Magnetic structure, magneto-caloric properties and magnetic critical behaviours of LaMn2Ge2 compounds

The structural and magnetic properties of LaMn2Ge2 compound in both the as-cast bulk and melt-spun ribbon forms have been investigated by a comprehensive set of x-ray/neutron powder diffraction, magnetic and heat capacity measurements as well as corresponding sets of data analyses. Our neutron diffraction study reveals that with decreasing temperature the magnetic state of bulk LaMn2Ge2 changes first from paramagnetic to incommensurate antiferromagnetism AFfs at TN ~ 430 K, and then gives way to incommensurate canted ferromagnetism Fmi below TC ∼ 320 K. No noticeable magnetoelastic effect was detected in the temperature dependence of lattice parameters derived from the refinement of the neutron diffraction patterns over the temperature range ~ 5–460 K. Detailed analyses of the magnetic data indicate that the magnetic phase transition around the ferromagnetic transition (TC ∼ 320 K) is second order. Under field changes of 2 T and 5 T, the maximum values of magnetic entropy change around the ferromagnetic transition respectively reach -ΔSmax = 1.65 J/kg K and -ΔSmax = 3.26 J/kg K for the bulk sample, compared with -ΔSmax = 1.21 J/kg K and -ΔSmax = 2.60 J/kg K, for the ribbon sample. The magnetic phase transition around TC has been investigated by Kouvel-Fisher analysis and the Modified Arrott Plot method with the critical exponent values indicating that the magnetic interactions in LaMn2Ge2 are long range. Moreover, it was found that the field- and temperature- magnetisation data around TC collapse onto two curves obeying the single scaling equation M(H,ε) = εβf ± (H/εβ+γ) for both the bulk and ribbon samples. With a field change of ΔB = 5 T and ΔB = 8 T, the relative cooling power for bulk sample around 320 K is derived to be RCP ~ 115 J/kg and RCP ~ 199 J/kg, respectively. These findings indicate that LaMn2Ge2 could be a promising candidate for magnetic refrigeration applications in the room temperature region.