Building on previous work finding a significant magnetocaloric effect (MCE) in manganite nanoparticles having the formula La0.60Cax Sr0.40-xMnO3 (0<x<0.4), which show promise for application in magnetic refrigeration systems. These were synthesized via a Pechini sol-gel method, as verified by powder x-ray diffraction and laser ablation-inductively coupled plasma-mass spectrometry. The temperature-dependence of the magnetic entropy change between 200K and 320K in a magnetic field of μH=3T was evaluated by means of Maxwell’s relations and isothermal MH curves. The reported values of ΔSmax increase from 1.5 J kg-1K-1 to 5.6 J kg-1K-1 as x increases from 0 to 0.4. Ms; crystallite size and b and c lattice parameters increase with x. The results show a clear positive correlation between calcium concentrations and the value of -ΔS, suggesting that a larger atom at the calcium site negatively impacts the magnetocaloric effect, thereby decreasing magnetic refrigeration viability.

According to the International Energy Agency (IEA), refrigeration systems for the cooling of residential and commercial buildings contribute 700 million metric tons of CO2 and are responsible for more than 15% of total energy consumption worldwide. To compensate for the increase in refrigeration system projected by the IEA, refrigeration energy consumption and carbon dioxide emission levels must be decreased by one third.1–3 Magnetic cooling devices utilizing magnetocaloric materials as solid refrigerants in place of environmentally unfriendly chlorofluorocarbons can exhibit up to 50% greater efficiency over condenser-evaporator systems.4–7 

There are several magnetic refrigeration device designs in existence today that are anticipated to be highly energy efficient, provided that appropriate working materials can be developed. Ideally, the ideal magnetocaloric material should possess a near room temperature Curie Temperature (Tc), high thermal conductivity, low electrical conductivity, a phase transition at the interface between first and second order, and a substantial magnetocaloric effect (MCE) at relatively low applied magnetic fields (μ0H∼ 2T).4,7 The MCE is defined as a magnetic material’s ability to undergo a temperature change upon application or removal of magnetic field in adiabatic conditions and it is indirectly quantified by evaluating the magnetic entropy change ΔSmag as calculated from Maxwell’s Relations:

ΔSmagH,T=μ00HmaxMTHdH
(1)

Here, µ0 is the permeability of free space, MT is the temperature derivative of the magnetization and Hmax is the maximum applied magnetic field.

Perovskite manganites of the formula La1-xCaxMnO3 (A=divalent alkaline-earth metal), have been explored as potential magnetic refrigeration materials due to their moderate MCE values (as high as ΔS=6.9 J/kgK at 2T in bulk materials of composition, La0.6Ca0.4MnO3); this value is tunable with dopants where the A-site may be occupied by Sr, Ca or Ba.4,6–10 Following the work of Zener et al., it is understood that the magnetic properties are determined by the manganese oxidation states and the Mn-O-Mn bond angles. A smaller bond angle increases orbital overlap, thereby increasing interactions. Size disorder at the calcium cation site affects this bond angle, thus enabling magnetic property tuning.11 It has been observed that nanoscale systems exhibit an increased MCE over the bulk.4,7 Using a Pechini sol-gel synthesis, La1-xCaxMnO3 nanoparticles can be synthesized simply, reliably, and inexpensively. In addition, this method allows for easy and dependable particle size tuning and A-site doping.4,6,12

Building on previous work, this study seeks to evaluate the impact of strontium doping on the MCE of La1-xCaxMnO3 nanoparticle systems synthesized via the Pechini sol-gel method. Here we demonstrate that the presence of the smaller calcium atom (rCa= 231 pm radius) in place of the larger strontium atom (rSr= 255 pm) in the La1-xCaxMnO3 lattice positively impacts ΔS values, thus increasing the magnetic refrigeration viability of the material.

The LC/SMO nanoparticles were synthesized using a Pechini sol-gel technique similar to previous work.12 Precursors were combined in stoichiometric amounts and dissolved in 4M nitric acid (10 mL), then heated to 70 °C for 3 hours. The resulting brown gels were calcined in a tube furnace open to the air at 1000 °C for 10 hours creating a sponge-like black solid which was ground with mortar and pestle into a fine powder.

Powder x-ray diffraction was performed using a PANalytical X’Pert Pro MPD diffractometer equipped with a spinner stage on a zero-background silicon holder (single scan; 20° ≤ 2θ ≥ 90°; 0.010° step size; 1 s/step). Patterns were compared to PDF-2 ICDD Library 2004 Release, PDF reference number 00-046-0513. Laser ablation-inductively coupled plasma-mass spectrometry was performed on pellets of each material using an NWRfemto laser ablation system and an Agilent 7900 ICP-MS (20 sec laser pulse; 30 μm diameter; 30 Hz rep rate; 8 J/cm3 power) using internal standards.13 The magnetic measurements were acquired using a Quantum Design Dynacool Physical Property Measurement System (PPMS). M(H) curves for MCE determination were collected from 220K to 320K (5K intervals; μH=3T). Zero Field Cooled and Field Cooled M(T) curves were collected in succession (100 Oe; 50K to 300K).

The Pechini modified sol-gel technique offers reliable, scalable, and environmentally friendly production of nanoparticles with tunable particle size and simple introduction of dopants.12,14 As strontium (which has an atomic radius of 255 pm) is introduced into the a-site of the lattice in place of calcium (which has an atomic radius of 231 pm), pXRD reveals the crystallite size decreases (x=0.4, 70.2 nm; x=0.393 ± 0.003, 37.0 nm; x=0.326 ± 0.003, 32.4 nm; x=0, 16.1 nm) and the crystal lattice is distorted, increasing side lengths and inducing strain. The b and c lattice parameters, and thus the Mn-O-Mn bond angle, decrease with the radius of the a-site cation (7.718≤b≤7.690 and 5.464≤c≤5.439). This leads to an increase in Ms, from 27 emu/g to 97 emu/g (Figure 1), and in the MCE (Figure 2).

FIG. 1.

M(H) curves at 220K across 3T.

FIG. 1.

M(H) curves at 220K across 3T.

Close modal
FIG. 2.

MCE plots showing ΔS versus Temperature calculated from MH curves obtained at 5K intervals with µH=3T (a) and ΔSmax dependence on x (b). The dotted line serves as a guide to the eye to indicate increase in ΔSmax with an increase in x. The values of x are reported with an error of ± 0.003.

FIG. 2.

MCE plots showing ΔS versus Temperature calculated from MH curves obtained at 5K intervals with µH=3T (a) and ΔSmax dependence on x (b). The dotted line serves as a guide to the eye to indicate increase in ΔSmax with an increase in x. The values of x are reported with an error of ± 0.003.

Close modal

There is a positive correlation between x and ΔS as calculated using Maxwell’s Relations and M(H) curves. The ΔS values are 5.6 J/kgK, 3.3 J/kgK, 2.5 J/kgK, and 1.5 J/kgK for the x=0.4, 0.39, 0.32 and 0 samples (Figure 2). The temperatures corresponding ΔSmax are constant and consistent with the Tc of 268K obtained from the intersection of the Zero Field Cooled (ZFC) and Field Cooled (FC) curves (Figure 3). The working temperature range of the MCE is tunable by varying the ratio of the Ca to Sr, increasing from 25K to 35K as x increases. In bulk systems, there is typically an increase in Tc as strontium replaces calcium. In these materials, the introduction of strontium leads to a decrease in particle size, which offsets the lattice effects on Tc.

FIG. 3.

Field Cooled (FC) plots obtained from 50K to 300K under a field of 100 Oe. ZFC and FC curves for the x=0.4 sample (inset).

FIG. 3.

Field Cooled (FC) plots obtained from 50K to 300K under a field of 100 Oe. ZFC and FC curves for the x=0.4 sample (inset).

Close modal

The decrease in ΔSmax as x decreases implies that a larger cation at the calcium site has a negative impact on the magnetocaloric response. These values, however, are still comparable to those of bulk LSMO, demonstrating the merits of nanomaterials in potential magnetic refrigeration systems (Table I).5,7,14–17

TABLE I.

Comparison of the La1-x AxMnO3 (A=Ca, Sr) nanoparticles synthesized in this work with representative nanoscale and bulk room temperature magnetocaloric materials systems in the literature.

MaterialMaterial TypeΔS (J/kgK)μH (T)Tc (K)Particle Size (nm)Ref.
Gd5Si2Ge2 Bulk 19 276 NA 7  
Gd5Si4 NPs 300 65 15  
(Fe70Ni30)99Cr1 NPs 1.6 300 12 16  
(Fe70Ni30)89B11 NPs 2.1 300 10 17  
La0.60Ca0.40MnO3 Bulk 4.5 264 NA 5  
La0.6Sr0.4MnO3 Bulk 2.50 372 NA 5  
La0.6Ca0.2Sr0.2MnO3 Bulk 2.0 342 NA 5  
La0.60Ca0.40MnO3 NPs 2.3 258 45 14  
La0.60Ca0.40MnO3 NPs 8.3 270 223 14  
La0.6Ca0.4MnO3 NPs 5.6 268 70.2 This work 
MaterialMaterial TypeΔS (J/kgK)μH (T)Tc (K)Particle Size (nm)Ref.
Gd5Si2Ge2 Bulk 19 276 NA 7  
Gd5Si4 NPs 300 65 15  
(Fe70Ni30)99Cr1 NPs 1.6 300 12 16  
(Fe70Ni30)89B11 NPs 2.1 300 10 17  
La0.60Ca0.40MnO3 Bulk 4.5 264 NA 5  
La0.6Sr0.4MnO3 Bulk 2.50 372 NA 5  
La0.6Ca0.2Sr0.2MnO3 Bulk 2.0 342 NA 5  
La0.60Ca0.40MnO3 NPs 2.3 258 45 14  
La0.60Ca0.40MnO3 NPs 8.3 270 223 14  
La0.6Ca0.4MnO3 NPs 5.6 268 70.2 This work 

Using a modified sol-gel technique, nanoparticles of La0.6CaxSr0.4-xMnO3 can be synthesized with tunable magnetic properties. As calcium levels increase, lattice parameters and strain decrease as a result of the smaller cation in the a-site. This decreases the distance between the Mn ions and enhances the magnetic properties leading to an increase in Ms and ΔSmax and a wide working temperature range. The decrease in particle size observed as calcium levels increase appears to offset changes in Tc typically observed in bulk materials. Future work confirming the effect of particle size and in determining the critical exponents and quantifying unambiguously the FOPT versus SOPT character, as has been done for other magnetocaloric systems,18 may be of merit as an ideal magnetocaloric material would exist at the intersection of first and second order.

Supplementary material contains powder XRD patterns and SEM micrographs.

Acknowledgement is made to the VCU Nanocharacterization Core Facility and to the VCU Chemistry Instrumentation Facility for use of instrumentation, to Dr. Joseph Turner for assistance with LA-ICP-MS measurements and data processing, and to Dr. Melissa Tsui for synthesis guidance.

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Supplementary Material