Magnetoresistance and field-induced phase transitions in the helical and conical states of holmium

Magnetoresistance measurements on single crystals of holmium are reported for the helical antiferromagnetic and conical ferromagnetic phases. Longitudinal and transverse measurements were made with the magnetic field applied in the $a$, $b$, and $c$ planes, including nine different field-current configurations. Data with the current parallel to the $c$ direction and the applied field parallel to the easy $b$ direction showed the magnetoresistance to be as large as 40%. Basal-plane resistance anomalies have been observed, which when compared to magnetostriction, magnetization, and neutron-diffraction data can be correlated with field-induced transitions, permitting the construction of schematic $H-T$ phase diagrams. The results of these measurements are in good general agreement with previous work, with the exception that evidence for an additional stable intermediate state has been obtained. The basal-plane data were isotropic with respect to the current direction, and highly anisotropic with respect to the direction of the applied magnetic field. In order to observe sharp resistance anomalies, it was essential to mount the samples in such a way as to provide for relief of thermal and magnetostrictive stresses. All samples studied were in the form of thin disks, with the magnetic field applied in the plane of the disk to minimize the demagnetizing effects.