The role of new materials in the development of magnetic sensors

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The role of new materials in the development of
magnetic sensors

• Chester C.H. Lo and David C. Jiles
• Ames Laboratory and Center for Nondestructive
  Evaluation
• Iowa State University
• Ames, Iowa
• Magnetic Sensors Roadmap Workshop
• National Institute of Standards and Technology
• November 7, 2003

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Classification of materials for magnetic sensors
and actuators

• Broadly magnetic sensors and actuators rely on
  only a few basic principles including-
• the Faraday law of induction,
• for magneto-inductive devices
• the Ampere force law,
• for magnetomechanical sensors
• changes in materials properties in a magnetic
  field,
• such as magnetoresistance, magneto-optics or
  magnetoelasticity

D.C. Jiles and C.C.H. Lo, The role of new
materials in the development of magnetic sensors
and actuators, Sensors and Actuators. A.
Physical, Vol. 106(1-3), pp. 3-7, 2003.
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New Development in Magnetoelastic Materials

• New developments in materials as applied to
  magnetoelastic sensors and actuators include-
• Magnetoelastic materials such as (CoO.Fe2O3)x.(Ag
  0.97 Ni 0.03 )1-x that are sensitive to stresses,
  including torsional stress
• magnetic-martensitic materials such as
  Gd5(SixGe1-x)4 (x ? 0.5)
• Potential applications include force and torque
  sensors, displacement/positioning devices, field
  sensors and magnetocaloric devices.

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Examples of inductive sensors
Fluxgate magnetometer
Magneto-inductive (MI) sensors

• Requires soft magnetic materials with high
  circumferential permeability
• e.g. FeCoSiB with ? -0.1?10-6
• Mohri, et al, IEEE Trans. Magn. 28, 3150, 1992

Field sensitivity 10-4 to 10-1 A/m (0.1-100 nT)
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Examples of force sensors

• Ampere force law
• Examples
• Torque magnetometer
• Force magnetometer
• Magnetic force microscope

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Magnetomechanical effect
Magnetostriction
Magnetomechanical Effect

• externally change M
• (e.g. by an applied field H)
• gt sample strains

• externally strain the sample
• gt changes M and Hsurface

Reversible thermodynamic relation
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Example Cobalt ferrite composite magnetoelastic
stress sensors

• Possible use of magnetoelastic material as
  contactless stress sensor for monitoring
  conditions of aerospace vehicles.

Boeing 707 wing skin with a lap splice
Caused by applied stresses
Sensor output

• Magnetic response of sensor material was detected
  remotely using a Hall device.

Time (sec)
Cobalt ferrite composite
Work currently supported by NASA through CNDE,
ISU
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Material requirements for stress sensor
applications

• Large magnetomechanical coupling
• high d?/dH (gt 110-9A-1m) needed for adequate
  sensitivity
• screen materials by measuring saturation
  magnetostriction (?s gt 100 x 10-6)
• must measure response to stress to predict sensor
  performance
• Small magnetic anisotropy
• Good mechanical properties (e.g. high shear
  strength)
• Able to be fastened to components
• Corrosion resistant
• Low cost (e.g. automobile steering wheel torque
  sensor for lt 10 )
• Enable wireless and smart sensors

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I. Cobalt ferrite (CoO.Fe2O3) composites
Inverse cubic spinel structure of CoOFe2O3

• Magnetic properties of CoOFe2O3
• Magnetic easy axes lt100gt
• K1 as high as 2 to 4 x 106 ergs/cm3
• (depends on stoichiometry)
• Compare to soft cubic ferrites 103 - 104
  erg/cm3
• Magnetostriction
• l100 -250 to -590 x 10-6 l111 ? -1/5 l100
• (l100 of soft cubic ferrites typically 1 to
  10 x 10-6)
• Compare to Terfenol l10090 ? 10-6
• l1111640 ?
  10-6

A sites
B sites
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Magnetostriction of metal-bonded cobalt ferrite
composites

• Ag and Ni were added as binder to improve
  mechanical properties
• Metal additives increase the maximum slope of
  magnetostriction

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Magnetostriction curves of cobalt ferrite-based
and Terfenol-based composites
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Comparison of ferrite and Terfenol composites
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Theoretical modeling of the Matteucci effect in
(CoOFe203)0.98(NiAg)0.02 composites
Modeled
Experimental
Read-out

• Desirable hysteresis level for torque sensor
  0.1 Nm

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II. Gd5(SixGe1-x)4 alloys

• Gd5(Si2Ge2) undergoes a magnetic-crystallographi
  c transformation at 280K

Orthorhombic
Monoclinic

• Atomic layers shear by 0.8Å
• Transition temperature depends on composition and
  magnetic field
• A candidate material for magnetic stress sensors,
  actuator and magnetic refrigeration applications

Gd
Si (Ge)
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Properties of Gd5 (Si2Ge2)

• Largest magnetocaloric effect to date

V. K. Pecharsky and K. A. Gschneidner Jr., Adv.
Mater. 13, 683 (2001).
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Thermal expansion of a-axis for single crystal
Gd5(Si1.95 Ge2.05)
Applied field 0 Tesla
2 Tesla
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Effect of applied magnetic field on transition
temperature
Average slope 5 Kelvin/Tesla
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Magnetostriction curves of Gd5(Si2.09Ge1.91)

• Polycrystalline Gd5(Si2.09Ge1.91) at room
  temperature

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Hysteresis loops at 260K

• Easy axis b-axis

Saturation field Ha 2K1/Ms?0
109 kA/m Measured
value ? 110 kA/m

• Uniaxial anisotropy K1 4.1 ? 0.2 ? 10-4 J/m3

• Comparable to that of Fe, favorable for sensor
  application

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Summary

• We have briefly reviewed some new developments in
  materials as applied to magnetoelastic sensors
  and actuators. These include
• Metal-bonded cobalt ferrite composites
• High stress sensitivity, low cost, good
  mechanical strength and corrosion resistance
• Stress and torque sensor applications
• Current work is aimed to reduce magnetomechanical
  hysteresis
• Gd5(Si2Ge2) alloys
• bulk magnetostriction 104 ppm ( 1) at phase
  transformation which can be triggered by changing
  composition, temperature, applied field and
  stress
• A candidate material for magnetoelastic sensors,
  actuator and magnetic refrigeration applications