Specific heat and magnetocaloric effect of Pr1−x Ag x MnO3 manganites

Abstract

The magnetocaloric effect in alternating magnetic fields has been investigated in Pr 1-x Ag x MnO 3 manganites with x=0. 05-0.25. The stepwise reversal of the sign of the magnetocaloric effect has been revealed in a weakly doped sample (x=0.05) at low temperatures (~80 K). This reversal is attributed to the coexistence of the ferromagnetic and canted antiferromagnetic phases with different critical temperatures.The physical properties of Pr 1-x Ag x MnO 3 praseodymium manganites (A is a univalent metal Na, K, Ag, etc.) are strongly different from the properties of manganites of other rare_earth metals (La, etc.). This system does not undergo a metalinsulator phase transition characteristic of other manganites and its existence region is much narrower, x≤0.25 [1][2][3]. Moreover, the Curie temperature T C of this system is lower than 136 K [2] owing to a large difference between the ion radii of Pr and substituting cations Na, K, and Ag (r Pr =0.099 nm, r Na =0.116 nm, r K =0.138 nm, and r Ag =0.115 nm), which causes local distortions of the crystal lattice; a decrease in the Mn-O-Mn valence angle; a weakening of the exchange interaction; and, as a result, a decrease in T C [4].The effect of the substitution of univalent alkali metal atoms Na and K for Pr atoms on the electric and magnetic properties was analyzed in [1,2]. The structure and magnetic and electric properties of Pr 1-x Ag x MnO 3 were investigated in [3]. According to [3], the behavior of the electric resistivity ρ(T) down to low temperatures is of a semiconducting character and the temperature dependence of the magnetic susceptibility has anomalies characteristic of the paramagnetferromagnet phase transition. For this reason, Pr 1-x Ag x MnO 3 was classified as a ferromagnetic insulator whose Curie temperature depends on the doping level.In this work, we report the experimental results for the specific heat C P and magnetocaloric effect ΔT of the Pr 1-x Ag x MnO 3 ceramic samples with x=0.05, 0.1, 0.15, and 0.25 for temperatures T=77-300 K. The technology of the manufacture of the samples and their magnetic and electric properties were described in [3]. According to the measurements of the magnetic susceptibility, the paramagnetferromagnet phase transition is observed in all of the samples and T C depends nonmonotonically on the doping level: it increases sharply at weak doping, reaches a maximum at x=0. 15-0.20, and decreases smoothly with a further increase in the doping level.The specific heat was measured by the ac calorimetry method [5] and, to directly measure the magnetocaloric effect, a special method based on the measurement of the amplitude of sample temperature oscillations in the presence of a weak ac magnetic field was developed [6].The essence of this method is as follows. The external alternating magnetic field H=H 0 cosωt (H 0 is the amplitude and ω is the cyclic frequency) applied to a magnetic material induces oscillations of its temperature T=T 0 cos(ωt+ϕ), where ϕ is the phase shift of oscillations of the temperature wit...

Thermal and magnetocaloric properties of manganites La-=SUB=-0.7-=/SUB=-Sr-=SUB=-0.3-x-=/SUB=-Ba-=SUB=-x-=/SUB=-MnO-=SUB=-3-=/SUB=-

“…The S * values for the La 0.7 Sr 0.3−x Ba x MnO 3 system vary from 1.20 to 2.38 J/K, which is much less ideal values for the Ising model ( S * = R ln 2 = 5.7 J/K) and Heisenberg model ( S * = R ln 4 = 11.52 J/K) [10]. Such discrepancies between experimental data and theoretical values of S * are typical for manganites and have been repeatedly discussed in the literature [7,[15][16][17].…”

Теплоемкость и магнитокалорический эффект в манганитах Pr-=SUB=-0.6-=/SUB=-Sr-=SUB=-0.4-=/SUB=-Mn-=SUB=-1-x-=/SUB=-Fe-=SUB=-x-=/SUB=-O-=SUB=-3-=/SUB=-

“…Для оценки изменения магнитной энтропии S M использовались данные по теплоемкости C P (T, H). ческого образца Pr 0.6 Sr 0.4 MnO 3 в работе [7] получено S = 3.16 J/mol · K. Такая разница экспериментальных данных и теоретических оценок величины энтропии перехода характерна для манганитов [12,14,[18][19][20].…”