“Magnets” adorn fridge doors and display boards. More seriously, in a world often dominated by the market, hard and soft magnets play crucial roles in motors, transformers, etc and magnet industry is a very important one. Hence, understanding of basics of magnetism and detailed analysis of magneto-chemistry would lead to exploration of various innovative fields. Magnets and magnetism always provide source of inspiration and excitement and can be executed towards the transformation of solar energy into mechanical energy.
Available magnets can be classified in two categories: “hard” magnets and “soft” magnets. “Hard” magnets remain magnetized permanently; they are magnetic even when there is no applied magnetic field. They have an important remnant magnetization in a zero applied magnetic field. Magnetite, whose chemical formula is written Fe3O4, is a hard magnet known for a long time. “Soft” magnets are not permanent magnets; they are attracted to hard magnets but lose their magnetization rapidly when the hard magnet is removed (their remnant magnetization is weak or zero).
‘Magnetic behaviour’ is intrinsic response of a material to an applied or external magnetic field. This is triggered by the electronic spin and mutual alignment of them as well as with respect to the adjacent atoms/molecules. Each electron has concomitant magnetic moment associated with its angular momentum (spin and orbital).Magnetic susceptibility(Χ) is fundamental property of magnetic materials.Magnetic susceptibility (χ) is being determined based on two principles (i) force and (ii) induction methods. Curie-Weiss behaviour (i.e. non-zero θ) is sometimes indicative of weak intermolecular interactions (i.e. weak magnetic interactions between neighbouring paramagnetic centres), in which case: positive θ implies ferromagnetic (FM) interaction and negative θ indicates antiferromagnetic (AFM) interaction. However, Curie-Weiss behaviour can also arise from e.g. low lying excited states, or zero-field splitting effects.several experimental protocols i.e. zro-field cooled measurements,field-cooled measurements,field scan,reduced magnetization have been designed to gain deeper insights into magnetism properties.
On application of magnetic field, the atomic dipoles of a ferromagnet are expected to align themselves with respect to the applied magnetic field. Sometimes, even after removal of magnetic field aligned spins will not relax back to zero magnetization (M), instead it retains the alignment giving rise to the magnetized material. In order to promote the bringing back of magnetization (M) to zero, an alternating magnetic field in the opposite direction needs to be applied whose magnetization (M) would trace out a loop known as hysteresis loop. Absence of retraceability of the magnetization curve is a characteristic known as hysteresis which can be correlated to the presence of magnetic domains in the material. The dynamics of magnetization can be investigated by varying the frequency through AC susceptibility(Χ) measurements.
Weak/strong/intermediate ligand field strength dictates the high-spin/low-spin/spin crossover mechanism in any given complex. This correlates to the number of lone electrons within a complex. This subsequently is correlated to the explore the strength of attraction the sample felt with respect to magnet. Spin only magnetic moment for first row transition elements/lighter elements of the periodic table changes to spin-orbit magnetic moment owing to the incorporation of relativistic effects associated with the heavier elements in the periodic table.Anomalous magnetic moment may also arise when a particular complex interacts with a coordinating solvent. Van Vleck equation has been deduced in order to gain deeper insights into the magnetic susceptibilities. Magnetic susceptibilities are dependent of molecular orientation with respect to the applied magnetic field; i.e. anisotropic in nature.Gyromagnetic tensor (g) values defined by magnetic susceptibility(Χ) measurements, best measured by EPR spectroscopy and most importantly anisotropic in nature.
When electronic spin (S)>1/2(i.e. more than one unpaired electron), the coupling of the ground state with excited states via spin-orbit coupling can also result in partially removing the degeneracy of the MS states in zero-field. The separation of the MS states in zero-field is the zero-field splitting (D). In magnetic studies this is often referred to as the single-ion anisotropy.Sign of D can be negative or positive depending upon the energy of the mS levels.For positive D, x and z directional contribution of the magnetic susceptibility tend to zero at T tends to zero. For negative D, z directional contribution of the magnetic susceptibility increases.
If the two paramagnetic centres A and B interact then the local spins SA and SB are not good quantum numbers for the system. The ions are said to be magnetically coupled, or are undergoing magnetic exchange. This exchange is described by a spin-Hamiltonian, which involves only spin operators and allows the wavefunctions (energy levels) of the system to be expressed in terms of spin functions only. Using the Bleaney-Bowers equation we can predict the behaviour of magnetic susceptibility and temperature for ferromagnetic (positive J) and antiferromagnetic (negative J) exchange interactions.
For antiferromagnetic exchange product of susceptibility and temprature(ΧT) decreases continuously with T while for ferromagnetic exchange ΧT increases on cooling, reaching a plateau at low temperatures (diamagnetic excited state is fully depopulated), i.e. Curie Law at low T with ΧT characteristic of the ground spin state.
Everything above concerns the “classical”, macroscopic, magnetism, at our scale, with various potential applications. We have crossed the quantum world towards transition to the macroscopic one, have seen the application of the theory, and taken the rigorous formula to the dream. It is a field persistently under development which is of essential interest not only in small, with “nano” but also with the complex. At a time when a Japanese Nobel laureate in Chemistry asserted that the synthesis of any molecule is now within reach of the chemist, the challenge was to conceive and synthesise increasingly complex systems in order to reach the requirements. These systems are not required to have only one property or function, but several functions: it is the world of multipurpose materials. These functions can exist alongside each other (magnetism and transparency) or they can interact with each other in order to create and highlight new properties.