Experimental proof of magnetic x-ray dichroism

Abstract

%hat we believe to be the first experimental results have been obtained on strong magnetic xray dichroism in the M4, 5 absorption spectra of magnetically ordered rare-earth materials, in accordance with recent predictions.The feasibility of using x-rays to determine the magnetic structure of magnetically ordered materials by magnetic dichroism has recently been predicted theoretically. ' Strong magnetic x-ray dichroism (MXD) is expected in the Mq, q absorption edge structure of rare-earth-metal compounds. Polarized synchrotron radiation can therefore be used to reveal information on the local rare-earth-metal magnetic moments in solids, thin films, and surfaces. In this Brief Report we will give what we believe is the first experimental proof of this effect.The M4, q absorption in rare-earth-metal compounds shows good agreement with the atomic Hartree-Fock calculations for the transitions from the 4f"(J) Hund's rule ground state to the manifold of 3d 4f"+'(J') final states.Although in x-ray absorption spectroscopy (XAS) hundreds of excited levels may be involved, one can distinguish between three different types of excitations, namely for J' -J -1, 0, and 1.In the presence of a magnetic field the 2J+1 degenerate ground state 4f"(J) splits into sublevels MJ -J, . . . , +J. The relative population of these sublevels depends on the temperature. That the polarization vector of the x rays has a drastic effect on the spectrum can be seen from the simple case where only M -J is populated (T 0 K). With the polarization direction parallel to the magnetization only the hM 0 transitions are allowed.The transitions J' -J -1 will then vanish, because the M' -Jsublevel is not present in the J' J -l state.Here, we will illustrate in more detail the use of MXD on terbium iron garnet (TbIG), which has a rather complicated magnetic structure. The rhombohedral (or trigonal distorted cubic) cell of ferrimagnetic rare-earth-metal iron garnets (R3Fe50t2) contains eight formula units. 5 The Fe + ions occupy the 24 d (tetrahedron) and the 16 a (octahedron) positions. The larger R + ions occupy the 6 c (dodecahedral) positions. Below the Weel temperature the spins of the 24 d and 16 a irons are ordered antiparallel along the [111]axis. The rare-earth-metal moments couple antiferromagnetically to the net iron moment. By symmetry the 6 c sites can be divided into c~, c2, c3 and cI c2 c3 where C2 (c2 ) and C3 (c3 ) are obtained from the c~(ct ) by rotation around the trigonal [111]axis. From neutron diffraction at 4.2 K the Tb moments in TbIG are known to form a double umbrella structure. 7s The magnetic moments on c~and cI are both in the (011) plane, having angles P 30. 79' and P' -28. 07' with the trigonal axis, and absolute values m 8.18ptt and m' 8.90ptt, respectively. Above 4.2 K the values of P, P', m and rn' are expected to decrease. In our experiment, a single crystal of TbIG was mounted on a rotatable helium-flow cryostat in an ultrahigh vacuum of -10 ' Torr. The temperature at the surface of the sample was 55+'5 K. A CosSm pe...

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