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Register a new userIsing-type Magnetic Anisotropy in CePd$_2$As$_2$
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Ajeesh Mukkattu Omanakuttan
Publication date
2017
fields
Physics
and research's language is
English
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We investigated the anisotropic magnetic properties of CePd$_2$As$_2$ by magnetic, thermal and electrical transport studies. X-ray diffraction confirmed the tetragonal ThCr$_2$Si$_2$-type structure and the high-quality of the single crystals. Magnetisation and magnetic susceptibility data taken along the different crystallographic directions evidence a huge crystalline electric field (CEF) induced Ising-type magneto-crystalline anisotropy with a large $c$-axis moment and a small in-plane moment at low temperature. A detailed CEF analysis based on the magnetic susceptibility data indicates an almost pure $langlepm5/2 rvert$ CEF ground-state doublet with the dominantly $langlepm3/2 rvert$ and the $langlepm1/2 rvert$ doublets at 290 K and 330 K, respectively. At low temperature, we observe a uniaxial antiferromagnetic (AFM) transition at $T_N=14.7$ K with the crystallographic $c$-direction being the magnetic easy-axis. The magnetic entropy gain up to $T_N$ reaches almost $Rln2$ indicating localised $4f$-electron magnetism without significant Kondo-type interactions. Below $T_N$, the application of a magnetic field along the $c$-axis induces a metamagnetic transition from the AFM to a field-polarised phase at $mu_0H_{c0}=0.95$ T, exhibiting a text-book example of a spin-flip transition as anticipated for an Ising-type AFM.
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We studied the physical properties of two Kondo-lattice compounds, CeRu$_2$As$_2$ and CeIr$_2$As$_2$, by a combination of electric transport, magnetic and thermodynamic measurements. They are of ThCr$_2$Si$_2$-type and CaBe$_2$Ge$_2$-type crystalline structures, respectively. CeRu$_2$As$_2$ shows localized long-range antiferromagnetic ordering below $T_N$=4.3 K, with a moderate electronic Sommerfeld coefficient $gamma_0$=35 mJ/mol$cdot$K$^2$. A field-induced metamagnetic transition is observed near 2 T below $T_N$. Magnetic susceptibility measurements on aligned CeRu$_2$As$_2$ powders suggest that it has an easy axis and that the cerium moments align uniaxially along $mathbf{c}$ axis. In contrast, CeIr$_2$As$_2$ is a magnetically nonordered heavy-fermion metal with enhanced $gamma_0$$>$300 mJ/mol$cdot$K$^2$. The initial onset Kondo temperatures of the two compounds are respectively 6 K and 30 K. We discuss the role of the crystal structure to the strength of Kondo coupling. This work provides two new dense Kondo-lattice materials for further investigations on electronic correlation, quantum criticality and heavy-electron effects.
A series of Sr(Co$_{1-x}$Ni$_x$)$_2$As$_2$ single crystals was synthesized allowing a comprehensive phase diagram with respect to field, temperature, and chemical substitution to be established. Our neutron diffraction experiments revealed a helimagnetic order with magnetic moments ferromagnetically (FM) aligned in the $ab$ plane and a helimagnetic wavevector of $q=(0,0,0.56)$ for $x$ = 0.1. The combination of neutron diffraction and angle-resolved photoemission spectroscopy (ARPES) measurements show that the tuning of a flat band with $d_{x^2-y^2}$ orbital character drives the helimagnetism and indicates the possibility of a quantum order-by-disorder mechanism.
The correlated electron material CePd$_2$P$_2$ crystallizes in the ThCr$_2$Si$_2$ structure and orders ferromagnetically at 29 K. Lai et al. [Phys. Rev. B 97, 224406 (2018)] found evidence for a ferromagnetic quantum critical point induced by chemical compression via substitution of Ni for Pd. However, disorder effects due to the chemical substitution interfere with a simple analysis of the possible critical behavior. In the present work, we examine the temperature - pressure - magnetic field phase diagram of single crystalline CePd$_2$P$_2$ to 25 GPa using a combination of resistivity, magnetic susceptibility, and x-ray diffraction measurements. We find that the ferromagnetism appears to be destroyed near 12 GPa, without any change in the crystal structure.
Spin wave dispersion in the frustrated fcc type-III antiferromagnet MnS$_2$ has been determined by inelastic neutron scattering using a triple-axis spectrometer. Existence of multiple spin wave branches, with significant separation between high-energy and low-energy modes highlighting the intrinsic magnetic frustration effect on the fcc lattice, is explained in terms of a spin wave analysis carried out for the antiferromagnetic Heisenberg model for this $S=5/2$ system with nearest and next-nearest-neighbor exchange interactions. Comparison of the calculated dispersion with spin wave measurement also reveals small suppression of magnetic frustration resulting from reduced exchange interaction between frustrated spins, possibly arising from anisotropic deformation of the cubic structure.
The ternary intermetallic compound Gd$_2$Cu$_2$In crystallizes in Mo$_2$Fe$_2$B type structure with the space group $P4/mbm$ and we study critical behaviour and magnetocaloric effect near the ferromagnetic transition ($T_C$ $approx$ 94 K) using the magnetic and heat capacity measurements. The maximum entropy change ($Delta S_m$) and adiabatic temperature change ($Delta T_{ad}$) for the field value of 7 T were observed to be 13.8 J/kg.K and 6.5 K respectively. We have employed modified Arrott plot (MAP), Kouvel-Fisher (KF) procedures to estimate the critical exponents near the FM-PM phase transition. Critical exponents $beta$ = 0.312(2), $gamma$ = 1.080(5) are self-consistently estimated from the non-linear fitting. The $beta$ value is close to the three dimensional (3D) Ising model where as $gamma$ and $delta$ values are close to mean field model. The estimated critical exponents for Gd$_2$Cu$_2$In suggest that the system may belong to different universal class. All the three critical exponent obey Widom scale and collapse the scaled magnetic isotherms into two distinct branches below and above $T_C$ in accordance with single scaling equation. Specific heat measurements show a $lambda$ type peak near 94 K confirming the bulk magnetic ordering. The data near $T_C$ was fitted using the non-linear function $C_{P} = B + Cepsilon + A^{pm}|epsilon|^{-alpha}(1 + E^{pm} |epsilon|^{0.5})$ between -0.025$<epsilon<$0.025 which yielded the fourth critical exponent $alpha$ value to be 0.11 (3). The value indicates possible 3D-Ising behavior where Gd$^{3+}$ moments arranged uniaxially along long tetragonel axis c as reported in literature.
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