J. Juraszek

Crafting the magnonic and spintronic response of BiFeO3 films by epitaxial strain.

Multiferroics are compounds that show ferroelectricity and magnetism. BiFeO3, by far the most studied, has outstanding ferroelectric properties, a cycloidal magnetic order in the bulk, and many unexpected virtues such as conductive domain walls or a low bandgap of interest for photovoltaics. Although this flurry of properties makes BiFeO3 a paradigmatic multifunctional material, most are related to its ferroelectric character, and its other ferroic property--antiferromagnetism--has not been investigated extensively, especially in thin films. Here we bring insight into the rich spin physics of BiFeO3 in a detailed study of the static and dynamic magnetic response of strain-engineered films. Using Mössbauer and Raman spectroscopies combined with Landau-Ginzburg theory and effective Hamiltonian calculations, we show that the bulk-like cycloidal spin modulation that exists at low compressive strain is driven towards pseudo-collinear antiferromagnetism at high strain, both tensile and compressive. For moderate tensile strain we also predict and observe indications of a new cycloid. Accordingly, we find that the magnonic response is entirely modified, with low-energy magnon modes being suppressed as strain increases. Finally, we reveal that strain progressively drives the average spin angle from in-plane to out-of-plane, a property we use to tune the exchange bias and giant-magnetoresistive response of spin valves.

Structural analysis of a (Pt/Co)3/IrMn multilayer: Investigation of sub-nanometric layers by tomographic atom probe

The structure of a Ta3 nm/[(Pt2 nm/Co0.4 nm)3/IrMn7 nm]7/Pt10 nm multilayer exhibiting perpendicular exchange bias has been investigated by x-ray reflectometry and laser-assisted tomographic atom probe (LATAP). A strong intermixing at the Co/IrMn interface is pointed out by x-ray reflectometry, this interface being more diffuse than the IrMn/Pt interface. A direct observation of this intermixing at the atomic scale is obtained thanks to the LATAP in real space. The three-dimensional reconstructions reveal the atomic planes in the Pt layers and the Pt–Co intermixing in the (Pt/Co)3 multilayer. The analysis of the concentration profiles allows to determine the chemical composition of the Co subnanometric layers; thus providing for the first time an accurate structural characterization of such layers leading to an estimation of their thickness, roughness, atomic concentration and width of their interfaces.

Control of ferroelectricity and magnetism in multi-ferroic BiFeO3 by epitaxial strain

Recently, strain engineering has been shown to be a powerful and flexible means of tailoring the properties of ABO3 perovskite thin films. The effect of epitaxial strain on the structure of the perovskite unit cell can induce a host of interesting effects, these arising from either polar cation shifts or rotation of the oxygen octahedra, or both. In the multi-ferroic perovskite bismuth ferrite (BiFeO3–BFO), both degrees of freedom exist, and thus a complex behaviour may be expected as one plays with epitaxial strain. In this paper, we review our results on the role of strain on the ferroic transition temperatures and ferroic order parameters. We find that, while the Néel temperature is almost unchanged by strain, the ferroelectric Curie temperature strongly decreases as strain increases in both the tensile and compressive ranges. Also unexpected is the very weak influence of strain on the ferroelectric polarization value. Using effective Hamiltonian calculations, we show that these peculiar behaviours arise from the competition between antiferrodistortive and polar instabilities. Finally, we present results on the magnetic order: while the cycloidal spin modulation present in the bulk survives in weakly strained films, it is destroyed at large strain and replaced by pseudo-collinear antiferromagnetic ordering. We discuss the origin of this effect and give perspectives for devices based on strain-engineered BiFeO3.

A setup combining magneto-optical Kerr effect and conversion electron Mössbauer spectrometry for analysis of the near-surface magnetic properties of thin films

We propose a system allowing the characterization of thin magnetic multilayer structures that combine conversion electron Mossbauer spectrometry (CEMS) under applied magnetic field with the magneto-optical Kerr effect (MOKE) technique. Measured hysteresis loops obtained from the MOKE part are used for investigation of sample surface magnetic properties. The CEMS part of such a system is suitable for studying the spatial spin distribution during magnetization reversal under applied magnetic field, whose values are established from the measured MOKE loop. The combined technique is demonstrated on the results obtained at 300 K on an exchange-coupled ferrimagnetic amorphous GdFe/TbFe bilayer, where the center of the GdFe layer is enriched in (57)Fe. Both techniques confirm in-plane uniaxial anisotropy. The spin structure at the position of the probe layer is analyzed for several values of the external magnetic field applied in the hard magnetization axis direction.

Strain and Magnetic Field Induced Spin‐Structure Transitions in Multiferroic BiFeO3

The magnetic-field-dependent spin ordering of strained BiFeO3 films is determined using nuclear resonant scattering and Raman spectroscopy. The critical field required to destroy the cycloidal modulation of the Fe spins is found to be significantly lower than in the bulk, with appealing implications for field-controlled spintronic and magnonic devices.

Insight into magnetic, ferroelectric and elastic properties of strained BiFeO3 thin films through Mössbauer spectroscopy

We have studied the magnetic order of highly strained (001)-oriented BiFeO3 (BFO) thin films using 57Fe Conversion Electron Mossbauer Spectrometry. From 90 K to 620 K the films exhibit a collinear antiferromagnetic structure, in contrast with the cycloidal structure observed in bulk BFO. Moreover, we find that both the planar magnetic anisotropy for compressive strain and out-of-plane anisotropy for tensile strain persist from 90 K up to the Neel temperature (TN), which itself shows only a weak strain dependence. An analysis of the line asymmetry of the paramagnetic doublet for temperatures above TN is used to reveal the strain-dependent rotation of the polarization direction, consistent with previous observations. Our results show that the lattice dynamics in BFO films are strongly strain-dependent, offering avenues toward acoustic phonon devices. Finally, we use the versatility of Mossbauer spectroscopy technique to reveal various multi-property features including magnetic states, polarization direction...

Magnetic composite materials obtained by swift heavy-ion irradiation of yttrium iron garnet ceramics

High-energy heavy-ion irradiation of magnetic insulators provides a unique way to generate reproducible random composites of ferrimagnetic crystals and paramagnetic amorphous latent tracks. Moreover, high-energy heavy-ion irradiation of the garnet ceramics Y3Fe5O12 is able to induce a third domain exhibiting a rotation of the hyperfine magnetic field parallel to the latent track axis. An analysis of the Mossbauer spectra allowed the intensities of the three contributions to be determined. A damage model, based on two shells surrounding an amorphous core, is proposed to estimate the volumes of the three components.

Selective FeTb and TbFe interface study: Swift uranium ion irradiation effect

Abstract Tb/bcc-Fe evaporated multilayers with 57Fe probe layers were irradiated with swift uranium ions. For the sharp Tb Fe interface, only continuous mixing is observed, but for the diffuse Fe Tb interface, an homogenization is first evidenced at low fluence. The mixing efficiency saturates at high fluence and it still remains a crystallized Fe part. The magnetic anisotropy was reported and interpreted versus the ion fluence.

Evidence for recrystallization of amorphous Fe/Tb multilayers under swift ion irradiation

Abstract Amorphous multilayers with 57 Fe probe layers were irradiated with swift heavy ions and investigated by 57 Fe Conversion Electron Mossbauer Spectroscopy. Partial recrystallization of iron has been evidenced in the amorphous Fe layers. Furthermore, a demixing effect between Fe and Tb atoms is observed at the interfaces. The electronic stopping power threshold value for the recrystallization and the demixing process has been estimated between 5 and 15 keV/nm.

Investigation of TbCo2/Fe Magnetostrictive Multilayers by Laser Assisted Tomographic Atom Probe (LATAP)

Laser assisted tomographic atom probe is used to analyze the atomic scale concentration data of TbCo2/Fe multilayers deposited through focused ion beam milling. Three dimensional reconstructions of the layers are obtained and compositional modulation is observed across each specimen