3.1 Introduction

One approach proposed to circumvent the superparamagnetic limit in magnetic recording is thermally or Heat Assisted Magnetic Recording (HAMR) [Ruigrok 00]. However, as pointed out by J. Thiele et al. [Thiele 03], the exponents of the power law variations of $K$ and $M_s$ with temperature are such that the anisotropy field varies more slowly than $K$, requiring temperatures close to or above the Curie temperature to write information. This leads to significant practical problems associated with the head-disk interface, and especially the loss of lubricant [Hsia 02]. As a solution J. Thiele et al. [Thiele 03] proposed the idea of a composite medium of FePt and FeRh. It has been established that the ordered b.c.c. alloy FeRh belongs to the class of materials with a metamagnetic transition [Kouvel 66]: at room temperature $Fe_{x}Rh_{1-x}$ alloy ($x$ $0.5$) is antiferromagnetic, undergoing a transition to the ferromagnetic state at temperatures around $300-400K$ depending on concentration $x$. The remarkable property of FeRh is that it has a similar lattice parameter to FePt. Therefore, the two materials could be grown epitaxially with good interfacial properties [Goto 04]. The advantage of the composite medium is that the antiferromagnetic character of FeRh at room temperature could provide additional thermal stability while the coupling between FePt and FeRh after the metamagnetic transition has occurred, could be used to lower the switching field via an ``exchange spring'' mechanism. J. Thiele et al. [Thiele 03,Thiele 04b] have measured that a coercivity reduction of FePt in $2-3$ times could be expected due to this coupling. The transition antiferro-ferro in FeRh has to be fast enough in order to implement a magnetic recording device. The heating through a laser pulse, in an similar way to 'pump-probe' experiments, yields transition times as low as $500 fs$ [Thiele 04a]. Therefore, it is possible to reverse the magnetization of the FePt in fields comparable to those provided by current write heads at temperatures considerably lower than the Curie point of FePt.

Figure 3.1: Temperature dependence of the coercive field and saturation magnetization of an FePt/FeRh bilayer. Extracted from [Thiele 03].

Figure 3.2: Mechanical analog of an ``exchange spring'' from [Goto 65].

On the other hand, exchange spring magnets have offered an extraordinary possibility to control hysteretic properties of composite media by tuning extrinsic and intrinsic parameters of different magnetic phases. The first work that explained the coercivity of composite thin films with an exchange spring mechanism was due to E. Goto et al. [Goto 65] (see Fig. 3.2), but Goto did not coin the ``exchange spring''. The fingerprint of an exchange spring is the reversible part of the hysteresis loop between the irreversible jumps that corresponds to the motion of the domain wall center towards the hard layer after the pinning of the domain wall at the interface. This reversible domain wall motion can be clearly observed in the experimental loop shown in Fig. 3.3. This part would be absent in the case of independent behavior of the two layers or strongly coupled bilayer. Posteriorly, the exchange spring mechanism was proposed by E. Kneller [Kneller 91] as a method to increase the energy product $BH_{max}$ in permanent magnets. This quantity needs to be maximized in order to obtain a good permanent magnet. The enhancement of the energy product was previously reported in experiments in Ref. [Coehoorn 89]. The hard phase $Nd_2Fe_{14}B$ was embedded in the soft phase $Fe_3B$ matrix. Hence, an important application of exchange spring magnets includes permanent magnets such as $SmCo$ or $NdFeB$ where hard magnetic phase provides high coercivity and soft magnetic phase provides high saturation magnetization.

Figure: Measured hysteresis loop for a Sm–Co/Fe(10 nm) bilayer. The exchange spring behavior of the sample is manifested in the reversible part of the curve before the irreversible jump of the Sm–Co layer at $H_{irr}$. Extracted from Ref. [Jiang 02].

Recently, the use of a general composite magnetic bilayer has been suggested for magnetic recording applications [Thiele 03,Guslienko 04,Victora 05a,Suess 05b]. Unlike conventional exchange spring media, perpendicular magnetic recording requires a new type of exchange spring - perpendicular exchange spring (see Fig. 3.4). Differently from permanent magnets, the properties of this composite media should be optimized to provide a relatively low coercive field suitable for conventional recording heads together with high thermal stability [Suess 05b,Dobin 06]. It has been shown that the properties of soft/hard phases could be optimized independently for coercive field and thermal energy barriers values, providing a real alternative for recording media of new generation [Suess 05b,Suess 07]. In comparison to FePt/FeRh material, this general proposal avoids heating process. However, there is an additional requirement of high energy barriers which does not exist in the FePt/FeRh case due to the anti-ferromagnetic character of FeRh at room temperature which itself provides additional thermal stability.

One can expect that the exchange spring mechanism depends on the coupling properties between the phases. Real concerns have been expressed on the possibility to grow soft/hard materials with grain matching in both layers [Thiele 04b]. Beside this, realistic interfaces between two materials present complicated properties, related to interface roughness, lattice parameters mismatch, diffusion of atoms across the interface, etc. All this situations contribute to the expectation of a reduced exchange value at the interface, which influences the exchange spring performance. The investigation of soft/hard magnetic layer performance as a function of the phenomenological interfacial exchange, $J_s$ constitutes the main goal of the present chapter.