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Structural studies by means of powder X-ray diffraction and single-crystal neutron diffraction show that URhGe crystallizes in the orthorhombic TiNiSi-type of structure. Combined magnetic, electrical-transport and thermal studies performed for the first time on a single crystal confirm ferromagnetic order of URhGe below 9.6 K. In contrast to previous powder neutron-diffraction experiments, our single-crystal results point to collinear ferromagnetic order of U moments of 0.35±0.08 μ B confined to the b–c plane, i.e. with no component along the a -axis, which is the hard-magnetization direction in URhGe. Our findings regarding the magnetic properties are in good agreement with theoretical first-principle calculations.

Specific-heat measurements have been performed on the tetragonal ThMn 12 -type RCr 4 Al 8 compounds with R=La, Ce, Pr, Gd and Er in the temperature range 1.5–200 K. For ErCr 4 Al 8 the measurements were extended down to 0.5 K and were also carried out in an applied field of 12 T. Anomalies associated with the magnetic ordering of the R sublattice were observed in the low-temperature part of the specific-heat curves for the compounds with R=Gd and Er below about 6 and at 0.5 K, respectively. No magnetic ordering was found in PrCr 4 Al 8 and CeCr 4 Al 8 .

Specific-heat measurements have been performed on the tetragonal ThMn 12 -type RFe 4 Al 8 compounds with R=Pr, Dy and Tm in the temperature range 1.5 to 200 K. The compound PrFe 4 Al 8 gives rise to a λ-type anomaly at 11.3 K. In DyFe 4 Al 8 we found anomalies at 8.6 K and at about 30 K, while in TmFe 4 Al 8 there is an anomaly below 1.5 K. From the temperature dependence of the magnetic entropy in the presently studied compounds and from data obtained in earlier investigations, it is concluded that the overall crystal-field splitting in the RFe 4 Al 8 compounds with R=Pr, Nd, Dy, Ho, Er and Tm is less than 200 K.

Specific-heat measurements have been performed on the tetragonal ThMn 12 -type RMn 4 Al 8 compounds with R=Y, La, Pr, Nd, Dy and Er in the temperature range 1.5 to 200 K. The compound PrMn 4 Al 8 gives rise to a λ-type anomaly at 14 K. No anomalies indicative of magnetic ordering were found above 1.5 K in the specific-heat curves of the RMn 4 Al 8 compounds with R=Y, La, Nd, Dy, and Er. From the temperature dependence of the magnetic entropy, it is concluded that the overall crystal-field splitting in the compounds with R=Pr, Nd, Dy, and Er is of the order of 200 K or even less.

Specific heat measurements on DyCu2 single crystals in magnetic fields of 0, 1.7 and 4.0 T parallel to the a-direction were performed between 0.3 and 50 K. Various second order transitions due to the strong magnetic anisotropy of the compound were found in accordance with the phase diagram that was derived from magnetization and thermal expansion measurements. The nearly vertical phase transition lines at 5 K in the ferrimagnetic F1 phase and at 6 K in the ferromagnetic F2 phase can be confirmed by the present data. Moreover, the temperature dependence of the Ising-axis conversion field of DyCu2 was complemented by specific heat and magnetocaloric investigations in fields parallel to the c-direction.

The magnetic structure of the compound HoFe2Ge2 (I4/mmm S.G) has been studied by neutron diffraction, magnetic and specific-heat measurements. The paramagnetic neutron data confirmed the ThCr2Si2 type of structure reported earlier for this compound. The analysis of the data collected in the magnetically ordered regime (TN=17 K) showed a uniaxial arrangement of the Ho moments along c associated with the wave vector q=(1/2,1/2,0). The moments of the atoms related by the non-primitive translation (1/2,1/2,1/2) are antiparallel. The refined moment value at 1.5 K is μHo=6.6(1) μB. In addition to the ordered phase the neutron data comprise diffuse magnetic peaks in the low-angle region demonstrating the coexistence of short-range order effects below 6 K.

We report on structure investigation and magnetization study of UNi 1− x Rh x Al solid solutions between an antiferromagnet UNiAl and a ferromagnet URhAl. The ZrNiAl-type hexagonal crystal structure of the parent compounds is preserved in the whole concentration range. Magnetization was measured as a function of magnetic field, temperature and external hydrostatic pressure. The observed complex evolution of magnetic phenomena with Rh substitution for Ni is discussed in terms of effects of the varying 5f-ligand hybridization. A tentative magnetic phase diagram is proposed.

We have performed specific-heat measurements on the heavy-fermion superconductor URu2Si2 in magnetic fields up to 17.5 T. A sharp peak in the specific heat signals the antiferromagnetic transition at TN517.5 K, which shifts to lower temperatures in applied magnetic fields. In order to describe the specific heat below TN , we have used the characteristic features of the excitation spectrum measured by neutron scattering. The relative field dependence of the antiferromagnetic transition temperature TN and the energy gap D in the magnetic excitation spectrum can be described by a single scaling relation of the form @12(B/B0) # . The scaling field of 48.5 T is close to the metamagnetic transition field B*540 T, where the heavy-fermion state is suppressed. @S0163-1829~97!05746-9#

By the use of a gold-plated silver sample holder, and a RuO2-Cernox combination thermometer, low-temperature specific-heat measurements in the temperature range from 0.35 to 90 K, and in magnetic fields up to 17.5 T, have become possible with remarkably improved accuracy. At 500 mK, the lower limit for the heat capacity that can be measured amounts to 0.02 mJ/K. The sensitivity in that case is about 0.3 μJ/K. The error margin is 1% for large heat capacities (larger than twice the sample-holder contribution) at higher temperatures, increasing to 3% for small heat capacities (half of the sample-holder contribution) around 500 mK. In high fields, for accurate measurements (error

URhGe crystallizes in the orthorhombic TiNiSi-type of structure and has been studied untill now only in polycrystalline form. Here, we report on the low-temperature specific heat of URhGe measured on a single crystal. The compound orders ferromagnetically at T C =9.6 K. Because of this low magnetic-ordering temperature, the specific heat c p cannot be described by the usual expression γT + aT 3 , also at low temperatures. Up to 7 K, c p is satisfactorily described by the relationship γT + bT 2 , which must be associated with the temperature depenedence of the magnetization in this temperature interval. The value of γ amounts to 164 mJ/mol U K 2 . We have also studied the effect on c p of an external magnetic field applied along the principal axes. The results are discussed in terms of the strong magnetic anisotropy of URhGe.

The magnetic properties and specific heat of ErMn4Al8 and ErCr4Al8 compounds have been studied between 0.5 and 30 K in magnetic fields up to 12 T. We find clear indications that the specific heat is dominated by CEF effects rather than by magnetic ordering. The very high low-temperature specific heat of ErMn4Al8 may be attributed to an isolated |Jz=±3/2〉 doublet, which is strongly affected by the applied magnetic field. For ErCr4Al8, the zero-field data very much resemble the ErMn4Al8 data but the influence of the magnetic field is quite different, which excludes a possible explanation in terms of a doublet ground state.

Specific-heat measurements have been performed on the tetragonal ThMn12-type compound ErCu4Al8 and YCu4Al8 in the temperature range 1.5–200 K. A sharp peak in the specific heat, signals the antiferromagnetic transitions at TN=5.18 K. In order to separate out the effect of the Er sublattice we have also performed measurements on the YCu4Al8 compound in the same temperature range. From the temperature dependence of the magnetic contribution to the specific heat of ErCu4Al8 we obtained the temperature dependence on the magnetic entropy and discuss our results in terms of the crystal field level scheme proposed earlier from inelastic neutron scattering.

Specific-heat measurements were performed on a single crystal of UNiGe in magnetic fields up to 17.5 T. This compound crystallizes in the orthorhombic TiNiSi structure and undergoes two magnetic phase transitions at 41.5 and 50 K. Besides these transitions, the specific heat also shows a Schottky contribution due to the crystal-field splitting. The effects of an applied magnetic field are compared with results from previous neutron diffraction, magnetoresistance, and magnetization measurements.

Specific-heat measurements have been performed on the tetragonal ThMn 12 -type compound GdMn 4 Al 8 in the temperature range 0.3–90 K. The compound GdMn 4 Al 8 shows a Schottky type anomaly at about 2 K. From the field dependence of the specific heat, it is concluded that this anomaly is due to short-range magnetic ordering.

We have studied the magnetic properties of the compound TbFe4Al8 on a single crystal and on polycrystalline material. When measured with the field applied along the c-direction, the magnetization is comparatively low. It slightly increases below about 50 K and there is a shallow maximum at about 160 K, a temperature at which earlier Mossbauer spectroscopy has indicated the onset of magnetic ordering of the Fe sublattice. When measured along the basal plane direction, the magnetization is much larger and its temperature dependence is characterized by pronounced thermal history effects. Measurements of the magnetic isotherms at various temperatures show the latter to be due to the occurrence of coercivities that are extremely large at low temperatures but decrease at higher temperatures. These coercivities develop after a field-induced transition, the easy direction of the net magnetization being along the [100] direction. The magnetic properties of the single crystal show marked differences with those of a polycrystalline sample. These differences are discussed in terms of concentation fluctuations.

Specific heat measurements have been performed on the tetragonal ThMn 12 -type compounds ErFe 4 Al 8 and YFe 4 Al 8 in the temperature range 1.5–200 K. There are only very small anomalies associated with the magnetic ordering of the Fe sublattice in the high-temperature part of the specific heat curves. A sharp peak in the specific heat, signalling the magnetic ordering of the Er moments at 5.5 K is observed in ErFe 4 Al 8 . In order to separate out the effect of the Er sublattice we have subtracted the data obtained for YFe 4 Al 8 in the same temperature range. From the temperature dependence of the magnetic contribution of the Er moments to the specific heat of ErFe 4 Al 8 we obtained the temperature dependence of the magnetic entropy. We discussed our results in terms of the crystal-field level scheme proposed earlier on the basis of inelastic neutron scattering.

The structure and magnetic ordering of the tetragonal TbFe 4 Al 8 compound were studied by neutron diffraction at temperatures between 1.5 and 170 K. The structure refinement in the paramagnetic state confirms the preference of the Fe atoms for the 8(f) positions. Magnetic reflections are only discernable below 110 K. The magnetic ordering is described by two wave vectors: q 1 =0 , and q 2 =(q x , q x , 0) , the latter having a temperature-dependent length. The ordering associated with these two wave vectors occurs simultaneously below 110 K. In the high-temperature range 40–110 K the best model obtained by refining neutron difference diagrams leads for the ordering associated with q 1 =0 to exclusively Tb ferromagnetic contributions with moments in the basal plane. For the ordering associated with q 2 =( q x , q x , 0 ) it leads to a cycloid spiral structure comprising Tb and Fe contributions. The moments of both sublattices rotate within the plane of the wave vector (0 0 1). The value of q x below 8 K is 0.125(1) r.l.u. Symmetry analysis shows that the Fe site splits into two independent sets (orbits) under the action of the in-plane wave vector q 2 =( q x , q x , 0 ). Various models are discussed. For the cycloid spiral, within the 3 σ limit, the phases of the Fe moments of both orbits are zero. The high-temperature structure is a distorted cycloid with elliptical envelope for the Tb sublattice which below 40 K induces a distortion to the Fe cycloid spiral because of the increasing in-plane magnetic field of the in-plane ferromagnetic Tb moment. Below 40 K the q 1 =0 structure comprises ferromagnetic Tb as well as antiferromagnetic Fe contributions in the basal plane.

Low-temperature specific heat data are reported on single-crystalline UPd2Al3 (TN≅14 K, Tc≅2 K) in magnetic fields up to 17.5 T, applied either along, or perpendicular to, the c-axis. The normal and superconducting properties have been extensively studied below the metamagnetic transition field, which has been detected previously by magnetization measurements at helium temperatures to be at 18 T, for the field applied in the basal plane. We discuss the anisotropic behavior of the antiferromagnetic transition (AFT) when applying magnetic fields. A small anisotropy for field applied in the basal plane was observed. Finally, we discuss the electronic contribution γ to the specific heat for B in the basal plane, we find γ increases quadratically with field.

K. Proke$ s*, T. Tahara, Y. Echizen, T. Takabatake, T. Fujita, I.H. Hagmusa, J.C.P. Klaasse E. Br . uck, F.R. de Boer, M. Divi$ s, V. Sechovsk$ y Hahn-Meitner-Institute, SF-2, Glienickerstrasse 100, Berlin D-141 09, Germany Department of Quantum Matter, ADSM, Hiroshima University, Higashi, Hiroshima 739-8526, Japan Van der Waals–Zeeman Institute, University of Amsterdam, 1018 XE, Amsterdam, The Netherlands Department of Electronic Structures, Charles University, 12116 Prague 2, Czech Republic

The quasiternary intermetallic compound URh 1/3 Ni 2/3 Al crystallizes in the hexagonal ZrNiAl-type of structure, like the parent compounds URhAl (ferromagnet, T C =27 K) and UNiAl (antiferromagnet, T N =19 K). A delicate balance between ferromagnetic and antiferromagnetic interactions leads to suppression of magnetic order. The low-temperature properties show all attributes of a non-Fermi-liquid system, i.e. divergence of the low-temperature specific heat divided by temperature, which behaves as -ln( T / T 0 ), divergence of the magnetic susceptibility and absence of a T 2 dependence in the electrical resistivity. URh 1/3 Ni 2/3 Al also exhibits an enhanced low-temperature specific heat of 225 mJ/mol U K 2 . The compound can be classified as a NFL system in which the magnetic phase transition is at zero kelvin.