Magnetic susceptibility

Introduction

An applied magnetic field can induce magnetic susceptibility (Latin: susceptibles, “receptive”; indicated) in materials. In other words, it’s the ratio of the applied magnetic field strength H to the magnetisation M (magnetic moment per unit volume). Most materials’ responses to an applied magnetic field may be divided into two categories: paramagnetism, where the magnetic field aligns with the material and diamagnetism, where the magnetic field does not align with the material at all. In a magnetic field, a material’s magnetic susceptibility determines whether or not it is drawn towards or away from the magnetic field. A larger magnetic field attracts paramagnetic materials, which align with the applied field. In the absence of a magnetic field, diamagnetic materials tend to drift apart and into areas with lower fields. Paramagnetism or diamagnetism occurs when the material’s intrinsic magnetisation creates an additional magnetic field on top of the imposed one. It is possible to gain insights into the structure of materials by measuring their magnetic susceptibility quantitatively. This provides information on bonding and energy levels. For paleomagnetic investigations and structural geology, it is commonly used.

Magnetic susceptibility formula

Xm=I/H

A magnetic susceptibility ratio does not have a unit because it is the ratio of two quantities expressed in the same units. Magnetic susceptibility is affected by material and temperature characteristics.

The mathematical term

In this case, Xm=I if H=1.

In other words, a material’s magnetic susceptibility is the amount of magnetisation it generates when exposed to a unit-strength magnet.

Interesting facts on magnetic susceptibility

It is possible to predict a material’s behaviour by its magnetic susceptibility. Using this technique, a magnetic field’s ability to attract or repel a material can be studied. When paramagnetic materials discover places with higher magnetic fields, they might be drawn to them. While aligned with the magnetic field, this is what happens. Depending on the situation, diamagnetic materials may exhibit unique behaviour. Magnetic fields cannot be aligned in these materials. As a result, materials are pushed away from locations with higher magnetic fields and toward regions with lower ones. Always, the magnetisation of the material is above the applied field. It is added to the magnetic field it already has. It can alter paramagnetism and diamagnetism using different types of field lines. It is possible to quantitatively measure magnetic susceptibility. All of them are capable of providing us with the necessary insights based on material structure. Besides that, it can reveal information about the material’s energy levels and the strength of its bonds.

Uses of magnetic susceptibility

In most cases, MS is utilised as a relative proxy indicator for changes in composition that can be connected to depositional processes governed by paleoclimates. Susceptibility loggers are highly beneficial for core-to-core and core-to-downhole log correlation because of their great precision and sensitivity. A more in-depth investigation of the magnetic characteristics of the sediment, the strength and direction of the ocean or wind currents, or the source of the MS is usually required in a shore-based laboratory. During thermal demagnetisation investigations, magnetic susceptibility is measured on isolated samples to detect changes in the magnetic mineralogy caused by phase transitions or oxidation. Ferromagnetic and paramagnetic minerals can be related in many situations to bottom currents, compaction or deformation using AMS, which measures the preferred orientation and distribution of these minerals. To correctly interpret AMS findings, it is usually necessary to do a thorough analysis of the magnetic characteristics of the sample.

Environmental effects of magnetic susceptibility

Temperature-dependent properties of most materials necessitate equilibration of cores to ambient temperature. For paramagnetic materials, the Curie-Weiss equation states that k = c/T, with c being the Curie constant and T being Kelvin temperatures. At 20°C, the MS of pure paramagnetic material is 1.7 percent (3.5 percent; 7.1 percent) greater than the room temperature susceptibility at 5°C (10°C; 20°C) below room temperature. Between 0°C and 20°C, other materials’ temperature dependence is less pronounced.

Because the magnetic field aligns the paramagnetic molecules’ magnetic moments somewhat, the magnetic susceptibility of a compartment containing Gd-DTPA chelates changes. The shape and orientation of the magnetic field in which the host molecule resides also play a role in this shift in susceptibility. The water protons’ local magnetic fields are modified by the magnetic susceptibility differential, which has an effect on the local resonance frequencies. Three things can happen to the signal from a certain voxel as a result of this.

• As a result of susceptibility differences, the signals from tissue water will occur over a range of frequencies and the net signal intensity will be diminished due to phase cancellation if the field distribution is non-uniform. When there is inhomogeneity in the static magnetic field, this will produce a T2 effect. Gradient echo sequences, for example, are particularly vulnerable to these effects because they do not target inhomogeneity dephasing.
•  The presence of intravoxel field gradients, such as spin-echo sequences, will indirectly contribute to signal loss during imaging time by diffusion in these gradients. This can be seen as a T2 effect that gets stronger as diffusion time gets longer (i.e., TE). Using relatively diffusion-insensitive sequences such as a Carr-Purcell-Meiboom-Gill (CPMG) multi-echo sequence with short echo periods, these effects can be differentiated from intrinsic spin-spin relaxation, as previously described.
• Because of the small voxel sizes and broad spatial extent of susceptibility differences, the field distribution within the voxel may still be uniform, but it has been skewed due to the net field and frequency offset. As TE rises, so will the signal phase changes caused by this shift in resonance frequency, which can be seen in phase-sensitive images.

Conclusion 

The volume distribution of superparamagnetic (SP) particles can be reconstructed using magnetic data as a function of temperature and time. Accurate observations can only be made when scientists understand how magnetic moments change with temperature. Bulk magnetic characteristics are commonly used to derive temperature dependences. The magnetic characteristics of microscopic particles, on the other hand, are greatly influenced by surface effects linked to low-temperature oxidation, reduced coordination of surface spins and interactions with neighbouring molecules. It is difficult to measure these effects, particularly in rocks and sediments. To overcome this issue, a method for re-creating the magnetic characteristics of SP particle assemblies is provided. Based on the inversion of data of magnetic susceptibility at a range of temperatures and frequencies, the approach was developed. It is possible to derive estimates of the temperature dependences, effective interaction field and pre-exponential factor in Néel–Brown relaxation theory without making any prior assumptions about the magnetic characteristics of the particles, thanks to the redundancy in data. Using numerical samples that approximate typical susceptibility measurements of both natural and artificial samples, the inversion method was evaluated and found to work. For palaeo- and environmental magnetism, susceptibility inversion studies can shed light on the magnetic characteristics of small particles.