Magnetic frustration and paramagnetic state transport anomalies in Ho4RhAl and Er4RhAl: Possble test cases for newly identified roles of itinerant electrons

We report the results of magnetic, heat-capacity, electrical and magnetoresistance measurements on Ho4RhAl and Er4RhAl, characterized by 3 sites for rare-earths (R). The main conclusions are: (i) Anti-ferromagnetic ordering sets in at (TN=) ~ 8.8 and ~ 4.0 K respectively. While Ho compound appears to enter into a complex spin-glass phase at T < TN (at nearly 5 K), spin-glass component appears to set in essentially almost at TN for the Er case. (ii) The loss of the spin-disorder contribution in the magnetically ordered state is not pronounced, mimicking that in Gd2PdSi3 [Ref. 1], a compound which now attracts interest in the area of topological Hall effect and magnetic skyrmions [2], indicating complex Fermi sur-face. (iii) There is a minimum in the temperature dependence of electrical resistivity in the case of only Ho above TN, but significant negative magnetoresistance is observed over a wide temperature range in the paramagnetic state increasing with decreasing temperature for both the cases. This finding establish-es that these compounds belong to a select group of intermetallics in which spin-disorder contribution apparently increases gradually as one approaches respective TN with decreasing temperature. This could be an experimental signature for the effect due to classical spin-liquid above TN. In view of these prop-erties analogous to those of Gd2PdSi3, it is of interest to investigate these 4:1:1 compounds further to un-derstand possible unconventional roles of itinerant electrons, not only in the magnetically ordered state but also in the paramagnetic state, predicted by some theories [3] in recent years for which this Gd com-pound is considered to be a classic example.
Besides, magnetoresistance and isothermal entropy change (magnetocaloric effect) track each other. Iso-thermal entropy change at the peak in its plot versus temperature is large in comparison with that of the Gd analogue, possible implication of which is pointed out.
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[2] E.V. Sampathkumaran, arXiv:1910.09194 and references cited therein; also, see the link https://science.sciencemag.org/content/365/6456/914/tab-e-letters.
[3] C.D. Batista, S. Lin, S. Hayami, and Y. Kamiya, Rep. Prog. Phys. 79, 084504 (2016); Z. Wang, K. Barros, G.-W. Chern, D.L. Maslov, and C.D. Batista, Phys. Rev. Lett. 117, 206601 (2016); Z.Wang and C.D. Batista, Phys. Rev. B 101, 184432 (2020).