Magnetocaloric effect as a signature of quantum level-crossing for a spin-gapped system

Abstract

Recent studies dealing with magnetocaloric response of antiferromagnetic (AFM) spin systems have established that being superior coolants they have the potential to replace the paramagnetic salts, which are presently abundant in cooling technology. Presently we report an enhanced magnetocaloric effect (MCE) in one such spin system: N H4CuP O4.H2O (NCP), an AFM spin 1/2 dimer. Experimental magnetization and specific heat data have been presented and these data have been employed to evaluate entropy, magnetic energy and magnetocaloric properties. We witness a sign change in magnetic Grüneisen parameter across the level-crossing field BC . An efficient adiabatic cooling is observed at low temperature by tracing the isentropic curves in temperature-magnetic field plane. Energy-level crossover characteristics in NCP interpreted through MCE analysis are well consistent with the observations made from magnetization and specific heat data.Studying low dimensional quantum antiferromagnets have been at the forefront of both experimental and theoretical condensed matter physics due to novel nature of their ground states. This has led to a number of fascinating physical properties [1][2][3][4][5][6][7][8]. Spin-gapped compounds are a class of quantum spin systems, which possess an energy gap in the excitation spectra [9]. Examples of widely investigated spin-gapped systems include spin-dimers [10,11], spin-Peierls compounds [12,13], spin ladders [14,15], alternating spin chains [16][17][18], frustrated spin systems [19,20] etc. Antiferromagnetically exchange-coupled spin dimers are interesting to study as depending on the nature of their inter-dimer coupling they exhibit various kinds of magnetic excitation under the influence of external tuning parameters like magnetic field. For instance, field induced magnetic ordering has been witnessed and the results have been explained using the theory of Bose-Einstein condensation of magnons for compounds like T lCuCl 3 [10] and BaCuSi 2 O 6 [11]. Appearance of magnetization plateaus and Wigner crystallization of magnons are investigated for the dimerized system SrCu 2 (BO 3 ) 2 [21,22]. Spin dimer systems with significantly weak interdimer interaction can be considered as independent spin clusters. The energy spectrum of a spin 1/2 dimer consists of a singlet ground state and a 3-fold degenerate state which upon application of external magnetic field split into three states and evolve as the field changes. A level crossing between the ground state and the first excited state occurs when the magnetic field is increased through a critical field value. Thus the ground state undergoes a qualitative change and the first excited state becomes the new ground state. Such level crossing happens between two pure quantum states and ideally it is a zero temperature phenomena, although it is possible to capture its evidence by measuring the physical properties of the system at finite temperature. The present report demonstrates investigation of magnetocaloric effect (MCE) at field-induced level ...

Magnetocaloric Effect in Cu5-NIPA Molecular Magnet: A Theoretical Study

“…The analytic expressions for the critical fields as a function of the exchange integrals are also given in Appendix A. The fact that the energy levels cross when the external magnetic field is varied might be termed as quantum level crossing [79] (for example, in some analogy to the phenomenon emerging in a simplest system with interacting spins-a spin dimer [33,80], studied experimentally in the systems of various degree of complexity [80,81]). The change of the ground state of the system as a result of the variation of external parameter should induce the most profound consequences at zero temperature and at low temperatures, whereas the increase in T would smear this effect due to increased mixing of all states by thermal fluctuations.…”

“…On the other hand, the maximization of MCE is searched in systems with magnetic frustration resulting from the interplay between the antiferromagnetic interactions and the geometry, where the relatively small changes of magnetic field can cause large variations of the magnetic entropy by lifting the quantum state degeneracy [30][31][32]. Another interesting route is utilizing the quantum level crossings [33]. In addition, such ideas as rotational MCE exploiting strong magnetic anisotropy [34,35] are investigated in molecular systems.…”

Low-Temperature Magnetocaloric Properties of V12 Polyoxovanadate Molecular Magnet: A Theoretical Study

“…The ground state of V12 in the absence of the field has (and it is non-degenerate); applying the external field causes the subsequent transitions to states with and (magnetic saturation of the tetramer). Let us note that the existence of quantum level crossings has been suggested as a route to the maximization of the magnetocaloric response [ 41 ]. Moreover, the quantum level crossings in molecular magnets of non-interacting cluster type have attracted the attention of experimentalists [ 40 , 73 , 74 , 75 ].…”

“…The optimization of magnetocaloric performance of molecular magnets [ 33 , 34 , 35 ] involves, for example, strategies based on exploiting the frustration [ 36 , 37 ], achieving particularly high spins [ 38 ] or utilizing magnetic anisotropy in rotational MCE [ 39 ]. One of the approaches can be connected with the pronounced sensitivity of the magnetic entropy to the magnetic field variation in the vicinity of the quantum level crossing [ 40 ] in magnetic clusters [ 41 ]. This can serve as a motivation for the exploration of the thermodynamic properties of spin clusters with various geometries from the magnetocaloric point of view, especially with the use of the exact methods.…”

Proximity-induced spin-polarized magnetocaloric effect in transition metal dichalcogenides

“…When a material is at constant temperature, and subjected to external magnetic field changes, it experiences entropy changes, and the magnetocaloric effect (MCE) arises cooling or heating the sample [17][18][19]. The MCE has been analyzed in diverse magnetic structures [20], including a spin-gapped material [21], a one-dimensional spin-1/2 system [22], frustrated magnets [23], spin-1/2 2D lattices [24], nanomagnets [25], and superlattices showing large entropy changes due to exchange interactions [26]. In 2D layers of graphene and gold, the MCE shows an oscillatory behavior [27,28], and an external magnetic field is capable to control the entropy in TMDs [29].…”