X-Ray Detected Magnetic Resonance: A Unique Probe of the Precession Dynamics of Orbital Magnetization Components

Abstract

X-ray Detected Magnetic Resonance (XDMR) is a novel spectroscopy in which X-ray Magnetic Circular Dichroism (XMCD) is used to probe the resonant precession of local magnetization components in a strong microwave pump field. We review the conceptual bases of XDMR and recast them in the general framework of the linear and nonlinear theories of ferromagnetic resonance (FMR). Emphasis is laid on the information content of XDMR spectra which offer a unique opportunity to disentangle the precession dynamics of spin and orbital magnetization components at given absorbing sites. For the sake of illustration, we focus on selected examples in which marked differences were found between FMR and XDMR spectra simultaneously recorded on ferrimagnetically ordered iron garnets. With pumping capabilities extended up to sub-THz frequencies, high-field XDMR should allow us to probe the precession of orbital magnetization components in paramagnetic organometallic complexes with large zero-field splitting. Even more challenging, we suggest that XDMR spectra might be recorded on selected antiferromagnetic crystals for which orbital magnetism is most often ignored in the absence of any supporting experimental evidence.

Site-selective couplings in x-ray-detected magnetic resonance spectra of rare-earth-substituted yttrium iron garnets

“…This immediately reminds us that, at high pumping power, Suhl's famous second-order instability process is expected to annihilate two uniform magnons (k = 0) while creating a pair of non-uniform magnons with opposite wavevectors (k, −k) [40,41]. Energy and momentum conservation implies that, in the latter nonlinear process, the non-uniform modes should have the same precession frequency as the uniform mode and opposite helicity [5,6]. It is therefore our interpretation that the spectral anomaly observed in figures 8 and 9 is basically the typical signature of the four-magnon nonlinear interaction process that is usually regarded as the main source of saturation of the FMR spectra [41].…”

“…Recall that the energy of the CP x-ray photons was tuned to the maximum of the Fe K-edge XMCD spectrum (E 1 = 7113.91 eV) and that the film was rotated by β Y 42 • in order to minimize the demagnetizing field anisotropy. Under such conditions, the XDMR peak intensity was found to increase linearly with the square root of the pumping power up to about 10 mW [5,6].…”

“…In recent years, XDMR emerged as a novel spectroscopy in which x-ray magnetic circular dichroism (XMCD) is used to probe the resonant precession of either spin or orbital magnetization components in a strong pump field typically oscillating at microwave frequencies [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17]. XDMR can be seen as a double-resonance experiment since two independent conditions need to be simultaneously satisfied: (i) either the frequency of the microwave pump field or the effective magnetic field acting locally on spin and orbital magnetization components has to be tuned to magnetic resonance; (ii) the energy of the monochromatic, circularly polarized (CP) x-ray photons has to be adjusted in order to maximize the amplitude of the XMCD probe signal.…”

“…XDMR spectra of high quality have been recorded in TRD geometry on YIG film 1 [6]. The spectra reproduced in figure 4 were obtained for a microwave pumping power as low as 1 mW.…”

“…A priori, one would guess that the XDMR spectra recorded at the Fe K-edge should look fairly similar for films 1 (YIG) and 2 (Y-La-LuiIG). In reality, we had some reasons to be suspicious because previous XDMR spectra recorded on film 2 in LOD geometry already led to puzzling results [5,6] when we tried to evaluate the precession cone angle (θ 0 ) using equation (5):…”