EFFECTIVE EXPONENTS FOR DE ALMEIDA-THOULESS LINE

The experimental irreversibility line at the magnetic phase diagram H- T of spin glasses is often approximated using a power law H alpha(T-G- T)(phi/2). The mean field theory of de Almeida and Thouless predicts p hi=3 for temperatures T close to the spin-glass transition temperature T-G For a range of reduced temperature involved in an experiment one should use, however, effective exponents (phi>3) obtained by fitting t he power law to the exact expression for de Almeida-Thouless line. The se compare favorably with experimental values of phi obtained for diff erent spin glasses. The increase of effective exponent up to phi=7 on approaching the spin-glass-ferromagnetism multicritical point is also considered.