| The magnetic property of the itinerant ferromagnet LaCoPO was investigated in the oriented (c||H_0) powder sample from 139La NMR Knight shift and the spin-lattice relaxation studies. The same measurements were also performed in random powder sample of LaFePO for comparison. The local magnetic field at the La nuclear site is isotropic and T independent in LaFePO while it is highly anisotropic and T dependent in LaCoPO. The dominant contribution to this anisotropy arises from the electron nuclear hyperfine interaction, producing a negative spin polarization at the La electronic orbital, possibly through the RKKY interaction, while the dipolar contribution to the same is positive in sign and one order of magnitude smaller. Moreover, the replacement of Fe by Co not only lowers the symmetry of the local magnetic field but also enhances the electric field gradient at the La nuclear site. This could arise due to some modification produced in the band structure, such that the contribution to the EFG from the non-s electronic orbitals is enhanced along with a lowering in symmetry of the local magnetic field at the La nuclear site. Detailed band structure calculation showing the involvement of La electronic orbitals, is necessary to understand these findings. The T dependence of 139La 1/T1T in LaCoPO shows the existence of non-negligible 3D spin fluctuations in both the paramagnetic and the ordered state over and above the dominant 2D spin fluctuations in the paramagnetic phase, observed from the 31P NMR relaxation data in the same oriented sample. The origin of this 3D spin fluctuations could be due to the existence of nonzero interlayer exchange interaction between the Co-P planes, which is communicated through the La-O plane situated in between. By applying the SCR theory of Moriya to the magnetization data of LaCoPO, we have also determined the different spinfluctuation parameters along with the same from La NMR relaxation data. The agreement was found to be quite satisfactory and suggests a more localized character of the 3d electrons in LaCoPO when compared with the same reported from the magnetization data in LaCoAsO. |
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