Exchange bias in a superspin glass system of ferromagnetic particles in an antiferromagnetic matrix

Transcription

1 Exchange bias in a superspin glass system of ferromagnetic particles in an antiferromagnetic matrix D. Fiorani Institute of Matter s Structure (ISM) Rome, CNR, Italy

2 I) OUTLINE Exchange Bias concepts (role of the disorder) II) Magnetic behaviour of granular Co-Mn films: Co nanoparticles embedded in Mn matrix (comparison with Co-Ag films) Exchange bias properties of Co-Mn films

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

4 Interface exchange coupling (IEC) IEC can be exploited as a tunable source of anisotropy (in magnetic recording spring magnets, magnetic sensors, spintronic devices) obtaining: - Improvement of thermal stability of FM magnetization through exchange anisotropy (increase of coercive field, nanoparticle blocking temperature) in FM/AF systems (Exchange Bias) - Control of the coercive field in FM(hard)/FM(soft) systems (exchange coupled magnetic recording media, spring magnets) - Supersoft and superhard properties in exchange coupled soft and hard nanograins

5 Composite exchange coupled perpendicular media 5 nm FeSiO 10 nm PdSi 1 nm (Variable thickness) SOFT PHASE (Co 0,26nm/PdSi 1nm) x 14 HARD PHASE GLASS PdSiO 2 4 nm RuAl 4 nm Tuning of coercivity, through the control of exchange coupling strength (via the non magnetic interlayer thickness) to improve media writability. Main advantage of exchange coupled media: Controlled reduction of coercivity, still keeping a high thermal stability factor K u V/K b T

6 FeSiO/PdSi(1-4nm)/(Co/PdSi) 14 Hard layer single t PdSi = 0.5 nm Proper coupling t PdSi = 4 nm decoupling K a V/K b T = 74 (single hard layer) = 58 (proper coupling) [(K a V/K b T)/H sf ] PC = 2 [(K a V/K b T)/H sf ] SHL

7 Beating the superparamagnetic limit with exchange bias Co(FM) particles in CoO(AFM) matrix (V.Skumriev et al, Nature 423, 850, 2003) FM(Co)/AFM(CoO) interface exchange coupling increases the thermal stability of FM magnetization particle (increase of T B ; Increase of H c ; exchange bias) 8 Co CORE CoO SHELL in CoO matrix T N m, J/T x FC ZFC 2 Co CORE CoO SHELL in Al 2 O 3 matrix AFM matrix T B T N CoO T, K Non-magneticmatrix: T B 30K Particle size: 3-4 nm

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

9 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 (c) (b) J. Nogues, I. Schuller,JMMM 203,192 (1999)

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

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

12 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. domain size: π(a AFM K AFM ) 1/2 H EB = (K eff A eff ) 1/2 /M FM t FM Randomness in exchange interactions at the interface Domain state model (U. Nowak et al, PRB 66, , 2002) Disorder and frustration induced by dilution in the AFM. FM layer coupled to a diluted (non magnetic substitution, defects) AFM layer (Co 1-x M x O; C 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)

13 Types of Exchange Bias Systems Layered films Patterned structures (dots..) Nanoparticles Inhomogeneous materials (inhomogeneous concentrated Spin Glasses)

14 Exchange Bias in an inhomogeneous SG 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

15 Nanoparticle Exchange Bias Systems Core/shell nanoparticles FM/AFM Co/CoO, Ni/NiO FM/FMI Fe/Fe 3 O 4, Fe/ γ-fe 2 O 3 FMI/SG NiFe 2 O 4, γ-fe 2 O 3 AFM/SG NiO Nanoparticles dispersed in a matrix FM/AFM FM/SG Co/CoO (core/shell NP in matrix) Fe/Cr 2 O 3, Fe/FeCl 2 Fe/FeO x (core/shell NP in matrix)

16 NiFe2O4 particles R. H. Kodama et al, JAP 81, 5552, 1997 Core shell FMI/SG particles: ordered core surrounded by a disordered shell

17 FM/AFM only two energetically equivalent spin configurations for the AFM phase FM(AFM,FI)/SG multiple equivalent spin configurations for the spin-glass phase

18 Spin glasses: multiminima energy structure

19 Expected EB features when one phase is spin glass Since : the SG phase is intrinsically disordered (random bonds: J ij distribution gaussian) magnetically frustrated ( competing F and AF magnetic interactions) characterized by aging effects and slow dynamics its multivalley energy structure is strongly modified by the magnetic field the following E.B. features are expected: Multiple random interfaces (random mixture of ferromagnetic and antiferromagnetic interface interactions): distribution of H ex and T B Different interface spin configurations are selected varying H cool (variation of H ex and H c ) Effect of the magneto-thermal history (H cool, T cool, cooling rate, aging) on exchange bias

20 Exchange bias in a superspin glass system of ferromagnetic particles in an antiferromagnetic matrix N. Domingo a,b, D. Fiorani a, Testa a C. Binns c, S. Baker c and J. Tejada b a Institute of Matter s Structure (ISM) CNR, Italy b Dept. Fisica Fonamental, Universitat de Barcelona, Spain c Department of Physics and AStronomy, University of Leicester, UK

21 Films of Cobalt particles in Mn matrix (Co-Mn) Films thickness: 200 nm (co-deposition by gas aggregation cluster source and MBE) Co Mn Co Ag φ 1.8 nm Co in Mn Co in Ag S1 S2 S3 S5 S4 S6 VFF 9.2% 4.7% 1.3% 8.9% 4.7% 1.5% Comparison with magnetic properties of Co particles in Ag matrix (same particle size, same volume fraction: 4.7% )

22 Co cluster size distribution Cluster Diameter Cluster Size Ion current (na) d (nm) Ion current (na) n atoms Co hcp (from EXAFS measurements on Co/Ag) Log-normal distribution of particle size

23 Mn matrix

24 Mn matrix ZFC/FC χ curves T = 5 K 7.50x10-5 M n film χ (emu/cm 3 ) Mn 8.0x10-3 ZFCFC Measurements: Mn film H = 100 Oe 7.0x x x10-3 m (emu) 5.00x x x x10-5 5K ZFC -7.50x H (Oe) 4.0x T (K) Disordered antiferromagnet (AFM domains of different size with uncompensated moments blocked at low temperature)

25 Mn-edge XMCD signal Drain current Transmission 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 )

26 DC susceptibility measurements

27 ZFC/FC susceptibility measurements H = 100 Oe χ (emu/cm 3 ) Co@Mn 1.5x x x10-3 FC FC ZFC ZFC Co@Mn Co@Ag χ (emu/cm 3 ) Co@Ag Co-Ag: assembly of non interacting particles Superparamagnetic blocking: individual particles blocking T max of χ(zfc): 17 K Curie-law behaviour of χ(fc) T irr /T max = T (K) Theoretical blocking temperature for Co- Ag,(1.8 nm hcp Co, magnetocrystalline bulk anisotropy, K a = 4.5x10 6 erg/cm 3 ): T B = 4k (from the Arrhenius law: K a V = 25 k b T B ) Evidence of strong surface anisotropy 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 behaviour

28 AC susceptibility measurements

29 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 χ'' α = 8.2; T g = 62 K; τ 0 = 10-9 s

30 Critical dynamics in spin - glasses Critical slowing down of dynamics approaching the phase transition from high temperature Critical power law: τ= τ 0 [T g /(T max (ν) T g )] α α = zν τ ~ ξ z ξ = [T g /(T T g )] ν In SG, α = 7-9 For Co/Mn: α =8.2 T g = 62 K; τ 0 = 10-9 s SG like behaviour: superspin glass (collectively frozen disordered state of Co particle moments) Long range interactions between Co particle moments propagated through the disordered Mn matrix (interface exchange interactions between Co particle moment and AFM Mn domains with uncompensated moments).

31 Hysteresis cycles measurements

32 M vs H measurements Co/Ag m (emu/cm 3 CoMn ) H (T) Nanospin Samples Co in Mn T = 5K VFF = 9,8% VFF = 4,7% VFF = 1,5% Co in Ag S5 S4 S H (T) Co/Mn T = 5K VFF=9,8% VFF=4,7% VFF=1,5% 8.9% 4.7% 1.5% Co in Mn S1 S2 S3 H c 350 Oe 275 Oe 200 Oe 9.2% 4.7% 1.3% T = 5 K H c 1800 Oe 1600 Oe 470 Oe M s 400 emu/cm emu/cm emu/cm 3 M s 39 emu/cm 3 29 emu/cm 3 6 emu/cm 3 M r /M s M rem /M s

33 Co/Mn and Co/Ag: Comparison between hysteresis cycles features M (emu/cm 3 ) Co@Mn Co@Mn ZFC 900 Co@Ag ZFC (a) H (koe) M (emu/cm 3 ) Co@Ag H C (Oe), H EB (Oe) Co@Mn H C ZFC Co@Ag H C ZFC (b) T (K) 1.0 Virgin curves 4.7% Co T = 5 K Co/Mn Co/Ag H c (Oe) M s (emu/cm 3 ) m / m sat CoMn CoAg Mn H / H sat

34 Co L-edge XMCD signal Pure Co clusters Co clusters in Ag or Mn (VF=4-5%) Itr Co in Ag Photon Energy (ev Photon energy (e Photon Energy Co in Mn 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

35 Mn-Edge XMCD signal Drain current Transmission Mn film T=8K ZFC Mn film T=8K ZFC %Co in Mn T=20K FC %Co in Mn T=20K FC %Co in Mn T=20K ZFC %Co in Mn T=20K ZFC %Co in Mn T=24K ZFC %Co in Mn T=24K ZFC Photon energy (ev) Photon energy (ev) Film thickness: 4.7% Co: 125 nm 22% Co: 23 nm

36 Co L-edge XMCD signal red: 4.7% Co black: 23.2% Co photon energy (ev)

37 Exchange Bias properties

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

39 Temperature dependence of H EB, H c and M r 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 Onset of exchange bias (T b ) and increase of coercivity and remanence after FC (4 koe) occur below 30 K, the temperature below which χ(fc) becomes temperature independent. M irm /M sat, M trm /M sat T (K) Co@Mn ZFC Co@Mn FC Co@Ag ZFC Co@Ag FC Co/Ag: FC, ZFC H c and M r coincide

40 Temperature dependence of H EB and H c H c FC Nanospin Sample2 Co in Mn at 4.7 % Hc, Hex (Oe) H c ZFC H EB FC Hc ZFC Hc FC 1T from 300 K Hex FC 1T from 300 K T (K) Exponential temperature dependence of H EB (as expected for non continuous AFM structure) and H c : H i = H i0 exp(-t/t 0 ) H EB0 = 2000 Oe; T 0 = 5 K H EB (5K) = 750 Oe H c0 = 3700 Oe; T 0 = 11 K H EB (5K, (Co(24A)/Mn(14)) 12 ) = 250 Oe (Stoner- Wolfarth temperature dependence of H c for Co/Ag: H c = H c0 (1-(T/T*) 1/2 )

41 Monte Carlo simulations FM/AFM core/shell spherical particles Random Anisotropy in the shell 0.25 Hc Hex Hex and Hc Y = Aexp(-x/b) Hc (A=0.2252; b=1.0774) Hex(A=0.1314; b=0.3305) temperature Exponential decrease of Hex with temperature (K. Trohidou, private communication)

42 Field cooling dependence of the Hex,H c(fc) and M r Dependence on the Field of Cooling (H FC ): Hex (Oe) Nanospin Sample2 Co in Mn at 4.7 % Hex M rem (% M sat ) Hc Mrem H (T) (H cooling )

43 Co/Mn: Relaxation of M r (from saturation) after ZFC from 300K m (emu) Sample 2 Nanospin Co in Mn at 4.7% From 4 T to 0 Oe Relaxations T = 5K T = 10K T = 15K T = 20K T = 25K T = 30K T = 35K H (T) m (emu) 1.3x10-4 Equation: Mo*(1-S*ln(x)) M = M0 (1-S ln(t)) Chi^2/DoF = E-13 Relax 4 T a 0 Oe R^2 = x10-4 Mo ±1.3226E-7 S ± x10-4 Mo_ ±1.5222E-7 S_ ± x10-4 Mo_ ±1.4745E-7 S_ ± Mo_ ±1.5383E-7 9.0x10-5 S_ ± Mo_ ±1.4849E-7 8.0x10-5 S_ ± Mo_ ±1.5374E-7 7.0x10-5 S_ ± Mo_ ±1.5554E-7 S_ ± x10-5 Mo_ ±1.5138E-7 S_ ± x10-5 Mo_ ±1.5448E-7 S_ ± x x x x K 4K 6K 8K 10K 12.5K 15K 17.5K 20K 25K 30K 35K 40K 45K 50K 55K lnt (s) Mo_ ±1.4997E-7 S_ ± Mo_ ±1.5788E-7 S_ ± Mo_ ±1.5376E-7 S_ ± Mo_ ±1.5475E-7 S_ ± Mo_ E-6 ±1.5345E-7 S_ ± Mo_ E-6 ±1.5296E-7 S_ ± Mo_ E-6 ±1.7828E-7 S_ ± Logarithmic relaxation Wide distribution of energy barriers M r = M o [1 S(T)ln(t/t o )]

44 Normalized viscosity vs T M ( t) = M (1 E c e t / 0 τ 0 n( E) de n(e) = S / T 1.0x10-2 Co@Mn Co@Ag 5.0x M(t) = Mo[1-n(<E>)KbTln(t/τo)] M(t) = Mo(1-S(T)ln(t/τo) S = -(1/Mo)dM/dlnt = n(<e>) KT S/T ~ n(<e>) T (K) Power law dependence of S/T~ n(e) = = (a+bt) -5 n(e) = E -1 barrier distribution for surface SG (R.H. Kodama et al, PRL 77, 394 (1996) <E> = KTln(<τ>/τ0) Mean energy barrier for the experimental time window characterized by a mean relaxation time <τ>

45 Field dependent remanence curves Normalized Remanence H cr IRM remanence curve I r (H) DCD remanence curve I d (H) Magnetic field (koe) δ M Sharp positive peak associated with exchange coupled systems Broader negative peak in dipolar coupled systems Magnetic field (H) Wolhfarth s relationship for an assembly of non interacting SD particles with UA anisotropy I d (H) = 1-2 I r (H) Deviations from linearity may be visualized through the di(dm) plots: Modified Wolhfarth s equation δi (δμ)= I d (H) - [1-2 I r (H)] (Henkel plot)

46 Henkel Plots m rem (H = 0) / m rem sat CoMn at 4.7% FC ZFC CoAg at 4.7% ZFC DCD remanence curves -1.0 I d (H) = 1-2 I r (H) M dcd (a.u.) CoAg at 4.7%VFF CoMn at 4.7%VFF M rem (a.u.) H inverse (T) δm = 0, no interaction Demagnetizing interactions much stronger in Co/Mn δm δi (δm)= I d (H) -[1-2 I r (H)] CoAg at 4.7%VFF H max = 270 Oe CoMn at 4.7%VFF H max = 4500 Oe H (Oe)

47 Irreversible susceptibility (derivative of the remanence curve), switching fields distribution (after ZFC and FC) m rem (H = 0) / m rem sat CoMn at 4.7% FC ZFC CoAg at 4.7% ZFC H inverse (T) χ irr = d(m rem /m sat )/dh inverse (a.u.) 1.0x x x x x10-4 CoMn at 4.7% FC ZFC CoAg at 4.7% ZFC Hr (χirr max) = 3800 Oe Hr (χirr max) = Oe Hr (χirr max) = 500 Oe H inverse (T) Oe Hr (max χ irr ) H rc (m rem = 0) Hc CoMn FC CoMn ZFC CoAg ZFC The switching fields distribution shifts to higher fields and flattens from Co/Ag to Co/Mn. FC induces a maximum (large) in the distribution.

48 Conclusions Superspin glass behavior in Co particles dispersed in a disordered Mn matrix Interparticle long range interactions mediated through Mn AFM domains, via interface exchange coupling With respect to Co-Ag (same particle size, same volume fraction): Increase of thermal stability of particle magnetization Exchange Bias (with coercivity and remanence enhancement after field cooling) Collective magnetic behaviour Broadening and shift of energy barrier and switching field distribution to larger values Activity within the Nanospin UE project (Coordinator: C. Binns)

49

50

51 Concentration dependence of H c(fc) and H EB Hc (FC 1T, from 300K) Heb (FC 1T, from 300K) Nanospin Samples Co in Mn S1 VFF = 9.2% S2 VFF = 4.7 % S2 VFF = 1.3 % 1000 Nanospin Samples Co in Mn S1 VFF = 9.2% S2 VFF = 4.7 % S2 VFF = 1.3 % Hc FC (Oe) Heb (Oe) T (K) T (K) H EB and H c(fc ) increase with Co particles concentration. T b is concentration independent

52 Exchange Bias: training effect T = 5 K H eb (Oe) n (number of cycles) Reduction of 46 % between the first and the second cycle

53 NiFe/CoO H ex for different CoO thicknesses T. Ambrose and C.L.Chien, JAP 83, 6822 (1998)

54 Sample Preparation for XMCD for XAS Measured in Transmission I 0 I d I t I t /I Circularly polarised soft X-rays ( eV) from synchrotron Cu mesh Thin magnetic sample L-edge absorption spectrum (shows dichroism) Applied magnetic field QuickTime and a TIFF (Uncompressed) decompressor are needed to see this picture. Deposit clusters and matrix Deposit 10nm carbon Deposit 10nm carbon TEM grid 60% transmission

55 Mn Edge XMLD Drain current Transmission Mn film T=8K ZFC Mn film T=8K ZFC %Co in Mn T=20K FC 23.2%Co in Mn T=20K ZFC %Co in Mn T=20K FC 23.2%Co in Mn T=20K ZFC Photon energy (ev) 4.7%Co in Mn T=24K ZFC Photon energy (ev) 4.7%Co in Mn T=24K ZFC

56 Ultrasoft magnetic properties Exchange-coupled nanograins v g =K 1 g α 6 =K 1 g α 6 V Anisotropy averaged over many particles : extremely small ξ = π K A with K = δ g α= D K g vg V K g = 10 4 J/m 3 D =10nm, δ g = 100nm α = J/m 3 K = Ultra-soft magnetic properties

57 Fe/FeOx : FM/SG system Aging effect on H ex D. Fiorani, L. Del Bianco, A. Testa, JMMM 179, 300 (2006) The magnetic field (4 koe) is applied at 50 K for different times (waiting time: t w ) before starting the cooling down to 5 K, where the cycle is measured T = 5 K H cool = 4 koe; T i = 50 K M (emu/g) T = 5 K H cool = 4 koe T i = 50 K t w = 60 s t w = s H ex (Oe) H (Oe) t w (s) t w = 60 s; H ex = 324 Oe; t w = s; H ex = 402 Oe

58 FM AFM Exchange Bias Effect Ideal system (Meiklejohn and Bean, 1956) for (FM) / AFM interfaces M H T Néel < T <T Curie Field Cooling H ex = σ/(m FM t FM ) = n J int S F S AF /(M FM t FM ) (a) (b) (c) H left T < T Néel M (a,c) H right H (b) J. Nogues, I. Schuller,JMMM 203,192 (1999) K AFM t AFM > J int σ: interfacial exchange energy density n: number of exchange bonds per unit area J int = exchange constant across the FM/AFM interface per unit area. M FM, t FM : magnetization and thickness of the FM layer K AFM, t AFM : anisotropy constant and thickness of the AFM layer