Magnetic Properties of S Based Bulk Ort Bibhuti Bhu

The focus of the thesis is the study of magnetization reversal and exchange bias behavior in rare earth orthochromites. The main goal of the thesis is to find the possible presence of magnetization reversal and exchange bias behavior in these compounds.

Introduction

Crystal Structure

The value of 't' is one (t = 1) for cubic structure, while any deviation from t = 1 causes structural distortion. 1.1(b) where a decrease in symmetry from the ideal cubic structure can be seen due to tilting in the CrO6 octahedron.

Crystal Field Effect

Another important aspect of the crystal field effect is the orbital quenching, commonly observed in the 3. Another important aspect of the crystal field effect is the orbital quenching, transition metal ions.

Figure 1.2: The electronic distribution of five d orbitals. In presence of cubic crystal field, the fivefold degeneracy of d orbital is lifted, and it splits into two e g

Magnetic Exchange Interaction

Direct Exchange Interaction
Superexchange Interaction
Double Exchange Interaction
Ruderman-Kittel-Kasuya-Yosida (RKKY) Interaction
Dzyaloshinskii-Moriya Interaction

This is an indirect exchange interaction seen in metals where the exchange interaction between the magnetic ions is mediated by the conduction electrons. Since the exchange interaction between the neighboring magnetic moments does not involve any direct coupling, it is indirect in nature.

Figure 1.5: Schematic diagram showing the spin arrangements in (a) an antiferromagnetic superexchange interaction and (b) a ferromagnetic superexchange interaction

Magnetic Orderings

Diamagnetic Materials
Paramagnetic Materials
Ferromagnetic Materials
Antiferromagnetic Materials
Ferrimagnetic Materials

The parallel alignment of the magnetic moments is due to the presence of a molecular field which is strong enough to magnetize the material even in the absence of an external field. Due to the difference in the molecular field of each sublattice, the temperature dependence of spontaneous magnetization of the sublattices differs from each other.

Figure 1.7: Different types of antiferromagnetic order in a magnetic cell. The up and down arrows represent the orientation of the spin

Magnetic Anisotropy

Magnetocrystalline Anisotropy
Shape Anisotropy
Stress Anisotropy
Exchange Anisotropy

Magnetocrystalline anisotropy arises mainly due to the presence of spin-orbit coupling in magnetic materials. Along with magnetocrystalline anisotropy, another effect related to spin-orbit coupling is magnetostriction which arises due to the strain dependences of the anisotropy constants.

Magnetization Reversal

Magnetization Reversal in Ferrimagnetic materials
Magnetization Reversal in Canted Antiferromagnetic Materials

Here, the origin of magnetization reversal is explained by considering the competition between the single-ion magnetocrystalline anisotropy and the antisymmetric DM interaction [ 66 ]. In some materials, the presence of a paramagnetic sublattice along with the FM/tilted AFM sublattice also leads to the magnetization reversal.

Figure 1.8: Reversal of magnetization observed in the temperature dependent field cooled magnetization of Co 2 VO 4 for H = 700 Oe

Exchange Bias

This leads to a reduction of the coercive field in the positive branch of the field axis. However, similar to magnetization sign reversal, exchange bias field sign reversal is quite rare.

Figure 1.9: Schematic representation of spin configuration of a FM-AFM couple at different stages of a shifted hysteresis loop for a system with large AFM anisotropy [18]

Dielectric Properties of Materials

Complex Electric Impedance
Complex dielectric constant
Complex AC conductivity

The grain and grain boundary resistance can be estimated from the diameters of the corresponding semicircles. Here, J is the frequency-independent direct current conductivity, A is the pre-exponential factor and n is the frequency exponent.

Rare Earth Orthochromites

SmCrO 3
GdCrO 3

Moreover, several other RCrO3 compounds (R=Dy, Ho, Tb, Lu, Y) are known to simultaneously exhibit magnetoelectric (ME) and multiferroic (MF) properties due to the coexistence of both ferroelectric and magnetic order. The presence of strong absorption compounds in the visible frequency range gives them an advantage compared to the conventional photocatalytic materials such as TiO2 [107]. compounds have emerged as a potential material for low-temperature magnetic cooling due to anisotropic magnetic interaction between the.

Figure 1.10: Magnetic spin configuration in from [10].

Motivation

For SmCrO3 and GdCrO3 compounds, many reports on their structural and magnetic properties are available, but a systematic study about the substitution effect on the rare earth site or Cr site is still limited. In similar orthochromites such as LaCrO3 and YCrO3, etc., substitution at the rare earth site or the Cr site by another rare earth or transition elements, respectively, is known to induce magnetization reversal.

Experimental Techniques

Sample Preparation

Sol-gel Method
High Temperature Furnaces

The resulting mixture is then maintained at a temperature of around 70 °C with constant stirring by placing it on a hot plate. Citric acid is added to the solution in a 2:1 molar ratio to the metal cation to convert the nitrates to citrates. Ethylene glycol is then added to the citrate solution after 1 hour and the solution is stirred for another 3-4 hours.

The mixture is then kept at 100 oC for 10-12 hours and finally a homogeneous gel is formed. The precursor is ground to obtain a fine powder which is then heated at different temperatures to obtain the final product. For heat treatment of the samples, high temperature homemade muffle furnaces were used with a maximum operating temperature of 1200 ºC and commercial high temperature furnaces with the maximum operating temperature of 1400 ºC.

Chromel-Alumel (Cr-Al) thermocouple and a commercial on/off type temperature controller are being used for temperature measurement and control.

Figure 2.1: Block diagram of furnace with maximum operating temperature of 1200 ˚C.

Characterization Techniques

X-ray Diffraction
Raman Spectroscopy
Field Emission Scanning Electron Microscope
Energy Dispersive X-ray Spectroscopy
Vibrating Sample Magnetometer
Dielectric Measurement

Magnetization Reversal and Exchange Bias in

Mn Substituted SmCrO 3

Sample Preparation and Characterization
Structural Properties
Magnetic Properties

It is due to increase in PM moment with increase in Mn concentration as a result of which it cancels the oppositely aligned weak FM component of tilted Cr3+ ions at a higher compensation temperature. The shift is due to the enhancement in the weak angular FM component of Cr3+ ions with the increase in the magnitude of the applied field. Thus, the increase in the magnitude of applied field during the cooling results in an increase in the size of the AFM domains, which in turn increases the MCr and HI values.

In many systems, the temperature-induced MR coexists with sign reversal in the exchange bias field above Tcomp. As mentioned above, the competition between the weak FM component of tilted Cr3+ ions and paramagnetic moments of Sm3+ and Mn3+ ions, each having different temperature dependencies, plays the main role in determining the net magnetization in the present samples. The observed reversal of the exchange bias field as a function of temperature in the present sample is comparable to that of NdMnO3 by Hong et al [85]. . as a function of cooling field measured at T = 30 K and 8 K.

For 000 Oe), they obtained negative HEB at T = 30 K, which switches to a as the temperature decreases to 8 K. The origin of the EB is due to AFM coupling between the bent FM component of the Mn3+ sublattice and the FM order .. sublattice while the reversal of its sign depends on the coupling intensity between the Nd FM component.

Figure 3.1: XRD patterns of SmCr 1-x Mn x O 3 samples for x = 0 – 0.50.

Fe Substituted SmCrO 3

Structural properties

Below Tmin, a rise in the FC magnetization towards positive magnetization is observed due to the SR transition. For T < Tmin, a sharp increase in the magnetization towards positive magnetization is observed in both samples due to the low-temperature spin reorientation transition. Thus, the samples exhibit several temperature-induced magnetization reversals with two magnetic compensation temperatures, Tcomp1 and Tcomp2. The value of Tcomp1 is found.

This is due to the field-induced enhancement in the weak ferromagnetic component of tilted Cr3+ ions leading to a reduction in the Tcomp1. The estimated values of θc are found to be in the range of -125 K to -190 K depending on the magnitude of applied magnetic field. Finally, the shift in the M-H loop disappears when the measuring temperature, T ≥ TN as in Fig.

This leads to a shift in the entire M-H loop towards the positive field axis and therefore a positive exchange bias is observed.

Figure 3.20: XRD patterns of SmCr 1-x Fe x O 3 compounds for x = 0 – 0.50.

Conclusions

Magnetic Properties of Mn and Fe Substituted

Mn Substituted GdCrO 3

The typical cationic ratio of the samples was estimated from the recorded EDS spectra and found to be comparable to the nominal starting composition. As H increases, both MCr and -HI values are found to increase, and the increase in MCr and -HI values with increase in applied field H is due to the possible increase in the size of AFM domains, and such behavior is reported in other orthochromites [94 ]. The loops are found to be unsymmetrical, indicating the presence of exchange anisotropy in the system.

The negative point of HEB observed in the interval Tcomp < T < TN for x = 0.05 sample is found to be suppressed compared to the x = 0 sample, which is due to the decrease in the strength of the ferromagnetic components of Cr3+ ions ( MCr) upon Mn doping. The reversal of the sign of the EB field in the present samples can be understood from the schematic diagram shown in Fig. 114] the competition between the DM interaction and the single ion anisotropy leads to the reversal of the sign of EB.

Therefore, in the present system, EB is attributed to the competition between the above two moments, and the dominance of one moment over the other with temperature change leads to a reversal of the sign of HEB.

Figure 4.1: XRD patterns of GdCr 1-x Mn x O 3 samples for x = 0 – 0.50.

Fe Substituted GdCrO 3

Impedance spectroscopy

4.17(b) where one can see the decrease in intensity of the Raman modes along with the significant broadening with increasing Fe concentration. The Curie temperature (θ) lies in the range from -15 K to -40 K implying the AFM nature of the samples. A clear hysteresis loop is observed in the low field region for samples x = 0 to x = 0.20 which arises due to the weak FM behavior of the samples.

The decrease in Mr value is attributed to decrease in the weak FM component of tilted Cr3+ ions. The linear increase in magnetization with magnetic field in the high field region is due to the PM moments of the rare earth sublattice and dominant AFM contribution from Cr3+. It is also observed that in the low frequency regime, the magnitude of Zʹ decreases with increase in temperature due to thermal activation of charge carriers across grains and grain boundaries.

The conductivity exhibits frequency distribution behavior in the mid-frequency region (kHz) which is due to the increase in successful hopping of charge carriers.

Figure 4.14: XRD patterns of x = 0.10, 0.30 and 0.50 samples along with their Rietveld refined data shown as solid lines

Conclusions

Magnetic Properties of (Gd,Y)CrO 3 Compounds

Yttrium (Y) Substituted GdCrO 3

As x increases, there are no noticeable changes in the position of the Ag(7) and B3g(3) modes. 5.6 (a), it can be seen that the magnetization (M) of the x = 0 sample measured in ZFC mode increases as the temperature decreases below TN as a typical paramagnet due to the presence of magnetic Gd3+ ions. In addition, it is found that the value of Mmax in the vicinity of TN increases with increasing concentration of Y.

The origin of negative magnetization in the current Gd1-xYxCrO3 (x samples can be explained by considering the competition between the weak ferromagnetic component of the tilted Cr3+ ions (MCr) and the paramagnetic moment of Gd3+ ions (MGd) and their different temperature dependence under the influence of the negative internal field (HI) of AFM-ordered Cr3+ ions. Interesting temperature-induced magnetization reversal is observed in the parent and Y-substituted samples up to x = 0.70 and the compensation temperature is found to decrease from 136 K for x = 0 to 42 K for x = 0.70. The samples with x = 0 to x = 0.20 exhibit a low-temperature spin-reorientation transition around T = 40 K due to the change in the magnetic spin configuration of Cr3+ ions.

A systematic decrease in the AFM transition temperature is observed with increase in Y concentration due to weakening of the superexchange networks.