magnetic and structural properties of mnrh thin films

Magnetic and structural properties of MnRh thin films , Anurag Chaturvedi and Takao Suzuki Citation: AIP Advances 6, 056121 (2016); doi: 10.1063/1.4944405 View online: http://dx.doi.org/10.1063/1.4944405 View Table of Contents: http://aip.scitation.org/toc/adv/6/5 Published by the American Institute of Physics AIP ADVANCES 6, 056121 (2016) Magnetic and structural properties of MnRh thin films Anurag Chaturvedi1,a and Takao Suzuki1,2,3 Center for Materials for Information Technology, The University of Alabama, Tuscaloosa, AL 35487, USA Department of Electrical and Computer Engineering, The University of Alabama, Tuscaloosa, AL 35487, USA Department of Metallurgical and Materials Engineering, The University of Alabama, Tuscaloosa, AL 35487, USA (Presented 15 January 2016; received November 2015; accepted 21 January 2016; published online 11 March 2016) A systematic study of magnetic and structural properties of MnRh thin films fabricated onto MgO substrates and amorphous SiO2 has been conducted All the MnRh thin films thus fabricated are found to be of the CsCl type structure, and exhibit the ferromagnetism at room temperature The coercivity of about 1.1 kOe was observed at K for films grown onto SiO2 substrates, while coercivity measured at 300 K in all the films were less than 200 Oe The temperature dependence of magnetization shows thermal hysteresis for all the samples ranging from 150 K to 250 K that varies with the substrates used The maximum of exchange bias field of 270 Oe and unidirectional magnetic anisotropy constant of 0.35 erg/cm2 at 5K was observed for films grown onto SiO2 substrates better than that observed for the films grown onto MgO substrates This enhanced exchange bias and unidirectional magnetic anisotropy constant in film grown onto SiO2 is attributed to the strong lattice distortion in such a case C 2016 Author(s) All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License [http://dx.doi.org/10.1063/1.4944405] I INTRODUCTION Manganese-based alloys are the attractive classes of materials for high magnetic anisotropy of the order of 1x107 erg/cc for both ferromagnetic and antiferromagnetic phases, thus attractive permanent magnets (MnBi, MnAl) and /or for giant magnetoresistance (GMR) and tunnel magnetoresistance (TMR) devices (MnIr, MnPd, MnNi).1–4 Bulk MnRh is reported to be antiferromagnetic at room temperature, the Néel point of which is 1330 K.5 Also, the magnetic anisotropy constant of MnRh (bulk) is reported to be -1.9x107 erg/cc, highest among the anti-ferromagnetic Mn-X compounds except for MnIr (2x108 erg/cc).6 Previous works showed that MnRh thin films exhibited the ferromagnetic behaviors.7 A recent work discussed the exchange bias mechanism in MnRh thin films fabricated onto amorphous SiO2 substrates at an ambient temperature.8 The temperature dependence of magnetization, together with the result of high resolution TEM observations suggested that the exchange bias effect is due to the coupling between the ferro- and antiferro-magnetic phases resulted from the compositional fluctuation within a MnRh grain of the CsCl structure Since the magnetic anisotropy of both the ferro- and antiferro-magnetic phases of MnRh is expected to be strongly dependent on the c/a ratio, where a and c are the lattice parameters, as found in other Mn-X systems such as MnIr, MnPd, the present has been motivated to explore the possibility to enhance the exchange bias effect by choosing single crystal substrates, so as to change the lattice constants of the films thus fabricated a Author to whom correspondence should be addressed: Anurag Chaturvedi (anukashi@gmail.com) 2158-3226/2016/6(5)/056121/6 6, 056121-1 © Author(s) 2016 056121-2 A Chaturvedi and T Suzuki AIP Advances 6, 056121 (2016) II EXPERIMENTAL The amorphous substrates of SiO2 and single crystal substrates of MgO with (100), (110), and (111) orientation were preheated at 550 ◦C for hours before deposition Multilayers of {Mn(2.0 nm)/Rh(1.0 nm)} × 25 were deposited at 550 ◦C in a multi-target Ultra High Vacuum (UHV) confocal sputtering system The base pressure during deposition was kept at 3.8 mTorr The samples were subsequently annealed at the same temperature for 30 minutes in-situ in a vacuum better than 10−8 Torr The films were capped with 50 nm Rh layer to prevent the oxidation Structural analyses were performed by X-ray diffraction (XRD) with cobalt target Magnetic measurements were carried out by using a vibrating sample magnetometer (VSM) over a temperature range from to 400 K in applied fields up to 50 kOe III RESULTS AND DISCUSSION Figure 1(a), 1(b), 1(c) shows the XRD data for the MnRh thin films fabricated on MgO substrates of (100), (110), and (111) orientations Fig 1(d) shows MnRh film grown onto amorphous SiO2 substrate The films thus fabricated are found to be of CsCl- type structure at room temperature The lattice constant was calculated from either (100) or (110) peaks for all the films from the XRD data The lattice constant were estimated to be 3.045 Å, 3.057 Å, and 3.052 Å for films grown onto MgO substrates of (100), (110), and (111) orientations respectively For the films grown onto SiO2 substrate, the lattice constant was estimated to be 3.022 Å The XRD data has an estimated error of ±0.002 Å When compared to the reported lattice constant for the bulk MnRh,9 lattice expansion of 0.05%, 0.42% and 0.25% was estimated in the case of all MgO substrates of (100), FIG XRD pattern of MnRh thin films deposited onto MgO substrates of (a) (100), (b) (110), (c) (111) orientations, and (d) SiO2 substrates 056121-3 A Chaturvedi and T Suzuki AIP Advances 6, 056121 (2016) FIG Field dependent magnetization measured at K for in plane configurations Inset of the figure shows the M-H loops measured at 300 K (110), and (111) orientations respectively However, in the case of films grown onto amorphous SiO2, the lattice contraction of about 0.71% was observed compared to its bulk counterpart Figure shows the magnetization vs magnetic field (M-H) loops measured at K for MnRh thin films with magnetic field parallel (in plane) to the film plane The inset shows the M-H Loops measured at 300 K for respective field direction The M-H measurements show the ferromagnetic (FM) behavior of films at all temperatures The in plane M-H measurements at K shows the coercivity (HC ) of about 1.1 kOe for films grown onto SiO2 substrates while the coercivity for films grown onto MgO substrates are about 250 Oe for (100) orientation, and 410 Oe for (110) and (111) orientation Figure shows the temperature dependence of magnetization (M-T) measured at 10 kOe with magnetic field parallel to the film plane The M-T curves show the thermal hysteresis for both the heating and cooling process In all cases, upon heating a gradual increase of M is observed, for all the films, which takes a maximum at corresponding transition temperatures This increase in M is believed to correspond to the ferromagnetic (FM) to antiferromagnetic (AF) transition The magnetization, upon cooling, attains a maximum and then decreases further cooling the sample The transition temperature during the cooling process were observed to be 190 K, 143 K, and 135 for the films grown FIG M-T curves measured during heating and cooling in T applied field in parallel to the film plane 056121-4 A Chaturvedi and T Suzuki AIP Advances 6, 056121 (2016) onto MgO substrates of (100), (110), and (111) orientation respectively, while that was observed to be 154 K for the films grown onto SiO2 substrate The thermal hysteresis of about 170 K was observed for films grown onto SiO2 substrates whereas the thermal hysteresis is estimated to be 150 K, 200 K and 250 K for films grown onto MgO substrates of (100), (110), and (111) orientation respectively It has been reported that the FM domains retained at the cooling process causes this large thermal hysteresis.10 It is interesting to note that the magnetization does not reduce to zero at low temperature, but remain at about 150 to 200 emu/cc It is believed that this “residual” magnetization is due to the ferromagnetic region present even at low temperatures In this study, the residual magnetization is defined as the constant magnetization retained in the system during the heating process It is to note that the magnetization in the case of films grown onto MgO (100) is much lower than those for the other cases It is believed that this is due to the loss of FM region in such a case Figure 4(a) shows the HC as a function of temperature from K to 300 K, for in plane measurements, for films grown onto different substrates It is to be noted that the HC increases and attains the maximum at K for the films deposited onto SiO2 substrates In the case of the films deposited onto MgO substrates the HC first increases and attains a maximum and further reducing the temperature the HC decreases It is believed that when temperature decreases, the anisotropy energies and exchange are enhanced gradually to give rise to the HC enhancement As the temperature is further decreased, the decrease in FM regions leads to reduction of HC Therefore, the variation of HC vs T is non-monotonic.11 The coercivity is an extrinsic property that depends on many factors, e.g morphology, grain sizes and lattice constants Despite the difference in the lattice constant of MnRh films grown onto Mgo (110) and (111) substrates, the almost same coercivity at K for these two samples might be attributed to one these factors Figure 4(b) shows the exchange bias field (H E B) as a function of temperature HEB increases with the decrease in the temperature for all the samples The maximum of the H E B of at K about 270 Oe was observed for the films grown onto SiO2, while the lowest HE B of about 50 Oe was observed for the films grown onto MgO substrate of FIG Temperature dependence of (a) coercivity and (b) exchange bias for MnRh thin films Inset of (b) shows the exchange bias field as a function of lattice constant a The arrow represents the reported a for bulk MnRh (Ref 9) 056121-5 A Chaturvedi and T Suzuki AIP Advances 6, 056121 (2016) (100) orientation The reason of this exchange bias has been explained elsewhere.8 It was proposed that within the films, the Mn-rich region remains ferromagnetic at low temperatures, while that with nearly equiatomic composition becomes antiferromagnetic.8 The inset of figure 4(b) shows the dependence of H E B on the lattice parameter a The maximum H E B of about 270 Oe was estimated for the case of films grown onto SiO2 substrates where the maximum of the lattice distortion was calculated In the case of films grown onto MgO substrates, the highest H E B was estimated for the case of films on (110) substrate that shows the maximum expansion of lattice parameter, a, among films grown onto all MgO substrates The unidirectional magnetic anisotropy constant (JK ) is defined by the relation, JK = HEB × MS × t FM, where MS the saturation magnetization of the FM region and t FM the thickness of FM region In the case of MgO substrates, the JK at K were estimated to be 0.04 erg/cm2, 0.2 erg/cm2, and 0.06 erg/cm2 for films grown onto substrates of (100), (110), and (111) orientations respectively The JK was estimated to be 0.35 erg/cm2 at 5K for MnRh films grown on SiO2 substrates The JK observed for films grown onto MgO are substantially smaller than that observed for the case of SiO2 The possible reason of higher JK in films grown onto SiO2 is attributed to the higher MS and H E B values than that of films grown onto MgO substrates Figure 5(a) shows the residual magnetization (Mres) as a function of lattice distortion (∆a/a%) calculated for all the films It can be seen that the residual magnetization decreases as the lattice distortion decreases and it further increase with increasing the distortion The maximum of Mres is FIG (a) Residual magnetization Mres at 200 K as a function of lattice distortion (b) Coercivity (HC) and exchange bias field (HEB) measured at K as a function of residual magnetization Mres 056121-6 A Chaturvedi and T Suzuki AIP Advances 6, 056121 (2016) observed for the case of SiO2 where the lattice contraction is about 0.7% Whereas, the minimum of Mres is observed for the case of films grown onto MgO (100) substrate The reason behind this trend is unclear and it might be related to the c-axis orientation of the films where the films the least lattice expansion of 0.05% among all the films Figure 5(b) shows the Mres dependence of HC and H E B The HC and H E B both increase with increasing the Mres It is believed that as the decrease in FM regions causes the decrease in the Mres and that results in the decrease in HC and H E B The structural and magnetic results indicate that the magnetic properties depend on the lattice constant of MnRh films The films grown onto MgO substrates exhibit lower H E B and HC than that observed in the films grown onto SiO2 substrate The lattice constant for the films grown onto SiO2 is contracted by 0.71% from that of bulk On the other hand, in the films grown onto MgO substrates the lattice constant expansion from the bulk is observed that varies from 0.05% to 0.42% depending upon the choice of substrate orientation IV SUMMARY In summary, the MnRh thin films grown onto SiO2 and MgO substrates show ferromagnetic behavior at room temperature The temperature dependence of magnetization shows the transformation between ferromagnetic and antiferromagnetic states At temperature below 150 K, all the samples show an exchange bias behavior and the exchange bias fields show an increase with the decrease in the temperature accompanied with increase in the respective coercivities This exchange bias field and coercivity depends on the lattice distortion from its bulk counterpart showing a relationship between the structure and the magnetic properties ACKNOWLEDGEMENT This work was partially supported by National Science Foundation for the G8 project (NSFCMMI #1229049) One of the authors A.C is supported by the postdoctoral fellowship from University of Alabama Y Nagata, H Sano, and K Ohta, “Hard magnetic MnAl and MnBi ribbons made from molten state by rapid cooling,” Japanese Journal of Applied Physics Part 1-Regular Papers Short Notes & Review Papers 23, 580-584 (1984) R F C Farrow, R F Marks, S Gider, A C Marley, S S P Parkin, and D Mauri, “Mn Pt x 1−x: A new exchange bias material for Permalloy,” Journal of Applied Physics 81, 4986-4988 (1997) J R Childress, M M Schwickert, R E Fontana, M K Ho, P M Rice, and B A Gurney, “Low-resistance IrMn and PtMn tunnel valves for recording head applications,” Journal of Applied Physics 89, 7353-7355 (2001) T Hozumi, P LeClair, G Mankey, C Mewes, H Sepehri-Amin, K Hono, and T Suzuki, “Magnetic and structural properties of MnBi multilayered thin films,” Journal of Applied Physics 115 (2014) R Y Umetsu, K Fukamichi, and 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properties of Rh-Mn phases,” Acta Chemica Scandinavica 26, 719 (1972) 11 W B Rui, Y Hu, A Du, B You, M W Xiao, W Zhang et al., “Cooling field and temperature dependent exchange bias in spin glass/ferromagnet bilayers,” Scientific Reports 5, 13640 (2015)  ... study of magnetic and structural properties of MnRh thin films fabricated onto MgO substrates and amorphous SiO2 has been conducted All the MnRh thin films thus fabricated are found to be of the... ? ?Magnetic and Structural Properties of MnRh ThinFilms,” Journal of Magnetism and Magnetic Materials 401, 144-149 (2016) J S Kouvel, C C Hartelius, and L M Osika, ? ?Magnetic properties and crystal-structure... the Mres and that results in the decrease in HC and H E B The structural and magnetic results indicate that the magnetic properties depend on the lattice constant of MnRh films The films grown