1.4 Evolution of Spintronics
7
What was actually
observed
Furnace
Classical
prediction
Inhomogeneous
magnetic field
Silver atoms
Fig. 1.1 Schematic diagram of Stern–Gerlach experimental setup
performing precession motion about its own axis and the magnitude of the corresponding angular momentum of self-rotation of electron is è/2. The fixed magnitude
of angular momentum associated with the spinning motion of electron suggested a
physical interpretation of particles spinning around their own axis. The mathematical
theory was worked out in depth by Pauli in 1927. Relativistic quantum mechanics,
derived by Paul Dirac in 1928, included electron spin as an integral vital part of it.
However, the electron spin was already accidently evidenced in the very wellknown Stern–Gerlach experiment, which is considered to be the turning point event
in the history of spin. An experiment of atom’s deflection named after Otto Stern
and Walther Gerlach (Gerlach and Stern 1922; Stern 1921) performed in Frankfurt,
Germany in 1920, was the first experiment to show the existence of an intrinsic
property of the electron called ‘spin’. Although their experimental result was not used
to proof the existence of the electron spin, nowadays it is widely used to illustrate
the existence of the spin and its quantization properties. Stern and Gerlach directed a
silver beam through an inhomogeneous magnetic field. Then the beam hits a screen
and shows how the Ag atoms are deflected after interacting with the inhomogeneous
magnetic field (Fig. 1.1). In order to suppress the effect of Lorentz force, the silver
beam was neutrally charged in the experiment.
Classically, the Ag atom is considered as spinning magnetic dipole. In the presence
of a homogeneous magnetic field the dipole will precess due to the torque exerted by
the magnetic field on it. If the magnetic field is inhomogeneous, the traversing dipole
through the magnetic field will be deflected depending on its orientation. According
to the dipole-magnetic field interaction, F = ∇(m · H), where m is the dipole and H is
the inhomogeneous magnetic field. Thus, one expects to see on the screen a smooth
distribution of the Ag atoms. On the other hand, Bohr–Sommerfeld predicted that
an atom of angular moment L = 1 would have a quantized magnetic moment with
two equal sizes and of opposite directions. The aim of Stern–Gerlach experiment
was to test the validity of this hypothesis (Stern 1921). Their result confirmed Bohr–
Sommerfeld hypothesis for they observed two spots on the detector screen relative to
two opposite magnetic moments. Later in 1927, a similar experiment using hydrogen
7
What was actually
observed
Furnace
Classical
prediction
Inhomogeneous
magnetic field
Silver atoms
Fig. 1.1 Schematic diagram of Stern–Gerlach experimental setup
performing precession motion about its own axis and the magnitude of the corresponding angular momentum of self-rotation of electron is è/2. The fixed magnitude
of angular momentum associated with the spinning motion of electron suggested a
physical interpretation of particles spinning around their own axis. The mathematical
theory was worked out in depth by Pauli in 1927. Relativistic quantum mechanics,
derived by Paul Dirac in 1928, included electron spin as an integral vital part of it.
However, the electron spin was already accidently evidenced in the very wellknown Stern–Gerlach experiment, which is considered to be the turning point event
in the history of spin. An experiment of atom’s deflection named after Otto Stern
and Walther Gerlach (Gerlach and Stern 1922; Stern 1921) performed in Frankfurt,
Germany in 1920, was the first experiment to show the existence of an intrinsic
property of the electron called ‘spin’. Although their experimental result was not used
to proof the existence of the electron spin, nowadays it is widely used to illustrate
the existence of the spin and its quantization properties. Stern and Gerlach directed a
silver beam through an inhomogeneous magnetic field. Then the beam hits a screen
and shows how the Ag atoms are deflected after interacting with the inhomogeneous
magnetic field (Fig. 1.1). In order to suppress the effect of Lorentz force, the silver
beam was neutrally charged in the experiment.
Classically, the Ag atom is considered as spinning magnetic dipole. In the presence
of a homogeneous magnetic field the dipole will precess due to the torque exerted by
the magnetic field on it. If the magnetic field is inhomogeneous, the traversing dipole
through the magnetic field will be deflected depending on its orientation. According
to the dipole-magnetic field interaction, F = ∇(m · H), where m is the dipole and H is
the inhomogeneous magnetic field. Thus, one expects to see on the screen a smooth
distribution of the Ag atoms. On the other hand, Bohr–Sommerfeld predicted that
an atom of angular moment L = 1 would have a quantized magnetic moment with
two equal sizes and of opposite directions. The aim of Stern–Gerlach experiment
was to test the validity of this hypothesis (Stern 1921). Their result confirmed Bohr–
Sommerfeld hypothesis for they observed two spots on the detector screen relative to
two opposite magnetic moments. Later in 1927, a similar experiment using hydrogen