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Interlayer coupling through a dimensionality-induced magnetic state.

Gibert M, Viret M, Zubko P, Jaouen N, Tonnerre JM, Torres-Pardo A, Catalano S, Gloter A, Stéphan O, Triscone JM - Nat Commun (2016)

Bottom Line: We show here that an induced antiferromagnetic order can be stabilized in the [111] direction by interfacial coupling to the insulating ferromagnet LaMnO3, and used to generate interlayer magnetic coupling of a nature that depends on the exact number of LaNiO3 monolayers.All three behaviours are explained based on the emergence of a (¼,¼,¼)-wavevector antiferromagnetic structure in LaNiO3 and the presence of interface asymmetry with LaMnO3.This dimensionality-induced magnetic order can be used to tailor a broad range of magnetic properties in well-designed superlattice-based devices.

View Article: PubMed Central - PubMed

Affiliation: Département de Physique de la Matière Quantique, University of Geneva, 24 Quai Ernest-Ansermet, 1211 Genève 4, Switzerland.

ABSTRACT
Dimensionality is known to play an important role in many compounds for which ultrathin layers can behave very differently from the bulk. This is especially true for the paramagnetic metal LaNiO3, which can become insulating and magnetic when only a few monolayers thick. We show here that an induced antiferromagnetic order can be stabilized in the [111] direction by interfacial coupling to the insulating ferromagnet LaMnO3, and used to generate interlayer magnetic coupling of a nature that depends on the exact number of LaNiO3 monolayers. For 7-monolayer-thick LaNiO3/LaMnO3 superlattices, negative and positive exchange bias, as well as antiferromagnetic interlayer coupling are observed in different temperature windows. All three behaviours are explained based on the emergence of a (¼,¼,¼)-wavevector antiferromagnetic structure in LaNiO3 and the presence of interface asymmetry with LaMnO3. This dimensionality-induced magnetic order can be used to tailor a broad range of magnetic properties in well-designed superlattice-based devices.

No MeSH data available.


Related in: MedlinePlus

Soft X-ray reflectivity at the Mn L3-edge of the(LNO7/LMO7)15 superlattice at 30 Kand in 0.05 T after cooling in the same field.(a) Reflectivities for circularly left (CL, blue line) and right (CR,red line) polarized light. Inset: extracted asymmetry ratio(CR−CL)/(CR+CL). (b) Reflectivity with linearly vertical(LV) polarized light. For simplicity, linear horizontal polarization is notshown. In all reflectivity curves, points are experimental measurements andsolid lines are fits. Inset: corresponding Mn L2,3 X-rayabsorption spectra. The arrow in the lower inset indicates the energy atwhich reflectivity measurements were performed. (c) Schematics of thescattering geometry for reflectivity measurements. (d) Sketch of theextracted magnetic configuration showing that the doubling of the magneticstructure along the normal direction corresponds to two LMO sublattices withtheir net magnetic moments oriented at 160° from each other.
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f3: Soft X-ray reflectivity at the Mn L3-edge of the(LNO7/LMO7)15 superlattice at 30 Kand in 0.05 T after cooling in the same field.(a) Reflectivities for circularly left (CL, blue line) and right (CR,red line) polarized light. Inset: extracted asymmetry ratio(CR−CL)/(CR+CL). (b) Reflectivity with linearly vertical(LV) polarized light. For simplicity, linear horizontal polarization is notshown. In all reflectivity curves, points are experimental measurements andsolid lines are fits. Inset: corresponding Mn L2,3 X-rayabsorption spectra. The arrow in the lower inset indicates the energy atwhich reflectivity measurements were performed. (c) Schematics of thescattering geometry for reflectivity measurements. (d) Sketch of theextracted magnetic configuration showing that the doubling of the magneticstructure along the normal direction corresponds to two LMO sublattices withtheir net magnetic moments oriented at 160° from each other.

Mentions: To investigate the nature of the magnetic state in the regime where EB hasvanished (T≥30 K), we performed polarization-dependent resonantX-ray reflectivity measurements at Mn L2,3-edges. This technique isparticularly well-suited to unveiling the depth-resolved magnetic profile ofsuch heterostructures. Figure 3a,b presents thereflectivity spectra measured on a(LNO7/LMO7)15 superlattice at the MnL3-edge (642.5 eV) at 30 K after field cooling in0.05 T. Reflectivity curves were acquired both with circular left andright, as well as with linear vertical and horizontal polarizations in speculargeometry with the magnetic field applied parallel to the intersection betweenthe sample surface and the scattering plane (Fig. 3c).Compared with the corresponding room temperature measurements (Fig. 1a), the emergence of ½-order peaks at q/2 and3q/2 is clearly visible in these low-temperature spectra. Thepositions of these resonant peaks correspond to a real space doubling of thesuperlattice periodicity, providing evidence for the existence of twomagnetically different LMO layers. The analysis of the full set of measurementsobtained with both circularly (Fig. 3a) and linearly(Fig. 3b) polarized light allows the directions ofindividual LMO sublattice magnetizations to be determined and their cantedantiferromagnetic arrangement to be confirmed. The structural parameters wereextracted from fits to the reflectivity curves at 300 K (Fig. 1a), that is, above the Curie point. Keeping the number of freeparameters as small as possible, layer thicknesses of LNO=1.31 nmand LMO=1.16 nm were obtained, in fair agreement with the nominalvalues, along with a typical interface intermixing of 0.4 nm (assumedconstant throughout heterostructure for simplicity of the fits). A magnetizationvalue of 2.3 μB per Mn was also inferred fromSQUID-magnetometry at saturation and single LMO layers. Keeping all theseparameters constant, the entire set of low-temperature curves obtained withcircularly and linearly polarized X-rays were fitted using only the twoindependent angles of the magnetization sublattices with respect to the appliedfield direction as free parameters. The best fits for the (7/7) periodmultilayer (Fig. 3a,b) reveal that when field cooled to∼30 K in 0.05 T the two LMO sublattice magnetizations areoriented, respectively, at 10° and −150° from the direction of theapplied magnetic field, thus making an angle of 160° between them (Fig. 3d). The inset in Fig. 3a showsthe agreement for the magnetic asymmetry, which is very sensitive to theorientation of the magnetic moment within the alternating layers. Fits assumingan antiparallel alignment between the two LMO sublattices rendered a totalmagnetization value much lower than the one extracted from SQUID-magnetometry.Further resonant magnetic reflectivity measurements showed that when a largermagnetic field is applied, the magnetizations of the two LMO sublattices foldprogressively and end up parallel near 0.3 T (this is shown in Supplementary Fig. 3). Asignificant coupling energy of 0.3 mJ m−2 canthen be inferred straightforwardly by considering that the Zeeman energy at thatfield compensates the interlayer coupling. Interestingly, the AF arrangement ofthe LMO layers is only obtained for the (111)-oriented superlattices withN=7 MLs and drops markedly once the LNO layer thicknessdeparts from this value; that is, superlattices with LNO thicknesses ofN≠7 do not show the ½-order peaks (Supplementary Fig. 4). As discussed below,the fact that the 7-ML-LNO thickness is a very special case is a central cluefor a possible explanation of the coupling behaviour through the (111)-LNOlayers.


Interlayer coupling through a dimensionality-induced magnetic state.

Gibert M, Viret M, Zubko P, Jaouen N, Tonnerre JM, Torres-Pardo A, Catalano S, Gloter A, Stéphan O, Triscone JM - Nat Commun (2016)

Soft X-ray reflectivity at the Mn L3-edge of the(LNO7/LMO7)15 superlattice at 30 Kand in 0.05 T after cooling in the same field.(a) Reflectivities for circularly left (CL, blue line) and right (CR,red line) polarized light. Inset: extracted asymmetry ratio(CR−CL)/(CR+CL). (b) Reflectivity with linearly vertical(LV) polarized light. For simplicity, linear horizontal polarization is notshown. In all reflectivity curves, points are experimental measurements andsolid lines are fits. Inset: corresponding Mn L2,3 X-rayabsorption spectra. The arrow in the lower inset indicates the energy atwhich reflectivity measurements were performed. (c) Schematics of thescattering geometry for reflectivity measurements. (d) Sketch of theextracted magnetic configuration showing that the doubling of the magneticstructure along the normal direction corresponds to two LMO sublattices withtheir net magnetic moments oriented at 160° from each other.
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Related In: Results  -  Collection

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Show All Figures
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f3: Soft X-ray reflectivity at the Mn L3-edge of the(LNO7/LMO7)15 superlattice at 30 Kand in 0.05 T after cooling in the same field.(a) Reflectivities for circularly left (CL, blue line) and right (CR,red line) polarized light. Inset: extracted asymmetry ratio(CR−CL)/(CR+CL). (b) Reflectivity with linearly vertical(LV) polarized light. For simplicity, linear horizontal polarization is notshown. In all reflectivity curves, points are experimental measurements andsolid lines are fits. Inset: corresponding Mn L2,3 X-rayabsorption spectra. The arrow in the lower inset indicates the energy atwhich reflectivity measurements were performed. (c) Schematics of thescattering geometry for reflectivity measurements. (d) Sketch of theextracted magnetic configuration showing that the doubling of the magneticstructure along the normal direction corresponds to two LMO sublattices withtheir net magnetic moments oriented at 160° from each other.
Mentions: To investigate the nature of the magnetic state in the regime where EB hasvanished (T≥30 K), we performed polarization-dependent resonantX-ray reflectivity measurements at Mn L2,3-edges. This technique isparticularly well-suited to unveiling the depth-resolved magnetic profile ofsuch heterostructures. Figure 3a,b presents thereflectivity spectra measured on a(LNO7/LMO7)15 superlattice at the MnL3-edge (642.5 eV) at 30 K after field cooling in0.05 T. Reflectivity curves were acquired both with circular left andright, as well as with linear vertical and horizontal polarizations in speculargeometry with the magnetic field applied parallel to the intersection betweenthe sample surface and the scattering plane (Fig. 3c).Compared with the corresponding room temperature measurements (Fig. 1a), the emergence of ½-order peaks at q/2 and3q/2 is clearly visible in these low-temperature spectra. Thepositions of these resonant peaks correspond to a real space doubling of thesuperlattice periodicity, providing evidence for the existence of twomagnetically different LMO layers. The analysis of the full set of measurementsobtained with both circularly (Fig. 3a) and linearly(Fig. 3b) polarized light allows the directions ofindividual LMO sublattice magnetizations to be determined and their cantedantiferromagnetic arrangement to be confirmed. The structural parameters wereextracted from fits to the reflectivity curves at 300 K (Fig. 1a), that is, above the Curie point. Keeping the number of freeparameters as small as possible, layer thicknesses of LNO=1.31 nmand LMO=1.16 nm were obtained, in fair agreement with the nominalvalues, along with a typical interface intermixing of 0.4 nm (assumedconstant throughout heterostructure for simplicity of the fits). A magnetizationvalue of 2.3 μB per Mn was also inferred fromSQUID-magnetometry at saturation and single LMO layers. Keeping all theseparameters constant, the entire set of low-temperature curves obtained withcircularly and linearly polarized X-rays were fitted using only the twoindependent angles of the magnetization sublattices with respect to the appliedfield direction as free parameters. The best fits for the (7/7) periodmultilayer (Fig. 3a,b) reveal that when field cooled to∼30 K in 0.05 T the two LMO sublattice magnetizations areoriented, respectively, at 10° and −150° from the direction of theapplied magnetic field, thus making an angle of 160° between them (Fig. 3d). The inset in Fig. 3a showsthe agreement for the magnetic asymmetry, which is very sensitive to theorientation of the magnetic moment within the alternating layers. Fits assumingan antiparallel alignment between the two LMO sublattices rendered a totalmagnetization value much lower than the one extracted from SQUID-magnetometry.Further resonant magnetic reflectivity measurements showed that when a largermagnetic field is applied, the magnetizations of the two LMO sublattices foldprogressively and end up parallel near 0.3 T (this is shown in Supplementary Fig. 3). Asignificant coupling energy of 0.3 mJ m−2 canthen be inferred straightforwardly by considering that the Zeeman energy at thatfield compensates the interlayer coupling. Interestingly, the AF arrangement ofthe LMO layers is only obtained for the (111)-oriented superlattices withN=7 MLs and drops markedly once the LNO layer thicknessdeparts from this value; that is, superlattices with LNO thicknesses ofN≠7 do not show the ½-order peaks (Supplementary Fig. 4). As discussed below,the fact that the 7-ML-LNO thickness is a very special case is a central cluefor a possible explanation of the coupling behaviour through the (111)-LNOlayers.

Bottom Line: We show here that an induced antiferromagnetic order can be stabilized in the [111] direction by interfacial coupling to the insulating ferromagnet LaMnO3, and used to generate interlayer magnetic coupling of a nature that depends on the exact number of LaNiO3 monolayers.All three behaviours are explained based on the emergence of a (¼,¼,¼)-wavevector antiferromagnetic structure in LaNiO3 and the presence of interface asymmetry with LaMnO3.This dimensionality-induced magnetic order can be used to tailor a broad range of magnetic properties in well-designed superlattice-based devices.

View Article: PubMed Central - PubMed

Affiliation: Département de Physique de la Matière Quantique, University of Geneva, 24 Quai Ernest-Ansermet, 1211 Genève 4, Switzerland.

ABSTRACT
Dimensionality is known to play an important role in many compounds for which ultrathin layers can behave very differently from the bulk. This is especially true for the paramagnetic metal LaNiO3, which can become insulating and magnetic when only a few monolayers thick. We show here that an induced antiferromagnetic order can be stabilized in the [111] direction by interfacial coupling to the insulating ferromagnet LaMnO3, and used to generate interlayer magnetic coupling of a nature that depends on the exact number of LaNiO3 monolayers. For 7-monolayer-thick LaNiO3/LaMnO3 superlattices, negative and positive exchange bias, as well as antiferromagnetic interlayer coupling are observed in different temperature windows. All three behaviours are explained based on the emergence of a (¼,¼,¼)-wavevector antiferromagnetic structure in LaNiO3 and the presence of interface asymmetry with LaMnO3. This dimensionality-induced magnetic order can be used to tailor a broad range of magnetic properties in well-designed superlattice-based devices.

No MeSH data available.


Related in: MedlinePlus