Magnetic properties of nanoscaled paramelaconite Cu4O3−x (x=0.0 and 0.5)

• The first successful preparation of pure paramelaconite Cu4O3−x.
• High resolution AC susceptibility measurements.
• Evidence is presented for two stoichiometric forms of paramelaconite; defect free Cu4O3 and Cu4O2.5.
• Additional magnetic transition is observed at T=120 K in the defect free Cu4O3.

Pure paramelaconite Cu4O3−x has been prepared in the form of nanoparticles with 56 nm in diameter. This mixed valency oxide crystallizes in a tetragonal lattice with 4 unit formulae and forms a pyrochlore structure which manifests in two stoichiometric forms; Cu4O3 and Cu4O2.5, the latter form having two oxygen vacancies per unit cell. Magnetic lattice consists of Cu spin 1/2, and both stoichiometric forms obey transition to the antiferromagnetic state at TN=45–55 K. Defect free Cu4O3 is indicated by an inversion symmetry and exhibits both antiferromagnetic and ferromagnetic state, where the latter is supposedly due to the superexchange interaction in Cu–O(1n)–Cu bonds. An additional magnetic transition was observed in Cu4O3 at T=120 K, probably as a result of an incommensurate ordering. Absence of an inversion symmetry in the oxygen defect Cu4O2.5 results in a long range Dzyaloshinsky–Moriya interaction accompanied by the strong superparamagnetism.