Exploring Remanent Magnetization in Dzyaloshinskii-Moriya Interaction Driven Weak Ferromagnets

Abstract

The phenomenon of weak ferromagnetism, observed in some otherwise antiferromagnets (AFMs) such as in α- Fe2O3, MnCO3, CoCO3, is associated with a spin canting mechanism related to the well-known Dzyaloshinskii-Moriya Interaction (DMI). The weak ferromagnetism in some of these AFMs is also concurrent with another functionality, namely, piezomagnetism, a magnetic counter part of the piezoelectric effect. However, unlike weak ferromagnetism, the phenomenon of piezomagnetism is a relatively less explored phenomenon. This thesis presents remanent magnetization measurements using SQUID magnetometry in some of the classic DMI driven weak ferromagnets (WFMs)/ piezomagnets (PzMs) including α- Fe2O3, MnCO3, CoCO3 and NiCO3. In addition to this, remanance in ultra small nano particles of FeCO3, a compound which is not a symmetry allowed WFM, has also been investigated. A core finding of the present thesis is that all the classic DMI driven compounds exhibit two distinct time scales in the magnetization relaxation measurements, one of which is short and, therefore, leads to a quick decay while the other is ultraslow leading to observation of a time-stable remanence, which can also be termed as quasi-static remanence. The time-stable part of the remanence also varies with the strength of the magnetic field in a counter intuitive way. These unique features of remanence are consistently observed in DMI driven WFMs, irrespective of shape, size and morphology. Our study also includes a single crystal of α- Fe2O3. We have divided the entire thesis into 7 chapters and 2 appendices. 1 Chapter 1 gives a brief introduction to various models of exchange interactions which gives rise to different types of long range magnetic ordering such as ferromagnetism, antiferromagnetism. The phenomenon of weak ferromagnetism, as observed in some otherwise AFM, symmetry requirements associated with Dzyaloshinskii Moriya Interaction as well as the phenomenon of piezomagnetism are introduced in this chapter. Remanent magnetization and its importance in identifying and exploring complex magnetic phases such as spin glasses and superparamagnets, which are known to exhibit slow magnetization relaxation phenomenon, is highlighted. The motivation and the objective behind experimental investigation of remanent magnetization in symmetry allowed weak ferromagnets and piezomagnets is outlined towards the end of chapter 1. Chapter 2 includes the details of sample synthesis and various techniques employed to characterize the samples. We have synthesized MnCO3, NiCO3, CoCO3, FeCO3 and αFe2O3 in the form of powders consisting of regular shaped crystallites using the hydrothermal technique. Here α- Fe2O3 has also been synthesized in five different morphologies by variations in the synthesis parameters of the hydrothermal technique. In this chapter we describe the hydrothermal technique and outline synthesis parameters for each sample. These samples are characterized using techniques such as X-Ray diffraction (XRD), temperature variation of synchrotron XRD along with Rietveld profile refinement and Scanning electron Microscopy (SEM). Neutron diffraction in remanent state has been performed on one representative sample and this technique is briefly described in this chapter. α- Fe2O3 in different morphologies has also been explored through dielectric and Raman spectroscopy across the WFM to AFM transition and both these techniques are 2 also discussed in chapter 2. In chapter 3, we present magnetization and remanent magnetization measurements conducted as a function of temperature in different magnetic fields in MnCO3, NiCO3, CoCO3 and FeCO3. Out of these four carbonates, the first three are symmetry allowed WFM. These data bring out the counter-intuitive magnetic field dependence of remanance. In addition, an ultra-slow magnetization relaxation phenomenon is observed in all these carbonates, which reveals quasi-static nature of the remanence. The presence of these unique features of remanence appear to be connected to the DMI driven spin canting phenomenon. This chapter also presents our study of the remanent magnetization in another member of this family, FeCO3, which is not a symmetry allowed WFM but reported to be a PzM. Remanence measurement in this compound is certainly interesting in particular with nano scaling. Chapter 4 includes remanent magnetization investigation in α- Fe2O3 which has a low temperature pure AFM phase and room temperature WFM phase. Considering the room temperature weak ferromagnetic property, remanent magnetization is explored in αFe2O3 with special interest to nano scaling. Corroborated by synchrotron measurements, the study provides crucial insights to optimize the magnitude of time-stable remanence in α- Fe2O3. In establishing the generality of the unique features of remanence, the chapter also discusses remanent magnetization study on a single crystal of α- Fe2O3. Chapter 5 includes the scaling behaviour of remanent magnetization in MnCO3, CoCO3, NiCO3. Qualitative and quantitative comparison of observed time-stable remanences in these symmetry allowed WFM, in conjuncture with their Neel temperature is put forward. 3 In this chapter we present a plausible mechanism behind the observation of ultra-slow magnetization dynamics in these canted systems. Finally this chapter concludes with a microscopic evidence for ultra-slow relaxation/time-stable remanence in one representative WFM sample. This is achieved by performing a neutron diffraction (ND) study in the presence of magnetic field, followed by ND in remanent state in CoCO3. In chapter 6 we have explored the AFM to WFM transition in α- Fe2O3 through dielectric and Raman spectroscopy. These studies performed on various shapes, sizes and morphologies bring out the presence of spin phonon coupling reflected as the anomaly in the dielectric constant around the AFM to WFM transition in α- Fe2O3. Chapter 7 gives the overall conclusion of this thesis and discusses the future scopes and aspects related to this field. Appendix 1 discusses the derivation of the ground state spin configurations and the direction of the net magnetic moment in α- Fe2O3 from the thermodynamic potential of Dzyaloshinskii. Appendix 2 contains the basic characterization and synthesis details of the samples which have not been investigated in the present work, but are interesting from the weak ferromagnetic and piezomagnetic points of view.