Memory and Chaos Effects in Spin Glasses

New low frequency ac susceptibility measurements on two different spin glasses show that cooling/heating the sample at a constant rate yields an essentially reversible (but rate dependent) $\chi \left(T\right)$ curve; a downward relaxation of $\chi$ occurs during a temporary stop at constant temperature (aging). Two main features of our results are (i) when cooling is resumed after such a stop, $\chi$ goes back to the reversible curve (chaos), and (ii) upon reheating, $\chi$ perfectly traces the previous aging history (memory). We discuss implications of our results for a real space (as opposed to phase space) picture of spin glasses.