Magnetic irreversibility and magnetization processes in Ni₅Al₃/NiO core/shell nanoparticle system

Area of Science:

• Condensed Matter Physics
• Materials Science
• Nanotechnology

Background:

• Core/shell nanoparticle systems offer unique magnetic properties.
• Understanding magnetic phase transitions in such systems is crucial for advanced applications.
• Ni5Al3/NiO nanoparticles present a complex magnetic behavior requiring detailed investigation.

Purpose of the Study:

• To explore the magnetic phase diagram of Ni5Al3/NiO core/shell nanoparticles.
• To identify and characterize magnetic transitions, including chiral glass (CG) and spin glass (SG) states.
• To investigate the influence of external magnetic fields on magnetic ordering and magnetization dynamics.

Main Methods:

• Experimental investigation of magnetic irreversibility lines (TWI(H) and TSI(H)).
• Comparison of experimental data with theoretical predictions for H-T phase diagrams.
• Analysis of magnetization behavior at various temperatures and magnetic fields (up to 70 kOe).

Main Results:

• Observed deviations from the theoretical phase diagram at magnetic fields above 30 Oe.
• Identification of simultaneous paramagnetic (PM)-CG and PM-SG transitions at T_CG+SG.
• Discovery of a new magnetic transition suppressed above 1 kOe, and induced long-range ferromagnetic order at 3 kOe.
• Anomalous upturn in saturation magnetization below 30 K, linked to spin-wave mode softening and magnon excitation.
• Bose-Einstein condensation of magnons observed at sub-Kelvin temperatures.

Conclusions:

• The Ni5Al3/NiO system exhibits complex magnetic behavior beyond simple theoretical models.
• External magnetic fields significantly influence phase transitions and can induce long-range order.
• Anomalous low-temperature magnetism is attributed to spin-wave dynamics and quantum phenomena like Bose-Einstein condensation.