We study the spin reorientation process in cobalt using capacitance dilatometric measurements of the strain along the a- and c-axes of a single crystal and then apply a phenomenological theory of magnetostrictive interactions together with a theory of spin reorientation. We find no evidence of singularities in the behaviour at either of the temperatures at the beginning and end of this process. We calculate the temperature dependence of the magnetoelastic fourth-order single-ion anisotropy coefficient K4, and draw attention to the importance of higher order terms in the anisotropy. PACS Nos.: 74.25.Ha, 75.30.Gw, 75.50.Cc, 75.80.+q

Capacitance dilatometry was used to measure the relative thermal expansivity, ΔL/L 0 , of single crystals of sodium nitrate (NaNO 3 ) as a function of temperature. From these measurements, the phase transition at T f = 550 K shows characteristics of a discontinuous transition. A theory based on the competition between long-range interactions and the entropy gained by rotating NO 3 – groups at high temperatures predicts a discontinuous transition at T = T f with ΔL/L 0 ∼ (T f – T) ζ with ζ = 1. Our measurements yield ζ ≈ 1. PACS Nos.: 23.23.+x, 56.65.Dy

We introduce a novel interpretation of the sequence of commensurate lock-ins of the spiral wave vector, tau, observed in the helimagnetic phase of holmium in the presence of c-axis magnetic fields. This is the first successful model for the prediction of these lock-ins. We show that a model which combines a spin-slip model with an assumption concerning the spin structure along the c-axis in finite size “domains”, yields excellent predictions when the energetics of this system with an applied c-axis magnetic field are considered.

In the region between 20 and 132 K, holmium is in a spiral anti-ferromagnetic phase, with the wave vector, τ, of the spiral increasing from 0.16 to 0.28 reciprocal lattice units. Near simple values of τ (such as 14, which occurs near 100 K), a magnetic field can cause a “lock-in”, where the variation of τ with temperature is arrested over a small temperature range. Recently, Jensen [Phys. Rev. B 54 (1996) 4021] predicted that the temperature width of this lock-in should be very strongly dependent on the magnetic field orientation in the plane containing the c- and b-axis. We have used the N5 triple-axis spectrometer at the NRU reactor, Chalk River, to measure the temperature width of the τ = 14 lock-in in holmium as a function of the orientation of a 2.6 T magnetic field applied in the (c–b) plane: Our results are substantially in agreement with Jensen's prediction.

We have found that the observed commensurate values at which the temperature dependent, incommensurate, spiral turn angle in the magnetic structures of holmium becomes locked-in, are predictable on the basis of the limitations which finite size places on the possible angular steps, which would form a quasi-continuous devil's staircase in an infinite sample.

We report measurements of the intensity and position of the satellites arising from the helimagnetic structure of Ho in 1.4 and 3 T b-axis magnetic fields. There are a number of lock-in effects at commensurate values of the spiral wave vector τ; here we report on those in the temperature range 115–132 K. We confirm our previous observation of the lock-in at τ=5/18 between 125 K and the Néel temperature (132 K). We have discovered a two-phase region in which the 5/18 phase coexists with an incommensurate phase. In this two-phase region, the proportion of the 5/18 phase increases with temperature from onset at 119 K to full lock-in at 125 K. The existence of this two-phase region may have significant influence on interpretation of the magnetic phase diagram.

We report measurements of the intensity and position of the satellites arising from the helimagnetic structure of holmium in magnetic fields applied along different crystal axes, concentrating on lock-in effects at commensurate values of the spiral wave vector τ. We have made the first observations of the lock-in at τ=1/5 near 42 K in a 3 T c-axis field as well as the lock-in at τ=1/4 in a 3 T b-axis field. We have also observed a lock-in at τ=5/18 in a b-axis field near the Néel temperature of 132 K, in contrast to an earlier reported value of 8/29. This illustrates the origin of the splitting previously reported by us in magnetization and dilatometric measurements.

The authors report neutron scattering observations of the stabilization of the locked-in phase with spiral pitch of 1/4c* in holmium by a magnetic field applied along the b-axis. In contrast to a theoretical suggestion that stabilization of this phase by a c-axis field was due to imperfect alignment causing a small component of field in the basal plane, it is shown that the effect of a field of 3 T along the b-axis is to stabilize the phase over a somewhat smaller temperature range than the stabilized range in an identical field along the c-axis. The centre of the locked-in phase moves from 96 K in zero field to 103 K in a b-axis field of 3 T, while in a similar c-axis field the phase remains centred at around 96 K. The authors also observe a locked-in phase at a wave-vector of 5/18, in the temperature range from 125 K to the Neel transition.

Neutron scattering has been used to investigate the behaviour of holmium in a c-axis magnetic field in the temperature range 90–130K. The variation of the position (τ) of the fundamental magnetic satellites with temperature is field-independent The intensities of the 2τ satellites show dramatic variation with temperature.

X-band (∼ 9.4 GHz) EPR and thermal-expansion measurements have been performed on a single crystal of the high-Tc superconducting material Bi2Sr2CaCu2O8+δ in the temperature range 77–295 K. Cu2+ EPR spectra due to at least four different Cu2+ centers located at about 0.3 T, were observed at room temperature. The EPR spectrum disappeared almost completely below the superconducting transition temperature 84 K. The temperature variation of the EPR line intensity indicated the occurrence of a second-order phase transition at 84 K. Thermal-expansion measurements confirmed the occurrence of this phase transition. The thermal-expansion coefficients in the phases of the crystal above and below 84 K have been estimated.

Neutron diffraction measurements have been made in the (h0l) plane of holmium in a c-axis magnetic field of 3 T. Thermal expansivity measurements have also been made by capacitance dilatometry. The main focus of our investigation has been a region about 2 K wide near 96 K, where the helimagnetic structure locks into a commensurate state. We have also been able to observe the ‘‘2-τ’’ satellites (at twice the fundamental magnetic ordering q vector) at temperatures as high as 3 K below the Néel transition. In the region between the lock-in transition and the Néel point, the intensities of these satellites show interesting behavior with temperature and may be showing effects due to the anomalies which have been identified in our magnetization measurements.

We have measured the thermal expansion of single crystals of several tetra-alkylammonium metal bromides. The number and nature of the phase transitions found in these compounds is compared to the situation in the corresponding chlorides.

We present the results of magnetization measurements on single-crystal holmium using a SQUID magnetometer in the temperature range from 4 to 140 K in magnetic fields up to 5.5 T. In low fields (0.01 T) the magnetization versus temperature data show a spiral to conical transition at Tc=16 K and the Néel temperature at 132 K. In addition, we observe new anomalies in the temperature dependence of the magnetization along the a, b, and c axes at 20, 24, 42, and 98 K. These new anomalies appear at the same temperatures as observed by Bates et al. [J. Phys. C 21, 4125 (1988); 21, 4113 (1988)] in ultrasonic velocity measurements on holmium. These anomalies could be accounted for within the frame work of the ‘‘spin-slip’’ model of Gibbs and co-workers. In the c axis magnetization we observe a splitting of the Néel temperature in magnetic fields greater than 0.5 T. The H-T phase diagrams of the magnetic phases of holmium for fields in three directions (along the a, b, and c axes) are presented.

Using thermal expansion measurements on single crystals of compounds with general formula A2BX4 (A=tetra-alkylammonium group, B=metal ions, X=halogen), several structural phase transitions have been detected. The nature of these transitions and their dependence upon the metal ions is discussed.

Measurements of the magnetic structure of holmium in c-axis applied magnetic fields were made by neutron diffraction in the (h0l) plane. In applied fields of 1.7 and 2.2 T directed along the c-axis, the overall periodicity of the spiral structure locks into the commensurate superlattice value 4c0 over a finite range of temperature (approximately 2 K at 2.2 T) near 98 K, whereas no such effect is seen in the crystal in zero field. The commensurate plateau extends between two transitions bracketing 98 K reported in magnetization and dilatometry by Steinitz et al. [Phys. Rev. B 40, 763 (1989)]. No new satellites were found in this temperature range, but the intensities of the fundamental magnetic Bragg peaks exhibited precursor behavior as the commensurate state was approached in temperature both from above and below.

We report thermal expansion measurements on single crystals of ((CH3)4N)2CdCl4, ((CH3)4N)2CdBr4, ((C2H5)4N)2CdCl4 and ((C2H5)4N)2CdBr4 using capacitance dilatometry. Several phase transitions have been detected for each compound on warming and cooling the samples in the temperature range 77–300 K.

We present the results of magnetization measurements on single-crystal holmium using a SQUID magnetometer in the temperature range from 4.2 to 140 K in magnetic fields from 0.01 to 3 T applied along the b axis. Our magnetization data shows the N\'eel temperature to be ${T}_{N}$=132 K. In addition, we observe anomalies in the temperature dependence of the magnetization at 21, 42, and 98 K. These anomalies can be accounted for within the spin-slip model; Bohr et al. [Physica (Amsterdam) 140A, 349 (1986)] have pointed out the existences of ferrimagnetic structures spin slip and Cowley and Bates [J. Phys. C 21, 4113 (1988)] additional structures that breaks the hexagonal symmetry.

We have examined the behavior of two well-characterized single crystals of holmium in a magnetic field applied along the c axis in a temperature range from 90 to 140 K, using magnetization and dilatometric measurements. We have found several new phases in this previously unexplored region of the phase diagram.

We report thermal expansion measurements using capacitance dilatometric methods on several polar and non-polar compounds of the langbeinite family, from liquid helium to room temperature. In K2Mn2(SO4)3 and Tl2Cd2(SO4)3 new thermal expansion anomalies indicative of phase transitions were detected. Similarly, two phase transitions were detected for the first time at 227.8 K and 330.8 K respectively in Tl2Mg2(SO4)3. In (NH4)2Mg2(SO4)3 two phase transitions were discovered at 220 K and 241 K with decreasing temperature, although with increasing temperature only one phase transition could be detected.