Abstract
Excitations in magnetic structures of the so-called spin-ice materials generate two different peaks in the specific heat and anomalies in entropy in the temperature interval between 0 and 1 K. These points are due to the existence of two low-energy excited global states which seem to transit from a bosonic condensate towards a magnetic neutral plasma in a narrow temperature interval between 0.05 ≤ T ≤ 1 K. In this paper, we determine the characteristic features of two states and we analyze the possibilities of existence of a BEC state and its phase transition to the magnetic plasma state from a model of two magnetic charge fluids. From the structural analysis of the many-body excitation states, we obtain theoretical results about entropy and specific heat since these two key physical magnitudes announce the phase transitions. We give criteria for distinguishing if some of these phase transitions is of either first or second order.
Highlights
The so-called “spin-ices”[1,2,3,4], the topological insulators[5] and the real magnets synthesized[6] can present excited low energy states represented in modifications in the magnetic structures that can be interpreted as entities whose behavior is similar to magnetic charges[1,2,3,4]
In order to describe the temperature evolution of the extensive state functions and their corresponding first and second temperature derivatives, in this scenario, we have considered the statistical thermodynamic theory in order to determine the specific heat and entropy starting from the thermodynamic potential or Helmholtz free energy F
Our research can be included within the genuine dumbbell model that is defined by the following logical segment: (i) in the first step, in vertices of tetrahedron pairs, spin-flips arise, which it implies the creation of confined magnetic dipoles under slight increases in temperature; (ii) in the second place Dirac’s strings are developed and in this stage, the second derivative of the thermodynamic potential increases and the specific heat decreases; (iii) in the third step, free magnetic charges progressively appear and the specific heat again increases up to a new maximum
Summary
These confined dipoles have complete similarity with microimagnets, or better said nanomagnets of dimensions of the height of each tetrahedron. The two energies corresponding to the fermionic phase are practically null, while that energy of the bosonic phase is fast increasing This difference is the cause for the existence of the intense peaks in the specific heat of Fig. 4a,b and the possible discontinuity in the derivative versus temperature of the Fig. 4b–d. A first physical reason is to consider the alternative idea that the change of spin symmetry from the boson channel to a fermion gas might occur in a continuous and progressive way; and a second reason is in to obtain an image for the specific heat more similar to that of experimental curves[26,27,42,43,44,47,48]
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