Magnetic properties of epitaxial-grown exchange-coupled FePt/FeRh bilayer films
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- Lu, W., He, C., Chen, Z. et al. Appl. Phys. A (2012) 108: 149. doi:10.1007/s00339-012-6862-1
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In this paper, (001) textured FeRh/FePt bilayer thin film was fabricated by sputtering and the temperature-dependent magnetic behavior of FePt/FeRh bilayers was investigated in detail. The magnetic regime passes from exchange bias to exchange spring when the temperature increases from low to high, resulting from the first-order antiferromagnetic (AFM) to ferromagnetic (FM) phase transition in ordered FeRh alloy layer. Controlling the temperature-allowed modification of the hysteresis loops of exchange-spring-like FeRh/FePt bilayer due to the nanoscale soft/hard interface exchange coupling, our experimental results clearly show that the coercive field decreases strongly at the temperature where FeRh completely transforms to ferromagnetic state. In an exchange-spring-like FeRh/FePt bilayer film, the out-of-plane magnetization reversal process was in two steps and resulted from domain wall nucleation and propagation from the FeRh layer into the FePt layer.
Perpendicular magnetic recording with an exchange coupled composite structure seems to be a way to move beyond the so-called “trilemma” in recording . High magnetocrystalline anisotropy (Ku) materials such as L10-ordered FePt alloy are used to overcome the thermal stability of magnetic recording media at small grain sizes. However, high Ku materials typically possess large coercivity (Hc), which raises the writability issue, because the maximum field that can be obtained using a perpendicular writing head is approximately 1.7 T . To address this problem, several methods have been proposed to reduce Hc to switch magnetization, such as thermally-assisted recording (TAR)  or heat-assisted magnetic recording (HAMR) , which was first proposed by Thiele et al. . The coercivity of the film is considerably reduced by heating the medium to a temperature which is close to its Curie temperature Tc, before writing signals are sent to it. Although the writing field can be greatly reduced in this way, side effects arise—especially for materials with high Tc such as FePt (Tc∼480 °C). One approach for improving the writability of a high-Ku media is to employ exchange-coupled or exchange-spring media in which isolated magnetic grains comprise both hard and soft magnetic phases. During switching, the soft layer rotates at small applied fields, exerting a torque on the hard layer because of strong exchange coupling between the soft and hard layers.
The magnetization reversal mechanism, switching characteristics, and thermal stabilities in exchange-spring or exchange-coupled composite films have been discussed extensively in theoretical work [5–7]. In experimental study, Thiele et al. have proposed the exchange-spring FePt/FeRh magnetic bilayer as a potential medium for magnetic recording . The advantage of this medium is that the antiferromagnetic character of FeRh at room temperature could provide additional thermal stability while the coupling between hard magnetic FePt layer and soft magnetic FeRh layer after the AFM to FM phase transition of FeRh layer could be used to lower the switching field via an exchange-spring mechanism . From a general point of view, the system represents a soft–hard magnetic structure, in which the understanding of the magnetic properties has a fundamental nature. This work aims to investigate the effect of temperature and phase transition behavior on the magnetic properties of FeRh/FePt bilayer.
Bilayer FeRh (25 nm)/FePt (25 nm) single-crystal thin film was sputter-deposited onto MgO (100) substrate by using Fe50Rh50 and Fe50Pt50 targets at substrate temperature of around 450 °C. The base pressure of the chamber was less than 1×10−8 Torr. The composition of the thin film was measured by an energy dispersion fluorescence X-ray spectrometer (EDX). The crystallographic structure was characterized by X-ray diffraction (XRD) using Cu Kα radiation. A vibrating sample magnetometer (VSM) with maximum applied field of 15 kOe was used to measure the magnetic properties in the temperature range from −25 to 250 °C. For the measurements of exchange biased hysteresis loops, the samples were firstly cooled from room temperature to −196 °C under an applied magnetic field of 10 kOe.
3 Results and discussion
3.1 Crystallographic structure
3.2 Magnetic properties
If the nucleation field HC1 (shown in Fig. 4(d)) marks the point at which the soft phase starts to deviate non-uniformly from the saturated state, a second critical field exists, i.e., the reversal field HC2 (shown in Fig. 4(d)), at which the hard phase becomes unstable and gives rise to the switching of the whole system. At high temperature, the FeRh/FePt bilayer shows positive nucleation field (HC1). Consequently, there is a decrease in squareness (from around 1 to around 0.9) with increasing temperature due to the start of the magnetization reversal at positive field values. The occurrence of positive nucleation field (HC1) is a peculiar prediction of a micromagnetic model developed for perpendicular bilayers where the shape anisotropy contribution is also taken into account . Also, as can be seen in Fig. 4, the reversal field HC2 is decreased with increasing temperature. This is mainly because the fraction of FM FeRh phase is increased with increasing temperature, causing the reduction of the switching field. As shown in Fig. 3, the antiferromagnetic phase in FeRh layer transforms to ferromagnetic phase with increasing temperature which is higher than Ttr. Higher temperature resulted in more transformed ferromagnetic FeRh phase, which interacts with hard FePt phase by exchange coupling. Thus, in present study, the exchange coupling strength in FeRh/FePt bilayer thin film is mainly controlled by the fraction of transformed ferromagnetic phase during the first-order AFM-FM phase transition of FeRh alloy. As a result, the coercivities of the FeRh/FePt bilayers decrease with the increasing of the fraction of ferromagnetic FeRh phase (with increasing temperature) due to the strong exchange coupling between FeRh and FePt layers. It clearly shows that the coercive field of FeRh/FePt bilayer decreases strongly at the temperature where AFM FeRh completely transforms to ferromagnetic state. It can be concluded that the soft ferromagnetic phase in FeRh layer is mainly used to manipulate the coercivities of the FeRh/FePt bilayers through the exchange coupling between the FeRh and FePt layers. The value of coercivity (switching field) is inversely proportional to the thickness of soft layer if the hard layer thickness has been fixed and theoretically depends on the ratio Msofttsoft/Mhardtthard, where M and t are the saturation magnetization and thickness of film layer, respectively .
In summary, (001) textured FeRh/FePt bilayer thin film was fabricated by sputtering. FeRh alloy layer in FeRh/FePt bilayer thin film shows a clear first-order antiferromagnetic–ferromagnetic phase transition in the temperature range from around 100 to 200 °C. The magnetic regime passes from exchange bias to exchange spring when the temperature increases from low to high, resulting from the first-order antiferromagnetic-to-ferromagnetic phase transition of ordered FeRh alloy layer. Controlling the temperature-allowed modification of the hysteresis loops of exchange-spring-like FeRh/FePt bilayer due to the nanoscale soft/hard interface exchange coupling, in an exchange-spring-like FeRh/FePt bilayer film, the out-of-plane magnetization reversal process was in two steps and resulted from domain wall nucleation and propagation from the FeRh layer into the FePt layer.
The present work was supported by National Natural Science Foundation of China (Grant No. 50901052), Program for Young Excellent Talents in Tongji University (Grant No. 2009KJ003) and “Chen Guang” Project (Grant No. 10CG21) supported by Shanghai Municipal Education Commission and Shanghai Education Development Foundation.