Oliveira João, Silva Bruna M, Rebelo Tiago, Rodrigues Pedro V, Baptista Rosa M F, Rodrigues Manuel J L F, Belsley Michael, Lekshmi Neenu, Araújo João P, Mendes Jorge A, Deepak Francis Leonard, Almeida Bernardo G
Center of Physics of Minho and Porto Universities (CF-UM-UP), Laboratory for Materials and Emergent Technologies (LaPMET), Departamento de Física, Universidade do Minho, Campus de Gualtar, 4710-057 Braga, Portugal.
Nanostructured Materials Group, International Iberian Nanotechnology Laboratory (INL), Avenida Mestre José Veiga, 4715-330 Braga, Portugal.
Materials (Basel). 2024 Nov 22;17(23):5707. doi: 10.3390/ma17235707.
Laser ablation was used to successfully fabricate multiferroic bilayer thin films, composed of BaTiO (BTO) and CoFeO (CFO), on highly doped (100) Si substrates. This study investigates the influence of BaTiO layer thickness (50-220 nm) on the films' structural, magnetic, and dielectric properties. The dense, polycrystalline films exhibited a tetragonal BaTiO phase and a cubic spinel CoFeO layer. Structural analysis revealed compression of the CoFeO unit cell along the growth direction, while the BaTiO layer showed a tetragonal distortion, more pronounced in thinner BTO layers. These strain effects, attributed to the mechanical interaction between both layers, induced strain-dependent wasp-waisted behavior in the films' magnetic hysteresis cycles. The strain effects gradually relaxed with increasing BaTiO thickness. Raman spectroscopy and second harmonic generation studies confirmed BTO's non-centrosymmetric ferroelectric structure at room temperature. The displayed dielectric permittivity dispersion was modeled using the Havriliak-Negami function combined with a conductivity term. This analysis yielded relaxation times, DC conductivities, and activation energies. The observed BTO relaxation time behavior, indicative of small-polaron transport, changed significantly at the BTO ferroelectric Curie temperature (Tc), presenting activation energies E in the 0.1-0.3 eV range for T < Tc and E > 0.3 eV for T > Tc. The BTO thickness-dependent Tc behavior exhibited critical exponents ν ~ 0.82 consistent with the 3D random Ising universality class, suggesting local disorder and inhomogeneities in the films. This was attributed to the composite structure of BTO grains, comprising an inner bulk-like structure, a gradient strained layer, and a disordered surface layer. DC conductivity analysis indicated that CoFeO conduction primarily occurred through hopping in octahedral sites. These findings provide crucial insights into the dynamic dielectric behavior of multiferroic bilayer thin films at the nanoscale, enhancing their potential for application in emerging Si electronics-compatible magneto-electric technologies.
采用激光烧蚀法在高掺杂（100）硅衬底上成功制备了由钛酸钡（BTO）和钴铁氧体（CFO）组成的多铁性双层薄膜。本研究考察了钛酸钡层厚度（50 - 220 nm）对薄膜结构、磁性和介电性能的影响。致密的多晶薄膜呈现出四方相的钛酸钡相和立方尖晶石钴铁氧体层。结构分析表明，钴铁氧体晶胞沿生长方向被压缩，而钛酸钡层呈现出四方畸变，在较薄的BTO层中更为明显。这些应变效应归因于两层之间的机械相互作用，在薄膜的磁滞回线中诱导出应变依赖的蜂腰行为。随着钛酸钡厚度的增加，应变效应逐渐松弛。拉曼光谱和二次谐波产生研究证实了BTO在室温下的非中心对称铁电结构。利用哈夫里利亚克 - 内加米函数结合电导率项对所显示的介电常数色散进行了建模。该分析得出了弛豫时间、直流电导率和活化能。观察到的BTO弛豫时间行为表明存在小极化子输运，在BTO铁电居里温度（Tc）处发生了显著变化，对于T < Tc，活化能E在0.1 - 0.3 eV范围内，对于T > Tc，E > 0.3 eV。BTO厚度依赖的Tc行为表现出与三维随机伊辛普适类一致的临界指数ν ~ 0.82，表明薄膜中存在局部无序和不均匀性。这归因于BTO晶粒的复合结构，包括内部块状结构、梯度应变层和无序表面层。直流电导率分析表明，钴铁氧体的传导主要通过八面体位置的跳跃发生。这些发现为纳米尺度下多铁性双层薄膜的动态介电行为提供了关键见解，增强了它们在新兴的与硅电子兼容的磁电技术中的应用潜力。
