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Tuning of Al doping on martensitic-like transition in phase separated manganites La0.6Ca0.4Mn1-xAlxO3-δ
Journal of Alloys and Compounds 810 (2019) 151863
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Tuning of Al doping on martensitic-like transition in phase separated manganites La0.6Ca0.4Mn1-xAlxO3-d C. Shang a, *, Z.C. Xia b, **, B. Zhao c, X.Z. Zhai a, D.W. Liu a, Y.Q. Wang a a
School of Physics and Electronics Engineering, Zhengzhou University of Light Industry, Zhengzhou, 450002, China Wuhan National High Magnetic Field Center, Huazhong University of Science and Technology, Wuhan, 430074, China c School of Science, Zhongyuan University of Technology, Zhengzhou, 450007, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 February 2019 Received in revised form 2 July 2019 Accepted 13 August 2019 Available online 13 August 2019
The tuning effects of Al3þ doping on the magnetic and magnetotransport properties of La0$6Ca0$4MnO33þ doping d have been studied by preparing series La0$6Ca0$4Mn1-xAlxO3-d (x ¼ 0, 0.1, 0.125 and 0.15). Al suppresses the ferromagnetic metal phase of La0$6Ca0$4MnO3-d by weakening the double-exchange interaction of Mn3þ-O2--Mn4þ, leading to a phase separation state. Al3þ doping dependent irreversible metamagnetic transitions are observed at lower temperatures, especially a martensitic-like transition appears in x ¼ 0.125 and 0.15 system below 4.2 K. Phase diagrams of La0$6Ca0$4Mn1-xAlxO3d (0.1 x 0.15) in the B-T plane are presented, in which the antiferromagnetic insulator and ferromagnetic metal phases boundary is figured out. The antiferromagnetic matrix and magnetic disorders caused by the random distribution of Al3þ ions and the spontaneous oxygen vacancies grow into a pinning center to pin the Mn moments. As magnetic field is increased to the critical field, the localized magnetic moments rotate to parallel to the magnetic field direction, leading to a martensitic-like transition. © 2019 Elsevier B.V. All rights reserved.
Keywords: Manganite Doping Martensitic-like transition Pinning effect
1. Introduction Perovskite manganites R1-xAxMnO3 (R ¼ trivalent rare-earth elements, A ¼ Ca, Sr, Ba) exhibit a great variety of unusual properties among which the most intriguing is colossal magnetoresistance effect (CMR) [1,2]. The phase separation (PS) scenario manifested by a competitive coexistence of antiferromagnetic (AFM) insulator matrix and ferromagnetic (FM) metal phase plays a significant role in the physics of manganites [3,4]. The PS phenomenon can be manipulated by temperature, magnetic field and doping at Mn sites. In phase separated manganites, step-like metamagnetic transitions associated with the conversion of AFM charge ordered insulating phase to FM metal phase in an applied field at low temperatures have been observed and studied a lot in previous researches [5e8]. A typical system for this study is Pr0.5Ca0$5Mn0$95Co0$05O3, which has been presumed that the AFM charge ordered state coexists with the ferromagnetism in the low
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Z.C. Xia).
(C.
https://doi.org/10.1016/j.jallcom.2019.151863 0925-8388/© 2019 Elsevier B.V. All rights reserved.
Shang),
[email protected]
temperature region [5]. With an application of magnetic field, amazing step-like transitions on the magnetization and resistivity curves were observed at low temperatures, indicating an abrupt AFM insulator to FM metal phase transition. The critical fields of the step-like transitions are depended on the average field sweep rate, which is a martensitic character associated with the strains in the interface of AFM and FM phases [7]. According to double-exchange (DE) mechanism, the magnetic and electrical transport properties are closely correlated in manganites [9]. Among all the CMR manganites, La1-xCaxMnO3 is particularly interesting. The phase diagram of La1-xCaxMnO3 shows that the ground state of the system changes from a FM metallic state for the samples with x ¼ 0.2e0.45 to an AFM charge ordered insulating state for the samples with x ¼ 0.5e0.9 [10]. Mn site substitution with other transition metal elements would create a much more various combinations, which has been quite well documented in recent studies on the tuning of the physical properties of manganites [11e15]. Indeed, doping at Mn sites does not only produce structural modification due to the difference of ionic size but also can change the Mn3þ/Mn4þ ionic ratio, which would strongly affect the exchange interactions among Mn ions including Mn3þ-O2--Mn4þ DE interaction and superexchange (SE)
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interactions of Mn3þ-O2--Mn3þ and Mn4þ-O2--Mn4þ. For example, the dilution of Mn3þ-O2--Mn4þ network with Al3þ and Ti4þ doping results in a suppression of the FM metallic conductivity in La0$7Ca0$3MnO3 [16]. The charge ordering in La0$5Ca0$5MnO3 is melted by Ti4þ substitution, leading to a phase separation phenomenon [17]. In this paper, the tuning effects of Al3þ doping at Mn sites in La0$6Ca0$4MnO3 system have been investigated. In accordance with previous studies, the system undergoes a paramagnetic (PM) to FM phase transition at the Curie temperature (TC) ~ 265 K [18,19]. As the non-magnetic Al3þ ions have no 3d electrons and do not participate in the magnetic exchange interactions, it is expected that Al3þ doping will block the conducting paths at the sites it occupies and partly destroy the Mn3þ-O2--Mn4þ FM coupling, leading to a significant effect on its intrinsic properties. In present work, the measurements of magnetization (M), ac susceptibility (c) and resistivity (r) for La0$6Ca0$4Mn1-xAlxO3-d (x ¼ 0, 0.1, 0.125 and 0.15) were performed over a wide temperature range. The experimental results show that Al3þ doping suppresses the FM metallic state in La0$6Ca0$4MnO3-d and induces a phase separation phenomenon. The magnetization and resistivity isotherms display metamagnetic transitions from the AFM insulator to FM metal phases over a wide temperature range, in which martensitic-like transitions were observed below 4.2 K.
Fig. 1. Room temperature XRD patterns of La0$6Ca0$4Mn1-xAlxO3-d (x ¼ 0, 0.1, 0.125 and 0.15) samples.
2. Experimental procedures La0$6Ca0$4Mn1-xAlxO3 (x ¼ 0, 0.1, 0.125 and 0.15) compounds were prepared by a sol-gel method. Stoichiometric ratio of La(NO3)3.nH2O, Ca(NO3)2$4H2O, Mn(NO3)2 and Al(NO3)3$9H2O were mixed and dissolved in distilled water with a continuous stirring. Citric acid with a ratio of 1:1 to metal nitrates was added as complexing agent. A homogenous wet gel was obtained after evaporating at 100 C and later it was decomposed at 150 C in an oven. The collected powder was calcined at 900 C in air for 10 h, and after grinding the powder was calcined at 1200 C for 12 h. After regrinding again, the powder was pelletized and calcined at 1350 C in air for another 12 h. Powder X-ray diffraction (XRD) patterns were collected to verify the purity of the samples. The Xray photoelectron spectra (XPS) were recorded to estimate the chemical valence state of Mn and Al ions, and the binding energy (B.E.) values were referenced to C 1s peak (284.8 eV). The average valence of manganese in La0$6Ca0$4Mn1-xAlxO3-d was determined by iodometric titration, then oxygen concentration was calculated by assuming no cation vacancy and constant 2 þ and 3 þ valences for calcium and for lanthanum and aluminum ions, respectively. The obtained oxygen stoichiometry is ~2.99, 2.97, 2.92 and 2.90 for x ¼ 0, 0.1, 0.125 and 0.15 samples, respectively. The details of the magnetization and electrical transport measurements were the same as those described in Ref. 15. 3. Results and discussion Fig. 1 displays the room temperature XRD patterns of La0$6Ca0$4Mn1-xAlxO3-d (x ¼ 0, 0.1, 0.125 and 0.15). It confirms that the prepared samples are in an orthorhombic structure with space group Pnma, and there is no any characteristic reflection peak derived from secondary or impurity phase. Fig. 2 shows the XPS of Mn 2p3/2 core level for x ¼ 0, 0.1 and 0.15 samples. The Mn 2p3/2 spectra are fitted into two peaks with binding energy values of 641.5 and 642.8 eV, which are corresponding to the Mn3þ and Mn4þ ionic states, respectively [20,21]. The inset of Fig. 2 shows the XPS of Al 2p core level for x ¼ 0.1 and 0.15 samples. A peak arises at about 75 eV (B E.), indicating the 3 þ valence state of Al [22]. The proportion of Mn3þ and Mn4þ ions in the samples is calculated by the
Fig. 2. X-ray photoelectron spectra of Mn 2p3/2 core level for La0$6Ca0$4Mn1-xAlxO3d (x ¼ 0, 0.1 and 0.15) samples. The spectra are fitted into two peaks corresponding to Mn3þ (open circles) and Mn4þ (open triangles) ions. The continuous line overlapping the observed XPS data (open squares) corresponding to the resulting curve fit. The bottom curve shows Shirley background. Inset shows the XPS spectra of Al 2p for x ¼ 0.1 and 0.15 samples.
areas under Mn3þ and Mn4þ peaks. The ratio values of Mn3þ/Mn4þ (rMn3þ =Mn4þ ) obtained by XPS analysis are about 3/2, 5/3 and 3/1 for x ¼ 0, 0.1 and 0.15 samples, respectively, which are larger than that of nominal compositions La0$6Ca0$4Mn1-xAlxO3
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Fig. 3. Temperature dependence of magnetization measured under various magnetic fields in ZFC (closed symbols) and FCC (open symbols) processes for La0$6Ca0$4Mn1-xAlxO3d with (a) x ¼ 0, (b) x ¼ 0.1, the inset shows the M-T curves measured at a field of 0.005 T in ZFC, FCC and FCW (field-cooled warming) processes, (c) x ¼ 0.125, the inset shows the M-T curves measured at a field of 0.01 T in ZFC, FCC and FCW processes, and (d) x ¼ 0.15, the inset shows the 0.01 T M-T curves for clarity. The arrows indicate the direction of temperature change.
(rMn3þ =Mn4þ ¼ ð0:6 xÞ=0:4). Presence of oxygen vacancies may affect the relative proportion of Mn3þ and Mn4þ ions to maintain the electroneutrality. In several previous publications, it was proposed that a preferential replacement of Mn4þ with increasing Al3þ doping was expected owing to the similar ionic radius of Al3þ (0.535 Å) and Mn4þ (0.530 Å) [23e26]. To preserve the charge
equilibrium, Al3þ doping at Mn sites may produce two effects: either an oxidation from Mn3þ to Mn4þ to keep the oxygen stoichiometry, or the formation of oxygen vacancies in the lattice. Take La2/3Ca1/3Mn1-xAlxO3-d system as an example, it has been confirmed that x 0.05 samples are nominally stoichiometric, and spontaneous oxygen vacancies are presented in x 0.1 samples, which is
Fig. 4. Time dependence of normalized magnetization M(t)/M(t ¼ 0) for La0$6Ca0$4Mn1-xAlxO3-d with (a) x ¼ 0.1, and (b) x ¼ 0.125 recorded at some selected temperatures. In each measurements, the samples were first cooled under zero field from 300 K to the desired temperatures, and then a field of (a) B ¼ 0.005 T, (b) B ¼ 1 T was applied. The time at which the applied field is stable is set as the initial time t ¼ 0.
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Fig. 5. Real component (c0 ) of the ac susceptibility as a function of temperature for x ¼ 0.15 sample measured at various frequencies. The arrow shows the peak shift with frequency. The inset shows the plot of ln(t) vs. Ln [(Tf-Tg)/Tg], and the solid line is the fit to the experimental data using t/t0 ¼ [(Tf-Tg)/Tg]zn.
intrinsic and not depending on the sample preparation [23]. Therefore, it may be the presence of oxygen vacancies that increases the proportion of Mn3þ ions for x ¼ 0.1 and 0.15 systems. Fig. 3 displays the temperature dependence of magnetization for La0$6Ca0$4Mn1-xAlxO3-d (x ¼ 0, 0.1, 0.125 and 0.15) measured in zerofield cooling (ZFC, solid symbol) and field-cooled cooling (FCC, open symbol) processes with applying different magnetic fields. We first discuss the variation of M-T curves with x under low magnetic field. For x ¼ 0 sample, as the temperature decreases, a PM-FM phase transition occurs at TC ¼ 265 K. For x ¼ 0.1 sample as shown in Fig. 3(b), a rapid decrease in TC (83 and 88 K on cooling and warming, respectively) is observed along with a thermal hysteresis. This thermal hysteresis behavior is also observed in x ¼ 0.125
system, but it becomes weaker with increasing Al3þ content. For x ¼ 0.1 sample, after the field cooling process (FCC), the FM phase remains unchanged in the FCW process until to Tb~75 K, above which the magnetization branches off from the cooling path, indicating that the system is blocked in a metastable state with a predominant FM phase at low temperatures. Tb can be identified as a blocking temperature. Similar magnetization behaviors have been seen in x ¼ 0.125 system. For x ¼ 0.1 and 0.125 samples, in the temperature window extending from Tb up to TC, the magnetization shows remarkable relaxation effects as shown in Fig. 4, which indicates a nonquilibrium nature and the growth of FM phase. The relaxation below Tb is significantly reduced. It is apparently that the growth of magnetization with time shows a stretched exponential behavior, implying that the magnetic state at low temperatures is similar to that of glassy system. For x ¼ 0.15 sample as shown in the inset of Fig. 3(d), a cusp-like peak at 41 K, recognized as freezing temperature Tf, arises in the ZFC curve. Subsequently, the FCC curves deviate from the ZFC curve. These features suggest a spinglass like transition occurring at Tf in x ¼ 0.15 system. Such a behavior is a signature of competing FM and AFM phase and, hence, points towards the phase separation state. This inhomogeneous magnetic phase is correlated to the magnetic disorders caused by Al3þ doping at Mn sites and the presence of the unexpected oxygen vacancies. Take the x ¼ 0.15 compound as an example, 15% of Al3þ dopants and 3% of oxygen vacancies may induce a structural disorder, which would lead to a considerable magnetic inhomogeneity. The DE mechanism is a process consisting of a MneMn indirect interaction through oxygen orbitals. Consequently, the ferromagnetic DE interaction through the Mn3þ-O2-Mn4þ bridges will be influenced by the presence of Al3þ dopants and oxygen vacancies. In the regions of high concentration of them, the DE interaction will be weakened. As the applied field is increased, the Al3þ-doped samples behaves as below: (1) the magnetization increases remarkably with increasing applied field, and the thermal hysteresis behavior decreases gradually until to disappear for x ¼ 0.1 and 0.125; (2) TC
Fig. 6. Temperature dependence of resistivity measured under different magnetic fields for La0$6Ca0$4Mn1-xAlxO3-d with (a) x ¼ 0, (b) x ¼ 0.1, (c) x ¼ 0.125 and (d) x ¼ 0.15. The insets display the resistivity as a function of magnetic field at 2 K. The arrows indicate the direction of magnetic field change.
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shifts to higher temperature with increasing magnetic field; (3) Under a high field of 7 T, the bifurcation of the FC cooling and ZFC heating curves almost disappear, and a nearly homogeneous ferromagnetism is achieved at low temperature region. These features suggest that high magnetic field suppresses the relaxation behavior of inhomogeneous phase and contributes to the FM phase. Spin-glass (SG) behavior can be further evidenced by the frequency dependent ac susceptibility. Fig. 5 displays the temperature variation of ac susceptibility for x ¼ 0.15 sample measured at several different frequencies (f ¼ 1e997 Hz) with an ac field of 2.5 Oe. It is obvious that the position of Tf is frequency dependent and shifts towards higher temperatures with an increase in frequency, which is a characteristic feature of SG behavior. The critical slowing-down power law: t/t0 ¼ (Tf/Tg-1)zn, which assumes a true equilibrium phase transition with a divergence of relaxation time near the freezing point, was employed to analyze the spin dynamics of the SG state [27]. Here Tf is the peak temperature measured at frequency f, Tg is the spin-glass phase transition temperature, t0 is the characteristic time scale for spin dynamics (the shortest relaxation time of the system), and zn is the dynamical critical exponent. A scaling of t with the reduced temperature ε ¼ (Tf/Tg-1) is shown in the inset of Fig. 5. The best fit with critical power law gives the value of zn ¼ 9.2, Tg ¼ 41.4 K and t0 ¼ 1.23 1012 s. This short flipping time (1012-1014 s for classical SG compounds) and relative large critical exponent zn indicate the nature typical of SG state in the samples [27]. Fig. 6 shows the temperature dependence of resistivity for La0$6Ca0$4Mn1-xAlxO3-d (x ¼ 0, 0.1, 0.125 and 0.15) measured under different magnetic fields. For the pure sample (x ¼ 0) as illustrated in the r-T curve in zero field, a sharp peak is observed at Tp1 ¼ 265 K accompanied with a very broad transition to metallic behavior at Tp2 ¼ 244 K as the temperature is decreased from room temperature, which is corresponding to and below the only magnetic transition at TC ¼ 265 K. This double maximum electrical transport behavior is ascribed to the disorders induced by the slight oxygen vacancies [28e30]. With applying an external field, the resistivity decreases a lot and the transition temperature of the insulatormetal (I-M) transition shifts towards higher temperature, which results in a MR effect. Results show that the Al3þ doping level and magnetic field have important effects on the electrical transport behavior: (1) the zero-field resistivity increases in several orders of magnitude with Al3þ doping, and even become insulating state for x ¼ 0.125 and 0.15 samples. (2) The insulating state is kept at low magnetic fields, and the field-induced metallic state appears with applying high magnetic field up to 5 T and 10 T for x ¼ 0.125 and 0.15 samples, respectively. The resistivity at 10 T remains a high value for x ¼ 0.15 system, which may be ascribed to the nature of SG state. (3) The I-M transition temperature Tp is decreased significantly with Al3þ doping and shifts towards higher temperatures with increasing the applied field. Since Al3þ is nonmagnetic, and there is no magnetic interaction between Al3þ-O-Mn3þ/4þ, Al3þ doping at Mn sites decreases the mobility of eg electrons, and further suppresses the metallic conduction. Furthermore, according to double exchange theory, oxygen ions are bridges through which the charge carriers transfer. Thus, the oxygen vacancies break the hopping bridges of Mn3þ-O2--Mn4þ for eg electrons. Hence, it is expected that the resistivity increases and Tp shifts towards lower temperatures. To quantify the field-induced I-M transition behavior, the magnetic field dependence of resistivity was measured at 2 K. For pure La0$6Ca0$4MnO3-d, the resistivity decreases continuously with increasing magnetic field, which indicates a pure phase and there is no obvious field-induced phase transition and irreversible phase in the system. For Al3þ-doped samples, an obvious field-induced I-M transition and irreversible phase are observed, and the
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irreversibility increases with increasing Al3þ content. Especially in the x ¼ 0.125 and 0.15 system, with increasing magnetic field, a sharp drop in the resistivity is observed at the critical fields BC at which a field-induced I-M transition occurs. Meanwhile, the hysteresis in x ¼ 0.125 is more pronounced and the resistivity almost keeps a low constant in field decreasing process, and the resistivity of x ¼ 0.15 grows again at a lower critical field when the field begins to decrease at the reverse run but its value is much lower than the original one. These results imply that multi-phases coexist in the Al3þ-doped samples, and phase separation scenario is induced with Al doping in La0$6Ca0$4MnO3-d. Fig. 7 displays the magnetic field dependence of magnetization for La0$6Ca0$4Mn1-xAlxO3-d at 2e4.2 K. The x ¼ 0 and 0.1 samples
Fig. 7. Magnetization of La0$6Ca0$4Mn1-xAlxO3-d as a function of magnetic field at 2e4.2 K with (a) x ¼ 0 and 0.1, (b) x ¼ 0.125, the inset shows the temperature dependence of critical field for x ¼ 0.125 and 0.15 samples, and (c) x ¼ 0.15, the inset shows the isothermal magnetization curves at 2.6 K and 2.7 K measured while stepping from B ¼ 0 Te7 T e 0 T.
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display a normal FM behavior as shown in Fig. 7(a). For x ¼ 0.125 sample, the gradual increase of magnetization with increasing magnetic field indicates an AFM ground state in the system. At the critical field BC, a step-like metamagnetic transition appears at lower temperatures such as 2 K, 3 K and 3.6 K. BC increases with increasing temperature as displayed in the inset of Fig. 7(b). At 4.2 K, the step-like metamagnetic transition changes into a continuous metamagnetic transition. This step-like transition is associated with the field-induced AFM insulator to FM metal phase transition. At 7 T and 2 K, a saturation moment ~91.76 emu/g is obtained, which is very close to the theory saturation moment~92.16 emu/g (calculated by considering only the spin contribution in Mn3þ and Mn4þ ions). The field-induced FM phase is irreversible and kept in the field-decreasing branch and the negative branch unless the sample is warmed into room temperature. For x ¼ 0.15 sample, the step-like metamagnetic transition is also observed, but the critical field is remarkably higher than that of x ¼ 0.125 system. The step-like metamagnetic transition appears at 2.6 K, but it vanishes at 2.7 K, indicating a highly sensitivity to the variation of temperature. Considering the M-B curves at 2 K, the percentage of the metamagnetic phase in x ¼ 0.125 and 0.15 systems is calculated by the jump of the magnetization at critical field BC over the measured saturation magnetization, which is about 68.2% and 38.9% respectively. The results indicate that nonmagnetic Al3þ doping at Mn sites weakens the DE interaction by diluting the magnetic Mn3þ-O2--Mn4þ network, and further suppresses the FM ordering. The presence of oxygen vacancies leads to a larger proportion of Mn3þ ions, which enhances the AFM superexchange interaction of Mn3þ-O2--Mn3þ.
In a canonical martensitic transition, the austenite phase transforms into the martensite phase upon lowering temperature or applying external stress. In manganites, the AFM and FM phases play the roles of austenite and martensite phases, respectively, while applying magnetic field just acts as lowering temperature or increasing stress. Therefore, it has been proposed that the step-like metamagnetic transition observed in phase separated manganites could be well explained as martensitic-like transition driven by the applied field [6,7]. In the framework of such a martensitic scenario, the occurrence of the magnetization step on M(B) curves could be described as follows: at low temperature and under zero field, the AFM (austenite) phase is extremely predominant as indicated by the linearity of the M(B) curves. As the magnetic field is applied, FM domains (martensite) start growing, and the competition develops between the magnetic energy promoting the development of the FM phase and the elastic energy associated with the strains created at the AFM/FM interfaces, which tends to block the transformation [6]. The magnetization step corresponds to a burstlike growth of the FM component when the driving force (here the magnetic field) overcomes the energy barriers associated with the strains. In order to study the dynamical behavior of the martensitic-like transition, we have investigated the influences of changing the average field sweep rate (dB/dt) on the magnetization behavior. Fig. 8 shows the isothermal magnetization curves measured with dB/ dt ¼ 50e200 Oe/s for x ¼ 0.125 and 0.15 system. The samples were heated to room temperature before each measurements. It is obvious that as dB/dt is increased, the step-like metamagnetic transition occurs at lower field. Similar phenomenon was observed in other phase-separated manganites, in which the field sweep rate
Fig. 8. Magnetic field dependence of magnetization measured with different field sweep rate (50e200 Oe/s) for La0$6Ca0$4Mn1-xAlxO3-d with (a) x ¼ 0.125 and (b) x ¼ 0.15.
Fig. 9. Magnetic field dependence of magnetization for La0$6Ca0$4Mn1-xAlxO3-d with (a) x ¼ 0.125 and (b) x ¼ 0.15 under various cooling fields.
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dependence of the step-like metamagnetic transition is regarded as a main feature of martensitic scenario [7]. The strain in the interface of the FM and AFM phases is responsible for the field sweep rate dependence. That is, a smaller dB/dt can facilitate the progressive accommodation of the martensitic strains and push the step to higher field [7]. Since field cooling could change the relative fraction of FM metal and AFM insulator states, field cooling dependent isothermal magnetization measurements were carried out under different cooling fields for x ¼ 0.125 and 0.15 system. As shown in Fig. 9(a), in the platform region, the magnetization increases apparently before the onset of the martensitic-like transition with increasing the applied cooling field, suggesting the increase of the FM fraction at the expenses of AFM matrix at low fields. A cooling field of 4 T is sufficiently high to fully convert the AFM matrix to FM phase for x ¼ 0.125, eliminating the martensitic-like transition. Similar behavior has been seen in Pr0.5Ca0$5Mn0$95Co0$05O3 [5]. In addition, the critical magnetic field of the martensitic-like transition increases with increasing the applied cooling field. Especially for x ¼ 0.15, the critical field is larger than the measured range of the instrument (7 T) in the cooling fields of 1 T and 4 T, so no step-like metamagnetic transition appears in the measurements. A higher applied cooling field could increase the FM fraction and decrease
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the fraction of AFM matrix, so a larger magnetic energy is required to overcome the strains at the interface region [31]. Fig. 10 shows the magnetization (left panels) and resistivity (right panels) isotherms at higher temperatures for x ¼ 0.1, 0.125 and 0.15 samples. The x ¼ 0.1 sample is in a FM metallic state below 85 K, and a low field-induced metamagnetic phase transition is observed at 85e110 K. For x ¼ 0.125 and 0.15 samples, initial magnetization increases linearly with increasing magnetic field accompanied with a sharp metamagnetic transition (that is, an AFM-to-FM) at the corresponding critical field. Meanwhile, the resistivity changes abruptly at the critical field Bþ C in the fieldincreasing branch, implying a field-induced I-M transition. This field-induced metallic state is irreversible and shows up even when the field is reduced to zero at low temperatures (below 40 K) for x ¼ 0.125 sample. While, for x ¼ 0.15 sample, the field-induced metallic state is partly reversible as evidenced by the jump of the resistivity in the field-decreasing branch. The hysteresis in magnetization and resistivity isotherms manifests the first-order nature of the AFM insulator-to-FM metal phase transition [32]. The hysteretic region is critically dependent on temperature and drastically expanded by the lowering of temperature. In order to study the magnetization behavior of x ¼ 0.15 sample in depth, isothermal magnetization measurements were performed
Fig. 10. Magnetization (left panels) and resistivity (right panels) vs. magnetic field isotherms at different constant temperatures for La0$6Ca0$4Mn1-xAlxO3-d with (a) (d) x ¼ 0.1, (b) (e) x ¼ 0.125 and (c) (f) x ¼ 0.15.
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Fig. 11. Magnetic field dependence of magnetization for x ¼ 0.15 sample measured at 4.2 K using pulsed magnetic field. The initial magnetization obtained under static field is shown with crosses. The inset shows the M-B plots measured at various temperatures, and the curves are offset for clarity.
under pulsed high magnetic field at some selected temperatures. Fig. 11 displays the magnetic field dependence of magnetization at 4.2 K. The magnetization was calibrated according to the data measured by SQUID. As the magnetic field is increased, the magnetization increases moderately and then jumps with a sharp metamagnetic transition, reaching a saturated behavior. In the field decreasing branch, the magnetization keeps a high value until to ~5 T. The saturation moment MS determined by extrapolating the M-B curve to zero field is estimated to be~90.00 emu/g, which is close to the theory saturation moment~90.93 emu/g. Therefore, this highly magnetized state can be regard as a fully spin-polarized phase. The inset of Fig. 11 shows the M-B curves recorded at various temperatures. The measurements were carried out after the sample was zero-field cooled from 300 K to the measured temperatures. The jump of the magnetization accompanied with hysteresis is observed below 50 K and vanishes above 77 K. Based on the magnetization and resistivity measurements, the phase diagrams of La0$6Ca0$4Mn1-xAlxO3-d (x ¼ 0.1, 0.125 and 0.15) in
the B-T plane have been constructed and are depicted in Fig. 12. The critical fields of the metamagnetic transition in field-increasing and -decreasing processes are defined as the points where the magnetization (resistivity) deviates from the nearly constant high (low) values as illustrated in the inset of Fig. 10 (b) (Fig. 10(e)), and represented by Bþ C (closed symbols) and BC (open symbols), respectively. It can be seen that the critical fields determined by the resistivity and magnetization isotherms are almost equal to each other. Apparently, the critical fields increase as the Al3þ content is increased due to the disorders induced by Al3þ doping and the spontaneous oxygen vacancies. The hatched area represents the hysteretic region, which suggests a first-order nature of the metamagnetic transitions. The low field zone is a region of phase separation between FM and AFM insulator phases. The high field zone, when the magnetization reaches the saturation behavior, corresponds to a fully FM metal system. The experimental results suggest that Al3þ doping weakens the FM double-exchange coupling postulated by Zener [9], although it still dominates in x ¼ 0.1 system. With increasing Al3þ content, i.e., x ¼ 0.125 and 0.15, an AFM spin ordering arises and coexists with the FM ordering. To get a better understanding of the results, a possible interpretation for the tuning effects of Al3þ doping on the magnetic and magnetotransport properties of La0$6Ca0$4Mn1xAlxO3-d is presented. The pure La0$6Ca0$4MnO3-d system is in a long range FM ordering below TC. When the Mn sublattice is substituted by nonmagnetic Al3þ, the long range FM ordering was broken into short range FM correlation, and an inhomogeneous magnetic system is obtained. That is, the FM coupling between Mn3þ and Mn4þ through the Mn3þ-O2--Mn4þ bridges is destroyed around the doped Al3þ ions because there is no magnetic exchange interaction between Al3þ-O2--Mn3þ/4þ. Meanwhile, the AFM superexchange interaction of Mn3þ-O2--Mn3þ is strengthened, which would decrease the mobility of eg electrons from Mn to Mn through the Mn3þ-O2--Mn4þ bonds, and favor the growing of the pinning center and the rotation of magnetic domains below TC. Moreover, the disorders were induced around the Al3þ dopants and enhanced by the presence of oxygen vacancies, leading to a pinning to the magnetization behavior. Thus, below TC, a coexistence of FM domains of La0$6Ca0$4MnO3-d and a pinning phase related to Al3þ ions is formed in the system. As the magnetic field is increased or the temperature is decreased, the number and size of FM domains
Fig. 12. Phase diagrams in the B-T plane determined by the magnetization and resistivity measurements on La0$6Ca0$4Mn1-xAlxO3-d with (a) x ¼ 0.1, (b) x ¼ 0.125 and (c) x ¼ 0.15. Closed and open symbols represent the critical fields (Bþ C and BC ) of the metamagnetic transitions determined from the field-up and -down sweep, respectively. The hysteretic region is represented by the hatched area. The crosses are the spin freezing temperature Tf determined by the ZFC M-T curves.
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increase. At the critical field, the FM domains orientate along with the magnetic field direction, thus leading to a FM metallic state via a sharp metamagnetic transition. Therefore, considering the pinning effects originated from the magnetic disorders and AFM matrix, the unpinning energy (here is the applied magnetic field) is necessary for a field-induced metamagnetic transition, especially for a martensitic-like transition. As the magnetic field is removed, the localization phase shows a pinning effect on the rotation of FM domains and impedes the FM decoupling due to the relaxation, leading to the system maintain in a higher magnetization and a lower resistivity. 4. Conclusions In summary, the effects of Al3þ doping on the phase evolution and the metamagnetic phase transition in La0$6Ca0$4Mn1-xAlxO33þ d (x ¼ 0, 0.1, 0.125 and 0.15) have been studied in detail. Al -doped samples become magnetically inhomogeneous due to the magnetic disorders caused by Al3þ doping and oxygen vacancies, which results in a phase separation scenario in the system. The irreversible martensitic-like transitions have been observed at very low temperatures for x ¼ 0.125 and 0.15 samples. Phase diagrams of La0$6Ca0$4Mn1-xAlxO3-d (0.1 x 0.15) in the B-T plane have been constructed based on the magnetization and resistivity measurements. Around the doped Al3þ ions, the DE interaction is weakened as a result of the dilution of Mn3þ-O2--Mn4þ networks, and the AFM matrix and the magnetic disorders grow into a pinning center, which shows a considerable pinning effect on the transition of the matrix. Acknowledgements This work was supported by National Natural Science Foundation of China (Grant Nos.: 11804311, 11805295 and 11674115) and Doctoral Fund of Zhengzhou University of Light Industry (Grant No.: 2017BSJJ063). The authors acknowledge the support of Wuhan National High Magnetic Field Center in magnetization measurements at pulsed high magnetic fields. References [1] R. von Helmolt, J. Wecker, B. Holzapfel, L. Schultz, K. Samwer, Giant negative magnetoresistance in perovskitelike La2/3Ba1/3MnOx ferromagnetic films, Phys. Rev. Lett. 71 (1993) 2331e2333. [2] S. Jin, T.H. Tiefel, M. McCormack, R.A. Fastnacht, R. Ramesh, L.H. Chen, Thousandfold change in resistivity in magnetoresistive La-Ca-Mn-O films, Science 264 (1994) 413e415. [3] M. Uehara, S. Mori, C.H. Chen, S.-W. Cheong, Percolative phase separation underlies colossal magnetoresistance in mixed-valent manganites, Nature 399 (1999) 560e563. [4] A. Moreo, S. Yunoki, E. Dagotto, Phase separation scenario for manganese oxides and related materials, Science 283 (1999) 2034e2040. bert, C. Martin, M. Hervieu, B. Raveau, [5] R. Mahendiran, A. Maignan, S. He J.F. Mitchell, P. Schiffer, Ultrasharp magnetization steps in perovskite manganites, Phys. Rev. Lett. 89 (2002) 286602. bert, A. Maignan, C. Martin, M. Hervieu, B. Raveau, Staircase [6] V. Hardy, S. He effect in metamagnetic transitions of charge and orbitally ordered manganites, J. Magn. Magn. Mater. 264 (2003) 183e191. , [7] V. Hardy, S. Majumdar, S.J. Crowe, M.R. Lees, D.McK. Paul, L. Herve bert, C. Martin, C. Yaicle, M. Hervieu, B. Raveau, FieldA. Maignan, S. He induced magnetization steps in intermetallic compounds and manganese oxides: the martensitic scenario, Phys. Rev. B 69 (2004), 020407.
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Tuning of Al doping on martensitic-like transition in phase separated manganites La0.6Ca0.4Mn1-xAlxO3-d C. Shang a, *, Z.C. Xia b, **, B. Zhao c, X.Z. Zhai a, D.W. Liu a, Y.Q. Wang a a
School of Physics and Electronics Engineering, Zhengzhou University of Light Industry, Zhengzhou, 450002, China Wuhan National High Magnetic Field Center, Huazhong University of Science and Technology, Wuhan, 430074, China c School of Science, Zhongyuan University of Technology, Zhengzhou, 450007, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 February 2019 Received in revised form 2 July 2019 Accepted 13 August 2019 Available online 13 August 2019
The tuning effects of Al3þ doping on the magnetic and magnetotransport properties of La0$6Ca0$4MnO33þ doping d have been studied by preparing series La0$6Ca0$4Mn1-xAlxO3-d (x ¼ 0, 0.1, 0.125 and 0.15). Al suppresses the ferromagnetic metal phase of La0$6Ca0$4MnO3-d by weakening the double-exchange interaction of Mn3þ-O2--Mn4þ, leading to a phase separation state. Al3þ doping dependent irreversible metamagnetic transitions are observed at lower temperatures, especially a martensitic-like transition appears in x ¼ 0.125 and 0.15 system below 4.2 K. Phase diagrams of La0$6Ca0$4Mn1-xAlxO3d (0.1 x 0.15) in the B-T plane are presented, in which the antiferromagnetic insulator and ferromagnetic metal phases boundary is figured out. The antiferromagnetic matrix and magnetic disorders caused by the random distribution of Al3þ ions and the spontaneous oxygen vacancies grow into a pinning center to pin the Mn moments. As magnetic field is increased to the critical field, the localized magnetic moments rotate to parallel to the magnetic field direction, leading to a martensitic-like transition. © 2019 Elsevier B.V. All rights reserved.
Keywords: Manganite Doping Martensitic-like transition Pinning effect
1. Introduction Perovskite manganites R1-xAxMnO3 (R ¼ trivalent rare-earth elements, A ¼ Ca, Sr, Ba) exhibit a great variety of unusual properties among which the most intriguing is colossal magnetoresistance effect (CMR) [1,2]. The phase separation (PS) scenario manifested by a competitive coexistence of antiferromagnetic (AFM) insulator matrix and ferromagnetic (FM) metal phase plays a significant role in the physics of manganites [3,4]. The PS phenomenon can be manipulated by temperature, magnetic field and doping at Mn sites. In phase separated manganites, step-like metamagnetic transitions associated with the conversion of AFM charge ordered insulating phase to FM metal phase in an applied field at low temperatures have been observed and studied a lot in previous researches [5e8]. A typical system for this study is Pr0.5Ca0$5Mn0$95Co0$05O3, which has been presumed that the AFM charge ordered state coexists with the ferromagnetism in the low
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Z.C. Xia).
(C.
https://doi.org/10.1016/j.jallcom.2019.151863 0925-8388/© 2019 Elsevier B.V. All rights reserved.
Shang),
[email protected]
temperature region [5]. With an application of magnetic field, amazing step-like transitions on the magnetization and resistivity curves were observed at low temperatures, indicating an abrupt AFM insulator to FM metal phase transition. The critical fields of the step-like transitions are depended on the average field sweep rate, which is a martensitic character associated with the strains in the interface of AFM and FM phases [7]. According to double-exchange (DE) mechanism, the magnetic and electrical transport properties are closely correlated in manganites [9]. Among all the CMR manganites, La1-xCaxMnO3 is particularly interesting. The phase diagram of La1-xCaxMnO3 shows that the ground state of the system changes from a FM metallic state for the samples with x ¼ 0.2e0.45 to an AFM charge ordered insulating state for the samples with x ¼ 0.5e0.9 [10]. Mn site substitution with other transition metal elements would create a much more various combinations, which has been quite well documented in recent studies on the tuning of the physical properties of manganites [11e15]. Indeed, doping at Mn sites does not only produce structural modification due to the difference of ionic size but also can change the Mn3þ/Mn4þ ionic ratio, which would strongly affect the exchange interactions among Mn ions including Mn3þ-O2--Mn4þ DE interaction and superexchange (SE)
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interactions of Mn3þ-O2--Mn3þ and Mn4þ-O2--Mn4þ. For example, the dilution of Mn3þ-O2--Mn4þ network with Al3þ and Ti4þ doping results in a suppression of the FM metallic conductivity in La0$7Ca0$3MnO3 [16]. The charge ordering in La0$5Ca0$5MnO3 is melted by Ti4þ substitution, leading to a phase separation phenomenon [17]. In this paper, the tuning effects of Al3þ doping at Mn sites in La0$6Ca0$4MnO3 system have been investigated. In accordance with previous studies, the system undergoes a paramagnetic (PM) to FM phase transition at the Curie temperature (TC) ~ 265 K [18,19]. As the non-magnetic Al3þ ions have no 3d electrons and do not participate in the magnetic exchange interactions, it is expected that Al3þ doping will block the conducting paths at the sites it occupies and partly destroy the Mn3þ-O2--Mn4þ FM coupling, leading to a significant effect on its intrinsic properties. In present work, the measurements of magnetization (M), ac susceptibility (c) and resistivity (r) for La0$6Ca0$4Mn1-xAlxO3-d (x ¼ 0, 0.1, 0.125 and 0.15) were performed over a wide temperature range. The experimental results show that Al3þ doping suppresses the FM metallic state in La0$6Ca0$4MnO3-d and induces a phase separation phenomenon. The magnetization and resistivity isotherms display metamagnetic transitions from the AFM insulator to FM metal phases over a wide temperature range, in which martensitic-like transitions were observed below 4.2 K.
Fig. 1. Room temperature XRD patterns of La0$6Ca0$4Mn1-xAlxO3-d (x ¼ 0, 0.1, 0.125 and 0.15) samples.
2. Experimental procedures La0$6Ca0$4Mn1-xAlxO3 (x ¼ 0, 0.1, 0.125 and 0.15) compounds were prepared by a sol-gel method. Stoichiometric ratio of La(NO3)3.nH2O, Ca(NO3)2$4H2O, Mn(NO3)2 and Al(NO3)3$9H2O were mixed and dissolved in distilled water with a continuous stirring. Citric acid with a ratio of 1:1 to metal nitrates was added as complexing agent. A homogenous wet gel was obtained after evaporating at 100 C and later it was decomposed at 150 C in an oven. The collected powder was calcined at 900 C in air for 10 h, and after grinding the powder was calcined at 1200 C for 12 h. After regrinding again, the powder was pelletized and calcined at 1350 C in air for another 12 h. Powder X-ray diffraction (XRD) patterns were collected to verify the purity of the samples. The Xray photoelectron spectra (XPS) were recorded to estimate the chemical valence state of Mn and Al ions, and the binding energy (B.E.) values were referenced to C 1s peak (284.8 eV). The average valence of manganese in La0$6Ca0$4Mn1-xAlxO3-d was determined by iodometric titration, then oxygen concentration was calculated by assuming no cation vacancy and constant 2 þ and 3 þ valences for calcium and for lanthanum and aluminum ions, respectively. The obtained oxygen stoichiometry is ~2.99, 2.97, 2.92 and 2.90 for x ¼ 0, 0.1, 0.125 and 0.15 samples, respectively. The details of the magnetization and electrical transport measurements were the same as those described in Ref. 15. 3. Results and discussion Fig. 1 displays the room temperature XRD patterns of La0$6Ca0$4Mn1-xAlxO3-d (x ¼ 0, 0.1, 0.125 and 0.15). It confirms that the prepared samples are in an orthorhombic structure with space group Pnma, and there is no any characteristic reflection peak derived from secondary or impurity phase. Fig. 2 shows the XPS of Mn 2p3/2 core level for x ¼ 0, 0.1 and 0.15 samples. The Mn 2p3/2 spectra are fitted into two peaks with binding energy values of 641.5 and 642.8 eV, which are corresponding to the Mn3þ and Mn4þ ionic states, respectively [20,21]. The inset of Fig. 2 shows the XPS of Al 2p core level for x ¼ 0.1 and 0.15 samples. A peak arises at about 75 eV (B E.), indicating the 3 þ valence state of Al [22]. The proportion of Mn3þ and Mn4þ ions in the samples is calculated by the
Fig. 2. X-ray photoelectron spectra of Mn 2p3/2 core level for La0$6Ca0$4Mn1-xAlxO3d (x ¼ 0, 0.1 and 0.15) samples. The spectra are fitted into two peaks corresponding to Mn3þ (open circles) and Mn4þ (open triangles) ions. The continuous line overlapping the observed XPS data (open squares) corresponding to the resulting curve fit. The bottom curve shows Shirley background. Inset shows the XPS spectra of Al 2p for x ¼ 0.1 and 0.15 samples.
areas under Mn3þ and Mn4þ peaks. The ratio values of Mn3þ/Mn4þ (rMn3þ =Mn4þ ) obtained by XPS analysis are about 3/2, 5/3 and 3/1 for x ¼ 0, 0.1 and 0.15 samples, respectively, which are larger than that of nominal compositions La0$6Ca0$4Mn1-xAlxO3
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3
Fig. 3. Temperature dependence of magnetization measured under various magnetic fields in ZFC (closed symbols) and FCC (open symbols) processes for La0$6Ca0$4Mn1-xAlxO3d with (a) x ¼ 0, (b) x ¼ 0.1, the inset shows the M-T curves measured at a field of 0.005 T in ZFC, FCC and FCW (field-cooled warming) processes, (c) x ¼ 0.125, the inset shows the M-T curves measured at a field of 0.01 T in ZFC, FCC and FCW processes, and (d) x ¼ 0.15, the inset shows the 0.01 T M-T curves for clarity. The arrows indicate the direction of temperature change.
(rMn3þ =Mn4þ ¼ ð0:6 xÞ=0:4). Presence of oxygen vacancies may affect the relative proportion of Mn3þ and Mn4þ ions to maintain the electroneutrality. In several previous publications, it was proposed that a preferential replacement of Mn4þ with increasing Al3þ doping was expected owing to the similar ionic radius of Al3þ (0.535 Å) and Mn4þ (0.530 Å) [23e26]. To preserve the charge
equilibrium, Al3þ doping at Mn sites may produce two effects: either an oxidation from Mn3þ to Mn4þ to keep the oxygen stoichiometry, or the formation of oxygen vacancies in the lattice. Take La2/3Ca1/3Mn1-xAlxO3-d system as an example, it has been confirmed that x 0.05 samples are nominally stoichiometric, and spontaneous oxygen vacancies are presented in x 0.1 samples, which is
Fig. 4. Time dependence of normalized magnetization M(t)/M(t ¼ 0) for La0$6Ca0$4Mn1-xAlxO3-d with (a) x ¼ 0.1, and (b) x ¼ 0.125 recorded at some selected temperatures. In each measurements, the samples were first cooled under zero field from 300 K to the desired temperatures, and then a field of (a) B ¼ 0.005 T, (b) B ¼ 1 T was applied. The time at which the applied field is stable is set as the initial time t ¼ 0.
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Fig. 5. Real component (c0 ) of the ac susceptibility as a function of temperature for x ¼ 0.15 sample measured at various frequencies. The arrow shows the peak shift with frequency. The inset shows the plot of ln(t) vs. Ln [(Tf-Tg)/Tg], and the solid line is the fit to the experimental data using t/t0 ¼ [(Tf-Tg)/Tg]zn.
intrinsic and not depending on the sample preparation [23]. Therefore, it may be the presence of oxygen vacancies that increases the proportion of Mn3þ ions for x ¼ 0.1 and 0.15 systems. Fig. 3 displays the temperature dependence of magnetization for La0$6Ca0$4Mn1-xAlxO3-d (x ¼ 0, 0.1, 0.125 and 0.15) measured in zerofield cooling (ZFC, solid symbol) and field-cooled cooling (FCC, open symbol) processes with applying different magnetic fields. We first discuss the variation of M-T curves with x under low magnetic field. For x ¼ 0 sample, as the temperature decreases, a PM-FM phase transition occurs at TC ¼ 265 K. For x ¼ 0.1 sample as shown in Fig. 3(b), a rapid decrease in TC (83 and 88 K on cooling and warming, respectively) is observed along with a thermal hysteresis. This thermal hysteresis behavior is also observed in x ¼ 0.125
system, but it becomes weaker with increasing Al3þ content. For x ¼ 0.1 sample, after the field cooling process (FCC), the FM phase remains unchanged in the FCW process until to Tb~75 K, above which the magnetization branches off from the cooling path, indicating that the system is blocked in a metastable state with a predominant FM phase at low temperatures. Tb can be identified as a blocking temperature. Similar magnetization behaviors have been seen in x ¼ 0.125 system. For x ¼ 0.1 and 0.125 samples, in the temperature window extending from Tb up to TC, the magnetization shows remarkable relaxation effects as shown in Fig. 4, which indicates a nonquilibrium nature and the growth of FM phase. The relaxation below Tb is significantly reduced. It is apparently that the growth of magnetization with time shows a stretched exponential behavior, implying that the magnetic state at low temperatures is similar to that of glassy system. For x ¼ 0.15 sample as shown in the inset of Fig. 3(d), a cusp-like peak at 41 K, recognized as freezing temperature Tf, arises in the ZFC curve. Subsequently, the FCC curves deviate from the ZFC curve. These features suggest a spinglass like transition occurring at Tf in x ¼ 0.15 system. Such a behavior is a signature of competing FM and AFM phase and, hence, points towards the phase separation state. This inhomogeneous magnetic phase is correlated to the magnetic disorders caused by Al3þ doping at Mn sites and the presence of the unexpected oxygen vacancies. Take the x ¼ 0.15 compound as an example, 15% of Al3þ dopants and 3% of oxygen vacancies may induce a structural disorder, which would lead to a considerable magnetic inhomogeneity. The DE mechanism is a process consisting of a MneMn indirect interaction through oxygen orbitals. Consequently, the ferromagnetic DE interaction through the Mn3þ-O2-Mn4þ bridges will be influenced by the presence of Al3þ dopants and oxygen vacancies. In the regions of high concentration of them, the DE interaction will be weakened. As the applied field is increased, the Al3þ-doped samples behaves as below: (1) the magnetization increases remarkably with increasing applied field, and the thermal hysteresis behavior decreases gradually until to disappear for x ¼ 0.1 and 0.125; (2) TC
Fig. 6. Temperature dependence of resistivity measured under different magnetic fields for La0$6Ca0$4Mn1-xAlxO3-d with (a) x ¼ 0, (b) x ¼ 0.1, (c) x ¼ 0.125 and (d) x ¼ 0.15. The insets display the resistivity as a function of magnetic field at 2 K. The arrows indicate the direction of magnetic field change.
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shifts to higher temperature with increasing magnetic field; (3) Under a high field of 7 T, the bifurcation of the FC cooling and ZFC heating curves almost disappear, and a nearly homogeneous ferromagnetism is achieved at low temperature region. These features suggest that high magnetic field suppresses the relaxation behavior of inhomogeneous phase and contributes to the FM phase. Spin-glass (SG) behavior can be further evidenced by the frequency dependent ac susceptibility. Fig. 5 displays the temperature variation of ac susceptibility for x ¼ 0.15 sample measured at several different frequencies (f ¼ 1e997 Hz) with an ac field of 2.5 Oe. It is obvious that the position of Tf is frequency dependent and shifts towards higher temperatures with an increase in frequency, which is a characteristic feature of SG behavior. The critical slowing-down power law: t/t0 ¼ (Tf/Tg-1)zn, which assumes a true equilibrium phase transition with a divergence of relaxation time near the freezing point, was employed to analyze the spin dynamics of the SG state [27]. Here Tf is the peak temperature measured at frequency f, Tg is the spin-glass phase transition temperature, t0 is the characteristic time scale for spin dynamics (the shortest relaxation time of the system), and zn is the dynamical critical exponent. A scaling of t with the reduced temperature ε ¼ (Tf/Tg-1) is shown in the inset of Fig. 5. The best fit with critical power law gives the value of zn ¼ 9.2, Tg ¼ 41.4 K and t0 ¼ 1.23 1012 s. This short flipping time (1012-1014 s for classical SG compounds) and relative large critical exponent zn indicate the nature typical of SG state in the samples [27]. Fig. 6 shows the temperature dependence of resistivity for La0$6Ca0$4Mn1-xAlxO3-d (x ¼ 0, 0.1, 0.125 and 0.15) measured under different magnetic fields. For the pure sample (x ¼ 0) as illustrated in the r-T curve in zero field, a sharp peak is observed at Tp1 ¼ 265 K accompanied with a very broad transition to metallic behavior at Tp2 ¼ 244 K as the temperature is decreased from room temperature, which is corresponding to and below the only magnetic transition at TC ¼ 265 K. This double maximum electrical transport behavior is ascribed to the disorders induced by the slight oxygen vacancies [28e30]. With applying an external field, the resistivity decreases a lot and the transition temperature of the insulatormetal (I-M) transition shifts towards higher temperature, which results in a MR effect. Results show that the Al3þ doping level and magnetic field have important effects on the electrical transport behavior: (1) the zero-field resistivity increases in several orders of magnitude with Al3þ doping, and even become insulating state for x ¼ 0.125 and 0.15 samples. (2) The insulating state is kept at low magnetic fields, and the field-induced metallic state appears with applying high magnetic field up to 5 T and 10 T for x ¼ 0.125 and 0.15 samples, respectively. The resistivity at 10 T remains a high value for x ¼ 0.15 system, which may be ascribed to the nature of SG state. (3) The I-M transition temperature Tp is decreased significantly with Al3þ doping and shifts towards higher temperatures with increasing the applied field. Since Al3þ is nonmagnetic, and there is no magnetic interaction between Al3þ-O-Mn3þ/4þ, Al3þ doping at Mn sites decreases the mobility of eg electrons, and further suppresses the metallic conduction. Furthermore, according to double exchange theory, oxygen ions are bridges through which the charge carriers transfer. Thus, the oxygen vacancies break the hopping bridges of Mn3þ-O2--Mn4þ for eg electrons. Hence, it is expected that the resistivity increases and Tp shifts towards lower temperatures. To quantify the field-induced I-M transition behavior, the magnetic field dependence of resistivity was measured at 2 K. For pure La0$6Ca0$4MnO3-d, the resistivity decreases continuously with increasing magnetic field, which indicates a pure phase and there is no obvious field-induced phase transition and irreversible phase in the system. For Al3þ-doped samples, an obvious field-induced I-M transition and irreversible phase are observed, and the
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irreversibility increases with increasing Al3þ content. Especially in the x ¼ 0.125 and 0.15 system, with increasing magnetic field, a sharp drop in the resistivity is observed at the critical fields BC at which a field-induced I-M transition occurs. Meanwhile, the hysteresis in x ¼ 0.125 is more pronounced and the resistivity almost keeps a low constant in field decreasing process, and the resistivity of x ¼ 0.15 grows again at a lower critical field when the field begins to decrease at the reverse run but its value is much lower than the original one. These results imply that multi-phases coexist in the Al3þ-doped samples, and phase separation scenario is induced with Al doping in La0$6Ca0$4MnO3-d. Fig. 7 displays the magnetic field dependence of magnetization for La0$6Ca0$4Mn1-xAlxO3-d at 2e4.2 K. The x ¼ 0 and 0.1 samples
Fig. 7. Magnetization of La0$6Ca0$4Mn1-xAlxO3-d as a function of magnetic field at 2e4.2 K with (a) x ¼ 0 and 0.1, (b) x ¼ 0.125, the inset shows the temperature dependence of critical field for x ¼ 0.125 and 0.15 samples, and (c) x ¼ 0.15, the inset shows the isothermal magnetization curves at 2.6 K and 2.7 K measured while stepping from B ¼ 0 Te7 T e 0 T.
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display a normal FM behavior as shown in Fig. 7(a). For x ¼ 0.125 sample, the gradual increase of magnetization with increasing magnetic field indicates an AFM ground state in the system. At the critical field BC, a step-like metamagnetic transition appears at lower temperatures such as 2 K, 3 K and 3.6 K. BC increases with increasing temperature as displayed in the inset of Fig. 7(b). At 4.2 K, the step-like metamagnetic transition changes into a continuous metamagnetic transition. This step-like transition is associated with the field-induced AFM insulator to FM metal phase transition. At 7 T and 2 K, a saturation moment ~91.76 emu/g is obtained, which is very close to the theory saturation moment~92.16 emu/g (calculated by considering only the spin contribution in Mn3þ and Mn4þ ions). The field-induced FM phase is irreversible and kept in the field-decreasing branch and the negative branch unless the sample is warmed into room temperature. For x ¼ 0.15 sample, the step-like metamagnetic transition is also observed, but the critical field is remarkably higher than that of x ¼ 0.125 system. The step-like metamagnetic transition appears at 2.6 K, but it vanishes at 2.7 K, indicating a highly sensitivity to the variation of temperature. Considering the M-B curves at 2 K, the percentage of the metamagnetic phase in x ¼ 0.125 and 0.15 systems is calculated by the jump of the magnetization at critical field BC over the measured saturation magnetization, which is about 68.2% and 38.9% respectively. The results indicate that nonmagnetic Al3þ doping at Mn sites weakens the DE interaction by diluting the magnetic Mn3þ-O2--Mn4þ network, and further suppresses the FM ordering. The presence of oxygen vacancies leads to a larger proportion of Mn3þ ions, which enhances the AFM superexchange interaction of Mn3þ-O2--Mn3þ.
In a canonical martensitic transition, the austenite phase transforms into the martensite phase upon lowering temperature or applying external stress. In manganites, the AFM and FM phases play the roles of austenite and martensite phases, respectively, while applying magnetic field just acts as lowering temperature or increasing stress. Therefore, it has been proposed that the step-like metamagnetic transition observed in phase separated manganites could be well explained as martensitic-like transition driven by the applied field [6,7]. In the framework of such a martensitic scenario, the occurrence of the magnetization step on M(B) curves could be described as follows: at low temperature and under zero field, the AFM (austenite) phase is extremely predominant as indicated by the linearity of the M(B) curves. As the magnetic field is applied, FM domains (martensite) start growing, and the competition develops between the magnetic energy promoting the development of the FM phase and the elastic energy associated with the strains created at the AFM/FM interfaces, which tends to block the transformation [6]. The magnetization step corresponds to a burstlike growth of the FM component when the driving force (here the magnetic field) overcomes the energy barriers associated with the strains. In order to study the dynamical behavior of the martensitic-like transition, we have investigated the influences of changing the average field sweep rate (dB/dt) on the magnetization behavior. Fig. 8 shows the isothermal magnetization curves measured with dB/ dt ¼ 50e200 Oe/s for x ¼ 0.125 and 0.15 system. The samples were heated to room temperature before each measurements. It is obvious that as dB/dt is increased, the step-like metamagnetic transition occurs at lower field. Similar phenomenon was observed in other phase-separated manganites, in which the field sweep rate
Fig. 8. Magnetic field dependence of magnetization measured with different field sweep rate (50e200 Oe/s) for La0$6Ca0$4Mn1-xAlxO3-d with (a) x ¼ 0.125 and (b) x ¼ 0.15.
Fig. 9. Magnetic field dependence of magnetization for La0$6Ca0$4Mn1-xAlxO3-d with (a) x ¼ 0.125 and (b) x ¼ 0.15 under various cooling fields.
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dependence of the step-like metamagnetic transition is regarded as a main feature of martensitic scenario [7]. The strain in the interface of the FM and AFM phases is responsible for the field sweep rate dependence. That is, a smaller dB/dt can facilitate the progressive accommodation of the martensitic strains and push the step to higher field [7]. Since field cooling could change the relative fraction of FM metal and AFM insulator states, field cooling dependent isothermal magnetization measurements were carried out under different cooling fields for x ¼ 0.125 and 0.15 system. As shown in Fig. 9(a), in the platform region, the magnetization increases apparently before the onset of the martensitic-like transition with increasing the applied cooling field, suggesting the increase of the FM fraction at the expenses of AFM matrix at low fields. A cooling field of 4 T is sufficiently high to fully convert the AFM matrix to FM phase for x ¼ 0.125, eliminating the martensitic-like transition. Similar behavior has been seen in Pr0.5Ca0$5Mn0$95Co0$05O3 [5]. In addition, the critical magnetic field of the martensitic-like transition increases with increasing the applied cooling field. Especially for x ¼ 0.15, the critical field is larger than the measured range of the instrument (7 T) in the cooling fields of 1 T and 4 T, so no step-like metamagnetic transition appears in the measurements. A higher applied cooling field could increase the FM fraction and decrease
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the fraction of AFM matrix, so a larger magnetic energy is required to overcome the strains at the interface region [31]. Fig. 10 shows the magnetization (left panels) and resistivity (right panels) isotherms at higher temperatures for x ¼ 0.1, 0.125 and 0.15 samples. The x ¼ 0.1 sample is in a FM metallic state below 85 K, and a low field-induced metamagnetic phase transition is observed at 85e110 K. For x ¼ 0.125 and 0.15 samples, initial magnetization increases linearly with increasing magnetic field accompanied with a sharp metamagnetic transition (that is, an AFM-to-FM) at the corresponding critical field. Meanwhile, the resistivity changes abruptly at the critical field Bþ C in the fieldincreasing branch, implying a field-induced I-M transition. This field-induced metallic state is irreversible and shows up even when the field is reduced to zero at low temperatures (below 40 K) for x ¼ 0.125 sample. While, for x ¼ 0.15 sample, the field-induced metallic state is partly reversible as evidenced by the jump of the resistivity in the field-decreasing branch. The hysteresis in magnetization and resistivity isotherms manifests the first-order nature of the AFM insulator-to-FM metal phase transition [32]. The hysteretic region is critically dependent on temperature and drastically expanded by the lowering of temperature. In order to study the magnetization behavior of x ¼ 0.15 sample in depth, isothermal magnetization measurements were performed
Fig. 10. Magnetization (left panels) and resistivity (right panels) vs. magnetic field isotherms at different constant temperatures for La0$6Ca0$4Mn1-xAlxO3-d with (a) (d) x ¼ 0.1, (b) (e) x ¼ 0.125 and (c) (f) x ¼ 0.15.
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Fig. 11. Magnetic field dependence of magnetization for x ¼ 0.15 sample measured at 4.2 K using pulsed magnetic field. The initial magnetization obtained under static field is shown with crosses. The inset shows the M-B plots measured at various temperatures, and the curves are offset for clarity.
under pulsed high magnetic field at some selected temperatures. Fig. 11 displays the magnetic field dependence of magnetization at 4.2 K. The magnetization was calibrated according to the data measured by SQUID. As the magnetic field is increased, the magnetization increases moderately and then jumps with a sharp metamagnetic transition, reaching a saturated behavior. In the field decreasing branch, the magnetization keeps a high value until to ~5 T. The saturation moment MS determined by extrapolating the M-B curve to zero field is estimated to be~90.00 emu/g, which is close to the theory saturation moment~90.93 emu/g. Therefore, this highly magnetized state can be regard as a fully spin-polarized phase. The inset of Fig. 11 shows the M-B curves recorded at various temperatures. The measurements were carried out after the sample was zero-field cooled from 300 K to the measured temperatures. The jump of the magnetization accompanied with hysteresis is observed below 50 K and vanishes above 77 K. Based on the magnetization and resistivity measurements, the phase diagrams of La0$6Ca0$4Mn1-xAlxO3-d (x ¼ 0.1, 0.125 and 0.15) in
the B-T plane have been constructed and are depicted in Fig. 12. The critical fields of the metamagnetic transition in field-increasing and -decreasing processes are defined as the points where the magnetization (resistivity) deviates from the nearly constant high (low) values as illustrated in the inset of Fig. 10 (b) (Fig. 10(e)), and represented by Bþ C (closed symbols) and BC (open symbols), respectively. It can be seen that the critical fields determined by the resistivity and magnetization isotherms are almost equal to each other. Apparently, the critical fields increase as the Al3þ content is increased due to the disorders induced by Al3þ doping and the spontaneous oxygen vacancies. The hatched area represents the hysteretic region, which suggests a first-order nature of the metamagnetic transitions. The low field zone is a region of phase separation between FM and AFM insulator phases. The high field zone, when the magnetization reaches the saturation behavior, corresponds to a fully FM metal system. The experimental results suggest that Al3þ doping weakens the FM double-exchange coupling postulated by Zener [9], although it still dominates in x ¼ 0.1 system. With increasing Al3þ content, i.e., x ¼ 0.125 and 0.15, an AFM spin ordering arises and coexists with the FM ordering. To get a better understanding of the results, a possible interpretation for the tuning effects of Al3þ doping on the magnetic and magnetotransport properties of La0$6Ca0$4Mn1xAlxO3-d is presented. The pure La0$6Ca0$4MnO3-d system is in a long range FM ordering below TC. When the Mn sublattice is substituted by nonmagnetic Al3þ, the long range FM ordering was broken into short range FM correlation, and an inhomogeneous magnetic system is obtained. That is, the FM coupling between Mn3þ and Mn4þ through the Mn3þ-O2--Mn4þ bridges is destroyed around the doped Al3þ ions because there is no magnetic exchange interaction between Al3þ-O2--Mn3þ/4þ. Meanwhile, the AFM superexchange interaction of Mn3þ-O2--Mn3þ is strengthened, which would decrease the mobility of eg electrons from Mn to Mn through the Mn3þ-O2--Mn4þ bonds, and favor the growing of the pinning center and the rotation of magnetic domains below TC. Moreover, the disorders were induced around the Al3þ dopants and enhanced by the presence of oxygen vacancies, leading to a pinning to the magnetization behavior. Thus, below TC, a coexistence of FM domains of La0$6Ca0$4MnO3-d and a pinning phase related to Al3þ ions is formed in the system. As the magnetic field is increased or the temperature is decreased, the number and size of FM domains
Fig. 12. Phase diagrams in the B-T plane determined by the magnetization and resistivity measurements on La0$6Ca0$4Mn1-xAlxO3-d with (a) x ¼ 0.1, (b) x ¼ 0.125 and (c) x ¼ 0.15. Closed and open symbols represent the critical fields (Bþ C and BC ) of the metamagnetic transitions determined from the field-up and -down sweep, respectively. The hysteretic region is represented by the hatched area. The crosses are the spin freezing temperature Tf determined by the ZFC M-T curves.
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increase. At the critical field, the FM domains orientate along with the magnetic field direction, thus leading to a FM metallic state via a sharp metamagnetic transition. Therefore, considering the pinning effects originated from the magnetic disorders and AFM matrix, the unpinning energy (here is the applied magnetic field) is necessary for a field-induced metamagnetic transition, especially for a martensitic-like transition. As the magnetic field is removed, the localization phase shows a pinning effect on the rotation of FM domains and impedes the FM decoupling due to the relaxation, leading to the system maintain in a higher magnetization and a lower resistivity. 4. Conclusions In summary, the effects of Al3þ doping on the phase evolution and the metamagnetic phase transition in La0$6Ca0$4Mn1-xAlxO33þ d (x ¼ 0, 0.1, 0.125 and 0.15) have been studied in detail. Al -doped samples become magnetically inhomogeneous due to the magnetic disorders caused by Al3þ doping and oxygen vacancies, which results in a phase separation scenario in the system. The irreversible martensitic-like transitions have been observed at very low temperatures for x ¼ 0.125 and 0.15 samples. Phase diagrams of La0$6Ca0$4Mn1-xAlxO3-d (0.1 x 0.15) in the B-T plane have been constructed based on the magnetization and resistivity measurements. Around the doped Al3þ ions, the DE interaction is weakened as a result of the dilution of Mn3þ-O2--Mn4þ networks, and the AFM matrix and the magnetic disorders grow into a pinning center, which shows a considerable pinning effect on the transition of the matrix. Acknowledgements This work was supported by National Natural Science Foundation of China (Grant Nos.: 11804311, 11805295 and 11674115) and Doctoral Fund of Zhengzhou University of Light Industry (Grant No.: 2017BSJJ063). The authors acknowledge the support of Wuhan National High Magnetic Field Center in magnetization measurements at pulsed high magnetic fields. References [1] R. von Helmolt, J. Wecker, B. Holzapfel, L. Schultz, K. Samwer, Giant negative magnetoresistance in perovskitelike La2/3Ba1/3MnOx ferromagnetic films, Phys. Rev. Lett. 71 (1993) 2331e2333. [2] S. Jin, T.H. Tiefel, M. McCormack, R.A. Fastnacht, R. Ramesh, L.H. Chen, Thousandfold change in resistivity in magnetoresistive La-Ca-Mn-O films, Science 264 (1994) 413e415. [3] M. Uehara, S. Mori, C.H. Chen, S.-W. Cheong, Percolative phase separation underlies colossal magnetoresistance in mixed-valent manganites, Nature 399 (1999) 560e563. [4] A. Moreo, S. Yunoki, E. Dagotto, Phase separation scenario for manganese oxides and related materials, Science 283 (1999) 2034e2040. bert, C. Martin, M. Hervieu, B. Raveau, [5] R. Mahendiran, A. Maignan, S. He J.F. Mitchell, P. Schiffer, Ultrasharp magnetization steps in perovskite manganites, Phys. Rev. Lett. 89 (2002) 286602. bert, A. Maignan, C. Martin, M. Hervieu, B. Raveau, Staircase [6] V. Hardy, S. He effect in metamagnetic transitions of charge and orbitally ordered manganites, J. Magn. Magn. Mater. 264 (2003) 183e191. , [7] V. Hardy, S. Majumdar, S.J. Crowe, M.R. Lees, D.McK. Paul, L. Herve bert, C. Martin, C. Yaicle, M. Hervieu, B. Raveau, FieldA. Maignan, S. He induced magnetization steps in intermetallic compounds and manganese oxides: the martensitic scenario, Phys. Rev. B 69 (2004), 020407.
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