Ferromagnetic resonance driven by an ac current: a brief review

Excitation of ferromagnetic resonance (FMR) by an ac current has been observed in macroscopic ferromagnetic films for decades and typically relies on the ac Oersted field of the current to drive magnetic moments into precession and classical rectification of ac signals to detect the resonance. Rec...

Full description

Saved in:

Bibliographic Details
Date:	2013
Main Authors:	Wang, C., Seinige, H., Tsoi, M.
Format:	Article
Language:	English
Published:	Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України 2013
Series:	Физика низких температур
Subjects:	К 75-летию со дня рождения И. К. Янсона
Online Access:	https://nasplib.isofts.kiev.ua/handle/123456789/118222
Tags:	Add Tag No Tags, Be the first to tag this record!
Journal Title:	Digital Library of Periodicals of National Academy of Sciences of Ukraine
Cite this:	Ferromagnetic resonance driven by an ac current: a brief review / C. Wang, H. Seinige, M. Tsoi // Физика низких температур. — 2013. — Т. 39, № 3. — С. 320–325. — Бібліогр.: 30 назв. — англ.

Institution

Digital Library of Periodicals of National Academy of Sciences of Ukraine

id	nasplib_isofts_kiev_ua-123456789-118222
record_format	dspace
spelling	nasplib_isofts_kiev_ua-123456789-1182222025-02-23T18:58:44Z Ferromagnetic resonance driven by an ac current: a brief review Wang, C. Seinige, H. Tsoi, M. К 75-летию со дня рождения И. К. Янсона Excitation of ferromagnetic resonance (FMR) by an ac current has been observed in macroscopic ferromagnetic films for decades and typically relies on the ac Oersted field of the current to drive magnetic moments into precession and classical rectification of ac signals to detect the resonance. Recently, current-driven ferromagnetic resonances have attracted renewed attention with the discovery of the spin-transfer torque (STT) effect due to its potential applications in magnetic memory and microwave technologies. Here STT associated with the ac current is used to drive magnetodynamics on the nanoscale that enables FMR studies in sample volumes smaller by a factor of 1000 compared to conventional resonance techniques. In this paper, we briefly review the basics of STT–FMR technique and the results of various STT–FMR experiments. This work was supported in part by NSF grants DMR- 0645377 and DMR-1207577. 2013 Article Ferromagnetic resonance driven by an ac current: a brief review / C. Wang, H. Seinige, M. Tsoi // Физика низких температур. — 2013. — Т. 39, № 3. — С. 320–325. — Бібліогр.: 30 назв. — англ. 0132-6414 PACS: 85.75.–d, 75.76.+j, 76.50.+g, 72.25.Pn https://nasplib.isofts.kiev.ua/handle/123456789/118222 en Физика низких температур application/pdf Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України
institution	Digital Library of Periodicals of National Academy of Sciences of Ukraine
collection	DSpace DC
language	English
topic	К 75-летию со дня рождения И. К. Янсона К 75-летию со дня рождения И. К. Янсона
spellingShingle	К 75-летию со дня рождения И. К. Янсона К 75-летию со дня рождения И. К. Янсона Wang, C. Seinige, H. Tsoi, M. Ferromagnetic resonance driven by an ac current: a brief review Физика низких температур
description	Excitation of ferromagnetic resonance (FMR) by an ac current has been observed in macroscopic ferromagnetic films for decades and typically relies on the ac Oersted field of the current to drive magnetic moments into precession and classical rectification of ac signals to detect the resonance. Recently, current-driven ferromagnetic resonances have attracted renewed attention with the discovery of the spin-transfer torque (STT) effect due to its potential applications in magnetic memory and microwave technologies. Here STT associated with the ac current is used to drive magnetodynamics on the nanoscale that enables FMR studies in sample volumes smaller by a factor of 1000 compared to conventional resonance techniques. In this paper, we briefly review the basics of STT–FMR technique and the results of various STT–FMR experiments.
format	Article
author	Wang, C. Seinige, H. Tsoi, M.
author_facet	Wang, C. Seinige, H. Tsoi, M.
author_sort	Wang, C.
title	Ferromagnetic resonance driven by an ac current: a brief review
title_short	Ferromagnetic resonance driven by an ac current: a brief review
title_full	Ferromagnetic resonance driven by an ac current: a brief review
title_fullStr	Ferromagnetic resonance driven by an ac current: a brief review
title_full_unstemmed	Ferromagnetic resonance driven by an ac current: a brief review
title_sort	ferromagnetic resonance driven by an ac current: a brief review
publisher	Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України
publishDate	2013
topic_facet	К 75-летию со дня рождения И. К. Янсона
url	https://nasplib.isofts.kiev.ua/handle/123456789/118222
citation_txt	Ferromagnetic resonance driven by an ac current: a brief review / C. Wang, H. Seinige, M. Tsoi // Физика низких температур. — 2013. — Т. 39, № 3. — С. 320–325. — Бібліогр.: 30 назв. — англ.
series	Физика низких температур
work_keys_str_mv	AT wangc ferromagneticresonancedrivenbyanaccurrentabriefreview AT seinigeh ferromagneticresonancedrivenbyanaccurrentabriefreview AT tsoim ferromagneticresonancedrivenbyanaccurrentabriefreview
first_indexed	2025-11-24T12:22:18Z
last_indexed	2025-11-24T12:22:18Z
_version_	1849674368440860672
fulltext	© C. Wang, H. Seinige, and M. Tsoi, 2013 Low Temperature Physics/Fizika Nizkikh Temperatur, 2013, v. 39, No. 3, pp. 320–325 Ferromagnetic resonance driven by an ac current: a brief review C. Wang, H. Seinige, and M. Tsoi Department of Physics, University of Texas at Austin, Austin TX 78712, USA E-mail: tsoi@physics.utexac.edu Received November 1, 2012 Excitation of ferromagnetic resonance (FMR) by an ac current has been observed in macroscopic ferromag- netic films for decades and typically relies on the ac Oersted field of the current to drive magnetic moments into precession and classical rectification of ac signals to detect the resonance. Recently, current-driven ferromagnet- ic resonances have attracted renewed attention with the discovery of the spin-transfer torque (STT) effect due to its potential applications in magnetic memory and microwave technologies. Here STT associated with the ac cur- rent is used to drive magnetodynamics on the nanoscale that enables FMR studies in sample volumes smaller by a factor of 1000 compared to conventional resonance techniques. In this paper, we briefly review the basics of STT–FMR technique and the results of various STT–FMR experiments. PACS: 85.75.–d Magnetoelectronics; spintronics: devices exploiting spin polarized transport or integrated magnetic fields; 75.76.+j Spin transport effects; 76.50.+g Ferromagnetic, antiferromagnetic, and ferrimagnetic resonances; spin-wave resonance; 72.25.Pn Current-driven spin pumping. Keywords: spintronics, spin-transfer torque, ferromagnetic resonance. 1. Introduction Ferromagnetic resonance (FMR) [1–3] is a unique spec- troscopic technique to study magnetodynamics in a variety of systems including magnetic nanostructures and spin- tronic devices. In conventional FMR, the sample is placed in a microwave resonant cavity in the presence of a static magnetic field and the precessional motion of the sample’s magnetization is resonantly excited by an applied ac (mi- crowave frequency) magnetic field. Absorption of micro- waves can be detected in the neighborhood of the reson- ance when the applied microwave frequency matches the natural frequency of the magnetization precession [1–5]. Interestingly, instead of detecting small variations in the absorption signal one can simply measure a dc voltage which appears across the sample at resonance when an ac (microwave) current is applied directly through the sample. This voltage is the result of the rectification processes in which the oscillating magnetic moments affect the electric current flow via, e.g., extraordinary Hall effect and/or ani- sotropic magnetoresistance [6–9]. In addition to greatly simplifying the detection of FMR, the applied ac current can be used as the driving force of FMR via the so-called spin-transfer torque (STT) effect. The new experimental technique of spin transfer torque driven FMR is the focus of this brief review. The STT effect refers to a novel method to control and manipulate magnetic moments by an electric current [10–12]. For instance, in a spin valve where two magnets are sepa- rated by a nonmagnetic spacer, the electric current flowing between the magnets can transfer spin angular momentum from one magnet to another and results in a STT torque on their magnetic moments. The direction of STT torque de- pends on the polarity of applied current that can be used for reversible switching of magnetic moments in magnet- ic memory applications [10]. When an ac current is flow- ing between the magnets, the resulting STT torque also oscillates and thus can be used to drive FMR. The combi- nation of STT with magnetic resonance techniques ex- tends the applicability of FMR to magnetic nanostruc- tures, thanks to the development of sub-micron fabrication and electrical probing techniques [13–15]. The spin transfer torque-driven FMR technique (STT–FMR) enables FMR studies in sample volumes smaller by a factor of 1000 compared to the conventional methods with resonant microwave cavities. Moreover, the electrical FMR detection based on rectification phenomena associated with anisotropic magnetoresistance (AMR) in single-layer structures, giant Ferromagnetic resonance driven by an ac current: a brief review Low Temperature Physics/Fizika Nizkikh Temperatur, 2013, v. 39, No. 3 321 magnetoresistance (GMR) in metallic spin valves and mag- netic multilayers, or tunneling magnetoresistance (TMR) in magnetic tunnel junctions provide the means to both excite and probe electrically the dynamical magnetic phenomena on the nanoscale [13–16]. This makes STT–FMR a unique and powerful tool for characterization of magnetodynamics in nanoscale devices. This review is organized as follows. In Sec. 2, the STT–FMR basics are discussed. In Sec. 3, we review a variety of experimental arrangements and setups used for STT–FMR observations. Sec. 4 summarizes this review about STT–FMR. 2. Spin transfer torque–ferromagnetic resonance basics The driving force of the precession of magnetic moments in a conventional FMR is the oscillating field-torque τB ex- erted by an oscillating rf magnetic field Brf (see Fig. 1(a)). The driving force of the magnetic precession in STT–FMR is the oscillating STT torque τSTT associated with an applied ac current Irf (see Fig. 1(b)). We will illustrate the STT ef- fects of current on a simple spin-valve structure where a free magnetic layer s is separated from a fixed magnet s* by a nonmagnetic spacer N (Figs. 2(a),(b)) and the fixed layer acts as a perfect spin filter, i.e., electron spins aligned with its magnetic moment are completely transmitted through the layer, while antiparallel spins are completely reflected. When electron current crosses the spin valve from left to right (Fig. 2(a)), electrons transmitted through the polarizer will be polarized along s* and exert a torque τ+ on s in a direction so as to align s with s. When the current crosses the spin valve from right to left (Fig. 2(b)), electrons reflect- ed from the polarizer will be polarized antiparallel to s and exert a torque τ– on s trying to align s antiparallel with s. Note that spin relaxation in N, spin filtering by the free layer and higher-order reflections between s and s, among other effects, can change the magnitudes of τ+ and τ– torques. In macrospin approximation, which treats the free and fixed layers as single magnetic domains, the current-driven dy- namics of the free layer can be described by the Landau– Lifshitz–Gilbert (LLG) equation with an addition due to the applied current — STT torque [10] ( ) , 2STT g j e = × ×s s sτ (1) where j is the current density, g is polarization factor, s and s are directions of the free- and fixed-layer magnetiza- tions. The STT torque (Eq. (1)) depends linearly on the densi- ty j of applied current, so a reversal of the current polarity would imply a reversal of the torque. When an ac current is applied to the spin valve, the STT torque oscillates and can drive the magnetic moment of the free layer into resonance if the ac frequency matches the natural frequency of the moment precession. At resonance, the oscillating STT tor- que acts in sync with the precessional motion of the mo- ment to compensate for the damping losses and conti- nuously drive the precessing moment away from its equilibrium. This mechanism is illustrated in Fig. 2(c) where two moment orientations correspond to two differ- ent (opposite) phases of the precessional motion. When the moment is to the left of equilibrium (dashed line in Fig. 2(c)) and the electron current flows from left to right (Fig. 2(a)), the STT torque will tend to increase the preces- sion angle as indicated by the vector τ+ in Fig. 2(c). Half a period later, the moment is to the right of equilibrium and τ– will continue increasing the precession due to the re- verse current flow (Fig. 2(b)). Such a synchronous varia- tion of the moment and STT torque directions provides a Fig. 1. Driving mechanisms of conventional field-driven FMR (a) and STT–FMR (b). An oscillating magnetic field (Brf) exerts an oscillating torque (τB) on a magnetic moment (a). An oscillating current (Irf) which flows between the fixed (bottom) and free (top) layers of a spin valve generates an oscillating STT torque (τSTT) on the magnetic moment of the free layer (b). (a) (b) �B �STT Brf Irf (a) (b) (c) (d) ss* N –e �+ �– � dc rf Bias T STT device N –e �+ �– s* s Fig. 2. STT-driven magnetodynamics in a simple spin-valve structure where a free magnetic layer s is separated from a fixed magnet s* by a nonmagnetic spacer N (a), (b). When electron current crosses the spin valve from left to right, electrons trans- mitted through the fixed polarizer will generate torque τ+ on the free layer (a). When the current flows from right to left, electrons reflected from the polarizer will generate torque τ– on the free layer (b). Magnetic precession driven by STT oscillating in sync with the precession (c). Schematic diagram of a STT–FMR expe- riment with dc and rf electronics connected to a STT device via a bias tee. Both dc and rf currents can be applied to the device and the resulting dc voltage across the device can be measured (d). C. Wang, H. Seinige, and M. Tsoi 322 Low Temperature Physics/Fizika Nizkikh Temperatur, 2013, v. 39, No. 3 means to continuously pump energy from the ac current into the magnetic system. Note that the in-sync mechanism described above re- quires a noncollinear configuration of magnetic moments in the two layers. In practice, this is usually achieved by making the free and fixed layers respond differently to an externally applied magnetic field, e.g., by using magneti- cally soft/hard materials and/or different magnetic aniso- tropies. In this case, STT–FMR is performed in an applied field well below the saturation field of the multilayer that favors a noncollinear orientation of the two layers. The later is beneficial for a stronger STT [10] but limits the range of magnetic fields (frequencies) for such experi- ments. STT–FMR experiments at higher frequencies would involve higher magnetic fields and therewith an almost parallel configuration of the two magnetic layers that im- plies a symmetric precession of the free layer about the fixed layer. In this case, the oscillating STT torque would promote the precession for one half of the period of pre- cession, but oppose it for the other half. This symmetry should be broken so that the ac STT could pump a non- zero net energy into the magnetic system over the preces- sional period. In real systems, the required asymmetry can come from a number of different sources, including differ- ent magnitudes of τ+ and τ– torques (see above) or a small non-collinearity of the magnetic moments (e.g., due to sample imperfections). Figure 2(d) shows a typical setup for STT–FMR expe- riments. Here a bias tee is used to connect both dc (current source, voltmeter, lock-in amplifier) and rf (microwave generator, oscilloscope) electronics to a STT device (spin valve); various devices will be discussed in the next sec- tion. The setup provides a means to apply both ac (micro- wave) and dc currents to the device and measure the result- ing voltage. As was discussed earlier, when a microwave current I(t) = Irf cos(ωt) at FMR frequency is applied to the device, the oscillating STT can drive the free layer magnet- ic moment into resonance. The moment precession results in a time-dependent spin-valve resistance due to giant magnetoresistance (GMR) effect as the relative orientation of the free and fixed layers changes continuously. Mixing of the time-dependent component of spin-valve resistance R(t) = ΔR cos(ωt + δ) with the microwave current I(t) con- tributes a dc term to the device voltage 1 cos . 2 rfV I Rω = Δ δ (2) This rectified voltage develops only at resonance and allows the detection of FMR by simply measuring a dc voltage across the STT device as a function of either the frequency of applied microwave current for a fixed mag- netic field, or as a function of the applied magnetic field for a fixed microwave frequency. Figure 3 shows the results of a typical room-temperature STT–FMR experiment. Figure 3(a) shows the rectified vol- tage Vω as a function of applied magnetic field B for differ- ent frequencies ω/2π of applied rf current in the range from 9–23.7 GHz. The Vω(B) spectra are shifted vertically for clarity. At higher fields there is a resonance structure (dip) in Vω(B) which shifts to higher Bs at higher frequencies. The resonance frequency vs field relation (solid circles in Fig. 3(b)) is well described by the Kittel’s equation [4] 0/2 ( )SB B Mω π = γ +μ (3) with μ0MS = 0.88 T for NiFe (see fit in Fig. 3(b)) that con- firms the FMR nature of the observed resonances. Fig. 3. Typical experimental results of STT–FMR. The rectified dc voltage (at zero dc bias) as a function of applied magnetic field B for different frequencies of applied rf current ω/2π = 9, 10, 11, 12, 13, 15, 16, 17, 18, 19, 20, 21, 22, 23, and 23.7 GHz. The spectra are shifted vertically for clarity (a). The experimental resonance frequency vs field relation (solid circles) fitted by the Kittel’s equation (b). DC bias dependence of FMR linewidth (c). (a) (b)23.7 9 0 300 600 1 V � � � /2 , G H z Magnetic field, mT 20 10 0 400 30 25 20 –5 0 5 dc current, mA 200 L in ew id th , m T c)( Magnetic field, mT Ferromagnetic resonance driven by an ac current: a brief review Low Temperature Physics/Fizika Nizkikh Temperatur, 2013, v. 39, No. 3 323 Direct evidence of spin-transfer effect in the observed FMR signals would be the dependence of the resonance linewidth on the dc bias, since the linewidth depends on damping which in turn depends on STT. Since the reson- ance linewidth is proportional to the effective damping constant which in turn depends linearly on the applied dc current, one can expect a linear dependence of the reson- ance linewidth on dc current. The dc bias dependence of the FMR linewidth shown in Fig. 3 (c) is consistent with these expectations. A direct outcome from the STT–FMR linewidth analysis is the determination of Gilbert damping in individual nanomagnets with only a million of Bohr magnetons and scalable to even smaller dimensions. The critical dc current of STT-induced dynamics can also be extracted from the linear fit for the bias dependence of the resonance linewidth [14,15]. 3. Spin transfer torque–ferromagnetic resonance experiments STT–FMR has been demonstrated in a variety of expe- riments. In this section, we would like to highlight several pioneering approaches to STT–FMR. Different ways of generating ac STT and probing STT–FMR in magnetic nano-domains are reviewed and discussed. Magnetic tunnel junctions. The first experimental demonstration of STT–FMR was conducted in 2005 by Tulapurkar et al. [13]. They performed experiments on a PtMn (15 nm)/CoFe (2.5 nm)/Ru (0.85 nm)/CoFeB (3 nm)/MgO (0.85 nm)/CoFeB(3 nm) magnetic tunnel junction (MTJ) patterned into oval-shaped pillars of dimen- sion 200 × 100 nm (see Fig. 4(a)), using electron-beam litho- graphy and ion milling techniques. Here the bottom syn- thetic antiferromagnetic CoFe/Ru/CoFeB layer acts as a fixed layer, while the top CoFeB acts as a free layer. The insulating MgO spacer serves as a tunneling barrier for the electrons traveling between the free and fixed layers. The dc voltage Vω produced across the MTJ device was measured as a function of the frequency ω of the rf current (0.55 mA) applied to MTJ using a setup similar to the one shown in Fig. 2(d). The Vω(ω) spectra showed a large re- sonance structure (~60 μV), whose position depends on the magnetic field in good agreement with the Kittel’s equa- tion (see Fig. 3 and Eq. (3)). Further studies of the ac-driven magnetodynamics in MTJs focused on the dependence of spin torque (torkance) on the dc voltage bias. It turns out that, in MTJs under high bias, there is a nonnegligible contribution to STT torque which is perpendicular to the plane formed by the fixed- and free-layer magnetic moments, also known as the “field-like” torque. The STT–FMR technique provided a means to study the bias dependence of STT torkances in great detail [17,18]. Metallic nanopillars. Soon after the first demonstration with MTJs, STT–FMR was observed in metallic spin-valve nanopillars [14]. Py (20 nm)/Cu (12 nm)/ PyCu (5.5 nm) spin valves were patterned into oval-shaped pillars (see Fig. 4(a)) of dimension 90 × 30 nm, using electron-beam lithography and ion milling techniques. Various modes of magnetic exci- tations in both the free (PyCu) and the fixed (Py) layers were found and the signals with the lowest frequency and largest amplitude were attributed to spatially uniform precession (FMR) modes in the two layers. The linear dc bias depen- dence of the resonance linewidth (see Fig. 3(c)) has con- firmed the STT nature of the resonances. STT–FMR was also observed in spin-valve nanopillars with perpendicular magnetic anisotropy (PMA) which can be used to achieve a non-collinear configuration of the free and fixed layers [19]. For instance, a free [Co (0.4 nm)/ Ni (0.8 nm)] × 3 synthetic PMA layer was used in combination with a fixed Co (12 nm) layer and a Cu (10 nm) spacer to investigate STT–FMR in the nonlinear regime [20]. It was found that at high power levels of applied rf current the na- nopillar behaves as a nonlinear forced oscillator whose fre- quency is a function of its amplitude. Point contacts. Metallic point contacts are convenient tools for experimental studies of electron kinetics in metals [21–23]. High current densities associated with their ex- tremely small size were instrumental for the first demonstra- tion of STT in spin-valve multilayers [10] and for high- frequency magnetodynamics studies [15,24,25]. STT–FMR was demonstrated in mechanical point contacts to ultrathin films of exchanged bias spin valves (EBSV) [15]. A stan- dard mechanical system [10,23] can be used to establish an Fig. 4. Examples of devices used in STT–FMR. EBL patterned nanopillars of spin valves (MTJs) with two magnetic — fixed and free — layers (dark grey) separated by a nonmagnetic metallic (insulating) spacer (a). Metallic point-contact devices to EBSV films (b). Magnetic nanowires electrodeposited into porous mem- brane (c). Ferromagnetic (dark grey)/ Pt (light grey) metal bilayer used in SHE driven FMR experiments. A transverse spin current IS is generated by a longitudinal charge current I in the Pt layer via SHE (d). (a) (b) (d) I I I I (c) IS C. Wang, H. Seinige, and M. Tsoi 324 Low Temperature Physics/Fizika Nizkikh Temperatur, 2013, v. 39, No. 3 electrical contact between an electrochemically sharpened Cu tip and EBSV film (see Fig. 4(b)). Point contacts as small as a few nanometers in size were used to inject high densities of ac and dc currents into IrMn (10 nm)/NiFe (10 nm)/Cu (5 nm)/NiFe (4 nm) spin valve, where the bot- tom (fixed) NiFe layer is exchange biased by the IrMn anti- ferromagnets [15]. The EBSV stack had a 30 nm thick Cu underlayer to produce a closely current-perpendicular-to- plane (CPP) current flow across the spin valve. Unlike most STT–FMR measurements made with strongly non-collinear free- and fixed-layer magnetizations, the point-contact experiment was performed in magnetic fields well above the saturation field of the spin valve with nearly collinear orientations of the two layers. Nevertheless, the resonance signal was clearly resolved (see Fig. 3) that may be associated with one of the symmetry breaking me- chanisms discussed above. The latter, along with the sim- plicity of the mechanical point-contact technique extends the applicability of STT–FMR to smaller sample volumes, higher/lower frequencies, and a broader range of materials. Electrodeposited nanowires. Very high current densi- ties can be also created in magnetic nanowires produced by an electrochemical deposition into porous membrane tem- plates. STT–FMR was successfully detected in single Co (40 nm)/ Cu (5 nm)/ Co(5 nm) [26] and CoCu (25 nm)/ Cu (10 nm)/ CoCu (7 nm)/ Cu (10 nm)/ CoCu (100 nm) [27] nanowires with diameters as small as 40 nm. An rf current was injected into individual nanowires through a Au tip controlled by a differential screw mechanism similar to the point-contact setup discussed above. Rectified dc voltages corresponding to STT–FMR in both free and fixed layers were electrically detected. Interestingly, the nanowire de- vice with three CoCu layers showed a resonance signal in zero applied magnetic field where STT–FMR presumably occurs in an effective field due to the magnetic coupling (e.g. dipolar) between the layers. Time-resolved STT–FMR. While majority of STT–FMR experiments to date measure the dc (i.e., time independent) voltage response of a system, time resolved techniques were recently developed for detecting the time evolution of the magnetic precession driven by an ac currents [28]. The idea is to send a square-shaped pulse of radiofrequency current and use an oscilloscope to detect a time resolved voltage response after the falling edge of the pulse. The maximum amplitude of the response is determined by the spin torque of rf current while its decay reflects on the damping of precession. This time resolved technique has paved the way to a more accurate understanding of the STT-driven magnetodynamics in MTJs under high bias voltages and allowed to go beyond the lowest order ap- proximation in the description of the bias dependence of STT torkances [28]. Alternative ways to generate STT–FMR. The driving force of magnetic precessions in STT–FMR is the oscillat- ing STT torque associated with the ac spin current crossing a magnetic multilayer (see STT–FMR basics section). In all of the experiments discussed above the spin current is generated by driving an ac charge current across the multi- layer. Recently, two alternative approaches were devel- oped to generate ac spin current via spin Hall effect (SHE) [16] or spin diffusion [29]. In SHE-driven FMR experiments, a single ferromagnet- ic layer adjacent to a Pt layer (see Fig. 4(d)) can be excited into a uniform precession by a transverse spin current gen- erated by a longitudinal charge current flowing in the Pt layer [16,30]. The resonance can be detected either (i) electrically by measuring a small rectified dc voltage, which appears at resonance when the longitudinal ac cur- rent mixes with the oscillating resistance of the magnet (controlled by AMR effect) [6,7], or (ii) by the Brillouin light scattering (BLS) technique [30] which probes the dynamic magnetization. In STT–FMR experiments mediated by spin diffusion, the charge current is driven across a ferromagnet- ic/nonmagnetic (F/N) metal interface to produce a spin accumulation in the N metal [29]. A pure spin current then diffuses away from the interface and drives FMR in a free layer of a MTJ device positioned in direct proximity of the F/N interface. The resonance is detected by measuring a rectified dc voltage which is a consequence of mixing be- tween the oscillating MTJ resistance and an ac leakage current flowing through the MTJ. 4. Summary The transfer of spin angular momentum from transport currents to local magnetic moments can drive the moments out of equilibrium. This STT effect provides a novel method to drive and characterize FMR in individual nanomagnets. The STT driven FMR has become a powerful tool to probe high-frequency magnetodynamics on the nanoscale. One of the direct outcomes of STT–FMR is the experimental deter- mination of effective magnetic damping parameters in indi- vidual nanomagnets (including in the presence of electric current) with only a million of Bohr magnetons and scalable to even smaller dimensions. Due to the universality and sca- lability of STT–FMR as well as the versatility and high sensi- tivity of its electrical detection, this technique will no doubt continue contributing to our understanding of high-frequency magnetodynamics in magnetic nanostructures and spintronic devices. STT–FMR is instrumental not only for the under- standing of fundamental physics involved in magnetodynam- ics on the nanoscale but also crucial to potential develop- ments of magnetic memory and rf device applications. This work was supported in part by NSF grants DMR- 0645377 and DMR-1207577. Ferromagnetic resonance driven by an ac current: a brief review Low Temperature Physics/Fizika Nizkikh Temperatur, 2013, v. 39, No. 3 325 1. V.K. Arkad'yev, J. Russ. Phys. -Chem. Soc. 44, 165 (1912). 2. J.H.E. Griffiths, Nature 158, 670 (1946). 3. S.V. Vonsovskii, Ferromagnetic Resonance: The Pheno- menon of Resonant Absorption of a High-Frequency Magnetic Field in Ferromagnetic Substances, Pergamon, Oxford (1966). 4. C. Kittel, Phys. Rev. 71, 270 (1947). 5. C. Kittel, Phys. Rev. 73, 155 (1948). 6. W.G. Egan and H.J. Juretschke, J. Appl. Phys. 34, 1477 (1963). 7. H.J. Juretschke, J. Appl. Phys. 31, 1401 (1960). 8. A. Yamaguchi, K. Motoi, A. Hirohata, H. Miyajima, Y. Miya- shita, and Y. Sanada, Phys. Rev. B 78, 104401 (2008). 9. M. Harder, Z.X. Cao, Y.S. Gui, X.L. Fan, and C.-M. Hu, Phys. Rev. B 84, 054423 (2011). 10. J.C. Slonczewski, J. Magn. Magn. Mater. 159, 1 (1996). 11. L. Berger, Phys. Rev. B 54, 9353 (1996). 12. M. Tsoi, A.G.M. Jansen, J. Bass, W.C. Chiang, M. Seek, V. Tsoi, and P. Wyder, Phys. Rev. Lett. 80 4281 (1998). 13. A.A. Tulapurkar, Y. Suzuki, A. Fukushima, H. Kubota, H. Maehara, K. Tsunekawa, D.D. Djayaprawira, N. Wata- nabe, and S. Yuasa, Nature 438, 339 (2005). 14. J.C. Sankey, P.M. Braganca, A.G.F. Garcia, I.N. Krivorotov, R.A. Buhrman, and D.C. Ralph, Phys. Rev. Lett. 96, 227601(2006). 15. T. Staudacher and M. Tsoi, J. Appl. Phys. 109, 07C912 (2011). 16. L. Liu, Takahiro Moriyama, D.C. Ralph, and R.A. Buhrman, Phys. Rev. Lett. 106, 036601 (2011). 17. H. Kubota, A. Fukushima, K. Yakushiji, T. Nagahama, Sh. Yuasa, K. Ando, H. Maehara, Y. Nagamine, K. Tsunekawa, D.D. Djayaprawira, N. Watanabe, and Y. Suzuki, Nature Physics 4, 37 (2008). 18. J.C. Sankey, Yong-Tao Cui, J.Z. Sun, J.C. Slonczewski, R.A. Buhrman, and Daniel C. Ralph, Nature Phys. 4, 67 (2008). 19. W. Chen, J.-M.L. Beaujour, G. de Loubens, A.D. Kent, and J.Z. Sun, Appl. Phys. Lett. 92, 012507 (2008). 20. W. Chen, G. de Loubens, J.-M.L. Beaujour, J.Z. Sun, and A.D. Kent, Appl. Phys. Lett. 95, 172513 (2009). 21. Y.V. Sharvin, Zh. Eksp. Teor. Fiz. 48, 984 (1965) [Sov. Phys. JETP 21, 655 (1965)]. 22. A.G.M. Jansen, A.P. Van Gelder, and P. Wyder, J. Phys. C 13, 6073 (1980). 23. Y.G. Naidyuk and I.K. Yanson, Point-Contact Spectroscopy, Springer (2005). 24. O.P. Balkashin, V.V. Fisun, I.K. Yanson, L.Yu. Triputen, A. Konovalenko, and V. Korenivski, Phys. Rev. B 79, 092419 (2009). 25. Y. Okutomi, K. Miyake, M. Doi, H.N. Fuke, H. Iwasaki, and M. Sahashi, J. Appl. Phys. 109, 07C727 (2011). 26. N. Biziere, E. Murè, and J.P. Ansermet, Phys. Rev. B 79, 012404 (2009). 27. A. Mourachkine, O.V. Yazyev, C. Ducati, and J.P. Ansermet, Nano Lett. 8, 3683 (2008). 28. C. Wang, Y.-T. Cui, J.A. Katine, R.A. Buhrman, and D.C. Ralph, Nature Phys. 7, 496 (2011). 29. L. Xue, C. Wang, Y.-T. Cui, L. Liu, A. Swander, J.Z. Sun, R.A. Buhrman, and D.C. Ralph, Phys. Rev. Lett. 108, 147201 (2012). 30. V.E. Demidov, S. Urazhdin, E.R.J. Edwards, and S.O. De- mokritov, Appl. Phys. Lett. 99, 172501 (2011).

Ferromagnetic resonance driven by an ac current: a brief review

Institution

Similar Items