Ever since 2008, I have been considering isotopes, and how to use them to improve processes and devices. It's been a lonely task, because virtually every chemist or physicist views "isotopes" as merely atoms with a different number of neutrons, and thus a different atomic weight. Yes, they are indeed that, but they are so much more.
...I've long been aware that elemental (stable) magnesium isn't merely "magnesium". Magnesium in nature consists of 78.99% Mg-24 isotope, 11.01% Mg-26, and 10.00% Mg-25. https://www.webelements.com/magnesium/isotopes.html
Moreover, I was well aware that it was only the Mg-25 isotope whose nucleus posses 'nuclear spin': Mg-24 and Mg-26 have both an even number of protons, and an even number of neutrons. But Mg-25 is different: its nucleus contains an odd (not even) number of neutrons, and so it has a slight 'wobble'. The unpaired neutron can be thought as orbiting around the positively-charged rest of the nucleus, so that rest of the nucleus behaves like a positive electric charge, itself spinning around the center-of-mass of the whole structure. And as every physicist should know (my degree is in Chemistry, from MIT), a charge travelling in a circle causes a magnetic dipole to exist.
From reading Mehta's description, I concluded that the problem is that not all of the magnesium 'worked'. I'll let you guess which one did. It seems fairly obvious to me.
Zinc, similarly, is made up of isotopes. https://www.webelements.com/zinc/isotopes.html Only 4.1% of natural, stable zinc is Zn-67 and it has a nuclear 'spin'. The rest is Zn-64, Zn-66, Zn-68, and Zn-70, and none of them have nuclear 'spin'. And I notice that some early work on GaN LEDs used Zinc as a p+ dopant. It worked, I suppose, but somehow it was abandoned early on, since magnesium worked better. Why? Could that be because 10% is greater than 4.1% ? Well, THAT can be fixed!
Doing some more research, I also notice that the radius of gallium atoms is 130 picometers. https://www.webelements.com/gallium/index.html The radius of zinc atoms is 135 picometers. https://www.webelements.com/zinc/ And the radius of magnesium atoms is 150 picometers. https://www.webelements.com/magnesium/ So I can certainly understand the difficulty they had packing a 150 picometer-radius magnesium atom into a position for suitable for a 130 picometer gallium atom. They must have used a shoe-horn to pack the magnesium into the spot! Zinc's 135 picometers looks far more easily matched!
Merry Christmas. And you're welcome!