Yves Henry | Université de Strasbourg (original) (raw)

Papers by Yves Henry

Research paper thumbnail of Magnetic anisotropy of epitaxial Co/Mn superlattices: An angular-dependent ferromagnetic resonance study

Physical review. B, Condensed matter, 1996

ABSTRACT

Research paper thumbnail of Magnetic anisotropy in giant magnetoresistive CoAg granular films

Research paper thumbnail of Structural changes in metastable epitaxial Co/Mn superlattices

Research paper thumbnail of Statistical analysis of the magnetization processes in arrays of electrodeposited ferromagnetic nanowires

Physical Review B, 2002

We present a statistical analysis of the magnetization processes in arrays of 22-m-long, 40-nm-wi... more We present a statistical analysis of the magnetization processes in arrays of 22-m-long, 40-nm-wide Co and Ni nanowires, with parallel-to-wire magnetic anisotropy, electrodeposited into porous polycarbonate membranes. This analysis is based on usual magnetization measurements taken with a magnetic field applied parallel to the average wire direction. It is shown that the magnetization curves may contain, in proportions which depend on the magnetic history of the arrays prior to the measurement, two contributions corresponding, respectively, to single-domain wires reversing their magnetization and to wires initially in a multidomain state which are remagnetized to saturation. Despite the extremely large number of wires involved, these two contributions exhibit clearly discernible substructures. These are related to the different and rather weakly distributed characteristic fields that describe the reversal and remagnetization processes: the nucleation and propagation fields. Numerical simulations of the magnetization curves are carried out which allow one to deduce the statistical distributions of these fields. From this modeling of the experimental data, it is shown that two distinct kinds of defects with very different pinning strength are certainly present in the nanowires. Finally, the analysis of the magnetization curves also provides accurate information concerning the distribution of wire orientation in the polycarbonate templates.

Research paper thumbnail of Structural and magnetic properties of semiepitaxial Co/Cr multilayers

Physical Review B, 1993

ABSTRACT

Research paper thumbnail of Spin Accumulation and Domain Wall Magnetoresistance in 35 nm Co Wires

Physical Review Letters, 2000

An enhancement of the resistance due to the presence of only one or two isolated domain walls is ... more An enhancement of the resistance due to the presence of only one or two isolated domain walls is clearly evidenced by transport measurements in 35 nm epitaxial Co wires, 20 &mgr;m long. The deduced relative change in the resistivity is at least 1 order of magnitude larger than the one predicted from a model based on the mixing of spin channels occurring over the length scale of the domain wall width [P. M. Levy and S. Zhang, Phys. Rev. Lett. 79, 5110 (1997)]. This inconsistency can be resolved by taking the effect of spin accumulation into account, which scales in the case of Co over the much larger distance of the spin diffusion length.

Research paper thumbnail of Training Effect in an Exchange Bias System: The Role of Interfacial Domain Walls

Physical Review Letters, 2006

Polarized neutron reflectivity (PNR) is used to obtain the magnetic depth profile of an antiferro... more Polarized neutron reflectivity (PNR) is used to obtain the magnetic depth profile of an antiferromagnetically coupled ferrimagnetic/ferrimagnetic bilayer, Gd 40 Fe 60 =Tb 12 Fe 88 . This system shows a transition from positive to negative exchange bias field H E as the cooling field H cf is increased from small to large positive value. It also exhibits training behavior upon field cycling which affects H E and the coercive field H C . From the PNR measurements at room temperature and at 15 K, we confirm that the magnetic configuration inside the TbFe layer is frozen when the sample is cooled in various H cf . The thickness and pitch of the magnetic twist inside the TbFe layer depend on H cf and give rise to the observed differences in the bias field. Irreversible reorganization of the TbFe magnetization at the interface occurs upon GdFe magnetization reversal and is found to explain the training effect as well as the overall reduction in coercivity.

Research paper thumbnail of Evidence for a Direct d-d Hybridization Mechanism for the Interlayer Exchange Coupling in Epitaxial Co/Mn Multilayers

Physical Review Letters, 1996

Research paper thumbnail of Current-induced magnetization reversal in terms of power dissipation

Physical Review B, 2011

ABSTRACT Magnetization excitation and reversal induced by spin-transfer torques is described in t... more ABSTRACT Magnetization excitation and reversal induced by spin-transfer torques is described in terms of power received or dissipated in a macrospin system. This approach provides a clear and intuitive understanding of the effect of both applied magnetic fields and injected spin-polarized currents on magnetization reversal. It is illustrated by solving the case of magnetization reversal in a nanopillar spin valve with perpendicular magnetizations although the approach can be applied more generally. The appearance of critical currents below which spin-transfer torque is no longer efficient is explained by a break in the uniaxial symmetry of such structures.

Research paper thumbnail of Magnetoresistance in an amorphous exchange-coupled bilayer

Physical Review B, 2009

ABSTRACT The effect of a magnetic domain wall on the electronic transport in disordered materials... more ABSTRACT The effect of a magnetic domain wall on the electronic transport in disordered materials is studied in an exchange-coupled amorphous Gd40Fe60/Gd10Fe90 bilayer. In this amorphous system, the size and the shape of an interfacial domain wall is controlled by an external magnetic field. Current-in-plane transport measurements are performed on single GdFe layers, Gd40Fe60/Gd10Fe90 bilayer, and on a Gd40Fe60/Si/Gd10Fe90 trilayer where the Si layer prevents the formation of the interfacial magnetic domain wall. Different contributions to the resistance are evidenced. In all types of samples, a linear positive magnetoresistance contribution is observed at high field which can be linked to the amorphous structure of the GdFe alloys. The comparison between the bilayer and the trilayer allows to eliminate this contribution and evidences that anisotropic magnetoresistance is the main effect induced by the interfacial domain wall. Beyond the anisotropic magnetoresistance signal, a supplementary negative magnetoresistance is evidenced. The origin of this effect is discussed qualitatively using previous theoretical predictions on magnetotransport through a magnetic domain wall in disordered metals.

Research paper thumbnail of Domain wall motion in nanopillar spin-valves with perpendicular anisotropy driven by spin-transfer torques

Physical Review B, 2012

ABSTRACT Using transport measurements and micromagnetic simulations we have investigated the doma... more ABSTRACT Using transport measurements and micromagnetic simulations we have investigated the domain wall motion driven by spin-transfer torques in all-perpendicular hexagonal nanopillar spin-valves. In particular, we probe domain walls nucleated in the free layer of the spin-valves, which are then pinned in the devices. We have determined both the field-current state diagrams for the domain-wall state and the thermally activated dynamics of the nucleation and depinning processes. We show that the nucleation process is well-described by a modified Néel-Brown model taking into account the spin-transfer torque, whereas the depinning process is independent of the current. This is confirmed by an analytical calculation which shows that spin-torques have no effect on the Arrhenius escape rate associated with thermally activated domain wall depinning in this geometry. Furthermore, micromagnetic simulations indicate that spin-transfer only weakly affects the domain wall motion, but instead modifies the inner domain wall structure.

Research paper thumbnail of Chirality reversal of the interface domain wall in a hard/soft magnetic bilayer

Physical Review B, 2004

... As 140401-3 RAPID COMMUNICATIONS Y. HENRY, S. MANGIN, AND F. MONTAIGNE PHYSICAL REVIEW B 69, ... more ... As 140401-3 RAPID COMMUNICATIONS Y. HENRY, S. MANGIN, AND F. MONTAIGNE PHYSICAL REVIEW B 69, 140401 R ... as in the case where the sample is initially in a single-configuration state major AMR curves ... 7 O. Hellwig, JB Kortright, K. Takano, and EE Fullerton, ...

Research paper thumbnail of Electrical switching in Fe/V/MgO/Fe tunnel junctions

Physical Review B, 2010

Bipolar hysteretic resistance switching in epitaxial Fe/V/MgO/Fe magnetic tunnel junctions is obs... more Bipolar hysteretic resistance switching in epitaxial Fe/V/MgO/Fe magnetic tunnel junctions is observed in highly reproducible I͑V͒ curves and found to be modified by the frequency of the bias voltage sweep. Observation of slow relaxation of the resistance state values is reported. A model is proposed that takes into account the incidence of time-dependent electric-field-induced migration of atomic species on the effective barrier thickness. This model provides a good qualitative agreement with experimental data.

Research paper thumbnail of Mechanism of chirality reversal for planar interface domain walls in exchange-coupled hard/soft magnetic bilayers

Physical Review B, 2008

ABSTRACT The mechanism of chirality reversal for a planar interface domain wall in a hard/soft ma... more ABSTRACT The mechanism of chirality reversal for a planar interface domain wall in a hard/soft magnetic bilayer has been identified by combining magnetoresistance measurements, modeling, and direct magnetic domain observations. The reversal occurs through IDW nucleation and lateral domain wall propagation. Over an unpredicted wide range of applied magnetic fields, the chirality transition takes place by an unwinding followed by a rewinding of the IDW. The chirality transition mechanism of phase transition could be identified from a micromagnetic analysis of the lateral magnetic domain wall orientation. Up to three magnetization phases coexist in the uniaxial material during reversal.

Research paper thumbnail of Positive exchange-bias induced by interface domain wall quenching in GdFe∕TbFe films

Physical Review B, 2006

Exchange-bias in a soft/hard ferrimagnetic (sFi/hFi) GdFe∕TbFe bilayer with antiferromagnetic int... more Exchange-bias in a soft/hard ferrimagnetic (sFi/hFi) GdFe∕TbFe bilayer with antiferromagnetic interface coupling has been studied as a function of the magnitude H cf and angle ψ cf of the cooling field. A continuous transition from negative exchange bias to positive exchange ...

Research paper thumbnail of Electrical transport across a structurally ordered phthalocyanine film: Role of defect states

Physical Review B, 2007

We present transport measurements from 10 to 290 K on a junction made of metallic electrodes sepa... more We present transport measurements from 10 to 290 K on a junction made of metallic electrodes separated by a structurally ordered, 100 nm-thick metal-free H 2 -phthalocyanine layer. Above 130 K, the junction is rectifying and the transport is thermally activated with a barrier of 0.148 eV. The conductance exhibits the signature of resonant tunneling across the energy levels of the molecules. Below 130 K, transport is activated with a barrier of 5.8ϫ 10 −4 eV and exhibits a zero-bias conductance anomaly at low temperature. We discuss these transport features, and the absence of magnetoresistance, in terms of the likely presence of defects due to the high structural quality of our organic spacer layer.

Research paper thumbnail of Distortion of the Stoner-Wohlfarth astroid by a spin-polarized current

Physical Review B, 2009

The Stoner-Wohlfarth astroid is a fundamental object in magnetism. It separates regions of the ma... more The Stoner-Wohlfarth astroid is a fundamental object in magnetism. It separates regions of the magnetic field space with two stable magnetization equilibria from those with only one stable equilibrium and it characterizes the magnetization reversal of nano-magnets induced by applied magnetic fields. On the other hand, it was recently demonstrated that transfer of spin angular momentum from a spin-polarized current provides an alternative way of switching the magnetization. Here, we examine the astroid of a nano-magnet with uniaxial magnetic anisotropy under the combined influence of applied fields and spin-transfer torques. We find that spin-transfer is most efficient at modifying the astroid when the external field is applied along the easy-axis of magnetization. On departing from this situation, a threshold current appears below which spin-transfer becomes ineffective yielding a current-induced dip in the astroid along the easy-axis direction. An extension of the Stoner-Wohlfarth model is outlined which accounts for this phenomenon.

Research paper thumbnail of Influence of lateral domains and interface domain walls on exchange-bias phenomena in GbFe∕TdFe bilayers

Physical Review B, 2006

The effect of lateral magnetic domains on exchange-bias phenomena in a soft/hard Gd40Fe60(100nm)/... more The effect of lateral magnetic domains on exchange-bias phenomena in a soft/hard Gd40Fe60(100nm)/Tb12Fe88(50nm) bilayer with antiferromagnetic interface exchange coupling is investigated. At low temperature, domains in the hard TbFe layer are found to behave independently of each other and to give rise to their own exchange-bias fields. Double hysteresis loops are observed when the sample is cooled with domains present in both the hard TbFe layer and the soft GdFe layer, whereas single hysteresis loops are observed when domains are only present in the hard layer. These features are understood by taking into account the presence of both lateral domains and planar interface domain walls in this strongly coupled system.

Research paper thumbnail of State diagram of nanopillar spin valves with perpendicular magnetic anisotropy

Physical Review B, 2012

The spin-torque switching of metallic nanopillar spin valves showing strong perpendicular anisotr... more The spin-torque switching of metallic nanopillar spin valves showing strong perpendicular anisotropy are studied. The magnetic states of the layers depend on extrinsic parameters such as the magnetic field and the dc current applied to the device. A state diagram presents a comprehensive graph of the role of those parameters on the spin-valve magnetic response. After explaining how state diagrams can be built and the different possible representation, experimental state diagrams are studied for perpendicular devices and the influence of lateral size, temperature, and field orientation are shown. An analytical model of a purely uniaxial system is presented. It is shown that this simple model does not properly reflect the experimental results, whereas if the symmetry is broken a qualitative agreement is obtained. Finally, the possible origins of the symmetry break are discussed in light of an analytical model and numerical simulations.

Research paper thumbnail of Size-induced enhanced magnetoelectric effect and multiferroicity in chromium oxide nanoclusters

Nature Communications, 2014

The control of the magnetization of a material with an electric field would make the design and t... more The control of the magnetization of a material with an electric field would make the design and the integration of novel electronic devices possible. This explains the renewed interest in multiferroic materials. Progress in this field is currently hampered by the scarcity of the materials available and the smallness of the magnetoelectric effects. Here we present a proof-of-principle experiment showing that engineering large strains through nanoscale size reduction is an efficient route for increasing magnetoelectric coefficients by orders of magnitude. The archetype magnetoelectric material, Cr2O3, in the form of epitaxial clusters, exhibits an unprecedented 600% change in magnetization magnitude under 1 V. Furthermore, a multiferroic phase, with both magnetic and electric spontaneous polarizations, is found in the clusters, while absent in the bulk.

Research paper thumbnail of Magnetic anisotropy of epitaxial Co/Mn superlattices: An angular-dependent ferromagnetic resonance study

Physical review. B, Condensed matter, 1996

ABSTRACT

Research paper thumbnail of Magnetic anisotropy in giant magnetoresistive CoAg granular films

Research paper thumbnail of Structural changes in metastable epitaxial Co/Mn superlattices

Research paper thumbnail of Statistical analysis of the magnetization processes in arrays of electrodeposited ferromagnetic nanowires

Physical Review B, 2002

We present a statistical analysis of the magnetization processes in arrays of 22-m-long, 40-nm-wi... more We present a statistical analysis of the magnetization processes in arrays of 22-m-long, 40-nm-wide Co and Ni nanowires, with parallel-to-wire magnetic anisotropy, electrodeposited into porous polycarbonate membranes. This analysis is based on usual magnetization measurements taken with a magnetic field applied parallel to the average wire direction. It is shown that the magnetization curves may contain, in proportions which depend on the magnetic history of the arrays prior to the measurement, two contributions corresponding, respectively, to single-domain wires reversing their magnetization and to wires initially in a multidomain state which are remagnetized to saturation. Despite the extremely large number of wires involved, these two contributions exhibit clearly discernible substructures. These are related to the different and rather weakly distributed characteristic fields that describe the reversal and remagnetization processes: the nucleation and propagation fields. Numerical simulations of the magnetization curves are carried out which allow one to deduce the statistical distributions of these fields. From this modeling of the experimental data, it is shown that two distinct kinds of defects with very different pinning strength are certainly present in the nanowires. Finally, the analysis of the magnetization curves also provides accurate information concerning the distribution of wire orientation in the polycarbonate templates.

Research paper thumbnail of Structural and magnetic properties of semiepitaxial Co/Cr multilayers

Physical Review B, 1993

ABSTRACT

Research paper thumbnail of Spin Accumulation and Domain Wall Magnetoresistance in 35 nm Co Wires

Physical Review Letters, 2000

An enhancement of the resistance due to the presence of only one or two isolated domain walls is ... more An enhancement of the resistance due to the presence of only one or two isolated domain walls is clearly evidenced by transport measurements in 35 nm epitaxial Co wires, 20 &mgr;m long. The deduced relative change in the resistivity is at least 1 order of magnitude larger than the one predicted from a model based on the mixing of spin channels occurring over the length scale of the domain wall width [P. M. Levy and S. Zhang, Phys. Rev. Lett. 79, 5110 (1997)]. This inconsistency can be resolved by taking the effect of spin accumulation into account, which scales in the case of Co over the much larger distance of the spin diffusion length.

Research paper thumbnail of Training Effect in an Exchange Bias System: The Role of Interfacial Domain Walls

Physical Review Letters, 2006

Polarized neutron reflectivity (PNR) is used to obtain the magnetic depth profile of an antiferro... more Polarized neutron reflectivity (PNR) is used to obtain the magnetic depth profile of an antiferromagnetically coupled ferrimagnetic/ferrimagnetic bilayer, Gd 40 Fe 60 =Tb 12 Fe 88 . This system shows a transition from positive to negative exchange bias field H E as the cooling field H cf is increased from small to large positive value. It also exhibits training behavior upon field cycling which affects H E and the coercive field H C . From the PNR measurements at room temperature and at 15 K, we confirm that the magnetic configuration inside the TbFe layer is frozen when the sample is cooled in various H cf . The thickness and pitch of the magnetic twist inside the TbFe layer depend on H cf and give rise to the observed differences in the bias field. Irreversible reorganization of the TbFe magnetization at the interface occurs upon GdFe magnetization reversal and is found to explain the training effect as well as the overall reduction in coercivity.

Research paper thumbnail of Evidence for a Direct d-d Hybridization Mechanism for the Interlayer Exchange Coupling in Epitaxial Co/Mn Multilayers

Physical Review Letters, 1996

Research paper thumbnail of Current-induced magnetization reversal in terms of power dissipation

Physical Review B, 2011

ABSTRACT Magnetization excitation and reversal induced by spin-transfer torques is described in t... more ABSTRACT Magnetization excitation and reversal induced by spin-transfer torques is described in terms of power received or dissipated in a macrospin system. This approach provides a clear and intuitive understanding of the effect of both applied magnetic fields and injected spin-polarized currents on magnetization reversal. It is illustrated by solving the case of magnetization reversal in a nanopillar spin valve with perpendicular magnetizations although the approach can be applied more generally. The appearance of critical currents below which spin-transfer torque is no longer efficient is explained by a break in the uniaxial symmetry of such structures.

Research paper thumbnail of Magnetoresistance in an amorphous exchange-coupled bilayer

Physical Review B, 2009

ABSTRACT The effect of a magnetic domain wall on the electronic transport in disordered materials... more ABSTRACT The effect of a magnetic domain wall on the electronic transport in disordered materials is studied in an exchange-coupled amorphous Gd40Fe60/Gd10Fe90 bilayer. In this amorphous system, the size and the shape of an interfacial domain wall is controlled by an external magnetic field. Current-in-plane transport measurements are performed on single GdFe layers, Gd40Fe60/Gd10Fe90 bilayer, and on a Gd40Fe60/Si/Gd10Fe90 trilayer where the Si layer prevents the formation of the interfacial magnetic domain wall. Different contributions to the resistance are evidenced. In all types of samples, a linear positive magnetoresistance contribution is observed at high field which can be linked to the amorphous structure of the GdFe alloys. The comparison between the bilayer and the trilayer allows to eliminate this contribution and evidences that anisotropic magnetoresistance is the main effect induced by the interfacial domain wall. Beyond the anisotropic magnetoresistance signal, a supplementary negative magnetoresistance is evidenced. The origin of this effect is discussed qualitatively using previous theoretical predictions on magnetotransport through a magnetic domain wall in disordered metals.

Research paper thumbnail of Domain wall motion in nanopillar spin-valves with perpendicular anisotropy driven by spin-transfer torques

Physical Review B, 2012

ABSTRACT Using transport measurements and micromagnetic simulations we have investigated the doma... more ABSTRACT Using transport measurements and micromagnetic simulations we have investigated the domain wall motion driven by spin-transfer torques in all-perpendicular hexagonal nanopillar spin-valves. In particular, we probe domain walls nucleated in the free layer of the spin-valves, which are then pinned in the devices. We have determined both the field-current state diagrams for the domain-wall state and the thermally activated dynamics of the nucleation and depinning processes. We show that the nucleation process is well-described by a modified Néel-Brown model taking into account the spin-transfer torque, whereas the depinning process is independent of the current. This is confirmed by an analytical calculation which shows that spin-torques have no effect on the Arrhenius escape rate associated with thermally activated domain wall depinning in this geometry. Furthermore, micromagnetic simulations indicate that spin-transfer only weakly affects the domain wall motion, but instead modifies the inner domain wall structure.

Research paper thumbnail of Chirality reversal of the interface domain wall in a hard/soft magnetic bilayer

Physical Review B, 2004

... As 140401-3 RAPID COMMUNICATIONS Y. HENRY, S. MANGIN, AND F. MONTAIGNE PHYSICAL REVIEW B 69, ... more ... As 140401-3 RAPID COMMUNICATIONS Y. HENRY, S. MANGIN, AND F. MONTAIGNE PHYSICAL REVIEW B 69, 140401 R ... as in the case where the sample is initially in a single-configuration state major AMR curves ... 7 O. Hellwig, JB Kortright, K. Takano, and EE Fullerton, ...

Research paper thumbnail of Electrical switching in Fe/V/MgO/Fe tunnel junctions

Physical Review B, 2010

Bipolar hysteretic resistance switching in epitaxial Fe/V/MgO/Fe magnetic tunnel junctions is obs... more Bipolar hysteretic resistance switching in epitaxial Fe/V/MgO/Fe magnetic tunnel junctions is observed in highly reproducible I͑V͒ curves and found to be modified by the frequency of the bias voltage sweep. Observation of slow relaxation of the resistance state values is reported. A model is proposed that takes into account the incidence of time-dependent electric-field-induced migration of atomic species on the effective barrier thickness. This model provides a good qualitative agreement with experimental data.

Research paper thumbnail of Mechanism of chirality reversal for planar interface domain walls in exchange-coupled hard/soft magnetic bilayers

Physical Review B, 2008

ABSTRACT The mechanism of chirality reversal for a planar interface domain wall in a hard/soft ma... more ABSTRACT The mechanism of chirality reversal for a planar interface domain wall in a hard/soft magnetic bilayer has been identified by combining magnetoresistance measurements, modeling, and direct magnetic domain observations. The reversal occurs through IDW nucleation and lateral domain wall propagation. Over an unpredicted wide range of applied magnetic fields, the chirality transition takes place by an unwinding followed by a rewinding of the IDW. The chirality transition mechanism of phase transition could be identified from a micromagnetic analysis of the lateral magnetic domain wall orientation. Up to three magnetization phases coexist in the uniaxial material during reversal.

Research paper thumbnail of Positive exchange-bias induced by interface domain wall quenching in GdFe∕TbFe films

Physical Review B, 2006

Exchange-bias in a soft/hard ferrimagnetic (sFi/hFi) GdFe∕TbFe bilayer with antiferromagnetic int... more Exchange-bias in a soft/hard ferrimagnetic (sFi/hFi) GdFe∕TbFe bilayer with antiferromagnetic interface coupling has been studied as a function of the magnitude H cf and angle ψ cf of the cooling field. A continuous transition from negative exchange bias to positive exchange ...

Research paper thumbnail of Electrical transport across a structurally ordered phthalocyanine film: Role of defect states

Physical Review B, 2007

We present transport measurements from 10 to 290 K on a junction made of metallic electrodes sepa... more We present transport measurements from 10 to 290 K on a junction made of metallic electrodes separated by a structurally ordered, 100 nm-thick metal-free H 2 -phthalocyanine layer. Above 130 K, the junction is rectifying and the transport is thermally activated with a barrier of 0.148 eV. The conductance exhibits the signature of resonant tunneling across the energy levels of the molecules. Below 130 K, transport is activated with a barrier of 5.8ϫ 10 −4 eV and exhibits a zero-bias conductance anomaly at low temperature. We discuss these transport features, and the absence of magnetoresistance, in terms of the likely presence of defects due to the high structural quality of our organic spacer layer.

Research paper thumbnail of Distortion of the Stoner-Wohlfarth astroid by a spin-polarized current

Physical Review B, 2009

The Stoner-Wohlfarth astroid is a fundamental object in magnetism. It separates regions of the ma... more The Stoner-Wohlfarth astroid is a fundamental object in magnetism. It separates regions of the magnetic field space with two stable magnetization equilibria from those with only one stable equilibrium and it characterizes the magnetization reversal of nano-magnets induced by applied magnetic fields. On the other hand, it was recently demonstrated that transfer of spin angular momentum from a spin-polarized current provides an alternative way of switching the magnetization. Here, we examine the astroid of a nano-magnet with uniaxial magnetic anisotropy under the combined influence of applied fields and spin-transfer torques. We find that spin-transfer is most efficient at modifying the astroid when the external field is applied along the easy-axis of magnetization. On departing from this situation, a threshold current appears below which spin-transfer becomes ineffective yielding a current-induced dip in the astroid along the easy-axis direction. An extension of the Stoner-Wohlfarth model is outlined which accounts for this phenomenon.

Research paper thumbnail of Influence of lateral domains and interface domain walls on exchange-bias phenomena in GbFe∕TdFe bilayers

Physical Review B, 2006

The effect of lateral magnetic domains on exchange-bias phenomena in a soft/hard Gd40Fe60(100nm)/... more The effect of lateral magnetic domains on exchange-bias phenomena in a soft/hard Gd40Fe60(100nm)/Tb12Fe88(50nm) bilayer with antiferromagnetic interface exchange coupling is investigated. At low temperature, domains in the hard TbFe layer are found to behave independently of each other and to give rise to their own exchange-bias fields. Double hysteresis loops are observed when the sample is cooled with domains present in both the hard TbFe layer and the soft GdFe layer, whereas single hysteresis loops are observed when domains are only present in the hard layer. These features are understood by taking into account the presence of both lateral domains and planar interface domain walls in this strongly coupled system.

Research paper thumbnail of State diagram of nanopillar spin valves with perpendicular magnetic anisotropy

Physical Review B, 2012

The spin-torque switching of metallic nanopillar spin valves showing strong perpendicular anisotr... more The spin-torque switching of metallic nanopillar spin valves showing strong perpendicular anisotropy are studied. The magnetic states of the layers depend on extrinsic parameters such as the magnetic field and the dc current applied to the device. A state diagram presents a comprehensive graph of the role of those parameters on the spin-valve magnetic response. After explaining how state diagrams can be built and the different possible representation, experimental state diagrams are studied for perpendicular devices and the influence of lateral size, temperature, and field orientation are shown. An analytical model of a purely uniaxial system is presented. It is shown that this simple model does not properly reflect the experimental results, whereas if the symmetry is broken a qualitative agreement is obtained. Finally, the possible origins of the symmetry break are discussed in light of an analytical model and numerical simulations.

Research paper thumbnail of Size-induced enhanced magnetoelectric effect and multiferroicity in chromium oxide nanoclusters

Nature Communications, 2014

The control of the magnetization of a material with an electric field would make the design and t... more The control of the magnetization of a material with an electric field would make the design and the integration of novel electronic devices possible. This explains the renewed interest in multiferroic materials. Progress in this field is currently hampered by the scarcity of the materials available and the smallness of the magnetoelectric effects. Here we present a proof-of-principle experiment showing that engineering large strains through nanoscale size reduction is an efficient route for increasing magnetoelectric coefficients by orders of magnitude. The archetype magnetoelectric material, Cr2O3, in the form of epitaxial clusters, exhibits an unprecedented 600% change in magnetization magnitude under 1 V. Furthermore, a multiferroic phase, with both magnetic and electric spontaneous polarizations, is found in the clusters, while absent in the bulk.