MAGNETIC INTERACTIONS IN AMORPHOUS MAGNETIC MULTILAYER THIN FILMS

Partial fulfillment of the requirement for the award of the degree of Doctor of Philosophy in Physics. Although thicker amorphous magnetic films are being explored for various applications, the magnetic properties of the films decrease as the film thickness increases.

Chapter 2 covers the fundamental aspects and theoretical models including basics of magnetism, intrinsic properties of magnetic materials, magnetic anisotropy, magnetic

Prologue

Introduction

Especially in magnetism and related transport properties, one of the most important milestones in the multilayers is the discovery of giant magnetoresistance in thin film structure consisting of two ferromagnetic layers separated by a metallic non-magnetic spacer layer [BAIB1988, BINA1989, HART2000, REIG2013 ] , which finds application in hard disks, where the magnetoresistive read heads convert information stored in recording media by a small magnetic stray field into electrical signal with a high spatial resolution [WOLF2001, PIRA2007, PIRA2011, MALL2012, HIRO2013]. Driven by technological applications, the study of thin films, interfaces, multilayer magnetism and magnetic interactions between layers is, in recent years, one of the most active research areas from both fundamental and applied research points of view.

Motivation behind the work

A careful literature review of these thin films reveals that the magnetic properties of the films are strongly dependent on the film thickness, ie. (i) films with thickness less than 7 nm exhibit a strong perpendicular magnetic anisotropy [IKED2010, MANT2013, WANG2014]. Furthermore, a comparison between multilayer thin films and single-layer thick films of similar thickness is essential to understand the advantage of multilayer films to improve the magnetic properties.

Objective of the thesis work

To investigate the effect of thickness on the surface topography, magnetic properties and magnetic domain structure of amorphous FeTaC thin films. To study the effect of the number of SiO2 spacer layers (n), the thickness of the SiO2 spacer layer (z), the magnetic domains of the FeTaC layers on the interlayer coupling between the FeTaC layers and their effects on the magnetic properties at room temperature and at higher temperatures low ( 30 K – 300 K) in [FeTaC (y nm)/SiO2 (z nm)]n/FeTaC (y nm) multilayer films.

Fundamental Properties and Theoretical Modeling

Introduction

Although the history of magnetism has been known for thousands of years and corresponds to the history of science, the underlying principles and mechanisms that explain the magnetic properties of the materials are still complex and mysterious. Therefore, understanding the development of magnetic properties in new materials, tuning their magnetic properties according to thickness, composition and layered structure and optimizing the magnetic properties for different applications is very essential.

Origin of magnetism

One of these is the orbital motion of the electron around the nucleus, which generates a magnetic field and has a magnetic moment along the axis of rotation (see Figure 2.01a) and (b). Therefore, the net magnetic moment for an atom is the sum of the magnetic moments of each electron component, including both orbital and spin contributions, taking into account moment cancellation.

Diamagnetism

This suggests that each electron in an atom can be thought of as a small magnet with permanent orbital and spin magnetic moments. For an atom with completely filled electron shells or subshells, when all electrons are taken into account, there is a total cancellation of both moments.

Paramagnetism

Some of the atoms or ions have a net magnetic moment due to the unpaired electrons in the partially filled orbital. In the presence of an applied magnetic field, there is a partial alignment of the atomic magnetic moments in the direction of the applied field.

Figure 2.02: Magnetic hysteresis loop of a ferromagnetic material.

Ferromagnetism

Another important parameter is the magnetic induction [B = μ0 (H + M), where μ0 is the permeability of free space], which is the total flux of magnetic field lines through a unit cross-sectional area of the material. These parameters indicate the amount of induction generated by the material in a given magnetic field and are useful for characterizing magnetic materials.

Figure 2.03: Typical magnetic hysteresis loops of soft and hard ferromagnetic materials

Antiferromagnetism

The phenomenon of FM can often be described by a mean field or molecular field model. The molecular field model simply assumes that all interactions of neighboring magnetic species can be described in terms of an effective internal or molecular field Bm, which is proportional to the magnetization (Bm . = λM, where λ is the Weiss molecular field constant).

Intrinsic properties of magnetic materials

Exchange Interaction

The origin of the Heisenberg exchange is electrostatic, but the explanation involves quantum mechanics. The variation of J as a function of the ratio of the atomic distance 'a' to the radius of the 3d orbit 'r' is shown in Fig. 2.05.

Figure 2.05: Bethe-Slater curve: Elements above (below) the horizontal axis are ferromagnetic (antiferromagnetic)

Anisotropy

Magnetocrystalline anisotropy
Shape anisotropy
Induced anisotropy
Magnetostrictive anisotropy
Magnetic surface anisotropy

If  is positive, applying a tensile stress to the rod creates an easy axis in the direction of the applied stress. When compressive stress is applied, the direction of the easy axis created will be perpendicular to the stress direction.

Figure 2.07: Magnetization of single crystals of Iron, Nickel and Cobalt.

Magnetic domains and domain walls

Similarly, Néel wall is defined by the magnetization rotating in the plane of the film which reduces the magnetostatic energy of the wall (see Figure 2.12b). These stripe domains are periodic oscillations of the magnetization within a laminar conventional domain structure [CRAU2002, KRON2003, AMOS2008, HUBE2014], as shown in Figure 2.13.

Figure 2.11: Schematic of 90, 180 domain walls inside any FM material and simulation of a 360 [ZHAN2016] domain walls inside a Co arc of 10 nm thick and 120 nm wide

Magnetic properties of amorphous and nanocrystalline materials

Interlayer coupling in multilayer thin films

Exchange coupling

Direct exchange coupling
Indirect exchange coupling

Magnetostatic coupling 1. Topological coupling

Stray field coupling
Domain wall stray field coupling

Increasing the thickness of the spacer layer between the magnetic layers results in interlayer coupling between the FM layers through the conduction electrons of the non-magnetic metallic spacer layer (see Figure 2.15b). Magnetostatic coupling arises due to the magnetic poles at the edges of the film, it is the coupling of the stray field.

Figure 2.15: Schematic representation of multilayer films with thin and thick interlayers

Experimental Methods

Introduction

In the course of the present investigations, various experimental techniques were used for the production and characterization of single and multilayer thin films.

Techniques used for sample preparation 1. Sputtering technique

DC sputtering technique
RF sputtering technique
Magnetron sputtering technique
Calibration of deposition rate

To increase the deposition rate and control the thermal load, magnetron sputtering is used to produce the films. The deposition of the films was carried out after the stabilization of the plasma and the completion of the pre-sputtering process. The deposition rate of the films studied in the present investigations was calibrated with a surface profilometer (Veeco, Dektak-150), as illustrated.

Figure 3.02: Schematic arrangement of DC sputtering technique.

Structural property characterization 1. X-ray diffraction

X-ray reflectivity
Transmission electron microscope

The period of the interference fringes and the decrease in intensity are related to the thickness and roughness of the layer(s). The interference between the rays reflected from the top and bottom of the film surfaces results in interference fringes. Transmission electron microscopy (TEM, TECNAI G2 F30; JEOL 2100) was used to study the amorphous nature of the films.

Figure 3.10: Schematic diagram of X-ray reflectivity arrangement.

Magnetic property characterization

Vibrating sample magnetometer

This signal has an amplitude proportional to the magnetic moment of the sample, the vibration amplitude and the vibration frequency. The exciter is vibrated at a frequency of 72 Hz (Lakeshore model 7410) and the signal received from the Hall probe and the pickup coils is converted into the magnetic moment of the sample. The oven was purged with nitrogen gas to prevent oxidation of the sample at high temperature.

Figure 3.13: Photographic view of vibrating sample magnetometer (Model: Lakeshore 7410, USA) used in the present investigation

Magnetic domain structure analysis 1. MOKE microscope

Magnetic force microscope

The heating rate and M–T ranges were programmed using the IDEASVSM software. parallel to the film plane but perpendicular to the incident plane of the light. The Kerr amplitude is then proportional to the sine of the angle of incidence ϑ, and therefore vanishes for perpendicular incidence. In addition, the point shape is important because of the long-range nature of magnetic forces.

Figure 3.14: Schematic drawing of MOKE set up.

Magnetic properties of single-layer and multilayer

Introduction

KIPG2012] reported the structural and magnetic properties of Co20Fe60B20 films in the thickness range of 5 - 50 nm, prepared by ion beam sputtering. The above studies show that most of the earlier reported works are focused on studying the magnetic properties of the films at random thicknesses using different manufacturing processes. However, no systematic study has been performed to understand the thickness-dependent magnetic properties of CoFeB alloy films over a wide range of thickness prepared by a single preparation technique.

Experimental details

Magnetic domain images and Kerr loops are obtained using a magneto-optical Kerr effect (MOKE) microscope (Evico Magnetics Ltd, Germany). Magnetic domain images are observed in both branches of hysteresis cycle in the longitudinal MOKE mode. Both hysteresis loops and domain images are recorded simultaneously for magnetic fields applied along in-plane directions [easy (0) and hard (90) axes].

Figure 4.01: Room temperature (a) XRD pattern, (b) bright field TEM image and selected area electron diffraction pattern and (c) AFM image for CoFeB (200 nm) film

Results and discussion

Room temperature magnetic properties
High temperature magnetic properties
Temperature dependent magnetic properties

The values of θ and n are extracted from the XRR curves and plotted as θ2 vs n2, typically for CoFeB films with x = 20 and 50 nm in the insets of Figure 4.02. This leads to the formation of stripe domain patterns (x  67 nm) in the as-deposited films. The observed results can be explained using change in the magnetic domain structure with increasing n and spacer layer-dependent interlayer coupling in multilayer thin films.

Figure 4.03: Room temperature normalized M-H loops measured along the film plane for CoFeB (x nm) films with different thicknesses (x = 10 – 200)

Summary

Interestingly, the magnitude of the increase in HC depends strongly on the values of n and z in multilayer films (Figure 4.14). The improvement of the magnetic properties of multilayer films is highly dependent on the number of multilayers and the optimal thickness of the spacer layer. The magnitude of the coercivity increase depends strongly on the values of n and z in multilayer films.

Tuning the magnetic properties of stripe domain

Introduction
Experimental details
Results and discussion

Properties of stack structured thin films 1. Structural properties

Summary

However, the amorphous nature of the deposited films changed to nanocrystalline with increasing annealing temperature. On the other hand, the introduction of a larger number of CoFeB layers in stack-structured films with different z changes substantially the nature of the M-H loops (see Figures 5.04b, 5.04c). On the other hand, the magnetic domain structure of stack-structured films varies strongly with z and n.

Figure 5.01: Schematic diagram of stack structure of [CoFeB (100 nm)/Ta (z nm)] n /CoFeB (100 nm) film with z = 0 – 1.5 and n = 0 – 3

Temperature dependent magnetization reversal and

Introduction

Thickness-dependent study of various amorphous thin films reveals that the soft magnetic properties degrade at higher film thicknesses (> 100 nm) due to the change in magnetic domains caused by the development of stress-induced effective magnetic anisotropy [CRAU2002, SHAR2006, LIXW20010, HUAN2001] reported that the enhancement of soft magnetic properties in HITPERM/SiO2 multilayer thin films strongly depends on the thickness of the ferromagnetic and spacer layers. URSE2005] reported the effect of multilayer number, composition, and heat treatment conditions on the structural, electrical, and magnetic properties of [FeNi/SiO2]n thin films and showed superior soft magnetic properties in multilayer films.

Experimental details

To understand the change in the magnetic properties of multilayer films, single layer FeTaC (x nm) films of different thicknesses (x = 5 – 100 nm) are prepared and studied for comparison. Magnetic domain images and Kerr loops were obtained using magneto-optical Kerr effect (MOKE) microscopy (Evico Magnetics Ltd, Germany) technique. Magnetic domain images were observed in both branches of hysteresis cycle in longitudinal MOKE mode.

Results and discussion

Room temperature magnetic properties
High temperature magnetic properties
Temperature dependent magnetic properties

With increasing z > 2 nm, a two-step magnetization curve is observed due to noncollective switching of the bilayer films caused by the reduction of interlayer coupling between FeTaC layers. It is clear from Figure 6.19a that a two-step reversal of magnetization is observed due to the individual switching of two FeTaC layers at two different fields. However, with increasing n (>1) and z (>2 nm), HCi of the bottom FeTaC layer decreases, mainly due to change in the magnetic domain structure and reduced interlayer coupling between FeTaC layers with increasing z.

Figure 6.02: Room temperature XRR curves for FeTaC (x nm) films with (a) x = 5 (b) x = 10, (c) x = 20, (d) x = 30 and the plot of θ 2 versus n 2 for films with (e) x = 10 and (f) x = 20

Summary

As the film thickness increases beyond the critical thickness (~50 nm), the soft magnetic properties deteriorate due to the transition of the in-plane magnetization to the dense ribbon domain. Temperature-dependent magnetic properties show that the number and nature of switching in multilayer films is strongly dependent on temperature. The overall increase in the temperature-dependent coercivity decreases with the increase in the number of SiO2 layers and its thickness, mainly due to the change in magnetic.

Summary and scope for future work

Summary of the results

The magnetic properties of CoFeB monolayer films (x nm) showed a strong thickness dependence in the thickness range between 10 nm and 200 nm. The improvements in the magnetic properties of structured array films compared to thick monolayer films were found to be strongly dependent on the thickness of the spacer layer and the number of spacer layers. Changes in the magnetic properties of single-layer and array-structured films were explained on the basis of the voltage-dependent band domain in single-layer films and the number of CoFeB layers and the spacer layer thickness-dependent interlayer coupling in array-structured thin films.

Scope for future work

Thickness-dependent surface topography, magnetic properties, and magnetic domain structure of amorphous FeTaC thin films. Magnetic properties and magnetic domain structures in single-layer amorphous Fe-Co-Zr-B-Cu thin films. Soft magnetic properties in multilayer amorphous Fe-Co-Zr-B-Cu / SiO2 thin films Camelia Das and Perumal Alagarsamy.