Experimental and Theoretical Investigations of the Magnetic Phases of Epitaxially Grown EuSe at Low Fields and Temperatu
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O4.14.1
Experimental and Theoretical Investigations of the Magnetic Phases of Epitaxially Grown EuSe at Low Fields and Temperatures 1
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K. Rumpf, 1 P. Granitzer, 1 H. Krenn Institut für Experimentalphysik, Universität Graz, A-8010 Graz, Austria
ABSTRACT Due to its metamagnetic behaviour the magnetic phases of EuSe are more complicated than for the other Eu-chalcogenides, e.g. EuTe. At very low temperatures (below 2K) there occurs an additional antiferromagnetic phase. This behaviour cannot be solely explained by exchange interaction of nearest neighbors and next nearest neighbors but requires more information about the NNSSNN, NSNS and NNSNNS spin arrangements. Because the nearest and next nearest neighbour exchange constants are nearly cancelled out, higher order interaction becomes important and the biquadratic exchange has also to be taken into account. Our measurements were carried out on an epitaxial grown 2.5 micrometers EuSe film on BaF2. Three regions predicted in the phase diagram of a EuSe bulk crystal are of special interest. For low temperatures (below 1.8K) and low magnetic fields (below 0.05T) a NSNS antiferromagnetic (type II) phase occurs. For higher temperatures from 1.8K to 4.6K and B = 0.1T to 2.5T a NNSNNS ferrimagnetic order exists whereas for lower magnetic fields a further antiferromagnetic (type I) NNSSNNSS-phase is formed above a temperature of 3.6-3.7K. The experimental data show that bilinear mean field calculations taking into account only nearest (JNN) and next nearest exchange (JNNN) interaction fail on this system. Therefore biquadratic exchange (K) has to be included. We present a 12x12 matrix MFA-formalism to extract the proper exchange parameters, which deviate slightly from the bilinear MFA-approach: JNN = 0.165 K, JNNN = -0.1209 K. The biquadratic exchange constant is taken as K = -0.0458 K. We prove this theoretical implication by testing Arrott plots for a varying slope. We made a fit to the experimental data even beyond MFA by taking the corresponding critical exponents of the Heisenberg model.
INTRODUCTION The Eu-chalcogenides (EuO, EuS, EuSe, EuTe) as magnetic semiconductors and classical Heisenberg antiferromagnets have been investigated intensively long years ago [1]. Despite the fact that the magnetic ordering temperature could not be raised up to temperatures of interest for industrial and commercial applications this substances are still of interest for studying magnetism on a very fundamental basis. Among all the Eu-chalcogenides with their fcc type rock-salt crystal structure EuSe is a special candidate concerning magnetic properties. EuSe has a metamagnetic behaviour and shows very interesting spin structures below the Neel temperature of 4.6 K. The metamagnetism of EuSe can be understood by the near-balance of the nearest (NN) and the next nearest neighbor (NNN) exchange constants, which means JNN + JNNN ≈ 0 for the 12 coordinating bonds in the fcc-lattice. The nearest neighbor exchange constant has been published [2] as ferromagnetic: JNN = 0.119 K, whereas the
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