Novel Magnetic Domain Structure in Iron Meteorite Induced by the Presence of L10-FeNi

Photoelectron emission microscopy has been carried out to study the magnetic properties of iron meteorite associated with the Widmanstätten structure for the first time. A magnetic circular dichroic image reveals a unique magnetic domain structure, resulting in the “head-on” magnetic coupling over the interface between the α and γ lamellae. Such a magnetic domain structure is unfavorable in any synthetic Fe–Ni alloys. Micromagnetics simulation reasonably explains that the formation of magnetic domains is induced by the L10-type FeNi (tetrataenite) phase segregated at the boundary in the Widmanstätten structure.

I ron meteorite shows an extraterrestrial pattern termed as the Widmanstätten structure [ Fig. 1(a)]. Its metallographic feature has been of great benefit to planetary scientists for studying the history of the solar system. [1][2][3][4] The scientists have believed that the Widmanstätten structure was formed by the long-range thermal diffusion of Fe and Ni in asteroid's core over a period of 4.6 billion years. 2) Meanwhile, the iron meteorite is also characterized by remarkable magnetic properties, namely large magnetic anisotropy and strong coercivity, differing from those of synthetic Fe-Ni alloys. 5) However, there is no explanation how the magnetic properties are associated with the Widmanstätten structure. From the viewpoint of materials science, the Widmanstätten structure is regarded as Fe-Ni alloy segregated (bcc-FeNi, Kamacite) and (fcc-FeNi, Taenite) lamellae on the micrometer scale [ Fig. 1(b)]. 1) The Ni concentration in the lamella rapidly increases toward the interface, 2) resulting in several laminated Fe-Ni alloys, namely invar alloy (Fe 65 Ni 35 ), 7) tetrataenite (Fe 50 Ni 50 ) [8][9][10][11] and permalloy (Fe 25 Ni 75 ). 12,13) The f110g bcc axis maintains its parallel orientation to f111g fcc , and either h001i bcc or h1 11i bcc orients parallel to h1 10i fcc known as the Nishiyama-Wassermann (NW) or Kurdjumov-Sachs (KS) orientation, respectively. 6) Such a heterogeneous structure near boundary can be considered as same sort of magnetic multilayer system. In this study, we investigate the magnetic properties of iron meteorite resulting from the Widmanstätten structure for the first time.
Among the Fe-Ni alloys, we pay particular attention to the tetrataenite phase, which is described as a chemically ordered FeNi alloy with L1 0 -type superstructure. 9,10) It is also well known that tetrataenite exhibits significantly different magnetic properties from synthetic Fe-Ni alloys. Néel et al. reported the magnetic anisotropy energy of tetrataenite as 3:2 Â 10 5 J/m 3 , 8) which is more than one order of magnitude larger than those of Fe, invar alloy, ordered permalloy and pure Ni (4:8 Â 10 4 , 5 Â 10 2 , À2:5 Â 10 3 , and À6 Â 10 3 J/m 3 , respectively 7,8,12,13) ). Our pilot magnetic hysteresis measurement for tetrataenite flake shows coercive force of more than 1:0 Â 10 5 A/m, which is entirely larger than common Fe-Ni alloys. 8,12) In other words, tetrataenite is characterized as a hard ferromagnet with a strong anisotropy despite the fact that common Fe-Ni alloys are classified as softmagnetic materials.
To elucidate the relationship between the Widmanstätten structure and the magnetic property, we used Gibeon iron meteorite, which is one of the typical iron meteorites showing a clear Widmanstätten structure. 1) Figure 1(c) shows the image of the Widmanstätten structure; a thin lamella of 4 m width is separated from a thick lamella of 40 m width by lamellae. Recent progress in photoelectron emission microscopy (PEEM) using synchrotron radiation enables us to obtain the spatial information on composition, electronic state, 14) crystallographic structure, 15,16) and magnetic domain structure [17][18][19] in the combination with X-ray absorption fine structure (XAFS) or magnetic circular dichroism (MCD). The spatial resolution of apparatus reaches well below 100 nm.
Specimen was sliced nearly parallel to the ð001Þ bcc plane of the lamella. The surface was carefully prepared using automatic mechanical polisher with a 6 m diamond slurry for rough treatment and finished by buff polishing with a 1 m diamond slurry. AC demagnetization field is applied to the specimen to cancel residual magnetization or improper magnetic treatment since 19th century. We examine the surface irregularity using AFM that shows the scratches with a typical width of 100 nm and depth of 10 nm. We adopted even deeper scratch in the numerical calculation, but the scratch does not influence observed magnetic domains. The influence of shape anisotropy energy is negligibly smaller than exchange energy to produce the scratch induced magnetic domain structure. PEEM measurement was performed in the region with the lowest density of scratches.
The spatial distribution of Ni is shown in Figs. 2(a) and 2(b) for the boundary regions indicated by circles in  The typical exposure time per image was 10 s. Ni composition was estimated by edge-jump of the XAFS spectrum. The Ni composition in lamella in Fig. 2(a) shows spatially homogeneous profile of 6.6 at. %. On the other hand, that in lamella is highly condensed of 28 at. %. Figure 2(c) is typical XAFS spectra obtained for and lamella. As denoted by arrows, the spectral change from a single to a double peak on the crest is ascribed to the structural alternation from bcc to fcc over the transition threshold at 25 at. % Ni. 21) Figure 2(d) shows the Ni line profile at the boundary region in Fig. 2(b), indicating that local Ni composition is rapidly increasing from 20 to 35 at. % toward the interface. L1 0 structure was not directly recognized here because of the limit of resolution, however such structural alternation associated with chemical composition suggests that tetrataenite is segregated at the boundary from the metallurgical viewpoint. 2) To confirm the presence of tetrataenite, chemical etching by 5% HCl for several minutes reveals the tetrataenite at this region. Scanning electron microscopy and electron probe microanalyzer (SEM-EPMA) estimates the chemical composition as to be Fe 50 Ni 50 . Next, the surface magnetic domain structure was probed by MCD-PEEM for the same area. The circularly polarized light from BL25SU 22) was used to illuminate the specimen along ½110 bcc of lamella as shown by a thick arrow in Figs. 2(e) and 2(f). The exposure time of PEEM was set to be about 20 min to accumulate the image at the Fe L 3 edge (708.4 eV). The red-to-blue color scale indicates the MCD intensity. As shown in Fig. 2(e), a transversal domain lying parallel to the interface is observed clearly at 6 and 12 m from the interface. In common bcc-Fe, such as whiskers, the magnetic domain shows a wide rectangular structure with a sharp domain wall; thus the transversal stripe in meteoritic iron shows a behavior different from that of the bcc Fe-Ni alloy. 12,13) By considering its direction, the striped domain may be associated with the boundary.
Over the = interface, as shown in Figs. 2(e) and 2(f), a fine structure of about 2 m width is also observed; this structure is characterized by an elongated shape oriented parallel to the ½110 bcc direction, and the direction of magnetization orients parallel or antiparallel to the ½110 bcc direction, as indicated by arrows in Fig. 2(f). The magnetizations on both sides of the interface align opposite to each other and orthogonal to the domain wall, and then this magnetic domain eventually forms a ''head-on'' structure, which requires a large amount of magnetostatic energy for demagnetizing field. 12,23) For a typical 180 domain structure, the magnetization orients parallel to the domain wall so as to reduce the static magnetic energy. For the polycrystalline Fe-Ni alloy, the magnetization over the grain boundary aligns in the same continuous direction; 23,24) thus the head-on configuration in the iron meteorite is completely different from the case of Fe-Ni alloy. In epitaxially grown Fe ultrathin films on Ni(111) system, Fe moment align perpendicularly or parallel to Ni moments. 25,26) However, this is also not the case of iron meteorite. The head-on configuration is not simply explained by interface mismatch or atomic relaxation. It is concluded, therefore, that the striped magnetic domain and head-on magnetic coupling are unique properties of the magnetic domain in iron meteorite.
To verify such a magnetic domain, we performed micromagnetics simulation solving the Landau-Lifshits-Gilbert (LLG) equation. 27,28) Numerical study is achieved fully three dimensional with a functional form of boundary condition. We used two simple theoretical models, namely Fe/Ni [ Fig. 3(a)] and Fe/tetrataenite/Ni [ Fig. 3(b)] interface. We assumed a spatially uniform composition for the Ni lamella here. Downward (Àz) of the specimen uses a continuous boundary, and upward (þz) free. Longitudinal direction (x) uses a continuous boundary, and transversal (y) periodic. 1:6 Â 6:4 Â 1:6 mm 3 with 100 nm grid is adopted for simulating boundary region. Magnetic moment is referred to 2.2 and 0.6 B /atom for Fe and Ni, respectively, and 1.33 B /atom evaluated by superconducting quantum interference device (SQUID) is used for tetrataenite. Exchange stiffness is adopted as 1:3 Â 10 À11 , 1:0 Â 10 À11 , and 0:8 Â 10 À11 J/m for Fe, tetrataenite and Ni respectively. Averaged value is used for the interface exchange stiffness here. Magnetic anisotropy energy is referred to the values as described above. 7,8,12,13) Calculation runs from random magnetization to a cooled equilibrium state under zero magnetic field. Numerical simulation was performed for both NW and KS configurations and for 1.4-, 1.2-, 1-, 0.8-, 0.6-, 0.4-, and 0.2-m-thick tetrataenite films to examine the dependences of orientation and thickness. Here, we present the NW configuration of the 1-m-thick tetrataenite film as a representative result. Top layer is responsible to experimental results. To confirm the entire region, we also performed the calculation for large area of 16 Â 72 Â 8 m 3 with 1 m gird, and the result was consistent with that for 100 nm grid and experimental result.
As indicated in Fig. 3(a), the Fe/Ni interface shows a simple magnetic domain, and no head-on magnetic domain is formed. Most magnetic moments in both Fe and Ni  lamellae align to the bulk-like easy axis as h100i bcc and h111i fcc . The Fe moment gradually cants while approaching the interface, because of the requirement for continuity of the magnetization in x-direction. On the other hand, magnetic domain is disarranged in the Fe/tetrataenite/Ni system [ Fig. 3(b)], and head-on structure definitely reveals at nearby interface. Such head-on domains are commonly formed at any tetrataenite film thickness and in both the KS and NW configurations.
According to technical magnetization, magnetic domain structure is determined so as to minimize the total energy. The magnetic anisotropy of tetrataenite is extremely larger than that of surrounding soft magnetic Fe and Ni. Thus, magnetization in tetrataenite remains in the direction of an easy axis. As shown in the inset, the magnetic pole with z component is produced at bare surface of tetrataenite, resulting in the increase of magnetostatic energy. In order to cancel the surface pole at tetrataenite (N-pole in Fig. 3), the S-pole is created at the surface of Ni. These upward and downward configurations of magnetization increase the exchange energy at the interface between Ni/tetrataenite. However, the exchange energy between Fe/tetrataenite is larger than that of Ni/tetrataenite. Accordingly, the configuration in Fig. 3 produces the lower energy. On the other hand, in x-direction, the S-pole is created at the interface between Ni/tetrataenite, because the magnetization of tetrataenite is larger than that of Ni. In order to cancel the influence of that pole, the generation of N-pole is required at the interface between tetrataenite/Fe. Therefore, the magnetization in Fe shows the opposite direction against the magnetization in tetrataenite. This configuration causes the head-on domain, and increases the exchange energy. However, the cost of the exchange energy in head-on domain wall is equivalent to that in 180 domain wall, assuming the same wall width. Thus, head-on domain is agreeable to reduce the magnetostatic energy in the system. Consequently, we can conclude that the observed magnetic domains in iron meteorite are induced by the large magnetic anisotropy of the tetrataenite phase at the boundary. Tetrataenite will play a key role in the magnetic anisotropy of iron meteorite.
Synthesis of tetrataenite phase is currently attracting new attention because of inexpensive and abundant resource of Fe and Ni, 29) thus tetrataenite phase will offer potential applications in magneto-electronic devices.  The tetrataenite thin film located at the boundary exhibits a high magnetic anisotropy, and behaves similarly to a permanent magnet against adjacent soft-magnetic FeNi alloys, resulting in the formation of the head-on and striped magnetic domains.