Functional Ferroic Materials, Films and Devices

Ferroic materials (e.g., ferroelectrics, ferromagnetics and multiferroics) have been extensively investigated due to their interesting physical properties. The ferroelectric and ferromagnetic properties of ferroic materials can be tuned by external stimuli (e.g., electric field, magnetic field, strain field), which are indispensable for applications in microelectronic devices. Ferroic materials and their coupling interactions can generate many physical phenomena, such as charge ordering, magnetoelectric coupling, etc. We can convert one of their properties into another form and develop materials with multifunctionalities. Therefore, systematical exploration of and summarization of knowledge regarding ferroic materials are crucial for the development and application of functional oxide materials [1]. This article’s objective is to provide an inventory of novel research in this field of functional oxide materials, focusing in particular on the preparation and properties of ferroelectric, ferromagnetic and multiferroic materials.

Ferroelectric materials are functional materials with spontaneous polarization, and spontaneous polarization is switchable and can be reversed by an external electric field. Therefore, these materials are technologically important for many applications (e.g., ferroelectric memory, sensor and energy storage devices), and they have thus received continuous attention [2]. For example, Hsu et al. found that the ferroelectric capacitor can dominate the transfer characteristics of a p-type SnO thin-film transistor through the modulation of series capacitance in the gate stack based on a one-transistor one-capacitor series configuration [3].

Lead-based ceramics such as (1 − x)Pb(Mg_1/3Nb_2/3)O₃–xPbTiO₃ and Pb_0.97La_0.02(Zr_0.97Ti_0.03)O₃ have been widely used in industry due to their suitable performance, such as piezoelectric and energy storage properties. However, as lead is harmful to the environment and to humans, lead-free materials have been studied in the last decade to substitute lead-based materials. For example, Li et al. prepared Bi₂O₃-doped (K_0.5Na_0.5)NbO₃ (KNN) lead-free ceramics and found that Bi₂O₃ doping not only enhanced remanent polarization(Pr) from 8 µC/cm² for KNN to 18 µC/cm² for KNN-0.3%Bi₂O₃ due to the creation of an orthogonal–tripartite phase at room temperature but also reduced the grain size and improved the grain size uniformity of ceramics [4]. Li et al. found that piezoelectric constant d₃₃ = 678 pC/N and electromechanical coupling coefficient K_p = 0.583 of (Ba_0.85Ca_0.15)(Zr_0.1Ti_0.9)O₃ ferroelectric ceramics could be achieved by the co-doping of CeO₂ and Y₂O₃ [5].

Although three-dimensional (3D) ferroelectric materials have been proposed for different applications and can be easily fabricated using various methods at the laboratory level, their practical applications are still limited by certain problems, such as their deep thickness, uneven grain size and their many crystal defects. Therefore, two-dimensional (2D) ferroelectric thin films with smooth surfaces and stable polarization are promising for the development of electronic applications. For example, Griesiute et al. obtained La-substituted BiFe_0.5Sc_0.5O₃ perovskite thin films with different crystal structures and roughness through fabricating films on different substrates [6]. Similarly, Magalhaes et al. grew Na_0.5Bi_0.5TiO₃ (NBT)-based epitaxial films on different single crystalline substrates and found that the films showed excellent ferroelectric and dielectric properties (Pr = 1.2 µC/cm², Ec = 58 kV/cm for 1 μm-NBT-BT films), and the roughness of films strongly was found to depend on film thickness [7]. Tkach et al. proved that variation of ferroelectric and dielectric properties of KNN-based thin films can be obtained by controlling the strain/stress level on the films by changing the substrates [8]. Furthermore, ferroelectric film/silicon carbide (Ba_0.4Sr_0.6TiO₃/SiC) structures were prepared and obtained a combination of high tenability, low losses and high dissipated power density of 125 W/mm² at microwaves [9].

Ferromagnetic materials, referring to the magnetic domain direction, can be controlled by an external magnetic field and have a certain level of hysteresis after the magnetic field is removed. Ferromagnetic materials have been widely applied for a long time, such as in ancient compasses and modern memories and sensors. Similar to ferroelectric materials, through the ages, it has been easy for 3D and nanocrystalline ferromagnetic materials with high Tc and magnetic intensity to be achieved [10]. For example, Ahmad et al. synthesized Ce³⁺-doped Ni_0.5Cd_0.5Fe₂O₄ nanoparticles using the sol–gel auto-combustion method, and the nanoparticles showed a nonlinear decrease in saturation magnetization (from 40.7 emu/g for Ni_0.5Cd_0.5Fe₂O₄ to 26.5 emu/g for Ni_0.5Cd_0.5Ce₀.₁₆Fe_1.84O₄) and coercivity (from 227 Oe for Ni_0.5Cd_0.5Fe₂O₄ to 170 Oe for Ni₀.₅Cd_0.5Ce_0.16Fe_1.84O₄) with the increase in Ce concentration [11]. In addition, nanograined La_0.7Sr_0.3MnO₃ (LSMO) and Nd_0.7Sr_0.3MnO₃ (NSMO) were produced using various methods. Sol–gel-synthesized NSMO was revealed to be a sample with single crystallite grains and exhibited excellent magnetic and electrical transport properties. The LSMO synthesized via solid-state reaction exhibited a magnetic hysteresis loop, and the NSMO exhibited a paramagnetic curve [12].

However, the absence of 2D ferromagnetics has always been a problem. In order to obtain a long-range magnetic order in 2D ferromagnetism materials at suitable temperature, researchers have established some theories, such as shape anisotropy, exchange anisotropy and magnetoelastic anisotropy, and many 2D ferromagnetic materials have also been successfully synthesized. For example, Albargi et al. prepared FeO, Fe₂O₃ and Fe₃O₄ doped In₂O₃ films under different oxygen pressures via pulsed laser deposition, and the effects of temperature on the magnetic properties of films were explored. FeO doped In₂O₃ films exhibited the largest Ms and Hc at H = 10,000 Oe compared with Fe₂O₃ and Fe₃O₄ doped In₂O₃ films due to the oxygen vacancies, and Fe²⁺ ions were necessary for ferromagnetism. With the increase in oxygen pressure, the number of oxygen vacancies was reduced, leading to a decrease in ferromagnetism. Furthermore, the separation of the zero-field cooled and the field cooled curves was observed due to the increase in the anisotropy field to a size comparable or larger than that of the measuring field.

Multiferroic materials, referring to multifunctional materials with multiple ferroic orders in a single phase, are quite crucial in both basic research and applications (e.g., actuators, nonvolatile memory and transducers). The unique properties of multiferroics have stimulated research interest for their use in mutual cross-coupling between ferroic orders by magnetoelectric and piezoelectric interactions [13]. BiFeO₃ (BFO) is a is a typical multiferroic material at room temperature and has two main ferroic orders, ferroelectricity and antiferromagnetism. Lin et al. explored the influence of Gd³⁺ doping on the ferroelectric and magnetic properties of BFO films. A Pr of 133.5 µC/cm² was achieved for Bi_0.9Gd_0.1FeO₃, and saturation magnetization (Ms) was enhanced from 4.9 emu/cm³ for BFO to 23.9 emu/cm³ for Bi_0.85Gd_0.15FeO₃. [14]. Wu et al. grew a (Ba_0.65Sr_0.35)TiO₃(BST)/(Bi_0.875Nd_0.125)FeO₃(BNF)/BST sandwich structured thin film on Pt-coated Si (100) substrates. Due to the coupling between BST and BNF thin films, the Pr values were enhanced from 3.77 µC/cm² for Au/BNF/Pt to 18.5 µC/cm² for Au/BST/BNF/BST/Pt thin-film capacitors. Furthermore, the BST/BNF/BST films had a higher Ms (10.1 emu/cm³) and lower Hc field (351 Oe) compared with single-layer BNF films [15]. Similarly, Hu et al. prepared multilayer BiFeO₃/BaTiO₃ samples and systemically studied their magneto-electric effect. With the increase in the magnetic field from 0 to 1200 Oe, the Pr of pure 100 nm BFO films was almost stable, but that of multilayer BiFeO₃/BaTiO₃ samples increased from 66 µC/cm² to 97 µC/cm² due to the interfacial effects and multiferroic coupling [16].

However, due to the limited number of multiferroic materials and low coupling strength, recent studies have focused on controlling the various magnetic properties of materials through electric fields in multiferroic heterostructures via a strain-driven magnetoelectric coupling effect, such as tunneling anisotropic magnetoresistance, magnetic domain wall motion and perpendicular magnetic as well as magnetoresistance properties [17]. For example, Fe₅₀Pt₅₀ thin films were fabricated on (011)-oriented 0.7Pb(Mg_1/3Nb_2/3)O₃-0.3PbTiO₃(PMN-PT) single crystal substrates. The Ms and Hc of the Fe₅₀Pt₅₀ film could be tuned by changing the electric fields. With the increase in the electric field, the magnetic properties of the Fe₅₀Pt₅₀ film along the in-plane [100] direction became harder and displayed a reduction in squareness ratio (M/Ms), but for the magnetic properties of the [1] direction, the change is the opposite. Furthermore, the magnetization changing rate (M_E−M₀)/M₀ is approximately +190% and −41% for the magnetic properties of the [100] and [1] directions, respectively. Simultaneously, Hc also shows an obvious response in the form of an increase in the electric field, and the change rate of (Hc(E)-Hc(0))/Hc(0) is approximately +137% for the magnetic properties of the [1] direction, while the change rate along the [100] direction is approximately −52%. Furthermore, the magnetic properties could be dynamically regulated by alternating the electric fields with the magnetic fields along the [1] direction, which indicates that the Fe₅₀Pt₅₀/PMN-PT structure is a promising method for developing magnetoelectric devices with lower power consumption [18]. The LaMnO_3−x thin film (~40 nm thick) was deposited on (001)-oriented PMN-PT single crystal substrates by Ni et al. By applying electric fields to PMN-PT single crystal substrates, they determined the resistance and photoresistance (enhanced by ~4.1%) of the LaMnO_3−x thin films. Under the action of a cycling electric field, the change curve of photoresistance shows a butterfly shape, which indicates the change induced by the piezoelectric strain effect. Furthermore, the resistance of the LaMnO_3−x film is suppressed under light irradiation, with an increase in E from 0 to +4.8 kV/cm, whereas ΔPR/PR gradually increased by ~4.1% and the falling time increased from 3.1 to 4 s. It is apparent that the electric-field-induced ferroelastic (in-plane compressive) strain significantly increases photoresistance. Both the electric-field-induced strain effect and the photoelectric effect are coupled to each other, which can change the magnetic and electrical transport properties of LaMnO_3−x. This method that exerts electric-field-tunable effects on electric properties in ferromagnetic/ferroelectric heterostructures provides opportunities for designing multifunctional devices by external stimuli [19].

This article presents a significant, interesting topic. It provides an update on current research in the field of ferroic materials, films and devices. I am convinced that all these studies will be a source of inspiration for the development of new multifunctional materials. However, more experimental and theoretical studies should be carried out to explore the fundamental physical and chemical properties of such materials in future.