321 ISSN 0021-3640, JETP Letters, 2016, Vol. 103, No. 5, pp. 321–327. © Pleiades Publishing, Inc., 2016. Effect of a Magnetic Field on the Intermediate Phase in Mn 1 – x Fe x Si: Spin-Liquid versus Fluctuations Scenario 1 S. V. Demishev a, b, *, I. I. Lobanova a, b , A. V. Bogach a , V. V. Glushkov a, b , V. Yu. Ivanov a , T. V. Ischenko a , N. A. Samarin a , N. E. Sluchanko a , S. Gabani c , E. Čižmár d , K. Flachbart c , N. M. Chubova e , V. A. Dyadkin e, f , and S. V. Grigoriev e a Prokhorov General Physics Institute, Russian Academy of Sciences, Moscow, 119991 Russia b Moscow Institute of Physics and Technology (State University), Dolgoprudnyi, Moscow region, 141700 Russia c Institute of Experimental Physics, Slovak Academy of Sciences, 040 01 Košice, Slovak Republic d P.J. Šafárik University in Košice, SK-04001 Košice, Slovak Republic e Petersburg Nuclear Physics Institute, Gatchina, 188300 Russia f Swiss–Norwegian Beamlines at the European Synchrotron Radiation Facility, 38000 Grenoble, France * e-mail: demis@lt.gpi.ru Received December 1, 2015; in final form, January 18, 2016 We report results of the magnetic field influence on the chiral spin liquid state in Mn 1 – x Fe x Si single crystal with iron content x = 0.108 in proximity of a hidden quantum critical point. The use of small angle neutron scattering data together with magnetic susceptibility measurements down to 0.4 K and precise magnetoresis- tance measurements in the temperature range 2–20 K in magnetic field up to 5 T allowed us to construct the magnetic phase diagram of this compound in which at low magnetic fields B < 0.15 T an intermediate phase with short-range magnetic order exists in a wide temperature range 0.62 K < T < 9.1 K. It was found that the increase in magnetic field first results in the suppression of a transition to the spiral phase with long-range magnetic order at very low temperatures, and then induces a transition of the intermediate phase to a spin- polarized (ferromagnetic) phase with lowering temperature. The temperature of this transition T SP increases with magnetic field logarithmically, T SP ~ log(B), and results in formation of a singular point on the magnetic phase diagram located at T ~ 8.5 K and B ~ 3.5 T, which may be either a triple or a critical point. The possible spin-liquid nature of the intermediate phase is discussed. DOI: 10.1134/S0021364016050027 1. Spiral magnets Mn 1 – x Fe x Si are characterized by presence of unusual magnetic phases, which are inter- mediate between common paramagnetic phase (PM) and spiral magnet (SM) phase with a long-range mag- netic order [1–4]. These intermediate phases (IP) may be detected either on B–T [1, 3] or on T –x [1, 2, 4] magnetic phase diagrams. Several theoretical studies also predict phases with an intermediate magnetic order for MnSi based solids [5, 6]. These results may be qualitatively understood following experimentally supported similarity between spiral magnets and cho- lesteric liquid crystals [7], in which the considered intermediate phases appear as a magnetic analog of the blue fog phases [5, 7, 8]. The IP are also referred as spin liquid (or chiral spin liquid) states due to straight- forward analogy from general physics, where the para- magnetic phase, spiral phase with long-range order (LRO) and intermediate phase may be considered as magnetic replicas of the common gas, solid and liquid phases [5]. Nevertheless, sometimes the aforemen- tioned description of the intermediate magnetic phases is disputed, and these specific states are treated as regions of pronounced magnetic f luctuations in the phase diagram [9]. In any case, these specific phases are of magnetic nature and thus may be affected by external magnetic field; however, the influence of magnetic field on the phases with intermediate mag- netic order has not yet been systematically studied. At the same time, an analogy with liquid crystals suggests that the external field may noticeably change the cor- responding phase diagram [8]. The aim of this work is to study experimentally the influence of magnetic field on the intermediate mag- netic phase in Mn 1 – x Fe x Si developing in proximity to the hidden quantum critical point at x* ~ 0.11 [4]. For this iron concentration the transition to the SM phase with long-range magnetic order is expected to be sup- pressed and an intermediate magnetic phase may be observed at temperatures below T s ~ 9–10 K in zero magnetic field, so that the quantum phase transition 1 The article is published in the original. CONDENSED MATTER