We present a study on the impact of Mn$^{ 3+}$ substitution in the geometrically frustrated Ising garnet Ho$_3$Ga$_5$O$_{ 12}$ using bulk magnetic measurements and low temperature powder neutron diffraction. We find that the transition temperature, $T_N$ = 5.8 K, for Ho$_3$MnGa$_4$O$_{ 12}$ is raised by almost 20 when compared to Ho$_3$Ga$_5$O$_{ 12}$. Powder neutron diffraction on Ho$_3$Mn$_x$Ga$_{ 5-x}$O$_{ 12}$ ($x$ = 0.5, 1) below $T_N$ shows the formation of a long range ordered ordered state with $\mathbf{ k}$ = (0,0,0). Ho$^{ 3+}$ spins are aligned antiferromagnetically along the six crystallographic axes with no resultant moment while the Mn$^{ 3+}$ spins are oriented along the body diagonals, such that there is a net moment along [111]. The magnetic structure can be visualised as ten-membered rings of corner-sharing triangles of Ho$^{ 3+}$ spins with the Mn$^{ 3+}$ spins ferromagnetically coupled to each individual Ho$^{ 3+}$ spin in the triangle. Substitution of Mn$^{ 3+}$ completely relieves the magnetic frustration with $f = \theta_{ CW}/T_N \approx 1.1$ for Ho$_3$MnGa$_4$O$_{ 12}$.
Categories: cond-mat.str-el