Role of reversal incoherency in reducing switching field and switching field distribution of exchange coupled composite bit patterned media

Thomas Hauet (1) , E Dobisz (1) , S Florez (1) , J Park (1) , B Lengsfield (1) , B.D. Terris (1) , O. Hellwig (1)

1 HGST San Jose Research Center

Model exchange coupled composite media, namely ͓Co/ Pd͔ 5 / Pd͑(t) / ͓Co/ Ni͔ 3 / Co multilayers, deposited onto prepatterned substrates have been investigated for potential use in bit patterned recording. Optimizing the thickness of the Pd interlayer allows decreasing the switching field ͑Hc and switching field distribution SFD ͒while maintaining thermal stability. The dependence of the remanent coercivity on the external field angle indicates that the gain in H C and SFD originates from the incoherent magnetization reversal introduced by slightly decoupling the hard and soft multilayer stacks. This tendency is confirmed by further reductions in H C and SFD , when inserting another Pd interlayer within the hard ͓Co/ Pd͔ 5 stack. Bit patterned recording ͑BPR͒ is a leading candidate to extend the densities in magnetic data storage beyond those achievable by conventional continuous perpendicular magnetic recording ͑PMR͒ based on granular media. BPR involves a lithographically patterned media, where each magnetic island corresponds to a single bit. At remanence, each discrete island maintains a single domain state with its uniform magnetization aligned to a well defined uni-axial aniso-tropy axis ͑commonly perpendicular to the disk͒. The thermal stability of a BPR magnetic bit is given in the macrospin approximation by the product K u V, where K u is the magnetic anisotropy energy density and V is the volume of one island. The bits are considered thermally stable for KuV = 60 k B T, where k B is the Boltzman constant and T is the temperature. Increasing the areal density implies decreasing V. However, any loss in V needs to be compensated for by an increase in K u in order to maintain a proper thermal stability. But the maximum anisotropy is limited by the maximum field that the write head can provide in a hard disk drive. Therefore, improving both thermal stability and writability of bit pat-terned media ͑BPM͒ is an important issue for reaching areal densities higher than 1 Tb/ in 2. Another critical issue for implementing BPM is that the switching field distribution ͑SFD͒, i.e., the bit-to-bit variation in coercive field needs to be narrow enough to secure exact addressability of individual predefined bits without overwrit-ing adjacent bits. The SFD has the two following main components: the dipolar interactions between neighboring islands within the array and the so-called intrinsic SFD of each individual island. 1,2 Depending on the head field gradient, the head-media spacing and the write-synchronization tolerances , a SFD value lower than 1 kOe is required for areal density beyond 1 Tb/ in 2 , considering a media with a static coercive field of around 5 to 7 kOe ͑Ref. 3͒. This value is about two times lower than the SFD measured in current PMR media. While in a single spin approximation, writability, and thermal stability are correlated via K u , it has been recently demonstrated that an improved PMR writability to thermal stability ratio can be achieved by changing the magnetization reversal mechanism. For instance, incoherent reversal is obtained in exchange-spring or exchange coupled composite ͑ECC͒ structures. 4,5 These heterogeneous systems consist of at least two coupled layers with different anisotropies. During the reversal process, the soft layer ͑SL͒ moment starts reversing first and forces the hard layer ͑HL͒ moment to follow via the interlayer exchange coupling. The torque or the spring produced by the SL moment allows decreasing the switching field of the HL stack, while conserving thermal stability. A large number of theoretical calculations 6 and experimental studies have been reported on PMR media.

Mots clés: switching field distribution; magnetic recording; magnetism; exchange-spring; material science; PVD; magnetometry

Domaines: Science des matériaux [cond-mat.mtrl-sci]

Thomas Hauet, E Dobisz, S Florez, J Park, B Lengsfield, et al.. Role of reversal incoherency in reducing switching field and switching field distribution of exchange coupled composite bit patterned media. Applied Physics Letters, 2009, ⟨10.1063/1.3276911͔⟩. ⟨hal-01345308⟩