Quantum bicriticality in Mn_{1-x}Fe_{x}Si solid solutions: exchange and percolation effects

S. V. Demishev^{+}, I. I. Lobanova^{+*}, V. V. Glushkov^{+*}, T. V. Ischenko^{+}, N. E. Sluchanko^{+}, V. A. Dyadkin, N. M. Potapova, S. V. Grigoriev

^{+}A.M.Prokhorov General Physics Institute of the RAS, 119991 Moscow, Russia

^{*}Moscow Institute of Physics and Technology, 141700 Dolgoprudny, Russia

Konstantinov Petersburg Nuclear Physics Institute, 188300 Gatchina, Russia

**Abstract**
The T-x magnetic phase diagram of Mn_{1-x}Fe_{x}Si
solid solutions is probed by magnetic susceptibility, magnetization and
resistivity measurements. The boundary limiting phase with short-range
magnetic order (analogue of the chiral liquid) is defined experimentally and
described analytically within simple model accounting both classical and
quantum magnetic fluctuations together with effects of disorder. It is shown
that Mn_{1- x}Fe_{x}Si system undergoes a sequence of two quantum phase
transitions. The first "underlying" quantum critical (QC) point corresponds to disappearance of the long-range magnetic order. This
quantum phase transition is masked by short-range order phase, however, it
manifests itself at finite temperatures by crossover between classical and
quantum fluctuations, which is predicted and observed in the paramagnetic
phase. The second QC point may have topological nature and
corresponds to percolation threshold in the magnetic subsystem of Mn_{1- x}Fe_{x}Si. Above x_{c} the short-range ordered phase is suppressed and
magnetic subsystem becomes separated into spin clusters resulting in
observation of the disorder-driven QC Griffiths-type phase characterized
by an anomalously divergent magnetic susceptibility with
the exponents .