Photoinduced Demagnetization and Insulator-to-Metal Transition in Ferromagnetic InsulatingBaFeO3Thin Films

Abstract

We studied the electronic and magnetic dynamics of ferromagnetic insulating BaFeO3 thin films by using pump-probe time-resolved resonant x-ray reflectivity at the Fe 2p edge. By changing the excitation density, we found two distinctly different types of demagnetization with a clear threshold behavior. We assigned the demagnetization change from slow (∼ 150 ps) to fast (< 70 ps) to a transition into a metallic state induced by laser excitation. These results provide a novel approach for locally tuning magnetic dynamics. In analogy to heat assisted magnetic recording, metallization can locally tune the susceptibility for magnetic manipulation, allowing to spatially encode magnetic information.PACS numbers: 71.30.+h, 73.61.-r, 75.70.-i, 78.66.-w Control of magnetic states by optical excitations in magnetically ordered materials has attracted considerable attention since the demonstration of ultrafast demagnetization in Ni within 1 ps, explored by timeresolved magneto-optical Kerr effect studies [1]. Ultrafast demagnetization was also observed for other elementary ferromagnetic transition metals such as Co and Fe, and intermetallic alloys [2,3].Several mechanisms have been proposed to understand the ultrafast demagnetization. Beaurepaire et al. proposed a phenomenological "three-temperature model" in order to understand the ultrafast demagnetization of Ni, which considers three interacting reservoirs of electrons, spins, and lattice, and suggested the importance of direct electron-spin interactions. Since the electron, spin, and lattice systems are quite tightly coupled to each other in strongly correlated 3d transition metal oxides, it is interesting to investigate the photoinduced dynamics with respect to the electronic states and magnetism [4][5][6][7][8][9][10][11][12][13][14][15].For this study, we chose fully oxidized single crystalline BaFeO 3 thin films, which show unusual behaviors of ferromagnetic and insulating properties with saturation magnetization and Curie temperature of 3.2 µ B /formula unit and 115 K, respectively [16]. The large magnetic moment of BaFeO 3 thin films results in quite large peak intensity of Fe 2p x-ray magnetic circular dichroism (XMCD), ∼ 18 % of the x-ray absorption peak * Electronic address: wadati@issp.u-tokyo.ac.jp; URL: http://www.geocities.jp/qxbqd097/index2.htm intensity [17]. Thus, BaFeO 3 thin films are appropriate samples to carry out time-resolved magnetic circular dichroism experiments. Furthermore, the investigation of the demagnetization dynamics of insulators allows one to relate electronic structure to magnetic dynamics.In order to investigate the magnetic dynamics of ferromagnetic insulating BaFeO 3 thin films, we performed time-resolved reflectivity studies at the Femtospex slicing facility at the synchrotron radiation source BESSY II [18], using circularly polarized x-ray pulses. Our experimental method has the advantage that, in one reflectivity experiment, we can probe electronic structure as well as magnetism. BaFeO 2.5 thin films were grown on SrTiO 3 (00...

Recent advances in ultrafast X-ray sources

“…Modern third-generation synchrotron facilities are designed to maximize both the average beam brightness and the e-bunch storage lifetime (see [174]), which typically results in rms bunch lengths of the order of a few 10 ps to a few 100 ps. This readily enables sensitive time-resolved X-ray studies of chemical [175,176], structural [177] and magnetic [178,179] dynamics on the subnanosecond timescale. In the following, we highlight a few key methods that have been developed and implemented to extend the time resolution at synchrotron sources to the few-picosecond and sub-picosecond regimes.…”

Capturing ultrafast magnetic dynamics by time-resolved soft x-ray magnetic circular dichroism

“…These results indicate that the photoinduced effect will not be a simple thermal effect. 2,19 The threshold-like behaviour, namely the abrupt increase of the change ratio at around a few tens ps scale with high fluence seems to be a consequence of the critical magnetization fluctuation near the Curie temperature, which was suggested in previous Tr-MOKE studies. 45,46 The demagnetization dynamics for general metallic magnets have been described by a so-called three-temperature model 3,4,46 in the Tr-MOKE studies, while Tr-MOKE detects macroscopic magnetizations and does not have an element selectivity.…”

“…The x-ray reflectivity and XMOKE measurements have also been performed for non-transparent thin films or bulk samples but with rather grazed settings for their magnetized axis. 11,12,19 Here, we presents a setting for Tr-XAS and Tr-XMCD in the partial electron yield (PEY) and total fluorescence yield (TFY) modes to measure non-transmissive as-grown samples at nearly normal incidence. The PEY mode, in which the emitted photoelectrons are measured, is rather surface sensitive but these spectra are known to usually be similar to those obtained in the TEY.…”

“…[15][16][17][18][19][20][21] The time-resolved x-ray magnetic circular dichroism (Tr-XMCD) and resonant soft x-ray scattering (Tr-RSXS) measurements for magnetic and electronic materials have been conducted in LCLS, 13,14 ALS, 9 and BESSY II slicing facilities. 10,11,[15][16][17][18][19] The timeresolved x-ray magneto optical Kerr effect (XMOKE) measurements have also been conducted in FERMI. 12 Tr-XMCD measurements at the M absorption below hv <100 eV have also been performed by using high harmonics generation from tabletop lasers in recent years.…”

Femtosecond resonant magneto-optical Kerr effect measurement on an ultrathin magnetic film in a soft X-ray free electron laser

“…In previous studies, a number of attempts have been made using various phenomenological models, such as atomistic calculations, (quasi)particle-mediated spinflip mechanisms, [4][5][6] and a framework of the superdiffusive transport. 7) Recently, the study of femtomagnetism has targeted more complicated magnetic materials such as strongly correlated systems 8,9) and magnetic heterostructures 10,11) with more than two magnetic components. Ultrathin magnetic heterostructures consisting of a (non)magnetic metal or an oxide layer have become one of the nextgeneration material groups in the femtomagnetism for unraveling the effect of spin-orbit interactions that induce perpendicular magnetic anisotropy.…”