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Gapped and Gapless Short-Range-Ordered Magnetic States with (1/2,1/2,1/2) Wave Vectors in the Pyrochlore Magnet Tb2+xTi2-XO7+δ



E. Kermarrec, D. D. Maharaj, J. Gaudet, K. Fritsch, D. Pomaranski, J. B. Kycia, Yiming Qiu, John R. Copley, M. M. P. Couchman, A. O. R. Morningstar, H. A. Dabkowska, B. D. Gaulin


Recent low temperature heat capacity (CP ) measurements on polycrystalline samples of the py- rochlore antiferromagnet Tb2+xTi2¿xO7+δ have shown a strong sensitivity to the precise Tb con- centration x, with a large anomaly exhibited for x0.005 at TC0.5 K and no such anomaly and corresponding phase transition for xless than or equal to}0. We have grown single crystal samples of Tbd2+x^Ti2¿xO7+δ, with approximate composition x = ¿0.001,+0.0042, and +0.0147, where the x = 0.0042 single crys- tal exhibits a large CdP^ anomaly at TC=0.45 K, but neither the x = ¿0.001 nor the x = +0.0147 single crystals display any such anomaly. We present new time-of-flight neutron scattering measure- ments on the x = ¿0.001 and the x = +0.0147 samples which show strong (1/2, 1/2, 1/2) quasi-Bragg peaks at low temperatures characteristic of short range antiferromagnetic spin ice (AFSI) order at zero magnetic field but only under field-cooled conditions, as was previously observed in our x = 0.0042 single crystal. Furthermore, the frozen AFSI state displays a gapped spin excitation spectrum around (1/2, 1/2, 1/2), with a gap of 0.1 meV, again similar to previous observations on the x = 0.0042 single crystal. These results show that the strong (1/2, 1/2, 1/2) quasi-Bragg peaks and gapped AFSI state at low temperatures under field cooled conditions are robust features of Tb2Ti2O7, and are not correlated with the presence or absence of the CP anomaly and phase transition at low temperatures. Further, these results show that the ordered state giving rise to the CP anomaly is confined to 0 less than or equal to} x less than or equal to} 0.01 for Tb2+xTi2¿xO^d7+δ, and is not obviously connected with conventional order of magnetic dipole degrees of freedom.
Physical Review B


pyrochlore magnet, short range order, neutron scattering


Kermarrec, E. , Maharaj, D. , Gaudet, J. , Fritsch, K. , Pomaranski, D. , Kycia, J. , Qiu, Y. , Copley, J. , Couchman, M. , Morningstar, A. , Dabkowska, H. and Gaulin, B. (2015), Gapped and Gapless Short-Range-Ordered Magnetic States with (1/2,1/2,1/2) Wave Vectors in the Pyrochlore Magnet Tb<sub>2+x</sub>Ti<sub>2-X</sub>O<sub>7+&#948;</sub>, Physical Review B, [online], (Accessed April 23, 2024)
Created December 9, 2015, Updated October 12, 2021