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1
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Cu_3(tmen)_3(tma)_2(H_2O)_2 ·6.5H_2O - New S = 1/2 "Sawtooth" Chain?

100%
Vrábel P., Orendáčová A., Černák J., Orendáč M.
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issue 5
993-994
EN
The results of the investigation of magnetic susceptibility, magnetization and specific heat of Cu_3(tmen)_3(tma)_2(H_2O)_2·6.5H_2O (tmen = N,N,N',N'-tetramethylethane-1,2 diamine; H_3tma = 1,3,5-benzenetricarboxylic acid) are reported. The spatial arrangement of magnetic Cu(II) ions and network of covalent bonds suggest that the studied material might be a representative of S = 1/2 sawtooth chain with moderate exchange coupling J/k_{B}. The investigation of the temperature dependence of susceptibility and magnetic field dependence of magnetization yielding J/k_{B} ≈ - 0.63 K is consistent with the structural features. In addition, specific heat data reveal short-range correlations in milikelvin temperature range and indicate long-range ordering below 150 mK.
2
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Thermodynamic Properties of Geometrically Frustrated S=1/2 XY Antiferromagnet Er_{2}Sn_{2}O_{7}

100%
Al Ghamdi N., Orendáčová A., Pavlík V., Orendáč M.
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vol. 126
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issue 1
264-265
EN
The magnetic susceptibility, specific heat and entropy of geometrically frustrated S=1/2 XY antiferromagnet Er_{2}Sn_{2}O_{7} are reported. It is shown, that temperature dependence of magnetic susceptibility above nominally 10 K is governed by crystal field effects. Systematic study of specific heat in magnetic fields up to 7 T enabled the estimation of exchange coupling, J/k<<4.6 K. Investigation of isothermal change of magnetic entropy upon magnetization from 1 T to 7 T revealed the absence of enhanced magnetocaloric effect. The obtained results suggest that frustration parameter f ≈140 proposed for Er_{2}Sn_{2}O_{7} earlier may be significantly overestimated.
3
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Experimental Study of Magnetic Properties of Alumina Ceramics

88%
Tarasenko R., Orendáčová A., Orendáč M., Kajňaková M., Feher A.
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issue 5
1067-1068
EN
Magnetic properties of alumina ceramics were studied within a program focused at design of microcalorimeters and experimental setup for specific heat and magnetocaloric measurements. The investigations were concentrated on the determination of amount of magnetic impurities that can significantly influence results of physical measurements at low temperatures. Our experimental studies of heat capacity, susceptibility and magnetization clearly indicate that alumina ceramics contains magnetic impurities, primarily ions with spin 5/2. The average concentration of magnetic ions was estimated, n ≈ 2000 ppm. More details of analysis of experimental data are discussed in the paper.
4
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Experimental Study of Magneto-Structural Correlations in Low-Dimensional Quantum Magnets Cu(en)Cl₂ and Cu(tn)Cl₂

88%
Tarasenko R., Lederová L., Orendáčová A., Orendáč M., Feher A.
Acta Physica Polonica A
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2018
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vol. 133
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issue 3
420-422
EN
The present work is focused on the investigation of magneto-structural correlations in Cu(en)Cl₂ and Cu(tn)Cl₂. A comparative study of powder susceptibility and magnetization of both compounds revealed that the replacement of tn by en ligand did not affect single-ion properties. On the other hand, the structure modification led to significant reduction of magnetic interactions as well as lowering the crystal symmetry. The impact of the high pressure on magnetic properties of the compounds is discussed.
5
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Experimental Study of Magnetocaloric Effect in Dipolar Magnet KEr(MoO_4)_2

88%
Hanko J., Orendáč M., Kajňaková M., Orendáčová A., Feher A.
Acta Physica Polonica A
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2008
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vol. 113
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issue 1
465-468
EN
We report on magnetothermodynamic properties of single crystal KEr(MoO_4)_2 which were investigated from nominally 0.4 K to 20 K in magnetic fields up to 4 T. Using the available specific heat data the diagram of temperature and magnetic field dependence of the total entropy was constructed. Predictions for temperature dependences of the relative temperature variations during the adiabatic demagnetization and the entropy variation during the isothermal demagnetization were calculated from the specific heat data. The obtained results suggest that unlike common refrigerants, in KEr(MoO_4)_2 the quantities describing magnetic cooling remain nearly constant in the temperature range 2-10 K.
6
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Magnetocaloric Study of S=1/2 Low-Dimensional Heisenberg Antiferromagnet [Cu(phen)(H_2O)_2]SO_4

88%
Sedláková L., Orendáč M., Kajňaková M., Orendáčová A., Feher A.
Acta Physica Polonica A
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2008
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vol. 113
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issue 1
473-476
EN
We present the experimental study of magnetocaloric effect on powder sample [Cu(phen)(H_2O)_2]SO_4 (phen = phenantroline = C_{12}H_8N_2) in the temperature range from nominally 0.2 K to 4 K and in magnetic fields up to 2 T. The values of total entropy were extracted from the experimental data obtained by adiabatic demagnetization. The comparison was performed with the temperature dependence of the entropy for S=1/2 paramagnet calculated at selected magnetic fields. The deviations observed below 0.6 T were attributed to internal field mediated by exchange coupling. In addition, the temperature response observed in [Cu(phen)(H_2O)_2]SO_4 revealed very short relaxation time below 1 K. The comparison of the behavior observed at the end of the demagnetization in [Cu(phen)(H_2O)_2]SO_4 and other systems suggests that the magnetocaloric effect can serve as an alternative tool for studying slow magnetic relaxation.
7
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Study of Niobium Thin Films under Pressure

88%
Pristáš G., Gabáni S., Orendáč M., Komanický V., Gažo E.
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vol. 126
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issue 1
346-347
EN
Niobium is widely used in many important superconducting applications. At ambient pressure, bulk Nb has the highest critical temperature, T_c ≈ 9.25 K among the superconducting elements. Thin films of Nb show several differences in behavior in comparison with bulk Nb, e.g. substantial increase in the upper critical field (H_{C2}). Critical temperature of superconducting transition is usually lower for thin films than in bulk sample and depends on thickness of the film, size of grains etc. We prepared 100 nm thick niobium thin films in the high vacuum DC magnetron sputtering system, with T_c=8.95 K at ambient pressure. In this study, we performed measurements of superconducting transition temperature by electrical resistivity measurements of Nb thin film under hydrostatic pressure of up to 30 kbar. We observed an increase of T_c with increasing value of pressure (dT_c/dp=7.3 mK/kbar). On the other side in the case of bulk sample of Nb we observed a decrease of T_c value (dT_c/dp=-2.5 mK/kbar) with increasing applied pressure. Difference in superconducting properties between niobium bulk and thin film under pressure is discussed.
8
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Spin Glass State in S=3/2 Kagomé Antiferromagnet Co(N_{3})_{2}(bpg)[(CH_{3})_{2}(NCOH)]_{4/3}

88%
Vrábel P., Čižmár E., Orendáčová A., Gao S., Orendáč M.
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vol. 126
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issue 1
266-267
EN
Specific heat, magnetic susceptibility and magnetization of Co(N_{3})_{2}(bpg)[(CH_{3})_{2}(NCOH)]_{4/3}, representing S=3/2 kagomé system are reported. Long-range ordering at 16 K was revealed, however, at lower temperatures slow spin dynamics is still found. The analysis of alternating susceptibility suggests the onset of glassy state. The study of the time dependence of magnetization revealed the existence of more relaxation channels with pronounced different relaxation times. The observed behaviour is consistent with the formation of topological spin glass in which relaxation is governed by both spin and chiral degrees of freedom.
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Experimental Studies of S=1 Heisenberg Planar Antiferromagnetic Chain Ni(C_{11}H_{10}N_{2}O)_{2}Ni(CN)_{4}

88%
Čizmár E., Černák J., Orendáč M., Herrmannsdörfer T., Feher A.
Acta Physica Polonica A
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2000
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vol. 97
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issue 3
579-582
EN
In this paper we discuss the results of experimental studies of specific heat and magnetic susceptibility of chain compound Ni(C_{11}H_{10}N_{2}O)_{2}Ni(CN)_{4}. We have observed a Schottky type anomaly at T=2.2 K in a temperature dependence of specific heat and the compound was identified as a S=1 Heisenberg antiferromagnetic chain with the subcritical exchange coupling D/k_{B}=5 K, D/|J|=5.2 and the strong in-plane anisotropy E/k_{B}=2.8 K. It has been found that the in-plane anisotropy affects the region of validity of a diluted exciton model [1] and a strong coupling model [2] for S=1 Heisenberg chains with the strong planar anisotropy. The analysis suggests that the in-plane anisotropy should be considered in any attempt to find a compound potentially located in the boundary of Haldane and "large-D" phases.
10
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Slow Magnetic Relaxation in the Highly Anisotropic Layered Crystal CsNd(MoO₄)₂

88%
Tkáč V., Orendáčová A., Tarasenko R., Orendáč M., Feher A.
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EN
The present work is devoted to the magnetic relaxation study of the single crystal CsNd(MoO₄)₂ a layered rare-earth dimolybdate. AC susceptibility measurements performed in magnetic fields up to B=5 T applied along the easy and hard axis with frequency f ≈1 kHz revealed rather complex field-induced slow magnetic relaxation. Two relaxation regimes in different magnetic field intervals connected with magnetic interactions and possible single-ion relaxation mechanism were observed.
11
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Magneto-Structural Correlations in Cu(en)_2SO_4 - a new Magnet with Two-Dimensional Triangular Arrangement of Cu(II) ions

76%
Kajňaková M., Orendáčová A., Orendáč M., Feher A., Mal'arová M., Trávníček Z.
Acta Physica Polonica A
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2008
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vol. 113
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issue 1
507-510
EN
Cu(en)_2SO_4 (en = 1, 2-diaminoethane) was synthesized and its crystal structure determined. From a crystallographic point of view, the compound might represent an example for the realization of a S =1/2 Heisenberg magnet on a two-dimensional spatially anisotropic triangular lattice. Magnetic susceptibility and magnetization studies were performed to analyze the magneto-structural correlations and to characterize the magnetic subsystem of the present compound. The results indicate the presence of antiferromagnetic interactions between magnetic ions with zJ/k_B≈-14.6 K and short-range order near 7 K.
12
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Frustrated Zig-Zag Spin Chains Formed by Hydrogen Bonds in [Cu(H₂O)(OH)(tmen)]₂[Pd(CN)₄]·2H₂O

76%
Čižmár E., Orendáčová A., Orendáč M., Kuchár J., Feher A., Park J., Meisel M.
Acta Physica Polonica A
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2017
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vol. 131
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issue 4
940-942
EN
The magnetic properties of the novel dimeric compound [Cu(H₂O)(OH)(tmen)]₂[Pd(CN)₄]·2H₂O (tmen=N,N,N',N'-tetramethylethylenediamine) with modulated crystal structure were studied in the temperature range from 95 mK to 300 K. Magnetic measurements revealed a presence of weak antiferromagnetic exchange coupling in the compound. The temperature dependence of specific heat is characterized by the presence of a Schottky-like maximum at 0.47 K and a λ-anomaly at 0.28 K, indicating the formation of long-range order in the system. The comparison of the experimental data with theoretical predictions revealed the presence of antiferromagnetic intradimer exchange coupling J/k_{B}= -1.2 K and interdimer coupling of a similar strength mediated via hydrogen bonds between dimeric units forming a frustrated magnetic zig-zag chain structure.
13
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Magnetocaloric Effect in CsDy(MoO₄)₂

76%
Tarasenko R., Tkáč V., Orendáčová A., Orendáč M., Sechovský V., Feher A.
Acta Physica Polonica A
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2018
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vol. 133
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issue 3
414-416
EN
CsDy(MoO₄)₂ belongs to the family of binary alkaline rare-earth molybdates, with detectable low-temperature structural phase transitions, caused by the cooperative Jahn-Teller effect. Magnetocaloric studies of a single crystal of CsDy(MoO₄)₂ have been performed in the temperature range from 2 K to 18 K in magnetic fields up to 7 T applied along the crystallographic a axis. The maximum value of the isothermal entropy change is about 8 J/(kgK) and was achieved already in the fields above 2 T. The observed results suggest that the studied system can be used as a magnetic refrigerant at helium temperatures.
14
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Growth of Pt-Ni Nanoparticles of Different Composition using Electrodeposition and Characterization of Their Magnetic Properties

76%
Kožejová M., Hložná D., Hua Liu Y., Ráczová K., Čižmár E., Orendáč M., Komanický V.
Acta Physica Polonica A
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2017
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vol. 131
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issue 4
839-841
EN
We prepared Pt₃Ni and PtNi₃ nanoparticles of various sizes on conductive and atomically smooth highly oriented pyrolytic graphite surfaces using potentiostatic electrodeposition. We can control the size of electrodeposited nanoparticles and their density on the surface by changing the deposition time. The morphology of nanoparticles was determined by scanning electron microscopy. PtNi₃ particles have spherical shape, while Pt₃Ni particles have more irregular shape. Composition of particles was confirmed by energy dispersive spectroscopy. We have measured magnetic properties of both systems with 100 s preparation time, superparamagnetic behavior was observed in PtNi₃ nanoparticles with blocking temperature T_{B}=225 K.
15
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Experimental Study of the Magnetocaloric Effect in Ni(en)(H₂O)₄SO₄·2H₂O - An S = 1 Molecular Magnet with Easy-Plane Anisotropy

76%
Tarasenko R., Orendáčová A., Čižmár E., Orendáč M., Potočňák I., Feher A.
Acta Physica Polonica A
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2017
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vol. 131
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issue 4
904-906
EN
Magnetocaloric properties of Ni(en)(H₂O)₄SO₄·2H₂O powder were investigated in temperature range from 2 K to 30 K in magnetic fields up to 7 T using isothermal magnetization measurements. The maximum value of the isothermal entropy change in the field 7 T is about 8 J/(kg K), with a refrigerant capacity of 55 J/kg. Temperature dependence of the isothermal entropy change under different magnetic fields is in good agreement with theoretical predictions from crystal electric field parameters.
16
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Magnetic-Field Induced Slow Relaxation in the Ising-Like Quasi-One-Dimensional Ferromagnet KEr(MoO₄)₂

76%
Tkáč V., Orendáčová A., Dlháň L., Orendáč M., Boča R., Feher A.
Acta Physica Polonica A
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2017
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vol. 131
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issue 4
893-895
EN
We present the study of spin dynamics of KEr(MoO₄)₂ in the magnetic field applied along the hard axis c. The temperature dependence of AC susceptibility in zero magnetic field studied at frequencies f=10, 100, and 1000 Hz indicated the absence of relaxation in the temperature range from 2 to 20 K. Application of magnetic field induced a slow magnetic relaxation, which was investigated in detail in the field 0.5 T. The highest intensity of the relaxation process, reflected by the values of imaginary susceptibility was observed at 2 K. With increasing temperature, the relaxation process is weaker and vanishes completely above 3.5 K. Corresponding Cole-Cole diagrams were constructed and analyzed within a single relaxation process which can be associated with a direct relaxation process with a bottleneck effect, τ ≈1/T^b, and b=1.4. The slow relaxation at 2 K intensifies with increasing magnetic field at least up to 1 T.
17
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EPR Study of the Two-Dimensional Quantum System Cu(en)(H_2O)_2SO_4

76%
Tarasenko R., Orendáčová A., Čižmár E., Orendáč M., Zvyagin S., Wosnitza J.
Acta Physica Polonica A
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2012
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vol. 121
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issue 5-6
1095-1097
EN
The angular dependence of electron paramagnetic resonance spectra of Cu(en)(H_2O)_2SO_4 single crystals was studied in the X-band frequency range at temperatures 4 and 300 K. Analysis of the linewidth at 300 K revealed nice agreement with the angular variation of the g-factor. This coincidence is the manifestation of the symmetric and antisymmetric exchange coupling, as main broadening mechanisms in Cu(en)(H_2O)_2SO_4 at high temperatures. The radical change of the angular dependence of the linewidth observed at 4 K can be ascribed to dipolar coupling.
18
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The Rare-earth Based Single-ion Magnet CsNd(MoO_{4})_{2}

76%
Tkáč V., Tibenská K., Orendáčová A., Orendáč M., Anders A., Pavlík V., Feher A.
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vol. 126
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issue 1
244-245
EN
Specific heat, magnetization and DC susceptibility of the single crystal CsNd(MoO_{4})_{2},a layered rare-earth dimolybdate, have been investigated nominally, in the temperature range from 100 mK to 300 K in the magnetic field up to 5 T, applied along the a axis. The analysis of the experimental data revealed the absence of a phase transition to the magnetic ordered state down to 100 mK. The application of a standard two-level model yielded an excellent agreement with the specific heat data above 2 K in nonzero magnetic field indicating a weakness of magnetic correlations and a predominant occupation of the ground-energy doublet. The latter indicates a large energy separation between the ground and first excited doublet. These measurements suggest that CsNd(MoO_{4})_{2} can represent a good realization of a single-ion magnet.
19
Content available remote

Thermal Conductivity of a Layered CsGd(MoO_4)_2 Crystal

76%
Tibenská K., Tkáč V., Orendáčová A., Orendáč M., Feher A., Šebek J., Sechovský V.
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issue 5
971-972
EN
The thermal conductivity of CsGd(MoO_4)_2 has been studied in the temperature range from 2 to 50 K in zero magnetic field. The analysis of the data performed within the Debye model with the relaxation-time approximation revealed the presence of the scattering of phonons by critical fluctuations. The behaviour of phonon mean free path at the lowest temperatures is discussed.
20
Content available remote

Experimental Study of the Magnetocaloric Effect in the Two-Dimensional Quantum System Cu(tn)Cl_2

76%
Baranová L., Tarasenko R., Orendáčová A., Orendáč M., Pavlík V., Feher A.
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vol. 126
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issue 1
256-257
EN
Magnetocaloric studies of a two-dimensional antiferromagnet Cu(tn)Cl_{2} (tn=1,3-diaminopropane=C_3H_{10}N_2) have been performed by adiabatic magnetization and demagnetization measurements, in the temperature range from 0.2 to 4 K and magnetic fields up to 2 T. The compound represents an S=1/2 spatially anisotropic triangular-lattice antiferromagnet. The magnetocaloric measurements were focused at the identification of the phase transition to the magnetically ordered state which was not indicated in the previous specific heat studies. Furthermore, the interplay of the magnetic-field induced easy-plane anisotropy and the intrinsic spin anisotropy present in the studied system should manifest in low magnetic fields. The obtained results of the magnetocaloric experiments of Cu(tn)Cl_{2} indicate a double crossover from the normal to inverse magnetocaloric effect (MCE). The first crossover from the normal to inverse MCE occurring at about 0.3 K can be attributed to the competition of the aforementioned anisotropies. The second crossover from the inverse to normal MCE observed at about 2.2 K might be ascribed to the formation of spin vortices stabilized by the easy-plane anisotropy introduced by magnetic field.
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