The study of hysteresis loop characteristics testing methods for core materials in extreme low-temperature environments is critical for advanced fields such as superconducting power equipment and deep-space exploration systems. In liquid nitrogen (-196°C) or even liquid helium temperature ranges (-269°C), the magnetic domain motion mechanisms and domain wall pinning effects of core materials undergo significant changes due to reduced lattice vibrations, leading to hysteresis losses that differ by orders of magnitude compared to room-temperature conditions. Conventional hysteresis loop testing setups face challenges in extreme low-temperature applications due to incompatibility with cryogenic vacuum chambers and thermal contraction interference. This necessitates redesigning the thermo-mechanical-magnetic coupling framework of the testing system to ensure synergistic stability among magnetic field excitation coils, flux detection probes, and the sample temperature zone.

The core design of the testing system revolves around low-temperature environment simulation and high-precision magnetic field control. A double-layered vacuum-insulated Dewar structure is employed, integrating niobium-titanium superconducting magnets to generate adjustable uniform magnetic fields (0–5 T) and achieving continuous temperature regulation (1.5 K–300 K) via G-M cryocoolers. To prevent deformation of flux detection coils caused by cryogenic contraction, sensors are encapsulated in zero-thermal-expansion composite materials (e.g., Invar alloys) and mechanically levitated at nanometer-level spacing from the sample surface. For diverse core materials like ferrites and amorphous nanocrystalline alloys, customized shaped yokes are required to suppress edge flux leakage, ensuring magnetic field uniformity errors below 0.3% within the test region.

Synchronized control of temperature and magnetic fields is pivotal for measurement accuracy. During cooling, PID algorithms dynamically adjust refrigeration power and magnet currents to limit sample temperature fluctuations to ±0.1 K. For instance, when testing Fe-Co-based amorphous alloys, a rapid temperature drop from 300 K to 4.2 K triggers a sudden shift in magnetostriction coefficients to the order of 10^-6, necessitating real-time correction of magnetic field orientation and pre-stress in sample clamping mechanisms to avoid measurement distortions from magneto-elastic coupling effects. Additionally, pulsed heating technology enables rapid local temperature calibration within millisecond windows, effectively isolating thermal relaxation impacts on magnetization curves.

Data acquisition must address weak signal extraction in extreme low temperatures. Flux detection employs SQUID (Superconducting Quantum Interference Device) arrays with sensitivities up to 10^-15 T/√Hz, combined with lock-in amplifiers to enhance signal-to-noise ratios above 80 dB. To counter eddy current delays at low temperatures, multi-frequency harmonic excitation and adaptive filtering algorithms are developed: stepwise frequency-modulated magnetic fields (10 Hz–1 kHz) are applied, and FFT decomposition extracts complex permeability at each frequency, while Cole-Cole models reconstruct dynamic loss components of hysteresis loops across wide temperature ranges. Experiments demonstrate that this approach reduces measurement uncertainty for hysteresis losses from 12% (conventional methods) to 3.5% at 4.2 K.

Engineering applications still require solutions to interfacial effects and multi-physical field interference. For example, in superconducting transformer core testing, contact thermal resistance between superconducting windings and cores induces temperature gradients, causing localized domain reversal lag. To address this, distributed refrigeration systems using pulse tube cold heads are developed, combined with infrared thermography to map micro-regional temperature differences as small as 0.01 K. Recent studies integrate in-situ scanning tunneling microscopy (STM) and magneto-optical Kerr effect (MOKE) techniques, achieving nanoscale spatially resolved measurements of single-domain hysteresis behavior at 4 K. This breakthrough provides atomic-scale experimental validation for intrinsic magnetic properties of core materials in extreme environments.