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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.


Exchange bias evolution for a superlattice with 7-ML-thick (111)-LNOlayers.Schematics of the field-cooling procedure: (a) at the FM orderingtemperature, the LMO layers induce a moment in the interfacial Ni, which(b) subsequently stabilizes a magnetic defect (orange triangles)in the (¼,¼,¼) AF order. This configuration freezes induring field cooling and gives the starting point for the fieldmeasurements. (c) At low temperature, the anisotropy in LNO is largeand the magnetic defect is frozen inside these layers. At negative field,both interfacial exchange energies JS andJI are frustrated, resulting in the existence ofnegative EB. As temperature increases, the anisotropy weakens and becomessmaller than the larger of the two interface exchanges(JI). (d) In this intermediate-temperature case, anegative field reverses the Ni spins on one side of the LMO interface andannihilates the magnetic defect in LNO. This configuration is stabilized ifJS is the smallest energy scale, thus inducing a signchange of the exchange bias field. (e) At higher temperature, theanisotropy is negligible and all the energy terms are minimized when the LMOlayers are AF-ordered.
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f5: Exchange bias evolution for a superlattice with 7-ML-thick (111)-LNOlayers.Schematics of the field-cooling procedure: (a) at the FM orderingtemperature, the LMO layers induce a moment in the interfacial Ni, which(b) subsequently stabilizes a magnetic defect (orange triangles)in the (¼,¼,¼) AF order. This configuration freezes induring field cooling and gives the starting point for the fieldmeasurements. (c) At low temperature, the anisotropy in LNO is largeand the magnetic defect is frozen inside these layers. At negative field,both interfacial exchange energies JS andJI are frustrated, resulting in the existence ofnegative EB. As temperature increases, the anisotropy weakens and becomessmaller than the larger of the two interface exchanges(JI). (d) In this intermediate-temperature case, anegative field reverses the Ni spins on one side of the LMO interface andannihilates the magnetic defect in LNO. This configuration is stabilized ifJS is the smallest energy scale, thus inducing a signchange of the exchange bias field. (e) At higher temperature, theanisotropy is negligible and all the energy terms are minimized when the LMOlayers are AF-ordered.

Mentions: The overall coupling between neighbouring LMO layers mediated by 7 MLs of the LNOAF structure is thus antiferromagnetic, as schematized in Fig.4c. This coupling is only possible along the [111]direction and for a LNO thickness of 7 MLs, in agreement with our data.Considering such a coupling through LNO, the challenge now is to explain themagnetic properties of the (LNO7/LMO7)15superlattices in the entire temperature range, including the EB and its signchange—sign change that is only observed for(LNO7/LMO7)15 superlattices as shown inFig. 2c. It is known that several magneticinteractions are at play in conventional FM/AF exchange-biased systems38, comprising the resulting magnetic ordering of the layers andtheir interface coupling. Interestingly, in our superlattices, transmissionelectron microscopy measurements indicate that the LNO/LMO and LMO/LNOinterfaces are not equivalent as can be seen in Fig. 1b(ref. 28). In the present case, X-ray absorptionspectroscopy (XAS) and EELS measurements performed on LMO/LNO heterostructuresshow that charge transfer is larger for the more intermixed interface28, which will likely unbalance the strength of interfacialcoupling on both sides of the ferromagnetic layer. Indeed, while intermixingleads to an alloy where strong Mn4+/Ni2+FM superexchange should dominate (as in the double perovskiteLa2MnNiO6), the smoother interface should give rise tocompeting AF contributions fromMn3+/Ni3+ superexchange. Thus, whilestill FM-coupled, the sharp interface should lead to a smaller exchange(JS) than the more intermixed LNO-on-LMO one(JI): JI>JS. Inaddition, there are two other relevant energy scales linked to theantiferromagnetic LNO structure. The first one is the single-atom anisotropy,KAF, and the second the energy of a planar AF defect,which is of the order of the second nearest-neighbour exchange in LNO,JSNN. Like in most conventional exchange-biased systems,the AF anisotropy and exchange are the quantities that vary most withtemperature and are responsible for the ‘freezing' of the AF statebelow the blocking temperature. Thus, one can imagine that KAFgoes from negligible at high temperatures to values larger than the interfaceexchanges at low temperature. During the field-cooling procedure, a likelyscenario is depicted on the top part of Fig. 5. At hightemperature, LMO becomes magnetic and drives the interfacial Ni moments to alignwith those of LMO (Fig. 5a), but the(¼,¼,¼) structure is not yet stable in LNO. Once itstabilizes, it has to adapt to the parallel LMO/LNO interfaces, which impose amagnetic phase shift in the 7-ML LNO. This would generate a magnetic defect inthe LNO layer, as sketched by the orange triangles in Fig.5b, which costs an energy of the order of JSNN.When the temperature decreases, this structure freezes in as the anisotropy ofthe Ni moments closer to the interface establishes a potential energy barrierpreventing the magnetic defect from moving. At very low temperatures (Fig. 5c), reversing the magnetization of the ferromagneticLMO layers does not affect the frozen AF-LNO configuration, and the total energyincreases through the additional frustration of the two interface couplings.This produces EB with the classic negative sign shift(HEB<0) of the hysteresis cycle. This scenario has commonpoints with the models of Mauri et al.39 and Kiwi38 for conventional exchange bias where an AF planar domain wall iswound in the AF. The main difference here is that the particular AF structure ofLNO is likely to allow for a magnetic phase slip on a single-unit-cell scale. Asthe temperature is raised (Fig. 5d), the AF anisotropydecreases below the larger interface exchange energy JI (butstill above JS). At this interface, the strongJI locks the interfacial Ni spins and forces them tofollow the Mn magnetization, at the (lower) cost of some anisotropy energy. Whenthe LMO magnetization reverses, the rotation of Ni moments annihilates the AFdefect. The total energy of this final state is decreased ifJS<JSNN, in which case the EB changessign (HEB>0). The observed sign reversal is noteworthy asreports of positive EB are scarce and its observation usually requires adifferent cooling procedure under a much larger field (for example, theFeF2/Fe system)40. Here the sign change resultsfrom a temperature-induced crossing of anisotropy energy with one (and only one)of the interfaces' exchanges. At higher temperature (Fig.5e), the anisotropy decreases further and the Ni moments at bothinterfaces become locked to those of Mn. The EB therefore disappears and thesystem can be considered to be above the blocking temperature. In this case, themost stable state is the one where no interaction is frustrated, that is, theAF-coupled LMO layers, and this is indeed what is unambiguously observed in thesynchrotron reflectivity measurements.


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)

Exchange bias evolution for a superlattice with 7-ML-thick (111)-LNOlayers.Schematics of the field-cooling procedure: (a) at the FM orderingtemperature, the LMO layers induce a moment in the interfacial Ni, which(b) subsequently stabilizes a magnetic defect (orange triangles)in the (¼,¼,¼) AF order. This configuration freezes induring field cooling and gives the starting point for the fieldmeasurements. (c) At low temperature, the anisotropy in LNO is largeand the magnetic defect is frozen inside these layers. At negative field,both interfacial exchange energies JS andJI are frustrated, resulting in the existence ofnegative EB. As temperature increases, the anisotropy weakens and becomessmaller than the larger of the two interface exchanges(JI). (d) In this intermediate-temperature case, anegative field reverses the Ni spins on one side of the LMO interface andannihilates the magnetic defect in LNO. This configuration is stabilized ifJS is the smallest energy scale, thus inducing a signchange of the exchange bias field. (e) At higher temperature, theanisotropy is negligible and all the energy terms are minimized when the LMOlayers are AF-ordered.
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Related In: Results  -  Collection

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Show All Figures
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f5: Exchange bias evolution for a superlattice with 7-ML-thick (111)-LNOlayers.Schematics of the field-cooling procedure: (a) at the FM orderingtemperature, the LMO layers induce a moment in the interfacial Ni, which(b) subsequently stabilizes a magnetic defect (orange triangles)in the (¼,¼,¼) AF order. This configuration freezes induring field cooling and gives the starting point for the fieldmeasurements. (c) At low temperature, the anisotropy in LNO is largeand the magnetic defect is frozen inside these layers. At negative field,both interfacial exchange energies JS andJI are frustrated, resulting in the existence ofnegative EB. As temperature increases, the anisotropy weakens and becomessmaller than the larger of the two interface exchanges(JI). (d) In this intermediate-temperature case, anegative field reverses the Ni spins on one side of the LMO interface andannihilates the magnetic defect in LNO. This configuration is stabilized ifJS is the smallest energy scale, thus inducing a signchange of the exchange bias field. (e) At higher temperature, theanisotropy is negligible and all the energy terms are minimized when the LMOlayers are AF-ordered.
Mentions: The overall coupling between neighbouring LMO layers mediated by 7 MLs of the LNOAF structure is thus antiferromagnetic, as schematized in Fig.4c. This coupling is only possible along the [111]direction and for a LNO thickness of 7 MLs, in agreement with our data.Considering such a coupling through LNO, the challenge now is to explain themagnetic properties of the (LNO7/LMO7)15superlattices in the entire temperature range, including the EB and its signchange—sign change that is only observed for(LNO7/LMO7)15 superlattices as shown inFig. 2c. It is known that several magneticinteractions are at play in conventional FM/AF exchange-biased systems38, comprising the resulting magnetic ordering of the layers andtheir interface coupling. Interestingly, in our superlattices, transmissionelectron microscopy measurements indicate that the LNO/LMO and LMO/LNOinterfaces are not equivalent as can be seen in Fig. 1b(ref. 28). In the present case, X-ray absorptionspectroscopy (XAS) and EELS measurements performed on LMO/LNO heterostructuresshow that charge transfer is larger for the more intermixed interface28, which will likely unbalance the strength of interfacialcoupling on both sides of the ferromagnetic layer. Indeed, while intermixingleads to an alloy where strong Mn4+/Ni2+FM superexchange should dominate (as in the double perovskiteLa2MnNiO6), the smoother interface should give rise tocompeting AF contributions fromMn3+/Ni3+ superexchange. Thus, whilestill FM-coupled, the sharp interface should lead to a smaller exchange(JS) than the more intermixed LNO-on-LMO one(JI): JI>JS. Inaddition, there are two other relevant energy scales linked to theantiferromagnetic LNO structure. The first one is the single-atom anisotropy,KAF, and the second the energy of a planar AF defect,which is of the order of the second nearest-neighbour exchange in LNO,JSNN. Like in most conventional exchange-biased systems,the AF anisotropy and exchange are the quantities that vary most withtemperature and are responsible for the ‘freezing' of the AF statebelow the blocking temperature. Thus, one can imagine that KAFgoes from negligible at high temperatures to values larger than the interfaceexchanges at low temperature. During the field-cooling procedure, a likelyscenario is depicted on the top part of Fig. 5. At hightemperature, LMO becomes magnetic and drives the interfacial Ni moments to alignwith those of LMO (Fig. 5a), but the(¼,¼,¼) structure is not yet stable in LNO. Once itstabilizes, it has to adapt to the parallel LMO/LNO interfaces, which impose amagnetic phase shift in the 7-ML LNO. This would generate a magnetic defect inthe LNO layer, as sketched by the orange triangles in Fig.5b, which costs an energy of the order of JSNN.When the temperature decreases, this structure freezes in as the anisotropy ofthe Ni moments closer to the interface establishes a potential energy barrierpreventing the magnetic defect from moving. At very low temperatures (Fig. 5c), reversing the magnetization of the ferromagneticLMO layers does not affect the frozen AF-LNO configuration, and the total energyincreases through the additional frustration of the two interface couplings.This produces EB with the classic negative sign shift(HEB<0) of the hysteresis cycle. This scenario has commonpoints with the models of Mauri et al.39 and Kiwi38 for conventional exchange bias where an AF planar domain wall iswound in the AF. The main difference here is that the particular AF structure ofLNO is likely to allow for a magnetic phase slip on a single-unit-cell scale. Asthe temperature is raised (Fig. 5d), the AF anisotropydecreases below the larger interface exchange energy JI (butstill above JS). At this interface, the strongJI locks the interfacial Ni spins and forces them tofollow the Mn magnetization, at the (lower) cost of some anisotropy energy. Whenthe LMO magnetization reverses, the rotation of Ni moments annihilates the AFdefect. The total energy of this final state is decreased ifJS<JSNN, in which case the EB changessign (HEB>0). The observed sign reversal is noteworthy asreports of positive EB are scarce and its observation usually requires adifferent cooling procedure under a much larger field (for example, theFeF2/Fe system)40. Here the sign change resultsfrom a temperature-induced crossing of anisotropy energy with one (and only one)of the interfaces' exchanges. At higher temperature (Fig.5e), the anisotropy decreases further and the Ni moments at bothinterfaces become locked to those of Mn. The EB therefore disappears and thesystem can be considered to be above the blocking temperature. In this case, themost stable state is the one where no interaction is frustrated, that is, theAF-coupled LMO layers, and this is indeed what is unambiguously observed in thesynchrotron reflectivity measurements.

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.