Magnetoresistance and anomalous Hall effect of magnesium single crystals with heavy rare earth impurities

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Magnetoresistance and anomalous Hall effect of magnesium single crystals with heavy rare earth

impurities

J. Bijvoet, G. Merlijn, P. Frings

To cite this version:

J. Bijvoet, G. Merlijn, P. Frings. Magnetoresistance and anomalous Hall effect of magnesium single crystals with heavy rare earth impurities. Journal de Physique Colloques, 1979, 40 (C5), pp.C5-38- C5-39. �10.1051/jphyscol:1979514�. �jpa-00218926�

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JOURNAL DE PHYSIQUE Colloque C5, suppliment au no 5, Tome 40, Mai 1979, page C5-38

Magnetoresistance and anomalous Hall effect of magnesium singIe crystals with heavy rare earth impurities

J. Bijvoet, G. Merlijn and P. Frings

Natuurkundig Laboratorium der Universiteit van Amsterdam, Valckenierstraat 65, 1018 XE Amsterdam, The Netherlands

Rkum6. - Le champ cristallin introduit une anisotropie de l'aimantation des ions des terres rares dans les monocristaux du magntsium. Cela rend difficile la stparation de I'effet de diffusion d'tchange et de I'effet de diffusion quadrupolaire dans les magnttoresistances mesurtes. La rtsistivitt de Hall extraordinaire de monocristaux de MgGd et de MgHo est anisotrope et ne peut pas Etre expliqute par un effet de champ cristallin du niveau 4f.

Abstract. - Because of crystal field splitting, the magnetization of rare earth ions in magnesium single crystals is anisotropic at low temperatures. This makes it difficult to separate the measured magnetoresistance into an exchange and a quadrupolar contribution. The anomalous Hall resistivity of MgGd and MgHo single crystals is anisotropic and cannot be explained by crystal field splitting of the 4f levels.

Single crystals of magnesium with small concentra- tions (0.1 to 0.4 at %) of heavy rare earth metals have been made. In these crystals we have observed the anisotropic magnetoresistance and the anomalous Hall effect which were first found in polycrystalline alloys of noble metals with rare earth impurities by Fert et al. [I]. In the h.c.p. lattice of magnesium the magnetization of the rare earth ions along the c-axis differs at low temperatures from the magnetisation in the basal plane. This is caused by the crystal field.

Because of this anisotropy it is not possible to separate the magnetoresistance into an isotropic part as caused by the exchange scattering and into a part which depends on the angle between current and magnetic field as caused by the quadrupolar scattering. As the crystal field splitting of the heavy rare earths in magnesium is not yet fully known, we can not compare our results with a calculation of the exchange scattering and quadrupolar scattering.

Meanwhile we want to present the magnetoresistance for various configurations of current magnetic field and crystal axis.

a ) When current and field are in the basal plane, turning the field with fixed current yields the longitudinal and a transverse magnetoresistance for a constant magnetization.

b) On another sample with current parallel to the c-axis the transverse magnetoresistance is measured and compared to the transverse magnetoresistance as measured in configuration a).

The results are : Transverse magnetoresistance

- Aplp in a field of 16 kG at 4.2 K : as obtained from a) and b) respectively.

and in a field of 16 kG at 1.2 K :

I l c ZYc

- -

Gd 8.4 x 8.0 x l o - 3

3 . 5 x 1 0 - 3 1.7 x

Dy 4.2 x 4.2 x

Ho 4.8 x 3.8 x

Er 2.0 x 0.2 x l o - 3

So we may conclude that fermi surface effects are not detected and that only crystal fields make the quadrupolar scattering dependent on crystal orien- tation.

c) Finally the longitudinal magnetoresistance I//c and BYc is compared with the transverse magnetoresistance Z I c, B/c. This yields the magnetoresistance anisotropy for magnetization along the c-axis.

At 4.2 K and in a field of 16 kG this anisotropy ( A P / P ~ - @PIP)= is :

B l c BHc

- .-

~b 1.8 10-3 2.1 10-3

DY 3.0 10-3 4.3 x 10-3

HO 1.7 10-3 2.1 1 0 - ~

Er - 0.4 x - 1.7 l o - 3

Tm - 0.2 x

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979514

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MAGNETORESISTANCE AND ANOMALOUS HALL EFFECT OF MAGNESIUM SINGLE CRYSTALS C5-39

The anomalous Hall effect is usually obtained as ^L ^, ^I ^, ^{, ,} ^,

,

^I ^, ^, ^,

^,

^, ^.,

P, (nRmm) .a

the temperature dependent part of the total Hall effect. This is not possible for dilute magnesium -20 alloys as the normal component is also strongly temperature dependent between room temperature and 10 K. However, we did not f h d any temperature

dependence for a number of dilute MgY alloys -

between 4.2 and 1.2 K so we shall take the difference / P

between 4.2 and 1.2 K for the magnetic rare earth alloys as a measure of the anomalous Hall effect.

The results for two dilute MgGd alloys are shown in the figure. The anomalous Hall effect is larger for I I c than for I Y c . As it is unlikely that this is caused by crystal field splitting of the 4f level, it must be due to crystal field splitting of the 5d level, perhaps in combination with fermi surface effects. - We have also measured the Hall resistivities of a MgHo 0.3 at % crystal. In addition to the 4f spin

dependent contribution we can expect to find a 4f _Fig. _1. _-Hall resistivities of two MgGd single crystals :

orbit dependent contribution of opposite sign. Fert 80.15at % Gd ; o ^{CI 0.07at}^{% G d ;} mB I caxis,z// caxis;

and Friederich have found that for holmium in gold ⁰ ^B^//^Câxis,ÎÎ^caxis full lines : 1.2 K, broken lines 4.2 K.

and silver the former prevails and for holmium in aluminium the latter. For holmium in magnesium

single crystals the sum of the two contributions is one. This is difficult to understand if one assumes highly anisotropic : for I l c the negative spin

contribution prevails and for I / / c the positive orbital that also in a crystal field - - remains constant.

2 - 9

References

[I] FERT, A,, FRIEDERICH, A., Phys. Rev. B 13 (1976) 397.

FERT, A., ASOMOZA, R., SANCHEZ, D . H., SPANJAARD, D., FRIEDERICH, A., Phys. Rev. B 16 (1977) 5040.