Enhancing Low Field Magnetoresistance in La0.7Ca0.25Sr0.05MnO3 /Mn3O4 Composite Nanoparticles: Unveiling its Transport Mechanism

doi:10.21203/rs.3.rs-5049998/v1

The low-field magnetoresistance of La0.7Ca0.25Sr0.05MnO3/Mn3O4 composite nanoparticles with an average particle size range of 28 to 32 nm is currently being investigated. These nanoparticles were synthesized using the sol-gel method, with sintering temperatures varying from 700°C to 900°C in 100°C intervals. The crystal structure, surface morphology, and electrical transport properties were analyzed. Rietveld refinement confirms the presence of a distorted monoclinic structure [ S.G. P21/c, COD:1525829] and a spinel structure in Mn3O4 [S.G. I41/amd, COD: 1514115]. As the sintering temperature increases, the crystallite and particle sizes of La0.7Ca0.25Sr0.05MnO3/Mn3O4 composite increase, with a corresponding increase in the Mn3O4 fraction. The temperature-dependent resistivity shows a magnetic transition from ferromagnetic-metallic to paramagnetic-insulator behavior with a transition temperature 170 K. The lowest resistivity, observed in 30 nm particles with 19% of Mn3O4, is attributed to the lowest grain boundary contributions and the smallest activation energy required for electron to hop between localized states. The low-field magnetoresistance of La0.7Ca0.25Sr0.05MnO3/Mn3O4 composite nanoparticles reaches up to 30% when the particle size is reduced to 28 nm with 17% of Mn3O4 by applying a magnetic field of 5 kOe at 5 K. This LFMR value is higher than that of previous La0.7Ca0.3MnO3/Mn3O4 composite nanocrystallites and is comparable to pure La0.7Ca0.3MnO3. The role of the insulating Mn3O4 phase and nanoparticle size in enhancing LFMR is discussed herein.

Composite

Electrical transport properties

Insulating phase

Low-field magnetoresistance

Nanoparticle

Sol-gel method

• TEM shows particle sizes increasing from 28 nm to 32 nm with sintering temperatures from 700–900°C.

• XRD confirms Mn₃O₄ phase increases as LCSMO phase decreases, verifying LCSMO/Mn₃O₄ formation.

• at 180 K for 30 nm with 19% Mn₃O₄; lowest resistivity due to enhanced conduction bandwidth.

• 30 nm with 19% Mn₃O₄ has the largest conduction bandwidth due to optimal bond length and bond angle.

• The nanosized effect and the addition of the insulator MnO to LCSMO cause an improvement in LFMR.

Extensive research has been conducted to investigate the phenomenon of colossal magnetoresistance (CMR) in the rare-earth manganese perovskite, La_1-xA_xMnO₃ (A = Ca, Sr) [1–8]. CMR in La_1-xA_xMnO₃ (A = Ca, Sr) is characterized by large magnetoresistance observed under moderate (H > 10 kOe) magnetic fields near the ferromagnetic-paramagnetic transition temperature, T_c. The intrinsic CMR originates from the double-exchange (DE) interaction, with significant values of CMR observed in single crystals [9]. Meanwhile, the extrinsic properties of the rare-earth manganese La_1-xA_xMnO₃ (A = Ca, Sr) observed under relatively low magnetic fields, which appeared only in polycrystalline form (both bulk and nanocrystalline), are recognized as low-field magnetoresistance (LFMR)[10], [11]. LFMR emerges in a relatively low magnetic field and is commonly observed far below the ferromagnetic-paramagnetic transition temperature, T_c [11], primarily attributed to the influence of the grain boundary and interfaces between the grains of the sample[7]. LFMR is an interesting phenomenon due to its potential for practical applications, such as magnetic sensor technology, which can detect variations in resistivity in response to small changes in magnetic fields[12].

In the La_1-xCa_xMnO₃ systems, the appearance of LFMR reaches approximately 22–32%, particularly notable in nanoparticle sizes from 25 to 50 nm[13–15]. LFMR is more pronounced in the nanocrystalline form than in the bulk form due to the significant contribution of grain boundaries and the inter-grain distances in the nanocrystalline form[15]. Aside from reducing the size to the nanoscale, another method to increase LFMR is by adding an insulator phase to manganite perovskite to create a composite [10], [14], [16–19]. Adding an insulator to the manganite perovskite promotes disruption of spin alignment within the grain boundary region, leading to spin disorder. These disordered spins tend to align with even small applied magnetic fields, thereby enhancing low-field magnetoresistance [20]. One potential insulator that can be utilized in perovskite manganese-insulator composite is Mn₃O₄, particularly within the La_1-xCa_xMnO₃ system, due to its capacity to enhance LFMR[10], [17]. Despite this benefit, the LFMR value observed in La_1-xCa_xMnO₃/Mn₃O₄ composite nanocrystalline remains inferior to that of pure La_1-xCa_xMnO₃ nanocrystalline, showing only 16% in [17] compared to 22% in [13] and 32% in [15]. Thus, the LFMR of La_1-xCa_xMnO₃/Mn₃O₄ composite still needs enhancement.

Concerning reaching high LFMR and increasing ferromagnetic-paramagnetic transition temperature, T_c, Sr is considered the best element to be doped into La_1-xCa_xMnO₃, as it exhibits a magnetoresistance ratio of up to 17% when exposed to a magnetic field of 1 T [5]. Furthermore, the ferromagnetic-paramagnetic transition temperature, T_c, for La_1-xSr_xMnO₃ is 370 K [21] while T_c for La_1-xCa_xMnO₃ is 260 K [22]. Therefore, Sr-doping in the La/Ca site could increase the ferromagnetic-paramagnetic transition temperature, T_c. As reported in [23–24], small Sr-doping in La_0.7Ca_0.25Sr_0.05MnO₃ can enhance T_c up to 275 K. Accordingly, introducing Sr to the La/Ca site in the La_1-xCa_xMnO₃/Mn₃O₄ nanocrystalline composite system is suggested to enhance the LFMR effect at the lowest possible external magnetic field and increase the ferromagnetic-paramagnetic transition temperature, T_c.

Although it is commonly acknowledged that the grain boundaries play a crucial role, as observed in La_1-xSr_xMnO₃/Glass composites[18], La_1-xCa_xMnO₃/Mn₃O₄ composites [10], [17], and La_1-xSr_xMnO₃/Manganite oxide composites [25], and that charge carrier mobility is suggested to influence LFMR in LCMO:xSiCN composites[19], the mechanisms underlying the transport properties of lanthanum manganite composites with the addition of insulating phases are still poorly understood and require further investigation. The percolation model is employed to elucidate the parameters that contribute to the transport mechanism in this complex composite system.

This work aims to investigate the effects of the insulator phase Mn₃O₄ and particle size reduction on the nanoscale on LFMR in La_0.7Ca_0.25Sr_0.05MnO₃/Mn₃O₄ (LCSMO/Mn₃O₄) composite nanoparticles. The investigation involves varying the sintering temperatures within the range of 700°C to 900°C, at intervals of 100°C, to achieve systematically different amounts of the insulator phase Mn₃O₄ and different nanoparticle sizes. LCSMO/Mn₃O₄ composites were synthesized by the sol-gel method to ensure sample homogeneity and to offer precise control over the stoichiometry of the samples[26–30]. The dependence of particle size on nanoparticle size and the insulator phase Mn₃O₄ on the crystal structure, transport properties, and LFMR is discussed herein.

The LCSMO/Mn₃O₄ composite nanoparticles were synthesized using the sol-gel method. Stoichiometric amounts of La₂O₃ (Merck, $\:\ge\:$99.95%), CaCO₃ (Merck, $\:\ge\:$99.95%), Sr(NO₃)₂ (Merck, $\:\ge\:$99.0%), and Mn(NO₃)₂.4H₂O (Merck, $\:\ge\:$98.5%), were used as starting precursors. La₂O₃ and CaCO₃ were dissolved separately in nitric acid (Merck, concentration of 65%) until homogeneous and transparent solutions were achieved. These solutions were then mixed, followed by the addition of Sr(NO₃)₂ and Mn(NO₃)₂·4H₂O. Citric acid monohydrate (Merck, $\:\ge\:$ 99.5%) was added to the solution as a catalyst, with a molar ratio of citric acid to total metal ions of 1.0. After achieving a homogenous transparent solution, ammonia solution (Merck, concentration of 25%) was introduced to adjust the pH to 7.0, as the catalyst is active and fastens the reaction at this pH. The solution was then heated on a hotplate at 80°C for six hours until a gel formed. The resulting gel was dried at 100°C for twelve hours to remove all water components, followed by calcination at 300°C for three hours to eliminate residual organic components. Finally, the sintering process was performed at three distinct temperatures, i.e., 700, 800, and 900°C for three hours to modify the particle size.

The sample quality, phase, and crystal structural properties were examined using X-ray diffraction (XRD) with a SMARTLAB RIGAKU system with Cu-Kα radiation, conducted at room temperature. The XRD pattern results were refined using the Rietveld refinement method with FULLPROF software[31]. The morphology and size of the particles were evaluated using transmission electron microscopy (TEM) with a Tecnai G2 20S-Twin Function, performed at room temperature. TEM characterizations were conducted by taking ten acquisitions from different areas of the sample to ascertain the distribution of particle sizes. The electrical resistivity and magnetoresistance were measured by a four-point probe (FPP) technique with a cryogenic magnet (Oxford Teslatron Instrument), in a temperature range of 5 to 300 K. To investigate its electrical resistivity and magnetoresistance properties, the powder samples were compacted into 10 mm diameter pellets, which were then heated at 500°C for three hours to ensure the inter-grains were well-connected.

The TEM images of LCSMO/Mn₃O₄ composite nanoparticles for samples sintered at temperatures of 700^oC, 800^oC, and 900^oC are depicted in Fig. 1(a), (b), and (c), respectively. All samples contain some agglomerated particles that overlap with each other. Additionally, the particle size distribution of each sample is presented in Fig. 1(d), (e), and (f), with black lines representing the fitting of the log-normal particle size distribution. It shows that the particle size is distributed up to 60 nm for all samples. The average particle size is 28 nm, 30 nm, and 32 nm for the samples sintered at 700^oC, 800^oC, and 900^oC, respectively. The TEM analyses confirmed that increasing sintering temperatures increased particle sizes, as in previous results [15], [32].

The XRD patterns of LCSMO/Mn₃O₄ composite nanoparticles at different sintering temperatures are presented in Fig. 3. The major peaks of all samples match those of monoclinic La_0.85Ca_0.15MnO₃ (COD no: 1525829 [33]), indicating that Sr has doped into the La/Ca site. The remaining peaks, represented by blue circles, correspond to tetragonal Mn₃O₄ (COD no: 1514115 [34]), confirming the formation of the LCSMO/Mn₃O₄ composite phase in all samples. Rietveld refinement analysis confirmed that the fraction of the LCSMO phase is 82.73%, 80.80%, and 76.80% for sintering temperatures of 700^oC, 800^oC, and 900^oC, respectively. Meanwhile, the fraction of the Mn₃O₄ phase is 17.27%, 19.20%, and 23.10% for the samples sintered at 700^oC, 800^oC, and 900^oC, respectively. This indicates that increasing the sintering temperature results in an increased fraction of the Mn₃O₄ composite phase. The increase in Mn₃O₄ composite fraction is likely due to interdiffusion between the LCSMO and Mn₃O₄ phases. The thermal energy from the sintering temperature might affect the mobility of ions in adjacent phases, facilitating diffusion across phase interface[10].

Figures 4 (a) and (b) illustrate the schematic crystal structures of LCSMO and Mn₃O₄, respectively. The La, Ca/Sr, Mn, and O atoms are represented by green, blue, purple, and red spheres. The LCSMO sample sintered at 800°C exhibits the largest lattice constants, a and b, along with the smallest lattice constant, c, resulting in the maximum volume of the unit cell. The d-spacing at the highest peak (002) is 0.27304, 0.27314, and 0.27314 nm for sintering temperatures 700, 800, and 900^oC, respectively. It is closely matching with the d-spacing estimated from TEM measurement. In the case of Mn-O bond length, notable alterations are observed in the Mn(1)-O(2) and Mn(2)-O(3), along with significant changes in the Mn(1)-O(2)-Mn(1) and Mn(2)-O(3)-Mn(2) bond angles. For the Mn₃O₄ phase, the sample sintered at 800^oC also shows the highest lattice constants a, b, and c, leading to the highest volume of the unit cell. This indicates that the volume of the unit cell in both LCSMO and Mn₃O₄ phases is highest in the sample sintered at 800^oC. The goodness of fit (GoF) value for all samples ranges between 1.50 and 1.90, suggesting good agreement between the fitting model and the data. The structural parameters, including lattice constants, volume of the unit cell, Mn-O bond lengths, and Mn-O-Mn bond angles for LCSMO and Mn₃O₄, are summarized in Table 1.

Table 1

Rietveld refinement crystallographic parameter of La_0.7Ca_0.25Sr_0.05MnO₃/Mn₃O₄ composite nanoparticles
Crystallographic parameters	Reference La_0.85Ca_0.15MnO₃ COD: 1525829 [33]	La_0.7Ca_0.25Sr_0.05MnO₃
Crystallographic parameters	Reference La_0.85Ca_0.15MnO₃ COD: 1525829 [33]	T_Sintering= 700^oC	T_Sintering= 800^oC	T_Sintering= 900^oC
Crystal structure	Monoclinic	Monoclinic	Monoclinic	Monoclinic
Space group	P 2₁/c	P 2₁/c	P 2₁/c	P 2₁/c
a (Å)	7.745	7.750	7.754	7.751
b (Å)	5.504	5.510	5.508	5.509
c (Å)	5.474	5.461	5.463	5.462
α = γ	90	90	90	90
β	90.090	90.51	90.30	90.50
V (Å³)	233.35	233.14	233.31	233.18
d-spacing at (002) (nm)	0.273675	0.27304	027314	0.27314
Fraction (%)		82.73	80.70	76.80
Bond length (Å)
Mn(1)-O(1)	1.941	1.857	2.016	2.048
Mn(1)-O(2)	1.912	1.920	2.083	1.834
Mn(1)-O(2)	2.013	2.048	1.837	2.084
Mn(2)-O(1)	1.994	2.005	1.926	1.888
Mn(2)-O(3)	1.938	1.916	2.136	1.937
Mn(2)-O(3)	1.991	2.008	1.769	1.998
Bond angle (°)
Mn(1)-O(1)-Mn(2)	159.500	161.717	159.146	159.994
Mn(1)-O(2)-Mn(1)	162.900	166.412	163.470	163.814
Mn(2)-O(3)-Mn(2)	162.200	162.421	166.653	160.543
Crystallographic parameter	Reference		Mn₃O₄
Crystallographic parameter	Mn₃O₄ COD:1514115 [34]	T_Sintering= 700^oC	T_Sintering= 800^oC	T_Sintering= 900^oC
Crystal structure	Tetragonal	Tetragonal	Tetragonal	Tetragonal
Space group	I41/amd	I41/amd	I41/amd	I41/amd
a (Å)	5.765	5.756	5.757	5.756
b (Å)	5.765	5.756	5.757	5.756
c (Å)	9.442	9.462	9.467	9.466
α = γ = β	90	90	90	90
V (Å³)	313.81	313.47	313.80	313.60
Fraction (%)		17.27	19.20	23.10
Reliability factors
R_wp		14.6	16.3	16
χ²		1.57	1.89	1.87

The crystallite size was determined from the XRD patterns using the Debye-Scherrer formula described in Eq. (1). In this equation, D represents the crystallite size, $\:K$ is a crystallite shape factor with a value of 0.9, $\:\beta\:$ is the full width at half maximum (FWHM), and $\:\lambda\:$ is the X-ray wavelength ($\:\lambda\:\:$= 1.5428 Å). The crystallite sizes are 32 nm, 33 nm, and 34 nm for the samples sintered at temperature of 700^oC, 800^oC, and 900^oC, respectively. Table 2 presents the crystallite size estimated from XRD characterization and the particle size obtained from TEM measurements for LCSMO/Mn₃O₄ composite nanoparticles. It confirms that the higher sintering temperature produced bigger particle and crystallite size.

$$\:D=\frac{K\lambda\:}{\beta\:cos\theta\:}$$

1

Table 2

Crystallite size and particle size of La_0.7Ca_0.25Sr_0.05MnO₃/Mn₃O₄ composite nanoparticles.
	La_0.7Ca_0.25Sr_0.05MnO₃/Mn₃O₄
	T_Sintering= 700^oC	T_Sintering= 800^oC		T_Sintering= 900^oC
Crystallite size (By XRD)	32 nm	33 nm	34 nm
Particle size (By TEM)	28 nm	30 nm	32 nm

Figures 5 (a) and (b) display the temperature dependence of resistivity for LCSMO/Mn₃O₄ composite nanoparticles, both without and with a 20 kOe applied magnetic field. Particle size and fraction of Mn₃O₄ are shown in the legend of the figure to illustrate their effects on the transport properties of this system. Ferromagnetic-metallic behavior is observed below 170 K, followed by paramagnetic-insulator behavior above 170 K. The temperature at which this transition occurs is defined as the metal-insulator transition temperature, T_MI. The T_MI is determined by selecting the temperature at which the resistivity reaches the maximum value. Without the applied magnetic field, T_MI is 172 K, 180 K, and 172 K for particle sizes of 28 nm with 17% Mn₃O₄, 30 nm with 19% Mn₃O₄, and 32 nm with 23% Mn₃O₄, respectively. Under the applied magnetic field of 20 kOe, T_MI is 184 K, 182 K, and 183 K for particle sizes of 28 nm, 30 nm, and 32 nm, respectively. It is inferred that without the magnetic field, T_MI is highest for the 30 nm particle size with 19% Mn₃O₄, while T_MI for the other particle sizes is the same. However, T_MI has no difference among all samples under the applied magnetic field in 20 kOe. In addition, the 28 nm particle size with 17% Mn₃O₄ exhibits the highest resistivity, both with and without an applied magnetic field.

The shift in T_MI to the higher temperature region and the change in resistivity suggests that the amount of Mn₃O₄ fraction disrupts electron transport in this system. The change in the Mn₃O₄ fraction might alter the ratio of Mn³⁺ to Mn⁴⁺ ions, affecting the double exchange (DE) interaction. This interaction involves electron hopping from a Mn³⁺ ion to neighboring Mn⁴⁺ ion via an O^2- ion[35]. The DE interaction relies on the oxygen 2p-orbital and the Mn-3d orbitals, making electron hopping in perovskite manganite sensitive to the conduction bandwidth, W (a.u.)[36] as described in Eq. (2), where $\:{\theta\:}_{Mn-O-Mn}$ and $\:{d}_{Mn-O}$ are the bond angle and bond distance of Mn-O-Mn, respectively[36].

$$\:W\propto\:\frac{cos\left[180-\left({\theta\:}_{Mn-O-Mn}\right)\right]}{{d}_{Mn-O}^{3.5}}$$

2

The conduction bandwidth is 0.0918, 0.0938, and 0.0905 (a.u.) for particle sizes of 28 nm with 17% Mn₃O₄, 30 nm with 19% Mn₃O₄, and 32 nm with 23% Mn₃O₄, respectively. This result suggests that the enhancement in the conduction bandwidth causes a reduction in resistivity and T_MI shifts towards the higher temperature region. In this regard, the 30 nm sample with 19% Mn₃O₄ exhibits the lowest resistivity due to the appearance of the highest conduction bandwidth that originates from the shortest Mn-O bond distance compared to other samples and Mn-O-Mn bond angle closer to 180°, facilitating easier electron hopping.

The upper left inset of Fig. 5 (a) and (b) shows an increase in resistivity in the low-temperature region, specifically below 50 K. The temperature at which the resistivity starts to increase in this region is referred to as T_min. T_min is about 50 K without an applied magnetic field and about 40 K with an applied magnetic field of 20 kOe. The increase in resistivity is more distinct in smaller particle sizes than in larger ones. The increase in resistivity at low temperatures likely originates from the Kondo-like scattering [37], [38]. However, further investigation about the Kondo effect in this system is needed.

To better comprehend the complex transport mechanism influenced by particle size reduction and variations in the Mn₃O₄ fraction, the resistivity is fitted using the percolation model. The resistivity is analyzed using Eq. (3) over the temperature range. The detailed derivation of the percolation model has been explained elsewhere [37–42]. In this model, the various contributions to resistivity are represented as follows: $\:{\rho\:}_{0}$ represents the contribution from grain boundaries, $\:{\rho\:}_{e}\:$represents the contribution from electron-electron interaction, $\:{\rho\:}_{s}$ represents the contribution from Kondo-like spin-dependent scattering, $\:{\rho\:}_{p}$ represents the contribution from electron-phonon interaction, $\:{\rho\:}_{2}$ represents the contribution from electron-electron scattering, $\:{\rho\:}_{\raisebox{1ex}{$9$}\!\left/\:\!\raisebox{-1ex}{$2$}\right.}$represents the contribution due to the combination of electron-electron, electron-magnon, and electron-phonon scattering, and $\:{\rho\:}_{\alpha\:}$represents the contribution from residual resistivity. T denotes temperature, U₀ represents the energy difference below $\:{T}_{C}^{mod}$ or between the ferromagnetic-metallic and paramagnetic-insulator phases, $\:{T}_{C}^{mod}$ stands for the theoretical Curie temperature, E_A denotes activation energy, and k_B denotes the Boltzmann constant. The percolation fitting function formula for the temperature dependence of resistivity is applied to the experimental data depicted in Fig. 6. The black solid lines represent the best-fitting results. In contrast, the colored markers represent the experimental data of three samples. Fitting parameters are summarized in Table 3.

$$\:\rho\:\left(T\right)=\:\left[{\rho\:}_{\alpha\:}Texp\left(\frac{{E}_{A}}{{k}_{B}T}\right)\frac{exp\left(\frac{-{U}_{0}\left(1-\frac{T}{{T}_{c}^{mod}}\right)}{{k}_{B}T}\right)}{1+exp\left(\frac{-{U}_{0}\left(1-\frac{T}{{T}_{c}^{mod}}\right)}{{k}_{B}T}\right)}\right]\left[{\rho\:}_{0}+{\rho\:}_{e}{T}^{\raisebox{1ex}{$1$}\!\left/\:\!\raisebox{-1ex}{$2$}\right.}-{\rho\:}_{s}\text{ln}T+{\rho\:}_{p}{T}^{5}+{\rho\:}_{2}{T}^{2}+{\rho\:}_{\raisebox{1ex}{$9$}\!\left/\:\!\raisebox{-1ex}{$2$}\right.}{T}^{\raisebox{1ex}{$9$}\!\left/\:\!\raisebox{-1ex}{$2$}\right.}\left(\frac{1}{1+exp\left(\frac{-{U}_{0}\left(1-\frac{T}{{T}_{c}^{mod}}\right)}{{k}_{B}T}\right)}\right)\right]\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\left(3\right)$$

3

Table 3
The best fitting parameters obtained from percolation model for La_0.7Ca_0.25Sr_0.05MnO₃/Mn₃O₄ composite nanoparticles.
Fitting parameters	La_0.7Ca_0.25Sr_0.05MnO₃/Mn₃O₄
Fitting parameters	28 nm with 17% of Mn₃O₄	30 nm with 19% of Mn₃O₄	32 nm with 23% of Mn₃O₄
$\:{\rho\:}_{0}$ (Ωm)	46.13	8.32	18.38
$\:{\rho\:}_{e}$ (ΩmK^-1/2)	0.11	0.12	1.42
$\:{\rho\:}_{s}$ (Ωm)	5.59	1.08	1.18
$\:{\rho\:}_{p}$ (×10^–10 ΩmK^-5)	42.74	3.14	54.23
$\:{\rho\:}_{2}$ (×10^− 4 ΩmK^-2)	14.80	1.79	7.59
$\:{\rho\:}_{9/2}$ (× 10^− 9 ΩmK^-2)	57.24	4.37	2.47
$\:{\rho\:}_{\alpha\:}$ (Ωm)	0.21	0.23	0.22
U₀/k_B (K)	1246.3	1880.6	1165.7
E_A/k_B (K)	335.9	52.4	303.4
$\:{T}_{c}^{mod}$ (K)	270.0	275.7	272.3
R² (%)	99.92	99.95	99.90

Among all fitting parameters, the contribution from grain boundaries is the most dominant compared to other parameters. The predominant role of grain boundaries in resistivity arises from electron tunneling between grains, as described by the core-shell model. According to this model, electron motion is significantly affected by the inter-grain distances, where the reduction in the inter-grain distances facilitates an enhancement of electrical conductivity[37–41]. Grain boundary contribution, $\:{\rho\:}_{0}$ is 46.13, 8.32, and 18.38 Ωm for particle sizes of 28 nm with 17% of Mn₃O₄, 30 nm with 19% of Mn₃O₄, and 32 nm with 23% of Mn₃O₄, respectively. The sample with a particle size of 30 nm with 19% of Mn₃O₄ has the lowest grain boundary contribution compared to other samples. The lower grain boundary contribution enhances inter-grain electron movement, leading to the lowest electrical resistivity in this sample. The grain boundary contribution has no linear relationship with particle size.

The 30 nm sample with 19% of Mn₃O₄ has the smallest activation energy (E_A/k_B), six times smaller than other samples. This lower activation energy means less energy is required for electron to hop between localized states [43]. The smallest activation energy observed in the 30 nm sample with 19% of Mn₃O₄ compared to the other samples is likely due to the larger conduction bandwidth. The conduction bandwidth significantly influences the double exchange (DE) interaction, with a larger bandwidth strengthening the DE interaction. This stronger DE interaction reduces the energy required for electrons to hop between localized states, resulting in lower activation energy [42]. Thus, the small activation energy in the 30 nm sample with 19% of Mn₃O₄ aligns with its large conduction bandwidth, where a larger bandwidth leads to lower activation energy.

It shows that the 30 nm sample with 19% of Mn₃O₄ exhibits superior electron conduction compared to the other samples due to the lowest contribution from grain boundaries and the smallest activation energy. The graph for the grain boundary contribution and activation energy (E_A/k_B) for all particle sizes in La_0.7Ca_0.25Sr_0.05MnO₃/Mn₃O₄ composite nanoparticles shows in Fig. 7 (a) and (b), respectively. In addition, Table 3 shows that the second-highest contribution to resistivity is from Kondo-like scattering, which increases as particle size decreases. This is consistent with the results in the upper left inset of Fig. 5 (a) and (b), where the increase in resistivity at low temperatures is more distinct for smaller particle.

Figure 8 shows the magnetic field dependence of magnetoresistance for the LCSMO/Mn₃O₄ composite nanoparticles. The magnetoresistance effect is calculated by using Eq. (4), where ρ(H) represents resistivity under an applied magnetic field and ρ(0) represents resistivity without an applied magnetic field.

$$\:MR\left(\%\right)=\frac{\rho\:\left(H\right)-\rho\:\left(0\right)}{\rho\:\left(0\right)}100\%$$

4

It demonstrates that the resistance decreases when applying a magnetic field below 5 kOe, indicating that LFMR is observed in this system only with a low magnetic field application. The magnetoresistance observed when applying a low magnetic field is called the LFMR phenomenon. The LFMR value of the LCSMO/Mn₃O₄ composite nanoparticles with a magnetic field of 5 kOe at 10 K increased from 25–27% by reducing the particle size from 32 nm to 30 nm. The LFMR value even reach up to 30% at magnetic field of 5 kOe at 5 K for particle size 28 nm. This value is higher than that reported by Vertruyen et al. in 2007 for La_0.7Ca_0.3MnO₃/Mn₃O₄ composite, which reached only 13% with a magnetic field of 5 kOe at 10 K [10] and comparable with 22‒28% MR ratio with a magnetic field of 5 kOe at 20 K by Vertruyen et al. in 2005 [13]. At 40 K, the LFMR value of the LCSMO/Mn₃O₄ composite nanoparticles is approximately 23%, still higher than that for pure LCMO, 22% MR Ratio with magnetic field of 3 kOe at 30 K by D. H. Manh. et al. 2010 [14].

If we compared LFMR ratio for our LCSMO/Mn₃O₄ at 100K for 5 kOe to La_0.7Ca_0.3MnO₃/Mn₃O₄ composite by Bhame et al. that shows 16% MR ratio at 77 K for 5 kOe[17] and LCMO by Gaur et al. 2010 that MR ratio reach up to 28% at 10 kOe in 80 K[15], our LFMR ratio of LCSMO/Mn₃O₄ composite nanoparticles is also still comparable that reached up to 20%. Lastly, at 170 K, all samples exhibit an MR ratio of approximately 10%. It can be concluded that reducing particle size into nanoparticle size and adding the insulator composite Mn₃O₄ to LCSMO to become LCSMO/Mn₃O₄ composite shows the tendency to improve LFMR. This supports our hypothesis that lowering the sintering temperature produces particle size that reduces to nanoparticle size and adding the insulator composite can enhance the low-field magnetoresistance (LFMR). The MR ratio for La_0.7Ca_0.25Sr_0.05MnO₃/Mn₃O₄ composite nanoparticles and the reference is summarized in Table 4.

For a clear view, we plot the temperature dependence of the MR ratio 5 kOe and MR at 20 kOe for all the particle sizes of LCSMO/Mn₃O₄ composite nanoparticles in Fig. 9. The temperature dependence of the magnetoresistance ratio with a magnetic field of 5 kOe and 20 kOe for LCSMO/Mn₃O₄ composite nanoparticles shows that the MR ratio decreases as the temperature increases for all particle sizes. This occurrence is expected due to the thermal energy disrupting the alignment of the magnetic moment, leading to a weakening of the MR ratio. The MR ratio with a magnetic field of 20 kOe is always higher than that of 5 kOe at all temperatures. This is because the higher magnetic field induced more spin alignment, resulting in a more significant decrease in resistivity so that the MR ratio increases[41]. In the case of particle size dependence, there is no significant difference in MR ratio with applied field 5 kOe in the range of particle size 28 until 32 nm below 170 K. The similar behavior also occurred when the applied field is 20 kOe.

Table 4
Comparison of MR ratio for La_0.7Ca_0.25Sr_0.05MnO₃/Mn₃O₄ composite nanoparticles from this study and reference data.
Magnetoresistance ratio
Present study			Reference data
28 nm	30 nm	32 nm	Reference data
30% (5 kOe at 5 K)	27% (5 kOe at 10 K)	25% (5 kOe at 10 K)	13% (5 kOe at 10 K) La_0.7Ca_0.3MnO₃/Mn₃O₄ [10]
30% (5 kOe at 5 K)	27% (5 kOe at 10 K)	25% (5 kOe at 10 K)	22‒28% (5 kOe at 20 K) La_0.7Ca_0.3MnO₃[13]
23% (5 kOe at 40 K)	24% (5 kOe at 40 K)	23% (5 kOe at 40 K)	22% (3 kOe at 30 K) La_0.7Ca_0.3MnO₃ with particle size of 16 nm [14]
17% (5 kOe at 100 K)	20% (5 kOe at 100 K)	15% (5 kOe at 100 K)	16% (5 kOe at 77 K) La_0.7Ca_0.3MnO₃/Mn₃O₄ with crystallite size of 35 nm [17]
17% (5 kOe at 100 K)	20% (5 kOe at 100 K)	15% (5 kOe at 100 K)	28% (10 kOe at 80 K) La_2/3Ca_1/3MnO₃ with particle size of 25 nm[15]
9% (5 kOe at 170 K)	9% (5 kOe at 170 K)	10% (5 kOe at 170 K)

The crystal structure, surface morphology, electrical resistivity, and LFMR properties of La_0.7C_0.25S_0.05MnO₃/Mn₃O₄ composite nanoparticles synthesized by the sol-gel method have been investigated. We observed a distorted monoclinic crystal structure with the largest volume of the unit cell at a sintering temperature of 800^oC, corresponding to particles of 30 nm with a 19% of Mn₃O₄ fraction. As the sintering temperature increases, the particle size increases from 28 nm to 32 nm, while the Mn₃O₄ fraction increases from 17–23%. The presence of Mn₃O₄ affects the Mn-O bond distance and bond angle, which in turn influences the conduction bandwidth, W (a.u.). The increase in the conduction bandwidth strengthens the double exchange (DE) interaction, resulting in a reduction in resistivity. Grain boundary contribution and activation energy also played crucial roles in the electron transport mechanism. An upturn in resistivity at low temperatures is likely originating from the Kondo-like scattering effect. However, further research is required to explore the Kondo effect in this system. The nanosized effect and the addition of the insulator Mn₃O₄ to La_0.7C_0.25S_0.05MnO₃ cause an improvement in LFMR.

Author contributions:

All authors contributed to the writing of the manuscript. Conceptualization was performed by Jumaeda Jatmika and Suci Winarsih. Data collection was carried out by Jumaeda Jatmika, Suci Winarsih, and Agung Imaduddin. Methodology and formal analysis were conducted by Jumaeda Jatmika and Suci Winarsih. Data visualization, writing, and original draft preparation were done by Jumaeda Jatmika. Project administration and supervision were handled by Suci Winarsih. Review and editing were completed by Jumaeda Jatmika, Suci Winarsih, and Risdiana. All authors read and validated the final manuscript.

Funding

This work was supported by Riset dan Inovasi untuk Indonesia Maju (RIIM) Research Grant (Contract Nos. B-3842/II.7.5/FR.06.00/11/2023 and B-3855/III.10/FR.06.00/11/2023).
Jumaeda Jatmika is supported by the National Research and Innovation Agency (BRIN) through Manajemen Talenta Post-doctoral Scheme (No. 86/II/HK/2023).
Jumaeda Jatmika and Suci Winarsih supported by the Rumah Program Nanoteknologi dan Material, National Research and Innovation Agency (BRIN) (No. 20/III.10/HK/2024).
The authors acknowledge XRD, TEM, and HRTEM characterization facilities at National Research and Innovation Agency (BRIN), E-Layanan Sains.

Compliance with Ethical Standards

Conflict of interest The authors declare no conflict of interest.

M. Pissas, G. Kallias, M. Hofmann, and D. M. Többens, “Crystal and magnetic structure of the La1-xCaxMnO3 compound (x = 0.8, 0.85),” Phys Rev B Condens Matter Mater Phys, vol. 65, no. 6, pp. 1–9, 2002, doi: 10.1103/PhysRevB.65.064413.
M. Pissas, I. Margiolaki, G. Papavassiliou, D. Stamopoulos, and D. Argyriou, “Crystal and magnetic structure of the La1-xCaxMnO3 compound (0.11 ≤ x ≤ 0.175),” Phys Rev B Condens Matter Mater Phys, vol. 72, no. 6, p. 064425, Aug. 2005, doi: 10.1103/PhysRevB.72.064425.
A. Szewczyk, M. Gutowska, B. Dabrowski, T. Plackowski, N. P. Danilova, and Y. P. Gaidukov, “Specific heat anomalies in La1-xSrxMnO3 (0.12 ≤ x ≤ 0.2),” Phys Rev B, vol. 71, no. 22, p. 224432, Jun. 2005, doi: 10.1103/PhysRevB.71.224432.
G. J. Snyder, R. Hiskes, and S. DiCarolis, “Intrinsic electrical transport and magnetic properties La0.67Ca0.33MnO3 and La0.67Sr0.33MnO3 MOCVD thin films and bulk material,” Phys Rev B Condens Matter Mater Phys, vol. 53, no. 21, pp. 14434–14444, Jun. 1996, doi: 10.1103/PhysRevB.53.14434.
F. Ji et al., “Effect of sintering temperature on room-temperature electrical and magnetic properties of La0.625(Ca0.315Sr0.06)MnO3 polycrystalline ceramics,” Mater Res Express, vol. 6, no. 8, p. 086326, Jun. 2019, doi: 10.1088/2053-1591/ab2512.
L. C. Rave-Osorio, V. Londoño-Calderón, J. Restrepo, O. Arnache, and E. Restrepo-Parra, “Structural and Hysteretic Properties of La0.7Ca0.3 – xSrxMnO3 Manganites Using the Hydrothermal Route,” J Supercond Nov Magn, vol. 32, no. 3, pp. 571–582, Mar. 2019, doi: 10.1007/s10948-018-4684-x.
H. Y. Hwang, S. W. Cheong, N. P. Ong, and B. Batlogg, “Spin-polarized intergrain tunneling in La2/3Sr1/3MnO3,” Phys Rev Lett, vol. 77, no. 10, pp. 2041–2044, Sep. 1996, doi: 10.1103/PhysRevLett.77.2041.
K. Das, P. Dasgupta, A. Poddar, and I. Das, “Significant enhancement of magnetoresistance with the reduction of particle size in nanometer scale,” Sci Rep, vol. 6, Feb. 2016, doi: 10.1038/srep20351.
T. M. Tank, A. Bodhaye, Y. M. Mukovskii, and S. P. Sanyal, “Crystallographic direction dependence of electrical-transport, magneto-transport, magnetic and thermal properties of La0.7Ca0.3MnO3 single crystal,” Mater Res Bull, vol. 83, pp. 250–258, Nov. 2016, doi: 10.1016/j.materresbull.2016.06.003.
B. Vertruyen, J.-F. Fagnard, Ph. Vanderbemden, M. Ausloos, A. Rulmont, and R. Cloots, “Electrical transport and magnetic properties of Mn3O4-La0.7Ca0.3MnO3 ceramic composites prepared by a one-step spray-drying technique,” J Eur Ceram Soc, vol. 27, no. 13–15, pp. 3923–3926, Jan. 2007, doi: 10.1016/j.jeurceramsoc.2007.02.061.
L. Balcells, B. Martı́nez, F. Sandiumenge, and J. Fontcuberta, “Magnetotransport properties of nanometric La2/3Sr1/3MnO3 granular perovskites,” J Magn Magn Mater, vol. 211, no. 1–3, pp. 193–199, Mar. 2000, doi: 10.1016/S0304-8853(99)00733-7.
T. Venkatesan, M. Rajeswari, Z. W. Dong, S. B. Ogale, and R. Ramesh, “Manganite-based devices: Opportunities, bottlenecks and challenges,” Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, vol. 356, no. 1742, pp. 1661–1680, Jul. 1998, doi: 10.1098/rsta.1998.0240.
B. Vertruyen et al., “Low-field magnetoresistance in La0.7Ca0.3MnO3 manganite compounds prepared by the spray drying technique,” J Mater Sci, vol. 40, no. 1, pp. 117–122, Jan. 2005, doi: 10.1007/s10853-005-5695-0.
D. H. Manh, P. T. Phong, T. D. Thanh, L. V. Hong, and N. X. Phuc, “Low-field magnetoresistance of La0.7Ca0.3MnO3 perovskite synthesized by reactive milling method,” J Alloys Compd, vol. 499, no. 2, pp. 131–134, Jun. 2010, doi: 10.1016/j.jallcom.2010.03.183.
A. Gaur, K. Yadav, U. K. Gaur, K. Yadav, and G. D. Varma, “Study of structural, magnetic and magneto-transport properties of nanocrystalline La2/3Ca1/3MnO3 manganite Study of structural, magnetic and magneto-transport properties of nanocrystalline La2/3Ca1/3MnO3 manganite,” Article in Journal of Optoelectronics and Advanced Materials, vol. 4, no. 7, pp. 989–994, 2010.
Y.-H. Huang, C.-H. Yan, Z.-M. Wang, C.-S. Liao, and G.-X. Xu, “Enhanced magnetoresistance in La0.7Sr0.3MnO3/Nd0.7Sr0.3MnO3 nanocomposites,” J Alloys Compd, vol. 349, no. 1–2, pp. 224–227, Feb. 2003, doi: 10.1016/S0925-8388(02)00864-2.
S. D. Bhame, J.-F. Fagnard, M. Pekala, P. Vanderbemden, and B. Vertruyen, “La0.7Ca0.3MnO3 / Mn3O4 composites: Does an insulating secondary phase always enhance the low field magnetoresistance of manganites?,” J Appl Phys, vol. 111, no. 6, p. 063905, Mar. 2012, doi: 10.1063/1.3694664.
S. Gupta, R. Ranjit, C. Mitra, P. Raychaudhuri, and R. Pinto, “Enhanced room-temperature magnetoresistance in La0.7Sr0.3MnO3-glass composites,” Appl Phys Lett, vol. 78, no. 3, pp. 362–364, Jan. 2001, doi: 10.1063/1.1342044.
D. Das, A. Saha, C. M. Srivastava, R. Raj, S. E. Russek, and D. Bahadur, “Magnetic and electrical transport properties of La 0.67Ca 0.33MnO 3 (LCMO): XSiCN composites,” in Journal of Applied Physics, Jun. 2004, pp. 7106–7108. doi: 10.1063/1.1667447.
B. Huang, Y. Liu, R. Zhang, Xiaobo-Yuan, C. Wang, and L. Mei, “Low-field MR behaviour in La0.67Ca0.33MnO3/ZrO2 composite system,” J Phys D Appl Phys, vol. 36, no. 16, pp. 1923–1927, Aug. 2003, doi: 10.1088/0022-3727/36/16/301.
J. Mira, J. Rivas, L. E. Hueso, F. Rivadulla, and M. A. López Quintela, “Drop of magnetocaloric effect related to the change from first- to second-order magnetic phase transition in La 2/3(Ca 1-xSr x) 1/3MnO 3,” J Appl Phys, vol. 91, no. 10 I, pp. 8903–8905, May 2002, doi: 10.1063/1.1451892.
X. Bohigas, J. Tejada, M. L. Marı́nez-Sarrión, S. Tripp, and R. Black, “Magnetic and calorimetric measurements on the magnetocaloric effect in La0.6Ca0.4MnO3,” J Magn Magn Mater, vol. 208, no. 1–2, pp. 85–92, Jan. 2000, doi: 10.1016/S0304-8853(99)00581-8.
M. H. Phan and S. C. Yu, “Review of the magnetocaloric effect in manganite materials,” Jan. 2007. doi: 10.1016/j.jmmm.2006.07.025.
M. H. Phan, S. C. Yu, and N. H. Hur, “Excellent magnetocaloric properties of La0.7 Ca0.3-x Srx MnO3 (0.05 ≤ x ≤ 0.25) single crystals,” Appl Phys Lett, vol. 86, no. 7, pp. 1–3, Feb. 2005, doi: 10.1063/1.1867564.
Y. M. Kang, H. J. Kim, and S. I. Yoo, “Excellent low field magnetoresistance properties of the La 0.7Sr0.3Mn1 + dO3 -manganese oxide composites,” Appl Phys Lett, vol. 95, no. 5, 2009, doi: 10.1063/1.3177192.
W. Xia, L. Li, H. Wu, P. Xue, and X. Zhu, “Structural, morphological, and magnetic properties of sol-gel derived La0.7Ca0.3MnO3 manganite nanoparticles,” Ceram Int, vol. 43, no. 3, pp. 3274–3283, Feb. 2017, doi: 10.1016/j.ceramint.2016.11.160.
L. E. Hueso, P. Sande, D. R. Miguéns, J. Rivas, F. Rivadulla, and M. A. López-Quintela, “Tuning of the magnetocaloric effect in La 0.67Ca 0.33MnO 3-δ nanoparticles synthesized by sol-gel techniques,” J Appl Phys, vol. 91, no. 12, pp. 9943–9947, Jun. 2002, doi: 10.1063/1.1476972.
R. D. Sánchez et al., “Giant magnetoresistance in fine particle of La0.67Ca 0.33MnO3 synthesized at low temperatures,” Appl Phys Lett, p. 134, 1995, doi: 10.1063/1.116780.
M. A. L pez-Quintela, L. E. Hueso, J. Rivas, and F. Rivadulla, “Intergranular magnetoresistance in nanomanganites,” Nanotechnology, vol. 14, no. 2, pp. 212–219, Feb. 2003, doi: 10.1088/0957-4484/14/2/322.
C. Vázquez-vázquez, M. C. Blanco, M. A. López-quintela, R. D. Sánchez, J. Rivas, and S. B. Oseroff, “Characterization of La0.67Ca0.33MnO3 ± δ particles prepared by the sol–gel route,” J Mater Chem, vol. 8, no. 4, pp. 991–1000, 1998, doi: 10.1039/a707226k.
H. M. Rietveld, “A profile refinement method for nuclear and magnetic structures,” J Appl Crystallogr, vol. 2, no. 2, pp. 65–71, Jun. 1969, doi: 10.1107/S0021889869006558.
M. F. Falhan et al., “Enhancement of magnetism by tailoring synthesis conditions in electron-doped superconducting nanoparticles,” Physical Chemistry Chemical Physics, vol. 26, no. 20, pp. 14787–14795, May 2024, doi: 10.1039/d4cp01072h.
M. V. Lobanov et al., “Structural and magnetic properties of the colossal magnetoresistance perovskite La0.85Ca0.15MnO3,” Phys Rev B, vol. 61, no. 13, pp. 8941–8949, Apr. 2000, doi: 10.1103/PhysRevB.61.8941.
D. Jarosch, “Crystal structure refinement and reflectance measurements of hausmannite, Mn3O4,” Mineral Petrol, vol. 37, no. 1, pp. 15–23, Aug. 1987, doi: 10.1007/BF01163155.
P. K. Siwach, H. K. Singh, and O. N. Srivastava, “Low field magnetotransport in manganites,” Journal of Physics: Condensed Matter, vol. 20, no. 27, p. 273201, Jul. 2008, doi: 10.1088/0953-8984/20/27/273201.
Z. Xu, C. Wei, L. Ai-Jun, M. Li-Mei, and Z. Yun, “Structural, Magnetic and Magnetocaloric Properties of La deficient La0.77 – xSrxCa0.2MnO3 Perovskites,” Chinese Physics Letters, vol. 26, no. 8, p. 087401, Aug. 2009, doi: 10.1088/0256-307X/26/8/087401.
D. S. Razaq, B. Kurniawan, A. Imaddudin, N. A. Sahara, and D. R. Munazat, “Electrical Transport and magnetoresistance properties of La0.8Ca0.2-xAgxMnO3 (x = 0 and 0.05),” in IOP Conference Series: Materials Science and Engineering, Institute of Physics Publishing, Jul. 2019. doi: 10.1088/1757-899X/546/4/042037.
B. Kurniawan, S. Winarsih, A. Imaduddin, and A. Manaf, “Correlation between microstructure and electrical transport properties of La0.7(Ba1-xCax)0.3MnO3 (x = 0 and 0.03) synthesized by sol-gel,” Physica B Condens Matter, vol. 532, pp. 161–165, Mar. 2018, doi: 10.1016/j.physb.2017.08.038.
X. Yu et al., “A comparative study on high TCR and MR of La0.67Ca0.33MnO3 polycrystalline ceramics prepared by solid-state and sol-gel methods,” Ceram Int, vol. 47, no. 10, pp. 13469–13479, May 2021, doi: 10.1016/j.ceramint.2021.01.205.
S. Jin et al., “A-site Na-doping to enhance room-temperature TCR of La1-xNaxMnO3 polycrystalline ceramics,” Mater Today Commun, vol. 28, p. 102496, Sep. 2021, doi: 10.1016/j.mtcomm.2021.102496.
A. Dhahri, M. Jemmali, E. Dhahri, and E. K. Hlil, “Electrical transport and giant magnetoresistance in La0.75Sr0.25Mn1-xCrxO3 (0.15, 0.20 and 0.25) manganite oxide,” Dalton Transactions, vol. 44, no. 12, pp. 5620–5627, Mar. 2015, doi: 10.1039/c4dt03662j.
K. Dhahri et al., “An in-depth insight at the percolation model and charge transport mechanism in La0.7Ca0.1Pb0.2Mn1-2xAlxSnxO3 manganite prepared by sol–gel route,” J Solgel Sci Technol, Jul. 2024, doi: 10.1007/s10971-024-06438-1.
G. Li, H. D. Zhou, S. J. Feng, X. J. Fan, X. G. Li, and Z. D. Wang, “Competition between ferromagnetic metallic and paramagnetic insulating phases in manganites,” J Appl Phys, vol. 92, no. 3, pp. 1406–1410, Aug. 2002, doi: 10.1063/1.1490153.

No competing interests reported.

Enhancing Low Field Magnetoresistance in La0.7Ca0.25Sr0.05MnO3 /Mn3O4 Composite Nanoparticles: Unveiling its Transport Mechanism

Status:

Version 1

Abstract

Figures

Highlights

I. INTRODUCTION

II. MATERIALS AND METHODS

III. RESULT AND DISCUSSION

IV. CONCLUSION

Declarations

References

Additional Declarations

Status:

Version 1