Exchange bias in nanogranular films. D. Fiorani. Institute for Matter s Structure (CNR, Rome, Italy)

Transcription

1 Exchange bias in nanogranular films D. Fiorani Institute for Matter s Structure (CNR, Rome, Italy)

2 Nanospin EU project D. Fiorani. E. Agostinelli, CNR, Rome, Italy A.Testa, D. Peddis J. Tejada, N. Domingo* CNR* Univ.Barcelona*, Spain A. C. Binns, S. Baker Univ. of Leicester, UK J. Blackmann Univ. of Reading, UK P. Trohidou Demokritos, Athens, Greece

3 Devices governed by interface exchange coupling HARD DISK Magnetoresistive read heads MRAMs Spin valve structure

4 Exchange bias FC Co/CoO particles ZFC Interface exchange interaction between the ferromagnetic Co core and the antiferromagnetic CoO surface shell After FC from T>T N to T<T N : - Shifted hysteresis loop (unidirectional anisotropy) µ 0 H E 0.2 T - increase of coercivity t FM FM Meiklejohn and Bean, Phys. Rev. 102 (1956) 1413, Phys. Rev. 105 (1957) 904 AFM

5 E THEORETICAL MODEL (W.H. Meiklejohn, J. Appl. Phys. 33 (1962) 1328) ASSUMPTIONS (IDEAL SYSTEM) MONOCRYSTALLINE COMPONENTS ABSENCE OF INTERFACE ROUGHNESS ABSENCE OF FM AND AFM DOMAINS FULLY UNCOMPENSATED AFM INTERFACE LAYER SPIN COLLINEARITY (PARALLEL FM AND AFM EASY AXES) FERROMAGNETIC INTERFACE SPIN COUPLING COHERENT SPIN ROTATION APPLIED FIELD ON FM LAYER = HM t cos( ϑ β ) + K t sin 2 ( β ) + FM FM FM ANISOTROPY FM FM t FM M FM M AFM β α K AFM, K FM H θ E AFM ANISOTROPY sin 2 ( α ) + K AFM t AFM J INT INTERFACIAL COUPLING cos( β α ) 2 = HM FMtFM cos( ϑ β ) + K AFMt AFM sin ( α) J INT cos( β α) K FM t FM << K AFM t AFM K t >> AFM AFM J INT H = E J M FM INT t FM

6 Realistic system: interfacial disorder (roughness,defects,grain boundary) and domains.

7 Role of disorder in EB Random field model (A.P. Malozemoff, PRB 35, 3679 (1987) Random fields at the interface, due to random roughness and defects, cause the AFM to break up into domains (with domain walls perpendicular to the interface) uncompensated at the surface, generating EB. Randomness in exchange interactions at the interface (frustration) Domain size: π(a AFM K AFM ) 1/2 H EB = (K eff A eff ) 1/2 /M FM t FM Domain state model (U. Nowak et al, PRB 66, , 2002) Disorder and frustration induced by dilution (non magnetic substitution, defects) in the AFM. (Co 1-x M x O; Co 1-y O). Increase of H EB with dilution, which promotes the formation of domains with net magnetization in the volume of AFM (Monte Carlo simulations and experiments)

8 Exchange Bias in a concentrated inhomogeneous spin glasses J.S. Kouvel, J.Phys. Chem. Solids 21, 57 (1961) Frustration of magnetic interactions (coexistence of FM and AFM interactions) and disorder (statistical fluctuations in the local Mn concentration) determine the formation of AFM and FM domains, exchange coupled at the interface (EB).

9 Exchange Bias Systems with Spin GLASS component The spin glass component can play either the role of the AFM: e.g. FMI/SG (core/shell nanoparticles) NiFe 2 O 4, γ-fe 2 O 3 FM/SG Co/Cu 94 Mn 6 or the role of the FM(FMI): e.g. AFM/SG (core/shell nanoparticles) NiO

10 Exchange Bias in Films of Cobalt particles in Mn matrix Co Mn Co φ 1.8 nm Ag Comparison with magnetic properties of Co particles in Ag matrix (same particle size: 1.8 nm; same volume fraction: 4.7%) Work supported by EU NANOSPIN Project

11 Films of Cobalt particles in Mn matrix (Co-Mn) Co-deposition by using a gas aggregation cluster source (Co) and MBE source (Mn) Cluster source Thickness: 200 nm Ag substrate; Ag capping layer MBE source C. Binns (Leicester, UK)

12 Co cluster size distribution Cluster Diameter Cluster Size Ion current (na) d (nm) Ion current (na) n atoms Log-normal distribution of particle size (measured in situ by a quadrupole filter) Mean size: 1.8 nm Mean number of atoms in a cluster: 260

13 Mn film X-ray diffraction Film: Ag(23nm)/Mn(127nm)/Ag(34nm) α - Mn (T N = 95 K) bcc (a = nm) Crystallite size: 11 nm

14 EXAFS data (Co-K edge); measurements in fluorescence) k 3 χ Fourier transform Co (MBE film) Co hcp r(n.n.) = 2.50 A Full lines: data; Dashed lines: fit 11.5% Co@Mn r 1 = 2.55(0.04)A; r 2 = 2.41 (0.04)A; Lower amplitude for Co@Mn films with respect to Co(hcp) film: Distribution in n.n. distance (CoMn alloying): Fits results: r 1 = 2.55(0.04)A; n.n. Co 1-x Mn x alloy fcc; a = 3.61(0.06)A; N1 = 8.7(2), (at the outer particle shell-af ) (Bulk Co 1-x Mn x : fcc, AF, for 0.32<x<0.52) r 2 = 2.41 (0.04)A; n.n.co bcc core (F); a = 2.78(0.05)A; N2 = 0.6(0.9) (significant alloying: 72% )

15 Co core, bcc, F Co 1-x Mn x alloy shell, fcc, AF α Mn matrix, AF

16 Mn L-edge XMCD signal for Mn matrix Drain current (surface sensitive) Transmission (bulk sensitive) Photon energy (ev) Photon energy (ev) Spin inbalance, mostly coming from the layers close to the surface. ( The drain current signal is generated mostly within the top 5 nm of the sample) (Film thickness: 50 nm )

17 Drain current Mn-Edge XMCD signal Transmission Mn Mn Mn(Co23%) FC Mn(Co23%) Photon energ Mn(Co5%) Mn(Co5%) Suppression of the uncompensated Mn moment for Co(5%)@Mn (disorder in the configuration of Mn spins, aligning according to the random orientation of Co cluster moments, washing out uncompensated spins)

18 Co L-edge XMCD signal Pure Co clusters Black: transmission; red: drain Co clusters in Ag or Mn (VF=4-5%) I tr Co in Ag Co in Mn Photon Energy (ev Photon Energy (ev) T=305 K B=0.6T T=24K (ZFC) B=0.6T m L = 0.31±0.02µ B m S = 2.2±0.3µ B Vanishing average Co moment for Co@Mn, unlike for Co@Ag.

19 Co 147 encapsulated in Mn shells (T.B. calculations) (J. Blackman; Reading, UK) Co core moments are quite insensitive to the thickness of Mn coating Co interface moments, much weaker, are sensitive to the thickness of Mn coating , 252, 362, 492

20 T = 5 K 4.7% Co T = 5 K Co/Mn Co/Ag H c (Oe) Much lower magnetization and much larger coercivity in Co@Mn (exchange anisotropy) N.Domingo et al, J. Phys D Appl. Phys. 41, (2008)

21 ZFC/FC susceptibility measurements: Co 4.7% H = 100 Oe χ (emu/cm 3 ) Co@Mn 1.5x x x10-3 FC FC ZFC ZFC Co@Mn Co@Ag T (K) χ (emu/cm 3 ) Co@Ag Co-Ag: assembly of non interacting particles Superparamagnetic behaviour: blocking of individual particle moments T max of χ(zfc): 17 K Curie-law behaviour of χ(fc) T irr /T max = 4.7 Co-Mn:spin-glass like behaviour T max of χ(zfc): 65 K Plateau of χ(fc) below 30 K T irr /T max = 1.2 Collective freezing of particle moments Much lower susceptibility (2 orders of magnitude) for Co@Mn with respect to Co@Ag

22 Other evidences of Spin-Glass- like behaviour Critical dynamics Memory effects

23 AC susceptibility measurements 1.2x10-6 Co/Mn(ν = Hz) Nanospin S2 Co en Mn at 4.7% 1.0x10-6 Co/Mn (ν = Hz) χ ' (emu) 8.0x x Co/Ag Arrhenius law τ = τ 0 exp(k a V/K b T) τ = t m =1/f f for T = T B τ 0 = 6x10-11 s K a = 2x10 7 erg/cm 3 ZFC FC w = 0.3 Hz w = 1 Hz w = 3 Hz w = 30 Hz w = 0.3Hz w = 1 Hz w = 3 Hz w = 30 Hz χ * AC (emu) 0.0 T (K) 12/12/ Much weaker frequency dependence for Co/Mn: T max (ν)/<t max > logν = 0.01 (0.10 for Co/Ag) χ' T (K) χ'' Co/Mn Power law τ = τ 0 [T g /(T max(ν) ) T g )] α (ν) α = 8.2; T g = 62 K; τ 0 = 10-9 s Collective dynamics

24 Critical dynamics in spin - glasses Critical slowing down of dynamics approaching the phase transition from high temperature Critical power law: τ= τ [T 0 g /(T max (ν) T g )] α α = zν (7 9) τ ~ ξ z ξ = [T g /(T T g )] ν For Co/Mn: α =8.2 T g = 62 K τ 0 = 10-9 s SG like behaviour: superspin glass (SSG) (collectively frozen disordered state of Co particle moments) Long range mixed (ferromagnetic and antiferromagnetic) interactions between Co particle moments propagated through the AF ordered Mn matrix

25 Memory effects : experimental procedure (stop and wait protocol) H = 0 H =100 Oe 150 K 5 K H = 100 Oe 5 K 80 K ZFC measurements H = 0 tw= 3 h 150 K 50 K 5 K H = 100 Oe H =100 Oe 5 K 80 K 2.8x x x x10-5 CoMn 4.7 % H = 100 Oe T stop =50 K (3h) 2.4x10-5 M (emu) 2.0x x x x10-5 Ref mem M (emu) 2.0x x x T (K) T (K)

26 H cooling = 2 T, from 300 K Co/Mn: : Exchange Bias After field cooling: Shift of the cycle (EB, H EB = 750 Oe) increase of H c M (emu/cm 3 ) Co@Mn increase of M r /M s (vertical shift) 30 Co@Mn ZFC Co@Mn FC H (koe) T = 5 K Co/Mn (ZFC) Co/Mn (FC) (a) H c (Oe) T = 5 K M r /M s

27 Mr off (vertical shift) is due to strongly pinned uncompensated AF interface moments Heb Mr off 7.0x x x10-5 H eb (Oe) x x x x Mr off (emu) T(K) The appearance of H eb (orizzontal shift) and Mr off (vertical shift) at the same temperature and the similar temperature dependence suggest that EB is due to strongly pinned uncompensated AF interface moments

28 Hc, Hex (Oe) Temperature dependence of H EB, H c H c FC H c ZFC H EB FC T (K) Nanospin Sample2 Co in Mn at 4.7 % Hc ZFC Hc FC 1T from 300 K Hex FC 1T from 300 K Below 30K (the temperature below which χ(fc) becomes temperature independent): onset of exchange bias (T b ): EB is related to the SG freezing features increase of coercivity after FC Hex (J FM /gµ B ) Hc(J FM /gµ B ) 0,5 0,4 0,3 0,2 0,1 0,0 MC simulations (P. Trohidou; ; Athens) (assuming a Co F core and a disordered AF shell) T(J/K B ) Hc Hex Co/Ag: FC, ZFC H c coincide

29 Origin of EB in Co core, bcc, F Co 1-x Mn x alloy shell, fcc, AF Mn matrix, AF Interface exchange coupling between the ferromagnetic Co core and the antiferromagnetic Co 1-x Mn x alloy shell Core/shell entities, whose total moment is randomly frozen, are coupled through the AF Mn matrix (collective behaviour).

30 Conclusions Frustrated and disordered system of Co particle moments, long range interacting through the AFM Mn matrix, collectively freezing in random directions at low temperature (SSG). Significant degree of alloying at the Co/Mn interface and strong reduction of the average Co moment, consistent with a metallic core surrounded by a Co x Mn 1-x fcc AF shell that fades into a pure AF Mn matrix. Exchange bias in a SSG system due to interface exchange coupling between Co (F) core and Co x Mn 1-x (AF) alloy shell (collective behaviour in a randomly frozen state) Enhancement of magnetothermal stability with respect Co@Ag, due to interface exchange coupling

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32 ZFC/FC susceptibility measurements: Co 4.7% H = 100 Oe χ (emu/cm 3 ) Co@Mn 1.5x x x10-3 FC FC ZFC ZFC Co@Mn Co@Ag T (K) χ (emu/cm 3 ) Co@Ag Co-Ag: assembly of non interacting particles Superparamagnetic behaviour: blocking of individual particle moments T max of χ(zfc): 17 K Curie-law behaviour of χ(fc) T irr /T max = 4.7 Co-Mn:spin-glass like behaviour T max of χ(zfc): 65 K Plateau of χ(fc) below 30 K T irr /T max = 1.2 Collective freezing of particle moments Much lower susceptibility (2 orders of magnitude) for Co@Mn with respect to Co@Ag

33 Co core, bcc, F Co 1-x Mn x alloy shell, fcc, AF α Mn matrix, AF The strong reduction of the average Co moment for Co@Mn is consistent with AF Co 1-x Mn x alloying

34 CoMn bulk alloy (31% Mn) J.Kouvel, J. Phys. Chem. Solids, 16, 107 (1960) Spin glass behaviour Exchange Bias Frustration of magnetic interactions (coexistence of FM and AFM interactions) and disorder (statistical fluctuations in the local Mn concentration) determine the formation of AFM and FM domains, exchange coupled at the interface (EB).

35 AC susceptibility measurements χ ' (emu) Co/Ag (ν = Hz) Co/Mn(ν = Hz) Co/Mn (ν = Hz) 1.2x x x x x x x x10-7 Co in Ag at 4.8% χ'(t) w = 0.1 Hz w = 0.3 Hz w = 1 Hz w = 3 Hz w = 10 Hz w = 30 Hz 8.0x x10-7 Nanospin S2 Co en Mn at 4.7% T (K) Co/Ag Arrhenius law τ = τ 0 exp(k a V/K b T) τ = t m =1/f f for T = T B τ 0 = 6x10-11 s K a = 2x10 7 erg/cm 3 χ ' (emu) ZFC FC w = 0.3 Hz w = 1 Hz w = 3 Hz w = 30 Hz w = 0.3Hz w = 1 Hz w = 3 Hz w = 30 Hz Much weaker frequency dependence for Co/Mn: T max (ν)/<t max > logν = 0.01 (0.10 for Co/Ag) χ * AC (emu) T (K) 12/12/05 χ' T (K) Co/Mn Power law τ = τ 0 [T g /(T max(ν) ) T g )] α (ν) χ'' α = 8.2; T g = 62 K; τ 0 = 10-9 s Collective dynamics

36 Co/Mn film X-ray diffraction

37 Interface exchange coupling (IEC) IEC can be exploited as a tunable source of anisotropy (in magnetic recording media, spring magnets, magnetic sensors, spintronic devices) obtaining: - Super-soft and super-hard properties in exchange coupled nanograins - Control of the coercive field in FM(hard)/FM(soft) systems (better writability in exchange coupled magnetic recording media; larger maximum energy product in spring-type magnets) Hard Soft - Improvement of thermal stability of FM magnetization (increase of nanoparticle blocking temperature and coercive field) in FM/AF systems (exchange anisotropy, Exchange Bias)

38 Monte Carlo simulation of the Hysteresis cycles for Co/Mn 1,0 0,8 0,6 Co/Mn Co/Ag C=5% 0,4 0,2 M/Ms 0,0-0,2-0,4-0,6-0,8-1, H(J FM /gµ B ) Total E = E (Intra-particle) + E (Inter-particle) E (Intra-particle) = Anisotropy + Zeeman + Exchange (core, shell) E (Inter-particle) = Dipole-Dipole Interactions Energy Minimisation by Metropolis Monte Carlo algorithm

39 Field cooling dependence of the Hex,H c(fc) and M r Dependence on the Field of Cooling (H FC ): M rem H c Hex (Oe) N anospin S am ple2 C o in M n at 4.7 % H e x M rem (% M sat ) H c M re m H (T ) (H cooling ) H ex

40 Mr off (vertical shift) is due to strongly pinned uncompensated AF interface moments Hex (Oe) x x x x x x x10-5 Mr off (emu)

41 Exchange Bias: training effect MC simulation by K. Trohidou, NCSRD-Athens, GR T = 5 K H cool = 1 T T=0.01(J/k B ) H eb (Oe) H ex (J fm / g µ b ) n (number of cycles) n( loop number) Experiment : H EB Reduction of ~ 50 % after the first cycle MC simulation for FM core/afm shell with random shell anisotropy : ~45% reduction

42 Temperature dependence of H EB and H c Experiment MC simulations (P. Trohidou; ; Athens) Hc, Hex (Oe) H c FC H c ZFC H EB FC T (K) Nanospin Sample2 Co in Mn at 4.7 % Hc ZFC Hc FC 1T from 300 K Hex FC 1T from 300 K Co/Ag: H c after ZFC and FC coincide Hex (J FM /gµ B ) Hc(J FM /gµ B ) 0,5 0,4 0,3 0,2 0,1 0,0 Hc Hex T(J/K B ) (assuming a Co F core and a disordered AF shell)

43 Memory effects : experimental procedure (stop and wait protocol) H = K 5 K H = 100 Oe H =100 Oe 5 K 80 K H = 0 tw= 3 h 150 K 50 K 5 K H = 100 Oe H =100 Oe 5 K 80 K M (emu) 2.8x x x x x x x x10-5 CoMn 4.7 % H = 100 Oe T stop =50 K (3h) Ref mem Μ (emu) 4.0x x x x x10-7 Co Mn 4.7 % Com 9.8 % 1.2x x T (K) -8.0x T (K)

44 What is known about Co/Mn system? Bulk Co 1-x Mn x alloys Magnetisation goes to zero at x~0.32 Antiferromagnetic fcc configurations at intermediate x Single Mn impurities in Co Mn aligns antiferromagnetically to Co in bulk Stepanyuk et al Phys. Rev. B 94, 5157 (1994) Mn aligns ferromagnetically to Co in small clusters Yin et al, Phys. Rev. Lett. 98, (2007); Knickelbein, Phys. Rev. B 75, (2007) Mn layers on Co(001) Complex spin arrangements M Passi-Mabiala et al, Surf. Sci. 518, 104 (2002); Demangeat and Parlebas, Rep. Prog. Phys. 65, 1679 (2002)

45 m rem (H = 0) / m rem sat CoMn at 4.7% FC ZFC CoAg at 4.7% ZFC Remanence curves vs field H rev (T) δm = I d (H) - [1-2 I r (H)] I d (DCD remanent magnetization after saturation) I r ( IRM remanent magnetization after demagnetization) δm CoAg at 4.7%VFF H max = 270 Oe CoMn at 4.7%VFF H max = 4500 Oe H (T) δm = 0, no interaction Demagnetizing interactions much stronger in Co/Mn

46 k 3 χ EXAFS data Fourier transform Co (MBE film) Full lines: data; Dashed lines: fit 11.5% Co@Mn N 1 = 8.7(2.9), r 1 = 2.55(0.04)A (Co 1-x Mn x, fcc) N 2 = 0.6(0.9), r 2 = 2.39(0.03)A (Co, bcc ) Lower amplitude for Co@Mn films with respect to Co(hcp) film: spread in n.n. distance. Fitting consistent with fcc (a = 3.59 A) Co 1-x Mn x alloy-af (at the outer particle shell) and Co (F) core. (Bulk Co 1-x Mn x : AF fcc for 0.32<x<0.52) Picture: Co core, bcc, F Co 1-x Mn x alloy shell, fcc, AF Mn matrix AF

47 Spin glasses: multiminima energy structure

48 Mn matrix: M vs T ZFC/FC curves 8.0x10-3 Mn film H = 100 Oe χ (emu/cm 3 ) Mn 7.0x x x x T (K) Presence of relaxing magnetic regions of different size with uncompensated magnetic moment Disorder

49 Mn matrix ZFC/FC χ curves T = 5 K 8.0x10-3 Mn film H = 100 Oe 7.50x10-5 Mn film χ (emu/cm 3 ) Mn 7.0x x x x T (K) m (emu) 5.00x x x x10-5 5K ZFC -7.50x H (Oe) Disordered antiferromagnet (AFM domains of different size with uncompensated moments blocked at low temperature)

50 Origin of EB in Interface exchange coupling between the ferromagnetic Co core and the antiferromagnetic fcc Co 1-x Mn x alloy Picture: Co core F Co 1-x Mn x alloy shell AF Mn matrix AF

51 Temperature dependence of H EB, H c and M r Hc, Hex (Oe) H c FC H c ZFC H EB FC Nanospin Sample2 Co in Mn at 4.7 % Hc ZFC Hc FC 1T from 300 K Hex FC 1T from 300 K M irm /M sat, M trm /M sat Co@Mn ZFC Co@Mn FC Co@Ag ZFC Co@Ag FC T (K) T (K) Below 30K (the temperature below which χ(fc) becomes temperature independent): onset of exchange bias (T b ): EB is related to the SG freezing features increase of coercivity after FC: Increase of remanence after FC (4 koe) Co/Ag: FC, ZFC H c and M r coincide

52 Titolo slide Testo slide figura Speaker Conferenza Luogo data

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54 Mn matrix T = 5 K after ZFC Long Moment (emu) M (emu) H (Oe) Uncompensated antiferromagnet: easily saturating (non compensated moment) and non saturating linear component (AFM)

55 Superspin Conclusions glass type behavior of Co particles dispersed in a Mn matrix (Long range interactions between Co particle moments mediated by interface exchange coupling between FM particles and uncompensated AFM domains of matrix) Exchange Bias producing an increase of thermal stability of particle magnetization (increase of Tmax, Hc and shift and broadening of the energy barrier distribution to larger values) Nanospin UE project

56 Co/Ag Co/Mn ln(τ) =ln(τ 0 ) -U/T 2 τ = τ 0 [T g / (T max (ν)- T g )] α 0 ln (τ) -1-2 Logτ /T (K -1 ) T max (K) Arrhenius law Power law

57 Numerical Modeling AFM Shell AFM Interface (alloying with the FM interface) FM Interface (alloying with the AFM interface) FM Core E (Inter-particle) Dipole-Dipole Interactions Multiscale Modeling : Beyond the single-spin spin (SW) model PR B71, (2005)

58 ZFC/FC curves for Co/Mn and Co/Ag Numerical Modelling Co/Mn Co/Ag M T T(J/K B ) C=5%

59 Comments on the slides Slide 1 describes the modelling, but I don t think you need to use it. The following comments are enough for slides 3-5: Developing the picture of your slide 18 (NY presentation) we have performed numerical modeling of the hysteresis loop and the zfc/fc curve for Co/Mn nanoparticles for concentration 5% including in our simulations dipolar interparticle interaction effects. The same for the following figure, that gives the temeperature dependence of Hc and Hex for Co/Mn and Hc for Co/Ag. The results agree with the experimental findings in slides 11,19,20 confirming the picture of fig 18 for Co/Mn nanoparticles. In the same figures we give numerical modelling for Co/Ag samples for concentration 5% For the Co/Ag samples we develop the single spin Stoner- Wohlfarth type picture for Co nanoparticles in Ag matrix including again dipolar interparticle interaction effects.

60 Coercivity and Exchange Bias Temperature dependence Numerical Modelling 0,5 0,5 Hex (J FM /gµ B ) Hc(J FM /gµ B ) 0,4 0,3 0,2 0,1 0,0 Hc Hex T(J/K B ) Hex(J FM /gµ B ) Hc(J FM /gµ B ) 0,4 0,3 0,2 0,1 0, T(J/K B ) Co/Mn Hc Hex Co/Ag Hc C=5% Exponential decay of Hex and Hc with temperature for the core/shell nanoparticles

61 ab initio results on 147 atom fcc clusters Co 1 Mn 146 Co 13 Mn 134 (0.06µ B ) (0.48µ B )

62 Exchange Bias Effect FM AFM M H T Néel < T <T Curie Field Cooling (a) T < T Néel M (a,c) (b) H left H right H J. Nogues, I. Schuller, JMMM 203,192 (1999) (c) (b)

63 Exchange Bias in Films of Cobalt particles in Mn matrix VFF S1 9.2% Co Mn Co Ag φ 1.8 nm Co in Mn Co in Ag S2 S3 S5 S4 4.7% 1.3% 8.9% 4.7% S6 1.5% Comparison with magnetic properties of Co particles in Ag matrix (same particle size, same volume fraction) Thickness: 200 nm Ag substrate; Ag capping layer Work supported by EU NANOSPIN Project

64 Mn-L edge XMCD with circularly polarised X-rays (sensitive to ferromagnetic moments) Drain Current (surface sensitive) Transmission (bulk-sensitive) Pure Mn ZFC Pure Mn-23%Co FC Pure Mn-23%Co ZFC Pure Mn-5%Co ZFC

65 Temperature dependence of H c and H ex Numerical Modelling (P. Trohidou; ; Athens) 0,5 Hex (J FM /gµ B ) Hc(J FM /gµ B ) 0,4 0,3 0,2 0,1 0,0 Hc Hex T(J/K B ) C=5% Exponential decay of Hex and Hc with temperature f core/shell nanoparticles (reproducing experimental findings)

66 Temperature dependence of H EB, H c and M r Hc, Hex (Oe) H c FC H c ZFC H EB FC Nanospin Sample2 Co in Mn at 4.7 % Hc ZFC Hc FC 1T from 300 K Hex FC 1T from 300 K M irm /M sat, M trm /M sat Co@Mn ZFC Co@Mn FC Co@Ag ZFC Co@Ag FC T (K) T (K) Below 30K (the temperature below which χ(fc) becomes temperature independent): onset of exchange bias (T b ): EB is related to the SG freezing increase of coercivity after FC: appearence of an extra anisotropy source (unidirectional anisotropy) Increase of remanence after FC (4 koe) Co/Ag: FC, ZFC H c and M r coincide

67 Irreversible susceptibility - switching fields distribution (SFD) after ZFC and FC m rem (H = 0) / m rem sat CoMn at 4.7% FC ZFC CoAg at 4.7% ZFC H rev (T) χ irr = d(m rem /m sat )/dh rev (a.u.) 1.0x x x x x10-4 CoMn at 4.7% FC ZFC CoAg at 4.7% ZFC Hr, fc (χirr max ) = 3800 Oe Hr (χirr max) = 500 Oe H rev (T) Oe CoMn FC CoMn ZFC H rc (m rem = 0) (Oe) Hc for Co/Mn increase of remanent coercivity After FC CoAg (ZFC, FC) for Co/Ag: narrow switching fields distribution (SFD) independent on the cooling procedure for Co/Mn: broadening of switching field (energy barrier) distribution coherent with SSG after ZFC, Flat SFD, covering a large field range After FC, SFD has a broad maximum, centered at larger magnetic fields with respect Co/Ag).

68