Influence of Size on the Properties of Materials

Transcription

1 Influence of Size on the Properties of Materials M. J. O Shea Kansas State University mjoshea@phys.ksu.edu If you cannot get the papers connected to this work, please me for a copy

2 1. General Introduction to finite size Why make things small? Physics reasons: 1) -finite size (electrons confined), -domain formation inhibited. 2) -more interface. Economic reasons more devices on a chip or more magnetic storage density

3 Why Make Things Small? 1) Modify/create new materials properties i) Finite size effects Quantization of energy levels (confinement) k F = (3 2 n) 1/3 F ~ 1/n 1/3 Reduced ordering temperatures Melting point Magnetic ordering Increased coercivity (single magnetic domains) ii) Surface/Interface effects. Layers, particles, < 10nm. Interface Anisotropy Interface Exchange Giant magnetoresistance in magnetic nanostructures

4 Economical to make current devices smaller. e.g. magnetic storage media (Himpsel, Adv. Phys. 47, 511 (1998)) Bit density for a hard disk

5 Electron density and size effects- general Finite size effects - Quantization of energy levels (confinement) Allowed states: -h 2 /2m (d 2 /dx 2 + ) = E k (x+l,y,z)= (x,y,z) From SE E = h 2 k 2 /2m Boundary condition = e ikr e ikxl = 1 or k x L = 2n k x =2 /L, 4 /L, E k = h 2 /2m(k x2 +k y 2 + k z2 ) Max k = kf 2 (4/3) k F 3 /(2 /L) 3 = N k f = (3 2 N/V) 1/3 E f = h 2 /2m k f 2 k F = (3 2 n) 1/3 F ~ 1/n 1/3 Semiconductor n << metal n L large energy levels closely spaces continuum L small Energy levels not closely spaced Semiconductor n small, F is large. There size effects more important for semiconductors at large size than for metals.

6 Electrons (and holes) in quantum wells Discuss the differences between a conductor, semiconductor, insulator Energy levels and bandgaps in bulk materials Room temp about 1/40 ev Conduction band Energy gap, E g Valence band

7 Band structure of a quantum well Al 0.3 Ga 0.7 As GaAs Al 0.3 Ga 0.7 As Cross-section of quantum well 5 nm E e 0.28 ev 1.85 ev 1.43 ev Bandstructure E h Quantum well 0.14 ev Energy of photon emitted when e - undergoes a transition: E e E h is E 1 + E h + E g

8 Semiconductor preparation Starting materials must be very pure then use a well defined amount of doping Si(As) Vapor phase epitaxy SiCl 4 + 2H 2 Si + 4HCL Add dopant gas e.g. one containing As (column 5), donates electron. Reaction chamber Si substrate (heated) pump Can use other gases such as Silane (SiH 4 ) Silicon tetrachloride reacts with H to give elemental Si. Occurs at surface deposit Si (+ any impurities). HCl does not disturb surface Reaction is reversible etch Si surface

9 2. Finite size in magnetic systems

10 Single magnetic domains Bulk material in the form of a single magnetic domain B outside is large so (B 2 /2 o ) dv is small Bulk material breaks up into magnetic domains to reduce magnetostatic energy B outside is small so (B 2 /2 o ) dv is small

11 Energetics of magnetic domains Magnetostatic energy E m = (B 2 /2 o ) dv volume V of magnetic domain. w E m /V = { (B 2 /2 o ) dv }V constant Domain wall energy, E w (J/m 2 ) surface area of wall V 2/3 w = E w (J/m2)/V surface area of wall/v V -1/3 Bulk: V -1/3 smaller than V, easier for domain walls to form Nanoscale: V -1/3 larger than V, difficult for domain walls to form So expect that for sufficiently small V, the sample should be a single magnetic domain. Important because domain walls no longer present coercivity is very high

12 Hc (koe) Hc (koe) Hc (koe) Hc (koe) Coercivity 1) Particles 3 dimensions in the nm range 2) layers 1 dimension in the nm range Isolated particles: Multilayers: Rare-earth/Mo layers Rare-earth/Ti matrix d Coercivity 20 Tb Particles become single domain 20 Tb 10 Onset of superparamagnetism 0 1 Gd T = 4.5 K log(d) (nm) log(d) (nm) Gd log(d) (nm) log(d) (nm)

13 Why make things small? 3) New devices: Spin Valve Low resistance state: (H up) Soft magnetic layer magnetic layers weakly coupled Hard magnetic layer High resistance state: (H down) soft layer reverses

14 Spin Valve Read Head Hard layer Soft layer MnFe (pins Co) Co (fixed) Cu NiFe (moveable) M Recording medium.

15 Superparamagnetism H = 0 kt (thermal energy) > KV (anisotropy energy), moments not pinned to lattice. When H = 0, M = 0 so H c = 0.

16 Magnetic Anisotropy Origin of Magnetic anisotropy 1) Interaction between a local quadrupole moment (L) and electric field gradients. 2) = (L, S) Magnetic anisotropy Strength of Magnetic anisotropy Rare-Earths large L strong magnetocrystalline anisotropy Transition metals small L weak magnetocrystalline anisotropy Metal K 1 (J/m 3 ) Co 7 x 10 5 Tb -5.6 x 10 7 All hexagonal : E A = K 1 sin 2 + K 2 sin 4. Dy -5.5 x 10 7 Gd -1.2x10 5 Note that for Fe, Ni: E A = (expression with cubic symmetry)

17 A Problem: Superparamagnetism Smaller bit size higher density magnetic recording (Co-Pt-Cr) Currently bit size is ~ 0.25 m, Density = 10 Gigabit/in 2 Superparamagnetism Flip rate due to thermal fluctuations: 1/ = exp(- E/k B T) 2.4x10 9 s -1 Depends on size of particle (KV) size (nm) week years Lower limit bit size 20 nm, Density = 1 Terabit/ in 2 Superparamagnetism ultimate limit for storage density

18 3. Tb (and other elemental rare-earth) particles in a Ti matrix fundamental studies

19 Sputter Deposition Ar gas ionized by electrical discharge, Ar + Ti Si subst. Tb sputter guns V - + -Control deposition rate via Ar gas pressure and V -build up particles or layers by moving substrate To pump Other methods -Evaporation -MBE (precisely controlled evaporation)

20 Structure I n t e n s i t y Equiv. layer thickness (nm) Particle size (nm) 1) Particles 3 dimensions in the nm range 2) layers 1 dimension in the nm range Isolated particles (Tb): Tb/Ti matrix Multilayers: Tb/Mo layers d (deg.)

21 X-ray diffraction of R/Mo multilayers

22 X-ray diffraction of R/Mo multilayers

23 As size changes for particles or layers, look at: Magnetic ordering temperature Coercivity Magnetic Anisotropy Time dependent magnetization

24 H/M (emu/g) M (emu/g) Gd particles, finite size and T c nm H = 200 Oe T (K) 500 nm T (K)

25 Tb Gd Magnetic layers: R(t nm)/mo(18 nm) Tb Gd

26 Tc (K) Tc (K) Tc (K) Tc (K) 1) Particles 3 dimensions in the nm range T c and finite size 2) layers 1 dimension in the nm range Isolated particles: Multilayers: Rare-earth/Mo layers Rare-earth/Ti matrix d Transition Temperature 300 Tb 300 Tb 200 Tb particles d (nm) Gd Gd d (nm) d (nm) d (nm)

27 Reduction of T C and Finite Size Scaling Reduction of T C with layer thickness described by: (T bulk - T C )/T C = ((d d )/t 0 ) -B where: T bulk - bulk transition temperature B - exponent to be determined d= - about thickness of one monolayer - constant to be determined t 0 System B(exponent) Ref Gd O Shea (1998) Tb Dy Ni F. Huang (1993) CoNi Co 1.34 C. Schneider (1990) B is expected to be from theory. [Ritchie et, Phys. Rev. B7, 480 (1973)]

28 Hc (koe) Hc (koe) Hc (koe) Hc (koe) Coercivity 1) Particles 3 dimensions in the nm range 2) layers 1 dimension in the nm range Isolated particles: Multilayers: Rare-earth/Mo layers Rare-earth/Ti matrix d Coercivity 20 Tb 20 Tb 10 T = 4.5 K log(d) (nm) Gd log(d) (nm) Gd log(d) (nm) log(d) (nm)

29 Magnetic Domains F = + E(domain wall) + dipolar energy ~ V 2/3 ~ V Small particle single domain Larger particle multi-domain domain wall

30 single domain multi-domain 0 size (nm) single magnetic domain size domain wall

31 Domain wall motion Multi-domain: H domain wall (stationary)

32 movement H is reversed

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34 Magnetic Domains single domain multi-domain 0 size (nm) single magnetic domain size domain wallpropagates reversing s as it moves.

35 Single domain: coherent rotation H

36 H is reversed

37 H Step-by-step

38

39

40 Magnetic Domains single domain multi-domain 0 size (nm) onset of superparamagnetism single magnetic domain size domain wallpropagates reversing s as it moves.

41 Superparamagnetism H = 0 kt (thermal energy) > KV (anisotropy energy), moments not pinned to lattice. When H = 0, M = 0 so H c = 0.

42 H = 0

43 H = 0

44 Hc (koe) superparamagnetism single domain multi-domain 0 10 nm 30 nm size (nm) 20 Tb log(d) (nm)

45 4. Rare-earth layers and interface anisotropy

46 Anisotropy and Interfaces Contributions to anisotropy(per unit area): Bulk: K V t Demagnetization: (-2 M s2 )t Interface: 2K s t Two interfaces/bilayer Interface anisotropy/area Anisotropy/volume K u = (Bulk + Demag + Interface)/ K u = 2K s / + (K V - 2 M s2 )t / Experiment: measure total anisotropy K u as a function of

47 Experimental Results DyNi 0.2 (t nm)/mo(1.2 nm) Er(t nm)/mo(1.2 nm) [Perera, O Shea, J. Appl. Phys. 70, 6212 (1991)]

48 Interface anisotropy discussion Note: K s > 0 favors perpendicular anisotropy Estimated enhancement of magnetic anisotropy using a point charge model, larger for Dy than Er. A. Fert, Magnetic and Transport Properties of Metallic Multilayers, Summer School on Metallic Multilayers, aussois, France (1989)

49 Interface anisotropy discussion Estimated enhancement of magnetic anisotropy using a point charge model, larger for Dy than Er. A. Fert, Magnetic and Transport Properties of Metallic Multilayers, Summer School on Metallic Multilayers, aussois, France (1989)

50 5. Co/CoO thin layers and inverted hysteresis

51 Interface Exchange Buffer layer (Al or Cu) provides protection A Co-CoO A substrate. t Gao, O Shea, J. Magn. Magn. Mater. 127, 181 (1993) O Shea, Al-sharif, J. appl. Phys. 75, 6673 (1994)

52 X-ray diffraction Homogeneous Co film Co-O film

53 Hysteresis loops [CoO-Co](t A)/Al(10 A)

54 H c vs t [Co-O](t A)/Cu(12 A) Co(t A)/Cu (34 A) [Co-O](t A)/Al(10 A)

55 Model of two-phase system E = -M A H cos A M B H cos B -J i cos( B - A ) -K A sin 2 ( A - P ) -K B sin 2 ( B - P ) M = M A cos A - M A cos A P angle between anisotropy axes and H A angle between M A and H B angle between M A and H (Co) (CoO)

56 Calculated magnetic phase diagrams M A = 2, M B = 1, K A = 1, K B = 1, P = 0

57 Hysteresis Loop (not inverted) A, B vs H Hysteresis loop

58 Co-CoO Results Can obtain inverted behavior in hysteresis with right choice of M s and K s Appear to correspond to phases with large M and small K (pure Co) and a second phase with small M and larger K (CoO).

59 Final comments Finite size effects exist in many nanostructured rare-earth based magnetic systems. These effects appear in the Magnetic ordering temperature Magnetic anisotropy Coercivity New physics at the interface also emerges with an interfacial contribution to magnetic anisotropy that can be measured multilayers.