M.C. Morón

The critical temperature and exchange interactions of an S = 5/2 Heisenberg antiferromagnet on an f.c.c. lattice

The critical temperature is calculated as a function of the ratio for an S = 5/2 Heisenberg spin lattice with antiferromagnetic ordering of types I and II on a face-centred cubic lattice. and represent, respectively, the nearest- and next-nearest-neighbour exchange constants. Both possibilities for ordering of type II, antiferromagnetic and ferromagnetic, are considered. The critical region is studied by applying the Pade approximant method to the corresponding high-temperature series expansion of the staggered susceptibility. The results presented here provide a useful tool for a straightforward interpretation and understanding of experimental data. The approach is applied to various experimental systems and the values obtained compared with those provided by other approximations.

Mössbauer study of the α and β forms of (NH4)2FeF5

Abstract The Mossbauer spectra of the two crystallographic forms α- and β-(NH 4 ) 2 FeF 5 have been studied in the temperature range 4.2–300 K. The α-form presents typical one-dimensional behavior showing a transition to magnetic ordering at T c = (7.5±0.5) K. The spectra of the β-form indicates the presence at low temperature of two different iron sites, while the room temperature crystal structure shows only one site for the iron ions. Differential scanning calorimetry measurements evidence the presence of a reversible structural phase transition at T s = (168.0±1) K on heating and T s = (187.0±3) K on cooling. This form also exhibits a magnetic ordering phase transition at T c = (13±1) K.

Crystal and magnetic structures of RbMnF4 and KMnF4 investigated by neutron powder diffraction : the relationship between structure and magnetic properties in the Mn3+ layered perovskites AMnF4 (A = Na, K, Rb, Cs)

The crystal and magnetic structures of RbMnF4 and KMnF4 have been determined by neutron powder diffraction. The crystal symmetry of both compounds belongs to the layered perovskite structure and exhibits a pseudo-tetragonal unit cell, space group P21/a. The (MnF2F42/)- octahedra show a distortion induced by both steric and Jahn-Teller effects. They are also tilted by an angle which depends on the size of the alkali ion. KMnF4 orders as a noncollinear antiferromagnet below 5.2 +or- 0.1 K exhibiting four magnetic sublattices with an angle between the two spin directions of 17 degrees . RbMnF4 is a collinear antiferromagnet below 3.7 +or- 0.1 K. Interestingly enough and contrary to what is found for the K compound, there are two active irreducible representations in the magnetic structure of the Rb derivative. Moreover, the relationship between crystal structure and magnetic behaviour has been investigated in the AMnF4 (A=Na, K, Rb, Cs) series. The sign of the isotropic magnetic interaction is studied as a function of the superexchange angle Mn-F-Mn and the degree of distortion of the octahedra.

Crystal structure and magnetic measurements of [Cr(en)3] [ZnCl4]Cl

The compound [Cr(en)3][ZnCl4]Cl has been synthesized by reaction of CrCl3·6H2O, Zn and [Cr(en)3]2(SO4)3 in HCl. Its molecular and crystalline structure was determined by X-ray diffraction methods, being monoclinic, P21/c, a=21.215(3), b=12.532(2), c=13.707(2) A, β=95.21°, V= 3629(2) A3, Dx=1.738g cm−3, MW=474.9, Z= 8, F(000)=1928, λ(Mo Kα)=0.71069 A, μ(Mo Kα)= 27.04 cm−1, 288 K. No significant exchange interactions between Cr(III) cations in the crystalline lattice were found. Curie-Weiss behavior was found in the three directions tested (g1=2.06±0.02,g2= 2.08±0.02,g3=2.09±0.01), T=1.2-1.4 K.

New low-temperature ferrimagnets

The research in Zaragoza and Barcelona has been supported respectively by Grants 3380/83 and 409/84, from the Comision Asesora de Investigacion Cientifica y Tecnica of the Ministerio de Educacion y Ciencia. The research in Chicago has been supported by Grant DMR-85 15224 fromthe SolidState Chemistry Program, Division of Materials Research of the National Science Foundation. Cooperative work has been supported by Grant CCB-8504/001 from the American—Spain Joint Committee for Technical and Scientific Cooperation. One of us (MCM) wants also to acknowledge a postgraduate student fellowship fromthe Ministerio de Educacion y Ciencia.

Single‐crystal ac susceptibility measurements on [Co(NH3)6][CuCl5], a 3D, S=1/2 Heisenberg antiferromagnet

Single‐crystal ac magnetic susceptibilities of [Co(NH3)6][CuCl5] along the three crystallographic axes in the temperature range from 1.1 to 90 K are presented. The magnetic behavior is characteristic of a three‐dimensional antiferromagnet, its ordering temperature being at Tc=3.8 K. Susceptibilty data can be fit to a Heisenberg S=1/2 simple cubic model using high‐temperature series expansions extrapolated with Pade approximants. Good agreement is found for an exchange constant J/kB=−3.13 K and values of g factor ga=2.09, gb=gc =2.04, a,b, and c being the crystallographic axes. This result makes [Co(NH3)6][CuCl5] one of the few examples of a 3D antiferromagnetic Heisenberg S=1/2 model. The magnetic behavior below Tc indicates the existence of crystallographic domains due to the structural transition from cubic to tetragonal symmetry that the system has at about 280 K.

Zn1 − xMnxGa2Se4 : a new series of diluted magnetic semiconductors

Abstract The magnetic properties of a new series of diluted magnetic semiconductors Zn 1 − x Mn x Ga 2 Se 4 with x ranging between 0 and 1 are reported from magnetization and AC magnetic susceptibility data. Antiferromagnetic as well as spin-glasslike behaviors have been detected showing a dependence with the Mn content as well as the cation distribution.

Magnetic behavior of CsMnF4 under high pressure

Abstract The magnetic behavior of the ferromagnetic layered-perovskite CsMnF 4 has been investigated up to pressures P of 4 GPa by using a diamond-anvil cell cooled down to 1.5 K and a SQUID vibrating magnetometer. As the pressure is increased up to 2 GPa the critical temperature T C decreases. At P ≳ 2 GPa, however, T C is almost independent of pressure. The correlation of these results with chemical pressure effects is discussed.

Neutron powder diffraction experiments on AMnF4 (A=K, Rb): nuclear and magnetic structures

Abstract Neutron powder diffraction experiments show that KMnF 4 is monoclinic, space group P2 1 /a, and RbMnF 4 orthorhombic, space group Pmab, between room temperature and 1.5K. The magnetic structure of both compounds is antiferromagnetic with T c =6.6K for KMnF 4 and T = 3.9K for RbMnF 4 .

Magnetic properties of the low-temperature ferrimagnet [Cr(H2O)(NH3)5][FeCl6]

Abstract We report on the magnetic properties of the complex bimetallic compound [Cr(H 2 O)(NH 3 ) 5 ][FeCl 6 ] studied by ac magnetic susceptibility and magnetization techniques in the range 1.5–30 K and 0–40 kOe. The crystal structure contains discrete [Cr(H 2 O)(NH 3 ) 5 ] 3+ and [FeCl 6 ] 3- ions, held together by ionic forces and by a network of hydrogen bonds. At temperatures below (2.32±0.01) K the compound seems to behave as a noncollinear ferrimagnet. In addition, the experimental H-T magnetic phase diagram has been derived.