纳米颗粒（CoFeB）$_{x}$ -（SiO $_{2}$）$_{1-x}$的反常低温磁弹性性质
Anomalous low-temperature magnetoelastic properties of nanogranular (CoFeB)$_{x}$-(SiO$_{2}$)$_{1-x}$
摘要：
本文报道了具有制造诱导面内各向异性的颗粒膜（CoFeB）$_{x}$ -（SiO $_{2}$）$_{1-x}$的静磁测量结果。测量是在4.5 $÷$ ~ 300 K的宽温度区间内进行的。他们表明，上述薄膜在低于电导率逾渗阈值的情况下具有低温异常。上述特性的实质是在100 K以下沿易磁化方向的矫顽场温度依赖性严重偏离Neel-Brown定律。随着温度的降低，薄膜的矫顽力急剧增加，并在薄膜平面内出现沿硬方向磁化的矫顽场。我们确定，所观察到的效果是与个别铁磁性颗粒的属性。当颗粒在高于逾渗阈值的$x$下合并成团块时，该效应减弱，并且在$x>1$下完全消失。我们解释了上述效果的结果，在颗粒和覆盖材料的热膨胀系数的差异。在温度降低时，这种差异减弱了单个颗粒被覆盖基质材料的包裹，从而允许实现颗粒的自发磁致伸缩。后者诱导了一个额外的各向异性与新的易磁化轴的颗粒沿外磁场方向。在$T$ = 300 K和$T$ = 77 K下的铁磁共振测量结果证实了我们的解释。
英文摘要：
We report magnetostatic measurements for granulated films (CoFeB)$_{x}$-(SiO$_{2}$)$_{1-x}$ with fabrication induced intraplanar anisotropy. The measurements have been performed in the film plane in the wide temperature interval 4.5$÷$300 K. They demonstrate that above films have low-temperature anomaly below the percolation threshold for conductivity. The essence of the above peculiarity is that below 100 K the temperature dependence of coercive field for magnetization along easy direction deviates strongly from Neel-Brown law. At temperature lowering, the sharp increase of coercivity is observed, accompanied by the appearance of coercive field for magnetization along hard direction in the film plane. We establish that observed effect is related to the properties of individual ferromagnetic granules. The effect weakens as granules merge into conglomerates at $x$ higher then percolation threshold and disappears completely at $x>1$. We explain the above effect as a consequence of the difference in thermal expansion coefficients of granule and cover material. At temperature lowering this difference weakens the envelopment of an individual granule by the cover matrix material, thus permitting to realize the spontaneous magnetostriction of a granule. The latter induces an additional anisotropy with new easy axis of a granule magnetization along the external magnetic field direction. Our explanation is tested and corroborated by the ferromagnetic resonance measurements in the films at $T$ = 300 K and $T$ = 77 K.
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