Spin glass

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A spin glass is a magnet with frustrated interactions, augmented by stochastic disorder, where usually ferromagnetic and antiferromagnetic bonds are randomly distributed. Its magnetic ordering resembles the positional ordering of a conventional, chemical glass.

Spin glasses display many metastable structures leading to a plenitude of time scales which are difficult to explore experimentally or in simulations.


Magnetic behavior

It is the time dependence which distinguishes spin glasses from other magnetic systems. Beginning above the spin glass transition temperature, Tc,[1] where the spin glass exhibits more typical magnetic behavior (such as paramagnetism as discussed here but other kinds of magnetism are possible), if an external magnetic field is applied and the magnetization is plotted versus temperature, it follows the typical Curie law (in which magnetization is inversely proportional to temperature) until Tc is reached, at which point the magnetization becomes virtually constant (this value is called the field-cooled magnetization). This is the onset of the spin glass phase. When the external field is removed, the spin glass has a rapid decrease of magnetization to a value called the remanent magnetization, and then a slow decay as the magnetization approaches zero (or some small fraction of the original value—this remains unknown). This decay is non-exponential and no simple function can fit the curve of magnetization versus time adequately.[citation needed] This slow decay is particular to spin glasses. Experimental measurements on the order of days have shown continual changes above the noise level of instrumentation.[citation needed]

If a similar test is run on a ferromagnetic substance, when the external field is removed there is a rapid change to a remanent value that then stays constant in time. For a paramagnet, when the external field is removed the magnetization rapidly goes to zero and stays there. In each case the decay is rapid and exponential.

If instead, the spin glass is cooled below Tc in the absence of an external field and then a field is applied, there is a rapid initial increase to a value called the zero-field-cooled magnetization followed by a slow upward drift toward the field cooled magnetization.

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