Chap. 4 Results and Discussions

a. Persistent Magnetoresistive Memory Effect (PMME) on LPCMO thin film

(a)Anisotropy of Persistent Mangetoresistive Memory Effect (PMME)

In order to delineate the intrinsic characteristic of the epitaxial thin films, whether the anisotropy of applied magnetic field existed deserved to be confirmed. The experiment procedure is as following: we first cooled the sample to T= 174 K, the middle point of the R(T) hysteresis in zero applied field, at a constant rate of 3 K/ min; next, after waiting for 90 minutes for the resistance to relax, a small magnetic field of 200Oe was applied for another 90 minutes; then the magnetic field was removed and completed as

one 180-minute cycle. Same cycles were repeated with larger and larger magnetic field up to 1 Tesla. Fig. 26(a) shows the results for in-plane applied field. Notice that below a field denoted as Hth the relaxation is descending while above that an ascending relaxation is evident. (See the circles shown in Fig. 26(a) and the enlarged version shown in Fig.

26(b).) For fields applied perpendicular to the film surface, similar behaviors were observed, except that Hth in this case is much larger (~1T) as compared to the in-plane field (Hth~0.1T).

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Fig.26 (a) The process of H-in-plane experiment with time evolution is illustrated. The drop of the resistance caused by magnetic field represents normal MR effect. We marked Hth as a threshold magnetic field that reversed the resistance relaxation tendency.

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Fig.26 (b) The enlarged diagram illustrates the reversed tendency of resistance relaxation after removal of Hth.

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Fig.27 (a) The process of H-out-of-plane experiment with time evolution is illustrated.

We discovered a larger Hth as shown.

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Fig.27 (b) The enlarged diagram illustrates the reversed tendency of resistance relaxation after removal of Hth.

The anisotropy of the change and hence the subsequent relaxation of the resistance is believed to be related to the degree of magnetization in the sample. Therefore, we’ve measured the magnetization-H (M(H)) dependence at various temperatures. The results are summarized in Fig. 28. From diagrams displayed in Fig. 28, especially at low temperature (e.g. T= 10 K), we can discern the easy axis parallel to H in plane by clear hysteresis loop. It is reasonable now to infer that the different Hth exhibited in the two applied directions is having the origin of the intrinsic anisotropy of our thin films. In the remaining of our discussion, we will focus on the results obtained for field applied in-plane.

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Fig. 28(a) By applying H in plane, the hysteresis of MH dependence is shown. At lower temperature, the M saturation is larger along with a clearer hysteresis.

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Fig. 28(b) By applying H out of plane, we can find no hysteresis behavior within same magnitude region of H. The lack of hysteresis indicates that when H is applied out of plane, it is hard-magnetization-direction for our sample.

(b)The Quenched Hth and The “Melting”Magnetic Filed Hm

It is notable that PMME only occurs at certain temperature with a cooling scheme.

Upon cooling, the FM phase might not have enough time to reach its equilibrium distribution. Thus, when field H is applied, it may enhance the growth of FM phase and even drive it to exceed the equilibrium proportion. It is under such premise that we can conduct PMME experiment and observe the reverse tendency of resistance relaxation after after removing H. However, if we first cooled LPCMO thin film to low temperature, say T= 10K, the proportion of FM phase at low temperature, though still may not reach its equilibrium proportion, is much larger than that at higher temperatures. Upon warming, due to transformation lagging, FM phase continuously exceeded the

equilibrium proportion at certain temperature. In that case, we will not be able to observe the reverse tendency of resistance relaxation, and naturally we cannot define Hth—the Hth is said to be quenched. The results at different temperatures at which R(T) displays hysteresis namely from T=160 K to T=180 K are shown in Fig.29 (a) (T= 174 K), Fig.

30 (a) (T= 177K), and Fig. 31 (a)(T= 180K, inset shows the enlarged vision).

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H(Oe)R(ohm)

Time(sec) T= 174 K

H_m

Fig.29 (a) At T = 174 K, as the time evolved, the resistance tended to increase clearly.

The noticeable sudden drop represented a strong MR effect, and the Hm is denoted.

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R(ohm)

Time(sec)

H = 4000 Oe

Fig.29 (b) When H < Hm , the relaxation tendency still increased.

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R(ohm)

Time(sec)

H = 1 tesla

Fig.29 (c) When H > Hm, the relaxation tendency decreased.

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H(Oe) R(ohm)

Time(sec) T= 177 K

Hm

Fig.30 (a) At T= 177 K, the sudden drop occurred with a smaller Hm. Below Hm the resistance relaxation displays an ascending tendency.

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R(ohm)

Time(sec) H= 1000 Oe

Fig. 30 (b) When H < Hm , the relaxation tendency still increased.

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R(ohm)

Time(sec)

H = 5000 Oe

Fig.30 (c) When H > Hm , the relaxation tendency decreased.

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Fig. 31 (b) When H > Hm , the relaxation tendency decreases.

Interestingly, at different temperatures, we needed to apply certain magnetic field which is strong enough to “melt” the CO phase. As just mentioned above, the FM phase always exceeded its equilibrium proportion upon the warming scheme. On the other hand, the CO phase continuously grows to reach its equilibrium proportion upon

warming protocol. Therefore, we need a strong H, namely Hm, to suppress the growth of CO phase or to “melt” the grown CO phases. This is how we define Hm: we observed the relaxation tendency of resistance while the applied magnetic field was on. With larger and larger H, the tendency changed from ascending to descending. When the tendency just reversed its sign (shown in Fig. 29(c), Fig. 30(c), and Fig.31(b)), the threshold H was defined as Hm.

To compare the trend of Hm at various temperatures in this experiment, we tabulated our data in Table 2. Recalling the resistance-temperature dependence shown in Fig. 15, we picked these three temperatures in the hysteresis region: at T= 174 K, the CO phases were still robust and far from its equilibrium proportion, so we needed larger magnetic force (Hm ~1T) to melt the CO phase; at T = 177 K, two phases compete with each other and CO phase is less robust, therefore smaller magnetic field (Hm ~0.5T) is needed to melt CO phase; at T= 180 K, because the CO phase is closer to its equilibrium proportion and is more vulnerable, thus, only a rather small magnetic filed (Hm ~0.1T) is needed to disturb the distribution of CO phase.

T = 174 K T = 177 K T = 180 K

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Table 2. The required H_m is smaller with increasing temperature.

b.Time-relaxation Data at Various Temperatures

In this study we tried to attain the metastable resistance-temperature dependence by utilizing PMME. Nevertheless, at temperature higher than T_CO (~200 K), the LPCMO film should be a single phase state. For this reason, we conducted PMME at T= 250 K (Fig. 32) and T = 210 K (Fig. 33).

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Time(sec) T= 250 K

H(Oe) R(Time)/R 250K

Fig.32 (a) At T = 250 K, even after the largest drop of resistance with H = 9 tesla, the resistance rejuvenated and kept the same value with time evolving.

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Fig.32 (b) This enlarged diagram reveals the constant value of the resistance before (blue open circles) and after (red open circles) a large applied H.

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0.0 resistance still rejuvenated and kept the same value with time evolving, similar to the situation at T= 250 K.

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Fig. 33 (b) This enlarged diagram reveals the constant value of the resistance before (blue open circles) and after (red open circles) a large applied H.

Based on the data displayed in Fig. 32 and Fig. 33, the absence of resistance relaxation after the removal of the applied field, indicates that the system is in a single phase regime.

In this sense the hysteresis of the electric transport properties described by the PS-model seems to require that there should be coexistence and competition between the

short-range CO phase and short-range FM phase for observing PMME. In order to further delineate the point, we carried out PMME experiments over a much wider temperature range (from 10 K to 190 K) instead of just in the hysteresis region (160 K <

T < 180 K).

Fig. 34 shows the PMME at T= 190 K. Except for a much larger Hth (~2T), the

resistance relaxation behaves similarly as previously described. It is suggestive that even at this temperature where noticeable hysteresis is still quite vague the PS has started to emerge. The ratio of the resistance as a function of time (R(T)) and the resistance at T=

190 K was enlarged in fig. 34 (b) to reveal clearer view of relaxation tendency.

Fig. 35, Fig. 36 and Fig. 37 display the PMME in the hysteric region, which is supposed to have the most pronounced effect. As expected, the modulation of resistance was huge and clear with a comparably small Hth = 1000 Oe. The pronounced response to a small perturbation of applied magnetic field, strongly suggest that in the hysterical region, the newly formed CO and FM phases are relative by vulnerable and a small perturbation can affect the competition between the phases.

Fig. 38, Fig.39 and Fig. 40, on the other hand, demonstrate the PMME in the LT region. In this region, generally CO phase and FM phase are both in long-range scale and are both more robust. The magnetic force must be stronger to disturb this metastable state. As we can see in table 3, the Hth at T=80 K is 2 tesla while the Hth at T= 50 K and T= 10 K is 3 tesla. The magnitude order is same as at T = 190 K and one-order larger than Hth in the hysteresis region.

Above T_CO Below T_CO In the hysteresis region

Low temperature region

T(K) 250 210 190 177 174 167 80 50 10

Hth(Tesla) None None 2 0.1 0.1 0.1 2 3 3

Table 3. The H_th at all temperatures are listed. The H_max applied at T= 210 K and T = 250 K was 9 tesla, and we still cannot find the PMME phenomena; therefore we cannot define H_th at these two temperatures.

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T= 190 K H(Oe) R(Time)/R190K

Time(sec)

H_th

Fig. 34 (a) The PMME experiment at T= 190 K. Notice the sudden drop of resistance accompanied with H_th.

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Fig.34 (b) The enlarged plot clarifies the decreasing and increasing relaxation tendency.

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