Z. Tun

Magnetic phase transitions in CsCoBr3

Elastic neutron‐scattering measurements have been performed on CsCoBr3, an Ising‐like antiferromagnet on a stacked triangular lattice. Three magnetic phase transitions are seen in both zero field and a magnetic field of 2.2 T applied along the stacking direction c. The field does not qualitatively change the behavior near the phase transitions and the transition temperatures are affected only slightly. The high‐temperature ordered phase is known to be partially paramagnetic and the transition from the completely paramagnetic phase to this ordered phase was carefully studied. The transition is found to be continuous and characterized by the critical exponent β=0.22±0.01, a result that disagrees with previous measurement. The disagreement can be attributed to a large amount of temperature‐dependent paramagnetic scattering that exists in both the paramagnetic and the partially paramagnetic ordered phase.

Lifting of the ground-state degeneracy by crystal-field interactions in the pyrochlore

The results of susceptibility and neutron inelastic scattering measurements on the pyrochlore compound are presented. The crystal-field ground state of the ion has zero spontaneous magnetization, and a virtually dispersionless crystal-field excitation is observed. It is concluded that the crystal-field interaction dominates the exchange interaction in , lifting the expected degeneracy of the frustrated ground state.

The ferromagnet to spin-glass cross-over: A neutron depolarisation study

Abstract Mossbauer spectroscopy, magnetisation, susceptibility and neutron depolarisation have been used to investigate the evolution of magnetic order in a-Fe 90− x Ru x Zr 10 . Ru additions cause exchange frustration leading to a rapid reduction in T c and the appearance of transverse spin freezing at T xy . At x c = 2.5, T c drops abruptly to meet T xy and the system becomes a spin glass with no long-range order. The magnetic phase diagram is fully consistent with results of mean field and Monte Carlo simulations. The anomalous peak in the neutron depolarisation signal observed close to x c in this and other systems is shown to be due to the formation of a uniformly magnetised state induced by the 1 mT guide field.

Magnetic and crystal phase transitions in KNiCl3

Neutron scattering measurements revealed that even a seemingly perfect crystal of KNiCl3 at room temperature may segregate into two phases with different lattice distortions in the basal ab plane at temperatures below T≊270 K. In phase A, distortion leads to an increase of the unit cell size by a factor of 3, giving rise to nuclear Bragg peaks of type (h/3,h/3,l). In phase B the distortion gives rise to Bragg peaks at (3h/4,0,l), with integer h and l. Two different magnetic structures have been observed with TN=12.5 and 8.6 K in phases A and B, respectively. Magnetization measurements confirm the values of TN found by neutron scattering for the two magnetic structures.

Anodic Oxide Growth and Hydrogen Absorption on Zr in Neutral Aqueous Solution: A Comparison to Ti

Anodization and subsequent cathodic polarization of a thin-film sample of Zr were studied with in situ neutron reflectometry and electrochemical impedance spectroscopy (EIS). The results show the originally 485 A thick sputter-deposited film generally behaved similar to a bulk electrode in neutral Na 2 SO 4 solution. The anodic oxide, grown by applying 0.5 V potential steps, contained a significant amount of hydrogen in one form or another. The observed anodization ratio was somewhat higher than the literature value determined by coulometry, while the Pilling Bedworth ratio was in good agreement with published data. Oxide layer thickening, accelerated immediately after an anodic polarization was applied, persisted for many hours, suggesting the migration of ions continues for an extended time. The oxide cracked when the applied potential reached 1.5 V and its thickness reached ∼100 A causing loss of passivation. Surprisingly, the EIS behavior of the sample with cracked oxide could still be well represented by a single time-constant equivalent circuit consisting of a capacitor and a leakage resistor, albeit with much lower resistance. This indicates that the cracked and intact regions of the electrode behave essentially independent as parallel electrodes. The proposed modified equivalent circuit can explain a number of other unusual observations, including those seen under cathodic polarization.

Ru-induced loss of long-range magnetic order in a-Fe90−xRuxZr10

Mosbauer spectroscopy, magnetization, susceptibility, and neutron depolarization have been used to investigate the loss of long-range ferromagnetic order in a-Fe90−xRuxZr10. Our depolarization results show a rapid reduction in domain size from 1.6 to 0.4 μm as x increases from 0 to 2, with no evidence for long-range order in either depolarization or magnetization data for x⩾3. Furthermore, for x=1 and 2, we observe a clear break in the temperature dependence of 〈Bhf〉 which may be associated with ordering of transverse spin components in these partially frustrated alloys.

Critical soliton dynamics in CsCoBr3

Abstract The low-energy magnetic excitation spectrum of the stacked triangular lattice Ising-like antiferromagnet, CsCoBr3, has been studied near the transition from the paramagnetic state to partially-paramagnetic long-range ordered state. The results explicitly show that the transition proceeds via the divergence of the soliton, or propagating domain wall contribution to the excitation spectrum.

Electrochemical modifications on the surface of a Ti film

Abstract An in situ neutron reflectometry experiment on a Ti film, sputter-deposited on a Si substrate, shows in detail how the native oxide thickens when the film is anodized. The resultant oxide has two distinct regions: the inner region with the same scattering length density (SLD) as the original oxide and the outer region of lower SLD. The latter must therefore be highly porous or contain significant amount of hydrogen, most probably in the form of (OH)− ions. Subsequent measurements under cathodic potential show that the low-SLD region becomes thicker at the expense of high-SLD region but the overall thickness of the oxide layer remains constant. Rapid increase in electrode current begins to occur when the remaining high-SLD region becomes very thin. We conclude that the low-SLD oxide is relatively more conductive and does not provide as much protection against hydrogen ingress as the native oxide.

Ferromagnetic order in U2NiSi3

Abstract Neutron scattering and magnetization measurements have been performed on a single crystal of U2NiSi3. Development of long-range ordered ferromagnetic domains, instead of spin glass freezing, was observed below 30 K. The ordered moment is (0.6 ±0.1)μB per uranium atom and lies perpendicular to the hexagonal axis.

Long range antiferromagnetic order and its coexistence with superconductivity in URu2Si2

Abstract Neutron scattering from a high quality single crystal of the heavy fermion superconductor URu 2 Si 2 shows an abrupt onset of antiferromagnetic order at T N = 17 K, unlike the gradual onset previously seen in lower quality samples. The magnetic peak intensity increases linearly down to 5 K, indicating mean-field behaviour and long range RKKY interactions. The intensity remains constant to within 7% between 3 and 0.2 K with no change at the superconducting transition temperature of 1.3 K. The resolution limited Bragg peak shows the order is long-range. The coexistence of antiferromagnetism and superconductivity seen in a lower quality crystal is confirmed and thus is intrinsic.