Fyzikální ústav Akademie věd ČR

Seminar Tuesday, 13/05/2014 15:00

Speakers: Soon Cheol Hong (Department of Physics and Energy Harvest-Storage Research Center, University of Ulsan, Ulsan 680-749, Republic of Korea)
Place: Na Slovance, main lecture hall
Presented in English
Organisers: Department of Condensed Matter Theory

Abstract: Ferromagnetic films that can provide perpendicular magnetocrytalline anisotropy (PMCA) are indispensable constituents in spin-transfer torque (STT) MRAM that utilizes spin polarized tunneling current to switch magnetization. Two criteria have to be satisfied for practical usage of high-density magnetic storage - low switching current (I_SW) and thermal stability. Small volume of a bit is favored to lower I_SW, but detrimental for the thermal stability. However, the small volume can be compensated by large PMCA, while retaining the thermal stability.
In this talk we will discuss roles of nonmagnetic elements in enhancement of PMCA energy. Significant enhancement of PMCA of an Fe(001) surface capped by 4d and 5d transition metal monolayers is found, in particular, an extremely large PMCA of +10 meV/Ir in Ir-capped Fe(001). The origin of the extremely large PMCA will be discussed by calculating the spin-channel decomposed contribution and electronic structure analyses. Strong 3d–5d band hybridization in the minority spin state is responsible for the sign changes of the MCA from parallel to perpendicular. A new metastable phase of the body-centered-tetragonal ruthenium (bct–Ru) is identified to exhibit a large PMCA, whose MCA energy is as large as 150 μeV/atom, two orders of magnitude greater than those of 3d magnetic metals. Further investigation over the range of tetragonal distortion suggests that the appearance of the magnetism in the bct–Ru is governed by the Jahn-Teller spit e_g orbitals. Moreover, from band analysis, MCA is mainly determined by an interplay between two e_g states, d_x2−y2 and d_z2 states, as a result of level reversal associated with tetragonal distortion.