Tuning the spontaneous exchange bias effect in ${\mathrm{La}}_{1.5}{\mathrm{Sr}}_{0.5}{\mathrm{CoMnO}}_6$ with sintering temperature

Here, we present a study of the influence of microstructure on the magnetic properties of polycrystalline samples of the ${\mathrm{La}}_{1.5}{\mathrm{Sr}}_{0.5}{\mathrm{CoMnO}}_6$ double perovskite, with primary attention to the spontaneous exchange bias effect, a fascinating recently discovered phenomena for which some materials exhibit unidirectional magnetic anisotropy after being cooled in zero magnetic fields. By sintering ${\mathrm{La}}_{1.5}{\mathrm{Sr}}_{0.5}{\mathrm{CoMnO}}_6$ at different temperatures, we obtained samples with distinct average grain sizes, ranging from 1.54–6.65 $\mu \mathrm{m}$. A detailed investigation of the material's structural, morphologic, electronic, and magnetic properties using x-ray powder diffraction, powder neutron diffraction, x-ray absorption near edge structure spectroscopy, scanning electron microscopy, and ac and dc magnetometry has revealed a systematic enhancement of the exchange bias effect with increasing the average grain size. This evolution is discussed in terms of changes in the material's porosity and grain morphology and its influence on the exchange couplings at the magnetic interfaces.

Figure 1

Rietveld refinement fittings of (a) PXRD and (b) PND patterns of LSCMO sample 1400. The vertical bars represent the Bragg reflections for the $R\overline{3}c$ space group. The top insets show magnified views of the PXRD patterns for all samples at the (102), (110), and (104) reflections.

Figure 2

SEM images, at 2000 times magnification, 10 mm working distance, and operating at 15 kV, for LSCMO samples (a) 1100, (b) 1200, (c) 1300, and (d) 1400.

Figure 3

ZFC-FC $M\left(T\right)$ curves measured with $H$ = 2.5 kOe for LSCMO samples (a) 1100, (b) 1200, (c) 1300, and (d) 1400. The insets show their respective ${\chi }^{-1}$ ($H$ = 2.5 kOe) curves measured up to 800 K, where the straight lines represent the best fit with the Curie-Weiss law.

Figure 4

(a) ${\chi }^{\prime }$ and (b) ${\chi }^{″}$ as a function of temperature for the LSCMO samples, measured with $H_{\text{ac}}$ = 5 Oe at five different frequencies. The insets show the evolution of $T_f$ with the frequency for each compound, where the solid lines represent the fits with Eq. (2).

Figure 5

IRM curves measured at 5 K on the LSCMO samples, normalized by their $t$ = 0 values. The solid lines represent the fits with Eq. (3).

Figure 6

ZFC $M\left(H\right)$ loops measured at 5 K on samples (a) 1100, (b) 1200, (c) 1300, and (d) 1400. The inset in (d) shows magnified views of the coercive fields for $M\left(H\right)$ curves measured on sample 1400 with field sweep rates of 100 and 200 Oe/s. (e) Evolution of $H_{\text{EB}}$ with $T_s$.