Lyubutin Igor S, Lin Chun-Rong, Starchikov Sergey S, Baskakov Arseniy O, Gervits Natalia E, Funtov Konstantin O, Tseng Yaw-Teng, Lee Wen-Jen, Shih Kun-Yauh, Lee Jiann-Shing
Shubnikov Institute of Crystallography of FSRC "Crystallography and Photonics" RAS , Moscow 119333, Russia.
Department of Applied Physics, National Pingtung University , Pingtung County 90003, Taiwan.
Inorg Chem. 2017 Oct 16;56(20):12469-12475. doi: 10.1021/acs.inorgchem.7b01935. Epub 2017 Oct 2.
A series of nickel-chromium-ferrite NiFeCrO (with x = 1.25) nanoparticles (NPs) with a cubic spinel structure and with size d ranging from 1.6 to 47.7 nm was synthesized by the solution combustion method. A dual structure of all phonon modes revealed in Raman spectra is associated with metal cations of different types present in the spinel lattice sites. Mössbauer spectra of small NPs exhibit superparamagnetic behavior. However, the transition into the paramagnetic state occurs at a temperature that is unusually high for small particles (T is about 240 K in the d = 4.5 nm NPs). The larger NPs with d > 20 nm do not exhibit superparamagnetic properties up to the Neel temperature. From the magnetic and Mössbauer data, the cation occupation of the tetrahedral (A) and octahedral [B] sites was determined (FeNi)[NiCr]O. The saturation magnetization M in the largest NPs is about (0.98-0.95) μ, which is more than twice higher the value in bulk ferrite (Fe)[CrNi]O. At low temperatures the total magnetic moment of the ferrite coincides with the direction of the B-sublattice moment. In the NPs with d > 20 nm, the compensation of the magnetic moments of A- and B-sublattices was revealed at about T = 360-365 K. This value significantly exceeds the point T in bulk ferrites NiFeCrO (about 315 K) with the similar Cr concentration. However, in the smaller NPs NiFeCrO with d ≤ 11.7 nm, the compensation effect does not occur. The magnetic anomalies are explained in terms of highly frustrated magnetic ordering in the B sublattice, which appears due to the competition of AFM and FM exchange interactions and results in a canted magnetic structure.
采用溶液燃烧法合成了一系列具有立方尖晶石结构、尺寸d在1.6至47.7nm之间的镍铬铁氧体NiFeCrO（x = 1.25）纳米颗粒（NPs）。拉曼光谱中揭示的所有声子模式的双重结构与尖晶石晶格位点中存在的不同类型的金属阳离子有关。小尺寸纳米颗粒的穆斯堡尔光谱表现出超顺磁行为。然而，转变为顺磁态的温度对于小颗粒来说异常高（在d = 4.5nm的纳米颗粒中T约为240K）。尺寸d > 20nm的较大纳米颗粒在奈尔温度之前不表现出超顺磁特性。根据磁性和穆斯堡尔数据，确定了四面体（A）和八面体[B]位点的阳离子占据情况为(FeNi)[NiCr]O。最大纳米颗粒中的饱和磁化强度M约为(0.98 - 0.95)μ，比块状铁氧体(Fe)[CrNi]O中的值高出两倍多。在低温下，铁氧体的总磁矩与B亚晶格磁矩的方向一致。在d > 20nm的纳米颗粒中，在约T = 360 - 365K时发现了A和B亚晶格磁矩的补偿。该值显著超过了具有相似Cr浓度的块状铁氧体NiFeCrO中的T点（约315K）。然而，在尺寸d≤11.7nm的较小纳米颗粒NiFeCrO中，没有出现补偿效应。磁异常是根据B亚晶格中高度受挫的磁有序来解释的，这是由于反铁磁（AFM）和铁磁（FM）交换相互作用的竞争而出现的，并导致倾斜的磁结构。
