M. Motokawa

Submillimeter ESR experiments using pulsed high magnetic field

A new method has been developed at Osaka University for producing a mega‐gauss field without destruction of the coil. As one of the applications of this system, submillimeter ESR experiments have been done at low temperatures by using HCN (λ=337μm) and H2O (λ=119μm) lasers in a field up to 0.5 MOe. We report some novel high field effects observed in these experiments. The g‐shift due to the SH3 term is expected to be appreciable in such a high magnetic field. This effect was found in a study of Co‐Tutton salt at 77 K. The experimental result showed 2% smaller g value at 119 μm than at 337 μm. This was explained quantitatively by an extensive calculation based on the Abragam‐Pryce theory. So‐called exchange splitting was observed in (NH4)  2CuCl4⋅2H2O, Cu(NH3)  4SO4⋅H2O (CTS) and CuCl2⋅2H2O. At 4.2 K the two observed lines were of quite different intensities and the weaker one showed a remarkable shift. These phenomena can be explained successfully by molecular field theory, and it is shown from the direct...

Recent Topics on the Generation and Application of Pulsed High Magnetic Fields at Osaka

There are a few ways to obtain very high magnetic fields of about 1 MOe mainly developed in the recent decades. The first success was obtained by FOWLER and others [1] by a flux compression due to chemical explosives and the second method developed by CNARE [2] is a similar compression due to electromagnetic forces. The flux compression methods can produce pulsed-fields of 2 or 3 MOe [3] inside the compressing liner. However, specimens and a part of the apparatus are necessarily destroyed in each run. Moreover, the pulse width is of the order of 1 μs so that the study of metals is very difficult due to both heating and skin effect. It is also not convenient for precise measurements to see the increasing field only.

Very high field measurements of magnetic materials

Abstract Typical experimental results mainly obtained in our High Magnetic Field Laboratory are shown after a short review of recent progress in the generation of very high magnetic fields. Magnetizations and magnetoresistances of various materials are shown together with some optical work such as the Rydberg-Landau problem.

Magnetic Resonances in the Ising Spin Ferro‐ and Antiferromagnets

Two new types of magnetic resonances in ferro‐ and antiferromagnetic Ising‐like spin system are reported. One of them is the spin‐cluster resonance first observed in CoCl22H2O. This compound is antiferromagnetic below 17°K with an antiferromagnetic arrangement of ferromagnetic Ising‐like linear chains. In such a system, one can expect localized spin‐cluster excitations in addition to the usual spin waves. We have observed the resonance with the selection rule Δm = ±1, where m is the number of spins in a short‐range‐order spin cluster directed opposite to the majority spins in a ferromagnetic chain of CoCl22H2O. In FeCl2, the spin‐cluster resonance in a ferromagnetic layer was observed.When an Ising‐like spin system contains some impurity spins, the impurity spin resonance can be observed under suitable conditions. We have observed the spin resonance of Fe3+ and Mn2+ spins in FeCl2. A more typical example was obtained in Mn‐doped CoCl22H2O whose resonance line shows a well‐separated fine structure splitting.

H-T phase diagram of randomly diluted FeF2

We have measured the H0-T phase diagrams of randomly diluted FexZn1−xF2. Samples with spin-flop (0.7 ⩽ x ⩽ 1.0) exhibit bicritical points decreasing with dilution much more rapidly in T than in H0. Samples with order-disorder transitions (0.51 ⩽ x < 0.7) exhibit smooth phase boundaries, connecting that low-T transition with TN at H0 = 0.

Temperature dependence of FeF2 spin flop field

Abstract The AF-SF and AF-P phase boundaries of FeF2 were measured using the Osaka high-field facility. The AF stability limit exhibits little temperature dependence before terminating at the bicritical point (HB = 483 kOe; TB ≈ 55 K). This result strongly disagrees with the prediction of exchange plus single-ion anisotropy induced, four-magnon renormalization of the k = 0 magnon energy.

New problems on high-field magnetization in magnetic materials

Abstract New experimental results for various magnetic materials are shown after a short review of the recent activities in the High Magnetic Field Laboratory of Osaka University. The main topics are as follows. A clear evidence of the four-spin exchange interaction is found in a graphite intercalation compound C 6 Eu by analyzing the magnetization process. The field-induced low-spin-high-spin transition is observed in MnAs and related compounds. Spin fluctuations are strongly suppressed by applying a high magnetic field and the typical examples are seen in a weakly ferromagnetic MnSi and low-dimensional magnets.

High‐field magnetization measurements of the metastability boundary in a d=2 random‐field Ising system

The d=2 random‐field Ising system Rb2Co0.85 Mg0.15F4 has been studied using pulsed, high‐field magnetization M measurements. A relatively sharp metastability boundary TF(H) has been discovered which separates a frozen region below from an equilibrium one above TF(H). TF(H) scales as does the (destroyed) phase transition ‘‘Tc(H)’’ as TN −TF(H)  ∝H2/φ, with the measured crossover exponent φ=1.8±0.1 in good agreement with the d=2 susceptibility exponent γ=7/4. As measured in the pulsed field experiment, TF(H) lies well below ‘‘Tc(H)’’ but above the value of TF(H) obtained from recent neutron scattering studies. The difference in the two values of TF(H) is attributed to the log t time dependence of the approach to equilibrium and to magnetocaloric effects.

Field‐induced ’’exchange flips’’ in a randomly diluted antiferromagnet (invited)

High field magnetization measurements of the randomly diluted, anisotropic antiferromagnet FexZn1−xF2 have revealed the behavior of novel single‐spin ’’exchange‐flips’’ as well as the concentration‐dependent spin‐flop field HSF. Measurements were made in pulsed field up to 550 kOe in the High Magnetic Field Laboratory in Osaka University on samples from pure FeF2 to beyond the percolation limit. The exchange flips occur when the applied field H0 exceeds the effective exchange field HE = nHE/z at sites on the down‐sublattice with n magnetic neighbors. Below either HSF or a ’’crossover’’ field Hcr, hysteretic behavior is seen with respect to the direction of field sweep, whereas above HSF or Hcr it is absent. Further, Hcr is marked by an anomalous peak in dM/dH versus H0. These effects are explained by a transition to a disordered state above Hcr, which possesses no long‐range antiferromagnetic order. A classical‐spin computer simulation has been found to accurately reproduce these effects.

Novel spin‐flop effects in FexMn1−xF2

High‐pulsed‐field magnetization measurements in the randomly mixed antiferromagnet FexMn1−xF2, with 0.15≤x≤0.40, have revealed an extremely broad spin‐flop transition, with a width of ΔH≂60 kOe. An unusual square shape, with rounded edges, is found in dM/dH, with an asymmetry which depends on x. For the largest values of x, an additional sharp peak is also seen. A four‐sublattice molecular‐field model has been used to interpret the results, and predicts a number of interesting effects. For 0.01≲x≲0.10, the usual first‐order spin‐flop is suppressed, and a new canted‐intermedi ate (CI) phase appears, in which the spins rotate continuously from the antiferromagnetic (AF) to spin‐flop (SF) phase. For 0.10≲x≲0.6, the CI–SF transition becomes first order, and for x≳0.6 the CI phase disappears, leaving a relatively normal first‐order spin‐flop. The model shows that small misalignments cause considerable broadening and rounding of the transition, and convincingly reproduces the experimental shapes and widths.