Double symmetry breaking in TmFe4Ge2 compared to RFe4Ge2 (R=Y, Lu, Er, Ho, Dy) magnetic behaviour

• Magnetic phase diagram of tetragonal TmFe4Ge2 compound studied by neutron diffraction.
• Unusual first order double complex magnetoelastic transition at TN,TC=44 °K.
• This phenomenon arises from strongly competing interactions of Tm–Tm, Fe–Fe and Fe–Tm with a different behaviour vs. T.
• The HT phase disproportionates into two distinct orthorhombic LT phases: P42/mnm→Cmmm (q1=0, 12, 0)+Pnnm (q2=0, qy,0) that vary portions linearly with T.
• The q1 and q2 phases have complex canted and incommensurate structures μTm||c. Typical for geometrical and exchange frustration systems.

TmFe4Ge2 undergoes a double magneto-elastic first order transition at TN,Tc where the high temperature (HT) tetragonal phase disproportionate into two distinct orthorhombic low temperature (LT) phases with commensurate and incommensurate magnetic wave vectors respectively:P42/mnm(HT)TN,Tc→Cmmmq1=(0,12,0)+Pnnm(q2=(0,qy,0),qy≈2/11(LT)Neutron diffraction shows the relative portions of the LT Cmmm and Pnnm competing phases change linearly with T. The amount of the majority HT phase Pnnm (54% at 30 K) decreases linearly to 30% down to 10 K in favour of the Cmmm phase that dominates the range 26–1.5 K. The Tm moments point along the c-axis in both phases while the Fe moments have canted arrangements. The μTm=3.54(3) μB/atom at 1.5 K is strongly reduced below the Tm3+ free ion value gJJ=7 μB for the q1 phase. The q2 phase corresponds to a 3D canted sinusoidal arrangement. The results are summarised on a phase diagram and compared to the findings in RFe4Ge2 (R=Y, Lu, Er, Ho, Dy) that are reviewed. The multitude of transition paths occurring in those systems arise from the competing magnetoelastic mechanisms involving the R-crystal field anisotropy, the exchange interactions R–R, R–Fe, Fe–Fe of the two sublattices and their coupling to the lattice strain. The geometrical frustration emerging from the compact tetrahedral Fe arrangement with antiferromagnetic interactions leads to 2D and 3D canted, incommensurate and non-magnetic states. The Cmmm transition is triggered by dominating R–R and R–Fe interactions becoming stronger at LT while the Pnnm phase is promoted by Fe–Fe and R–Fe interactions that prevail at HT. Included is also the magnetic structure of the ferromagnetic impurity phase Fe3Ge.