Influence of exchange bias on magnetic losses in CoFeB/MgO/CoFeB tunnel junctions

Transcription

1 Influence of exchange bias on magnetic losses in CoFeB/MgO/CoFeB tunnel junctions Ryan Stearrett Ryan Stearrett, W. G. Wang, Xiaoming Kou, J. F. Feng, J. M. D. Coey, J. Q. Xiao, and E. R. Nowak, Physical Review B, (2012). Center for Spintronics and Biodetection, Department of Physics and Astronomy, University of Delaware

2 DESG Support (2010 to 2012) Publications Stearrett et al., Applied Physics Letters 97, (2010). Published in Virtual Journal of Nanoscale Science and Technology Volume 22 Issue 26 (12/20/2010). Presentations 55 th Conference on Magnetism and Magnetic Materials 2011 American Physical Society March Meeting 56 th Conference on Magnetism and Magnetic Materials 2012 American Physical Society March Meeting 12 th Joint Magnetism and Magnetic Materials-Intermag

3 Fluxgate magnetometers Watts Low-frequency resistance noise limits their performance TMR magnetic sensors low-power small inexpensive low-frequency ultra-sensitive milliwatts

4 Magnetic Tunnel Junctions (MTJs) MgO CoFeB CoFeB Ferromagnetic Layer Insulating Layer Ferromagnetic Layer

5 TMR (%) Tunneling Magnetoresistance (TMR) TMR R AP R P R P CoFeB CoFeB Parallel State Low Resistance m 0 H (mt) H (mt) Parallel Antiparallel State State Low High Resistance

6 Exchange Bias M few mt to switch H Dieny et al., Phys. Rev. B 43, 1297 (1991) H ex Ferromagnetic Antiferromagnetic

7 Exchange bias layers and noise Shaw et al., Appl. Phys. Lett. 89, (2006) Easy Axis Loop Lorentz microscopy 200 nm Magnetic Fluctuation

8 Stack Structures DC magnetron sputtering Capping Free Barrier Reference Spacer Pinned AF Seed Buffer Substrate Bottom-Pinned MTJ Capping AF Pinned Spacer Reference Capping Barrier Free Barrier Reference Spacer Pinned AF Seed Buffer Substrate Double-Barrier MTJ Symmetric about free layer Barrier Reference Spacer Pinned AF Provides desirable growth texture

9 Battery (0-25 V) Voltage Noise Measurement MTJ Ballast Resistor (0 10 MW) AC Coupling SR-552 Preamplifier SR-640 Antialias Filter ADC NI 16 to 24-bit

10 Types of noise W.F. Egelhoff Jr. et al., Sens. Actuators, A 155, 217 (2009) Amplifier noise Thermal and shot noise Electronic 1/f noise (charge traps in barrier) Magnetic 1/f noise (magnetization fluctuations) Power Spectral Density S V ( f ) amp S V ev 2k B T R 2 V + 2eI coth d + a 2 elec a Af Wf V 2 amp S V thermal/shot S V S V S V + a mag elec V 2 Wf

11 Fluctuation-Dissipation (FD) Theorem a mag ( H ) = W f mag S V 2 V e ( H ) a mag ( H ) e ( H ) k B T pm 0 M s DR R MSP 1 dr R dh e ( H ) c R c R Phase lag of reference layer Probe magnetization dynamics of the system CoFeB m 0 M s = 1.6 T CoFeB Saturation Magnetization of CoFeB MSP DR R 1 dr R dh Magnetoresistance- Sensitivity Product

12 Magnetic Field Dependence reference free Broad, asymmetric Sharp peak H ref P AP Magnetic losses largest in AP state

13 Magnetic Noise and Sensitivity Linear Correlation a mag MSP 43 min P AP 43 min e ref 48 hr MSP peak of reference layer sharper with annealing Shifts towards zero field Indicates loss of exchange bias coupling

14 Loss of exchange bias coupling MSP / MSP max Reversal of reference layer After Before 20 s 43 min 5 hr 48 hr -1-1 m 0 (H-H ref ) (mt) Triangular profiles resemble free layer 48 hr MSP sharper and more symmetric about H ref

15 Seed Layer Thickness Bottom-pinned 5 nm Seed e ref = 1.4 deg H ref = -80 mt Above 3 nm Under 3 nm Capping AF Pinned Spacer Reference Barrier Free Barrier Reference Spacer Pinned AF Seed Buffer Substrate Thicker seed layers Larger exchange bias More magnetic losses Top-pinned 2.5 nm Seed e ref = 0.68 deg H ref = -28 mt

16 Conclusions MR noise probes magnetic disorder Strong exchange bias (thick seed layers) Larger values of e ref and H ref Broad MSP peak of reference layer Weakened exchange bias (prolonged annealing) Reduction in e ref and H ref Sharpened MSP curve of reference layer

17 Synthetic Antiferromagnetic (SAF) Layer Reference Layer Spacer forms (001)- oriented bcc layer Pinned Layer Metallic Spacer 361 % TMR with spacer 181 % TMR without spacer Lee et al., Appl. Phys. Lett. 89, (2006)

18 Annealing Ar Environment N S 380 C Cool to Room Temperature Set Exchange Pinning