Open accessJournal Article

# Nutation in antiferromagnetic resonance

02 Mar 2021-Physical Review B (American Physical Society)-Vol. 103, Iss: 10, pp 104404
Abstract: The effect of inertial spin dynamics is compared between ferromagnetic, antiferromagnetic, and ferrimagnetic systems. The linear response to an oscillating external magnetic field is calculated within the framework of the inertial Landau-Lifshitz-Gilbert equation using analytical theory and computer simulations. Precession and nutation resonance peaks are identified, and it is demonstrated that the precession frequencies are reduced by the spin inertia, while the lifetime of the excitations is enhanced. The interplay between precession and nutation is found to be the most prominent in antiferromagnets, where the timescale of the exchange-driven sublattice dynamics is comparable to inertial relaxation times. Consequently, antiferromagnetic resonance techniques should be better suited for the search for intrinsic inertial spin dynamics on ultrafast timescales than ferromagnetic resonance.

##### Citations
More

6 results found

Journal Article
27 Apr 2021-Physical Review B
Abstract: The appropriate magnetic Langevin equation, namely the inertial Landau-Lifshitz-Gilbert equation augmented by thermal noise, is written starting from an analogy with the dynamics of the magnetic dipole moment of a circular current-carrying loop (wire) viewed as a symmetric top. Hence the corresponding Fokker-Planck equation for the evolution of the probability density function in the phase space of angular velocities and orientations and its stationary solution are derived. Moreover, the inertial stochastic magnetization dynamics of ferromagnetic nanoparticles is also seen to be analogous to the stochastic dynamics of the electric dipole moment of a polar molecule visualized as a dipole lying along the axis of symmetry of a symmetric top ignoring friction about that axis--a conclusion reached by replacing electrical parameters with their magnetic analogs in the respective Langevin equations. Therefore, existing results from gyroscopic theory, as applied to dielectric relaxation of polar molecules, may with appropriate modifications be used to study inertial magnetization effects in ferromagnetic nanoparticles.

3 Citations

Open accessJournal Article
28 May 2021-Physical Review B
Abstract: Inertia effects in magnetization dynamics are theoretically shown to result in a different type of spin waves, i.e., nutation surface spin waves, which propagate at terahertz frequencies in in-plane magnetized ferromagnetic thin films. Considering the magnetostatic limit, i.e., neglecting exchange coupling, we calculate dispersion relation and group velocity, which we find to be slower than the velocity of conventional (precession) spin waves. In addition, we find that the nutation surface spin waves are backward spin waves. Furthermore, we show that inertia causes a decrease of the frequency of the precession spin waves, namely magnetostatic surface spin waves and backward volume magnetostatic spin waves. The magnitude of the decrease depends on the magnetic properties of the film and its geometry.

Topics: Spin wave (67%), Nutation (57.99%), Dispersion relation (56.99%) ... show more

1 Citations

Open accessJournal Article
Abstract: The magnetic inertial dynamics have previously been investigated for one sublattice ferromagnets. Here, we develop the magnetization dynamics in two-sublattice ferromagnets including the intra- and inter-sublattice inertial dynamics. First, we derive the magnetic susceptibility of such a ferromagnet. Next, by finding the poles of the susceptibility, we calculate the precession and nutation resonance frequencies. Our results suggest that while the resonance frequencies show decreasing behavior with the increasing intra-sublattice relaxation time, the effect of inter-sublattice inertial dynamics has an opposite effect.

Topics: , Nutation (52%),

1 Citations

Open accessJournal Article
Abstract: We analyze dispersion relations of magnons in ferromagnetic nanostructures with uniaxial anisotropy taking into account inertial terms, i.e. magnetic nutation. Inertial effects are parametrized by damping-independent parameter $\beta$, which allows for an unambiguous discrimination of inertial effects from Gilbert damping parameter $\alpha$. The analysis of magnon dispersion relation shows its two branches are modified by the inertial effect, albeit in different ways. The upper nutation branch starts at $\omega=1/ \beta$, the lower branch coincides with FMR in the long-wavelength limit and deviates from the zero-inertia parabolic dependence $\simeq\omega_{FMR}+Dk^2$ of the exchange magnon. Taking a realistic experimental geometry of magnetic thin films, nanowires and nanodiscs, magnon eigenfrequencies, eigenvectors and $Q$-factors are found to depend on the shape anisotropy. The possibility of phase-matched magneto-elastic excitation of nutation magnons is discussed and the condition was found to depend on $\beta$, exchange stiffness $D$ and the acoustic velocity.

Topics: Magnon (57.99%), Nutation (53%), Dispersion relation (52%)

1 Citations

Open accessJournal Article
18 Aug 2021-Physical Review B
Abstract: We analyze dispersion relations of magnons in ferromagnetic nanostructures with uniaxial anisotropy taking into account inertial terms, i.e., magnetic nutation. Inertial effects are parametrized by the damping-independent parameter $\ensuremath{\beta}$, which allows for an unambiguous discrimination of inertial effects from Gilbert damping parameter $\ensuremath{\alpha}$. The analysis of magnon dispersion relation shows its two branches are modified by the inertial effect, albeit in different ways. The upper nutation branch starts at $\ensuremath{\omega}=1/\ensuremath{\beta}$, the lower branch coincides with ferromagnetic resonance (FMR) in the long-wavelength limit and deviates from the zero-inertia parabolic dependence $\ensuremath{\simeq}{\ensuremath{\omega}}_{\text{FMR}}+D{k}^{2}$ of the exchange magnon. Taking a realistic experimental geometry of magnetic thin films, nanowires, and nanodiscs, magnon eigenfrequencies, eigenvectors, and $Q$-factors are found to depend on the shape anisotropy. The possibility of phase-matched magnetoelastic excitation of nutation magnons is discussed and the condition was found to depend on $\ensuremath{\beta}$, exchange stiffness $D$, and the acoustic velocity.

Topics: Magnon (53%)

1 Citations

##### References
More

35 results found

Journal Article
Abstract: In 1955, a phenomenological theory of ferromagnetism was well established and had been corroborated by a considerable amount of experimental data. However, there were problems in the phenomenological theory of the dynamics of the magnetization field. The Landau-Lifshitz equation for damping of the motion of the magnetization field could not account for the large noneddy-current damping in thin Permalloy sheets. The problem undertaken herein is a reformulation of the theory in a way that is more consistent with the theory of damping in other physical systems in order to be able to take large damping into account.

Topics: Field (physics) (50%)

1,855 Citations

Open accessReference Book
15 Dec 2007-
Abstract: VOLUME 1: Fundamentals and Theory Part 1: Electron Theory of Magnetism Density Functional Theory of Magnetism Hubbard Model Dynamical Mean Field Theory of Itinerant Electron Magnetism Quantum Monte Carlo Methods Part 2: Strongly Correlated Electronic Systems Heavy Fermions: electrons at the edge of magnetism The Kondo Effect Orbital physics in transition metal oxides: Magnetism and optics Part 3:Theory of Magnetic Spectroscopy and Scattering Magnetic Spectroscopy X-ray and Neutron Scattering by Magnetic Materials Part 4:Spin Dynamics and Relaxation Spin Waves History and A Summary of Recent Developments Dissipative Magnetization dynamics close to the adiabatic regime Part 5:Phase Transitions and Finite Temperature Magnetism Experiment and Analysis Electron Theory of Finite Temperature Magnetism Theory of Magnetic Phase Transitions Disordered and Frustrated Spin Systems Quantum Phase Transitions Part 6: Theory of Magneocrystalline Anisotropy and Magnetoelasticity Theory of Magnetocrystalline Anisotropy and Magnetoelasticity in transition metal systems Theory of Magnetocrystalline Anisotropy and Magnetoelasticity for 4f and 5f Metals Magnetostriction and Magnetoelasticity Theory: a Modern View Part 7: Theory of Transport and Exchange Phenomena in Layer Systems Exchange Coupling in Magnetic Multilayers Enhanced Magnetoresistance Berry phase in magnetism and the anomalous Hall effect Theory of Spin-Dependent Tunneling Part 8: Magnetism of Low Dimensions Magnetism of Low-dimensional Metallic Structures Magnetism of Low-Dimensional Systems: Theory Part 9: Molecular Magnets: Phenomenology and Theory Molecular Magnets: Phenomenology and Theory Part 10: Magnetism and Superconductivity Interplay of Superconductivity and Magnetism Magnetic Superconductors VOLUME 2: Micromagnetism Part 1: Fundamentals of Micromagnetism and Discrete Computational Models General Micromagnetic Theory Numerical Micromagnetics : Finite Difference Methods Numerical Methods in Micromagnetics (FEM) Magnetization dynamics including thermal fluctuations: basic phenomenology, fast remagnetization processes and transitions over high energy barriers Nonlinear Magnetization Dynamics in Nanomagnets Classical Spin Models Part 2: Micromagnetics Applications: Distribution of Equilibrium Configurations, Phase Diagrams and Hysteretic Properties- Small Objects Magnetization Configurations and reversal in small magnetic elements Magnetic Properties of Systems of Low Dimensions Part 3: Micromagnetics Applications: Distribution of Equilibrium Configurations, Phase Diagrams and Hysteretic Properties- Wall in Nanowires Domain Wall Propagation in Magnetic Wires Current Induced Domain-Wall Motion in Magnetic Nanowires The Motion of Domain Walls in Nano-Circuits and its Application to Digital Logic Part 4: Micromagnetics Applications: Distribution of Equilibrium Configurations, Phase Diagrams and Hysteretic Properties- Microstructure and Magnetization Processes Guided Spin Waves Micromagnetism-Microstructure Relations. Micromagnetism of the Hysteresis Loop Modelling of Non-linear Behaviour and Hysteresis in Magnetic Materials Part 5: Magnetization dynamics, solitons, Modes and Thermal Excitations Magnetization Dynamics: Thermal Driven Noise in Magnetoresistive Sensors Modes, Theory and Experiment Nonlinear Multi-dimensional Spin Wave Excitations in Magnetic Films Part 6: Micromagnetics of Spin angular transfer Theory of Spin-Transfer Torque Microwave Generation in Magnetic Multilayers and Nanostructures VOLUME 3: Novel Techniques for Characterizing and Preparing Samples Part 1: X-Ray and Neutron Diffraction Techniques Spin Structures and Spin Wave Excitations Domain States determined by Neutron Refraction and Scattering Polarized neutron reflectivity and scattering of magnetic nanostructures and spintronic materials Part 2: Synchrotron Radiation Techniques, Circular Dichroism of Hard & Soft X-Rays Synchrotron radiation techniques based on X-ray magnetic circular dichroism Part 3: Time and Space Resolved Magnetization Dynamics Ultrafast Magnetodynamics with Lateral Resolution: A View by Photoemission Microscopy Part 4: Electron Microscopy and Electron Holography Lorentz Microscopy of Thin Film Systems Electron Holography Of Ferromagnetic Materials Spin-Polarized Low Energy Electron Diffraction Spin-polarized Low Energy Electron Microscopy (SPLEEM) Scanning Electron Microscopy with Polarisation Analysis Part 5: Magneto-optical Techniques Investigation of Domains and Dynamics of Domain Walls by the Magneto-optical Kerr-effect Magnetization-induced second harmonic generation technique Investigation of Spin Waves and Spin Dynamics by Optical Techniques Time-resolved Kerr-effect and spin dynamics in itinerant ferromagnets Part 6: Spin Polarized Electron Spectroscopies Investigation of Ultrathin Ferromagnetic Films by Magnetic Resonance Spin-Polarized Photoelectron Spectroscopy as a probe of Magnetic Systems High-energy surface spin-waves studied by Spin-polarized Electron Energy Loss Spectroscopy Part 7: Nano Magnetism- Application and Charaterisation Scanning Probe Techniques: MFM and SP-STM Alternative Patterning Techniques : Magnetic Interactions in Nanomagnet Arrays Chemical Synthesis of Monodisperse Magnetic Nanoparticles Nanoimprint Technology for Patterned Magnetic Nanostructures Part 8: Growth Techniques Growth of Magnetic Materials using Molecular Beam Epitaxy Epitaxial Heusler alloys on III-V semiconductors Crystal Growth of magnetic materials VOLUME 4: Novel Materials Part 1: Soft Magnetic Materials Amorphous Alloys Soft Magnetic Materials - Nanocrystalline Alloys Soft Magnetic Bulk Glassy and Bulk Nanocrystalline Alloys Advanced Soft Magnetic Materials for Power Applications Part 2 : Hard Magnetic Materials Rare earth intermetallics for permanent magnet applications Rare-earth (RE) Transition-Metal (T M) Magnets Rare earth nanocrystalline and nanostructured magnets Current Status of Magnetic Industry in China & its Future Part 3: Ferro- and ferrimagnetic oxides and alloys Ferrimagnetic Insulators Crystallography and Chemistry of Perovskites Chalcogenides and Pnictides Dilute Magnetic Oxides and Nitrides Heusler alloys Half Metals Part 4: Ferro- and ferrimagnetic particles Superparamagnetic Particles Novel Nanoparticulate Magnetic Materials and Structures Part 5: Micro- and Nanowires Advanced Magnetic Microwires Template-based Synthesis and Characterization of High-Density Ferromagnetic Nanowire Arrays Magnetic Carbon Part 6: Magnetic Thin Films Magnetic Ultra-hyphen thin Films Magnetic Thin Films Hard Magnetic Films Part 7: Magnetic Materials with outstanding properties Magneto-optical materials Magnetocaloric Materials Magnetostrictive Materials and Magnetic Shape Memory Materials Ferroelectricity in Incommensurate Magnets Magnetism and Quantum Critically in Heavy-Fermion Compounds: Interplay with Superconductivity Molecular nanomagnets Part 8: Biomagnetic Materials Spintronic Biochips For Biomolecular Recognition Application of Magnetic Particles in Medicine and Biology VOLUME 5: Spintronics and Magnetoelectronics Part 1: Metal Spintronics Magnetic Tunnel Junctions including Applications Spin angular momentum transfer in magnetoresistive nano-junctions Spin-transfer in high magnetic fields and single magnetic layer nanopillars Microwave Excitations in Spin Momentum Transfer Devices Theory of Spin-Polarized Current and Spin-Transfer Torque in Magnetic Multilayers Part 2: Exotic Materials High Temperature Superconductivity- Magnetic Mechanisms Ferromagnetic Manganite Films Magnetic Polarons Kondo Effect in Mesoscopic Quantum Dots Ferromagnetic Semiconductors Diluted ferromagnetic semiconductors - theoretical aspects Part 3: Semiconductor spintronics Spin Engineering in Quantum Well Structures Hot Electron Spintronics Spin-dependent transport of carriers in semiconductors Spintronic devices/spin relaxation Theory of Spin Hall Effects in Semiconductors Manipulation of Spins and Coherence in Semiconductors Quantum computing with spins in solids Part 4: Quantum computation The Magnetic Resonance Force Microscope Part 5: Magnetoresistance Tunneling Magnetoresistance in Semiconductors Spin-dependent Tunneling: Role of Evanescent and Resonant States Unusual magnetoresistance including extraordinary and Ballistic

Topics: Magnetic domain (69%), Magnetism (68%), Spin polarization (66%) ... show more

1,190 Citations

Open accessJournal Article
C.D. Stanciu1, Fredrik Hansteen1, Alexey Kimel1, Andrei Kirilyuk1  +3 moreInstitutions (2)
Abstract: We experimentally demonstrate that the magnetization can be reversed in a reproducible manner by a single 40 femtosecond circularly polarized laser pulse, without any applied magnetic field. This optically induced ultrafast magnetization reversal previously believed impossible is the combined result of femtosecond laser heating of the magnetic system to just below the Curie point and circularly polarized light simultaneously acting as a magnetic field. The direction of this opto-magnetic switching is determined only by the helicity of light. This finding reveals an ultrafast and efficient pathway for writing magnetic bits at record-breaking speeds.

1,013 Citations

Open accessBook
30 Nov 1996-
Abstract: Isotropic Ferromagnet Magnetized to Saturation Ferromagnetism Elementary Magnetic Moments Paramagnetism Weiss Theory Exchange Interaction Equation of Motion of Magnetization High-Frequency Magnetic Susceptibility Solution of the Linearized Equation of Motion Peculiarities of the Susceptibility Tensor High-Frequency Permeability Allowance for Magnetic Losses Dissipative Terms and Dissipation Parameters Susceptibility Tensor Components Uniform Oscillations in a Small Ellipsoid Internal and External Magnetic Fields Eigenoscillations Forced Oscillations Anisotropic Ferromagnet Landau-Lifshitz Equation Generalization of Equation of Motion Methods of Analysis of Ferromagnetic Resonance in Anisotropic Ferromagnet Magnetocrystalline Anisotropy Origins of Magnetocrystalline Anisotropy Phenomenological Description Equilibrium Orientations of Magnetization Ferromagnetic Resonance in a Single Crystal Sphere of Uniaxial Ferromagnet Sphere of a Cubic Ferromagnet Simultaneous Allowance for Different Kinds of Anisotropy Ferromagnetic Resonance in a Polycrystal Independent-Grain and Strongly-Coupled Grain Approximations Influence of Porosity Antiferromagnets and Ferrites Antiferromagnetism and Ferrimagnetism Crystal and Magnetic Structures Equations of Motion and Energy Terms Ground States and Small Oscillations Antiferromagnetic Resonance Antiferromagnet with an Easy Axis of Anisotropy: Steady States Oscillations in Antiparallel State Oscillations in Noncollinear State Oscillations in Transverse and Arbitrarily Oriented Fields Antiferromagnet with Easy Plane of Anisotropy Magnetic Oscillations in Ferrimagnets Ground States of Two-Sublattice Ferrimagnet Oscillations in Antiparallel Ground State Oscillations in Noncollinear Ground State Damped and Forced Oscillations Fundamentals of Electrodynamics of Gyrotropic Media Equations General Equations and Boundary Conditions Equations for Bigyotropic Media Uniform Plane Waves General Relations Longitudinal Magnetization Transverse Magnetization Nonreciprocity Energy Relations Equation of Energy Balance Energy Losses Perturbation Method Gyrotropic Perturbation of a Waveguide Gyrotropic Perturbation of a Resonator Quasistatic Approximation Resonator with Walls of Real Metal Waveguides and Resonators with Gyrotropic Media. Microwave Ferrite Devices Waveguide with Longitudinally Magnetized Medium Circular Waveguide Circular Waveguide with Ferrite Rod Faraday Ferrite Devices Waveguide with Transversely Magnetized Ferrite Rectangular Waveguide Filled with Ferrite Rectangular Waveguide with Ferrite Plates Microwave Ferrite Devices Resonators with Gyrotropic Media Eigenoscillations and Forced Oscillations Waveguide Resonators Ferrite Resonators Use of Perturbation Method Waveguides and Waveguide Junctions with Ferrite Samples Ferrite Ellipsoid in a Waveguide Coupling of Orthogonal Waveguides. Ferrite Band-Pass Filters General Properties of Nonreciprocal Junctions Magnetostatic Waves and Oscillations Magnetostatic Approximation Nonexchange Magnetostatic Waves in Plates and Rods Volume Waves in Plates Surface Waves Magnetostatic Waves in Waveguides with Finite Cross Section Energy Flow and Losses Magnetostatic Waves in Ferrite Films: Excitation, Applications Magnetostatic Oscillations (Walker's Modes) Metallized Cylinder Sphere and Ellipsoid of Revolution Damping, Excitation, and Coupling Spin Waves Spin Waves in Unbounded Ferromagnet Energy and Effective Field of Exchange Interaction Dispersion Law Magnetization, Field Components, and Damping Spin Waves in Bounded Bodies Exchange Boundary Conditions Standing Spin Waves in Films Propagating Spin Waves in Films Spin Waves in Nonuniform Magnetic Fields Magnons Quantization of Magnetic Oscillations and Waves Thermal Magnons Elements of Microscopic Spin-Wave Theory Diagonalization of the Hamiltonian Discussion of the Dispersion Law Allowance for Dipole-Dipole Interaction and Anisotropy Interaction of Magnons Magnetic Oscillations and Waves in Unsaturated Ferromagnet Oscillations of Domain Walls Domain Walls and Domain Structures Equation of Motion of a Domain Wall Dynamic Susceptibility Ferromagnetic Resonance in Samples with Domain Structure Ellipsoid of Uniaxial Ferromagnet Sphere of Cubic Ferromagnet Nonuniform Modes in Unsaturated Samples Nonlinear Oscillations of Magnetization Ferromagnetic Resonance in Strong Alternating Fields Rigorous Solution of Equation of Motion Approximate Methods Harmonic Generation and Frequency Conversion Frequency Doubling Frequency Mixing Parametric Excitation of Magnetic Oscillations and Waves Nonlinear Coupling of Magnetic Modes Thresholds of Parametric Excitation under Transverse Pumping First-Order and Second-Order Instabilities Threshold Fields Effect of Pumping-Field Polarization Longitudinal and Oblique Pumping Longitudinal Pumping Effect of Nonuniformities Oblique Pumping Instability of Nonuniform Modes and Nonuniform Pumping Parametric Excitation of Magnetostatic Oscillations and Waves Ferrite Parametric Amplifier Nonuniform Pumping Above-Threshold State Reaction of Parametric Spin Waves on Pumping Phase Mechanism Nonlinear Damping Stability of the Above-Threshold State Nonlinear Microwave Ferrite Devices Spin-Spin Relaxation Relaxation Processes in Magnetically Ordered Substances Kinds of Relaxation Processes Methods of Theoretical Study Inherent Spin-Spin Processes Three-Magnon Splitting Three-Magnon Confluence Four-Magnon Scattering Inherent Processes for Uniform Precession Experimental Data Two-Magnon Processes Theory of Two-Magnon Processes Disorder in Distribution of Ions over Lattice Sites Anisotropy-Field Variation and Pores in Polycrystals Surface Roughness Magnetoelastic Coupling Elastic Properties and Magnetoelastic Interaction Elastic Waves and Oscillations Magnetoelastic Energy and Equations of Motion Effect of Elastic Stresses on Ferromagnetic Resonance Magnetoelastic Waves Normal Waves Damping and Excitation Magnetoelastic Waves in Nonuniform Steady Magnetic Field Parametric Excitation of Magnetoelastic Waves Longitudinal Pumping of Magnetoelastic Waves Parametric Excitation Caused by Magnetoelastic Coupling Elastic Pumping Spin-Lattice Relaxation Ionic Anisotropy and Relaxation Anisotropy Caused by Impurity Ions Energy Levels of Ions One-Ion Theory of Ferromagnetic-Resonance Anisotropy Near-Crossing Energy Levels Experimental Data Ion Relaxation Processes Transverse Relaxation Longitudinal (Slow) Relaxation Relaxation of Ionic-Level Populations Experimental Data Interaction of Magnetic Oscillations and Waves with Charge Carriers Effect of Charge Carriers in Semiconductors Damping of Magnetic Oscillations Caused by Conductivity Influence of Interionic Electron Transitions Interaction of Spin Waves with Charge Carriers Ferromagnetic Resonance and Spin Waves in Metals Thin-Film Model Theory without Allowance for Exchange Interaction Influence of Exchange Interaction Antiresonance Processes of Magnetic Relaxation Appendices Units and Constants Demagnetization Factors Dirac Delta Function and Kronecker Delta Symbol Bibliography Subject Index to the Bibliography Index

Topics: Spin wave (64%), Polarization (waves) (59%), Ferromagnetic resonance (57.99%) ... show more

963 Citations

Journal Article
Ilie Radu1, K. Vahaplar1, Christian Stamm2, Torsten Kachel2  +13 moreInstitutions (6)
14 Apr 2011-Nature
Abstract: The dynamics of spin ordering in magnetic materials is of interest for both fundamental understanding and progress in information-processing and recording technology. Radu et al. study spin dynamics in a ferrimagnetic gadolinium–iron–cobalt (GdFeCo) alloy that is optically excited at a timescale shorter than the characteristic magnetic exchange interaction between the Gd and Fe spins. Using element-specific X-ray magnetic circular dichroism spectroscopy, they show that the Gd and Fe spins switch directions at very different timescales. As a consequence, an unexpected transient ferromagnetic state emerges. These surprising observations, supported by simulations, provide a possible new concept of manipulating magnetic order on a timescale of the exchange interaction. Ferromagnetic or antiferromagnetic spin ordering is governed by the exchange interaction, the strongest force in magnetism1,2,3,4. Understanding spin dynamics in magnetic materials is an issue of crucial importance for progress in information processing and recording technology. Usually the dynamics are studied by observing the collective response of exchange-coupled spins, that is, spin resonances, after an external perturbation by a pulse of magnetic field, current or light. The periods of the corresponding resonances range from one nanosecond for ferromagnets down to one picosecond for antiferromagnets. However, virtually nothing is known about the behaviour of spins in a magnetic material after being excited on a timescale faster than that corresponding to the exchange interaction (10–100 fs), that is, in a non-adiabatic way. Here we use the element-specific technique X-ray magnetic circular dichroism to study spin reversal in GdFeCo that is optically excited on a timescale pertinent to the characteristic time of the exchange interaction between Gd and Fe spins. We unexpectedly find that the ultrafast spin reversal in this material, where spins are coupled antiferromagnetically, occurs by way of a transient ferromagnetic-like state. Following the optical excitation, the net magnetizations of the Gd and Fe sublattices rapidly collapse, switch their direction and rebuild their net magnetic moments at substantially different timescales; the net magnetic moment of the Gd sublattice is found to reverse within 1.5 picoseconds, which is substantially slower than the Fe reversal time of 300 femtoseconds. Consequently, a transient state characterized by a temporary parallel alignment of the net Gd and Fe moments emerges, despite their ground-state antiferromagnetic coupling. These surprising observations, supported by atomistic simulations, provide a concept for the possibility of manipulating magnetic order on the timescale of the exchange interaction.