Materials with PMA have larger anisotropy energy compared to most of in-plane anisotropy materials. In addition, for the patterned device, the magnetization of PMA materials is more uniform and does not suffer from thermal instability due to magnetization curling observed at the edge of in-plane case 14. From the point of view of STT switching current, it is easier to reverse the magnetization of PMA free layer than the in-plane anisotropy one 15, 16. As choice for materials with PMA to be used in GMR or TMR devices, three main categories have been explored.
Multilayer-based spin valve and magnetic tunnel junctions
Multilayers based on transition metal (TM) like Co, Fe and CoFe and noble metal such as Pt, Pd and Au show perpendicular magnetic anisotropy for a certain range of thickness and number of periods 17–20. Although these materials have been extensively studied for magneto-optics and magnetic recording, especially bit-patterned media 21–27, they have also gained interest for spintronics applications 15, 16, 28–35. The magnetic properties of these materials such as saturation magnetization and anisotropy energy can be easily tailored changing the number of bilayers and also by adjusting the thicknesses of TM and Pt (or Pd). The commonly used multilayers so far are Co/Pd, Co/Pt and Co/Ni due to their relatively larger PMA compared to Co/Au or Co/Ag for example. Firstly, multilayers were used as part of spin valves in the soft layer and hard layer without the need of anti-ferromagnetic layer which is required for the in-plane magnetoresistive case. In an earlier work on Co/Pd multilayers, Carcia et al. demonstrated that for Co thickness less than 0.8 nm the multilayer showed PMA which has been attributed to the surface anisotropy at Co and Pd interfaces and the strain in Co thin layers 17.
For magnetoresistive structures, Joo et al. used FeMn as an antiferromagnetic (AFM) layer to induce exchange bias to one of the Co/Pd multilayers 36. Their structure was similar to what was used in conventional in-plane spin valve (SV). However, since it is easy to tailor the coercive field of the multilayer by changing the thickness of TM and Pd (or Pt) and the number of periods, the AFM layer is not required. This makes the structure simpler and sometimes called pseudo-spin-valve. Law et al. used Co and CoFe to further improve the softness of the soft layer and also to achieve a better magnetoresistance ratio (MR) 29.
Although much work has been done on spin valve, there is a growing interest in using multilayers with PMA in MTJ devices. The first work started with MTJ based on amorphous AlOx tunnel barrier, as AlOx is easy to be deposited in comparison to crystalline MgO. Actually, for the cases of MTJ with MgO barrier, coherent tunnelling which provides high TMR is possible only for the (001) phase of MgO. This is a challenging requirement from the materials point of view as there is a need for a judicious choice of all layers deposited below the tunnel barrier. Co/Pd and Co/Pt multilayers grow with (111) fcc structure on selected seedlayers such as Ta, Pd, Pt or Ru. So far, MTJ based on amorphous AlOx with Co/Pt as soft and hard layers showed a relatively low TMR of about 25% in the best case 37–40. This is mainly due to the low spin polarization of the Co/Pt multilayer compared to CoFe or CoFeB alloys as a consequence of the insertion of Pt or Pd non-magnetic layer.
The MTJ based on MgO requires a tight control of growth of the magnetic electrode deposited below. The best choice is a ferromagnetic layer that can favour the growth of MgO with (001) phase so that coherent tunnelling could be obtained 41. Unfortunately, the multilayers studied so far based on the alteration of TM and noble metal grow in fcc (111) phase. This is the main reason for low TMR obtained with these structures even with the insertion of very thin CoFeB at both MgO interfaces to help improve the growth of MgO tunnel barrier 42.
More recently, Yakushiji et al. showed that by using very thin Co and Pd (or Pt) of less than 0.2 nm, it is possible to achieve a high TMR in MgO based junctions 43. The multilayer called superlattice can be annealed at high temperature exceeding 350 °C which is important for promoting the (001) phase of MgO, essential for high TMR. In comparison, the conventional multilayers (referring to relatively thicker Co and Pd) will face degradation of their intrinsic properties at such high temperature because of the Co and Pd (or Pt) intermixing. As can be seen in Fig. 4, a TMR of 62% could be obtained in a structure where the soft layer is composed of [Co(0.2 nm)/Pd(0.2 nm)]4 and 0.8 nm thick Co60Fe20B20. The hard layer is composed of 1.3 nm CoFeB and 16 nm TbFeCo. Although Co/Pd and Co/Pt superlattices can be annealed at 350 °C without degradation of their magnetic properties, the existence of TbFeCo as part of the hard layer limits the annealing temperature. It is then expected that larger TMR could be obtained if one could find a replacement material for the hard layer.
Figure 4. (a) Magnetization versus field and (b) resistance versus field for perpendicular MTJ-bsed MgO with a free layer made of [Co(0.2 nm)/ Pd(0.2 nm)]4/Co60Fe20B20(0.8 nm). Reprint with permission from Fig. 3 of Ref. 43. Copyright (2010), American Institute of Physics.
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FeCoB with PMA
As discussed earlier, STT-MRAM with PMA has a better scalability in comparison with in-plane STT MRAM in terms of low critical switching current density (Jc) and high thermal stability. In addition, the device with PMA has improved shape uniformity, which is critical to control the driving current distribution for STT induced switching.
In the early development stage, rare earth (RE) and transition metal (TM) alloys, such as TbFeCo and GdFeCo 44, 45, were adopted to obtain PMA in MTJ devices. However, the magnetic property of such materials is sensitive to oxidation during the fabrication process, especially for sub-100 nm size. This unavoidable oxidation is still a challenge to integrate RE materials with STT-MRAM. Another method to obtain PMA is to use multilayer structures, such as TM/Pd or TM/Pt, as discussed in the previous section. However, due to the strong spin–orbit coupling of Pd and Pt, the damping constant α is found to be several times larger than in regular TM and results in a large STT switching current Ic according to
where H is the applied field, Hk is the effective anisotropy field, Ms and t are the saturation magnetization and thickness of the soft (free) layer, respectively. In Eq. (1), A is the device area, e is the charge of electron, ħ = h /2π (where h is the Planck constant) and η is the spin polarization.
In 2010, first principle calculations conducted by Shimabukuro predicted that the hybridization of Fe-3d and O-2p orbitals may introduce PMA in an MgO/Fe/MgO system 46. In the same year, Ikeda et al. demonstrated PMA in CoFeB–MgO based MTJ devices as can be seen from Fig. 547. The easy axis of magnetization of a CoFeB layer is in-plane of the film for a relatively large thickness (more than 1.5 nm or so) while it is oriented in the out-of-plane direction for small thickness. The PMA energy density (Ku) of the CoFeB–MgO system increases with reducing CoFeB thickness. The achieved Ku could support a memory cell size as small as 40 nm with 10 years thermal stability. On the other hand, the STT study shows that Jc0 of the system has a fairly low value of 4 MA/cm2, despite the thermal stability factor of 46. Jc0 is the value of Jc when an STT current pulse of 1 ns is applied. Since the system is based on crystallized CoFeB–MgO structure, a TMR of more than 100% was achieved. The demonstrated perpendicular CoFeB–MgO MTJ system combines sufficient thermal stability, low switching current and relatively high TMR signal. Having these characteristics at the same time is a real breakthrough for STT-MRAM.
Figure 5. Resistance versus magnetic field for CoFeB/MgO/CoFeB with 150 nm device size annealed at 300 °C. (a) and (b) are cases where the magnetic field is applied out-of-plane and in-plane, resepectively. (c) TMR as function of annealing temperature. Reprinted by permission from Macmillan Publishers Ltd: Nature Materials (Fig. 4 of Ref. 47), copyright (2010).
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Besides the CoFeB thickness factor, PMA of a CoFe–MgO system also relies on stack structures and process conditions. Recently, Worledge et al. found that a Ta seed layer is essential to obtain PMA in the system 48. Structures with Ru seed layer were not favourable to obtain PMA in the same CoFeB thickness range. Hao et al. reported that the annealing process is also a critical step to build up PMA and needs to be optimized in order to achieve simultaneously good TMR, low resistance and low Jc049. Although the demonstrated PMA in thin CoFeB layer shows a strong potential application for STT-MRAM, it should be noted that this will result in a larger Jc. In addition, TMR is also degraded as the CoFeB thickness decreases. Thus, the CoFeB thickness in a perpendicular MTJ stack should be optimized in order to make a trade off among PMA, TMR and Jc.
To reduce Jc and yet avoid the decay of TMR, the interfacial PMA can be also applied to in-plane MTJ devices. For an MTJ with easy axis in the film plane, the STT current Ic is defined by
where H⊥ is the PMA component which partially cancels the effect of the out-of-plane demagnetization field. Although the easy axis of the free layer is still in the film plane, its PMA component is expected to reduce Jc. Based on this design, Amiri et al. reported a Jc reduction by a factor of 2.4, which is more than the corresponding reduction in the free layer volume alone 50.
FePt materials for ultra-high density
The cell size of STT MRAM has to keep shrinking in order to catch up with the increase of density. However, the thermal stability factor Δ of each cell, defined by KuV /kBT, should be above a value of 40 to resist thermal fluctuation and secure 10 years thermal stability 16. Since V is reduced with size shrinking, Ku has to be increased to maintain the value of Δ. For an MTJ with a diameter d of 20 nm, if the soft layer thickness is 2 nm for example, the required Ku should be larger than 2.5 Merg/cm3 to secure Δ above 40. The energy Ku will dramatically increase to 10 Merg/cm3 for a diameter of 10 nm and further break 40 Merg/cm3 for 5 nm diameter. One of the material candidates to provide such high Ku is L10-FePt. In 2006, Seki et al. integrated L10-FePt in a GMR device and achieved a Ku value of 34 Merg/cm3 at room temperature 51. However, growing FePt with L10 ordered phase requires a substrate temperature as high as 500 °C, which brings a process integration issue with CMOS transistor. In addition, since a special seed layer/substrate such as MgO is required to introduce texture growth before FePt deposition, the material and structure options are very limited. The same group further demonstrated the STT effect in an FePt/Au/FePt GMR structure, where Jc was up to 108 A/cm2 due to the high Ku value, weak spin polarization and large damping constant of FePt 51. Two years later, a lower Jc of 2.7 × 106 A/cm2 and a TMR of 60% were demonstrated in MgO based MTJ consisting of CoFeB/(Pd/Co) free layer and FePt/CoFeB reference layer 52. The relatively low Jc and high TMR signal are mainly attributed to the crystallized CoFeB–MgO system. However, FePt in the demonstrated MTJ stack only serves as the reference layer, which does not contribute to the thermal stability of the storage layer (free layer).