Disentangling Highly Asymmetric Magnetoelectric Effects in Engineered Multiferroic Heterostructures

Enric Menéndez, Veronica Sireus, Alberto Quintana, Ignasi Fina, Blai Casals, Rafael Cichelero, Mikko Kataja, Massimiliano Stengel, Gervasi Herranz, Gustau Catalán, Maria Dolors Baró, Santiago Suriñach, and Jordi Sort

Phys. Rev. Applied 12, 014041 – Published 23 July 2019

Abstract

One of the main strategies to control magnetism by voltage is the use of magnetostrictive-piezoelectric hybrid materials, such as ferromagnetic-ferroelectric heterostructures. When such heterostructures are subjected to an electric field, piezostrain-mediated effects, electronic charging, and voltage-driven oxygen migration (magnetoionics) may simultaneously occur, making the interpretation of the magnetoelectric effects not straightforward and often leading to misconceptions. Typically, the strain-mediated magnetoelectric response is symmetric with respect to the sign of the applied voltage because the induced strain (and variations in the magnetization) depends on the square of the ferroelectric polarization. Conversely, asymmetric responses can be obtained from electronic charging and voltage-driven oxygen migration. By engineering a ferromagnetic-ferroelectric hybrid consisting of a magnetically soft 50-nm thick ${Fe}_{75} {Al}_{25}$ (at. %) thin film on top of a 〈110〉-oriented $Pb ({Mg}_{1 / 3} {Nb}_{2 / 3}) O_{3}$ - $32 {Pb Ti O}_{3}$ ferroelectric crystal, a highly asymmetric magnetoelectric response is obtained and the aforementioned magnetoelectric effects can be disentangled. Specifically, the large thickness of the ${Fe}_{75} {Al}_{25}$ layer allows dismissing any possible charge accumulation effect, whereas no evidence of magnetoionics is observed experimentally, as expected from the high resistance to oxidation of ${Fe}_{75} {Al}_{25}$ , leaving strain as the only mechanism to modulate the asymmetric magnetoelectric response. The origin of this asymmetric strain-induced magnetoelectric effect arises from the asymmetry of the polarization reversal in the particular crystallographic orientation of the ferroelectric substrate. These results are important to optimize the performance of artificial multiferroic heterostructures.

Received 6 April 2019
Revised 30 May 2019

DOI:https://doi.org/10.1103/PhysRevApplied.12.014041

Physics Subject Headings (PhySH)

Electric polarization Magneto-dielectric effect Piezoelectricity

Ferromagnets Multiferroics

Magnetization measurements X-ray diffraction

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Enric Menéndez ^1,*, Veronica Sireus¹, Alberto Quintana^1,2, Ignasi Fina³, Blai Casals³, Rafael Cichelero³, Mikko Kataja³, Massimiliano Stengel^3,4, Gervasi Herranz³, Gustau Catalán^4,5, Maria Dolors Baró¹, Santiago Suriñach¹, and Jordi Sort^1,4

¹Departament de Física, Universitat Autònoma de Barcelona, E-08193 Bellaterra, Cerdanyola del Vallès, Spain
²Department of Physics, Georgetown University, Washington DC, 20057, USA
³Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Campus UAB, Bellaterra, E-08193 Barcelona, Spain
⁴Institució Catalana de Recerca i Estudis Avançats (ICREA), Pg. Lluís Companys 23, E-08010 Barcelona, Spain
⁵Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Campus UAB, Bellaterra, E-08193 Barcelona, Spain

^*enricmenendez@gmail.com

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Vol. 12, Iss. 1 — July 2019

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Images

Figure 1
(a) θ/2θ x-ray diffraction patterns corresponding to a pristine (i.e., as-deposited) 50-nm thick ${Fe}_{75} {Al}_{25}$ /〈110〉 PMN-PT sample (black line) and a bare 〈110〉 PMN-PT substrate (red line). (b) Scanning electron microcopy image taken using secondary electrons of the surface of a 50-nm thick ${Fe}_{75} {Al}_{25}$ /〈110〉 PMN-PT sample. (c) TEM image of the cross section along [1–10] of a pristine 50-nm thick ${Fe}_{75} {Al}_{25}$ /〈110〉 PMN-PT sample. (d) High resolution TEM image of the interface layer formed between the ${Fe}_{75} {Al}_{25}$ film and the PMN-PT substrate. The inset shows the FFT of the area marked with a red rectangle.
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Figure 2
(a) Schematic representation of the out-of-plane poling applied to the samples. (b) Ferroelectric hysteresis loop (electric polarization P vs applied electric field E) of an as-deposited 50-nm thick ${Fe}_{75} {Al}_{25}$ /〈110〉 PMN-PT sample. (c) Square of P (which is proportional to the piezoelectric strain) vs E.
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Figure 3
(a) Hysteresis loops recorded along in-plane direction [001] (0^o configuration) by vibrating sample magnetometry at a given applied electric field applied out-of-plane. The piezoelectric is always brought to ferroelectric saturation at −6 kV/cm before registering the magnetic measurement. (b) and (c) dependences of the magnetic moment at remanence normalized by the magnetic moment at saturation (m_R/m_S) and coercivity ( $H_{C}$ ) with the applied electric field. Ascending (A) and descending (D) refer to the branches of the ferroelectric hysteresis loop as sketched in the inset of panel (b).
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Figure 4
(a) Hysteresis loops recorded along in-plane direction [1–10] (90^o configuration) by vibrating sample magnetometry at a given applied electric field out-of-plane. The piezoelectric is always brought to ferroelectric saturation at −6 kV/cm before registering the magnetic measurement. (b) and (c) dependences of the magnetic moment at remanence normalized by the magnetic moment at saturation (m_R/m_S) and coercivity ( $H_{C}$ ) with the applied electric field. Ascending (A) and descending (D) refer to the branches of the ferroelectric hysteresis loop as sketched in the inset of panel (b).
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Figure 5
(a) and (b) dependences of the magnetic moment at saturation with the applied electric field when the magnetic field is applied along the [001] (0^o configuration) or [1–10] (90^o configuration) directions, respectively.
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Figure 6
(a) and (b) in-plane reciprocal space mappings around the (001) and (1–10) planes of a pristine substrate (i.e., 0 kV/cm), respectively. (c) and (d) in-plane reciprocal space mappings around the (001) and (1–10) planes of a PMN-PT substrate at negative ferroelectric remanence (measured at 0 kV/cm after being subjected to −6 kV/cm), respectively.
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