Magnetic properties of NiMn2O4−δ (nickel manganite): Multiple magnetic phase transitions and exchange bias effect

• We have successfully synthesized NiMn2O4−δ sample by complex polymerization synthesis.
• Magnetic measurements reveal complex properties and triple magnetic phase transitions.
• Magnetic measurements of M(H) show hysteretic behavior below 120 K.
• Hysteresis properties after cooling of the sample in magnetic field show exchange bias effect.

We present magnetic properties of NiMn2O4−δ (nickel manganite) which was synthesized by complex polymerization synthesis method followed by successive heat treatment and final calcinations in air at 1200 °C. The sample was characterized by using X-ray powder diffractometer (XRPD), scanning electron microscopy (SEM), field-emission scanning electron microscopy (FE-SEM) and superconducting quantum interference device (SQUID) magnetometer. The XRPD and FE-SEM studies revealed NiMn2O4−δ phase and good crystallinity of particles. No other impurities have been observed by XRPD. The magnetic properties of the sample have been studied by measuring the temperature and field dependence of magnetization. Magnetic measurements of M(T) reveal rather complex magnetic properties and multiple magnetic phase transitions. We show three magnetic phase transitions with transition temperatures at TM1 = 35 K (long-range antiferromagnetic transition), TM2 = 101 K (antiferromagnetic-type transition) and TM3 = 120 K (ferromagnetic-like transition). We found that the TM1 transition is strongly dependent on the strength of the applied magnetic field (TM1 decreases with increasing applied field) whereas the TM3 is field independent. Otherwise, the TM2 maximum almost disappears in higher applied magnetic fields (H = 1 kOe and 10 kOe). Magnetic measurements of M(H) show hysteretic behavior below TM3. Moreover, hysteresis properties measured after cooling of the sample in magnetic field of 10 kOe show exchange bias effect with an exchange bias field |HEB|=196 Oe. In summary, the properties that distinguish the investigated NiMn2O4-δ sample from other bulk, thin film, ceramic and nanoparticle NiMn2O4-δ systems are the triple magnetic transitions with sharp increase of the ZFC and FC magnetizations at 120 K and the exchange bias effect. The analysis of the results and comparison with literature data allowed us to conjecture that the mixed oxidation states of Mn ions, ferromagnetic and antiferromagnetic sublattice orders and surface effects in the sample tailor these interesting magnetic properties.

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