Theoretical and numerical model for estimating unknown magnetic parameters in studying ferromagnetic and antiferromagnetic coupled films Influence of exchange energy and magnetic anisotropy on the nanocrystalline alloy A magnetic cluster is a group of magnetic ions ͑''spins''͒ that interact with each other but which, to a good approximation, do not interact with other magnetic ions. Such clusters are responsible for many of the interesting and useful properties of a large number of molecular crystals, and of dilute magnetic materials below the percolation concentration. In a molecular crystal the magnetic clusters are usually all of one type. In a dilute magnetic material, on the other hand, many cluster types are present. The magnetization-step ͑MST͒ method is a relatively new form of spectroscopy for measuring intracluster magnetic interactions, mainly exchange constants and anisotropy parameters. In dilute magnetic materials this method also yields the relative populations of different cluster types. This review focuses on the principles and applications of the MST method to relatively small clusters, no more than a dozen spins or so. It covers only MSTs from spin clusters in which the dominant exchange interaction is antiferromagnetic ͑AF͒, and MSTs from isolated magnetic ions. Such MSTs are the result of changes of the magnetic ground state, caused by energy-level crossings in a magnetic field H. At a sufficiently low temperature, each change of the ground state leads to a MST. Magnetic clusters may be classified by size. The smallest is a ''single,'' consisting of one isolated magnetic ion. Next are ''pairs'' ͑dimers͒, followed by ''triplets'' ͑trimers͒, ''quartets'' ͑tetramers͒, etc. Although the classification by size is useful, clusters of the same size may have different intracluster interactions, and also different geometrical shapes. More detailed classifications of magnetic clusters are therefore also needed. A cluster ''type'' specifies both the size of the cluster and the set of all intracluster magnetic interactions which are nonzero. Different geometries of clusters of the same type correspond to different ''configurations.'' MSTs from isolated spins ͑singles͒ are discussed first. When subjected to certain types of single-ion anisotropy, e.g., uniaxial hard-axis anisotropy, singles give rise to MSTs. Examples of anisotropy parameters which were determined from such MSTs are presented. An interesting application of MSTs from singles is the determination of the populations of Jahn-Teller distortions which are energetically equivalent at Hϭ0 but are inequivalent at finite H. For clusters larger than singles, the strongest intracluster interaction is usually the isotropic exchange. Using a model with one isotropic exchange constant J, predictions for MSTs from pairs, open and closed triplets, and the six possible types of quartets, are presented. Observations of some of these MSTs, and the exchange constants derived from them, are discussed. Recent studies of MSTs from AF rings in molecular crystals ...

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Measurements of the transverse differential susceptibility in MnP suggest that the paraferro-fan triple point is a Lifshitz point (LP). At this point all phase boundaries are tangent to each other, and the A line exhibits an inflection point. These features agree with predictions for a LP characterized by d = 3, n=m = l i which is expected for this crystal symmetry. A crossover exponent 0=O.634± 0.03 is obtained, in agreement with theory.

Magnetization and neutron-diffraction measurements were performed on a single crystal of Cu 2 MnSnS 4. This quartenary magnetic semiconductor has the stannite structure ͑derived from the zinc-blende structure which is common to many II-VI dilute magnetic semiconductors͒, and it orders antiferromagnetically at low temperature. The neutron data for the nuclear structure confirm that the space group is I42m. Both the neutron and magnetization data give T N ϭ8.8 K for the Néel temperature. The neutron data show a collinear antiferromagnetic ͑AF͒ structure with a propagation vector kϭ͓1/2,0,1/2͔, in agreement with earlier neutron data on a powder. However, the deduced angle between the spin axis and the crystallographic c direction is between 6°and 16°, in contrast to the earlier value of 40°. The magnetization curve at TӶT N shows the presence of a spin rotation ͑analogous to a spin flop͒, which indicates that the spin axis is indeed close to the c direction. The deduced magnetic anisotropy gives an anisotropy field H A Х2 kOe. At high magnetic fields the magnetization curve at TӶT N shows the transition between the canted ͑spin-flop͒ phase and the paramagnetic phase. The transition field, Hϭ245.5 kOe, yields an intersublattice exchange field H E ϭ124 kOe. The exchange constants deduced from H E and the Curie-Weiss temperature ⌰ϭϪ25 K show that the antiferromagnetic interactions are an order of magnitude smaller than in II-VI dilute magnetic semiconductors ͑DMS's͒. The much weaker antiferromagnetic interactions are expected from the difference in the crystal structures ͑stannite versus zinc-blende͒. A more surprising result is that the exchange constant which controls the AF order below T N is not between Mn ions with the smallest separation. This result contrasts with a prediction made for the related II-VI DMS, according to which the exchange constants decrease rapidly with distance. ͓S0163-1829͑97͒04234-3͔

The differential susceptibility dM /dH of ͓Fe͑salen͒Cl͔ 2 where salen is N,NЈ-ethylenebis͑salicylideneiminato͒, was measured in pulsed magnetic fields up to 550 kOe. The samples were in a capsule that was immersed in a liquid-helium bath maintained at a temperature 1.5рT bath р4.2 K. Three magnetization steps ͑MST's͒ arising from energy-level crossings for the Fe 3ϩ dimer were observed as peaks in dM /dH. The intradimer exchange constant obtained from the MST's is JϭϪ8.4Ϯ0.2 cm Ϫ1 , assuming a g factor of 2.00. The line shapes of the peaks in dM /dH strongly suggest the existence of weak interactions that are not included in the conventional model of independent dimers with isotropic intradimer exchange only. The missing weak interactions are yet to be identified. The narrow widths of the peaks in dM /dH indicate strong departures from thermal equilibrium with the helium bath during the field pulse ͑milliseconds duration͒. Such narrow widths are the result of cooling by the magnetocaloric effect. The cooling, and therefore the line shapes of the dM /dH peaks, depended on the arrangement for the sample-to-bath heat flow during the field pulse. With one heat-flow arrangement hysteresis was observed. Computer simulations of the line shape resulting from the magnetocaloric effect during the field pulse reproduce the narrow widths and the hysteresis. Data for the low-field magnetic susceptibility between 2 and 300 K, measured in steady fields, gave JХϪ8 cm Ϫ1 .

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