Spin‐Chain Incipient Magnetocaloric Effect and Rare‐Earth(R) Controlled Switching in the Haldane‐Chain System, R ₂ BaNiO ₅

ABSTRACT

We have experimentally investigated the magnetocaloric effect (MCE) of a prototype spin‐frustrated one‐dimensional spin‐chain system, the famous Haldane‐chain system, R ₂ BaNiO ₅ (R = Dy, Nd, Gd, Er). The compounds exhibit antiferromagnetic ordering with T _N of 58, 48, 55, and 42 K for Dy, Nd, Gd, and Er, respectively. The significant MCE is observed far above long‐range ordering, even in the paramagnetic region, which is attributed to the change in magnetic entropy due to short‐range spin correlation arising from (low‐dimensional) magnetic frustration. Such a spin‐chain incipient MCE above long‐range ordering is rarely reported. Interestingly, multiple magnetocaloric switching between conventional and inverse MCE (and vice versa) is observed below long‐range magnetic ordering, as a function of temperature and magnetic field, for the R = Nd, Dy, and Er members. However, such MCE switching is absent in the Gd member, which is an S‐state atom (L = 0). Our systematic investigation reveals that the interplay among crystal electric field (CEF), spin‐orbit coupling (SOC), and rare‐earth anisotropy governs spin reorientation and drives complex magnetic and lattice entropy changes. The maximum change of entropy for Er, Gd, Dy, and Nd is 7.8, 6.8, 4.0, and 1.0 J Kg ⁻¹ K ⁻¹ respectively.