Applied Physics A

, Volume 108, Issue 1, pp 149–153

Magnetic properties of epitaxial-grown exchange-coupled FePt/FeRh bilayer films

Authors

    • School of Materials Science and Engineering, Shanghai Key Lab of D&A for Metal-Functional MaterialsTongji University
  • Chenchong He
    • School of Materials Science and Engineering, Shanghai Key Lab of D&A for Metal-Functional MaterialsTongji University
  • Zhe Chen
    • School of Materials Science and Engineering, Shanghai Key Lab of D&A for Metal-Functional MaterialsTongji University
  • Junwei Fan
    • School of Materials Science and Engineering, Shanghai Key Lab of D&A for Metal-Functional MaterialsTongji University
  • Biao Yan
    • School of Materials Science and Engineering, Shanghai Key Lab of D&A for Metal-Functional MaterialsTongji University
Article

DOI: 10.1007/s00339-012-6862-1

Cite this article as:
Lu, W., He, C., Chen, Z. et al. Appl. Phys. A (2012) 108: 149. doi:10.1007/s00339-012-6862-1

Abstract

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.

1 Introduction

Perpendicular magnetic recording with an exchange coupled composite structure seems to be a way to move beyond the so-called “trilemma” in recording [1]. 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 [2]. To address this problem, several methods have been proposed to reduce Hc to switch magnetization, such as thermally-assisted recording (TAR) [3] or heat-assisted magnetic recording (HAMR) [4], which was first proposed by Thiele et al. [3]. 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 [57]. In experimental study, Thiele et al. have proposed the exchange-spring FePt/FeRh magnetic bilayer as a potential medium for magnetic recording [8]. 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 [8]. 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.

2 Experimental

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

Figure 1(a) shows XRD patterns (θ–2θ) of FeRh thin film. The (001) superlattice peak and (002) fundamental peak of bcc-ordered FeRh phase are clearly observed and this result indicates that the (001) oriented FeRh thin film was successfully fabricated on MgO (100) substrate. Figure 1(b) shows XRD patterns (θ–2θ) of FeRh/FePt bilayer thin film. The (001) superlattice peak and (002) fundamental peak of bcc-ordered FeRh phase and L10-ordered FePt phase are clearly observed in the XRD patterns. Both phases displayed similar growth textures. Characteristic reflections of L10 FePt phase and B2 FeRh phase appeared at angles of 23.95 and 29.68, respectively. The other peaks with no indication in Fig. 1 come from the MgO (100) substrate. Furthermore, the ϕ-scan diffraction patterns of FeRh (001) peak and FePt (001) peak are shown in Fig. 2. As seen in the spectra, very sharp peaks were observed at every 90. This result indicates that the FeRh and FePt possess a fourfold symmetry along the film plane. So the FeRh layer and FePt layer in FeRh/FePt bilayer film can be considered as single crystal films.
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Fig. 1

XRD patterns (θ–2θ) of (a) FeRh and (b) FeRh/FePt thin film

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Fig. 2

ϕ-scan diffraction patterns of (a) FeRh (001) peak and (b) FePt (001) peak

3.2 Magnetic properties

Figure 3 plots the normalized temperature-dependent magnetization curves (MT curves; the normalization factor is defined as the highest saturated magnetization in the temperature-dependent magnetization curve and the value is about 3493) for FeRh/FePt thin film with an applied magnetic field in the direction of perpendicular to film plane. Herein, we define the transition temperature, Ttr, as the temperature at half-change of the magnetization. A sharp rise in magnetization slightly above 100 °C during heating indicates the onset of the first-order AFM to FM phase transition of FeRh alloy layer in FeRh/FePt thin film. During cooling, the onset of the FM to AFM phase transition takes place at temperature slightly below 200 °C. It is also seen that the transition temperature (Ttr) in heating process is higher than that in cooling process. The hysteresis width is about 15 °C, which is smaller than that of FeRh single-layer thin film [9]. This is one of classical intrinsic features of a first-order phase transition. Hysteresis across a first-order phase transition occurs due to the supercooling and/or superheating of the parent phase in the product matrix across the transition.
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Fig. 3

Normalized temperature-dependent magnetization curves (MT) of FeRh/FePt thin film

Figure 4 shows the out-of-plane and in-plane hysteresis loops of FeRh/FePt bilayer thin film at different temperatures. During measurements, the applied magnetic field is fixed at 15 kOe while temperature is changed for each MH loop. The hysteresis loop measured at low temperature (which is less than phase transition temperature of FeRh alloy) is square (Mr/Ms = ∼1) and shows a typical perpendicular magnetization with a single switching field in the reversal process. The value of out-of-plane coercivity is around 7 kOe and it is clear that the coupling between FeRh layer and FePt layer is antiferromagnetic/ferromagnetic interaction. Consequently, there is an exchange bias effect in the hysteresis loops at low temperature, as reported in previous studies [10]. It can be seen from Fig. 4(a) that the exchange bias field is about 300 Oe for FeRh/FePt bilayer film at −25 °C. The exchange bias effect is also observed at temperatures of 25 and 75 °C with reduced exchange bias field. When the temperature is higher than phase transition temperature of FeRh alloy, the AFM phase of FeRh alloy starts to transform to FM phase and the magnetization behavior of FeRh/FePt bilayer becomes completely different, as shown in Fig. 4. The hysteresis loops measured at high temperature (>75 °C) are of two-step processes and the coupling between FeRh layer and FePt layer change to exchange-spring-like behavior. The magnetization reversal process changes from a single switching field process to a two-step reversal process.
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Fig. 4

Out-of-plane and in-plane hysteresis loops of FeRh/FePt bilayer thin film at different temperatures

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 [11]. 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 [12].

The FeRh/FePt bilayer reveals strong interlayer coupling at the soft/hard magnetic interface. To identify the out-of-plane switching behavior of exchange-spring-like FeRh/FePt bilayers, the soft layer domain wall width can be calculated by the formula [1, 13]:
$$ L_{\mathrm{dw}}^{\mathrm{soft}} = \sqrt{\frac{2A_{\mathrm{soft}}}{H_{\mathrm{appl}}M_{\mathrm{soft}}}},$$
(1)
where Happl is the applied field, Asoft and Msoft are the stiffness constant and saturation magnetization of the soft layer, respectively. Figure 4(d) shows the MH loops of FeRh/FePt bilayer measured at 130 °C with typical exchange-spring-like behavior. For the out-of-plane MH loop, soft-layer switching occurred at the applied field ∼0.9 kOe (point A). In this field, the domain wall is pinned near the hard/soft interface mostly in the soft layer. The domain wall width at the switching field (point A: ∼0.9 kOe) is about \(L_{\mathrm{dw}}^{\mathrm{soft}}=15\ \mbox{nm}\) by the experimental parameters of the FeRh soft layer. As the applied field increases, the domain wall in the soft layer is compressed and penetrates more into the hard layer. Once the applied field reaches 4.8 kOe (point B), the domain wall de-pins from the hard/soft interface and propagates through the entire hard layer.

4 Conclusions

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.

Acknowledgements

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.

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© Springer-Verlag 2012