29 Magnetic Storage Materials

Contents of this Unit

1. Introduction

2. Magnetic Storage Materials

2.1.Material responds to an applied magnetic field in two ways.

2.2.Paramagnetic and diamagnetic materials.

2.3.Magnetic susceptibility as a function of temperature.

2.4.Ferro, anti-ferro and ferri-magnetizations.

2.5.The conditions for super-para-magnetism.

2.6.The magnetic core shell.

3. Summary

Learning Outcomes

After studying this module, you shall be able to
Learn a few basic processes which show how the memory storage devices function and what are the inner components of magnetic storage devices, with visualizations of elementary processes in which discrete unit of memory called bytes and bites are formed. Know, how a paramagnetic and diametric material can be used for storage devices.
What are the prevalent magnetic properties of magnetic materials such spin, magnetic moment and magnetic susceptibility, magnetization and demagnetizations and magnetic coersivity.
What are ferro, anti-ferro and ferri-magnetizations and their distinctions due to their different nature of spins and magnetic domains alignments.
We will learn the conditions for super-para-magnetic materials.

1. INTRODUCTION

The topic such as nano-devices requires an exhaustive study of nanophysics and nanotechnology, to comprehend the phenomena which occur at microscopic to Nano dimensional scale, which is the order of atomic radii. This module will be presenting an outline of the fundamental concepts of nanophysics and nano-magnetism which will facilitate the platform for Nano-devices and its applications.

It is more important to detect small magnetic fields rather than merely making smaller storage devices. Advancements in miniaturizations of devices has opened up into the massive possibility for highly efficient devices like “QUANTUM COMPUTER”. Today’s computer-storage discrete unit, called bytes and bites while for quantum computers, qubits are the quantization of memory storage.

There are plenty of other astounding possibilities for developing electronic devices & it’s industries. Through nanophysics and nanotechnology scientists are trying to harness the properties of magnetism to develop more efficient memory storage devices.

The thorough & exhaustive study of magnetism has already been done by various prestigious research institutes across the world, which has revealed that magnetic properties of a material is explicitly dependent on the spin property of electrons and on the spin properties of atomic nuclei.

It is an electron which is also responsible for the formation of covalent bond in a molecules and due to its electrostatic exchange interaction, leads to the ferromagnetic state. This ferromagnetic state leads to ferromagnetic property of material.

Here, the space function u(x)and the spin function v of the wave function, are given by w = u(x)v.

2. MAGNETIC MATERIALS: –

Magnetism is a universal phenomenon associated with all materials that are composed of charged particles. The local magnetic field in a magnetically susceptible material is A very large local magnetic field can result from a small applied field; in these cases is called the “magnetic permeability”.

Large values of km exceeding 104 are obtained in alloys of Ni and Fe (e.g. Perm alloy and Supermalloy) and are fundamental to the operation of magnetic sensors used in computer hard disks and in iPod memory devices.

2.1 MATERIAL RESPONDS TO AN APPLIED MAGNETIC FIELD IN TWO WAYS: –

(i) it may get attracted or

(ii) repelled.

2.2 PARAMAGNETIC AND DIAMAGNETIC MATERIALS: –

Para-magnetism is observed in materials that contain atoms/ions with unpaired electrons, whereas paired electronic systems behave as diamagnetic.
Diamagnetism can be regarded as originating from the shielding currents induced by an applied magnetic field in the filled electron shells. The magnitude of the diamagnetic response is very small in most of the materials except for superconductors.

In a uniform magnetic field H, if a magnetic pole of strength p is placed at an angle θ to another pole and separated by a distance l, then a couple acts on the magnet, called as magnetic moment m.

The magnetic moment per unit volume is called intensity of magnetization or simply magnetization and is given by

M = m/V,

where V is the volume of the material.

The magnetization per unit magnetic field is called the magnetic susceptibility χ = M/H.

Commonly, the origin of magnetic moment in the atoms of a material is the motion of electrons. In any material, the origin of magnetism lies in the orbital and spin motions of electrons and how the electrons are distributed in the orbitals and interact with one another.

The magnetic moment due to the unpaired electrons is regarded as arising from spin and orbital motion of the electrons. The atoms or ions containing unpaired electrons have an overall magnetic moment given by,

μJ = [4S(S+1)+L(L+1)]1/2,

where S and L are the spin and angular momentum quantum numbers, respectively, and J is the total quantum number resulting from the coupling of the spin and orbital angular momentum.

In most cases, especially in the case of the transition metal ions, L is negligible and the spin only moment can be calculated as

μS = g[S(S+1)]1/2,

where g is the gyromagnetic ratio ≈ 2. For a single electron, μS = 1.73 Bohr Magneton (BM), which is given by 1 BM = eh/4πmc = 0.927 × 1020 erg/e, where e and m are charge and mass of the electron, respectively, h is the Planck’s constant and c is the velocity of light. Bohr magneton is also represented by the symbol μB and 1 μB is equal to the magnetic moment of an electron in the first Bohr orbit.

Fig: 1 – (A) Orientation of magnetic moments in the individual atoms or ions in a paramagnetic substance in the absence of a field (H = 0) and in a strong applied magnetic field H. (B) Magnetization as a function of applied field for a paramagnetic substance. The magnetization is saturated only at very high applied fields.

According to the first systematic measurements of the susceptibility of a large number of substances over a wide range of temperature, made by the French physicist Pierre Curie, χ was found to be independent of temperature for diamagnetic substances, but it varied inversely with the absolute temperature for paramagnetic substances.

2.3 MAGNETIC SUSCEPTIBILITY AS A FUNCTION OF TEMPERATURE: –

Magnetic susceptibility can be expressed as,

χ = Nμeff2/3kT,

where N is the Avogadro number, k is the Boltzmann constant and μeff is the effective magnetic moment and is given by

μeff = g [J(J+1)]1/2. Nμeff2/3k is a constant, and therefore, the magnetic susceptibility equation can be written as

χ = C/T.

This relation is called the Curie’s Law and C is the Curie constant. The effective magnetic moment, μeff, can be calculated from the slope of the plot of 1/χ vs T, which will be linear for a paramagnetic substance.

There are systems, which are paramagnetic only above a critical temperature and below that temperature they undergo spontaneous ordering of magnetic moments. Based on the types of ordering, such magnetic systems may be classified as ferromagnetic, antiferromagnetic, and ferrimagnetic.

Ferromagnetic metals like Fe, Co, Ni, and compounds of the transition metal ions (iron group), rare-earth, and actinide elements, their atomic magnetic moments tend to line up in a common direction even in the absence of a magnetic field below a certain temperature. This critical temperature is called the Curie temperature (TC). And ferromagnetic materials, below Curie temperature exhibit a magnetic moments alignment parallel to each other. All the atomic moments are essentially aligned giving a net magnetic moment even after the removal of the external magnetic field.

2.4 FERRO, ANTI-FERRO AND FERRI-MAGNETIZATIONS: –

Ferromagnetic materials are like paramagnets above the TC.

Fig: 2- Alignments of magnetic domain of ferro, anti-ferro and ferromagnetic materials.

For antiferromagnetic substances, the individual magnetic moments align in a regular pattern with neighboring moments pointing in the opposite directions (moments in opposite direction) in the unit cell of a solid. In the case of antiferromagnetism, moments cancel out each other due to antiparallel alignment, below a critical temperature called the Neel temperature, TN. Hence, the net magnetization is zero due to the opposing orientation of the sub lattice’s magnetization.

Antiferromagnetic materials are like paramagnets above the TN.

Ferrimagnetism is a phenomenon where there can be incomplete cancellation of antiferromagnetically arranged moments happens. Ferrimagnetism is also a situation of antiparallel alignment of moments, but the moments are of unequal magnitudes, so that there is a net magnetization.

They are like paramagnets above the TN. Ferrimagnetic materials are like ferrites (oxides containing more than one metal and Fe is one of the constituents) and magnetic garnets.

It has always been into assumptions that the state of lowest free energy of ferromagnetic particles or single domain particles have uniform magnetization. Single domain particles are particles with size lesser than a certain critical limit While in the case of multi-domain particles where particles size are relatively larger there will be nonuniform magnetization associated with them.

Any change in quantity of magnetization requires coherence in rotational symmetry of spins of electrons as well as of magnetic nano-particles’ domains. This will result in massive increase in coercivities.

Nano sized magnetic particles exhibit behavior similar to paramagnetism at temperatures below the Curie or the Neel temperature. Below the Curie or Neel temperature, the thermal energy is sufficient to change the direction of the magnetization of the nano-particles.

The individual atom has a negligible magnetic influence, to observe and calculate, after being influenced by an external magnetic field. While the magnetic moment of the entire group of nano-particle tends to align with the externally applied magnetic field. Therefore, after that these particles will be called as “superparamagnetic” particles. This is because the property of Super-para-magnetism only can occur when there are materials composed of very small particles.

Fig: 3 – Superposition of spins to form superspin particles.

A number of interesting magnetic phenomena arise when one or more of the dimensions of a magnetic particle are reduced to the atomic size, that are of the order of single domain. Some changes in the magnetic properties like saturation magnetization, Curie temperature, coercive force and magnetic anisotropy could be recognized as the effects of reduced dimensions.

Apart from the magnetocrystalline and magnetostatic anisotropies present in the bulk materials, which are also operative in nanoparticles, there are several other kind of anisotropy which are contributing to effective and un-effective magnetic properties. These are the surface anisotropy due to the changes in the coordination, broken bonds and magnetic exchanges at the surface of a particle. Since the surface area to volume ratio of a fine particle is larger than that of the bulk, the surface anisotropy contributions to the total anisotropy is considerably very high.

Again, due to the smaller size of the particles, there will be considerable strain on the surface and this also contributes in the form of strain anisotropy. Finally, if the magnetic nanoparticles are closer together, there will be magnetic dipolar interactions between the particles and different types of magnetic exchange interactions at the interface between the particles. These two interactions also contribute to magnetic anisotropy. The additional magnetic anisotropy contributes to determine the overall magnetic properties.

In a nano-magnetic material, the coercivity is small when the particles are very large. At the starting the coercivity increases drastically as the particle size is decreased due to the decreasing size of the domains and their number. After reaching a maximum value or the saturation point of the coercivity, it decreases further as the particle size is decreased. The coercivity can be zero below certain dimensions in nano-scale.

Fig: 4 -Particle size dependence and coercivity.

Here, as we discussed EA = KV Sin2θ, where K is the magnetic anisotropy constant, V is volume of a particle and θ is the angle between the direction of magnetization.

EA is comparable to thermal activation energy kT, where k is the Boltzman constant, the magnetization vectors as flips randomly and goes through rapid super-para-magnetic relaxation.

Interestingly, at this temperature of thermal activation energy it overcomes the magnetic anisotropy energy barrier and the nanoparticles become super-para-magnetically relaxed. This temperature is known as the superparamagnetic blocking temperature and it is denoted by TB. The relaxation time of nano-magnetic particles are as follows: –

It is also essential to note that at f0 ≈ 109 Hz, τ ≈ 100 s for typical super-para-magnets and KV ≈ 25kTB. KV is very appealing parameter for designing a high anisotropy magnetic nanoparticles for data storage.

According to the above mentioned equation, TB depends on the volume of the particle and on particle size. And we know that gradually blocking temperature decreases with decreasing particle size.

2.5 THE CONDITIONS FOR SUPER-PARA-MAGNETISM: –

A. The coercivity is zero.

B. The magnetization measured as a function of field, at different temperatures, superimposes when plotted

as a function of H/T.

C. In the zero field cooled (ZFC) magnetic measurements, the sample requires cooling to the lowest possible

temperature, at which a maximum value is observed. This temperature is known as superparamagnetic

blocking temperature TB.

D. The shapes of the zero-field-cooled curves are determined by the particle size, shape and size

distribution. The shape of the curves will be different in the two cases; broader in one case and narrower in

the other depending on the size distributions of particles and their domain.

Fig: 5 – Hysterias curve of ferromagnetic, superparamagnetic and paramagnetic materials.

Similarly, in an another measurement FC measurement, a sample is cooled to the lowest possible temperature in the presence of a small magnetic field and measurements are made while cooling or heating in the same field after cooling. When the same sample is cooled under a magnetic field, the magnetization remains almost constant below the blocking temperature. Therefore, FC and ZFC magnetizations deviate below TB and overlap when the temperature rises above TB.

Such temperature dependence of the ZFC magnetization and the divergence of ZFC and FC magnetizations below TB are the characteristic features of super-para-magnetism.

In magnetization measurements, superparamagnetic particles show a typical behavior as shown. The characteristics of the FC magnetization curve depend on the nature of the interaction between the particles. If the particles are well separated in a non-magnetic matrix or by proper surface coatings, the different types of interactions between the particles, such as dipolar interactions or exchange interactions are suppressed. In this case, the FC magnetization continuously increases below TB as the temperature is decreased whereas the FC magnetization remains constant or decrease slightly below TB depending on the strength of the interactions between the particles. The dynamics of the nanoparticle systems are governed by the distribution of the relaxation times of the individual particles, arising from the particle size distribution.

Fig: 6 – Typical field cooled (FC) and zero field cooled (ZFC) magnetization curves of magnetic nanoparticles showing the blocking temperature.

2.6 THE MAGNETIC CORE SHELL: –

In the case of nanostructured magnetic materials, a significant reduction in saturation magnetization is observed. Therefore, an extremely large magnetic fields are usually required for saturation of the magnetization in case of nano-magnetic particles.

The possible explanation for the reduced magnetization of the nanoparticles is the core shell morphology of the Nano crystallites. In this model, the magnetization is originating only from the core of the particles, and from predominantly on the surface of each particle. The core shell layer of thickness ‘t’, which is a constant, and the magnetization varies with size by the relation.

Ms(d) = Ms (1- β/ d)

where Ms is the saturation magnetization of the bulk, Ms(d) is the saturation magnetization of the nanoparticles of diameter d, and β is a constant related to the thickness of the dead layer, β ≈ 6t, where ‘t’ is the thickness.

Thus the saturation magnetization is inversely proportional to the particle size.

Fig: 7 – The magnetic core shell structure. The shell is assumed as a magnetically dead layer of thickness t. t ≈ 1-unit cell dimension.

Hence we can say that small particles offer the potential of exhibiting special effects that are not observed in bulk materials and a great deal of research effort is devoted to this field. Surface effects arise due to spin canting anomaly.

Saturation magnetization less than the bulk value may indicate some changes in the magnetic spin structure.

Evidence for the magnetic spin structure of fine particles, which defers from that of the bulk is obtained by using Mössbauer spectroscopic studies.

Mössbauer spectroscopy technique provides a direct way of investigating the nano-sized magnetic particles and their spin structures. And for studying noncollinear spin structure, an external magnetic field is applied. This orients the magnetization in the direction of external fields.

On the reduction of particle size of magnetic materials to the nanometer regime an unusual change in the properties are observed, especially for anti-ferromagnetic nanomaterials also. It is the surface contribution of magnetism, which determines the resulting magnetic behavior.

It necessary to be recalled that the net magnetic moment is zero below the Neel temperature in the case of antiferromagnetic materials due to the opposite alignment of the moments of adjacent magnetic ions/atoms in a lattice.

We also know that if size of the magnetic particles will be reduced to nanometer dimensions this will give rise to enhanced net magnetic moment for the nano-particles. Such as in the case of nickel oxide, (NiO) it is paramagnetic above 250 °C and anti- ferromagnetic below this Neel temperature. When the size of NiO particles is reduced below 10 nm, it exhibits huge increase in the magnetic susceptibility below the Neel temperature and show superparamagnetic behavior at room temperature.

3. SUMMARY

1. A material may respond to an applied magnetic field in two ways,

(i) it may get attracted or

(ii) repelled.

2. Commonly, the origin of magnetic moment in the atoms of a material is the motion of electrons. In any material, the origin of magnetism lies in the orbital and spin motions of electrons and how the electrons are distributed in the orbitals and interact with one another.

3. M = m/V,