D. A. Tindall

Simple, versatile, and compact capacitance dilatometer

We describe a novel capacitance dilatometer, which is simple to construct, compact in size, and which can be used with samples of any shape, size, conductivity, or dielectric constant.

Observation of transitions to spin‐slip structures and splitting of the Néel temperature of holmium in magnetic fields

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 Neel 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 Neel 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.

Observation of a commensurate temperature plateau induced by c -axis applied magnetic fields in holmium

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.

Studies of the magnetic phase diagram of holmium using neutron diffraction

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 Neel 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.

Investigation of the helimagnetic phases of holmium in a c -axis magnetic field

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 Neel transition. In the region between the lock‐in transition and the Neel 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.

Lock-ins in a b-axis field in holmium

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.

First- and second-order magnetic phase transitions in terbium and dysprosium

High-resolution capacitance dilatometric measurements on the a, b and c axes of terbium and dysprosium indicate that, to a resolution of 1*10-6, the Neel transitions of both methods are second order. This is both in contrast to the prediction of Barak and Walker (1982), and also in contrast to the case of holmium, which has a first-order Neel transition.

Discovery of τ=2/9 lock‐in in holmium

The complex magnetic behavior of holmium is a well‐established experimental fact and has been studied by a variety of experimental techniques, including magnetization, x‐ray diffraction, thermal expansion, and heat capacity. Our recent studies have focused on using neutron diffraction to study the temperature and field dependence of the helimagnetic structure. The pitch of the helix is described by τ, the spiral wave vector. τ usually varies smoothly with temperature but tends to lock in at various values that are commensurate with the lattice when a field is applied. These lock‐ins provide clues to the nature of the different magnetic phases and, up to now, all the lock‐ins have corresponded to features of the magnetic phase diagram obtained from magnetization measurements. However, recent experiments have revealed a previously unobserved lock‐in at τ=2/9 rlu in a 1.4 T b‐axis field. It shares the same general features of other lock‐ins except that it does not correspond to any feature of the magnetic ph...

Details of the magnetic phase diagram of holmium from neutron diffraction in b‐axis fields

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 Neel 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.

Finite size effects and the devil's staircase in holmium

Abstract 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 devils staircase in an infinite sample.