The 2010 measurement by Endo and co-workers on sputtered MgO/Co40Fe40B20/Ta structures established that the magnetic easy axis becomes perpendicular as the CoFeB thickness is reduced below about 1.3 nm, that an electric field applied across the MgO layer modifies the perpendicular anisotropy energy and coercivity at room temperature, and that CoFeB layers below roughly 0.9 nm are superparamagnetic, with the electric-field effect surviving below their blocking temperature. The paper left the two underlying microscopic questions open, and they remain identifiable open questions in the later literature: what structural or chemical nonuniformity produces the superparamagnetic clusters in sub-nanometer sputtered CoFeB and sets their blocking temperature, and which microscopic mechanism - interfacial charge-induced d-state occupation change, ionic migration, or interfacial chemistry - actually acts when a gate field is applied to this sputtered stack. Substantial adjacent progress exists (magnetometric cluster characterization in the same stack, interface TEM of 1 nm CoFeB, and operando X-ray spectroscopy on epitaxial Fe, Co and Ni at MgO), but there is as yet no direct structural identification of the superparamagnetic clusters and no operando spectroscopic mechanism determination for the sputtered CoFeB/MgO system itself, so the structural origin of the superparamagnetic clusters and the field-acting mechanism at the sputtered MgO/CoFeB interface remain unresolved.

Background

Co40Fe40B20 (CoFeB) is an amorphous ferromagnetic alloy used as the electrode of magnetic tunnel junctions (MTJs), devices in which two ferromagnetic layers are separated by an MgO insulating barrier and whose tunneling magnetoresistance is the basis of magnetic random-access memory (MRAM). In ultrathin films the magnetostatic (shape) energy favors magnetization lying in the film plane, but the MgO/CoFeB interface contributes a perpendicular magnetic anisotropy (PMA) that grows in relative importance as the film thickness shrinks; below a critical thickness the easy axis turns out of plane. A superparamagnetic film is one in which the ferromagnetic material forms isolated clusters small enough that thermal fluctuations overcome the anisotropy energy barrier, so the assembly shows no hysteresis above its blocking temperature (the temperature below which each cluster's magnetization is frozen on the measurement timescale) and responds like a Langevin paramagnet with a giant moment. Voltage-controlled magnetic anisotropy (VCMA) is the change of the magnetic anisotropy energy, per unit interface area, produced by an electric field applied across an adjacent insulator; theory predating the source paper (Duan, Velev, Sabirianov, Zhu, Chu, Jaswal, Tsymbal, Phys. Rev. Lett. 101, 137201 (2008), DOI 10.1103/PhysRevLett.101.137201, https://doi.org/10.1103/PhysRevLett.101.137201) predicted that the surface charge induced in a ferromagnetic metal by such a field changes the occupation of transition-metal dd orbitals and hence the magnetocrystalline anisotropy, and Maruyama, Shiota, Nozaki, Ohta, Toda, Mizuguchi, et al. had demonstrated the effect experimentally in atomic layers of Fe on MgO (Nat. Nanotechnol. 4, 158 (2009), DOI 10.1038/nnano.2008.406, https://doi.org/10.1038/nnano.2008.406). Against this background, M. Endo, S. Kanai, S. Ikeda, F. Matsukura and H. Ohno (Appl. Phys. Lett. 96, 212503 (2010), DOI 10.1063/1.3429592, https://doi.org/10.1063/1.3429592) studied sputtered stacks in which a CoFeB layer of nominal thickness t≤2t \le 2 nm is sandwiched between MgO and Ta, with the electric field EGE_G applied across the MgO. Using anomalous Hall measurements (the Hall voltage of a ferromagnet contains a term proportional to its out-of-plane magnetization, from which hysteresis loops and anisotropy energies can be extracted), they found: the easy axis is in-plane for t≥1.33t \ge 1.33 nm and perpendicular at t=1.16t = 1.16 and 1.021.02 nm; at t=1.33t = 1.33 nm the perpendicular anisotropy energy per area changes roughly linearly with EGE_G (about 33 μ33\ \muJ/m2^2 per V/nm) at room temperature; at t=1.16t = 1.16 nm the perpendicular coercivity is modulated by EGE_G; and at t≈0.75t \approx 0.75 nm the film is superparamagnetic with a blocking temperature near 100 K, the electric-field effect being observable at 30 K. Superparamagnetism of ultrathin CoFeB layers was known earlier in CoFeB/MgO/CoFeB junctions (Y. Jang, C. Nam, J. Y. Kim, et al., Appl. Phys. Lett. 2006, DOI 10.1063/1.2370876, https://doi.org/10.1063/1.2370876), but its microscopic origin in sputtered films, and the identity of the field-acting mechanism at the MgO/CoFeB interface, were not established.

Problem Statement

Determine, at the microscopic level, the two unresolved origins of electric-field-tunable magnetism in ultrathin sputtered MgO/Co40Fe40B20/Ta structures, as flagged by Endo, Kanai, Ikeda, Matsukura and Ohno (Appl. Phys. Lett. 96, 212503 (2010)): (1) the structural and chemical origin of the superparamagnetism observed in CoFeB films of roughly 0.9 nm and below - identify whether structural discontinuity (island growth), thickness nonuniformity, or phase segregation/interfacial chemistry produces the magnetic clusters, and establish how that structure sets the blocking temperature (about 100 K in the source samples) and the cluster relaxation dynamics; and (2) the microscopic mechanism by which an electric field applied across the MgO modifies the perpendicular magnetic anisotropy and coercivity of this sputted stack - identify which of interfacial charge-induced change of CoFeB d-state occupation, ionic migration/redox, or interfacial chemical modification acts. The two parts share one objective, the microscopic understanding of the same ultrathin sputtered CoFeB/MgO(Ta) system on which the electric-field effect was measured.

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