Time-resolved magnetic domain patterns of (Co/Pt) and (CoFeB/Pd) multilayers with perpendicular magnetic anisotropy are observed by means of magneto-optical microscopy, from which magnetic relaxation curves are determined for. Interestingly, it has been observed that the relaxation processes not only from the saturated state, but also with pre-existing domains, are well explained based on the Fatuzzo-Labrune model [1,2]. Full details of the relaxation behavior and subsequent microscopic domain patterns evolving from the pre-existing nucleation sites originated from the sub-structured magnetic domains are discussed. It has been well known that the Fatuzzo-Labrune model is applicable in explaining magnetic relaxation processes involved with a thermally activated magnetization reversal in various magnetic thin films [3,4]. According to the model, the magnetic relaxation is quantied by a reversal parameter k = v/Rr c , where v is the domain wall velocity, R the nucleation rate, and r c the initial domain radius. The shape of the curve and the value of k reflect a competition between domain wall motion and domain nucleation during the magnetic relaxation process. For k >> 1, the magnetization reversal mechanism is known to be dominated by domain wall propagation and the curve has a so-called S-shaped one, while, for k << 1, the reversal mechanism is considered to be dominated by nucleation with the curve of so-called L-shape meaning an exponential decay. However, so far, no study has been addressed to explore the validity of the Fatuzzo-Labrune model to relaxation behavior starting not from the saturated state but from the partially saturated state with non-reversed pre-existing sub-structured domains, which corresponds to the minor loop field cycling. In this work, we explore the validity of the Fatuzzo-Labrune model even for the case of relaxation behavior with pre-existing sub-structured magnetic domains using magneto-optical microscopy, which is capable of quantitatively characterizing magnetization reversal behaviors directly determined from the real-time domain observation. We have investigated magnetic relaxation behaviors of (3.1-Å Co/7.7-Å Pt) 10 and (4-Å CoFeB/10-Å Pd) 4 multilayers with a perpendicular magnetic anisotropy to explore how the reversal mechanism changes itself even for the same sample when beginning with pre-existing nucleation sites. After saturated the films by a very high negative field -H sat about of 1 kOe, the field is switched to a positive holding field +H hold in 5s to let magnetization appear to reach the reversed state of +M S . Then the field is switched again to a negative reversing field -H rev and relaxation curve is monitored. As an example, relaxation processes under a constant negative reversing field of H rev = 220 Oe with variation of holding field H hold , in case of (3.1-Å Co/7.7-Å Pt) 10 film, are presented in Fig. 1(a). With increasing H hold , the relaxation process becomes slower, which is characterized by a decrease of half-reversal time as seen in the inset of Fig....

Currently, hole-doped perovskite-type manganites with a generally chemical formula Ln 1-x A x MnO 3 (Ln 3+ and A 2+ are rare-earth and alkaline-earth ions, respectively) are still of intensive interest due to many intriguing features, typically the colossal magnetoresistance and magnetocaloric effects 1,2) . In order to understand their unusual magnetic and transport properties, many studies have been done through doping on the A site 1,3) . Other research works also carried out the substitution into the Mn site, which dramatically affects the magnetic and transport properties of manganites 4,5) . It has been found that the substitution into the Mn site not only changes the ratio of Mn 3+ /Mn 4+ , but also introduces lattice distortion . In an attempt further understand the nature of magnetic interactions in these compounds, we prepared a polycrystalline sample La 0 .7 Sr 0 .3 Mn 0 .9 Ti 0 .1 O 3 , and then studied carefully their magnetocaloric effect and critical behavior based on analyzing the magnetization versus magnetic-field data, M(H) . Here, La 0 .7 Sr 0 .3 Mn 0 .9 Ti 0 .1 O 3 was prepared by conventional solid-state reaction at 1300 0C for 24 h in air . Magnetic measurements were performed on a superconducting quantum interference device magnetometer . Spin dynamics was also investigated by means of electron spin resonance (ESR) spectroscopy (a JES-TE300) working at 9 .2 GHz . Our studies revealed the inverse susceptibility, χ -1 (T), obtained from magnetization versus temperature data above the Curie temperature (T C ), departing from the linear Curie-Weiss behavior . This is due to the presence of ferromagnetic (FM) clusters in the paramagnetic (PM) region . Assessments based on Banerjee's criteria indicate La 0 .7 Sr 0 .3 Mn 0 .9 Ti 0 .1 O 3 undergoing a second-order magnetic phase transition . Using the modified Arrott plot method and the scaling hypothesis, we have determined the critical parameters T C ≈ 234 K, β = 0 .372±0 .012, γ = 1 .171±0 .112 and δ = 4 .15±0 .01, as shown in Fig . 1 . Comparing with the theoretical models, we have found that β = 0 .372 is close to that expected for the 3D Heisenberg model (β = 0 .365), while γ = 1 .171 is close to the 3D Ising model value (γ = 1 .241) . These results reflect the existence of short-range exchange interactions in our unconventional ferromagnet of La 0 .7 Sr 0 .3 Mn 0 .9 Ti 0 .1 O 3 , which are related to FM clusters and/or magnetic inhomogeneity persisting in a wide temperature range even above T C . To prove the existence of FM clusters above T C , we have studied ESR spectra of La 0 .7 Sr 0 .3 Mn 0 .9 Ti 0 .1 O 3 . The spectra shown in Fig . 2 indicate that asymmetrical signals at low temperatures become Lorentzian at temperatures higher than T min ≈ 1 .3T C . Below T min , two typical peaks are observed . One peak is located at ~2 .5 kOe associated with the FM clusters, and the other at ~3 .5 kOe is associated with the PM phase . The sample is completely PM at temperatures above T min . Clearly, in the temperature range from T C to T min , t...