Negative Spin Temperatures Research Articles

Studies of nuclear magnetism in silver at positive and negative nanokelvin temperatures

We have measured the entropy and susceptibility of silver nuclei down to T = 0.8 nK and T = - 4.3 nK. A ferromagnetic Curie-Weiss law was found at negative absolute spin temperatures, in contrast to the antiferromagnetic law at T > 0. For the Curie-Weiss θ we obtain −4.4 ± 1.0 nK. These results provide the first direct demonstration that the antiferromagnetic alignment at positive temperatures transforms into a ferromagnetic preference at T < 0 in a nuclear system where exchange interaction dominates.

Production of Negative Spin Temperature in Pulsed Magnetic Field

Adiabatic demagnetization of Cs3CoCl5 is performed in a pulsed magnetic field. Cooling of the electronic spin system and the resulting spin ordering are observed in the course of the demagnetizing process. By increasing the sweep rate, a time constant corresponding to the antiferromagnetic arrangement, residual magnetization after the reversal of the magnetic field and the subsequent anomalous relaxation phenomena are observed.

Ferromagnetic ordering of the proton spins in Ca(OH)2at negative spin temperatures

Experimental evidence is presented for a ferromagnetic ordering with domains of the proton spins in Ca(OH)2. This ordering was produced by dynamic nuclear polarisation (DNP) and adiabatic demagnetisation in the rotating frame (ADRF) and occurred at negative spin temperature. The transverse and longitudinal susceptibilities were measured and were in agreement with theoretical predictions for a ferromagnetic ordering with domain structure. Further evidence for such a type of ordering is contained in the relaxation behaviour of the dipole energy after ADRF. The observed relaxation behaviour was found to be due to spin diffusion between the ferromagnetic domains. From these relaxation data the size of the domains could be calculated.

Ferromagnetic ordering of the proton spins in Ca(OH)2

Presents experimental evidence for a ferromagnetic ordering of the proton spins in Ca(OH)2. This ordering was produced by adiabatic demagnetisation in rotating frame (ADRF) and occurred at negative spin temperature. The transverse and longitudinal susceptibilities were measured and were in agreement with theoretical predictions. Further evidence for the ferromagnetic type of ordering is contained in the relaxation behaviour of the dipolar energy after ADRF. The observed relaxation behaviour was found to be due to spin diffusion between the ferromagnetic domains. From these relaxation data the size of these domains could be calculated.

Nuclear antiferromagnetic susceptibilities of systems with two spin species

The static properties of a spin system with two species are investigated by the Monte Carlo technique. The Hamiltonian of the system is similar to the effective Hamiltonian describing nuclear spin-spin interactions under the experimental circumstances designed to produce nuclear dipolar magnetic ordering. The spin structures are determined at negative spin temperatures for various ratios, $R$, of the magnetic moments. The structures are of the antiferromagnetic type. For two values of $R$ we have calculated the temperature variation of the staggered magnetization of the two species as well as components of the bulk and staggered susceptibilities. It is shown that the mean-field theory is qualitatively---but not quantitatively---adequate to describe the transverse and longitudinal bulk susceptibilities for the two species in the antiferromagnetic phase. The results are discussed in relation to recent nuclear-magnetic-resonance experiments on LiF and neutron scattering experiments on LiH.

Static properties of spin systems with truncated secular dipolar coupling

The Monte Carlo technique is used to calculate the static properties of spin systems with an interaction described by a secular dipolar Hamiltonian with a finite range of interaction. The spins are arranged in a simple cubic lattice with periodic boundary conditions. The static spin structures are determined for the magnetic field along the three directions [001], [110], and [111], both at positive and at negative spin temperatures. The spin structures, which are of antiferromagnetic type, are compared to predictions based on the mean-field theory and to NMR experiments on Ca${\mathrm{F}}_{2}$. Two of the antiferromagnetic spin systems are investigated in detail and the following properties are calculated: the critical temperature, spontaneous sublattice magnetization, internal energy, heat capacity, bulk and staggered susceptibilities, and pair- and autocorrelation functions. Series expansions of a number of properties in the paramagnetic phase support the results of the Monte Carlo calculations. The Monte Carlo data are used to derive the critical exponents and critical amplitudes for the spontaneous sublattice magnetization, longitudinal staggered susceptibility, and inverse correlation length. The critical parameters for the two antiferromagnets are discussed within the concepts of the static scaling and universality hypotheses. The parameters are consistent with the classification of the antiferromagnets within the universality class of the three-dimensional Ising model.

Dipole magnetic ordering in nuclear spin-spin systems

Magnetic dipole-dipole interactions in a nuclear-spin system can lead to ordering of the spins only at temperatures ~10–6 °K. For nuclear antiferro- and ferromagnetic states to occur in this case, however, only the nuclear-spin system must be cooled to this temperature. This becomes possible in a number of cases, since the spin-lattice relaxation time becomes large enough already at a crystal-lattice temperature ~0.1–0.3 °K, and the spin system is well insulated from the lattice. We discuss dynamic methods of obtaining near-zero positive and negative spin temperatures in a solid by adiabatic demagnetization of the nuclear-spin system. A simple theory is described, which yields the possible ordered configurations of the spins in the crystals. Methods of observing ordered states are described, together with the results of recent experiments that confirm the onset of nuclear antiferro- and ferromagnetic states.

Theory of a proton spin maser with two emission coils

A nuclear maser oscillator with two emission coils, coupled solely by a stream of a prepolarized liquid which is initially at negative spin temperature, is investigated theoretically. The second emission coil is assumed to be tuned by a condenser to the proton spin resonance, while the first emission coil can be detuned. If the spin-spin interaction time is sufficiently long, then the voltage amplitude available from the terminals of the second coil can go through two maxima as the first coil is detuned. For maximum detuning of the first coil, a self-modulation effect can arise in the second resonance circuit.