Ferromagnetic Cr x Ti 1 − x N solid solution nitride thin films grown by pulsed laser deposition and their magnetoresistance Kei Inumaru, Kunihiko Koyama, Yuji Miyaki, Kazuma Tanaka, and Shoji Yamanaka Citation: Applied Physics Letters 91, 152501 (2007); doi: 10.1063/1.2776853 View online: http://dx.doi.org/10.1063/1.2776853 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/91/15?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Ferromagnetism and electronic transport in epitaxial Ge1−xFexTe thin film grown by pulsed laser deposition Appl. Phys. Lett. 102, 102402 (2013); 10.1063/1.4795312 Ferromagnetic and electrical properties in ferromagnetic semiconductor Ti 1 − x Co x O 2 − δ : Sb thin films grown by pulsed laser deposition J. Appl. Phys. 97, 10D326 (2005); 10.1063/1.1855455 Substrate effects on the room-temperature ferromagnetism in Co-doped TiO 2 thin films grown by pulsed laser deposition J. Appl. 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The films showed ferromagnetism for 0.28�x �0.5 and the TC showed a maximum �140 K� at x=0.5. When x exceeded 0.5�x=0.58� ferromagnetism disappeared completely. At x=0.5, the nitride film showed a large magnetoresis- tance �6%–7% at 5 T�, which had a maximum at 130 K. © 2007 American Institute of Physics. �DOI: 10.1063/1.2776853� Synthesis and physical properties of metal nitrides are a current field of study in the inorganic synthetic chemistry and physics of condensed matter. Many experimental and theoretical studies that reflect the wide variety of physical properties of transition-metal nitrides have been reported.1–27 For example, the 3d transition of metal nitrides show anti- ferromagnetism �MnN,1–4 CrN,5–8 and FeN �Refs. 9 and 10��, Pauli paramagnetism �CoN�,1 and superconductivity �TiN, VN�.26 The discovery of the electron-doped layered hafnium nitride superconductor ��-HfNCl� with a TC as high as 25.5 K �Ref. 27� has stimulated interest in the properties of group-IV metal nitrides such as HfN, ZrN, and TiN. Another current topic is the dilute magnetic semiconductor; transition-metal doped GaN exhibits substantial carrier spin polarization and is attracting much attention.28–30 In1−xCrxN is reported to show ferromagnetism at room temperature.31 These findings indicate that metal doping is a promising strategy to induce characteristic magnetic and electronic properties of metal nitrides. Perovskite metal oxides based on LaMnO3 �Refs. 32 and 33� have attracted much attention due to their potential ap- plication for various devices utilizing the colossal magne- toresistance �CMR� effect. A theoretical study6 suggested similarity between CrN and LaMnO3; in both, the configu- rations of magnetic ions were close to d4, and the Cr–N–Cr interaction in CrN is governed by the double-exchange mechanism to be ferromagnetic �FM�. Thus, chemical dop- ing and/or dilution of CrN in a nonmagnetic medium is an interesting subject for experimental studies. In this study, epitaxial thin films of CrxTi1−xN solid so- lution with a wide range of x were grown by pulsed laser deposition �PLD�. We found that the material showed ferro- magnetism for a certain range of x and a characteristic de- pendence of the magnetism on the x values. The ferromag- netic film showed large magnetoresistance. The PLD system used in this study was equipped with a KrF excimer laser �COMPex 102, Lamda Physics, Goettin- gen, Germany� and a rf-plasma radical source �model RF 4.5, SVTA, Inc., MN, USA�. The residual pressure of the chamber was less than 10−8 Torr. MgO �001� substrates �Advanced Film Technology, Inc., Tokyo, Japan� were cleaned ultrasonically in methanol and then annealed under O2 flow. Ti–Cr metal alloy targets with variant Ti/Cr ratios were prepared by the arc melting method. The CrxTi1−xN epitaxial thin films were deposited using these targets under nitrogen radical irradiation at a substrate temperature of 973 K. The Cr/Ti molar ratios of the films analyzed using scanning electron microscopy-energy dispersive x-ray spec- troscopy �JSM-6320FS/ED-2110� were close to those of the metal targets. X-ray photon spectroscopy �Shimadzu, ESCA- 850� measurements confirmed that the nitrogen contents in the films were almost stoichiometric. X-ray diffraction recip- rocal space mapping was carried out using an XPert MRD diffractometer �Philips, The Netherlands� equipped with a Ge �220� 4-crystal monochrometer. Thicknesses of the films were measured by an atomic force microscope �Digital In- struments, D-3100�. The thicknesses were in a range of 10–15 nm. Magnetization of thin films was measured with a magnetometer �MPMS-5S, Quantum Design, USA�. A mag- netic field was applied parallel to the substrate surface and to the MgO �1 0 0� direction. Electronic conductivity of the samples was measured using a four-probe method in the tem- perature range of 2–250 K, where the current flowed parallel to the �1 0 0� direction. The magnetic field was parallel to the current flow. The x-ray diffraction pattern �2�-� scan� and x-ray dif- fraction reciprocal space map �data not shown� revealed that CrxTi1−xN films grew epitaxially with the �0 0 1� plane par- allel to the substrate surface. Figure 1 shows the Cr content �x� dependence of lattice constants for the CrxTi1−xN films. The inset shows that the unit cell volume monotonically de- creased as the Cr content increased, demonstrating that Cr atoms were incorporated into the nitride lattice. As shown in Fig. 1, the lattice constant a parallel to the substrate surface coincided with the lattice parameter of MgO �0.4213 nm� for 0�x�0.5. This shows that the films grew in a mode coher- ent to the substrate lattice; film lattices shrunk or expanded in directions parallel to the substrate surface to fit the MgO substrate lattice. For 0.5�x, the lattice constant a varied depending on x and the lattice of the films relaxed due to a large mismatch with the substrate lattice. On the other hand, the lattice constant c of the films monotonically changeda�Electronic mail:
[email protected] APPLIED PHYSICS LETTERS 91, 152501 �2007� 0003-6951/2007/91�15�/152501/3/$23.00 © 2007 American Institute of Physics91, 152501-1 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 155.33.120.209 On: Sun, 30 Nov 2014 15:04:05 http://dx.doi.org/10.1063/1.2776853 http://dx.doi.org/10.1063/1.2776853 http://dx.doi.org/10.1063/1.2776853 depending on x when 0�x�0.8. The unit cell volume �the inset of Fig. 1� changed monotonically and continuously for 0�x�1 and showed no anomaly, demonstrating that the Cr and Ti ions were incorporated into the NaCl-type nitride lat- tices of the films. Figure 2 shows typical magnetization behaviors of the thin films. The films with 0.28�x�0.5 showed ferromag- netic behavior. Pure TiN is a metallic �Pauli paramagnetic� compound, and Cr ions are believed to bring about the fer- romagnetism. Among the films having different Cr contents x, Cr0.5Ti0.5N showed the highest TC and the strongest mag- netization. The magnetization per Cr ion was calculated to be 2.8�B, but we do not discuss it quantitatively because a large error is expected due to the error in film thickness measure- ment �the thickness was approximately 10 nm�. Figure 3 shows the dependence of TC and magnetization �M, at 100 Oe and 2 K� on x in CrxTi1−xN. As the Cr content x increased, ferromagnetism was observed at 0.28�x. TC and M increased as the Cr content x increased to 0.5. TC and M reached maxima at x=0.5. However, the ferromagnetism disappeared completely when x exceeded 0.5 �0.58�x�1�, irrespective of the strongest ferromagnetism which occurs at x=0.5. Electronic conductivity was measured for the films with varying x. Figure 4 shows the temperature dependence of resistivity � for the thin films. The ordinate was normalized using resistivity at 200 K. The resistivity of the films was in the range of 10−5–10−3 � cm. They showed corresponding metallic behaviors in the temperature dependence of � shown in Fig. 4. It should be noted that CrN �x=1.0�, an antiferro- magnetic compound with TN�270 K, also showed metallic behavior, being consistent with the results for CrN�001� on MgO previously reported by Constantin et al.8 Ferromag- netic films with x=0.5 and 0.36 showed a steep decline of � when the samples were cooled across their Currie tempera- tures. Since the film with x=0.5 �Cr0.5Ti0.5N� showed the high- est TC and the largest magnetization, we then focused on this film. As is discussed later, a theoretical study suggested that Cr ions in CrN have a valency close to 2+, i.e., 3d4 states, and that there are similarities in the electronic structure be- tween CrN and LaMnO3, a parent compound of CMR mate- rials. Figure 5 shows magnetoresistance behavior of the Cr0.5Ti0.5N film. The main panel of Fig. 5 compares � of the film at H=0 T and that at H=5 T. The � at 0 T steeply decreased when the sample was cooled below TC ��140 K�, while the temperature dependence of � above TC was smaller. This behavior corresponds well to the temperature dependence of magnetization for the film shown in Fig. 2. Note that the magnetoresistance was observed also at tem- peratures above TC ��140 K� to approximately 200 K. As shown in the inset of Fig. 5, the film showed a large magne- toresistance ��7% � maximum at 135 K, which is just below the TC of the film. FIG. 1. Lattice constants a �circles� and c �squares� as functions of x in CrxTi1−xN. a coincides with the lattice constant of MgO �dashed line� for x�0.5, demonstrating coherent growth of the films to the substrate lattice. The inset shows the cell volume as a function of x. Solid symbols represent the values for bulk compounds. FIG. 2. Magnetization of CrxTi1−xN thin films as a function of temperature. Magnetic field is 100 Oe. FIG. 3. Currie temperature �TC� and magnetization at 2 K �100 Oe� as func- tions of x in CrxTi1−xN. FIG. 4. Temperature dependence of resistivity for CrxTi1−xN thin films. Open circles, x=1; open squares, x=0.9; diamonds, x=0.6; solid circles, x =0.5; solid squares, x=0.36. The solid symbols represent results for ferro- magnetic films. 152501-2 Inumaru et al. Appl. Phys. Lett. 91, 152501 �2007� This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 155.33.120.209 On: Sun, 30 Nov 2014 15:04:05 Filippetti et al.6,7 carried out detailed band calculations for CrN. According to the calculation, the Cr configuration in CrN is nearly Cr2+, i.e., d4. Competition among spin order- ings was discussed with a two-parameter Heisenberg model involving the nearest neighbor Cr–Cr �J1� and the nearest next neighbor Cr–N–Cr interactions �J2�. The estimated pa- rameters were J1=−9.5 meV and J2=4 meV. For the Cr– N–Cr interaction, there is competition between superex- change �antiferromagnetic �AFM�� and double-exchange �FM� mechanisms involving Cr eg and N p orbitals, and the resulting coupling is FM due to the double-exchange inter- action, as for CMR manganese oxides. Filippetti et al. high- lighted another similarity of CrN and LaMnO3, in that for both, the magnetic ions have a formally equivalent configu- ration �Cr2+ and Mn3+, i.e., 3d4� and are octahedrally coordi- nated by neigbor atoms. A major difference between the two compounds is that, in CrN, the Cr–N–Cr coupling is smaller than the direct Cr–Cr AFM coupling, whereas for LaMnO3 the Mn–O–Mn coupling is the only relevant interaction. In this study we investigated the CrxTi1−xN films with varying x and found that Cr0.5Ti0.5N showed the strongest ferromagnetism. Since CrxTi1−xN films had the same NaCl- type crystal structure for the whole range of 0�x�1, the CrxTi1−xN solid solutions may be regarded as CrN diluted in a nonmagnetic nitride medium TiN. The ferromagnetism in this system is considered to be the result of competition be- tween J1 �AFM direct interaction between half-filled Cr t2g orbitals� and J2 �FM interaction governed by Cr–N–Cr double-exchange mechanism� discussed by Filippetti et al. Considering that ferromagnetism occurred via the formation of the solid solution, the dilution of CrN by TiN may reduce the Cr–Cr neighboring interaction �AFM� to a larger extent than the Cr–N–Cr interaction �FM�, although this point should be discussed on the basis of a theoretical quantitative calculation. The important point here is that the origin of the ferromagnetism is the Cr–N–Cr double-exchange interaction. 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The inset shows magnetoresistance �5 T� as a function of temperature. 152501-3 Inumaru et al. Appl. Phys. Lett. 91, 152501 �2007� This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 155.33.120.209 On: Sun, 30 Nov 2014 15:04:05