Magnetics Business & Technology - Summer 2017 - 6

FEATURE ARTICLE

Magnetometry Measurements:

Considerations for Magnetic and First-Order-Reversal-Curve Measurements
Brad C. Dodrill, Lake Shore Cryotronics, Inc.
Magnetometers are used to characterize magnetic material properties. Magnetometry techniques can be broadly classified into two
categories: inductive and force-based. Common inductive methods
include vibrating sample magnetometry (VSM), extraction magnetometry, AC susceptometry and superconducting quantum interference device (SQUID) magnetometry. The two most commonly used
inductive techniques are VSM and SQUID magnetometry. Alternating gradient magnetometry (AGM) is the most often used forcebased technique. The measurement most commonly performed
to characterize a material's magnetic properties is that of a major
hysteresis loop. The hysteresis or M(H) loop is typically used to
determine a material's saturation magnetization Ms (the magnetization at maximum applied field), remanence Mr (the magnetization
at zero applied field after applying a saturating field) and coercivity
Hc (the field required to demagnetize the material). More complex
magnetization curves covering states with field and magnetization
values located inside the major hysteresis loop, such as first-orderreversal-curves (FORCs), can provide additional information that
can be used to characterize magnetic interactions and coercivity
distributions in magnetic materials[1].
Three important considerations in determining which type of
magnetometer is best suited to specific magnetic materials are; a)
sensitivity as this determines the smallest magnetic moment that
may be measured with acceptable signal-to-noise, b) measurement

speed, e.g., the time required to measure a hysteresis loop, as this
determines sample throughput, c) temperature and field range over
which measurements are to be performed. Measurement speed is
particularly important for FORC measurements because a typical
series of FORCs can contain thousands to tens of thousands of data
points. This article discusses some of the techniques currently used
in the characterization of magnetic materials, and presents typical
measurement results that demonstrate the sensitivity and measurement speed of an electromagnet-based VSM.

First-Order-Reversal-Curve

A FORC is measured by saturating a sample in a field Hsat, decreasing the field to a reversal field Ha, then sweeping the field back to
Hsat in a series of regular field steps Hb. This process is repeated for
many values of Ha yielding a series of FORCs. The measured magnetization at each step as a function of Ha and Hb gives M(Ha,Hb).
The FORC distribution ρ(Ha,Hb) is the mixed second derivative, that
is, ρ(Ha,Hb) = -(1/2)∂2 M(Ha,Hb)/∂Ha∂Hb. A FORC diagram is a 2D or
3D contour plot of ρ(Ha,Hb) with the axis rotated by changing coordinates from (Ha,Hb) to Hc=(Hb-Ha)/2 and Hu=(Hb+Ha)/2 where Hu
corresponds to the distribution of interaction or reversal fields and
Hc the distribution of switching or coercive fields.
FORC has been extensively used by earth and planetary scientists studying the magnetic properties of natural samples because
FORC can distinguish between single-domain (SD), multi-domain
(MD), and pseudo single-domain (PSD) behavior, and because it

From Earth

The Standard in Magnetic Measurements
RM100 Nanotesla Meter
*
*
*
*
*
*

µMAG-01 Handheld
Magnetometer

*
*
*
*
*
*
*

MEDA.COM
6

FVM400 Vector Magnetometer
*
*
*
*
*

Single Axis
200,20 and 2µT Ranges
1 nT Resolution on 2µT Range
+/-0.5% Accuracy
3 1/2 dig LCD Display
Ambient Field Cancellation
Analog Output

SALES@MEDA.COM

Magnetics Business & Technology * Summer 2017

±200,000 nT Range
±0.01% Accuracy
0.1 nT Resolution
RS232 & Ethernet Connectivity
Ambient Field Cancellation
LabView Driver

703.996.8990

3-Axis
±100,000 nT Range
±0.25% Accuracy
1 nT Resolution
RS232 Interface

FAX: 703.996.8770
www.MagneticsMagazine.com
http://www.MEDA.COM http://www.MagneticsMagazine.com
Table of Contents for the Digital Edition of Magnetics Business & Technology - Summer 2017
