Perspective of Spintronics Applications Based on the Extraordinary Hall Effect

Abstract

Extraordinary Hall effect (EHE) is a spin-dependent phenomenon that generates voltageproportional to magnetization across a current carrying magnetic film. Magnitude of the effect can be artificially increased by stimulating properly selected spin-orbit scattering events. Already achieved sensitivity of the EHE-based sample devices exceeds 1000 Ω/T, which surpasses the sensitivity of semiconducting Hall sensors. Linear field response, thermal stability, high frequency operation, sub-micron dimensions and, above all, simplicity, robustness and low cost manufacture are good reasons to consider a wide scale technological application of the phenomenon for magnetic sensors and memory devices.2 Anisotropic magnetoresistance [1,2], planar Hall effect [3,4], spin-dependent tunneling [5,6] and the extraordinary or anomalous Hall effect (EHE) [7,8] are spin-dependent electronic transport phenomena known for many years. However, it is the discovery of the giant magnetoresistance (GMR) [9,10] that gave birth to the term spintronics and triggered a world-wide outburst of the spin-related research. Extraordinary Hall Effect (EHE) in magnetic materials was discovered more than a century ago [7], extensively studied both theoretically and experimentally [8], and left out of the mainstream research for the last thirty years. The possibility to use the effect for technical applications, such as magnetic sensors and nonvolatile magnetic random access memories (MRAM), has been mentioned more than three decades ago [11], but no significant progress was reported until recently. A probable reason for this is that although EHE in bulk magnetic materials can be significantly higher than the ordinary Hall effect in normal metals, its magnitude remained far beyond the sensitivity of semiconductors and magnetic sensors based on the anisotropic magnetoresistance [12]. The renewed interest in EHE has only recently arisen when some recipes to enhance the effect were found [13][14][15].The Hall effect in magnetic materials is commonly described [8,16] by the phenomenological equationwhere ρ H is the Hall resistivity, B, H and M are components of the magnetic induction, applied field and magnetization normal to the film plane, and D is the demagnetization factor. R 0 is the ordinary Hall coefficient related to the Lorentz force acting on moving charge carriers. R EHE , the extraordinary Hall coefficient, is associated with a break of the right-left symmetry at spin-orbit scattering in magnetic materials.. Demagnetization factor D is equal to 1 when field is applied perpendicular to a homogeneous magnetic film. In this case Eq.1 is simplified toVoltage measured between Hall contacts located perpendicular to the direction of an electric current is given by:where I is current and t thickness of the film. Fig.1 presents a typical field dependence of the Hall resistance (in a 4 nm thick Ni film at room temperature with field applied normal to the film plane.Magnetization normal to the film increases with field till saturation at about ±0.2 T. The EH...

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