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Register a new userMagnetization dynamics in helical-ordered spin systems. A case study of EuFe$_2$As$_2$
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Added by
Igor A Golovchanskiy
Publication date
2021
fields
Physics
and research's language is
English
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In this work, magnetization dynamics in helical-ordered spin systems is studied theoretically and experimentally using magnetization measurements and ferromagnetic resonance measurement technique. EuFe2As2 is considered as the case study. Theoretically two types of resonance responses are examined for helical-ordered systems: the molecular field response and collective spin resonance modes. Both types of responses demonstrate clear dependence on helicity. It is demonstrated that combination of magnetization measurements and ferromagnetic resonance studies define the helicity completely. Experimentally for EuFe2As2 the molecular field response is observed. The defined helical angle is about 2pi/5.
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We report the measurements of anisotropic magnetization and magnetoresistance on single crystals of EuFe$_2$As$_2$, a parent compound of ferro-arsenide high-temperature superconductor. Apart from the antiferromagnetic (AFM) spin-density-wave transition at 186 K associated with Fe moments, the compound undergoes another magnetic phase transition at 19 K due to AFM ordering of Eu$^{2+}$ spins ($J=S=7/2$). The latter AFM state exhibits metamagnetic transition under magnetic fields. Upon applying magnetic field with $Hparallel c$ at 2 K, the magnetization increases linearly to 7.0 $mu_{B}$/f.u. at $mu_{0}H$=1.7 T, then keeps at this value of saturated Eu$^{2+}$ moments under higher fields. In the case of $Hparallel ab$, the magnetization increases step-like to 6.6 $mu_{B}$/f.u. with small magnetic hysteresis. A metamagnetic phase was identified with the saturated moments of 4.4 $mu_{B}$/f.u. The metamagnetic transition accompanies with negative in-plane magnetoresistance, reflecting the influence of Eu$^{2+}$ moments ordering on the electrical conduction of FeAs layers. The results were explained in terms of spin-reorientation and spin-reversal based on an $A$-type AFM structure for Eu$^{2+}$ spins. The magnetic phase diagram has been established.
Ternary iron arsenide EuFe$_2$As$_2$ with ThCr$_2$Si$_2$-type structure has been studied by magnetic susceptibility, resistivity, thermopower, Hall and specific heat measurements. The compound undergoes two magnetic phase transitions at about 200 K and 20 K, respectively. The former was found to be accompanied with a slight drop in magnetic susceptibility (after subtracting the Curie-Weiss paramagnetic contribution), a rapid decrease in resistivity, a large jump in thermopower and a sharp peak in specific heat with decreasing temperature, all of which point to a spin-density-wave-like antiferromagnetic transition. The latter was proposed to be associated with an A-type antiferromagnetic ordering of Eu$^{2+}$ moments. Comparing with the physical properties of the iso-structural compounds BaFe$_2$As$_2$ and SrFe$_2$As$_2$, we expect that superconductivity could be induced in EuFe$_2$As$_2$ through appropriate doping.
We have carried out high-field resistivity measurements up to 27,T in EuFe$_2$As$_2$ at $P$,=,2.5,GPa, a virtually optimal pressure for the $P$-induced superconductivity, where $T_mathrm{c}$,=,30,K. The $B_mathrm{c2}-T_mathrm{c}$ phase diagram has been constructed in a wide temperature range with a minimum temperature of 1.6 K ($approx 0.05 times T_mathrm{c}$), for both $B parallel ab$ ($B_mathrm{c2}^mathrm{ab}$) and $B parallel c$ ($B_mathrm{c2}^mathrm{c}$). The upper critical fields $B_mathrm{c2}^mathrm{ab}$(0) and $B_mathrm{c2}^mathrm{c}$(0), determined by the onset of resistive transitions, are 25 T and 22 T, respectively, which are significantly smaller than those of other Fe-based superconductors with similar values of $T_mathrm{c}$. The small $B_mathrm{c2}(0)$ values and the $B_mathrm{c2}(T)$ curves with positive curvature around 20 K can be explained by a multiple pair-breaking model that includes the exchange field due to the magnetic Eu$^{2+}$ moments. The anisotropy parameter, $Gamma=B_mathrm{c2}^{ab}/B_mathrm{c2}^{c}$, in EuFe$_2$As$_2$ at low temperatures is comparable to that of other 122 Fe-based systems.
$^{57}$Fe Mossbauer spectra at different temperatures between $sim 5$ K and $sim 300$ K were measured on an oriented mosaic of single crystals of CaKFe$_4$As$_4$ . The data indicate that CaKFe$_4$As$_4$ is a well formed compound with narrow spectral lines, no traces of other, Fe - containing, secondary phases in the spectra and no static magnetic order. There is no discernible feature at the superconducting transition temperature in any of the hyperfine parameters. The temperature dependence of the quadrupole splitting approximately follows the empirical $T^{3/2}$ law. The hyperfine parameters of CaKFe$_4$As$_4$ are compared with those for KFe$_2$As$_2$ measured in this work, and the literature data for CaFe$_2$As$_2$, and were found to be in between those for these two, ordered, 122 compounds, in agreement with the gross view of CaKFe$_4$As$_4$ as a structural analog of KFe$_2$As$_2$ and CaFe$_2$As$_2$ that has alternating Ca - and K - layers in the structure.
We present the magnetic and superconducting phase diagram of EuFe$_2$As$_2$ for $B parallel c$ and $B parallel ab$. The antiferromagnetic phase of the Eu$^{2+}$ moments is completely enclosed in the superconducting phase. The upper critical field vs. temperature curves exhibit strong concave curvatures, which can be explained by the Jaccarino-Peter compensation effect due to the antiferromagnetic exchange interaction between the Eu$^{2+}$ moments and conduction electrons.
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