*Magnetocaloric System*

Magnetic refrigeration at ambient temperatures has attracted interest with the
discovery of new materials with improved efficiencies and advantages, as
potential replacements for the classical vapor compression systems in use today.
In particular, Pecharsky *et al*
reported that Gd_{5}(Ge_{2}Si_{2})
has a giant magnetocaloric effect (MCE) between 270 and 300 *K*, while
Tegus *et al.* found that MnFe(P_{1-x}As_{x}) with the
hexagonal Fe_{2}P-type structure has a paramagnetic-ferromagnetic phase
transition that is strongly first order and exhibits a huge MCE. In addition,
the Curie temperature (*T _{c}*) and hence optimal operating
temperature of this latter material can be varied from 200 to 350

For the
particular material chosen to study here, we find that Mn_{1.1}Fe_{0.9}P_{0.8}Ge_{0.2
}is single phase and paramagnetic at higher temperature, single phase and
ferromagnetic at lower temperature, and in between the system undergoes a
strongly first-order phase transition as a function of temperature or applied
magnetic field. Both phases possess the same symmetry space group (*P-62m*)
but have distinctly different structures; the *a*- and *b*-axes are
~1.3% longer while the *c*-axis is contracted by ~2.6% in the ferromagnetic
(FMP) phase compared to the paramagnetic (PMP) phase. Therefore the
diffraction peaks come at distinctly different scattering angles as shown in the
figure below, where we follow the intensity of the (001) Bragg peak as a
function of applied field. Initially we are in the paramagnetic phase since
T > T_{C}, and we find that there is an abrupt phase transition between
the paramagnetic phase at low field and the ferromagnetic phase in applied
field. This field-dependent transition can be used in a magnetic
refrigeration cycle.

(001) Bragg peak diffraction data for the paramagnetic (PMP) and
ferromagnetic (FMP) phases as a function of field. The difference in *c*-axis
lattice parameter means that the peak occurs at a distinctly different
diffraction angle.

We have found that the large MCE in this material
originates from simply converting one phase to the other, with most of the
entropy change coming from the structural transition rather than the magnetic
phase change. The interesting result, however, is that both transitions
can be simply controlled by the magnetic field. Using the standard Maxwell
relations method, we obtain the maximum magnetic entropy changes
from the isothermal magnetization in both magnetic field increasing and
decreasing mode, of 74 and 78 J/kg-K, respectively, for a field change of 5 T in
a bulk Mn_{1.1}Fe_{0.9}P_{0.8}Ge_{0.2} compound.
This is
twice the previous value for this system and the highest MCE for any material
presently known. The improved properties and overall advantages of this material
open the possibility for its use in wide scale magnetic refrigerant applications.

Temperature
dependence of the magnetic entropy change of the bulk Mn_{1.1}Fe_{0.9}P_{0.8}Ge_{0.2}
compound as a function of applied magnetic field up to 5 T.

Very
recent work has now been carried out to further improve the properties, using
neutron diffraction, differential scanning calorimetry, and magnetization
measurements to study the effects of both crystallite size and Mn and Ge
location in the structure on the ordered magnetic moment, MCE, and hysteresis of
the material. This work demonstrates how to tune the properties, and shows that
the composition Fe_{0.83}Mn_{0.17}(P_{0.74}Ge_{0.26})
has a giant magnetocaloric effect (MCE) (35.5< MCE<46.5 J/kg-K) near room
temperature and in low magnetic fields (<1.2 T), and small thermal hysteresis
(<2 K). This material therefore is a good candidate for commercial
magnetic refrigeration.
This work has been submitted for publication.

Origin of
the Magnetocaloric and Magnetoelastic Properties of the Magnetic Refrigerant
Mn_{1.1}Fe_{0.9}P_{0.8}Ge_{0.2}, Danmin
Liu, Ming Yue, Tyrel M. McQueen, Jeffrey W. Lynn, Xiaolu Wang, Ying Chen,
Jiying Li, Robert J. Cava, Xubo Liu, Zaven Altounian, and Qing-zhen Huang,
*Phys. Rev.* B** 79**, 014435 (2009).

**Temperature,
Magnetic Field, and Pressure Dependence of the Crystal and Magnetic
Structure of the Magnetocaloric Compound Mn _{1.1}Fe_{0.9}P_{0.8}Ge**

Neutron Diffraction Study of the Mn_{1.1}Fe_{0.9}P_{0.76}Ge_{0.24}
Compound, L. J. Liu, D. M. Liu, Q. Z. Huang, M. Yue, J. W. Lynn, J. X.
Zhang,
Powder Diffraction
25, 525 (2010).

**Tuning the range, magnitude, and sign of the
thermal expansion in intermetallic Mn**_{3}**(Zn/M)N,
M=Ag, Ge, and Co**, Cong Wang, Lihua Chu, Qingrong Yao,^{ }
Ying Sun, Meimei Wu, Lei Ding, Jun Yan, Yuanyuan Na, Weihua Tang, Guannan
Li, Qingzhen Huang, and Jeffrey W. Lynn, *
Phys. Rev.* B

**Structure Evolution and Entropy Change of
Temperature and Magnetic Field Induced Magneto-structural Transition in the
Mn**_{1.1}**Fe**_{0.9}P_{0.76}**Ge**_{0.24}, Ming Xue, Danmin Liu, Qingzhen Huang, Tong Wang, Fengxia
Hu, Jingbo Li, Guanghui Rao, Baogen Shen, Jeffrey W. Lynn, and Jiuxing
Zhang,* J. Appl. Phys*.
**113**, 043925 (2013).

**Magnetic Structure and Lattice Contraction in Mn**_{3}**NiN**,
Meimei Wu, Cong Wang, Ying Sun, Lihua Chu, Jun Yan, Dongfeng Chen, Qingzhen
Huang, and Jeffrey Lynn, *J. Appl. Phys.*
**114**, 123902 (2013).

**The effect of Al doping on the crystal
structure and magnetocaloric behavior of Mn**_{1.2}**Fe**_{0.8}P_{1-x}**Ge**_{x}**
compounds**, D.M. Liu, H.Zhang, S.B.Wang, W.Q. Xiao, Z.L. Zhang,
N.Tian, C.X.Liu, M .Yue, Q. Z.Huang, J. X. Zhang, and J. W. Lynn,
*J. Alloys
& Compounds* **633**, 120 (2015).

**A pathway to optimize the properties of magnetocaloric Mn**_{x}**Fe**_{2-x}**(P**_{1-y}**Ge**_{y}**)
for magnetic refrigeration**,
D. M. Liu, Z. L.Zhang, S. L. Zhou, Q. Z. Huang, X. J.Deng, M. Yue, C. X.
Liu, J. X. Zhang, and J. W. Lynn,
*J. Alloys &
Compounds* **666**, 108 (2016).

**Floating zone growth of **
**a****-Na**_{0.90}**MnO**_{2}**
single crystals**, Rebecca Dally,
Raphaële J. Clémentc,
Robin
Chisnell, Stephanie Taylor, Megan Butala, Vicky Doan-Nguyen, Mahalingam
Balasubramanian, Jeffrey W. Lynn, Clare P. Grey, and Stephen D. Wilson,
(submitted).

**
NIST Tech Beat**

http://www.nist.gov/ncnr/refrigeration_012709.cfm