K. S. White

Ferromagnetism of 3He films in the low-field limit.

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.

Magnetization of Materials Used in Cryostats

We measure the magnetic susceptibility of a group of materials commonly used in the construction of low temperature cryostats. These materials are considered non-magnetic at room temperature. We use a variable temperature susceptometer to measure samples at temperatures between 2.0 K and 12.0 K and at magnetic fields between 0.25 Tesla and 4 Tesla. The samples we use include Aluminum, Beryllium Copper, fused silica, Grafoil, stainless steel, filled and unfilled epoxy, Macor, polycarbonate, aerogel, silver powder, Teflon, and Vespel. We present plots of the magnetization vs. temperature and field, and a table of the parameters to a fitting function which can be used to interpolate the data.

Liquid 3He quasiparticle free-path distribution function in simulated aerogels

Conflicting results between different laboratories of measurements of the phase diagram of superfluid 3 He when immersed in aerogel, even with nominally identical aerogels, suggest that details of the aerogels other than the porosity are important in determining superfluid properties. We have simulated the growth of small clusters ðNB10 5 Þ by offlattice single-particle diffusion limited aggregation (DLA). Measurements of the free-path distribution function of 3 He quasiparticles scattered by these clusters—truncated and repeated to simulate experimental conditions—show significant differences from uncorrelated scatterers at the same density, which differences may be significant for superfluid properties. r 2003 Elsevier Science B.V. All rights reserved.

SQUID-based pulsed and CW NMR for the study of 3He monolayers

Abstract The use of a SQUID coupled to a sample by a superconducting circuit allows NMR measurements over wide bandwidths. For our recent NMR experiments on 3He monolayers we developed a sample cell that allows for indirect cooling of the grafoil without excessive eddy currents. The details of the cell design are discussed. In addition, we have found that the 13C signal from the graphite is observable in this system. The FID time constant for this signal is comparable to the field homogeneity and the amplitude follows the Curie law over our temperature range ( 15 mK >T>0.2 mK ). Since the NMR signal in 13C is observed simultaneously with the 3He signal, this signal can act as a thermometer for the graphite substrate.

Thermodynamic Magnetization of Liquid 3He

We have measured the absolute thermodynamic magnetization of 3He in all of its liquid phases while using a new calibration technique which relies on the nuclear paramagnetism over the entire temperature range below 1 K as measured by Ramm, et al. We have directly measured the temperature dependence separately of the nuclear paramagnetism via NMR and of the total magnetism via our free-surface modulation technique. We report substantial agreement with Ramm, et al. In consequence, we find that the diamagnetism of liquid 3He is significantly smaller than expected theoretically, a condensed matter effect not seen previously. We have also remeasured the magnitude of the magnetization discontinuity at the superfluid A-B transition and find close agreement at saturated vapor pressure with NMR-deduced values from Scholz.

SQUID NMR Studies of the Second Layer of 3He at High Density

Nuclear magnetism in the second layer of 3He films on graphite has been studied as a function of density in the zero-field limit at temperatures down to 300 µK. Total surface coverages in this study ranged from 20 atoms/nm2 (the lowest coverage at which the effective exchange parameter turns ferromagnetic) through the two-phase region (ending at about 24 atoms/nm2) and into the single-phase imcommensurate solid region up to 31.2 atoms/nm2. The surface layers were studied using both pulsed and cw NMR measured with our SQUID NMR system in magnetic fields of 500 µT and lower. As the surface density continues to increase in the single-phase region, the exchange constant decreases consistently with a picture of steric hindrance impeding ferromagnetic exchange. Several different observational features scale with this inferred exchange: low-field magnetization, NMR frequency shift, and the temperature of onset of significant magnetization.

SQUID NMR studies of the dipole field in ferromagnetic 3He films

Abstract We use SQUID NMR to observe the magnetization of 3 He films with densities between 20 and 24 atoms / nm 2 in the zero field limit. Ferromagnetism in these nearly 2D Heisenberg exchange systems is stabilized by weak anisotropies. In the ferromagnetic phase, the NMR line becomes very broad and shifts to lower frequencies, consistent with a large dipolar field opposing a perpendicular applied field. Grafoil can be modeled by a Gaussian distribution of platelet angles centered on the normal to the Grafoil plus a randomly distributed set of platelets. Using the spin dynamics of a 2D polarized sheet with reasonable assumptions for the distributions of platelet angles, we show that the magnetization, frequency shift and line shape form a consistent picture of a highly polarized sheet.

Magnetic Susceptibility of Liquid 3He

We report measurements of the magnetic susceptibility of liquid 3He at saturated vapor pressure in the normal state from the superfluid transition temperature up to 1 K. Our results are comparable to those of Ramm, et al.1 at temperatures above 200 mK, but are systematically higher as T→0. We conclude that the zero-temperature limiting susceptibility is approximately 5.9% larger than reported by Ramm, et al. Consequently, the discrepancy in the liquids diamagnetism between values expected theoretically and our experimentally measured results is essentially eliminated.

The absolute magnetization of 3He at zero pressure

Abstract We have measured the absolute magnetization of liquid 3He at saturated vapor pressure over the temperature range 0.3–500 mK. The technique compares the magnetization of the liquid to that of vacuum by passing the liquid–vacuum interface through a SQUID coil. We find that, contrary to expectations, the electronic diamagnetism of the normal liquid is approximately 6% less than the calculated atomic value. This appears to be the first measurement of the diamagnetic susceptibility of any noble gas in a condensed state.

A new look at ferromagnetic two-dimensional magnetism of 3He

Abstract 3He films on graphite at densities above 20 atoms / nm 2 have a strong ferromagnetic tendency. Models of multiple spin exchange are believed to describe the dominant interaction. However, other much smaller interactions can significantly alter the low-temperature behavior of these systems, especially in very low magnetic fields. Our recent experiments have used a SQUID system for both pulsed and continuous wave NMR at very low applied fields (B mT ) . Our results show that the polarization of this system grows much more rapidly than is expected from the Heisenberg exchange models, even approaching saturation. Possible explanations are the influence of finite size effects and dipolar interactions.