Chapter 6 is devoted to a comprehensive overview of the overall work carried out in this thesis. The linear trend of MH loop in the vicinity of Tcomp ≈43.8 K indicates the quasi AFM behavior of the sample. Insets in all panels are the temperature-dependent effective coercive field for the respective samples.
Shade of the orange color in the combined phase of long distance FIM &.
Basic aspects of magnetism
According to Heisenberg theory of magnetism [46], the magnetic exchange energy of a magnetic system can be expressed as. According to N'eel, the magnetic susceptibility above the magnetic ordering temperature can be written as,.
Structural and magnetic properties of CoCr 2 O 4
- Crystal structure of CoCr 2 O 4
- Magnetic frustration due to geometric constrains
- Magnetic phase diagram of CoCr 2 O 4
- Other ferroic properties in CoCr 2 O 4
The Curie constantC is the sum of the magnetic ions residing in the different sublattices. As shown in Fig.1.6, all moments are limited to the surface of the cones.
Exchange bias effect
Tunable exchange bias
The magnitude of the HF C required to change the sign in the shift of the M-H loop from negative to positive depends strongly on the microstructure of the sample, and thus on the interface coupling at the interface. As shown in the main panel of Figure 1.11, HEB is small at the small HF C. As shown in Figure 1.12(b), the EB field in the immediate vicinity of Tcomp changes sign about Tcomp, with the magnetization of the sample also changing of sign.
The two schemes in Figure 1.12(d) represent the orientations of the local Nd/Ho moments and the CEP moment with respect to the applied field in two temperature domains i) above Tcomp and ii) below Tcomp. It should be noted here that the reversal in the orientation of the various subcomponents in Tcomp is induced by cooling the sample in a sufficiently large external field. Therefore, the analogy between CEP in rare earth alloys and FM work in multilayer and composite materials is counterintuitive [84].
The existence of the EB effect near Tcomp indicates that the contributions of the local magnetic moments of both rare earth ions are almost compensated (as we see the collapse of the HCef f(T) of the M-H loop in Figure 1.12 (a)), and the pinning to CEP becomes significant [17]. In this way, the soft CEP plays an important role in the existence of the EB effect and its sign change over Tcomp in rare earth intermetallic compounds. The difference between the multilayers [84] and single crystalline samples [17] in which the sign change in the EB field is compelling varies.
Motivation of the thesis
Solid state reaction
The stoichiometric amounts of these starting materials are ground in the organic solvents for several hours using an agate mortar pestle to ensure homogeneity. Thermal energy initially decomposes the starting materials and then provides the energy required for the reaction between the constituent elements [96]. Initially, the starting materials begin to react with each other so that the products begin to form, and at high enough temperatures the reaction is complete.
Therefore, a complete grinding is necessary not only to obtain homogeneity, but also to reduce the particle size of the starting materials in order to increase the surface area. Pressing the samples into pellets with the aid of a hydraulic press further helps to increase the contact area between the particles of the starting materials. The thermodynamic kinetics of the reaction can also be controlled by the rate of temperature heating/cooling.
In this method, most oxide materials require a temperature in the range of 1100 ◦C−1500 ◦C to form in one phase. The initial required amount of input substances in the form of oxides/acetonates was calculated from the stoichiometric equation and weighed with an electronic balance to the nearest 1 mg. These materials are ground in an organic medium of acetone or methanol using an agate motor and a pestle for several hours to obtain a homogeneous mixture.
Sol-gel method
These thoroughly ground mixers of these materials were heated to 600–700 ◦C to evaporate the remaining organic solvents. Then, the powder was pressed into pellets and sintered at C for several hours to obtain single-phase polycrystalline samples. The resulting solution is kept at the same temperature for several hours until the water and acids evaporate.
The gel was then baked at high temperatures to remove volatile components trapped in the gel's pores and organic ligands, and to crystallize the final product. This process allows obtaining the required particle sizes by sintering the samples at different temperatures. In general, metal chlorides and nitrates are widely used as starting materials due to their solubility in water.
Here also the required amount of nitrates/chlorides along with citric acid has been calculated using the stoichiometric equation. These materials were dissolved in distilled water/acid, the resulting solutions were mixed together and kept under stirring at 60 ◦C for several hours to ensure homogeneous mixing. The black powder obtained after combustion was preheated to 600◦C, then pressed into pellets and sintered at 1100 ◦C for several hours to obtain large single-phase polycrystalline samples.
Measurement Techniques
X-ray Diffraction
Citric acid is a chelating agent, carboxylate groups in it act as ligands to form complexes with metal cations. Other organic acids such as tartaric acid and polyacrylic acid, and polyhydroxyalcohols and poly(ethylene glycol) can also be used as chelating agents. When the angle of incidence is changed, maximum intensity peaks are observed if different planes are exposed, otherwise no peak is observed.
In this way, XRD patterns are obtained as a function of 2θ for all possible (h,k,l) planes. To investigate the phase purity of the sample, these patterns are fitted with the theoretical pattern using the Rietveld refinement method [97] in a computer program called FULLPROF [98–100]. From this analysis one can obtain information about the crystal structure, such as lattice parameters, different bond lengths, bond angles, atomic positions and occupancies.
Scanning electron microscopy
Secondary electrons are ejected from the inner shells (lower energy) of the atoms in the sample (mostly within a depth of 5 nm from the surface) in the process of electron beam impact. Due to the holes created in the process of secondary electron production, electronic transition from outer sell to lower shell takes place. Energy dispersive analysis of X-rays (EDAX) is used to detect which elements are present in the sample.
Differential scanning calorimetry
The temperature in both the sample and the reference is monitored by thermocouples placed alongside them.
Superconducting Quantum Interference Device (SQUID) mag-
The superconducting sensing coils are configured as a second-order gradiometer, with counter-wound outer loops that ensure that the set of coils does not respond to uniform magnetic fields and linear magnetic field gradients. The detection coils only generate current in response to local (sample) magnetic field disturbances. The effect of the magnetic field is therefore only on the sample and not on the coil.
Sample dimension is much smaller than the dimensions of the detection coils, the current signal in the detection coils is maximum at the center as shown in the Fig. - sure. As the sample moves through the detection coils, the magnetic moment of the sample induces an electric current in the coils and is transferred to the SQUID via a transformer. SQUID works as a linear current-to-voltage converter, the variations in the current in the detection coils produce corresponding variations in the SQUID output voltage which is proportional to the magnetic moment of the sample.
In this way, the flux generated in the detection coil due to the magnetic moment of the sample is directly measured in the SQUID. But in a conventional VSM, the rate of change of current caused by the movement of the sample causes induced voltages in the pickup coils surrounding the sample and is sensed by the lock-in amplifier. If the sample induces a weak signal, it is very difficult to measure the flux level, the VSM fails to measure the weak signal, but in SQUID this is not a problem as it measures the flux directly.
Physical Property Measurement System
It exhibits nearly one order of magnitude change in the magnetization compared to that of the CoCr2O4. We can see that in the temperature range TS≤T≤TC,HEB(T) appears only in a temperature window located in the vicinity of Tcomp, in which HCef f(T) shows a sharp drop[Fig.3.15(a), ( d) & (g)]. As shown in the Fig.3.16(b) as the HF C increases from zero, the magnitude of the negative HEB increases sharply up to HF C ~2 kOe.
As shown in Fig. 3.21(a), below TC, for small applied fields the moments of Cr1, Cr2 and Fe at site B begin to align in such a way that the resultant moment of site B toward the parallel to site A moment (which is already aligned along the field direction). Cooling the sample through Tcomp induces SR, resulting in configuration (iii) for TS < T < Tcomp. The decrease of ∆Smag(T) involved in SR/spin-flop over Tcomp/TS with Fe concentration is well supported by the decrease in HEB(T) size as predicted earlier.
It is clear that the lattice parameter 'a' decreases with the increase in 'Co' concentration. The decrease in the unit cell parameter may be due to the smaller ion size of Al+3(3d0) compared to that of Cr+3(3d3). Long-range non-ordering also results in the change of the tilt angles of the magnetic moments.
In addition, the ionic disorder and the frozen transverse components of the moments also lead to the glassy behavior of the samples. Furthermore, it may also mean that there is no presence of Co+2 at the B-site. At a glance, one can appreciate the different magnetic phases that appear as a function of Al concentration in the range 0 ≤ x ≤ 1.
In compensated stoichiometry, the coercive field of the loop exhibits an unusual temperature dependence.
