P. de V. du Plessis

Elastic constants of a NiO single crystal: I (Magnetic transitions)

The elastic constants of a single crystal of NiO are determined by an ultrasonic measuring technique. Near the Neel temperature clearly defined anomalies are observed in 1/2 (C11-C12) and 1/2(C11+C12+2C44) but not in C44. These results are discussed in terms of the thermodynamic theory for second order phase transitions and the necessity for further measurements are emphasized.

Large magnetic anisotropy in single crystal us

Abstract It is shown for ferromagnetic US that an extremely large anisotropy restrains magnetic moments to 〈111〉 easy axes, resulting in a near cos θ angular dependence of the magnetization away from the 〈111〉 axes. This is further confirmed by torque measurements, which in addition show large hysteresis effects upon rotation through the hard axes. It is illustrated through torque and magnetization measurements that a near stable domain configuration can be established by field rotation through decreasing angular amplitude around a hard 〈001〉 direction. The anisotropy constant K 1 ( T ) is estimated by computing the small angle through which the magnetization deviates from the 〈111〉 axes when a field is applied along the [001] direction.

A study of magnetic behaviour in us single crystals

Abstract The temperature and field dependences of the magnitization of ferromagnetic single crystal US along its easy 〈111〉 axis and hard 〈110〉 and 〈100〉 axes are reported. A considerable anisotropy is observed. The behaviour near the Curie temperature is described in terms of the Belov method of thermodynamic coefficients, as well as in terms of critical exponents. The temperature dependence of the spontaneous magnetization is investigated.

Ultrasonic and neutron diffraction studies of dilute Cr-Re and Cr-Ru alloy single crystals

Measurements are reported on the temperature dependence of the elastic constants of dilute Cr-Ru and Cr-Re alloy single crystals, containing respectively 0.3 at.% Ru, 0.5 at.% Ru, 0.3 at.% Re and 0.5 at.% Re. Neutron diffraction experiments were also performed on the two Cr-Ru crystals. Well defined anomalies were observed in all the elastic constants as well as in the integrated neutron intensities at the magnetic phase transitions of the alloys. The commensurate (C) spin density wave (SDW) to paramagnetic (P) and incommensurate (I). SDW to P transitions are second-order phase transitions, while the ISDW-CSDW and CSDW-ISDW transitions are of first order. A thermodynamic model describes the temperature dependence of the magnetic contributions ( Delta B) to the bulk modulus (B) reasonably well, but fails to give the observed relationship between Delta B and the magnetization.

Anomalies in the elastic constants of antiferromagnetic uranium mononitride

Abstract Measured anomalies in the elastic constants of a single crystal of cubic anti-ferromagnetic uranium mononitride are reported. A dip of about 10% occurs in C 11 at 6 K below the Neel point T N ⋍53 K . Starting at T N a renormalization in C 44 which is proportional to the square of the sublattice ,magnetization also occurs. The results are in general agreement with model calculations of the sound velocity renormalization due to spin-phonon interactions.

Critical behaviour of helical ordered dysprosium and terbium

Abstract Neutron measurements of the order parameter near the planar helical ↔ paramagnetic transition of dysprosium and terbium are reported. For Dy a critical exponent β = 0.39 (+0.04, -0.02) is obtained in agreement with the expectation from a renormalization group calculation of β − 0.39 for a n = 4-component model. For Tb, due to factors discussed in the text, no definitive conclusions about the value of β can be made.

Magnetic properties of single-crystal uranium mononitride

The susceptibility of a single crystal of antiferromagnetic uranium mononitride (TN ≃ 53 K) is reported between 4 and 1000 K. The paramagnetic susceptibility follows a modified Curie-Weiss law with a temperature-independent susceptibility χ0=8.0×10−6 emu cm−3 added, ϑ=−247 K, and 2.66 Bohr magnetons/U atom. Nearest and next nearest neighbor exchange constants are evaluated. The anisotropy in the ordered region is discussed on the basis of an equal distribution of antiferromagnetic domains with spin direction along any of the three 〈100〉 axes. Such an assumption is corroborated by thermal expansion measurements along the three 〈100〉 axes.

A neutron scattering investigation of the order of the magnetic transition in NiO

Abstract The measured sublattice magnetization of NiO indicates, in contradiction to theoretical expectations, a continuous second-order phase transition with a critical exponent β =0.38.

Hydrostatic pressure dependences of elastic constants and vibrational anharmonicity of uranium nitride

Measurements of the effect of hydrostatic pressure on ultrasonic wave velocities have been used to determine the pressure derivatives of the elastic stiffness of uranium nitride at room temperature.∂C44/∂P, and hence the Grüneisen parameter for the transverse mode propagating down an 〈001〉 axis, is negative; however, this mode softening is not anomalous for rocksalt structure crystals. The Grüneisen gammas of the acoustic modes obtained in the long wavelength limit have a pronounced anisotropy which accrues largely from the presence or absence of contributions to modes of vibration from nearest-neighbour repulsive forces. The compression of uranium nitride calculated from the Murnaghan equation of state is much smaller than those of the alkali halides or IV–VI compounds because this compound is much stiffer.

Magnetic susceptibility, electrical resistivity and elastic constants of antiferromagnetic un single crystals

Abstract Susceptibility and electrical resistivity measurements on UN indicate T N ⋍ 53 K. The spin-disorder resistivity is mainly proportional to 1- m 2 n ( m n is the reduced sublattice magnetization). The elastic constant C 44 shows a renormalization proportional to m 2 n , whereas C 11 exhibits an anomalous softening of 10% well below T N at 47 K.