Magnetocaloric effect and critical behavior in a disordered ferromagnet La<inf>0.7</inf>Sr<inf>0.3</inf>Mn<inf>0.9</inf>Ti<inf>0.1</inf>O<inf>3</inf>

Abstract

Currently, hole-doped perovskite-type manganites with a generally chemical formula Ln 1-x A x MnO 3 (Ln 3+ and A 2+ are rare-earth and alkaline-earth ions, respectively) are still of intensive interest due to many intriguing features, typically the colossal magnetoresistance and magnetocaloric effects 1,2) . In order to understand their unusual magnetic and transport properties, many studies have been done through doping on the A site 1,3) . Other research works also carried out the substitution into the Mn site, which dramatically affects the magnetic and transport properties of manganites 4,5) . It has been found that the substitution into the Mn site not only changes the ratio of Mn 3+ /Mn 4+ , but also introduces lattice distortion . In an attempt further understand the nature of magnetic interactions in these compounds, we prepared a polycrystalline sample La 0 .7 Sr 0 .3 Mn 0 .9 Ti 0 .1 O 3 , and then studied carefully their magnetocaloric effect and critical behavior based on analyzing the magnetization versus magnetic-field data, M(H) . Here, La 0 .7 Sr 0 .3 Mn 0 .9 Ti 0 .1 O 3 was prepared by conventional solid-state reaction at 1300 0C for 24 h in air . Magnetic measurements were performed on a superconducting quantum interference device magnetometer . Spin dynamics was also investigated by means of electron spin resonance (ESR) spectroscopy (a JES-TE300) working at 9 .2 GHz . Our studies revealed the inverse susceptibility, χ -1 (T), obtained from magnetization versus temperature data above the Curie temperature (T C ), departing from the linear Curie-Weiss behavior . This is due to the presence of ferromagnetic (FM) clusters in the paramagnetic (PM) region . Assessments based on Banerjee's criteria indicate La 0 .7 Sr 0 .3 Mn 0 .9 Ti 0 .1 O 3 undergoing a second-order magnetic phase transition . Using the modified Arrott plot method and the scaling hypothesis, we have determined the critical parameters T C ≈ 234 K, β = 0 .372±0 .012, γ = 1 .171±0 .112 and δ = 4 .15±0 .01, as shown in Fig . 1 . Comparing with the theoretical models, we have found that β = 0 .372 is close to that expected for the 3D Heisenberg model (β = 0 .365), while γ = 1 .171 is close to the 3D Ising model value (γ = 1 .241) . These results reflect the existence of short-range exchange interactions in our unconventional ferromagnet of La 0 .7 Sr 0 .3 Mn 0 .9 Ti 0 .1 O 3 , which are related to FM clusters and/or magnetic inhomogeneity persisting in a wide temperature range even above T C . To prove the existence of FM clusters above T C , we have studied ESR spectra of La 0 .7 Sr 0 .3 Mn 0 .9 Ti 0 .1 O 3 . The spectra shown in Fig . 2 indicate that asymmetrical signals at low temperatures become Lorentzian at temperatures higher than T min ≈ 1 .3T C . Below T min , two typical peaks are observed . One peak is located at ~2 .5 kOe associated with the FM clusters, and the other at ~3 .5 kOe is associated with the PM phase . The sample is completely PM at temperatures above T min . Clearly, in the temperature range from T C to T min , t...