Magnetic interactions and interface phenomena on nanogranular thin films

Transcription

1 Magnetic interactions and interface phenomena on nanogranular thin films J. Alonso, Mª Luisa Fdez-Gubieda, A. García Prieto, A. Svalov, I. Orue. Department Electricidad y Electrónica. Fac. Ciencia y Tecnología. Univ. Basque Country

2 collaborators L. Fernández Barquín, D. Alba Venero J. C. Hernández Garrido; D. Ortega Ponce T. Neisius

3 Introduction Outline Motivation and aims Sample preparation Structural characterization Magnetic behaviour Conclusions

4 Introduction Nanogranular thin films Different preparation techniques (PLD, sputtering ) Magnetic interactions are very important Rich variety of magnetic behaviours Crucial role in practical applications Magnetic nanoparticles Fe Solid matrix Ag

5 Low concentrations Introduction Non-interacting monodomain uniaxial magnetic nanoparticles: k B T < KV Magnetic moments are blocked along their easy axes k B T > KV Magnetic moments spontaneously reverse Superparamagnetism: [ ] M = nµ L( a) = nµ cotanh ( a) -1/ a (i) superparamagnetism (ii) Interacting superparamagnetism a = µ µ H k T B Volume density of nanoparticles

6 Introduction Increasing concentrations Long range interaceons of intermediate strength (dipolar, RKKY) collec%ve magne%c freezing. Spin glass like behaviour: Superspin Glass Randomness Frustra%on (iii) Spin-glass like (iv) Superspin glass J.L. Dormann et al, Adv. Chem. Phys. 98 (1997) 283 S. Sahoo et al; Phys. Rev. B 63 (22) F. Jimenez-Villacorta et al; Phys. Rev. B 82 (21) Volume density of nanoparticles

7 Higher concentrations Introduction Near or above the percolaeon. Long range ferromagneec correlaeons. Direct exchange interaceons, superexchange. The term superferromagne%sm was introduced by Mørup et al. In J. Magn. Magn. Mater. 4, 163 (1983). Sankar et al; J. Magn. Magn. Mat. 1 (2) 221 W. Kleemann et al; Phys. Rev. B 63 (21) Bedanta et al; J. Phys. D: Appl. Phys. 42 (29) 131 (v) Superferromagnetism Volume density of nanoparticles

8 Mo#va#on Aim of the work 1- Role of MagneEc InteracEons RKKY interaceon Dipolar interac%on Direct exchange interac%on SPUTTERING TECHNIQUE: Fe x Ag 1- x (15 x 55)) 2. Interface phenomena Direct exchange interaceons Greatly depend on the nature of the interface PULSED LASER DEPOSITION: Fe 5 Ag 5

9 Role of magnetic interactions Sample prepara#on Fe x Ag 1- x granular thin films: 15 x 55 DC magnetron spuhering: increasing Fe content increases nanopar%cle concentra%on Au coa%ng (1-2 nm) Fe nanoparecles Fe- Ag thin film (1-2 nm) Ag matrix Si substrate (5 μm) C. Binns et al; Phys. Rev. B 66 (22) J. Q. Wang and G. Xiao; Phys. Rev. B 49 (1994) 3982 P. Allia et al; Phys. Rev. B 73 (26)

10 Intensidad (u.a.) Dark field- TEM Univ. Cambridge Fe 5 Ag 5 Ag (111) Si (4) ! (º) Particles number Structural characteriza#on XRD Nanogranular Ag matrix ( D = 8-12 nm) textured in the fcc (111) direceon. Log-Normal D (nm) Fe nanoparecles present a Log- Normal size distribueon with D = 2.5 nm (x = 2)

11 Fe K-edge Structural characteriza#on EXAFS SPECTROSCOPY Fe bcc High disorder due to surface effect BM29 ESRF

12 Structural characteriza#on GISAXS 3% Fe 4% Fe 5% Fe BM16 ESRF Granular state up to 5%

13 MagneEzaEon vs temperature Magnetic behaviour M (emu/cm 3 ) ZFC H=5Oe FC x = 2 x = 25 x = 35 x = 4 x = 45 x = T (K) Maximum in the ZFC curves blocking/freezing of the magneec moments. Maximun of ZFC displace to higher T with increasing Fe content Above the maximum Curie- Weiss transi%on

14 MagneEc behaviour T P- FC MagneEzaEon vs temperature M (emu/cm 3 ) T B x = 35 T irr T (K) Maximum in the FC curves strong dipolar interac%on. spin glass like behaviour

15 M (emu/cm 3 ) x = T (K) x=15 MagneEzaEon vs temperature x=2 x=25 x=3 x = 35 x=4 θ(k) 47(2) 75(2) 119.4(2) 152(2) 222(2) 287(1) D (nm) 2.7(5) 2.7(5) 2.74(5) 2.95(5) 2.56(5) 2.52(5) θ! ZFC (emu/(cm 3 Oe)) MagneEc behaviour χ= C T-θ experimental fit T (K) C= x ' VM 3k 2 S B µ

16 MagneEc behaviour CharacterisEc temperatures Magne%c Percola%on

17 EvoluEon of the ZFC- FC curves with H MagneEc behaviour With increasing H: ZFC- FC curves tend to overlap. Faster response for x 35 longer range ferromagne%c order

18 EvoluEon of the ZFC- FC curves with H MagneEc behaviour Modified random anisotropy model: T P$ ZFC K! % D + " ( LH $ D )& ( H ) = ' ( ) /2 6kB ln( # m /# ) % ' 1 + x( LH $ D ) / D & ( 1/2 1.5 % HM % S 1 + " ( LH $ D ) / D & & * 1$ ' ( + * 2K + ' ( Nunes et al. Phys. Rev. B (25)

19 EvoluEon of the ZFC- FC curves with H MagneEc behaviour Modified random anisotropy model: T P$ ZFC K! % D + " ( LH $ D )& ( H ) = ' ( ) /2 6kB ln( # m /# ) % ' 1 + x( LH $ D ) / D & ( x = 2 K (J/m 3 ) 3.(8) 1 5 L (nm) 24(3) 1/2 1.5 % HM % S 1 + " ( LH $ D ) / D & & * 1$ ' ( + * 2K + ' ( x = (8) (3) x = (1)1 5 1(17)

20 Magnetization vs magnetic field MagneEc behaviour L = 25(nm) L = 39(nm) T = 5K L = 1(nm)

21 MagneEc behaviour Spin dynamic- AC suscepebility!' (emu cm 3 / Oe)!'' (emu cm 3 / Oe) Fe 24 Ag 76!' (emu / cc Oe)!'' (emu / cc Oe) T (K) 25 Fe35 Ag T (K)!' (emu / cc Oe)!'' (emu / cc Oe) 25 Fe39 Ag T (K).3 Hz.7 Hz.3 Hz.7 Hz 1 Hz 3 Hz 1 Hz 3 Hz 1 Hz 3 Hz 1 Hz Maximum in χ freezing of the magneec moment Maximum displace to higher T with increasing ν Above the maximum, χ decays in a dispersionless Curie- Weiss way, and χ becomes null magneec transieon mediated by interaceons.

22 AC suscepebility MagneEc behaviour EvoluEon of the maximum T max in χ with frecuency: log (!/! ) Dynamical cri%cal exponent ac%va%on law: x = 24 x = 35 x = 39 " " $ T # T % max g = & T ' g ( ) # z! Cri#cal exponent x = 24 zv = 8.3(6) T g = 132.5(3) K x = 35 zv = 4.7(4) T g = 192.7(1) K x = 39 zv = 4.3(3) T g = 278.9(3) K SSG SFM log [(T -T )/T ] max g g * D. Ucko et al; Phys. Rev. B 64 (21) 14433

23 Summary 3 25 INTERACTING SUPERPARAMAGNET SUPERFERROMAGNET T (K) 2 15 MAGNETIC PERCOLATION SUPERSPIN GLASS % at. de Fe J. Alonso, M.L. Fdez-Gubieda et al; Phys. Rev. B. 82 (21) 5446 M.L. Fdez-Gubieda, et al Nanoparticles features Electromagnetic properties: From science to engineering 212 Research Signpost. Editors A. Chilerio and P. Allia.

24 Interface Phenomena Pulsed Laser Deposition (PLD): Fe 5 Ag 5

25 Pulsed Laser Deposition (PLD): High energies reached during deposieon allow some Fe- Ag miscibility modify the Fe/Ag interface Fe 5 Ag 5 Crystalline nanopar%cles: Fe Crystalline nanoparecles Ag - Small bcc Fe nanoparecles (2-4 nm). - Bigger fcc Ag nanoparecles (1-12 nm). 2 nm Ag Fe Amorphous interface Amorphous interface: - Occupies around a 2 %. - Variable thickness of a few nm. Ins8tut Matériaux Microelectronique Nanosciences (Universités Paul Cézanne, France) Fei TITAN Ultra High ResoluEon

26 Magnetization vs temperature Magnetic behaviour M (emu/cm 3 ) M (emu/cm 3 ) 6 5 PLD sputtered T (K) Fe 5 Ag 5 T 1 x = 5 T T (K) The interface modify the magnetic behaviour 2 nm Ag Fe Crystalline nanoparecles Ag Fe Amorphous interface Two anomalous magnetic transitions

27 T<T 1 Magnetic behaviour M (emu/cm 3 ) T (K) H C (Oe) T 1 T 1 M r/ M s T (K) x = 5 High coerci%vity low correlaeon length. Magne%cally disordered interface disables coupling between Fe nanoparecles. J. Alonso, M.L. Fdez-Gubieda et al., Nanotechnology 23 (212) 2575

28 M (emu/cm 3 ) T<T 1 8 x = H (Oe) Magnetic behaviour T=5K Exchange bias aver cooling at remanence: H ex = Oe Unsaturated magneec behaviour. M (emu/cm 3 ) K 3 K 5 K 85 K 145 K 2 K 25 K x = H (Oe) Spin glass amorphous interface J. Alonso, M.L. Fdez-Gubieda et al., Nanotechnology 23 (212) 2575

29 M (emu/cm 3 ) T 1 T T (K) H C (Oe) T 1 <T<T 2 T 1 T 2 M r/ M s T (K) x = 5 Magnetic behaviour FerromagneEc behaviour: Smooth variaeon of the M(T) Low coerci%vity and high remanence Mr/Ms=.9 high correlaeon length. Ferromagne%c interface enables coupling between Fe nanoparecles. J. Alonso, M.L. Fdez-Gubieda et al., Nanotechnology 23 (212) 2575

30 M (emu/cm 3 ) T (K) H C (Oe) T>T 2 T 2 M r/ M s T (K) x = 5 T 2 Magnetic behaviour Coerci%vity increases and remanence decreases low correlaeon length. Paramagne%c interface disables coupling between Fe nanoparecles. Ms decreases 15%. J. Alonso, M.L. Fdez-Gubieda et al., Nanotechnology 23 (212) 2575

31 Interface Phenomena J. Alonso, M.L. Fdez-Gubieda et al., Nanotechnology 23 (212) 2575

32 Conclusions Fe x Ag 1-x granular thin film composed of magnetic nanoparticules (2-3nm) inside a Ag matrix have been prepared by sputtering technique. Crossover from a SSG to a SFM sate around 35 at. % More information: 1. J. Alonso, M.L. Fdez-Gubieda et al; Phys. Rev. B. 82 (21) M.L. Fdez-Gubieda, et al Nanoparticles features Electromagnetic properties: From science to engineering 212 Research Signpost. Editors A. Chilerio and P. Allia. Pulsed Laser Deposition Technique allow us to modify the FeAg interface The magnetic ordering/disordering of the interface acts as a switch in the coupling/decoupling of the magnetic nanoparticules More information: 1. J. Alonso, M.L. Fdez-Gubieda et al., Nanotechnology 23 (212) 2575

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