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Evidence of anomalous conventional and spontaneous exchange bias, high coercivity in Fe doped NiCr2O4 spinel
Dalton Transactions ( IF 3.5 ) Pub Date : 2020/03/04 , DOI: 10.1039/d0dt00124d G. C. Pandey 1, 2, 3, 4 , K. Nemkovski 5, 6, 7, 8 , Y. Su 5, 6, 7, 8 , Chandana Rath 1, 2, 3, 4
Dalton Transactions ( IF 3.5 ) Pub Date : 2020/03/04 , DOI: 10.1039/d0dt00124d G. C. Pandey 1, 2, 3, 4 , K. Nemkovski 5, 6, 7, 8 , Y. Su 5, 6, 7, 8 , Chandana Rath 1, 2, 3, 4
Affiliation
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NiCr2−xFexO4 (x = 0 and 0.2) polycrystalline ceramics have been synthesized successfully through a simple co-precipitation technique to study the evolution of structural and magnetic properties by doping Fe. X-ray diffraction (XRD) reveals that the high-temperature cubic phase (space group Fd![[3 with combining macron]](https://www.rsc.org/images/entities/char_0033_0304.gif)
m) observed at 320 K in bulk NiCr2O4 is stabilized at room temperature by decreasing the particle size to nanometer in x = 0 as well as after incorporating 20 at% Fe in the NiCr2O4 lattice. The cation distribution obtained from X-ray absorption fine structure (XAFS) analysis illustrates that while in x = 0, Ni2+ and Cr3+ ions occupy the tetrahedral (A) and octahedral (B) sites, respectively, x = 0.2, Fe3+ and Cr3+ ions occupy the A and B sites, respectively, and Ni2+ ions are distributed among the A and B sites. This transformation from the normal to mixed spinel structure strongly affects the magnetic properties. While the paramagnetic to long-range ferrimagnetic ordering temperature TC is enhanced from 71 to 192 K, significantly large coercive field (HC) of ∼29 kOe is observed for x = 0.2 as compared to the HC ∼13 kOe for x = 0. Moreover, unusually large conventional and spontaneous exchange bias fields of ∼26 and ∼2.6 kOe are observed for x = 0.2, which is absent for x = 0. The presence of anomalous exchange bias field is ascribed to the unidirectional exchange anisotropy between the two magnetic sublattices at A and B sites. The training effect of the exchange bias field is discussed using a phenomenological model, which considers the contribution from irreversible uncompensated spins that modify the exchange anisotropy at the interface between A and B magnetic sublattices. In addition, diffuse neutron scattering (DNS) with XYZ analysis is employed for both compositions to clearly illustrate the low-temperature peculiar magnetic phase transitions such as spin spiral transition, TS and spin lock-in transition, Tl. The DNS demonstrates that while Tl decreases from 10 K to 7 K with the incorporation of Fe in the NiCr2O4 lattice, TS significantly increases from 28 K to 50 K.
中文翻译：
掺铁的NiCr2O4尖晶石中常规和自发交换偏向异常，高矫顽力的证据
通过一种简单的共沉淀技术成功地合成了NiCr 2- x Fe x O 4（x = 0和0.2）多晶陶瓷，研究了掺杂Fe后结构和磁性能的变化。X射线衍射（XRD）显示，在320 K的块状NiCr 2 O 4中观察到的高温立方相（空间群Fdm）在室温下可通过将x = 0的粒径减小至纳米来稳定，以及在NiCr 2 O 4中掺入20 at％Fe之后格子。通过X射线吸收精细结构（XAFS）分析获得的阳离子分布说明，当x = 0时，Ni 2+和Cr 3+离子分别占据四面体（A）和八面体（B）的位置，x = 0.2， Fe 3+和Cr 3+离子分别占据A和B位，Ni 2+离子分布在A和B位之间。从正常的尖晶石结构转变为混合的尖晶石结构会极大地影响磁性能。尽管顺磁至远距离亚铁磁有序温度T C从71 K提高到192 K，但矫顽场（H C）〜29千奥斯特的用于观察X = 0.2相比，ħ Ç为〜13千奥斯特X为被观察的~26 = 0。此外，非常大的常规的和自发的交换偏磁场和~2.6千奥斯特X = 0.2，其x = 0时不存在。交换异常场的存在归因于A和B位置两个磁子晶格之间的单向交换各向异性。使用现象学模型讨论了交换偏置场的训练效果，该模型考虑了不可逆的无补偿自旋的贡献，这些自旋改变了A和B磁子晶格之间界面处的交换各向异性。此外，采用XYZ的扩散中子散射（DNS）采用分析两种组合物以清楚地示特有磁性相变低温如旋螺旋过渡，Ť小号和旋转锁定过渡，Ť升。DNS表明，随着在NiCr 2 O 4晶格中掺入铁，T 1从10 K降低到7 K ，而T S从28 K显着增加到50K。
更新日期：2020-04-08
m) observed at 320 K in bulk NiCr2O4 is stabilized at room temperature by decreasing the particle size to nanometer in x = 0 as well as after incorporating 20 at% Fe in the NiCr2O4 lattice. The cation distribution obtained from X-ray absorption fine structure (XAFS) analysis illustrates that while in x = 0, Ni2+ and Cr3+ ions occupy the tetrahedral (A) and octahedral (B) sites, respectively, x = 0.2, Fe3+ and Cr3+ ions occupy the A and B sites, respectively, and Ni2+ ions are distributed among the A and B sites. This transformation from the normal to mixed spinel structure strongly affects the magnetic properties. While the paramagnetic to long-range ferrimagnetic ordering temperature TC is enhanced from 71 to 192 K, significantly large coercive field (HC) of ∼29 kOe is observed for x = 0.2 as compared to the HC ∼13 kOe for x = 0. Moreover, unusually large conventional and spontaneous exchange bias fields of ∼26 and ∼2.6 kOe are observed for x = 0.2, which is absent for x = 0. The presence of anomalous exchange bias field is ascribed to the unidirectional exchange anisotropy between the two magnetic sublattices at A and B sites. The training effect of the exchange bias field is discussed using a phenomenological model, which considers the contribution from irreversible uncompensated spins that modify the exchange anisotropy at the interface between A and B magnetic sublattices. In addition, diffuse neutron scattering (DNS) with XYZ analysis is employed for both compositions to clearly illustrate the low-temperature peculiar magnetic phase transitions such as spin spiral transition, TS and spin lock-in transition, Tl. The DNS demonstrates that while Tl decreases from 10 K to 7 K with the incorporation of Fe in the NiCr2O4 lattice, TS significantly increases from 28 K to 50 K.
中文翻译：
掺铁的NiCr2O4尖晶石中常规和自发交换偏向异常，高矫顽力的证据
通过一种简单的共沉淀技术成功地合成了NiCr 2- x Fe x O 4（x = 0和0.2）多晶陶瓷，研究了掺杂Fe后结构和磁性能的变化。X射线衍射（XRD）显示，在320 K的块状NiCr 2 O 4中观察到的高温立方相（空间群Fd
m）在室温下可通过将x = 0的粒径减小至纳米来稳定，以及在NiCr 2 O 4中掺入20 at％Fe之后格子。通过X射线吸收精细结构（XAFS）分析获得的阳离子分布说明，当x = 0时，Ni 2+和Cr 3+离子分别占据四面体（A）和八面体（B）的位置，x = 0.2， Fe 3+和Cr 3+离子分别占据A和B位，Ni 2+离子分布在A和B位之间。从正常的尖晶石结构转变为混合的尖晶石结构会极大地影响磁性能。尽管顺磁至远距离亚铁磁有序温度T C从71 K提高到192 K，但矫顽场（H C）〜29千奥斯特的用于观察X = 0.2相比，ħ Ç为〜13千奥斯特X为被观察的~26 = 0。此外，非常大的常规的和自发的交换偏磁场和~2.6千奥斯特X = 0.2，其x = 0时不存在。交换异常场的存在归因于A和B位置两个磁子晶格之间的单向交换各向异性。使用现象学模型讨论了交换偏置场的训练效果，该模型考虑了不可逆的无补偿自旋的贡献，这些自旋改变了A和B磁子晶格之间界面处的交换各向异性。此外，采用XYZ的扩散中子散射（DNS）采用分析两种组合物以清楚地示特有磁性相变低温如旋螺旋过渡，Ť小号和旋转锁定过渡，Ť升。DNS表明，随着在NiCr 2 O 4晶格中掺入铁，T 1从10 K降低到7 K ，而T S从28 K显着增加到50K。
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