Abstract
We have investigated the rotating magnetocaloric effect (R-MCE) of TmB4 - an anisotropic magnetic system with geometrical frustration of Shastry-Sutherland type. The R-MCE was obtained from detailed temperature dependencies of heat capacity in various magnetic fields of a single crystalline sample for crystal axes orientations c || B and c ⊥ B. The received results exhibit rather complex distributions of positive and negative entropy ΔS(T, B) and temperature ΔT(T, B) differences below and above TN when the direction of the magnetic field changes between directions c || B and c ⊥ B. The calculated results were confirmed by direct R-MCE measurements which, moreover, show an interesting angular dependence of R-MCE in the ordered phase, which seems to be related with the change of the effective magnetic field along the c axis during sample rotation. Thus, our study presents a new type of magnetic refrigerant with a rather large R-MCE for low temperature magnetic refrigeration, and points to further interesting magnetic features in the ordered phase of this frustrated system.
Highlights
The magnetocaloric effect (MCE) represents a magneto-thermodynamic phenomenon in which the temperature variation in magnetic material is caused by the change of external magnetic field[1,2,3,4,5]
In our contribution we present results of rotating magnetocaloric effect (R-MCE) investigations carried out on thulium tetraboride (TmB4), an anisotropic geometrically frustrated magnetic system
In our case the R-MCE investigation of TmB4 was based on detailed heat capacity C(T, B0) measurements in a wide T and B range from which the entropy difference ΔS and temperature ΔT were calculated using the method described in ref.[34]
Summary
Single crystals of TmB4 were grown by an inductive, crucible-free zone melting method. The difference between ΔTexp(T, B) and ΔT(T, B) layouts (e.g. the cooling minimum of ΔTexp(T, B) is observed at lower temperatures than this of ΔT(T, B)) is most probably associated with the fact that the sharp C(T, B) changes near TN (Fig. 2) which determine the ΔT(T, B) values, are in case of ΔTexp(T, B) estimation (due to the rather large heat capacity of the rotary calorimeter) considerably reduced. These results point to further interesting magnetic features in the ordered phase of this anisotropic frustrated system, and it will be interesting to investigate e.g. how the phase diagram and the plateaus will change depending on angle φ
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