CoFeAlB alloy with low damping and low magnetization as a candidate for spin transfer torque switching

Abstract

We investigate the effect of Al doping on the magnetic properties of the alloy CoFeB. Comparative measurements of the saturation magnetization, the Gilbert damping parameter α and the exchange constant as a function of the annealing temperature for CoFeB and CoFeAlB thin films are presented. Our results reveal a strong reduction of the magnetization for CoFeAlB in comparison to CoFeB. If the prepared CoFeAlB films are amorphous, the damping parameter α is unaffected by the Al doping in comparison to the CoFeB alloy. In contrast, in the case of a crystalline CoFeAlB film, α is found to be reduced. Furthermore, the x-ray characterization and the evolution of the exchange constant with the annealing temperature indicate a similar crystallization process in both alloys. The data proves the suitability of CoFeAlB for spin torque switching properties where a reduction of the switching current in comparison with CoFeB is expected.The alloy CoFeB is widely used in magnetic tunneling junctions in combination with MgO barriers due to the large magnetoresistance effect originating in the spin filtering effect [1][2][3][4]. For the application in magnetic random access memories, the switching of the magnetization of the free layer via spin transfer torque (STT) with spin polarised currents is a key technology. However, the required currents for the switching process are still large and hinder the applicability of this technique. The critical switching current density for an in-plane magnetized system is given by [5]where e is the electron charge, α is the Gilbert damping parameter, M S is the saturation magnetization, t f is the thickness of the free layer, H ext is the external field, H K is the effective anisotropy field and η is the spin transfer efficiency. From the expression it is clear that, concerning material parameters, J c0 is ruled by the product αM 2 S . For out-of-plane oriented layers, the term 2πM S vanishes and then J C0 is proportional to αM S [6]. Even in the case of using pure spin currents created by the Spin Hall effect, the required currents are proportional to factors of the form α n M S with n = 1, 1/2 [7]. A proper strategy to reduce the critical switching currents is then defined by reducing the saturation magnetization. This can be achieved by the development of new materials or the modification of known materials with promising properties. Since the compatibility with a MgO tunneling barrier and the spin filtering effect must be guaranteed together with industrial applicability, the second option is clearly an advantage by reducing M S in the CoFeB alloy. In this case, a critical point is that this reduction must not be associated with an increase of the damping parameter α.

Reduced Exchange Interactions in Magnetic Tunnel Junction Free Layers with Insertion Layers

“…Our data reveal that the exchange constant of the MgO/ CoFeB/MgO samples is much lower than that for thicker CoFeB samples, i.e., 19−28 pJ m −1 . 13,14 This indicates that the exchange constant is significantly affected by interfaces and reduced dimensions. Moreover, insertion of the W layer further reduces the exchange constant of the CoFeB layer, as seen in Figure 3a.…”

“…The exchange constant can be determined using dynamic methods such as Brillouin light scattering (BLS) and spin-transfer ferromagnetic resonance (ST-FMR). , However, these methods have significant limitations when applied to the study of magnetic tunnel junction materials and devices. For example, BLS cannot typically be used to study ultrathin films.…”

“…19-28 pJ m -1 . [12,13] This indicates that the exchange constant is significantly affected by interfaces and reduced dimensions. Moreover, insertion of the W layer further reduces the exchange constant of the CoFeB layer as seen in Figure 3a.…”

High-temperature Magnetodielectric Bi(Fe0.5Mn0.5)O3 Thin Films with Checkerboard-Ordered Oxyg

“…Furthermore, although the magnetic nature of the highly strained material is quite well established to date, the magnetic damping of the BFMO is not. Magnetic relaxation processes play an important role in the magnetization switching speed and switching power of spin-transfertorque-(STT-)based devices, especially for materials where the total magnetization is relatively low, since it directly translates into low switching currents [55,56]. Possibly more exciting are the important and intrinsic applications that a ferromagnetic material with strong magnetodielectric coupling could provide in the spin-wave propagation field.…”

Spin-torque switching mechanisms of perpendicular magnetic tunnel junction nanopillars

“…All the above mentioned parameters such as 𝑀 𝑠 , 𝛼, 𝐾 𝑢 , 𝐴, and 𝑡 are the experimental values for the pMTJ free layers studied in [12]. For comparison, we have also studied free layers with 19 pJ/m (as this value is reported for thick CoFeB films [15]). Given these parameters, the domain wall width, 𝜆 = 2√A/K eff , and the critical diameter 𝑑 𝑐 , 𝑑 𝑐 = 16 𝜋 √𝐴 K eff ⁄ , the diameter below which the thermally activated switching is expected to be coherent, are listed in Table 1.…”