Abrupt field-induced transition triggered by magnetocaloric effect in phase-separated manganites

Abstract

The occurrence at low temperatures of an ultrasharp field-induced transition in phase separated manganites is analyzed. Experimental results show that magnetization and specific heat step-like transitions below 5 K are correlated with an abrupt change of the sample temperature, which happens at a certain critical field. This temperature rise, a magnetocaloric effect, is interpreted as produced by the released energy at the transition point, and is the key to understand the existence of the abrupt field-induced transition. A qualitative analysis of the results suggests the existence of a critical growing rate of the ferromagnetic phase, beyond which an avalanche effect is triggered. PACS numbers: 75.30.Kz, 75.30.Sg, 75.30.Vn Mixed valent manganites show a great deal of fascinating properties, arising from the strong interplay between spin, charge, orbital, and lattice degrees of freedom [1]. The most intriguing one is the existence of a phase separated state, the simultaneous coexistence of submicrometer ferromagnetic (FM) metallic and charge ordered (CO) insulating regions [2]. The phase separation scenario has its origin in the unusual proximity of the free energies of these very distinct FM and CO states, and in the fact that the competition between both phases is resolved in mesoscopic length scales, giving rise to real space inhomogeneities in the material.Yet another surprising result more recently found in manganites is the appearance of ultrasharp magnetization steps at low temperatures (below ∼ 5 K) in the isothermal magnetization M (H) curves [3,4,5,6,7]. This effect, the field induced transition of the entire compound from one phase to the other of the coexisting states, is included in the category of metamagnetic transitions [8]. However, unlike the broad continuous transitions expected for inhomogeneous granular systems, in this case it occurs in an extremely narrow window of magnetic fields. These ultrasharp steps were observed in both single crystals and polycrystalline samples, indicating that it is not related to a particular micro-structure of the material.The actual existence of a phase separated state was recognized as a key parameter for the observation of these magnetization jumps [7]. The effect was first reported in manganites doped at the Mn site, and the disorder in the spin lattice was thought to play a relevant role [4]. However, a similar behavior was also found in Pr 0.6 Ca 0.4 MnO 3 , and the qualitative interpretation of the phenomenon shifted to the martensitic character of the phase separated state [6]. Accommodation strains were shown to be relevant in the stabilization of phase separation [9, 10], but their role in the magnetization steps is not clear, since it is expected that grain boundaries would act as a sort of "firewall" for the movement of the domain walls, stopping the avalanche process. Additionally, despite its intrinsic first-order character, the martensitic transformation is spread over a large range of the external parameter driving the transition, the magnetic ...

Avalanche-like metamagnetic transition in (LaNd)CaMnO manganites

“…This is confirmed by our Rietveld analysis, which yield an Mn-O-Mn bond angle changing from 164° in the sample with y = 0.1, to 155° for the y = 0.5 compound. identified as one of the main ingredients for the occurrence of abrupt metamagnetic transitions at low temperatures [11]. We further investigate this issue by measuring the temperature dependence of the magnetization of the y = 0.5 sample, with applied fields from 1 to 4 T, as shown in Fig 3. With a moderate field of 1 T (Fig.…”

“…In manganites both phases coexist in low magnetic fields, and the broad metamagnetic transitions observed, are the result of an increasing field dependent fraction of the ferromagnetic phase within the phase-separated material. But in addition, measurements at very low temperatures (< 4 K) have revealed a surprising and beautiful effect: the observation of a discontinuous metamagnetic transition at a critical magnetic field [9,10,11]. This phenomenon, referred to as ultrasharp magnetization jumps, avalanche-like or abrupt metamagnetic transition, has puzzled the scientific community since it was first reported [12].…”

“…It has been established that the critical field of the avalanche-like transition depends strongly on the magnetic field sweep rate [18,19,20,21]. This is a consequence of relaxation effects, which play a major role in the avalanche process, and yield the observation of a spontaneous transition [10,11,13] at a fixed magnetic field. The main question was, and to some extent still is, what causes these discontinuous magnetization jumps?…”

“…The explanation more widely accepted is that the avalanches are a consequence of the frozen disorder within the phase separated state, and the sudden release of an excess entropy is in large part responsible for triggering the magnetization jump [18]. It is also known that one necessary ingredient for the observation of avalanche-like magnetization jumps is the presence of blocked metastable states at low temperatures [8,11,18]. This can be experimentally observed through the zero field cooled magnetization, which must start well below the field cooled data due to frozen quenched disorder.…”

The Breaking of Brittle Materials

“…Transitions of mixed first order and continuous character have become something of a theme in the past decade or so. Recently we noted that the Mott transition and dielectric breakdown have a mixed character [31]; similar findings have been reported in a variety of fields such as jamming transitions, rigidity percolation [32], and phase-separated manganites [33].…”