Download 2.5. Types of Materials

Advanced Metal Detectors
Dr. Serkan Aksoy
2.5. Types of Materials
Conductors (metals) can be fundamentally classified as ferrous and nonferrous materials for their
detecting sense by the electromagnetic induction sensors.
The ferrous metals are magnetic because they contain iron, but they may have small amounts of other
metals/elements for the required properties. They produce a strong magnetic field because a large part of the
individual atomic magnetic moments line up. This can be done by an external magnetic field. The magnetic
moments remain in the same direction when the external magnetic field is removed. They give little
resistance to corrosion (mild steel, cast iron, wrought iron).
The nonferrous metals are not magnetic because they do not contain any iron. Generally, they are more
resistant to corrosion than ferrous metals (aluminum, copper, lead. zinc and tin). They produce a weak
magnetic field because a large part of the individual atomic magnetic moments is in opposite directions.
Magnetization comes from the orbit motion and spin motion of the electrons. The domain theory is stated
to explain the magnetic behavior of the materials. According to this, the magnetic directions of all the atoms
(atomic magnetic moments) in materials are aligned in the same or opposite directions by settling up small
regions known as magnetic domains. In principal, the particles in these domains can be classified as
- Stable single domain particles: They are physically separated and not joined together as in the multidomain iron. Therefore, they are not direct magnetically interacted. Most of them are not viscous and
too large to be thermally reducing the magnetic alignment. Therefore, they exhibit stable magnetism.
- Pseudo-single domain particles,
- Multi-domain particles.
Relation between a magnetic induction vector ⃗ (Weber/m2 or Tesla) and a magnetic field vector ⃗
(Amper/m or Amper-turn/m) is stated as (in mks unit)
⃗
)⃗
(
⃗
where
and
are the magnetic susceptibility and relative magnetic permeability, respectively (
(
)). They are dimensionless parameters (
for ferromagnetic materials.
H/m
is free space permeability)1.
Magnetic permeability
Magnetic Moment
Before external H
After external H
1
can be used to classify magnetic materials as2
Diamagnetic,
0
extremely weak
Cu, Ag, Zn, Au, Al2O3
In cgs unit, ⃗ (Gauss), ⃗ (Oersted) and ⃗
(
Paramagnetic,
0
weak
Al, Ti, Zr, Cr, Na
Ferromagnetic,
0
strong
Fe (alfa), Ni, Co
)⃗
and
⃗ ; susceptibility
[Won et al, 1998].
2
Linearity between ⃗ and ⃗ breaks down for high
dependent.
materials. It means that
are dimensionless
will be not a constant, but ⃗
Advanced Metal Detectors
Dr. Serkan Aksoy
After removing the external magnetic field, there are no magnetizations in diamagnetic and paramagnetic
materials. However, the magnetization is present in ferromagnetic materials after removing the external
magnetic field. They used to fabricate permanent magnets.






Diamagnetic: They have two sets of magnetic dipole moments in opposite directions. These moments
almost cancel each other. Therefore, they have an extremely weak magnetic property (weaker than air).
is negative (copper), therefore
.
Paramagnetic: They show a weak magnetic property when placed near a magnet. In a strong magnetic
field, they become magnetic and stay magnetic while the field is present. Because temperature competes
simultaneously to randomize the particles, the atoms can’t align well magnetic force of an applied field
(
, temperature). A subclass of this is superparamagnetic materials.
is positive (aliminum),
therefore
.
Superparamagnetic: The mechanism is similar to the paramagnetic material. However, much longer
single domain particles are responsible for the magnetic enhancement. Therefore much stronger
paramagnetic like effect causes the magnetic moment is gigantic. Time constant is highly depending on
size and temperature. They ideally exhibit no hysteresis (no remnant magnetism) and magnetically
saturate, easily. This means that the received signal is not linearly related to the transmitted magnetic
field. The saturation causes the soil decay responses lose accuracy. The different sized particles
saturates differently. This means that no straightforward way of accurate ground balancing.
Ferromagnetic: They show a very strong magnetic property when placed near a magnet (related to the
nonlinearity of the magnetic field). Two more extra groups in this class:
Antiferromagnetic: The moments are exactly equal but opposite, the net moment is zero (Hematite). It
means that the two types of strong magnetic moments, but in inverse direction are induced as similar to
the ferromagnetic material. Therefore, the net magnetization is zero in this type.
Ferrimagnetic: A compound form of ferromagnetic and antiferromagnetic materials. The magnetic
moments of the sublattices are not equal and result in a net magnetic moment. Therefore, they are
similar to ferromagnetism (ionic compounds, magnetite).
Viscous superparamagnetic particles of a specific size and shape have an exponential decay time constant
that is highly dependent on size and temperature. Magnetite, maghemite and titano-hematite particles are
most known superparamagnetic materials which exhibit no hysteresis and thus no remanent magnetism.
They magnetically saturate easily than multi-domains or even pseudo-single domains. The saturation means
that no linear relation between the transmitted field and the received
response. Although most of the
superparamagnetic particles in the goldfields require intense transmitted field to cause saturation, some
benign mineralized soils saturate more easily that cause worse spurious signals than the highly mineralized
goldfields. The saturation can be observed when a mono-coil is pumped causing varying filed strength.
Generally, all soils (particularly hot rocks) exhibit very small frequency dependence of the signal
because the viscous superparamagnetic decay time distribution is not perfectly log-uniform over the search
range. The some class soils and hot rocks with larger frequency dependence of the signal
exhibit
noticeably temperature dependent permeability frequency spectrum of . As a rare example, “super hot
rocks” exhibit the significant signal
frequency dependent permeabilities. These are thus highly
temperature dependent because the time constant spectrum is not log-uniform.
Types of targets can also be classified as [Huang and Won, 2003]
- Isolated objects: UXO, drums,
- Long linear objects: Pipes, trenches,
- Sheet-like objects with large horizontal extends: High salinity soils, contaminant ground water.
The first two of them needs 2D or 3D modeling. 1D modeling is enough for the last one.