Complex Magnetism of Lanthanide Intermetallics and the Role of their Valence Electrons: Ab Initio Theory and Experiment.

We explain a profound complexity of magnetic interactions of some technologically relevant gadolinium intermetallics using an ab initio electronic structure theory which includes disordered local moments and strong f-electron correlations. The theory correctly finds GdZn and GdCd to be simple ferromagnets and predicts a remarkably large increase of Curie temperature with a pressure of +1.5 K kbar(-1) for GdCd confirmed by our experimental measurements of +1.6 K kbar(-1). Moreover, we find the origin of a ferromagnetic-antiferromagnetic competition in GdMg manifested by noncollinear, canted magnetic order at low temperatures. Replacing 35% of the Mg atoms with Zn removes this transition, in excellent agreement with long-standing experimental data.

Anomalous Schottky specific heat and structural distortion in ferromagnetic PrAl2.

Unique from other rare earth dialuminides, PrAl(2) undergoes a cubic to tetragonal distortion below T = 30 K in a zero magnetic field, but the system recovers its cubic symmetry upon the application of an external magnetic field of 10 kOe via a lifting of the 4f crystal field splitting. The nuclear Schottky specific heat in PrAl(2) is anomalously high compared to that of pure Pr metal. First principles calculations reveal that the 4f crystal field splitting in the tetragonally distorted phase of PrAl(2) underpins the observed unusual low temperature phenomena.

Controlling magnetism of a complex metallic system using atomic individualism.

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--Gd(5)Ge(4)--that resulted in a controlled alteration of the magnetism of this compound using precise chemical tools.

Magnetostructural behavior in the non-centrosymmetric compound Nd7Pd3.

A systematic study of the physical properties and microscopic magnetism of Nd7Pd3 compound, which in the paramagnetic state crystallizes in the non-centrosymmetric hexagonal Th7Fe3-type structure (hP20-P63 mc; with a = 10.1367(1) Å and c = 6.3847(1) Å at 300 K), confirms multiple magnetic ordering transitions that occur upon cooling. Antiferromagnetic transition is observed at T N = 37 K, which is followed by ferromagnetic transformation at T C = 33 K. The first-order magnetic transition at T C is magnetoelastic: it involves a change of crystal symmetry from P63 mc to Cmc21 and leads to anisotropic changes of the unit cell parameters. While the antiferromagnetic structure is symmetry allowed in P63 mc, the ferromagnetic structure with magnetic moments along the a-direction of the original hexagonal unit cell induces the first order transition to Cmc21. Density functional theory calculations confirm the experimentally observed ground state with the a-axis as the easy magnetization direction.

Non-hysteretic first-order phase transition with large latent heat and giant low-field magnetocaloric effect.

First-order magnetic transitions (FOMTs) with a large discontinuity in magnetization are highly sought in the development of advanced functional magnetic materials. Isosymmetric magnetoelastic FOMTs that do not perturb crystal symmetry are especially rare, and only a handful of material families, almost exclusively transition metal-based, are known to exhibit them. Yet, here we report a surprising isosymmetric FOMT in a rare-earth intermetallic, Eu2In. What makes this transition in Eu2In even more remarkable is that it is associated with a large latent heat and an exceptionally high magnetocaloric effect in low magnetic fields, but with tiny lattice discontinuities and negligible hysteresis. An active role of the Eu-5d and In-4p states and a rather unique electronic structure borne by In to Eu charge transfer, altogether result in an unusual exchange mechanism that both sets the transition in motion and unveils an approach toward developing specific magnetic functionalities ad libitum.

Experimental investigation of the electronic structure of Gd(5)Ge(2)Si(2) by photoemission and x-ray absorption spectroscopy.

The electronic structure of the magnetic refrigerant Gd(5)Ge(2)Si(2) has been experimentally investigated by photoemission and x-ray absorption spectroscopy. The resonant photoemission and x-ray absorption measurements performed across the Gd N(4,5) and Gd M(4,5) edges identify the position of Gd 4f multiplet lines, and assess the 4f occupancy (4f(7)) and the character of the states close to the Fermi edge. The presence of Gd 5d states in the valence band suggests that an indirect 5d exchange mechanism underlies the magnetic interactions between Gd 4f moments in Gd(5)Ge(2)Si(2). From 175 to 300 K the first 4 eV of the valence band and the Gd partial density of states do not display clear variations. A significant change is instead detected in the photoemission spectra at higher binding energy, around 5.5 eV, likely associated to the variation of the bonding and antibonding Ge(Si) s bands across the phase transition.

Crystal, magnetic, calorimetric and electronic structure investigation of GdScGe1-x Sb x compounds.

Experimental investigations of crystal structure, magnetism and heat capacity of compounds in the pseudoternary GdScGe-GdScSb system combined with density functional theory projections have been employed to clarify the interplay between the crystal structure and magnetism in this series of RTX materials (R = rare-earth, [Formula: see text] = transition metal and X = p-block element). We demonstrate that the CeScSi-type structure adopted by GdScGe and CeFeSi-type structure adopted by GdScSb coexist over a limited range of compositions [Formula: see text]. Antimony for Ge substitutions in GdScGe result in an anisotropic expansion of the unit cell of the parent that is most pronounced along the c axis. We believe that such expansion acts as the driving force for the instability of the double layer CeScSi-type structure of the parent germanide. Extensive, yet limited Sb substitutions [Formula: see text] lead to a strong reduction of the Curie temperature compared to the GdScGe parent, but without affecting the saturation magnetization. With a further increase in Sb content, the first compositions showing the presence of the CeFeSi-type structure of the antimonide, [Formula: see text], coincide with the appearance of an antiferromagnetic phase. The application of a finite magnetic field reveals a jump in magnetization toward a fully saturated ferromagnetic state. This antiferro-ferromagnetic transformation is not associated with a sizeable latent heat, as confirmed by heat capacity measurements. The electronic structure calculations for [Formula: see text] indicate that the key factor in the conversion from the ferromagnetic CeScSi-type to the antiferromagnetic CeFeSi-type structure is the disappearance of the induced magnetic moments on Sc. For the parent antimonide, heat capacity measurements indicate an additional transition below the main antiferromagnetic transition.

Magnetic structures of R5Ni2In4 and R11Ni4In9 (R = Tb and Ho): strong hierarchy in the temperature dependence of the magnetic ordering in the multiple rare-earth sublattices.

The magnetic properties and magnetic structures of the R 5Ni2In4 and the microfibrous R 11Ni4In9 compounds with R = Tb and Ho have been examined using magnetization, heat capacity, and neutron diffraction data. Rare earth atoms occupy three and five symmetrically inequivalent rare earth sites in R 5Ni2In4 and R 11Ni4In9 compounds, respectively. As a result of the intra- and inter-magnetic sublattice interactions, the magnetic exchange interactions are different for various rare earth sites; this leads to a cascade of magnetic transitions with a strong hierarchy in the temperature dependence of the magnetic orderings. A transition at T C = 125 K in Tb5Ni2In4 [κ 1 = (0, 0, 0)] leads to a ferro/ferrimagnetic order where the magnetic ordering in one of the three R-sublattices leads to the ordering of another one; the third sublattice stays non-magnetic. New magnetic Bragg peaks appearing below T N = 20 K can be indexed with the incommensurate magnetic propagation vector κ 2 = (0, 0.636, ½); at T N = 20 K a cycloidal spin order, which acts mostly upon the third R-sublattice, occurs. Ho5Ni2In4 establishes first antiferromagnetism [κ = (0, 0, 0)] at T N = 31 K on two R-sublattices; then the system becomes ferro/ferrimagnetic at T C = 25 K with the third sublattice ordering as well. Tb11Ni4In9 has three magnetic transitions at T C = 135 K, T N1 = 35 K and at T N2 = 20 K; they are respectively coupled to the appearance of different propagation vectors [κ 1 = (0, 0, 0), κ 2 = (0, 0, ½), κ 3 = (0, 1, ½)], which themselves are operating differently on the five different R-sublattices. Two sublattices remain mostly ferromagnetic down to lowest temperature while the three others are predominantly coupled antiferromagnetically. In Ho11Ni4In9 a purely antiferromagnetic order, described by four different magnetic propagation vectors [κ 1 = (0, 0.62, 0), κ 2 = (0, 1, 0), κ 3 = (0, 0, ½), κ 4 = (0, 1, ½)], succeedingly includes all five different sublattices on cooling through transitions at T N1 = 22 K, T N2 = 12 K, T N3 = 8 K and T N4 = 7 K. The strength of the magnetic interactions of the different sublattices can be linked to structural details for both R 5Ni2In4 and R 11Ni4In9 compounds.

Magnetic and magnetothermal properties, and the magnetic phase diagram of single-crystal holmium along the easy magnetization direction.

The magnetic and magnetothermal properties of holmium single crystal have been investigated from 4.2 to 300 K in magnetic fields up to 100 kOe using magnetization and heat capacity data measured along the easy magnetization direction, which is the crystallographic b-axis, i.e. [112¯0] direction. The magnetic phase diagram of Ho has been refined by examining data measured using a high purity single crystal.

Spin-glass behavior in a giant unit cell compound Tb117Fe52Ge113.8(1).

In this paper we demonstrate evidence of a cluster spin glass in Tb117Fe52Ge113.8(1) (a compound with a giant cubic unit cell) via ac and dc magnetic susceptibility, magnetization, magnetic relaxation and heat capacity measurements. The results clearly show that Tb117Fe52Ge113.8(1)) undergoes a spin glass phase transition at the freezing temperature, ~38 K. The good fit of the frequency dependence of the freezing temperature to the critical slowing down model and Vogel-Fulcher law strongly suggest the formation of cluster glass in the Tb117Fe52Ge113.8(1) system. The heat capacity data exhibit no evidence for long-range magnetic order, and yield a large value of Sommerfeld coefficient. The spin glass behavior of Tb117Fe52Ge113.8(1) may be understood by assuming the presence of competing interactions among multiple non-equivalent Tb sites present in the highly complex unit cell.

Magnetic and magnetothermal properties and the magnetic phase diagram of high purity single crystalline terbium along the easy magnetization direction.

The magnetic and magnetothermal properties of a high purity terbium single crystal have been re-investigated from 1.5 to 350 K in magnetic fields ranging from 0 to 75 kOe using magnetization, ac magnetic susceptibility and heat capacity measurements. The magnetic phase diagram has been refined by establishing a region of the fan-like phase broader than reported in the past, by locating a tricritical point at 226 K, and by a more accurate definition of the critical fields and temperatures associated with the magnetic phases observed in Tb.

Understanding and prediction of electronic-structure-driven physical behaviors in rare-earth compounds.

Rare-earth materials, due to their unique magnetic properties, are important for fundamental and technological applications such as advanced magnetic sensors, magnetic data storage, magnetic cooling and permanent magnets. For an understanding of the physical behaviors of these materials, first principles techniques are one of the best theoretical tools to explore the electronic structure and evaluate exchange interactions. However, first principles calculations of the crystal field splitting due to intra-site electron-electron correlations and the crystal environment in the presence of exchange splitting in rare-earth materials are rarely carried out despite the importance of these effects. Here we consider rare-earth dialuminides as model systems and show that the low temperature anomalies observed in these systems are due to the variation of both exchange and crystal field splitting leading to anomalous intra-site correlated-4f and itinerant-5d electronic states near the Fermi level. From calculations supported by experiments we uncover that HoAl2 is unique among rare-earth dialuminides, in that it undergoes a cubic to orthorhombic distortion leading to a spin reorientation. Calculations of a much more extended family of mixed rare-earth dialuminides reveal an additional degree of complexity: the effective quadrupolar moment of the lanthanides changes sign as a function of lanthanide concentration, leading to a change in the sign of the anisotropy constant. At this point the quadrupolar interactions are effectively reduced to zero, giving rise to lattice instability and leading to new phenomena. This study shows a clear picture that accurate evaluation of the exchange, crystal field splitting and shape of the charge densities allows one to understand, predict and control the physical behaviors of rare-earth materials.

Role of Ge in bridging ferromagnetism in the giant magnetocaloric Gd5(Ge1-xSix)4 alloys.

X-ray magnetic circular dichroism (XMCD) measurements and density functional theory (DFT) are used to study the electronic conduction states in Gd5(Ge(1-x)Si(x))4 materials through the first-order bond-breaking magnetostructural transition responsible for their giant magnetocaloric effect. Spin-dependent hybridization between Ge 4p and Gd 5d conduction states, which XMCD senses through the induced magnetic polarization in Ge ions, enables long-range Ruderman-Kittel-Kasuya-Yosida ferromagnetic interactions between Gd 4f moments in adjacent Gd slabs connected by Ge(Si) bonds. These interactions are strong below but weaken above the Ge(Si) bond-breaking transition that destroys 3D ferromagnetic order.

Massive magnetic-field-induced structural transformation in Gd5Ge4 and the nature of the giant magnetocaloric effect.

A massive magnetic-field-induced structural transformation in Gd5Ge4, which occurs below 30 K, was imaged at the atomic level by uniquely coupling high-resolution x-ray powder diffraction with magnetic fields up to 35 kOe. In addition to uncovering the nature of the magnetic field induced structural transition, our data demonstrate that the giant magnetocaloric effect, observed in low magnetic fields, arises from the amplification of a conventional magnetic entropy-driven mechanism by the difference in the entropies of two phases, borne by the concomitant structural transformation.

Decoupling of the magnetic and structural transformations in Er5Si4.

Er5Si4 is a member of the R5(Si(4-x)Gex) family of alloys, where R=rare earth metal. Many of these compounds display a strong coupling between the magnetic and crystal lattices. In the naturally layered R5(Si(4-x)Gex) materials, inter- and intralayer interactions can be controlled by chemical and physical means; thus their physical properties can be tailored within wide limits. The Er5Si4 is unique in that the temperature dependent structural sequence is opposite that of other representatives of this family. The magnetism of Er5Si4 is reflective of its exceptional place within the series.

Anisotropy of the magnetoresistance in Gd5Si2Ge2.

The observed magnetoresistance of single crystalline Gd5Si2Ge2 is negative and strongly anisotropic. The absolute values measured along the [100] and [010] directions exceed those parallel to the [001] direction by more than 60%. First principles calculations demonstrate that a structural modification is responsible for the anisotropy of the magnetoresistance, and that the latter is due to a significant reduction of electronic velocity in the [100] direction and the anisotropy of electrical conductivity.