ISSN 0021-3640, JETP Letters, 2014, Vol. 99, No. 1, pp. 32–36. © Pleiades Publishing, Inc., 2014. Original Russian Text © V.B. Efimov, A.V. Lokhov, L.P. Mezhov-Deglin, C. Dewhurst, V.V. Nesvizhevsky, G.V. Kolmakov, 2014, published in Pis’ma v Zhurnal Eksperimental’noi i Teoreticheskoi Fiziki, 2014, Vol. 99, No. 1, pp. 35–39. 32 1. INTRODUCTION When the flux of gaseous 4 He, which contains ≤2% of the impurity of vapors of atomic or molecular gases or vapors of a liquid under normal conditions inside the working cell filled with He-II, is condensed on the surface of superfluid He-II cooled below 1.8 K, a porous impurity–helium condensate impregnated by the superfluid liquid—impurity gel—is formed [1–3]. The studies of the structure and properties of impu- rity-gel samples that are formed when various impuri- ties are introduced in He-II were reviewed in [4–6]. It was found that the dispersive system (backbone) of an as-prepared impurity gel sample is formed by aggregates consisting of impurity nanoclusters, more precisely, of impurity van der Waals complexes, which appear in dense helium vapor at the vapor–superfluid liquid interface and consist of impurity nanoclusters coated with one or two layers of atoms of solid 4 He. The helium layer prevents the coalescence and col- lapse of impurity clusters. Liquid helium filling pores between impurity aggregates serves as a dispersive medium of the sample formed in He-II. It is notewor- thy that the properties of liquid helium in restricted geometry, in particular, the transition temperature of the liquid in nanopores to the superfluid state, can sig- nificantly differ from the properties of the bulk liquid [7]. This can be a reason for the appearance of thermal instability in the bulk of helium: an irreversible increase in the absorption of ultrasound, rise of lumi- nescence in the condensate containing nitrogen atoms in the molecular matrix of nitrogen helium [1, 4, 6], and a growth of the content of small clusters in deute- rium gel [5, 8] at temperatures T ≈ 2.0 K noticeably below the transition point of the liquid surrounding the sample from the superfluid to normal state (T λ = 2.172 K). Savich and Shalnikov [9] long ago observed the thermal instability of the hydrogen-condensate layer that was formed on the surface of He-II at the introduction of the gaseous hydrogen impurity into the cryostat (solid hydrogen is lighter than liquid helium) and “exploded with the formation of a halo of hydrogen particles” at the heating of the liquid and transition from He-II to He-I. Thus, impurity gels constitute a new class of non- crystalline materials (quantum soft matter), which are formed in a quantum liquid at low temperatures owing to the relatively weak van der Waals interaction of neu- tral impurity nanoclusters between each other and with the surrounding superfluid liquid. These gels can be applied in modern science and technologies, e.g., in the development of new power-consuming low- temperature materials with a high content of free rad- icals (in the simplest case, atoms of molecular materi- als) in a molecular matrix [1, 2, 4, 6], and in the study of the features of chemical reactions and phenomena on the surface of nanocrystalline catalysts at low and ultralow temperatures. Nanocluster gels of materials weakly absorbing neutrons, primarily, gels of heavy water and deuterium, can be used as moderators of cold neutrons [5, 10–14]. Coatings made of nanopo- wders that appear at the decay of such gels [15, 16] can be used in neutron guides as reflectors of cold and ultracold neutrons [17–21]. The interaction of cold neutrons with nanocluster oxygen gel has not yet been studied. Our study was Nanocluster Magnetic Gel in Superfluid He-II V. B. Efimov a , A. V. Lokhov a , L. P. Mezhov-Deglin a , C. Dewhurst b , V. V. Nesvizhevsky b , and G. V. Kolmakov c a Institute of Solid State Physics, Russian Academy of Sciences, Chernogolovka, Moscow region, 142432 Russia e-mail: mezhov@issp.ac.ru b Institut Laue-Langevin, Grenoble 38000, France c New York City College of Technology, City University of New York, Brooklyn, 11201 NY, USA Received December 11, 2013 The first results of the study of the structure of an impurity oxygen gel in superfluid He-II and in normal liquid helium have been obtained by the small-angle neutron scattering (SANS) method with cold neutrons. The angular dependence of the neutron scattering intensity I(q) indicates that the characteristic sizes of nanoclus- ter aggregates forming a dispersive system (backbone) of an oxygen gel sample are distributed from 1 to ≈100 nm. According to the estimates made, if the working cell with superfluid helium cooled below 1.8 K is placed in a magnetic field of H ≥ 200 G, the magnetic structure of the nanocluster sample of oxygen gel, which is formed at the condensation of the flow of gaseous 4 He with the impurity of O 2 vapor on the surface of He-II, will be close to ferromagnetic. DOI: 10.1134/S0021364014010044