We present a search for ultrarelativistic magnetic monopoles with the Pierre Auger observatory. Such
particles, possibly a relic of phase transitions in the early Universe, would deposit a large amount of energy
along their path through the atmosphere, comparable to that of ultrahigh-energy cosmic rays (UHECRs).
The air-shower profile of a magnetic monopole can be effectively distinguished by the fluorescence
detector from that of standard UHECRs. No candidate was found in the data collected between 2004 and
2012, with an expected background of less than 0.1 event from UHECRs. The corresponding
90% confidence level (C.L.) upper limits on the flux of ultrarelativistic magnetic monopoles range from
10^ 1219(cm2 sr s)^ 121 for a Lorentz factor \u3b3 = 10^9 to 2.5
7 10 1221(cm2 sr s)^ 121 for \u3b3 = 10^12. These results\u2014the
first obtained with a UHECR detector\u2014improve previously published limits by up to an order of
magnitude
