Exchange bias in a nanocrystalline hematite/permalloy thin film investigated with polarized neutron reflectometry
Exchange bias in a nanocrystalline hematite/permalloy thin film investigated with polarized neutron reflectometry
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DOI：
10.1103/physrevb.86.054408
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发表时间：
2012-08-07
影响因子：
3.7
通讯作者：
Klose, F.
中科院分区：
文献类型：
--
作者：
Cortie, D. L.;Lin, K. -W.;Klose, F.
我们研究了赤铁矿 α-Fe2O3/坡莫合金 Ni80Fe20 双层薄膜，其中反铁磁层由 2 至 16 nm 范围内的小赤铁矿晶粒组成。在 40 K 的封闭温度以下，发生了明显的交换偏倚效应。相对于大晶粒外延膜中相同化合物的报道，交换偏倚的幅度有所增强。然而，封闭温度显着降低。由于已知块状 α-Fe2O3 的尼尔温度非常高 (860 K)，我们将交换偏压的低温起始归因于众所周知的有限尺寸效应，该效应抑制了纳米结构赤铁矿的莫林转变。偏振中子反射计用于对反铁磁界面处未补偿力矩层的浓度和长度尺度设定上限。发现数据与反铁磁界面处每 Fe 原子 1-2 nm 深度 0.5-1.0 mu(B) 的感应磁区一致。在偏置状态下分析了中子自旋翻转信号和自旋不对称性的场依赖性，发现第一次和第二次磁反转是通过不对称机制发生的。对于经过充分训练的坡莫合金环，通过铁磁域的面内自旋旋转，在两个矫顽场处对称地发生反转。
We investigated a hematite alpha-Fe2O3/permalloy Ni80Fe20 bilayer film where the antiferromagnetic layer consisted of small hematite grains in the 2 to 16 nm range. A pronounced exchange bias effect occurred below the blocking temperature of 40 K. The magnitude of exchange bias was enhanced relative to reports for identical compounds in large grain, epitaxial films. However, the blocking temperature was dramatically reduced. As the Neel temperature of bulk alpha-Fe2O3 is known to be very high (860 K), we attribute the low-temperature onset of exchange bias to the well-known finite-size effect which suppresses the Morin transition for nanostructured hematite. Polarized neutron reflectometry was used to place an upper limit on the concentration and length scale of a layer of uncompensated moments at the antiferromagnetic interface. The data were found to be consistent with an induced magnetic region at the antiferromagnetic interface of 0.5-1.0 mu(B) per Fe atom within a depth of 1-2 nm. The field dependence of the neutron spin-flip signal and spin asymmetry was analyzed in the biased state, and the first and second magnetic reversal were found to occur by asymmetric mechanisms. For the fully trained permalloy loop, reversal occurred symmetrically at both coercive fields by an in-plane spin rotation of ferromagnetic domains.