1. Department of Physics, Stanford University, Stanford, California 94305, USA
   
2. Laboratoire Léon Brillouin, CEA-CNRS, CEA-Saclay, 91191 Gif sur Yvette, France
   
3. Stanford Synchrotron Radiation Laboratory, Stanford, California 94309, USA
   
4. 1. Physikalisches Institut, Universität Stuttgart, Pfaffenwaldring 57, 70550 Stuttgart, Germany
   
5. BK21 Team of Nano Fusion Technology, Pusan National University, Busan 609-735, Korea
   
6. State Key Lab of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, 2699 Qianjin Street, Changchun 130012, China
   
7. Department of Applied Physics, Stanford University, Stanford, California 94305, USA

Correspondence to: M. Greven^3,7 Correspondence and requests for materials should be addressed to M.G. (Email: greven@stanford.edu).

The pseudogap region of the phase diagram is an important unsolved puzzle in the field of high-transition-temperature (high-T _c) superconductivity, characterized by anomalous physical properties^{1, 2}. There are open questions about the number of distinct phases and the possible presence of a quantum-critical point underneath the superconducting dome^{3, 4, 5}. The picture has remained unclear because there has not been conclusive evidence for a new type of order. Neutron scattering measurements for YBa₂Cu₃O₆₊ (YBCO) resulted in contradictory claims of no^{6, 7} and weak^{8, 9} magnetic order, and the interpretation of muon spin relaxation measurements on YBCO^{10, 11} and of circularly polarized photoemission experiments on Bi₂Sr₂CaCu₂O₈₊(refs 12, 13) has been controversial. Here we use polarized neutron diffraction to demonstrate for the model superconductor HgBa₂CuO₄₊ (Hg1201) that the characteristic temperature T* marks the onset of an unusual magnetic order. Together with recent results for YBCO^{14, 15}, this observation constitutes a demonstration of the universal existence of such a state. The findings appear to rule out theories that regard T* as a crossover temperature^{16, 17, 18} rather than a phase transition temperature^{19, 20, 21}. Instead, they are consistent with a variant of previously proposed charge-current-loop order^{19, 20} that involves apical oxygen orbitals²², and with the notion that many of the unusual properties arise from the presence of a quantum-critical point^{3, 4, 5, 19}.