does the exchange coupling exists between atoms or between domains in magnetic substances?
To my knowledge, the exchange interaction is between spin magnetic moments of unpaired electrons. As the result, any small piece ( in order of " exchange length" ) of a ferromagnetic material is a single domain as the size of the material becomes larger, other factors break the large domain in several domains. Between domains, we have thin domain walls. Through the domain wall, the magnetization ( in fact spin moments) change direction gradually from one domain to the neighboring domain. This rotation may take tens of atoms because due to the exchange interaction, sharp changes of direction of spins is not possible.
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