Search

Binary intermetallic ${\mathrm{Eu}}_{2}\mathrm{In}$ was recently reported to exhibit a giant anhysteretic magnetocaloric effect due to a first-order magnetic phase transition between paramagnetic and ferromagnetic states. Experimentally, the transition occurs with a small phase volume change, $\mathrm{\ensuremath{\Delta}}V/V$, of approximately 0.1% around ${T}_{C}$ of $ca.$ 55 K. We represent magnetic and compute magnetocaloric properties of a ${\mathrm{Eu}}_{2}\mathrm{In}$ compound using a microscopic description based on a model Hamiltonian that takes into account magnetic exchange and magnetoelastic interactions. In the model the thermodynamic nature of the transition is conveniently represented by a single magnetoelastic interaction parameter. A good agreement between the theoretical results and earlier published experimental data confirms the effectiveness of our approach.

While binary $R{E}_{2}\mathrm{In}$, where $RE=\mathrm{rare}\phantom{\rule{0.16em}{0ex}}\mathrm{earth}$, have been reported a few decades ago, recent investigations revealed intriguing new physical insights. For instance, the discovery of a nearly ideal first-order ferromagnetic transition in ${\mathrm{Eu}}_{2}\mathrm{In}$ calls for further exploration of structures and properties of $R{E}_{2}\mathrm{In}$, in particular for the least-documented $RE=\mathrm{Eu}$ and Yb cases. Here, we investigate ${\mathrm{Eu}}_{2\text{\ensuremath{-}}x}{\mathrm{Yb}}_{x}\mathrm{In}$ pseudobinaries with nominal values of $x=0.25$, 0.5, 0.75, 1, 1.5, 2 by powder x-ray diffraction (including as function of temperature from 100 to 375 K for ${\mathrm{Yb}}_{2}\mathrm{In}$), magnetization (5--300 K), as well as electrical resistivity (5--300 K) and calorimetric (2--150 K) measurements for ${\mathrm{Yb}}_{2}\mathrm{In}$. Compared to other RE, Yb or Eu always raise challenging questions linked to their valence states. From average atomic volume, Yb is anticipated to be divalent in ${\mathrm{Yb}}_{2}\mathrm{In}$, at least between 100 and 375 K, which is in line with the absence of $4f$ magnetism. In agreement with x-ray diffraction and magnetization data, the resistivity of ${\mathrm{Yb}}_{2}\mathrm{In}$ is rather featureless and typical of a metal. Establishing ${\mathrm{Yb}}_{2}\mathrm{In}$ as a nonmagnetic isostructural reference for ${\mathrm{Eu}}_{2}\mathrm{In}$ allows one to use its heat capacity to revisit that of the latter, and get experimental insights into the exceptional magnetocaloric effect of the compound with Eu. In particular, we show that a third of the total magnetic entropy (${S}_{\mathrm{m}}\ensuremath{\approx}35.6\phantom{\rule{0.16em}{0ex}}\mathrm{J}\phantom{\rule{0.16em}{0ex}}\mathrm{mo}{\mathrm{l}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ at $T=100\phantom{\rule{0.16em}{0ex}}\mathrm{K}$) is concentrated in a 3 K temperature window around the ${T}_{\mathrm{C}}$ of ${\mathrm{Eu}}_{2}\mathrm{In}$. Starting from the ferromagnetic compound ${\mathrm{Eu}}_{2}\mathrm{In}$ $[{T}_{\mathrm{C}}=55.2(5)\phantom{\rule{0.16em}{0ex}}\mathrm{K}]$, we show that Yb substitutions in ${\mathrm{Eu}}_{2\text{\ensuremath{-}}x}{\mathrm{Yb}}_{x}\mathrm{In}$ lead to a decrease in both the Curie temperature [${T}_{\mathrm{C}}=41(2)$ and 32(2) K for $x=0.25$ and 0.5] and magnetic saturation, while weakening the first-order character of the transition as $x$ increases. A significant isothermal entropy change of $5.1(4)\phantom{\rule{0.16em}{0ex}}\mathrm{J}\phantom{\rule{0.16em}{0ex}}\mathrm{mo}{\mathrm{l}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ for $\mathrm{\ensuremath{\Delta}}B=2\phantom{\rule{0.16em}{0ex}}\mathrm{T}$ is found at 44 K in ${\mathrm{Eu}}_{1.75}{\mathrm{Yb}}_{0.25}\mathrm{In}$, demonstrating that the giant magnetocaloric effect of ${\mathrm{Eu}}_{2}\mathrm{In}$ can be tuned to lower temperatures by Yb substitutions.

Using first-principles calculations, based on disordered local moment (DLM) theory combined with the self-interaction corrected local spin density approximation (SIC-LSDA), we study magnetic correlations in the paramagnetic state of $\mathrm{Gd}X\phantom{\rule{4pt}{0ex}}(X$=Cu, Zn, Ga, Ag, Cd, In, Au, Hg, and Tl) intermetallics and their alloys. The predicted magnetic orders and ordering temperatures that these correlations lead to are in overall good agreement with available experiments. The interactions between the Gd $f$-electron local moments are mediated by the valence electrons of the intermetallics which comprise both Gd and $X\phantom{\rule{4pt}{0ex}}d$ bands as well as $sp$ bands. There are RKKY-like features such as dependence on the number of $sp$-valence electrons but other variations manifest themselves in the phase diagrams as regions of incommensurate magnetic ordering, the origin and range of which are related to the binding energies of the alloying anion $d$ states, and their propensity to hybridize with the Gd states at the Fermi level.

The functionality of many magnetic materials critically depends on first manipulating and then taking advantage of highly nonlinear changes of properties that occur during phase transformations. Unique to lanthanides, property-defining $4f$ electrons are highly localized and, as commonly accepted, play little to no role in chemical bonding. Yet here we demonstrate that the competition between $4f$-electron energy landscapes of Dy $(4{f}^{9})$ and Er $(4{f}^{11})$ is the key element of the puzzle required to explain complex interplay of magnetic and structural features observed in $\mathrm{E}{\mathrm{r}}_{1\ensuremath{-}x}\mathrm{D}{\mathrm{y}}_{x}\mathrm{C}{\mathrm{o}}_{2}$, and likely many other mixed lanthanide systems. Unlike the parent binaries---$\mathrm{DyC}{\mathrm{o}}_{2}$ and $\mathrm{ErC}{\mathrm{o}}_{2}$---$\mathrm{E}{\mathrm{r}}_{1\ensuremath{-}x}\mathrm{D}{\mathrm{y}}_{x}\mathrm{C}{\mathrm{o}}_{2}$ exhibits two successive magnetostructural transitions: a first order at ${T}_{\mathrm{C}}$, followed by a second order in the ferrimagnetically ordered state. Supported by first-principles calculations, our results offer new opportunities for targeted design of magnetic materials with multiple functionalities, and also provide a critical insight into the role of $4f$ electrons in controlling the magnetism and structure of lanthanide intermetallics.

Rare-earth intermetallics play a critical yet often obscure role in numerous technological applications, including sensors, actuators, permanent magnets, and rechargeable batteries; therefore, understanding their basic science is of utmost importance. Here we report structural behaviors, specific heat, and magnetism of $\mathrm{P}{\mathrm{r}}_{1\text{--}x}\mathrm{E}{\mathrm{r}}_{x}\mathrm{A}{\mathrm{l}}_{2}$ studied by means of temperature-dependent x-ray powder diffraction, heat capacity, and magnetization measurements, in addition to first-principles calculations. Although the cubic lattice of $\mathrm{PrA}{\mathrm{l}}_{2}$ distorts tetragonally at the Curie temperature ${T}_{C}$, the distortion is rhombohedral in $\mathrm{ErA}{\mathrm{l}}_{2}$, creating a potential for instability in the pseudobinary $\mathrm{PrA}{\mathrm{l}}_{2}\text{\ensuremath{-}}\mathrm{ErA}{\mathrm{l}}_{2}$ system. When $0.05\ensuremath{\le}x\ensuremath{\le}0.5$, materials show complex magnetization behaviors, including metamagnetic transitions and Griffith-like phase. Unique among other mixed-lanthanide dialuminides, the substitution of Er for Pr in $\mathrm{P}{\mathrm{r}}_{1\text{--}x}\mathrm{E}{\mathrm{r}}_{x}\mathrm{A}{\mathrm{l}}_{2}$ results in unexpected ferrimagnetic behavior, and the ferrimagnetic interactions become strongest around $x=0.25$, where the compound shows unusual metamagnetic like transitions observed only in the odd-numbered quadrants of the full magnetic field cycles. The electronic structure calculations, including exchange interactions and crystal field splitting, magnetic moments, anisotropic $4f$ energy density, and magnetic surface potentials rationalize the interesting physics observed experimentally.

Magnetization of DyFe${}_{4}$Ge${}_{2}$ measured as function of temperature in a 1 kOe magnetic field indicates antiferromagnetic (AFM) ordering at ${T}_{\mathrm{N}}$ $=$ 62 K followed by two spin reorientation transitions at ${T}_{f}$${}_{1}$ $=$ 52 and ${T}_{f}$${}_{2}$ $=$ 32 K and one unusual anomaly at 15 K (${T}_{f}$${}_{3}$). Three transitions (${T}_{f}$${}_{1}$, ${T}_{f}$${}_{2}$, and ${T}_{\mathrm{N}}$) are further confirmed by heat capacity measurement in a zero magnetic field. The two low-temperature magnetic transitions are broadened and gradually vanish when the applied magnetic field exceeds 30 kOe, and the AFM transition shifts toward low temperatures with an increasing magnetic field. The reentrant magnetic glassy state is observed below the freezing point of ${T}_{f}$${}_{3}$ $=$ 15 K. Two field-induced metamagnetic phase transitions are observed between 2 and 50 K in fields below 140 kOe. A temperature-magnetic-field phase diagram has been constructed. The first-principles electronic structure calculations show that the paramagnetic tetragonal structure of DyFe${}_{4}$Ge${}_{2}$ is stable at high temperatures. The calculations with collinear Dy spins confirm ferrimagnetic orthorhombic DyFe${}_{4}$Ge${}_{2}$ as the ground-state structure.

The magnetization of the oriented Er${}_{5}$Si${}_{4}$ single crystal, measured along the three principal crystallographic directions, reveals strong magnetocrystalline anisotropy. The $b$ axis is the easy magnetization direction. The possible presence of the crystal-field effect and the noncollinear alignment of magnetic moments result in a lower than $gJ$ magnetization along all crystallographic directions, even in a 70-kOe applied magnetic field, with the lowest moment (4.22 $\ensuremath{\mu}$${}_{B}/$Er${}^{3+}$) recorded along the $a$ axis. The magnetization measurements show that even in the true paramagnetic state there is a weak magnetic field dependence of the structural-only transition when the field is applied along the $a$ and $c$ axes, but this transition is magnetic field independent along the $b$ axis in fields of 70 kOe or less. The temperature- and magnetic-field-dependent x-ray powder diffraction study of the powdered single crystal confirms the temperature-driven structural orthorhombic-monoclinic transition in the paramagnetic state and the low-temperature magnetic-field-driven monoclinic-orthorhombic transition in the magnetically ordered state. The x-ray powder diffraction indicates that the high-temperature transition is magnetic field independent below 40 kOe in a polycrystalline sample while the low-temperature transition requires a high magnetic field for its completion.

The composition- and pressure-dependent magnetostructural properties of Tb{sub 5}(Si{sub x}Ge{sub 1-x}){sub 4} (x = 0.4, 0.485, 0.625, and 0.7) were investigated using x-ray powder diffraction and x-ray magnetic circular dichroism in a diamond anvil cell, respectively. Substituting the smaller-size Si for Ge stabilizes a single-phase, ferromagnetic (FM) orthorhombic O(I) structure for x ? 0.7. Similarly, application of external pressure causes a canted antiferromagnetic orthorhombic O(II) sample (x = 0.4) to transform into an FM O(I) phase at 4 GPa. The element- and orbital-specific x-ray absorption data indicate that the Tb 4f orbital occupation changes with external pressure, likely through 4f-5d electronic mixing, yet no changes in Tb 4f electronic structure are observed with Si doping. The results point to different mechanisms behind the enhancement of FM exchange interactions in Tb{sub 5}(Si{sub x}Ge{sub 1-x}){sub 4} with chemical and applied pressure, respectively.

When the complexity of a metallic compound reaches a certain level, a specific location in the structure may be critically responsible for a given fundamental property of a material while other locations may not play as much of a role in determining such a property. The first-principles theory has pinpointed a critical location in the framework of a complex intermetallic compound---${\mathrm{Gd}}_{5}{\mathrm{Ge}}_{4}$---that resulted in a controlled alteration of the magnetism of this compound using precise chemical tools.

$L{\text{V}}_{4}{\text{O}}_{8}(L=\text{Yb},\text{Y},\text{Lu})$ compounds are reported to crystallize in a structure similar to that of the orthorhombic ${\text{CaFe}}_{2}{\text{O}}_{4}$ structure type and contain four inequivalent V sites arranged in zigzag chains. We confirm the structure and report the magnetic, thermal, and transport properties of polycrystalline ${\text{YV}}_{4}{\text{O}}_{8}$ and ${\text{LuV}}_{4}{\text{O}}_{8}$. A first-order-like phase transition is observed at 50 K in both ${\text{YV}}_{4}{\text{O}}_{8}$ and ${\text{LuV}}_{4}{\text{O}}_{8}$. The symmetry remains the same with the lattice parameters changing discontinuously. The structural transition in ${\text{YV}}_{4}{\text{O}}_{8}$ leads to partial dimerization of the V atoms resulting in a sudden sharp drop in the magnetic susceptibility. The V spins that do not form dimers order in a canted antiferromagnetic state. The magnetic susceptibility of ${\text{LuV}}_{4}{\text{O}}_{8}$ shows a sharp peak at $\ensuremath{\sim}50\text{ }\text{K}$. The magnetic entropies calculated from heat capacity versus temperature measurements indicate bulk magnetic transitions below 90 K for both ${\text{YV}}_{4}{\text{O}}_{8}$ and ${\text{LuV}}_{4}{\text{O}}_{8}$.

Magnetic and crystallographic properties of ${\text{Gd}}_{5}{\text{Sb}}_{0.5}{\text{Ge}}_{3.5}$ were investigated using dc magnetization, ac magnetic susceptibility, and heat capacity of an oriented single crystal, combined with temperature and magnetic field dependent x-ray powder diffraction. The compound undergoes an unusual magnetostructural transition at 40 K and a nonmagnetic second-order transition around 63 K. The detailed crystallographic study of ${\text{Gd}}_{5}{\text{Sb}}_{0.5}{\text{Ge}}_{3.5}$ shows that contrary to the ${R}_{5}{({\text{Si}}_{x}{\text{Ge}}_{1\ensuremath{-}x})}_{4}$ systems ($R$ is a rare-earth metal), the structural transition occurs without shear displacements of the $_{\ensuremath{\infty}}^{2}[{R}_{5}{T}_{4}]$ slabs ($T=\text{Si}$, Ge, and Sb), and a substantial volume change $(\ensuremath{-}0.5%)$ does not lead to a change in crystallographic symmetry. The first-principles electronic structure calculations show higher interslab than intraslab ferromagnetic exchange interaction indicating that ${\text{Sm}}_{5}{\text{Ge}}_{4}$ type of structure supports a ferromagnetic ground state in ${\text{Gd}}_{5}{\text{Sb}}_{0.5}{\text{Ge}}_{3.5}$.

The metamagnetic-like transitions and giant magnetocaloric effect were observed with the magnetic field applied parallel to the $a$ and $c$ axes, but not the $b$ axis in a ${\mathrm{Tb}}_{5}{\mathrm{Si}}_{2.2}{\mathrm{Ge}}_{1.8}$ single crystal. The in situ x-ray powder diffraction study indicates that these metamagnetic-like transitions are coupled to crystallographic phase transformations occurring via strong magnetoelastic interactions. The magnetocrystalline anisotropy plays an important role in this system. Magnetic fields less than $40\phantom{\rule{0.3em}{0ex}}\mathrm{kOe}$ cannot drive either the magnetic or the crystallographic phase transition to completion for ${\mathrm{Tb}}_{5}{\mathrm{Si}}_{2.2}{\mathrm{Ge}}_{1.8}$ powder due to the strong single ion anisotropy of Tb.

The atomic scale details of the pressure-induced polymorphism of Gd{sub 5}Si{sub 2}Ge{sub 2} have been established by in situ x-ray powder diffraction. At room temperature, the monoclinic Gd{sub 5}Si{sub 2}Ge{sub 2} phase ({beta}) is transformed to the orthorhombic {alpha}-Gd{sub 5}Si{sub 2}Ge{sub 2}, observed previously as the low temperature, high magnetic field, or high silicon content polymorph. The transition occurs between {approx}10 kbar and {approx}20 kbar. Diffraction data provide the missing link in order to achieve a more complete understanding of how a structural change in a material can be induced by a variety of thermodynamic variables.