La0.67 Ca 0.33 MnO 3纳米颗粒中一级到二级磁相变的尺寸诱导转变
Size induced change-over from first to second order magnetic phase transition in La0.67Ca0.33MnO3 nanoparticles
摘要：
在这份报告中，我们表明，在钙钛矿锰氧化物La_{1-x} Ca_{x} MnO_3为固定的x \approx 0.33，磁性转变从一阶到二阶的颗粒尺寸减少到近几十纳米。这种转换仅通过减小尺寸而不改变化学计量来实现。尺寸减小到约15 nm的平均尺寸保持铁磁状态，尽管具有稍微更小的饱和磁化强度，并且铁磁转变温度T_{C}被少量（4%）抑制。在T_{C}附近的纳米颗粒的磁化遵循标度方程$M/|ε|^β= f_\pm(H/|ε|^{γ+β})$，其中，$ε= |T-T_C|/T_C$。由修正的Arrott图得到了与相变有关的临界指数，它们是$β=0.47\pm 0.01$和$γ=1.06\pm 0.03$。从T_{C}处的M与H的曲线图中，我们可以找到指数$δ=3.10 \pm 0.13$。所有的指数都接近平均场值。转变顺序的改变归因于由于尺寸减小引起的纳米颗粒中的压力增加而导致的衍生物dT_{C} /dP的值降低。该效应与由于纳米颗粒中的随机应变而引起的舍入效应串联起作用。
英文摘要：
In this report we show that in the perovskite manganite La_{1-x}Ca_{x}MnO_3 for a fixed x \approx 0.33, the magnetic transition changes over from first order to second order on reducing the particle size to nearly few tens of a nanometer. The change-over is brought about only by reducing the size and with no change in the stoichiometry. The size reduction to an average size of about 15 nm retains the ferromagnetic state albeit with somewhat smaller saturation magnetization and the ferromagnetic transition temperature T_{C} is suppressed by a small amount (4%). The magnetization of the nanoparticles near T_{C} follow the scaling equation $M/|ε|^β= f_\pm(H/|ε|^{γ+β})$, where, $ε= |T-T_C|/T_C$. The critical exponents, associated with the transition have been obtained from modified Arrott plots and they are found to be $β=0.47\pm 0.01$ and $γ=1.06\pm 0.03$. From a plot of M vs H at T_{C} we find the exponent $δ=3.10 \pm 0.13$. All the exponents are close to the mean field values. The change-over of the order of the transition has been attributed to a lowering of the value of the derivative dT_{C}/dP due to an increased pressure in the nanoparticles arising due to size reduction. This effect acts in tandem with the rounding off effect due to random strain in the nanoparticles.
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