Perpendicular Magnetic Anisotropy of CoFeB\Ta bilayers on ALD HfO2

This contribution reports on the Perpendicular Magnetic Anisotropy (PMA) of a thin film stack of the common high-κ dielectric ALD HfO2 on which a CoFeB\Ta bilayer was deposited. So far, PMA has only been reported for film stacks with Ox\CoFe(B) interfaces making use of MgO [1] or TaOx [2] as oxide or with an adjacent crystalline heavy metal like Pt [3,4]. Currently, standard MTJ materials stacks with PMA using MgO as a dielectric are most commonly used for Voltage Control of Magnetic Anisotropy (VCMA) [5,6]. However, MgO has a relatively low dielectric permittivity (κ). A higher κ- value of the gate dielectric allows an increase in injected charge, thus likely enhancing the VCMA effect. A necessary first step toward enhanced VCMA is the control of the anisotropy on high-κ materials. CoFeB was deposited on ALD HfO2 at room temperature and capped with Ta. As deposited stacks do not show PMA. However, annealing in N2 at temperatures above 200°C is shown to induce PMA. Anisotropy is analyzed using Kerr and Vibrating Sample Magnetometry (VSM). The CoFeB thickness and the annealing temperature dependence of the anisotropy are studied in the ranges of 0.6 – 2.4 nm and 200°C to 350°C. The structural properties are probed with X-Ray Diffraction and Transmission Electron Microscopy. The impact of diffusion and crystallization onto the anisotropy is assessed. The VCMA effect is analyzed through Hall measurements. The present results indicate that PMA can be induced in a CoFeB\Ta bilayer with no recourse to MgO. The MgO - ferromagnet interface is thought to contribute to the PMA through the hybridization of the ferromagnet 3dz orbitals and the oxide 2pxy and 2pyz orbitals (corresponding to Fe-O and Co-O bonds) [7,8]. Such hybridization is a possible explanation for our observations of PMA of high-κ Ox\CoFeB\Ta systems. A variety of high-κ dielectric/ferromagnet interfaces might now be investigated for PMA applications. This is of potential interest for the next generation of spintronics and MRAM devices. [1] S. Ikeda et al., Nature Materials 9, 9 (2010) [2] G. Yu et al., Applied Physics Letters 105, 102411 (2014) [3] U. Bauer et al., Applied Physics Letters 101, 192408 (2012) [4] B. Rodmacq et al., Physical Review B 79, 024423 (2009) [5] T. Maruyama et al. Nature nanotechnology 4, 158 (2009) [6] K. Kita et al., J. Appl. Phys 112, 033919 (2012) [7] H. Yang et al., Physical Review B 84, 054401 (2011) [8] B. Rodmacq et al., Journal of applied physics 93, 7513 (2003)status: publishe