Journal of Physics Condensed Matter: Preface

Materials with a coexistence of magnetic and ferroelectric order — multiferroics — provide an efficient route for the control of magnetism by electric fields. The study of multiferroics dates back to the 1950s, but in recent years, key discoveries in theory, synthesis and characterization techniques have led to a new surge of interest in these materials. Different mechanisms, such as lone-pair, geometric, charge-ordering and spin-driven effects, can support multiferroicity. The general focus of the field is now shifting into neighbouring research areas, as we discuss in this Review. Multiferroic thin-film heterostructures, device architectures, and domain and interface effects are explored. The violation of spatial and inversion symmetry in multiferroic materials is a key feature because it determines their properties. Other aspects, such as the non-equilibrium dynamics of multiferroics, are underrated and should be included in the topics that will define the future of the field. Multiferroic materials exhibit magnetic and ferroelectric order at the same time and provide a way to control magnetism with electric fields. We discuss the mechanisms supporting multiferroicity, multiferroic thin films and heterostructures, the non-equilibrium dynamics of multiferroics, fundamental symmetry issues and the impact of multiferroics on other research areas.

The evolution of multiferroics

Computational Materials Science X

Electric-field-induced switching of material’s magnetization is a promising approach for achieving energy-efficient memory devices. By taking advantage of the strong magnetoelectric coupling with a BaTiO3 substrate, a small electric field is used to switch a FeRh thin film from anti- to ferromagnetic above room temperature.

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Electric-field control of magnetic order above room temperature

We review the recent developments in the electric field control of magnetism in multiferroic heterostructures, which consist of heterogeneous materials systems where a magnetoelectric coupling is engineered between magnetic and ferroelectric components. The magnetoelectric coupling in these composite systems is interfacial in origin, and can arise from elastic strain, charge, and exchange bias interactions, with different characteristic responses and functionalities. Moreover, charge transport phenomena in multiferroic heterostructures, where both magnetic and ferroelectric order parameters are used to control charge transport, suggest new possibilities to control the conduction paths of the electron spin, with potential for device applications.

https://iopscience.iop.org/article/10.1088/0953-8984/24/33/333201/pdf

Electric field control of magnetism in multiferroic heterostructures

Spintronic devices currently rely on magnetic switching or controlled motion of domain walls by an external magnetic field or spin-polarized current. Achieving the same degree of magnetic controllability using an electric field has potential advantages including enhanced functionality and low power consumption. Here we report on an approach to electrically control local magnetic properties, including the writing and erasure of regular ferromagnetic domain patterns and the motion of magnetic domain walls, in CoFe-BaTiO3 heterostructures. Our method is based on recurrent strain transfer from ferroelastic domains in ferroelectric media to continuous magnetostrictive films with negligible magnetocrystalline anisotropy. Optical polarization microscopy of both ferromagnetic and ferroelectric domain structures reveals that domain correlations and strong inter-ferroic domain wall pinning persist in an applied electric field. This leads to an unprecedented electric controllability over the ferromagnetic microstructure, an accomplishment that produces giant magnetoelectric coupling effects and opens the way to electric-field driven spintronics.

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Electric-field control of magnetic domain wall motion and local magnetization reversal

IO N The ability to tailor magnetic properties via coupling to ferroelectric domains is of great interest for the design of electric-fi eld-tunable magnetic devices. Imprinting of ferroelectric domains into continuous magnetic fi lms would enable local control over magnetization dynamics but requires interfacial coupling to overcome exchange and magnetostatic interactions within the ferromagnet. Here, we demonstrate full pattern transfer and electric-fi eld-induced magnetic domain formation in multiferroic heterostructures consisting of a ferroelectric substrate and a thin magnetic fi lm. Simultaneous imaging of ferroelectric and ferromagnetic domains and local magnetization reversal analysis using polarization microscopy reveals strong lateral modulations of magnetic hysteresis due to strain coupling to the underlying ferroelectric substrate. While the magnetic confi guration is an exact copy of the ferroelectric domain structure in the as-deposited state, new magnetic domains form when an external electric fi eld is applied. In fact, the electric-fi eld response of the magnetic fi lm is characterized by a superposition of two patterns, one that mirrors the current ferroelectric domain confi guration (electric-fi eld induced pattern) and another that refl ects the ferroelectric domain structure during deposition (growth-induced pattern). This ability to electrically write magnetic domains in continuous magnetic fi lms opens up new avenues for electric-fi eld control of magnetic functionalities and provides a framework for the exploration of ferroelectric, ferroelastic, and ferromagnetic domain interactions in multiferroic heterostructures. Domain formation and hysteresis in magnetic thin fi lms depend on internal material parameters (exchange stiffness, magnetization), fi lm thickness and shape (magnetostatics), and various other sources of magnetic anisotropy including crystal symmetry, lattice deformations (strain), and interfaces. [ 1 ] The interplay between these fi lm properties and an external magnetic fi eld usually results in a laterally uniform energy landscape with random dispersions due to defects and interface roughness. The magnetic hysteresis therefore depends little on sample position. Area-specifi c magnetic responses in continuous fi lms require a local modifi cation of the magnetic properties. This can be accomplished, for example, by the growth of ferromagnetic fi lms onto prepatterned substrates [ 2 ] or by local ion irradiation. [ 3–5 ] In these cases, the magnetic anisotropy remains fi xed after sample preparation. Interface strain

https://www.onlinelibrary.wiley.com/doi/pdf/10.1002/adma.201190178

Pattern Transfer and Electric‐Field‐Induced Magnetic Domain Formation in Multiferroic Heterostructures

We report on domain formation and magnetization reversal in epitaxial Fe films on ferroelectric BaTiO3 substrates with ferroelastic a–c stripe domains. The Fe films exhibit biaxial magnetic anisotropy on top of c domains with out-of-plane polarization, whereas the in-plane lattice elongation of a domains induces uniaxial magnetoelastic anisotropy via inverse magnetostriction. The strong modulation of magnetic anisotropy symmetry results in full imprinting of the a–c domain pattern in the Fe films. Exchange and magnetostatic interactions between neighboring magnetic stripes further influence magnetization reversal and pattern formation within the a and c domains.

/pdf/alternating-domains-with-uniaxial-and-biaxial-magnetic-ea7oviy7kr.pdf

Alternating domains with uniaxial and biaxial magnetic anisotropy in epitaxial Fe films on BaTiO3

We report on domain formation and magnetization reversal in epitaxial Fe films on ferroelectric BaTiO3 substrates with ferroelastic a-c stripe domains. The Fe films exhibit biaxial magnetic anisotropy on top of c domains with out-of-plane polarization, whereas the in-plane lattice elongation of a domains induces uniaxial magnetoelastic anisotropy via inverse magnetostriction. The strong modulation of magnetic anisotropy symmetry results in full imprinting of the a-c domain pattern in the Fe films. Exchange and magnetostatic interactions between neighboring magnetic stripes further influence magnetization reversal and pattern formation within the a and c domains.

/pdf/alternating-domains-with-uniaxial-and-biaxial-magnetic-4xgubq99dj.pdf

We report on the evolution of ferromagnetic domain walls during magnetization reversal in elastically coupled ferromagnetic-ferroelectric heterostructures. Using optical polarization microscopy and micromagnetic simulations, we demonstrate that the spin rotation and width of ferromagnetic domain walls can be accurately controlled by the strength of the applied magnetic field if the ferromagnetic walls are pinned onto 90${}^{\ensuremath{\circ}}$ ferroelectric domain boundaries. Moreover, reversible switching between magnetically charged and uncharged domain walls is initiated by magnetic field rotation. Switching between both wall types reverses the wall chirality and abruptly changes the width of the ferromagnetic domain walls by up to $1000%$.

Tuomas H. E. Lahtinen

Papers

Electric-field control of magnetic domain wall motion and local magnetization reversal

Pattern Transfer and Electric‐Field‐Induced Magnetic Domain Formation in Multiferroic Heterostructures

Alternating domains with uniaxial and biaxial magnetic anisotropy in epitaxial Fe films on BaTiO3

Alternating domains with uniaxial and biaxial magnetic anisotropy in epitaxial Fe films on BaTiO3

Field tuning of ferromagnetic domain walls on elastically coupled ferroelectric domain boundaries