Magnetic anisotropy of quaternary GaMnAsP ferromagnetic semiconductor Hakjoon Lee, Jihoon Chang, Phunvira Chongthanaphisut, Sangyeop Lee, Seonghoon Choi, Seul-Ki Bac, Alviu R Nasir, Sanghoon Lee, A Pardo, Sining Dong, X Li, X Liu, J K Furdyna, and M Dobrowolska Citation: AIP Advances 7, 055809 (2017); doi: 10.1063/1.4972856 View online: http://dx.doi.org/10.1063/1.4972856 View Table of Contents: http://aip.scitation.org/toc/adv/7/5 Published by the American Institute of Physics Articles you may be interested in Robust spin-current injection in lateral spin valves with two-terminal Co2FeSi spin injectors AIP Advances 7, 055808055808 (2016); 10.1063/1.4972852 Temperature-dependent shape anisotropy in patterned ferromagnetic (Ga,Mn)As films with low Mn concentration AIP Advances 7, 055810055810 (2016); 10.1063/1.4973201 A comparative study of ultra-low-temperature thermal conductivity of multiferroic orthoferrites RFeO3 (R = Gd and Dy) AIP Advances 7, 055806055806 (2016); 10.1063/1.4973293 Nanopatterning spin-textures: A route to reconfigurable magnonics AIP Advances 7, 055601055601 (2016); 10.1063/1.4973387 AIP ADVANCES 7, 055809 (2017) Magnetic anisotropy of quaternary GaMnAsP ferromagnetic semiconductor Hakjoon Lee,1 Jihoon Chang,1 Phunvira Chongthanaphisut,1 Sangyeop Lee,1 Seonghoon Choi,1 Seul-Ki Bac,1 Alviu R Nasir,1 Sanghoon Lee,1,a A Pardo,2 Sining Dong,3 X Li,3 X Liu,3 J K Furdyna,3 and M Dobrowolska3 Physics Department, Korea University, Seoul 136-701, South Korea Department, Universidad del Atlantico, Km Antigua via a Puerto Colombia, Barranquilla, Colombia Department of Physics, University of Notre Dame, Notre Dame, Indiana 46556, USA Physics (Presented November 2016; received 23 September 2016; accepted 29 September 2016; published online 27 December 2016) We report a systemeatic investigation of magnetic anisotropy of quaternary GaMnAsP ferromagnetic semiconductor films by magneto-transport Hall measurements showed a transition of the easy magnetization direction from in-plane to out-of plane with incorporation of the P into the GaMnAs films Quantitative information on magnetic anisotropy of the films is obtained by fitting the angular dependence of Hall resistance data to magnetic free energy using the coherent rotation model Values of magnetic anisotropy parameters show that in-plane anisotropy decreases and out-ofplane anisotropy increases with increasing P content in these films The out-of-plane magnetic anisotropy in GaMnAsP layers is further enhanced by low temperature annealing By optimizing the growth and annealing conditions, we were able to obtain a Curie temperature of 125 K in such quaternary films, with strong out-ofplane anisotropy This study showed that the magnetic anisotropy of the GaMnAsP films can be controlled by adjusting the concentration of the P, and by appropriate post-growth annealing © 2016 Author(s) All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) licenses (http://creativecommons.org/licenses/by/4.0/) [http://dx.doi.org/10.1063/1.4972856] INTRODUCTION It is now well established that incorporation of Mn ions in III-V semiconductors results in hole-mediated ferromagnetism, GaMnAs being the best-known example.1–4 An important property of these ferromagnetic materials is their magnetic anisotropy, which determines the orientation of magnetization in the absence of magnetic field The easy direction of magnetization in III-Mn-V films can either be in-plane or out-of-plane, depending on strain conditions arising from lattice mismatch between the III-Mn-V layer and the substrate on which it is grown For example, in GaMnAs the easy axis of magnetization is in-plane under compressive strain, and out-of-plane under tensile strain.5,6 The most extensively studied case is that of GaMnAs grown directly on GaAs, where the GaMnAs layer is compressively strained, thus exhibiting in-plane magnetic easy axes However, magnetic films with out-of-plane easy axes have recently become increasingly interesting, owing to their potential for improving the density of magnetic devices GaMnAs films with out-of-plane magnetic anisotropy can be obtained by using buffer layers having larger lattice constants than that of GaMnAs, such as InGaAs and ZnCdSe.7,8 This approach, however, requires deposition of additional layers during growth, which often degrades magnetic properties of the ferromagnetic film Since the incorporation of P into the GaMnAs lattice reduces the lattice parameters below that of GaAs, a Correspondence: slee3@korea.ac.kr 2158-3226/2017/7(5)/055809/6 7, 055809-1 © Author(s) 2016 055809-2 Lee et al AIP Advances 7, 055809 (2017) the growth of quaternary systems such as GaMnAsP on GaAs provides a promising alternative for ferromagnetic semiconductor films with out-of-plane magnetic anisotropy Successful growth of GaMnAsP film with an out-of-plane magnetic easy axis has already been reported by several groups.9–13 In this paper, we carry out a detailed quantitative investigation of magnetic anisotropy of GaMnAsP films using Hall effect measurements, which allow us to quantify both in-plane and out-of-plane anisotropy constants This in turn allows us to construct 3-dimensional (3-D) magnetic free energy density profiles of this material, thus providing a useful guide for understanding its magnetic behavior EXPERIMENTS A set of Ga1-x Mnx As1-y Py film was grown by molecular beam epitaxy (MBE) on (001) GaAs substrates, with the value of x kept near 0.04, and y varied from to 0.14 The thicknesses of the Ga1-x Mnx As1-y Py films ranged from 60 nm to100 nm Each sample was cut into two pieces, one of which was annealed at 250 ◦ C for hours Hall bars were then fabricated by photolithography and dry etching from each sample, with lengths of 1000 µm and widths of 100 µm, with the long dimension along [110] direction of the GaAs substrate, as shown in Fig 1(a) Hall resistance measurements were performed using a sample holder which allows a magnetic field to be applied in arbitrary directions The electromagnet used for this purpose was mounted on a rotatable base, so that the field could either be swept along a fixed direction or could be continuously rotated with a fixed field magnitude All magnetic-field-dependent experiments were done at 10 K In discussing the results, we used angles θ and ϕ to indicate the direction of magnetization, and θ H and ϕH for the direction of the applied magnetic field Angles θ and θ H are measured from the [001] direction to the (001) plane, and ϕ and ϕH are measured counterclockwise from the [110] direction (i.e., from direction of positive current in the Hall device) in the (001) plane RESULTS AND DISCUSSION Figure 1(b) shows temperature dependences of zero-field resistance observed on as-grown and annealed samples The data are typical for ferromagnetic semiconductors, exhibiting a peak that occurs around the Curie temperature Tc 14 The positions of the peak indicate a decrease of Tc as the concentration of P increases Low-temperature annealing is seen to significantly improve both the conductivity and the Curie temperature of the samples, suggesting that the magnetic properties of GaMnAsP films can be improved by this process To investigate magnetic anisotropy in our films, Hall resistance was measured with magnetic field applied perpendicular to the film, i.e., at θ H = Hall resistance data taken at 10 K for both as-grown and annealed samples are shown in Fig While a sharp transition behavior is absent in FIG (a) Schematic diagram of the Hall device patterned on a Ga1-x Mnx As1-y Py film Directions of the external field and magnetization are shown by arrows Measurement schemes for angles defining field and magnetization directions are also shown, together with crystallographic directions Directions of field rotation in (001) and (110) planes are shown as circled arrows (b) Temperature dependence of zero field resistance for three Ga1-x Mnx As1-y Py films, with data for as-grown samples plotted as black squares, and for annealed samples as red circles Note that the peaks in resistance shift to lower temperatures with increasing y 055809-3 Lee et al AIP Advances 7, 055809 (2017) FIG Anomalous Hall resistance (AHR) obtained for Ga1-x Mnx As1-y Py films by sweeping the field applied perpendicular to the film Black squares and red circles correspond to as-grown and annealed samples, respectively The data in panel (a) show a typical AHR behavior of a film with in-plane easy axes, while those in panels (b) and (c) show a magnetization reversal behaviors typical for out-of-plane easy axes the GaMnAs film data in Fig 2(a), well-defined hystereses centered at zero field are observed for both GaMnAsP films, as shown in Figs 2(b) and 2(c), indicating that easy magnetization direction is in-plane in the GaMnAs films and out-of-plane in the GaMnAsP films Thus the incorporation of P into GaMnAs clearly changes the magnetic anisotropy of the film, consistent with the fact that tensile strain has been introduced in the films by incorporation of P Magnetic anisotropy of the Ga1-x Mnx As1-y Py films was investigated by angle-dependent dependent Hall measurements, in which magnetic field direction was continuously rotated at fixed strength Hall data obtained by rotating the field direction in the (110) plane at field strengths between 100 Oe and 3000 Oe are shown in Fig The Hall resistance remains nearly constant during the rotation of a weak field, shown in Fig 3(e) and (f), due to a strong magnetic anisotropy that locks the magnetization along one direction, so that it cannot be rotated by the applied field As the strength of the rotating field is increased, the Hall resistance varies, and hysteresis loops form However, as the field is increased further, the hysteretic behavior eventually disappears in all samples due to coherent rotation of magnetization with the field.15 A fundamental difference between the y = (GaMnAs) and y (GaMnAsP) films is the angle at which the hysteresis occurs in the form of abrupt transitions between clockwise (CW) and counterclockwise (CCW) field rotations While the hysteresis occurs at θ H = and 180◦ for the GaMnAs sample (panels (a) and (d)), it occurs at θ H = 90◦ and 270◦ for GaMnAsP (panels (b), (c), (e), and (f)) Since the hysteresis in such angular scans occurs when the magnetization makes a transition across hard axes, this observation indicates that the magnetic hard axis is out-of-plane for the GaMnAs film, and in-plane for GaMnAsP The hystereses appearing at θ H = 90◦ and 270◦ in GaMnAsP samples become more abrupt and more pronounced after annealing, thus indicating that annealing strengthens the out-of-plane magnetic anisotropy of GaMnAsP films The hysteresis is systematically weakened with increasing field strength due to Zeeman energy, at sufficiently strong fields the magnetization simply following a magnetic energy minimum as the field rotates, without hysteresis Such angular-dependent Hall resistance can be analyzed using the magnetic free energy given as16,17 F = M −2H cos θ cos θ H + sin θ sin θ H cos(ϕ − ϕH ) + 4πM(cos θ)2 − H2⊥ (cos θ)2 1 − H4⊥ (cos θ)4 − H (3 − cos 4ϕ)(sin θ)4 − H (sin θ)2 (sin ϕ)2 2 (1) where H2|| and H4|| are the in-plane uniaxial and cubic anisotropy fields; H2⊥ and H4⊥ are the perpendicular uniaxial and cubic anisotropy fields, respectively; and θ, ϕ, θ H , and ϕH are defined in Fig 1(a) When the angular dependence of the Hall resistance is measured in the film plane, both the external field and the magnetization rotate in the plane One can then set θ = θ H = π2 , and Eq (1) is 055809-4 Lee et al AIP Advances 7, 055809 (2017) FIG Angular dependence of Hall resistance for Ga1-x Mnx As1-y Py films obtained by rotating θH clockwise (red) or counterclockwise (black) in the (110) plane (see Fig 1a) at several magnetic field strengths Hysteresis appears at θH = and 180◦ for the sample with y = (panels (a) and (d)), and at θH = 90◦ and 270◦ for samples with y (panels (b), (c), (e), and (f)) Note abruptness of the hystereses at θH = 90◦ and 270◦ in the y samples in panels (e) and (f) after annealing simplified into a form containing only in-plane anisotropy constants These anisotropy constants can then be obtained by fitting the Hall resistance to minimum energy conditions By analogy with the in-plane data analysis, one can also set ϕ = ϕH = π2 in Eq (1) for out-of-plane measurements, i.e., when the field is rotated in the (110) plane, and extract the out-of-plane components of magnetic anisotropy by fitting the data obtained with out-of-plane rotation, as described in the literature.15 Using the anisotropy fields obtained, 3-dimensional (3-D) free energy density diagrams can then be constructed, as shown for as-grown and annealed samples in Fig For the GaMnAs film, both as-grown and annealed samples show energy minima, and thus the easy axes, only in the (001) plane, consistent with the behavior seen in the Hall resistance obtained by both field scans and angular scans, plotted in Figs and The appearance of an energy minimum in the [001] direction in GaMnAsP samples, on the other hand, signals the presence of an out-of-plane easy axis Interestingly, even though an out-of-plane energy minimum appears in the [001] direction, in-plane energy minima still exist in both as-grown GaMnAsP samples, indicating a coexistence of in-plane and out-of-plane magnetic anisotropies Note, however, the rather dramatic change of the energy diagrams of GaMnAsP samples after annealing The energy minima in the (001) plane now completely disappear, with a clear energy minimum only in the [001] direction, as shown in Figs 4(e) and 4(f), indicating a single well-defined out-of-plane magnetic easy axis in annealed samples Based on the results described above, we optimized growth and annealing conditions for GaMnAsP films so as to improve their magnetic properties Magnetotransport measurements obtained on a representative GaMnAsP film after optimization are shown in Fig The temperature scan of resistance shows a Curie temperature near 125 K, similar to that of high-quality GaMnAs films The 055809-5 Lee et al AIP Advances 7, 055809 (2017) FIG 3-D plots of free energy density for as-grown (upper panels) and annealed (lower panels) Ga1-x Mnx As1-y Py samples The as-grown and annealed samples with no P show energy minima, and thus easy axes, only in the (001) plane Samples with y show energy minima along the [001] direction, indicating out-of-plane magnetic easy axes This feature becomes enhanced when the y samples are annealed, as seen in panels (e) and (f) FIG Magnetotransport data and 3-D plot of free energy density for annealed Ga1-x Mnx As1-y Py sample with x = 0.07 and y = 0.09 Temperature dependence of resistance shows a Tc of ∼125 K The inset at the bottom shows AHR data taken with an external magnetic field applied perpendicular to the film plane The 3-D magnetic energy diagram for this film is shown at upper left AHR results plotted in the inset of Fig show a square hysteresis that indicates an out-of-plane easy axis The 3-D magnetic energy diagram obtained for the new sample also shows a well-defined energy minimum in the [001] direction, consistent with AHR data CONCLUSIONS We investigated the magnetic anisotropy of Ga1-x Mnx As1-y Py films using Hall resistance measurements Hall resistances obtained with the field rotating from the normal to the in-plane orientation reveal that magnetic easy axis of our Ga1-x Mnx As1-y Py films is in-plane for y = 0, and lies out-of-plane for y This fact was further confirmed in AHR measurements, which showed a linear dependence in the samples with y = 0, and abrupt hysteretic behavior for the sample with y 3-D free energy diagrams obtained from the analysis of the angular dependence of the Hall resistance clearly show the changes of magnetic anisotropy with increasing incorporation of P in the Ga1-x Mnx As1-y Py films 055809-6 Lee et al AIP Advances 7, 055809 (2017) ACKNOWLEDGMENTS This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2015R1D1A1A01056614); by Korea University Grant; and by the National Science Foundation Grant DMR 1400432 T Dietl, H Ohno, F Matsukura, J Cibert, and D Ferrand, Science 287(5455), 1019–1022 (2000) Dietl, H Ohno, and F Matsukura, Phys Rev B 63(19), 195205 (2001) T Hayashi, M Tanaka, T Nishinaga, H Shimada, H Tsuchiya, and Y Otuka, J Cryst Growth 175, 1063–1068 (1997) H Ohno, A Shen, F Matsukura, A Oiwa, A Endo, S Katsumoto, and Y Iye, Appl Phys Lett 69(3), 363–365 (1996) A Shen, H Ohno, F Matsukura, Y Sugawara, N Akiba, T Kuroiwa, A Oiwa, A Endo, S Katsumoto, and Y Iye, J Cryst Growth 175, 1069–1074 (1997) J Zemen, J Kucera, K Olejnik, and T Jungwirth, Phys Rev B 80(15), 155203 (2009) X Liu, Y Sasaki, and J K Furdyna, Phys Rev B 67(20), 205204 (2003) H Lee, S Chung, S Lee, X Liu, and J K Furdyna, J Appl Phys 105(7), 07C505 (2009) A Lemaitre, A Miard, L Travers, O Mauguin, L Largeau, C Gourdon, V Jeudy, M Tran, and J M George, Appl Phys Lett 93(2), 021123 (2008) 10 P R Stone, J W Beeman, K M Yu, and O D Dubon, Physica B 401, 454–457 (2007) 11 M Kopecky, J Kub, F Maca, J Masek, O Pacherova, A W Rushforth, B L Gallagher, R P Campion, V Novak, and T Jungwirth, Phys Rev B 83(23), 235324 (2011) 12 A W Rushforth, M Wang, N R S Farley, R P Campion, K W Edmonds, C R Staddon, C T Foxon, and B L Gallagher, J Appl Phys 104(7), 073908 (2008) 13 M Cubukcu, H J von Bardeleben, K Khazen, J L Cantin, O Mauguin, L Largeau, and A Lemaitre, Phys Rev B 81(4), 041202(R) (2010) 14 S U Yuldashev, H Im, T W Kang, S H Lee, Y Sasaki, X Liu, J K Furdyna, and Y D Woo, J Korean Phys Soc 45, S572–S576 (2004) 15 H Son, S J Chung, S Y Yea, S Lee, X Liu, and J K Furdyna, J Appl Phys 103(7), 07F313 (2008) 16 X Liu, W L Lim, L V Titova, M Dobrowolska, J K Furdyna, M Kutrowski, and T Wojtowicz, J Appl Phys 98(6), 063904 (2005) 17 M Farle, Rep Prog Phys 61(7), 755–826 (1998) T ... such as GaMnAsP on GaAs provides a promising alternative for ferromagnetic semiconductor films with out -of- plane magnetic anisotropy Successful growth of GaMnAsP film with an out -of- plane magnetic. .. systemeatic investigation of magnetic anisotropy of quaternary GaMnAsP ferromagnetic semiconductor films by magneto-transport Hall measurements showed a transition of the easy magnetization direction... K in such quaternary films, with strong out-ofplane anisotropy This study showed that the magnetic anisotropy of the GaMnAsP films can be controlled by adjusting the concentration of the P, and