Mingzhong Wu

In spin-transfer-torque (STT) magnetic random-access memory (MRAM), both switching current and switching time depend on the damping of the free layer. It is known that the magnetic damping constant (α) varies with temperature; because STT MRAM cells operate well above room temperature, there is an urgent need to determine and understand damping properties in STT MRAM free layers at elevated temperatures. This paper reports on comparative studies of high-temperature frequency-dependent ferromagnetic resonance (FMR) in two STT MRAM free layers:(i) a low-α free layer with split W layers, and (ii) a conventional free layer with a single W layer. Comprehensive FMR measurements and analyses show that (1) the damping constant of the low-α free layer is always smaller than that in the conventional free layer, as the temperature increases from 300 to 520 K;(2) the damping constant increases …

We present evidence for and characterization of a≈ 4− nm-thick (Y 1− x Gd x) 3 Fe 5 O 12 layer with x≥ 0.4 formed at the interface between a gadolinium gallium garnet (GGG) substrate and a sputtered Y 3 Fe 5 O 12 (YIG) epitaxial film with nominal thickness of 20 nm. Temperature-dependent polarized neutron reflectometry (PNR) and superconducting quantum interference device (SQUID) magnetometry show antiferromagnetic alignment of this interfacial layer with the bulk of the YIG film at low T that persists to at least 3 tesla. These experiments also show that this interfacial alignment switches from antiparallel to parallel between 100 and 200 K in small applied magnetic fields. Simple modeling suggests correlation of this crossover with the ferrimagnetic compensation point of the mixed garnet that forms this interfacial layer.

The unidirectional spin Hall and Rashba− Edelstein magnetoresistance is of great fundamental and practical interest, particularly in the context of reading magnetization states in two-terminal spin–orbit torque memory and logic devices due to its unique symmetry. Here, we report large unidirectional spin Hall and Rashba− Edelstein magnetoresistance in a new material family—magnetic insulator/topological insulator Y 3 Fe 5 O 12/Bi 2 Se 3 bilayers. Such heterostructures exhibit a unidirectional spin Hall and Rashba− Edelstein magnetoresistance that is about an order of magnitude larger than the highest values reported so far in all-metal Ta/Co bilayers. The polarized neutron reflectometry reveals a unique temperature-dependent magnetic intermediary layer at the magnetic insulator–substrate interface and a proximity layer at the magnetic insulator–topological insulator interface. These polarized neutron …

Understanding of damping processes in ferromagnetic thin films at elevated temperatures has significant implications for heat-assisted magnetic recording, spin-transfer torque memory, and magnetic sensors operating at high temperatures. Through cavity-based high-temperature ferromagnetic resonance (FMR) measurements, this work examined the FMR linewidth and damping properties of continuous cubic FePt thin films at elevated temperatures. The data show that the FMR linewidth and the Gilbert damping constant both increase monotonically when temperature is increased from room temperature toward the Curie temperature. This temperature dependence is opposite to that observed previously in FePt thin films that are granular, rather than continuous, and have L10 structure, rather than cubic structure; in those films, the FMR linewidth decreases monotonically with an increase in temperature [PR Applied …

Magnonics addresses the physical properties of spin waves and utilizes them for data processing. Scalability down to atomic dimensions, operation in the GHz-to-THz frequency range, utilization of nonlinear and nonreciprocal phenomena, and compatibility with CMOS are just a few of many advantages offered by magnons. Although magnonics is still primarily positioned in the academic domain, the scientific and technological challenges of the field are being extensively investigated, and many proof-of-concept prototypes have already been realized in laboratories. This roadmap is a product of the collective work of many authors, which covers versatile spin-wave computing approaches, conceptual building blocks, and underlying physical phenomena. In particular, the roadmap discusses the computation operations with the Boolean digital data, unconventional approaches, such as neuromorphic computing, and …

The spin-momentum locking of surface states in topological materials can produce a resistance that scales linearly with magnetic and electric fields. Such a bilinear magnetoelectric resistance (BMER) effect offers a new approach for information reading and field sensing applications, but the effects demonstrated so far are too weak or for low temperatures. This article reports the first observation of BMER effects in topological Dirac semimetals; the BMER responses were measured at room temperature and were substantially stronger than those reported previously. The experiments used topological Dirac semimetal α-Sn thin films grown on silicon substrates. The films showed BMER responses that are 106 times larger than previously measured at room temperature and are also larger than those previously obtained at low temperatures. These results represent a major advance toward realistic BMER applications …

Antiferromagnetic insulators (AFIs) are of substantial interest because of their potential in the development of next-generation spintronic devices. One major effort in this emerging field is to harness AFIs for long-range spin information communication and storage. Here, we report a noninvasive method to optically access the intrinsic spin transport properties of an archetypical AFI α-Fe2O3 via nitrogen-vacancy (NV) quantum spin sensors. By NV relaxometry measurements, we successfully detect the frequency-dependent dynamic fluctuations of the spin density of α-Fe2O3 along the Néel order parameter, from which an intrinsic spin diffusion constant of α-Fe2O3 is experimentally measured in the absence of external spin biases. Our results highlight the significant opportunity offered by NV centers in diagnosing the underlying spin transport properties in a broad range of high-frequency magnetic materials such as two …

We report the clean experimental realization of cubic–quintic complex Ginzburg–Landau (CQCGL) physics in a single driven, damped system. Four numerically predicted categories of complex dynamical behavior and pattern formation are identified for bright and dark solitary waves propagating around an active magnetic thin film-based feedback ring:(1) periodic breathing;(2) complex recurrence;(3) spontaneous spatial shifting; and (4) intermittency. These nontransient, long lifetime behaviors are observed in self-generated spin wave envelopes circulating within a dispersive, nonlinear yttrium iron garnet waveguide. The waveguide is operated in a ring geometry in which the net losses are directly compensated for via linear amplification on each round trip (of the order of 100 ns). These behaviors exhibit periods ranging from tens to thousands of round trip times (of the order of μs) and are stable for 1000s of …

We present images of spin-wave excitations in a patterned yttrium iron garnet (YIG) thin film obtained by use of near-field microwave microscopy, which can achieve spatial resolution as high as 50 nm. Visualization of magnetic excitations is an enticing prospect for high-speed, high-density magnetic logic and storage applications, which has spurred the development of new magnetic microscopy and imaging techniques in recent years. Here we present a novel approach for local imaging of magnetic modes excited at room temperature with sub-diffraction-limited spatial resolution. This approach is based on a special atomic force microscope with broadband GHz capability for imaging the spatial distribution of dynamic magnetic excitations in patterned magnetic structures. Due to the inherent sub-wavelength, nanometer-scale spatial resolution of near-field scanning probe techniques, this approach has potential for a …

This article reports damping enhancement in a ferromagnetic NiFe thin film due to an adjacent α‐Sn thin film. Ferromagnetic resonance studies show that an α‐Sn film separated from a NiFe film by an ultrathin Ag spacer can cause an extra damping in the NiFe film that is three times bigger than the intrinsic damping of the NiFe film. Such an extra damping is absent in structures where the α‐Sn film interfaces directly with a NiFe film, or is replaced by a β‐Sn film. The data suggest that the extra damping is associated with topologically nontrivial surface states in the topological Dirac semimetal phase of the α‐Sn film. This work suggests that, like topological insulators, topological Dirac semimetal α‐Sn may have promising applications in spintronics.

The topological Hall effect (THE) as a powerful probe for the experimental observation of topological spin textures, such as magnetic skyrmions, has been observed in a wide variety of distinct material systems. However, limited experimental observations have been reported for antiferromagnetic (AFM) materials. Here, the THE signals in the AFM state were observed in compensated ferrimagnetic thin films interfaced with heavy metals at the magnetization compensation temperature (TM). Ferrimagnetic CoTb thin films grown on Pt thin films were used in the experiments. The CoTb films exhibited a magnetization compensation point at which the moments of Co and Tb sublattices canceled each other, giving rise to the AFM state. The temperature (T)-dependent Hall measurements showed anomalous Hall effect (AHE) and THE responses at T ≠ TM but pure THE responses at T = TM. Control measurements and …

Spin pumping is a technique widely used to generate pure spin current and characterize the spin-charge conversion efficiency of heavy metals. Upon microwave excitation, the sample may also be heated, and the parasitic thermoelectric signals could contaminate the spin pumping results. Owing to their identical angular dependences with respect to the magnetic field, it is difficult to isolate one from the other. In this paper, we present a quantitative method to separate thermoelectric contributions from spin pumping signals in both Py (Ni 80 Fe 20)/Pt and YIG (Y 3 Fe 5 O 12)/Pt bilayers through microwave photoresistance measurements. We find that the microwave absorption indeed can raise the temperature of samples, resulting a field-dependent thermoelectric hysteresis loop. However, the additional heat dissipation due to the resonant precession of the magnetization in the ferromagnet is negligibly small …

Spin waves can transfer information free of electron transport and are promising for wave-based computing technologies with low-power consumption as a solution to severe energy losses in modern electronics. Logic circuits based on the spin-wave interference have been proposed for more than a decade, while it has yet been realized at the nanoscale. Here, we demonstrate the interference of spin waves with wavelengths down to 50 nm in a low-damping magnetic insulator. The constructive and destructive interference of spin waves is detected in the frequency domain using propagating spin-wave spectroscopy, which is further confirmed by the Brillouin light scattering. The interference pattern is found to be highly sensitive to the distance between two magnetic nanowires acting as spin-wave emitters. By controlling the magnetic configurations, one can switch the spin-wave interferometer on and off. Our …

Nanomagnets are widely used to store information in non-volatile spintronic devices. Spin waves can transfer information with low-power consumption as their propagations are independent of charge transport. However, to dynamically couple two distant nanomagnets via spin waves remains a major challenge for magnonics. Here we experimentally demonstrate coherent coupling of two distant Co nanowires by fast propagating spin waves in an yttrium iron garnet thin film with sub-50 nm wavelengths. Magnons in two nanomagnets are unidirectionally phase-locked with phase shifts controlled by magnon spin torque and spin-wave propagation. The coupled system is finally formulated by an analytical theory in terms of an effective non-Hermitian Hamiltonian. Our results are attractive for analog neuromorphic computing that requires unidirectional information transmission …

Recent experiments show that topological surface states (TSS) in topological insulators (TI) can be exploited to manipulate magnetic ordering in ferromagnets. In principle, TSS should also exist for other topological materials, but it remains unexplored as to whether such states can also be utilized to manipulate ferromagnets. Herein, current‐induced magnetization switching enabled by TSS in a non‐TI topological material, namely, a topological Dirac semimetal α‐Sn, is reported. The experiments use an α‐Sn/Ag/CoFeB trilayer structure. The magnetization in the CoFeB layer can be switched by a charge current at room temperature, without an external magnetic field. The data show that the switching is driven by the TSS of the α‐Sn layer, rather than spin‐orbit coupling in the bulk of the α‐Sn layer or current‐produced heating. The switching efficiency is as high as in TI systems. This shows that the topological Dirac …

Nitrogen-vacancy (N-V) centers, optically active atomic defects in diamond, have been widely applied to emerging quantum sensing, imaging, and network efforts, showing unprecedented field sensitivity and nanoscale spatial resolution. Many of these advantages derive from their excellent quantum-coherence, single-spin addressability, and remarkable functionality over a broad temperature range, enabling opportunities to outperform their classical counterparts. To date, N-V measurements of spin fluctuations have mainly focused on systems with in-plane magnetization, but applications to material systems with a spontaneous out-of-plane magnetization remain largely unexplored despite their technological importance. Here, we report N-V sensing of intrinsic spin fluctuations of magnetic insulator Y 3 Fe 5 O 12 thin films with perpendicular magnetic anisotropy. The measured field dependence of N-V relaxation …

Tunable microwave and millimeter wave oscillators and bandpass filters with ultra-low phase noise play a critical role in electronic devices, including wireless communication, microelectronics, and quantum computing. Magnetic materials, such as yttrium iron garnet (YIG), possess ultra-low phase noise and a ferromagnetic resonance tunable up to tens of gigahertz. Here, we report structural and magnetic properties of single-crystal 60 and 130 nm-thick YIG films prepared by metal-organic decomposition epitaxy. These films, consisting of multiple homoepitaxially grown monolayers, are atomically flat and possess magnetic properties similar to those grown with liquid-phase epitaxy, pulsed laser deposition, and sputtering. Our approach does not involve expensive high-vacuum deposition systems and is a true low-cost alternative to current commercial techniques that have the potential to transform the industry.

Spin-orbit torque (SOT) has been proposed as an efficient mechanism to switch the magnetization in heavy metal/ferromagnet heterostructures. Several experiments show controversial results on the relationship between the magnetic proximity effect (MPE) and the SOT efficiency in heavy metal/ferromagnet structures. In this work, we use first-principles calculations to investigate the dependence of the MPE and the SOT efficiency on the Pt/CoFe interfacial strain in a model structure of Pt/CoFe/MgO. We have found that the interfacial strain can effectively reduce electron hybridization between Pt and CoFe and thereby significantly suppress the MPE while enhance the SOT efficiency. Thermodynamic magnetics analysis further confirmed these findings. Our results have pointed out a new research direction in which one uses interfacial strain as an effective route to design efficient spintronic devices.

Nitrogen vacancy (NV) centers, optically active atomic defects in diamond, have attracted tremendous interest for quantum sensing, network, and computing applications due to their excellent quantum coherence and remarkable versatility in a real, ambient environment. One of the critical challenges to develop NV-based quantum operation platforms results from the difficulty in locally addressing the quantum spin states of individual NV spins in a scalable, energy-efficient manner. Here, we report electrical control of the coherent spin rotation rate of a single-spin qubit in NV-magnet based hybrid quantum systems. By utilizing electrically generated spin currents, we are able to achieve efficient tuning of magnetic damping and the amplitude of the dipole fields generated by a micrometer-sized resonant magnet, enabling electrical control of the Rabi oscillation frequency of NV spins. Our results highlight the potential of …

S47. 00001: Topological Insulator/Magnetic Insulating Oxide: A Platform for Efficient Spin Current Transport*AbstractPresenter:Peng Li(Auburn University)Authors:Peng Li(Auburn University)Steven S-L Zhang(case western reserve university)Lauren Riddiford(Stanford University)Timothy Pillsbury(Penn Sate University)Jinjun Ding(Colorado State University)James Kally(Penn Sate University)Alexander Grutter(National Institute of Standards and Technology)Gaurab Rimal(University of Wyoming)Chong Bi(Stanford University)Gyorgy Csaba(Pazmany Peter Catholic University)J Samuel Jiang(Argonne National Laboratory)Junjia Ding(Argonne National Laboratory)Wei Zhang(Oakland University)Jinke Tang(University of Wyoming)Weigang Wang(University of Arizona)Olle Heinonen(Argonne National Laboratory)Valentyn Novosad(Argonne National Laboratory)Axel Hoffmann(University of Illinois at Urbana-Champaign …

The discovery of interfacial Dzyaloshinskii-Moriya interaction (DMI) enables development of novel ultra-compact non-reciprocal devices for microwave signal processing. Such devices are based on control of spin waves and magneto-elastic waves by electric field and addition of ultra-thin layers of heavy metals to the devices. Here we discuss recent advances in the development of such systems, which can be used for implementation of on-chip non-reciprocal microwave devices.

L41. 00008: Overcoming the spectroscopic limitations of inelastic light scattering by sub-diffraction light confinement*AbstractPresenter:Ryan M Freeman(Department of Physics, Emory University)Authors:Ryan M Freeman(Department of Physics, Emory University)Vladislav Demidov(Institute for Applied Physics and Center for Nonlinear Science, University of Muenster)Sergej Demokritov(Institute for Applied Physics and Center for Nonlinear Science, University of Muenster)Jinjun Ding(Department of Physics, Colorado State University)Mingzhong Wu(Department of Physics, Colorado State University)Hayk Harutyunyan(Department of Physics, Emory University)Sergei Urazhdin(Department of Physics, Emory University)The small momentum of light strongly limits inelastic light scattering techniques. For instance, only long-wavelength spin waves are accessible to Brillouin light spectroscopy (BLS) widely utilized in …

Y 3 Fe 5 O 12 (YIG) thin films having a thickness of several nanometers and showing both strong perpendicular magnetic anisotropy (PMA) and low magnetic damping are reported. The films are deposited by magnetron sputtering at room temperature first and then annealed in O 2 at high temperature. The substrates are Gd 3 (Sc 2 Ga 3) O 12, which share the same crystalline structure as YIG, but have a lattice constant slightly larger than that of YIG; the lattice mismatching gives rise to an out-of-plane compressive strain and PMA in the YIG films. The PMA is confirmed by vibrating sample magnetometer, magneto-optical Kerr effect, anomalous Hall effect, and angle-dependent ferromagnetic resonance (FMR) measurements. The damping of the films is analyzed through frequency-dependent FMR measurements. As an example, an 8-nm-thick YIG film shows an effective PMA field of about 2800 Oe, a nearly square …

A topological insulator (TI) interfaced with a magnetic insulator (MI) may host an anomalous Hall effect (AHE), a quantum AHE, and a topological Hall effect (THE). Recent studies, however, suggest that coexisting magnetic phases in TI/MI heterostructures may result in an AHE-associated response that resembles a THE but in fact is not. This Letter reports a genuine THE in a TI/MI structure that has only one magnetic phase. The structure shows a THE in the temperature range of T = 2–3 K and an AHE at T = 80–300 K. Over T = 3–80 K, the two effects coexist but show opposite temperature dependencies. Control measurements, calculations, and simulations together suggest that the observed THE originates from skyrmions, rather than the coexistence of two AHE responses. The skyrmions are formed due to a Dzyaloshinskii–Moriya interaction (DMI) at the interface; the DMI strength estimated is substantially higher …

Understanding of damping processes in ferromagnetic thin films at temperatures (T) near the Curie temperature (T c) has significant implications for heat-assisted magnetic recording and magnetic sensors operating at elevated temperatures. Recent ferromagnetic resonance (FMR) studies [PR Applied 10, 054046 (2018)] using out-of-plane fields showed that there are two major relaxation processes in granular L1 0-odered FePt thin films at 10-45 K below T c: two-magnon scattering and spin-flip magnon-electron scattering; with a decrease in T, the FMR linewidth increases due to the enhancement of the two-magnon scattering. This presentation reports high-T FMR studies on continuous FePt thin films with cubic structures, rather than L1 0 structures. The films are 6-nm thick and have T c≈ 680 K; the FMR measurements were performed over 300-620 K in out-of-plane fields. As opposed to the L1 0 FePt films, the …

We report the modification of magnetism in a magnetic insulator Y 3 Fe 5 O 12 thin film by topological surface states (TSS) in an adjacent topological insulator Bi 2 Se 3 thin film. Ferromagnetic resonance measurements show that the TSS in Bi 2 Se 3 produces a perpendicular magnetic anisotropy, results in a decrease in the gyromagnetic ratio, and enhances the damping in Y 3 Fe 5 O 12. Such TSS-induced changes become more pronounced as the temperature decreases from 300 to 50 K. These results suggest a completely new approach for control of magnetism in magnetic thin films.

We report the direct observation of the fingerprint of the inverse Rashba-Edelstein effect (IREE) via investigations of the spin-to-charge conversion in Ta (Pt)/Cu and Cu/Ta (Pt) as a function of the heavy-metal thickness. The converted charge voltages have opposite signs for samples with reversed stacking orders in the ultrathin regime and have the same sign at higher thickness. The effect is demonstrated with two independent experimental approaches and supported by first-principles calculations. Our observations unambiguously demonstrate the existence of the IREE at heavy-metal/Cu interfaces and provide a framework for manipulating the spin-charge conversion via interface engineering.

Heterostructures made of strongly correlated oxides host various fundamentally interesting and potentially useful emergent phenomena.(LaMn O 3) n/(SrMn O 3) n− 1 superlattices that consist of an A-type antiferromagnetic insulator LaMn O 3 and a G-type antiferromagnetic insulator SrMn O 3 were investigated in this work. Several very intriguing effects were observed in such superlattices that include (1) the coexistence of a strong exchange bias effect and a giant vertical magnetization shift in superlattices with intermediate periods,(2) an insulator-to-metal transition associated with a change in the superlattice thickness, and (3) a large nontrivial negative magnetoresistance around the insulator-to-metal transition. To understand these phenomena, microscopic preferential orbital occupancy in different superlattices was studied through measurements of x-ray linear dichroism at Mn L edges. This study facilitated the …

This letter reports the development of low-damping yttrium-iron-garnet (YIG) thin films via sputtering. The films were deposited by sputtering at room temperature first and were then annealed in O 2 at high temperature. It is found that the annealing temperature critically affects the structural properties of the YIG films and thereby dictates the static and dynamic properties of the films. A 75 nm thick YIG film annealed at 900 °C shows an rms surface roughness of 0.08 nm, a coercivity of only 14 A/m (or 0.18 Oe), a saturation induction of 0.1778 T (or 1778 G), which is very close to the bulk value, a gyromagnetic ratio of 2.82 x 10 4 MHz/T (or 2.82 MHz/Oe), which almost matches the standard value, and a Gilbert damping constant of α≈5.2 x 10 -5 , which is the lowest among the values reported so far for magnetic films in the nanometer thickness range. Frequency-dependent ferromagnetic resonance measurements with …

Microfocus Brillouin light scattering is a powerful technique for the spectroscopic and spatial characterization of elementary excitations in materials. However, the small momentum of light limits the accessible excitations to the center of the Brillouin zone. Here we utilize a metallic nanoantenna fabricated on the archetypal ferrimagnet yttrium iron garnet to demonstrate the possibility of Brillouin light scattering from large-wave-vector, high-frequency spin wave excitations that are inaccessible with free-space light. The antenna facilitates subdiffraction confinement of the electromagnetic field, which enhances the local field intensity and generates momentum components significantly larger than those of free-space light. Our approach provides access to high-frequency spin waves important for fast nanomagnetic devices, and can be generalized to other types of excitations and light-scattering techniques.