D. Exchange Bias Effect

Transcription

1 D. Exchange Bias Effect 6.9 Real time temperature dynamics in exchange coupled double layers upon photoexcitation M.C. Weber, H. Nembach, and B. Hillebrands 1 Recently, significant effort has been focused on the development of magnetic media and methods to increase areal densities in magnetic recording above 1Tb/in 2 [1]. One promising way is to make use of hybrid recording, e.g., lowering the magnetic anisotropy of a ferromagnetic storage medium by laser light. This so-called heat-assisted magnetic recording [2] involves temperatures close to or even above the Curie temperature of ferromagnetic films. Key issues are still the questions of how fast the storage medium can be heated, and how fast the heat gets out of the system. An alternative approach is presented here exploiting the so-called exchange bias effect, which is found, for instance, in bilayers consisting of a ferromagnetic (F) and an adjacent antiferromagnetic (AF) layer magnetic field cooled from above the blocking temperature of the AF layer. For recent reviews of the exchange bias effect see [3, 4]. In the present article, we address the temperature dynamics in an exchange coupled bilayer due to a short laser excitation including an optical unpinning of the exchange coupling. This is an ideal test system to study temperature assisted dynamics in ferromagnets, since the exchange bias anisotropy is known to depend strongly on temperature. The exchange bias field is therefore a direct measure of the temperature at the F/AF heterointerface, i.e., it can be used as an ultrafast thermometer. It has been shown that short laser pulses may be used to study hot non-equilibrium electrons, spins and phonons in ferromagnetic thin films [5, 6] on an ultrafast time scale. Within 1 ps the electron and spin systems thermalize, whereas Vaterlaus et al. have shown that typical spinlattice relaxation times of ferromagnets are less than 100 ps [7]. In our case, the excitation is in the low picosecond range, and thus, in the time regime where we can start to consider temperature dynamics phenomena in contrast to spin-lattice relaxation phenomena which are finished after a few tens of picoseconds. Temperature dynamics were mainly probed by time domain reflectivity measurements which deliver indirect information about the electron temperature of the studied samples [8]. However, a deep understanding of thermal transport in magnetic storage media, in nanoscale magnetic films or even in multilayer structures is still lacking [9]. In order to exploit the potential for fast thermo-magnetic writing with temperatures involved far below the Curie temperatures of single ferromagnetic films, the time constants of the temperature dynamics have to be investigated. The polycrystalline exchange bias samples have been prepared by UHV electron-beam evaporation. As a growth template a 15nm thick Cu buffer on top of a thermally oxidized Si substrate has been used. The exchange bias system itself consists of a 5nm thick Ni 81 Fe 19 (F) and a 10nm thick Fe 50 Mn 50 (AF) layer. To protect the bilayer from oxidation it was finally covered with a 2nm Cr cap layer. In order to initialize the bias effect, the sample was magnetic field cooled from above the blocking temperature (160 C) after deposition. For further details, see [10]. 1 In collaboration with: J. Fassbender, Institut für Ionenstrahlphysik und Materialforschung, Forschungszentrum Rossendorf, Dresden; D. Hoffmann and M. Aeschlimann, AG Ultraschnelle Phänomene an Oberflächen und Laser in der Medizin, TU Kaiserslautern, Kaiserslautern. 55

2 exchange bias field H eb (norm.) 1,0 0,8 0,6 0,4 0,2 0,0 T B temperature T ( C ) Fig. 1: Exchange bias field as a function of temperature, H eb (T ). The decrease with temperature and the blocking temperature T B = 160 C can clearly be identified. A real time thermometer has been built up to study fast temperature dynamics in exchange coupled bilayers exploiting all-optical pump-probe experiments which have been performed in the following way. A picosecond mode-locked laser oscillator [11] running at a repetition rate of 80 MHz and a wavelength of 1064nm is used as a pulsed laser source. After amplification the laser beam is inserted into a SHG unit which delivers laser pulses with a pulse duration of 8.3ps and maximum pulse energy of 50nJ. The pump pulse is directed nearly normal to the sample surface and focused to a diameter of about 30µm, yielding a power density of about 0.5GW/cm 2. The weak time delayed probe pulse is used to sense the thermally induced changes of magnetization by means of the longitudinal magnetooptic Kerr effect. In order to investigate the temperature associated magnetization dynamics of the exchange coupled bilayer in the time domain, quasistatic hysteresis loops are sensed by a probe pulse with a fixed time delay to the excitation pulse. First, temperature dependent quasistatic hysteresis loops along the easy magnetization direction are investigated using only the weak probe pulse. Hysteresis loops for different sample temperatures ranging from room temperature up to 200 C are recorded. The exchange bias field H eb has been extracted for each temperature, and after normalization, plotted as a function of temperature in Fig. 1. At room temperature the sample shows an absolute bias field value of H eb,rt = 123Oe and a coercivity of H c,rt = 24Oe. The bias field decreases monotonously with temperature according to (1 T /T B ) 1 (see solid line). Next, transient hysteresis loops along the easy magnetization axis are investigated using the above described setup. Details of the measurements are reported elsewhere [12]. The extracted bias field values are plotted as a function of the pump-probe delay time in Fig. 2. Within the pump pulse width (see inset of Fig. 2) a clear reduction of the bias field to about 50 percent of its initial value is observed which can be understood in terms of a fast thermal decoupling at the F/AF interface upon arrival of the pump pulse [12, 13]. The unpinning is followed by a slower recovery to the initial bias field value. A phenomenological model taking thermal activation into account [12, 14] accounts for these data H eb (t)=h eb,rt (1 m exp( t/τ)) (1) 56

3 exchange bias field H eb (Oe) H eb (Oe) Fig. 2: Exchange bias field as a function of pump-probe delay, H eb (t). The dashed line is a fit to Eq. (1) yielding τ = 160ps. The inset shows the time evolution around 0 ps delay together with a pump pulse autocorrelation. where m is the modulation depth and τ represents the relaxation time comprising spin-lattice and heat diffusion effects. The fit, shown as a dashed line in Fig. 2, yields m = 0.45and τ = 160ps. Taking the temperature dependence of the bias field H eb (T ) from Fig. 1 and the measured time evolution of the bias field H eb (t) from Fig. 2, we easily obtain the time evolution of the spin and lattice temperature T (t) at the F/AF interface (exchange bias is mainly an interface effect) in a calibrated way. Upon the short picosecond photoexcitation the spin and phonon systems are in nonequilibrium. Within the first 10 to 20ps the spin temperature at the F/AF interface is elevated up to about 100 C. A relative temperature rise T with respect to room temperature can be evaluated from the measured T (t) evolution, which is plotted in Fig. 3. These data are now compared to the well known heat diffusion equation [15] with a source term P(t) and C the heat capacity of the exchange bias system: C dt dt = (K T )+P(t). (2) The first term on the right hand side of Eq. (2) describes cooling by diffusion (K: heat conductivity perpendicular to the sample surface), whereas the second term acts as a pulsed heat source. Assuming that the typical sech 2 pump pulse leads to a Gaussian temperature profile and that onedimensional heat diffusion dominates the heat flow (pump laser spot is much larger than multilayer stack thickness), we can solve Eq. (2) with the so-called diffusivity D eff = K/C ( ) T max T (z,t)= π Deff t exp z2 4 D eff t (3) where z is the direction perpendicular to the stack. The temperature difference T ( ) λ 0 T max t τ spin-lattice = π D eff t + λ 2 0 (4) 57

4 temperature difference T (a.u.) 1,0 0,8 0,6 0,4 0,2 0,0 T/ T max heat diffusion ( 0 =5nm) Fig. 3: Temperature dynamics extracted out of Fig. 1 and Fig. 2, thus leading to a temperature difference T as a function of pump-probe delay. The solid line represents a fit to Eq. (4) with an effective diffusivity. is deduced, with the thermal diffusion length λ 0. Figure 3 summarizes the time evolution of the temperature rise at the F/AF interface together with the best fit to Eq. (4) which was performed with the thermal diffusion length λ 0 and the diffusivity D eff as free parameters. A thermal diffusion length of λ 0 = 5nm is deduced which is at the lower limit for metals [9] and larger than the Cr cap layer thickness which is homogeneously heated. The fit yields an effective diffusivity of D eff = 4.7(3) 10 5 m 2 /s for the system stack/substrate. Taking the composition weighted specific heat (C = J/Km 3 ) of the measured exchange bias stack into account a heat conductivity of about K eff = 172 ± 20W/Km is derived. Since Eq. (4) fits only for delay times larger than 20ps, this value is also estimated to be the spinlattice relaxation time. This value corresponds to the lower boundary determined by Vaterlaus et al. [7]. The heat diffusion analysis reveals that the heat flow is dominated by the Cu buffer layer (K bulk = 400W/Km) and that phonon scattering at the different interfaces of our multilayer system is not as significant as one might expect [16]. The latter applies since the Debye temperatures of the respective magnetic materials are quite close to each other, i.e., there is little acoustic mismatch. A maximum contribution to Kapitza like phonon scattering only exists for the Cu/substrate interface leading to a reduction of the heat conductivity. In summary, the observed temperature evolution can be understood in terms of heating the spin and lattice system close to the blocking temperature of the bilayer, leading to a reduction of the exchange coupling. The recovery process can be described by energy dissipation, e.g., the spinlattice system is cooled by heat flow into the substrate setting the ultimate limit for the speed of recovery. By a detailed heat diffussion analysis crucial parameters such as the effective diffusivity and the heat conductivity for the whole layer system have been evaluated employing the inherent temperature dependence of the exchange bias effect. M.C.W. would like to acknowledge support by the Graduiertenkolleg 792 Nichtlineare Optik und Ultrakurzzeitphysik of the DFG. The work is supported in part by the European Communities Human Potential program under contract number HPRN-CT ULTRASWITCH. 58

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