Session S52: Magnetization and Spin Dynamics III: AF, Ferro-, Ferri-Magnets & High Spin Systems

Temperature Dependent Magneto-Optical Studies of Dynamic Spin Alignment in FeGd Thin Films

We examine temperature dependent spin dynamics in ferrimagnetic Fe_1-xGd_x thin films using pump-probe, ultrafast time-resolved magneto-optic Kerr effect (tr-MOKE). The Gd content (x) was varied to alter the magnetic compensation temperature (T_MC).  Room temperature scans reveal a rapid demagnetization which occurs on the sub-ps timescale, followed by a return to equilibrium that is accompanied by coherent oscillatory motions of the Fe/Gd spins with differing timescales.  The demagnetization signal reverses sign as a function of Gd content, indicating that we move through the Fe/Gd ratio that produces T_MC at room temperature.  The GHz-scale ferromagnetic resonance oscillations damp out relatively quickly (~200 ps) and exhibit a non-monotonic relationship with Gd content. In a sample which has T_MC ~ 225 K, we observe a similar variation of the GHz range FMR oscillations as a function of temperature and we also track much faster dynamics closer to time zero.  These quickly-damped oscillations occur up closer to ~600 GHz, and the frequency of this mode also varies with temperature, having a peak close to T_MC.  We attribute this  faster, near-THz scale mode to an exchange mode oscillation where the angle between the normally anti-ferromagnetically aligned Fe and Gd moments deviates from 180 deg.   

Anomalous temperature dependence of phonon pumping by ferromagnetic resonance in Co/Pd multilayers with perpendicular anisotropy

The generation of phonons by ferromagnetic resonance in magnetic thin films and subsequent propagation of phonons into the substrate is an effect known as phonon pumping [1,2]. This effect can be utilized in spintronic devices that rely on magnetoelastic coupling for information transfer. The phonon pumping effect is most efficient in the perpendicular configuration, and can therefore be suitably integrated in devices composed of materials with perpendicular magnetic anisotropy (PMA). Here we demonstrate the pumping of phonons by ferromagnetic resonance in a series of [Co(0.8 nm)/Pd(1.5 nm)]_n multilayers (n = 6, 11, 15, and 20) with large magnetostriction and PMA. The effect is shown using broadband ferromagnetic resonance over a range of temperatures (10 to 300 K), where a resonant damping enhancement is observed at frequencies corresponding to standing wave phonons across the multilayer. The strength of this effect is strongly dependent on temperature—approximately a factor of 4 larger at 10 K compared to room temperature. Lastly, we demonstrate that the damping enhancement coincides with a shift in the ferromagnetic resonance field, and further that it is a complementary effect that can be predicted from the resonant damping enhancements. We emphasize that [Co/Pd]_n multilayers are an ideal platform for studying this effect due to their large magnetostriction and PMA. Importantly, the PMA does not show significant dependence on multilayer thickness, so devices can be engineered that have phonon pumping resonances at zero applied field. 

[1] S. Streib, H. Keshtgar, and G. E. W. Bauer, Damping of Magnetization Dynamics by Phonon Pumping, Phys. Rev. Lett. 121, 027202 (2018).

[2] K. An et al., Coherent long-range transfer of angular momentum between magnon Kittel modes by phonons, Phys. Rev. B 101, 060407(R) (2020).   

Magnetic Profile of Ferrimagnetic Fe-Gd Alloy Thin Films with Varying Thickness

Rare earth (RE) / transition metal (TM) ferrimagnet alloys exhibit an intriguing magnetic compensation of the RE/TM magnetic sublattices as a function of composition and temperature and this tunability of magnetization and electronic structure makes these alloys suitable in spintronics. We developed compositionally fixed Fe_1-xGd_x [DA1] [JS2] [JS3] films with x ≈ 28% that vary in thickness from 100nm to 25nm.  While the composition is fixed in these films, the magnetization compensation temperature T_MagComp, determined from bulk magnetometry, varies from 25K to over 300K.  We utilized polarized neutron reflectometry (PNR) to examine this effect.  As PNR probes the magnetization parallel to the field as a function of depth, it is an ideal technique to investigate the temperature evolution of these films as the magnetic scattering length density (mSLD) changes drastically as the temperature is varied through T_MagComp.  We conducted PNR studies of our Fe-Gd samples as a function of temperature and film thickness.  The PNR scans suggest that large changes in the mSLD occur near the interfaces with the seed and cap layers and the observations indicate that the evolution of the magnetic interfaces is nontrivial.   

Bistability in dissipatively coupled cavity magnonics

Dissipative coupling is characterized by the cooperation of external dampings to a common reservoir and shows energy level attraction in the dispersion. In this study, a magnetic material (yttrium iron garnet) is placed at the node of rf magnetic field in a Fabry-Perot-like microwave cavity such that the magnons and cavity photons are dissipatively coupled, and nonlinear effect is directly observed by driving the cavity with high power and measuring through its transmission. Such a dissipatively coupled hybridized system results in bistable behaviors, manifesting themselves as clockwise, counterclockwise, and butterflylike hysteresis loops by modulating the frequency detuning with respect to the coupling strength. The experimental results are well fitted and explained as a nonlinear Duffing oscillator dissipatively coupled with a harmonic oscillator. In addition, the coupled oscillators model could determine the critical condition required to produce bistability effects in a dissipatively coupled system. The model describing the coupling between magnon and cavity subsystems is generic and can be widely used in dissipatively coupled nonlinear systems involving Duffing oscillator and harmonic oscillator. This research sheds light upon potential applications of dissipatively coupled system such as creating a lower power threshold for nonlinearity and making enhanced anharmonicity sensors.   

Spin-1 magnets — a u(3) formalism

Spin-1 magnets include dipolar and quadrupolar moments on a single site, which allow for novel properties as seen in spin nematic phases [1], Fe-based superconductors [2] and cold atom systems [3].

However, such unconventional phases can be found hard to probe experimentally, and therefore require new theoretical tools to describe and interpret their ground state and excitation properties.

In this talk, we extend the commonly used algebra for spin-1 moments from su(3) to u(3) [4], and derive equations of motions well suited for numerical implementation.

We benchmark our method by studying the ferroquadrupolar phase of the spin-1 bilinear-biquadratic (BBQ) model on the triangular lattice, and find a perfect match between our formalism and analytical flavour-wave theory complemented by low-temperature expansion.

Furthermore, we show the application of our method to dynamics of topological defects and spin liquids in spin-1 magnets.

 

[1] H. Tsunetsugu and M. Arikawa, J. Phys. Soc. Jpn 75, 083701 (2006)

[2] R. M. Fernandes, A. V. Chubukov, and J. Schmalian, Nature Physics 10, 97 EP (2014)

[3] E. Demler and F. Zhou, Phys. Rev. Lett. 88, 163001 (2002)

[4] N. Papanicolaou, Nuclear Physics B 305, 367 (1988)   

Anisotropic magnetoresistance reversal in a two-dimensional helical antiferromagnet

The helimagnetic order describes a non-collinear spin texture of antiferromagnets, arising from competing exchange interactions. Although collinear antiferromagnets are elemental building blocks of antiferromagnetic (AFM) spintronics, the potential of implementing spintronic functionality in non-collinear antiferromagnets has not been clarified thus far. In this study, we propose an AFM helimagnet of EuCo₂As₂ as a novel single-phase spintronic material that exhibits an unconventional reversal behavior of the anisotropic magnetoresistance (AMR) effect. The contrast in the AMR arises from two electrically distinctive magnetic phases with spin-flop-driven switching from positive to negative magnetoresistance along the easy axis. Remarkably, an easy-plane anisotropic spin model reproduces the aspects of magnetic and magnetotransport properties, indicating that the highly two-dimensional magnetocrystalline anisotropy is essential in the observed spintronic functionality. The switchable antiferromagnetic memory states formed by integrating intrinsic bulk properties simplifies the stacking geometry and thus provides a more expandable platform to construct sophisticated heterostructures.   

Angular Dependence of Spin Torques in Current-in-Plane Exchange-Biased Spin Valves

An in-plane charge current through a magnetic heterostructure with sizable spin-orbit coupling can generate spin torques (e.g. via the spin Hall effect). Recent studies have reported reduced crystal symmetry [1] or magnetic ordering [2,3] as a means of generating spin torques that are not possible with the conventional spin Hall effect.

Here, we investigate spin torques in current-in-plane NiFe/Cu/CoFe/IrMn spin valves. The exchange-biased CoFe layer provides the symmetry breaking necessary for unconventional spin torques. By employing spin-torque ferromagnetic resonance, we have measured spin torques in the NiFe free layer magnetized along different directions with respect to the CoFe fixed layer. Our angle-dependent results show expected torques from the Oersted field and spin/anomalous Hall effects, along with an additional torque that cannot be explained by existing predictions. Our work points to the possibility of generating a new family of unconventional torques in spin valves compatible with well-established device fabrication processes.

 

[1] D. MacNeill, et al., Nat. Phys. 13, 300 (2017)

[2] Y. Hibino, et al., Phys. Rev. B 101, 174441 (2020)

[3] S. C. Baek, et al., Nat. Mater. 17, 509 (2018)   

Spin-resolved unoccupied states of NiCo2O4thin films

Theoretical calculations indicate that NiCo₂O₄ is a half-metal with fully spin polarized carriers [1], although realization of half metallicity is in fact very doubtful [2] especially at finite temperatures. It remains important, nonetheless, to ascertain whether NiCo₂O₄ is a high polarization material and whether the spin polarization has a projection in the plane of the thin film. We observed in-plane net spin polarized unoccupied states of NiCo₂O₄ near Fermi level using spin-polarized inverse photoemission spectroscopy (SPIPES) which is an extremely surface sensitive technique. Although NiCo₂O₄ is thought to possess a perpendicular magnetic anisotropy, the presence of in-plane spin-resolved unoccupied states could indicate the influence of the surface on magnetic ordering.

[1] P. F. Ndione et al., Adv. Funct. Mater. 24, 610 (2014).

[2] P.A. Dowben and R. Skomski, J. Appl. Phys. 95, 7453 (2004).   

Proximity-induced magnetic moments in Pt by Co and Gd in [Co/Gd/Pt]n multilayers

The proximity effect, which describes the tendency for a heavy metal (HM) to develop a magnetic moment near an interface with a magnetic material, has attracted interest recently due to its relevance to spintronics. Thus far more attention has been paid to proximity effects at HM/3d transition metal interfaces, and HM/rare earth structures have been largely overlooked. Here we use element specific x-ray magnetic circular dichroism (XMCD) and first-principles density-functional theory (DFT) to study the proximity-induced Pt moment in  [Co(0.5)/Gd(1)/Pt(1)]_n=10(nm) multilayers. XMCD measurements show that the temperature dependence of the induced Pt moment follows the trend of the magnitude of Co magnetization (|M_Co|) from 300 K to 50 K, but follows that of Gd (|M_Gd|) from 50 to 5 K. DFT calculations reveal that there are induced Pt moments at both the Pt/Co and the Pt/Gd interfaces with the directions parallel (antiparallel) to that of the Co (Gd). Moreover, the Pt moment at the Pt/Gd interface is induced dominantly by the neighboring Gd layer. Our results demonstrate that a magnetic moment can be induced in Pt not only by a 3d transition metal but also by a rare earth element, which is an important consideration for heterostructure spintronic systems.    

Intrinsic and Extrinsic Damping in Polycrystalline Fe Thin Films

Advancing the understanding of magnetic relaxation is essential for energy-efficient spintronic memories, oscillators, etc. Here, we examine the intrinsic and extrinsic damping in two series of polycrystalline Fe thin films with distinct structural properties: (1) strongly textured, smooth Fe films grown on Cu and (2) weakly textured, rough Fe films grown on Ag.

Out-of-plane FMR measurements reveal that intrinsic Gilbert damping parameters for all Fe films thicker than 4 nm are in quantitative agreement with that for epitaxial Fe [1]. The remarkable invariance with microstructure strongly suggests that intrinsic Gilbert damping is a local property of nanoscale crystal grains with limited impact from film microstructure. By contrast, the in-plane FMR linewidths of the Fe films exhibit pronounced nonlinear frequency dependencies indicating the presence of strong two-magnon scattering. These in-plane FMR results cannot be reproduced quantitatively by a standard grain-to-grain two-magnon scattering model [2], even with different types of correlation functions. Our finding points to the need to further examine the fundamental impact of film microstructure on extrinsic damping.

[1] Khodadadi et al., Phys. Rev. Lett. 124, 957201 (2020)

[2] Kalarickal et al., Phys. Rev. B 77, 054427 (2008)   

Magnetic excitations in a metallic spin chain Ti4MnBi2

One-dimensional spin chain systems provide platforms to explore, both experimentally and theoretically, a wealth of novel quantum phenomena. So far, almost all spin chain systems studied have been insulating. We investigated a new metallic compound, Ti₄MnBi₂, whose structure consists of chains of S = 1/2 Mn ions that extend along the c-axis. In Ti₄MnBi₂, the bands involving the Mn d_x²_-y² and d_xz orbitals are spin-polarized and AFM order is predicted, consistent with the inferred Weiss temperature of -9 K and broad peaks in the specific heat and magnetic susceptibility. We have performed inelastic neutron scattering measurements on single-crystal Ti₄MnBi₂, finding a dispersionless magnetic excitation mode around ~0.9 meV. There is no evidence for spin waves for T<T_N, or other dispersing excitations at any temperature. With decreasing temperature, there is a marked increase in the scattered intensity that increases with decreasing wave vector Q. While indicating that the excitations are magnetic, this Q-dependence is much stronger than would be expected from the Mn²⁺ form factor. We propose that clusters of spins are becoming increasingly correlated as the temperature decreases, although their fluctuations persist within the ordered state to the lowest temperatures.   

Classical spin dynamics based on SU(N) coherent states

We introduce a classical limit of the dynamics of quantum spin systems based on coherent states of SU(N), where N is the dimension of the local Hilbert space. This approach, which generalizes the well-known Landau-Lifshitz dynamics (LLD) from SU(2) to SU(N), provides a better approximation to the exact quantum dynamics for a large class of realistic spin Hamiltonians, including S≥1 systems with large single-ion anisotropy and weakly-coupled multi-spin units, such as dimers or trimers. We illustrate this idea by applying the SU(3) LLD to model the finite-temperature inelastic neutron scattering data of a spin-one antiferromagnet Ba₂FeSi₂O₇. Unlike the traditional LLD based on SU(2) coherent states, the results  obtained with classical SU(3) spins reproduce the measured spectrum in the whole temperature range.   

DJ Resonances: Conditions for Anisotropy and Exchange-Driven Entangled State Switching in a Three Coupled Spin Particle Model

Recent work has begun to uncover the utility of coupling higher-spin centers, such as molecular magnets and many-electron artificial molecules, as qubits. In this theoretical work, we investigate a representative model of a spin ½ particle coupled to two spin S particles to answer a vital question in conceptualization of such devices: can an electron prepare, manipulate, and interrogate the entanglement state of two-spin particles? By analyzing the model’s spin space, we report entangled state switching resonance conditions stemming from the interplay of exchange coupling (J) and magnetic anisotropy (D) and demonstrate the general resonance existence formula for any spin S for experimentally-relevant conditions. Using this general formula, we compare the entanglement switching mechanisms for the S_2,3 = ½ and S_2,3 = 1 models. We find that the inclusion of magnetic anisotropy in the S_2,3 = 1 model enables an additional control mechanism of a representative Bloch vector in several spin subspaces.   

Entanglement manipulation with a tunable magnonic beamsplitter

Generation and detection of entanglement by two-photon interference, i.e., a photonic Hong-Ou-Mandel (HOM) effect [1], is central to many quantum technologies [2]. Here, we propose a hybrid magnonic system [3,4] for two-magnon interference, i.e., a magnonic HOM. The system is a set of 2 magnonic resonators with coupling rate κ and frequencies ω₁ and ω₂. A dynamic magnetic field B(t) can tune the frequency separation Δω=│ω₁–ω₂│ of the system.

A magnonic beamsplitter (BS) operation forms when the system is strongly coupled (Δω≪κ) for a time such that the probability for single magnon scattering between resonators is ½. Applying the BS pulse to a two-magnon initially unentangled state │11〉 generates a maximally entangled N00N state (│20〉+e^iθ│02〉)/√2. Shaping the magnetic field profile can tune the sensitivity of the magnonic BS to generate and detect (being a unitary operation) different relative phases θ of the N00N state. Such tunable magnonic BS that can manipulate N00N states could be useful for the quantum metrology of hybrid magnonic systems.

[1] C. K. Hong et al., Phys. Rev. Lett., 59, 2044–2046 (1987)

[2] F. Bouchard et al., Rep. Prog. Phys., 84, 012402 (2021)

[3] Y. Li et al., J. Appl. Phys., 128, 130902 (2020)

[4] D. D. Awschalom et al., IEEE Trans. Quantum Eng., 2, 1-36 (2021)