Europium titanate has the cubic perovskite structure containing divalent Eu (7 μB) and tetravalent Ti. From magnetic measurements we find that EuTiO3 is one of the few antiferromagnetic materials with a positive θ (TN=5.3°K, θ=3.8°K). At 1.3°K the magnetic moment (σ) increases linearly with field to 10 kOe; above 14 kOe the moment saturates and σ=156 emu/gm (6.93 μB) at 20 kOe. Powder neutron-diffraction work indicates that EuTiO3 has the Type G magnetic structure in which a given Eu++ has six nearest-neighbor europium ions antiparallel and 12 next-nearest-neighbor europium ions parallel. In a perovskite structure where only the 12-coordinated ion is magnetic, i.e., Eu++, the molecular field relations for a two sublattice model yield J1/k=−0.021°K, where J1 is the effective intersublattice exchange interaction, and J2/k=0.040°K, where J2 is the effective intrasublattice exchange interaction. The signs of J1 and J2 are opposite to those found in the europium chalcogenide series. The chalcogenides, however, have the rocksalt structure in which the number of 90° cation-anion-cation interactions differs from the perovskite structure.

The thermodynamic properties of cubic or uniaxial antiferromagnets are examined using spin wave theory. The specific heat, magnetization, and parallel susceptibility are shown to be exponentially increasing functions of applied field for values of H Q less than the critical spin-flopping field. Since this field dependence suggests that an antiferromagnet can be cooled by the adiabatic application of a magnetic field, the theory of adiabatic magnetization is investigated. Field-dependent nuclear spin effects are evaluated on an effective field model by perturbation theory and are included in the analysis. It is found that when spin wave effects are dominant, cooling should be observed; at lower temperatures, when nuclear effects are non-negligible, either cooling or heating may be observed, depending on the initial temperature and final value of the magnetic field. The dependence of the cooling on the physical parameters of the antiferromagnet is discussed and detailed calculations are made for MnF 2 .

We have measured the moment of a pressed powder sample of monoclinic CuF2 from 4.2° to 300°K in fields to 25 kOe. χ(T) indicates an AFM transition at TN=69°K, and analysis gives ḡ≈2.3 and Θ≈−200°K. TN and Θ are consistent with magnetic ordering of the first kind with AFM exchange interactions J1(corner-bc)≈34°K and J2≈22°K. Weak ferromagnetism of anisotropic exchange origin is expected on the grounds of symmetry; the measured ferromagnetic moment is σ0≈1 G cm3/mole, corresponding to a canting angle φ≈0.01°. Assuming magnetic dipolar anisotropy and estimating principal axes and values for the anisotropic g tensor, we find the spin axis to be about 2° from c and 99° from a in the ac plane with anisotropy field HA∼2000 Oe, and the weak moment to be parallel to the monoclinic axis b with canting field HDM∼1000 Oe. Pseudodipolar anisotropy may be expected to change the orientation, but not the order of magnitude of these fields. The uniform mode AFM resonances are not degenerate in general, but both would be expected to occur at v/c∼9 cm−1 using these values.

Spin-wave theory was used to determine the magnetic field dependence of the thermodynamic properties of cubic or uniaxial antiferromagnets at temperatures well below the Néel point. These properties were found to be exponentially increasing functions of applied field for values of H0 less than the critical spin-flopping field. This field dependence suggests that if spin wave effects are dominant an antiferromagnet can be cooled by adiabatic magnetization in contradistinction to the paramagnetic case of cooling by demagnetization. (The phenomenon of antiferromagnetic cooling by isentropic magnetization was recognized many years ago.) Both lattice and nuclear effects can alter the situation in a real material and were included in this investigation. The dependence of adiabatic magnetization cooling on the physical quantities characterizing the antiferromagnet is discussed and theoretical temperature changes for several uniaxial antiferromagnets are given.

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