Enhanced Magnetic Probes for Spintronic Materials of Tomorrow

1
Enhanced Magnetic Probes for Spintronic Materials
of Tomorrow
G.J. Mankey P. Mani, Z. Zhao, I. Zoto, M.
Walock, Z. Lu V.V. Krishnamurthy, H. Fujiwara,
W.H. Butler MINT Center, The University of
Alabama J.L. Robertson (ORNL), F. Klose (ORNL) J.
Fenske (GKSS), D. Lott (GKSS)
These projects are funded by grants from NSF, ARO
and DOE/EPSCoR
2
Center for Materials for Information Technology
(MINT) at The University of Alabama

• A multidisciplinary research program focusing on
  new materials for advanced data storage.
• 22 faculty, 10 postdocs, and 40 graduate students
  from 7 academic programs in science and
  engineering.
• Support federal grants (including an NSF
  Materials Research Science Engineering Center
  grant), industry (IBM, Seagate, Quantum, Sony,
  Fujitsu, Hitachi Maxell, INSIC), and university
  support.

3
1956 Worlds First Hard Drive (5 MB) (Scientific
American Nov, 1956)
4
Toshiba Makes Smallest HDD (4 Gb)

• Toshiba Corporation showed off its 0.85-inch hard
  disk drive (HDD) at the headquarters in Tokyo, 16
  March 2004.
• Toshiba announced that the Guinness World Records
  has certified it as the smallest HDD in the
  world.
• On average, hard drives today are 1000X Smaller
  and 1000X less expensive than the RAMAC.
• Capacities beyond 100 Gb are typical for personal
  computer drives.

5
Materials Science Today
Design
Synthesis
Synthesis
Synthesis
Characterization
Ref E.W. Plummer
6
Materials Science Today
Design
Science DrivenSynthesis
Synthesis
Characterization
Ref E.W. Plummer
7
Materials Science Today
Design
Technology Driven Processing
Synthesis
Characterization
Ref E.W. Plummer
8
Whoever Controls the Materials Controls the
Science and Technology
Design
Science DrivenNanofabrication
Synthesis
Characterization
Ref E.W. Plummer
9
Head, Media and Servo

• The read/wire head, media and servomechanism with
  associated signal processing electronics are
  integrated into the disk drive system.

Ref Hitachi Global Storage Website
10
Research 20 more years?

• Moores Law for semiconductors has density of
  transistors doubling every 18 months.
• Kryders Law has magnetic recording storage
  density doubling every 12 months.
• Current demo 400 Gigabit per square inch1 bit
  is 20 nm x 100 nm.
• 2-3 years 1 Tb on a 3.5 disk.
• 10 years 10 Terabit per square inch.
• 20 years 1 Petabit per square inch
• 1 bit will be 1 nm x 1 nm!

Ref IBM Website
Scientific American, August 2005
11
The Trilemma
Transition position jitter dominated noise

• To overcome the superparamagnetic effect and
  increase the thermal stability, the magnetic
  anisotropy of the media can be increased.
• Higher anisotropy materials require higher fields
  to switch.
• The signal to noise ratio of the readback signal
  decreases as the grain size is reduced.

Courtesy of Stella Wu, Seagate
12
Antiferromagnets for Spintronics

• Thin film antiferromagnets
• Exchange bias effect
• Spin ordering transitions
• Exchange inversion materials
• New tools for future work
• Focus is fundamental physics with possible link
  with applications
• Low Néel temperature antiferromagnetic materials
• Learning to control spin ordering behavior spin
  engineering

13
Quality Samples

• The samples are fabricated by ultra-clan
  sputtering in our home-built vacuum deposition
  system.
• We work with equipment vendors to produce
  economical systems with unique capabilities.
• Extensive characterization before analysis with
  national lab facilities insures positive results
  and future invitations.

14
F/AF Exchange Bias

• When a ferromagnet (F) is deposited on an
  antiferromagnet (AF) in an applied field, the
  hysteresis loop of the F film is altered in two
  ways
• There is a bias (or shift) of the hysteresis loop
  by an amount called Hp or the pinning field.
• There is an enhancement of the coercive field,
  Hc, particularly along the direction of the
  applied field.
• The origin of this effect is FM defects in the
  antiferromagnet.
• The anisotropy of the antiferromagnet controls
  the magnitude of the effect.
• This is the first commercial application of an
  antiferromagnet.

15
The Basic Spin Valve
16
The Magnetoresistance Curve
Anti-parallel gives high resistance
f
f
p
p
DR/R
Parallel gives low resistance
f
f
p
p
Happ
Happ
17
Band Engineering

• The magnitude of the GMR effect is determined by
  the relative alignment of spin-polarized energy
  bands in the ferromagnet and the conduction
  electrons in the copper spacer layer.

• Inserting a thin Co layer on both sides of the Cu
  layer aligns the energy bands better and improves
  the magnitude when using permalloy which has a
  better field sensitivity.

Ref Hitachi Global Storage Website
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The UA MR Sensor

• Separate current inputs and voltage outputs allow
  measurements of magnetotransport independent of
  contact resistance.
• Effect of Sense current on MR ratio was studied.
• What are the important fundamental properties of
  the antiferromagnetic layer?

19
Antiferromagnetic Spin Arrangementsin FeMn
Neutron scattering studies can reveal the
detailed spin structure. The theoretical and
experimental studies are performed in
collaboration with T. Schulthess, W.H. Butler,
G.M. Stocks and J.L. Robertson at ORNL.
2Q mFe 1.78 mB mMn 2.18 mB E 73 meV / atom
1Q mFe 1.11 mB mMn 2.01 mB E 156 meV / atom
Compare EEX 1 eV / atom
Ref T. Schulthess et al., J. Appl. Phys. 85,
4842 (1999).
20
Crystal Diffraction
a2

• A Bravais lattice is an infinite array of
  discrete points with an arrangement an
  orientation which appears exactly the same, from
  whichever of the points the array is viewed.
• There are 14 Bravais lattices with primitive
  vectors a1, a2, and a3.
• The set of all wave vectors k that yield plane
  waves with the periodicity of a given Bravais
  lattice is known as the reciprocal lattice.
• The primitive vectors of the reciprocal lattice
  are found from

a1
REAL
b22p/a2
b12p/a1

• Where cyclic permutations of i, j, and k generate
  the three primitive vector components.

RECIPROCAL
Ref Ashcroft and Mermin, Solid State Physics
(1976).
21
Period Doubling in Reciprocal Space
a2

• Doubling the periodicity in real space produces
  twice as many diffraction spots in reciprocal
  space.
• This effect can be produced chemically with an
  ordered binary alloy or magnetically with
  antiparallel spins in an antiferromagnet..
• The primitive vectors of the reciprocal lattice
  are found from

a1
2a2
REAL
b2p/a2

• A doubling of the periodicity in real space due
  to chemical order or antiferromagnetism will
  produce half-order spots in reciprocal space.
• The magnetic scattering of neutrons gives them a
  unique property of scattering from spins in
  antiferromagnetic ordered structures.

b1p/2a1
RECIPROCAL
Ref Ashcroft and Mermin, Solid State Physics
(1976).
22
Chemical Ordering in Crystalline FePt3 Films

• FePt3(110) peak is absent in disordered FePt3
  since disordered phase is fcc.
• Ratio of (220) and (110) integrated peak
  intensities is a direct measure of the order
  parameter.

23
Intensities of Bragg Peaks
Structure factor Complete disorder Each lattice
point is occupied by an average Fe Pt atom.
Structure is fcc, so no mixed hkl reflections
(all even or odd) are present.
Complete order The unit cell consists of 1 Fe
atom at (0 0 0) and 3 Pt atoms at (½ ½ 0), (½ 0
½), (0 ½ ½). Structure is simple cubic. Mixed hkl
allowed.
Ref B.D. Cullity, Elements of X-ray diffraction.
Lorentz polarization factor
Intensity ratio
Single Crystal
Geometrical factors and x-ray polarization
effects are included
24
Calculated vs. Experimental X-Ray Intensities

• RBS measurements were carried out IBM Almaden
  Research Center.
• The deposition temperature was varied to optimize
  the chemical order as determined by X-ray
  analysis.
• 500nm FePt3 samples were analyzed with RBS and
  Particle Induced X-ray Emission (PIXE) and the
  compositions were found to be accurate to 1
  --Precise control of film composition and
  crystallinity by magnetron sputtering.

25
Measuring Antiferromagnetic Order with Neutron
Diffraction

• Neutrons have zero charge highly penetrating.
• Neutrons possess a magnetic moment can interact
  with sample magnetic moment to produce
  diffraction effects.
• Neutron diffraction is conceptually similar to
  X-ray diffraction the magnetic unit cell is
  double the chemical unit cell in the case of
  antiferromagnets.
• Neutrons can distinguish isotopes basis of
  biological structure determinations.
• The available neutron beam fluxes are rather low
  when compared to X-ray sources to do this
  measurement we need high-quality samples and high
  brightness .

(100) Bragg peak
(½ 0 0) Bragg peak
Period doubling in real space by spin orientation
produces half ordered diffraction spots in
reciprocal space.
26
Neutron-Nucleus Interaction

• The scattering amplitude f(?) is related to the
  nuclear potential V(r) by
• s-wave scattering approximation can be applied
  since the interaction is short range
• f is independent of angle.
• V(r) does not show a systematic variation with
  atomic mass unlike X-ray scattering where it is
  related to the electron density (atomic number).
• The scattering length has an imaginary part which
  is related to the nuclear absorption.

The unit of scattering length b is fm 10-15 m.
27
Chemical Contrast

• The contrast mechanism for neutrons scattering
  length density rather than electron density for
  x-rays.

ref T.E. Mason
28
Magnetic Interaction Vector

• Magnetic neutron scattering is represented by the
  magnetic interaction vector
• It is non-zero only in the presence of a
  component of sample moment perpendicular to the
  scattering vector.
• The magnetic scattering is zero if the unit
  scattering vector is equal to the unit sample
  moment vector.
• This is in contrast to chemical or nuclear
  scattering which is angle independent.

O. Halpern, and M. H. Johnson, Phys. Rev. 55,
898 (1939).
29
Antiferromagnetic Spin Ordering of FePt3 Films

• The epitaxial films are 300 nm thick and (111)
  oriented on a-sapphire.
• The Fe30Pt70 film has only 1/2 0 0 spin
  ordering with a Néel temperature of 140 K.
• The Fe27Pt73 film exhibits a spin ordering
  transition from 1/2 0 0 to 1/2 1/2 0 at 100 K
  and a Néel temperature of 160 K.
• Thin films have different spin ordering behavior
  than bulk samples.

Ref. S. Maat et al., Phys. Rev. B 63, 134426
(2001).
30
Antiferromagnetic Spin Ordering in FePt3

• Two types of antiferromagnetic spin ordering are
  observed.
• There is a spin ordering transition from 1/2 0
  0 to 1/2 1/2 0 at 100 K.
• The Néel temperature is 165 K for the
  stoichiometric material and is composition
  dependent.
• We were the first to directly show that
  spin-ordering phase diagrams for
  antiferromagnetic films is different than bulk
  materials.

S. Maat et al., Phys. Rev. B 63, 134426 (2001).
31
The High Flux Isotope Reactor (2007)
32
HB1A Triple Axis Spectrometer

• Three axes of rotation monochromator, sample
  and analyzer each angle adjusted such that
  Braggs law select the momentum transfer and
  energy.
• HB1A is a fixed initial energy spectrometer with
  Ei14.61 meV (2.37 Å).
• The sample mounted on a goniometer that allows
  translations in 2? (s2), ? (s1), ? (sgl) and f
  (sgu).
• The beam was monochromated using a vertically
  focusing Pyrolitic Graphite monochromator (PG
  002).
• Flux at sample position 6.3 x106 neutrons/cm2/s.

Picture obtained from Dr. J. L . Zarestky, ORNL
33
MgO(100)/ Fe25Pt75 (200 nm)

• The (½ 0 ½) phase has a Néel temperature of 155 K
  which is close to that observed in bulk FePt3.
• At T 120 K which is the TN of the (½ 0 0) phase,
  the (½ 0 ½) integrated intensity reaches a
  maximum indicating a reordering of transition of
  the spins.
• The values of the phenomological power law
  exponents obtained are characteristic of bulk
  FePt3.

34
Magnetic Structure of FePt3 Alloys

• All samples investigated exhibit
  antiferromagnetism below 150 K.
• Both substrate and composition affect the AF
  ordering in FePt3 alloys. The films on MgO have a
  different behavior
• Can we control the spin ordering behavior in
  FePt3 alloys?
• Experiments are planned to formulate a more
  complete description of the observed behavior.

Mani et al., J. Appl. Phys. 99, 08C109 (2006).
35
Neutron Scattering of Antiferromagnetic Films

• We have learned how neutrons can probe the
  fundamental properties of antiferromagnetic
  materialsand proved that thin films exhibit
  different spin ordering behavior than bulk
  systems.
• Our goal is to perform inelastic measurements on
  antiferromagnetic thin films to determine spin
  dynamicsCan spin dynamics be controlled with
  defects, too?
• To make these measurements routine we will
• Improving the sensitivity of the neutron analyzer
  by horizontal focusing.
• Use cooler neutrons.

36
Spin Waves in PSMO Measured at HFIR

• Spin wave dispersion for q perpendicular to FM
  planes.
• HFIR HB3 beamline was used it has a 40x
  improvement in count rates over RITA II.
• The broad excitation around 14 meV is from Pr
  crystal fields.
• The challenge is to perform this measurement on
  thin films.

Krishnamurthy et al., Phys. Rev. B 73, 060404
(2006)
37
The High Flux Isotope Reactor (2007)
38
Liquid Hydrogen vs. Water Moderator

• Use of a liquid hydrogen moderator significantly
  enhances the neutron flux for energies below 7
  meV.
• For diffraction experiments, a beryllium filter
  can be used with a cutoff energy of 5 meV.
• This corresponds to an incident wavelength of 4 Å
  vs. 2.4 Å for a HOPG filter and a 2X gain in
  flux.
• Lower energy neutrons also have a higher cross
  section for inelastic scattering from magnons.
• To be continued.

39
New Challenge Get Perpendicular

• For perpendicular recording, the media becomes
  part of the write head.
• Now, additional criteria are added to media
  design
• Soft underlayer
• Perpendicular anisotropy
• Even smaller grain sizes
• New ideause an underlayer that has a spin
  ordering phase transition with applied field.

40
The Seagate Barracuda

• Perpendicular Recording Technology--320GB 7200
  RPM, 3.0Gb/s Hard Drive
• 84.99 on the internet.
• The technology is viable and robust.
• Will it keep up with Kryders Law?

41
Perpendicular Recording
Picture ref Erik Riedel, Seagate

• FePt in the L10 crystalline structure exhibits a
  strong perpendicular anisotropy possible
  recording media.
• A soft underlayer is necessary to support the
  closure field through the larger, trailing
  magnetic pole.

Smaller bit size (Smaller grain size) ? Higher
anisotropy (Better thermal stability) ? A higher
write field is required though (so writeability
may be a problem) ? Possible solution Write the
bit curie temperature (grains switch easily )
HAMR
42
Heat Assisted Magnetic Recording
Picture ref Erik Riedel, Seagate

• To overcome the high anisotropy and the resulting
  higher write field required, heat assisted
  recording is currently being investigated.
• A laser is used to heat the media in the region
  leading the write head.
• The coercive field of ferromagnetic layer is
  significantly reduced at temperatures close to
  the Curie point.

43
Two Layer Recording Scheme

• Fe50Pt50-xRhx films can be used as soft
  underlayers since they become ferromagnetic when
  heated.
• A thinner layer of Fe50Pt40Rh10 may be sufficient
  to provide the same closure field as a
  conventional underlayer like permalloy.
• The low temperature antiferromagnetic phase will
  also help stabilize the FePt media via exchange
  interactions.

• FeRh/FePt bilayers have been investigated as a
  new media material for perpendicular recording.
• There is mismatch between the lattice parameters
  of FePt and FeRh which gives rise to tilted
  grains.

J. U. Thiele, S. Maat, and E. E. Fullerton,
Appl. Phys. Lett. 82, 2859 (2003).
44
Magnetic Phase Diagram of Fe50Pt50-xRhx

• Fe50Pt50-xRhx alloys in the bulk have an
  interesting magnetic phase diagram dependent on
  composition and temperature as shown in the
  figure on left.
• Fe50Pt38Rh12 has a ferromagnetic to
  antiferromagnetic transition above room
  temperature.
• The detailed nature of the antiferromagnetic
  phase has not yet been determined either in bulk
  or in films by techniques like neutron
  diffraction.

S. Yuasa, H. Miyajima, and Y. Otani, J. Phys.
Soc. Jpn. 63, 3129 (1994).
45
Crystal Structure

• The unit cell of Fe50Pt50-xRhx alloys is centered
  tetragonal.
• afct has excellent lattice match with L10 FePt
  lattice constant (3.88 Å).
• The AF-FM transitions can be correlated to
  discontinuities in the variation of c/a ratio
  with temperature.
• Although bcc and fcc lattice have been well
  studied, the effect of lattice distortion on the
  magnetic properties of ct lattice is not well
  known.

46
X-ray Diffraction

• The alloy films were co-sputtered in a vacuum
  chamber with base pressure less than 1x10-8 mbar
  with the following structure.
• MgO(100)/Cr(3 nm)/Pt(12 nm)/ Fe50Pt50-xRhx (200
  nm)/ Pt (2 nm).
• The films are epitaxial and chemically
  well-ordered.
• The ordering was quantified using structure
  factors, Lorentz factors and the intensities of
  the Bragg peaks.
• All films had an order parameter S 0.95
  indicating nearly perfect chemical ordering.

a 2.75 Å c 3.63 Å c/a 1.32
47
Variation of Lattice Parameters with Composition

• The lattice parameters of the film are slightly
  larger than those observed in the bulk.
• The difference may be due to strain MgO has a
  larger lattice constant than the film.
• RBS measurements have confirmed the stoichiometry.

S. Yuasa, H. Miyajima, and Y. Otani, J. Phys.
Soc. Jpn. 63, 3129 (1994).
48
Geometry of the Neutron Diffraction Experiment

• The sample oriented such that the (0 0 ½), (½ ½
  ½) and ( ½ ½ 0) reflections were in the
  scattering plane.
• The spectrometer is limited in travel for
  in-plane rotation and ? tilt but has good degree
  of freedom with the ? angle.
• Good knowledge of sample crystal orientations
  beforehand is crucial.

49
Neutron Diffraction of Fe50Pt25Rh25

• The 2p/a (0 0 ½) antiferromagnetic Bragg
  intensity in the Fe50Pt25Rh25 film showed a
  maximum at 275 K which corresponds to a measured
  anomaly in the bulk susceptibility.
• The Bragg peak intensity was fit to a power law
  equation to estimate the Néel temperature (376
  K).

S. Yuasa et al., J. Phys. Soc. Jpn. 63, 3129
(1994).
50
Fe50Pt40Rh10

• In Fe50Pt40Rh10, the (0 0 ½) antiferromagnetic
  phase exhibits a Néel temperature 338 K.
• ! The transition is right where we want it!

51
Antiferromagnetic to Ferromagnetic Transition
AF
F
Neutrons
Magnetometry
52
There is more Field alone can induce the
transition

• Field induced AF-F transition (figure on left) in
  Fe50Pt38Rh12 (bulk) has been characterized by
  magnetostriction measurements.
• The field necessary for the transition from AF to
  F decreases with increasing temperature.
• This phenomena conceivably may be applied to a
  ultra-high density two layer magnetic recording
  system as described before.
• This suggests a scheme to use reasonable write
  fields in high anisotropy perpendicular media by
  appropriate optimization of composition and write
  temperature.
• Is heating really necessary?

P. A. Algarabel et al., J. Appl. Phys. 79, 4659
(1996).
53
Magnetic Field Dependent NeutronDiffraction on
(001) and (100) Peaks

• (001)- only observed with the onset of magnetic
  field
• - intensity stronger with increasing temperature
• Since b(Fe) b(PtRh) growth in intensity is due
  to ferromagnetic structure with moment
  perpendicular to the scattering plane.
• (100) increasing intensity with increasing
  temperature
• - magnetic field has only little effect
• Polarized neutron diffraction experiments were
  performed at ILL to extract more information
  about the spin structure. (3/07)

b(Fe) 9.45, b(Rh) 5.88, b(Pt) 9.60 9.45 vs.
8.88
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What happens in the multilayer?

• The exchange interaction at the interface
  modifies the magnetic structure of the
  antiferromagnet.
• This changes the design parameters for the media
  application.
• The nature of the interaction is not well
  understood.
• We will apply polarized neutron reflectometry to
  determine the magnetic structure of the
  interfaces.

?
58
The Spallation Neutron Source (2006)
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60
The SNS Magnet

• We designed a 500 Oe variable field electromagnet
  for initial studies at the magnetism
  reflectometer.
• It will be used for low-field studies and
  commissioning of the reflectometer.

61
What is the Relation Between Fe and Pt(Rh)
Magnetization Vectors

• The neutron scattering experiment does not
  discriminate between Fe and Pt or Rh magnetic
  moments.
• X-ray Magnetic Circular Dichroism and X-ray
  Magnetic Linear Dichroism are Element-Specific
  Magnetic Probes.
• Combined with spin analysis, element-specific
  spin directions in antiferromagnets can be
  determined (neutrons give spin ordering behavior).

62
Spin Resolved Electron Spectrometer

• This experimental technique directly probes the
  spin-dependent Fermi surface.
• We can identify the nature of the spin
  polarization in the conduction electrons in
  ferromagnets and antiferromagnets.
• We apply this technique to the study of the
  spin-split energy bands involved in
  magnetotransport.

Ref K.N. Altmann, et al., Phys. Rev. B 61,
15661 (2000).
63
Spin and Orbital Contributions to the Magnetic
Moment

• Analysis of the spectral lineshapes allows the
  determination of element specific contributions
  to the spin and orbital components of the
  magnetization.
• This technique will be applied to the study of
  (enhanced) magnetic moments in films and
  multilayers.
• Combining polarization with spin analysis allows
  the determination of the complete set of quantum
  numbers of photoelectrons.
• This information is useful for designing better
  sensor materials.

http//www-ssrl.slac.stanford.edu/stohr/xmcd.htm
64
Relevant Research

• Industry needs X
• Better antiferromagnetic materials and artificial
  antiferromagnetic coupled layers (sensors).
• 3-6 nm decoupled magnetic grains (media).
• High moment materials (writer).
• Pinning without the AF material (sensor).
• 30-60 nm magnetic nanorods (tape).
• High anisotropy self assembled magnetic particle
  arrays (media).

Hard bias
Anisotropy too high
Demag field
Spin transfer torque (noise)
Coupling too high
Switching speed
Conductivity Too low
Contact resistance
MRAM and/or CPP GMR Sensor
65
Staying on Kryders Curve

• Make it TINY!
• Areal Density Environment -- Process
  ImplicationsFor the last 5 years the areal
  density of recording head structures has
  sustained annual increases as high as 100. From
  a process perspective this requires that the
  width of the critical read sensor and write pole
  tip decrease by 20 to 30 each year. By
  comparison, integrated circuit dimension decrease
  at an annual rate of 10 to 12. This figure (5)
  shows a historical plot of integrated circuit
  dimensions and thin film head dimensions over the
  last 20 years. Projections shows that the
  dimensional requirements for the thin film head
  will merge with those of the integrated circuit
  in the middle of this decade. The Advanced
  Recording Head Processing Department is
  developing processing techniques which will
  sustain the areal density growth through this
  decade.

Ref Hitachi Global Storage Website
66
Confined Current Path Current Perpendicular to
the Plane Spin Valve
SV pillar (stack) D 9 mm
Top electrode
Contact hole (d 100 nm)
Bottom electrode
Reference mark for e-beam lithography
67
New Paradigm MRAM?
SE image
SEMPA images
Y component
X component
Sample 6, 400nm elements, 13, 5µm? 5µm

• We are applying scanning electron microscopy with
  polarization analysis to measure the magnetic
  switching behavior of MRAM elements.
• SEMPA is a vector magnetometer with sub-50 nm
  spatial resolution.

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Sponsors
69
Laboratories