M.F. Collins

Magnetic Excitations in the Heavy-Fermion Superconductor Uru2si2

Antiferromagnetic order and fluctuations in the heavy-fermion superconductor URu{sub 2}Si{sub 2} have been studied by magnetic neutron scattering. Below {ital T}{sub {ital N}}=17.5 K, URu{sub 2}Si{sub 2} is a type-I antiferromagnet with an anomalously small ordered moment of (0.04{plus minus}0.01){mu}{sub {ital B}} polarized along the tetragonal {ital c} axis. Dispersive resonant excitations exist in the ordered state with a zone-center gap of 0.43 THz. The excitations are polarized along the ordered moment and have a large dipolar matrix element, which suggests that they are coupled transitions between singlet crystal-field-like states. For energy transfer above 3 THz, peaks have not been identified in the magnetic excitation spectra, but instead a continuous spectrum of scattering peaked around the ordering wave vector indicates the presence of overdamped antiferromagnetically correlated spin fluctuations. Upon heating above {ital T}{sub {ital N}}, the resonant excitations abruptly become heavily damped but the magnetic scattering at higher energies does not change at {ital T}{sub {ital N}}. Instead, the disappearance of the antiferromagnetic modulation of the higher-energy scattering coincides with the maximum in the resistivity of URu{sub 2}Si{sub 2}.

Magnetic excitations in the heavy-fermion superconductor URu sub 2 Si sub 2

Antiferromagnetic order and fluctuations in the heavy-fermion superconductor URu{sub 2}Si{sub 2} have been studied by magnetic neutron scattering. Below {ital T}{sub {ital N}}=17.5 K, URu{sub 2}Si{sub 2} is a type-I antiferromagnet with an anomalously small ordered moment of (0.04{plus minus}0.01){mu}{sub {ital B}} polarized along the tetragonal {ital c} axis. Dispersive resonant excitations exist in the ordered state with a zone-center gap of 0.43 THz. The excitations are polarized along the ordered moment and have a large dipolar matrix element, which suggests that they are coupled transitions between singlet crystal-field-like states. For energy transfer above 3 THz, peaks have not been identified in the magnetic excitation spectra, but instead a continuous spectrum of scattering peaked around the ordering wave vector indicates the presence of overdamped antiferromagnetically correlated spin fluctuations. Upon heating above {ital T}{sub {ital N}}, the resonant excitations abruptly become heavily damped but the magnetic scattering at higher energies does not change at {ital T}{sub {ital N}}. Instead, the disappearance of the antiferromagnetic modulation of the higher-energy scattering coincides with the maximum in the resistivity of URu{sub 2}Si{sub 2}.

Spin waves in the triangular antiferromagnet CsMnBr3

We report neutron scattering measurements of the spin‐wave dispersion relations in the hexagonal triangular antiferromagnet CsMnBr3. The data in the easy plane are described satisfactorily in terms of a model with Heisenberg exchange parameter  J2=−0.0019 ±0.0002 meV between a magnetic atom and its six nearest neighbors and a single‐ion anisotropy parameter D=0.014±0.002 meV. In a magnetic field of 3.7 T along a 〈100〉 direction the variation of the staggered magnetization, measured at (1/3 1/3 1), as a function of temperature shows two critical phase transitions separated by about 1.7 K. The triangular antiferromagnetic phase that occurs below 8.6 K in zero field is split by the field into two phases with transitions at 8.9 and 7.2 K. The intermediate phase below 8.9 K is of an unknown nature.

Magnetic structures of the rare earth orthotitanites RTiO3; R = Tb, Dy, Tm and Yb

Abstract The magnetic structures of the rare earth orthotitanites, RTiO 3 , R = Tb, Dy, Tm and Yb, have been solved using neutron powder diffraction techniques. Two different types of magnetic structure have been found. One has the titanium and rare earth moments antiparallel along the c axis. The other structure has the rare earth moments in the ab plane with both ferromagnetic and antiferromagnetic components. In TbTiO 3 , the terbium moment of (8.1 ± 0.4)μ β has ferromagnetic and antiferromagnetic components along the [100] and [010] directions, respectively, with the moments lying at an angle of (36 ± 3)° to the [100] direction. In DyTiO 3 , the dysprosium moment of (9.7 ± 0.7)μ β has ferromagnetic and antiferromagnetic components along the [010] and [100] directions, respectively, with the moments making an angle of (31 ± 5)° to the [010] direction. TmTiO 3 has a thulium moment of (6.0 ± 0.4)μ β in a ferromagnetic array along the [001] direction. The average titanium moment in the orthotitanites is (0.7 ± 0.3)μ β in a direction antiparallel to the ferromagnetic component of the rare earth moment. The ytterbium moment in YbTiO 3 is quenched. It is found to be (1.7 ± 0.2)μ β assuming a moment direction along [001]. The rare earth moment directions are found to be remarkably consistent in the series RMO 3 , M = Ti, Cr, Fe and Al.

The magnetic phase diagram of UNi2Si2

Abstract Neutron diffraction and magnetization measurements as a function of applied magnetic field are reported for single crystal UNi 2 Si 2 . The combined results are used to construct a magnetic phase diagram for applied fields along c . The simple body-centered antiferromagnetic phase, stable in zero field from 55 K to 103 K, is suppressed completely by fields greater than 3.8 T. There is a reentrant triple point at 102.8 K and 2.1 T.

Antiferromagnetism and superconductivity in URu2Si2

Abstract We have performed neutron scattering measurements of the temperature dependence of the antiferromagnetic Bragg peak intensity for the heavy fermion system URu 2 Si 2 , as it enters the superconducting phase. Temperature scans were carried out in zero magnetic field and field scans through the upper critical field were also made with the field along the c -axis. The behaviour of the ordered moment upon entering the superconducting phase limits the possible types of pairing. Our results exclude isotropic singlet state pairing but d wave pairing is possible.

Magnetic phase transitions in UNi2Si2

We report neutron diffraction measurements on a single crystal of UNi2 Si2. Three magnetically ordering phases are observed below 123 K with the magnetic moment parallel to the c‐axis in all three phases. The low temperature (T<53 K) phase is a superposition of a commensurate spin density wave with wave vector q = (0 0 0.667±0.002) and a ferromagnetic ordered moment of 1.1±0.3μB, the intermediate (53 K<T<103 K) is a simple body‐centered tetragonal antiferromagnet with an ordered magnetic moment of 1.8±0.2μB and the high‐temperature phase is an incommensurate spin density wave with wave vector q = (0 0  0.744±0.002) at 110 K. The phase transitions at 53 and 103 K are first order.

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.

The magnetic structures of HoTiO3 and ErTiO3

Abstract The magnetic structures of rare-earth titanium perovskites, ErTiO 3 and HoTiO 3 , have been determined at 4.2 K by neutron diffraction. The Er 3+ moment of (8.5 ± 0.5) μ B lies along [001] and is colinear with the titanium moment of (-0.7 ± 0.3) μ B . The Ho 3+ moment of (8.1 ± 0.5) μ B is inclined at an angle of 24° to the bc plane and 32° to the ab pla so as to produce an antiferromagnetic ordering of the x component and a ferromagnetic ordering of the y and the z components. The titanium moment of (-0.55 ± 0.3) μ B lies in the bc plane but its precise direction has not been determined.

Magnetic excitations in CePd2Si2

The results of inelastic neutron scattering measurements on a single crystal of CePd2Si2 are presented. A broad magnetic contribution was observed near 20 meV at 20 K. Two different models were used to analyze this response: a single crystal‐field excitation with a slightly Q‐dependent energy, and two nearly degenerate crystal‐field excitations without dispersion. In both cases the excitations have an appreciable intrinsic width due to the strong coupling between the 4f electrons of Ce and the conduction electrons. It is remarkable that, despite the presence of this strong coupling, the reduction of the Ce moment to 0.66μB at 4.2 K seems to be solely due to crystal‐field effects.