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Localization of Conduction-band Electrons inβ”-(BEDT-TTF)4 NH4 [Cr (C2O4)3]⋅DMF Single Crystals
Institute for Molecular Science, Myodaiji, Okazaki 444-8585, Japan

A. A. Baskakov, L. R. Dunin-Barkovskiy, T. Kato, ○R. B. Morgunov, Y. Tanimoto, E. Yagubskiy
Preface. Recently synthesized β”-(BEDT-TTF)4 NH4 [Cr (C2O4)3]⋅DMF [1] single crystals represent
a compound that could be considered as a material for spintronics since crystals of this type possess giant
magnetoresistance and, under condition of magnetic ordering in the subsystem of localized spins, could bean analog of heterostructures providing spin polarization of conduction electrons emitted from the magneticlayer to the conducting one [2]. This makes investigation of interactions between conduction electrons andlocalized magnetic moments in crystals to be attractive. The purpose of the presented work was in design ofthe conditions for separation of contributions of localized magnetic moments and conduction electrons into
magnetic properties of single crystals of the mentioned compound as well as in an attempt of finding andinvestigation of mutual influence of s- and conduction π-electrons.
Experimental.

Singlecrystalsonthebaseoforganic

donor

moleculesofbis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) and chromium oxalate Cr(C2O4)3 were obtained byelectrocrystallization from solution (for details see [1]). According to X-ray structural analysis [1], thesingle crystals are characterized by the C2/c symmetry and consist of alternating layers of β”-phase of
BEDT-TTF and approximately hexagonal net of (C2O4)3 and Cr3+ with dimethylformamide (DMF)
molecules. X-band EPR spectrometers Bruker ESP300E and E500 (rectangular Н102 cavity) with themodulation frequency 100 kHz and the range of static magnetic field sweep B0 = 0–15 kOe were used. The
measured EPR signal was proportional to the first derivative of the real part of magnetic susceptibility χ ofthe specimen. The temperature was varied within the range 1.5–300 К. During the measurements thespecimen investigated was placed into a fused quartz ampoule, pumped out and sealed.
Resultsofexperiments.

EPR

spectrumof

theinvestigated single crystals consists of two lines (figure). One,consisting of three Gaussian shape components, is the EPR
response of Cr3+ ions. The other line at temperatures close to theroom one represents a spectrum of clearly Lorentzian shape if
the electrical component Е of the microwave field is directed
normal to the (ab) plane of the crystal. In the orientationЕ ⊥ (ab) an asymmetric Dysonian shape line is observed. It is
logical to associate its origin with resonance in the subsystem ofconduction electrons of BEDT-TTF. So, presence of two Figure. Angular dependence of ESR
magnetic subsystems in the crystal, predicted by X-ray structural spectrumofβ”-(BEDT-TTF)4

analysis [1], becomes apparent in the form of two separated EPR NH4 [Cr (C2O4)3]⋅DMF single crystal.
lines. This gives one the possibility of independent investigationof magnetic features of each the subsystem and interaction between them.
Decreasing of the temperature from 300 K down to 1.5 K causes changes in the EPR spectra for thelines of both the magnetic subsystems. The real part of magnetic susceptibility of chromium ions χCr isclose to the Curie-Weiss law χCr = СCr/(T-ΘCr), where ΘCr = - 2 К, however the temperature dependence ofthe value χCrТ, proportional to the number of spins in the specimen, allows one to find out deviations fromthe Curie-Weiss law. At the temperature Т ≈ 20 К an abrupt decrease of the value χCrТ is observed.
The temperature dependence of the effective g-factor depends on orientation of Н0 relative to thecrystal: if Н0 is normal to the BEDT-TTF layer, at Т ≈ 20 К increase of gЕТ occurs, whereas under parallelorientation of Н0 relative to the BEDT-TTF layer an abrupt decrease of gЕТ is observed. In the case whenthe Н0 vector is directed at the angle of ~ 20° to the (ab) plane, the gЕТ value does not depend on
temperature in all the investigated temperature range.
At temperatures Т > 20 К the ЕТ spectrum in the orientation Е ⊥ (ab) has the Lorentzian shapewithin the accuracy 99%. At Т < 20 К, when jump-like decrease of the number of Cr3+ spins occurs, anabrupt broadening on the wings of the BEDT-TTF spectrum is observed, what does not allow one toapproximate it by a Lorentzian line any more. In order to quantitatively characterize this deviation, we used
the relation δ = Ymax’/Y’(3/2∆HppET), that is ratio of amplitude of the first derivative of the EPR signal
Ymax’ = (dY/dH0)max at the width HppET to its amplitude Y’(3/2∆HppET) at the width 3/2∆HppET. According
to [9], this value has to be equal to 0.33 for a purely Lorentzian line and becomes equal to 0.05 for a purely
Gaussian one. It was found that δ = 0.33 at Т > 20 К and it abruptly decreases down to 0.1 as thetemperature is decreased from 20 to 3.3 K. Since the described above change of the line shape does not
disturb its symmetry, decomposition of the BEDT-TTF spectrum into two lines of Lorentzian and Gaussian
shapes was carried out with account of the fact that the effective g-factors of both the components coincide.
It was found that at Т < 20 К relative contribution of the Lotentzian component χL decreases and theportion of the magnetic susceptibility corresponding to the Gaussian line χG increases. This occurs mosteffectively for the crystal orientations in which the BEDT-TTF layers are normal to the static magneticfield. Change of the line shape is the mostly pronounced under the orientation of the magnetic field Н0along the BEDT-TTF layers. As one approaches the mutually perpendicular orientation of Н0 and theBEDT-TTF layers, line distortions caused by the temperature decreasing turn to be weaker.
Conclusion.
1. Separated ESR lines of BEDT-TTF radicals and Cr3+ ions were found in spectra ofβ”-(BEDT-TTF)4 NH4 [Cr (C2O4)3]⋅DMF single crystals. Comparison of angular dependencies of Cr3+
g-factor with anisotropy of ESR spectra in Cr (C2O4)3 revealed influence of BEDT-TTF layers on splittingof Cr3+ spin sublevels. Angular variation of line-width and its mean value at the room temperature are inagreement with the BEDT-TTF packing and predictions of the Elliot theory.
2. It was found that decrease of temperature below Т ≈ 20 К leads to abrupt reorganization of ESRspectrum. Transformation of the ВЕDT-TTF Lorentzian line-shape to the Gaussian one indicates
localization of conduction-band electrons. Interdependence between changes of the ВЕDT-TTF and Сr3+lines showed that temperature variations of Сr3+ susceptibility are accompanied by changes inconduction-band electrons behavior.
References
1.

T. G. Prokhorova et al., Advanced Functional Materials, 13, 1 (2003).

2.

M. Ziese, M. J. Thornton, “Spin electronics”, Berlin: Springer, 2001, p. 500.