The phase diagram of La2 Sr Cu04 is quite rich, including the antiferromagnetic phase for x~0.02, an intermediate region for 0.02 (x ( 0.05 characterized by 2D weak localization and unusual short range 2D magnetism, and the high T, superconduc. ting phase for x ) 0.05 [1].Each of these ranges contains new and important physics issues. Further, because the magnetic and electronic properties evolve continuously as a function of doping, a thorough characterization of the entire phase diagram may provide clues about the origin of the superconductivity.We focus here on La] 96Srp p4Cu04, which is in the intermediate region with neither long range antiferromagnetic order nor superconductivity.Aharony et al.[2] predicted a novel spin-glass (SG) phase at such intermediate x.However, although experimental evidence for slowing down of the spin fiuctuations has been found in neutron [3], muon spin relaxation (p, SR) [4 -6], and nuclear quadrupole resonance (NQR) [7] measurements, there has been no report of the magnetization behavior typical of canonical SGs: irreversibility and remnant magnetization below the glass transition and scaling behavior above and below it [8]. We report here measurements of the dc magnetization of a single crystal of La] 96Srpp4Cu04 that show all these features. Notably, although neutron measurements show that there is a staggered magnetization corresponding to one spin per Cu atom, the system has a uniform magnetization characterized by a very low density of effective free spins that undergo a conventional three-dimensional SG transition.The single crystal used for this experiment was a small piece (2 X 2 X 4 mm with the c axis normal to the largest face) cut from the same crystal used for the neutron scattering and conductivity measurements of Keimer et al. [3]. It was grown by the top-seeded solution method using CuO as a IIux [9]. The neutron results show short range antiferromagnetic order with a correlation length that is about 40 A. and independent of T below 250 K. There is indirect evidence [3] that, although the spins behave like three-component Heisenberg ones at high T, they may cross over to XY-like behavior at T -20 K. This is also the temperature below which a central (near zero-energy transfer) peak begins to grow with decreasing T, showing that the spin fluctuations slow down on the time scale of picoseconds, determined by the resolution of the neutron spectroscopy. Magnetization measurements were made with a Quantum Design Superconducting Quantum Inter-ference Device (SQUID) magnetometer at fields between 0.02 and 5.5 T applied parallel (H~~ah) and perpendicular (H~~c) to the CuOz planes.The zero-field-cooled (ZFC) and field-cooled (FC) magnetizations M at H = 0.02 T for the two field directions are plotted in Fig. 1. History dependence sets in at the irreversibility temperature, identified by the splitting of the ZFC and FC curves. This irreversibility temperature is the same for H((ab and H()c. A peak is evident 300 XT = X, T+C 250 9~~~g~g ply $1 150 . 20 40 60 T (K)~~~~H~~I~8 ...