Mesoscopic correlations inTb2Ti2O7spin liquid

Abstract

We have studied the spin correlations with k= ( 1 2 , 1 2 , 1 2 ) propagation vector which appear below 0.4 K in Tb2Ti2O7 spin liquid by combining powder neutron diffraction and specific heat on Tb2+xTi2−xO7+y samples with x=0, 0.01, -0.01. The k= ( 1 2 , 1 2 , 1 2 ) order clearly appears on all neutron patterns by subtracting a pattern at 1.2(1) K. Refining the subtracted patterns at 0.07 K yields two possible spin structures, with spin-ice-like and monopole-like correlations respectively. Mesoscopic correlations involve Tb moments of 1 to 2 µB ordered on a length scale of about 20Å. In addition, long range order involving a small spin component of 0.1 to 0.2 µB is detected for the x= 0 and 0.01 samples showing a peak in the specific heat. Comparison with previous single crystals data suggests that the ( 1 2 , 1 2 , 1 2 ) order settles in through nanometric spin textures with dominant spin ice character and correlated orientations, analogous to nanomagnetic twins.Rare earth pyrochlores with geometrical magnetic frustration are model compounds to investigate classical spin ice as well as quantum spin ice physics. The short range ordered ground states with large degeneracy give rise to a large variety of interesting phenomena such as magnetic monopoles 1,2 , first order transition 3 , or unconventional magnetoelastic excitations 4,5 . These frustrated ground states are highly sensitive to small perturbations and to the presence of defects. Already in the 80s, Villain 6 predicted that for an arbitrary small concentration of defects the spinel lattice with pyrochlore structure could switch its ground state from spin liquid to spin glass. More recently many studies focus on the role of a minute amount of defects in pyrochlores. The crystal structure of R 2 Ti 2 O 7 pyrochlores (where R 3+ is a rare earth ion, Ti 4+ is non magnetic) accommodates oxygen vacancies, small off-stoechiometries in the R 3+ or Ti 4+ lattice, "stuffing" or site inversion. Oxygen vacancies introduce Ti 3+ defects instead of Ti 4+ ions, modifying the R 3+ crystal field and inducing local distortions, with various consequences on the low temperature magnetic properties and spin ice physics. In classical spin ice Dy 2 Ti 2 O 7 , oxygen depletion or a low level Dy stuffing in the Ti sites strongly slows down the monopole dynamics at very low temperature 7,8 and affects the nonequilibrium macroscopic properties 9,10 . In the quantum spin ice Yb 2 Ti 2 O 7 , small off-stoechiometry clearly affects the magnetic ground state 11 . It appears that precise studies of the magnetic defects are needed to resolve controversies concerning these unconventional ground states.Among the pyrochlores, Tb 2 Ti 2 O 7 is one of the most complex, the subject of numerous investigations since the discovery of its spin liquid character in 1999. The Tb 3+ short range correlated moments fluctuate 12 down to 20 mK at the time scale of the muon probe (10 −6 s). The origin of these fluctuations has been highly debated [13][14][15][16][17][18] . Several mechanisms have been pr...

Spin correlations of quantum spin liquid and quadrupole-ordered states ofTb2+xTi2−xO7+yKadowaki1, Wakita2, Fåk3 et al. 2019Phys. Rev. B121250View full textAdd to dashboardCite

“…8(s), which shows that it disappears at 0.4 K. We note that detector gaps of AMATERAS prohibited us from measuring Q = ( 1 2 , 1 2 , 1 2 ) and (0, 0, 2) reflections. The appearance of tiny magnetic Bragg reflections at Q = ( 1 2 , 1 2 , 3 2 ), ( 1 2 , 1 2 , 1 2 ), and (0, 0, 2) was reported only for samples with large quadrupole orders [26,28,38]. In order to complement our previous experimental data of the magnetic Bragg reflections shown in Fig.…”

Thermodynamic properties of quadrupolar states in the frustrated pyrochlore magnet Tb2Ti2O7

“…It is thus apparently different from the magnetic dipole order inferred by earlier theories [7,8]. This mysterious, or hidden, order is an important subject for the study of the actual nature of the ground state of Tb 2 Ti 2 O 7 [9][10][11][12][13][14][15][16][17][18].…”

Double vibronic process in the quantum spin ice candidate Tb2Ti2O7 revealed by terahertz spectroscopy

“…Despite effective antiferromagnetic interactions, Tb 2 Ti 2 O 7 features no long-range order [12,13], however short-range correlations are present over a broad temperature range [14][15][16]. Remarkably, the ground state supports elastic power law spin correlations [17,18], bearing some resemblance to the pinch point pattern observed in spin ices [19].…”