14 research outputs found
Structure and magnetic interactions in the solid solution Ba3-xSrxCr2O8
Solid solutions of the magnetic insulators Ba3Cr2O8 and Sr3Cr2O8
(Ba3-xSrxCr2O8) have been prepared in polycrystalline form for the first time.
Single crys- talline material was obtained using a mirror image floating zone
technique. X-ray diffraction data taken at room temperature indicate that the
space group of Ba3-xSrxCr2O8 remains unchanged for all values of x, while the
cell parameters depend on the chemical composition, as expected. Magnetization
data, measured from 300 K down to 2 K, suggests that the interaction constant
Jd within the Cr5+ dimers varies in a peculiar way as a function of x, starting
at Jd = 25K for x = 0, then first slightly dropping to Jd = 18K for x = 0.75,
before reaching Jd = 62K for x = 3
A Supercooled Spin Liquid State in the Frustrated Pyrochlore Dy2Ti2O7
A "supercooled" liquid develops when a fluid does not crystallize upon
cooling below its ordering temperature. Instead, the microscopic relaxation
times diverge so rapidly that, upon further cooling, equilibration eventually
becomes impossible and glass formation occurs. Classic supercooled liquids
exhibit specific identifiers including microscopic relaxation times diverging
on a Vogel-Tammann-Fulcher (VTF) trajectory, a Havriliak-Negami (HN) form for
the dielectric function, and a general Kohlrausch-Williams-Watts (KWW) form for
time-domain relaxation. Recently, the pyrochlore Dy2Ti2O7 has become of
interest because its frustrated magnetic interactions may, in theory, lead to
highly exotic magnetic fluids. However, its true magnetic state at low
temperatures has proven very difficult to identify unambiguously. Here we
introduce high-precision, boundary-free magnetization transport techniques
based upon toroidal geometries and gain a fundamentally new understanding of
the time- and frequency-dependent magnetization dynamics of Dy2Ti2O7. We
demonstrate a virtually universal HN form for the magnetic susceptibility, a
general KWW form for the real-time magnetic relaxation, and a divergence of the
microscopic magnetic relaxation rates with precisely the VTF trajectory. Low
temperature Dy2Ti2O7 therefore exhibits the characteristics of a supercooled
magnetic liquid; the consequent implication is that this translationally
invariant lattice of strongly correlated spins is evolving towards an
unprecedented magnetic glass state, perhaps due to many-body localization of
spin.Comment: Version 2 updates: added legend for data in Figures 4A and 4B;
corrected equation reference in caption for Figure 4
The Anderson-Mott transition induced by hole-doping in Nd1-xTiO3
The insulator/metal transition induced by hole-doping due to neodymium
vacancies of the Mott- Hubbard antiferromagnetic insulator, Nd1-xTiO3, is
studied over the composition range 0.010(6) < x < 0.243(10). Insulating p-types
conduction is found for x < 0.071(10). Anderson localization in the presence of
a Mott-Hubbard gap, is the dominant localization mechanism for the range of
0.074(10) < x < 0.089(1) samples. For x < 0.089(1), n-type conduction is
observed and the activation energy extrapolates to zero by x < 0.1. The
0.095(8) < x < 0.203(10) samples are Fermi-liquid metals and the effects of
strong electronic correlations are evident near the metal-to-insulator
boundaries in features such as large Fermi liquid T2 coefficients. For 0.074(9)
< x < 0.112(4), a weak negative magnetoresistance is found below ~ 15 K and it
is attributed to the interaction of conduction electrons with Nd3+ magnetic
moments. Combining information from our companion study of the magnetic
properties of Nd1-xTiO3 solid solution, a phase diagram is proposed. The main
conclusions are that long range antiferromagnetic order disappears before the
onset of metallic behavior and that the Anderson-Mott transition occurs over a
finite range of doping levels. Our results differ from conclusions drawn from a
similar study on the hole doped Nd1-xCaxTiO3 system which found the
co-existence of antiferromagnetic order and metallic behavior and that the Mott
transition occurs at a discrete doping level
Phase diagram of the Shastry-Sutherland Compound SrCu2(BO3)2 under extreme combined conditions of field and pressure
Motivated by the intriguing properties of the Shastry-Sutherland compound
SrCu2(BO3)2 under pressure, with a still debated intermediate plaquette phase
appearing at around 20 kbar and a possible deconfined critical point at higher
pressure upon entering the antiferromagnetic phase, we have investigated its
high-field properties in this pressure range using tunnel diode oscillator
(TDO) measurements. The two main new phases revealed by these measurements are
fully consistent with those identified by infinite Projected Entangled Pair
states (iPEPS) calculations of the Shastry-Sutherland model, a 1/5 plateau and
a 10 x 2 supersolid. Remarkably, these phases are descendants of the
full-plaquette phase, the prominent candidate for the intermediate phase of
SrCu2(BO3)2. The emerging picture for SrCu2(BO3)2 is shown to be that of a
system dominated by a tendency to an orthorhombic distortion at intermediate
pressure, an important constraint on any realistic description of the
transition into the antiferromagnetic phase
Common glass-forming spin-liquid state in the pyrochlore magnets Dy2Ti2O7 and Ho2Ti2O7
Despite a well-ordered pyrochlore crystal structure and strong magnetic interactions between the Dy3+ or Ho3+ ions, no long-range magnetic order has been detected in the pyrochlore titanates Ho2Ti2O7 and Dy2Ti2O7. To explore the actual magnetic phase formed by cooling these materials, we measure their magnetization dynamics using toroidal, boundary-free magnetization transport techniques. We find that the dynamical magnetic susceptibility of both compounds has the same distinctive phenomenology, which is indistinguishable in form from that of the dielectric permittivity of dipolar glass-forming liquids. Moreover, Ho2Ti2O7 and Dy2Ti2O7 both exhibit microscopic magnetic relaxation times that increase along the super-Arrhenius trajectories analogous to those observed in glass-forming dipolar liquids. Thus, upon cooling below about 2 K, Dy2Ti2O7 and Ho2Ti2O7 both appear to enter the same magnetic state exhibiting the characteristics of a glass-forming spin liquid
Optical Observation of Striations in Y<sub>2</sub>Ti<sub>2</sub>O<sub>7</sub> Single Crystals
RE2Ti2O7 (RE = Y, Yb, Ho, Er) pyrochlores are very interesting as potential candidates for host materials for applications in transition-metal ions lasers. Y2Ti2O7 crystals were grown by the optical floating zone (OFZ) method. The shape of the growth interface is of paramount importance for the growth of single crystals. As striation and the growth interface have the same shape, we observed the striations in as-grown crystals under polarized light. The degree of overheating of the molten zone influences the shape of the growth interface. An increase of power supplied to the molten zone combined with a decrease of both, thermal conductivity and the amount of heat dissipated by the seed-rod, causes an increase in the degree of overheating of the floating zone. Under a high degree of overheating, the interface of the crystal grown is less convex, with smaller curvature. With the speed of rotation of these crystals decreasing from 30 to 7 rpm, the curvature of striations decreases and the shape of the growth interface changes from convex to less convex, and finally to concave
Magnetic nanopantograph in the SrCu[subscript 2](BO[subscript 3])[subscript 2] Shastry-Sutherland lattice
Magnetic materials having competing, i.e., frustrated, interactions can display magnetism prolific in intricate structures, discrete jumps, plateaus, and exotic spin states with increasing applied magnetic fields. When the associated elastic energy cost is not too expensive, this high potential can be enhanced by the existence of an omnipresent magnetoelastic coupling. Here we report experimental and theoretical evidence of a nonnegligible magnetoelastic coupling in one of these fascinating materials, SrCu[subscript 2](BO[subscript 3])[subscript 2] (SCBO). First, using pulsed-field transversal and longitudinal magnetostriction measurements we show that its physical dimensions, indeed, mimic closely its unusually rich field-induced magnetism. Second, using density functional-based calculations we find that the driving force behind the magnetoelastic coupling is the [^ over CuOCu] superexchange angle that, due to the orthogonal Cu[superscript 2+] dimers acting as pantographs, can shrink significantly (0.44%) with minute (0.01%) variations in the lattice parameters. With this original approach we also find a reduction of ~10% in the intradimer exchange integral J, enough to make predictions for the highly magnetized states and the effects of applied pressure on SCBO
Comparing Magnetism in Isostructural Oxides A0.8La1.2MnO4.1: Anisotropic Spin Glass (A = Ba) Versus Long Range Order (A = Sr)
This study presents the strikingly distinct magnetic properties of two isostructural compounds, Ba0.8La1.2MnO4.1 and Sr0.8La1.2MnO4.1 (K2NiF4 – type, I4/mmm). Spectroscopic studies have shown that Mn is in a +3.0(1) oxidation state only, in both compounds; therefore, the charge is balanced by accommodating extra oxygen at interstitials sites, as confirmed by neutron powder diffraction. We found that the Ba compound exhibits an exceedingly rare anisotropic spin glass behaviour, Tg = 26.4 K, with the moment freezing along the c-axis only while the in-plane spin components remain dynamic well below Tg. Experimental results including neutron diffraction, heat capacity, and magnetic (dc and ac) measurements performed on an oriented single crystal support this conclusion. This is a remarkable result, the only other known example of an anisotropic spin glass being Fe2TiO5. The spin glass state in Ba0.8La1.2MnO4.1 is argued to arise due to competing antiferromagnetic and ferromagnetic 180º Mn3+−O−Mn3+ superexchange interactions. In contrast, the Sr analogue shows 2D antiferromagnetic correlations and long range antiferromagnetic order below 95 K with a remarkably reduced ordered moment of 1.4 μB/Mn3+ instead of the ~ 4 μB expected for an S = 2 ion