Magnetoresistivity as a probe to the field-induced change of magnetic entropy inRAl2compounds(R=Pr,

J.C.P. Campoy

¹

,

E.J.R. Plaza

²

,

A. A. Coelho

³

et al.

Abstract

The heat capacity C P ͑T͒ of the ferromagnetic compounds RAl 2 ͑R =Pr,Nd,Tb,Dy,Ho,Er͒ was measured at zero and applied magnetic field of 5 T in the temperature interval from 2 to 200 K. From these results are calculated the magnetic component of the entropy change, −⌬S mag ͑T͒ = S͑0,T͒ − S͑H , T͒. From resistivity measurements, ͑H , T͒, from 2 to 300 K in the same compounds, we calculated the resistivity change due to the applied magnetic field, −⌬ mag ͑T͒ = ͓ mag ͑0,T͒ − mag ͑H , T͔͒. The results are compared and we observed a similar dependence between −⌬ mag ͑T͒ and ͑T / T C ͒ m ⌬S mag ͑T͒ with m = 0 for T ജ T C and m = 1 for T ഛ T C. A simple model using a Hamiltonian considering molecular and crystalline electric fields, in a mean field approximation, is adopted for the calculus. Our results show that theory and experiment are in good agreement showing that the magnetoresistivity is a probe to the field-induced change of magnetic entropy in these compounds and can be extended to other materials. A model for the factor connecting both quantities, −⌬S mag ͑T͒ and −⌬ mag ͑T͒, is developed. This factor contains mainly the effective exchange integral which is related to Fermi energy that in turn is related to the electron effective mass.

Thermal and Magnetic Properties in Ce1−xErxAl2 Intermetallic Compounds

“…If we choose R such that RAl 2 shows a ferromagnetic transition, interesting magnetic properties are expected to be observed in the pseudobinary system Ce 1−x R x Al 2 because of the competition between the ferromagnetic interaction of RAl 2 (x = 1) and the antiferromagnetic interaction of CeAl 2 (x = 0). In particular, ErAl 2 exhibits ferromagnetic ordering at approximately T C =13 K, 11) which is relatively close to the T N of CeAl 2 compared with the T C of the other RAl 2 compounds. Moreover, the easy axis of ErAl 2 is [111], 12,13) which is the same as that of CeAl 2 in the antiferromagnetic ground state.…”

Magnetocaloric Effect and Transition Order in HoAl

“…where magnetic eld H 0 = 0 and H j max = H with j = 1, ..., j max while σ i and σ i+1 were the values of magnetization at temperature T i and T i+1 , respectively [8]. Magnetic entropy change vs temperature, for several external magnetic elds, is shown in Fig.…”

Anisotropic magnetoresistivity and magnetic entropy change in HoAl2

“…They reported a satisfactory experimental correlation between ÀDS mag ¼ [S mag (0,T) À S mag (H,T)] and ÀDr mag ¼ [r mag (0,T) À r mag (H,T)] in the measured T-range along the [110] direction and above T C in the [100] direction. Such a correlation was first proposed by Campoy et al [1] for the RAl 2 polycrystalline series; they showed that DS mag $ T TC m ¼ F$Dr mag , where F is an adjust factor and m ¼ 1 for T < T C and m ¼ 0 for T > T C . This power-T-correction would be valid in the absence of elastic effects, negligible dependence of phonon spectra and Fermi energy, and carrier concentration with a magnetic field.…”

“…Also, one can relate some quantities with similar behavior to the evolution of the state of the system. For instance, the magnetoresistivity change (Dr mag ) of magnetic materials is of great interest, among other reasons, due to the possibility of connecting with the magnetic entropy change (DS mag ) [1]. In this respect, one subject of interest is to study the possibility of this connection in the presence of SR.…”