Ferromagnetism in the boundary layer of <sup>3</sup>He on graphite

Friedman, L. J.; Ytterboe, S. N.; Bozler, H. M.; Thomson, A. L.; Cross, M. C.

doi:10.1139/p87-214

Cited by 5 publications

(3 citation statements)

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“…The latest results 16 on the magnetization of liquid 3 He confined in Grafoil suggest the occurrence of a surface ferromagnetic transition below 1 mK in very low magnetic fields (<6G).…”

Section: Nuclear Ferromagnetism Of Two-dimensional 3 Hementioning

confidence: 99%

Nuclear Ferromagnetism of Two-DimensionalHe3

1986

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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...

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“…The latest results 16 on the magnetization of liquid 3 He confined in Grafoil suggest the occurrence of a surface ferromagnetic transition below 1 mK in very low magnetic fields (<6G).…”

Section: Nuclear Ferromagnetism Of Two-dimensional 3 Hementioning

confidence: 99%

Nuclear Ferromagnetism of Two-DimensionalHe3

1986

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“…With the exception of the studies by Friedman et al [3,12] on 3 He which used a SQUID NMR technique [13] with filled pores, all of the previous NMR studies used conventional techniques with applied fields greater than 6 mT. The Larmor frequencies in those (conventional NMR) experiments were much smaller than the exchange rates, but nevertheless the applied fields were large enough to substantially enhance the magnetization in the vicinity of T ∼ J, effectively masking any spontaneous magnetization -if it should occur.…”

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confidence: 99%

“…Reported values of the random fraction range from 50% [4,17] down to 33% [19]. In order to calculate the distribution of frequencies that simulate our NMR lines we need to calculate the angle of the magnetization (which is not exactly the angle of the applied field at low fields) and then the resulting NMR frequency assuming 1) a temperature dependent average dipolar field (λM 0 ), 2) the equations for spin dynamics from Friedman et al [12], and 3) a temperature dependent intrinsic linewidth. We can approximately fit our NMR absorption lines to this model, however this process works best when we assume that only 20-25% of the spins are on the randomly distributed plates.…”

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Ferromagnetism ofH3eFilms in the Low-Field Limit

Bozler

Zhang

et al. 2002

Phys. Rev. Lett.

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We provide evidence for a finite-temperature ferromagnetic transition in two dimensions as H -->0 in thin films of 3He on graphite, a model system for the study of two-dimensional magnetism. We perform pulsed and cw NMR experiments at fields of 0.03-0.48 mT on 3He at areal densities of 20.5-24.2 atoms/nm(2). At these densities, the second layer of 3He has a strongly ferromagnetic tendency. With decreasing temperature, we find a rapid onset of magnetization that becomes independent of the applied field at temperatures in the vicinity of 1 mK. Both the dipolar field and the NMR linewidth grow rapidly as well, which is consistent with a large (order unity) polarization of the 3He spins.

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