Tuning of the spin gap transition of spin dimer compound Ba₃Mn₂O₈ by doping with La and V

We have successfully synthesized the coupled spin dimer systems La_xBa_3−xMn₂O₈ (x = 0, 0.2, 0.5, 1) and Ba₃Mn_2−yV_yO₈ (y = 0.5, 1.0, 2.0). The magnetic properties have been investigated as a function of magnetic field and temperature down to 2 K. The susceptibility increases and the intradimer spin exchange interaction decreases with increase of La concentration. The most important finding in higher La doped systems reveals hysteresis in magnetization as a function of magnetic field. The substitution of La (x = 0.5, 1.0) for Ba induces ferromagnetism due to the formation of a mixed valence state of Mn and enhancement of the inter-bilayer ferromagnetic interaction. The replacement of Mn by non-magnetic V destroys the spin gap. La and V doping significantly affect the magnetic properties of the quantum antiferromagnetic compound Ba₃Mn₂O₈.