Multiferroic phase competition and magnetoelectric switching in La-ion substituted bismuth ferrite thim films = 란타늄 이온 치환된 비스무스 철 산화물 박막의 다강성 상태 경쟁 및 자기전기 스위칭 연구

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Control of an order parameter by a non-conjugated field has been an interesting subject of research for applications of information storage. Multiferroics, compounds possessing equal or more than two ferroic orders, have been extensively studied for the last two decades under the spotlight of the control of an order parameter by a non-conjugate field, \textit{e.g.}, the electric control of magnetization. To realize the inter-coupling and cross-control, many researchers have devoted to synthesize single phase materials and composite structures. However, the compelling result of the reliable electric control of magnetic order at room temperature has still been challenging. Single phase multiferroic materials are classified into two classes, so called, type-I and type-II multiferroics. The type-I multiferroic has two different origins separately responsible for electric and magnetic orders. Due to the innate difference in their origins, mutual coupling is poor. However the magnitudes of the order parameters are large enough to be utilized and the ordering temperature is also high above room temperature. On the other hand, the type-II multiferroics show ferroelectric and magnetic orders from a single origin, \textit{e.g.}, a non-collinear modulation of magnetic ordering induces a ferroelectric order by breaking spatial inversion symmetry. The exchange striction and the geometric effect have been proposed to induce the type-II multiferroics. For this intimate nature of sharing the same origin, the magnetic and electric orders in the type-II multiferroics show considerable inter-coupling effects. Nevertheless, the type-II multiferroics have limitations for the realistic use because they usually have small magnitudes of order parameters and operate at low temperatures. To overcome the limitations, there have been attempts to enhance the inter-coupling effect based on the type-I multiferroics. Bismuth ferrite is the representative type-I multiferroic. The ferroelectric and antiferromagnetic order parameters are well above room temperature. At the meantime, the magnitude of the ferroelectric polarization is 90 ${\mu C/cm^2}$ which is comparable to the ferroelectric material PbTi$_{1-x}$Zr$_x$O$_3$ (PZT) with 80 to 100 ${\mu C/cm^2}$. This material is expected to have a meta-stable structure which shows a huge lattice change. The meta-stable phase can be stabilized as a highly-elongated tetragonal-like structure by growing bismuth ferrite on LaAlO$_3$ substrate with a small lattice parameter which offers a large misfit strain of about 4\% as compared to the bulk phase. The highly-elongated phase accompanies enhancement of ferroelectric polarization up to 150 ${\mu C/cm^2}$ as theoretically predicted and concurrent phase transitions of structural/ferroelectric reorientation and magnetic order-disorder transition at a temperature near room temperature. We studied the concurrent phase transition to understand the origin of simultaneity and answer the question whether it is an indicative of the strong spin-lattice coupling effect or an accidental coincidence. We argue the phase proximity effect can offer a useful pathway in the tantalizing efforts for realistic magnetoelectric applications. Phase proximity effect has been emergent for the giant piezoelectric effect in PZT and the colossal magnetoresistance in La$_{1-x}$Ca$_x$MnO$_3$ (LCMO). The two phenomena emerge at the phase boundary between two or more phases are competing. In this thesis, we study the phase diagram of La-ion substituted bismuth ferrite thin films to address the multiferroic phase proximity effect, thereby realizing electrically addressable magnetic phases. A phenomenological Landau model is proposed to understand the phase diagram. The phenomenological model not only explains the experimentally observed phase competing features but also predicts a reversible electrical switching between the paramagnetic state and anti-ferromagnetic states in a non-volatile manner. This theoretical prediction is proved by tip-based electrical writing and spatially-resolved photoemission microscopy. In addition, we applied tip-based electrical writing to the control of the mixed phase boundary and consequent magnetic easy axis, thereby controlling the anisotropic axis of the ferromagnetic layer on top of the mixed phase boundary via exchange coupling at room temperature.
Yang, Chan-Horesearcher양찬호researcher
한국과학기술원 :물리학과,
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학위논문(박사) - 한국과학기술원 : 물리학과, 2017.2,[v, 96 p. :]


Multiferroics▼abismuth ferrite▼aphase proximity effect▼amultiferroic triple phase point▼amagnetoelectric switching▼aexchange anisotropy; 다강체▼a비스무스 철 산화물▼a상근접 효과▼a다강성 삼상점▼a자기전기 효과▼a교환 자기이방성

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