Giant moments of Fe and Co on and in rubidium and potassium films

Abstract

Thin quench-condensed films of Rb and K are covered with 1/100 of a mono-layer of Fe or Co. Then the impurities are covered with several atomic layers of the host. The magnetization of the films is measured by means of the anomalous Hall effect (AHE). The magnetization follows a Brillouin function with a magnetic moment of more than 10 Bohr magnetons for bulk Co and Fe impurities. These moments are much larger than the moments of the atomic configurations of Fe and Co and suggest enhanced magnetic moments of the impurities. PACS. 75.20.Hr Local moment in compounds and alloys; Kondo effect, valence fluctuations, heavy fermions – 71.20.Dg Alkali and alkaline earth metals – 73.50.-h Electronic transport phenomena in thin films The alkali metals are rather distinct from all other metals in the periodic system. For one thing they are very open metals, i.e. the volume of the alkali ions take only a small fraction of the total metal volume. Furthermore the Coulomb interaction plays an important role in the alkali metals. This has been studied in particular for Cs, which has the smallest electron concentration of any metallic element (rs = 6.6). Already Wigner [1] showed that a Hartree-Fock calculation for Cs yields a ferro-magnetic ground state. Overhauser [2] discovered that a spinand charge density state has even lower energy (in HartreeFock) and developed the charge-density wave model of the alkali metals. Our group recently discovered a number of surprising properties of thin Cs films [3,4]. Of particular interest are transition metal impurities in the alkali metals. Their properties are difficult to investigate since the d-impurities do not dissolve in the alkali host. There are only a few methods to obtain alkali metals with magnetic impurities such as (i) ion-implantation, (ii) quenched condensation and (iii) nuclear reactions. Riegel et al. [5] investigated the properties of Fe in the alkali hosts Cs, Rb, K and Li. These authors introduced the magnetic d-impurities by nuclear reactions or recoil from nuclear reactions into the alkali metals. For the investigation of the magnetic properties they used the experimental method of “time-differential perturbed angular γ-ray-distribution” in the temperature range from 20 K to 350 K. This method measures the hyperfine field at the Fe nucleus due to the susceptibility of the electrons a Mohamed Hossain performed these experiments as a junior b e-mail: bergmann@usc.edu in the Fe. They observed a Curie law for the temperature dependence of the local susceptibility, β (T ) − 1 = gJμB (J + 1)B (0)/3kBT , where gJ is the Lande factor for the total spin J , μB=Bohr magneton, kB=Boltzmann constant and B (0) is the hyperfine field constant of the Fe atom. From the positive sign of B (0) they concluded that the orbital angular momentum of the d-electrons contribute strongly to the Fe moment. They suggested a 3d-configuration for the Fe atom dissolved in the alkali host (with exception of the Li host) and found a good agreement forB (0) between a simple calculation and their experimental result. Our group used the method of quenched condensation onto a substrate at helium temperature to obtain alkali films with d-impurities. The magnetization of the 3d-impurities was measured by means of the anomalous Hall effect. At the present time this is the only experimental method to measure the magnetization as a function of both the temperature and the magnetic field. We observed giant magnetic moments for Fe and Co in thin Cs films [6] with a magnetic moment of the order of 7−8 μB (μB=Bohr magneton). In comments on our results Gruyters and Riegel [7] and Mohn et al. [7] emphasized that the Fe and Co impurities in Cs possess their atomic electronic structure for the dshell which are 3d for Fe and 3d for Co. The Fe has a total angular momentum of J = 4 and a Lande-factor g = 32 while the Co has J = 92 and g = 4 3 . In both cases the total magnetic moment should be μ = Jg μB = 6 μB. Both groups emphasized that there should be no polarization of the Cs host. Guo [8], stimulated by our experiments, performed “orbital-polarization corrected relativistic spindensity-functional” calculations for Fe and Co in the 8 The European Physical Journal B alkali hosts K, Rb and Cs. He obtained a similar electronic structure for the Fe and Co impurities and arrived at the conclusion that the host should not be polarized by the impurities. McHenry et al. in an earlier paper [9] obtained a different electronic structure for Fe impurities in alkali hosts (simulated by clusters). They found a 3d shell for the Fe impurities (which yields the same moment of 6 μB). In this paper we investigate the properties of magnetic impurities on the surface and in the bulk of Rb and K films. Our experimental method is the anomalous Hall effect (AHE). In metals or alloys with magnetic moments one observes, besides the normal Hall effect, also an “anomalous” component which results from the asymmetric scattering of the conduction electrons by the magnetic moments. This anomalous Hall resistance (AHR) is proportional to the magnetization of the magnetic atoms (see for example [10]).

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Cite this paper

@inproceedings{HossainGiantMO, title={Giant moments of Fe and Co on and in rubidium and potassium films}, author={M. F. Hossain and Gerd Bergmann} }