Influence of temperature on the magnetic properties of Mn 3 O 4 nanowires

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Introduction
One of the recent developments in the field of nanomaterials is the ability to tune the physical/chemical properties as a function of synthesis temperature, morphology, and homogeneity of the samples.This process enables us to extract the properties reversibly in a controlled and more systematic manner. 1 In addition, magnetic properties of nanoparticles can be altered by several factors such as particle size, inter-particle interactions, 2 core-shell exchange interactions, 3 and surface effects. 4anganese-based oxides have been used extensively in the field of rechargeable batteries, catalysis, gas sensors, solar cells, photo-electrochemical, and magnetic resonance imaging due to their low costs, elemental availability (5 th Most abundant metal in the earth's crust) and environmental friendliness. 5he manganese oxides, MnO, and Mn3O4 can attain different crystalline structures depending on the synthesis condition.In the case of Mn3O4, two polymorphs are possible: the stable tetragonal (α-Mn3O4) and cubic (γ-Mn3O4) phases.The most stable α-Mn3O4 phase at room temperature has a tetragonal spinel structure containing both di-and trivalent manganese which is represented as Mn 2+ (Mn 3+ )2O4.Where the Mn 3+ ions are located at the octahedral crystallographic sites and Mn 2+ ions are located at the tetrahedral crystallographic sites within the spinel structure. 6][9][10][11] For example; Zhang et al. reported a low-temperature solvothermal route and made nanocrystalline Mn3O4.However, the method involves an intermediate preparation step of -MnOOH nanowires. 12Yang et al. reported a controlled synthesis of Mn3O4 nanocrystals by solvothermal route at 140-160⁰/24 h.When the reaction time prolonged to 48 h a secondary phase of MnCO3 appeared. 13For using a solvothermal method, one should focus on reaction temperature and time, which eventually leads to non-agglomerated homogeneous nanoparticles.The present study mainly focuses on the improvement of a solvothermal route for the preparation of homogenous, single crystalline Mn3O4 nanowires.In addition, the study involves an understanding of the basic structure, morphology, and magnetic properties under different synthesis conditions as they are of a prime requisite for the large-scale production of phase pure samples with controlled structure.We report the presence of Mn 3+ by ESR spectroscopy, and the antiferromagnetic behavior of Mn3O4 nanowires.

Materials and Methods
Mn3O4 nanowires were synthesized using a solvothermal technique; all the chemicals used in this synthesis were obtained from Sigma-Aldrich with purity greater than 99%.A stoichiometric amount of alcoholic MnCl2 solution was prepared using ethanol and ethylene glycol as a surfactant in aqueous solution.The pH of the solution mixture was adjusted to 11 using NaOH and transferred into a 60 ml Teflon lined stainless steel autoclave.The sealed autoclave was heated at temperatures 150, 175, 200, and 250 °C for 20 h.The product obtained was separated using a centrifuge and washed with water/ethanol mixture and finally dried at 60 ⁰C for 24 h.Powder X-ray diffraction pattern (PXRD) of nanopowders was obtained using Phillips X'PERT PRO diffractometer with CuKα radiation source.The nanowire morphology and phase were obtained using a JEOL 2010 high-resolution transmission electron microscope with an accelerating voltage of 200 kV.ESR spectra of nanowires were obtained at room temperature using X-Band JEOL, JES PX 2300 spectrometer.The magnetic measurements were carried out using SQUID, Quantum Design MPMS-XL7.

Results and discussion
The PXRD patterns of Mn3O4 nanowires synthesized at different temperatures are shown in Fig. 1.All the diffraction peaks were indexed to the tetragonal hausmannite Mn3O4 structure (JCPDS card No 24-0734).The diffraction peak becomes sharp and the intensity of the peak increases with increase in annealing temperature from 150 to 250 ⁰C because of the increase in crystallite size.The crystallite sizes calculated from PXRD data are 28.4 to 43.2 nm for the samples annealed at 150 to 250 ⁰C.Fig. 2(a) shows a typical TEM image of Mn3O4 nanowires made at 150 °C/20 h.The diameter and length of the single nanowire Mn3O4 is around 5 nm and more than 2 µm, respectively.TEM image of the sample showed that the Mn3O4 nanowires are structurally uniform without crystalline defects; such as stacking faults and dislocations.Mn3O4 wires have a needle-like morphology without sharp edges, which is expected from the solvothermal method.The concentration of the solution was found to have a significant effect on the formation of the nanowires, high Mn concentration leads to aggregation of nanowires.SAED patterns of the nanowire revealed perfectly crystalline nature with tetragonal structure (Fig. 2b) and showed bright lattice fringes with d-spacing 0.273 nm, which is equal to the inter planar distance of the (200) plane of Mn3O4 (Fig. 2c).

ESR and Magnetism
Fig. 3 shows the room-temperature ESR spectra of Mn3O4 nanowires annealed at different temperatures between 150 to 250 °C.Intensity of the ESR signal decreases as the temperature increases due to the induced magnetic field (exchange anisotropy field), which is the primary source of magnetic moment for these systems.The ESR spectra composed of a single and relatively broad peak for all samples, whereas the absence of hyperfine splitting suggests that Mn 3+ cations are well separated.The observed single broad peak is isotropic, suggesting that Mn 3+ present in octahedral site symmetry.The broadening and shift of the center of resonance to the lower fields are due to the presence of the nonhomogenous local magnetic field, which modifies the resonance field and signals shapes.The neutron diffraction studies of Mn3O4 show that Mn 2+ ions are located at the tetrahedral sites, whereas the Mn 3+ ions are located at the octahedral sites. 14The calculated Lande 'g' values using g = hυ/µBHr (where υ, µ, and Hr are microwave frequency, Bohr magnetron, and resonance field respectively) are given in Table 1.The zero-field-cooled (ZFC) and field-cooled (FC) magnetization versus temperature (M-T) curves are shown in Fig. 4. The FC-ZFC and M-T curves show weak bifurcation below 50 K.Fig. 4(C) shows a small kink around 10 K, which is attributed to the antiferromagnetic Neel transition temperature TN and magnetization is attributed to the specific directional (200) growth and morphology of the as synthesized Mn3O4 nanowires.The increase in magnetization below 80 K is attributed to the suppression effect of thermal agitation of non-interacting magnetic moments causing the associated paramagnetic contribution. 15The Curie-Weiss temperature 'θ' estimated from the 1/χ curves (shown in the inset of Fig. 4) is found to be 32 K for sample annealed at 150 ºC, 29 K for the sample annealed at 175 ºC, and -14 K for the sample annealed at 250 ºC.The sample synthesized at 150 and 175 ºC have positive Curie-Weiss temperature indicating positive susceptibility due to the presence of spin canting in the samples.Curie-Weiss temperature shifts to the negative value with synthesis temperature 250 ºC indicating the antiferromagnetic ordering.

Conclusions
Single-crystalline Mn3O4 nanowires were synthesized by the solvothermal method.The structure, morphological, ESR, and magnetization studies were carried out as a function of synthesis temperature.The powder X-ray diffraction revealed the tetragonal hausmannite structure, whereas ESR spectra confirms that the Mn3+ ions present in the octahedral environment.The intensity of the ESR resonance signal decreases with the increasing synthesis temperature due to the enhanced exchange interaction that arises from the inhomogeneous Mn distribution.Curie-Weiss temperature provided the evidence for the existence of antiferromagnetic interactions in the sample, which evolved as a function of synthesis temperature.Through the current study, we propose a simple, safe, and low-cost solvothermal method to synthesize large-scale homogenous single crystalline nanowires of manganese oxide.In addition, the magnetic properties can be tuned as a function of synthesis temperature.

Fig. 1 .
Fig. 1.Powder X-ray diffraction patterns of the Mn3O4 nanowires synthesized at various temperatures

Table 1 .
Synthesis temperature, g-values, line width (∆H) of Mn3O4 nanowires are listed