Second order anisotropy contribution in perpendicular magnetic tunnel junctions.

Area of Science:

• Materials Science
• Condensed Matter Physics
• Nanotechnology

Background:

• Perpendicular magnetic tunnel junctions (pMTJs) are key components in magnetic storage devices.
• Understanding magnetic anisotropy is crucial for optimizing pMTJ performance.
• Conventional models often use uniaxial anisotropy, which may not fully capture complex magnetic behaviors.

Purpose of the Study:

• To investigate the temperature dependence of magnetic anisotropy in pMTJ pillars.
• To identify and characterize higher-order anisotropy terms beyond the conventional uniaxial model.
• To explore the impact of this anisotropy on the magnetic ground state of pMTJ layers.

Main Methods:

• Fabrication and measurement of pMTJ pillars with diameters from 50 to 150 nm.
• Analysis of hard-axis magnetoresistance loops using an analytical model.
• Temperature-dependent measurements from 5 K to 340 K.
• Ferromagnetic resonance experiments on FeCoB/MgO sheet films for validation.

Main Results:

• A second-order anisotropy term (-K2cos(4)θ) is necessary to accurately model experimental data.
• This higher-order anisotropy was observed in both free and reference layers of the pMTJs.
• The ratio of second-order to first-order anisotropy (-K2/K1) increases significantly at lower temperatures.
• The reference layer transitions from an easy-axis to an easy-cone magnetic ground state below 300 K.
• The easy-cone regime exhibits distinct signatures in the hard-axis magnetoresistance loops.

Conclusions:

• The study reveals the presence and significance of higher-order magnetic anisotropy in pMTJs.
• This anisotropy is of interfacial origin, likely stemming from nanoscale fluctuations at the FeCoB/MgO interface.
• The temperature-dependent shift to an easy-cone state in the reference layer has implications for magnetic device stability and operation.
• The findings necessitate incorporating higher-order anisotropy terms in models for advanced magnetic devices.