Dale T. Teaney

Critical Point of the Cubic Antiferromagnet RbMnF3

We have studied the critical region of RbMnF3 (TN=83°K) using specific heat, x‐ray, and strain‐gauge measurements. Because RbMnF3 so closely resembles an ideal Heisenberg antiferromagnet, experimental results may be especially relevant to theoretical predictions. The specific heat is found to diverge logarithmically over three decades for T<TN, but only over two decades for T<TN. No distortion from cubic symmetry is detectable in the range 20° to 300°K. Finally a remarkably small thermal expansion anomaly is found in a narrow region near TN.

Mn55 Nuclear Magnetic Resonance in Antiferromagnetic RbMnF3

The observation and study of the Mn55 nuclear magnetic resonance in antiferromagnetic RbMnF3 at low temperatures are reported. The direct detection is possible because of a large enhancement of the signal from one of the NMR modes in a low anisotropy antiferromagnet above the critical field for spin flopping. Large frequency pulling effects are observed, and found to be in excellent quantitative agreement with theory. The unpulled NMR frequency has been determined as 687 Mc/sec at 2°K, and the corresponding linewidth is 60 kc/sec. An anomalous increase in linewidth with decreasing field and temperature is described.

Current‐ and Magnetic‐Field‐Induced Order and Disorder in Ordered Nematic Liquid Crystals

The voltage‐magnetic field phase diagram is explored for a nematic liquid crystal aligned between rubbed conductive glass plates. New patterns of long‐range order are found for ac excitation and nonzero magnetic field. The variation of conductance with changes of order and with the angle with respect to the nematic director have been measured. An unexpected onset of turbidity is found at a voltage proportional to applied field when current is flowing parallel to the director.

Crown Ether Complex Salts as Conductive Dopants for Nematic Liquids

Abstract series of crown ether complex salts was demonstrated to increase significantly the electrical conductivity of mesomorphic liquids for the purpose of inducing dynamic scattering. The cations of these salts, synthesized by well-known procedures, were the complexes of sodium and potassium primarily with dicyclohexyl- 18-crown-6, and organic anions of varying size, shape and flexibility were employed. The salts exhibited both satisfactory solubility arid degree of dissociation to be useful as charge carries in chlorostilbenes, Schiff bases and azoxybenzenes. The conductivity, cut-off frequency and threshold voltage for the (low frequency) electrohydrodynamic instability were measured for several combinations. The concentration dependence of the equivalent conductivity indicates that a large portion of the charge is carried by associated (triple) ions. In chlorostilbones, the available range of conduct, ivities with these dopants is about 10−12 to 10−7ohm−1cm−1.

Antiferromagnetic Resonance in Cubic RbMnF3

Antiferromagnetic resonance has been observed in cubic RbMnF3, and the results are in excellent agreement with a theoretical treatment of the resonance conditions. Because a more detailed publication has recently appeared,1 we present in this abstract a brief background and comments on the observed temperature dependence.Cubic RbMnF3 is characterized by a strong exchange interaction (TN≃54.5°K)2 and low anisotropy. The anisotropy energy has the form EA = K(α4+β4+γ4), where K is positive. When the sublattice magnetization is close to the preferred {111} direction, the anisotropy may be represented by an effective field of 5.0 Oe at 4.2°K. The exchange field is found to be 710 kOe. The exchange and anisotropy fields may be determined separately because the nuclear anisotropy field, HN, may be calculated and the exchange field obtained from 2H EHN = 2HE× (9.43/T).We are presently investigating the temperature dependence of the resonance field in the helium range. Within the experimental uncertainty of 2%, we...

Field and Temperature Dependence of 153Eu Spin‐Lattice Relaxation in Ferromagnetic EuO

Nuclear spin‐lattice relaxation times, T1, of 153Eu have been measured in an irregularly shaped single crystal of EuO over the temperature range of 1.2∘≦T≦4.2∘K and magnetic field range of 0≦H0≦14 kOe for the crystal orientation H0⊥Hrf∥〈110〉 by use of the spin‐echo technique. The relaxation process was found to satisfy T1 ∝ T−n, where n increases from approximately 2–3 as H0 increases from zero to 10 kOe. The T1 is essentially field independent up to H0≈2 kOe, Above this field there is a general increase in T1 but with an intermediate peak value at H0≈6 kOe. The analysis of these data is complicated by the irregular shape of the sample and the high saturation magnetization, 4πM0=24 kOe, for EuO. Within these limitations these results have been compared with the theory developed by Honma for EuS.