (FM) [Co/Ni]N multilayer have been investigated. As compared with pure TbxCo100–x alloy, the magnetization compensation composition of the heterostructures shift to a higher Tb content, implying Co/Ni also serves to compensate the Tb moment in TbCo layer. The net magnetization switching field Hc⊥ and interlayer interfacial coupling field Hex, are not only sensitive to the magnetization and thickness of the switched TbxCo100–x or [Co/Ni]N layer, but also to the perpendicular magnetic anisotropy strength of the pinning layer. By tuning the layer structure we achieve simultaneously both large Hc⊥ = 1.31 T and Hex = 2.19 T. These results, in addition to the fundamental interest, are important to understanding of the interfacial coupling interaction in the FM/FI heterostructures, which could offer the guiding of potential applications in heat-assisted magnetic recording or all-optical switching recording technique.

tunable, and T-dependent interfacial coupling and net magnetization switching fields (H c⊥ ) 20,21 . Such FI-based heterostructures might possess potential applications in HAMR and AOS, since its large and temperature-sensitive H c⊥ and H ex can be used to store information while the strongly coupled FM layer can improve the properties of readout. Therefore, understanding and clarifying the related coupling and switching mechanism in the FM/FI heterostructures should be of great importance for ultrahigh density recording in these new storage techniques.
Due to the great potential applications in data storage technology, in recent years some research work has been performed regarding the FM/FI composite structures 12,[15][16][17] . For instance, S. Romer et al. investigated the temperature dependence of large exchange bias effect in TbFe/[Co/Pt] system 12 . The dependence of interfacial exchange coupling on the stoichiometry of TbFe layer and repetition numbers of Co/Pt was analyzed by C. Schubert et al. 17 . However, the net magnetization switching properties were not well analyzed in these studies. Moreover, except Co/Pt (Pd) multilayer, no other FM layer material has been employed. In our previous work, the [Co/Ni] N multilayer has been employed to couple with TbCo as the reference layer of perpendicular spin valves, by which we have achieved high GMR signal and large switching plateau 20 . The Co/Ni multilayer, which owns relatively higher spin polarization and smaller Gilbert damping factor than Co/Pt, has been considered as a potential material in MRAMs for high spin torque efficiency 22 . In order to thoroughly understand the interfacial exchange coupling, magnetization reversal, and their relationship in FM/FI heterostructures, in this work we have fabricated several series of samples of glass /Ta(3) /Cu(3) /[Co(0.28)/Ni(0.58)] N /Co(t Co ) /Tb x Co 100-x (t) /Ta(5) (layer thickness in unit of nm). The influences of Tb contents x, Co/Ni repetition number N, and thicknesses (t Co and t) of the additional Co and TbCo layers will be discussed. Note that for all these samples the easy axes of both the FI TbCo and FM [Co/Ni] N layers are maintained perpendicular to the film plane. Figure 1 displays the out-of-plane magnetic hysteresis loops measured by Physical Property Measurement System (PPMS) for the heterostructure samples of [Co/Ni] 5 /Tb x Co 100-x (12) with various Tb content x. As defined in the loops, the magnetic coercivity H c⊥ in the central loop corresponds to the total net magnetic moment switching, while the antiferromagnetic coupling field H ex denotes the magnetization switching of either TbCo or Co/Ni layer that has lower magnetic moment. Clearly, for different x the loops exhibit different H ex and H c⊥ . Especially for x = 33.0%, no central switching loop can be detected, indicating that the magnetization of 12-nm thick Tb 33 Co 67 layer is balanced with that of Co/Ni and thus the sample has nearly zero net magnetic moment.

Results
In order to clearly see the variation trends, Tb content dependence of H c⊥ and H ex are shown in Fig. 2, the H c⊥ values of pure Tb x Co 100-x are also given for comparison. Note that, for the pure TbCo alloy film, the H c⊥ firstly increases with increasing Tb content, at x ≈ 22% it starts to decrease. It is noticed that the largest H c⊥ occurs when the magnetic moments of Tb and Co are approaching compensated according to the inverse relation to the magnetization 23 , where K eff and M net denote the effective magnetic anisotropy and net saturation magnetization of the heterostructure, respectively. As a result, for our 12-nm thick TbCo film, the magnetization compensation composition at RT is approximately 22%. Interestingly, for the perpendicularly exchange-coupled [Co/ Ni] 5 /TbCo composite film, a similar relationship between Tb content and H c⊥ happens. Nevertheless, the RT compensation composition has been moved to a higher Tb content of x ≈ 33%, verifying that the Co/Ni atoms also serve to compensate the magnetic moment of Tb atoms in the Tb-rich TbCo layer 20,21 . Therefore, for x < 33%, the heterostructure is [Co/Ni] 5 rich in magnetic moment and the H ex comes from magnetization switching of the TbCo layer. On the contrary, for x > 33% it becomes TbCo rich and the H ex corresponds to the switching field of Co/Ni multilayer. As shown in Fig. 2(b), the H ex reaches a value as high as 3.22 T at x = 25.5%, significantly larger than the exchange field observed in normal FM/AFM systems. With the increase of x, it firstly decreases rapidly until x reaches the compensation point, after that it becomes nearly stable. The observed H ex tendency of the heterostructure can be well interpreted by the following formula 17,24 , where J ex is the interlayer coupling strength, M s and t represent the saturation magnetization and thickness of the switched layer, while K p is the magnetic anisotropy energy of the pinning layer, respectively. At x < 33%, the TbCo layer is switched since it has a lower moment than the Co/Ni layer, so the strong decrease of H ex with the increase of x is mainly caused by the increased magnetization of TbCo layer. At x > 33%, the Co/Ni layer is switched, further increase of Tb content will only affect the K p of TbCo slightly 25 , thus giving rise to the observed slight reduction of H ex .
Considering that the Tb moment enlarges much more rapidly than the transition metal as the measurement temperature decreases, the H c⊥ and H ex will have different temperature-dependent variation trends for samples with different Tb contents, which can be clearly seen in Fig. (3). As shown in Fig. 3(a), the H c⊥ value for x = 25.5% decreases slowly with the increase of temperature when T is below 200 K, above which it drops dramatically. Here T = 200 K is the transition point of magnetization compensation temperature (T Mcomp ) for the sample of x = 25.5%, i.e. the sample is Co/Ni rich at T > T Mcomp and TbCo rich as T < T Mcomp . According to our analyses in the preceding parts, maximum H c⊥ should take place at the T Mcomp where magnetic moments are compensated. However, the H c⊥ value keeps increasing as T decreases from 200 K to 50 K, we attribute this increase to the enhanced PMA strength of TbCo pinning layer at reduced temperatures. For the x = 30.0% sample the temperature dependence of H c⊥ is similar to that of x = 25.5% case, except that it has a higher T Mcomp of about 260 K. However, for the sample of x = 33.0% which has a T Mcomp of RT, the varying trend of H c⊥ is distinctly different from the other two samples. Instead of a monotonic slow increase as T decreases from T Mcomp = RT, the H c⊥ value firstly decreases and subsequently increases after reaching a minimum at about 200 K. We attribute such behavior to the combined action of enhanced PMA strength and net magnetic moment. The initial decrease of H c⊥ originates from the enlarged uncompensated moment of the heterostructure due to the much rapidly increased Tb moment in TbCo layer, whereas with further decreasing temperature the PMA enhancement plays a dominant role which leads to the slow increasing behavior, similar to the varying trend of samples of x = 25.5% and 30.0% at T < 200 K. Interestingly, as shown in Fig. 3(b), the H ex value at T = T Mcom is always the smallest for all the three samples. The fast increase of H ex with T at T > T Mcomp can be ascribed to the reduced magnetization of TbCo switched layer. Nevertheless, at T < T Mcomp the switching field change of Co/Ni layer is very likely related to the enhanced PMA of TbCo pinning layer.
In addition to the Tb content, the TbCo layer thickness also plays an important role on the magnetization switching of [Co/Ni] 5 /TbCo heterostructures. Figure 4(a) shows the H c⊥ and H ex values for samples with a fixed Tb content of x = 30.0% but various Tb 30 Co 70 layer thicknesses. Three representative out-of-plane magnetic loops of t = 8.0, 13.5, and 20 nm, measured by Vibrating Sample Magnetometer (VSM), are inserted in Fig. 4(a). With the increase of Tb 30 Co 70 thickness, we find the H c⊥ varies also non-monotonically. It firstly increases with increasing t and then begins to decrease at t = 13.5 nm, which means that t ≈ 13.5 nm is the magnetization compensation thickness for the [Co0.28/Ni0.58] 5 /Tb 30 Co 70 (t) heterostructure. Meanwhile, no central switching loop takes place from the magnetic hysteresis loop of t = 13.5 nm sample, confirming that the total net magnetic moment is compensated. Accompanied with the change of H c⊥ , the exchange coupling field H ex also varies but in a different way. At t < 13.5 nm, the sample is Co/Ni rich, so the coupling field comes from the TbCo layer switching, which certainly increases with the decrease of TbCo layer thickness. As soon as t is over 13.5 nm, the Co/Ni layer with fixed magnetization and thickness will be switched. Therefore, owing to the increased PMA of thicker TbCo layer, the H ex exhibits a weak increase. Note that the H ex value for the samples of t < 12 nm is not given here because it is far beyond the highest magnetic field of 2.2 T supplied by our VSM. Figure 4  Based on these results, we calculate the interfacial coupling strength J ex is up to 4.4 ± 0.3 mJ/m 2 , such magnitude is comparable to the value found in other exchange-coupled FI/FM structures 12,17 , but greatly higher than that of the FM/AFM systems. Furthermore, we selected 12 nm-thick Tb 30 Co 70 as the switching layer and investigated the PMA effect of the FM pinning layer on the H ex and H c⊥ . The perpendicular anisotropy energy K p of the FM pinning layer was firstly modulated by changing the repetition number N of the [Co/Ni] N multilayer. Figure 5(a) shows the H c⊥ and H ex of the heterostructure, as well as the effective uniaxial anisotropy energy K p of the single Co/Ni layer as a function of N for the [Co/Ni] N /Tb 30 Co 70 (12.0) samples. The K p was calculated according to K p = M s H k /2, where H k is the saturation magnetic field of in-plane magnetic loops. Apparently, the K p increases monotonically with N due to the increased Co/Ni interfaces. From the maximum H c⊥ we can conclude that the magnetization is compensated at N = 4. Therefore, the magnetization of Co/Ni is dominant and the TbCo layer will be switched at N > 4. Although the magnetization and layer thickness of TbCo layer are fixed, we can still see an obvious H ex increase, which can be ascribed to the enhanced K p of the FM layer. By optimizing the layer structure, we achieved large H c⊥ up to 1.31 T and H ex of 2.19 T simultaneously at the N = 4 case, which will be of great importance for practical applications. In addition, the PMA strength of Co/Ni layer was manipulated by tuning the thickness of an additional Co interlayer inserted between the [Co/Ni] 5 and Tb 30 Co 70 (12.0) layers as well. The interlayer Co thickness t Co is kept below 1.2 nm to ensure the easy axis of Co/Ni along perpendicular direction. A maximum K p occurs at t Co = 0.28 nm, further increasing t Co will give rise to K p decrease. Such non-monotonic variation is the result of competition between the interfacial PMA and in-plane shape anisotropy. For this sample structure, the net magnetization is always dominated by [Co/Ni]/Co and increases with t Co , thus leading to the monotonic reduction of H c⊥ , as shown in Fig. 5(b). Meanwhile, the H ex follows a similar variation trend to the K p , again demonstrating that the exchange coupling field is strongly dependent on K p of the pinning layer.
In conclusion, we have investigated the antiferromagnetic exchange coupling interactions and net magnetization switching in perpendicular [Co/Ni] N /TbCo composite structures. The magnetization compensation composition, compensation thickness of FM or FI layer, as well as compensation temperature for the coupled heterostructures have been clarified. By controlling the magnetization, thickness and PMA strength of the FM and FI layers, a wide range of variation in both net magnetization switching field H c⊥ and exchange coupling field H ex are realized. The calculated interfacial coupling strength at RT is as strong as 4.4 ± 0.3 mJ/m 2 , leading to a large H ex value even exceeding 3.0 T, which is significantly higher than the normal FM/AFM systems. These results offer us valuable information for practical applications in data storage technology and profound understanding of the fundamental exchange coupling mechanism.

Methods
Different series of samples, in a structure of glass/Ta(3)/Cu(3)/[Co0.28/Ni0.58] N /Co (t Co )/Tb x Co 100-x (12)/Ta(5) (layer thickness in unit of nm), were deposited sequentially at ambient temperature in a Kurt J. Lesker magnetron sputter system with a base pressure better than 1 × 10 −8 Torr. The TbCo alloy layer was fabricated by co-sputtering from pure Tb and Co targets, their relative atomic concentration was controlled by varying the sputtering power of Tb and determined by X-ray Photoelectron Spectroscopy (XPS). Magnetic properties were characterized by Vibrating Sample Magnetometer and Physical Property Measurement System.  (12) and (b) the interlayer Co thickness t Co for the samples of [Co0.28/Ni0.58] 5 /Co(t Co )/Tb 30 Co 70 (12).