R. T. Vuorinen

Nuclear ferromagnetic ordering in silver at negative nanokelvin temperatures

Nuclear ferromagnetic ordering in silver, caused by the antiferromagnetic Ruderman-Kittel interactions, has been observed at negative absolute temperatures T>−1.9 nK. The ordered states, reached by adiabatic demagnetization after population inversion at initial entropies S<Sc=0.82 ℜ ln2, are described by a saturation of susceptibility to −1 and by nonhysteretic response to an external field. These observations are consistent with the domain configurations calculated by Viertiö and Oja in the mean field theory. The experimental critical entropy, however, does not agree with theoretical calculations. In addition to our NMR measurements leading to these findings, we also discuss basic thermodynamics at T<0 and the theoretically calculated domain configurations in simple terms.

Susceptibility and relaxation measurements on rhodium metal at positive and negative spin temperatures in the Nanokelvin range

We have measured the susceptibility of polycrystalline rhodium foils, down to nuclear spin temperaturesT = 280 pK and “up” toT = −750 pK. AtT > 0, the static susceptibility follows the antiferromagnetic Curie-Weiss law with θ = −1.8±0.3 nK. AtT < 0, the expected ferromagnetic behavior in the vicinity ofT = −0 changes into antiferromgnetic tendency around −6 nK. If only nearest and next nearest neighbor interactions are assumed, our data yieldJn/h = −17±3 Hz andJnnn/h = 10±3 Hz for the exchange interaction coefficients. We have also investigated the field and polarization dependence of the spin-lattice relaxation time τ1 at positive and negative nanokelvin temperatures; surprisingly τ1 is longer whenT< 0. Infields below 1 mT, the Korringa constant shows a strong decrease, which becomes more pronounced as the conduction electron temperature decreases. Observed behavior is consistent with conduction electrons scattering from magnetic impurities, provided that the impurity-induced contribution to spin-lattice relaxation is proportional to the inverse of the nuclear spin temperature in small fields.

Anomalous Spin-Lattice Relaxation in Dilute RhFe at Positive and Negative Nanokelvin Spin Temperatures

We have investigated at ultralow temperatures the spin-lattice relaxation in rhodium samples with about 10 p.p.m. of iron impurities. A strong decrease in the Korringa constant κ, dependent on the sign of the nuclear spin temperature T, was observed below 1 mT. Our data on the spin-lattice relaxation time τ1, measured at the electronic temperature Te = 120 μK, fit the equation 1/τ1 - 1/(κ0/Te) = 1/τimp1 (B, T), where the impurity term τimp1 (B, T) has a contribution proportional to the inverse nuclear spin temperature in small fields.

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,

Neutron studies of nuclear magnetism at ultralow temperature

{\text{S}}_c = 0.54R

Neutron diffraction determination of the nuclear spin ordering in Cu and Ag at nano‐ and subnano‐K temperatures (invited)

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

Neutron experiments on nuclear order in silver at pK temperatures

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