(604d) Engineered Multiferroic PZT-CFO and BFO-CFO Thin Films for Large Magnetoelectric Coefficient by Atomic Layer Deposition

Multiferroic materials simultaneously possess two or more of ferroic order parameters such as ferroelectricity, ferroelasticity, ferromagnetism, and antiferromagnetism. They hold considerable promise because of their potential applications in information storage and spintronics, such as magneto-electric sensors, magneto-capacitive devices, and electrically driven magnetic data storage. The revival of modern multiferroics was possible not only due to recent theoretical breakthroughs in understanding the coexistence of magnetic and electrical ordering but also advances in thin film growth techniques. The ultimate goal for practical device application of multiferroic materials is dependent on how to create strong coupling between different types of ferroic order. In order to explore new degrees of freedom for achieving stronger magnetoelectric coupling, the research has been directed towards designing engineered multiferroic composite materials in the form of horizontal multilayer (2-2), vertical superstructures (3-1) or other nanoparticle composite structures (3-0) in a precise controlled manner.

In this work, the intrinsic multiferroic materials such as BiFeO₃ (BFO) and ferroelectric materials Pb(ZrTi)O₃ (PZT) thin film were synthesized by RE-ALD. RE-ALD is a gas-phase technique in which precursor vapors are pulsed alternately into the reaction chamber and the thin film growth proceeds through surface reactions in a self limiting manner. The advantages of ALD include excellent conformality, simple and accurate thickness control and good uniformity on large areas. More effort on nanostructure composites have been made to produce large magnetoelectric (ME) coefficients through a strictive interaction between the piezoelectricity (PE) of ferroelectric (FE) phase and the magnetostriction of ferromagnetic (FM) or antiferromagnetic (AFM) phase. In order to demonstrate conformal deposition of engineered multiferroic materials in the form of 3-0 configuration, PZT (FE, PE) and BFO (FE, AFM) was deposited onto a mesoporous CoFe₂O₄ (CFO, AFM) substrate by radical enhanced atomic layer deposition (RE-ALD).

The mesoporous CFO films were found to be fully filled by ALD PZT and BFO. The composition and crystal structure of PZT-CFO and BFO-CFO were confirmed by X-ray Photon Spectroscopy and X-ray Diffraction (XRD), respectively. More detail crystal structure were investigated synchrotron XRD and extended x-ray absorption fine structure spectroscopy (EXAFS). The magnetic and ferroelectric properties for the PZT-CFO or BFO-CFO thin film were characterized by a superconducting quantum interference device (SQUID) magnetometer and piezoresponse force microscopy (PFM). Composite systems were found to have higher coercive magnetic field and lower saturation moment than that of the pure CFO substrate.