IEEE TRANSACTIONS ON MAGNETICS, VOL. 50, NO. 11, NOVEMBER 2014
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Magnetoresistance and Resistance Relaxation of Nanostructured La-Ca-MnO Films in Pulsed Magnetic Fields Nerija Žurauskien˙e1, Saulius Baleviˇcius1, Dainius Pavilonis1 , Voitech Stankeviˇc1, Valentina Plaušinaitien˙e1, Sergei Zherlitsyn2 , Thomas Herrmannsdörfer2 , Joseph M. Law2 , and Joachim Wosnitza2 1 Center
for Physical Sciences and Technology, Semiconductor Physics Institute, Vilnius LT-01108, Lithuania Dresden, Helmholtz-Zentrum Dresden-Rossendorf, Dresden D-01314, Germany
2 Hochfeld-Magnetlabor
The results of magnetoresistance (MR) and resistance relaxation of nanostructured La1−x Cax MnO3 films, with different composition x grown by metal–organic chemical vapor deposition technique, are presented and compared with the La0.83 Sr0.17 MnO3 films. The MR was investigated in pulsed magnetic fields up to 60 T in the temperature range 1.5–294 K while the relaxation processes were studied in pulsed fields up to 10 T and temperatures in the range of 80–300 K. It was demonstrated that at low temperatures the MR has higher values in the LCMO films in comparison with the LSMO ones, while at room temperatures, the highest MR values are obtained for the LSMO films. The fast (∼100 µs) and slow (∼ms) resistance relaxation processes were observed after the magnetic field pulse was switched off. It was shown that the fast process could be analyzed using the Kolmogorov–Avrami– Fatuzzo model, considering the reorientation of magnetic domains into their equilibrium state, while the slow process—by the Kohlrausch–Williams–Watts model considering the interaction of the magnetic moments in disordered grain boundaries having spin-glass properties. It was concluded that La1−x Cax MnO3 films having a higher sensitivity and lower memory effects and should be favored for the development of fast pulsed magnetic field sensors operating at low temperatures. Index Terms— Colossal magnetoresistance (CMR), magnetic field sensors, manganites, resistance relaxation processes, thin films.
I. I NTRODUCTION
R
ECENTLY, it was demonstrated that nanostructured (nanograined polycrystalline) La1−x Srx MnO3 (LSMO) films which exhibit the colossal magnetoresistance effect (CMR) can be successfully used for the development of CMR-B-scalar sensors [1], [2]. These small-volume sensors allow for measurements of absolute magnitude of magnetic flux density B, in the time scale of milliseconds, during high magnetic field pulses. They were used at room temperature to measure the magnetic diffusion processes and B-field dynamics during railgun launch [3], [4] and the distribution of transient magnetic fields in non-destructive dual-coil pulsed-field magnets [5]. However, for plasma science, condensed matter physics and other special applications sensors operating at cryogenic temperatures while measuring magnetic fields in a wide range of amplitudes (0.1–100 T) in the time scale of milliseconds or shorter are required. In such cases, it is important to avoid or minimize the magnetic memory effects, which limits the speed of such sensors. It was demonstrated that polycrystalline films reveal high MR values in a wide temperature range down from paramagnetic (PM)-ferromagnetic (FM) phase transition temperature TC to low temperatures. However, the highest MR values usually are obtained close to TC . Therefore, for low temperature applications the La1−x Cax MnO3 (LCMO) films having lower TC in comparison with the LSMO ones might be of greater interest. The memory effects in manganite films are related to magnetization relaxation of these films upon removal or
Manuscript received March 6, 2014; revised April 26, 2014; accepted May 9, 2014. Date of current version November 18, 2014. Corresponding author: N. Žurauskien˙e (e-mail:
[email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMAG.2014.2324895
reversal of the external magnetic field. The numerous investigations of these processes proposed to use different mathematical expressions, which can characterize the time-dependences of the magnetization and the resistance relaxation: logarithmic, power-law-like, exponential (Debye), and stretched or compressed exponential [6]–[9]. Recently, it was demonstrated that the dynamics of resistance relaxation in nanostructured La0.83 Sr0.17 MnO3 films could be analyzed mainly by two models [10]: the Kolmogorov–Avrami–Fatuzzo model [11]–[13], considering the reorientation of the magnetic domains into their equilibrium state and by the Kohlrausch–Williams–Watts [14], [15] model considering the short-range interaction of the magnetic moments in disordered grain boundaries as having spin-glass properties. Therefore, these models could be applicable for the analysis of relaxation processes in LCMO films revealing similar microstructure as with LSMO. In this paper, the results of the investigation of the MR and the resistance relaxation of La1−x Cax MnO3 films are presented and compared with the La0.83 Sr0.17 MnO3 ones. II. E XPERIMENT La1−x Cax MnO3 films having different composition, x, with a thickness of 320 nm were deposited using the pulsedinjection metal–organic chemical vapor deposition technique onto a polycrystalline lucalox (99.9% Al2 O3 + 0.1% MgO) substrate. The reference film of La0.83 Sr0.17 MnO3 , used for comparison, was also grown by this technique on the same substrate. As it was demonstrated for LSMO/lucalox films [1], the change of composition x or deposition temperature has a profound influence on dimensions of crystallites and their clusters, and properties of grain boundaries. The morphology of the LCMO films investigated using atomic force microscopy
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IEEE TRANSACTIONS ON MAGNETICS, VOL. 50, NO. 11, NOVEMBER 2014
also demonstrated the complex structure of these films: crystallites with dimensions of ∼10 nm were cumulated in larger clusters. It was observed that by increasing x, the average dimension of crystallite clusters, D, increases. It could be explained by a possible decrease of growth rate due to the change of Ca/La ratio in the vapor during film deposition. For the films with x = 0.218, 0.296, and 0.41, D was 110, 140, and 210 nm, respectively. For the LSMO film, D was 170 nm. The resistivity, ρ, dependence on temperature was investigated in a closed cycle helium gas cryo-cooler in the temperature range of 5–300 K. It was obtained that the resistivity maximum (ρ0m ) decreases by increasing x and D: 8.2 cm, 5.4 cm, and 2.8 cm. This is a result of larger relative amount of crystallites material having lower conductivity in comparison with the grain boundaries. The transition temperature (Tm ) from metal-like to an insulator-like state increased going from x = 0.41 to x = 0.296 as usually observed in epitaxial films: from 150 to 185 K, respectively. However, the further increase of Tm (220 K) for the film with x = 0.218 is unusual. The comparison with the epitaxial films grown on LaAlO3 substrate during the same deposition process showed that for these two particular x (0.296 and 0.218), the Tm values of epitaxial films were very close. The shift of Tm in polycrystalline films could be the result of possible off stoichiometry resulting in different level of magnetic disorder of the films. For the LSMO film, Tm and ρ0m were found to be 235 K and 1.2 cm, respectively. The MR of the LCMO and LSMO films was investigated in pulsed magnetic fields up to 60 T in the temperature range 1.5–294 K (for setup details see [5]). The resistance relaxation was studied in pulsed magnetic fields up to 10 T in the temperature range 80–300 K (for setup details see [10]). III. R ESULTS AND D ISCUSSION A. MR of Nanostructured LCMO Films The dependence of MR on the magnetic flux density B is shown in Fig. 1 both in the FM (77 K) and PM (294 K) phases for LCMO films with various doping and compared with the LSMO one. The MR is defined as: MR = [ρ(B)/ρ(0) − 1] × 100%, where ρ(B) and ρ(0) correspond to electrical resistivity in magnetic field and without it, respectively. It is evident that the MR at 77 K has higher magnitude values in the LCMO films with larger crystallites (larger x). At 294 K, the highest MR magnitude values are obtained for the LSMO films. Therefore, for the development of magnetic field sensors operating at low temperatures, the LCMO films are preferable. It was demonstrated that a modified Mott’s hopping model [16] could be used to analyze MR of such films in a wide range of magnetic fields by considering the contributions of crystallite clusters and intercluster boundaries (CB). According to this model, the MR in the FM state is supposed to scale with the Brillouin function, B, while in the PM state with B 2 . It was shown that a simple approach where both nanocrystallites (NC) and inter-CB are connected in series could be used for the LSMO films [1]. We applied such model for the LCMO films and analyzed the MR by the sum of the
Fig. 1. MR-magnetic flux density dependences in the (a) FM (77 K) and (b) PM (294 K) phases of LCMO and LSMO films having different composition x. Thin solid curves represent fitting results to the modified Mott’s hopping model: using (1) at 77 K, and using (2) at 294 K.
two contributions MR = f × ANC × B(x NC ) + (1 − f ) × ACB ×B(x CB ) + L F M R MR = f × ANC ×B 2 (x NC )+(1− f )× ACB ×B 2 (x CB).
(1) (2)
Here, (1) and (2) are given for the FM and PM states, respectively, f is the cluster-material fraction, (1− f ) is the intercluster fraction. B(x) is the Brillouin function, x NC(CB) = g×μ B × JNC(CB) ×B/k B ×T is the ratio of magnetic and thermal energy, g is the Lande factor, μ B is the Bohr magneton, k B is the Boltzman constant, B is the magnetic flux density, and T is the temperature. The MR amplitudes ANC and ACB as well as spin-orbit quantum numbers JNC and JCB are treated as fitting parameters. Low-field magnetoresistance (LFMR) was obtained from low-field measurements in magnetic fields aligned parallel to the film plane. Thin solid curves in Fig. 1 show fitting results both for the FM (77 K) and PM (294 K) states. The fitting parameters for the LCMO films were obtained in the following ranges: f × ANC = −(31.3–45.4)%, (1− f ) × ACB = −(44.9–45.7)%, JNC = 7.5–11.8, JCB = 1.8–3.1, LFMR = −(5–7)% at T = 77 K; f × ANC = −(28.8–30.7)%, (1− f ) × ACB = −(52.9–57)%, JNC = 27.2–37.8, JCB = 7.8–8.6 at T = 290 K. For the refence sample of LSMO film, the f × A values are similar, but J values are ∼1.5 times higher. The obtained J values indicate that the clusters of NC and intercluster material in these films behave like a superparamagnet of magnetically aligned polarons [16]–[18].
ŽURAUSKIENË et al.: MR AND RESISTANCE RELAXATION OF NANOSTRUCTURED La-Ca-MnO FILMS
Fig. 2. Magnetic-field pulse (left scale) and resistivity change (right scale) of LCMO film (x = 0.41) during this pulse. Maximal resistivity change and remnant resistivities of fast and slow relaxation processes after the pulse is switched off are indicated by arrows (T = 100 K).
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Fig. 3. Conductivity of the fast relaxation process of LCMO film. The thick black curve shows the experimental results, the light thin curve represents the results when fitted to the KAF model.
B. Resistance Relaxation of Nanostructured LCMO Films The resistance relaxation was investigated after the magnetic field pulse was switched off (Fig. 2). It is observed that the relaxation processes in the LCMO films are similar as in the LSMO ones [10] and take place in three different time scales: ultrafast (
