2022 Basic of Spintronics (Giant Magnetoresistance Basics) By Management Practicalintroduction

Electrons have a charge and a spin, but until recently, charges and spins have been considered separately. In conventional electronics, the charges are manipulated by electric fields but the spins are ignored. Other classical technologies, magnetic recording, for example, are using the spin but only through its macroscopic manifestation, the magnetization of a ferromagnet. This picture started to change in 1988 when the discovery Baibich et al., 1988 and Binash et al., 1989 of the giant magnetoresistance GMR of the magnetic multilayers opened the way to an efficient control of the motion of the electrons by acting on their spin through the orientation of a magnetization. This rapidly triggered the development of a new field of research and technology, today called spintronics and, like the GMR, exploiting the influence of the spin on the mobility of the electrons in ferromagnetic materials. Actually, the influence of the spin on the mobility of the electrons in ferromagnetic metals, first suggested by Mott 1936, had been experimentally demonstrated and theoretically described by Albert Fert on his Ph.D. thesis almost 20 years before the discovery of 1988. The GMR was the first step on the road of the exploitation of this influence to control an electrical current. Its application to the read heads of hard disks greatly contributed to the fast rise in the density of stored information and led to the extension of the hard disk technology to consumer’s electronics. Then, the development of spintronics revealed many other phenomena related to the control and manipulation of spin currents. Today this field of research is expanding considerably, with very promising new axes like the phenomena of spin transfer, spintronics with semiconductors, molecular spintronics, or single-electron spintronics. GMR and spintronics take their roots from previous research on the influence of the spin on the electrical conduction in ferromagnetic metals Mott, 1936; Fert and Campbell, 1968, 1971, 1976; Loegel and Gautier, 1971. 

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Fig. 1

The spin dependence of the conduction can be understood from the typical band structure of a ferromagnetic metal shown in Fig. 1. The splitting between the energies of the “majority spin” and “minority spin” directions spin up and spin down in the usual notation makes that the electrons at the Fermi level, which carry the electrical current, are in different states for opposite spin directions and exhibit different conduction properties. This spin-dependent conduction was proposed by Mott 1936 to explain some features of the resistivity of ferromagnetic metals at the Curie temperature. However, in 1966, when Albert Fert started in his Ph.D. thesis, the subject was still almost completely unexplored. With his supervisor, Ian Campbell, proposed that he investigate it with experiments on Ni- and Fe-based alloys and had the privilege to be at the beginning of the study of this topic. He could confirm that the mobility of the electrons was spin dependent and, in particular, showed that the resistivities of the two channels can be very different in metals doped with impurities presenting a strongly spin-dependent scattering cross section. In Fig. 2, the example of the spin up majority spin and spin down minority spin resistivities of nickel doped with 1% of different types of impurities.

Fig. 2

It can be seen that the ratio alpha of the spin down resistivity to the spin up one can be as large as 20 for Co impurities or, as well, smaller than 1 for Cr or V impurities, consistent with the theoretical models developed by Jacques Friedel for the electronic structures of these impurities. The two-current conduction was rapidly confirmed by other groups and, for example, extended to Co-based alloys by Loegel and Gautier 1971 in Strasbourg.

reference :

Awschalom, D. D., and M. E. FlattΓ©, 2007, Nat. Phys. 3, 153.

Baibich, M. N., J. M. Broto, A. Fert, F. Nguyen Van Dau, F. Petroff, P. Etienne, G. Creuzet, A. Friederich, and J. Chazelas, 1988, Phys. Rev. Lett. 61, 2472.

A. Fert, 2008, Rev. Mod. Phys. 80, 1517.

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