Thermally Assisted MRAM How does it work ? Cristian PAPUSOI, Bernard DIENY SPINTEC, Grenoble, France Acknowledgments: O.Redon, A.Astier, (LETI), J.P.Nozières (CROCUS Tech.), S.Cardoso, P.Freitas (INESC-MN Portugal)

Introduction EDUCATION 1993 – 1998 1987 – 1992 EDUCATION Ph. D., Faculty of Physics, “Al.I.Cuza” University, Iasi, Romania Diploma in Physics, Faculty of Physics, “Al.I.Cuza” University, Iasi, Romania 2006 – 2007 2004 – 2006 2003 – 2004 2000 – 2003 1992 – 2000 2006 ACADEMIC AND PROFESSIONAL EXPERIENCE Postdoctoral Research Fellow, Grenoble, France Investigation of RAM devices with thermal assisted switching Postdoctoral Research Fellow, Center for Materials for Information Technology (MINT) Center, University of Alabama, Tuscaloosa, USA Fabrication and characterization of CPP (Current Perpendicular to the Plane) spin valves Postdoctoral Research Fellow, RWTH University,2 Physikalisches Institut A, Aachen, Germany Role of non-magnetic defects inserted in metallic antiferromagnets on exchange bias Postdoctoral Research Fellow, Information Storage Materials Laboratory, Toyota Technological Institute, Nagoya, Japan  Thermal stability and recording performance of hard-disk media Lecturer, Department of Electricity and Physical Electronics, Faculty of Physics, "Alexandru Ioan Cuza" University, 11 Blvd. Carol I, 700 506 Iasi, Romania AWARDS Outstanding REU student/postdoc mentor, University of Alabama (11/8/2006)

SPINTEC - location SPINTEC MINATEC LETI

Main topics of research at SPINTEC GOAL Bridge fundamental research and advanced technology in spin electronics Modeling Analytical models Micromagnetism Finite elements CAD tools Basic phenomena GMR TMR Spin transfer Functional materials Ferro/Antiferro coupling Materials with perpendicular magnetization Nanoparticles Data storage Patterned media GMR/TMR sensors Recording heads Thermally assisted recording Spintronics MRAM Microwave oscillators Magnetic logic gates Created January 2002 as joint CEA/CNRS laboratory affiliated to MINATEC R&D center 27 permanent staff members, 16 PhD students and 7 post-docs

Why MRAM ? • Non-Volatility of FLASH • Density competitive with DRAM • Speed competitive with SRAM

Why Thermally Assisted MRAM ? Problems in conventional MRAM Selectivity –> difficulty in writing a single junction Scalability –> electromigration in magnetic field lines with decreasing in-plane size Thermal stability -> reduced life-time of written information New approaches 1. Thermally assisted MRAM (Spintec Patent + lab. demo) - good thermal stability ensured by exchange coupling of the storage layer with an Antiferromagnet; - high selectivity; - low power consumption during writing at high temperature. 2. Current induced magnetization switching - linear decrease of power consumption with decreasing junction in-plane area 3. Possibility to integrate 1 and 2

Outline TA-MRAM. Definition, structure and principle of operation. Electric characterization Regimes of operation. Power of the electric pulse PHP vs. junction temperature TAF. Exchange bias as a temperature probe. Electric pulse width d vs. junction temperature TAF. Conclusions

Thermally Assisted MRAM (TA-MRAM) – principle  Status 0 OFF  Writing ON Heating Cooling Hw IHP Iw  Status 1 R H Hex PHP = 0 Hw = 0 T = Room Temperature Hw < 0 T  Blocking Temperature 1

Thermally Assisted MRAM (TA-MRAM) – structure TMR = 30% Vbias = 40 mV 250 nm PtMn (20nm) CoFe (2.5nm) Ta (50nm) Ru (0.8 nm) CoFe (3nm) NiFe (3nm) Al (20nm) Ta (90nm) CoFe (1.5nm) IrMn (5nm) AlOX (0.5 nm) Hard mask (thermal barrier) Reference layer Tunnel barrier Storage layer 450 nm

Thermally Assisted MRAM (TA-MRAM) – electric characterization MTJ PPB I MR Tester H (I>60 mA, |H|<1000 Oe) Pulse Generator (d>1.5 ns, UPG<3.8 V) Switch 1 2 Trigger OSC Equivalent electric circuit of the MRAM device (by network analyser) UAPPL UMTJ RMTJ ≈ 800 Ω 4.2 Ω 54 pH 54 pF ~ ZPG = 50 Ω 2UPG ZOSC = 50 Ω No amplitude attenuation (UAPPL=UMTJ) for d > 1 ns pulse width UMTJ = 2UPG – (ZPG+ZOSC)I I = UOSC / ZOSC PHP = UMTJ I What do we measure ?

Decrease pulse width d. Set PHP to PHP,min Thermally Assisted MRAM (TA-MRAM) – measurement procedure Set pulse width d to dMAX. Set pulse power PHP to PHP, MIN How do we measure ? d Time PHP HSET Magnetic field pulse Electric pulse Saturate HEX (negative magnetic field pulse -HSET and heating pulse of power PHP, MAX) Attempt to reverse HEX (positive magnetic field pulse HSET and heating pulse of power PHP) Increase heating pulse power PHP Decrease pulse width d. Set PHP to PHP,min Quasi-Static MR cycle (HEX, HC, R) HEX<HEX ? WRT NO YES PHP (d) WRT

Stationary regime (TAF=TRT+aPHP) Thermally Assisted MRAM (TA-MRAM) – operation regimes 1-D model of heat diffusion in the MTJ stack Thermal barrier 1 (cTB, rTB, dTB) Magnetic stack (ci, ri, di) Thermal barrier 2 (cTB, rTB, dTB) Layer position z Temperature T TAF TAF PHP Time Stationary regime (TAF=TRT+aPHP) Transient regime c - specific heat capacity J/(Kg K) k – thermal conductivity W/(K m) r - density Kg/m3 d - layer thickness m

120 MHz maximum writing frequency Thermally Assisted MRAM (TA-MRAM) – transient regime dPUMP dPROB t Time PPUMP HSET Magnetic field pulse PPROB Electric pulse tTR=2.7 ns 120 MHz maximum writing frequency tTR=2.7 ns

Thermally Assisted MRAM (TA-MRAM) – stationary regime Measurement time per point is 50 ms >> tTR -> always in stationary regime a=105 K/W   - measured as a function of temperature   - measured as a function of PHP and converted into temperature according to TAF = TRT + a PHP

Theory – 1D model of heat diffusion Thermally Assisted MRAM (TA-MRAM) – consistency of results Theory – 1D model of heat diffusion Material r kg/m3 c* J/(K kg) k** W/(K m) Ta*** (b) 16327 144 4.3 PtMn 12479 247 4.9 CoFe 8658 446 37 Ru 12370 239 120 AlOx 3900 900 27 IrMn 10181 316 5.7 NiFe 8694 447 Al 2700 904 235 SiO2 2200 730 1.4 rTB = 15846 kg/m3 cTB = 154.1 J/(K kg) kTB = 4.38 W/(K m) Al Experiment (a = 105 K/W, tTR = 2.7 ns) *for metallic alloys calculated according to Dulong-Petit law **for metallic alloys calculated according to Widemann-Franz law ***confirmed by electrical resistivity measurements (170 mWxcm) and XRD scan

3-D simulations of heat diffusion in the MTJ stack Thermally Assisted MRAM (TA-MRAM) – consistency of results 3-D simulations of heat diffusion in the MTJ stack Electric pulse (measured write power is used as input) 300 oC 200 oC 100 oC 25 oC SiO2 electrodes junction stack Ta SiO2 231 nm tTR = 2.3 ns a = 0.84 x 105 K/W

Thermally Assisted MRAM (TA-MRAM) – conclusions Two temperature regimes of the MTJ evidenced: transient temperature regime for pulse widths d < 9 ns; stationary temperature regime for longer pulse widths; in this regime, the relationship between the temperature of the storage layer TAF and the power of the electric pulse PHP is linear: TAF = TRT + 105 (K/W) PHP. Use of thermal barrier layers reduces the electric power density required for writing but also decreases the writing frequency The writing power density increases with decreasing pulse width from 0.6 mW for d = 1 s up to 6 mW for d = 2 ns; even in the range of pulse widths 1 s - 10 ns, where the storage layer reaches the stationary temperature regime by the end of the electric pulse, the writing power shows a 500 % increase.

Thermally Assisted MRAM (TA-MRAM) – exchange bias as temperature probe AFM layer = collection of non-interacting single domain grains FM layer = single domain (-) (+) D mAFM q HE S (-) (+)

Thermally Assisted MRAM (TA-MRAM) – exchange bias as temperature probe Ni80Fe20/Ir20Mn80 Ferromagnetic/Antiferromagnetic bilayer TNeel=350 oC, a0=2.7 Å, l=0.33 JINT = 8.11x10-15 erg KAF = 2.44x105 erg/cm3 R.White et al, J.Appl.Phys. 92, 4828 (2002) TB = 275 oC TB = 50 oC Decreasing the pulse width d may require a considerable increase of the writing temperature !!!

Thermally Assisted MRAM (TA-MRAM) – exchange bias as temperature probe 200 nm a TAF = TRT + 8.97 x103 PHP KAF = 7.3 x 105 erg/cm3 KF = 1.3 x 104 erg/cm3 JINT = 8.5 x 10-15 erg PHP= () 0.38 mW -> 58 oC () 0.69 mW -> 85 oC (▲) 0.92 mW -> 105 oC (▼) 1.24 mW -> 133 oC (◄) 1.47 mW -> 153 oC (►) 1.7 mW -> 173 oC () 2.24 mW -> 220 oC

Thermally Assisted MRAM (TA-MRAM) – exchange bias as temperature probe HEX = 0 Oe WRT  3-D simulation of heat diffusion – experimental write power PHP (d) is used as input WRT Exchange bias model - temperature required to set HEX = 0 for heating time equal to the pulse duration d; temperature pulse shape is assumed rectangular. WRT Exchange bias model - temperature required to set HEX = 0 for heating time equal to the pulse duration d; temperature pulse shape is calculated by 3-D simulation of heat diffusion. WRT

Conclusion Writing temperature increases with decreasing pulse width d as a consequence of thermal relaxation in the Antiferromagnetic storage layer and approaches the Neel temperature in the limit d -> 0. Exemple: writing with 2 ns pulses imply heating at about 300 oC with possible negative effects on the integrity of tunnel barrier and storage layer antiferromagnet. Solution: decrease the writing temperature by using antiferromagnets of lower Nèel temperature than IrMn (TN ≈ 350 oC) for pinning the storage layer.