Material Dependence of Thermally Assisted Magnetization Reversal Properties in Microstructured Co/Pd Multilayers Budi Purnama, Terumitsu Tanaka, Yukio Nozaki, and Kimihide Matsuyama Department of Electronics, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan Received December 5, 2008; accepted February 1, 2009; published online February 20, 2009 Using extraordinary Hall resistance (R H ) measurements, the material dependence of thermally assisted magnetization reversal (TAMR) was investigated for microstructured multilayers of [Co (0.17 nm)/Pd (0.80 nm)] N with N ¼ 7 and 20, which exhibit markedly different magnetic properties. The threshold values of the external field (H w,th ) necessary for controlling the magnetization direction in TAMR, obtained by direct application of a current pulse to the sample, were 220 Oe for the N ¼ 7 and 710 Oe for N ¼ 20 samples. The values of H w,th are found to be related to the magnetization saturation field at a critical temperature at which apparent coercivity decays. # 2009 The Japan Society of Applied Physics DOI: 10.1143/APEX.2.033001 M agnetoresistive random access memory (MRAM) is expected to be a universal memory that satisfies the demands for high bit density, fast access time and practically unlimited durability. Thermally assisted magnetization reversal (TAMR) has been proposed for application in MRAM 1–6) as a promising approach to realize the maximum possible bit density using high perpendicular magnetic anisotropy (PMA) materials, as already demon- strated in hard disc drive (HDD) technology. Since the magnetization reversal in MRAM is performed with current- induced local magnetic fields, reduction of the reversal field is essential for low-power memory operation. Unlike conventional magnetization reversal, TAMR is a field- induced magnetic ordering process under significant thermal disturbance. Thus, a threshold value of the external field (H w,th ) to define the final magnetization direction and its dependence on the material parameters are important for material optimization and the prospects for scaling in thermally assisted MRAM (TA-MRAM). Among the various PMA materials available, Co/Pd multilayers exhibit a wide variety of magnetic properties, including variation in the strength of PMA and its temperature dependence, which can be controlled by varying the individual layer thickness, layer repetition number, and deposition conditions. 7,8) A correlation between the H w,th and the strength of PMA, reflected in the coercivity, was predicted in micromagnetic simulations. 9) The present work compares TAMR perfor- mance in microstructured Co/Pd films with different magnetic properties. Co/Pd multilayers were deposited on glass substrates by tandem-type magnetron sputtering with a multi-cathode system (Anelva SPC-350). Background pressure was below 5:0  10 7 Torr, and Ar pressure during deposition was fixed at 20 mTorr. The substrate was mounted on a sample holder and rotated at 50 rpm around the Co and Pd targets using a PC-controlled shutter system. The sputtering rates for Co and Pd were 0.10 and 0.036 nm/s, respectively. Microstructured Co/Pd multilayers with a lateral size of 50  50 m 2 were fabricated by photolithography and followed by a lift-off process. Two pairs of 10-m-wide underlie electrodes of Ti (20 nm)/Au (100 nm), used for Joule heating and extraordinary Hall resistance (R H ) measurements, were fabricated at each edge of the Co/Pd pattern as shown in Fig. 1. In the TAMR experiments, the sample temperature was raised by applying a current pulse to the Co/Pd pattern through one pair of electrodes with a fast pulse generator. The sample resistance was matched to a coaxial cable impedance of 50  with an additional series resistance. A pulse width of 300 ns was chosen, so that the entire structure could attain a thermal stationary state, 10) as was evidenced from the pulse width dependence of TAMR properties. The magnetization reversal behavior was in- vestigated by measuring R H , which is related to the perpendicular magnetization. 11) The R H hysteresis for the perpendicular field sweep was also measured at various temperatures with a Cu heat block system. Among the fabricated samples, [Co (0.17 nm)/Pd (0.80 nm)] N with N ¼ 7 and 20, referred to N7 and N20 hereafter, are the focus of the present study since they exhibit significantly different coercivity, which is related to the strength of PMA. Figure 2 shows R H hysteresis curves for N7 and N20, measured at various temperatures. The plotted R H values are normalized with that measured at a room temperature (RT) of 23  C. As can be seen in the figure, superior squareness and remanence are realized for the N7 up to 90  C, suggesting that the nucleation-type magnetization reversal is dominant at elevated temperatures. In contrast, a gradual increase of R H without hysteresis is observed at 120  C, which can be attributed to the enhanced thermal fluctuation effect. Another critical change in the R H behavior is observed at 190  C for the sample N20, as shown in Fig. 2(b). The observed hysteresis-free R H curve with a notable knee is typical of that measured along the effective hard axis direction, which can be explained as follows. The net uniaxial perpendicular anisotropy K u is a summation of the surface anisotropy (2K s =t Co ) and the volume anisotropy Fig. 1. Schematic top view of microstructured Co/Pd multilayers for TAMR experiments. Applied Physics Express 2 (2009) 033001 033001-1 # 2009 The Japan Society of Applied Physics