We have measured the magnetic torque of YBa2Cu4O8in the a-axis and b-axis rotation for revealing the intrinsic physics in the underdoped cuprates. So far, we have shown that the first peak below 40 K in b-axis rotation appears at θca∼90°±10°, where θcais the angle between the c-axis and the field direction towards the a-axis. We consider that the origin of first peak is the spiral spin density wave along a-axis with a long period. We have also measured the magnetic torque of YBa2Cu4O8around the  axis. The first peaks are again confirmed to appear in the torque curve. The NMR spectra are systematically investigated by using the field-aligned powder sample. The NMR line width shows an appreciable broadening at temperatures below 30 K when H⊥c axis. We consider that this might be subsequent evidence for the occurrence of the spin density wave in YBa2Cu4O8. © 2004 Elsevier B.V. All rights reserved.
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