Nuclear Ferromagnetism of Two-DimensionalHe3

Abstract

The magnetization of 3 He adsorbed on Grafoil has been measured as a function of coverage (two to three layers) at millikelvin temperatures. The observation of a well-defined peak substantially above the free-spin (Curie) value proves unambiguously the existence of a surface ferromagnetic effect.PACS numbers: 67.70. + n, 67.65.+z, 67.80Jd Surface magnetic phenomena have been observed by several groups in liquid 3 He in confined geometries. 1 For different systems the magnetization has a ferromagnetic Curie-Weiss contribution with a characteristic temperature 9 -0.5 mK which has its origin in the first adsorbed layers. In these experiments, cooling of the substrate was achieved indirectly, by the liquid 3 He in contact with a heat exchanger. Therefore, the effect of the adsorbed layers could not be separated from that of the liquid. Moreover, heterogeneous substrates were often used, making the data analysis ambiguous.We have developed the techniques necessary to cool down to millikelvin temperatures a well characterized and homogeneous substrate (Grafoil 2 ) in order to perform high-sensitivity NMR measurements on monolayers of adsorbed 3 He. In a recent Rapid Communication 3 we reported the measurement of a Curie-Weiss constant of 1.8 mK (larger than that observed in confined 3D liquid) at a coverage of 43.9-cm 3 STP (of the order of 2.6 layers). Accurate data in a narrow coverage range were therefore desirable.We report in this Letter new results obtained with the experimental cell described in Ref. 3. A detailed account of the novel techniques we have developed to obtain good thermal contact between copper and Grafoil and to minimize the heat leaks will be given in a technical publication. The present measurements were made with a new cw NMR spectrometer using lock-in detection. The NMR frequency was 886.0 kHz, the corresponding value of the magnetic field 27.3 mT, and the width of the magnetic field sweep 0.5 mT. The 3 He absorption line was integrated numerically by a microcomputer. It was found convenient to calibrate the amplitude of the measured absorption by comparison with that caused by a calibrated attenuator (0.2%). This reference signal was generated immediately after each field sweep. In this way the NMR absorption was measured independently of the gain of the amplifier.We have checked that the usual method (keeping track of the gain during the measurements) and the present procedure agree within 2%. The area of the normalized absorption signal is proportional to the magnetization, and is expressed here in arbitrary units which are the same throughout the experiment. Magnetization measurements as a function of temperature were performed in the temperature range 3-50 mK. The temperature was given by carbon resistors and a cerium magnesium nitrate mutual inductance calibrated at zero field and at 27.3 mT. The measurements were performed at the following coverages: 22. 05, 36.67, 37.95, 39.22, 40.49, 41.76, 43.04, 44.31, 45.58, 46.85, 48.11, 49.38, 50.65, 51.92, 54.48, and 57.03 cm 3 STP. From ne...

Nuclear Magnetism in Two-Dimensional Solid Helium Three on Graphite

“…The magnetization also does not change appreciably in this region. 8,75) This is hard to occur in solid 3 He where only a small density difference results in a sizable variation of the exchange interactions due to the strong density dependence of J P .…”

“…73) Two of them survived after Franco et al 8) who opened the door for low temperature experiments on thin films adsorbed on Grafoil without facing bulk liquid 3 He. One is the RKKY model to consider indirect exchange interactions among localized spins in the second layer mediated by interlayer exchanges with itinerant spins in overlayers.…”

“…Since the pioneering work by the Grenoble group, 8) the origin of ferromagnetic (FM) anomaly observed at high densities has been a matter of debate for long time. In earlier days, the ferromagnetism emerged when Grafoil or fine particles are immersed in liquid 3 He 66-68) had been discussed …”

“…Solid monolayer of 3 He adsorbed on atomically flat graphite surface provides us good opportunities to address such a question. 8) Unlike layered materials, it is a perfect two-dimensional (2D) system at temperatures below 1 K. By investigating 2D solid 3 He we should basically be able to test fundamental problems characteristic of low dimensional quantum spin systems such as the Mermin-Wagner theorem, 9) the resonating valence bond (RVB) state, 10) etc. Eventually, an exotic gapless spin-liquid ground state has been discovered 11,12) in a low density commensurate (C) phase, which is attracting many theoretical interests.…”

Spin-liquid phase due to competing classical orders in the semiclassical theory of the Heisenberg model with ring exchange on an anisotropic triangular lattice

“…solid 3 He; for an extensive review of magnetism in solid 3 He, see Roger [35] and references therein. Since the early studies by Thouless [36], single monolayers of solid 3 He have been absorbed on graphite [37,38] and several studies were performed to develop a theoretical understanding of the experiments in terms of multiple spin-exchange models [39][40][41][42][43][44][45][46][47]. Additionally, multiple spin-exchange models have gained interest in frustrated spin systems, in particular the parent cuprate high-temperature superconductors [48][49][50].…”