Anisotropic magnetoresistance: A 170-year-old puzzle solved
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nuary 2021 Vol. 64 No. 1: 217531 https://doi.org/10.1007/s11433-020-1618-7
Anisotropic magnetoresistance: A 170-year-old puzzle solved Gerrit E.W. Bauer 1
1,2*
Advanced Institute for Materials Research, Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan; 2 Zernike Institute for Advanced Materials, University of Groningen, Groningen 9747 AG, Netherlands Received August 18, 2020; accepted September 8, 2020; published online November 17, 2020
Citation:
G. E. W. Bauer. Anisotropic magnetoresistance: A 170-year-old puzzle solved, Sci. China-Phys. Mech. Astron. 64, 217531 (2021), https://doi.org/ 10.1007/s11433-020-1618-7
Thomson [1], the later Lord Kelvin, reported an increased electric resistance in iron and nickel when the magnetization is parallel rather than normal to the current direction, an effect now called anisotropic magnetoresistance (AMR). The AMR allows sensing magnetic fields by simply measuring the resistance change when the magnetization realigns. While the magnetoresistance of magnetic tunnel junctions is larger, AMR sensors are attractive by their simplicity and robustness. Alloys of the transition metals Fe, Ni, and Co are the materials of choice because they combine a large AMR ratio with other convenient properties [2]. In a convergence and culmination of methods and ideas developed in the past decades, a collaboration led by Zhe Yuan from Beijing Normal University (theory) and Yizheng Wu from Fudan University (experiments) recently solved the conundrum of the AMR for the generic alloy CoxFe1−x [3]. The AMR is a relativistic effect, caused by the Zeemanlike interaction of the electron spin with velocity-dependent magnetic fields transverse to the electron wave vector, or, on average, the electric current direction. In metallic magnets, the majority and minority-spin electrons carry the current in parallel. The low-resistance channel dominates transport by short-circuiting the high resistive one. When the magnetization is parallel to the current, however, electron spins precess in the relativistic magnetic fields. Rotating the magnetization relative to the current direction mixes up spin channels, blocks the short circuit, and increases resistance. *Corresponding author (email: [email protected])
Atomic-like s-d model theories of the AMR capture the essentials of the AMR for various materials [4], but depend on adjustable parameters and therefore do not have predictive power. First-principles calculations based on density functional theory, on the other hand, are very successful in quantitatively capturing many properties of transition metal magnetic systems, including the AMR [5]. However, without a deeper analysis such “numerical experiments” often provide only limited insight into the underlying physics. The topology of the electronic band structure throughout the Brillouin zone is a very active research subject. “Accidental” degeneracies and the associated geometric Berry phase acquired by electronic wave packets affect many physical properties. In this ve
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