Exchange coupled composite FePt/TbCo/[Co/Ni]_N films with an TbCo interlayer

Heat-assisted magnetic recording (HAMR) is regarded as the next-generation storage technology, which will replace the current perpendicular magnetic recording.^1,2 L1₀-FePt based films with very high coercivity are the proposed recording media. In order to decrease their coercivity in HAMR, the films have to be heated above 475 °C, the Curie temperature of FePt, so that they can be written by the current magnetic head field.³ However, high temperature is a big challenge for the disk.

Much effort has been made to decrease the Curie temperature by doping a third element, such as Ni and Cu in FePt.^4,5 However, magnetic anisotropy decreases also, partially attenuating the advantage of FePt, which has very high magnetic anisotropy. Exchange coupled composite (ECC) is another approach to decrease the coercivity. By covering a soft layer on FePt, the coercivity of the exchange coupled composite decreases greatly.^6,7 Among the ECC media, FePt/FeRh is unique.⁸ FeRh has a transition temperature T_t. Below T_t, FeRh is antiferromagnetic, but it becomes a soft magnetic material when the temperature is above T_t. Thus, above the transition temperature FePt/FeRh is an ECC bilayer, and low coercivity is expected. Nevertheless, at the room temperature the coercivity is large as FeRh becomes antiferromagnetic. Since the transition temperature can be modified by doping Ir, FePt/FeRhIr is an excellent HAMR media. However, FeRh is difficult to epitaxially grow on small FePt grains. Graded magnetic media comprises more than two soft layers plus the hard L1₀-FePt layer.^9,10 From the softer layer in the case of two soft layers (or the softest layer of more than three soft layers) to the hard layer, the anisotropy gradually changes, following z² where z is the distance from the soft layer to the hard layer. This decreases the pinning field between the soft and the hard layers, which is dominated by the anisotropy difference. Further decrease of coercivity is demonstrated.

In a ferrimagnetic TbCo, the magnetic moments of Tb and Co are antiparallel. Usually, there is a compensation temperature T_M, which changes with the composition of Tb and Co. At T_M, the moment of Tb is equal to that of Co because of their different temperature dependent magnetization, and then the film is antiferromagnetic (AFM) without net magnetization, as shown in Fig. 1(a). When the temperature is above T_M, the moment of Tb is smaller than that of Co because the magnetization of Tb decreases faster than that of Co with the temperature increase, thereafter the net magnetization and its direction are determined by Co. The film is TM-dominated. When the temperature is below T_M, the net magnetization and its direction are determined by Tb. The film is then RE-dominated. When a RE-dominated film is inserted between FePt and [Co/Ni]_N, the magnetization of Co in TbCo is parallel to that of FePt and [Co/Ni]_N, but the magnetization of Tb is anti-parallel to that of FePt and [Co/Ni]_N. At that point, the net magnetization of TbCo is antiparallel to the magnetizations of FePt and [Co/Ni]_N. As shown in Fig. 1(b), a ferrimagnetic configuration is formed in the tri-layer structure, and then the TbCo layer can pin the domain wall. With the temperature increases up to the compensation temperature of the TbCo layer, the antiferromagnetic TbCo layer suppresses the domain wall motion, and large coercivity can be expected. However, if the temperature is higher than the compensation temperature, the net magnetization of the TbCo layer is parallel to those of FePt and [Co/Ni]_N (Fig. 1(b)). The anisotropy of TbCo is larger than [Co/Ni]_N, so a graded perpendicular ECC composite forms. Small coercivity is then expected. In the paper, we demonstrated that a TM-dominated TbCo interlayer could greatly decrease the coercivity of FePt/TbCo/[Co/Ni]_N when the temperature increases from room temperature up to 200 °C.

All the samples were prepared on the single crystal MgO substrates. A 5-nm-thick FePt layer was deposited at 550 °C by co-sputtering. The TbCo layer was deposited at room temperature, with fixed Co sputtering power and varied Tb power. The thickness of TbCo is about 10 nm. [Co/Ni]_N multilayers were then also deposited at room temperature.¹¹ The periodic numbers are 5 and 10, and the corresponding thicknesses are 4 nm and 8 nm, respectively. Finally, a 3-nm-thick Pt layer was covered to protect the composite from oxidation. The magnetic properties of the composite were measured by vibrating sample magnetometry (VSM) and Magneto-optical Kerr effect (MOKE) in the polar mode.

The hysteresis loops of the TbCo films are shown in Fig. 2. All the films are perpendicular. For Tb₃₁Co₆₉, the coercivity H_C is 2.63 kOe. With the Tb content decreasing, the coercivity increases and reaches the maximum in the current experiments at Tb₂₈Co₇₂, H_C = 9.94 kOe, nearly the maximum of the magnetic field of the MOKE system. There is no hysteresis loop for Tb₂₇Co₇₃. It seems the coercivity of the sample is too large to be measured, and/or the net magnetization is too small to be detected. After that, coercivity decreases with further Tb content decreases, and H_C is 2.15 kOe for Tb₂₅Co₇₅. Also, it is noted that the loop direction of the saturation magnetization faces left for the TbCo film, whose Tb content is no less than 28 at %. However, the loop faces right for TbCo films whose Tb is no more than 26 at%. Thus, Tb₂₇Co₇₃ is very close to an antiferromagnetic film without net magnetization at room temperature, and the compensation composition is close to 27 at% at room temperature.¹² Then, the films whose Tb content are larger than 27 at % are RE-dominated, and their compensation temperatures are higher than the room temperature. But the films whose Tb content are smaller than 27 at % are TM-dominated, and their compensation temperatures are lower than the room temperature. Further studies shows that the compensation temperature depends on the films’ thickness and the buffer layers. The temperature becomes smaller for thinner films.

In order to confirm the exchange coupling between the antiferromagnetic-like Tb-Co and ferromagnetic [Co/Ni]₅, the trilayers of Tb₂₈Co₇₂/Pt(t)/[Co/Ni]₅ have been prepared by inserting a non-ferromagnetic Pt. The hysteresis loops change with the thickness of the Pt layer. For Tb₂₈Co₇₂/[Co/Ni]₅ coercivity is about 1.16 kOe, largely reduced form Tb₂₈Co₇₂ by coupling with the soft [Co/Ni]₅. When a thin Pt of 0.5 nm is inserted in the interface of Tb₂₈Co₇₂ and [Co/Ni]₅, coercivity further decreases down to 0.76 kOe. However, two-step loop appears when the Pt thickness is 2.5 nm. The magnetization reversals of Tb₂₈Co₇₂ and [Co/Ni]₅ occur at the magnetic field of 9.97 kOe and 0.46 kOe, respectively. The coercivity of single [Co/Ni]₅ is 0.52 kOe.

As shown in Fig. 3(a) and (b), the coercivity of FePt is 18.3 kOe, and that of FePt/[Co/Ni]_N is 11.2 kOe. When the Tb₂₈Co₇₂ is inserted in the interface of the FePt and [Co/Ni]₅, coercivity of the trilayer is nearly the same, about 11.3 kOe, but the slope of the hysteresis loop of FePt/Tb₂₈Co₇₂/[Co/Ni]₅ is 3 times smaller than that of FePt/[Co/Ni]₅. This implies that Tb₂₈Co₇₂ suppresses the exchange coupling between FePt and [Co/Ni]₅, fastening the magnetization reversal of [Co/Ni]₅, but delaying the magnetization reversal of FePt. However, in the case of inserted Tb₃₂Co₆₈, coercivity is reduced to 8.24 kOe, and the slope is 3 times larger than that of FePt/[Co/Ni]₅. The rapid magnetization reversal and coercivity reduction accord with the synthesis ferrimagnetic sandwich structure, and the small coercivity of Tb₃₂Co₆₈.¹³ 

Usually there is an inter-diffusion in the interface between two layers, which will result in slight change of the composition. Also, slight temperature increase is expected in usage. In the current experiments, Tb-rich Tb₃₂Co₆₈ is then used to prepare the ECC trilayers. Figure 4 summarizes the magnetic properties of FePt/Tb₃₂Co₆₈/[Co/Ni]₅. As shown in Fig. 4(a), the saturation magnetization of FePt/Tb₃₂Co₆₈/[Co/Ni]₅ at first increases while the temperature increases, and then decreases with further increase of the temperature. Since Tb₃₂Co₆₈ is RE-dominated, its net magnetization is antiparallel to those of FePt and [Co/Ni]₅, as indicated in Fig. 1(b). At that point, the net magnetization of FePt/Tb₃₂Co₆₈/[Co/Ni]₅ is small because the magnetization is the algebraic sum of FePt, Tb₃₂Co₆₈ and [Co/Ni]₅. With the temperature increasing, the magnetization of Tb₃₂Co₆₈ decreases quickly, but those of FePt and [Co/Ni]₅ change slightly because of their high Curie temperature.^14,15 Then the net magnetization of FePt/Tb₃₂Co₆₈/[Co/Ni]₅ increases. If Tb₃₂Co₆₈ crosses over its compensation temperature, it becomes TM-dominated and its magnetization is parallel to that of FePt and [Co/Ni]₅. Thus, further magnetization increase is observed at 450 K. When the temperature is higher than 500 K, Tb₃₂Co₆₈ is paramagnetic with very small magnetization. Also, the magnetization of FePt and [Co/Ni]₅ decrease with the temperature. Then, the magnetization of FePt/Tb₃₂Co₆₈/[Co/Ni]₅ decreases greatly.

Coercivity of FePt/Tb₃₂Co₆₈/[Co/Ni]₅ decreases quickly with the temperature increase, two times faster than the coercivity reduction of FePt/[Co/Ni]₅ (Fig. 4(b)). Coercivity of FePt/Tb₃₂Co₆₈/[Co/Ni]₅ at 450K, is one third of the coercivity at room temperature. At high temperature, the Tb₃₂Co₆₈ is TM-dominated and its magnetization is parallel to that of FePt and [Co/Ni]₅. The trilayer becomes a graded exchange coupled composite, and then small coercivity is realized.

The recoil curve of FePt/Tb₃₂Co₆₈/[Co/Ni]₅ is measured at 200 °C, and shown in Fig. 4(c). When a negative magnetic field is removed, the magnetization returns back along the hysteresis loop. Even when the negative field is larger than the composite’s coercivity, remanence magnetization is nearly the same. Moreover, the nearly flat dependence of hard layer coercivity on the angle between the applied field and the film’s normal direction is shown in Fig. 4(d). It indicates good angle tolerance of the composite film. Both recoil curves and angle tolerance demonstrate the exchange-spring characteristics of the graded ECC FePt/Tb₃₂Co₆₈/[Co/Ni]₅ film at high temperature.

In summary, a RE-dominated ferrimagnetic Tb₃₂Co₆₈ is inserted in the interface of FePt/[Co/Ni]₅. Then a synthesized ferrimagnetic configuration is formed in the FePt/Tb₃₂Co₆₈/[Co/Ni]₅ composite at room temperature, and it has high coercivity. When the temperature of the composite increases, the coercivity of FePt/Tb₃₂Co₆₈/[Co/Ni]₅ decreases greatly, to a level only one third of the coercivity at room temperature. This is due to the formation of a graded exchange coupled composite, because Tb₃₂Co₆₈ becomes a TM-dominated magnetic structure at high temperature. Small coercivity is expected. Therefore, the ferrimagnetic TbCo layer can be used to improve the temperature dependent magnetic properties of FePt/[Co/Ni]₅ for heat assisted magnetic recording.

This work is supported by Seagate Technology.