Magnetic structures in the beta -TbH(D)2+x system.

Abstract

We have measured the electrical resistivity, the susceptibility, and the neutron diffraction spectra of several \ensuremath{\beta}-TbH(D${)}_{2+\mathit{x}}$ specimens in the range 0\ensuremath{\le}x\ensuremath{\le}0.245. The samples present, in general, two magnetic phases at low temperature, one commensurate antiferromagnetic (AF) (phase 2), below T=${\mathit{T}}_{2}$, the other incommensurate AF (phase 1), between ${\mathit{T}}_{2}$ and ${\mathit{T}}_{1}$=${\mathit{T}}_{\mathit{N}}$, with an overlap region between the two of a relative width \ensuremath{\Delta}T/${\mathit{T}}_{2}$\ensuremath{\sim}0.1. For the pure dihydride, x=0, the propagation vector of phase 2 is ${\mathbf{k}}_{\mathit{c}}$=1/4(1,1,3) for T\ensuremath{\le}${\mathit{T}}_{2}$=16.0 K, that of phase 1 is ${\mathbf{k}}_{\mathit{i}\mathit{c}}$\ensuremath{\sim}(0.123,0.137,0.754). In the case x=0.18, the phase 2 has a ${\mathbf{k}}_{\mathit{c}}$=1/4(1,1,4) for T\ensuremath{\le}${\mathit{T}}_{2}$=32.5 K and phase 1 a ${\mathbf{k}}_{\mathit{i}\mathit{c}}$\ensuremath{\sim}(0.25,0.18,1). The ${\mathbf{k}}_{\mathit{i}\mathit{c}}$ for both concentrations vary slightly with temperature in their respective existence ranges 14.8\ensuremath{\le}T\ensuremath{\le}${\mathit{T}}_{1}$=19.0 K (for x=0) and 29\ensuremath{\le}T\ensuremath{\le}42 K for (x=0.18). The magnetic structures for x=0.245 are the same as for x=0.18, the existence ranges shifting to T\ensuremath{\le}${\mathit{T}}_{2}$=36 K and 31\ensuremath{\le}T\ensuremath{\le}${\mathit{T}}_{1}$=42 K. The magnetic manifestations are very sensitive to possible ordering of the excess hydrogens (x) into an octahedral sublattice of the type ${\mathit{DO}}_{22}$ occurring around 200 K for x\ensuremath{\gtrsim}0.1, as evidenced by quenching effects upon the magnetic lines. A magnetic phase diagram in the existence range of the \ensuremath{\beta} phase is proposed using the present results and the available data from the literature, showing the disappearance of the phase 2 in the intermediate x range 0.02\ensuremath{\lesssim}x\ensuremath{\lesssim}0.1.