Magnetic vortices are topological spin structures frequently found in ferromagnets, yet novel to ... more Magnetic vortices are topological spin structures frequently found in ferromagnets, yet novel to antiferromagnets. By combining experiment and theory, we demonstrate that in a nanostructured antiferromagnetic-ferromagnetic NiO(111)-Fe(110) bilayer, a magnetic vortex is naturally stabilized by magnetostatic interactions in the ferromagnet and is imprinted onto the adjacent antiferromagnet via interface exchange coupling. We use micromagnetic simulations to construct a corresponding phase diagram of the stability of the imprinted antiferromagnetic vortex state. Our in depth analysis reveals that the interplay between interface exchange coupling and the antiferromagnet magnetic anisotropy plays a crucial role in locally reorienting the Néel vector out-of-plane in the prototypical in-plane antiferromagnet NiO and thereby stabilizing the vortices in the antiferromagnet.
We demonstrate a practical possibility of loss compensation in measured photocounting statistics ... more We demonstrate a practical possibility of loss compensation in measured photocounting statistics in the presence of dark counts and background radiation noise. It is shown that satisfactory results are obtained even in the case of low detection efficiency and large experimental errors.
We investigate magnetization dynamics of Mn 2 Au/Py (Ni 80 Fe 20 ) thin film bilayers using broad... more We investigate magnetization dynamics of Mn 2 Au/Py (Ni 80 Fe 20 ) thin film bilayers using broadband ferromagnetic resonance (FMR) and Brillouin light scattering spectroscopy. Our bilayers exhibit two resonant modes with zero-field frequencies up to almost 40 GHz, far above the singlelayer Py FMR. Our model calculations attribute these modes to the coupling of the Py FMR and the two antiferromagnetic resonance (AFMR) modes of Mn 2 Au. The coupling-strength is in the order of 1.6 T nm at room temperature for nm-thick Py. Our model reveals the dependence of the hybrid modes on the AFMR frequencies and interfacial coupling as well as the evanescent character of the spin waves that extend across the Mn 2 Au/Py interface. Ferromagnets have a net magnetic moment and uniform spin dynamics in the GHz range . In contrast, collinear antiferromagnets have two equal but opposite sublattice magnetizations with vanishing net magnetic moment [2, 3] and spin dynamics that can reach the THz range . In addition to THz spin dynamics, antiferromagnets can exhibit significant stability of their magnetic moments against external magnetic perturbations [6]. The difference in the magnetization dynamics of ferromagnetic (FM) and antiferromagnetic (AFM) materials could potentially be exploited in applications that integrate AFM materials in high-frequency spintronic devices. A promising approach to enhance the FM spin dynamics frequencies and control FM spin-wave dispersions might be the combination of FM and AFM thin-film layers with interfacial exchange coupling. As a result of interfacial exchange coupling, a pronounced increase in the coercivity of the FM layer and exchange bias can be observed. Interfacial coupling also modifies GHz spin dynamics in FM/FM, chiral FM/FM and AFM/FM heterostructures, in particular the magnetic damping and anisotropy . However, the role of the THz frequency spin dynamics of AFMs for the hybrid spin dynamics in AFM/FM bilayers has not been revealed so far. Consequently, AFM/FM bilayers have so far not been leveraged to control hybrid mode frequencies or study AFM dynamics without requiring THz spectroscopy tools. Here we show that by coupling AFM and FM modes, we can make the AFM dynamics visible in the GHz range and use the exchange-enhancement of AFM modes to elevate the FM spin dynamics frequencies. The existence of such a coupling between FM and AFM spin dynamics opens new possibilities to design next generation magneto-electronic devices that exploit AFM materials beyond exchange bias. The Mn 2 Au/Py system
Possible mechanisms of the external magnetic field influence on exchange coupling in ultrathin M/... more Possible mechanisms of the external magnetic field influence on exchange coupling in ultrathin M/N/M film structures are discussed. It is shown that the most reasonable mechanism is due to variation of the translational motion of spacer electrons in the external magnetic field. In this case in-plane components of the electron momentum should be quantized according to Landau levels. The most drastic effect should be observed in the case when magnetic field is perpendicular to the film plane. Comparison of calculations according to the proposed model with the experimental data is presented, showing very good correlation for the cases of presence and absence of external magnetic field.
INTERMAG Asia 2005. Digests of the IEEE International Magnetics Conference, 2005., 2005
ABSTRACT In the present work a theoretical ground for the influence of external magnetic field on... more ABSTRACT In the present work a theoretical ground for the influence of external magnetic field on coupling oscillations behaviour is proposed and corresponding modification of Bruno's formula was made.
We investigate magnetization dynamics of Mn 2 Au/Py (Ni 80 Fe 20 ) thin film bilayers using broad... more We investigate magnetization dynamics of Mn 2 Au/Py (Ni 80 Fe 20 ) thin film bilayers using broadband ferromagnetic resonance (FMR) and Brillouin light scattering spectroscopy. Our bilayers exhibit two resonant modes with zero-field frequencies up to almost 40 GHz, far above the singlelayer Py FMR. Our model calculations attribute these modes to the coupling of the Py FMR and the two antiferromagnetic resonance (AFMR) modes of Mn 2 Au. The coupling-strength is in the order of 1.6 T nm at room temperature for nm-thick Py. Our model reveals the dependence of the hybrid modes on the AFMR frequencies and interfacial coupling as well as the evanescent character of the spin waves that extend across the Mn 2 Au/Py interface. Ferromagnets have a net magnetic moment and uniform spin dynamics in the GHz range . In contrast, collinear antiferromagnets have two equal but opposite sublattice magnetizations with vanishing net magnetic moment [2, 3] and spin dynamics that can reach the THz range . In addition to THz spin dynamics, antiferromagnets can exhibit significant stability of their magnetic moments against external magnetic perturbations [6]. The difference in the magnetization dynamics of ferromagnetic (FM) and antiferromagnetic (AFM) materials could potentially be exploited in applications that integrate AFM materials in high-frequency spintronic devices. A promising approach to enhance the FM spin dynamics frequencies and control FM spin-wave dispersions might be the combination of FM and AFM thin-film layers with interfacial exchange coupling. As a result of interfacial exchange coupling, a pronounced increase in the coercivity of the FM layer and exchange bias can be observed. Interfacial coupling also modifies GHz spin dynamics in FM/FM, chiral FM/FM and AFM/FM heterostructures, in particular the magnetic damping and anisotropy . However, the role of the THz frequency spin dynamics of AFMs for the hybrid spin dynamics in AFM/FM bilayers has not been revealed so far. Consequently, AFM/FM bilayers have so far not been leveraged to control hybrid mode frequencies or study AFM dynamics without requiring THz spectroscopy tools. Here we show that by coupling AFM and FM modes, we can make the AFM dynamics visible in the GHz range and use the exchange-enhancement of AFM modes to elevate the FM spin dynamics frequencies. The existence of such a coupling between FM and AFM spin dynamics opens new possibilities to design next generation magneto-electronic devices that exploit AFM materials beyond exchange bias. The Mn 2 Au/Py system
Antiferromagnetic materials have been proposed as new types of narrowband THz spintronic devices ... more Antiferromagnetic materials have been proposed as new types of narrowband THz spintronic devices owing to their ultrafast spin dynamics. Manipulating coherently their spin dynamics, however, remains a key challenge that is envisioned to be accomplished by spin-orbit torques or direct optical excitations. Here, we demonstrate the combined generation of broadband THz (incoherent) magnons and narrowband (coherent) magnons at 1 THz in low damping thin films of NiO/Pt. We evidence, experimentally and through modeling, two excitation processes of spin dynamics in NiO: an off-resonant instantaneous optical spin torque in (111) oriented films and a strain-waveinduced THz torque induced by ultrafast Pt excitation in (001) oriented films. Both phenomena lead to the emission of a THz signal through the inverse spin Hall effect in the adjacent heavy metal layer. We unravel the characteristic timescales of the two excitation processes found to be < 50 fs and > 300 fs, respectively, and thus open new routes towards the development of fast optospintronic devices based on antiferromagnetic materials. Antiferromagnetic spintronics has recently become an important research field from both a fundamental viewpoint and its strong applicative potential 1,2. Antiferromagnets (AFM) have key advantages linked to their magnetic ordering: they are insensitive to perturbative external magnetic fields, stray fields are absent, and magnon modal frequencies reach the terahertz (THz) regime 1-3. This renders AFMs prime candidates for ultrafast spintronic devices 4 compared to their ferromagnetic counterparts. A recent work demonstrated the writing of AFM memory states with picosecond excitations 5. In parallel, narrowband sub-THz detection has been achieved using spin-pumping in AFMs 6,7. Another spintronic application is spintronic-based broadband THz emission that currently relies on ferromagnet/heavy metal
Shape effects in magnetic particles are widely studied, because of the ability of the shape and t... more Shape effects in magnetic particles are widely studied, because of the ability of the shape and the size to control the parameters of a sample during its production. Experiments with nanosized samples show that the shape can affect also the properties of antiferromagnetic (AFM) materials. However, the theoretical interpretation of these effects is under discussion. A model to study the shape-induced effects in AFM particles at the AFM resonance frequency is proposed. The Lagrange function method is used to calculate the spectrum of resonance oscillations of the AFM vector in a synthetic multiferroic (piezoelectric + antiferromagnet). The influence of the specimen shape on the AFM resonance frequency in the presence of an external magnetic field is studied. Conditions for a resonance under the action of an external force or for a parametric resonance to arise in the magnetic subsystem are considered.
Antiferromagnets are promising materials for spintronics because they show fast magnetic dynamics... more Antiferromagnets are promising materials for spintronics because they show fast magnetic dynamics, low susceptibility to magnetic fields, and produce no stray fields. In addition, the antiferromagnetic dynamics can be efficiently manipulated by spin and charge currents. Here we discuss spin and/or charge current induced dynamics of the antiferromagnetic textures (domain walls, skyrmions) and nanoparticles. We consider and analyse four types of torques which (spin) current can generate in an antiferromagnet with two magnetic sublattices. These torques can be classified as the staggered/nonstaggered (S/ NS) according to the effective spin accumulation at the magnetic sublattices and the field-like/antidamping-like (FL/ADL) according to the produced energy dissipation (Fig. 1). The NS-FL torque is similar to the torque created by the magnetic field. The recently predicted [1] Néel spin orbit torque gives an example of the S-FL torque, while the spin transfer and antidamping Néel spin orbit torques [2, 3] represent the NS-ADL case. The S-FL and NS-ADL torques can efficiently move the antiferromagnetic domain walls [4, 5]. The symmetry matching between these torques and staggered magnetization provides high mobility compared to other torques. Due to the absence of the Walker breakdown, the velocity of the domain wall motion can reach 30 km/s at the reasonable values of currents. The NS-ADL is sensitive to the structure of the domain wall and pushes all the domain walls in the same direction, which is suitable for the race-track memory. On the contrary, the S-FL torque splits degeneracy of the domains, pushes the domain walls toward the unfavourable domain and thus is suitable for switching. Moreover, this torque is the only physical field which allows to distinguish domains with the opposite orientation of the Néel vector and to manipulate 1800 domain walls in antiferromagnets. The dynamics of the Néel vector induced by the S-FL and NS-ADL torques is assisted by large internal torques of the exchange origin which allow to achieve an ultrafast (compared to ferromagnets) magnetic dynamics. In particular, NS-ADL torque induced by dc spin current induces stable autooscillations of the Néel vector with THz frequency. This paves a way to the antiferromagnetic-based devices fitting THz gap, e.g., emmiters and detectors of THz radiation, which couples to the magnetic dynamics via Néel spin orbit (S-FL) torque [6]. The S-ADL torque, whose effect up to our knowledge was not yet discussed in the literature, can induce fast and reliable 1800 switching of the Néel vector, similar to the spin transfer torque in the ferromagnets. In contrast to NS-ADL which always competes with the internal damping and thus supports precession rather than switching of the Néel vector, the S-ADL torque can either compensate or enhance the internal damping depending on the geometry. The time-dependent S-ADL torque can also induce oscillations with THz frequency. However, the dynamics induced by this torque is not assisted by the internal exchange torques and thus is less efficient than S-FL and NS-ADL torques. From the symmetry point of view, the S-ADL torque can also compensate the Bloch damping which controls relative orientation of the magnetic sublattices and their length. Thus, this torque can play an important role in the vicinity of the Néel point. We also discuss the effects which different torques produce on an antiferromagnetic skyrmion. The field-like torques (both S-FL and NS-FL) influence the skyrmion stability. They allow to control uniaxial (via S-FL) and unidirectional (via NS-FL) anisotropy of the Néel vector. The NS-ADL torque creates a force which can induce linear motion of a skyrmion [7]. To summarise, (spin) current induced dynamics of antiferromagnets is much richer compared to ferromagnetic materials due to the variety of torques and magnetic degrees of freedom. By tuning and combining different types of torques is it possible to induce switching, precession, domain wall motion and other nontrivial dynamical regimes which give antiferromagnetic-based deivces new functionalities.
Present-day spintronics uses electric field to manipulate with magnetic moments of ferromagnetic ... more Present-day spintronics uses electric field to manipulate with magnetic moments of ferromagnetic (FM) layers. However, utilization of antiferromagnets (AFM) as active parts of spintronic units promises transition to ultra-high-speed and low-energy devices. Recent experiments demonstrated sensitivity of AFM layers to spin-polarized current [1] and their ability to change the state of adjacent FM layer [2]. In the present paper we study magnetic dynamics of FM and AFM layers separated with thin nonmagentic layer (NM) in the presenence of electric and/or spin current. The model includes standard dynamic equations for FM and AFM moments supplemented with the balance equation for nonequilibrium spin density carried by free electrons. We show that spin pumping from AFM layer increases damping of the whole structure and analyse back-action of AFM on the state of FM layer in the presence of electric current. References:
Using antiferromagnets as active elements in spintronics requires the ability to manipulate and r... more Using antiferromagnets as active elements in spintronics requires the ability to manipulate and read-out the Néel vector orientation. Here we demonstrate for Mn 2 Au, a good conductor with a high ordering temperature suitable for applications, reproducible switching using current pulse generated bulk spin-orbit torques and read-out by magnetoresistance measurements. Reversible and consistent changes of the longitudinal resistance and planar Hall voltage of star-patterned epitaxial Mn 2 Au(001) thin films were generated by pulse current densities of ≃10 7 A/cm 2. The symmetry of the torques agrees with theoretical predictions and a large read-out magnetoresistance effect of more than ≃6% is reproduced by ab initio transport calculations.
It is shown that the magnetoelastic interaction can play a substantial role in the formation of t... more It is shown that the magnetoelastic interaction can play a substantial role in the formation of the equilibrium domain structure in antiferromagnets having two or more equivalent easy directions of the axis of magnetic anisotropy. It is demonstrated with the use of the methods of nonlinear theory of elasticity in the framework of a phenomenological model that the surface of the crystal can and does serve as a source of fictitious ''elastic charges'' of incompatibility, which have consequences analogous to those of the magnetostatic charges that arise on the surface of ferromagnetic crystals and lead to the formation of domains. Here the surface of the crystal is treated as a special phase with its own magnetic, elastic, and magnetoelastic properties. The field of the ''elastic charges'' is long-ranged, and, accordingly, its contribution to the stray ͑''detwinning''͒ energy, which plays the main role in the formation of the equilibrium domain structure, is proportional to the volume of the sample rather than its surface area. The formation of the domain structure is nothing more than the restoration of the initial global symmetry of the crystal in those cases when the phase transformation that breaks this symmetry is described by an order parameter which is conjugate to the shear strain. The conditions for the absence of ''elastic charges'' inside the sample impose certain restrictions on the morphology of the magnetoelastic domain structure in antiferromagnets. It is shown how the character of the equilibrium state of the crystal is influenced by disclinations arising at places where three or more domains come together.
A theoretical model is proposed for describing the irreversible behavior of the magnetoresistance... more A theoretical model is proposed for describing the irreversible behavior of the magnetoresistance of underdoped cuprates RBa 2 Cu 3 O x in an external magnetic field over a wide range of temperatures. The underlying assumption of the model is a magnetoelastic mechanism for the formation of the domain structure of the collinear antiferromagnet owing to long-range ''stray'' fields resulting from the difference in the elastic properties of the bulk and surface. The law of corresponding states in reduced variables is established for the magnetoresistance, external magnetic field, and temperature, and the temperature dependence of the ''monodomainization'' field, the field at which the sample is brought to a single-domain state, is determined. The theoretical relations obtained give a comprehensive description of the features of the magnetoresistance of RBa 2 Cu 3 O x and are in quantitative agreement with the experimental data.
A phenomenological model in the spirit of Landau theory is constructed and used to analyze the ph... more A phenomenological model in the spirit of Landau theory is constructed and used to analyze the phase stability conditions and the conditions for phase transitions between the various magnetocrystalline structures of solid oxygen over a wide range of pressure, temperature, and external magnetic field.
In antiferromagnetic spintronics, the read-out of the staggered magnetization or Néel vector is t... more In antiferromagnetic spintronics, the read-out of the staggered magnetization or Néel vector is the key obstacle to harnessing the ultra-fast dynamics and stability of antiferromagnets for novel devices. Here, we demonstrate strong exchange coupling of Mn2Au, a unique metallic antiferromagnet that exhibits Néel spin-orbit torques, with thin ferromagnetic Permalloy layers. This allows us to benefit from the well-estabished read-out methods of ferromagnets, while the essential advantages of antiferromagnetic spintronics are retained. We show one-to-one imprinting of the antiferromagnetic on the ferromagnetic domain pattern. Conversely, alignment of the Permalloy magnetization reorients the Mn2Au Néel vector, an effect, which can be restricted to large magnetic fields by tuning the ferromagnetic layer thickness. To understand the origin of the strong coupling, we carry out high resolution electron microscopy imaging and we find that our growth yields an interface with a well-defined morphology that leads to the strong exchange coupling.
Magnetic vortices are topological spin structures frequently found in ferromagnets, yet novel to ... more Magnetic vortices are topological spin structures frequently found in ferromagnets, yet novel to antiferromagnets. By combining experiment and theory, we demonstrate that in a nanostructured antiferromagnetic-ferromagnetic NiO(111)-Fe(110) bilayer, a magnetic vortex is naturally stabilized by magnetostatic interactions in the ferromagnet and is imprinted onto the adjacent antiferromagnet via interface exchange coupling. We use micromagnetic simulations to construct a corresponding phase diagram of the stability of the imprinted antiferromagnetic vortex state. Our in depth analysis reveals that the interplay between interface exchange coupling and the antiferromagnet magnetic anisotropy plays a crucial role in locally reorienting the Néel vector out-of-plane in the prototypical in-plane antiferromagnet NiO and thereby stabilizing the vortices in the antiferromagnet.
We demonstrate a practical possibility of loss compensation in measured photocounting statistics ... more We demonstrate a practical possibility of loss compensation in measured photocounting statistics in the presence of dark counts and background radiation noise. It is shown that satisfactory results are obtained even in the case of low detection efficiency and large experimental errors.
We investigate magnetization dynamics of Mn 2 Au/Py (Ni 80 Fe 20 ) thin film bilayers using broad... more We investigate magnetization dynamics of Mn 2 Au/Py (Ni 80 Fe 20 ) thin film bilayers using broadband ferromagnetic resonance (FMR) and Brillouin light scattering spectroscopy. Our bilayers exhibit two resonant modes with zero-field frequencies up to almost 40 GHz, far above the singlelayer Py FMR. Our model calculations attribute these modes to the coupling of the Py FMR and the two antiferromagnetic resonance (AFMR) modes of Mn 2 Au. The coupling-strength is in the order of 1.6 T nm at room temperature for nm-thick Py. Our model reveals the dependence of the hybrid modes on the AFMR frequencies and interfacial coupling as well as the evanescent character of the spin waves that extend across the Mn 2 Au/Py interface. Ferromagnets have a net magnetic moment and uniform spin dynamics in the GHz range . In contrast, collinear antiferromagnets have two equal but opposite sublattice magnetizations with vanishing net magnetic moment [2, 3] and spin dynamics that can reach the THz range . In addition to THz spin dynamics, antiferromagnets can exhibit significant stability of their magnetic moments against external magnetic perturbations [6]. The difference in the magnetization dynamics of ferromagnetic (FM) and antiferromagnetic (AFM) materials could potentially be exploited in applications that integrate AFM materials in high-frequency spintronic devices. A promising approach to enhance the FM spin dynamics frequencies and control FM spin-wave dispersions might be the combination of FM and AFM thin-film layers with interfacial exchange coupling. As a result of interfacial exchange coupling, a pronounced increase in the coercivity of the FM layer and exchange bias can be observed. Interfacial coupling also modifies GHz spin dynamics in FM/FM, chiral FM/FM and AFM/FM heterostructures, in particular the magnetic damping and anisotropy . However, the role of the THz frequency spin dynamics of AFMs for the hybrid spin dynamics in AFM/FM bilayers has not been revealed so far. Consequently, AFM/FM bilayers have so far not been leveraged to control hybrid mode frequencies or study AFM dynamics without requiring THz spectroscopy tools. Here we show that by coupling AFM and FM modes, we can make the AFM dynamics visible in the GHz range and use the exchange-enhancement of AFM modes to elevate the FM spin dynamics frequencies. The existence of such a coupling between FM and AFM spin dynamics opens new possibilities to design next generation magneto-electronic devices that exploit AFM materials beyond exchange bias. The Mn 2 Au/Py system
Possible mechanisms of the external magnetic field influence on exchange coupling in ultrathin M/... more Possible mechanisms of the external magnetic field influence on exchange coupling in ultrathin M/N/M film structures are discussed. It is shown that the most reasonable mechanism is due to variation of the translational motion of spacer electrons in the external magnetic field. In this case in-plane components of the electron momentum should be quantized according to Landau levels. The most drastic effect should be observed in the case when magnetic field is perpendicular to the film plane. Comparison of calculations according to the proposed model with the experimental data is presented, showing very good correlation for the cases of presence and absence of external magnetic field.
INTERMAG Asia 2005. Digests of the IEEE International Magnetics Conference, 2005., 2005
ABSTRACT In the present work a theoretical ground for the influence of external magnetic field on... more ABSTRACT In the present work a theoretical ground for the influence of external magnetic field on coupling oscillations behaviour is proposed and corresponding modification of Bruno&#39;s formula was made.
We investigate magnetization dynamics of Mn 2 Au/Py (Ni 80 Fe 20 ) thin film bilayers using broad... more We investigate magnetization dynamics of Mn 2 Au/Py (Ni 80 Fe 20 ) thin film bilayers using broadband ferromagnetic resonance (FMR) and Brillouin light scattering spectroscopy. Our bilayers exhibit two resonant modes with zero-field frequencies up to almost 40 GHz, far above the singlelayer Py FMR. Our model calculations attribute these modes to the coupling of the Py FMR and the two antiferromagnetic resonance (AFMR) modes of Mn 2 Au. The coupling-strength is in the order of 1.6 T nm at room temperature for nm-thick Py. Our model reveals the dependence of the hybrid modes on the AFMR frequencies and interfacial coupling as well as the evanescent character of the spin waves that extend across the Mn 2 Au/Py interface. Ferromagnets have a net magnetic moment and uniform spin dynamics in the GHz range . In contrast, collinear antiferromagnets have two equal but opposite sublattice magnetizations with vanishing net magnetic moment [2, 3] and spin dynamics that can reach the THz range . In addition to THz spin dynamics, antiferromagnets can exhibit significant stability of their magnetic moments against external magnetic perturbations [6]. The difference in the magnetization dynamics of ferromagnetic (FM) and antiferromagnetic (AFM) materials could potentially be exploited in applications that integrate AFM materials in high-frequency spintronic devices. A promising approach to enhance the FM spin dynamics frequencies and control FM spin-wave dispersions might be the combination of FM and AFM thin-film layers with interfacial exchange coupling. As a result of interfacial exchange coupling, a pronounced increase in the coercivity of the FM layer and exchange bias can be observed. Interfacial coupling also modifies GHz spin dynamics in FM/FM, chiral FM/FM and AFM/FM heterostructures, in particular the magnetic damping and anisotropy . However, the role of the THz frequency spin dynamics of AFMs for the hybrid spin dynamics in AFM/FM bilayers has not been revealed so far. Consequently, AFM/FM bilayers have so far not been leveraged to control hybrid mode frequencies or study AFM dynamics without requiring THz spectroscopy tools. Here we show that by coupling AFM and FM modes, we can make the AFM dynamics visible in the GHz range and use the exchange-enhancement of AFM modes to elevate the FM spin dynamics frequencies. The existence of such a coupling between FM and AFM spin dynamics opens new possibilities to design next generation magneto-electronic devices that exploit AFM materials beyond exchange bias. The Mn 2 Au/Py system
Antiferromagnetic materials have been proposed as new types of narrowband THz spintronic devices ... more Antiferromagnetic materials have been proposed as new types of narrowband THz spintronic devices owing to their ultrafast spin dynamics. Manipulating coherently their spin dynamics, however, remains a key challenge that is envisioned to be accomplished by spin-orbit torques or direct optical excitations. Here, we demonstrate the combined generation of broadband THz (incoherent) magnons and narrowband (coherent) magnons at 1 THz in low damping thin films of NiO/Pt. We evidence, experimentally and through modeling, two excitation processes of spin dynamics in NiO: an off-resonant instantaneous optical spin torque in (111) oriented films and a strain-waveinduced THz torque induced by ultrafast Pt excitation in (001) oriented films. Both phenomena lead to the emission of a THz signal through the inverse spin Hall effect in the adjacent heavy metal layer. We unravel the characteristic timescales of the two excitation processes found to be < 50 fs and > 300 fs, respectively, and thus open new routes towards the development of fast optospintronic devices based on antiferromagnetic materials. Antiferromagnetic spintronics has recently become an important research field from both a fundamental viewpoint and its strong applicative potential 1,2. Antiferromagnets (AFM) have key advantages linked to their magnetic ordering: they are insensitive to perturbative external magnetic fields, stray fields are absent, and magnon modal frequencies reach the terahertz (THz) regime 1-3. This renders AFMs prime candidates for ultrafast spintronic devices 4 compared to their ferromagnetic counterparts. A recent work demonstrated the writing of AFM memory states with picosecond excitations 5. In parallel, narrowband sub-THz detection has been achieved using spin-pumping in AFMs 6,7. Another spintronic application is spintronic-based broadband THz emission that currently relies on ferromagnet/heavy metal
Shape effects in magnetic particles are widely studied, because of the ability of the shape and t... more Shape effects in magnetic particles are widely studied, because of the ability of the shape and the size to control the parameters of a sample during its production. Experiments with nanosized samples show that the shape can affect also the properties of antiferromagnetic (AFM) materials. However, the theoretical interpretation of these effects is under discussion. A model to study the shape-induced effects in AFM particles at the AFM resonance frequency is proposed. The Lagrange function method is used to calculate the spectrum of resonance oscillations of the AFM vector in a synthetic multiferroic (piezoelectric + antiferromagnet). The influence of the specimen shape on the AFM resonance frequency in the presence of an external magnetic field is studied. Conditions for a resonance under the action of an external force or for a parametric resonance to arise in the magnetic subsystem are considered.
Antiferromagnets are promising materials for spintronics because they show fast magnetic dynamics... more Antiferromagnets are promising materials for spintronics because they show fast magnetic dynamics, low susceptibility to magnetic fields, and produce no stray fields. In addition, the antiferromagnetic dynamics can be efficiently manipulated by spin and charge currents. Here we discuss spin and/or charge current induced dynamics of the antiferromagnetic textures (domain walls, skyrmions) and nanoparticles. We consider and analyse four types of torques which (spin) current can generate in an antiferromagnet with two magnetic sublattices. These torques can be classified as the staggered/nonstaggered (S/ NS) according to the effective spin accumulation at the magnetic sublattices and the field-like/antidamping-like (FL/ADL) according to the produced energy dissipation (Fig. 1). The NS-FL torque is similar to the torque created by the magnetic field. The recently predicted [1] Néel spin orbit torque gives an example of the S-FL torque, while the spin transfer and antidamping Néel spin orbit torques [2, 3] represent the NS-ADL case. The S-FL and NS-ADL torques can efficiently move the antiferromagnetic domain walls [4, 5]. The symmetry matching between these torques and staggered magnetization provides high mobility compared to other torques. Due to the absence of the Walker breakdown, the velocity of the domain wall motion can reach 30 km/s at the reasonable values of currents. The NS-ADL is sensitive to the structure of the domain wall and pushes all the domain walls in the same direction, which is suitable for the race-track memory. On the contrary, the S-FL torque splits degeneracy of the domains, pushes the domain walls toward the unfavourable domain and thus is suitable for switching. Moreover, this torque is the only physical field which allows to distinguish domains with the opposite orientation of the Néel vector and to manipulate 1800 domain walls in antiferromagnets. The dynamics of the Néel vector induced by the S-FL and NS-ADL torques is assisted by large internal torques of the exchange origin which allow to achieve an ultrafast (compared to ferromagnets) magnetic dynamics. In particular, NS-ADL torque induced by dc spin current induces stable autooscillations of the Néel vector with THz frequency. This paves a way to the antiferromagnetic-based devices fitting THz gap, e.g., emmiters and detectors of THz radiation, which couples to the magnetic dynamics via Néel spin orbit (S-FL) torque [6]. The S-ADL torque, whose effect up to our knowledge was not yet discussed in the literature, can induce fast and reliable 1800 switching of the Néel vector, similar to the spin transfer torque in the ferromagnets. In contrast to NS-ADL which always competes with the internal damping and thus supports precession rather than switching of the Néel vector, the S-ADL torque can either compensate or enhance the internal damping depending on the geometry. The time-dependent S-ADL torque can also induce oscillations with THz frequency. However, the dynamics induced by this torque is not assisted by the internal exchange torques and thus is less efficient than S-FL and NS-ADL torques. From the symmetry point of view, the S-ADL torque can also compensate the Bloch damping which controls relative orientation of the magnetic sublattices and their length. Thus, this torque can play an important role in the vicinity of the Néel point. We also discuss the effects which different torques produce on an antiferromagnetic skyrmion. The field-like torques (both S-FL and NS-FL) influence the skyrmion stability. They allow to control uniaxial (via S-FL) and unidirectional (via NS-FL) anisotropy of the Néel vector. The NS-ADL torque creates a force which can induce linear motion of a skyrmion [7]. To summarise, (spin) current induced dynamics of antiferromagnets is much richer compared to ferromagnetic materials due to the variety of torques and magnetic degrees of freedom. By tuning and combining different types of torques is it possible to induce switching, precession, domain wall motion and other nontrivial dynamical regimes which give antiferromagnetic-based deivces new functionalities.
Present-day spintronics uses electric field to manipulate with magnetic moments of ferromagnetic ... more Present-day spintronics uses electric field to manipulate with magnetic moments of ferromagnetic (FM) layers. However, utilization of antiferromagnets (AFM) as active parts of spintronic units promises transition to ultra-high-speed and low-energy devices. Recent experiments demonstrated sensitivity of AFM layers to spin-polarized current [1] and their ability to change the state of adjacent FM layer [2]. In the present paper we study magnetic dynamics of FM and AFM layers separated with thin nonmagentic layer (NM) in the presenence of electric and/or spin current. The model includes standard dynamic equations for FM and AFM moments supplemented with the balance equation for nonequilibrium spin density carried by free electrons. We show that spin pumping from AFM layer increases damping of the whole structure and analyse back-action of AFM on the state of FM layer in the presence of electric current. References:
Using antiferromagnets as active elements in spintronics requires the ability to manipulate and r... more Using antiferromagnets as active elements in spintronics requires the ability to manipulate and read-out the Néel vector orientation. Here we demonstrate for Mn 2 Au, a good conductor with a high ordering temperature suitable for applications, reproducible switching using current pulse generated bulk spin-orbit torques and read-out by magnetoresistance measurements. Reversible and consistent changes of the longitudinal resistance and planar Hall voltage of star-patterned epitaxial Mn 2 Au(001) thin films were generated by pulse current densities of ≃10 7 A/cm 2. The symmetry of the torques agrees with theoretical predictions and a large read-out magnetoresistance effect of more than ≃6% is reproduced by ab initio transport calculations.
It is shown that the magnetoelastic interaction can play a substantial role in the formation of t... more It is shown that the magnetoelastic interaction can play a substantial role in the formation of the equilibrium domain structure in antiferromagnets having two or more equivalent easy directions of the axis of magnetic anisotropy. It is demonstrated with the use of the methods of nonlinear theory of elasticity in the framework of a phenomenological model that the surface of the crystal can and does serve as a source of fictitious ''elastic charges'' of incompatibility, which have consequences analogous to those of the magnetostatic charges that arise on the surface of ferromagnetic crystals and lead to the formation of domains. Here the surface of the crystal is treated as a special phase with its own magnetic, elastic, and magnetoelastic properties. The field of the ''elastic charges'' is long-ranged, and, accordingly, its contribution to the stray ͑''detwinning''͒ energy, which plays the main role in the formation of the equilibrium domain structure, is proportional to the volume of the sample rather than its surface area. The formation of the domain structure is nothing more than the restoration of the initial global symmetry of the crystal in those cases when the phase transformation that breaks this symmetry is described by an order parameter which is conjugate to the shear strain. The conditions for the absence of ''elastic charges'' inside the sample impose certain restrictions on the morphology of the magnetoelastic domain structure in antiferromagnets. It is shown how the character of the equilibrium state of the crystal is influenced by disclinations arising at places where three or more domains come together.
A theoretical model is proposed for describing the irreversible behavior of the magnetoresistance... more A theoretical model is proposed for describing the irreversible behavior of the magnetoresistance of underdoped cuprates RBa 2 Cu 3 O x in an external magnetic field over a wide range of temperatures. The underlying assumption of the model is a magnetoelastic mechanism for the formation of the domain structure of the collinear antiferromagnet owing to long-range ''stray'' fields resulting from the difference in the elastic properties of the bulk and surface. The law of corresponding states in reduced variables is established for the magnetoresistance, external magnetic field, and temperature, and the temperature dependence of the ''monodomainization'' field, the field at which the sample is brought to a single-domain state, is determined. The theoretical relations obtained give a comprehensive description of the features of the magnetoresistance of RBa 2 Cu 3 O x and are in quantitative agreement with the experimental data.
A phenomenological model in the spirit of Landau theory is constructed and used to analyze the ph... more A phenomenological model in the spirit of Landau theory is constructed and used to analyze the phase stability conditions and the conditions for phase transitions between the various magnetocrystalline structures of solid oxygen over a wide range of pressure, temperature, and external magnetic field.
In antiferromagnetic spintronics, the read-out of the staggered magnetization or Néel vector is t... more In antiferromagnetic spintronics, the read-out of the staggered magnetization or Néel vector is the key obstacle to harnessing the ultra-fast dynamics and stability of antiferromagnets for novel devices. Here, we demonstrate strong exchange coupling of Mn2Au, a unique metallic antiferromagnet that exhibits Néel spin-orbit torques, with thin ferromagnetic Permalloy layers. This allows us to benefit from the well-estabished read-out methods of ferromagnets, while the essential advantages of antiferromagnetic spintronics are retained. We show one-to-one imprinting of the antiferromagnetic on the ferromagnetic domain pattern. Conversely, alignment of the Permalloy magnetization reorients the Mn2Au Néel vector, an effect, which can be restricted to large magnetic fields by tuning the ferromagnetic layer thickness. To understand the origin of the strong coupling, we carry out high resolution electron microscopy imaging and we find that our growth yields an interface with a well-defined morphology that leads to the strong exchange coupling.
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Papers by Helen Gomonay