{'fraction_non_alphanumeric': 0.03600702576112412, 'fraction_numerical': 0.016978922716627636, 'mean_word_length': 4.993859649122807, 'pattern_counts': {'":': 0, '<': 0, '<?xml version=': 0, '>': 0, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
Anomalous Ferromagnetism in TbMnO3 Thin Films

Magnetometry, x-ray, and neutron scattering have been used to study the structural and magnetic properties of a TbMnO3 thin film grown on a [001] SrTiO3 substrate by pulsed laser deposition. Although bulk TbMnO3 is a low temperature antiferromagnet, magnetometry measurements indicate the presence of low temperature ferromagnetism. Depth profiling by x-ray and polarized neutron reflectometry reveals a net sample magnetization that is commensurate with the film thickness, indicating that the observed ferromagnetism is not due to an altered surface phase (such as Mn3O4), or external impurities that might give rise to an artificial magnetic signal. Instead, these results show that the ferromagnetism is an intrinsic property of the TbMnO3 film.

Multiferroic materials, which feature simultaneous coupled ordering of magnetic moments and electric polarization, are the subject of widespread research interest. Multiferroics hold the promise of novel devices in which the the magnetic properties are electrically controlled, and vice versa. One example is that of an antiferromagnetic multiferroic thin film used to produce a preferred magnetization orientation in an adjacent ferromagnetic thin film via exchange bias [1,2] coupling. For such a structure, it is possible to alter the nature of the exchange coupling (and therefore the magnetic orientation of the ferromagnet) by applying an electric field to change the interfacial magnetic domain structure of the multiferroic antiferromagnet. For example, it has recently been demonstrated that the magnitude of the low temperature exchange bias field in YMnO3 / permalloy multilayers changes as a function of the electric field applied during cooling. [3] To further pursue the technological possibilities of exchange biasing multiferroics, other materials need to be explored. In bulk, TbMnO3 is a multiferroic material which in bulk exhibits a complex magneto-electric phase diagram, including antiferromagnetism below 30 K -making it a candidate for exchange-bias applications. [4][5][6] However, scaling down manganite crystals into thin film form causes strain that can drastically affect the magnetic properties, [7] and relatively little work has been done to study the magnetic properties of TbMnO3 thin films. As a first step towards determining the utility of TbMnO3,for exchange-bias applications, we have produced a thin film of the orthorhombic phase of TbMnO3, and studied it with magnetometry, x-ray, and neutron scattering measurements. These measurements show that the TbMnO3 film exhibits an intrinsic ferromagnetic order.
A TbMnO3 film was laser ablated from a stoichiometric TbMnO3 target, onto a [001] oriented SrTiO3 substrate. Figure 1 shows results of θ-2θ x-ray diffraction (XRD) scans, performed with a commercial diffractometer. [8] The fabricated film exhibits the [00l] peak of the perovskite structure, with an out-of-plane lattice parameter c = 0.372 nm. No peaks identifiable as originating from additional phases were observed. X-ray reciprocal space mapping measurements around the SrTiO3 [103] Bragg reflection (not shown) reveal that the in-plane lattice parameter of the TbMnO3 film closely matches that of the cubic SrTiO3 substrate (0.391 nm). Bulk TbMnO3 has an orthorhombic perovskite structure which (in the pseudo-cubic convention) has lattice parameters of 0.393 nm in-plane, and 0.370 nm out-of-plane. [9] Therefore, we conclude the fabricated film is compressively strained by the substrate, resulting in the tetragonally distorted orthorhombic phase of TbMnO3.
Net magnetization measurements of the sample were performed using superconducting quantum interference device (SQUID) magnetometry. Figure 2 shows the field H dependent magnetic moment m at temperature T = 5 K after cooling from 100 K in zero field. [10] Hysteretic behavior is observed, clearly indicating ferromagnetism -a drastic departure from the antiferromagnetic order observed for bulk TbMnO3. m(T) in a 1.5 T field, is shown in the Fig. 1 inset, and reveals that a net magnetization persists above 100 K. However, standard magnetometry measurements alone cannot determine whether the observed ferromagnetism is present throughout the film, or if it is instead due to a departure from the TbMnO3 stoichiometry in a small region of the film.
To determine if a net magnetization is truly intrinsic to the TbMnO3 film, we determined the structural and magnetic depth profiles via x-ray and polarized neutron reflectometry [11] (XRR and PNR, respectively) measurements at the NIST Center for Neutron Research. XRR measurements were conducted at room temperature with wavelength λ = 0.154 nm x-rays (Cukα), and PNR measurements were conducted at cryogenic temperatures with λ = 0.475 nm cold neutrons. The neutron beam was polarized by Fe/Si supermirror and a Mezei spin-flipper to be alternately spin-up (+) or spin-down (-) relative to an in-sample-plane magnetic field H, and was incident on the sample. The reflected beam was intercepted by an analyzer supermirror/flipper assembly so that all four PNR cross sections (non spin-flip: R + + , R --, and spin-flip: R -+ , R + -) could be measured with a 3 He pencil detector.
In reflectometry, depth (z) dependent sample properties are determined from the scattering vector (Q) dependent reflectivities. [12] The x-ray and neutron beams are both highly penetrating, and probe the entirety of the TbMnO3 thin film. XRR is sensitive to the sample's x-ray scattering length density ρX(z) (a function of electron density), [13] while PNR is sensitive to the sample's nuclear scattering length density ρN(z) and volume magnetization M(z). [14,15] Specifically, the non spin-flip scattering R + + and Further, the common frequency of the R + + and R -oscillations is essentially the same as that of the XRR oscillations shown in Figure 3, suggesting that the magnetized film thickness is similar to the total film thickness. At 100 K (b), the splitting between R + + and R -persists, indicating that the sample is still magnetized. The higher frequency oscillations observed in panel a) and Fig. 3 are much weaker at 100 K. This indicates that while neutrons are very sensitive to the magnetic interface between the film and the substrate, they are not very sensitive to the nuclear interface. With the high frequency oscillations damped, a very weak lower frequency oscillation becomes apparent, suggesting some depth dependence of ρN within the TbMnO3 film.
Results of model fitting the XRR and PNR data are shown in Figure 5 Figure 5: Models used to fit the data in Fig. 2-3. a) Structural profiles: real and imaginary components of the x-ray scattering length density, and the nuclear scattering length density (multiplied 10). b) Magnetic profiles at 6 K and 100 K.