Fe-Si System Spintronics

The Fe-Si system has various phases such as the semiconducting β-FeSi2, amorphous and nanocrystalline FeSi2, non-magnetic metallic FeSi, and ferromagnetic Fe3Si. We adopted Fe3Si and FeSi2 for the ferromagnetic and semiconducting layers, respectively. This combination has the following merits: (i) the electric conductivity mismatch in this combination is within an order of magnitude as after-mentioned; (ii) DO3-type Fe3Si can be regarded as a Heusler alloy, and high spin polarization is expected at the Fermi level; (iii) FeSi2 has an extremely large optical absorption coefficient, which is two orders of magnitude greater than that of Si at 1 eV, and thus a response with a high sensitivity for irradiation is expected;4 (iv) the d-electrons contribute to the electric conduction in both layers; Moreover, Fe3Si is suitable for practical use since it has a Curie temperature of 840 K and a half saturation magnetization of Fe. The Fe3Si/FeSi2 superlattice is a promising candidate for the ferromagneto/semiconductor heterostructure in spintronics.

● Our recent reports ●

"Temperature-dependent Magnetoresistance Effects in Fe3Si/FeSi2/Fe3Si Trilayered Spin Valve Junctions"

Kazuya Ishibashi, Kazuki Kudo, Kazutoshi Nakashima, Yuki Asai, Ken-ichiro Sakai, Hiroyuki Deguchi, and Tsuyoshi Yoshitake

JJAP Conf. Proc. 5, 011501 (2017)

Fe3Si/FeSi2/Fe3Si trilayered junctions were fabricated by facing targets direct-current sputtering combined with a mask method, and the spin valve signals of the junctions were studied in the temperature range from 50 to 300 K. Whereas the magnetoresistance ratio of giant magnetoresistance and tunnel magnetoresistance junctions monotonically increases with decreasing temperature, that of our samples has the maximum value around 80 K and decreases with decreasing temperature at lower than 80 K, which might be due to an increase in the electrical conductivity mismatch between the metallic Fe3Si layers and semiconducting FeSi2 interlayer in the low temperature range .

	(a) Schematic diagram of deposition procedure of Fe3Si/FeSi2/Fe3Si junction, (b) top view of the junction, and (c) electrical circuit for measuring electrical resistance of the junction.	MR curves of Fe3Si/FeSi2/Fe3Si junction, measured at 300, 200, 100, 77, and 5 K.

"Fabrication of Spin Valve Junctions Based on Fe3Si/FeSi2/Fe3Si Trilayered films"

Yuki Asai, Ken-ichiro Sakai, Kazuya Ishibashi, Kaoru Takeda, and Tsuyoshi Yoshitake

JJAP Conf. Proc. (2015) 011501

Fe3Si/FeSi2/Fe3Si trilayered junctions were prepared on Si(111) by facing targets direct-current sputtering combined with a mask method, and spin valve signals in current-perpendicular-to-plane (CPP) geometry was investigated for the change of the magnetization alignment. The shape of magnetization curves evidently exhibited that an antiparallel alignment is realized owing to a difference in the coercive force between the top and bottom Fe3Si layers. The electrical resistance was alternately changed for the formation of parallel and antiparallel alignments with the magnetic field. The spin valve signals in the Fe3Si/FeSi2 trilayered junctions were experimentally demonstrated.

	(a) Preparation procedure of CPP trilayered junctions with masks, (b) top view of CPP trilayered junction	MR curve of CPP trilayered film, measured at room temperature.

"Current-induced Magnetization Switching at Low Current Densities in CPP Structural Fe3Si/FeSi2 Artificial Lattices"

Ken-ichiro Sakai, Yūta Noda, Takeshi Daio, Daiki Tsumagari, Aki Tominaga, Kaoru Takeda, and Tsuyoshi Yoshitake

Jpn. J. Appl. Phys. Vol. 53, Issue 2S (2014) 02BC15.

Current-perpendicular-to-plane (CPP) junctions of Fe3Si/FeSi2 were fabricated from Fe3Si/FeSi2 artificial lattice films, which were prepared by facing-target direct-current sputtering, by employing a focused ion beam (FIB) technique. CPP structurization was confirmed by scanning electron microscopy. The CPP junctions, in which antiferromagnetic interlayer coupling is induced between the Fe3Si layers, exhibited a clear hysteresis loop in the electrical resistance for current injection, which is probably due to current-induced magnetization switching. The critical current density for it is approximately 3.3 ' 101A/cm2, which is at least four orders smaller than the values that have ever been reported.

	(a) Schematic of entire CPP junction and (b) planeview SEM images of CPP junction and tungsten electrode deposited on Fe3Si bottom layer.	Electrical resistance-injection current characteristics of CPP junction.

"Temperature-dependent interlayer coupling in Fe3Si/FeSi2 artificial lattices"

Ken-ichiro Sakai, Yuta Noda, Daiki Tsumagari, Hiroyuki Deguchi, Kaoru Takeda, and Tsuyoshi Yoshitake

Phys. Status Solidi A 211, No. 2 (2014) 323–328.

Fe3Si/FeSi2 artificial lattices, wherein ferromagnetic (F)/antiferromagnetic (AF) interlayer coupling between the Fe3Si layers were induced by controlling the thickness of FeSi2 layers, were prepared on Si(111) substrates by facing targets direct-current sputtering. The interlayer couplings were investigated at different temperatures by measuring the magnetization curves. The AF coupling at room temperature was gradually weakened with a decrease in the temperature, and it finally became ferromagnetic or noncoupled at temperatures lower than 77 K. We consider that the FeSi2 layers act as semiconductors and their change in the electric state for the temperature induces the interlayer coupling switching.

	Change in magnetization curve of [Fe3Si(25 Å)/FeSi2(10 Å)]20 artificial lattice for different temperature.	Change in interlayer coupling strength (J) with temperature.

"Current-Induced Magnetization Switching in Fe₃Si/FeSi₂ Artificial Lattices"

Ken-ichiro Sakai, Takayuki Sonoda, Shin-ichi Hirakawa, Kaoru Takeda, and Tsuyoshi Yoshitake

Jpn. J. Appl. Phys. Vol. 51, No. 2 (2012) 028004.

Current-induced magnetization switching in Fe3Si/FeSi2 artificial lattices, wherein ferromagnetic Fe3Si layers were antiferromagnetically coupled, was studied in a current perpendicular to plane geometry. While the electrical resistance-injected current (R-I) curve exhibited a clear hysteresis loop under zero magnetic field, the hysteresis loop in the curve disappeared under magnetic fields larger than the saturation magnetic field. This implies that the interlayer coupling was altered from antiferromagnetic to ferromagnetic by the current injection under zero magnetic field; on the other hand, under the magnetic fields, the coupling was fixed to be ferromagnetic regardless of the current injection.

"Temperature-Dependent Current-Induced Magnetization Switching in Fe₃Si/FeSi₂/Fe₃Si trilayered films"

Shin-ichi Hirakawa, Takayuki Sonoda, Ken-ichiro Sakai, Kaoru Takeda, and Tsuyoshi Yoshitake

Jpn. J. Appl. Phys., Vol. 50, No. 8 (2011) 08JD06.

Fe3Si/FeSi2/Fe3Si trilayered films were grown on Si(111) substrates at a substrate temperature of 300 ºC by facing targets direct-current sputtering, and current-induced magnetization switching in current-perpendicular-to-plane geometry was studied for the films wherein an antiferromagnetic interlayer coupling perpendicular to the plane was probably formed at room temperature. The appearance of a hysteresis loop in the electrical resistance-injection current curve well coincided with that of a hysteresis loop in the magnetization curve perpendicular to the plane. In addition, the hysteresis loop in the electrical resistance-injection current curve disappeared under large magnetic fields. The origin of the change in the electrical resistance for the injection current might be attributable to the change in the interlayer coupling.

"Enhanced interlayer coupling and magnetoresistance ratio in Fe3Si/FeSi2 superlattices"

Kaoru TAKEDA, Tsuyoshi YOSHITAKE , Yoshiki SAKAMOTO, Tetsuya OGAWA, Daisuke HARA, Masaru ITAKURA, Noriyuki KUWANO, Toshinori KAJIWARA1, and Kunihito MAGAYAMA

Appl. Phys. Express, Vol. 1, No. 2 (2008) 021302.

[Fe3Si/FeSi2]20 superlattices were prepared on Si(111) at an elevated substrate temperature of 300 ºC, and the magnetoresistance ratio and interlayer coupling strengths were enhanced by approximately 100% and 34%, respectively, as compared to those of superlattices deposited at room temperature. While the elevated substrate temperature degraded the interface sharpness, the crystalline orientation and the crystallinity of the Fe3Si layers were apparently enhanced. The latters strongly influence on the interlayer coupling and the magnetoresistance ratio. This implies that quantum well states are tightly formed under the well-ordered crystalline planes, and the spin diffusion lengths are improved due to the enhanced crystallinity.

	Change in Ms for Ts. The inset (a) shows the temperature dependence of M/Ms of Fe3Si films deposited at various Ts values. The inset (b) shows X-ray diffraction patterns of Fe3Si thin films deposited at RT before and after the Tc measurement.	Cross-sectional TEM image of the [Fe3Si(25 Å)/FeSi2(60 Å)]20 superlattice. (a) Bright field image, (b) dark field image obtained using a Fe3Si spot, and (c) SAED pattern for the area including the substrate and the superlattice.

"Interlayer coupling in ferromagnetic epitaxial Fe3Si/FeSi2 superlattices"

Kaoru TAKEDA, Tsuyoshi YOSHITAKE, Dai NAKAGAUCHI, Tetsuya OGAWA, Daisuke HARA, Masaru ITAKURA, Noriyuki KUWANO, Yoshitsugu TOMOKIYO, Toshiyuki KAJIWARA, and Kunihito NAGAYAMA

Jpn. J. Appl. Phys. Part 1, Vol. 46, No. 12, 2007, pp. 7846–7848.

Fe3Si/FeSi2 superlattices were prepared on Si(111) at two deposition rates by facing target direct-current sputtering. For the deposition rates of 2.0 nm/min for Fe3Si and 1.3 nm/min for FeSi2, the Fe3Si layers were nonoriented. On the other hand, for half-deposition rates, the Fe3Si layers were epitaxially grown not only on Si(111) but also up to the top layer across the FeSi2 layers. The antiferromagnetic interlayer coupling between the Fe3Si layers was induced in the epitaxial superlattices, whereas it disappeared in the nonepitaxial superlattices. The regular accumulation of highly oriented Fe3Si layers is crucial for the interlayer coupling induction.

	XRD patterns of [Fe3Si (25 Å)/FeSi2 (X Å)]20 superlattices, which were deposited at deposition rate A, measured in (a) 2θ-θ mode and (b) 2θ mode with fixed incidence angle of 4°.	XRD patterns of [Fe3Si (25 Å)/FeSi2 (X Å)]20 superlattices, which were deposited at deposition rate B, measured in (a) 2q-q mode and (b) 2q mode with fixed incidence angle of 4°. The inset shows typical f-scan patterns of the superlattice with X = 7.5 Å, which was deposited at the deposition rate B, and the Si(111) substrate.

"Interlayer coupling in ferromagnetic epitaxial Fe3Si/FeSi2 superlattices"

T. Yoshitake, T. Ogawa, D. Nakagauchi, D. Hara, M. Itakura, N. Kuwano, Y. Tomokiyo, K. Takeda, T. Kajiwara, M. Ohashi, G. Oomi, and K. Nagayama

Appl. Phys. Lett. Vol. 89, 253110, 2006.

Ferromagnetic epitaxial B2-type Fe3Si/FeSi2 superlattices were prepared on Si(111) at room temperature by facing targets direct-current sputtering. The bilinear and biquadratic coupling constants J1 and J2 of the antiferromagnetically coupled superlattice were comparable to those of the similar superlattices using Fe layers although the saturation magnetization of Fe3Si is approximately half as large as that of Fe. We believe that this is due to the formation of a well-ordered quantum well in the spacers, which is mainly caused by the regular accumulation of highly oriented Fe3Si layers.

	Low-angle X-ray diffraction patterns of the [Fe3Si(25 Å)/ FeSi2(X Å)]20 superlattices. The inset shows the cross-sectional TEM image of the [Fe3Si(25 Å)/FeSi2(17.5 Å)]20 superlattice.
	Typical magnetization curves of (a) antiferromagnetically and (b) ferromagnetically or non-coupled superlattices (X = 7.5 Å and 17.5 Å) at room temperature. (c) Change in Mr/Ms and Hs for the FeSi2 layer thickness X. The inset shows typical MR curves of superlattices with X = 7.5 Å and 17.5 Å at room temperature.	Cross-sectional TEM image of the [Fe3Si(25 Å)/FeSi2(7.5 Å)]20 superlattice. (a) bright field image, (b) dark field image using a Fe3Si-011 spot. (c) SAED pattern for the area including the substrate and the film, (d) SAED pattern for the film.