VOLUME 82, NUMBER 18 P H Y S I C A L R E V I E W L E T T E R S 3 MAY 1999 Ultrafast Time Resolved Photoinduced Magnetization Rotation in a Ferromagnetic/Antiferromagnetic Exchange Coupled System Ganping Ju and A. V. Nurmikko Department of Physics and Division of Engineering, Brown University, Providence, Rhode Island 02912 R. F. C. Farrow, R. F. Marks, M. J. Carey, and B. A. Gurney IBM Research Division, Almaden Research Center, 650 Harry Road, San Jose, California 95120-6099 (Received 17 November 1998) Ultrashort pulse laser techniques are applied to study optically induced modulation in exchange biased ferromagnetic/antiferromagnetic (FM/AF) thin bilayer films (NiFe NiO). Photoexcitation of the FM/ AF interface with subpicosecond laser pulses induces large modulation in the unidirectional exchange bias field (Hex) on an ultrashort time scale. The "unpinning" of the exchange bias leads to coherent magnetization rotation in the permalloy film which is time resolved by the experiment and corresponds to a large modulation in the magnetization component (DMZ MS 0.5), on a time scale of 100 psec. [S0031-9007(99)09042-0] PACS numbers: 75.70.Ak, 78.20.Ls A problem of contemporary interest at both fundamen- regime of (large) magnetization modulation, depend on the tal and applied levels concerns the ultimate speed "limit" microstructure at the FM/AF interface. In practical terms, for magnetization reversal under external control. Re- the sensitivity of the exchange interaction to photoexcita- cently, there have been several types of experiments prob- tion has allowed us to obtain significant modulation in the ing the question. For example, Lederman et al. [1] studied magnetization of the NiFe NiO system (DMZ MS 0.5) the thermal activation switching of small Stoner-Wolfarth- on a 100 psec time scale. type FeO particles, while Doyle and co-workers [2] and Our samples were dc magnetron sputtering polycrys- Freeman with co-workers [3] applied a short transient talline Ni81Fe19 NiO bilayers on glass substrate, with magnetic field pulse created by a microstripline (on a Hex 100 Oe [9]. The transparency of NiO makes it pos- nanosecond time scale) to study magnetization switching sible to photoexcite the interface between the FM and AF in magnetic thin films. Most recently, Siegmann and co- directly. The low blocking temperature Tb 220 ±C of workers [4] applied high energy beam generated picosec- the NiO/NiFe system is convenient for breaking the ex- ond magnetic field pulses on CoPt films. change bias nonthermally. We seek to create conditions A different avenue to the study of fast spin and magneti- across the FM/AF interface such that the interfacial spin/ zation in ferromagnetic (FM) metals has emerged in which electron temperature Te,s t is elevated close to or above ultrashort laser pulses create a nonequilibrium distribution Tb, but kept below the Curie point, while the lattice tem- of electrons and their spins on a subpicosecond time scale perature Tl ø Tb. The hot electrons are unable to diffuse [5,6]. The ultrafast optical approach raises the prospect of into the insulating NiO AF layer. In addition, any domain inducing and investigating basic magnetization switching wall motion is slow on the time scales considered here so and relaxation phenomena with the system driven by the that the bulk magnetic structure of NiO is "frozen." absorbed photons. As a specific material test system, we have chosen the exchange coupled NiFe NiO FM/AF bi- layer, characterized by its distinct unidirectional magnetic anisotropy [7] which has found important recent use in giant magnetoresistive and spin-valve sensors [8]. The magnetic characteristics of such coupled FM/AF systems show both an effective exchange bias field Hex (shifted hysteresis loop) and an increased coercivity (HC). The idea in this paper, sketched in Fig. 1, is to create spin exci- tations by photons at the FM/AF interface so as to abruptly reduce the exchange coupling. When the magnetization of the FM layer is initially biased antiparallel to the exter- nal applied field HA, the optically induced "unpinning" of the exchange is shown below to activate ultrafast switch- FIG. 1. The concept of optically induced "unpinning" of the ing of the internal field H exchange bias (field) at FM/AF exchange coupled interface. ex, which provides the driving With the magnetization of the FM layer is initially biased force for the study of subsequent coherent magnetization antiparallel to external field, ultrashort pulse photoexcitation dynamics. These dynamics, studied here in the reversible triggers a coherent rotation process. 3705 0031-9007 99 82(18) 3705(4)$15.00 © 1999 The American Physical Society VOLUME 82, NUMBER 18 P H Y S I C A L R E V I E W L E T T E R S 3 MAY 1999 In our experiments, excitation pulses from a mode- locked Ti:sapphire laser (tp 120 fsec, hn 1.4 eV) were directed normal to the sample through the transpar- ent glass substrate and the NiO layer Eg . 5 eV , and absorbed within the NiFe layer [9]. To reduce average lattice heating, we use a pulse repetition rate of 1.9 MHz. The optically induced changes in the magnetization of the samples were recorded by measuring the transient changes in the (longitudinal) Kerr rotation of the NiFe layer, labeled below as u0K t , using time delayed weak (,0.2 mW) probe pulses in the blue hn 2.8 eV . The Kerr instrumentation employed a polarization-sensitive optical bridge and a low-noise differential detector [6,9]. The time-resolved longitudinal Kerr effect u0K t probes the off-diagonal component of the conductivity tensor sxy, which is proportional to magnetization and the net spin po- larization [6,9]. Two contributions are found in our pump probe experiments, distinguished by their different time constants and field dependence. First, following the subpi- cosecond impulse of photoexcitation, the hot electron and spin distributions are displaced from thermal equilibrium, leading to modulation in uK because of spin occupancy ef- fects and modulation in the net spin polarization. Recent studies in thin FM films have given information about the dynamics of spin relaxation [5,6]. The proportionality of the longitudinal Kerr effect to the inplane component of magnetization makes the transient experiment also sensi- FIG. 2. (a) Easy-axis transient Kerr loops for a photoexcited NiFe NiO exchange biased bilayer at t 1, 50, and 200 psec tive to photoinduced changes in the direction of M due to following pulse photoexcitation at t 0. The open circles coherent rotation, a key feature in this paper. inserted in the bottom trace show expected behavior in the Figure 2(a) displays "snapshots" of the easy-axis tran- absence of coherent magnetization rotation. (b) Transient sient Kerr hysteresis loops acquired from measurements Kerr loops at t 1 and 200 psec for a thermally quenched of u0 NiFe NiO bilayer companion sample. (c) Transient Kerr loops K t on a 100 Å 400 Å NiFe NiO bilayer sample at at t 1 and 200 psec for a NiFe thin film. Both control a probe delay of t 1, 50, and 200 psec, following the samples show no sign of magnetization rotation. impulse of pulsed laser excitation at t 0 ( 1 nJ fo- cused to a 30-mm-diameter spot). The results are typi- cal of several different samples studied, and should be zation, and contains a contribution which is proportional compared with the static (unperturbed) Kerr loop for the to the changes of magnetization component in the plane sample (dashed lines in the top panel). First, we find of incidence, DMZ. The large "negative Kerr loop anom- that within about 1 psec the transient Kerr loops show a aly" implies directly that coherent magnetization rotation significant reduction of the Hex. Some "softening" of is triggered upon photoexcitation of the bilayer. When the loop is seen, demonstrating direct electronic access calibrated against the static Kerr rotation, this magneti- to both the exchange coupling and the spins in the NiFe zation modulation, induced by each pump laser pulse in layer (increased TS). On the other hand, the coercivity Fig. 2(a) and first peaking at t 150 psec, corresponds remains nearly unaffected. to approximately DMZ MS 0.4 under our experimental With increasing time delay, the photoexcited, coupled conditions, implying an average magnetization rotation of FM/AF bilayer systems relaxes via electron, spin, and about 53±. We have achieved 100% switching at higher lattice interaction. Over several tens of psec, the tran- excitation levels; however, in that case irreversible effects sient Kerr loop, while being reduced in amplitude due take place. Here, the system returns to its initial equilib- to these relaxation processes, begins to display a pro- rium state after each pulse as verified by measuring the nounced change in its shape. An asymmetric distortion static hysteresis loops after each run. sets in against the trend of a monotonically decreasing Important supporting information was provided by two u0K t , the effect being concentrated in the lower right cor- types of control samples which showed no evidence for ner of the loop, which becomes quite dramatic at t magnetization rotation, even in a considerably higher 200 psec; a more detailed discussion of the time depen- excitation regime than in Fig. 2(a). First, Fig. 2(b) dence is given below. We emphasize that in the time- shows two transient Kerr loop snapshots for a NiFe/NiO resolved longitudinal Kerr configuration u0K t provides a bilayer sample from the same wafer, whose exchange bias measure of the pump-induced modulation of the magneti- had been intentionally quenched by thermal annealing. 3706 VOLUME 82, NUMBER 18 P H Y S I C A L R E V I E W L E T T E R S 3 MAY 1999 Neither were anomalies seen in a 100 Å thick NiFe riod of 280 psec. The distinct time dependent feature, for single epitaxial layer, shown in Fig. 2(c). Accordingly, which we found no counterpart in the control samples, give we interpret the data in Fig. 2(a) as direct evidence direct insight into the magnetization dynamics that are trig- for magnetization switching on an ultrafast time scale gered by the photomodulation of the exchange coupling. ( 100 psec), triggered by photoinduced unpinning of the For a physical description of a possible optically exchange coupling. The microscopies of the unpinning induced magnetization reversal, we first consider the are presumed to involve the uncompensated spins at energetics of the coupled NiFe NiO system by a "meso- the NiO surface and the adjacent ferromagnetic spins in scopic" two-level model, used recently to describe the NiFe [10]. Note that the large magnetization modulation stability of exchange bias against thermal fluctuations is observed only in easy-axis configuration where the [10]. In this model, which recognizes the importance of external field HA is antiparallel to both the built-in Hex the surface morphology and (columnar) microstructure of and the static magnetization of the FM layer, i.e., in the the AF medium (NiO) [11], an ensemble of independent "third quadrant" of the hysteresis loop. This is consistent single-domain AF grains (typical size of 10 nm) interacts with the constraints imposed by the exchange coupled with the FM layer. The energy per unit area for the system as shown below. coupled system is written as [10,12] The time evolution of the transient magnetization in E 2 AK 1 2 1 2 cos u 2 c 1 K the bilayer is illustrated in Figs. 3(a) and 3(b), for both I 1 KU tf sin2u easy and hard axis directions, respectively, at H 1 K A 80 AFtAF sin2c 2 HAMStf cosu , (1) and 2220 Oe. [The calculations leading to Figs. 3(c) and where the first term is the energy associated with the ex- 3(d) will be discussed below.] Focusing on Fig. 3(a), change bias field Hex 2 AK 1 2 MStF; the second term the u0K t traces show a very short (,1 psec) initial hot represents the uniaxial anisotropy (Ku of the FM layer and spin transient due to the contribution of occupancy factor/ the uniaxial interfacial anisotropy KI [12]); the third term spin effects already mentioned. For the applied field of is the Zeeman energy. The angle between a positive ap- HA 2220 Oe, optically induced magnetization switch- plied field and the (sublattice) magnetization of the FM and ing is not possible since in negative full saturation M AF layer is given by u and c, respectively. Figure 4(a) is already parallel to HA and Hex. On the other hand, shows the "configurational coordinate" diagram for the for HA 80 Oe where the conditions for magnetization NiFe NiO bilayer with fixed values of c 180±, and switching are optimized, one clearly identifies a strongly HA 80 Oe (along the easy axis), but with varying Hex. damped oscillating feature with an approximately 150 psec The initial magnetization bias is set antiparallel to the rise time and 300 psec period. Figure 3(b) shows the applied field (i.e., u 2180±); hence, by modulating the corresponding case for HA oriented along the hard axis, exchange coupling only, the relative energy for the two where the time evolution of magnetization component stable magnetization states is varied. With a decrease along the applied field [DMY t ] is probed. The oscil- in Hex from 100 to 50 Oe, taken here to be rapidly time latory nature of u0K t is more accentuated with a pe- varying due to the optically induced modulation, the state for parallel magnetization (u 0±) becomes energetically favored over the antiparallel (u 2180±) configuration. For the parameters appropriate to our case, note how the initial energy barrier disappears, so that magnetization of the FM layer can begin to coherently rotate towards its new, energetically favorable, direction. By contrast, Fig. 4(b) shows how, when initial magnetization is parallel to the external field (HA 200 Oe), the parallel mag- netization remains always energetically favorable. This prediction is consistent with our experimental results, including the qualitative shape of the transient Kerr hysteresis loops. Note that the model is used here to describe nonadiabatic behavior in that all other energy terms in Eq. (1), including those due to magnetocrystalline anisotropy and domain wall structure, are assumed to be frozen on the time scale of interest (ø1 nsec). Finally, Figs. 4(c) and 4(d) show the "configurational" coordinate diagrams for the hard axis case with HA 80 and 200 Oe, respectively. In both cases, the magnetization vector can FIG. 3. Time-resolved transient Kerr effect u0K t for the rotate towards the direction of its new in-plane energy photoexcited NiFe NiO bilayer in an external field of HA 80 Oe, and H minimum upon the modulation of the exchange coupling. A 2220 Oe. (a) applied along the easy axis, and (b) along the hard axis. Traces (c) and (d) show the results We now consider the temporal details of the opti- of a model calculation described in the text. cally induced magnetization "switching" in Fig. 3. The 3707 VOLUME 82, NUMBER 18 P H Y S I C A L R E V I E W L E T T E R S 3 MAY 1999 similar to that in Ref. [4] such that the thin film lies in the y-z plane, while HA is applied at angle b to the easy axis. The optically induced modulation of the exchange field is entered as a time dependent driving term of the form Hex t 0, 0, H0ex 1 2 m exp 2t t0 , with the rise time approximated as a step function (,1 psec) and the relaxation time t0 150 psec corresponding to an empirical spin lattice relaxation time at the FM/AF inter- face. The results of the calculations for the dynamics of coherent magnetization rotation DMY,Z t MS are shown in Fig. 3(c) and 3(d) for a modulation depth m 0.6 and HA 80 Oe, HA 2220 Oe applied along the easy and hard axes, respectively. A satisfactory fit is obtained for the overall damped oscillatory behavior a 0.05, entered as a parameter. The damping is higher than reported for permalloy films [3] but involves the coupled interface sys- tem. The model supports semiquantitatively the picture in which the photoinduced modulation of Hex provides FIG. 4. Energy diagrams for the bilayer system with the the system the possibility for spontaneous magnetization exchange bias field Hex t modulated between 100 and 50 Oe. The external field of field H reversal from a simple energy argument, but where the A 80 Oe and HA 200 Oe is applied along the easy axis in (a) and (b), and along the hard temporal details of the magnetization dynamics are gov- axis in (c) and (d). erned by driven equations of motion, with Hex t as the time dependent "power supply." The microstructure at the NiFe NiO interface defines a key spatial scale, bridging temporal shape of the photomodulated exchange bias field between the atomic scale associated with the optically in- Hex t consists of a fast rise time due to the unpinning duced spin excitations and the macroscopic magnetization of the exchange coupling by optically excited interfacial which is measured via u0K t in our experiments. spins. Subsequently, Hex t returns to its equilibrium In summary, we have demonstrated an ultrafast, opti- state, with a time constant approximately given by the cally modulated magnetization response in a FM thin film, spin-lattice relaxation time. Transient Kerr measure- unidirectionally exchange coupled to an AF layer. The ments, performed on quenched NiFe NiO bilayers and approach provides a possibility for very high speed con- NiFe thin films, suggest that this time constant is on the trol and coherent switching of magnetization through mi- order of 150 psec at room temperature. If the observed croscopic access to spins in an exchange coupled system. magnetization modulation is due to coherent rotation of This research was supported by National Science Foun- local moments defined by each AF grain, the driving dation Grant No. DMR-970159. term in the relevant equations of motion is provided by Hex t . On the one hand, as already noted, energetically the disappearance of the barrier for magnetization reversal at t 1 psec [Fig. 4(a)] suggests the possibility for a [1] M. Lederman et al., Phys. Rev. Lett. 73, 1986 (1994). spontaneous magnetization reversal at some characteristic [2] W. D. Doyle et al., IEEE Trans. Magn. 29, 3634 (1993). frequency of spin reversal y [3] W. K. Hiebert et al., Phys. Rev. Lett. 79, 1134 (1997). 0. Presumably, this rate is [4] C. H. Back et al., Phys. Rev. Lett. 81, 3251 (1998); related to the spin-lattice relaxation time as well [4,5], H. C. Siegmann et al., J. Magn. Magn. Mater. 151, L8 and could at least partly shape the 150 psec rise time (1995); L. He and W. D. Doyle, J. Appl. Phys. 79, 6489 of the u0K t signal. With the concurrency recovery of (1996). Hex T , the local magnetization within the grain-sized [5] E. Beaurepaire et al., Phys. Rev. Lett. 76, 4250 (1996); single domains rotate back. On the other hand, the A. Vaterlaus et al., Phys. Rev. Lett. 67, 3314 (1991). coherent (macroscopic) magnetization dynamics are [6] G. Ju et al., Phys. Rev. B 57, 700 (1998). generally considered by the Gilbert-Landau-Lifshitz [7] W. H. Meiklejohn and C. P. Bean, Phys. Rev. 102, 1413 equations of motion in the presence of a time varying (1956); A. P. Malozemoff, Phys. Rev. B 37, 7673 (1988). magnetic field. We have applied this formalism to test [8] B. Dieny et al., Phys. Rev. B 43, 1297 (1991); S. Soeya the details of the oscillatory behavior in the u0 et al., J. Appl. Phys. 74, 6297 (1993). K t traces in Fig. 3(a) and 3(b). The total effective field is taken as [9] G. Ju et al., Phys. Rev. B 58, 11 857 (1998). [10] P. A. A. van der Heijden et al., Appl. Phys. Lett. 72, 492 HT HA 1 HD 1 HK 1 Hex t , with an applied field (1998). HA, a uniaxial anisotropy field HK 0, 0, HK cosu , [11] K. Takano et al., Phys. Rev. Lett. 79, 1130 (1997); and a demagnetization field HD 24pMX, 0, 0 due to D. H. Han et al., J. Appl. Phys. 81, 340 (1997). the shape anisotropy. The coordinate system is defined [12] Z. Qian et al., J. Appl. Phys. 83, 6825 (1998). 3708