THE MAGNETO-OPTICAL PROPERTIES OF HEUSLER ALLOYS OF THE TYPE Coz_xCuxMnSn

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1 Philips J. Res. 42, , 1987 R 1165 THE MAGNETO-OPTICAL PROPERTIES OF HEUSLER ALLOYS OF THE TYPE Coz_xCuxMnSn by P.P.J. VAN ENGE~EN and K.H.J. BUSCHOW Philips Research Laboratories, 56 JA Eindhoven, The Netherlands Abstract The magnetic and magneto-optical properties of compounds of the series CO 2 _xcu x MnSn have been investigated. The polar magneto-optical Kerr effect was measured at room temperature in the energy range.5-5 ev. It was found that the Kerr intensity below 1.5 ev is due to transitions which involve the 3d electron states of Co and Cu, whereas the Kerr-rotation peak observed near 2.5 ev is attributed to a charge transfer transition between the Mn 3d states and the Sn p states. Keywords: Heusier compounds, magnetic properties, magneto-optical Kerr effect. 1. Introduetion In continuation of a program dealing with the magnetic and magneto-optical properties of Heusier alloys 1-3) we have studied the polar Kerr-effect spectra of compounds of the series Co2_ x Cu x MnSn at room temperature in the energy range.5-5ev. The purpose of this investigation is to obtain a better understanding of the Kerr-effect spectra known for CU2MnSn 1) and C2MnSn 3). The Kerr rotation!'pk! of CU2MnSnwas found to be rather small ( ::;;;.7 ), whereas!'pk! of C2MnSn is in excess of.4 at the low-energy side. A better understanding of the origin of these spectra might enable us to use chemical substitutions to shift the high Kerr intensity into a photon energy range suitable for technical applications, such as magneto-optical recording. 2. Experimental details The samples of the series CO2_ x Cu x MnSn were prepared by are melting, followed by vacuum annealing at 45 C for 4 weeks. The only exception was the compound CU2MnSn which was annealed for 3 weeks at 65 C and subsequently quenched to room temperature. Phillps Journol of Research Vol.42 No

2 P.P.J. van Engelen and K.H.J. Buschow Magnetic measurements were made in the range 4.2 to 9 K with magnetic field strengths up to 144kAlm by means of the Faraday method. The Curie temperatures were determined from the temperature dependence of. the magnetization u measured in low fields (H:O;:;;8 kalm) by plotting u 2 versus T and extrapolating the linear part to u 2 = O. The polar Kerr rotation 'PK and the ellipticity ek were measured at room temperature in the energy range.5 to 5 ev with an accuracy better than.1 A Faraday modulator was used for polarization modulation, allowing phase-sensitive detection of the output signal. The Kerr ellipticity was determined by using a number of quarter-wave plates. The retardation of these plates had been carefully measured as a function of the wavelength. Measurements were made on flat portions of the samples, polished with diamond paste. The magnetization direction was kept perpendicular to the mirror surface by means of an external magnetic field of 925 kalm. This field was sufficiently large to allow complete or nearly complete alignment of the magnetization. In practice the mean angle ofincidence of the light beam on the sample surface had to be 4.5, which, according to numerical estimates, caused an error in the Kerr-effect data of at most 1.5 %. a (nm) "- Tc CO 2 -x Cu x Mn Sn ~.61 '".", A> " " " 3 /... " "'" 2 "'" '"'".6 t/a " " x Tc (K) M (,us/f U) Fig. 1. Lattice constant a, saturation moment M at 4.2 K, and Curie temperature Tc as a function of the concentration x in Heusier alloys of the type Co 2 _xcu x MnSn. 43 Phlllps Journalof Research Vol.42 No

3 The magneto-optical properties of Heusier alloys of the type CO 2 xcuxmnsn 3. Expertmental results and discussion X-ray diffraction measurements made on CO 2 _ xcu x MnSn showed that the samples were almost single phase, the amount of second phases being of 'the order of 5 %. The cubic L2 1 structure was found for all concentrations. The lattice constant a is plotted as a function of concentration x in fig. LA perfectly linear relation is observed. The magnetic isotherms measured at 4.2K were used to obtain values for the saturation magnetization. These values, expressed in Bohr magnetons per formula unit, are plotted as a 'function of concentration in the middle part of fig. 1. The substitution of Cu for Co leads to a smalllinear reduction in saturation moment. The concentration dependence of the Curie temper-.4, " \ a~---4\ ~--~~~ CflK,EK (deg) x= hv(ev) Fig. 2. Kerr rotation 'PK as a function of the energy hv in several HeusIer compounds of the type Coz_xCuxMnSn. The ellipticity BK is also plotted (broken line) for the compound CUzMnSn. A dotted line has been drawn in the spectrum of the compound with x = 1.8 to indicate the overlapping of two separate closely spaced peaks. Phillps Journalof Research Vol.42 No

4 P.P.!. van Engelen and K.H.J. Buschow ature in CO2_ x Cu x MnSn is shown in the top part of fig. 1. There is some uncertainty as to the exact location of Tc for CU2MnSn due to a phase transition above 5 K. The Curie temperature is seen to first decrease linearly with x, but after reaching a minimum at x = 1.4 to increase again. This behaviour was already observed by UhI 4 ). Since the magnetic coupling mechanism in this class of materials appears to be fairly complicated we are not able to offer an explanation for the occurrence of the minimum in Tc. All the values of Tc shown in fig. 1 are sufficiently high above room temperature so that no special corrections need to be made when comparing the room. temperature Kerr-effect spectra of these materials.' Results of Kerr-effect measurements made at room temperature are given for several representative concentrations in figs 2 and 3. For practical reasons the rotation spectra are given for all compounds studied, but the energy dependence of the ellipticity is shown by way of example only for the end members of the series CO2_ x Cu x MnSn. In the Kerr-rotation spectrum of CU2MnSn (fig. 2) two negative peaks can be distinguished, centered around 1.5 and 2.5 ev. Inspection of fig. 2 shows that decreasing Cu concentration o~ ~ x= IPK,E K (deg) L--L~ L-~~~ h'1l(ev) Fig. 3. Kerr rotation 'PK as a function of the energy hij in several Heusier compounds of the type Co2_ x Cu"MnSn. The ellipticity 6K is also plotted (broken line) for the compound Ç2MnSn. 432 Philip. Journal of Research Vol. 42 No

5 The magneto-optical properties of Heusier alloys of the type Co 2 _ x Cu x MnSn leads to a gradual decrease of the intensity of the peak at 1.5 ev. At the same time there is a gradual increase in intensity at the lowest photon energy considered here (about.6 ev). This increase in intensity is strong, in particular for high Co concentrations, as can be,seen in fig. 3. These results suggest that the (negative) peak near 1.5eV is associated with the presence of Cu atoms while the strong Kerr intensity below O.6eV is to be attributed to the presence pf Co atoms. It is obvious then that the 3d electron states of Cu and Co will be involved. No particular assignment with respect to Cu or Co can be made with respect to the peak centered around 2.5 ev. This peak persists for all concentrations, although there is a substantial increase in intensity in going from CU2MnSn to Co2MnSn.. It is interesting to compare the interpretation of the Kerr-rotation spectra given above with those of Heusier alloys made before. Starting with the persistent peak at 2.5 ev we note that a similar peak was found in the Kerrrotation spectra of CU2MnZ (Z = Sn, In, AI). This peak was ascribed to a charge transfer transition occurring between the Mn ~d states and the p states of the Z element 1). Evidently, this holds also for the Co2_ x Cu x MnSn series. The increase in intensity of this peak when going from CU2MnSn to Co 2 MnSn may be' the result of an increasing difference in the density of Mn 3d states between majority and minority spin electrons when going in the same direction. The assignment of the peak located at 1.5 ey to Cu and the peak located below O.6eV to Co implies that one expects a peak in Ni2MnSn to occur in between O.6eY and 1.5 ev. This agrees with the observation of a (negative) peak in the spectrum of Ni2MnSn at 1.ey 5 ). 4. Conclusions We have analysed the Kerr-rotation spectra of Heusier compounds of the series Co2_ x Cu x MnSn and have shown that the Kerr intensity in the photon energy range below 1.5 ev points to transitions which involve the 3d electron states of Co and Cu. In all spectra of the above series a second (negative) Kerr-rotation peak is observed near 2.5 ev. This peak is attributed to a charge transfer transition between the Mn 3d states and the Sn p states. REFERENCES 1) K.H.J. Buschow, P.G. van Engen and R. Jongebreur, J. Magn. Magn. Mater. 38, 1 (1983). 2) R.A. de Groot, F.M. Mueller, P.G. van Engen, P.P.J. van Engelenand K.H.J. Buschow, to be published. 3) P. G. van Engen, Thesis Technical University Delft, ) E. Uhl, J. Magn. Magn. Mater. 25,221 (1981). - 5) K.H.J. Buschow, P.G. van Engen and D.B. de Mooij, J. Magn. Magn. Mater. 4, 339 (1984). Phlllps Journalof Research Vol.42 No

6 P.P.J. van Engelen and K.R.J. Buschow Authors P.P.J. van Engelen; Ir. degree (Physics), Technical University Delft, The Netherlands, 1966; Ph. D., University of Utrecht, The Netherlands, 198; Philips Research Laboratories, Eindhoven, His thesis work was on magnetic resonance spectroscopy of impurities in semiconductors. He is presently involved in the research on the magneto-optical properties of ferromagnetic metallic materials. Kurt Heinz Jurgen Buschow; Drs degree, Free University, Amsterdam, The Netherlands, 196; Ph.D., Free University, Amsterdam, 1963; Philips Research Laboratories, Eindhoven, He is a Senior Research Scientist. Since 1986 he is also a Professor of Materials Science at the University of Leiden. The subject of his thesis work was on ion pair formation with polyacene mono-and divalent ions. At Philips his interests are related to the occurrence, crystal structure and physical properties of intermetallic compounds. He has made many contributions of a fundamental nature, like crystal field effects, the origin of magnetic moments and exchange interaction but also of a more applied nature, like permanent magnets, hydrogen absorption and magneto-optics. 434 Philip, Journal of Research Vol.42 No

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