Klaer, P.

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Lawrence Berkeley National LaboratoryLawrence Berkeley National LaboratoryTitleDisentangling the Mn moments on different sublattices in the half-metallic ferrimagnet Mn3?xCoxGaPermalinkhttps://escholarship.org/uc/item/4s75m9rmAuthorKlaer, P.Publication Date2011-05-04eScholarship.org Powered by the California Digital LibraryUniversity of CaliforniaDisentangling the Mn moments on di!erent sublattices in thehalf-metallic ferrimagnet Mn3!xCoxGaP.Klaer,1 C.A. Jenkins,2, 3 V.Alijani,3 J.Winterlik,3 B.Balke,3 C.Felser,3 and H.J. Elmers1, !1Institut für Physik, Johannes Gutenberg-Universität Mainz, D-55128 Mainz, Germany2Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA, 947203Institut für Anorganische Chemie und Analytische Chemie,Johannes Gutenberg-Universität Mainz, D-55128 Mainz, GermanyAbstractFerrimagnetic Mn3"xCoxGa compounds have been investigated by magnetic circular dichroismin x-ray absorption (XMCD). Compounds with x > 0.5 crystallize in the CuHg2Ti structure. Atetragonal distortion of the cubic structure occurs for x " 0.5. For the cubic phase, magnetometryreveals a linearly increasing magnetization of 2x Bohr magnetons per formula unit obeying the gen-eralized Slater-Pauling rule. XMCD confirms the ferrimagnetic character with Mn atoms occupyingtwo di!erent sublattices with antiparallel spin orientation and di!erent degrees of spin localizationand identifies the region 0.6 < x " 0.8 most promising for a high spin polarization at the Fermilevel. Individual Mn moments on inequivalent sitesare compared to theoretical predictions.PACS numbers: 61.82.Bg,75.20.En, 75.30.Cr, 75.50.Gg! EMail: elmers@uni-mainz.de1Heusler compounds are receiving considerable interest due to their predicted half-metalliccharacter.[1, 2] Half-metals have a metallic band structure for one spin channel and aninsulating band gap at the Fermi level for the opposite spin thus providing a completespin polarization of conducting electrons. [3] This is highly advantageous for spintronicapplications, where the performance of devices is directly related to the spin polarizationof the current emitted and collected by the ferromagnetic electrodes.[2, 4] For applicationsin spin transfer torque devices, a small magnetization combined with high spin polarizationwould considerably enhance the e!ective torque induced by a spin polarized current. [5]The combination of low magnetization and large local moments can be achieved by theconcept of half-metallic ferrimagnetism as proposed by von Leuken and de Groot. [6] Half-metallic ferrimagnetism has been predicted for Mn2VZ (Z=Al, Si) [7] and Co2TiAl [8] Heusleralloys. Ferrimagnetism has been proven experimentally for Mn2VAl and Co2TiAl with,however, a very small magnetic moment located at the V and Ti atom, respectively. [9–11] Ferrimagnets with large local moments and half-metallic properties have recently beenpredicted for compounds that form the CuHg2Ti-type structure. [12] This crystal structureis based on a cubic structure with four interpenetrating fcc sublattices A, B, C, D [Fig. 1]. InMn2CoGa the Mn atoms occupy sublattices A and B, Co and Ga atoms form sublattices Cand D, respectively. In this structure two Mn atoms are in nearest neighbor positions to eachother invoking an antiparallel alignment of their magnetic moments. Liu et al. [12] studiedthe Mn2CoZ (Z=Al, Ga, In, Si, Ge, Sn, Sb) compounds resulting in a mean magnetizationthat obeys the generalized Slater-Pauling rule. Mn on the A site has a predicted magneticmoment of -2.2 µB which is antiparallel to the B site moment of +3.3 µB and the Comoment of +0.9 µB. Moreover, according to Ref. [13] these compounds possess high Curie-temperatures of more than 800 K.In this work we investigated a series of Mn3"xCoxGa compounds where the Co atom onthe C site is successively substituted by Mn. Thus the magnetization can be tuned to smallervalues. The partial replacement of Co by Mn also avoids an occupation of B sites by Co athigh Co concentrations leading to a rapid degradation of the half-metallic properties. X-rayabsorption spectroscopy (XAS) reveals element-specific magnetic moments versus composi-tion confirming the theoretical prediction. An analysis of the Mn X-ray magnetic circulardichroism (XMCD) spectra reveals the antiparallel alignment of the itinerant magnetic mo-ment on the A site and the localized moment on the B site [Fig. 3(b)]. The di!erent degree2FIG. 1. (Color online) The generalized cubic structure with four interpenetrating fcc sublatticesA, B, C, and D. Mn3"xCoxGa crystallizes in the CuHg2Ti-type structure with Mn occupying theA and B sublattice, Co the C site and Ga the D site.of localization as revealed by comparison to spectra from reference compounds agrees wellwith calculated results [12].The polycrystalline samples were prepared by arc melting of stoichiometric quantities ofthe constituents in an argon atmosphere. Oxygen contamination was avoided by evaporatingTi inside the fabrication chamber before melting the constituents, as well as by additionalpurification of the process gas. The melting procedure was repeated three times to geta homogeneous material. After melting, the polycrystalline ingots were annealed in anevacuated quartz tube at 1073 K for 8 days. For powder investigations, a part of thesample was crushed by hand using a mortar. The structure was then investigated by x-raydi!raction (XRD) using excitation by Mo-K radiation of powder samples. The magneticproperties were investigated by a super conducting quantum interference device (SQUID).Energy dispersive X-ray spectroscopy (EDX) with an accuracy of ± 2-atomic percent wasused to check the homogeneity and stoichiometry of the samples.The XAS/XMCD experiments were performed on beamlines UE56/1 -SGM at the Ger-man synchrotron light source (BESSY II) and BL6.3.1 at the Advanced light source (ALS).The samples were cleaved in-situ in UHV directly before the measurement in order to avoidsurface contamination. An external field of 2 T was applied parallel to the incident X-raybeam and perpendicular to the surface and reversed at each 0.1 eV photon energy step.X-ray di!raction measurements of Mn3"xCoxGa indicate a transition from the cubic3FIG. 2. (Color online) (a) Magnetization (full circles) extrapolated to 20 K from SQUID magne-tometry at room temperature for Mn3"xCoxGa. Open circles denote the orbital to spin momentratio resulting from the sum rule analysis [14, 15]. The dotted line indicates the magnetizationaccording to the Slater-Pauling rule. (b) Element-specific spin moments for Co in Mn3"xCoxGameasured at 20 K and normalized to the average magnetization as measured by SQUID magnetom-etry. (c) Similar data for the Mn moments. The shaded area indicates the tetragonally distortedphase.structure for x > 0.5 to a tetragonally distorted phase for x " 0.5. For x = 1, the formationof the cubic CuHg2Ti-type structure as shown in Fig. 1 was confirmed in Ref. [12]. Thesimilar scattering factors for the 3d transition elements prevents us from a confirmation ofthis structure for all values of x. The tetragonal phase has been investigated in detail forthe case of x = 0 [16]. This phase cannot be produced without a high degree of disorder andinstead of the predicted fully compensated half-metallic ferrimagnet of the cubic phase, thetetragonal DO22 phase leads to a magnetization of roughly 1 µB per formula unit combinedwith a reduced spin polarization.4FIG. 3. (Color online) (a) XAS spectra for Mn in Mn3"xCoxGa with x = 0.8 compared to theMn spectra in Mn2VAl [11] and Co2MnSi [10]. The thin (green) line indicates the average of bothreference spectra. (b) Corresponding data for the XMCD data. The thin (green) line is a linearcombination of the reference spectra.The structural change from cubic to tetragonal with decreasing x considerably changesthe magnetic hysteresis. The coercive field rapidly increases for x " 0.5 indicating a largeincrease of the magnetic anisotropy for the tetragonal phase. In contrast the deviation ofthe magnetization from the Slater-Pauling behavior is less sensitive and appears only forx " 0.3 as shown in Fig. 2.Element-specific spin and orbital magnetic moments were determined from XAS/XMCDspectra using the sum rule analysis. For x > 0.5, we find a good agreement of the totalmagnetization determined by the sum rules and by SQUID magnetometry, while XMCDresults in smaller values for x " 0.5. We assume that this is caused by the limited external5field (2 T) thus failing to reach magnetic saturation. Element-specific values shown in Fig. 3were normalized in order to match the measured magnetization in saturation.The Co moment increases with its concentration to a maximum value at x = 0.7 andassumes a nearly constant value of 1 µB per atom for larger concentrations. In contrastthe Mn moment shows a constant up to x > 0.5. In the range of 0.6 " x " 0.8 the Mnmoment increases proportional to x as expected for the half-metallic phase. The decreasingMn moment for high Co concentrations might indicate an increased disorder with Mn-Coantisite defects which would be prohibitive for half-metallic properties. The dependenceof the spin moments on the Co concentrations identifies the region 0.6 " x " 0.8 mostpromising for a high spin polarization at the Fermi level.Interestingly, the orbital magnetic moments of Mn and Co reveals opposite behavior withincreasing Co concentration: For small values of x, the Co orbital moment nearly vanisheswhile Mn has a fairly large orbital moment. For large x, the Mn orbital moment approacheszero and the Co orbital moment increases. The decrease of the orbital to spin moment ratiofor the summed moments [see Fig. 2(a)] near x = 0.5 is related to the symmetry increase inthe cubic phase.In order to prove the predicted ferrimagnetism in highly ordered cubic Mn3"xCoxGa weanalyze the Mn XAS/XMCD spectra for x = 0.8 by comparison with reference spectrafrom Heusler alloys where Mn occupies a single sublattice (Fig. 3). Co2MnSi is seen asthe prototype half-metallic Heusler alloy with a localized Mn moment on the B site in thenotation of Fig. 1. The localization of the Mn d-bands in this compound causes the multipletfeatures in the XAS/XMCD spectra that can be simulated quite well using an atomic model.In contrast, the magnetization in Mn2VAl originates from itinerant Mn states occupying theA and C sublattice according to theoretical and experimental studies. [11]The linear combination with equal weight of the Mn spectra from Co2MnSi and Mn2VAlfits the Mn XAS spectra for Mn2.2Co0.8Ga fairly well (Fig. 3(a)). Prominent features of theCo2MnSi spectra namely the L3 pre-edge shoulder and the small branching ratio is cut inhalf, thus indicating the equal occupation of both sites. The small nominal deviation fromequal occupation of A and B sites by Mn (because of x = 0.8 instead of x = 1) cannotbe resolved by the fit. The XMCD spectra of Mn2.2Co0.8Ga can also be fitted by a linearcombination of the reference spectra [Fig. 3(b)]. In particular the positive L3 pre-edge peakis indicative of the presence of antiparallel Mn moments. The deviation at the L2 pre-edge6peak likely originates from a shift of the Mn multiplet feature from Mn2.2Co0.8Ga to Co2MnSicaused by the di!erent crystal field parameters.Using the values for the averaged Mn moment (µspin = 0.4 µB, µorb = 0.01 µB) peratom and for the ratio of the Mn moment on the A and B site (µspin(B)/µspin(A) = !3.9)and (µorb(B)/µorb(A) = !1.9) determined from the fit of the reference spectra, individualMn moments result: µspin(A) = !0.33 µB, µspin(B) = +1.3 µB, µorb(A) = !0.03 µB andµorb(B) = +0.05 µB. Similar ratios of magnetic moments are observed for 0.7 " x " 0.9.The Mn moments are much smaller than values calculated by ab initio theory. This mayindicate a residual anti-site disorder that can be sensitively detected by XMCD. For x < 0.7,the Mn moment on the A site vanishes completely indicating a strong deviation from theCuHg2Ti structure.In summary, we have demonstrated the preparation of ferrimagnetic Mn3"xCoxGa com-pounds with the cubic CuHg2Ti structure for x > 0.5. Disentangling contributions of Mnatoms on di!erent sublattices reveals fairly large antiparallel oriented Mn moments althoughthe average magnetization is small. The dependence of magnetic moments on the Co concen-tration identifies the composition 0.6 < x " 0.8 most promising for a high spin polarisationat the Fermi level and as a potential candidate for the preparation of highly e"cient spintorque devices given that the atomic order can be increased.We thank S. Cramm for support at BESSY. This project was financially supported bythe DFG Research Unit 559, ”New Materials with High Spin Polarization” (Germany) andBMBF (Grant No. ES3XBA/5). CAJ acknowledges support of MAINZ, the German FederalGraduate School Of Excellence.”[1] C. Felser, G. 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Disentangling the Mn moments on different sublattices in the half-metallic ferrimagnet Mn3?xCoxGa

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