serf, under feudalism, peasant laborer who can be generally characterized as hereditarily attached to the manor in a state of semibondage, performing the servile duties of the lord (see also manorial system). Although serfs were usually bound to the land, many exceptions are found in the medieval economy of Western Europe, and, serfdom, as an institution, assumed a number of different forms in Western Europe and Eastern Europe. Serfdom also appeared with feudalism in China, Japan, India, pre-Columbian Mexico, and elsewhere.

Serfdom is distinguished from slavery chiefly by the body of rights the serfs held by a custom generally recognized as inviolable, by the strict arrangement that made the peasants servile in a group rather than individually, and by the fact that they could usually pass the right to work their land on to a son. In Western Europe during the Middle Ages the status of manorial peasants was regulated by local custom, and a wide diversity of names was applied to the various types of tenancy, which extended from the completely servile tenant to the freeholder who paid only a form of rent. Many serfs were theoretically subject to labor service at the will of the lord and in many cases the lord had the right to arrange the marriage of his serfs, but all such matters came to be governed by set customs. In legal theory the serf's holding was granted at the will of the lord, but in practice the right to hold came to be hereditary.

Serfdom sometimes arose from the conquest of a people by victors who did not reduce the natives to slavery but only depressed them to tributaries; these tributaries held their lands as of old, but paid dues (especially labor dues) to the conquerors. Thus serfdom was established in some Aegean regions by Greek conquests. More generally it may be said that serfdom arose only under a local agricultural economy, connected with a political system based on personal contract—some form of feudalism.

See also slavery; peonage.

History

Serfdom was known in the Hellenistic civilization, and in the Roman Empire economic maladjustment led to the appearance of the servile class, the coloni. In the Middle Ages, serfdom developed in France, Italy, and Spain, later spread to Germany, and in the 15th cent. was carried to Slavic countries. It developed separately in England (where serfs were more commonly referred to as villeins), and became widespread by the end of the 10th cent. While the majority of peasants were serfs during the Middle Ages, free peasants continued to exist and in some regions whole villages did not come under the rule of a lord. In Western Europe the breakdown of the manorial system allowed peasants to obtain more freedom in the 14th and 15th cent.

Serfdom disappeared in England before the end of the Middle Ages. In the Hapsburg monarchy, it was ended (1781) by Emperor Joseph II, but feudal labor service (robot) continued in some provinces until 1848. In France, where it survived in outlying provinces, serfdom was swept away by the French Revolution. The repercussions of the Napoleonic Wars helped to destroy it elsewhere, the most notable example being the reforms of Karl vom und zum Stein in Prussia. In Russia and the other Slavic countries serfdom took different forms and persisted in some cases as late as the 19th cent.

In Russia serfdom originated during the 16th cent. when Ivan IV created a new landholding aristocracy, the pomiestchiks, whose tenure was based on service to the czar. Beginning in 1581, laws were passed inhibiting the free movement of the peasant tenants of the pomiestchiks; however, at this time the peasants still retained their civil rights. In the reign of Peter I the peasants were definitely bound to the landowner rather than to the land; their condition became virtual slavery. There were also real slaves in the Muscovite state, and in the 18th cent. all real distinction between slaves and serfs was abolished. As can be seen, the institution was more akin to slavery in the United States than to serfdom under feudalism.

Serfdom reached its peak in the late 18th cent. under Catherine II but was somewhat limited by reforms under Alexander I and Nicholas I. It was regarded by the majority of Russians as the major defect in the Russian state and as contrary to the interests of the rising industrial class and of the great landowners. It was the small landowners who risked losing everything if serfdom were abolished, and it was that class that most stubbornly resisted reform. The serfs were freed only in 1861 by Alexander II (see Emancipation, Edict of).

A spin-exchange relaxation-free (SERF) magnetometer achieves very high magnetic field sensitivity by monitoring a high density vapor of alkali metal atoms precessing in a near-zero magnetic field. The sensitivity of SERF magnetometers improves upon traditional atomic magnetometers by eliminating the dominant cause of atomic spin decoherence caused by spin-exchange collisions among the alkali metal atoms. SERF magnetometers are among the most sensitive magnetic field sensors and in some cases exceed the performance of SQUID detectors of equivalent size. They are vector magnetometers capable of measuring all three components of the magnetic field simultaneously.

Spin-exchange relaxation

Spin-exchange collisions preserve total angular momentum of a colliding pair of atoms but can scramble the hyperfine state of the atoms. Atoms in different hyperfine states do not precess coherently and thereby limit the coherence lifetime of the atoms. However, decoherence due to spin-exchange collisions can be completely eliminated if the spin-exchange collisions occur much faster than the precession frequency of the atoms. In this regime of fast spin-exchange, all atoms in an ensemble rapidly change hyperfine states, spending the same amounts of time in each hyperfine state and causng the spin ensemble to precess more slowly but remain coherent. This so-called SERF regime can be reached by operating with sufficiently high alkali metal density (at higher temperature) and in sufficiently low magnetic field.

The spin-exchange relaxation rate $R\_\{se\}$ for atoms with low polarization experiencing slow spin-exchange can be expressed as follows:

$$

R_{se} = frac{1}{2 pi T_{se}} left(frac{2 I(2 I -1)}{3(2I+1)^2} right) where $T\_\{se\}$ is the time between spin-exchange collisions, $I$ is the nuclear spin, $nu$ is the magnetic resonance frequency, $gamma\_e$ is the gyromagnetic ratio for an electron.

In the limit of fast spin-exchange and small magnetic field, the spin-exchange relaxation rate vanishes for sufficiently small magnetic field:

$$

R_{se} = frac{gamma_e^2 B^2 T_{se} }{2 pi} frac{1}{2}left(1-frac{(2I+1)^2}{Q^2} right) where $Q$ is the "slowing-down" constant to account for sharing of angular momentum between the electron and nuclear spins:

$Q(I=3/2)=4left(2\; -\; frac\{4\}\{3+P^2\}\; right)^\{-1\}$

$Q(I=5/2)=6left(3\; -\; frac\{48(1+P^2)\}\{19+26\; P^2+3\; P^4\}\; right)^\{-1\}$

$Q(I=7/2)=8left(frac\{4(1+7P^2+7P^4+P^6)\}\{11+35P^2+17P^4+P^6\}\; right)^\{-1\}$

where $P$ is the average polarization of the atoms. The atoms suffering fast spin-exchange precess more slowly when they are not fully polarized because they spend a fraction of the time in different hyperfine states precessing at different frequencies (or in the opposite direction).

Sensitivity

The sensitivity $delta\; B$ of atomic magnetometers are limited by the number of atoms $N$ and their spin coherence lifetime $T\_2$ according to

$delta\; B\; =\; frac\{1\}\{gamma\}\; sqrt\{frac\{2\; R\_\{tot\}\; Q\}\{F\_z\; N\}\; \}$

where $gamma$ is the gyromagnetic ratio of the atom and $F\_z$ is the average polarization of total atomic spin $F\; =\; I+S$.

In the absence of spin-exchange relaxation, a variety of other relaxation mechanisms contribute to the decoherence of atomic spin:

$R\_\{tot\}\; =\; R\_D\; +\; R\_\{sd,self\}\; +\; R\_\{sd,mathrm\{He\}\}\; +\; R\_\{sd,mathrm\{N\_2\}\}$

where $R\_D$ is the relaxation rate due to collisions with the cell walls and $R\_\{sd,X\}$ are the spin destruction rates for collisions among the alkali metal atoms and collisions between alkali atoms and any other gasses that may be present.

In an optimal configuration, a density of 10¹⁴ cm^-3 potassium atoms in a 1 cm³ vapor cell with ~3 atm helium buffer gas can achieve 10 aT Hz^-1/2 (10^-17 T Hz^-1/2) sensitivity with relaxation rate $R\_\{tot\}$ ≈ 1 Hz.

Typical operation

Alkali metal vapor of sufficient density is obtained by simply heating solid alkali metal inside the vapor cell. A typical SERF atomic magnetometer can take advantage of low noise diode lasers to polarize and monitor spin precession. Circularly polarized pumping light tuned to the $D\_1$ spectral resonance line polarizes the atoms. An orthogonal probe beam detects the precession using optical rotation of linearly polarized light. In a typical SERF magnetometer, the spins merely tip by a very small angle because the precession frequency is slow compared to the relaxation rates.

Advantages and disadvantages

SERF magnetometers compete with SQUID magnetometers for use in a variety of applications. The SERF magnetometer has the following advantages:

• Equal or better sensitivity per unit volume
• Cryogen-free operation
• All-optical measurement limits enables imaging and eliminates interference.

Potential disadvantages:

• Can only operate near zero field.
• Sensor vapor cell must be heated.

Applications

Applications utilizing high sensitivity of SERF magnetometers potentially include:

• High-performance magnetoencephalographic imaging.
• Sample magnetization measurement, especially rock samples.

History

The SERF magnetometer was developed by Michael V. Romalis at Princeton University in the early 2000s. The underlying physics governing the suppression spin-exchange relaxation was developed decades earlier by William Happer but the application to magnetic field measurement was not explored at that time. The name "SERF" was partially motivated by its relationship to SQUID detectors in a marine metaphor.

References

External links

• Photographs of a SERF magnetometer from the Romalis Group at Princeton University.