Nuclei possess a property called spin, which makes them act like tiny magnets. In a magnetic field, the spins perform a wobbling motion known as Larmor precession. Bloch observed the resonance of nuclei in liquid water, while Purcell did so in solid paraffin, thereby establishing nuclear magnetic resonance (NMR). By measuring the resonance frequency with high precision, they determined nuclear magnetic moments far more accurately than before. NMR has become indispensable for chemical analysis, protein structure determination, and materials development. The same principle is applied in MRI scanners, a safe medical tool that images the inside of the body without X-rays. Thus, their research influences everything from basic physics to industry and healthcare.

Nuclear magnetic resonance is an experimental technique in which nuclei with spin (e.g., spin-½) are placed in a static magnetic field B0, excited by a radio-frequency (RF) pulse, and their free induction decay (FID) signal is detected. Bloch introduced the “free precession” method that detects the collective magnetization of the sample rather than individual particle beams, and he formulated the Bloch equations describing the time response of the magnetization vector M. Purcell, on the other hand, placed a solid sample in a resonant cavity and observed the resonance as a change in induced voltage. Their methods quantified longitudinal (T1) and transverse (T2) relaxation times and enabled determination of nuclear magnetic moments, chemical shifts, and quadrupole interactions. The achievable accuracy exceeded that of contemporary optical spectroscopy and directly tested models of nuclear structure. From the 1960s onward, pulse and Fourier-transform NMR were developed, dramatically improving resolution and sensitivity. Assignment of NMR spectra has become a cornerstone for elucidating reaction mechanisms in organic chemistry and for quality control in drug discovery. Advances in superconducting magnets have realized high-field NMR that can resolve the three-dimensional structures of large biomolecules. In medicine, MRI—combining magnetic-field gradients with reconstruction algorithms—has become a clinical standard, while fMRI and MRS extend the technique to functional measurements. The pioneering work of Bloch and Purcell can thus be regarded as the starting point of today’s entire “spin science” field.

Bloch magnetized a water sample in B0 ≈ 520 G, applied an RF pulse, and detected the free-induction signal with a pickup coil, extracting T2 ≈ 1.1 s from the decay of the transverse component M⊥. He also analyzed the exponential recovery of the longitudinal magnetization and determined T1 ≈ 5 s, incorporating both processes into the differential form dM/dt = γ(M × B) − (M⊥/T2) ê⊥ − (M∥ − M0)/T1 ê∥ that became the Bloch equations. Purcell employed a 450 MHz RF circuit with a high-Q (~80) resonator, observing ^1H spins in solid paraffin and discussing lattice mobility from the absorption linewidth. Both measurements achieved Δν/ν ≈ 10^-5 precision, paving the way for detecting anomalies such as g-factor deviations and nuclear crossover phenomena. The techniques directly influenced later developments like pre-saturation, Hahn spin echoes, and ultimately high-resolution solid-state NMR and dynamic nuclear polarization (DNP). They also formed the basis for detecting secondary effects such as the Bloch–Siegert shift and diamagnetic shielding. Precision NMR enabled tensor analysis of magnetic dipole and electric quadrupole interactions, constraining nuclear structure parameters and hadron polarizabilities. These concepts are inherited today in spin-based quantum sensors and nuclear-spin qubit manipulation.