K. K. Nummila

Phase Diagram and NMR Studies of Antiferromagnetically Ordered Polycrystalline Silver

We have studied magnetic ordering in the frustrated antiferromagnetic spin-(1/2) system composed of the natural isotopic mixture of silver nuclei. The dynamic susceptibility of polycrystalline foils was measured down to about 0.5 nK (S > 0.4R ln 2) using SQUID-NMR techniques. Two antiferromagnetic resonance branches were observed, which is consistent with ordering into a simple 1-k state. We obtain Tc = (560 ± 60) pK for the transition temperature at B = 0 and Bc = 100 μT for the critical field extrapolated to T = 0.

A Versatile Nuclear Demagnetization Cryostat for Ultralow Temperature Research

A new cascade nuclear demagnetization cryostat has been designed and constructed. Our aim was to build a versatile, modular cryostat, with a large experimental space providing an excellent platform for various types of ultralow temperature measurements. A powerful dilution refrigerator, combined with a massive copper nuclear cooling stage, enables us to reach lattice temperatures below 100μK continuously for more than two months. The cryostat is equipped with a second magnet for operating a double-stage nuclear demagnetization setup. Details of the design and performance are presented.

Experiments with negative ions in3He superfluids

The interaction between negative ions and3He superfluids was studied by using a time-of-flight spectrometer having a 10−3 relative resolution. The ions guided through a sample of rotating3He-A were found to be focused into the cores of the vortices present in this rotating superfluid. Two different types, with different core structures and presumably different numbers of circulation quanta associated with them, were found. The ion mobility along the vortex cores, μc, observed in the absence of external magnetic field, can be explained by the mobility anisotropy of the A phase, which indicates that the vortices are continuous, i.e., consist of the A phase even in the core. In finite magnetic fields, a qualitatively similar continuous vortex structure was found, a result compatible with the earlier nuclear magnetic resonance (NMR) experiments, but also a new vortex type with an anomalously high μc was seen when the vortex sample was prepared “adiabatically.” The observed strong ion focusing indicates broken w-symmetry in the core textures. This is in contradiction with the earlier numerical calculations on the optimal continuous vortex texture. Experiments to observe trapping of ions into the vortex cores were performed both in A and B phases. No trapping could be detected. Mobility measurements in stationary liquid were extended down toT=0.3Tc. Comparison with the calculated transport coefficients yieldsΔA(T)=1.32ΔBCS(T) for the maximal A phase gap atP=29.3 bar, andΔB(T)=1.12ΔBCS(T) for the B phase gap at 4.8 bar. In the normal Fermi liquid a shallow minimum of ion mobility was detected aroundT≃2Tc=5 mK.

Polarized Nuclei in Normal and Superconducting Rhodium

We performed SQUID-NMR measurements on a rhodium single crystal at ultra-low nuclear-spin temperatures. With initial polarizations up to p=0.95, the antiferromagnetic tendency was clear, but surprisingly no indication of actual nuclear magnetic ordering was obtained. The lowest nuclear temperatures achieved were below 100 pK, whereas the lowest directly measured temperature was 280 pK. Double-spin-flip and evidence for triple-spin-flip resonance lines were detected, yielding direct information of the interactions between the nuclear spins. The superconducting transition of rhodium was observed with the critical values, Tc=210 μK and Bc(0)=3.4 μT. For the first time, measurements with substantially correlated nuclei were performed in the superconducting state, where the effect of the coherent electron system on the spin-lattice relaxation rate was studied. The spin-lattice relaxation time was longer in the superconducting state at all temperatures and displayed a strong dependence on nuclear susceptibility.

Measurements of the Dipolar Velocity in Superfluid 3He-B.

We have measured the frequency shift of the counterflow-induced peak in the NMR spectrum of the flare-out texture in rotating 3He in a vortex-free state. From a comparison with the calculated shifts, the temperature dependence of the dipolar velocity at P = 5.0 and 29.3 bar has been deduced. Our results are in good agreement with theoretical predictions.

Neutron Diffraction Studies of Nuclear Magnetic Ordering in Silver

AbstractNuclear antiferromagnetism in fcc silver metal, already investigated by NMR measurements, has been studied in a single crystal of109Ag by neutron absorption and diffraction techniques. Below the Neel temperature TN, a (001) Bragg reflection with a resolution limited width demonstrates long range order in a simple type-I AFM structure with the ordering vectork = (2π/a)(001) (“up-down” structure). The entropy at the transition,

Effect of heat treatment of pure and carbon-polluted rhodium samples on the low-temperature resistivity

{\text{S}}_c = 0.54R

Neutron studies of nuclear magnetism at ultralow temperature

ln 2 in zero magnetic field, corresponding to a critical polarization Pc= 0.75 and TN= 700 ± 80 pK. Magnetic field B versus entropy S phase diagrams of the (001) structure have been constructed for two directions ofB: [001] and