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3Al single crystal probed by polarized neutron diffraction
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 272–276 (2004) e67–e69
Magnetic properties of UNi2=3 Rh1=3Al single crystal probed by polarized neutron diffraction K. Prokes$a,b,*, A. Gukasovc, V. Sechovsky! b, A.V. Andreevd b
a Hahn-Meitner-Institute, SF-2, Glienicker Strasse 100, Berlin 141 09, Germany Department of Electronic Structures, Charles University, Ke Karlovu 5, Prague 2 121 16, The Czech Republic c Laboratoire L"eon Brillouin, CEA-Saclay, Gif sur Yvette 91191, France d Institute of Physics ASCR, Na Slovance 2, Prague 8 18221, The Czech Republic
Abstract A delicate balance between ferromagnetic and antiferromagnetic interactions in URh1=3 Ni2=3 Al leads to the absence of a long-range magnetic order and anomalous bulk properties of this material in zero magnetic field. The reported polarized neutron diffraction experiment in fields up to 6 T indicates that the positional dependence of Ni/Rh atomic distribution is responsible for the abnormal zero field profiles (most reflections have a significant Lorentzian contribution), while the field-induced U moments of 0:2 mB (at 2 K) cause a magnetic contribution to be of Gaussian type. r 2003 Elsevier B.V. All rights reserved. PACS: 75.25.+z; 75.50.Cc; 75.30.m Keywords: Magnetic structure determination; Neutron diffraction; Field-induced ferromagnetic order; UNi2=3 Rh1=3 Al; Magnetic frustration
The quasiternary intermetallic compound URh1=3 Ni2=3 Al crystallizes in the hexagonal ZrNiAl type of structure, like the parent compounds URhAl (ferromagnet (F), TC ¼ 27 K) and UNiAl (antiferromagnet (AF), TN ¼ 19 K) [1]. Recently, it has attracted interest owing to its peculiar magnetic, transport and thermodynamic properties. Speculations based on data obtained on polycrystals [2] regarding a possible nonFermi liquid (NFL) behavior appeared. Although bulk magnetic measurements revealed that also the single crystalline sample has some attributes of a NFL system, the overall zero-field bulk properties were ascribed to freezing of U moments with antiferromagnetic correlations [3]. While the magnetic susceptibility and electrical resistivity show upon application of a magnetic field restoration of Fermi-liquid type of behavior, the *Corresponding author. Hahn-Meitner-Institute, SF-2, Glienicker Strasse 100, Berlin 141 09, Germany. Tel.: +49-30-80622804; fax: +49-30-8062-3172. E-mail address: [email protected] (K. Proke$s).
temperature dependence of specific heat remains abnormal up to 14 T [3] although above 12 T a field-forced ferromagnetic state is derived from neutron-diffraction and magnetization data [3]. The same single crystal of UNi2=3 Rh1=3 Al as reported in Ref. [3] was used for present study. Previous nonpolarized neutron experiments [3] revealed that some of the nuclear reflections have significant Lorentzian contribution suggesting short-range order in this material, which can be in principle either of crystallographic (e.g. position dependence of the Ni/Rh atom distribution) or magnetic origin (frustration of magnetic interactions). From the negligible temperature dependence of the ð1 0 0Þ reflection that shows the strongest deviation from the ordinary Gaussian profile, we have concluded that this profile is most probably connected with details of the crystal structure. Polarized neutron diffraction experiments using the normal-beam 5c1 spectrometer with lifting counter installed at the ORPHEE reactor at CEA Saclay were undertaken on the UNi2=3 Rh1=3 Al crystal in fields up to 6 T applied
0304-8853/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.12.1233
ARTICLE IN PRESS e68
K. Proke$s et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) e67–e69
nearly along the c-axis in order to clarify the peculiar magnetic properties. The incident neutron beam had a ( and a polarization degree of wavelength of 0:846 A 92%. First, we recorded at 1:8 K profile of few reflections in a field of 0:1 T (necessary to keep polarization of neutrons) and in 6 T in order to clarify whether the Lorentzian profile is of magnetic or crystalographic origin. In Fig. 1a we show scans made at 0:1 T around the ð1 0 0Þ reflection, which was found to have the most pronounced Lorentzian shape [3]. Clearly, at low fields the intensities for the spin-up and -down polarizations of the incident beam are almost identical. The observed flipping ratio close to 1 suggests absence of magnetic contribution. As the applied magnetic field increases (see inset of Fig. 1c), the spin-up and -down intensities start
Fig. 1. Rocking curves of the ð1 0 0Þ reflection of URh1=3 Ni2=3 Al consisting of two grains measured with spinup and spin-down polarization of incoming neutrons at 1:8 K and 0:1 T (a), 6 T (b) and the inverse flipping ratio fitted to a Gaussian profile (c). In the inset, the field dependence of the square root of the inverse flipping ratio is shown.
to deviate from each other and in a field of 6 T the flipping ratio R amounts to Rð1 0 0Þ ¼ I þ =I ¼ 0:5970:02 (Fig. 1b). The dependence of the square root of R is linear, i.e. the development of field-induced magnetic moments is proportional to the applied field. The shape of the magnetic signal at 6 T (here represented by the angular dependence of R) is of Gaussian profile in contrast to the Lorentzian profile of the whole reflection at low fields suggesting that the field-forced ferromagnetic order is of long-range type and uniform. This is in agreement with the expectation that only the U atoms, which are occupying only one crystallographic site, are capable to carry sizable induced magnetic moment. In order to determine the magnitude of the induced U moment, we have collected flipping ratios of 133 reflections (56 inequivalent ones) at 1:8 K and 6 T: A direct non-linear refinement of the measured flipping ratios was performed using programs of the Cambridge Crystallography Subroutine Library CCSL [4], which allow to refine magnitude and directions of magnetic moments. The flipping ratios were fit to the atomic model in which the magnetic centers correspond to the atomic positions. The dipolar approximation /j0 S þ c2 /j2 S [5], where j0 and j2 are the Bessel functions and the coefficient c2 corresponds to 3þ valence of the U ion and intermediate coupling (c2 ¼ 1:64 in this case), has been used for the U magnetic form factor. The best fit leads to the conclusion that indeed only the U moments are significantly developed. In subsequent fits we therefore put induced moments on Ni/Rh equal to zero. The best fit leads to moments of 0:20270:008 mB =U with the deviation angle with respect to the c-axis of 0:8 : This finding is in excellent agreement with magnetization studies which revealed at 2:0 K and 6 T a magnetization along the c-axis of 0:2 mB =U whereas a 10 times smaller magnetization is indicated along the a-axis [3]. From our polarized neutron experiment, it clearly follows that unusual profiles of nuclear reflections are connected with crystal structure details (positional variation of Rh/Ni composition) of our sample. Upon application of magnetic field, magnetic moments on U sites are linearly induced with magnetic field. At 1:8 K and 6 T directed along the c-axis, a moment of 0:2 mB =U can be determined. No significant moments could be detected on Ni/Rh sites. To our understanding, a delicate balance between F and AF interactions in zero field leads to a suppression of magnetic order in UNi2=3 Rh1=3 Al: Some similarities of this material with respect to the itinerant 5f electron metamagnet UCoAl [1] suggest that it is very close to magnetic order. Peculiar electrical transport properties can be caused by the hybridization of 5f electron states with conduction electrons that is reduced in magnetic fields. However, the abnormal low-temperature specific-heat dependence in
ARTICLE IN PRESS K. Proke$s et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) e67–e69
low fields suggests that AF correlations play very important role even in rather elevated fields. This work is a part of the research program MSM113200002 financed by the Ministry of Education of the Czech Republic. Support to K.P. for his work at LLB Saclay in the framework of the ‘‘Improving Human Potential Program’’ of the EU is acknowledged.
e69
References [1] V. Sechovsky, L. Havela, in: K.H.J. Buschow (Ed.), Handbook of Magnetic Materials, Vol. 11, North-Holland, Amsterdam, 1998, p. 1. [2] K. Proke$s, et al., Physica B 281–282 (2000) 377. [3] A.V. Andreev, et al., Philos. Mag. 83 (2003) 1614. [4] P.J. Brown, J.C. Matthewman, CCSL Mark 4, RAL 1993. [5] A.J. Freeman, et al., Phys. Rev. B 13 (1976) 1168.
Journal of Magnetism and Magnetic Materials 272–276 (2004) e67–e69
Magnetic properties of UNi2=3 Rh1=3Al single crystal probed by polarized neutron diffraction K. Prokes$a,b,*, A. Gukasovc, V. Sechovsky! b, A.V. Andreevd b
a Hahn-Meitner-Institute, SF-2, Glienicker Strasse 100, Berlin 141 09, Germany Department of Electronic Structures, Charles University, Ke Karlovu 5, Prague 2 121 16, The Czech Republic c Laboratoire L"eon Brillouin, CEA-Saclay, Gif sur Yvette 91191, France d Institute of Physics ASCR, Na Slovance 2, Prague 8 18221, The Czech Republic
Abstract A delicate balance between ferromagnetic and antiferromagnetic interactions in URh1=3 Ni2=3 Al leads to the absence of a long-range magnetic order and anomalous bulk properties of this material in zero magnetic field. The reported polarized neutron diffraction experiment in fields up to 6 T indicates that the positional dependence of Ni/Rh atomic distribution is responsible for the abnormal zero field profiles (most reflections have a significant Lorentzian contribution), while the field-induced U moments of 0:2 mB (at 2 K) cause a magnetic contribution to be of Gaussian type. r 2003 Elsevier B.V. All rights reserved. PACS: 75.25.+z; 75.50.Cc; 75.30.m Keywords: Magnetic structure determination; Neutron diffraction; Field-induced ferromagnetic order; UNi2=3 Rh1=3 Al; Magnetic frustration
The quasiternary intermetallic compound URh1=3 Ni2=3 Al crystallizes in the hexagonal ZrNiAl type of structure, like the parent compounds URhAl (ferromagnet (F), TC ¼ 27 K) and UNiAl (antiferromagnet (AF), TN ¼ 19 K) [1]. Recently, it has attracted interest owing to its peculiar magnetic, transport and thermodynamic properties. Speculations based on data obtained on polycrystals [2] regarding a possible nonFermi liquid (NFL) behavior appeared. Although bulk magnetic measurements revealed that also the single crystalline sample has some attributes of a NFL system, the overall zero-field bulk properties were ascribed to freezing of U moments with antiferromagnetic correlations [3]. While the magnetic susceptibility and electrical resistivity show upon application of a magnetic field restoration of Fermi-liquid type of behavior, the *Corresponding author. Hahn-Meitner-Institute, SF-2, Glienicker Strasse 100, Berlin 141 09, Germany. Tel.: +49-30-80622804; fax: +49-30-8062-3172. E-mail address: [email protected] (K. Proke$s).
temperature dependence of specific heat remains abnormal up to 14 T [3] although above 12 T a field-forced ferromagnetic state is derived from neutron-diffraction and magnetization data [3]. The same single crystal of UNi2=3 Rh1=3 Al as reported in Ref. [3] was used for present study. Previous nonpolarized neutron experiments [3] revealed that some of the nuclear reflections have significant Lorentzian contribution suggesting short-range order in this material, which can be in principle either of crystallographic (e.g. position dependence of the Ni/Rh atom distribution) or magnetic origin (frustration of magnetic interactions). From the negligible temperature dependence of the ð1 0 0Þ reflection that shows the strongest deviation from the ordinary Gaussian profile, we have concluded that this profile is most probably connected with details of the crystal structure. Polarized neutron diffraction experiments using the normal-beam 5c1 spectrometer with lifting counter installed at the ORPHEE reactor at CEA Saclay were undertaken on the UNi2=3 Rh1=3 Al crystal in fields up to 6 T applied
0304-8853/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.12.1233
ARTICLE IN PRESS e68
K. Proke$s et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) e67–e69
nearly along the c-axis in order to clarify the peculiar magnetic properties. The incident neutron beam had a ( and a polarization degree of wavelength of 0:846 A 92%. First, we recorded at 1:8 K profile of few reflections in a field of 0:1 T (necessary to keep polarization of neutrons) and in 6 T in order to clarify whether the Lorentzian profile is of magnetic or crystalographic origin. In Fig. 1a we show scans made at 0:1 T around the ð1 0 0Þ reflection, which was found to have the most pronounced Lorentzian shape [3]. Clearly, at low fields the intensities for the spin-up and -down polarizations of the incident beam are almost identical. The observed flipping ratio close to 1 suggests absence of magnetic contribution. As the applied magnetic field increases (see inset of Fig. 1c), the spin-up and -down intensities start
Fig. 1. Rocking curves of the ð1 0 0Þ reflection of URh1=3 Ni2=3 Al consisting of two grains measured with spinup and spin-down polarization of incoming neutrons at 1:8 K and 0:1 T (a), 6 T (b) and the inverse flipping ratio fitted to a Gaussian profile (c). In the inset, the field dependence of the square root of the inverse flipping ratio is shown.
to deviate from each other and in a field of 6 T the flipping ratio R amounts to Rð1 0 0Þ ¼ I þ =I ¼ 0:5970:02 (Fig. 1b). The dependence of the square root of R is linear, i.e. the development of field-induced magnetic moments is proportional to the applied field. The shape of the magnetic signal at 6 T (here represented by the angular dependence of R) is of Gaussian profile in contrast to the Lorentzian profile of the whole reflection at low fields suggesting that the field-forced ferromagnetic order is of long-range type and uniform. This is in agreement with the expectation that only the U atoms, which are occupying only one crystallographic site, are capable to carry sizable induced magnetic moment. In order to determine the magnitude of the induced U moment, we have collected flipping ratios of 133 reflections (56 inequivalent ones) at 1:8 K and 6 T: A direct non-linear refinement of the measured flipping ratios was performed using programs of the Cambridge Crystallography Subroutine Library CCSL [4], which allow to refine magnitude and directions of magnetic moments. The flipping ratios were fit to the atomic model in which the magnetic centers correspond to the atomic positions. The dipolar approximation /j0 S þ c2 /j2 S [5], where j0 and j2 are the Bessel functions and the coefficient c2 corresponds to 3þ valence of the U ion and intermediate coupling (c2 ¼ 1:64 in this case), has been used for the U magnetic form factor. The best fit leads to the conclusion that indeed only the U moments are significantly developed. In subsequent fits we therefore put induced moments on Ni/Rh equal to zero. The best fit leads to moments of 0:20270:008 mB =U with the deviation angle with respect to the c-axis of 0:8 : This finding is in excellent agreement with magnetization studies which revealed at 2:0 K and 6 T a magnetization along the c-axis of 0:2 mB =U whereas a 10 times smaller magnetization is indicated along the a-axis [3]. From our polarized neutron experiment, it clearly follows that unusual profiles of nuclear reflections are connected with crystal structure details (positional variation of Rh/Ni composition) of our sample. Upon application of magnetic field, magnetic moments on U sites are linearly induced with magnetic field. At 1:8 K and 6 T directed along the c-axis, a moment of 0:2 mB =U can be determined. No significant moments could be detected on Ni/Rh sites. To our understanding, a delicate balance between F and AF interactions in zero field leads to a suppression of magnetic order in UNi2=3 Rh1=3 Al: Some similarities of this material with respect to the itinerant 5f electron metamagnet UCoAl [1] suggest that it is very close to magnetic order. Peculiar electrical transport properties can be caused by the hybridization of 5f electron states with conduction electrons that is reduced in magnetic fields. However, the abnormal low-temperature specific-heat dependence in
ARTICLE IN PRESS K. Proke$s et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) e67–e69
low fields suggests that AF correlations play very important role even in rather elevated fields. This work is a part of the research program MSM113200002 financed by the Ministry of Education of the Czech Republic. Support to K.P. for his work at LLB Saclay in the framework of the ‘‘Improving Human Potential Program’’ of the EU is acknowledged.
e69
References [1] V. Sechovsky, L. Havela, in: K.H.J. Buschow (Ed.), Handbook of Magnetic Materials, Vol. 11, North-Holland, Amsterdam, 1998, p. 1. [2] K. Proke$s, et al., Physica B 281–282 (2000) 377. [3] A.V. Andreev, et al., Philos. Mag. 83 (2003) 1614. [4] P.J. Brown, J.C. Matthewman, CCSL Mark 4, RAL 1993. [5] A.J. Freeman, et al., Phys. Rev. B 13 (1976) 1168.