arXiv subjects

B. G. Ueland

Publications and source records attributed to B. G. Ueland.

50 records · Page 3Linked to original sources

arXiv2007

Spin ice behavior in Dy2Sn2-xSbxO7+x/2 and Dy2NbScO7

We report the magnetic and thermal properties of Dy2Sn2-xSbxO7+x/2, x = 0, 0.25, and 0.5, and Dy2NbScO7. We find evidence for Ising-like single ion ground states in the Dy2Sn2-xSbxO7+x/2 materials. These materials possess nearly the same zero point entropy as the canonical spin ices Ho2Ti2O7 and Dy2Ti2O7, strongly suggesting that they have spin ice states at low temperatures. We also observe a somewhat reduced zero point entropy in Dy2NbScO7, which is possibly associated with the higher level of cation disorder. The ice-like states in these materials with cation disorder on the B-sites of the pyrochlore lattice provide new evidence for the robust nature of spin ice behavior in the presence of disorder.

cond-mat.str-el↗

arXiv2007

Non-monotonic zero point entropy in diluted spin ice

Water ice and spin ice are important model systems in which theory can directly account for zero point entropy associated with quenched configurational disorder. Spin ice differs from water ice in the important respect that its fundamental constituents, the spins of the magnetic ions, can be removed through replacement with non-magnetic ions while keeping the lattice structure intact. In order to investigate the interplay of frustrated interactions and quenched disorder, we have performed systematic heat capacity measurements on spin ice materials which have been thus diluted up to 90%. Investigations of both Ho and Dy spin ices reveal that the zero point entropy depends non-monotonically on dilution and approaches the value of Rln2 in the limit of high dilution. The data are in good agreement with a generalization of Pauling's theory for the entropy of ice.

cond-mat.str-el↗

arXiv2007

Structural Disorder and Properties of the Stuffed Pyrochlore Ho2TiO5

We report a structural and thermodynamic study of the "stuffed spin ice" material Ho2TiO5 (i.e., Ho2(Ti1.33Ho0.67)O6.67), comparing samples synthesized through two different routes. Neutron powder diffraction and electron diffraction reveal that the previously reported defect fluorite phase has short-range pyrochlore ordering, in that there are domains in which the Ho and Ho/Ti sublattices are distinct. By contrast, a sample prepared through a floating zone method has long range ordering of these sublattices. Despite the differences in crystal structures, the two versions of Ho2TiO5 display similar magnetic susceptibilities. Field dependent magnetization and measured recovered entropies, however, show a difference between the two forms, suggesting that the magnetic properties of the stuffed pyrochlores depend on the local structure.

cond-mat.mtrl-sci↗

arXiv2007

Magnetic Structure and Properties of the S = 5/2 Triangular Antiferromagnet $α$-NaFeO$_2$

The magnetic properties of $α$-NaFeO$_2$ are studied by neutron diffraction and magnetization measurements. An ordered phase with spins aligned along the b$_hex$ axis exists at low temperatures (T < 4 K). At intermediate temperatures (4 K < T < 11 K), the system passes through an incommensurate ordered phase before transforming into a short range ordered state at higher temperatures that persists up to at least 50 K. Although the short range ordering does not persist to room temperature according to neutron diffraction, the magnetic susceptibility does not follow Curie-Weiss behavior, even up to 320 K. This rich magnetic behavior can be understood qualitatively as a competition between different magnetic exchange interactions that are similar in magnitude. The delicate balance between these interactions makes $α$-NaFeO$_2$ a candidate for more detailed theoretical work to understand magnetic behavior in frustrated magnetic systems.

cond-mat.str-el↗

arXiv2006

Unconventional Dynamics in Triangular Heisenberg Antiferromagnet NaCrO2

We report magnetization, specific heat, muon spin rotation and Na NMR measurements on the S=3/2 rhombohedrally stacked Heisenberg antiferromagnet NaCrO2. This compound appears to be an ideal candidate for the study of triangular Heisenberg antiferromagnets with very weak interlayer coupling. While specific heat and magnetization measurements indicate the occurrence of a transition in the range 40-46 K, both muon spin rotation and NMR reveal a fluctuating regime extending well below T_c, with a peak of relaxation rate 1/T1 around 30 K. This novel finding is discussed within the context of excitations in the triangular Heisenberg antiferromagnets.

cond-mat.str-el↗

arXiv2006

Zero Point Entropy in Stuffed Spin Ice

The third law of thermodynamics dictates that the entropy of a system in thermal equilibrium goes to zero as its temperature approaches absolute zero. In ice, however, a "zero point" or residual entropy can be measured - attributable to a high degeneracy in the energetically preferred positions of the hydrogen ions associated with the so-called "ice rules".1,2 Remarkably, the spins in certain magnetic materials with the pyrochlore structure of corner-sharing tetrahedra, called "spin ice", have an equivalent degeneracy of energetically preferred states and also have been shown to display a zero point entropy.3,4,5,6,7 Here we report that we have chemically altered Ho2Ti2O7 spin ice by stuffing extra Ho magnetic moments into normally non-magnetic Ti sites surrounding the Ho tetrahedra. The resulting series, Ho2(Ti2-xHox)O7-x/2, provides a unique opportunity to study the effects of increased connectivity between spins on a frustrated lattice. Surprisingly, the measured zero point entropy per spin appears unchanged by these excess spins, and the dynamic freezing of the spins is suppressed to lower temperatures. The results challenge our understanding of the spin ice state, and suggest a new avenue for using chemistry to study both ice-like frustration and the properties of the broad family of geometrically frustrated magnets based on the pyrochlore structure.

cond-mat.str-el↗

arXiv2005

Slow spin relaxation in a highly polarized cooperative paramagnet

We report measurements of the ac susceptibility of the cooperative paramagnet Tb2Ti2O7 in a strong magnetic field. Our data show the expected saturation maximum in chi(T) and also an unexpected low frequency dependence (< 1 Hz) of this peak, suggesting very slow spin relaxations are occurring. Measurements on samples diluted with nonmagnetic Y3+ or Lu3+ and complementary measurements on pure and diluted Dy2Ti2O7 strongly suggest that the relaxation is associated with dipolar spin correlations, representing unusual cooperative behavior in a paramagnetic system.

cond-mat.stat-mech↗

arXiv2005

Magnetic frustration in the sawtooth lattice of ZnLn2S4 (Ln = Er, Tm, Yb) olivines

We report the magnetic properties of the ZnLn2S4 (Ln = Er, Tm, Yb) olivines, in which the magnetic lanthanide ions are in a potentially frustrated geometry consisting of sawtooth chains of corner-sharing triangles. Fits to the high temperature magnetic susceptibility yielded Curie-Weiss temperatures of theta ~ -4, -13, and -75 K for the Er, Tm, and Yb compounds respectively. Antiferromagnetic interactions indicated by these negative theta values, along with the absence of magnetic long range order above T = 1.8 K, suggest that geometrical frustration of magnetic ordering exists in these sulfide olivines as has been observed previously in the rare earth chalcogenide spinels.

cond-mat.str-el↗

arXiv2005

Geometrical Magnetic Frustration in Rare Earth Chalcogenide Spinels

We have characterized the magnetic and structural properties of the CdLn2Se4 (Ln = Dy, Ho), and CdLn2S4 (Ln = Ho, Er, Tm, Yb) spinels. We observe all compounds to be normal spinels, possessing a geometrically frustrated sublattice of lanthanide atoms with no observable structural disorder. Fits to the high temperature magnetic susceptibilities indicate these materials to have effective antiferromagnetic interactions, with Curie-Weiss temperatures theta ~ -10 K, except CdYb2S4 for which theta ~ -40 K. The absence of magnetic long range order or glassiness above T = 1.8 K strongly suggests that these materials are a new venue in which to study the effects of strong geometrical frustration, potentially as rich in new physical phenomena as that of the pyrochlore oxides.

cond-mat.str-el↗

arXiv2004

Quantum and thermal spin relaxation in diluted spin ice: Dy(2-x)MxTi2O7 (M = Lu, Y)

We have studied the low temperature a.c. magnetic susceptibility of the diluted spin ice compound Dy(2-x)MxTi2O7, where the magnetic Dy ions on the frustrated pyrochlore lattice have been replaced with non-magnetic ions, M = Y or Lu. We examine a broad range of dilutions, 0 <= x <= 1.98, and we find that the T ~ 16 K freezing is suppressed for low levels of dilution but re-emerges for x > 0.4 and persists to x = 1.98. This behavior can be understood as a non-monotonic dependence of the quantum spin relaxation time with dilution. The results suggest that the observed spin freezing is fundamentally a single spin process which is affected by the local environment, rather than the development of spin-spin correlations as earlier data suggested.

cond-mat.mtrl-sci↗

arXiv2003

Specific heat study of the Na(0.3)CoO(2).1.3H(2)O superconductor: influence of the complex chemistry

We report results of specific heat measurements on polycrystalline samples of the layered superconductor, Na(0.3)CoO(2).1.3H(2)O. The electronic contribution to the specific heat, gamma, is found to be 12.2 mJ/mol-K2. The feature at the superconducting transition is rather sharp, becoming broad and strongly suppressed in an applied magnetic field. The data indicate a residual normal state electronic specific heat at low temperatures, implying that there is a sizable population of normal state electrons in the samples even below Tc. Inhomogeneity in the Na content, to which the superconducting state is exquisitely sensitive, appears to be the most likely explanation for these results. These results further indicate that special sample handling is required for an accurate characterization of the superconducting state in this material.

cond-mat.supr-con↗

arXiv2003

Low Temperature Spin Freezing in Dy2Ti2O7 Spin Ice

We report a study of the low temperature bulk magnetic properties of the spin ice compound Dy2Ti2O7 with particular attention to the (T < 4 K) spin freezing transition. While this transition is superficially similar to that in a spin glass, there are important qualitative differences from spin glass behavior: the freezing temperature increases slightly with applied magnetic field, and the distribution of spin relaxation times remains extremely narrow down to the lowest temperatures. Furthermore, the characteristic spin relaxation time increases faster than exponentially down to the lowest temperatures studied. These results indicate that spin-freezing in spin ice materials represents a novel form of magnetic glassiness associated with the unusual nature of geometrical frustration in these materials.

cond-mat.mtrl-sci↗

arXiv2003

Large positive magnetocaloric effect in a perovskite manganite

We investigate the magnetothermodynamic properties of the perovskite manganite Pr0.46Sr0.54MnO3. Our data imply that the paramagnetic to antiferromagnetic transition at TN = 210 K is first order with an abrupt decrease of volume (~ 0.14 % in zero magnetic field) and is accompanied by a sharp anomaly in specific heat. Upon application of a sufficiently large magnetic field, the antiferromagnetic phase transforms into a ferromagnetic phase with a sharp increase in sample volume. This field induced transition results in a large positive magnetocaloric effect just below TN (∆Sm = 10.4 Jkg-1K-1 at H = 5.5 T), which is associated with the increasing stability of the antiferromagnetic state with decreasing temperature.

cond-mat.mtrl-sci↗

arXiv2003

Quantum-Classical Reentrant Relaxation Crossover in Dy2Ti2O7 Spin-Ice

We have studied spin relaxation in the spin ice compound Dy2Ti2O7 through measurements of the a.c. magnetic susceptibility. While the characteristic spin relaxation time is thermally activated at high temperatures, it becomes almost temperature independent below Tcross ~ 13 K, suggesting that quantum tunneling dominates the relaxation process below that temperature. As the low-entropy spin ice state develops below Tice ~ 4 K, the spin relaxation time increases sharply with decreasing temperature, suggesting the emergence of a collective degree of freedom for which thermal relaxation processes again become important as the spins become highly correlated.

cond-mat.mtrl-sci↗