Imprinting domain/spin configurations in antiferromagnets. A way to tailor hysteresis loops in ferromagnetic-antiferromagnetic systems

Transcription

1 Imprinting domain/spin configurations in antiferromagnets A way to tailor hysteresis loops in ferromagnetic-antiferromagnetic systems Dr. J. Sort Institució Catalana de Recerca i Estudis Avançats (ICREA) and Dept. de Física, Univ. Autònoma de Barcelona, Spain 1

2 Collaborators S. Brück, G. Salazar-Álvarez, M.D. Baró, S. Suriñach, Universitat Autònoma de Barcelona, Spain V. Baltz, B. Dieny SPINTEC, Grenoble, France J. Nogués ICREA at Universitat Autònoma de Barcelona, Spain A. Hoffmann, S.H. Chung, K. Buchanan, M. Grimsditch, V. Novosad Materials Science Division, Argonne National Lab. 2

3 OUTLINE Introduction - Fundamentals of exchange bias (single shifted loops) - Occurrence of double-shifted loops Experimental - Sample compositions and preparation Results and discussion Conclusions - Double-shifted loops with perpendicular anisotropy - Magnetization Reversal and Asymmetric vortex-like loops in circular dots 3

4 Introduction. Hysteresis loop shift Ferromagnetic (F) Antiferromagnetic (AF) Exchange Coupling F AF H T C > T > T N F AF F AF M F AF Field Cooling T < T N F AF H E H F AF H E : Exchange Bias Field J. Nogués and I.K. Schuller, J. Magn. Magn. Mater. 192, 203 (1999) 4

5 INTRODUCTION TO EXCHANGE BIAS Spin Valves, Tunnel Junctions Cu, Al 2 O 3 Spin Valves 5

6 Read Heads WHY IS IT INTERESTING? Submicron AF-F structures Magnetic RAM Write Head Read Head S.S.P. Parkin et al., Proc. IEEE. 91, 661 (2003) J. Åkerman et al., IEEE Trans. Dev. Mat. Reliab. 4, 428 (2004) 6

7 Occurrence of dual loops in F-AF bilayers Sometimes double-shifted loops are obtained instead of a single, shifted, hysteresis loop H.-W. Zhao et al., J. Appl. Phys. 91 (2002) 6893 P. Miltényi et al., Appl. Phys. Lett. 75 (1999) 2304 Only for systems with in-plane anisotropy. The exchange bias (shift of the loop) is the same for each sub-loop (only vertical tailoring). This is ascribed to the coupling between the F and differently oriented domains in the AF 7

8 New types of double-shifted loops? Are dual loops possible in systems with out-of-plane anisotropy? Is it possible to tailor both the loop shift and the vertical magnetization amplitude of each sub-loop separately? What happens in patterned structures? Is it possible to imprint inhomogeneous magnetization states (e.g., vortices) into an AF? 8

9 Experimental. Sample Preparation SYSTEM 1: Continuous multilayer with perpendicular anisotropy [Pt (2nm)/Co (0.65nm)] 3 / Pt(0.1nm)/IrMn(5nm)/Pt(2nm) F multilayer with perpendicular-to-plane anisotropy AF with high blocking temperature (T B = 500 K) SYSTEM 2: Array of patterned disks with in-plane anisotropy Ta (5 nm)/ Permalloy (12 nm)/ Pt (2 nm) ; Ta (5 nm)/ Py (12 nm)/ IrMn (5 nm)/pt (2nm) Buffer F Capping ; Buffer F AF Capping - Continuous films deposited by magnetron sputtering - Arrays of disks prepared by e-beam lithography - Heat treatments from above the blocking temperature The blocking temperature is the temperature at which the loop shift is locally set. 9

10 Experimental. Sample Preparation Resin Electron Beam Lithography Exposure e - Development AF F Si(100) Ar + Mask Deposition Mask Lift off Ion Milling 10

11 Results and Discussion SYSTEM 1: Continuous multilayer with perpendicular anisotropy [Pt (2nm)/Co (0.65nm)] 3 / Pt(0.1nm)/IrMn(5nm)/Pt(2nm) 11

12 How to imprint domains into the AF? AC -Demag. Apply Field H FC Sample Heat Sample Sample Sample 20 mm to T Dem = 520 K T dem > T B Cool in zero - field to RT (c) (a) (b) m 0 H FC = 0 M R /M S = 0 m 0 H FC = 3.5 mt M R /M S = 0.45 m 0 H FC = 800 mt M R /M S =

13 M Tailoring of the magnetization amplitude of each sub-loop Applied field: 1 0 mt There is a correlation between the F magnetisation after imprint and the vertical amplitude ratio ΔM: mt M (a) 0.9 (c) (a) M / M s mt mt (b) (b) M r / M s mt 0 MFM domain coverage corresponds to vertical amplitude ratio AF exchange interaction direction mimics the FM domain pattern m 0 H (T) (c) AF regions with less or equal size than F domains 13

14 Distribution (a.u.) Tailoring of the loop shift of each sub-loop AC - Demagnetisation H ext Apply 800 mt H ext H ext Sample Heat to T Dem = 520 K Sample Cool in zero - field to RT Heat to T ANN Sample Cool in field to RT Sample Vertical Tailoring after ZFC at T = 300 K Horizontal Tailoring AF F A distribution of local blocking temperatures in the AF is considered. This distribution of T B stems from a distribution of crystallite sizes and/or Local T B < T 300 ANN local anisotropies in the AF T ANN at T = T T (K) ANN AF 14

15 Distribution (a.u.) Tailoring of the loop shift of each sub-loop 1 (a) after ZFC at T = 300 K 0 H + E H Ē AF -1 1 no T ANN F M / M s m 0 H (T) T ANN = 350K T ANN = 370K T ANN = 390K T ANN = 520K AF F Local T B < T ANN T ANN (b) at T = T T (K) ANN In one of the two sub-systems, the direction of the AF is locally reversed H ext The resultant loop shift in one of the two sub-loops progressively averages out as T ANN is increased. This implies that the AF regions have to be smaller than the F domains 15

16 Possible to combine both procedures (full tailoring)? Vertical tuning followed by: (i) (ii) M/M S m 0 H (T) m 0 H (T) Horizontal tuning of the subloop on the right Horizontal tuning of the subloop on the left 16

17 Conclusions SYSTEM 1: Continuous multilayer with perpendicular anisotropy [Pt (2nm)/Co (0.65nm)] 3 / Pt(0.1nm)/IrMn(5nm)/Pt(2nm) Dual-loop in system with perpendicular anisotropy Full, post-deposition tailoring of the loop Prove for the imprint model Tuning as probe of length scales 17

18 Results and Discussion SYSTEM 2: Array of patterned disks (400 nm / 1 mm diameter) with in-plane anisotropy Ta (5 nm)/ Py (12 nm)/ IrMn (5 nm)/pt (2nm) Ta (5 nm)/ Py (12 nm)/ Pt (2 nm) 18

19 SIZE EFFECTS IN FERROMAGNETS SIZE EFFECTS ASPECT RATIO EFFECTS SHAPE EFFECTS Vortex J.I. Martín et al., J. Magn. Magn. Mater. 256, 449 (2003) 19

20 What is a vortex state? Circular or Elliptical Magnets H N - Nucleation field H A - Annihilation field x In normal vortices: M R /M S = 0 R. Cowburn et al., J. Phys. D Appl. Phys. 33, R1 (2000) 20

21 400 nm Unbiased Dots FC in a strong field 1.0 H N M/M s H A VORTEX LOOP H (koe) Ta (5 nm)/permalloy (12 nm)/pt (2 nm) NO ANGULAR DEPENDENCE 21

22 What happens if we FC the F-AF dots in a strong field? (along FC) M/M s H (koe) BIASED 400 nm dots SHIFTED VORTEX LOOP Exchange Bias Ta (5 nm)/permalloy (12 nm)/irmn(5 nm)/pt (2 nm) 22

23 ANGULAR DEPENDENCE EXCHANGE BIAS EXPECTED cos( ) BEHAVIOR 23

24 SPIN STRUCTURE?? After Field Cooling Induced AFM Spin Structure H FC During Vortex Formation FM AFM x FM AFM Energetically unfavorable to have a vortex at H = 0! 24

25 ANGULAR DEPENDENCE Rotate Field H with respect to Exchange Bias K E K E H M/M s Critical angle c sin 1 H N,0 /H E M/M s H (koe) H (koe) Nucleation field Exchange bias at 0 degrees H (koe) H (koe) 25

26 MAGNETIC FORCE MICROSCOPY REVERSAL MODES M/M s H (koe) M/M s H (koe) Shifted Vortex! (along 0º) Coherent Rotation (along 90º) 26

27 ANGULAR DEPENDENCE INTUITIVE PICTURE H EFF H EX H EFF H EX H APP L H APP L If the effective field, H EFF, always larger than H N a vortex can NOT be nucleated! 27

28 nucleation field (Oe) annihilation field (Oe) ANGULAR DEPENDENCE NUCLEATION and ANNIHILATION FIELDS Assume exchange bias acts like offset-field: H N ( ) H N 2 (0) H E 2 sin 2 H A ( ) H A 2 (0) H E 2 sin 2 75 Nucleation Annihilation (degrees) (degrees) 28

29 MICROMAGNETIC SIMULATIONS H E ASSUMED AS A CONSTANT FIELD Vortex! Coherent Rotation! 29

30 H A (Oe) MICROMAGNETIC SIMULATIONS ANNHILATION FIELD 450 Two vortices 300 One vortex Coherent Rotation of S-state (deg) MORE COMPLEX REVERSAL STATES?? 30

31 For larger dots ( 1 mm) after FC in strong field: M/M S Py Py-IrMn m 0 H (mt) The result is a vortex-like loop, shifted along the magnetic field axis 31

32 1 mm Dots: Angular Dependence after FC from 550 K C H arcsin H N,0 E,0 C 20º (a) Py disks (b)-(e) Py-IrMn disks after FC in strong field, measured along several in-plane angles ( m = 0º, 22º, 30º and 90º) (f) Py-IrMn disks after ZFC from 550 K. Vortex Imprint? 32

33 nº grains H E [Oe] Tailoring of the exchange bias in the 1 mm dots At random Ordered T D T N T B Field Cooling (H negativo) T Bi T[K] Standard cooling procedure under positive field + cooling from T D in negative field + measurement at 300 K => H E vs. T D => T B + distribution of T B 33

34 Tailoring the critical angle: C H arcsin H N,0 E,0 Several hysteresis loops after field-cooling from 550 K using a positive field, H FC = 2.5 koe, and subsequently field cooling again but now using H FC = koe from T D = 350 K. The exchange bias decreases from 88 Oe to 56 Oe and, consequently, the critical angle increases from 20º to 40º. 34

35 sin ( C ) H E,0 (Oe) Relationship exchange bias critical angle C (deg) T D (K) /H E,0 (Oe-1) H arcsin C H N,0 E,0 35

36 How to imprint a vortex state into an AF? AC -Demag. Apply Field H FC Sample Heat Sample Sample Cool Sample to T Dem = 550 K T dem > T B in zero - field to RT m 0 H FC = 0 FC = 2.5 mt m 0 H FC m 0 H = 10 mt (a) (b) (c) 36

37 The result of such imprint. Loops along 0º M/M S The result is an asymmetric vortex-like loop The remanence depends on H FC H (mt) 37

38 Core Position (nm) Origin of the asymmetric loop shape Micromagnetic simulations performed. The core describes a straight trajectory but curvature is modified due to the AF. m 0 H FC = 0 mt m 0 H FC = 2.5 mt m 0 H FC = 10 mt (a) (b) (c) M/M S y Py-IrMn m 0 H FC = 10 mt x 1.0 M x Py M x M 0.5 x Py-IrMn ZFC M y M y (e) (d) (e) (f) (g) (f) y, Py y, Py-IrMn ZFC x y x 0 m 0 H appl (0 º ) m 0 H (mt) m 0 H (mt) 38

39 What happens after FC along 90º? 1.0 m 0 H FC = 10 mt y M x M y M/M S M y M x m 0 H (mt) (d) (a) (h) (b) x Imprinted Configurations States at Remanence Core pos. M/M S y x m 0 H (mt) m 0 H appl The simulations reveal that the core follows a curved trajectory in this case. A transversal hysteresis loop can be measured. 39

40 Probing the core trajectories by MFM FC along 0º FC along 90º 40

41 Conclusions SYSTEM 2: Ta (5 nm)/ Py (12 nm)/ IrMn (5 nm)/pt (2nm) Array of patterned disks (1 mm diameter) with in-plane anisotropy Tuneable magnetization reversal mode. Vortex-like loop with asymmetric reversible central part. Remanence tunable. Prove for the imprint model. Possible to imprint inhomogeneous magnetization states in an AF. 41

42 References: S. Brück, J. Sort, V. Baltz, S. Suriñach, J. S. Muñoz, B. Dieny, M. D. Baró, J. Nogués, Adv. Mater. 17 (2005) J. Sort, A. Hoffmann, S. H. Chung, K. S. Buchanan, M. Grimsditch, M. D. Baró, B. Dieny, J. Nogues, Phys.Rev. Lett. 95 (2005) J. Sort, G. Salazar, M. D. Baró, B. Dieny, A. Hoffmann, V. Novosad, J. Nogués, Appl. Phys. Lett. 88 (2006) Acknowledgements: Financial support from the HPRN-CT , the 2001-SGR-00189, the MAT projects and the U. S. DOE-BES, W ENG-38 contract is acknowledged. 42