Magnetic anisotropies in epitaxial Fe3O4/GaAs(100) patterned structures
W. Zhang, P. K. J. Wong, D. Zhang, S. J. Yuan, Z. C. Huang, Y. Zhai, J. Wu, and Y. B. Xu
Citation: AIP Advances 4, 107111 (2014); doi: 10.1063/1.4897963 View online: http://dx.doi.org/10.1063/1.4897963
View Table of Contents: http://scitation.aip.org/content/aip/journal/adva/4/10?ver=pdfcov Published by the AIP Publishing
Articles you may be interested in
Extraordinarily large perpendicular magnetic anisotropy in epitaxially strained cobalt-ferrite CoxFe3−xO4(001) (x = 0.75, 1.0) thin films
Appl. Phys. Lett. 103, 162407 (2013); 10.1063/1.4824761
Multiferroic PbZrxTi1−xO3/Fe3O4 epitaxial sub-micron sized structures Appl. Phys. Lett. 100, 102903 (2012); 10.1063/1.3692583
Origin of uniaxial magnetic anisotropy in epitaxial MnAs film on GaAs(001) substrate Appl. Phys. Lett. 92, 082503 (2008); 10.1063/1.2844856
Strain induced magnetic anisotropy in highly epitaxial Co Fe 2 O 4 thin films Appl. Phys. Lett. 89, 262506 (2006); 10.1063/1.2424444
Magnetic anisotropy and domain structure in epitaxial CoFe 2 O 4 thin films J. Appl. Phys. 93, 8143 (2003); 10.1063/1.1541651
Magnetic anisotropies in epitaxial Fe
3O
4/GaAs(100)
patterned structures
W. Zhang,1,aP. K. J. Wong,2,3D. Zhang,1S. J. Yuan,1Z. C. Huang,1Y. Zhai,1
J. Wu,4and Y. B. Xu5
1Department of Physics, Southeast University, Nanjing 211189, China
2NanoElectronics Group, MESA+ Institute for Nanotechnology, University of Twente,
P. O. Box 217, 7500 AE Enschede, The Netherlands
3Department of Chemistry, National University of Singapore, 3 Science Drive 3,
117543, Singapore
4Department of Physics, University of York, York YO10 5DD, UK
5Spintronics and Nanodevice Laboratory, Department of Electronics, University of York,
York, YO10 5DD, UK
(Received 22 July 2014; accepted 1 October 2014; published online 8 October 2014)
Previous studies on epitaxial Fe3O4 rings in the context of spin-transfer torque
effect have revealed complicated and undesirable domain structures, attributed to the intrinsic fourfold magnetocrystalline anisotropy in the ferrite. In this Letter, we report a viable solution to this problem, utilizing a 6-nm-thick epitaxial Fe3O4
thin film on GaAs(100), where the fourfold magnetocrystalline anisotropy is negli-gible. We demonstrate that in the Fe3O4 planar wires patterned from our thin
film, such a unique magnetic anisotropy system has been preserved, and rela-tively simple magnetic domain configurations compared to those previous reports can be obtained. C 2014 Author(s). All article content, except where otherwise
noted, is licensed under a Creative Commons Attribution 3.0 Unported License. [http://dx.doi.org/10.1063/1.4897963]
The well-known spin-transfer torque effect,1–4 via which the motion of magnetic domain walls can be realized by injection of spin-polarized currents,5–8has become one of the most pro-mising approaches for future information storage with low power consumption.9,10 Before such a spin-based phenomenon could be implemented for practical device applications, a number of funda-mental issues have to be tackled. A major current challenge is to reduce the critical current density that is required to trigger the domain wall motion in artificial magnetic micro- or nano-structures. Theoretical calculations have suggested that, in the adiabatic limit and in the absence of extrinsic pinning, the critical current density, jC, has a simple relation with the spin polarization, P, of a given
magnetic material, as jC∝ 1/P.11,12 With this notion, Fe3O4, a theoretically predicted fully
spin-polarized material with high Curie temperature of 858 K13and good chemical stability, appears to be especially promising.
Even though previous experiments have indeed verified the above belief, i.e., achieving a spin polarization of ∼80% at best near the Fermi level in the ferrite thin film at room temperature,14the utilization of Fe3O4in domain wall devices remains scarce.15This is in part attributed to the
forma-tion of undesirable domain structures that could result from an interplay between the shape- and the intrinsic cubic magnetocrystalline anisotropies in Fe3O4.16For instance, using x-ray photoemission
electron microscopy, Fonin et al. observed the appearance of zig-zag shaped domain walls in their 1-µm-wide Fe3O4 rings,17 as a consequence of the dominant in-plane cubic magnetic anisotropy
in the ferrite structures. These zigzag domain walls, with complex wall structure and boundary, could possibly complicate the wall motion, which are certainly setbacks for integrating highly spin-polarized Fe3O4into the context of spin-transfer torque applications.
aElectronic mail:xiaotur@gmail.com
107111-2 Zhang et al. AIP Advances 4, 107111 (2014)
In this Letter, we report a viable approach to circumvent the above-mentioned complication by taking advantage of a unique in-plane magnetic anisotropy system in high-quality Fe3O4ultrathin
film grown on GaAs(100) by postgrowth oxidation of epitaxial Fe(100). As we have already shown elsewhere,18our Fe
3O4thin film is very unique in that it exhibits
a strong in-plane uniaxial magnetic anisotropy (two-fold), superimposed with a negligible intrinsic cubic anisotropy (four-fold).18–20 Moreover, such a magnetic anisotropy system does not exist in epitaxial Fe3O4 thin films prepared by other techniques.21–23 On the other hand, from the charge
transport measurements in our high-quality epitaxial Fe3O4film, a resistivity of about 5 × 10−3Ωcm
has been demonstrated, which is very close to that of bulk single crystal24–26and much lower than those of the commonly reported Fe3O4thin. films.27–29This largely benefits from our growth method,
thereby resulting in the lack of anti-phase boundaries, which has been clearly evidenced by various characterisation techniques.30,31Considering the comprehensive advantages of our Fe3O4thin film,
our present work should therefore lead to a renewed interest, and offer an unprecedented opportunity to acquire controllable and well-defined domain wall configurations in Fe3O4patterned structures,
without suffering from the intrinsic cubic magnetocrystalline anisotropy of the ferrite.
Following the growth recipe developed in our previous work, epitaxial Fe thin film with a thick-ness of 3 nm was first deposited on GaAs(100) substrate at room temperature and at a rate of 2 Å/min, using electron-beam evaporation in a molecular beam epitaxy system with a base pressure of 6 × 10−9
mbar. Afterwards the Fe film was oxidized in situ in an O2partial pressure of 5 × 10−5mbar for 180 s
at 500 K,32,33thereby forming a stoichiometric Fe
3O4film with its thickness twice that of the original
Fe film, i.e., 6 nm.34,35The oxidized film was further patterned by focused ion beam lithography (30 keV Ga+, 10 pA ion beam current) into several sets of planar wires, as shown in Fig.1. These wires are 10 µm long with the width ranging from 0.5 µm to 1 µm. The long axes of the wires are either parallel or perpendicular to the easy axis of the uniaxial anisotropy in the film. Specifically, patterns 1 and 2 contain a series of 1-µm-wide wires, with their long axes along the [0-11] and [011] directions of the GaAs substrate, respectively; the wires in pattern 3, with a width of around 0.5 µm, have their long axes along the [0-11] direction. It is noteworthy that the inter-wire distances in all these patterns are no less than 1 µm, which are sufficiently large to rule out magnetostatic interactions between the wires in the discussions later on.36–38
We first illustrate, in Fig.2, the magnetic hysteresis loops of the planar wires in patterns 1-3, in order to examine the evolution of the magnetic anisotropies originally existing in the continuous film. The hysteresis loops were acquired by a home-built focused magneto-optical Kerr effect (f-MOKE) magnetometer, with a 1 µm laser spot on the samples, and the magnetic fields were applied along the two major crystallographic axes, [0-11] and [011], of the GaAs(100) substrate. From Figs.2(a)-(b)one can see that the Fe3O4continuous film exhibits a strong uniaxial anisotropy, KU, having its easy and
hard axes along the [0-11] and [011] directions, respectively, as expected. Simultaneously a negligible cubic magnetocrystalline anisotropy, K1, is superimposed, as shown in TableI.18Therein KU and
K1of the continuous film were obtained from our previous work on numerical fitting of the angular
dependences of the ferromagnetic resonance field of the film.18
Figure2(c)–(h)show the hysteresis loops of pattern 1-3, which share the same easy- and hard magnetization axes in the continuous film, regardless of the alignment between their long axes and the [0-11] direction. The high remnant magnetization in that direction has been retained as well. Particularly, even if the wires in pattern 2 have their long axes normal to the [0-11] direction, which
FIG. 1. SEM images of (a) pattern 1, (b) pattern 2, and (c) pattern 3. The white arrows indicate the directions of the easy ([0-11]) and hard ([011]) axes of the uniaxial magnetic anisotropy in the Fe3O4thin film, prior to focused ion beam lithography.
FIG. 2. (Color online) Representative f-MOKE hysteresis loops of (a)-(b) Fe3O4continuous film, planar wires in (c)-(d) pattern 1,(e)-(f) pattern 2, and (g)-(h) pattern 3, respectively. The LLG simulations are shown in (i)-(j) as well. Inset in (j): Modified simulation for pattern 1 along the [011] direction, with interfacial defects taken into account. The directions of the applied magnetic field are labeled above each column with respect to the crystallographic axes of the GaAs(100) substrate.
is expected to reorient the easy axis into the [011] direction due to the large demagnetizing field along the [0-11] direction, they still follow the easy- and hard magnetization axes of the thin film, as shown in Fig.2(e)-(f). This observation suggests that the uniaxial anisotropy has been well pre-served after the lithographic process, and plays a decisive role over the cubic- and the shape anisot-ropy in determining the overall switching behavior of the 1-µm-wide wires. One more interesting observation is the smaller coercivity in pattern 3 than that in pattern 1 along the [0-11] direction, when comparing Fig. 2(g)to Fig.2(c). This seems to contradict to one’s intuition that larger de-magnetizing field in narrower wires should be able to provide larger coercivity. The only plausible
TABLE I. Magnetic anisotropy constants, KUand K1, used for LLG simulations of pattern 1-3. The experimental values for continuous film are also listed in the first column for ease of comparison. W is the wire width, x denotes the direction of the wire long-axis relative to the GaAs(100) substrate.
Film Pattern1 Pattern2 Pattern3
W (µm) - 1 1 0.5
x - [0-11] [011] [0-11]
KU(104erg/cm3) 4.0 4.0 4.0 0.1
107111-4 Zhang et al. AIP Advances 4, 107111 (2014)
reason for this reduction should, therefore, be the strong relaxation of the original uniaxial anisot-ropy in the 0.5-µm-wide wires.
We have attempted to reproduce the hysteresis loops by means of the LLG Micromagnetics Simulator,39as shown in Fig.2(i)-(j). The material parameters used for the simulations are as below: saturation magnetization MS= 480 emu/cm3, exchange stiffness constant A = 1.3 × 10−11erg/cm,
and damping coefficient α = 0.3; the cell size was 10 × 10 × 6 nm3. The two most important
param-eters, KU and K1, for best-fitting are shown in Table I, where we can see that the KU of the
continuous film has been largely retained in patterns 1 and 2, while has a strong relaxation in pattern 3. Although an in-depth analysis of such a decrease in the KUis out of the scope of this work, we
would like to point out that the relaxation of the uniaxial anisotropy in this case might be closely related to the chemical bonding at the Fe3O4/GaAs(100) interface, which has been discussed by
some of us elsewhere.18Equally important is the negligible K1values observed in both of our film
and patterned wires, which are significantly lower than those in bulk Fe3O4and in other reported
Fe3O4thin films.17,40
Another aspect one may notice in the second column of Fig.2is that the experimental magneti-zation curves along the hard axis look more difficult to be saturated than the calculated ones do. This is primarily attributed to the existence of localized nonuniform magnetization caused by or close to the interfacial defects,41which requires much higher field for saturation. We attempted to model this by treating the film as two layers, namely, a thick layer with only domain-rotation considered and a thin one at the interface with interfacial defects involved, and modified the simulation accordingly. One such calculated result by the modified simulation for pattern 1 is illustrated in the inset of Fig.
2(j), which indeed seems to better match the experimental curve.
From the fitting of the magnetization curves in Fig.2, we found that no matter what dimension the wire has, the cubic magnetic anisotropy is always much smaller than the uniaxial- and the shape anisotropy. In the following, we will address the main issue on whether this negligible K1would
indeed lead to a more desirable domain configurations for spin-transfer torque applications. To answer so, micromagnetic simulations have been performed by the LLG simulator for Fe3O4rings,
with the width of 1 µm and 0.5 µm, respectively. The ring shape was chosen for its high geom-etry symmgeom-etry, guaranteeing the ease of trapping magnetic domain walls by applying an external magnetic field,42–44 and also for ease of comparison to the previous reports.16,17 The simulation results, by taking the parameters used for Fig.2, are shown in Fig.3, where we find that the zig-zag domain walls, as a consequence of the strong fourfold in-plane magnetocrystalline anisotropy of the film in the previous studies,17can be avoided in both rings and far more simplified domain struc-tures16,17are revealed. Specifically, when being saturated along the hard axis ([011]), followed by relaxation at zero field, our 1-µm-wide Fe3O4ring exhibits four 180◦walls with a narrow boundary
in the equilibrium state at remanence, as shown in Fig.3(a). While in Fig.3(b), only two domain walls have been observed in the 0.5-µm-wide Fe3O4ring, i.e., one head-to-head and one tail-to-tail
walls, dividing the ring into two magnetic domains, with the in-plane magnetization following the perimeter of the ring. Most importantly, the reported additional 90◦reorientations arising from the
FIG. 3. (Color online) LLG simulations of the magnetic domain structures of Fe3O4rings with the linewidth W= 1 µm (a) and 0.5 µm (b), respectively, in remanent states after saturation.
influence of magnetocrystalline anisotropy17have not been observed. The 0.5-µm-wide Fe
3O4ring,
therefore, exhibits an onion-state magnetic configuration as found in 3d metal rings,43,45which have been widely used as unit cells for domain wall devices, profiting from ease of the shape tailoring with negligible intrinsic magnetic anisotropies in those materials.
It is noteworthy that the simulations with K1= 0 have also been carried out, which essentially
show identical domain configurations to Fig.3(a)and(b), further suggesting that the marginal K1in
our epitaxial thin film has nearly no influence in the domain formation and is therefore negligible. These above findings form the key results of the present work, which provide a possibility of avoid-ing the complex domain structures caused by K1via utilization of our Fe3O4thin film. In particular,
when the width of the ring is reduced to 0.5 µm, simple domain configuration as found in 3d metal rings can be obtained, due to relaxation of both the uniaxial and cubic anisotropies. For the strong uniaxial anisotropy preserved in the 1-µm-wide ring, its influence on the domain wall structures during the motion requires further current-driven experiment to evaluate.
The present study demonstrates a remarkable approach to generate simple domain configura-tions in half-metallic Fe3O4patterned structures, based on the unique magnetic anisotropy system in
epitaxial Fe3O4/GaAs(100). Combined with the high spin polarization and Curie temperature, and
good chemical stability of Fe3O4, the patterned structures from our high-quality magnetite thin film
are promising candidates for future current-driven applications in the domain wall devices, where small threshold currents as well as simple wall structures are strongly preferred.
This work is supported by National Natural Science Foundation for Young Scientists of China (Grant No. 61306121) and China Postdoctoral Science Foundation (Grant No. 2013M541580). Y.Z. acknowledges the financial support from National Basic Research Program (Grant No. 2010CB923404) and National Natural Science Foundation of China (Grant Nos. 50871029 and 11074034). S.J.Y. acknowledges the financial support from National Natural Science Foundation of China (Grant No. 11104027). P.K.J.W. is financially supported by the EU FP7 Project SpinValley under Grant No. PIOF-GA-2013-628063.
1G.A. Prinz,Science282, 1660 (1998).
2S.A. Wolf, D.D. Awschalom, R.A. Buhrman, J.M. Daughton, S. Von Molnár, M.L. Roukes, A.Y. Chtchelkanova, and D.M. Treger,Science294, 1488 (2000).
3J.C. Slonczewski,J. Magn. Magn. Mater.159, L1 (1996). 4L. Berger,Phys. Rev. B54, 9353 (1996).
5M. Kläui, C.A.F. Vaz, J.A.C. Bland, W. Wernsdorfer, G. Faini, E. Cambril, and L.J. Heyderman,Appl. Phys. Lett.83, 105 (2003).
6A. Yamaguchi, T. Ono, and S. Nasu,Phys. Rev. Lett.92, 077205 (2004). 7S. Lepadatu and Y.B. Xu,Phys. Rev. Lett.92, 127201 (2004).
8W. Zhang, P.K.J. Wong, P. Yan, J. Wu, S.A. Morton, X.R. Wang, X.F. Hu, Y.B. Xu, A. Scholl, A. Young, I. Barsukov, M. Farle, and G. van der Laan,Appl. Phys. Lett.103, 042403 (2013).
9S.S.P. Parkin, U.S. Patent No. 309,6,834,005 (2004).
10S.S.P. Parkin, M. Hayashi, and L. Thomas,Science320, 190 (2008). 11G. Tatara and H. Kohno,Phys. Rev. Lett.92, 086601 (2004).
12G. Tatara, T. Takayama, H. Kohno, J. Shibata, Y. Nakatani, and H. Fukuyama,J. Phys. Soc. Jpn.75, 064708 (2006). 13M.L. Glasser and F.J. Milford,Phys. Rev.130, 1783 (1963).
14Y.S. Dedkov, U. Rudiger, and G. Guntherodt,Phys. Rev. B65, 064417 (2002).
15O. Cespedes, S.M. Watts, J.M.D. Coey, K. Dörr, and M. Ziese,Appl. Phys. Lett.87, 083102 (2005).
16M. Kläui, in Magnetism and Synchrotron Radiation, Springer Proceedings in Physics (Springer-Verlag, Berlin, Heidelberg, 2010), Ch. 13, Vol. 133, p. 367.
17M. Fonin, C. Hartung, U. Rüdiger, D. Backes, L. Heyderman, F. Nolting, A. Fraile Rodrjuez, and M. Kläui,J. Appl. Phys. 109, 07D315 (2011).
18W. Zhang, J.Z. Zhang, P.K.J. Wong, Z.C. Huang, L. Sun, J.L. Liao, Y. Zhai, Y.B. Xu, and G. van der Laan,Phys. Rev. B84, 104451 (2011).
19P.K.J. Wong, Y. Fu, W. Zhang, Y. Zhai, Y.B. Xu, Z.C. Huang, Y.X. Xu, and H.R. Zhai,IEEE Trans. Magn.44, 2907 (2008). 20P.K.J. Wong, W. Zhang, Y.B. Xu, S. Hassan, and S.M. Thompson,IEEE Trans. Magn.44, 2640 (2008).
21M. Luysberg, R.G.S. Sofin, S.K. Arora, and I.V. Shvets,Phys. Rev. B80, 024111 (2009). 22E.J. Preisler, J. Brooke, N.C. Oldham, and T.C. McGill,J. Vac. Sci. Technol. B21, 1745 (2003).
23F.C. Voogt, T. Fujii, P.J.M. Smulders, L. Niesen, M.A. James, and T. Hibma,Phys. Rev. B60, 11193 (1999). 24R. Ramos, S.K. Arora, and I.V. Shvets,Phys. Rev. B78, 214402 (2008).
25N.F. Mott, 1974 Metal–Insulator Transitions (London: Taylor and Francis). 26P.A. Miles, W.B. Westphal, and A. Von Hippel,Rev. Mod. Phys.29, 279 (1957).
27J.M.D. Coey, A.E. Berkowitz, LI. Balcells, F.F. Putris, and F.T. Parker,Appl. Phys. Lett.72, 734 (1998). 28K. Aoshima and S.X. Wang,J. Appl. Phys.91, 7146 (2002).
107111-6 Zhang et al. AIP Advances 4, 107111 (2014)
29A.V. Ramos, J.-B. Moussy, and M.-J. Guittet et al.,J. Appl. Phys.100, 103902 (2006). 30S.S.A. Hassan, Y.B. Xu, and J. Wu et al.,IEEE Trans. Magn.45, 4357 (2009). 31S.S.A. Hassan, Y.B. Xu, and J. Wu et al.,IEEE Trans. Magn.45, 4360 (2009).
32Y.X. Lu, J.S. Claydon, Y.B. Xu, S.M. Thompson, K. Wilson, and G. van der Laan,Phys. Rev. B70, 233304 (2004). 33P.K.J. Wong, W. Zhang, and X.G. Cui et al.,Phys. Rev. B81, 035419 (2010).
34Z.C. Huang, Y. Zhai, Y.X. Lu, G.D. Li, P.K.J. Wong, Y.B. Xu, Y.X. Xu, and H.R. Zhai,Appl. Phys. Lett.92, 113105 (2008). 35Y. Zhai, L. Sun, Z.C. Huang, Y.X. Lu, G.D. Li, Q. Li, Y.B. Xu, J. Wu, and H.R. Zhai,J. Appl. Phys.107, 09B110 (2010). 36M. Grimsditch, Y. Jaccard, and I.K. Schuller,Phys. Rev. B58, 11539 (1998).
37G. Gubbiotti, S. Tacchi, G. Carlotti, P. Vavassori, N. Singh, S. Goolaup, A.O. Adeyeye, A. Stashkevich, and M. Kostylev,
Phys. Rev. B72, 224413 (2005).
38B. Borca, O. Fruchart, E Kritsikis, F. Cheynis, A. Rosusseau, Ph. David, C. Meyer, and J.C. Toussaint,J. Magn. Magn.
Mater.322, 257 (2010).
39The commercial LLG Micromagnetics Simulator software is developed by M. R. Scheinfein. For details, see the Web site
http://llgmicro.home.mindspring.com.
40M. Ziese, R. Höhne, P. Esquinazi, and P. Busch,Phys. Rev. B66, 134408 (2002).
41L. Sun, P.K.J. Wong, W. Zhang, X. Zou, L.Q. Luo, Y. Zhai, J. Wu, Y.B. Xu, and H.R. Zhai, J. Appl. Phys. 109, 07B910 (2011).
42S.P. Li, D. Peyrade, M. Natali, A. Lebib, Y. Chen, U. Ebels, L.D. Buda, and K. Ounadjela,Phys. Rev. Lett.86, 1102 (2001). 43J. Rothman, M. Kläui, L. Lopez-Diaz, C.A.F. Vaz, A. Bleloch, J.A.C. Bland, Z. Cui, and R. Speaks,Phys. Rev. Lett.86,
1098 (2001).
44L. Huang, M.A. Schofield, and Y. Zhu,Appl. Phys. Lett.95, 042501 (2009). 45M. Kläui, J. Phys.: Condens. Matter 20, 313001 (2008).
