Structural Modification of Magnetic Properties in Perpendicularly Magnetized Co / Pd Multilayer for M RAM

⎯ The structural modification of magnetic properties in as grown films of Co/Pd multilayer with perpendicular magnetic anisotropy (PMA) has been systematically studied. The thickness dependence of element multilayer for both the Co thickness tCo and the Pd thickness tPd as parameters was investigated. Occurred PMA should be correlated to between bi-layers and following the enhancement of interface interaction due to the increase of repetition number bi-layers. The optimal experimental result was obtained for Co = 1.7 Å and Pd = 8.0 Å realizing very good square shape hysteresis curve and high coercive field Hc of 1.5 kOe.


I. INTRODUCTION
ince it was firstly introduced by Suna, et al., in 1985 [1] Co/Pd multilayer with perpendicular magnetic anisotropy (PMA) has been intensively investigated due to their potential for high recording medium as well as spin electronic device materials. For ultra high-density magnetic recording media application [2], recently the heat assisted magnetization reversal is believed as one prospective scheme. The concept can be adapted to a magnetic random access memory (MRAM) [3,4]. In thermally assisted MRAM, the switching field is temporally reduced by heating the selected memory cell in the writing process. Consequently the magnetic anisotropy of the storage layer should be large enough to insure thermal stability when is patterned into very small lateral dimension without lose their energy barrier i.e. one of essential key for practical read-write process in spin electronic device. Thus, PMA which suitable for thermally assisted MRAM becomes an essential issue of material magnetic for spin electronic application.
Various efforts for the PMA mechanism have been reported, that the atomic scale is broken symmetry at the interface [1], the stress induced anisotropy in Co and/or Co-Pd alloy [5,6], hybridization of Co 3d and Pd 4d [7]. However, the origin mechanism for perpendicularly anisotropy appearance is still unclear. After that, major researcher reported on enhancement of perpendicular anisotropy by using some treatments such as inserted various under layer (single or multi), buffer layer, using selected substrate e.g. Si (111), MgO (111) and annealing at air condition.
In case for TAMR application, high PMA is one criterion and material magnetic owing medium Curie T C temperature ranging from 120°C to 200°C is required.
The medium of the T C ensure low power consuming in practical application. It is believed that the reduction of field required for magnetization reversal correspond to the reduced of coercive field H c at elevating temperature. However, recent report indicate that the reduced of field required for switching in TAMR is not simple which material dependence of switching field is one of the variables. Moreover, the strength of medium PMA is closer related to MRAM application [8,9]. In this paper, the structural control of magnetic properties in Co/Pd multilayer is investigated. By tailoring Co layer, not only fundamental magnetic property of Co/Pd multilayer system can be easily modified, but also thermo-magnetic property material [10]. Here, preliminary material investigation is reported and is discusses on magnetic property at room temperature.

II. THEORY
The perpendicular anisotropy constant (K ⊥ ) is generally described (Carcia et al, 1984) as: Where K s is the interface anisotropy energy at each of the two interacting bi-layer period, volume anisotropy (K v ) and the shape anisotropy of the Co layers (2πM 0 2 d). Clearly here, the balanced of three element anisotropy define the strength PMA.
The interface anisotropy are mainly attributed from interaction between bi-layer in multi-layer system. In case of ultra thin film with perpendicular anisotropy has one possibility to improve interfacing used under layer film. Later, many experiment reports successfully improve the strength of PMA by using under layer film.
The second element for PMA rises from the K v . The magnitude of the K v is larger than the crystallographic value for hexagonal Co and may be due to magnetostrictive effects [11]. A tensile stress in Co due to lattice mismatch of Co and Pd brings into being a large magnitude of K v . Here, tailoring Co thickness in Co/Pd structure will modify PMA.
The third element for PMA comes from the shape anisotropy of Co layer. The factor is origin from Co element and magnetization tends to in-plane film.

III. EXPERIMENTAL
The structured multilayer films were fabricated on glass substrates by a tandem type DC magnetron sputtering with a multi-cathode system (Anelva SPC-350). The background pressure before deposition was below 5.0×10 −7 Torr and the Ar pressure during the deposition was 20 m Torr at 1.0×1.0 cm glass substrate. where t Co = 1.0 Å; 1.7 Å; 2.0 Å; 3.0 Å and 3.6 Å In order to evaluate the magnetic property, vibrating sample magnetometer (VSM) is performed and measurement carried out in room temperature for as grown sample. It is noted that the applied field is perpendicular to the films, which is the case for all measurements presented in this paper. However, it is compensated by poor square shape and a large field required for switching due to complicated multi domain configuration as shown at Fig. 1(a). The H c gradually decreases with the increase of the t Co exception for very thin Co thickness layer of 1.0 Å. It is can be attributable to an imperfect interface between Co and Pd layer. However, samples loss their square shape of the M-H curve when t Co > 2.0 Å. As alloying multilayered Co/Pd dependence of elements layer is discussed, so there is a condition for both Co and Pd layers element completely compounded to be alloy realization. Here, stress induced anisotropy promote a perpendicular anisotropy magnetic realization. In increasing number of bi-layer, the huge interface anisotropy energy occur breakthrough symmetry, thus realizing a large perpendicular anisotropy magnetic. Furthermore, the raise of individually element cannot change the alloying composition; however only enrich Co (transition metal element = TM) or Pd (rare earth element = RE) contain in Co/Pd multilayer structure.

IV. RESULT AND DISCUSSION
Regarding Equation 1 for case of TM rich multilayer, the enhancement of both volume and shape anisotropy is suggested dominantly compete with interface anisotropy energy. Since the K shape of Co tends to in-plane anisotropy so the poor square shape of M-H curve is obtained as shown in Fig. 1b. (t Co ≥ 2.0 Å). Contrast for RE rich multilayer, remaining RE element layer reduces interface interaction between bi-layer. Here, the reduced of strength PMA hardly change square shape of M-H curve. The highest H c of 1.5 kOe for t Co = 1.7 Å can be suggested with the minimum miscible elements of alloying Co/Pd in surface.
The structural composition dependence of saturated magnetization (4πM S ) for two series is depicted in Fig. 2. For all case, saturated magnetization of t Pd = 5.5 Å series is much larger than t Pd = 8.0 Å. This is can be related to the volume anisotropy (K v ) and the shape anisotropy (K shape ) of the remaining Co layers after alloying bi-layer which is represented to ratio of TM and RE element in bi-layer structure. Minimum value of ratio TM-RE for t Pd = 5.5 Å series is 0.44 (t Co /t Pd = 2.4/5.5) and 0.125 (t Co /t Pd = 1.0/8.0) for t Pd = 8.0 Å series. From this data, the remaining Co layer for t Pd = 5.5 Å series were much larger than t Pd = 8.0 Å series within result, a large magnetization saturated of t Pd = 5.5 Å series.  Fig. 3 show the coercive field H c as a function of structural composition in two various Co/Pd multilayer systems. In general, the H c decrease with the increase of Co thickness layer except for t Co = 1.0 Å of t Pd = 8.0 Å series. The H c increases with the decrease of t Co reflecting the property of the surface anisotropy enhanced at the thinner thickness. The highest coercive value is obtained 1.56 kOe for structural composition t Co = 1.7 Å of t Pd = 8.0 Å. The rapid reduction of H c at t Co = 1.0 Å of t Pd = 8.0 Å series can be attributable to an imperfect interface between Co and Pd layers V. CONCLUSION The structural control of magnetic properties in as grown films of Co/Pd multilayer has been systematically studied. The thickness dependence of element multilayer for both the Co thickness t Co and the Pd thickness t Pd as parameters was investigated. PMA occurred should be correlated to alloying between bi-layer and following the enhancement of interface interaction due to the increase of repetition number bi-layer. The optimal experimental result was obtained for Co = 1.7 Å and Pd = 8.0 Å realizing very good square shape hysteresis curve and high coercive field H c of 1.5 kOe.