### Abstract

The site dilution problems on a square lattice antiferromagnet with spin S = 5/2, Mn(HCOO)_{2}2(NH_{2})_{2}CO (T_{N} = 3.77K), have been studied for a wide range of magnetic concentration x, covering the percolation threshold x_{p} = 0.59 for the square lattice, by the measurements of magnetic heat capacity and susceptibility. The Néel temperature T_{N}(x) has been found to decrease as d/dx{T_{N}(x)/T_{N}(1)} = R = 2.7 just below x = 1 for the three kinds of non-magnetic impurities, Mg^{2+}(2p^{6}), Zn^{2+}(3d^{10}) and Cd^{2+}(4d^{10}), at almost the same reduction rate as in the cases of the antiferromagnet K_{2}MnF_{4} with S = 5/2 and the ferromagnet K_{2}CuF_{4} with S = 1/2. The present value R = 2.7, however, makes a contrast to R = 3.1 for the diluted La_{2}CuO_{4}, an S = 1/2 Heisenberg antiferromagnet. The extrapolation of T_{N}(x), with R = 2.7(3.1) down to TN(x) = 0 gives a pseudo-critical concentration x = x_{c} = 0.63(0.68) deviating from x_{p} = 0.59. This indicates that T_{N}(x) must draw concavely near T_{N} (x_{p}) = 0 at low temperatures, making a contrast to the convex curve of decreasing T_{N}(x) down to zero at x = x_{p} for Ising systems. As x decreases, dilution effects reflected on the reduction of lattice dimensionality, the Curie-Weiss temperature and the Curie constant are discussed on the base of the absolute values of the magnetic heat capacity and susceptibility. An anomalous enhancement of the heat capacities has been found in the external field of 50 kOe for the system with x = 0.12, which was theoretically reproduced by the contribution from isolated ions and clusters computer-generated on the square lattice with the same magnetic concentration.

Original language | English |
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Pages (from-to) | 3696-3703 |

Number of pages | 8 |

Journal | journal of the physical society of japan |

Volume | 69 |

Issue number | 11 |

DOIs | |

Publication status | Published - Nov 2000 |

### All Science Journal Classification (ASJC) codes

- Physics and Astronomy(all)

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## Cite this

*journal of the physical society of japan*,

*69*(11), 3696-3703. https://doi.org/10.1143/JPSJ.69.3696