Why are electronic spins parallel?

Electronic spins can be parallel or antiparallel depending on their arrangement within an atom or a molecule. However, if you are referring to why spins in certain contexts tend to align parallel to each other, particularly in materials or systems exhibiting ferromagnetism or paramagnetism, it's because of a phenomenon known as exchange interaction.

Exchange interaction arises from the Pauli exclusion principle, which states that no two electrons in an atom can have the same set of quantum numbers. In other words, electrons cannot occupy the same quantum state simultaneously.

In a system with parallel spins, electrons tend to have lower energy compared to those with antiparallel spins. This is because the wave functions of electrons with parallel spins do not overlap as much, reducing the overall electrostatic repulsion between them. As a result, the system achieves a lower energy state when spins are parallel.

In ferromagnetic materials, such as iron, nickel, and cobalt, the exchange interaction between neighboring atoms tends to align the spins of electrons parallel to each other, resulting in a net magnetic moment. This alignment persists even after an external magnetic field is removed, making ferromagnetic materials capable of retaining magnetization.

In paramagnetic materials, which are weakly attracted by magnetic fields, the parallel alignment of spins arises due to thermal energy disrupting the alignment. However, when an external magnetic field is applied, the spins tend to align parallel to the field, resulting in a net magnetic moment.

In summary, electronic spins tend to align parallel to each other due to the exchange interaction, which minimizes energy in certain materials and systems, leading to phenomena like ferromagnetism and paramagnetism.