The anisotropic magnetoresistance (AMR) effect is a fundamental phenomenon in which the electrical resistivity
depends on the relative angle between the magnetization direction and the electric current direction. The efficiency of the effect ``AMR ratio'' is defined by (r_parallel - r_perp)/r_perp, where r_parallel (r_perp) is a resistivity for the case of the electrical current parallel to the magnetization (a resistivity for the case of the current perpendicular to the magnetization). The AMR effect has been experimentally studied for various ferromagnets since about 150 years ago. The intuitive explanation about the AMR effect, however, has scarcely been reported. In this study, we first derive a general expression of the AMR ratio extending the conventional model  to a more general one [2 - 4]. Using the general expression, we next give the intuitive explanation about the AMR effect . In addition, we show that the negative AMR ratio is a necessary condition for half-metallic ferromagnets [2,3,5,6,7].
 I. A. Campbell et al., J. Phys. C 3, S95 (1970).
 S. Kokado, M. Tsunoda et al., J. Phys. Soc. Jpn. 81, 024705 (2012).
 S. Kokado and M. Tsunoda, Adv. Mater. Res. 750-752, 978 (2013).
 S. Kokado and M. Tsunoda, J. Phys. Soc. Jpn. 84, 094710 (2015).
 F. Yang, Y. Sakuraba, S. Kokado et al., Phys. Rev. B 86, 020409 (2012).
 Y. Sakuraba, S. Kokado et al., Appl. Phys. Lett. 104, 172407 (2014).
 S. Kokado, Y. Sakuraba, and M. Tsunoda, Jpn. J. Appl. Phys. 55, 108004 (2016).