Driving magnetic order in a manganite by ultrafast lattice excitation

Abstract

At low-temperatures (T < T N =110 K < T CO/OO =220 K), single-layer La 0.5 Sr 1.5 MnO 4 exhibits CE-type charge, spin and orbital order, characterized by in-plane "zig-zag" ferromagnetic chains. These chains are antiferromagnetically coupled with one another, in and out of plane [11,12,13]. Resonant soft Xray diffraction is directly sensitive to this spin and orbital order, when the incident photon energy is tuned to the 2p→3d transitions (Mn L 2,3 edges), and provides both momentum-dependent and spectroscopic information [14,15]. Figure 1 The temporal evolution of the integrated diffraction spot intensity at the magnetic (¼ ¼ ½) wave vector, obtained from the fits as described above, is reported in Figure 2(a). Diffraction was reduced by 8%, with a single time constant of 12.2 ps. For comparison, we display the significantly faster response response measured after excitation with 5-mJ/cm 2 pulses at 800-nm wavelength [23,24,25], which reveals a prompt collapse of magnetic order on the 250 fs time resolution of the experiment.This observation of different timescales is evidence that lattice driven magnetic disordering must follow a different physical path than for electronic excitation in the near infrared.In Figure 2 timescale and amplitude, with the orbital order only reduced by only 3% with a single-exponential decay time of 6.3 ps. We note that this lattice-driven orbital disordering is slower than was observed previously by time-dependent optical birefringence [19]. However, time dependent optical birefringence, proportional to the orbital order parameter squared in equilibrium [18], is a less direct method than the resonant x-ray diffraction used here.Throughout these dynamics, we see no transient change in the position and width of the scattered diffraction spots for either order and conclude that the correlation lengths are not perturbed. This is shown in Figure 2(c) where we exemplarily plot the transient width of the magnetic diffraction spot together with its peak position. The latter is constant within < 1×10 -5 (calculated standard deviation). , among which we find the Raman-active Jahn-Teller mode depicted in Fig. 3(c). Thus, according to the IRS model, rectification of the mid-infrared mode is able to relax the cooperative Jahn-Teller distortion, which has no infrared activity and thus cannot be driven directly by mid-infrared excitation. Importantly, the Jahn-Teller mode shown in Fig. 3(c) relaxes the splitting between crystal field levels and reduces the ordering of the orbitals. In turn, this weakens the exchange interaction that stabilizes the CE-type order and would thus lead to a smaller equilibrium magnetization, or to a lower equivalent Neel temperature.We stress that in contrast to the case of La 0.7 Sr 0.3 MnO 3 [27], in which the envelope of the infraredactive E u mode drives a low-frequency 1.2-THz rotational (E g ) mode impulsively, the Jahn-Teller A g mode has a higher frequency (15 THz) than the inverse 130-fs envelope of the infrared-active mode.Thus, in La 0.5 Sr 1.5 MnO 4 , the A...

Magnetic order dynamics in optically excited multiferroicTbMnO3

“…Residual diffraction peak intensity for even the highest fluence is likely a result of a small misalignment between pump and probe beams and relatively similar spot sizes. At low fluences, the decay time appears to be nearly constant, but then decreases as the decay amplitude saturates with complete melting of the long-range magnetic order; extracted time constants range from 22.4 ps at the lowest fluence to 7.5 ps at the highest fluence, much slower than time constants of <1 ps recorded for melting of magnetic order in other materials [10,12,16]. …”

“…For example, we have recently shown that THz excitation can resonantly drive a magnon, which leads to a tilt of the spin cycloid structure in TbMnO3 due to magnetoelectric coupling [13]. In other materials, such as CuO or La0.5Sr1.5MnO4, it was observed that pumping with 800 nm light leads to an ultrafast suppression of magnetic order in less than 1 ps and that the magnetic phase transformation is delayed by approximately ¼ of the period of the low lying spin gap excitation [10,12]. Such ultrafast changes in magnetization (less than 1 ps) have also been observed in metallic ferromagnets via all optical pump/probe schemes and optical pump/XMCD (X-ray magnetic circular dichroism) probe experiments [14][15][16].…”

“…Time-resolved resonant X-ray diffraction has been used to probe ultrafast changes in magnetic order in several oxides [10][11][12][13]. For example, we have recently shown that THz excitation can resonantly drive a magnon, which leads to a tilt of the spin cycloid structure in TbMnO3 due to magnetoelectric coupling [13].…”

Spatially resolved ultrafast magnetic dynamics initiated at a complex oxide heterointerface

“…Switching has also been demonstrated in these materials using femtosecond optical excitation, at near-visible [9,10,11,12,13,14,15], mid-infrared [16,17,18,19,20], or THz [21,22,23] wavelengths.…”

Single-pulse terahertz coherent control of spin resonance in the canted antiferromagnet YFeO3, mediated by dielectric anisotropy

“…However, due to the lack of efficient ultrafast sources of direct magnetic excitation, the ultrafast magnetization dynamics in magnetically ordered materials was until recently initiated and controlled indirectly, using various optically activated mechanisms. For example, the inverse Faraday effect based on Raman-type nonlinear optical process, [2][3][4] dynamic momentum-space distribution of photoexcited spinpolarized carriers, 5 optical Stark effect, 6 thermal magnetic anisotropy change, 7 and lattice distortion 8 has been employed to excite the spin systems on femtosecond and picosecond timescales. In most of these cases, the spin excitation by ultrashort laser illumination of materials is accompanied by an undesirable thermal contribution, usually associated with electron dynamics that mediate the excitation of spin waves in the laser-induced process.…”