Synthesis and physical properties of magnetically frustrated R2NiSi3 (R = Rare-earth) intermetallic compounds
Magnetically frustrated systems are a topic of great interest both from experimental and theo-retical points of view, because of their unique and intriguing structural and physical properties. Additionally, it has been recently predicted theoretically that frustrated magnetic systems can also be used for application purposes, due to its possible exhibition of large magnetocaloric ef-fect (MCE). However, only on very rare occasions, this particular theoretical prediction has been verified experimentally. We have synthesized new polycrystalline compounds of R2NiSi3 (R = Pr, Nd, Sm, Gd - Tm) type using arc melting technique. The X-ray diffraction data reveals that only Gd- and Er- analogues form in single phase having essentially full-stoichiometric composition. In our work, we have shown that the other compounds, when synthesized with deliberate and appropriate structural voids in the Ni and Si sites, could form in single phase. The detailed analysis of dc and ac magnetization as well as heat capacity data confirm the coexistence of long-range magnetic ordering and magnetically frustrated spin glass behavior for most the new magnetic analogues of R2NiSi3 series. However, neutron diffraction experi-ment for Ho2Ni0.95Si2.95 reveal much lower magnetic coherence length ( ˘ 35Å) in the system, which further confirms the absence of any true long-range ordering in the system. Neutron diffraction measurements for Er2NiSi3 and Tb2Ni0.90Si2.94 also support the coexistence of dif-ferent magnetic phases in these systems, where the magnetic coherence length remain finite. On the basis of dynamical scaling of ac susceptibility and theoretical analysis of detailed ex-perimentally observed non-equilibrium dynamics, viz., wait-time, temperature and field depen-dent magnetic relaxation behaviour, aging phenomena, and magnetic memory effects, it is sug-gested that Gd2NiSi3 is a canonical spin glass, Pr2Ni0.95Si2.95, Sm2Ni0.87Si2.87, Tb2Ni0.90Si2.94, & Dy2Ni0.87Si2.95 are spin cluster glass material and Nd2Ni0.94Si2.94 & Er2NiSi3 are reentrant spin cluster glass material. Ho2Ni0.95Si2.95 is an anomalous one, which shows neither long-range ordering nor spin freezing. We have estimated magnetocaloric effect for all the compounds that found to be quite large over a wide temperature range in most cases (except for Sm2Ni0.87Si2.87). Among all these compounds, Ho2Ni0.95Si2.95 exhibits giant magnetocaloric effect despite the absence of neither true long-range ordering nor spin freezing behaviour, which is rather hard to find in nature. Experimentally, we have shown that R2NiSi3 series of materials are one of the rare examples of magnetically frustrated materials, that support the theoretical prediction of getting large MCE in such systems.
Host: Prof. Je-Geun Park