Enhanced grain surface effect on magnetic properties of nanometric La0.7Ca0.3MnO3 manganite: Evidence of surface spin freezing of manganite nanoparticles

Abstract

We have investigated the effect of nanometric grain size on magnetic properties of single phase, nanocrystalline, granular La 0.7 Ca 0.3 MnO 3 (LCMO) sample. We have considered core-shell structure of our LCMO nanoparticles, which can explain its magnetic properties. From the temperature dependence of field cooled (FC) and zero-field cooled (ZFC) dc magnetization (DCM), the magnetic properties could be distinguished into two regimes: a relatively high temperature regime T > 40 K where the broad maximum of ZFC curve (at T = T max ) is associated with the blocking of core particle moments, whereas the sharp maximum (at T = T S ) is related to the freezing of surface (shell) spins. The unusual shape of M (H) loop at T = 1.5 K, temperature dependent feature of coercive field and remanent magnetization give a strong support of surface spin freezing that are occurring at lower temperature regime (T < 40 K) in this LCMO nanoparticles. Additionally, waiting time (t w ) dependence of ZFC relaxation measurements at T = 50 K show weak dependence of relaxation rate [S(t)] on t w and dM/dln(t) following a logarithmic variation on time. Both of these features strongly support the high temperature regime to be associated with the blocking of core moments.2 At T = 20 K, ZFC relaxation measurements indicates the existence of two different types of relaxation processes in the sample with S(t) attaining a maximum at the elapsed time very close to the wait time t w = 1000 sec, which is an unequivocal sign of glassy behavior.This age-dependent effect convincingly establish the surface spin freezing of our LCMO nanoparticles associated with a background of superparamagnetic (SPM) phase of core moments.

Size effects and interactions in La0.7Ca0.3MnO3 nanoparticles

“…* Corresponding author. The magnetic behavior of the particle surface differs from that corresponding to the core [13][14][15], which usually displays a spin arrangement similar to that of the bulk material. A much higher magnetic disorder is present in the surface, giving rise to magnetic behaviors which cover from that of a dead magnetic layer to that of a spin glass-like.…”

Magnetic susceptibility studies of the spin-glass and Verwey transitions in magnetite nanoparticles

“…This behavior could be, partially, attributed to the Fe 3 O 4 nanoparticles crystallite size distribution which affects the magnitude of disordered surface area; but this is still under discussion. [14][15][16] Finally, because the spin-glass is a magnetic transition, when spin-glass transition occurs at the grain surface, the surface magnetic anisotropy at the grain would significantly increase, leading to an increment in coercivity at the T sg due to the strong coupling between the ordered spins inside the grain and the disordered spins at the surface. 9 To probe this, Figure 4 shows the hysteresis loops measured at temperatures of 5, 50, 90, 120, and 300 K. The inset shows that coercivity decreases to almost half its value when temperature goes from 5 to 50 K. This decrement at low temperatures ratifies the presence of a spin-glass transition in the Fe 3 O 4 nanoparticles.…”

Deviation from Bloch’s T3/2 Law and Spin-Glass-Like Behavior in La0.7Ca0.3MnO3 Nanoparticles

“…It was believed that the origin and nature of SG state in the nanostructured LCMO system were similar to the picture of hierarchical organization of metastable states. Evidences related to surface-spin freezing of nanopolycrystalline LCMO samples were observed by Dey and coworkers [4].…”

“…It is known that for perovskite manganites, when their size is reduced to the nanometer scale, they exhibit a number of outstanding physical properties; for example, low-field magnetoresistance, surface spin-glass (SG) behavior, superparamagnetism (SPM), and low-field saturation magnetization [1][2][3][4]. These phenomena have promoted many research works on nanosized manganite materials.…”